CuFeS₂ as a Very Stable High-Capacity Anode Material for Sodium-Ion Batteries: A Multimethod Approach for Elucidation of the Complex Reaction Mechanisms during Discharge and Charge Processes

Highly crystalline CuFeS2 containing earth-abundant and environmentally friendly elements prepared via a high-temperature synthesis exhibits an excellent electrochemical performance as an anode material in sodium-ion batteries. The initial specific capacity of 460 mAh g–1 increases to 512 mAh g–1 in the 150th cycle and then decreases to a still very high value of 444 mAh g–1 at 0.5 A g–1 in the remaining 550 cycles. Even for a large current density, a pronounced cycling stability is observed. Here, we demonstrate that combining the results of X-ray powder diffraction experiments, pair distribution function analysis, and 23Na NMR and Mössbauer spectroscopy investigations performed at different stages of discharging and charging processes allows elucidation of very complex reaction mechanisms. In the first step after uptake of 1 Na/CuFeS2, nanocrystalline NaCuFeS2 is formed as an intermediate phase, which surprisingly could be recovered during charging. On increasing the Na content, Cu+ is reduced to nanocrystalline Cu, while nanocrystalline Na2S and nanosized elemental Fe are formed in the discharged state. After charging, the main crystalline phase is NaCuFeS2. At the 150th cycle, the mechanisms clearly changed, and in the charged state, nanocrystalline CuxS phases are observed. At later stages of cycling, the mechanisms are altered again: NaF, Cu2S, and Cu7.2S4 appeared in the discharged state, while NaF and Cu5FeS4 are observed in the charged state. In contrast to a typical conversion reaction, nanocrystalline phases play the dominant role, which are responsible for the high reversible capacity and long-term stability.