Influence of the Cation on the Reaction Mechanism of Sodium Uptake and Release in Bivalent Transition Metal Thiophosphate Anodes: A Case Study of Fe2P2S6

Abstract

The layered active material Fe2P2S6 was examined as anode material in sodium-ion batteries (SIBs) and compared to previously investigated Ni2P2S6. A reversible specific capacity of 540 mAh g−1 was achieved after 250 cycles, depicting similar electrochemical performance as observed for Ni2P2S6. The rate capability and long-term behavior of these two materials are also very similar. Another objective was to elucidate the reaction mechanism during discharging and charging by applying several techniques such as X-ray diffraction, pair distribution function analysis as well as X-ray absorption and solid state NMR spectroscopy. The results clearly demonstrate that the majority of Fe2+ is reduced to elemental Fe during the uptake of 5 Na/f.u., while an amorphous intermediate is generated, which was identified as Na4P2S6 by solid state NMR spectroscopy. Completely discharging against a Na metal counter electrode leads to the formation of nanocrystalline Na2S and indications of the formation of polymeric phosphorus were found. In sum, the Na uptake reaction process observed for Fe2P2S6 coincides with the previously unraveled reaction pathway of Ni2P2S6. We therefore conclude that a universal reaction takes places for bivalent transition metal thiophosphate (M2P2S6) electrodes in SIBs.