Revealing Na⁺-coordination induced Failure Mechanism of Metal Sulfide Anode for Sodium Ion Batteries

Metal sulfide (MS) is regarded as a promising candidate of the anode materials for sodium-ion battery (SIB) with ideal capacity and low cost, yet still suffers from the inferior cycling stability and voltage degradation. Herein, the coordination relationship between the discharge product Na₂S with the Na⁺ (NaPF₆) in the electrolyte, is revealed as the root cause for the cycling failure of MS. Na⁺-coordination effect assistants the dissolution of Na₂S, further delocalizing Na₂S from the reaction interface under the function of electric field, which leads to the solo oxidation of the discharge product element metal without the participation of Na₂S. Besides, the higher highest occupied molecular orbital of Na₂S suggest the facilitated Na₂S solo oxidation to produce sodium polysulfides (NaPSs). Based on these, lowering the Na⁺ concentration of the electrolyte is proposed as a potential improvement strategy to change the coordination environment of Na₂S, suppressing the side reactions of the solo-oxidation of element metal and Na₂S. Consequently, the enhanced conversion reaction reversibility and prolonged cycle life are achieved. This work renders in-depth perception of failure mechanism and inspiration for realizing advanced conversion-type anode.