HighlyImproved CyclicStability of High Voltage LiNi_0.6Co_0.2Mn_0.2O₂/Graphite PouchCells via a Silicon-Based Electrolyte Additive

The LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622)/graphite composite has gained considerable traction in the realm of lithium-ion batteries owing to its favorable cost-performance ratio, high energy density, and inherent structural stability. However, the unstable cathode and anode interface at high voltage represents a significant challenge to further development. In this study, we propose an electrolyte additive, allyl trimethylsiloxysilane (TMSS), featuring dual-functional siloxane and vinyl groups, to construct low-resistance and electrochemically stable interfaces on both electrodes. Remarkably, the NCM622/graphite pouch cell cycled at 4.35 V and 1 C demonstrates significantly improved capacity retention, increasing from 58.8 to 80.0% after 800 cycles and from 18.8 to 73.0% after 1000 cycles compared to the baseline system. Theoretical calculations and electrochemical characterizations reveal that TMSS with an enriched siloxane group can preferentially adsorb on the NCM622 electrode surface and can be oxidized to construct a polysiloxane, which endows cathode electrolyte interface with exceptional stability; meanwhile, the oxidation intermediates can capture F^– and HF during the oxidation process, which significantly mitigates the leaching of transition metal ions from the cathode. As for the anode, TMSS can be preferentially reduced and utilize the vinyl group to auto-polymerize, forming a siloxane-framed polysiloxane, which enables the development of a solid electrolyte interface with smaller resistance on the anode. Such an exceedingly steady and lower impedance interface layer on both the cathode and anode can effectively improve the cycling stability of the NCM622/graphite batteries.