Revisit sodium-storage mechanism of metal selenides in ether-based electrolytes: Electrochemically-driven Cu permeation to the formation of Cu2-xSe

Metal chalcogenides as anode materials of sodium ion batteries show excellent performances. However, there is still much confusion about sodium storage mechanisms. Here, CoSe2 is used as a model of metal selenides to disclose these electrochemical reactions in ether-based electrolytes, where Cu species from Cu foil gradually permeate into the electrode and replace Co to bind with Se upon cycling. The driving force of this transition in thermodynamics can be qualitatively illustrated by Hard-Soft-Acid-Base theory, as confirmed by a series of metal selenides. Because Cu species need to pass through solid-electrolyte-interphase (SEI) during this transition, electrolytes and voltages, which control the formation of SEI, become important in kinetics. To our knowledge, it is the first time to realize the crucial role of SEI in this transition. These results are important to understand the electrochemical properties of metal selenides in sodium ion batteries.