CA1149452A - Battery cathode and methods of making same - Google Patents

Abstract

BATTERY CATHODE AND METHODS OF MAKING SAME

Abstract A battery cathode comprising MoO2 or MoS2 particles coated with MoO2, methods of making same, and cells having a lithium anode together with such cathodes are disclosed.

Description

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BATTERY CATHODE AND METHODS OF MAKING SAME

Field to Which the Invention Relates This invention relates to battery cathodes and methods of making same for use in secondary battery cells. More particularly, this invention relates to battery cathodes and methods of making same, the cathode-active material comprising molybdenum dioxide (MoO2) or molybdenum dioxide coating molybdenum disulphide (MoS2).
MOO2 is one of a class of compounds having a so-called "rutile-related" structure, the crystallo-graphy of which has been well characterized in the literature. Further, MoO2 is known to be a metal with room temperature electrical conductivity approximately ten times higher than that of carbon. Some investiga-tion of the electrochemical properties of MO2 has been done, mainly directed to the use of the material as an oxygen-reduction catalyst.
MoS2 is one of a class of compounds referred to as layered transition metal dichalcogenides, and use of MoS2, as well as other layered transition metal dichalcogenides, as cathode-active materials have been reported in the literature. From the point of view of cost, MoS2 is a desirable substance to use as a cathode-active material - it occurs in nature in sig-nificant quantities and is one of the less expensive layered transition metal dichalcogenides.
A number of techniques for producing coatings on metallic substrates are known. For example, it is ~1~945Z

well known to produce a coating on a metallic substrate by painting a mixture of the coating material dissolved or suspended in a carrier onto the substrate to be coat-ed, and then heating to decompose the carrier, leaving the coating material adherent to the substrate.
U.S. Patent No. 2,819,962 teaches a method of producing sintered plates for galvanic cells by prepar-ing a suspension of metallic powder having intercalating properties in water in which a viscosity-increasing agent has been dissolved. The substrate to be coated is dipped in the suspension, the thickness of the coating so obtained is adjusted and then the substrate is heated to dry the coating. The coated substrate is then sintered in a nonoxidizing atmosphere and cut into individual cell plates.
According to U.S. Patent No. 2,905,574, a sur-face coating of MoS2 may be obtained by coating the surface with a saturated ammoniacal solution of mono-methylammonium tetrathiomolybdate and then heating to 480C in a stream of nitrogen.
U.S. Patent No. 3,047,419 teaches a method of producing a corrosion-resistant silicide coating on titanium by painting the titanium to be coated with a carrier liquid in which an organic binder has been dis-solved, and in which fine particles of silicon andtitanium are suspended. The coated body is dried, then heated in an inert atmosphere to decompose and vaporize the carrier and sinter the silicon to leave a silicon-titanium alloy coating.

Some molybdenum compounds have been considered as possible cathode-active materials. A significant amount of effort has been directed to the study of molybdenum trioxide (MoO3) as a cathode-active mate-rial. In connection with these studies, lithium has often been used as the opposing anode-active material.
Although MoO3 is of commercial interest because of the possibility that it may be used to fabricate a high energy density battery, it is a relatively poor electri-cal conductor - a characteristic which imposes limita-tions on high current discharge performance. To improve such performance, MoO3 powder is often mixed with a conducting additive such as powdered graphite. However, the charge-discharge cycle performance of the Li/MoO3 couple appears to be relatively limited. Although the action of MoO3 with lithium is not well understood, it is generally believed to react or tend to react in a non-reversible manner to form other oxides of molybdenum or compounds such as Li2MoO4.
Recently, M823 has been examined for possible use as a cathode-active material versus an anode which includes lithium as the anode-active material: see High Energy Density Batteries Based on Lithium Anodes and Substoichiometric Oxide Cathodes in Organic Electrolytes, Power Sources 1977, Vol. VI~ pp.
527-536, Pietro et al. However, the Li/MogO23 couple is believed to involve an irreversible reaction resulting in production of lithium molybdates (Ibid).
An object of the present invention is to faci-litate the production of a battery cathode using rela-tively inexpensive MoS2 while achieving a high degree of reversibility and discharge rate capability.
Summary of the Invention In one aspect of the present invention, there is provided a method of making a battery cathode having a cathode-active material at least a portion of which is MoO2, the method comprising the steps of applying a suspension of finely divided MoS2 particles as a film on a selected substrate, and baking the substrate and applied film in an oxygen-containing atmosphere to con-vert a selected proportion of the MoS2 to an oxide of molybdenum coating the substrate. The foregoing baking step may be controlled so that a selected proportion of or substantially all the MoS2 converts to MoO2.
Alternatively, the foregoing baking step may be control-led so that substantially all the MoS2 converts to MoO3. In the latter case a further step of baking the coated substrate in a reducing atmosphere is then taken to reduce substantially all of the MoO3 to MoO2.
In another aspect of the present invention, there is provided a method of making a battery cathode having a cathode-active material which is predominantly MoO2, the method comprising the steps of baking MoS2 particles in an oxygen-containing atmosphere to convert substantially all of the MoS2 particles to MoO3 particles, further baking the MoO3 particles in a re-ducing atmosphere to convert substantially all of the MoO3 particles to MO2 particles and then applying a suspension of the MO2 particles as a film on a selected substrate and baking the substrate and applied 11~9452 film in an inert atmosphere to drive off the suspending media for the MO2 particles, leaving the MO2 particles adherent on the substrate.
Hence, the present invention contemplates the fabrication of battery cathodes, the cathode-active material of which comprises MO2 or MO2 coating MoS2 .
It has been found that MO2 itself (viz.
without any substantial amount of MoS2) behaves as a good cathode-active material, and that its presence as a coating on MoS2 particles will improve the discharge rate characteristics over that obtainable where the cathode-active material consists essentially of MoS2 particles. Cells having such cathodes have been found to exhibit a high degree of reversibility.
The electrical conductivity of a cathode is improved where a relatively thin coating of MO2 appears on MoS2. As the mole percentage of MO2 coating MoS2 is increased in relation to the mole per-centage of MoS2, the electrical conductivity of thecathode improves with only limited sacrifice of energy density characteristics. MO2 has energy density characteristics somewhat inferior to those of MoS2, but conversely has superior electrical conductivity characteristics.
In given applications, the amount of MoO2 present will be a function of a trade-off between energy density requirements and desired high current discharge characteristics. If high current discharge performance is not a primary consideration, then only a relatively 11~945Z
small amount of MO2 may be present. Conversely, if high current discharge performance is of primary import-ance, then a relatively large amount of MO2 may be present. Indeed, as indicated above, in some applica-tions the cathode-active material may consist essen-tially of MO2 with little or no MoS2 present.
As described in more detail hereinafter, the present invention lends itself to fabrication using MoS2 as a raw material whether the eventually result-ing cathode-active material comprises MO2 or MO2 coating MoS2.
Drawings Figure 1 is a cross-sectional front view show-ing a typical tube furnace having installed a quartz tube containing an aluminum support slab bearing a glass slide on which a foil substrate with applied film to be baked is mounted. Also shown are baking atmosphere in-let and exhaust ports.
Figure 2 is a graph showing discharge and re-charge characteristics of a cell constructed as describ-ed hereinafter in Example 1. Cell voltage is plotted as the ordinate with time in hours plotted as the abscissa.
Figure 3 is a graph showing discharge and re-charge characteristics of a cell constructed as describ-ed hereinafter in Example 3. Cell voltage is plotted asthe ordinate with time in hours plotted as the abscissa.
Detailed Description The description which follows deals primarily with the fabrication of battery cathodes which have a cathode-active material consisting essentially of 11~9452 MOO2 particles or MoS2 particles coated with MoO2.
The fabrication of complete cells which include a lith-ium anode and a selected electrolyte is discussed in the Examples which appear at the end of the description.
The construction of lithium anodes and complete cells is not discussed in detail because the techniques involved are purely conventional and well known to those skilled in the art.
To fabricate from MoS2 a battery cathode which comprises MO2 as a cathode-active material, particles of MoS2 are oxidized to form MO2 or MoO3. In cases where MoO2 is formed, it will not necessarily be the case that the MoS2 is completely oxidized to MoO2. In fact, for some applications, it is contemplated that only a relatively small proportion of the MoS2 present will be oxidized to MoO2. In cases where MoO3 is formed by oxidization of the MoS2, substantially all the MoS2 present is oxidized and there then follows a subsequent reduction step dur-ing which sutstantially all the MoO3 is reduced to M02 .
Herein, conversion of MoS2 to MO2 or to amixture of MoS2 and MoO2 will be referred to as "direct conversion~ as distinct from ~indirect conver-sion", which contemplates conversion from MoS2 toMoO3 followed by conversion of MoO3 to MoO2.

11~9452 (a) MoS2 to MO2 or a mixture of MoS2 and MO2 ("direct conversion") A suspension of finely divided MoS2 parti-cles in a suitable suspending media is applied to a metallic substrate.
Various suspending media are suitable, the primary requisite being that the viscosity be suffi-ciently high to allow handling of the substrate when coated without significant loss of the suspending media from the substrate surface. Preferably, a liquid having a boiling point below the baking temperature (discussed hereinafter) is chosen so that the liquid will evaporate before the baking temperature is reached, thus reducing the possibility that the liquid may interfere with the oxidization process. The inventors suggest the use of a liquid such a propylene glycol as the suspending media.
A variety of metals or metal alloys are suit-able for use as a substrate, the primary requisites be-ing that they do not adversely react with MoS2 or MO2 and do not themselves oxidize to produce unac-ceptable side effects. Preferred substrate materials include aluminum or titanium or alloys thereof, or stainless steel. The inventors have found aluminum foil used for ordinary household purposes to be quite accept-able as a substrate. Platinum may be used, but its costwill likely be considered prohibitive for most commer-cial applications. Nickel may also be used; however, problems may be encountered with excessive oxidation of the nickel when the MoS2 particles are oxidized on the substrate. The nickel substrate may lose mechanical rigidity and its electrical conductivity may be degrad-ed. Although the other substrate materials may be oxi-dized at least in part during the oxidization of MoS2 particles, they do not appear as susceptible to the problems which have been encountered with nickel.
The suspension of finely dividied MoS2 particles may be applied to only one or to both faces of the substrate. If the suspension is appied to only one face, then the substrate may be placed with the coated face up on a slab of material such as aluminum which acts as a support during the baking procedure described hereinafter. If the suspension is applied to both faces of the substrate, the substrate should preferably be suspended to permit a free flow of the baking atmosphere past both substrate faces.
The inventors have found it convenient to use as a substrate a strip of aluminum foil having a width approximately equal to the width of a standard micro-scope slide. One of the narrower ends of the strip is bent around one of the narrower ends of the slide. The aluminum foil strip thus mounted on a microscope slide is easy to handle during subsequent steps of cathode preparation. The inventors advise against bending both ends of the strip around corresponding ends of the microscope slide bacause glass and aluminum have dif-ferent coefficients of thermal expansion which might result in buckling during baking of the strip.
The suspension is preferably applied to the substrate to yield a film-like coating which is suffi-ciently thin to allow diffusion of oxygen throughout the coating in a time which is relatively short compared to ~1~9452 the reaction time of oxygen with MoS2. This will en-courage the formation of a homogeneously oxidized cath-ode. The time required for oxygen to diffuse throughout the coating has been found to depend upon the average MoS2 particle size in the coating, the packing density of MoS2 particles in the coating and the baking tem-perature. It has been found that coatings of up to at least 20 mg of MoS2 per cm2 having an average parti-cle size of about 20 microns may be uniformly oxidized onto an aluminum substrate at tempertures ranging be-tween about 400C to 650C. The thickness of the coat-ing which may be oxidized onto a given substrate will to some extent be governed by the substrate material chosen. The substrate and the coating will likely have different coefficients of thermal expansion, which, de-pending upon the relative difference between these co-efficients, may result in buckling or cracking of the coating during baking if an attempt is made to oxidize a coating which is too thick.
The substrate with applied film is placed on the support slab (or suspended) in a closed tube made from a heat resistant material such a quartz, the whole of which is then inserted into a tube furnace.
~y way of example, Figure 1 illustrates a quartz tube 1 having single-hole neoprene stoppers 8 in-serted in both ends. The substrate 2 with the applied film is mounted on microscope slide 3 which rests on support slab 4. The support slab, microscope slide and substrate with applied film are inserted in quartz tube 1 which is then placed in a standard Lindberg tube 114g452 furnace 5 which has been preheated to a temperature below the melting point of the metal foil substrate.
(For aluminum substrates which melt at about 650C, the furnace is preferably preheated to about 525C to 610C.
Above this range, problems of differential thermal ex-pansion of the aluminum may be encountered, possibly resulting in buckling or cracking of the substrate coat-ing. At lower temperatures, correspondingly longer oxi-dization times are required - which may detract from the commerical suitability of the method.) An inert gas flow is maintained at a fixed rate with the aid of flowmeter 6 and needle valve regu-lator 7. Gases which have flowed through quartz tube 1 may be passed through bubbler apparatus 11 to assist in preventing backflow of air into quartz tube 1. Various gases are suitable as the inert gas. Both purified nitrogen and argon have proven to be acceptable. It is expected that helium would also perform satisfactorily.
The substrate and applied film is baked in the inert atmosphere, thereby driving off the suspending media for the MoS2 particles. After the substrate and applied film has been allowed to reach thermal equil-ibrium (at which point substantially all of the suspend-ing media should have evaporated), oxygen is admitted to closed tube 1 and is caused to flow with the aid of the flowmeter 9 and the needle valve regulator 10 past the substrate to oxidize the MoS2 particles.
Gas flow rate is adjusted with the aid of flowmeters 6 and 9 and needle valve regulators 7 and 10, such that the flow rate is fast enough to prevent a 11~94S2 backflow of air into the tube through the tube gas out-let port, but also slow enough to prevent cooling of the substrate due to the flow of gas past the substrate. If the gas flow rate is too low, an oxygen concentration gradient may be set up along the length of the substrate such that more oxygen will diffuse into the coating at the end of the substrate closest to the source of the oxygen flow than will diffuse into the coating at the end of the substrate farthest from the source of the oxygen flow. The gas flow rate must therefore be ad-justed to minimize the effect of any such oxygen concen-tration gradient.
It is considered that the problem with oxygen concentration gradient and the resultant requirement for careful control over the gas flow rate may be alleviated by adapting the method to the production of a continuous cathode by moving a continuous strip of substrate with applied film past a stationary oxygen source which bath-es the moving strip in oxygen for a time period depend-ent upon the rate at which the strip is moving. The in-ventors believe that production of cathodes by such a moving strip method may result in an economically viable means for mass production of cathodes.
Use of a vacuum furnace may also alleviate the oxygen concentration gradient problem. If a vacuum furnace is used then the partial pressure of oxygen within the furnace may be monitored to yield a concen-tration of oxygen equivalent to that which would have been required if oxygen in an inert gas atmosphere had been used. A "cold trap" may also be used to remove 11~9~52 sulphur dioxide produced during oxidization of MoS2 in the vacuum furnace.
Where direct conversion from MoS2 to MO2 or to a mixture of MoS2 and MO2 is to be achieved, careful regulation of oxygen concentration and oxidiza-tion time is required. For a given average MoS2 particle diameter and a given oxidization time, the maximum allowable concentration of oxygen in the inert atmosphere which will impede the formation of molybdenum oxides rather than MO2 has been found to be dependent upon temperature and upon the thickness of the film desired to be oxidized. The inventors are not able to provide generalized conclusions respecting the condi-tions under which the formation of molybdenum oxides other than MO2 will be impeded. Reference should be made to the Examples which follow. Of course, in adjusting the concentration of oxygen in combination with the parameters discussed below, care should be taken not to establish an oxygen concentration gradient along the length of the substrate, as mentioned above.
Once a baking temperature is selected, and a corresponding suitable concentration of oxygen in a given inert gas determined, a suitable oxidization time must be determined. The rate of oxidization of MoS2 has been found to vary approximtely exponentially with temperature and approximately linearly with oxygen con-centration (below the maximum allowable oxygen concen-tration above which formation of molybdenum oxides other than MO2 may occur). The oxidization time should be selected to be sufficiently long to allow the desired il~9452 proportion of MoS2 to be converted to MoO2, but sufficiently short to prevent the reaction of MO2 with oxygen to form other molybdenum oxides. An appro-priate oxidization time may be determined empirically.
For example, several substrates with applied films which have been baked for varying lengths of time may be examined through x-ray diffraction analysis to determine the amounts of MO2 and/or other molybdenum oxides produced. Oxidization times of a few minutes have been found appropriate at higher temperatures (about 550C), while oxidization times of several hours have been found to be required at lower temperatures.
By direct conversion, it is possible to pro-duce cathodes in which the cathode-active material is a Hmixture" of both MO2 and MoS2 in ratios which may vary substantially over the entire compositional range (i.e. about 100% MO2 to about 100% MoS2). Such cathodes may be produced by baking the substrate and applied coating for a time sufficient to allow only a selected proportion of MoS2 to be converted to MoO2.
Cathodes containing a "mixture" of MoS2 and MO2 may not be produced by the n indirect" conversion discussed hereinafter slnce substantially all of the MoS2 is converted to MoO3 which is then converted to MoO~.
The inventors advise against simply mixing selected proportions of MoS2 particles and MO2 particles and then adhering the mixed particles onto the substrate in some manner to fabricate a cathode in which the cathode-active material is a "mixtureH of both MoS2 and MoO2. The inventors believe that when a cathode con ll~9~S2 taining such a "mixture" is produced by direct conver-sion, then an MO2 coating forms on the MoS2 parti-cles. Thus, individual MoS2 particles are given a metallic coating which improves electrical conductivity between adjacent particles. If MoS2 particles are simply mixed indiscriminately with MO2 particles, it is believed that problems of electrical conductivity may be experienced in the completed cathode.
By way of summary and further explanation, the following procedure has been found acceptable for fabri-cating by direct conversion a battery cathode which in-cludes MoO2 as a cathode-active material:
MoS2 concentrate is washed in organic solv-ents and water to remove substantially all traces of organic impurities. Inorganic impurities may also be removed by a leaching process or in any known manner.
The MoS2 concentrate is then mixed with a viscous liquid. The mixture should comprise approximately equal parts by volume of MoS2 and viscous liquid. Propylene glycol has been found to be an acceptable viscous liquid. The resulting slurry is applied as a film to a metal substrate.
A screening process may be used to yield a film of uniform desired thickness. The substrate and applied film may be dried in an oven at 100C for a few minutes to simplify handling. The substrate may be, for example, a piece of aluminum foil. The inventors have used an aluminum foil strip having an area of approximately 20 cm2 and approximately 11~9452 20 microns thick as a substrate. In using an aluminum substrate, the inventors usually apply a film to yield a distribution of up to about 20 mg/cm2 of MoS2 on the substrate.
The substrate with applied film is then placed directly on a support slab with the coated surface of the substrate away from the support slab. An aluminum slab measuring approximately 12 inches x 2 inches x 1/4 inch may be used as a support. The support and substrate are then placed in a quartz tube such that the substrate with applied film is longitudinally aligned with the tube axis. An inert gas is caused to flow through the tube. Once the tube has been flushed of air (approximately 10 minutes after the inert gas begins to flow), the tube is placed in a standard Lindberg-type tube furnace which has been preheated to the pre-selected baking temperature and the substrate is allowed to reach thermal equilibrium in the furnace. Once the substrate has reached thermal equilibrium, oxygen is added to the inert gas flow-ing through the tube at a rate governed as describ-ed above. The oxygen concentration and a corres-ponding baking temperature are predetermined as described above to discourage formation of molyb-denum oxides other than MO2 during baking. The tube is left in the furnace with the gas mixture flowing for an oxidization time (which has been predetermined as described above) which is suffi-cient to convert a selected proportion, or substan-11~9452 tially all of the MoS2 to MoO2, but not long enough to encourage the further conversion of MOO2 to other molybdenum oxides. The oygen flow is turned off at the end of the oxidization time, the inert gas flow maintained and the tube is re-moved from the furnace. After the tube has cooled (approximately 5 minutes) the inert gas flow is turned off and the completed cathode is removed from the tube.
~b) MoS2 to MoO3 to MO2 ("indirect conversion") As indicated above, to produce an MoO2 cath-ode "indirectly" from MoS2 there is first an oxidiza-tion step, then a reduction step.

The oxidization step may be performed by fol-lowing the procedure generally as described for direct conversion. However, since the object of the oxidiza-tion step now is to encourage conversion of the MoS2 to MoO3 and not to MoO2, care~ul regulation of oxygen concentration and oxidization time is not as critical as it is in the case of direct conversion when the object is to encourage conversion of the MoS2 to MOO2 and not to MoO3.
Thus, to effect the oxidization step, an MoS2 film may be applied to a metal foil substrate and the substrate and applied film then placed on a support slab and inserted into the quartz tube as described in the case of direct conversion. The tube containing the support slab, substrate and applied film is then placed in a standard Lindberg tube furnace which has been pre-9~5~

heated (preferably to about 525C to 610C if an alum-inum substrate is used), and an oxygen containing atmos-phere is then caused to flow through the tube. The tube is left in the furnace until substantially all of the MoS2 has been conver~ed MoO3 (the substrate coating should be pale yellow or white in color when this has happened). Then, the reduction step follows.
To effect reduction, the oxygen flow is turned off and, preferably, the tube is flushed with an inert gas (e.g. nitrogen) for several minutes before the re-ducing atmosphere is introduced. Then, a reducing atmosphere such as hydrogen mixed with the inert gas is caused to flow through the tube, the furnace temperature having been lowered to a temperature in the neighbour-hood of 430C to 450C. The substrate is baked in the reducing atmosphere for several hours until substan-tially all of the MoO3 has been reduced to MoO2.
The hydrogen flow is then turned off, and the tube re-moved from the furnace and allowed to cool for approxi-mately 5 minutes before the completed cathode is removed.
Examples The following examples are provided to give those skilled in the art a better understanding of the invention:
Example 1:
A cathode which included MO2 as the cathode-active material was constructed on a platinum foil strip using direct conversion as follows:

1149L~SZ

(a) 3 milligrams of a 10% by weight suspension in heavy lubricating oil of MoS2 particles hav-ing an average particle diameter of about .25 microns was applied as a film to the platinum substrate.
(b) The coated substrate was inserted into a quartz tube through which nitrogen gas was caused to flow at about .8 litres per minute.
(c) The tube was then placed in a Lindberg tube furnace which had been preheated to about 575C. The tube was allowed to reach thermal equilibrium in the furnace.
(d) A mixture of about 0.1 mole percent oxygen in nitrogen was then caused to flow through the tube at about .8 litres per minute for 2 minutes. Then, pure nitrogen was again caused to flow through the tube for a further 3 minutes.
(e) The tube was then removed from the furnace and allowed to cool for about 5 minutes after which time the nitrogen flow was turned off and the completed cathode removed from the tube.
The completed cathode was used to construct a cell in a glass beaker which was sealed with a neoprene stopper. A lithium anode and the prepared cathode were suspended in the beaker from wires fitted through holes drilled into the stopper. The beaker was filled with about 20 cc of a .7M solution of LiBr in propylene car-bonate which served as the cell electrolyte. An argon 11~9'~52 atmosphere was introduced into the airtight beaker. The cell was then discharged and recharged at 200 micro-amperes. Figure 2 is a graph in which the cell voltage discharge and recharge characteristics are plotted versus time.
Example 2:
An MoO2 cathode was made using indirect con-version as follows:
(a) A coating of about 1.6 mg/cm2 MoS2 was applied as a film to a 19 cm2 piece of 20 micron thick aluminum foil.
(b) The substrate with applied film was placed on an aluminum support slab and inserted in a quartz tube through which a pure nitrogen atmosphere was caused to flow. The tube was placed in a Lindberg tube furnace which had been preheated to about 575C, and allowed to reach thermal equilibrium in the furnace.
(c) A gas mixture of about 0.3 mole percent oxygen in nitrogen was then caused to flow through the tube for about 17 minutes. This time was found to be sufficient to convert the approxi-mately 1 micron MoS2 particles to MoO3.
(d) The furnace temperature was then reduced to 440C and a hydrogen atmosphere caused to flow through the tube. The baking was continued for 9 hours before the completed cathode was removed from the furnace and tube.
X-ray diffraction analysis revealed that the substrate was coated with essentially pure MO2 con-ll'~g~SZ

taining relatively small trace amounts of molybdenum metal.
A cell was constructed as an Example 1 using a

2.5 cm2 piece of the prepared cathode. Discharge characteristics similar to those of Example 1 (as shown in Figure 2) were obtained.
Example 3:
A cathode was constructed on a 20 micron thick piece of aluminum foil using direct conversion as follows:
(a) MoS2 powder having an average particle dia-meter of about 20 microns was mixed in a 1 to 1 volume ratio with propylene glycol and a film of the resulting slurry applied to the aluminum foil substrate.
(b) The substrate with applied film was baked at 580C in an atmosphere containing about 0.4 mole percent oxygen in nitrogen for about 10 minutes to form a cathode containing approxi-mately 20 mole percent MO2 and approximate-ly 80 mole percent MoS2.
A cell was constructed using two stainless steel flanges separated by a neoprene O-ring sealer.
The anode consisted of a 6 cm2 sheet of lithium. A 6 cm2 piece of the prepared cathode (on which had been deposited approximately 43 milligrams of cathode-active material (MO2 + MoS2)) was used as the cell cath-ode. A porous polypropylene separator sheet which had been soaked in a lM solution of lithium perchlorate in ll'l9'~S2 propylene carbonate was inserted between the anode and cathode.
The newly constructed cell was conditioned by initially discharging it at 4 milliamperes to a lower cutoff voltage of about 0.85 volts. During this initial discharge, the cell voltage dropped in about 20 minutes to a plateau of about 1 volt and then decreased approxi-mately linearly to about 0.85 volts in a further 2 hours. The cell thus prepared and conditioned was cycl-ed through 66 discharge-charge cycles at about 4 milli-amperes between a minimum voltage of about 0.85 volts and a maximum voltage of about 2.7 volts. Figure 3 is a graph which shows the cell discharge-charge character-istic beginning with the fifth discharge.
Alternatively, particulate MoS2 may be con-verted to particulate MO2 using either direct or in-direct conversion. For example, if indirect conversion is used MoS2 particles may be stirred or tumbled while baking them in an oxygen-containing atmosphere to yield particulate MoO3 and then further stirring or tumbling the MoO3 particles while baking them in a reducing atmosphere to yield particulate MoO2. Then, when it is desired to produce a cathode which comprises MO2 as a cathode-active material, a suspension of some of the MO2 particles may be applied as a film to a metallic substrate as described above and then baked in an inert atmosphere to drive off the suspending media for the particles, leaving a coating of MoO2 adherent on the substrate. In the case of direct conversion, MoS2 particles are converted to MO2 or MoS2 coat-9~SZ

ed with MoO2 while stirring or tumbling in an oxygen-containing atmosphere. The following is an example where direct conversion was used:
(a) MoS2 powder having an average particle dia-meter of about 20 microns was placed inside a quartz tube which was then inserted into a Lindberg tube furnace.
(b) A mixture of oxygen gas flowing at about 4 c.c./min. and nitrogen gas flowing at about 2 litres/min. was caused to flow through the tube for about one hour during which time the furnace temperature was held at about 550C.
The quartz tube was continually rocked during this time to stir the particles.
(c) The quartz tube was then removed from the furnace and allowed to cool.
(d) A sample of the oxidized powder was suspended in propylene glycol and applied as a film to an aluminum substrate which was then baked in a nitrogen atmosphere at about 550C for about 15 minutes to yield a 6 cm2 cathode bearing about 30 mg. of cathode-active material.
(e) The completed cathode was used to construct a cell as in example 3.
(f) The newly constructed cell was cooled to about 0C and then conditioned at 0C by initially discharging it at 1 milliampere to a lower cutoff voltage of about .65 volts. During this initial discharge, the cell voltage drop-ped to a plateau of about 1 volt and then 11~945Z

decreased approximately linearly to about .65 volts.
(g) The cell thus prepared and conditioned was cycled at ambient temperature at about 2 milliamperes between about 1.1 volts and about 2.7 volts.

Claims (16)

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

1. A method of making a battery cathode having a cathode-active material at least a portion of which material is molybdenum dioxide, said method comprising the steps of:
(a) applying a suspension of finely divided molyb-denum disulphide particles in a viscous liquid as a film on a selected substrate; and, (b) baking the substrate and applied film in an oxygen-containing atmosphere to convert a selected proportion of the molybdenum disul-phide to an oxide of molybdenum coating said substrate.

2. A method as defined in claim 1, wherein before said substrate and applied film are baked in said oxygen-containing atmosphere, said substrate and applied film are first baked in an inert atmosphere to drive off substantially all of the viscous liquid.

3. A method as defined in claim 2, wherein said substrate and applied film is baked to convert molyb-denum disulphide to molybdenum dioxide such that the cathode-active material consists essentially of molyb-denum disulphide and molybdenum dioxide.

4. A method as defined in claim 3, wherein the mole percentage of molybdenum disulphide is greater than the mole percentage of molybdenum dioxide.

- Page 1 of Claims -

5. A method as defined in claim 3, wherein the mole percentage of molybdenum dioxide is greater than the mole percentage of molybdenum disulphide.

6. A method as defined in claim 3, wherein the mole percentage of molybdenum disulphide is relatively small in relation to the mole percentage of molybdenum dioxide.

7. A method a defined in claim 3, wherein the mole percentage of molybdenum dioxide is relatively small in relation to the mole percentage of molybdenum disulphide.

8. A method as defined in claim 2, wherein said substrate and applied film is baked to convert molyb-denum disulphide to molybdenum dioxide such that sub-stantially all the molybdenum disulphide applied as a film on the substrate is converted to molybdenum dioxide coating said substrate.

9. A method as defined in claim 1, wherein said substrate and applied film is baked such that substan-tially all the molybdenum disulphide applied as a film on said substrate is converted to molybdenum trioxide coating said substrate, and comprising a further step wherein said coated substrate is then further baked in a reducing atmosphere to convert substantially all the molybdenum trioxide coating said substrate to molybdenum dioxide coating said substrate.

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10. A batter cathode, comprising:
(a) a substrate; and, (b) a cathode-active material consisting essen-tially of molybdenum disulphide and molybdenum dioxide formed on the substrate by applying a suspension of finely divided molybdenum di-sulphide particles in a viscous liquid as a film on the substrate, then baking the sub-strate and applied film in an oxygen-containing atmosphere to convert a selected proportion of the molybdenum disulphide to molybdenum dioxide.

11. A battery cathode, comprising:
(a) a substrate; and, (b) a cathode-active material consisting essen-tially of molybdenum dioxide formed on the substrate by applying a suspension of finely divided molybdenum disulphide particles in a viscous liquid as a film on the substrate, then baking the substrate and applied film in an inert atmosphere to drive off substantially all of the viscous liquid, and then baking the substrate and applied film in an oxygen-containing atmosphere to convert substantially all the molybdenum disulphide to molybdenum dioxide coating said substrate.

12. A battery cathode, comprising:

(a) a substrate; and, - Page 3 of Claims -(b) a cathode-active material consisting essentially of molybdenum dioxide formed on the substrate by applying a suspension of finely divided molybdenum disulphide particles in a viscous liquid as a film on the substrate, then baking the substrate and applied film in an oxygen-containing atmosphere to convert substantially all the molybdenum disulphide to molybdenum trioxide coating said substrate, then baking the coated substrate in a reducing atmosphere to convert substantially all the molybdenum trioxide coating said substrate to molybdenum dioxide coating said substrate.

13. A method of making a battery cathode having a predominantly molybdenum dioxide cathode-active mate-rial, said method comprising the steps of:
(a) baking finely divided molybdenum disulphide particles in an oxygen-containing atmosphere to convert substantially all of the molybdenum disulphide particles to finely divided molyb-denum trioxide particles;
(b) further baking the molybdenum trioxide parti-cles in a reducing atmosphere to convert sub-stantially all of the molybdenum trioxide particles to finely divided molybdenum dioxide particles;
(c) applying a suspension of said molybdenum dioxide partices in a viscous liquid as a film on a selected substrate; and, - Page 4 of Claims -(d) baking the substrate and applied film in an inert atmosphere to drive off substantially all of the viscous liquid.

14. A method of making a battery cathode having a cathode-active material which comprises molybdenum dioxide, said method comprising the steps of:
(a) baking finely divided molybdenum disulphide particles in an oxygen-containing atmosphere to convert a selected proportion of the molyb-denum disulphide to molybdenum dioxide;
(b) applying a suspension of the converted parti-cles in a viscous liquid as a film on a selected substrate; and (c) baking the substrate and applied film in an inert atmosphere to drive off substantially all of the viscous liquid.

15. A battery cathode as defined in claim 10, wherein the mole percentage of molybdenum disulphide is relatively small in relation to the mole percentage of molybdenum dioxide.

16. A battery cathode as defined in claim 10, wherein the mole percentage of molybdenum dioxide is relatively small in relation to the mole percentage of molybdenum disulphide.

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