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 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.