Sodium-Storage Behavior of Exfoliated MoS2 as an Electrode Material for Solid-State Batteries with Na3PS4 as the Solid Electrolyte

Abstract

MoS2 nanosheets prepared by chemical exfoliation are studied as electrode materials for sodium solid-state batteries (Na-SSBs). Sodium thiophosphate (Na3PS4) is used as the solid electrolyte and is synthesized by high-energy ball milling. The stability of Na3PS4 in contact with the Na metal and a Na–Sn alloy is compared, with the latter showing more favorable properties. The configuration of the Na-SSB is Na–Sn|Na3PS4|(MoS2, Na3PS4, carbon) with the positive electrode containing 60 wt % MoS2, 30 wt % Na3PS4, and 10 wt % carbon. Assuming full conversion, the theoretical capacity of MoS2 is 670 mA h g–1 (5 mA h cm–2), but only a fraction is achieved in the experiment. Cells are cycled at 33.5 mA g–1 at temperatures of 22, 40, and 60 °C for which initial discharge capacities of 243, 319, and 373 mA h g–1 are found. The increase in capacity can be linked to a decrease in resistance as determined by electrochemical impedance spectroscopy. Results are compared with analogue Li-SSBs (as recently discussed, see http://dx.doi.org/10.1021/acs.jpcc.9b01816) and the corresponding cells with a liquid electrolyte. Although SSBs show lower capacity compared to cells with the liquid electrolyte, their initial coulomb efficiency is much higher. For both electrolytes, the capacity in the case of sodium is smaller compared to lithium. All cells show a voltage hysteresis typical for conversion reactions. Interestingly, the volume expansion of the MoS2 electrode in the SSBs during initial sodiation/lithiation is about 70% in both cases, indicating that the discharge capacity is limited by mechanical constraints. Nevertheless, the redox activity of MoS2 electrodes is demonstrated over 100 cycles at different temperatures, with capacities exceeding most previous studies on the use of metal sulfides in Na-SSBs.