Chemistry of inorganic compounds – Phosphorus or compound thereof – Sulfur containing
Patent
1984-08-06
1986-04-01
Doll, John
Chemistry of inorganic compounds
Phosphorus or compound thereof
Sulfur containing
429194, 429218, 429221, 429223, C01B 2514
Patent
active
045797242
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a cathode material for an alkali-metal rechargeable cell, and to a method of preparing the material. There is a need for cells storing as much electricity as possible per unit weight or volume. Some of the most promising systems use alkali-metals, especially lithium, as the anode active material which is transferred to the cathode as discharge and returns to the anode on recharge. While many substances could serve as cathode materials for such cells, certain divalent metal phosphorus trisulphides (M.sup.II PS.sub.3) have the combined advantages of high capacity and rechargeable operation. The nickel and iron compounds NiPS.sub.3 and FePS.sub.3 in particular show desirable battery characteristics, because of the nature of their cell reactions which involve reversible insertion or intercalation of alkali metal ions from the anode into the crystal lattice of the cathode.
Compounds M.sup.II PS.sub.3 can be synthesised by direct reaction of the pure elements at 600.degree. to 800.degree. C. but this is costly in energy and leads to crystals which, we find, do not show the best performance. This high-temperature direct synthesis is totally unsuited for large-scale production.
The present invention provides a lower-temperature method (e.g. 0.degree. to 100.degree. C.) of making M.sup.II PS.sub.3 compounds in a somewhat amorphous (i.e. microcrystalline or partially ordered) form which is particularly well-suited to their use in cathode fabrication and also offers improved battery behaviour.
Thus, according to the invention, a method of preparing the compound M.sup.II PS.sub.3 (empirical formula), where M is a metal, comprises bringing together a source of hexathiohypophosphate anion and a solution of M.sup.2+ cation, whereby the compound M.sup.II PS.sub.3 is formed, and heating the M.sup.II PS.sub.3 in a non-reactive environment for 1 to 6 hours at 150.degree. to 500.degree. C.
The advantages of this method include a large saving in energy, and a much greater measure of thermal control, since the heat changes in the reactions are small. As the initial product is amorphous and may be of tiny particle size, the heating can cause some particle growth and ordering, to give a material of optimum sub-division and degree of crystallinity for efficient cathode preparation, especially for thin-film electrodes. To obtain material of a sufficiently high specific surface area and of sufficient disorder for good cathode power density and kinetic reversibility is difficult by the known high-temperature direct synthesis.
It has been found that the reaction in solution of various divalent metal ions M.sup.2+ with hexathiohypophosphate anion (P.sub.2 S.sub.6).sup.4- leads to amorphous, and in some cases colloidal, products with the empirical formula M.sup.II PS.sub.3. The reaction may be expressed stoichiometrically thus: not in simple molecular form as depicted.
M is preferably a transition metal (i.e. of atomic number 22-30, 40-48 or 72-80), preferably of atomic number 22-30, such as Fe, Co or Ni.
The reaction between the above ions may be at 0.degree. to 100.degree. C., such as room temperature or slightly above.
The solution of M.sup.2+ cation optionally originates by contact of a solution of another cation (such solution possibly also being the said source of hexathiohypophosphate anion) with an M.sup.2+ -cation-exchange resin.
The M.sup.II PS.sub.3 formed may be removed from the solution and washed, and the washing liquid and any residual solvent are removed, before the heating. The heating may be for from 2 to 4 hours (e.g. substantially 3 hours), and at a temperature of from 300.degree. to 450.degree. C., preferably 320.degree. to 400.degree. C., such as substantially 350.degree. C.
After removal of the solvent and some heating, some of the compounds (for example NiPS.sub.3) will function as a cathode material in lithium-organic electrolyte cells. Thus, a method of making a cathode for an alkali-metal rechargeable cell comprises applying a compound prepared as set forth above, mixed with a vehicle
REFERENCES:
patent: 4267157 (1981-05-01), Maas et al.
patent: 4439301 (1984-03-01), Reichman et al.
Foot Peter J. S.
Nevett Brian A.
Doll John
Langel Wayne A.
National Research Development Corporation
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