CA1257648A - Constant volume lithium battery cell and process - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A battery cell and a method for preparing a battery cell are disclosed in which a substantially fixed volume container includes the cell components therein in voltaic relationship, with the cell components including an anode comprising lithium metal, a non-aqueous electro-lyte and a cathode in a spatial relationship to the anode within the fixed volume container, wherein the cathode comprises cathode active material which upon discharge intercalates lithium and undergoes a phase transition to a distinct structural phase in which phase the cathode active material can reversibly operate and which phase provides a cathode expansion greater than the anode volume decrease upon discharge, and wherein the spatial relation-ship and the cathode expansion within the substantially fixed volume are sufficient to produce a compressive load on the anode which inhibits the formation of a porous deposit of exterior, irregularly oriented, amalgamated lithium grains on the anode when the cell is reversibly operated with the cathode in said phase.

Description

The present invention rela-tes -to a non-aqueous, lithium ba-t-tery cell and to a process -for preparing such a battery cell. More par-ticularly, the invention rela-tes to such a battery cell employing a ca-thode comprising an active material which, upon discharge, intercalates and undergoes a phase -transition -to a distinct s-tructural phase, which phase transition produces a cathode expansion.
U.S. Patent No. 4,224,390, assigned to the presen-t applicant, discloses a non-aqueous, lithium battery cell including as the ca-thode active ma-terial MoS2. In particular, this patent discloses that a secondary lithium ba-ttery having good reversible characteristics can be provided by discharging a non-aqueous, lithium/MoS2 battery cell under certain conditions so -that certain phase transitions occur wi'thin the lithium intercalated MoS2. The patent discloses advan-tageous phases referred -to as "Phase 2" and "Phase 3"
and discloses that, when -the cathodes of such batteries are opera-ted with the ca-thodes maintained within these phases, -the cathodes provide good reversibility.
In Offenlegungsschrif-t No. 32 30 249 A1, it is taught tha-t, if a compressive load is applied to a li-thium electrode especially during recharging thereof in a non-aqueous battery cell system, it is possible -to inhibit the formation of a porous deposi-t of exterior, irregularly oriented, amalgamated lithium grains on the anode, which grains have been recognized in the art as decreasing the reversibility of the lithiurn anodes. The compressive load provides a subs-tantially non-porous lithium deposit on the anode, which allows enhanced reversibility for the lithium anode. Offenlegungsschrif-t No. 32 30 249 Al also teaches the use of MoS2 as a cathode active material in connection wi-th -tha-t invention. While this broadly discloses and clairns -the principle of using a compressive load on -the li-thium electrode to provide the desired inhibition, the specific embodimen-ts disclosed therein employ mechanical elemen-ts separate from -the cell components -themselves for providing -the compressive load, e.g., a spring, C-clamp, etc.

~ -ray da-ta had been developed during -the course of the work in connection with the invention disclosed in U.S. Paten-t No. 4,224,390 tha-t indicated that the unit cell volume of -the LixMoS2 in "Phase 2" as described in U.S. Patent No. 4,224,390 increased in volume, but less than the decrease in volume of lithium during discharge, i.e., the net change in the volume of the cell from such information would be expected to decrease. Thus, it could not have been expected that ca-thode expansion with an MoS2 cathode active material could be employed to create a compressive load on the lithium anode, let alone a sufficient compressive load -to provide the results as disclosed in Offenlegungsschrif-t No. 32 30 249 Al.
Certain other cathode materials are known -to expand upon cell discharge in non-aqueous, lithium batteries. For example, Kaduboski U.S. Paten-t No .
4,129,686 discloses that solid cathodes such as FeS2 expand upon discharge. The Kaduboski patent, however, employs such cathode expansion in a lithium primary cell as shown in Figures 1 and 2 of -the patent in combination with a conduc-tive member 20 having protusions 26 embedded in the anode which will provide a shorting ou-t of the cell when the anode is discharged to a predetermined extent, i.e., the ~rotrusions break through the separator and contact the expanding cathode to short the cell. This is said to avoid distortion of the overall dirnensions of the cell. In fact, the Kaduboski patent suggests that in instances where the negative electrode maintains its contour during discharge or for cells that may bulge prematurely, only a portion of the protusions may be embedded in the anode (i.e., that a gap be left) so that upon expansion of the cathode, the anode will be forced back against the base of the conduc-tive member.
It has now surprisingly been found that the inhibition of the forma-tion of a porous deposit of exterior, irregularly oriented, amalgamated lithium grains on the anode, e.g., during charging, can be obtained ~,, B

withou-t the need for separate mechanical compressing means, such as springs, etc., separate from -the cell components. More specifically, i-t has now been found that the general principle disclosed in Offenlegungsschrif-t No.
32 30 249 A1 can be accomplished by a battery cell comprising a substantially fixed volume container containing the cell components therein in vol-taic rela-tion-ship, wherein the cell componen-ts include an anode comprising lithium metal, a non-aqueous elec-trolyte and a cathode in a spa-tial relationship wi-th the anode wi-thin the fixed volume container, wherein -the cathode comprises cathode active material which upon discharge in-tercala-tes li-thium and undergoes a phase transi-tion to a distinc-t structural phase in which phase the cathode active material can reversibly operate, and which phase provides a ca-thode expansion within the subs-tantially fixed volume spatial rela-tionship which is sufficiently greater -than the anode volume decrease upon discharge -to produce a compressive load on the anode which inhibits -the formation of a porous deposit of exterior, irregularly oriented, amalgamated lithium grains on -the anode when -the cell is reversibly operated with the cathode in the distinct structural phase. Rather than such a porous deposi-t, wi-th the invention a substantially non-porous deposit s-tructure is provided which enhances the reversibility of the lithium anode. The preferred non~porous lithium metal deposit comprises close packed, lithium grains having columns wi-th the axes aligned substantially perpendicular to the substrate, i.e., -the underlying layer of li-thium.
This type of lithium plating morphology is obtained by pressures of above abou-t 3.5 kg/cm2, preferably from abou-t 3.5 kg/cm2 to about 35 kg/cm2. Preferably, the bat-tery cell is pre-conditioned for the desired reversible opera-tion by discharging the cell -to provide the desired phase -transition along with the expansion of the cathode active material and the compressive load on -the anode.

~2S~8 With particular regard to battery cells employing MoS2 as a ca-thode active material, it has been surprisingly found that, although the uni-t cell volume of "Phase 2" MoS2 cathode active material intercalated with li-thium indicates an insufficient volume expansion even -to counterbalance the li-thium anode volume decrease, the "Phase 2" MoS2 ca-thode on a macroscopic level does expand sufficiently upon discharge -to more -than compensate for the decrease in lithium volume such -tha-t the expansion of the MoS2 ca-thode during discharge wi-thin such a fixed volume can maintain a sufficient compressive load on the lithium anode to inhibi-t the forma-tion of the porous deposit of ex-terior, irregularly orien-ted, amalgam~ated lithium grains on the anode.
The present invention provides significan-t advantages in that a ba-t-tery cell can be produced which provides -the desired effect of inhibiting -the forma-tion of -the porous deposi-t of a exterior, irregularly orien-ted, amalgamated grains of lithium metal on the anode without the need for any ex-ternal applied force such as a spring, C-clamp, or swaging, e-tc. Rather, the compressive load is supplied by the operation of the cell components themselves. All that is needed is that the appropriate cell componen-ts be confined in a spatial relationship withir a constant volume and allowed -to discharge so that a phase -transition occurs in -the cathode active material such that the volume of the cathode increases to the extent needed -to provide the required compressive load on the anode when the cell is cycled (charged and discharged) with -the cathode ac-tive material in such phase. Thus, the compressive load applied to the anode is greater than the "cri-tical" pressure necessary -to achieve the desired plating characteristics for lithium metal on the anode during recharging and subsequent discharging.
Moreover, upon recharging of the battery cell with -the cathode maintained in the desired phase, the pressure wi-thin the cons-tan-t volume will increase due -to 6~

the increase in volume of lithium pla-ted on the anode.
This further enhances the inhibition of the porous deposit upon recharging of -the cell.
Still further, since it is the characteristics of a cell component, i.e., the ca-thode active material, which provide the desired result and not any external element, the cathode might be "engineered" to achieve an optimum result, e.g., by crea-ting lattice vacancies or crystal faults in the cathode active material, by doping it or perhaps by thermal treatment such as annealing.
In order that the invention may be fully understood, embodiments thereof will now be described with reference to the accompanying drawings, in which: ~
Figures 1-5 are graphical representations for a series of five non-aqueous, Li/MoS2 cells showing how the vol-tage (solid line) and the effective pressure on the cell componen-ts (dashed line) change with time as the cell is discharged so tha-t a phase transition occurs from "Phase 1" to "Phase 2". In each instance, the cell components were contained within a constant volume in accordance with the present invention, but each had a differen-t initial pressure applied to the cell components.
Figure 6 is a graphical represen-tation showing how the stack pressure varies with cycling of a non-aqueous Li/MS2 ("Phase 2~') cell with the cell componen-ts contained wi-thin a constant volume in accordance with the present invention, the lower line representing -the stack pressure at the end of discharge and the upper llne representing the stack pressure at the end of recharge.
The cathode of the present invention includes a cathode active material which upon discharge intercalates lithium and undergoes a phase transitlon to a distinct structural phase in which phase the cathode active material can reversibly operate, and which phase provides a cathode expansion sufficient to produce with the spatial rela-tionship between the anode and cathode components the ,, ~

desired compressive load on -the anode. Suitable cathode active materials include -transition metal chalcogenides which have such charac-teristics. In this regard, a number of transition metal chalcogenides are known to undergo a first order phase transition upon in-tercala-tion with lithium. Molybdenum disulfide is a preferred cathode active ma-terial, but materials such as -tungs-ten disulfide and molybdenum diselenide may also work. The disclosure of U.S. Pa-tent No. 4,224,390 may be referred to for its disclosure of suitable cathode ma-terials, their prepara-tion and conditioning of such materials -to provide phase transitions suitable for use in the present invention.
In a preferred embodimen-t of the invention, the ca-thode active material is MoS2. Preferably? the MoS2 is of the crys-talline 2H-poly type. In another preferred embodiment of the inven-tion, the cathode active material also contains MOO2 deposited on or coated on the MoS2. In par-ticular, in such embodiment, a por-tion of the MoS2 material is oxidized to M02 so as to form some M02 on the surface of the MoS2 material, e.g., 2% to 50% by weight, preferably from abou-t 5% to about 20% by weight.
This M02 surface treated MoS2 has advan-tages in -the invention as explained in U.S. Paten-t No. 4,251,606, the disclosure of which may be referred to for purposes of describing such M02 surface treated MoS2 and the prepa-ration thereof.
The ca-thode active ma-terial is -typically granular or particulate ma-terial coated on a substra-te.
For example, with MoS2 as the cathode active material, -the grain size can typically be from about 1 -to about 30 microns, preferably abou-t 10 microns in diame-ter. Any of the conven-tional binder ma-terials can be used -to bind the cathode active par-ticles. Such particles can be employed on a suitable substrate, such as an aluminum substra-te as described in U.S. Paten-t Nos. 4,224,390 and 4,251,606.
Typically, the cathode active grains are presen-t in a -thickness of from about 0.5 -to abou-t 5 microns, with the ~2~ 8 total cathode -thickness being, for example, from about .003 inches (.08 mm) to about .01 inches (.25 mm). The active material is thus used -to coat the subs-trate at -the rate of from about 20 to about 50 mg/cm , more preferably from about 25 to abou-t 35 mg/cm2 of active material. The cathode active material preferably has a porosity of from abou-t 30% -to about 70%. Typically, for a C-cell in a jelly roll structure, cathodes having abou-t 800 cm2 can be employed.
The anode in the presen-t invention comprises lithium metal. In a preferred embodirnent, the anode is a lithium foil. However, a lithium alloy could be employed such as li-thium/aluminum, li-thium/magnesium, or lithium/silicon alloys. The lithium me-tal could also be included on a non-active subs-trate. Typically, the li-thium anode is included in a battery cell of -the inven-tion in a thickness of from about 0.003 inch (.076 mm) to abou-t 0.02 inch (.51 rnm), more preferably from about 0.005 inch (.13 mm) -to about 0.01 inch (.25 mm). For a C-cell in a jelly roll s-tructure, anodes having about 400 cm2 on each side can be employed.
The process and battery of -the invention can employ any of the non-aqueous elec-trolytes conventional in the art. For example, propylene carbonate, ethylene carbonate and dimethoxyethane are suitable solvents. In one embodirnent, a mixture of propylene carbonate and e-thylene carbonate in about a 50/50 mixture are employed.
As -the solute, any of the conventional lithium salts used in the ar-t for this purpose can be employed.
For example, suitable lithium salts include LiI, LiBr, LiCl04, LiAsF6, LiBF4 and LiAlCl~. Such solu-tes can be employed in the non-aqueous electrolyte typically in a range of from about 0.1 to about 1.5 molar (or up to the limi-t of solubility of the salt in the solvent), preferably from about 0.5 to abou-t ] molar.

;7~i48 A separator ls preferably employed between the anode and cathode components of -the battery cell. The separator is preferably chemically inert in -the battery system, e.g., i-t will not reac-t with the electrodes or electrolyte and will not dissolve in the elec-troly-te. The separator should have a uniform pore structure, with the pore size small relative -to the cathode par-ticle size which allows the elec-trolyte to permeate throuyh the separator. A suitable separator ma-terial is polypropylene.
The separator is suitably a thin sheet, preferably about .001 inch (.025 mm) thick. The separator should be flexible or pliable, especially when used in connec-tion with a jelly roll type struc-ture. The separator in~-the jelly roll -type structure also preferably has a sufficient elasticity so that, when it is wound together in a jelly roll type s-tructure with the anode and ca-thode materials, -the separator can be pulled taut so as to apply an initial compressive load to the elec-trodes. In such a jelly roll -type structure, the separator is also preferably longer than the electrode components so -that it overlaps itself and can be heat sealed so as to enclose the electrode elements therein, wi-th the electrical connections being provided by sui-table -tabs as conventional in -the art.
The subs-tantially cons-tant or fixed volume in which the cell components are contained is provided by a container whose walls have a sufficient resilience in -the direc-tion which the ca-thode expands, i.e., the radial direc-tion in a cylindrical container or perpendicular to the elec-trodes in a button cell, such that when the cathode expands, any expansion in the container is much smaller than the volume change in the cell components so that the desired compression of -the li-thium anode is ob-tained. Preferably, there is essentially no expansion of the container. Sui-table fixed volume containers include containers made of mild steel or stainless steel ei-ther of which could be nickel plated, or for that ma-t-ter, any other container which will provide -the desired fixed volume for cell componen-ts.

~2~

The battery cell of -the present invention can be prepared, for example, as a button cell or as a spiral or jelly roll type cell. In a preferred jelly roll -type structure, the cathode, separator and anode are provided in a sequence of cathode (with -the ca-thode active material facing the separator)/separator/anode/separator/ca-thode (again with -the cathode ac-tive ma-terial facing the separator). These components are wound about a mandrel as is conven-tional with jelly roll s-truc-tures. As no-ted above, the separa-tor is normally greater in length than the anode or cathode.
The cell components (typically anode, cathode and separator) are placed in -the substan-tially f.ixed volume container in a spatial relationship to each other so -that, when -the cell is discharged and the phase transition and cathode expansion -take place, the desired compressi~ve load on the anode is ob-tained. In one embodi-ment, prior to -the initial discharge -to cause the phase transi-tion, at least a portion of the cell componen-ts is subjec-t -to a compressive load less -than -the "critical"
pressure and -the "critical" pressure is generated by discharging the cell components in the confined volume. As indicated by results shown in Figure 5, when an Li/MoS2 non-aqueous cell wi-th a polypropylene separa-tor are placed in a fixed volume container at about zero initial pressure (i.e., so tha-t essen-tially no ini-tial pressure is applied to -the anode but so -tha-t substantially no space is left in the fixed volume to allow expansion of the cell components), a compressive load above the desired "cri-tical" pressure is ob-tained when the cell is discharged -to provide the first order phase transition from "Phase 1" -to "Phase 2". However, it is preferred to apply an ini-tial compressive load to -the cell componen-ts wi-thin the substan-tially fixed volume con-tainer. This can be accomplished by use of the cell components -themselves without -the need for any ex-ternal force component, e.g., a spring.

~257Çi~

For example, with a jelly roll -type s-tructure, -the cathode and/or separator can be pulled during winding of -the cell components about the mandrel so as to provide an initial compressive load on -the cell components. The elastici-ty of a separator ma-terial such as polypropylene can be employed to provide the initial compressive load by pulling on the separator as the cell componen-ts are being wound. Likewise, a cathode comprising MoS2 on a aluminum foil subs-trate of -thickness .0018 cm can be pulled by a force of about 100 to 1000 grams per 5.08 cm of wid-th as -the cell componen-ts are being wound about the mandrel.
Once the cell components are wound, the extra length of the separator allows it to be heat sealed to itself so -that the cell components can be held together abou-t -the mandrel to maintain the desired ini-tial compressive load.
The cell componen-ts are then placed into the fixed volume container which is shaped so that substantially no space is lef-t therein (with -the cell components in place) to allow expansion of the cell components. In other words, -the cell components (wi-th or wi-thout -the initial compressive load) are placed in -the substantially fixed volume container so tha-t there is substan-tially no unused space wi-thin -the container to allow for expansion of -the cell componen-ts in the relevant direc-tion, i.e., the radial direc-tion in a jelly roll s-tructure. Thus, -the cell components of the jelly roll would be placed in a cylinder having a radius so as not to provide substantially any space for expansion of the cell componen-ts radially. While it is preferred -that the axial dimensions of -the cylinder also provide substantially no space for expansion of the cell components, this is not as critical, since the expansion necessary to provide the compressive load is taking place in -the radial direc-tion.
Typically, a top element is then placed on -the container and -the top is welded shu-t. A suitable amount of electrolyte is filled into the con-tainer -through a hole in the top. The hole in -the top is then welded shut.

7fi~8 Once the cell components are contained in the fixed voLume container in the desired spatial relation-ship, the cell is discharged un-til the desired phase transition occurs which provides -the cathode expansion and the compressive load on -the anode. This discharge is normally conducted a-t ambien-t temperatures or below, preferably below 0C.
In such a phase transition, the structure of the cathode active material changes. This change does not necessarily only have to take place on the molecular or atomic level. For example, as noted above, a definite phase transi-tion -takes place between "Phase 1" and "Phase 2" for li-thium in-tercalated MoS2 at a voltage of from abou-t 0.7 to about l.l vol-ts. However, by only looking at the molecular or atomic struc-tural changes, one would not expec-t that such a phase -transition could provide a sufficient volume increase -to produce the desired compressive load or "critical" pressure on the li-thium anode. Although we do not wish to be bound by any theory, what is believed to happen is tha-t -the MoS2 cathode active particles or grains change -their macroscopic configuration in -the phase -transition from "Phase 1" to "Phase 2", e.g., the particles bend, curl, or cup so as to further increase -the volume of -the cathode.
This increased volume in addi-tion to -that expec-ted from the changes in the unit cell volume in -the phase transi-tion provides the necessary expansion within the fixed volume container to produce the desired compressive load on the lithium anode.
This change in -the mechanical configuration of -the cathode particles may also lead to further advantages in -the control or op-timization of -the pressure applied wi-thin the constant volume. For example, i-t may be possible -to vary -the porosi-ty, the particle size, the particle size dis-tribution, thickness/diame-ter ratio of -the par-ticles, orien-tation distribution of the particles, and the MOO2 concen-tra-tion within the particles -to provide 76~

con-trol of the overall s-truc-tural changes which take place in the phase transi-tion. Moreover, the change in mechanical configura-tion is likely to be affected by defect densities in -the MOS2 crystals, e.g., lattice vacancies, crystal faults, doping wi-th ma-terials such as Nb, etc. Further, thermal treatmen-t could change the mechanical configura-tion characteristics of the cathode active particles e.g., by annealing.
With a jelly roll type struc-ture, it is difficult -to obtain a uniform ini-tial pressure on the electrodes as the cell components are wound into a spiral.
Thus, in general, i-t is though-t that -the pressure at the inside of the cell is greater than towards the radially outward por-tion of -the cell. Wha-t is believed -to occur in a jelly roll -type of struc-ture is -that during the initial discharge to cause -the phase transi-tion, a relatively high current density is obtained about -the mandrel where a higher pressure exists. This results in a relatively rapid conversion of -the cathode active material in that region and -therefore leads -to expansion of -the jelly roll star-ting from the center radially ou-twardly agains-t the boundary of -the constant volume container. As the stack pressure increases, hitherto unconverted cathode active material toward -the ou-tside of the spiral undergoes the deSired phase transi-tion, e.g., from "Phase 1" to "Phase 2" for MoS2. This hypo-thesis is supported by the observation that there is an initial sharp drop in voltage of a jelly roll cell in a constant volume situa-tion followed by recovery of the voltage as conversion proceeds with -time Another significant advantage of the invention employing non-aqueous Li/MoS2 cell componen-ts is tha-t -the pressure wi-thin the subs-tantially fixed volume con-tainer generally increases wi-th cycling of the cell with the ca-thode in "Phase 2". See for example the results illustrated in Figure 6 which are discussed further below.
Similar results would be expec-ted wi-th other ca-thode active ma-terial tha-t expand like MoS2 upon an inter-calation phase transition. Thus, the bat-tery cell will no-t lose i-ts effec-tiveness -for application of the desired compressive load upon repeated use of -the bat-tery cell and may even provide be-tter lithium reversibili-ty wi-th cycling.
An initial pro-totype C-cell in a jelly roll structure was prepared using MOO2 surface treated MoS2 (about 30 mg/cm ) on an aluminum subs-tra-te as a cathode, a lithium metal foil as an anode, and a polypropylene separator. These componen-ts were employed in a sequence of cathode/separator/anode/separator/cathode and wound about a mandrel. The separa-tor and cathode were pulled during the winding and the extra length of separator was hea-t sealed to i-tself so as to hold the cell componen-ts in -the desired cylindrical jelly roll shape. Tabs were used for the necessary elec-trical contac-ts. The jelly roll was placed in a -tight fi-tting stainless s-teel cylinder so tha-t substantially no unused space remained for expansion of the cell components. The top (with a hole for insertion of electroly-te) was welded on-to -the cylinder, 1 molar LiAsF6 in propylene carbonate was added -through -the hole, and -then -the hole was welded shut. The cell was then discharged to a vol-tage of abou-t .6 vol-ts to provide the ca-thode transition to "Phase 2". The cell could then be readily cycled in "Phase 2" between about 2.4 volts and 1.3 vol-ts.
The following examples are intended to illustra-te, but no-t to limi-t the scope of the presen-t invention. For example, the inven-tion is illustrated with respect to MoS2 as a cathode active material, but other materials may similarly work.

A special cell and cell holder were constructed which allow cycling of a cell sandwich under constan-t height, i.e., constant volume. The constant volume. The cons-tan-t volume arrangemen-t consists of a pair of flanges with knife-edge seals, an elec-trical feed-through and a ~;7~

piston that is sealed using a stainless steel bellows. The cell holder consists of a rigid press frame and a spindle, which is used via the pis-ton -to cons-train the cell height.
A load cell is used to measure the force exerted on the cell sandwich. This arrangement allows the measurement of the cell sandwich height with a resolution of about .5 ~
and the stack pressure wi-th a resolu-tion of .0703 kg/cm .
A series of five cell sandwiches were prepared employing four ca-thodes and -two anodes with separators between -the electrodes in the sequence: cathode/sep-arator/anode/separator/cathode/cathode/separa-tor/anode/sep-arator/cathode. Each cathode and anode was 2 cm by 2 cm in size. The separators were polypropylene. The anodes were lithium foils of 0.013 + .0013 cm thickness. The five cathode ma-terials employed in generating the da-ta shown in Figures 1-5 con-tained, respectively, 32.4, 31.0, 32.6, 31.1 and 31.0 rng/cm Of MoS2 on an aluminum foil substrate. I`he porosity of -the ca-thode coa-ting was about 50%. Each assembled cell sandwich was confined in the fixed volume of the arrangemen-t and subjected to an initial pressure, e.g-, 9.7 kg/cm , 4.78 kg/cm2, 4.08 kg/cm2, 1.703 kg/cm2 and 0 kg/cm for the cells relating -to Figures 1-5 respectively.
Each of these assembled cell sandwiches were then discharged under the same conditions. Two measure-ments were taken wi-th respec-t to -time during this discharge, namely, the voltage of the cell and the pressure on the cell components. Plo-ts of voltage vs. time (solid line) and pressure vs. time (dashed line) for the five cells are shown in Figures 1-5. Also, the change in cell thickness af-ter discharge was measured for each cell and found to be +.002 inch (.051 mm), +.006 inch (.15 mm), -~.001 inch (.025 mm), -.005 inch (-.13 mm) and +.005 inch (.13 rnm), respectively for the cells of Figures 1-5.
As can be observed from the plo-ts of voltage vs.
-time, each of the assembled cells underwent a first order phase -transition from "Phase 1" to "Phase 2" as described in U.S. Patent No. 4,224,390 as is indicated by the fla-t portion of the curve designa-ted 1 --~ 2 in Figures 1-5.
Moreover, as can be seen from the pressure vs. -time plo-ts in these figures, the pressure of the cell sandwich within the constant volume increases during -the phase -transition in going from "Phase 1" -to "Phase 2". Fur-ther, in each ins-tance, the final pressure after the phase -transi-tion within the constant volume cell is greater than the "criti-cal" pressure of abou-t 3.5 kg/cm -that is required to provide the inhibition of the forma-tion of -the porous deposi-t of ex-terior, irregularly oriented, amalgamated lithium metal grains during repla-ting on the lithium anode.

The arrangement described in Example 1 above was also used to determine how the pressure varied on -the same -type of cell sandwich (with the ca-thode conditioned to operate in "Phase 2" as in Example 1) when the cell was cycled (charged and discharged) repeatedly be-tween voltages of from about 2.4 to abou-t 1.3 volts. A plot of pressure on such a cell sandwich vs. cycle number is shown in Figure 6, wi-th the pressure a-t discharge being shown by the lower plot (solid line) and the pressure at recharge being shown by the upper plot (dashed line) of Figure 6.
The ini-tial pressure on the cell sandwich in the cons-tan-t volume of the cell was about 14.76 kg/cm2 at discharge, i.e., -the pressure after the cell was converted to "Phase 2".
As shown in Figure 6, the stack pressure increases significantly upon recharging of the cell with cathode maintained in "Phase 2". Moreover, as the number of cycles for the cell increased, the pressure a-t the discharge end of the cycle generally becomes higher and higher so that the s-tack pressures a-t the end of discharge and at the end of recharge are moving closer and closer together. Thus, the pressure within the cons-tant volume of the cell maintains -the desired compressive load on -the lithium so that the forma-tion of a porous deposit of ~ 3~ ~

exterior, irregularly oriented, amalgama-ted grains of lithium me-tal on -the anode is inhibited, even after repeated cycling.
It will be understood tha-t -the embodiments de-5 scribed above are merely exemplary and -that persons skilled in the art may make many variations and modifica-tions wi-thout depar-ting from the spirit and scope of the invention. All such modifications and variations are intended -to be included within the scope of -the invention as defined by -the appended claims.

Claims (19)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A battery cell, comprising a substantially fixed volume container containing cell components therein in voltaic relationship, said cell components including an anode comprising lithium metal, a non-aqueous electrolyte, and a cathode in a spatial relationship to the anode within the fixed volume container, said cathode comprising cathode active material which upon discharge intercalates lithium and undergoes a phase transition to a distinct structural phase in which phase the cathode active material can reversibly operate, and which phase provides a cathode expansion greater than the anode volume decrease upon discharge, wherein the interrelationship between said spatial relationship and said cathode expansion within said substantially fixed volume are such that there will be produced a sufficient compressive load on the anode to inhibit the formation of a porous deposit of exterior, irregularly oriented, amalgamated lithium grains on the anode when the cell is reversibly operated with the cathode in said phase.

2. A battery cell according to Claim 1, wherein said cell has been conditioned by discharging said cell to provide said phase transition, said expansion of said cathode active material and said compressive load on said anode.

3. A battery cell according to Claim 1 or 2, wherein the cathode active material comprises a transition metal chalcogenide providing such phase transition upon discharge.

4. A battery cell according to Claim 1 or 2, wherein the cathode active material comprises MoS2.

5. A battery cell according to Claim 2, wherein the cathode active material comprises MoS2 which has been discharged so as to undergo a phase transition to "Phase 2".

6. A battery cell according to Claim 1 or 2, wherein the cathode active material comprises MoO2 surface treated MoS2.

7. A battery cell according to Claim 1 or 2, wherein the cathode active material comprises MoO2 surface treated MoS2 which has been discharged so as to undergo a phase transition to "Phase 2".

8. A battery cell according to Claim 1, wherein the anode is lithium foil.

9. A battery cell according to Claim 8, wherein a separator is disposed between the anode and the cathode.

10. A battery cell according to Claim 9, wherein the anode and cathode are wound in a spiral with the separator therebetween.

11. A process for preparing a battery cell, said process comprising the steps of constructing a battery cell of a substantially fixed volume container and cell components within said substantially fixed volume including an anode comprising lithium metal, a non-aqueous electrolyte, and a cathode comprising cathode active material which upon discharge intercalates lithium and undergoes a phase transition to a distinct structural phase in which phase the cathode active material can reversibly operate, and which phase provides a cathode expansion greater than the anode volume decrease upon discharge, said cathode being contained in said substantially fixed volume container in a spatial relation-ship to the anode such that said spatial relationship and said cathode expansion within said substantially fixed volume are sufficient to produce a compressive load on the anode which inhibits the formation of a porous deposit of exterior, irregularly oriented, amalgamated lithium grains on the anode when the cell is reversibly operated with the cathode in said phase; and discharging the cell to provide said phase transition, said expansion of said cathode active material and said compressive load on said anode.

12. A process according to Claim 11, wherein the cathode active material comprises a transition metal chalcogenide providing such phase transition upon discharge.

13. A process according to Claim 11, wherein the cathode active material comprises MoS2.

14. A process according to Claim 11, wherein the cathode active material comprises MoS2 and said discharge causes the MoS2 to undergo a phase transition to "Phase 2".

15. A process according to Claim 11, wherein the cathode active material comprises MoO2 surface treated MoS2.

16. A process according to Claim 11, wherein the cathode active material comprises MoO2 surface treated MoS2 and said discharge causes the MoS2 to undergo a phase transition to "Phase 2".

17. A process according to Claim 14, wherein the anode is lithium foil.

18. A process according to Claim 17, wherein a separator is disposed between the anode and the cathode.

19. A process according to Claim 18, wherein the anode and cathode are wound in a spiral with the separator therebetween.