Sulfur is a New High-Performance Additive toward High-Voltage LiNi_0.5Co_0.2Mn_0.3O₂ Cathode: Tiny Amount, Huge Impact

Increasing working voltage of cathode has been identified as one of the most promising strategies to increase energy density of the lithium-ion batteries. It is of crucial importance to suppress side reactions and control the formation of a cathode electrolyte interface (CEI) on the cathode surface in a high voltage range. In this work, sulfur is utilized to increase the working voltage of LiNi_0.5Co_0.2Mn_0.3O₂(NCM 523) to 4.5 V as demonstrated by both the NCM523/Li half-cell and NCM 523/graphite full cell. When a tiny amount of sulfur (0.1 mg mL^-1) is added to the blank electrolyte of ethylene carbonate (EC) and dimethyl carbonate (DMC) (3:7 by volume), the cycling stability and rate performance are greatly improved in the NCM523/Li half-cell. The capacity retention over 200 cycles at 170 mA g^-1 (1.0 C) is increased from 61.2 to 82.0%. The capacity at a high current density of 850 mA g^-1 (5.0 C) is increased from 92 mAh g^-1 to 120 mAh g^-1. Because the addition of sulfur also enhances the performance of the Li/graphite half-cell, improved performance is demonstrated by the NCM 523/graphite full cell as well. The mechanism is interpreted based on various characterizations. It is revealed that the preferential oxidation of sulfur at the cathode surface suppress decomposition of electrolyte solvent. Because only a tiny amount of sulfur is added into the electrolyte solution, excessive decomposition of sulfur is avoided, leading to improved electrochemical performance.