WO2007124401A2

WO2007124401A2 - Method for making electrode active material

Info

Publication number: WO2007124401A2
Application number: PCT/US2007/067051
Authority: WO
Inventors: Titus Faulkner
Original assignee: Valence Technology Inc
Current assignee: Valence Technology Inc
Priority date: 2006-04-21
Filing date: 2007-04-20
Publication date: 2007-11-01
Anticipated expiration: 2008-10-21
Also published as: JP5467863B2; US20070248520A1; CN101426721B; CA2649747A1; KR20080111077A; EP2010454A2; CA2649747C; US20100303701A1; KR101392012B1; WO2007124401A3; CN101426721A; JP2009534288A; EP2010454A4; US7790319B2; US8318353B2

Abstract

The present invention is directed to a method for making electrode active materials represented by the general formula: Aa(VO)bXO4l wherein: (a) A is an alkali metal or mixture of alkali metals, and 0 < a < 4; (b) 0 < b < 2; (c) X is selected from the group consisting of phosphorous (P), sulfur (S), arsenic (As), silicon (Si), and combinations thereof; and wherein A, X, a and b are selected to maintain the electroneutrality of the electrode active material in its nascent (as prepared or synthesized) state.

Description

METHOD FOR MAKING ELECTRODE ACTIVE MATERIAL

FIELD OF THE INVENTION

[0001] This invention relates to a novel method for synthesizing electrode

active materials for use in secondary electrochemical cells.

BACKGROUND OF THE INVENTION

[0002] A battery pack consists of one or more secondary (rechargeable)

electrochemical cells or batteries, wherein each cell typically includes a positive

electrode, a negative electrode, and an electrolyte or other material for facilitating

movement of ionic charge carriers between the negative electrode and positive

electrode. As the cell is charged, cations migrate from the positive electrode to

the electrolyte and, concurrently, from the electrolyte to the negative electrode.

During discharge, cations migrate from the negative electrode to the electrolyte

and, concurrently, from the electrolyte to the positive electrode.

[0003] Compounds of the formula VOXO₄ (X = S, P or As) are known to be

suitable for use as electrode active materials in lithium anode-based secondary

electrochemical cells (Li//VOXO₄). However, these materials must be lithiated in

order to make these materials useful in a lithium-ion cell (e.g. a cell containing a

graphitic negative electrode). Lithiation of these electrode active materials can be accomplished chemically using lithiating agents such as butyl lithium and

lithium iodide. However, such chemical lithiation techniques are expensive, slow

and produce low yields, therefore making them unsuitable for commercial use.

[0004] Accordingly, there is a current need for a method of making

compounds of the formula AVOXO₄ (A = an alkali metal, and X = S, P or As),

which is easier, faster, less expensive and/or produces greater yields, than those

methods known in the art.

SUMMARY OF THE INVENTION

[0005] The present invention is directed to a novel method for making

electrode active materials represented by the general formula:

A_a(VO)_bXO₄, wherein:

(a) A is an alkali metal or mixture of alkali metals and 0 < a < 4;

(b) 0 < b < 2;

(c) X is selected from the group consisting of phosphorous (P), sulfur

(S), arsenic (As), silicon (Si), and combinations thereof; and

(d) A, X, a and b are selected to maintain the electroneutrality of the

electrode active material in its nascent (as prepared) state;

wherein the method includes the step of precipitating the compound from

an aqueous solution containing vanadyl ions (VO²⁺), alkali metal ions and ions of

moiety XO₄. ■ BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 is a schematic cross-sectional diagram illustrating the

structure of a non-aqueous electrolyte cylindrical electrochemical cell of the

present invention.

[0007] Figure 2 is a schematic cross-sectional diagram illustrating the

structure of another embodiment of an electrochemical cell of the present

invention.

[0008] Figure 3 shows the results of an x-ray powder diffraction analysis of

a first sample of LiVOPO₄ prepared according to the invention, as well as an

analysis of the electrochemically delithiated form of the first sample.

[0009] Figure 4 is a plot of cathode specific capacity vs. cell voltage for a

first Li / 1 M LiPF₆ (EC/DMC) / LiVOPO₄ cell.

[0010] Figure 5 shows the results of an x-ray powder diffraction analysis of

a second sample of LiVOPO₄ prepared according to the invention, both before

and after dehydration.

[0011] Figure 6 is an EVS differential capacity plot for a second Li / 1 M

LiPF₆ (EC/DMC) / LiVOPO₄ cell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] It has been found that the novel methods of this invention affords

benefits over such methods among those known in the art. Such benefits include, without limitation, one or more of ease of manufacture, greater yields,

faster reaction times, and reduced costs. Specific benefits and embodiments of

the present invention are apparent from the detailed description set forth herein

below. It should be understood, however, that the detailed description and

specific examples, while indicating embodiments among those preferred, are

intended for purposes of illustration only and are not intended to limit the scope

of the invention.

[0013] The present invention is directed to a method for making electrode

active materials represented by the general formula (I):

A_a(VO)_bXO₄, (I)

wherein:

(a) A is an alkali metal or mixture of alkali metals, and 0 < a < 4;

(b) 0 < b < 2;

(c) X is selected from the group consisting of phosphorous (P), sulfur

(S), arsenic (As), silicon (Si), and combinations thereof; and

wherein A, X, a and b are selected to maintain the electroneutrality of the

electrode active material in its nascent (as prepared or synthesized) state.

[0014] Unless otherwise specified, a variable described herein algebraically

as equal to ("="), less than or equal to ("<"), or greater than or equal to (">") a

number is intended to subsume values or ranges of values about equal or

functionally equivalent to said number. [0015] As referred to herein "electroneutrality" is the state of the electrode

active material wherein the sum of the positively charged species (e.g., A and

moiety (VO)) in the material is equal to the sum of the negatively charged

species (e.g., XO₄) in the material. Preferably, the XO₄ moiety is an anion having

a charge of -2, -3, or -4, depending on the selection of X. Upon the selection of X

for moiety XO₄, selection of the values for variables a (moiety A) and b (moiety

(VO)) are governed by the formula (A):

a + 2b = V^XO4, (A)

wherein V^XO4 is the oxidation state for moiety XO₄.

[0016] Electrode active materials represent by general formula (I), made by

the methods described herein, are characterized has having VOXO₄ host layers

composed of corner-sharing VO₆ octahedra and XO₄ tetrahedra, intercalated with

guest species (A) between the host layers.

[0017] For all embodiments described herein, A is selected from the group

consisting of lithium (Li), sodium (Na), potassium (K), and mixtures thereof. In

one subembodiment, A is selected from the group consisting of Li, a mixture of Li

with Na, a mixture of Li with K, and a mixture of Li, Na and K. In another

subembodiment, A is Li. In another subembodiment, A is Na. For all

embodiments herein, O < a < 4. In one subembodiment, O < a < 2. In one

subembodiment, a = 1. In another subembodiment, a = 2. [0018] For all embodiments described herein, 0 < b < 2. In one

subembodimenti 0 < b ≤ 1. In another subembodiment, 0 < b < 0,5. In another

subembodiment, b = 1.

[0019] For all embodiments described herein, X is selected from the group

consisting of phosphorous (P), sulfur (S), arsenic (As), silicon (Si), and

combinations thereof. In one subembodiment, X is P. In another

subembodiment, X is selected from the group consisting of Si, P, and mixtures

thereof. In another subembodiment, X = (Si_1-x,P_x), wherein 0 < x < 1.

[0020] As used herein, the recitation of a genus of elements, materials or

other components, from which an individual component or mixture of

components can be selected, is intended to include all possible sub-generic

combinations of the listed components, and mixtures thereof.

[0021] Electrode active materials represent by general formula (I) are

prepared by precipitating the electrode active material from an aqueous solution

containing vanadyl ions (VO²⁺), alkali metai ions and ions of moiety XO₄.

[0022] In one embodiment, the electrode active materials represent by

general formula (I) are prepared by first preparing an aqueous solution containing

one or more ion species selected from the group consisting of vanadyl ions, alkali

metal ions and ions of moiety XO₄. The aqueous solution is prepared by

dissolving one or more vanadyl precursor compounds, one or more alkali metal

precursor compounds, and/or one or more precursor compounds containing

moiety XO₄, in an aqueous solvent. In one embodiment, one or more alkali metal precursor compounds and the one or more precursor compounds containing

moiety XO₄, are the same. Stated differently, the alkali metal ions and moiety

XO₄ derive from the same compound. In another embodiment, the alkali metal

ions are present in solution with either the vanadyl ions or the ions of moiety XO₄,

wherein the third ion species is then added and the electrode active material is

formed by precipitation.

[0023] The precursor compounds can be dissolved separately and then

combined to form a single aqueous solution, dissolved simultaneously in one

step (single step dissolution reaction), or dissolved using a step-wise dissolution

reaction (e.g. by dissolving the first precursor in an aqueous solution, followed by

the addition and dissolution of the second, and so forth until all the requisite

precursors have been dissolved and the precipitant material forms).

[0024] During the precipitation step, the aqueous solution is acidic to assist

in complete dissolution of the vanadyl ions. In one subembodiment, pH < 5 at

standard temperature and pressure or "STP" (O⁰C and 1 atmosphere (atm) of

absolute pressure). In one subembodiment, pH < 4 at STP. In another

subembodiment, 0 < pH < 5 at STP. In another subembodiment, 0 < pH < 4 at

STP. In yet another subembodiment, 2 < pH < 4.

[0025] Vanadyl precursor compounds useful herein for preparing an

aqueous solution containing vanadyl ions include V₂O₅, V₂O₃, VOSO₄, VOC₂O₄,

and mixtures thereof. Alkalized forms of the aforementioned precursor

compounds may also be used. [0026] Alkali metal precursor compounds useful herein for preparing an

aqueous solution containing alkali metal ions include AOH, A₂CO₃, A₃C₆H₅O_7,

ANO₃, CH₃AO, Al, ACI, and mixtures thereof, wherein A is an aikali metal.

Precursor compounds useful herein for preparing an aqueous solution containing

alkali metal ions as well as one of the other two ion species include A₂SO₄,

A₄SiO₄, AH₂PO₄, A₃PO₄, HAsA₂O₄, and mixtures thereof, wherein A is an alkali

metal.

[0027] Precursor compounds containing moiety XO₄ which are useful

herein for preparing an aqueous solution containing ions of moiety XO₄ include

H₃PO₄, H₂SO₄, and mixtures thereof.

[0028] The precursor compounds selected preferably disassociate in the

aqueous solution to produce counter-ions (e.g. Cl^", I^", and SO₄ ^2") which do not

compete with the precipitation of the electrode active material or form insoluble

impurities which could otherwise become entrained within the electrode active

material precipitant.

[0029] Where two of the three ions participating in the formation of the

electrode active material tend to form other compounds and therefore compete

with the precipitation of the electrode active material, steps should be taken to

avoid formation of those other, unwanted compounds to ensure the electrode

active material is substantially pure. For example, in one embodiment where

alkali metal ions (e.g. Li⁺) and phosphate ions (PO₄ ^3") are present in solution,

alkali metal phosphates (e.g. A₃PO₄) may form, typically when pH is greater than 5 at STP. Formation of A₃PO₄ can be substantially avoided by conducting the

precipitation reaction in an acidic media buffered by an acidic buffer and/or by

excess acidic precursor. Preferably, the acidic buffer yields ions that do not

compete with or participate in, the precipitation reaction. Suitable buffers include

HCI, HNO₃, H₂O₂, and combinations thereof.

[0030] If an aqueous solution containing one or two ion species selected

from the group consisting of vanadyl ions, alkali metal ions and ions of moiety

XO₄ is prepared, the remaining ion species are subsequently added (preferably

after the pH has been adjusted, if necessary, in order to substantially avoid

competing reactions). Thereafter, the solution is stirred and the active material

precipitate is collected.

[0031] The present invention also provides a secondary electrochemical

cell containing an electrode active material made by the method described

herein. The present invention also provides for batteries containing the novel

electrode active material described by general formulas (I), (IV) and (V), wherein

the battery includes:

(a) a first electrode (also commonly referred to as a positive electrode or

cathode) which includes an active material of the present invention;

(b) a second electrode (also commonly referred to as a negative

electrode or anode) which is a counter-electrode to the first

electrode; and (c) an electrolyte in ion-transfer communication with the first and second

electrodes.

[0032] The electrode active material of this invention may be incorporated

into the first electrode, the second electrode, or both. Preferably, the electrode

active material is employed in the cathode. The architecture of a battery of the

present invention is selected from the group consisting of cylindrical wound

designs, wound prismatic and flat-plate prismatic designs, and polymer laminate

designs.

[0033] Referring to Figure 1 , in one embodiment, a novel secondary

electrochemical cell 10 having an electrode active material of the present

invention, includes a spirally coiled or wound electrode assembly 12 enclosed in

a sealed container, preferably a rigid cylindrical casing 14 as illustrated in Figure

1. In one subembodiment, the cell 10 is a prismatic-type cell, and the casing has

a substantially rectangular cross-section (not illustrated).

[0034] Referring again to Figure 1 , the electrode assembly 12 includes; a

positive electrode 16 consisting of, among other things, an electrode active

material represented by general formula (I); a counter negative electrode 18; and

a separator 20 interposed between the first and second electrodes 16,18. The

separator 20 is preferably an electrically insulating, ionically conductive

microporous film, and composed of a polymeric material selected from the group

consisting of polyethylene, polyethylene oxide, polyacrylonitrile and poiyvinylidene fluoride, polymethyl methacrylate, polysiloxane, copolymers

thereof, and admixtures thereof.

[0035] Each electrode 16,18 includes a current collector 22 and 24,

respectively, for providing electrical communication between the electrodes 16,18

and an external load. Each current collector 22,24 is a foil or grid of an

electrically conductive metal such as iron, copper, aluminum, titanium, nickel,

stainless steel, or the like, having a thickness of between 5 μm and 100 μm,

preferably 5 μm and 40 μm. Optionally, the current collector may be treated with

an oxide-removing agent such as a mild acid and the like, and coated with an

electrically conductive coating for inhibiting the formation of electrically insulating

oxides on the surface of the current collector 22,24. Examples of a suitable

coatings include polymeric materials comprising a homogenously dispersed

electrically conductive material {e.g. carbon), such polymeric materials including:

acrylics including acrylic acid and methacrylic acids and esters, including poly

(ethylene-co-acrylic acid); vinylic materials including polyvinyl acetate) and

poly(vinylidene fluoride-co-hexafluoropropylene); polyesters including poly(adipic

acid-co-ethylene glycol); polyurethanes; fluoroelastomers; and mixtures thereof.

[0036] The positive electrode 16 further includes a positive electrode film 26

formed on at least one side of the positive electrode current collector 22,

preferably both sides of the positive electrode current collector 22, each film 26

having a thickness of between 5 μm and 150 μm, preferably between 25 μm an

125 μm, in order to realize the optimal capacity for the cell 10. The positive electrode film 26 is composed of between 80% and 95% by weight of an

electrode active material represented by general formula (I), between 1% and

10% by weight binder, and between 1% and 10% by weight electrically

conductive agent.

[0037] Suitable binders include: polyacrylic acid; carboxymethylcellulose;

diacetylcellulose; hydroxypropylcellulose; polyethylene; polypropylene; ethylene-

propylene-diene copolymer; polytetrafluoroethylene; polyvinylidene fluoride;

styrene-butadiene rubber; tetrafluoroethyiene-hexafluoropropylene copolymer;

polyvinyl alcohol; polyvinyl chloride; polyvinyl pyrrolidone; tetrafluoroethylene-

perfluoroalkylvinyl ether copolymer; vinylidene fluoride-hexafluoropropylene

copolymer; vinylidene fluoride-chlorotrifluoroethylene copolymer;

ethylenetetrafluoroethylene copolymer; polychlorotrifluoroethylene; vinylidene

fluoride-pentafluoropropylene copolymer; propylene-tetrafluoroethylene

copolymer; ethylene-chlorotrifluoroethylene copolymer; vinylidene fiuoride-

hexafluoropropylene-tetrafluoroethylene copolymer; vinylidene fluoride-

perfluoromethylvinyl ether-tetrafiuoroethylene copolymer; ethylene-acryiic acid

copolymer; ethylene-methacrylic acid copolymer; ethylene-methyl acrylate

copolymer; ethylene-methyl methacrylate copolymer; styrene-butadiene rubber;

fluorinated rubber; polybutadiene; and admixtures thereof. Of these materials,

most preferred are polyvinylidene fluoride and polytetrafluoroethylene.

[0038] Suitable electrically conductive agents include: natural graphite (e.g.

flaky graphite, and the like); manufactured graphite; carbon blacks such as acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal

black, and the like; conductive fibers such as carbon fibers and metallic fibers;

metal powders such as carbon fluoride, copper, nickel, and the like; and organic

conductive materials such as polyphenylene derivatives.

[0039] The negative electrode 18 is formed of a negative electrode film 28

formed on at least one side of the negative electrode current collector 24,

preferably both sides of the negative electrode current collector 24. The negative

electrode film 28 Is composed of between 80% and 95% of an intercalation

material, between 2% and 10% by weight binder, and (optionally) between 1%

and 10% by weight of an electrically conductive agent.

[0040] Intercalation materials suitable herein include: transition metal

oxides, metal chalcogenides, carbons (e.g. graphite), and mixtures thereof. In

one embodiment, the intercalation material is selected from the group consisting

of crystalline graphite and amorphous graphite, and mixtures thereof, each such

graphite having one or more of the following properties: a lattice interplane (002)

d-value (d_(O02)) obtained by X-ray diffraction of between 3.35 A to 3.34 A,

inclusive (3.35 A < d₍₀₀₂) ≤ 3.34 A), preferably 3.354 A to 3.370 A, inclusive

(3.354 A < d(oo₂₎ ≤ 3.370 A; a crystallite size (L_c) in the c-axis direction obtained

by X-ray diffraction of at least 200 A, inclusive (L_c > 200 A), preferably between

200 A and 1 ,000 A, inclusive (200 A < L_c < 1 ,000 A); an average particle

diameter (P_d) of between 1 μm to 30 μm, inclusive (1 μm < P_d < 30 μm); a

specific surface (SA) area of between 0.5 m²/g to 50 m²/g, inclusive (0.5 m²/g < SA < 50 m²/g); and a true density (p) of between 1.9 g/cm³to 2.25 g/cm³,

inclusive (1.9 g/cm³ < p < 2.25 g/cm³).

[0041] Referring again to Figure 1 , to ensure that the electrodes 16,18 do

not come into electrical contact with one another, in the event the electrodes

16,18 become offset during the winding operation during manufacture, the

separator 20 "overhangs" or extends a width "a" beyond each edge of the

negative electrode 18. In one embodiment, 50 μm < a < 2,000 μm. To ensure

alkali metal does not plate on the edges of the negative electrode 18 during

charging, the negative electrode 18 "overhangs" or extends a width "b" beyond

each edge of the positive electrode 16. In one embodiment, 50 μm < b < 2,000

μm.

[0042] The cylindrical casing 14 includes a cylindrical body member 30

having a closed end 32 in electrical communication with the negative electrode

18 via one or more negative electrode leads 34, and an open end defined by

crimped edge 36. In operation, the cylindrical body member 30, and more

particularly the closed end 32, is electrically conductive and provides electrical

communication between the negative electrode 18 and an external load (not

illustrated). An insulating member 38 is interposed between the spirally coiled or

wound electrode assembly 12 and the closed end 32. The insulating member 38

may be provided with apertures (not illustrated) for pressure equalization and/or

electrolyte permeation. [0043] A positive terminal subassembly 40 in electrical communication with

the positive electrode 16 via a positive electrode lead 42 provides electrical

communication between the positive electrode 16 and the external load (not

illustrated). Preferably, the positive terminal subassembly 40 is adapted to sever

electrical communication between the positive electrode 16 and an external

load/charging device in the event of an overcharge condition (e.g. by way of

positive temperature coefficient (PTC) element), elevated temperature and/or in

the event of excess gas generation within the cylindrical casing 14. Suitable

positive terminal assemblies 40 are disclosed in U.S. Patent No. 6,632,572 to

Iwaizono, et al., issued October 14, 2003; and U.S. Patent No. 6,667,132 to

Okochi, et al., issued December 23, 2003. A gasket member 44 sealingly

engages the upper portion of the cylindrical body member 30 to the positive

terminal subassembly 40.

[0044] A non-aqueous electrolyte (not shown) is provided for transferring

ionic charge carriers between the positive electrode 16 and the negative

electrode 18 during charge and discharge of the electrochemical cell 10. The

electrolyte includes a non-aqueous solvent and an alkali metal salt dissolved

therein. Suitable solvents include: a cyclic carbonate such as ethylene

carbonate, propylene carbonate, butylene carbonate or vinylene carbonate; a

non-cyclic carbonate such as dimethyl carbonate, diethyl carbonate, ethyl methyl

carbonate or dipropy! carbonate; an aliphatic carboxylic acid ester such as

methyl formate, methyl acetate, methyl propionate or ethyl propionate; a .gamma.-lactone such as γ-butyrolactone; a non-cyclic ether such as 1 ,2-

dimethoxyethane, 1 ,2-diethoxyethane or ethoxymethoxyethane; a cyclic ether

such as tetrahydrofuran or 2-methyltetrahydrofuran; an organic aprotic solvent

such as dimethylsulfoxide, 1 ,3-dioxolane, formamide, acetamide,

dimethylformamide, dioxolane, acetonitrile, propylnitrile, nitromethane, ethyl

monoglyme, phospheric acid triester, trimethoxymethane, a dioxolane derivative,

suifolane, methylsulfolane, 1 ,3-dimethyl-2-imidazolidinone, 3-methyl-2-

oxazolidinone a propylene carbonate derivative, a tetrahydrofuran derivative,

ethyl ether, 1 ,3-propanesuitone, anisole, dimethylsulfoxide and N-

methylpyrrolidone; and mixtures thereof. A mixture of a cyclic carbonate and a

non-cyclic carbonate or a mixture of a cyclic carbonate, a non-cyclic carbonate

and an aliphatic carboxylic acid ester, are preferred.

[0045] Suitable alkali metal salts include: LiCIO₄; LiBF₄; LiPF₆; LiAICI₄;

LiSbF₆; LiSCN; LiCI; LiCF₃ SO₃; LiCF₃CO₂; Li(CF₃SO₂)₂; LiAsF₆; LiN(CF₃SO2)₂;

LiB₁₀CI₁₀; a lithium lower aliphatic carboxylate; LiCI; LiBr; LiI; a chloroboran of

lithium; lithium tetraphenylborate; lithium imides; sodium and potassium

analogues of the aforementioned lithium salts; and mixtures thereof. Preferably,

the electrolyte contains at least LiPF₆.

[0046] Referring to Figure 2, in another embodiment, a polymer laminate-

type secondary electrochemical cell 50 having an electrode active material

represented by general formula (I), includes a laminated or polymer stacked cell

structure, having a negative electrode 52, a positive electrode 54, and an electrolyte/separator 56 there between. The negative electrode 52 includes a

current collector.60 (preferably, a copper foil or grid) in electrical communication

with a negative electrode membrane or film 62; and the positive electrode 54

includes a current collector 58 {preferably, an aluminum foil or grid) in electrical

communication with a positive electrode membrane or film 64. Protective

bagging material 66 covers the cell and prevents infiltration of air and moisture.

Such structures are disclosed in, for example, U.S. Patent 4,925,752 to Fauteux

et ai; U.S. Patent 5,01 1 ,501 to Shackle et al.; and U.S. Patent 5,326,653 to

Chang; all of which are incorporated by reference herein.

[0047] The relative weight proportions of the components of the positive

electrode 54 are generally: about 50-90% by weight active material represented

by general formula (I); 5-30% carbon black as the electric conductive diluent; and

3-20% binder chosen to hold all particulate materials in contact with one another

without degrading ionic conductivity. Stated ranges are not critical, and the

amount of active material in an electrode may range from 25-95 weight percent.

The negative electrode 52 includes about 50-95% by weight of a preferred

intercalation material, with the balance constituted by the binder. In a preferred

embodiment, the negative electrode intercalation material is graphite. For test

purposes, test cells are often fabricated using lithium metal electrodes.

[0048] Those skilled in the art will understand that any number of methods

are used to form films from the casting solution using conventional meter bar or

doctor blade apparatus. It is usually sufficient to air-dry the films at moderate temperature to yield self-supporting films of copolymer composition. Lamination

of assembled cell structures is accomplished by conventional means by pressing

between metal plates at a temperature of about 120-160⁰C. Subsequent to

lamination, the battery cell material may be stored either with the retained

plasticizer or as a dry sheet after extraction of the plasticizer with a selective low-

boiling point solvent. The plasticizer extraction solvent is not critical, and

methanol or ether are often used.

[0049] Separator membrane element 56 is generally polymeric and

prepared from a composition comprising a copolymer. A preferred composition

is the 75 to 92% vinylidene fluoride with 8 to 25% hexafluoropropylene copolymer

(available commercially from Atochem North America as Kynar FLEX) and an

organic solvent plasticizer. Such a copolymer composition is also preferred for

the preparation of the electrode membrane elements, since subsequent laminate

interface compatibility is ensured. The plasticizing solvent may be one of the

various organic compounds commonly used as solvents for electrolyte salts, e.g.,

propylene carbonate or ethylene carbonate, as well as mixtures of these

compounds. Higher-boiling plasticizer compounds such as dibutyl phthalate,

dimethyl phthalate, diethyl phthalate, and tris butoxyethyl phosphate are

particularly suitable. Inorganic filler adjuncts, such as fumed alumina or silanized

fumed silica, may be used to enhance the physical strength and melt viscosity of

a separator membrane and, in some compositions, to increase the subsequent

level of electrolyte solution absorption. [0050] Electrolyte solvents are selected to be used individually or in

mixtures, and include dimethyl carbonate (DMC), diethylcarbonate (DEC),

dipropylcarbonate (DPC)₁ ethylmethylcarbonate (EMC), ethylene carbonate (EC),

propylene carbonate (PC), butylene carbonate, lactones, esters, glymes,

sulfoxides, suifolanes, and mixtures thereof. The preferred solvents are

EC/DMC, EC/DEC, EC/DPC and EC/EMC. The salt content ranges from 5% to

65% by weight, preferably from 8% to 35% by weight. One example is a mixture

of EC: D MC: Li P F₆ in a weight ratio of about 60:30:10. Desirable solvents and

salts are described in U.S. Patent Nos. 5,643,695 to Barker et al. and 5,418,091

to Gozdz et al.

Examples of forming laminate and polymer stacked cells are disclosed in U.S.

Patent No. 4,668,595 to Yoshino et al,; U.S. Patent No. 4,830,939 to Lee et al.;

U.S. Patent No. 4,935,317 to Fauteux et a!.; U.S. Patent No. 4,990,413 to Lee et

al.; U.S. Patent No. 4,792,504 to Schwab et al.; U.S. Patent No. 5,037,712 to

Shackle et al.; U.S. Patent No. 5,262,253 to to Golovin; U.S. Patent No.

5,300,373 to Shackle; U.S. Patent No. 5,435,054 to Tonder et al.; U.S. Patent

No. 5,463,179 to Chalonger-Gill et al.; U.S. Patent No. 5,399,447 to Chalonger-

GiII et al.; U.S. Patent No. 5,482,795 to Chaionger-Gill and U.S. Patent No.

5,411 ,820 to Chalonger-Gill; each of which is incorporated herein by reference in

its entirety. Note that the older generation of cells contained organic polymeric

and inorganic electrolyte matrix materials, with the polymeric being most

preferred. The polyethylene oxide of 5,411 ,820 is an example. More modern examples are the VdF:HFP polymeric matrix. Examples of casting, lamination

and formation of cells using VdF:HFP are as described in U.S. Patent No.

5,418,091 to Gozdz; U.S. Patent No. 5,460,904 to Gozdz; U.S. Patent No.

5,456,000 to Gozdz et al.; and U.S. Patent No. 5,540,741 to Gozdz et al.; each of

which is incorporated herein by reference in its entirety.

[0051] The following non-limiting examples illustrate the compositions and

methods of the present invention.

EXAMPLE 1

[0052] An electrode active material of formula LiVOPO₄, was made as

follows. At room temperature, 4.438 g (0.5 moles) of V₂O₃ (commercially

available from Stratcor) was added to 2 equivalents plus 10% of HCI to form a

solution. Subsequently, 2 equivalents plus 10% of HNO₃ were slowly added to

the solution, and the solution was diluted with water to 100 milliliters (mL) to yield

a dark blue solution. 12.310 g (1 mole) of LiH₂PO₄ as a 20% solution in water

was added to the diluted dark blue solution, followed by 3 equivalents of LiOH,

yielding a solution having a pH of 4.5. The resulting solution was stirred at room

temperature, yielding a dark green precipitate. Thereafter, the solution was

boiled to concentrate the solution to about 100 mL. The concentrated solution

was filtered and washed three times, each time with 150 mL of water, yielding a

dark green cake. The cake was dehydrated at 25O⁰C in air for 1 hour, yielding

7.4 g of a dark green powder. [0053] CuKa (λ = 1.5405A) powder x-ray diffraction patterns were collected

for the as-made. dehydrated precipitate material and the electrochemically

delithiated material. The pattern shown in Figure 3 indicates the precipitate

material to be Ct-LiVOPO₄, in fair agreement with the structural analysis for α-

LiVOPO₄ described by Journal of Solid State Chemistry, vol. 177 (2004), pgs. 2896-

2902. The pattern for the electrochemically delithiated material was not in

agreement with α-VOPO₄; however, this is believed to be due to intercalation of solvent into the material.

[0054] Electrodes were formulated from a slurry with a weight ratio of

77:10:13 of active material synthesized per this Example: Super-P carbon :

PVDF binder, respectively, which were mixed in a NMP-based slurry. The slurry

was cast onto aluminum foil, dried to remove the solvent and cut to form an

electrode coupon. A cell was assembled using the coupon as a cathode, lithium

metal as the anode, LiPF₆ in 2:1 ethylene carbonate : dimethylene carbonate

(EC:DMC) as the electrolyte and glass fiber as the separator.

[0055] Figure 4 shows the ceil cycled at constant current between 3.2 V

and 4.4 V for one and a half cycles at a C/12 rate. The voltage curves for the

charge and discharge scans are similar in shape. The cell exhibited a discharge

capacity of 77 mAh/g {milli-amp hours per gram), which was lower than expected

and is believed to be due to the electrode formulation. After the final half cycle,

the cathode coupon was removed from the cell in the fully charged state and the

XRD pattern was scanned as previously described. EXAMPLE 2

[0056] An electrode active material of formula LiVOPO₄, was made

substantially per the teachings of Example 1. At room temperature, 4.628 g (0.5

moles) of V₂O₃ (commercially available from Stratcor) was added to a mixture of

2 equivalents of HCI and 4 equivalents of HNO₃ to form a solution. 12.838 g of

LiH₂PO₄ as a 30% solution in water was added to the solution. 20 mL of 30%

H₂O₂ in water was slowly added, after which an aqueous solution of 8% LiOH

monohydrate was added until the pH reached about 5.0. The resultant

precipitate was then filtered and washed three times with 140 mL of water. The

filter cake was then dried at 70°C under partial vacuum for 15 minutes. The as-

dried material was then dehydrated in air at 25O⁰C for 1 hour.

[0057] Referring to Figure 5, CuKa (λ = 1.540δA) powder x-ray diffraction

patterns were collected for the as-dried precipitate and the dehydrated material.

[0058] Electrodes were formulated from a slurry with a weight ratio of

77:10:13 of active sample : super-p carbon : PVDF binder, which were mixed in a

NMP-based slurry. The slurry was cast onto aluminum foil, dried to remove the

solvent and cut to form an electrode coupon. A cell was assembled using the

coupon as a cathode, lithium metal as the anode, LiPF₆ in 2:1 EC:DMC as the

electrolyte and glass fiber as the separator.

[0059] High-resolution electrochemical measurements were performed

using the Electrochemical Voltage Spectroscopy (EVS) technique. EVS is a voltage step method, which provides a high-resolution approximation to the open

circuit voltage curve for the electrochemical system under investigation. Such

technique is known in the art as described by J. Barker in Synth. Met 28, D217

(1989); Synth. Met. 32, 43 (1989); J. Power Sources, 52, 185 (1994); and

Electrochemica Acta, Vol. 40, No. 11 , at 1603 (1995).

[0060] Figure 6 shows an EVS scan of the material made by this means.

There are two distinct reversible processes, at about 3.9V and 3.75V on discharge.

The specific discharge capacity for this cell was calculated to be 112.8 mAh/g.

[0061] The examples and other embodiments described herein are

exemplary and not intended to be limiting in describing the full scope of

compositions and methods of this invention. Equivalent changes, modifications

and variations of specific embodiments, materials, compositions and methods

may be made within the scope of the present invention, with substantially similar

results.

Claims

What is claimed is:

1. A method for making a compound represented by the general formula:

A_a(VO)_bXO₄, wherein:

(a) A is an alkali metal or mixture of alkali metals and 0 < a ≤ 2;

(b) X is selected from the group consisting of phosphorous (P), sulfur

(S), arsenic (As), silicon (Si), and combinations thereof; and

(c) A, X, a and b are selected to maintain the eiectroneutrality of the

compound;

the method comprising the steps of:

forming an acidic aqueous solution comprising vanadyl ions, ions of an

alkali metal, and ions of XO₄; and

precipitating the compound from the acidic aqueous solution.

2. The method of Claim 1 , wherein the aqueous solution has a pH < 5 at

standard temperature and pressure.

3. The method of Claim 2, wherein 0 < pH < 4 at standard temperature and

pressure.

4. The method of Claim 3, wherein 2 < pH < 4.

5. The method of Claim 2, wherein the step of forming the acidic aqueous

solution comprises the step of dissolving one or more vanadyl precursor

compounds, one or more alkali metal precursor compounds, and one or more

precursor compounds containing moiety XO₄, in an aqueous solvent.

6. The method of Claim 5, wherein:

the one or more vanadyl precursor compounds is selected from the group

consisting of V₂O₅, V₂O₃, VOSO₄, VOC₂O₄, and mixtures thereof;

the one or more alkali metal precursor compounds is selected from the

group consisting of AOH, A₂CO₃, A₃C₆H₅O_7, ANO₃, CH₃AO, Al, ACI, and mixtures

thereof, wherein A is an alkali metal; and

the one or more precursor compounds containing moiety XO₄ is selected

from the group consisting of H₃PO₄, H₂SO₄, and mixtures thereof.

7. The method of Claim 5, wherein:

the one or more vanadyl precursor compounds is selected from the group

consisting of V₂O₅, V₂O₃, VOSO₄, VOC₂O₄, and mixtures thereof; and

the one or more alkali metal precursor compounds and the one or more

precursor compounds containing moiety XO₄ are the same compound and is

selected from the group consisting of A₂SO₄, A₄SiO₄, AH₂PO₄, A₃PO₄, HAsA₂O₄,

and mixtures thereof, wherein A is an alkali metal.

8. The method of Claim 5, wherein the step of dissolving one or more vanadyl

precursor compounds, one or more alkali metal precursor compounds, and one

or more precursor compounds containing moiety XO₄, in an aqueous solvent, is a

single step dissolution reaction.

9. The method of Claim 5, wherein the step of dissolving one or more vanadyl

precursor compounds, one or more alkali metal precursor compounds, and one

step-wise dissolution reaction.

10. The method of Claim 5, wherein the step of dissolving one or more vanadyl

precursor compounds, one or more alkali metal precursor compounds, and one

or more precursor compounds containing moiety XO₄, in an aqueous solvent,

further comprises the step of adding an acidic buffer in an amount sufficient to

adjust the pH to ≤ 5.

11. The method of Claim 1 , wherein A is selected from the group consisting of

Li, Na, K, and mixtures thereof.

12. The method of Claim 1 , wherein A is Li.

13. The method of Claim 1 , wherein the compound is LiVOPO₄.