Synthetically Produced Isocubanite as an Anode Material for Sodium-Ion Batteries: Understanding the Reaction Mechanism During Sodium Uptake and Release.

Bulk isocubanite (CuFeS) was synthesized via a multistep high-temperature synthesis and was investigated as an anode material for sodium-ion batteries. CuFeS exhibits an excellent electrochemical performance with a capacity retention of 422 mA h g for more than 1000 cycles at a current rate of 0.5 A g (0.85 C). The complex reaction mechanism of the first cycle was investigated via PXRD and X-ray absorption spectroscopy. At the early stages of Na uptake, CuFeS is converted to form crystalline CuFeS and nanocrystalline NaFeS simultaneously. By increasing the Na content, Cu is reduced to nanocrystalline Cu, followed by the reduction of Fe to amorphous Fe while reflections of nanocrystalline NaS appear. During charging up to -5 Na/f.u., the intermediate NaFeS appears again, which transforms in the last step of charging to a new unknown phase. This unknown phase together with NaFeS plays a key role in the mechanism for the following cycles, evidenced by the PXRD investigation of the second cycle. Even after 400 cycles, the occurrence of nanocrystalline phases made it possible to gain insights into the alteration of the mechanism, which shows that CuS phases play an important role in the region of constant specific capacity.