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

摘要

Highly crystalline CuFeS_(2) 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 ~(23)Na NMR and M?ssbauer 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/CuFeS_(2), nanocrystalline NaCuFeS_(2) 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 Na_(2)S and nanosized elemental Fe are formed in the discharged state. After charging, the main crystalline phase is NaCuFeS_(2). At the 150th cycle, the mechanisms clearly changed, and in the charged state, nanocrystalline Cu_(x) S phases are observed. At later stages of cycling, the mechanisms are altered again: NaF, Cu_(2)S, and Cu_(7.2)S_(4) appeared in the discharged state, while NaF and Cu_(5)FeS_(4) 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.