SOLID STATE ELECTROLYTES FOR ELECTROCHEMICAL CELLS

Abstract: A solid state electrolyte for solid state electrochemical cells comprising, in mole percent, a combination of from 8 to 47% lithium moiety, from 3 to 16% phosphate moiety, from 10 to 50% iodide and from 5 to 30% in total of one or both of calcium or aluminum moieties, the non-metallic moieties being in stoichiometric relationship with the metallic moieties. An advantageous electrolyte is prepared by reacting calcium iodide and lithium orthophosphate.

Description: Solid State Electrolytes For Electrochemical Cells The present invention is concerned with solid state electrolytes containing lithium and iodine suitable for use in solid state electrochemical cells having lithium anodes and operating, at least in part, through lithium ion transport in the electrolyte. The present invention is also concerned with solid state electrochemical cells employing lithium metal (or alloy) anodes. In a solid state electrochemical cell the utility of the cell with a given cathode and anode is greatly influenced by the characteristics of the electrolyte. A suitable electrolyte must be an electronic insulator and an electrolytic conductor. The greater the electrolytic conductivity of the electrolyte, all other factors being equal, the greater is the allowable current drain from the cell and the less is the internal resistance of the cell. The present invention provides solid state electrolytes having high electrolytic conductivity as well as high electronic resistivity. It has now been discovered that by means of a combination of lithium, phosphate and iodide moieties and either calcium or aluminum or both moieties a novel, highly advantageous solid state electrolyte can be provided. According to the present invention, there is provided a solid state electrolyte for use in solid state electrochemical cell having a lithium anode, which electrolyte comprises, in mole percent, from 10 to 50% iodine moiety, from 8 to 47% lithium moiety, from 3 to 16% phosphate moiety, and from 5 to.30% in total of one or both of calcium and aluminium moieties, said electrolyte being in the form of an interdiffused body of sources of said moieties and said moieties being in stoichiometric relationship metallic to non-metallic. The solid state electrolytes of the present invention include as essential ingredients lithium, phosphate and iodine moieties and either calcium or aluminium or both moieties in stoichiometric relationship metallic to non-metallic moieties. The term "moieties" is used as being generic to "atoms" and "radicals" and any combination of "atoms" or "radicals". The term "solid state is employed in the sense that the electrolyte, as a whole, has a stable shape and volume at ambient temperatures at which the cell is used. The use of the term "solid state" does not imply the total absence of any phase or phases in the electrolyte composition which might be technically describable as liquid or pasty at the temperature of use of the electrolyte. The electrolytes are in the form of an interdiffused body (or compressed mass) of chemical compounds (or the elements) which are the sources of the moieties. In addition to the aforelisted moieties the electrolytes of the present invention may contain from 0 to 38 mole percent of the oxygen moiety (based on atomic rather than molecular oxygen). The solid electrolytes of the present invention may be made by dry blending powders of lithium orthophosphate, together with one or more of calcium iodide, lithium aluminium iodide and aluminium iodide and, if desired, one or both of aluminium oxide and lithium iodide. The dry blended powder should then be heated to cause interdiffusion or reaction between or among the ingredients (usually at a temperature above the melting point of the lowest melting of the ingredients) for a period of time of about 0.25 to about 3 or more hours. The thus reacted material is then cooled and ground to a powder and pressed to a suitable shape. If the electrolyte is to be free of oxygen moiety (except for the oxygen present in the phosphate moiety), it is preferably prepared by interdiffusing a mixture of 20 to 55 mole percent lithium orthophosphate with the balance being one or more of aluminium iodide, lithium aluminium iodide and calcium iodide. If the electrolyte is to contain oxygen moiety (again apart from that present in the phosphate) up to 60 mole percent aluminium oxide may be diffused with a mixture of lithium orthophosphate and one or more of calcium iodide, lithium aluminium iodide and aluminium iodide, and optionally with lithium iodide. For the purposes of the above ranges, the aluminium moiety is to be treated as a single entity even though, in compositions made from aluminium iodide and aluminium oxide, the aluminium is divided among the species aluminium phosphate, aluminium oxide and perhaps other aluminium-containing phases. All operations starting from blending dried powders are generally carried out in an inert dry atmosphere. It will be appreciated that the composition of the final electrolyte will not necessarily be the same as that of the initial ingredients. For example, if lithium phosphate (having a lithium ion conductivity of about 10 14 (ohm-cm) 1) and calcium iodide (not a lithium ion conductor) are initial ingredients, the final composition will likely contain not only these ingredients but also calcium orthophosphate and lithium iodide, the formation of which is indicated by the following equation 2Li P04 + 3Ca12 > Ca3(P04)2 + 6LiI The actual final composition of the electrolyte will vary depending not only on the proportions of the original reactants but also on the degree of approach to equilibrium permitted by the heating operation described hereinbefore and any side reactions which may occur. When making electrolytes according to the present invention, it is advantageous to employ an initial powder mixture containing,in mole percent, from 27% to 50% of lithium orthophosphate, and from 50% to 73% of calcium iodide. These proportions of moiety sources produce electrolytes containing, in mole percent, from 25 to 43% lithium moiety, from 8% to 14.5% phosphate moiety, about 14 to 23% calcium moiety and 28 to 45% iodine moiety. It is to be noted that if aluminium oxide is to be added to the composition, it is not necessary that this material be present in the original powder mixture. One can, if desired, initially heat the other ingredients of the powder mixture together, cool the heated mixture, grind the heated and cooled mixture to powder and thereafter blend that powder with aluminium oxide and reheat the blend to interdiffuse or interreact the originally heat treated powder with the alumina. Another advantageous electrolyte of the present invention is made by blending and heating (amounts in mole percent) from 18% to 50% of lithium phosphate, from 18% to 50% lithium aluminium iodide, and up to 60% aluminium oxide. Again, aluminium oxide can be included originally in the powder blend or it can be separately heated with the other pre-reacted (or pre-diffused) ingredients. Furthermore, the blended ingredients can contain lithium and iodide moieties in excess of those present in the lithium aluminium iodide composition. Examples of electrolytes according to the present invention together with processes for making them will now be given; Example I 7.85 parts-by-weight of a dry, powdered calcium iodide was blended with 2.15 parts-by-weight of dry, powdered lithium orthophosphate to give a composition containing, in mole percent, 35.3% of lithium moiety, 11.8% of phosphate moiety, 17.6% calcium moiety and 35.3% iodine moiety. The blended mixture was heated at 8000C for about 1/2 an hour and ground to a powder. Conductivity of the thus formed electrolyte was about 8 x 10 6 (ohm cm) 1 at room temperature (250C). Example II A1203 powder (grade H51 Alcoa) was blended and ground with a portion of the electrolyte as prepared in Example I and the ground blend was heated for 20 minutes at 5000C quenched and powdered. The final composition comprised, in mole percent, about 24.5% of lithium moiety, 8.2% of phosphate moiety, 12.2% of calcium moiety, 24.5% of iodine moiety, 12.2% of aluminium moiety and 18.4% of oxygen (on an atom basis) moiety not combined in the phosphate radical. conductivity of this electrolyte at room temperature wasabout 2 x -l at room temperature was about 2 x 10 -5 (ohm cm) -1. A 250 mA hr solid state cell was fabricated using this novel electrolyte with lithium as an anode and a mixture of BiBr3, TiS2 and Bi as the cathode. The room temperature open circuit potential of this cell was about 2.6 volts. Upon discharge at l000C under a 9.8 kilo-ohm external load, the cell exhibited a potential of greater than 2.0 volts after 100 mA hr of discharge. Example III 1.39 parts-by-weight of LiI, 4.21 parts-byweight of A1I3, 1.2 parts-by-weight of Li3P04 and 3.2 0 parts-by-weight of Awl 203 were blended, heated at 300 C for 1 hour then at 5000C for 1/2 an hour, quenched and powdered, and finally reheated at 8000c for 1/2 hour more and again quenched and powdered. The final composition was, in mole percent, 16% of lithium moiety, 4% of phosphate moiety, 16% of iodine moiety, 28% of aluminium moiety and 36% of oxygen (on an atom basis) moiety not combined in the phosphate radical. Conductivity at room -6 was 8.5 x -l temperature was 8.5 x 10 (ohm-cm) -1. The conductivity cell (Li/electrolyte/Li) was kept at 150 C for several days with no change in conductivity (measured at room temperature), thus indicating stability of the electrolyte to lithium. Example ;IV A number of electrolyte compositions made by reacting lithium orthophosphate and calcium iodide were tested for conductivity. These compositions were made by drying lithium phosphate in a vacuum at 120 C and drying calcium iodide at a sequence of temperatures, the last being 150 C in a vacuum. Appropriate quantities of these dried materials were ground together for two hours. The ground mixtures were then fired in alumina crucibles at 9000C for 1 to 2 hours. After firing and cooling the resultant masses were crushed, ground to fine powders capable of passing 100% through sieves having openings of about 0.044 mm and pressed to thin tablet form. Conductivities were measured on these pressed tablets between two lithium electrodes at 29 C. The following Table sets forth the results. TABLE Mole % of Cal2 Conductivity (ohm-cm) 1 in CaI-Li PO4 mixture Minimum maximum 40 1.25 x 10 7 2.3 x 10 50 9.6 x 10 -7 2.3 x 10 6 55 2.6 x 10 6 3 x 10 6 60 4.3 x 10 6 5.4 x 10 6 65 6.2 x 10 6 6.3 x 10 6 70 1.9 x 10 6 2.2 x 10 6 80 6.6 x 10 -7 8 x 10 By interpolating from the results of the foregoing Table it can be seen that in the range of 45 to 80 mole percent of CaI2 and 20 to about 55 mole percent of Li3P04 in binary mixtures employed in making the electrolytes of the present invention, the room temperature conductivi ties exceed about 5 x 10 7 (ohm-cm) 1. An even more advantageous range of CaI2 in these binary mixtures to give room temperature conductivities greater than about 1.5 x 10 6 (ohm-cm) 1 is 50 to 70 or 73 mole percent. Maximum conductivities were obtained at about 65 mole percent of Cay,, that is, in mole percentages of electrolyte, about 31.3% of lithium moiety, about 10.4% of phosphate moiety, about 19.4% of calcium moiety and about 38.88 of iodine moiety.

Claims: Claims 1. A solid state electrolyte for use in a solid state electrochemical cell having a lithium anode, which electrolyte comprises, in mole percent, from 10 to 50% iodine moiety, from 8 to 47% lithium moiety, from 3 to 16% phosphate moiety, and from 5 to 30% in total of one or both of calcium and aluminium moieties, said electrolyte being in the form of an interdiffused body of sources of said moieties and said moieties being in stoichiometric relationship metallic to non-metallic. 2. An electrolyte as claimed in claim 1, which includes up to 38 molar % oxygen moiety (calculated on the basis of atomic oxygen) apart from the oxygen contained in the phosphate moiety. 3 An electrolyte as claimed in claim 1, which consists of, in mole percentages, from 25% to 43% lithium moiety, from 8% to 14.5% phosphate moiety, from 14% to 23% calcium moiety and from 28E to 45t iodine moiety. 4. An electrolyte as claimed in claim 3, which consists, in mole percent, of about 31.3% lithium moiety, about 10.4% phosphate moiety, about 19.4% calcium moiety and about 38.8% iodine moiety. 5. A solid state electrochemical cell having a lithium anode, a cathode and an electrolyte as defined in any one of claims 1 to 4. 6. A process of making an electrolyte for use in a solid state electrochemical cell, which process comprises interdiffusing quantities of lithium orthophosphate, one or more of calcium iodide, lithium aluminium iodide, and aluminium iodide to provide an interdiffused composition comprising, in mole percent, from 10% to 50E iodine moiety, from 8% to 47% lithium moiety, from 3 to 16% phosphate moiety, and from 5 to 30% in total of one or both of calcium and aluminium moieties. 7. A process as claimed in claim 6, wherein lithium iodide is interdiffused with the other ingredients to produce the interdiffused composition. 8. A process as claimed in claim 7 or claim 8, wherein the interiffused composition contains up to 38% oxygen moiety (apart from the oxygen in the phosphate moiety) and wherein aluminium oxide is interdiffused with the other ingredients to produce the interdiffused composition. 9. A process as claimed in claim 6, wherein the interdiffused composition comprises, in mole percent, from 25% to 43% lithium moiety, from 8% to 14.5% phosphate moiety, from 14% to 23% calcium moiety and from 28% to 45% iodine moiety. 10. A process as claimed in any one of claims 6 to 9, wherein the interdiffusicn takes place at a temperature in excess of the melting point of the lowest melting ingredient but below that temperature at which all ingredients melt for up to about 2 hours.

Inventors: JATKAR, ARUN D., JOSHI, ASHOK VENDIMADHAV, SHOLETTE, WILLIAM PAUL
Applicants: RAYOVAC CORPORATION
Application year: 1981
Main classification: H01M6
IPC codes: C04B35, C01B25, H01M6