Disordered rocksalt material and method of forming it

Abstract

A composition comprising a disordered rocksalt having a cation comprised of lithium and at least one other metal and an anion comprised of oxygen having a coating comprised of a borate comprised of lithium may be made by mixing a disordered rocksalt with a borate to form a mixture, and heating the mixture to an annealing temperature of 400 °C to 700 °C to form the coated disordered rocksalt.

Description

DISORDERED ROCKSALT MATERIAL AND METHOD OF FORMING ITFIELD

[0001] The present invention is in the field of battery technology. In particular, it relates to a disordered rocksalt having a coating.BACKGROUND

[0002] Lithium metal oxides have been used to formulate cathode materials for lithium ion batteries. The cathodes are derived from a few basic crystallographic structure types, such as spinels, olivines, and layered oxide structures. The layered oxide structures have included lithium- excess type structures, where additional lithium is present in the structure.

[0003] Recently, attention has been focused on disordered rocksalt structures, such as those formed from particular lithium metal oxides. Compounds represented by the formula: xLi3NbO4*(l-x)LiMO2(1) where M is a divalent or trivalent cation, have been shown to be a promising class of transition metal oxides for use as cathodes in lithium ion batteries. The compounds of formula (1) are considered a disordered rocksalt in which a random atomic arrangement of lithium and transition metal ions are packed in a closely-packed cubic structure. These disordered rocksalt compositions offer the ability to contain up to 3 lithium atoms per formula unit, which is more than the conventional lithium-excess layered materials. Formula (1) can be transformed and represented as LixMyNzOw.

[0004] The disordered rocksalt structure is an attractive cathode material for next generation lithium ion batteries due to a greater specific energy density (e.g., a higher theoretical energy density) than state-of-the-art cathode materials, such as layered lithium metal oxide structures. For example, certain disordered rocksalt structure materials have a theoretical gravimetric energy density of about 1120 Wh / kg, while a LiMn2O4 active material has a theoretical gravimetric energy density of about 492 Wh / kg and a LiMm.5Nio.5O4 has a theoretical gravimetric energy density of about 691 Wh / kg. This energy density is especially appealing when lower cost raw materials are used as components in the disordered rocksalt structure, such as manganese. As such, thedisordered rocksalt materials can achieve relatively high energy density with relatively low material cost. In order to achieve comparable energy density, known cathode materials require higher-cost raw materials, such as cobalt or nickel.

[0005] Disordered rocksalt materials have tended to have a shorter cycle life compared to incumbent lithium ion batteries. Recently, attempts to improve the cycle life of disordered rocksalt batteries has been described by substituting some of the oxygen with fluorine such as described in U.S. Pat. No. 10,280,092. Nevertheless, it would be desirable to provide a battery comprised of disordered rocksalt cathodes having longer life and other desirable attributes such as safer batteries.BRIEF SUMMARY

[0006] We discovered a coated disordered rocksalt (coated DR). The coated DR may exhibit longer long cycle life and inhibition of resistance growth over the lifetime of the battery.

[0007] Herein, when a majority is specified of a component, it means more than 50% by mole or (readily understood from the context used) to essentially all of that component (99% or less). That is, the majority specified constituent of a component is present in an amount greater than 50% to 99%, 90, 80%, 70% or 60% of that component. When a minority of a component is a specified constituent, it is present in an amount less than 50% to about 1% with the balance being the majority specified constituent.

[0008] An illustration is a composition comprising a disordered rocksalt having a cation comprised of lithium and at least one other metal and an anion comprised of oxygen having a coating comprised of a borate comprised of lithium.

[0009] Another illustration is a method of forming a coated disordered rocksalt, comprising, (i) mixing a disordered rocksalt with a borate to form a mixture, and (ii) heating the mixture to an annealing temperature of 400 °C to 700 °C to form the coated disordered rocksalt.

[0010] The coated DR may be used in primary and secondary batteries comprised of lithium. The coated DR may be used with any suitable electrolyte, separator and anode such as those known in the ait.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 shows the initial capacity of half cells comprised of coated disordered rocksalts of the invention and rocksalts not of the invention.

[0012] Figure 2 shows first cycle coulombic efficiency of half cells comprised of a coated disordered rocksalt of the invention and rocksalts not of the invention.

[0013] Figure 3 shows first cycle energy density of half cells comprised of a coated disordered rocksalt of the invention and rocksalts not of the invention.

[0014] Figure 4 shows the 1C discharge capacity of half cells comprised of a coated disordered rocksalt of the invention and a rocksalt not of the invention.

[0015] Figure 5 shows the total resistance of half cells comprised of a coated disordered rocksalt of the invention and rocksalts not of the invention as a function of discharge cycle.

[0016] Figure 6 shows the capacity retention of half cells comprised of a coated disordered rocksalt of the invention and rocksalts not of the invention.DETAILED DESCRIPTION

[0017] The following definitions apply to some of the aspects described with respect to some embodiments of the invention. These definitions may likewise be expanded upon herein. Each term is further explained and exemplified throughout the description, figures, and examples. Any interpretation of the terms in this description should take into account the full description, figures, and examples presented herein.

[0018] The singular terms “a,” “an,” and “the” include the plural unless the context clearly dictates otherwise. Thus, for example, reference to an object can include multiple objects unless the context clearly dictates otherwise.

[0019] A rate “C” refers to either (depending on context) the discharge current as a fraction or multiple relative to a “1 C” current value under which a battery (in a substantially fully charged state) would substantially fully discharge in one hour, or the charge current as a fraction or multiplerelative to a “1 C” current value under which the battery (in a substantially fully discharged state) would substantially fully charge in one hour.

[0020] To the extent certain battery characteristics can vary with temperature, such characteristics are specified at 30 degrees C, unless the context clearly dictates otherwise.

[0021] Ranges presented herein are inclusive of their endpoints. Thus, for example, the range 1 to 3 includes the values 1 and 3 as well as the intermediate values.

[0022] The DR compositions and morphology (e.g., structure) are useful in formulating electrodes of electrochemical cells. More specifically, the coated DR may be used to form the cathode. The lithium ion battery includes an electrolyte formulation with a lithium salt present at a concentration suitable for conducting the lithium ions through the electrolyte formulation between the cathode and an anode during the discharge and recharge operations.

[0023] In a disordered rocksalt both lithium and a transition metal occupy a cubic close-packed lattice of octahedral sites. In electrochemical reactions, lithium diffusion proceeds by the lithium hopping from one octahedral site to another octahedral site via an intermediate tetrahedral site. Lithium in the intermediate tetrahedral site is the activated state in lithium diffusion. The activated tetrahedral lithium ion shares faces with four octahedral sites as follows: (i) the site previously occupied by the lithium ion itself; (ii) the vacancy the lithium ion will move into; and (iii & iv) two sites that can be occupied by lithium, a transition metal, or a vacancy.

[0024] The composition may be represented byLix y zO2-(a+b)Fa Zb where 1.0<x<1.75; 0.01 <y<0.55; 0.1 <z<l; 0<(a+b)<0.7; (b>0) M’ is one of Ti, Ta, Zr, W, Nb, or Mo; M is one or more of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Zr, Y, Mo, Ru, Rh and Sb; Z is one or more of P, N and S.

[0025] If Z is present it may be desirable for b to be 0.05 to 0.25. Z may be any combination of P, N and S, or may be just one of them.

[0026] The composition may have any desirable Li of 1 or above, but it may be desirable for Li as represented by x to be at least 1.1, 1.15, 1.2 to 1.65, 1.5 or 1.4.

[0027] The cation of the composition may be the metals described, but desirably, at least one of the metals as represented by M is comprised of one or more of Ti, Mn, Fe, Co,V, Cr, Ni and Cu. It may be desirable for M to be comprised of Ti and Mn. The composition may illustratively be one where M’ is comprised of Nb. When Nb is present, it may desirable for M to be comprised of Mn. Illustratively, M’ may be Nb and M may be Mn. When Nb and Mn are present with or without other metals, they may be present in a ratio of Mn / Nb of 1 or 2 to 5 or 10 by mole.

[0028] The DR may be made by any suitable method such as those known in the art to make disordered rocksalts. Illustrative methods are described in U.S. Pat. Nos. 10,280,092, 10,978,706, copending U.S. application 63 / 348,797, and ACS Appl Mater Interfaces. 2019 Oct 2;11(39):35777-35787, each incorporated herein by reference.

[0029] Illustratively, the DR desirably is comprised of micro-sized clusters or agglomerations of sub-micro-sized particles, which may be useful to increase the capacity and energy density of a battery cathode. The micro-sized clusters are also referred to herein as secondary particles. The secondary particles desirably have average particle sizes (e.g., diameters) in the micrometer scale, such as between 1 micrometer and 20 micrometers. The sub-micro-sized particles cluster to form the secondary particles. The sub-micro-sized particles are also referred to herein as primary particles. The terms “primary” and “secondary” indicate that the primary particles are formed before the secondary particles, and the secondary particles are agglomerations of the primary particles. The primary particles have average particle sizes (e.g., diameters) in the nanometer scale, such as less than 400 nanometers. The sub-micro primary particles of disordered rocksalt material may provide desirable conductivity and the micro-sized secondary particles of the disordered rocksalt material yield high electrode energy density.

[0030] The composition comprises a DR having a cation of lithium and another metal and anion comprised of oxygen having a coating comprised of a lithium borate. The coating is desirably comprised of lithium borate, which may arise from the reaction product of a deposited lithium compound on the DR or the lithium present in the DR and a deposited borate (e.g., boric acid). The metal of the coating may be comprised of one or more of Cu, Mn, Mg, Ca and Co andthe borate. The metal, including lithium, of the coating may be a metal in the DR (e.g., reaction product of the DR and a borate deposited thereon). The presence of the metal borate may be determined by a surface analysis method such as X-ray photo-spectroscopy (XPS).

[0031] Illustratively, the coated DR may be formed by a wet or dry method. For example, the coated DR may be formed by intermixing, in the absence of a liquid, a disordered rocksalt with a boron containing compound such as boric acid a metal borate salt such as a lithium metal borate or transition metal borate to make a mixture and heating the mixture to an annealing temperature of 400 °C to 700 °C to form the coated disordered rocksalt. The coated DR may be made by mixing a disordered rocksalt with a transition metal ion and a borate ion dissolved in water or polar solvent such as a protic polar solvent (e.g., alcohol, such as methanol, ethanol, propyl alcohol and butanol) to form a slurry at a coating temperature, removing the water to form a dried coated powder and, heating the dried coated powder to an annealing temperature of 400 °C or 500 °C to 700 °C or 650 °C to form the coated disordered rocksalt.

[0032] The mixing may be any suitable method to realize the desired coating with examples being a milling method such as micromedia mill, ball mill, planetary mill or attrition mill. If desired, the borate and DR may be separately milled and intermixed using conventional mixing of dry powders. The intermixing when employing a liquid (wet method) may use any suitable mixer such as those known in the art (e.g., colloid mill, axial and radial paddle mixers) and the aforementioned milling. An Example of a suitable micro bead mill is a Buhler PML2 mill (Buhler Group). The intermixing may be for any time useful to realize the desired coated DR. The time, illustratively, may be from 5, 10, 30 minutes, 1 hour, 2 hours to 24, 12 or 6 hours.

[0033] The coating may further comprise depositing a compound comprised of lithium on the surface of the DR and then subsequently intermixing such Li coated DR with the borate such as boric acid followed by annealing to form the coated DR. Illustratively, the lithium compound may be dissolved in a polar solvent including a nonaqueous polar solvent such as a protic polar solvent with alcohol having 1 to 6, 5 or 4 carbons being useful and the DR immersed therein and the solvent removed to form a Li coated DR that is then contacted with the borate and annealed to form the coated DR.

[0034] The lithium compound may be any useful lithium compound (e.g., LiOH) to deposit on to the DR that may then react with the borate to form a lithium borate, which may have further metal cations. The amount of lithium compound may be any useful amount to form the desired coating. Generally, the amount of the lithium compound is about 1% or 2% to 10%, 8% or 5% by weight of the lithium compound and coated DR.

[0035] The temperature found to be useful to form the coated DR having desired performance is from 400 °C to 700 °C. It is understood that slightly less temperature of 10 or 20 °C may be useful with 550 °C ± 100 °C being generally suitable. The atmosphere may be any useful to form the coated DR. Any suitable atmosphere may be used, with an inert atmosphere being useful such as a noble gas (e.g., argon).

[0036] The coated DR may be used to form a cathode by any suitable method such as those known in the art. For example, the coated DR powder may be mixed with a binder such a polymer useful to make cathodes (e.g., polyfluoropolymer such as poly vinylidene fluoride) one or more solvents to form a slurry. Non-limiting examples of the one or more solvents may be an aprotic polar solvent such as methyl-2-pyrrolidinone (NMP). The slurry may then be deposited on a metal current collector (e.g., stainless steel, copper, or any suitable conductive metal foil) and the solvent removed to form the cathode.

[0037] Desirably the coated DR of the cathode has an average secondary particle size of 1 to 20 micrometers. Each of the secondary particles is an agglomeration of primary particles. The DR primary particles desirably have an average particle size as described and may contain other particles that may be useful such as increasing the electrical conductivity (e.g., carbon or other inorganic high ionic conductive particles).

[0038] The coated DR cathode may be used in a rechargeable lithium ion battery cell. The battery cell includes the cathode, an anode, separator and electrolyte. The battery or battery cell may be formed in any suitable atmosphere such as common in the aid. For example, a high purity argon atmosphere may be used to limit any undesirable contamination from species present in atmospheric air.Embodiments

[0039] Embodiment 1. A composition comprising a disordered rocksalt having a cation comprised of lithium and at least one other metal and an anion comprised of oxygen having a coating comprised of a borate comprised of lithium.

[0040] Embodiment 2. The composition of embodiment 1 wherein the disordered rocksalt is represented by chemical formula (i):

[0041] LixM’yMzO2-(a+b)FaZb

[0042] where 1.0<x<1.75; 0.01<y<0.55; 0.1<z<l; 0<(a+b)<0.7; (b>0) M’ is one of Ti, Ta, Zr, W, Nb, or Mo; M is one or more of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Zr, Y, Mo, Ru, Rh and Sb; and Z is one or more of P, N and S.

[0043] Embodiment 3. The composition of embodiment 1 or 2, wherein the metal of the coating is further comprised of a transition metal.

[0044] Embodiment 4. The composition of any one of embodiment 1 to 3, wherein the metal of the coating is comprised of a metal present in the disordered rocksalt.

[0045] Embodiment 5. The composition of any one of the preceding embodiments, wherein the coating is present in a coating amount from 0.5% to 5% by weight of the composition.

[0046] Embodiment 6. The composition of embodiment 5, wherein the coating amount is from 1 to 2% by weight of the composition.

[0047] Embodiment 7. A method of forming a coated disordered rocksalt comprising,

[0048] (i) mixing a disordered rocksalt with a borate to form a mixture, and

[0049] (ii) heating the mixture to an annealing temperature of 400 °C to 700 °C to form the coated disordered rocksalt.

[0050] Embodiment 8. The method of embodiment 7, wherein the annealing is at an annealing temperature of 500 °C to 650 °C.

[0051] Embodiment 9. The method of embodiment 7 or 8, wherein the annealing temperature is for an annealing time of 15 minutes to 6 hours.

[0052] Embodiment 10. The method of any one of embodiment 7 to 9, further comprising contacting the disordered rocksalt with a nonaqueous liquid having a lithium compound dissolved therein and removing the nonaqueous liquid.

[0053] Embodiment 11. The method of embodiment 11, wherein the nonaqueous liquid is a polar- solvent.

[0054] Embodiment 12. The method of embodiment 11, wherein the nonaqueous liquid is a polar protic solvent.

[0055] Embodiment 13. The method of embodiment 12, wherein the polar protic solvent is an alcohol.

[0056] Embodiment 14. The method any one of embodiments 10 to 13, wherein the lithium compound is comprised of lithium hydroxide.

[0057] Embodiment 15. The method of any one of embodiments 10 to 14, wherein the amount of the lithium compound dissolved is from greater than 0% to 2% by weight of the nonaqueous liquid and lithium compound.

[0058] Embodiment 16. The method of any one of embodiments 7 to 15, wherein the heating is under an inert atmosphere.

[0059] Embodiment 17. The method of any one of embodiment 7 to 15, wherein the borate is boric acid.

[0060] Embodiment 18. The method of any one of embodiments 7 to 17, wherein the borate is present in an amount of 1.5% to 2.5% by weight of the borate and disordered rock

[0061] Embodiment 19. A cathode comprising the composition of any one of embodiments 1 to 8.

[0062] Embodiment 20. A battery comprising the cathode of embodiment 19.Examples

[0063] Disordered rocksalt (DR) are synthesized via a conventional solid state reaction. Typically, stoichiometric amounts of precursors (M112O3, TiCh, bfeOs, Li2CO3 andLiF) are mixed in deionized water to make a suspension, which are ball milled with a planetary ball mill to decrease the particle size as well as to obtain a homogeneous mixture of all precursors. The mixture is dried at 100°C for 12h under air before being annealed between 700-900 °C for 1-24 hours under argon flow to obtain the disordered rocksalt phase.

[0064] The as-made DR is pre-milled for 3 hours (20g / 3h). The milled DR is mixed with boric acid by milling to realize 1 %, 2%, or 3% by weight of boric acid (H3BO3) and the DR. The mixing is by planetary milling as described above. The dried DR is annealed at 400 °C, 500 °C, or 650 °C for 3 hours under argon (20 ml / min) atmosphere to form the coated DR. In another set ofExamples, the milled DR is first immersed in ethanol having LiOH dissolved therein at a concentration to realize about 1 % by weight of the LiOH and DR. The immersion is for 3 hours while stirring at 500 rpm by an axial paddle mixer and the removal of the ethanol is by heating to 80 °C for about 12 to 18 hours to form a lithium compound coated DR. The lithium compound coated DR is then mixed with boric acid and annealed as described above. A control material (Comparative) was also pre-milled and re-annealed at 500 °C for comparison.

[0065] The coated DR powder is then milled with a carbon nanotube in a 93:7 (powder to carbon weight ratio) as active material to use for cathode casting and testing. The resulting material is washed with deionized water and dried at 150 °C for 12 hours under vacuum.

[0066] Battery cells are assembled in a high purity argon fdled glovebox (M-Braun, O2 and humidity content <0.1 ppm). The cathode is prepared by mixing the coated disordered rocksalt powder with poly( vinylidene fluoride) (Sigma Aldrich) and l-methyl-2-pyrrolidinone (Sigma Aldrich), and the resulting slurry is deposited on a stainless steel current collector and dried to form a composite cathode film. For the anode, a thin Li foil is cut into the required size. Each battery cell included the composite cathode film, a polypropylene separator, and a lithium foil anode. An electrolyte (1.0M LiPF6 in EC / EMC (1:2 v / v)) containing lithium hexafluororophosphate in a mixture of ethylene carbonate and ethyl methyl carbonate with an additive is used. The battery cell is sealed and cycled between 1.5-4.6V at 30 °C at C / 20 formation rate and C / 3 cycling rate, where 1C = 300 mAh / g. For full cell testing graphite anode has been used and battery has been cycled between 2 - 4.55V at 30 °C at C / 20 formation rate and C / 3 cycling rate.

[0067] The initial capacity is shown in Figure 1. Figure 1 shows initial capacity is relatively stable with the addition of the borate coating of this invention. The initial capacity of the half cells coated with 2% by weight borate coating display less sensitivity to annealing temperature with the greatest capacity being shown for annealing at 650 °C, which is not expected. The cycle 1 coulombic efficiency is shown in Figure 2. Figure 2 shows that the LiOH pretreated examples have higher coulombic efficiency (CE). The CE displays a modest decrease in CE at higher temperature with the highest CEs being displayed by the 2% borate coated Examples. The first cycle energy density is shown in Figure 3, in which the trends are the same as for the initial capacity. Figure 4 shows the 1C capacity discharge. The 1C capacity discharge shows that for1% and 3% borate coatings the higher annealing temperatures display a decrease in this capacity, but at 2%, the same behavior is not displayed. Figure 5 shows the total resistance of the half cells as a function is discharge cycle. The half cells having coated rocksalts annealed at 500 °C and 650 °C display lower total resistance compared to those annealed at lower temperatures with the 2% coated DR cells being more consistent as a function of annealing temperature. The capacity as a function of discharge cycle is shown in Figure 6, which shows that higher annealing temperatures and borate concentration may be desirable.

Claims

WHAT IS CLAIMED IS:

1. A composition comprising a disordered rocksalt having a cation comprised of lithium and at least one other metal and an anion comprised of oxygen having a coating comprised of a borate comprised of lithium.

2. The composition of claim 1 wherein the disordered rocksalt is represented by chemical formula (i):LixM yMzO2-(a+b)Fa Zb where 1.0<x<1.75; 0.01<y<0.55; 0.1<z<l; 0<(a+b)<0.7; (b>0) M’ is one of Ti, Ta, Zr, W, Nb, or Mo; M is one or more of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Zr, Y, Mo, Ru, Rh and Sb; and Z is one or more of P, N and S.

3. The composition of claim 1, wherein the metal of the coating is further comprised of a transition metal.

4. The composition of claim 1, wherein the metal of the coating is comprised of a metal present in the disordered rocksalt.

5. The composition of claim 4, wherein the coating is present in a coating amount from 0.5% to 5% by weight of the composition.

6. The composition of claim 5, wherein the coating amount is from 1 to 2% by weight of the composition.

7. A method of forming a coated disordered rocksalt comprising,(i) mixing a disordered rocksalt with a borate to form a mixture, and(ii) heating the mixture to an annealing temperature of 400 °C to 700 °C to form the coated disordered rocksalt.

8. The method of claim 7, wherein the annealing is at an annealing temperature of 500 °C to 650 °C.

9. The method of claim 7, wherein the annealing temperature is for an annealing time of 15 minutes to 6 hours.

10. The method of claim 7, further comprising contacting the disordered rocksalt with a nonaqueous liquid having a lithium compound dissolved therein and removing the nonaqueous liquid.

11. The method of claim 10, wherein the nonaqueous liquid is a polar solvent.

12. The method of claim 11, wherein the nonaqueous liquid is a polar protic solvent.

13. The method of claim 12, wherein the polar protic solvent is an alcohol.

14. The method of claim 10, wherein the lithium compound is comprised of lithium hydroxide.

15. The method of claim 10, wherein the amount of the lithium compound dissolved is from greater than 0% to 2% by weight of the nonaqueous liquid and lithium compound.

16. The method of claim 7, wherein the heating is under an inert atmosphere.

17. The method of claim 7, wherein the borate is boric acid.

18. The method of claim 7, wherein the borate is present in an amount of 1.5% to 2.5% by weight of the borate and disordered rock19. A cathode comprising the composition of any one of claims 1 to 6.

20. A battery comprising the cathode of claim 19.

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