CA1046578A - Lithium-metal chromate organic electrolyte cell and improvements thereof - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE A method for preparing metal chromate cathode electrodes for use in non-aqueous lithium/chromate cells for medium to high discharge rate service is described. Cells utilizing such cathodes are also described. Chromates used include the chromates, dichromates and basic chromates of silver, copper, mercury and lead. The cathodes can be prepared without internal conductive elements or cohesive binders.

Description

~0~578 F I E LD OF THE INVENT I ON ~-This invention relates to high energy density organic electrolyte cells and more particularly to improvements in the j method for the manufacture of such cells based upon active metal anodes and metal chromate cathodes.
BACKGROUND OF THE INVENTION
In U.S. Patent 3,658,592 there is described a high energy density cell comprising a positive electrode, composed of a metal chromate and graphite in weight rat~os of 1 to 24 parts of metal chromate to 1 part of graphite, and a binder in an ~1 amount of 1 to 10 percent by weight of the electrode material, said metal chromate being selected from the group consisting of silver~ copper, iron, cobalt, mercury, thallium, lead and bismuth , ~
chromates, and mixtures thereof; negative electrodes composed of light metals selected from the group consisting of Li, Na, X, Ca, Be, Ng, and Al; said electrodes being disposed in an electrolyte comprising an organic solvent selected ~rom the group consisting of tetrahydrofuran, N-nitrosodimethylamine, dimethyl-sulfite, propylene carbonate, gamma-butyrolactone, dimethyl carbonate, ~;
dimethoxyethane, acetonitrile, dimethyl sulfoxide, dimethyl formamide, and mixtures thereof and having dissolved therein ;~
soluble salts of said light metals.
Such cells have an excellent open circuit voltage (OCV=3.35-0.05~) and pérform with reasonable efficienty at low discharge rates (ma/cm2). However, at even moderate discharge rates (4ma/cm2) such cells rapidly polarized, the voltage drop-ped, and the discharge efficiency was less than 10 percent.

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OBJECTS OF THE INVENTION
It is an object of this invention to provide a modi-_ fied method of preparing a metal chromate cathode active material into cathodes which when utilized in the organic electrolyte cells according to the aforementioned pa-tent will permit oper-ation of such cells at medium rates at reasonably high efficien-cies.
It is a further object of this invention to provide ancillary cell components which when utilized with such cathodes will provide commercially useful electrochemical cells.
In the foresaid Patent, U.S. 3,658,592, there is described the method of preparation of the cathodes by the forma-~ tion of a dough of the metal chromate, graphite as a conductive j diluent and about 3 percent by weight of a binder, preferably colloidal Teflon ~a trademark for polytetrafluoro~ethylene). A
paste is formed of this mixture by the addition thereto of an easily volatilized organic solvçnt. The paste is formed into a pliable dough which is formed around a metallic current collector and the assembly is then pressed in a die at pressures in the ~ 20 range of 70,000-80,000 psi. As a result of pressing the com-N posite at such pressures, the resultant cathode material has adequate mechanical integrity~ The pressed cathode is then ~-! dried and cured at temperatures of about 300C.
! THE INVENTION
The present invention is based upon the discovery that by the use of a simplified cathode construction procedure, it is possible to eliminate both the graphite and the binder and obtain !

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. : . ~ ~, ~ 6S7~3 cells with good performance at high efficiencies and that the _ cells prepared with only the graphi-te added to the chromate and pressed at pressures in the range 1500-5000 psi and preferably at approximately 2000 psi, will yield cathodes which will per-form in cells according to the aforementioned patent at medium to high discharge rates at efficiencies approaching and exceed-ing 70 percent.
The cathodes prepared from pure silver chromate and from silver chromate containing up to 10 percent by weight of 1-0 graphite, when pressed at pressures of about 2000 psi yield cathodes of adequate mechanical integrity for use in commercial button cells. Similarly, cells using chromates, dichromates and ¦ basic chromates of metals from the group consisting of a silver, copper, mercury, and lead yielded adequate cathodes for use in cells according to the aforementioned patent. These cells were adequate with regard to mechanical integrlty and for use at medium discharge rates.
While the use of a binder is not absolutely necessary, binders can be included in the cathode mix. However, it is not necessary to heat the bound electrodes pressed at the lower pressures or at the more elevated pressures. Curing step is no longer necessary, no matter whether the electrodes are pressed at low or high pressures.
Satisfactory cathodes can be prepared from such metal chromates as silver chromates (Ag2CrO4), mercury chromate~ ;~
r (HgCrO4), copper chromate (CuCrO4), silver dichromate (Ag2Cr2O7), mercury dichromate (HgCr2O7), copper dichromate (CuCr2O7) and ' basic mercuric chromates of the general formula Hgx(CrO4)y where-¦ in the '' .

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mole ratio of x/y varies from about l:l to 1:0.22.
The cathodes prepared at the low pressures described above can be fashioned into high efficiency, medium discharge rate, electrochemical cells which consist of these metal chromate cathode prepared as set forth above, an anode and separator means therebetween in a non-aqueous electrolyte system comprising an organic solvent selected from the group consisting of tetrahydrofuran, N-nitrosodimethylamine, dimethyl sulfoxide, dimethyl sulfite, propylene carbonate, gamma-butyrolactone, dimethyl carbonate, dimethoxyethane, acetonitrile and dimethyl formamide having dissolved therein, as electrolytes, at least one -solvent-soluble ionic salt of an anode metal. The anode can be selected from the active light-metal anode metals of the group ! consisting of lithium, sodium, potassium, calcium, beryllium, magnesium, and aluminum. The solvent-soluble salt being selected from the group of soluble salts of said light anode metals. Th~
cathodes are those above-described comprising powered metal chromates containing up to lO weight/percent of powdered graphite and compressed at pressures not exceeding 5000 psi but in the range from about 1500 psi - 5000 psi. About 2000 psi is pre-ferred.
~' In general, it has been found that for the cells of this invention, the organic electrolyte should include sufficient dissolved ionic salts so that the electrolyte has a specific con-ductivity of 10 5ohms lcm l or higher. The conductive electro-lyte solvent-soluble salts should consist of salts of said active . .
metals with such anions as C104 AsF6 , BF~ , SbF6 , AlC14 , Cl , Br , and I anions.
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The invention will be more particularly described with , .. . . . ............ . . .. .. .. ..
~ 30 ~4~57~3 reference to the follwoing examples and the accompanyiny drawings relating ~o thes~ specific examples in which:
Figure 1 is a side elevation partially in section of a ,' lithium silver chromate cell in accordance with one embodiment of the present invention;
Figure 2 is a graph showiny the current voltage characteristics of the cell of Figure 1 with various cathode components;
Figure 3 is a yraph showing the discharge characteristicS
of the cell of Figure 2;
Figure 4 is a graph showing the discharge characteristic~
of the cell of Figure 2 with various electrolytes;
Figure 5 is a graph showing the discharge curve of a lithium CuCro4 cell of the configuration of Figure l;
Figure 6 is a graph showing the discharge characteristic~ :
of a lithium HgCrO2O7 (HgO:CrO3 1:2) cell of the configuration of Figure l;
Figure 7 is a graph showing the discharge characteristic~
of a lithium Hgx (CrO4)y (HgO:CrO3 1:0.93) cell of the configuratio~
of Figure l; and j Figure 8 is a graph showing the discharge characteristics of a lithium Hgx (CrO~)y (HgO:CrO3 1:0.3) cell of the configuration ~
of Figure 1. -SILVER CHROMATES
~ Example 1 .j Lithium silver chromate cells were constructed in the button cell cans, having thë configuration of (Fig. 1) using pure ~` ;
powdered silver chromate pressed onto -the cathode can (1) with a force of approximately 2000 lbs. The cathode made in the above 30 manner retained its mechanical integrity. The lithium anode was constructed by prewelding a disc of expanded copper (3) on the anode can and subsequent pressing of a disc of lithium (2) on to b ~ ~ ~ - 6 -- ' . : ,, ' . , ' :
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~0~ 8 the expanded metal. The separator (4) consisted of a disc of filter paper (cellulosic material) placed between the anode top j and the ca-thode can. The cell was crimped closed after addition of the electrolyte consisting of 1 molar LiC104 in a equivolume - mixture of tetrahydrofuran and propylene carbonate.
The open clrcuit voltage of the cell was 3.3 volt. The ~ current-voltage characteristics of the cell is shown in Fig. 2 j (curve l). The cell was discharged at a constant current of 4 ma.
q The discharge curve is shown in Fig. 3 (curve 1). The lithium metal chromate cells can apparently operate successfully without 1 any graphite or binder in the cathode, because the discharge of ! metal chromates such as Ag2CrO4 or HgCrO4 results in the in situ 'i`5 formation of the electronically conductive metal according to the ~, cell reaction:

b 4Li + Ag2CrO4~____ ~2Ag + 2 Li20 + CrO2 :~ `
The metal provides the electronic conductivity needed for further discharge of the metal chromate.
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;Si7~3 E ample 2 Lithium/silver chromate cells were prepared as in Example l but the cathode fabrication procedure was modified to the extent that 5~ of graphite was added to the chromate mix without any binder. Cells were constructed in button cell cans as in Example l above, except the cathodes were made by pressing mixtures of 20 parts of powder silver chromate and l part of graphite, at a force of 2000 lbs. The open circuit voltage of ¦ these cells were 3.3 volts. The current-voltage characteristics of the cells and the discharge curve at 4 ma current are shown in Fig. 2 (curves 2 and 3) and Fig. 3 (curve 2) respectively.
The test data demonstrate that the lithium-metal chromate cells can be operated without any ~inder in the cathode.
Example 3 i The metal chromate cathodes were also found to operate well when these were constructed with graphite and a colloidal Teflon binder, without any curing at temperatures above 25C.
Lithium-mercuric chromate 'D' cells were constructed using ~ ~
HgCrO4 cathodes made from a paste of a slugged mixture of HgCrO4: ;
graphite:collodial Teflon in the ratio 17:2:1. The paste was prepared with isopropanol as a vehicle as described in U.S. ;~
3,658,592. The cathodes (12" x 1.75") were made by rolling the paste on an expanded tantalum grid by means of a dough rolling machine. The cathodes were then dried under vacuum at room , j temperature. 'D' size cells were constructed using these cathodes and a lithium anode. The cells were fitted with an~aluminum top and a titanium cathode tab coated with a solution-deposited polyethylene coating to provide protection against corrosion.
The electrolyte used was 1 M LiC104 in a equivolume mixture of tetrahydrofuran and propylene carbonate. The 'D' cells were dis~
charged at currents of 0.25 amp, 0.5 amp and 0.75 amp. The re-covered cell capacities and the ca-thode utilization efficiencies ~
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_ ~'ABLE I
Li~HgCrO4 'D' Cell Performance , No.Discharge Capacities A.Hr. Cathode Utilization Current_ to 1.5 volt cutoff Efficiency , I 10.25 Amp 14 99%
i~ 20.50 Amp 5.6 40~
~ 30.75 ~mp 4.5 33%
;''`l The good cell performance as shown above, demonstrates that the metal chromate cathodes can be operated without any curing step in the cathode fabrication.
Example 4 Cells were prepared utilizing the configuration of ~' ~
Figure 1 and the cathode materials of Example 2. The cells were modified only to the extent that the following electrolytes were used in the cells.
(a) 1 M LiAsF6 (lithium hexafluoroarsenate) in a equi volume mixture of tetrahydrofuran, and propylene carbonate, (b) 1 M LiPF6 (Lithium hexafluorophosphate) in an ~, 20 equi-volume mixture of tetrahydrofuran and propylene carbonate, and ,~ (c) 1 M LiBF4 (lithium tetrafluoroborate) in a equi-!~ volume mixture,of tetrahydrofuran and propylene carbonate.
The cells were discharged at 4 ma current. The dis-., charge curves are shown in Figure 4. It is evident that the , cells performed quite well. This demonstrates that the electro-lyte salts such as LiAsF6, LiPF6, LiBF4 in conjunction with the previously disclosed solvents can be used as electrolytes for the lithium-metal chromate organic electrolyte cells. Other salts with anions such as AsF6 , PF6 , C104 ,BF4 , AlC14 ,Cl ~Br , , , " I andcations such,as Li+,Na , K , Mg , Ca , Ba , quarternary ammonium ions which provide conducting solutions (of specific , '''' ' -8-7l 3 ~ :

conductivit~ of 10 5 ohm 1 cm 1 or higher~ in conjunction with the organic solvents and solvent mixtures are also useful as non-aqueous organic electrolytes for the lithium-metal chromate cells.
In U.S. Patent 3,658,592 the use of metal chromates (MCrO4) is described. The salts of the metal-chromium-oxygen combinations other than M CrO4 can also be used to formulate cells. This includes higher valent metal cation and/or dichro-mate anion, and also basic chromates. The use of these salts provides certain improvements. These are illustrated in the ~-following preparations and tests.
Example 5 , Silver dichromate is prepared by the addition o a solution of alkali metal dichromate solution to a silver nitrate solution to precipitate Ag2Cr2O7. The open circuit voltage of Li/Ag2Cr2O7 cells prepared according to Example 2 using Ag2Cr2O
prepared by this procedure is 3.4-0.05V.
Example 6 : ~:
; The following preparative procedures are used to pre~
pare the dichromates instead of the procedure of Example 5, the open circuit voltage of Li/AgCr2O7 cells assembled as in Example

2, are improved to 3.8-0.05V from 3.4+0.05V observed in Example 5.
6A. 2.47 g of AgO (solid) is mixed with 2.0 g CrO3 in _ pestle and morter, and the mixture is set at 50C overnight.
X-ray analysis of the resultant brownish red solid indicated the presence of Ag2Cr2O7 and absence of AgO and CrO3. Use of the Ag2Cr2O7 sample as cathode in lithium cell provides open circuit voltage of 3.8+0.05V.

~ 30 6B. To a solution of 15 g K2S2O8 in 150 ml water, a r~ 100 ml solution of 16.~ g AgNO3 is added, to precipitate AgO.
. : , To the in situ precipitated AgO solution a 100 ml solution of - ~ ~ . ':
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il~4t~578 19.4 g K2CrO4 is added and a precipitate is obtained. The pre-cipitate is analyzed to be mainly Ag2Cr2O7, which performed in cells substantially as the material of 6A.
! 6C. 12.4 g AgO was suspended in 250 ml water. To this suspension, 100 ml solution of 10.1 g CrO3 is slowly added and stirred. I'he resultant precipitate is analyzed to be mainly Ag2Cr2O7 and the dried material was substantially similar in performance to the material of 6A in cell performance.
COPPER CHROMATES
Preparation of CuCrO4 and CuCr2O7 is generally a problem due to the high solubility of CuCr2O7 and/or its hydrolytic equilibrium.
For example, intermixing of equimolar quantities of CuO and CrO3 in aqueous medium does not lead to the formation of a CuCrO4 or CuCr2O7 prec~pitate as in the case of silver salts.
- Instead 2 Cu CrO4.3 Cu (OH)2.H2O and CuCrO4.Cu (OH)2 results.
Even if the salts are formed by suitable preparative procedure, it is difficult to make a graphite-copper salt mix suitable for lithium cell cathodes, because of the tendency for the formation of the hydrated salt, as in CuCrO4.2H2O (ref.: S.H.C. Briggs, J.
Chem. Soc. 1929, 242-245) and deliquescent nature of CuCr2O7.2H2O
;~ (Handbook of Chemistry and Physics, (see Physical Constants of Inorganic Compounds under CuCr2O7.2H2O, CuCrO4.2CuO etc)~
Included within the ambit of this invention is the discovery that suitable copper chromate or copper dichromate can be prepared for use in lithium cells by the following procedure which includes the graphite addition and in si*u chromates crystallization process:
Example 7 A sample of 11.54 grams of CuCO3Cu(OH)2 ti.e. green-basic copper carbonate (assay: 55~ Cu) is slowly added to 100 ml mixture of H2CrO4 solution containing 11.83 g CrO3 and 2.1 g ~, ~

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graphite powder. Upon addition of the green copper carbonate to the mixture, CuCrO4 is formed in presence of the graphite. Also, C2 is evolved and H20 is formed in the reaction. The reacted 1 mixture is slowly evaporated to crystallize the copper chromate in presence of the graphite. The mixture may be powdered and i stored for direct use in lithium cells. Composition of CuCrO4 is !I confirmed for the sample by x-ray diffraction.
The open circuit voltage of the lithium-CuCrO4 cell ¦ formed by using this cathode material formed by the above described procedure and prepared into cathodes and cells by the procedure of Example 2 is 3.9V. The discharge curve of a ~ ;
Li/CuCrO4 cell made into a button cell is as in Fig. 5.
Similar procedure was developed for the ln situ forma-j tion of CuCr2O7-graphite mixture for use as lithium cell cathode.
In this case, instead of 11.5 g CrO3, 23 g of CrO3 is used.
Lithium-CuCr2O7 cells formed using this mixture showed an open circuit voltage of 3.85-3.9V.
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A similar procedure of ln situ graphite with modified proportions may be adopted for the preparation of basic copper ¦ 20 chromate salts.

MERCURY CHROMATES

-~` In U.S. 3,658,592 patent, the use of mercuric chromate , , .
is taught. In this compound the ratio of Hg:Cr is 1:1. Accord-ing to the present invention, compounds of various Hg:Cr ratios can be formulated by procedures as described below, and the ;
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~, compounds thus formed can be successfully employed as cell cathodes with lithium. The general preparative procedure con-~:. ,. : :
sisted of the following:

Example 8 ~ :
A known weight (5 to 40 grams~ of CrO3 was dissolved in 100 ml distilled water. To this solution 43.32 grams of HgO
was added, and the mixes s-tirred for one day. The resultant -11~

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mercury-chromate sample is filtered, washed with propylene carbonate or tetrahydrofuran, and dried in vacuum. The dried sample ls weighed and also chemically analyzed to de~ermine the yield and the HgO:CrO3 ratio. The result of various preparations ! carried out is yiven in Table 1.
TABLE I
Preparation of HgO~CrO3 Salts of Various Composition 43.43 grams HgO is added in each case ¦ SampleWt. of CrO Starting ObservedObserved Mole taken in 3 Mole Ratio Yield % Ratio 10G ml water HgO:CrO HgO:CrO3 (srams) 3 1 40 1:2 71.8 101.08 ' 2 30 1:1.5 86.3 1:1.08

3 25 1:1.25 92.0 1:0.85

4 20 1:1 86.6 1:0.63 18 1:0.9 88.3 1:0.51 `~
6 15 1:0.75 87.2 1:0.41 , 7 13 1:~.65 71.0 1:0.43 8 10 1:0.5 92.4 1:0.3 9 5 1:0.25 98.9 1:0.22 1:1.75 78.0 1:0.93 11 27 1:1.35 88.5 1:0.88 12 23 1:1.15 93.4 1:0.87 Table 1 shows that the HgO:CrO3 ratio of the product can be varied to a wide extent (e.g. from 1:1 for sample 1 to t 1:0.3 for sample 8) by varying the starting mole ratio of HgO
~, and CrO3.
Example 9 , For the preparation o~ Hg2Cr2O7, the following proced~

ure was adopted.
~ 30 J 1/4 lb HgCrO4 was added to a solution of 150 grams CrO3 in 100 ml. The material was stirred overnight, then filtered and washed with acetone. 125 y of product is recovered.

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HgO:CrO3 mixes of various ratios xanging from mercuric ¦
dichromate, mercuric chromates to basic mercuric chromates were .
employed as cathodes in lithium anode cells, and the discharge characteristics of such cells are shown in Figs. 6-8.
Figure 6: Discharge characteristics of Li-HgCr'2O
(HgO:CrO3::1:2).
Figure 7: Discharge characteristics of Li-Hgx(CrO4)y ~:
(HgO : CrO3 : : l : O.93).
Figure 8: Discharge characteristics of Li-Hgx(CrO4)y, 10basic chromates, (HgO : CrO3 : : l : 0.3). .
The results demonstrate that salts of various ratios of HgO:CrO3 can be used as satisfactory depolarizers for the high energy density organic electrolyte cells.

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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for preparing cathodes of metal chromates for use in high energy density organic-electrolyte, electro-chemical cells which comprises the steps of compressing a mater-ial consisting of powdered metal chromate into cathode form at pressures substantially in the range of 1500-5000 psi, wherein said metal chromate is selected from the group consisting of metal chromates, metal dichromates and metal basic chromates.

2. The method according to claim 1 wherein the metal is selected from the group consisting of silver, copper, mer-cury and lead.

3. The method according to claim 1 wherein said cathode is compressed at pressures of about 2000 psi.

4. The method according to claim 2 wherein said metal chromate is Ag2CrO4.

5. The method according to claim 2 wherein said metal cathode is HgCrO4.

6. The method according to claim 2 wherein said metal chromate is CuCrO4.

7. The method according to claim 2 wherein said metal chromate is a dichromate of a metal selected from the group con-sisting of silver, mercury and copper.

8. A method for preparing cathodes of metal chro-mates for use in high energy density, organic-electrolyte, elec-trochemical cells which consists essentially of the steps of compressing powdered metal chromate and uncured binder into cathode form at a pressure in the range 1500-5000 psi, and maintaining said cathodes at a temperature below about 25 C
throughout the formation of the cathodes, wherein said metal chromate is selected from the group consisting of metal chro-mates, metal dichromates and metal basic chromates.

9. The method according to claim 8 wherein said cathode in final form contains up to 5 percent of uncured fluorinated hydrocarbon binder.

10. The method according to claim 8 wherein said chromate is basic mercuric chromate having the formula (HgO)x(CrO3)y wherein the mole ratio of x/y varies from about 1:1 to 1:0.22.