Electrochemically stable elastomer-encapsulated particles of anode active materials for lithium batteries
11005094 · 2021-05-11
Assignee
Inventors
Cpc classification
H01M4/62
ELECTRICITY
H01M4/485
ELECTRICITY
H01M12/08
ELECTRICITY
H01M4/505
ELECTRICITY
Y02E60/10
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
H01M10/0525
ELECTRICITY
International classification
H01M10/0525
ELECTRICITY
H01M4/485
ELECTRICITY
H01M4/505
ELECTRICITY
H01M4/58
ELECTRICITY
H01M4/36
ELECTRICITY
H01M4/62
ELECTRICITY
Abstract
Provided is a lithium battery anode electrode comprising multiple particulates of an anode active material, wherein at least a particulate is composed of one or a plurality of particles of an anode active material being encapsulated by a thin layer of inorganic filler-reinforced elastomer having from 0.01% to 50% by weight of an inorganic filler dispersed in an elastomeric matrix material based on the total weight of the inorganic filler-reinforced elastomer, wherein the encapsulating thin layer of inorganic filler-reinforced elastomer has a thickness from 1 nm to 10 μm, a fully recoverable tensile strain from 2% to 500%, and a lithium ion conductivity from 10.sup.−7 to S/cm to 5×10.sup.−2 S/cm and the inorganic filler has a lithium intercalation potential from 1.1 V to 4.5 V (preferably 1.2-2.5 V) versus Li/Li.sup.+. The anode active material is preferably selected from Si, Ge, Sn, SnO.sub.2, SiO.sub.x, Co.sub.3O.sub.4, Mn.sub.3O.sub.4, etc.
Claims
1. An anode electrode for a lithium battery, said electrode comprising multiple particulates of an anode active material, wherein at least a particulate is composed of one or more particles of said anode active material being encapsulated by a thin layer of inorganic filler-reinforced elastomer having from 0.01% to 50% by weight of an inorganic filler dispersed in an elastomeric matrix material based on the total weight of the inorganic filler-reinforced elastomer, wherein said encapsulating thin layer of inorganic filler-reinforced elastomer has a thickness from 1 nm to 10 μm, a fully recoverable tensile strain from 2% to 500%, and a lithium ion conductivity from 10.sup.−7 S/cm to 5×10.sup.−2 S/cm and said inorganic filler has a lithium intercalation potential from 1.1 V to 4.5 V versus Li/Li.sup.+.
2. The anode electrode of claim 1, wherein said inorganic filler is selected from an oxide, carbide, boride, nitride, sulfide, phosphide, or selenide of a transition metal, a lithiated version thereof, and combinations thereof.
3. The anode electrode of claim 2, wherein said transition metal includes Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Pd, Ag, Cd, La, Ta, W, Pt, Au, Hg, combinations thereof, or combinations thereof with Al, Ga, In, Sn, Pb, Sb, or Bi.
4. The anode electrode of claim 1, wherein said inorganic filler is nanodiscs, nanoplatelets, or nanosheets of (a) bismuth selenide or bismuth telluride, (b) transition metal dichalcogenide or trichalcogenide, (c) sulfide, selenide, or telluride of niobium, zirconium, molybdenum, hafnium, tantalum, tungsten, titanium, cobalt, nickel, manganese, or any transition metal; (d) boron nitride, or (e) combinations thereof, wherein said nanodiscs, nanoplatelets, or nanosheets have a thickness from 1 nm to 100 nm.
5. The anode electrode of claim 1, wherein said elastomeric matrix material contains a material selected from natural polyisoprene, synthetic polyisoprene, polybutadiene, chloroprene rubber, polychloroprene, butyl rubber, styrene-butadiene rubber, nitrile rubber, ethylene propylene rubber, ethylene propylene diene rubber, metallocene-based poly(ethylene,-co-octene) elastomer, poly(ethylene-co-butene) elastomer, styrene-ethylene-butadiene-styrene elastomer, epichlorohydrin rubber, polyacrylic rubber, silicone rubber, fluorosilicone rubber, perfluoroelastomers, polyether block amides, chlorosulfonated polyethylene, ethylene-vinyl acetate, thermoplastic elastomer, protein resilin, protein elastin, ethylene oxide-epichlorohydrin copolymer, polyurethane, urethane-urea copolymer, sulfonated versions thereof, and combinations thereof.
6. The anode electrode of claim 1, wherein said inorganic filler-reinforced elastomer further contains an electron-conducting filler dispersed in said elastomer matrix material wherein said electron-conducting filler is selected from a carbon nanotube, carbon nanofiber, carbon nanoparticle, metal nanoparticle, metal nanowire, electron-conducting polymer, graphene, or a combination thereof, wherein said graphene is selected from pristine graphene, graphene oxide, reduced graphene oxide, graphene fluoride, graphene chloride, nitrogenated graphene, hydrogenated graphene, doped graphene, functionalized graphene, or a combination thereof and said graphene comprise single-layer graphene or few-layer graphene, wherein said few-layer graphene is defined as a graphene platelet formed of less than 10 graphene planes.
7. The anode electrode of claim 6, wherein said electron-conducting polymer selected from polyaniline, polypyrrole, polythiophene, polyfuran, a bi-cyclic polymer, a sulfonated derivative thereof, and combinations thereof.
8. The anode electrode of claim 1, wherein said anode active material is (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), nickel (Ni), cobalt (Co), and cadmium (Cd); (b) alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, or Cd with other elements; (c) oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Fe, Ni, Co, V, or Cd, and their mixtures, composites, or lithium-containing composites; (d) salts and hydroxides of Sn; (e) lithium titanate, lithium manganate, lithium aluminate, lithium-containing titanium oxide, lithium transition metal oxide; (f) prelithiated versions thereof; (g) particles of Li, Li alloy, or surface-stabilized Li having at least 60% by weight of lithium element therein; or (h) combinations thereof.
9. The anode electrode of claim 8, wherein said Li alloy contains from 0.1% to 10% by weight of a metal element selected from Zn, Ag, Au, Mg, Ni, Ti, Fe, Co, V, and combinations thereof.
10. The anode electrode of claim 1, wherein said anode active material contains a prelithiated Si, prelithiated Ge, prelithiated Sn, prelithiated SnO.sub.x, prelithiated SiO.sub.x, prelithiated iron oxide, prelithiated VO.sub.2, prelithiated Co.sub.3O.sub.4, prelithiated Ni.sub.3O.sub.4, lithium titanate, and combinations thereof, wherein 1≤x≤2.
11. The anode electrode of claim 1, wherein said anode active material is in a form of nanoparticle, nanowire, nanofiber, nanotube, nanosheet, nanoplatelet, nanodisc, nanobelt, nanoribbon, or nanohorn having a thickness or diameter from 0.5 nm to 100 nm.
12. The anode electrode of claim 1, wherein said one or more particles is coated with a layer of carbon prior to being encapsulated.
13. The anode electrode of claim 1, wherein said particulate further comprises a carbon material encapsulated therein.
14. The anode electrode of claim 13, wherein said carbon material is selected from polymeric carbon, amorphous carbon, chemical vapor deposition carbon, coal tar pitch, petroleum pitch, mesophase pitch, carbon black, coke, acetylene black, activated carbon, fine expanded graphite particle with a dimension smaller than 100 nm, artificial graphite particle, natural graphite particle, and combinations thereof.
15. The anode electrode of claim 11, wherein said nanoparticle, nanowire, nanofiber, nanotube, nanosheet, nanoplatelet, nanodisc, nanobelt, nanoribbon, or nanohorn is pre-intercalated or pre-doped with lithium ions to form a prelithiated anode active material having an amount of lithium from 0.1% to 54.7% by weight of said prelithiated anode active material.
16. The anode electrode of claim 1, wherein said inorganic filler is in a form of nanoparticle, nanowire, nanofiber, nanotube, nanosheet, nanoplatelet, nanodisc, nanobelt, nanoribbon, or nanohorn.
17. The anode electrode of claim 1, wherein said elastomeric matrix material further contains from 0.1% to 40% by weight of a lithium ion-conducting additive dispersed in said elastomeric matrix material.
18. The anode electrode of claim 17, wherein said lithium ion-conducting additive is selected from Li.sub.2CO.sub.3, Li.sub.2O, Li.sub.2C.sub.2O.sub.4, LiOH, LiX, ROCO.sub.2Li, HCOLi, ROLi, (ROCO.sub.2Li).sub.2, (CH.sub.2OCO.sub.2Li).sub.2, Li.sub.2S, Li.sub.xSO.sub.y, and combinations thereof, wherein X=F, Cl, I, or Br, R=a hydrocarbon group, and 0≤x≤1, 1≤y≤4.
19. The anode electrode of claim 17, wherein said lithium ion-conducting additive contains a lithium salt selected from lithium perchlorate (LiClO.sub.4), lithium hexafluorophosphate (LiPF.sub.6), lithium borofluoride (LiBF.sub.4), lithium hexafluoroarsenide (LiAsF.sub.6), lithium trifluoro-metasulfonate, (LiCF.sub.3SO.sub.3), bis-trifluoromethyl sulfonylimide lithium (LiN(CF.sub.3SO.sub.2).sub.2), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF.sub.2C.sub.2O.sub.4), lithium oxalyldifluoroborate (LiBF.sub.2C.sub.2O.sub.4), lithium nitrate (LiNO.sub.3), Li-fluoroalkyl-phosphates, (LiPF.sub.3(CF.sub.2CF.sub.3).sub.3), lithium bisperfluoro-ethysulfonylimide (LiBETI), lithium bis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), ionic liquid-based lithium salts, and combinations thereof.
20. The anode electrode of claim 17, wherein said lithium ion-conducting additive contains a lithium ion-conducting polymer selected from poly(ethylene oxide) (PEO), polypropylene oxide (PPO(Original)), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVDF), poly bis-methoxy ethoxyethoxide-phosphazene, polyvinyl chloride, polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a sulfonated derivative thereof, and combinations thereof.
21. A powder mass of an anode active material for a lithium battery anode electrode, said powder mass comprising multiple particulates of an anode active material, wherein at least one particulate is comprised of one or a plurality of particles of said anode active material being encapsulated by a thin layer of inorganic filler-reinforced elastomer having from 0.01% to 50% by weight of an inorganic filler dispersed in an elastomeric matrix material based on the total weight of the inorganic filler-reinforced elastomer, wherein said encapsulating thin layer of inorganic filler-reinforced elastomer has a thickness from 1 nm to 10 μm, a fully recoverable tensile strain from 2% to 500%, and a lithium ion conductivity from 10.sup.−7 S/cm to 5×10.sup.−2 S/cm and said inorganic filler has a lithium intercalation potential from 1.1 V to 4.5 V versus Li/Li.sup.+.
22. The powder mass of claim 21, wherein said inorganic filler is selected from oxides, carbides, borides, nitrides, sulfides, phosphides, or selenides of a transition metal, lithiated versions thereof, and combinations thereof.
23. The powder mass of claim 22, wherein said transition metal includes Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Pd, Ag, Cd, La, Ta, W, Pt, Au, Hg, combinations thereof, or combinations thereof with Al, Ga, In, Sn, Pb, Sb, or Bi.
24. The powder mass of claim 21, wherein said inorganic filler is nanodiscs, nanoplatelets, or nanosheets of (a) bismuth selenide or bismuth telluride, (b) transition metal dichalcogenides or trichalcogenides, (c) sulfide, selenide, or telluride of niobium, zirconium, molybdenum, hafnium, tantalum, tungsten, titanium, cobalt, nickel, manganese, or any other transition metal; (d) boron nitride, or (e) combinations thereof, wherein said nanodiscs, nanoplatelets, or nanosheets have a thickness from 1 nm to 100 nm.
25. The powder mass of claim 21, wherein said elastomeric matrix material contains a material selected from natural polyisoprene, synthetic polyisoprene, polybutadiene, chloroprene rubber, polychloroprene, butyl rubber, styrene-butadiene rubber, nitrile rubber, ethylene propylene rubber, ethylene propylene diene rubber, metallocene-based poly(ethylene-co-octene) elastomer, poly(ethylene-co-butene) elastomer, styrene-ethylene-butadiene-styrene elastomer, epichlorohydrin rubber, polyacrylic rubber, silicone rubber, fluorosilicone rubber, perfluoroelastomers, polyether block amides, chlorosulfonated polyethylene, ethylene-vinyl acetate, thermoplastic elastomer, protein resilin, protein elastin, ethylene oxide-epichlorohydrin copolymer, polyurethane, urethane-urea copolymer, a sulfonated version thereof, or a combination thereof.
26. The powder mass of claim 21, wherein said inorganic filler-reinforced elastomer further contains an electron-conducting filler dispersed in said elastomer matrix material wherein said electron-conducting filler is selected from a carbon nanotube, carbon nanofiber, carbon nanoparticle, metal nanoparticle, metal nanowire, electron-conducting polymer, graphene, or a combination thereof, wherein said graphene is selected from pristine graphene, graphene oxide, reduced graphene oxide, graphene fluoride, graphene chloride, nitrogenated graphene, hydrogenated graphene, doped graphene, functionalized graphene, or a combination thereof and said graphene comprises single-layer graphene or few-layer graphene, wherein said few-layer graphene is defined as a graphene platelet formed of less than 10 graphene planes.
27. The powder mass of claim 21, wherein said anode active material is (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), nickel (Ni), cobalt (Co), and cadmium (Cd); (b) alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, or Cd with other elements; (c) oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Fe, Ni, Co, V, or Cd, and their mixtures, composites, or lithium-containing composites; (d) salts and hydroxides of Sn; (e) lithium titanate, lithium manganate, lithium aluminate, lithium-containing titanium oxide, lithium transition metal oxide; (f) prelithiated versions thereof; (g) particles of Li, Li alloy, or surface-stabilized Li having at least 60% by weight of lithium element therein; or (h) combinations thereof.
28. The powder mass of claim 21, further comprising graphite particles, carbon particles, mesophase microbeads, carbon or graphite fibers, carbon nanotubes, graphene sheets, and combinations thereof mixed with said multiple particulates.
29. The powder mass of claim 21, wherein said elastomeric matrix material further contains from 0.1% to 40% by weight of a lithium ion-conducting additive dispersed in said elastomeric matrix material.
30. The powder mass of claim 29, wherein said lithium ion-conducting additive is selected from Li.sub.2CO.sub.3, Li.sub.2O, Li.sub.2C.sub.2O.sub.4, LiOH, LiX, ROCO.sub.2Li, HCOLi, ROLi, (ROCO.sub.2Li).sub.2, (CH.sub.2OCO.sub.2Li).sub.2, Li.sub.2S, Li.sub.xSO.sub.y, and combinations thereof, wherein X=F, Cl, I, or Br, R=a hydrocarbon group, and 0≤x≤1, 1≤y≤4.
31. The powder mass of claim 29, wherein said lithium ion-conducting additive contains a lithium salt selected from lithium perchlorate (LiClO.sub.4), lithium hexafluorophosphate (LiPF.sub.6), lithium borofluoride (LiBF.sub.4), lithium hexafluoroarsenide (LiAsF.sub.6), lithium trifluoro-metasulfonate, (LiCF.sub.3SO.sub.3), bis-trifluoromethyl sulfonylimide lithium (LiN(CF.sub.3SO.sub.2).sub.2), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF.sub.2C.sub.2O.sub.4), lithium oxalyldifluoroborate (LiBF.sub.2C.sub.2O.sub.4), lithium nitrate (LiNO.sub.3), Li-fluoroalkyl-phosphates, (LiPF.sub.3(CF.sub.2CF.sub.3).sub.3), lithium bisperfluoro-ethysulfonylimide (LiBETI), lithium bis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), ionic liquid-based lithium salts, and combinations thereof.
32. The powder mass of claim 21, wherein said anode active material is prelithiated to contain from 0.1% to 54.7% by weight of lithium.
33. The powder mass of claim 26, wherein said electron-conducting polymer selected from polyaniline, polypyrrole, polythiophene, polyfuran, a bi-cyclic polymer, a sulfonated derivative thereof, or a combination thereof.
34. A lithium battery containing an optional anode current collector, the anode electrode as defined in claim 1, a cathode active material layer, an optional cathode current collector, an electrolyte in ionic contact with said anode active material layer and said cathode active material layer, and an optional porous separator disposed between said anode active material layer and said cathode active material layer.
35. The lithium battery of claim 34, which is a lithium-ion battery, lithium metal battery, lithium-sulfur battery, lithium-selenium battery, or lithium-air battery.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
(12) The following includes definitions of various terms and phrases used throughout this specification.
(13) The term “graphene sheets” means a material comprising one or more planar sheets of bonded carbon atoms that are densely packed in a hexagonal crystal lattice in which carbon atoms are bonded together through strong in-plane covalent bonds, and further containing an intact ring structure throughout a majority of the interior. Preferably at least 80% of the interior aromatic bonds are intact. In the c-axis (thickness) direction, these graphene planes may be weakly bonded together through van der Waals forces. Graphene sheets may contain non-carbon atoms at their edges or surface, for example OH and COOH functionalities. The term graphene sheets includes pristine graphene, graphene oxide, reduced graphene oxide, halogenated graphene including graphene fluoride and graphene chloride, nitrogenated graphene, hydrogenated graphene, doped graphene, functionalized graphene, and combinations thereof. Typically, non-carbon elements comprise 0 to 25 weight % of graphene sheets. Graphene oxide may comprise up to 53% oxygen by weight. Graphene sheets may comprise single-layer graphene or few-layer graphene, wherein the few-layer graphene is defined as a graphene platelet formed of less than 10 graphene planes. Graphene sheets may also comprise graphene nanoribbons. “Pristine graphene” means a graphene sheet having substantially no non-carbon elements. “Nanographene platelet” (NGP) refers to a graphene sheet having a thickness from less than 0.34 nm (single layer) to 100 nm (multi-layer).
(14) The term “substantially” and its variations are defined as being largely but not necessarily wholly what is specified as understood by one of ordinary skill in the art, and in one non-limiting embodiment substantially refers to ranges within 10%, within 5%, within 1%, or within 0.5% of a referenced range.
(15) Other objects, features and advantages of the present disclosure may become apparent from the following figures, description, and examples. It should be understood, however, that the figures, description, and examples, while indicating specific embodiments of the disclosure, are given by way of illustration only and are not meant to be limiting. In further embodiments, features from specific embodiments may be combined with features from other embodiments. This disclosure is directed at the anode active material layer (negative electrode layer or anode, not including the anode current collector) containing a high-capacity anode material for a lithium secondary battery, which is preferably a secondary battery based on a non-aqueous electrolyte, a polymer gel electrolyte, an ionic liquid electrolyte, a quasi-solid electrolyte, or a solid-state electrolyte. The shape of a lithium secondary battery can be cylindrical, square, button-like, etc. The present disclosure is not limited to any battery shape or configuration. For convenience, we will primarily use Si, Sn, and SnO.sub.2 as illustrative examples of a high-capacity anode active material. This should not be construed as limiting the scope of the disclosure.
(16) As illustrated in
(17) In a less commonly used cell configuration, as illustrated in
(18) In order to obtain a higher energy density cell, the anode in
(19) In other words, there are several conflicting factors that must be considered concurrently when it comes to the design and selection of an anode active material in terms of material type, shape, size, porosity, and electrode layer thickness. Thus far, there has been no effective solution offered by any prior art teaching to these conflicting problems. We have solved these challenging issues that have troubled battery designers and electrochemists alike for more than 30 years by developing the elastomer-protected anode active material.
(20) The present disclosure provides an anode electrode comprising multiple particulates of an anode active material (plus an optional resin binder and/or an optional conductive additive), wherein at least a particulate is composed of one or a plurality of particles of an anode active material being encapsulated by a thin layer of inorganic filler-reinforced elastomer (the encapsulating shell) that has thickness from 1 nm to 10 μm. Preferably, the inorganic filler-reinforced elastomer has a fully recoverable tensile strain from 2% to 500% (more typically from 5% to 300% and most typically from 10% to 150%), a thickness from 1 nm to 10 μm (preferably less than 100 nm and most preferably <10 nm), and a lithium ion conductivity from 10.sup.−7 S/cm to 10.sup.−2 S/cm (more typically from 10.sup.−5 S/cm to 10.sup.−3 S/cm). When an electron-conducting additive is dispersed in the elastomer matrix material, the filled elastomer has an electrical conductivity from 10.sup.−7 S/cm to 100 S/cm (more typically from 10.sup.−3 S/cm to 10 S/cm) when measured at room temperature on a separate cast thin film 20 μm thick. Preferably, the anode active material is a high-capacity anode active material having a specific lithium storage capacity greater than 372 mAh/g (which is the theoretical capacity of graphite).
(21) In certain embodiments, the inorganic filler is selected from an oxide, carbide, boride, nitride, sulfide, phosphide, or selenide of a transition metal, a lithiated version thereof, or a combination thereof. Preferably, the transition metal is selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Pd, Ag, Cd, La, Ta, W, Pt, Au, Hg, a combination thereof, or a combination thereof with Al, Ga, In, Sn, Pb, Sb, or Bi.
(22) These inorganic fillers are preferably selected to have an intercalation potential (the electrochemical potential at which lithium intercalates into these materials) higher than the intercalation potential of the anode active material encapsulated in the particulate. For instance, lithium intercalates into Si at approximately 0.4-0.5 V (vs. Li/Li.sup.+) and the intercalation potential of lithium titanate (Li.sub.4Ti.sub.5O.sub.12) is 1.1-1.5 V. The lithium titanate may be considered as a lithiated version of titanium oxide (TiO.sub.2), which has a lithium intercalation potential >2.5 V. The inorganic filler must have a lithium intercalation potential higher than 1.1 V versus Li/Li.sup.+, preferably higher than 1.2 V, more preferably higher than 1.4 V, and most preferably higher than 1.5 V. These electrochemical potential conditions are found to be surprisingly capable of avoiding the formation of SEI on/in the encapsulating shell and preventing repeated formation and breakage of SEI on anode active material particles, which otherwise would result in continued and rapid decay of battery capacity.
(23) Other examples of metal oxide are NbO.sub.2 and its lithiated version and titanium-niobium composite oxide (e.g. represented by a general formula TiNb.sub.2O.sub.7) and its lithiated versions. They typically have a lithium intercalation potential higher than 1.1 V versus Li/Li.sup.+.
(24) The niobium-containing composite metal oxide for use as an inorganic filler in the encapsulating shell may be selected from the group consisting of TiNb.sub.2O.sub.7, Li.sub.xTiNb.sub.2O.sub.7 (0≤x≤5), Li.sub.xM.sub.(1-y)Nb.sub.yNb.sub.2O.sub.(7+δ) (wherein 0≤x≤6, 0≤y≤1, −1≤δ≤1, and M=Ti or Zr), Ti.sub.xNb.sub.yO.sub.7 (0.5≤y/x<2.0), TiNb.sub.xO.sub.(2+5x/2) (1.9≤x<2.0), M.sub.xTi.sub.(1-2x)Nb.sub.(2+x)O.sub.(7+δ) (wherein 0≤x≤0.2, −0.3≤δ≤0.3, and M=a trivalent metal selected from Fe, Ga, Mo, Ta, V, Al, B, and a mixture thereof), M.sub.xTi.sub.(2-2x)Nb.sub.(10+x)O.sub.(29+δ) (wherein 0≤x≤0.4, −0.3≤δ≤0.3, and M=a trivalent metal selected from Fe, Ga, Mo, Al, B, and a mixture thereof), M.sub.xTiNb.sub.2O.sub.7 (x<0.5, and M=B, Na, Mg, Al, Si, S, P, K, Ca, Mo, W, Cr, Mn, Co, Ni, and Fe), TiNb.sub.2-xTa.sub.xO.sub.y (0≤x<2, 7≤y≤10), Ti.sub.2Nb.sub.10-vTa.sub.vO.sub.w (0≤v<2, 27≤y≤29), Li.sub.xTi.sub.(1-y)M1.sub.yNb.sub.(2-x)M2.sub.zO.sub.(7+δ) (wherein 0≤x≤5, 0≤y≤1, 0≤z≤2, −0.3≤δ≤0.3, M1=Zr, Si, and Sn, and M2=V, Ta, and Bi), P-doped versions thereof, B-doped versions thereof, carbon-coated versions thereof, and combinations thereof. In such a niobium-containing composite metal oxide, niobium oxide typically forms the main framework or backbone of the crystal structure, along with at least a transition metal oxide.
(25) Transition metal oxide is but one of the suitable inorganic filler materials. The inorganic filler may be selected from an oxide, carbide, boride, nitride, sulfide, phosphide, or selenide of a transition metal, a lithiated version thereof, or a combination thereof. Preferably, these and other inorganic fillers are in a form of nanoparticle, nanowire, nanofiber, nanotube, nanosheet, nanobelt, nanoribbon, nanodisc, nanoplatelet, or nanohorn having a dimension (diameter, thickness, or width, etc.) less than 100 nm, preferably less than 10 nm. These inorganic filler materials typically have a lithium intercalation potential from 1.1 V to 3.5 V versus Li/Li.sup.+, and more typically and preferably from 1.1 V to 2.5 V, and most preferably from 1.1 V to 1.5 V. The lithium intercalation potential of a filler dispersed in the elastomeric matrix material must be higher than the lithium intercalation potential of the anode active material encapsulated by the filled elastomer.
(26) The inorganic filler material for reinforcing an elastomer matrix material may also be selected from nanodiscs, nanoplatelets, or nanosheets (having a thickness from 1 nm to 100 nm) of: (a) bismuth selenide or bismuth telluride, (b) transition metal dichalcogenide or trichalcogenide, (c) sulfide, selenide, or telluride of niobium, zirconium, molybdenum, hafnium, tantalum, tungsten, titanium, cobalt, nickel, manganese, or any transition metal; (d) boron nitride, or (e) a combination thereof. These nanodiscs, nanoplatelets, or nanosheets preferably have a thickness less than 20 nm, more preferably from 1 nm to 10 nm.
(27) In certain preferred embodiments, the inorganic filler-reinforced elastomer further contains an electron-conducting filler dispersed in the elastomer matrix material wherein the electron-conducting filler is selected from a carbon nanotube, carbon nanofiber, nanocarbon particle, metal nanoparticle, metal nanowire, electron-conducting polymer, graphene, or a combination thereof. The graphene may be preferably selected from pristine graphene, graphene oxide, reduced graphene oxide, graphene fluoride, graphene chloride, nitrogenated graphene, hydrogenated graphene, doped graphene, functionalized graphene, or a combination thereof and the graphene preferably comprises single-layer graphene or few-layer graphene, wherein the few-layer graphene is defined as a graphene platelet formed of 2-10 graphene planes. More preferably, the graphene sheets contain 1-5 graphene planes, most preferably 1-3 graphene planes (i.e. single-layer, double-layer, or triple-layer graphene). The electron-conducting polymer is preferably selected from (but not limited to) polyaniline, polypyrrole, polythiophene, polyfuran, a bi-cyclic polymer, a sulfonated derivative thereof, or a combination thereof.
(28) As illustrated in
(29) As schematically illustrated in the upper portion of
(30) Alternatively, referring to the lower portion of
(31) The anode active material may be selected from the group consisting of: (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), nickel (Ni), cobalt (Co), and cadmium (Cd); (b) alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, or Cd with other elements; (c) oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Fe, Ni, Co, V, or Cd, and their mixtures, composites, or lithium-containing composites; (d) salts and hydroxides of Sn; (e) lithium titanate, lithium manganate, lithium aluminate, lithium-containing titanium oxide, lithium transition metal oxide; (f) prelithiated versions thereof; (g) particles of Li, Li alloy, or surface-stabilized Li; and (h) combinations thereof. Particles of Li or Li alloy (Li alloy containing from 0.1% to 10% by weight of Zn, Ag, Au, Mg, Ni, Ti, Fe, Co, or V element), particularly surface-stabilized Li particles (e.g. wax-coated Li particles), were found to be good anode active material per se or an extra lithium source to compensate for the loss of Li ions that are otherwise supplied only from the cathode active material. The presence of these Li or Li-alloy particles encapsulated inside an elastomeric shell was found to significantly improve the cycling performance of a lithium cell.
(32) Prelithiation of an anode active material can be conducted by several methods (chemical intercalation, ion implementation, and electrochemical intercalation). Among these, the electrochemical intercalation is the most effective. Lithium ions can be intercalated into non-Li elements (e.g. Si, Ge, and Sn) and compounds (e.g. SnO.sub.2 and Co.sub.3O.sub.4) up to a weight percentage of 54.68% (see Table 1 below). For Zn, Mg, Ag, and Au encapsulated inside an elastomer shell, the amount of Li can reach 99% by weight.
(33) TABLE-US-00001 TABLE 1 Lithium storage capacity of selected non-Li elements. Intercalated Atomic weight Atomic weight of Max. wt. % compound of Li, g/mole active material, g/mole of Li Li.sub.4Si 6.941 28.086 49.71 Li.sub.4.4Si 6.941 28.086 54.68 Li.sub.4.4Ge 6.941 72.61 30.43 Li4.4Sn 6.941 118.71 20.85 Li.sub.3Cd 6.941 112.411 14.86 Li.sub.3Sb 6.941 121.76 13.93 Li.sub.4.4Pb 6.941 207.2 13.00 LiZn 6.941 65.39 7.45 Li.sub.3Bi 6.941 208.98 8.80
(34) The particles of the anode active material may be in the form of a nanoparticle, nanowire, nanofiber, nanotube, nanosheet, nanoplatelet, nanodisc, nanobelt, nanoribbon, or nanohorn. They can be non-lithiated (when incorporated into the anode active material layer) or prelithiated to a desired extent (up to the maximum capacity as allowed for a specific element or compound.
(35) Preferably and typically, the inorganic filler reinforced elastomer has a lithium ion conductivity from 10.sup.−7 S/cm to 5×10.sup.−2 S/cm, more preferably and typically greater than 10.sup.−5 S/cm, further more preferably and typically greater than 10.sup.−4 S/cm, and most preferably no less than 10.sup.−3 S/cm. In some embodiments, the composite further contains from 0.1% to 40% (preferably 1% to 35%) by weight of a lithium ion-conducting additive dispersed in an elastomer matrix material.
(36) The inorganic filler-reinforced elastomer must have a high elasticity (high elastic deformation value). By definition, an elastic deformation is a deformation that is fully recoverable upon release of the mechanical stress and the recovery process is essentially instantaneous (no significant time delay). An elastomer, such as a vulcanized natural rubber, can exhibit a tensile elastic deformation from 2% up to 1,000% (10 times of its original length). Sulfonation of the rubber reduces the elasticity to 800%. With the addition of 0.01%-50% of inorganic filler particles and/or conductive filler (e.g. CNT and graphene sheets), the tensile elastic deformation of a sulfonated elastomer/rubber is reduced to typically from 2% to 500%. It may be noted that although a metal typically has a high ductility (i.e. can be extended to a large extent without breakage), the majority of the deformation is plastic deformation (non-recoverable) and elastic deformation occurs to only a small extent (typically <1% and more typically <0.2%).
(37) A broad array of inorganic filler reinforced elastomers can be used to encapsulate an anode active material particle or multiple particles. Encapsulation means substantially fully embracing the particle(s) without allowing the particle to be in direct contact with electrolyte in the battery. The elastomeric matrix material may be selected from a sulfonated or non-sulfonated version of natural polyisoprene (e.g. cis-1,4-polyisoprene natural rubber (NR) and trans-1,4-polyisoprene gutta-percha), synthetic polyisoprene (IR for isoprene rubber), polybutadiene (BR for butadiene rubber), chloroprene rubber (CR), polychloroprene (e.g. Neoprene, Baypren etc.), butyl rubber (copolymer of isobutylene and isoprene, IIR), including halogenated butyl rubbers (chloro butyl rubber (CIIR) and bromo butyl rubber (BIIR), styrene-butadiene rubber (copolymer of styrene and butadiene, SBR), nitrile rubber (copolymer of butadiene and acrylonitrile, NBR), EPM (ethylene propylene rubber, a copolymer of ethylene and propylene), EPDM rubber (ethylene propylene diene rubber, a terpolymer of ethylene, propylene and a diene-component), metallocene-based poly(ethylene-co-octene) (POE) elastomer, poly(ethylene-co-butene) (ME) elastomer, styrene-ethylene-butadiene-styrene (SEBS) elastomer, epichlorohydrin rubber (ECO), polyacrylic rubber (ACM, ABR), silicone rubber (SI, Q, VMQ), fluorosilicone rubber (FVMQ), fluoroelastomers (FKM, and FEPM; such as Viton, Tecnoflon, Fluorel, Aflas and Dai-El), perfluoroelastomers (FFKM: Tecnoflon PFR, Kalrez, Chemraz, Perlast), polyether block amides (PEBA), chlorosulfonated polyethylene (CSM; e.g. Hypalon), and ethylene-vinyl acetate (EVA), thermoplastic elastomers (TPE), protein resilin, protein elastin, ethylene oxide-epichlorohydrin copolymer, polyurethane, urethane-urea copolymer, and combinations thereof.
(38) The urethane-urea copolymer film usually consists of two types of domains, soft domains and hard ones. Entangled linear backbone chains consisting of poly(tetramethylene ether) glycol (PTMEG) units constitute the soft domains, while repeated methylene diphenyl diisocyanate (MDI) and ethylene diamine (EDA) units constitute the hard domains. The lithium ion-conducting additive can be incorporated in the soft domains or other more amorphous zones.
(39) The electron-conducting filler may be selected from a carbon nanotube (CNT), carbon nano-fiber, graphene, nanocarbon particles, metal nanowires, etc. A graphene sheet or nanographene platelet (NGP) composed of one basal plane (graphene plane) or multiple basal planes stacked together in the thickness direction. In a graphene plane, carbon atoms occupy a 2-D hexagonal lattice in which carbon atoms are bonded together through strong in-plane covalent bonds. In the c-axis or thickness direction, these graphene planes may be weakly bonded together through van der Waals forces. An NGP can have a platelet thickness from less than 0.34 nm (single layer) to 100 nm (multi-layer). For the present electrode use, the preferred thickness is <10 nm, more preferably <3 nm (or <10 layers), and most preferably single-layer graphene. Thus, the presently disclosed sulfonated elastomer/graphene composite shell preferably contains mostly single-layer graphene, but could make use of some few-layer graphene (less than 10 layers or 10 graphene planes). The graphene sheet may contain a small amount (typically <25% by weight) of non-carbon elements, such as hydrogen, nitrogen, fluorine, and oxygen, which are attached to an edge or surface of the graphene plane.
(40) Graphene sheets may be oxidized to various extents during their preparation, resulting in graphite oxide (GO) or graphene oxide. Hence, in the present context, graphene preferably or primarily refers to those graphene sheets containing no or low oxygen content; but, they can include GO of various oxygen contents. Further, graphene may be fluorinated to a controlled extent to obtain graphite fluoride, or can be doped using various dopants, such as boron and nitrogen.
(41) Graphite oxide may be prepared by dispersing or immersing a laminar graphite material (e.g., powder of natural flake graphite or synthetic graphite) in an oxidizing agent, typically a mixture of an intercalant (e.g., concentrated sulfuric acid) and an oxidant (e.g., nitric acid, hydrogen peroxide, sodium perchlorate, potassium permanganate) at a desired temperature (typically 0-70° C.) for a sufficient length of time (typically 30 minutes to 5 days). In order to reduce the time required to produce a precursor solution or suspension, one may choose to oxidize the graphite to some extent for a shorter period of time (e.g., 30 minutes) to obtain graphite intercalation compound (GIC). The GIC particles are then exposed to a thermal shock, preferably in a temperature range of 600-1,100° C. for typically 15 to 60 seconds to obtain exfoliated graphite or graphite worms, which are optionally (but preferably) subjected to mechanical shearing (e.g. using a mechanical shearing machine or an ultrasonicator) to break up the graphite flakes that constitute a graphite worm. The un-broken graphite worms or individual graphite flakes are then re-dispersed in water, acid, or organic solvent and ultrasonicated to obtain a graphene polymer solution or suspension.
(42) The pristine graphene material is preferably produced by one of the following three processes: (A) Intercalating the graphitic material with a non-oxidizing agent, followed by a thermal or chemical exfoliation treatment in a non-oxidizing environment; (B) Subjecting the graphitic material to a supercritical fluid environment for inter-graphene layer penetration and exfoliation; or (C) Dispersing the graphitic material in a powder form to an aqueous solution containing a surfactant or dispersing agent to obtain a suspension and subjecting the suspension to direct ultrasonication.
(43) In Procedure (A), a particularly preferred step comprises (i) intercalating the graphitic material with a non-oxidizing agent, selected from an alkali metal (e.g., potassium, sodium, lithium, or cesium), alkaline earth metal, or an alloy, mixture, or eutectic of an alkali or alkaline metal; and (ii) a chemical exfoliation treatment (e.g., by immersing potassium-intercalated graphite in ethanol solution).
(44) In Procedure (B), a preferred step comprises immersing the graphitic material to a supercritical fluid, such as carbon dioxide (e.g., at temperature T>31° C. and pressure P>7.4 MPa) and water (e.g., at T>374° C. and P>22.1 MPa), for a period of time sufficient for inter-graphene layer penetration (tentative intercalation). This step is then followed by a sudden de-pressurization to exfoliate individual graphene layers. Other suitable supercritical fluids include methane, ethane, ethylene, hydrogen peroxide, ozone, water oxidation (water containing a high concentration of dissolved oxygen), or a mixture thereof.
(45) In Procedure (C), a preferred step comprises (a) dispersing particles of a graphitic material in a liquid medium containing therein a surfactant or dispersing agent to obtain a suspension or slurry; and (b) exposing the suspension or slurry to ultrasonic waves (a process commonly referred to as ultrasonication) at an energy level for a sufficient length of time to produce the separated nanoscaled platelets, which are pristine, non-oxidized NGPs.
(46) NGPs can be produced with an oxygen content no greater than 25% by weight, preferably below 20% by weight, further preferably below 5%. Typically, the oxygen content is between 5% and 20% by weight. The oxygen content can be determined using chemical elemental analysis and/or X-ray photoelectron spectroscopy (XPS).
(47) The laminar graphite materials used in the prior art processes for the production of the GIC, graphite oxide, and subsequently made exfoliated graphite, flexible graphite sheets, and graphene platelets are, in most cases, natural graphite. However, the present disclosure is not limited to natural graphite. The starting material may be selected from the group consisting of natural graphite, artificial graphite (e.g., highly oriented pyrolytic graphite, HOPG), graphite oxide, graphite fluoride, graphite fiber, carbon fiber, carbon nanofiber, carbon nanotube, mesophase carbon microbead (MCMB) or carbonaceous microsphere (CMS), soft carbon, hard carbon, and combinations thereof. All of these materials contain graphite crystallites that are composed of layers of graphene planes stacked or bonded together via van der Waals forces. In natural graphite, multiple stacks of graphene planes, with the graphene plane orientation varying from stack to stack, are clustered together. In carbon fibers, the graphene planes are usually oriented along a preferred direction. Generally speaking, soft carbons are carbonaceous materials obtained from carbonization of liquid-state, aromatic molecules. Their aromatic ring or graphene structures are more or less parallel to one another, enabling further graphitization. Hard carbons are carbonaceous materials obtained from aromatic solid materials (e.g., polymers, such as phenolic resin and polyfurfuryl alcohol). Their graphene structures are relatively randomly oriented and, hence, further graphitization is difficult to achieve even at a temperature higher than 2,500° C. But, graphene sheets do exist in these carbons.
(48) Graphene sheets may be oxidized to various extents during their preparation, resulting in graphite oxide or graphene oxide (GO). Hence, in the present context, graphene preferably or primarily refers to those graphene sheets containing no or low oxygen content; but, they can include GO of various oxygen contents. Further, graphene may be fluorinated to a controlled extent to obtain graphene fluoride.
(49) Pristine graphene may be produced by direct ultrasonication (also known as liquid phase production) or supercritical fluid exfoliation of graphite particles. These processes are well-known in the art. Multiple pristine graphene sheets may be dispersed in water or other liquid medium with the assistance of a surfactant to form a suspension.
(50) Fluorinated graphene or graphene fluoride is herein used as an example of the halogenated graphene material group. There are two different approaches that have been followed to produce fluorinated graphene: (1) fluorination of pre-synthesized graphene: This approach entails treating graphene prepared by mechanical exfoliation or by CVD growth with fluorinating agent such as XeF.sub.2, or F-based plasmas; (2) Exfoliation of multilayered graphite fluorides: Both mechanical exfoliation and liquid phase exfoliation of graphite fluoride can be readily accomplished [F. Karlicky, et al. “Halogenated Graphenes: Rapidly Growing Family of Graphene Derivatives” ACS Nano, 2013, 7 (8), pp 6434-6464].
(51) Interaction of F.sub.2 with graphite at high temperature leads to covalent graphite fluorides (CF).sub.n or (C.sub.2F).sub.n, while at low temperatures graphite intercalation compounds (GIC) C.sub.xF (2≤x≤24) form. In (CF).sub.n carbon atoms are sp3-hybridized and thus the fluorocarbon layers are corrugated consisting of trans-linked cyclohexane chairs. In (C.sub.2F).sub.n only half of the C atoms are fluorinated and every pair of the adjacent carbon sheets are linked together by covalent C—C bonds. Systematic studies on the fluorination reaction showed that the resulting F/C ratio is largely dependent on the fluorination temperature, the partial pressure of the fluorine in the fluorinating gas, and physical characteristics of the graphite precursor, including the degree of graphitization, particle size, and specific surface area. In addition to fluorine (F.sub.2), other fluorinating agents may be used, although most of the available literature involves fluorination with F.sub.2 gas, sometimes in presence of fluorides.
(52) For exfoliating a layered precursor material to the state of individual layers or few-layers, it is necessary to overcome the attractive forces between adjacent layers and to further stabilize the layers. This may be achieved by either covalent modification of the graphene surface by functional groups or by non-covalent modification using specific solvents, surfactants, polymers, or donor-acceptor aromatic molecules. The process of liquid phase exfoliation includes ultra-sonic treatment of a graphite fluoride in a liquid medium.
(53) The nitrogenation of graphene can be conducted by exposing a graphene material, such as graphene oxide, to ammonia at high temperatures (200-400° C.). Nitrogenated graphene could also be formed at lower temperatures by a hydrothermal method; e.g. by sealing GO and ammonia in an autoclave and then increased the temperature to 150-250° C. Other methods to synthesize nitrogen doped graphene include nitrogen plasma treatment on graphene, arc-discharge between graphite electrodes in the presence of ammonia, ammonolysis of graphene oxide under CVD conditions, and hydrothermal treatment of graphene oxide and urea at different temperatures.
(54) In some embodiments, the inorganic filler-reinforced elastomer further contains a lithium ion-conducting additive dispersed in an elastomer matrix material. The lithium ion-conducting additive may be selected from Li.sub.2CO.sub.3, Li.sub.2O, Li.sub.2C.sub.2O.sub.4, LiOH, LiX, ROCO.sub.2Li, HCOLi, ROLi, (ROCO.sub.2Li).sub.2, (CH.sub.2OCO.sub.2Li).sub.2, Li.sub.2S, Li.sub.xSO.sub.y, or a combination thereof, wherein X═F, Cl, I, or Br, R=a hydrocarbon group, 0≤x≤1, 1≤y≤4.
(55) Alternatively, the lithium ion-conducting additive may contain a lithium salt selected from lithium perchlorate (LiClO.sub.4), lithium hexafluorophosphate (LiPF.sub.6), lithium borofluoride (LiBF.sub.4), lithium hexafluoroarsenide (LiAsF.sub.6), lithium trifluoro-metasulfonate, (LiCF.sub.3SO.sub.3), bis-trifluoromethyl sulfonylimide lithium (LiN(CF.sub.3SO.sub.2).sub.2), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF.sub.2C.sub.2O.sub.4), lithium oxalyldifluoroborate (LiBF.sub.2C.sub.2O.sub.4), lithium nitrate (LiNO.sub.3), Li-fluoroalkyl-phosphates, (LiPF.sub.3(CF.sub.2CF.sub.3).sub.3), lithium bisperfluoro-ethysulfonylimide (LiBETI), lithium bis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), ionic liquid-based lithium salts, and combinations thereof.
(56) In some embodiments, the lithium ion-conducting additive or filler is a lithium ion-conducting polymer selected from poly(ethylene oxide) (PEO), Polypropylene oxide (PPO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVdF), Poly bis-methoxy ethoxyethoxide-phosphazene, Polyvinyl chloride, Polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a derivative thereof (e.g. sulfonated versions), or a combination thereof.
(57) The elastomeric matrix material may contain an electron-conducting polymer selected from polyaniline, polypyrrole, polythiophene, polyfuran, a bi-cyclic polymer, derivatives thereof (e.g. sulfonated versions), or a combination thereof.
(58) Some elastomers are originally in an unsaturated chemical state (unsaturated rubbers) that can be cured by sulfur vulcanization to form a cross-linked polymer that is highly elastic (hence, an elastomer). Prior to vulcanization, these polymers or oligomers are soluble in an organic solvent to form a polymer solution. Graphene sheets can be chemically functionalized to contain functional groups (e.g. —OH, —COOH, NH.sub.2, etc.) that can react with the polymer or its oligomer. The graphene-bonded oligomer or polymer may then be dispersed in a liquid medium (e.g. a solvent) to form a solution or suspension. Particles of an anode active material (e.g. SnO.sub.2 nanoparticles and Si nanowires) can be dispersed in this polymer solution or suspension to form a slurry of an active material particle-polymer mixture. This suspension can then be subjected to a solvent removal treatment while individual particles remain substantially separated from one another. The graphene-bonded polymer precipitates out to deposit on surfaces of these active material particles. This can be accomplished, for instance, via spray drying.
(59) Unsaturated rubbers that can be vulcanized to become elastomer include natural polyisoprene (e.g. cis-1,4-polyisoprene natural rubber (NR) and trans-1,4-polyisoprene gutta-percha), synthetic polyisoprene (IR for isoprene rubber), polybutadiene (BR for butadiene rubber), chloroprene rubber (CR), polychloroprene (e.g. Neoprene, Baypren etc.), butyl rubber (copolymer of isobutylene and isoprene, IIR), including halogenated butyl rubbers (chloro butyl rubber (CIIR) and bromo butyl rubber (BIIR), styrene-butadiene rubber (copolymer of styrene and butadiene, SBR), nitrile rubber (copolymer of butadiene and acrylonitrile, NBR),
(60) Some elastomers are saturated rubbers that cannot be cured by sulfur vulcanization; they are made into a rubbery or elastomeric material via different means: e.g. by having a copolymer domain that holds other linear chains together. Graphene sheets can be solution- or melt-dispersed into the elastomer to form a graphene/elastomer composite. Each of these graphene/elastomer composites can be used to encapsulate particles of an anode active material by one of several means: melt mixing (followed by pelletizing and ball-milling, for instance), solution mixing (dissolving the anode active material particles in an uncured polymer, monomer, or oligomer, with or without an organic solvent) followed by drying (e.g. spray drying), interfacial polymerization, or in situ polymerization of elastomer in the presence of anode active material particles.
(61) Saturated rubbers and related elastomers in this category include EPM (ethylene propylene rubber, a copolymer of ethylene and propylene), EPDM rubber (ethylene propylene diene rubber, a terpolymer of ethylene, propylene and a diene-component), epichlorohydrin rubber (ECO), polyacrylic rubber (ACM, ABR), silicone rubber (SI, Q, VMQ), fluorosilicone rubber (FVMQ), fluoroelastomers (FKM, and FEPM; such as Viton, Tecnoflon, Fluorel, Aflas and Dai-El), perfluoroelastomers (FFKM: Tecnoflon PFR, Kalrez, Chemraz, Perlast), polyether block amides (PEBA), chlorosulfonated polyethylene (CSM; e.g. Hypalon), and ethylene-vinyl acetate (EVA), thermoplastic elastomers (TPE), protein resilin, and protein elastin. Polyurethane and its copolymers (e.g. urea-urethane copolymer) are particularly useful elastomeric shell materials for encapsulating anode active material particles.
(62) Several micro-encapsulation processes require the elastomer materials to be dissolvable in a solvent. Fortunately, all the elastomers used herein are soluble in some common solvents. Even for those rubbers that are not very soluble after vulcanization, the un-cured polymer (prior to vulcanization or curing) can be readily dissolved in a common organic solvent to form a solution. This solution can then be used to encapsulate solid particles via several of the micro-encapsulation methods to be discussed in what follows. Upon encapsulation, the elastomer shell is then vulcanized or cured. Some examples of rubbers and their solvents are polybutadiene (2-methyl pentane+n-hexane or 2,3-dimethylbutane), styrene-butadiene rubber (toluene, benzene, etc.), butyl rubber (n-hexane, toluene, cyclohexane), etc. The SBR can be vulcanized with different amounts sulfur and accelerator at 433° K in order to obtain different network structures and crosslink densities. Butyl rubber (IIR) is a copolymer of isobutylene and a small amount of isoprene (e.g. about 98% polyisobutylene with 2% isoprene distributed randomly in the polymer chain). Elemental sulfur and organic accelerators (such as thiuram or thiocarbamates) can be used to cross-link butyl rubber to different extents as desired. Thermoplastic elastomers are also readily soluble in solvents.
(63) There are three broad categories of micro-encapsulation methods that can be implemented to produce elastomer-encapsulated particles of an anode active material: physical methods, physico-chemical methods, and chemical methods. The physical methods include pan-coating, air-suspension coating, centrifugal extrusion, vibration nozzle, and spray-drying methods. The physico-chemical methods include ionotropic gelation and coacervation-phase separation methods. The chemical methods include interfacial polycondensation, interfacial cross-linking, in-situ polymerization, and matrix polymerization.
(64) Pan-coating method: The pan coating process involves tumbling the active material particles in a pan or a similar device while the encapsulating material (e.g. elastomer monomer/oligomer, elastomer melt, elastomer/solvent solution) is applied slowly until a desired encapsulating shell thickness is attained.
(65) Air-suspension coating method: In the air suspension coating process, the solid particles (core material) are dispersed into the supporting air stream in an encapsulating chamber. A controlled stream of a polymer-solvent solution (elastomer or its monomer or oligomer dissolved in a solvent; or its monomer or oligomer alone in a liquid state) is concurrently introduced into this chamber, allowing the solution to hit and coat the suspended particles. These suspended particles are encapsulated (fully coated) with polymers while the volatile solvent is removed, leaving a very thin layer of polymer (elastomer or its precursor, which is cured/hardened subsequently) on surfaces of these particles. This process may be repeated several times until the required parameters, such as full-coating thickness (i.e. encapsulating shell or wall thickness), are achieved. The air stream which supports the particles also helps to dry them, and the rate of drying is directly proportional to the temperature of the air stream, which can be adjusted for optimized shell thickness.
(66) In a preferred mode, the particles in the encapsulating zone portion may be subjected to re-circulation for repeated coating. Preferably, the encapsulating chamber is arranged such that the particles pass upwards through the encapsulating zone, then are dispersed into slower moving air and sink back to the base of the encapsulating chamber, enabling repeated passes of the particles through the encapsulating zone until the desired encapsulating shell thickness is achieved.
(67) Centrifugal extrusion: Anode active materials may be encapsulated using a rotating extrusion head containing concentric nozzles. In this process, a stream of core fluid (slurry containing particles of an anode active material dispersed in a solvent) is surrounded by a sheath of shell solution or melt. As the device rotates and the stream moves through the air it breaks, due to Rayleigh instability, into droplets of core, each coated with the shell solution. While the droplets are in flight, the molten shell may be hardened or the solvent may be evaporated from the shell solution. If needed, the capsules can be hardened after formation by catching them in a hardening bath. Since the drops are formed by the breakup of a liquid stream, the process is only suitable for liquid or slurry. A high production rate can be achieved. Up to 22.5 kg of microcapsules can be produced per nozzle per hour and extrusion heads containing 16 nozzles are readily available.
(68) Vibrational nozzle encapsulation method: Core-shell encapsulation or matrix-encapsulation of an anode active material can be conducted using a laminar flow through a nozzle and vibration of the nozzle or the liquid. The vibration has to be done in resonance with the Rayleigh instability, leading to very uniform droplets. The liquid can consist of any liquids with limited viscosities (1-50,000 mPa.Math.s): emulsions, suspensions or slurry containing the anode active material. The solidification can be done according to the used gelation system with an internal gelation (e.g. sol-gel processing, melt) or an external (additional binder system, e.g. in a slurry).
(69) Spray-drying: Spray drying may be used to encapsulate particles of an active material when the active material is dissolved or suspended in a melt or polymer solution. In spray drying, the liquid feed (solution or suspension) is atomized to form droplets which, upon contacts with hot gas, allow solvent to get vaporized and thin polymer shell to fully embrace the solid particles of the active material.
(70) Coacervation-phase separation: This process consists of three steps carried out under continuous agitation: (a) Formation of three immiscible chemical phases: liquid manufacturing vehicle phase, core material phase and encapsulation material phase. The core material is dispersed in a solution of the encapsulating polymer (elastomer or its monomer or oligomer). The encapsulating material phase, which is an immiscible polymer in liquid state, is formed by (i) changing temperature in polymer solution, (ii) addition of salt, (iii) addition of non-solvent, or (iv) addition of an incompatible polymer in the polymer solution. (b) Deposition of encapsulation shell material: core material being dispersed in the encapsulating polymer solution, encapsulating polymer material coated around core particles, and deposition of liquid polymer embracing around core particles by polymer adsorbed at the interface formed between core material and vehicle phase; and (c) Hardening of encapsulating shell material: shell material being immiscible in vehicle phase and made rigid via thermal, cross-linking, or dissolution techniques.
(71) Interfacial polycondensation and interfacial cross-linking: Interfacial polycondensation entails introducing the two reactants to meet at the interface where they react with each other. This is based on the concept of the Schotten-Baumann reaction between an acid chloride and a compound containing an active hydrogen atom (such as an amine or alcohol), polyester, polyurea, polyurethane, or urea-urethane condensation. Under proper conditions, thin flexible encapsulating shell (wall) forms rapidly at the interface. A solution of the anode active material and a diacid chloride are emulsified in water and an aqueous solution containing an amine and a polyfunctional isocyanate is added. A base may be added to neutralize the acid formed during the reaction. Condensed polymer shells form instantaneously at the interface of the emulsion droplets. Interfacial cross-linking is derived from interfacial polycondensation, wherein cross-linking occurs between growing polymer chains and a multi-functional chemical groups to form an elastomer shell material.
(72) In-situ polymerization: In some micro-encapsulation processes, active materials particles are fully coated with a monomer or oligomer first. Then, direct polymerization of the monomer or oligomer is carried out on the surfaces of these material particles.
(73) Matrix polymerization: This method involves dispersing and embedding a core material in a polymeric matrix during formation of the particles. This can be accomplished via spray-drying, in which the particles are formed by evaporation of the solvent from the matrix material. Another possible route is the notion that the solidification of the matrix is caused by a chemical change.
(74) A variety of synthetic methods may be used to sulfonate an elastomer or rubber: (i) exposure to sulfur trioxide in vapor phase or in solution, possibly in presence of Lewis bases such as triethyl phosphate, tetrahydrofuran, dioxane, or amines; (ii) chlorosulfonic acid in diethyl ether; (iii) concentrated sulfuric acid or mixtures of sulfuric acid with alkyl hypochlorite; (iv) bisulfites combined to dioxygen, hydrogen peroxide, metallic catalysts, or peroxo derivates; and (v) acetyl sulfate.
(75) Sulfonation of an elastomer or rubber may be conducted before, during, or after curing of the elastomer or rubber. Further, sulfonation of the elastomer or rubber may be conducted before or after the particles of an electrode active material are embraced or encapsulated by the elastomer/rubber or its precursor (monomer or oligomer). Sulfonation of an elastomer or rubber may be accomplished by exposing the elastomer/rubber to a sulfonation agent in a solution state or melt state, in a batch manner or in a continuous process. The sulfonating agent may be selected from sulfuric acid, sulfonic acid, sulfur trioxide, chlorosulfonic acid, a bisulfate, a sulfate (e.g. zinc sulfate, acetyl sulfate, etc.), a mixture thereof, or a mixture thereof with another chemical species (e.g. acetic anhydride, thiolacetic acid, or other types of acids, etc.). In addition to zinc sulfate, there are a wide variety of metal sulfates that may be used as a sulfonating agent; e.g. those sulfates containing Mg, Ca, Co, Li, Ba, Na, Pb, Ni, Fe, Mn, K, Hg, Cr, and other transition metals, etc.
(76) For instance, a triblock copolymer, poly(styrene-isobutylene-styrene) or SIBS, may be sulfonated to several different levels ranging from 0.36 to 2.04 mequiv./g (13 to 82 mol % of styrene; styrene being 19 mol % of the unsulfonated block copolymer). Sulfonation of SIBS may be performed in solution with acetyl sulfate as the sulfonating agent. First, acetic anhydride reacts with sulfuric acid to form acetyl sulfate (a sulfonating agent) and acetic acid (a by-product). Then, excess water is removed since anhydrous conditions are required for sulfonation of SIBS. The SIBS is then mixed with the mixture of acetyl sulfate and acetic acid. Such a sulfonation reaction produces sulfonic acid substituted to the para-position of the aromatic ring in the styrene block of the polymer. Elastomers having an aromatic ring may be sulfonated in a similar manner.
(77) A sulfonated elastomer also may be synthesized by copolymerization of a low level of functionalized (i.e. sulfonated) monomer with an unsaturated monomer (e.g. olefinic monomer, isoprene monomer or oligomer, butadiene monomer or oligomer, etc.).
EXAMPLE 1
Sol-Gel Process for Producing Li.SUB.x.TiNb.SUB.2.O.SUB.7 .(TNO)
(78) The synthesis method involves precipitating the precursor to niobium-based composite metal oxide nanoparticles from a solution reactant mixture of Nb(OH).sub.5 (dissolved in citric acid) and water-ethanol solution containing Ti(OC.sub.3H.sub.7).sub.4. Specifically, Nb.sub.2O.sub.5 was dissolved in hydrofluoric acid to form a transparent solution. In order to remove the F ions from the solution, ammonia was added to obtain a white Nb(OH).sub.5 precipitate. After the precipitate was washed and dried, the Nb(OH).sub.5 was dissolved in citric acid to form a Nb(V)-citrate solution. A water-ethanol solution containing Ti(OC.sub.3H.sub.7).sub.4 was added to this solution while the pH value of the solution was adjusted using ammonia. This final mixture containing Nb(V) and Ti(IV) ions was then stirred at 90° C. to form a citric gel. This gel was then heated to 140° C. to obtain a precursor, which was annealed at 900° C. and at 1350° C. to obtain the Li.sub.xTiNb.sub.2O.sub.7 (TNO) powder.
(79) The powder was ball-milled in a high-intensity ball mill to obtain nanoparticles of TNO, which were then dispersed in monomers/oligomers of several different elastomers (e.g. polyurethane, polybutadine, etc.) to form reacting suspensions. The monomers/oligomers were then polymerized to a controlled extent without allowing for any significant cross-linking of chains. This procedure often enables chemical bonding between the composite metal oxide particles and other inorganic filler species (particles of transition metal carbide, sulfide, selenide, phosphide, nitride, boride, etc.). These non-cured or non-crosslinked polymers were then each separately dissolved in an organic solvent to form a suspension (polymer-solvent solution plus bonded metal oxide particles). Particles of anode active materials (Si, Sn, SnO.sub.2, SiO.sub.x, etc., respectively) were then dispersed into this suspension to form a slurry. The slurry was then spray-dried to form particulates containing anode active material particles being embraced by an encapsulating shell of metal oxide-reinforced elastomer.
EXAMPLE 2
Preparation of TiNb.SUB.2.O.SUB.7., TiMoNbO.SUB.7., and TiFe.SUB.0.3.Nb.SUB.1.7.O.SUB.7
(80) A niobium-titanium composite oxide represented by the general formula TiNb.sub.2O.sub.7 was synthesized, by following the following procedure: Commercially available niobium oxide (Nb.sub.2O.sub.5) and a titanate proton compound were used as starting materials. The titanate proton compound was prepared by immersing potassium titanate in hydrochloric acid at 25° C. for 72 hours. In the process, 1M hydrochloric acid was replaced with a 1M of fresh acid every 24 hours. As a result, potassium ions were exchanged for protons to obtain the titanate proton compound.
(81) The niobium oxide (Nb.sub.2O.sub.5) and the titanate proton compound were weighed such that the molar ratio of niobium to titanium in the synthesized compound was 3. The mixture was dispersed in 100 ml of pure water, followed by vigorous mixing. The obtained mixture was placed in a heat resistant container and was subjected to hydrothermal synthesis under conditions of 180° C. for a total of 24 hours. The obtained sample was washed in pure water three times, and then dried. The sample was then subjected to a heat treatment at 1,100° C. for 24 hours to obtain TiNb.sub.2O.sub.7.
(82) Additionally, a niobium-molybdenum-titanium composite oxide was synthesized in the same manner as above except that niobium oxide (Nb.sub.2O.sub.5), molybdenum oxide (Mo.sub.2O.sub.5), and a titanate proton compound were weighed such that the molar ratio of niobium to titanium and that of molybdenum to titanium in the synthesized compound was 1.5 and 1.5, respectively. As a result, a niobium-molybdenum-titanium composite oxide (TiMoNbO.sub.7) was obtained.
(83) In addition, a niobium-Iron-titanium composite oxide was synthesized in the same manner as above except that niobium oxide (Nb.sub.2O.sub.5), a titanate proton compound, and iron oxide (Fe.sub.2O.sub.3) were weighed such that the molar ratio of niobium to titanium and of iron to titanium in the synthesized compound was 3 and 0.3, respectively. As a result, a niobium-titanium composite oxide (TiFe.sub.0.3Nb.sub.1.7O.sub.7) was obtained.
(84) The above niobium-containing composite metal oxide powders (TiNb.sub.2O.sub.7, TiMoNbO.sub.7, and TiFe.sub.0.3Nb.sub.1.7O.sub.7) were separately added into a monomer of synthetic polyisoprene and a mixture of monomers for urethane-urea copolymer, respectively. Polymerization of the respective reacting mass was initiated and proceeded to obtain linear chains without crosslinking. This step was found to create some bonding between the composite metal oxide particles. Subsequently, these substantially linear chains were dissolved in a solvent (e.g. benzene and DMAc) to form a solution and particles of selected anode active materials (Si, Ge, and Co.sub.3O.sub.4) were dispersed in the solution to form a slurry. The slurry was then made into particulates using the vibration nozzle method.
EXAMPLE 3
Preparation of Ga.SUB.0.1.Ti.SUB.0.8.Nb.SUB.2.1.O.SUB.7
(85) In an experiment, 0.125 g of GaCl.sub.3 and 4.025 g of NbCl.sub.5 were dissolved in 10 mL of anhydrous ethanol under an inert atmosphere (argon) and magnetic stirring. The solution was transferred under air. Then, added to this solution was 6.052 g solution of titanium oxysulfate (TiOSO.sub.4) at 15% by mass in sulfuric acid, followed by 10 mL of ethanol to dissolve the precursors under a magnetic stirring. The pH of the solution was adjusted to 10 by slow addition of ammonia NH.sub.3 at 28% by mass into water.
(86) The paste was transferred into a Teflon container having a 90-mL capacity, which was then placed in an autoclave. The paste was then heated up to 220° C. for 5 hours with a heating and cooling ramp of 2 and 5 degrees C./min, respectively. The paste was then washed with distilled water by centrifugation until a pH between 6 and 7 was obtained. The resulting compound was heated at 60° C. for 12 hours and then ball-milled for 30 min at 500 rpm (revolutions per minute) in hexane. After evaporation of the solvent, the powder was calcinated at 950° C. for 1 hour with a heating/cooling ramp of 3 degrees C./min to produce crystals of Ga.sub.0.1Ti.sub.0.8Nb.sub.2.1O.sub.7.
EXAMPLE 4
Preparation of Fe.SUB.0.1.Ti.SUB.0.8.Nb.SUB.2.1.O.SUB.7 .Powder as a Reinforcement for Elastomer
(87) In a representative procedure, 0.116 g of FeCl.sub.3 and 4.025 g of NbCl.sub.5 were dissolved in 10 mL of anhydrous ethanol under an inert atmosphere (argon) and magnetic stirring. The resulting solution was transferred under air. Then, added to this solution was 6.052 g of titanium oxysulfate (TiOSO.sub.4) at 15% by mass in sulfuric acid and 10 mL of ethanol to dissolve the precursors under a magnetic stirring. The pH of the solution was adjusted to 10 by slow addition of ammonia NH.sub.3 at 28% by mass into water.
(88) The paste was transferred into a Teflon container having a 90-mL capacity, which was then placed in an autoclave. The paste was then heated up to 220° C. for 5 hours with a heating and cooling ramp of 2 and 5 degrees C./min, respectively. The paste was then washed with distilled water by centrifugation until a pH between 6 and 7 was obtained. The compound was heated at 60° C. for 12 hours and then ball-milled for 30 min at 500 rpm in hexane. After evaporation of hexane, the powder was calcinated at 950° C. for 1 hour with a heating/cooling ramp of 3 degrees C./min to obtain Fe.sub.0.1Ti.sub.0.8Nb.sub.2.1O.sub.7 crystals.
EXAMPLE 5
Production of Molybdenum Diselenide Nanoplatelets Using Direct Ultrasonication
(89) A sequence of steps can be utilized to form nanoplatelets from many different types of layered compounds: (a) dispersion of a layered compound in a low surface tension solvent or a mixture of water and surfactant, (b) ultrasonication, and (c) an optional mechanical shear treatment. For instance, dichalcogenides (MoSe.sub.2) consisting of Se—Mo—Se layers held together by weak van der Waals forces can be exfoliated via the direct ultrasonication process disclosed by our research group. Intercalation can be achieved by dispersing MoSe.sub.2 powder in a silicon oil beaker, with the resulting suspension subjected to ultrasonication at 120 W for two hours. The resulting MoSe.sub.2 platelets were found to have a thickness in the range from approximately 1.4 nm to 13.5 nm with most of the platelets being mono-layers or double layers.
(90) Other single-layer platelets of the form MX.sub.2 (transition metal dichalcogenide), including MoS.sub.2, TaS.sub.2, ZrS.sub.2, and WS.sub.2, were similarly exfoliated and separated. Again, most of the platelets were mono-layers or double layers when a high sonic wave intensity was utilized for a sufficiently long ultrasonication time.
EXAMPLE 6
Production of ZrS.SUB.2 .Nanodiscs
(91) In a representative procedure, zirconium chloride (ZrCl.sub.4) precursor (1.5 mmol) and oleylamine (5.0 g, 18.7 mmol) were added to a 25-mL three-neck round-bottom flask under a protective argon atmosphere. The reaction mixture was first heated to 300° C. at a heating rate of 5° C./min under argon flow and subsequently CS.sub.2 (0.3 mL, 5.0 mmol) was injected. After 1 h, the reaction was stopped and cooled down to room temperature. After addition of excess butanol and hexane mixtures (1:1 by volume), 18 nm ZrS.sub.2 nanodiscs (˜100 mg) were obtained by centrifugation. Larger sized nanodiscs ZrS.sub.2 of 32 nm and 55 nm were obtained by changing reaction time to 3 h and 6 h, respectively otherwise under identical conditions.
EXAMPLE 7
Preparation of Boron Nitride Nanosheets
(92) Five grams of boron nitride (BN) powder, ground to approximately 20 μm or less in sizes, were dispersed in a strong polar solvent (dimethyl formamide) to obtain a suspension. An ultrasonic energy level of 85 W (Branson 5450 Ultrasonicator) was used for exfoliation, separation, and size reduction for a period of 1-3 hours. This is followed by centrifugation to isolate the BN nanosheets. The BN nanosheets obtained were from 1 nm thick (<3 atomic layers) up to 7 nm thick.
EXAMPLE 8
Sulfonation of Triblock Copolymer poly(styrene-isobutylene-styrene) or SIBS
(93) An example of the sulfonation procedure used in this study is summarized as follows: a 10% (w/v) solution of SIBS (50 g) and a desired amount of graphene oxide sheets (0.15 TO 405 by wt.) in methylene chloride (500 ml) was prepared. The solution was stirred and refluxed at approximately 40 8 C, while a specified amount of acetyl sulfate in methylene chloride was slowly added to begin the sulfonation reaction. Acetyl sulfate in methylene chloride was prepared prior to this reaction by cooling 150 ml of methylene chloride in an ice bath for approximately 10 min. A specified amount of acetic anhydride and sulfuric acid was then added to the chilled methylene chloride under stirring conditions. Sulfuric acid was added approximately 10 min after the addition of acetic anhydride with acetic anhydride in excess of a 1:1 mole ratio. This solution was then allowed to return to room temperature before addition to the reaction vessel.
(94) After approximately 5 h, the reaction was terminated by slowly adding 100 ml of methanol. The reacted polymer solution was then precipitated with deionized water. The precipitate was washed several times with water and methanol, separately, and then dried in a vacuum oven at 50 8 C for 24 h. This washing and drying procedure was repeated until the pH of the wash water was neutral. After this process, the final polymer yield was approximately 98% on average. This sulfonation procedure was repeated with different amounts of acetyl sulfate to produce several sulfonated polymers with various levels of sulfonation or ion-exchange capacities (IECs). The mol % sulfonation is defined as: mol %=(moles of sulfonic acid/moles of styrene)×100%, and the IEC is defined as the mille-equivalents of sulfonic acid per gram of polymer (mequiv./g).
(95) After sulfonation and washing of each polymer, the S-SIBS samples were dissolved in a mixed solvent of toluene/hexanol (85/15, w/w) to form solutions having polymer concentrations ranging from 5 to 2.5% (w/v). Desired amounts of transition metal oxides prepared in Examples 1-4 were added into these solutions and the resulting slurries were ultrasonicated for 0.5-1.5 hours. Particles of a desired anode active material, along with a desired amount of conducting additive (e.g. graphene sheets or CNTs) were then added into the slurry samples. The slurry samples were separately spray-dried to form sulfonated transition metal oxide-reinforced elastomer-embraced particles.
(96) Alternatively, sulfonation may be conducted on the reinforced elastomer layer after this encapsulating layer is form. (e.g. after the anode active material particle(s) is/are encapsulated.
EXAMPLE 9
Synthesis of Sulfonated Polybutadiene (PB) by Free Radical Addition of Thiolacetic Acid (TAA) Followed by in Situ Oxidation with Performic Acid
(97) A representative procedure is given as follows. PB (8.0 g) was dissolved in toluene (800 mL) under vigorous stirring for 72 h at room temperature in a 1 L round-bottom flask. Benzophenone (BZP) (0.225 g; 1.23 mmol; BZP/olefin molar ratio=1:120) and TAA (11.9 mL; 0.163 mol, TAA/olefin molar ratio=1.1) and a desired amount of inorganic filler particles (0.1%-40% by wt.) were introduced into the reactor, and the polymer solution was irradiated for 1 h at room temperature with UV light of 365 nm and power of 100 W.
(98) The resulting inorganic material-reinforced thioacetylated polybutadiene (PB-TA) was isolated by pouring 200 mL of the toluene solution in a plenty of methanol and the polymer recovered by filtration, washed with fresh methanol, and dried in vacuum at room temperature (Yield=3.54 g). Formic acid (117 mL; 3.06 mol; HCOOH/olefin molar ratio=25), along with a desired amount of anode active material particles, from 10 to 100 grams) were added to the toluene solution of PB-TA at 50° C. followed by slow addition of 52.6 mL of hydrogen peroxide (35 wt %; 0.61 mol; H.sub.2O.sub.2/olefin molar ratio=5) in 20 min. We would like to caution that the reaction is autocatalytic and strongly exothermic. The resulting slurry was spray-dried to obtain reinforced sulfonated polybutadiene (PB-SA-encapsulated anode active material particles (Si, SiO, Sn, SnO.sub.2, Co.sub.3O.sub.4, separately).
(99) It may be noted that inorganic filler material particles may be added at different stages of the procedure: before, during or after BZP is added or before/during/after the anode active material particles are added.
EXAMPLE 10
Synthesis of Sulfonated SBS
(100) Sulfonated styrene-butadiene-styrene triblock copolymer (SBS) based elastomer was directly synthesized. First, SBS (optionally along with graphene sheets) is first epoxidized by performic acid formed in situ, followed by ring-opening reaction with an aqueous solution of NaHSO.sub.3. In a typical procedure, epoxidation of SBS was carried out via reaction of SBS in cyclohexane solution (SBS concentration=11 g/100 mL) with performic acid formed in situ from HCOOH and 30% aqueous H.sub.2O.sub.2 solution at 70° C. for 4 h, using 1 wt % poly(ethylene glycol)/SBS as a phase transfer catalyst. The molar ratio of H.sub.2O.sub.2/HCOOH was 1. The product (ESBS) was precipitated and washed several times with ethanol, followed by drying in a vacuum dryer at 60° C.
(101) Subsequently, ESBS was first dissolved in toluene to form a solution with a concentration of 10 g/100 mL, into which was added 5 wt % TEAB/ESBS as a phase transfer catalyst and 5 wt % DMA/ESBS as a ring-opening catalyst. Herein, TEAB=tetraethyl ammonium bromide and DMA=N,N-dimethyl aniline. An aqueous solution of NaHSO.sub.3 and Na.sub.2SO.sub.3 (optionally along with graphene sheets, if not added earlier) was then added with vigorous stirring at 60° C. for 7 h at a molar ratio of NaHSO.sub.3/epoxy group at 1.8 and a weight ratio of Na.sub.2SO.sub.3/NaHSO.sub.3 at 36%. This reaction allows for opening of the epoxide ring and attaching of the sulfonate group according to the following reaction:
(102) ##STR00001##
The reaction was terminated by adding a small amount of acetone solution containing antioxidant. The mixture was washed with distilled water three times, then precipitated by ethanol, followed by drying in a vacuum dryer at 50° C. It may be noted that particles of an inorganic filler and an electrode active material may be added during various stages of the aforementioned procedure (e.g. right from the beginning, or prior to the ring opening reaction). Preferably, the inorganic filler (along with the optional electron-conducting additive or lithium ion-conducting additive) is added before or during the ring opening reaction and the anode active material is added afterwards.
EXAMPLE 11
Synthesis of Sulfonated SBS by Free Radical Addition of Thiolacetic Acid (TAA) Followed by in Situ Oxidation with Performic Acid
(103) A representative procedure is given as follows. SBS (8.000 g) in toluene (800 mL) was left under vigorous stirring for 72 hours at room temperature and heated later on for 1 h at 65° C. in a 1 L round-bottom flask until the complete dissolution of the polymer. Thus, benzophenone (BZP, 0.173 g; 0.950 mmol; BZP/olefin molar ratio=1:132) and TAA (8.02 mL; 0.114 mol, TAA/olefin molar ratio=1.1) were added, and the polymer solution was irradiated for 4 h at room temperature with UV light of 365 nm and power of 100 W. To isolate a fraction of the thioacetylated sample, 20 mL of the polymer solution was treated with plenty of methanol, and the polymer was recovered by filtration, washed with fresh methanol, and dried in vacuum at room temperature. The toluene solution containing the thioacetylated polymer was equilibrated at 50° C., and 107.4 mL of formic acid (2.84 mol; HCOOH/olefin molar ratio=27.5) and 48.9 mL of hydrogen peroxide (35 wt %; 0.57 mol; H.sub.2O.sub.2/olefin molar ratio=5.5) were added in about 15 min. It may be cautioned that the reaction is autocatalytic and strongly exothermic! Particles of the desired anode active materials were added before or after this reaction. The resulting slurry was stirred for 1 h, and then most of the solvent was distilled off in vacuum at 35° C. Finally, the slurry containing the sulfonated elastomer was coagulated in a plenty of acetonitrile, isolated by filtration, washed with fresh acetonitrile, and dried in vacuum at 35° C. to obtain sulfonated elastomers.
(104) Other elastomers (e.g. polyisoprene, EPDM, EPR, polyurethane, etc.) were sulfonated in a similar manner. Alternatively, all the rubbers or elastomers can be directly immersed in a solution of sulfuric acid, a mixture of sulfuric acid and acetyl sulfate, or other sulfonating agent discussed above to produce sulfonated elastomers/rubbers. Again, both the inorganic filler material and anode active material particles may be added at various stages of the procedure. However, the inorganic filler material is preferably added before or immediately after addition of TAA and the anode active material particles are added at a later stage.
EXAMPLE 12
Graphene Oxide from Sulfuric Acid Intercalation and Exfoliation of MCMBs
(105) MCMB (mesocarbon microbeads) were supplied by China Steel Chemical Co. This material has a density of about 2.24 g/cm.sup.3 with a median particle size of about 16 μm. MCMBs (10 grams) were intercalated with an acid solution (sulfuric acid, nitric acid, and potassium permanganate at a ratio of 4:1:0.05) for 48 hours. Upon completion of the reaction, the mixture was poured into deionized water and filtered. The intercalated MCMBs were repeatedly washed in a 5% solution of HCl to remove most of the sulfate ions. The sample was then washed repeatedly with deionized water until the pH of the filtrate was neutral. The slurry was dried and stored in a vacuum oven at 60° C. for 24 hours. The dried powder sample was placed in a quartz tube and inserted into a horizontal tube furnace pre-set at a desired temperature, 800° C.-1,100° C. for 30-90 seconds to obtain graphene samples. A small quantity of graphene was mixed with water and ultrasonicated at 60-W power for 10 minutes to obtain a suspension. A small amount was sampled out, dried, and investigated with TEM, which indicated that most of the NGPs were between 1 and 10 layers. The oxygen content of the graphene powders (GO or RGO) produced was from 0.1% to approximately 25%, depending upon the exfoliation temperature and time.
EXAMPLE 13
Oxidation and Exfoliation of Natural Graphite
(106) Graphite oxide was prepared by oxidation of graphite flakes with sulfuric acid, sodium nitrate, and potassium permanganate at a ratio of 4:1:0.05 at 30° C. for 48 hours, according to the method of Hummers [U.S. Pat. No. 2,798,878, Jul. 9, 1957]. Upon completion of the reaction, the mixture was poured into deionized water and filtered. The sample was then washed with 5% HCl solution to remove most of the sulfate ions and residual salt and then repeatedly rinsed with deionized water until the pH of the filtrate was approximately 4. The intent was to remove all sulfuric and nitric acid residue out of graphite interstices. The slurry was dried and stored in a vacuum oven at 60° C. for 24 hours.
(107) The dried, intercalated (oxidized) compound was exfoliated by placing the sample in a quartz tube that was inserted into a horizontal tube furnace pre-set at 1,050° C. to obtain highly exfoliated graphite. The exfoliated graphite was dispersed in water along with a 1% surfactant at 45° C. in a flat-bottomed flask and the resulting graphene oxide (GO) suspension was subjected to ultrasonication for a period of 15 minutes to obtain a homogeneous graphene-water suspension.
EXAMPLE 14
Preparation of Pristine Graphene Sheets
(108) Pristine graphene sheets were produced by using the direct ultrasonication or liquid-phase exfoliation process. In a typical procedure, five grams of graphite flakes, ground to approximately 20 μm in sizes, were dispersed in 1,000 mL of deionized water (containing 0.1% by weight of a dispersing agent, Zonyl® FSO from DuPont) to obtain a suspension. An ultrasonic energy level of 85 W (Branson 5450 Ultrasonicator) was used for exfoliation, separation, and size reduction of graphene sheets for a period of 15 minutes to 2 hours. The resulting graphene sheets were pristine graphene that had never been oxidized and were oxygen-free and relatively defect-free. There are substantially no other non-carbon elements.
EXAMPLE 15
Preparation of Graphene Fluoride (GF) Sheets
(109) Several processes have been used by us to produce GF, but only one process is herein described as an example. In a typical procedure, highly exfoliated graphite (HEG) was prepared from intercalated compound C.sub.2F.xClF.sub.3. HEG was further fluorinated by vapors of chlorine trifluoride to yield fluorinated highly exfoliated graphite (FHEG). A pre-cooled Teflon reactor was filled with 20-30 mL of liquid pre-cooled ClF.sub.3, and then the reactor was closed and cooled to liquid nitrogen temperature. Subsequently, no more than 1 g of HEG was put in a container with holes for ClF.sub.3 gas to access the reactor. After 7-10 days, a gray-beige product with approximate formula C.sub.2F was formed. GF sheets were then dispersed in halogenated solvents to form suspensions.
EXAMPLE 16
Preparation of Nitrogenated Graphene Sheets
(110) Graphene oxide (GO), synthesized in Example 12, was finely ground with different proportions of urea and the pelletized mixture heated in a microwave reactor (900 W) for 30 s. The product was washed several times with deionized water and vacuum dried. In this method graphene oxide gets simultaneously reduced and doped with nitrogen. The products obtained with graphene/urea mass ratios of 1/0.5, 1/1 and 1/2 are designated as N-1, N-2 and N-3 respectively and the nitrogen contents of these samples were 14.7, 18.2 and 17.5 wt. % respectively as determined by elemental analysis. These nitrogenated graphene sheets remain dispersible in water.
EXAMPLE 17
Cobalt Oxide (Co.SUB.3.O.SUB.4.) Anode Particulates
(111) An appropriate amount of inorganic salts Co(NO.sub.3).sub.2.6H.sub.2O and ammonia solution (NH.sub.3.H.sub.2O, 25 wt. %) were mixed together. The resulting suspension was stirred for several hours under an argon flow to ensure a complete reaction. The obtained Co(OH).sub.2 precursor suspension was calcined at 450° C. in air for 2 h to form particles of the layered Co.sub.3O.sub.4. Portion of the Co.sub.3O.sub.4 particles was then encapsulated with a composite metal oxide reinforced urea-urethane copolymer with the encapsulating elastomer shell thickness being varied from 25 nm to 220 nm.
(112) For electrochemical testing, the working electrodes were prepared by mixing 85 wt. % active material (elastomer composite encapsulated or non-encapsulated particulates of Co.sub.3O.sub.4, separately), 7 wt. % acetylene black (Super-P), and 8 wt. % polyvinylidene fluoride (PVDF) binder dissolved in N-methyl-2-pyrrolidinoe (NMP) to form a slurry of 5 wt. % total solid content. After coating the slurries on Cu foil, the electrodes were dried at 120° C. in vacuum for 2 h to remove the solvent before pressing. Then, the electrodes were cut into a disk (ϕ=12 mm) and dried at 100° C. for 24 h in vacuum. Electrochemical measurements were carried out using CR2032 (3V) coin-type cells with lithium metal as the counter/reference electrode, Celgard 2400 membrane as separator, and 1 M LiPF.sub.6 electrolyte solution dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (EC-DEC, 1:1 v/v). The cell assembly was performed in an argon-filled glove-box. The CV measurements were carried out using a CH-6 electrochemical workstation at a scanning rate of 1 mV/s.
(113) The electrochemical performance of the particulates of nano Li.sub.4Ti.sub.5O.sub.12-reinforced sulfonated elastomer-encapsulated Co.sub.3O.sub.4 particles and that of non-protected Co.sub.3O.sub.4 were evaluated by galvanostatic charge/discharge cycling at a current density of 50 mA/g, using a LAND electrochemical workstation. The results indicate that the charge/discharge profiles for the encapsulated Co.sub.3O.sub.4 particles and un-protected Co.sub.3O.sub.4 particle-based electrodes show a long voltage plateau at about 1.06 V and 1.10 V, respectively, followed by a slopping curve down to the cut-off voltage of 0.01 V, indicative of typical characteristics of voltage trends for the Co.sub.3O.sub.4 electrode.
(114) As summarized in
(115) As the number of cycles increases, the specific capacity of the bare Co.sub.3O.sub.4 electrode drops precipitously. Compared with its initial capacity value of approximately 752 mAh/g, its capacity suffers a 20% loss after 150 cycles and a 43.9% loss after 440 cycles. By contrast, the presently disclosed elastomer-encapsulated particulates provide the battery cell with a very stable and high specific capacity for a large number of cycles, experiencing a capacity loss of less than 4% after 440 cycles. These data have clearly demonstrated the surprising and superior performance of the presently disclosed particulate electrode materials compared with prior art un-encapsulated particulate-based electrode materials.
(116) It may be noted that the number of charge-discharge cycles at which the specific capacity decays to 80% of its initial value is commonly defined as the useful cycle life of a lithium-ion battery. Thus, the cycle life of the cell containing the non-encapsulated anode active material is approximately 150 cycles. In contrast, the cycle life of the presently disclosed cells (not just button cells, but large-scale full cells) is typically from 1,000 to 4,000.
EXAMPLE 18
Inorganic Filler Reinforced Elastomer-Encapsulated Tin Oxide Particulates
(117) Tin oxide (SnO.sub.2) nanoparticles were obtained by the controlled hydrolysis of SnCl.sub.4.5H.sub.2O with NaOH using the following procedure: SnCl.sub.4.5H.sub.2O (0.95 g, 2.7 m-mol) and NaOH (0.212 g, 5.3 m-mol) were dissolved in 50 mL of distilled water each. The NaOH solution was added drop-wise under vigorous stirring to the tin chloride solution at a rate of 1 mL/min. This solution was homogenized by sonication for 5 m in. Subsequently, the resulting hydrosol was reacted with H.sub.2SO.sub.4. To this mixed solution, few drops of 0.1 M of H.sub.2SO.sub.4 were added to flocculate the product. The precipitated solid was collected by centrifugation, washed with water and ethanol, and dried in vacuum. The dried product was heat-treated at 400° C. for 2 h under Ar atmosphere. A dilute elastomer-solvent solution (0.01-0.1 M of cis-polyisoprene in cyclohexane and 1,4-dioxane) was used as a coating solution in an air-suspension method to produce elastomer-encapsulated SnO.sub.2 particles having a shell thickness of 2.3 nm to 124 nm.
(118) The battery cells from the TiNb.sub.2O.sub.7 reinforced elastomer-encapsulated SnO.sub.2 particles and non-coated SnO.sub.2 particles were prepared using a procedure described in Example 1.
EXAMPLE 19
Tin (Sn) Nanparticles Encapsulated by a Reinforced Sulfonated Styrene-Butadiene Rubber (SBR)/Graphene Composite
(119) Nanparticles (76 nm in diameter) of Sn were encapsulated with a thin layer of SBR shell via the spray-drying method, followed by curing of the butadiene segment of the SBR chains to impart high elasticity to the SBR. For comparison, some amount of Sn nanoparticles was encapsulated by a carbon shell. Carbon encapsulation is well-known in the art. Un-protected Sn nanoparticles from the same batch were also investigated to determine and compare the cycling behaviors of the lithium-ion batteries containing these particles as the anode active material.
(120) Shown in
EXAMPLE 20
Si Nanowire-Based Particulates
(121) Si nanowires were supplied from Angstron Energy Co. (Dayton, Ohio). Some Si nanowires were encapsulated with graphene-reinforced cis-polyisoprene elastomer. Some Si nanowires were coated with a layer of amorphous carbon and then encapsulated with graphene-reinforced cis-polyisoprene elastomer. For comparison purposes, Si nanowires unprotected and protected by carbon coating (but no elastomer encapsulation), respectively, were also prepared and implemented in a separate lithium-ion cell. In all four cells, approximately 25-30% of graphite particles were mixed with the protected or unprotected Si nanowires (SiNW), along with 5% binder resin, to make an anode electrode. The cycling behaviors of these 4 cells are shown in
(122)
EXAMPLE 21
Effect of Lithium Ion-Conducting Additive in an Elastomer Shell
(123) A wide variety of lithium ion-conducting additives were added to several different sulfonated elastomer composites to prepare encapsulation shell materials for protecting core particles of an anode active material. We have discovered that these filled elastomer materials are suitable encapsulation shell materials provided that their lithium ion conductivity at room temperature is no less than 10.sup.−7 S/cm. With these materials, lithium ions appear to be capable of readily diffusing in and out of the encapsulation shell having a thickness no greater than 1 μm. For thicker shells (e.g. 10 μm), a lithium ion conductivity at room temperature no less than 10.sup.−4 S/cm would be required.
(124) TABLE-US-00002 TABLE 2 Lithium ion conductivity of various sulfonated elastomer composite compositions as a shell material for protecting anode active material particles. Li.sub.4Ti.sub.5O.sub.12-elastomer (1-2 μm thick); 5-10% Sample Lithium-conducting Li.sub.4Ti.sub.5O.sub.12 unless No. additive otherwise noted Li-ion conductivity (S/cm) E-1s Li.sub.2CO.sub.3 + (CH.sub.2OCO.sub.2Li).sub.2 70-99% polyurethane, 5.2 × 10.sup.−6 to 4.8 × 10.sup.−3 S/cm 2% RGO E-2s Li.sub.2CO.sub.3 + (CH.sub.2OCO.sub.2Li).sub.2 65-99% polyisoprene, 1.8 × 10.sup.−5 to 7.5 × 10.sup.−4 S/cm 8% pristine graphene E-3s Li.sub.2CO.sub.3 + (CH.sub.2OCO.sub.2Li).sub.2 65-80% SBR, 15% RGO 8.9 × 10.sup.−6 to 8.7 × 10.sup.−4 S/cm D-4s Li.sub.2CO.sub.3 + (CH.sub.2OCO.sub.2Li).sub.2 70-99% urethane-urea, 1.6 × 10.sup.−6 to 6.6 × 10.sup.−4 S/cm 12% nitrogenated graphene D-5s Li.sub.2CO.sub.3 + (CH.sub.2OCO.sub.2Li).sub.2 75-99% polybutadiene 2.2 × 10.sup.−5 to 7.9 × 10.sup.−3 S/cm B1s LiF + LiOH + Li.sub.2C.sub.2O.sub.4 80-99% chloroprene 1.7 × 10.sup.−6 to 6.8 × 10.sup.−4 S/cm rubber B2s LiF + HCOLi 80-99% EPDM 5.6 × 10.sup.−6 to 4.4 × 10.sup.−3 S/cm B3s LiOH 70-99% polyurethane 3.9 × 10.sup.−5 to 4.5 × 10.sup.−3 S/cm B4s Li.sub.2CO.sub.3 70-99% polyurethane 5.4 × 10.sup.−5 to 5.3 × 10.sup.−3 S/cm B5s Li.sub.2C.sub.2O.sub.4 70-99% polyurethane 2.4 × 10.sup.−5 to 3.2 × 10.sup.−3 S/cm B6s Li.sub.2CO.sub.3 + LiOH 70-99% polyurethane 2.6 × 10.sup.−5 to 4.1 × 10.sup.−3 S/cm C1s LiClO.sub.4 70-99% urethane-urea 5.7 × 10.sup.−5 to 4.8 × 10.sup.−3 S/cm C2s LiPF.sub.6 70-99% urethane-urea 4.7 × 10.sup.−5 to 1.7 × 10.sup.−3 S/cm C3s LiBF.sub.4 70-99% urethane-urea 3.2 × 10.sup.−5 to 4.4 × 10.sup.−4 S/cm C4s LiBOB + LiNO.sub.3 70-99% urethane-urea 8.7 × 10.sup.−6 to 3.5 × 10.sup.−4 S/cm S1s Sulfonated polyaniline 85-99% SBR 8.2 × 10.sup.−6 to 9.3 × 10.sup.−4 S/cm S2s Sulfonated SBR 85-99% SBR 7.8 × 10.sup.−6 to 5.8 × 10.sup.−4 S/cm S3s Sulfonated PVDF 80-99% chlorosulfonated 5.4 × 10.sup.−6 to 5.7 × 10.sup.−4 S/cm polyethylene (CS-PE) S4s Polyethylene oxide 80-99% CS-PE 6.6 × 10.sup.−6 to 4.7 × 10.sup.−4 S/cm
EXAMPLE 22
Cycle Stability of Various Rechargeable Lithium Battery Cells
(125) In lithium-ion battery industry, it is a common practice to define the cycle life of a battery as the number of charge-discharge cycles that the battery suffers 20% decay in capacity based on the initial capacity measured after the required electrochemical formation. Summarized in Table 3 below are the cycle life data of a broad array of batteries featuring presently disclosed elastomer-encapsulated anode active material particles vs. other types of anode active materials.
(126) TABLE-US-00003 TABLE 3 Cycle life data of various lithium secondary (rechargeable) batteries. Protective means; 1- Initial Sample 25% graphene; Type & % of anode capacity Cycle life (No. ID sulfonated elastomer active material (mAh/g) of cycles) Si-1i SBR-encapsulation, 25% by wt. Si 1,120 1,644-1,855 2% Gn nanparticles (80 nm) + 67% graphite + 8% binder Si-2i Carbon encapsulation 25% by wt. Si 1,242 251 nanparticles (80 nm) SiNW-1i Urea-Urethane 35% Si nanowires 1,258 1,720 encapsulation, 5% Gn (diameter = 90 nm) SiNW-2i ethylene oxide- 45% Si nanparticles, 1,766 1,880 (prelithiated); epichlorohydrin prelithiated or non- 1,355 (no prelithiation) copolymer, 8% Gn prelithiated (no pre-Li) VO.sub.2-1i Polyurethane 90%-95%, VO.sub.2 255 1886 encapsulation, 3% Gn nanoribbon Co.sub.3O.sub.4-2i Polyisoprene 85% Co.sub.3O.sub.4 + 8% 720 2,650 (Prelithiated); encapsulation, 10% Gn graphite platelets + 1,980 (no pre-Li) binder Co.sub.3O.sub.4-2i No encapsulation 85% Co.sub.3O.sub.4 + 8% 725 266 graphite platelets + binder SnO.sub.2-2i polybutadiene 75% SnO.sub.2 particles (3 740 1,675 encapsulation, 2% Gn μm initial size) SnO.sub.2-2i EPDM encapsulation, 75% SnO.sub.2 particles (87 738 3,565 (Pre-Li); 7% Gn nm in diameter) 2,250 (non pre-Li) Ge-1i butyl rubber 85% Ge + 8% graphite 850 1,666 encapsulation of C- platelets + binder coated Ge, 2% Gn Ge-2i Carbon-coated 85% Ge + 8% graphite 856 120 platelets + binder Al—Li-1i Polyurethane Al/Li alloy (3/97) 2,850 2,118 encapsulation, 25% Gn particles Al—Li-2i None Al/Li alloy particles 2,856 155 Zn—Li-1i Cis-polyisoprene C-coated Zn/Li alloy 2,626 1,755 encapsulation, 1% Gn (5/95) particles Zn—Li-2i None C-coated Zn/Li alloy 2,631 146 (5/95) particles
(127) These data further confirm the following: (1) The inorganic filler reinforced elastomer encapsulation strategy is surprisingly effective in alleviating the anode expansion/shrinkage-induced capacity decay problems. (2) The encapsulation of high-capacity anode active material particles by carbon or other non-elastomeric protective materials does not provide much benefit in terms of improving cycling stability of a lithium-ion battery (3) Prelithiation of the anode active material particles prior to elastomer encapsulation is beneficial. (4) Sulfonation further improves the lithium ion conductivity of an elastomer and, hence, power density of the resulting battery. (5) SEM examination of the anodes in various battery cells opened after hundreds of charge/discharge cycles reveal no significant amount of SEI formed on surfaces of anode active material particles or the inorganic filler reinforced elastomer shell containing an inorganic filler having a lithium intercalation potential higher than 1.1 V versus Li/Li.sup.+. This filled elastomer encapsulation strategy prevents continued consumption of the electrolyte and Li ions to repeatedly form additional SEI on surfaces of either the anode active material particles or the encapsulating shell.