Passivated pre-lithiated micron and sub-micron group IVA particles and methods of preparation thereof

Abstract

A method of producing a negative electrode, including comminuting Li-Group IVA alloy particles in a solvent to a desired particle size distribution range, exposing surfaces of the Li-Group IVA alloy particles to at least one surface modifier present during the comminution process, the at least one surface modifier forming at least one continuous coating on at least one of the exposed surfaces of the Li-Group IVA alloy particles, removing the solvent, and adding the surface-modified Li-Group IVA alloy particles to a negative electrode material by a coating process.

Claims

1. A method of producing a negative electrode, the method comprising: comminuting Li Group IVA alloy particles in a solvent to a particle size distribution range of about 50 to about 1,000 nm; exposing surfaces of the Li Group IVA alloy particles to at least one surface modifier present during the comminution process, wherein the at least one surface modifier comprises a Li salt selected from the group consisting of LiF and Li.sub.2CO.sub.3, wherein the at least one surface modifier forms at least one continuous coating on at least one of the surfaces of the Li Group IVA alloy particles; removing the solvent; and adding the surface-modified Li Group IVA alloy particles to a negative electrode material by a coating process.

2. The method according to claim 1, wherein comminuting includes using a circulating bead mill.

3. The method according to claim 2, wherein the milling bead is a hard ceramic material having a diameter from about 500 microns to about 700 microns.

4. The method according to claim 3, wherein an agitator tip-speed during comminuting is about 12.5 m/s for about 5 hours.

5. The method according to claim 1, wherein the solvent is selected from the group consisting of cycloalkanes, hexanes, heptanes, octanes, cyclohexanes, and any other saturated alkane.

6. The method according to claim 1, wherein the at least one surface modifier comprises a monomer additive or a polymer additive, wherein the monomer additive is selected from the group consisting of alkenes, alkynes, aromatics, heteroaromatics, cycloalkenes, alcohols, glycols, polyglycols, ethers, polyethers, thiols, disulfides, amines, amides, pyridines, pyrroles, imides, imidazoles, imidazoline, furans, thiophenes, cyanates, isocyanates, isothiocyanates, ketones, carboxylic acids, esters, amino acids, aldehydes, acrylates, methacrylates, oxylates, organic carbonates, lactones, and the gases H.sub.2, O.sub.2, CO.sub.2, N.sub.2O, and HF, and fluorinated analogs thereof when applicable; and wherein the polymer additive is selected from the group consisting of polystyrene, polyacrylonitrile, polyacrylic acid, lithium polyacrylate, and polyaniline, wherein the polymer additive covalently bonds directly to at least one of the surfaces of the Li Group IVA particles.

7. The method according to claim 1, wherein the at least one surface modifier comprises a metal-oxide or a metal-alkoxide; wherein the metal-oxide is selected from the group consisting of Al.sub.2O.sub.3, TiO.sub.2, Li.sub.4Ti.sub.5O.sub.12, MgO, NiO, and borates; and wherein the metal-alkoxide is selected from the group consisting of magnesium alkoxides, aluminum alkoxides, titanium alkoxides, zinc alkoxides, and lithium alkoxides.

8. The method according to claim 1, wherein the at least one surface modifier comprises an inorganic carbon surface modifier selected from the group consisting of single-walled carbon nanotubes (SWCN), multi-walled carbon nanotubes (MWCN), nanospherical carbon, fullerenes, graphene, graphite, and carbon black.

9. The method according to claim 1, wherein the continuous coatings form protective shells capable of impeding diffusion of oxygen and/or water to cores of the Li Group IVA alloy particles, wherein the continuous coatings are capable of allowing Li.sup.+ ion mobility and/or facilitate electrical charge transfer from the Li Group IVA alloy particles to an electrode current collector; and wherein the continuous coatings cover the surfaces of the Li Group IVA particles with layers greater than or equal to about 2 nm and less than or equal to about 500 nm thick.

10. The method according to claim 1, wherein removing the solvent includes an evaporation process performed by spray drying and/or evaporation under reduced pressure in a suitable atmosphere.

11. The method according to claim 10, further comprising: after exposing surfaces of the Li Group IVA alloy particles to at least one surface modifier, heating the Li Group IVA alloy particles at about 150° C. to about 1200° C. for about 30 minutes to about 24 hours under the suitable atmosphere, wherein the suitable atmosphere comprises air, Ar, Ar/H.sub.2, and/or vacuum.

12. The method according to claim 1, wherein a Li-organic complex or a Na-organic complex delivers Li to a silicon particle during comminution to form at least one of the Li Group IVA alloy particles; and wherein the Li-organic or the Na-organic complex is a polycyclic aromatic compound selected from the group consisting of pyrene, perylene, and naphthalene.

13. The method according to claim 12, further comprising: applying a current with Li foil as a counter-electrode and with a Si/graphite electrode laminated on a Cu current collector as a working electrode, wherein the polycyclic aromatic compound functions as an electrolyte and the laminate becomes partially charged with Li.

14. The method according to claim 1, wherein at least one of the Li Group IVA alloy particles is formed during comminuting from Li-active solar grade Si wafer kerf, and/or Li-active metallurgical silicon.

15. The method according to claim 1, further comprising: incorporating the negative electrode material into a slurry production process for producing a negative electrode for a battery.

16. The method according to claim 1, wherein the at least one continuous coating comprising the polymer surface modifier is thermally processed or crosslinked with a reagent; and wherein the reagent is a monomer additive selected from the group consisting of alkenes, alkynes, aromatics, heteroaromatics, cycloalkenes, alcohols, glycols, polyglycols, ethers, polyethers, thiols, disulfides, amines, amides, pyridines, pyrroles, imides, imidazoles, imidazoline, furans, thiophenes, cyanates, isocyanates, isothiocyanates, ketones, carboxylic acids, esters, amino acids, aldehydes, acrylates, methacrylates, oxylates, organic carbonates, lactones, and the gases H.sub.2, O.sub.2, CO.sub.2, N.sub.2O, HF, and fluorinated analogs thereof when applicable.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a first charge-discharge plot comparison between prelithiated laminate (green) and non-prelithiated laminate (blue). The First Cycle Efficiency (FCE) without prelithiation is about 83.5%, whereas with prelithiation the FCE is about 90%. It can be made to be about 100% with added prelithiated surface-modified silicon.

(2) FIG. 2 shows the first 50 charge-discharge cycles after the formation cycle (FCE about 88.5%) full cell with Nickel/Cobalt/Alumina (NCA) cathode. The anode/cathode ratio is about 1.1.

(3) FIG. 3 shows images of laminate made in aqueous binder. (a) About 5% prelithiated surface-modified silicon with protected shell. (b) About 5% prelithiated surface-modified silicon with non-protected shell. (c) Slurry of about 5% prelithiated Si with non-protected shell. Comparing the panels shows the stability of prelithiated surface-modified silicon product in aqueous binder.

(4) FIG. 4 shows a plot of the non-spherical particle size distributions (PSD) of prelithiated surface-modified silicon. D.sub.50 is about 500 nm, and the PSD is narrow. PSD is measured using dynamic light scattering (DLS) particle size analysis is based on Brownian motion light scattering.

(5) FIG. 5 shows an SEM image of the non-spherical surface-modified silicon represented in FIG. 3.

DETAILED DESCRIPTION

(6) Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways.

(7) The inventors of the present application have previously developed a flexible, scalable process (U.S. Pat. No. 9,461,304 incorporated herein by reference) to produce sub-micron surface-modified particles of Group IVA elements (U.S. Pat. No. 9,461,309 incorporated herein by reference). By employing this general process, it is possible to produce micron or sub-micron Li.sub.xSi.sub.y alloy particles encased with an inactive protective shell. This protective shell may function much like an artificial SEI layer.

(8) The inactive protective shell allows safe handling of Li.sub.xSi.sub.y alloy particles in air and aqueous environments. Without this protective shell, Li metal reacts exothermically with water and both Li and Si will quickly oxidize in air to form an oxide shell. While an oxide shell impedes the diffusion of oxygen to the reactive Li.sub.xSi.sub.y alloy core, it is insufficient to prevent water from reacting violently with Li.sub.xSi.sub.y alloy. By applying a continuous inactive protective shell that impedes the diffusion of water, the passivated Li.sub.xSi.sub.y alloy particles can be combined with binders and other components of the anode composite in common commercial slurry production processes used in conventional LIB manufacturing.

(9) The coated Li.sub.xSi.sub.y alloy may increase the cycle stability of the LIB. Si and other Group IVA elements are known to undergo large volume expansion during lithiation/delithiation (i.e. cycling). The volume expansion causes physical break down of the SEI, which leads to capacity loss as Li is consumed to form new SEI. However, the shell may be inactive and may not undergo significant volume change during cycling. Consequently, much less Li is consumed during lithiation and delithiation, leading to higher overall Columbic efficiencies and cycle life.

(10) This disclosure describes, among other things, how to produce coated Li.sub.xSi.sub.y alloys designed to enhance the performance of LIB negative electrodes. It should be understood that Ge and Sn or some combination of Si, Ge, and Sn could also be used to form alloys with Li that would function similarly as components of LIB negative electrodes. There are several possible methods in which these materials can be combined to create a composite material with the attributes of each component while also gaining synergies that the individual components lack by themselves. Examples used in the following discussion are intended to be non-limiting with respect to the reagents used in the examples.

(11) In the present disclosure, methods of producing micron or submicron scale Li.sub.xSi.sub.y alloy with an inactive coating are described. The methods described herein use a Li.sub.xSi.sub.y alloy as the feedstock for comminution in alkane or cycloalkane solvents, such as hexanes, heptanes, octanes, cyclohexanes, or any saturated alkane solvents. Other Li alloys can also be used with the methods of the present invention, including LiGe, LiSn, or other combinations of Si, Ge, and Sn with Li to form an alloy, for example. One exemplary alloy is Li.sub.15Si.sub.4. Depending on the desired particle size distribution (PSD) range of the Li.sub.xSi.sub.y alloy product, comminution conditions can be chosen from a combination of parameters that one skilled in the art of comminution will be able to select. Any suitable method of comminution may be used in the process of the present invention, including, but not limited to, milling, wet milling, crushing, grinding, cutting, vibrating, or other processes. If a milling process is used, agitator speeds and other conditions during comminution are chosen depending on the size of the mill, batch size, bead selection, solids loading, solvent selection circulation rate, and secondary reagents employed.

(12) For example, in some embodiments, Li.sub.xSi.sub.y alloy is comminuted by a circulating bead mill in an alkane solvent (including, but not limited to, cycloalkane solvents, hexanes, heptanes, octanes, cyclohexanes, or any saturated alkane solvents) with a polymer additive (including, but not limited, to polystyrene, polyacrylonitrile, polyacrylic acid (and its neutral Li salt), and polyaniline). Alternately or additionally, polymer coatings may be added post-milling on top of Li.sub.xSi.sub.y alloy particles with any surface coatings that were applied during comminution processing.

(13) The Li.sub.xSi.sub.y alloy particle size distribution is reduced to the desired PSD range by comminution, preferably about 1,000-44,000 nm, or more preferably about 50-1,000 nm, and more preferably between about 400-600 nm. Highly reactive surfaces of the Li.sub.xSi.sub.y alloy that are exposed on the fractured Li.sub.xSi.sub.y alloy particles will form covalent bonds to the polymer and any desired surface modifiers present during the comminution process. The polymer and co-reagents form a continuous coating (protective shell) on the Li.sub.xSi.sub.y alloy particles, covering all surfaces with an at least 2 nm to about 500 nm layer. The polymer coating may be thermally processed or crosslinked with added reagents, all part of the protective shell. The milling solvent is then removed by an evaporation process. This could be done by any number of methods known in the art, such as spray drying or evaporation under reduced pressure in a suitable atmosphere. Optionally, the particle can be heated, for example, to about 150-1200° C. for about 30 minutes to about 24 hours under a suitable gas (including, but not limited to, air, Ar, or Ar/H.sub.2) or in a vacuum to cure the protective shell. Curing is considered to be partial to complete cross-linking of polymer precursors or carbonization of the hydrocarbon mass on the Li.sub.xSi.sub.y alloy surfaces. Heating temperatures and durations will depend on what polymer coatings are used to form a protective shell. The resulting coated Li.sub.xSi.sub.y alloy material can then added to a conventional LIBs negative electrode composite using conventional coating processes (including, but not limited to, the addition of carbon black, graphite, or other additives used with aqueous binder systems in conventional anode slurries, or non-aqueous binder systems, such as NMP/PVdF).

(14) In one non-limiting example, 325-mesh Li.sub.15Si.sub.4 (about 48:52 wt %) was prepared thermochemically as the feedstock for wet milling processes in alkane or cycloalkane solvents, such as hexanes, heptanes, octanes, cyclohexanes, or any saturated alkane solvents with a boiling range well above the operation temperature of the milling slurry (typically between about 25-60° C.). Depending on the desired PSD range of the Li.sub.xSi.sub.y alloy product, comminution conditions can be determined. Milling beads and materials are typically selected from hard ceramic materials. Beads range in diameter from about 100-1,000 microns, preferably about 300-900 microns. The agitator tip-speeds during comminution are typically running between about 2-15 m/s for about 15-1200 minutes, depending on the size of the mill, batch size, bead selection, solids loading, solvent selection circulation rate, and secondary reagents employed.

(15) In another non-limiting example, Li.sub.15Si.sub.4 alloy was stirred under Ar for about 48 hours in an about 5% polyacrylonitrile (PAN) solution in dimethylsulfoxide (DMSO). The weight equivalent ratio of Li.sub.xSi.sub.y to PAN was about 7:3. The DMSO was stripped by vacuum distillation, and the remaining solids were dried for about 6 hours under dynamic vacuum at about 80° C. The remaining solid was heated for about 2 hours under Ar atmosphere to cure the protective coating. In an alternate embodiment, dimethylformamide (DMF) is used as the solvent in place of DMSO.

(16) In some embodiments, monomeric surface modifiers may be added in place of or in addition to polymers. The surface modifiers may be monomers with functional groups that react with the Li.sub.xSi.sub.y alloy particle surfaces or with the polymer to form chemical bonds. Because Li.sub.xSi.sub.y alloy is so reactive, almost any organic compound with heteroatoms and/or unsaturated bonds are potential surface modifiers. Reactive monomers could be selected from the group consisting of alkenes, alkynes, aromatics, heteroaromatics, cycloalkenes, alcohols, glycols, polyglycols, ethers, polyethers, thiols, disulfides, amines, amides, pyridines, pyrroles, imides, imidazoles, imidazoline, furans, thiophenes, cyanates, isocyanates, isothiocyanates, ketones, carboxylic acids, esters, amino acids, aldehydes, acrylates, methacrylates, oxylates, organic carbonates, lactones, and gases, such as H.sub.2, O.sub.2, CO.sub.2, N.sub.2O, and HF. Various fluorinated analogs of these compounds can also be used, such as trifluoroacetone, bis(2,2,2-trifluoroethyl) carbonate, 2,2,2-trifluoroethyl acrylate, 2,2,2-trifluoroethyl methacrylate, and 1,3,5-trifluorobenzene. The comminution solvent is then removed by any suitable method, and the coated Li.sub.xSi.sub.y alloy particle may or may not require heating under a suitable gas to cure the coating and form a protective shell. This protective shell may prevent water or other solvents used to combine elements of the electrode coating process from reacting with the Li.sub.xSi.sub.y core material. This coated Li.sub.xSi.sub.y alloy can be added to conventional LIBs negative electrode composite using conventional coating processes.

(17) Table 1 below shows several mass equivalents of reagents used for comminution, compared with product yields of comminution process and heat processing steps. Table 1 (supplemented by the drawings) demonstrates composition of matter of products after comminution and after heat treatment, as well as the processes disclosed herein.

(18) TABLE-US-00001 TABLE 1 Reagent Mass Yield Mass Equivalent Reagents Mass Equivalent Equivalent after heat treatment Li.sub.15Si.sub.4 4 13 13 M(i-OPr).sub.4 1 Alkane 400 Li.sub.15Si.sub.4 4 7 7 Polyether 1 Alkane 400

(19) In some embodiments, the Li.sub.xSi.sub.y alloy is comminuted in an inert alkane solvent in the presence of a metal-oxide or a metal-alkoxide reagent. Some examples of metal oxides are Al.sub.2O.sub.3, TiO.sub.2, Li.sub.4Ti.sub.5O.sub.12, MgO, NiO, and borates. Metal alkoxides constitute an important class of compounds often used in sol-gel processes. They are characterized by a metal-oxygen-carbon bonding system, including such metals as magnesium, aluminum, titanium, zinc, or lithium. Metal alkoxides in particular have proven to be especially beneficial during comminution as they modify particle surfaces to keep particles well suspended in the slurry with alkane solvents. If required, subsequent curing of the coated Li.sub.xSi.sub.y particles will form a protective shell comprised of metal oxides that sufficiently impedes ingress of solvents to the Li.sub.xSi.sub.y alloy particle core.

(20) In some embodiments, inorganic carbon (non-hydrocarbon) surface modifiers can be added at some stage during or following comminution of Li.sub.xSi.sub.y alloy and allowed to contact and form covalent bonds on highly reactive sites on newly fractured surfaces of the Li.sub.xSi.sub.y alloy particles. For example, the surface modifiers can be present at the beginning of the comminution process, or added after most of the particle size reduction has taken place but while surfaces are still very reactive (while there are many non-passivated sites). The progress of the comminution can be monitored by particle size distribution measurements and/or by monitoring slurry temperatures, viscosity, or power input. Inorganic carbon surface modifiers may be comprised from the group: carbon nanotubes (SWCNT, MWCNT), nanospherical carbon, fullerenes, graphene, graphite, or carbon black. Optionally, other hydrocarbons (monomers or polymers) may be added to help provide complete coverage of the Li.sub.xSi.sub.y particle surface and provide passivation of the Li.sub.xSi.sub.y alloy core from reactive solvents.

(21) In some embodiments, the coated Li.sub.xSi.sub.y alloy particles are blended with natural flake graphite (NFG). Prolonged blending of these dry powders under inert atmosphere can imbed sub-micron Li.sub.xSi.sub.y alloy particles into surface pores and crevices of NFG particles (typically about 10-20 micron). Subsequent coating of the NFG particles with a polymer or polymer precursors to provide a continuous coating on the NFG and over imbedded Li.sub.xSi.sub.y particles is a means of passivating the Li.sub.xSi.sub.y alloy while also spacing the Li.sub.xSi.sub.y particles in a Li-active matrix that will tolerate the expansion and contraction of Li.sub.xSi.sub.y alloys without breaking critical covalent bonds with its surrounding framework. Subsequent heat treatment may be required, particularly for polymer precursors to form cross-linked covalent bonds that impart added strength to the electrode composite.

(22) In some embodiments, the Li.sub.xSi.sub.y alloy is comminuted in an inert alkane solvent in the presence of hydrogen, which serves as a forming gas. A forming gas is an industrial gas comprised of inert gas blended with typically about 5% H.sub.2 or less. Forming gas can be used in the place of purified argon to blanket the slurry during comminution. Molecular hydrogen is reactive toward silicon surfaces. It can also and will migrate into silicon and other metals as atomic hydrogen and will form LiH on the surface of Li.sub.xSi.sub.y alloy particles. H.sub.2 is also known to cap (passivate) Si surfaces by forming Si—H bonds with “dangling” Si—Si bonds created from fracturing Si particles. This condition may be preferred when it is desired to produce Li.sub.xSi.sub.y alloy particles with no oxides.

(23) In some embodiments, comminution of Li.sub.xSi.sub.y alloy produces submicron or nanoparticle distributions (preferably about 44,000-1,000 nm, or more preferably about 50-1,000 nm, and more preferably between about 400-600 nm). Surface modifiers may be applied to the particles, which induces aggregation of the nanoparticles into micron-sized clusters. Grain structure in the micron-sized clusters is created from the coatings on the nanoparticles. These coatings can be heat-processed (cured) to form tight, porous covalently bonded masses of carbon and metal oxides in grains between the Li.sub.xSi.sub.y alloy core nanocrystals. The same coating that resides in grains between the Li.sub.xSi.sub.y alloy nanocrystals form a continuous protective shell around the micron-sized cluster that impedes ingress of solvents, but allows Li.sup.+ ion mobility and facilitates electrical charge transfer from the Li.sub.xSi.sub.y alloy particle core to the electrode current collector. The surface modifiers used in this process of making aggregated Li.sub.xSi.sub.y alloy clusters could be any of the organic reagents, metal oxides, or metal alkoxides disclosed herein.

(24) In some embodiments, Li salts (for example from the group LiF, Li.sub.2O.sub.2, Li.sub.2CO.sub.3, LiBF.sub.2(C.sub.2O.sub.4), Li.sub.2(C.sub.2O.sub.4)) can be added during comminution of Li.sub.xSi.sub.y alloy alone or with other surface modifiers, which can be heat processed and cured as described above to form a protective shell from a covalently bonded continuous layer of the additives that impedes ingress of solvents, but will allow Li.sup.+ ion mobility and will facilitate electrical charge transfer from the particle Li.sub.xSi.sub.y alloy core to the electrode current collector.

(25) In some embodiments, Li- or Na-organic complexes may be used with any source of Li-active Group IVA elements (e.g., solar grade Si or Ge wafer kerf or metallurgical silicon) to prepare Group IVA particles with partial insertion (prepared in-situ and added during the comminution process) of the alkali metals. For example, polycyclic aromatic (PA) compounds, such as pyrene, perylene, and naphthalene, form ion-paired Li.sup.+PA.sup.− complexes that can deliver Li to the Group IVA particle during comminution.

(26) The Li.sup.+PA.sup.− complexes described above can also function as an electrolyte in an appropriate solvent (such as gamma butyrolactone) whereupon a current is applied to a cell with Li foil as the counter electrode and with a Si/graphite electrode laminated on a Cu current collector as the working electrode. The Si particles in the Si/graphite electrode will undergo Li insertion. This electrode laminate is then partially charged with Li and can be used to make a battery with a partially charged negative electrode.

(27) The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (for example, it includes at least the degree of error associated with the measurement of the particular quantity). The modifier “about” should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression “from about 2 to about 4” also discloses the range “from 2 to 4”. The term “about” may refer to plus or minus 10% of the indicated number. For example, “about 10%” may indicate a range of 9% to 11%, and “about 1%” may mean from 0.9-1.1. Other meanings of “about” may be apparent from the context, such as rounding off, so, for example “about 1” may also mean from 0.5 to 1.4.

(28) For the recitation of numeric ranges herein, each intervening number there between with the same degree of precision is explicitly contemplated. For example, for the range of 6-9, the numbers 7 and 8 are contemplated in addition to 6 and 9, and for the range 6.0-7.0, the number 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, and 7.0 are explicitly contemplated.

(29) Thus, the invention provides, among other things, a method of producing a negative electrode. Various features and advantages of the invention are set forth in the following claims.