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

Abstract 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 Na 2 S with the Na + (NaPF 6 ) in the electrolyte, is revealed as the root cause for the cycling failure of MS. Na + ‐coordination effect assistants the dissolution of Na 2 S, further delocalizing Na 2 S 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 Na 2 S. Besides, the higher highest occupied molecular orbital of Na 2 S suggest the facilitated Na 2 S 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 Na 2 S, suppressing the side reactions of the solo‐oxidation of element metal and Na 2 S. 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.