Multivalent metal ion battery having a cathode layer of protected graphitic carbon and manufacturing method

Abstract

Provided is a method of producing a multivalent metal-ion battery comprising an anode, a cathode, and an electrolyte in ionic contact with the anode and the cathode to support reversible deposition and dissolution of a multivalent metal, selected from Ni, Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr, Nb, Mn, V, Co, Fe, Cd, Cr, Ga, In, or a combination thereof, at the anode, wherein the anode contains the multivalent metal or its alloy as an anode active material and the cathode comprises a cathode active layer of graphitic carbon particles or fibers that are coated with a protective material. Such a metal-ion battery delivers a high energy density, high power density, and long cycle life.

Claims

1. A method of manufacturing a multivalent metal-ion battery, comprising: (a) providing an anode containing a multivalent metal or its alloy, wherein said multivalent metal is selected from Ni, Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr, Mn, V, Co, Fe, Cd, Cr, Ga, In, or a combination thereof; (b) providing a cathode active layer of graphitic carbon particles or fibers as a cathode active material that intercalates/de-intercalates ions; and (c) providing an electrolyte capable of supporting reversible deposition and dissolution of said multivalent metal at the anode and reversible adsorption/desorption and/or intercalation/de-intercalation of ions at the cathode; wherein said graphitic carbon particles or fibers are coated with a protective layer selected from carbonized resin, an ion-conducting polymer, an electrically conductive polymer, or a combination thereof; wherein said ion-conducting polymer is selected from the group consisting of sulfonated polymers, polypropylene oxide (PPO), poly bis-methoxy ethoxyethoxide-phosphazene, polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), and combinations thereof; wherein said electrically conducting polymer is selected from the group consisting of polyfuran, bi-cyclic polymers, derivatives thereof, and combinations thereof; wherein said protective layer is permeable to ions of said multivalent metal or ions dissolved in said electrolyte and said protective layer prevents or reduces exfoliation of graphitic planes in said graphitic carbon particles or fibers during a charge/discharge cycle of said battery, wherein said graphitic carbon particles or fibers have a hard carbon or amorphous carbon surface that is at least partially removed prior to being coated with said protective layer.

2. The method of claim 1, further including providing a porous network of electrically conductive nano-filaments to support said multivalent metal or its alloy.

3. The method of claim 1, wherein said graphitic carbon particles or fibers in said cathode active layer are selected from meso-phase pitch, meso-phase carbon, mesocarbon micro-beads (MCMB), coke particles/needles, expanded graphite flakes, artificial graphite particles, natural graphite particles, amorphous graphite containing graphite micro-crystallites, highly oriented pyrolytic graphite, soft carbon particles, hard carbon particles, multi-walled carbon nanotubes, carbon nano-fibers, carbon fibers, graphite nano-fibers, graphite fibers, carbonized polymer fibers, or a combination thereof.

4. The method of claim 1, wherein said step of providing a cathode active layer includes a procedure of cutting needle coke, carbon nano-fiber, carbon fiber, graphite nano-fiber, graphite fiber, or multi-walled carbon nanotube to obtain graphitic carbon fibers having an average length shorter than 10 μm.

5. The method of claim 1, wherein said graphitic carbon particles or fibers have a hard carbon or amorphous carbon surface that is removed prior to being coated with said protective layer.

6. The method of claim 1, wherein said electrolyte contains an aqueous electrolyte, an organic electrolyte, a polymer electrolyte, a molten salt electrolyte, an ionic liquid, or a combination thereof.

7. A method of manufacturing a multivalent metal-ion battery, comprising: (d) providing an anode containing a multivalent metal or its alloy, wherein said multivalent metal is selected from Ni, Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr, Mn, V, Co, Fe, Cd, Cr, Ga, In, or a combination thereof; (e) providing a cathode active layer of graphitic carbon particles or fibers as a cathode active material that intercalates/de-intercalates ions; and (f) providing an electrolyte capable of supporting reversible deposition and dissolution of said multivalent metal at the anode and reversible adsorption/desorption and/or intercalation/de-intercalation of ions at the cathode; wherein said graphitic carbon particles or fibers are coated with a protective layer selected from carbonized resin, an ion-conducting polymer, an electrically conductive polymer, or a combination thereof; wherein said ion-conducting polymer is selected from the group consisting of sulfonated polymers, polypropylene oxide (PPO), poly bis-methoxy ethoxyethoxide-phosphazene, polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), and combinations thereof; wherein said electrically conducting polymer is selected from the group consisting of polyfuran, bi-cyclic polymers, derivatives thereof, and combinations thereof; wherein said protective layer is permeable to ions of said multivalent metal or ions dissolved in said electrolyte and said protective layer prevents or reduces exfoliation of graphitic planes in said graphitic carbon particles or fibers during a charge/discharge cycle of said battery, wherein said multivalent metal-ion battery has an average discharge voltage no less than 1.0 volts and a cathode specific capacity no less than 125 mAh/g, wherein said graphitic carbon particles or fibers have a hard carbon or amorphous carbon surface that is removed prior to being coated with said protective layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1(A) Schematic of a multivalent metal secondary battery, wherein the anode layer is a thin multivalent metal coating or foil and the cathode active material layer contains a layer of graphitic carbon particles or fibers having a protective coating; and

(2) FIG. 1(B) Schematic of a multivalent metal secondary battery cell, wherein the anode layer is a thin multivalent metal coating or foil and the cathode active material layer is composed of graphitic carbon particles or fibers having a protective coating, a conductive additive (not shown), and a resin binder (not shown).

(3) FIG. 2 The discharge curves of two Zn foil anode-based cells; one containing a cathode layer of original graphite fibers and the other a cathode layer of surface-treated graphite fibers having hard carbon skin removed.

(4) FIG. 3 The discharge curves of two Ca-ion cells: one containing a cathode layer of carbon nanofibers (CNFs) having no hard carbon skin (skin having been chemically etched away) and the other a cathode layer of CNFs having a hard carbon skin.

(5) FIG. 4 The discharge curves of two Ni mesh anode-based cells; one containing a cathode layer of original MCMB particles and the other a cathode layer of surface-treated MCMB particles.

(6) FIG. 5 The specific capacity of two V-needle coke cells (one containing a cathode of sulfonated PVDF-protected needle coke and the other un-protected) plotted as a function of the number of charge/discharge cycles.

(7) FIG. 6 The specific capacity of two Mg-ion cells, one containing a cathode layer of MWCNTs protected by carbonized phenolic resin and the other un-protected. The electrolyte used was 1 M of MgCl.sub.2:Al Cl.sub.3 (2:1) in monoglyme.

(8) FIG. 7 Ragone plots of two Ti-ion cells, one having a cathode of surface-treated MCMB and the other having untreated MCMB.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(9) As schematically illustrated in the upper portion of FIG. 1(A), bulk natural graphite is a 3-D graphitic material with each graphite particle being composed of multiple grains (a grain being a graphite single crystal or crystallite) with grain boundaries (amorphous or defect zones) demarcating neighboring graphite single crystals. Each grain is composed of multiple graphene planes that are oriented parallel to one another. A graphene plane or hexagonal carbon atom plane in a graphite crystallite is composed of carbon atoms occupying a two-dimensional, hexagonal lattice. In a given grain or single crystal, the graphene planes are stacked and bonded via van der Waal forces in the crystallographic c-direction (perpendicular to the graphene plane or basal plane). The inter-graphene plane spacing in a natural graphite material is approximately 0.3354 nm.

(10) Artificial graphite materials, such as highly oriented pyrolytic graphite (HOPG), also contain constituent graphene planes, but they have an inter-graphene planar spacing, d.sub.002, typically from 0.336 nm to 0.365 nm, as measured by X-ray diffraction. Both the natural graphite and artificial graphite have a physical density being typically >2.1 g/cm.sup.3, more typically >2.2 g/cm.sup.3, and most typically very close to 2.25 g/cm.sup.3.

(11) Many carbon or quasi-graphite materials (herein referred to as graphitic carbon) also contain graphite crystals (also referred to as graphite crystallites, domains, or crystal grains) that are each composed of stacked graphene planes. However, the structure typically has a high proportion of amorphous or defect zones. These include meso-carbon micro-beads (MCMBs), meso-phase carbon, soft carbon, hard carbon, coke (e.g. needle coke), and carbon or graphite fibers (including vapor-grown carbon nano-fibers or graphite nano-fibers). The multi-walled carbon nanotubes (MW-CNT) does have very little defect or amorphous portion, but each CNT has a tubular structure. Hence, the multi-walled CNTs have a physical density of approximately 1.35 g/cm.sup.3. Other types of graphitic carbon have a typical density lower than 2.1 g/cm.sup.3, and more typically lower than 2.0 g/cm.sup.3, further more typically <1.9 g/cm.sup.3, and most typically <1.8 g/cm.sup.3.

(12) It may be noted that the “soft carbon” refers to a carbon material containing graphite domains wherein the orientation of the hexagonal carbon planes (or graphene planes) in one domain and the orientation in neighboring graphite domains are not too different or mis-matched from each other so that these domains can be readily merged together when heated to a temperature above 2,000° C. (more typically above 2,500° C.). Such a heat treatment is commonly referred to as graphitization. Thus, the soft carbon can be defined as a carbonaceous material that can be graphitized. In contrast, a “hard carbon” can be defined as a carbonaceous material that contain highly mis-oriented graphite domains that cannot be thermally merged together to obtain larger domains; i.e. the hard carbon cannot be graphitized.

(13) The present disclosure provides a multivalent metal secondary battery comprising an anode, a cathode, an optional porous separator electronically separating the anode and the cathode, and an electrolyte in ionic contact with the anode and the cathode to support reversible deposition and dissolution of the multivalent metal at the anode, wherein the anode contains multivalent metal or its metal alloy as an anode active material and the cathode comprises a layer of graphitic carbon particles or fibers (filaments), preferably selected from meso-phase carbon particles, mesocarbon micro-beads (MCMB), coke particles or needles, soft carbon particles, hard carbon particles, amorphous graphite containing graphite micro-crystallites, multi-walled carbon nanotubes, carbon nano-fibers, carbon fibers, graphite nano-fibers, graphite fibers, or a combination thereof. These graphitic carbon fibers or particles are coated with a thin layer of a protective material.

(14) We have observed that some graphitic carbon materials, such as meso-phase carbon particles, mesocarbon micro-beads (MCMB), coke particles or needles, soft carbon particles, hard carbon particles, carbon nano-fibers, carbon fibers, graphite nano-fibers, and graphite fibers, have a thin skin layer of hard carbon naturally formed in their surfaces when these synthetic graphitic carbon particles or fibers are produced. We have surprisingly observed that it is highly beneficial to subject these particles or fibers to a surface treatment (e.g. surface chemical etching, surface plasma cleaning, etc.) to remove some or all of the hard carbon on their exterior surface.

(15) In certain preferred embodiments, the graphitic carbon (e.g. meso-phase carbon particles, MCMBs, coke particles or needles, soft carbon particles, hard carbon particles, amorphous graphite, multi-walled carbon nanotubes, and carbon nano-fibers), with or without the above-stated surface treatment, may be coated with a protective layer that is permeable to multivalent metal ions or ions dissolved in the electrolyte and that prevents or reduces exfoliation of graphitic planes in the graphitic carbon particles or fibers. We have surprisingly observed that, upon repeated intercalation/de-intercalation of multivalent metal ions and other electrolyte-derived ions into and out of the graphitic crystallites or domains could cause expansion of inter-planar spaces between graphene planes and exfoliation of graphene planes (hexagonal carbon atom planes). This effect, although can initially increase the charge storage capacity of the cathode material, later causes severe graphene plane exfoliation to the extent that the cathode layer structural integrity is compromised and the charge storage capability rapidly decays. By depositing a thin layer of protective material on surfaces of the graphitic carbon particles or fibers, one could significantly improve the structural integrity and cycle stability of the cathode layer.

(16) This protective material may be selected from reduced graphene oxide (that wraps around the graphitic carbon particles), carbonized resin (or polymeric carbon), ion-conducting polymers (e.g. sulfonated polymers), and electrically conductive polymers. Reduced graphene oxide sheets have many naturally occurring surface defects (pores) that are permeable to all the ions of interest. The polymeric carbon may be selected from a polymer of low carbon content (e.g. epoxy resin or polyethylene) or high carbon content (e.g. phenolic resin or polyacrylonitrile). The electrically conducting polymer may be selected from polyaniline, polypyrrole, polythiophene, polyfuran, a bi-cyclic polymer, derivatives thereof (e.g. sulfonated versions), or a combination thereof.

(17) In some embodiments, the ion-conducting polymer is selected from a sulfonated polymer, poly(ethylene oxide) (PEO), Polypropylene oxide (PPO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVdF), Poly bis-methoxy ethoxyethoxide-phosphazenex, Polyvinyl chloride, Polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a combination thereof.

(18) Sulfonation also generates pores that are permeable to metal ions. The sulfonated polymer may be selected from the group consisting of sulfonated poly(perfluoro sulfonic acid), sulfonated poly (tetrafluoroethylene), sulfonated perfluoroalkoxy derivatives of polytetrafluoroethylene, sulfonated polysulfone, sulfonated poly(ether ketone), sulfonated poly (ether ether ketone), sulfonated polyimide, sulfonated styrene-butadiene copolymers, sulfonated polystyrene, sulfonated poly chloro-trifluoroethylene (PCTFE), sulfonated perfluoroethylene-propylene copolymer (FEP), sulfonated ethylene-chlorotrifluoroethylene copolymer (ECTFE), sulfonated poly vinylidenefluoride (PVDF), sulfonated copolymers of polyvinylidenefluoride with hexafluoropropene and tetrafluoroethylene, sulfonated copolymers of ethylene and tetrafluoroethylene (ETFE), polybenzimidazole (PBI), their chemical derivatives, copolymers, blends, and combinations thereof.

(19) The configuration of a multivalent metal secondary battery is now discussed as follows:

(20) A multivalent metal-ion battery includes a positive electrode (cathode), a negative electrode (anode), and an electrolyte typically including a metal salt and a solvent. The anode can be a thin foil or film of a multivalent metal or its alloy with another element(s); e.g. 0-10% by weight of Sn in Zn. The multivalent metal may be selected from Ni, Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr, Nb, Mn, V, Co, Fe, Cd, Cr, Ga, In, or a combination thereof. The anode can be composed of particles, fibers, wires, tubes, or discs of the multivalent metal or metal alloy that are packed and bonded together by a binder (preferably a conductive binder) to form an anode layer.

(21) We have observed that a select multivalent metal (e.g. Ni, Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr, Mn, V, Co, Fe, Cd, Ga, or Cr), when coupled with a presently invented graphite or carbon material having expanded inter-graphene planar spaces, can exhibit a discharge curve plateau or average output voltage at approximately 1.0 volt or higher. This plateau regime of a discharge voltage vs. time (or capacity) curve enables the battery cell to provide a useful constant voltage output. A voltage output lower than 1 volt is generally considered as undesirable. The specific capacity corresponding to this plateau regime is typically from approximately 100 mAh/g (e.g. for Zr or Ta) to above 600 mAh/g (e.g. for Zn or Mg).

(22) A desirable anode layer structure is composed of a network of electron-conducting pathways (e.g. mat of graphene sheets, carbon nano-fibers, or carbon-nanotubes) and a thin layer of the multivalent metal or alloy coating deposited on surfaces of this conductive network structure. Such an integrated nano-structure may be composed of electrically conductive nanometer-scaled filaments that are interconnected to form a porous network of electron-conducting paths comprising interconnected pores, wherein the filaments have a transverse dimension less than 500 nm. Such filaments may comprise an electrically conductive material selected from the group consisting of electro-spun nanofibers, vapor-grown carbon or graphite nanofibers, carbon or graphite whiskers, carbon nano-tubes, nano-scaled graphene platelets, metal nanowires, and combinations thereof. Such a nano-structured, porous supporting material for the multivalent metal can significantly improve the metal deposition-dissolution kinetics at the anode, enabling high-rate capability of the resulting multivalent metal secondary cell.

(23) Illustrated in FIG. 1(A) is a schematic of a multivalent metal secondary battery, wherein the anode layer is a thin multivalent metal coating or foil and the cathode active material layer contains a layer of graphitic carbon fibers or particles, an optional resin binder (not shown), and an optional conductive additive (not shown). Alternatively, FIG. 1(B) shows a schematic of a multivalent metal secondary battery cell wherein the cathode active material layer is composed of particles or fibers of a graphitic carbon material and a resin binder (not shown) that helps to bond the particles or fibers together to form a cathode active layer of structural integrity.

(24) The surface treated and/or surface-protected graphitic carbon materials, when implemented as a cathode active material, enable the multivalent metal-ion cell to exhibit a voltage plateau portion in a discharge voltage-time or voltage-capacity curve obtained at a constant current density. This plateau portion typically occurs at a relatively high voltage value intrinsic to a given multivalent metal, and typically lasts a long time, giving rise to a high specific capacity.

(25) The composition of the electrolyte, which functions as an ion-transporting medium for charge-discharge reaction, has a great effect on battery performance. To put multivalent metal secondary batteries to practical use, it is necessary to allow metal ion deposition-dissolution reaction to proceed smoothly and sufficiently even at relatively low temperature (e.g., room temperature).

(26) In the invented multivalent metal-ion battery, the electrolyte typically contains a metal salt dissolved in a liquid solvent. The solvent can be water, organic liquid, ionic liquid, organic-ionic liquid mixture, etc. In certain desired embodiments, the metal salt may be selected from NiSO.sub.4, ZnSO.sub.4, MgSO.sub.4, CaSO.sub.4, BaSO.sub.4, FeSO.sub.4, MnSO.sub.4, CoSO.sub.4, VS0.sub.4, TaSO.sub.4, CrSO.sub.4, CdSO.sub.4, GaSO.sub.4, Zr(SO.sub.4).sub.2, Nb.sub.2(S.sup.O.sub.4).sub.3, La.sub.2(SO.sub.4).sub.3, MgCl.sub.2, AlCl.sub.3, Mg(C10.sub.4).sub.2, Mg(BF.sub.4).sub.2, Alkyl Grignard reagents, magnesium dibutyldiphenyl, Mg(BPh2Bu2)2, magnesium tributylphenyl Mg(BPhBu3)2), or a combination thereof.

(27) The electrolyte may in general comprise at least a metal ion salt selected from a transition metal sulfate, transition metal phosphate, transition metal nitrate, transition metal acetate, transition metal carboxylate, transition metal chloride, transition metal bromide, transition metal nitride, transition metal perchlorate, transition metal hexafluorophosphate, transition metal borofluoride, transition metal hexafluoroarsenide, or a combination thereof.

(28) In certain embodiments, the electrolyte comprises at least a metal ion salt selected from a metal sulfate, phosphate, nitrate, acetate, carboxylate, chloride, bromide, nitride, or perchlorate of zinc, aluminum, titanium, magnesium, calcium, manganese, cobalt, nickel, iron, vanadium, tantalum, gallium, chromium, cadmium, niobium, zirconium, lanthanum, or a combination thereof.

(29) In the multivalent metal-ion battery, the electrolyte comprises an organic solvent selected from ethylene carbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), methyl butyrate (MB), ethyl propionate, methyl propionate, propylene carbonate (PC), γ-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methyl formate (MF), tetrahydrofuran (THF), toluene, xylene, methyl acetate (MA), or a combination thereof.

(30) This disclosure is directed at the cathode active layer (positive electrode layer) containing a high-capacity cathode material for the multivalent metal secondary battery. The disclosure also provides such a battery based on an aqueous electrolyte, a non-aqueous electrolyte, a molten salt electrolyte, a polymer gel electrolyte (e.g. containing a metal salt, a liquid, and a polymer dissolved in the liquid), or an ionic liquid electrolyte. The shape of a multivalent metal secondary battery can be cylindrical, square, button-like, etc. The present disclosure is not limited to any battery shape or configuration.

(31) The following examples are used to illustrate some specific details about the best modes of practicing the instant disclosure and should not be construed as limiting the scope of the disclosure.

EXAMPLE 1

(32) Cathode Layer Containing Needle Coke Commercially available needle coke (Jinzhou Petrochemical Co.) was used to prepare cathode active material layers. Both surface treated and non-treated needle coke powders were studied. A sample of surface-treated needle coke (needle-shape coke filaments) was prepared by immersing the filaments in concentrated sulfuric acid for 2 hours to remove the hard carbon skin. The rinsed and dried powder was then mixed with a PVDF binder in a solvent (NMP) to form a slurry, which was coated on a sheet of carbon paper (as a current collector) to form a cathode layer.

EXAMPLE 2

Various Graphitic Carbon and Graphite Materials

(33) Several cathode layers were prepared according to the same procedure as used in Example 1, but the starting graphite materials were powders of highly oriented pyrolytic graphite (HOPG), natural graphite powder, pitch-based graphite fiber, vapor-grown carbon nano-fiber (VG-CNF), and amorphous graphite, respectively.

EXAMPLE 3

Preparation of Graphite Oxide Using a Modified Hummers' Method and Subsequent Wrapping of Amorphous graphite with graphene oxide sheets

(34) Graphite oxide was prepared by oxidation of natural graphite flakes with sulfuric acid, sodium nitrate, and potassium permanganate according to the method of Hummers [U.S. Pat. No. 2,798,878, Jul. 9, 1957]. In this example, for every 1 gram of graphite, we used a mixture of 22 ml of concentrated sulfuric acid, 2.8 grams of potassium permanganate, and 0.5 grams of sodium nitrate. The graphite flakes were immersed in the mixture solution and the reaction time was approximately 4 hours at 35° C. It is important to caution that potassium permanganate should be gradually added to sulfuric acid in a well-controlled manner to avoid overheat and other safety issues. Upon completion of the reaction, the sample was then washed repeatedly with deionized water until the pH of the filtrate was approximately 5. The solution was ultrasonicated for 30 minutes to produce graphene oxide suspension.

(35) Powder of amorphous graphite containing micro-crystallites was poured into the graphene oxide suspension to form a slurry. The slurry was spray-dried to form graphene oxide-wrapped amorphous graphite particles (protected particulates). We have observed that the cycle life of protected amorphous graphite particulates (defined as the number of charge/discharge cycles when a 20% reduction of capacity is reached) is significantly longer than that of the unprotected amorphous graphite particles (>3,000 cycles for protected particulates vs. <1,000 cycles for un-protected particles).

EXAMPLE 4

Cathode Active Layer Containing Soft Carbon Particles

(36) Particles of soft carbon were prepared from a liquid crystalline aromatic resin. The resin was ground with a mortar, and calcined at 900° C. for 2 h in a N.sub.2 atmosphere to prepare the graphitizable carbon or soft carbon. Soft carbon particles were then surface treated with a 30% aqueous solution of sulfuric acid at room temperature for 2 hours to remove hard carbon skin. The rinsed and dried soft carbon particles were then coated with sulfonated PEEK.

EXAMPLE 5

Petroleum Pitch-Derived Hard Carbon Particles

(37) A pitch sample (A-500 from Ashland Chemical Co.) was carbonized at 900° C. for 2 hours, followed by carbonization at 1,200° C. for 4 hours. A solution of KOH in water (5% concentration) was used to surface-treat the hard carbon particles for the purpose of removing the skin carbon layer of the pitch-based hard carbon particles.

EXAMPLE 6

Meso-Phase Carbon

(38) Optically anisotropic spherical carbon (average particle size: 25 μm, quinoline soluble: 5%) was prepared from coal-based meso-phase pitch by heat treating the pitch at 500° C. for 2 hours, carbonized at 900° C. for 2 hours and then partially graphitized at 2,500° C. for 1 hour. The graphitic carbon particles were then coated with sulfonated polyaniline.

EXAMPLE 7

(39) Multi-Walled Carbon Nanotubes (MW-CNTs) of Different Tube LVengths Powder samples of MW-CNTs (5% by weight) were dispersed in water with a 0.5% by weight of a surfactant to form several suspensions. The suspensions were then ultrasonicated for 30 minutes, 1 hour, and 3 hours, respectively. One of the samples (3 hours) was further ball-milled in a high-intensity mill for 5 hours. The resulting CNT samples have different average CNT lengths (43.5 μm, 3.9 μm, and 0.32 μm, respectively). Some CNTs were protected with phenolic resin which was carbonized.

EXAMPLE 8

Preparation and Testing of Various Multivalent Metal-Ion Cells

(40) The particles or fibers of graphitic carbon materials prepared in Examples 1-7 were separately made into a cathode layer and incorporated into a metal-ion secondary battery. The cathode layer was prepared in the following way. As an example, first of all, 95% by weight of the graphitic carbon fibers or particles with or without surface treatments or coatings were mixed together with PVDF (a binder) in NMP to obtain a slurry mixture. The slurry mixture was then cast onto a glass surface to make a wet layer, which was dried to obtain a cathode layer.

(41) Two types of multivalent metal anode were prepared. One was metal foil having a thickness from 20 μm to 300 μm. The other was metal thin coating deposited on surfaces of conductive nano-filaments (e.g. CNTs) or graphene sheets that form an integrated 3D network of electron-conducting pathways having pores and pore walls to accept a multivalent metal or its alloy. Either the metal foil itself or the integrated 3D nano-structure also serves as the anode current collector.

(42) Cyclic voltammetry (CV) measurements were carried out using an Arbin electrochemical workstation at a typical scanning rate of 0.5-50 mV/s. In addition, the electrochemical performances of various cells were also evaluated by galvanostatic charge/discharge cycling at a current density from 50 mA/g to 10 A/g. For long-term cycling tests, multi-channel battery testers manufactured by LAND were used.

(43) FIG. 2 shows the charge and discharge curves of two Zn foil anode-based cells: one Zn-ion cell containing a cathode layer of original graphite fibers and the other a cathode layer of surface-treated graphite fibers having hard carbon skin removed. The discharge curve of the Zn-ion cell featuring skin-free graphite fibers exhibits a longer plateau regime at 1.15-1.35 volts and a higher specific capacity (plateau ending at 150 mAh/g and overall capacity being 180 mAh/g) relative to the cell having a cathode of original untreated graphite fibers (plateau ending at 20 mAh/g and overall capacity being 35 mAh/g). The resulting cell-level energy density is approximately 100 Wh/kg, higher than those of nickel metal hydride and very close to those of lithium-ion batteries. Zinc is more abundant, safer, and significantly less expensive than lithium, nevertheless.

(44) Shown in FIG. 3 are the discharge curves of two Ca-ion cells: one containing a cathode layer of carbon nanofibers (CNFs) having no hard carbon skin (skin having been chemically etched away) and the other a cathode layer of CNFs having a hard carbon skin. The skin-free CNFs enable a Ca-ion cell to deliver a discharge curve plateau up to 80 mAh/g, as opposed to the mere 30 mAh/g of the cell featuring un-treated CNFs.

(45) FIG. 4 shows the discharge curves of two Ni mesh anode-based cells; one Ni-ion cell containing a cathode layer of original MCMB particles and the other a cathode layer of surface-treated MCMB particles. Again, by removing the hard carbon skin from a graphitic carbon particle, one can significantly increase the ion storage capability, 105 mAh/g vs. 52 mAh/g in this case.

(46) Summarized in Table 1 below are the typical plateau voltage ranges of the discharge curves of a broad array of multivalent metal-ion cells using skin-free artificial graphite, graphite fibers, and CNFs as a cathode active material. The specific capacity is typically from 100 to 250 mAh/g. In contrast, for each type of battery cell, the corresponding graphitic carbon having hard carbon skin provides very limited ion storage capability (typically <50 mAh/g).

(47) TABLE-US-00001 TABLE 1 Plateau voltage ranges of the discharge curves in multivalent metal-ion cells. Anode Metal Voltage range Ba 3.45-3.55 V Ca 3.25-3.35 V La 2.84-3.05 V Mg 2.85-3.01 V Be 2.36-2.51 V Ti 2.15-2.22 V Zr 1.97-2.07 V Mn 1.77-1.85 V V 1.74-1.82 V Nb 1.67-1.73 V Zn 1.20-1.35 V Cr 1.16-1.31 V Ta 1.14-1.25 V Ga 1.10-1.18 V Fe 0.96-1.13 V Cd 0.95-1.10 V Co 0.88-0.98 V Ni 0.86-0.95 V

(48) FIG. 5 shows the specific capacity of two V-needle coke cells (one V-ion cell containing a cathode of sulfonated PVDF-protected needle coke and the other un-protected) plotted as a function of the number of charge/discharge cycles. These data indicate that the V-ion cell can maintain a 90% capacity over 2500 cycles if the needle coke particles are protected by a select coating. In contrast, the V-ion cell containing un-protected needle coke suffers a 20% reduction in capacity after approximately 1,000 charge/discharge cycles.

(49) Similarly, FIG. 6 shows the specific capacity of two Mg-ion cells, one containing a cathode layer of MWCNTs protected by carbonized phenolic resin and the other un-protected. The protected version enables a significantly higher level of cycling stability.

(50) Summarized in FIG. 7 are the Ragone plots of two Ti-ion cells, one having a cathode of surface-treated MCMB and the other one untreated MCMB. The treated MCMB beads having their hard carbon skin substantially removed enable the Ti-ion cell to deliver a higher energy density and higher power density.

(51) We have also observed that shorter carbon nanotubes or carbon nano-fibers, when implemented as a cathode active material, lead to a higher energy density and higher power density.

(52) Additionally, we have discovered that by supporting the multivalent metal (in a thin film or coating form) on a nano-structured network composed of interconnected carbon or graphite filaments (e.g. carbon nanotubes or graphene sheets) one can significantly increase the power density and high-rate capability of a metal-ion cell. This nano-structured network of interconnected carbon nano-fibers provides large surface areas to support multivalent metal and facilitate fast and uniform dissolution and deposition of metal cations at the anode side. Other nano-filaments or nano-structures that can be used to make such a network include electro-spun nanofibers, vapor-grown carbon or graphite nanofibers, carbon or graphite whiskers, carbon nano-tubes, metal nanowires, or a combination thereof.