Influence of the Cation on the Reaction Mechanism of Sodium Uptake and Release in Bivalent Transition Metal Thiophosphate Anodes: A Case Study of Fe 2 P 2 S 6

The layered active material Fe 2 P 2 S 6 was examined as anode material in sodium‐ion batteries (SIBs) and compared to previously investigated Ni 2 P 2 S 6 . A reversible specific capacity of 540 mAh g −1 was achieved after 250 cycles, depicting similar electrochemical performance as observed for Ni 2 P 2 S 6 . 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 Fe 2+ is reduced to elemental Fe during the uptake of 5 Na/f.u., while an amorphous intermediate is generated, which was identified as Na 4 P 2 S 6 by solid state NMR spectroscopy. Completely discharging against a Na metal counter electrode leads to the formation of nanocrystalline Na 2 S and indications of the formation of polymeric phosphorus were found. In sum, the Na uptake reaction process observed for Fe 2 P 2 S 6 coincides with the previously unraveled reaction pathway of Ni 2 P 2 S 6 . We therefore conclude that a universal reaction takes places for bivalent transition metal thiophosphate (M 2 P 2 S 6 ) electrodes in SIBs.