Transition Metal Phosphorous Trisulfides as Cathode Materials for High Temperature Batteries

The challenging environments of high temperature and high pressure prevalent on the Venus surface limit the battery options for Venus landers and surface probes. High temperature batteries employing Li alloy anodes, molten salt electrolytes and FeS cathodes were demonstrated to be resilient and operational for several days. For further improvements in performance, i.e., both specific energy and operational life, new high-capacity cathode materials are needed. Transition metal phosphorus trisulfides (TMPS3) are promising with considerably higher (2X) specific capacity, specific energy and energy density, by virtue of their ability to react with more than two lithium ions. This papers describes the assessment of these cathodes for high temperature batteries to power future Venus landers and probes. Various metal phosphorus trisulfides, manganese, iron, cobalt and nickel phosphorus trisulfides were synthesized and characterized by Scanning Electron Microscopy (SEM)/Energy Dispersive X-Ray Spectroscopy (EDAX) and X-Ray Diffraction (XRD) and tested in our high-temperature laboratory cells at 475 oC using cyclic voltammetry (CV) and galvanostatic discharges at different rates. All these cathodes, except Co, showed reversible behavior in cyclic voltammetric measurements. In the discharge tests, NiPS3 displayed the best capacity in both C/20 and C/720 rates, with higher voltages and slightly higher capacity than FeS, followed by FePS3, while MnPS3 displayed relatively poor performance at C/20. Cathodes extracted from the discharged cells were analyzed using XRD and shown contain transition metal (Fe or Ni) and Li2S, as expected from the reaction scheme.