Systemic analysis to reveal the improved stability of LiNi0.6Co0.2Mn0.2O2 electrode–electrolyte interface modulated by N-methylpyrrolidone

Owing to their high specific capacity and low cost as well as their ability to considerably alleviate mileage issues in electric vehicles, nickel-rich layered cathode materials are widely used in state-of-the-art lithium-ion batteries. However, severe material degradation caused by high contents of nickel limits their large-scale commercial utilization. Additives provide a simple and considerably effective means of improving the interfacial instability of nickel-rich cathode materials. Herein, we propose the use of a multifunctional additive N-methylpyrrolidone (NMP) to modulate the electrode–electrolyte interface of the LiNi0.6Co0.2Mn0.2O2 (NCM622) cathodes, thereby improving battery performance. In particular, the specific capacity retention of the battery containing this additive was 72.2% after 500 cycles, which was only 31.1% for the baseline electrolyte. Systemic analysis of the electrode surface shows that the improved performance of the additive-containing battery originated from the fact that NMP can scavenge trace water from the cell system, which inhibits the attack of harmful fluorine species on the electrode. Moreover, the hydrolysis products of NMP form a protective film on the electrode surface, thereby inhibiting the electrode–electrolyte side reactions. In addition, charge/discharge and cyclic voltammetric tests under overcharge conditions reveal that upon introduction of the NMP additive, a highly resistive NMP-derived protective film covered the NCM622 cathode surface, inhibiting a rapid increase in voltage and protecting the battery components to a certain extent.