Promoting Interfacial Reaction via Quantifying NMC811 CEI Evolution and Li⁺ Desolvation Using Single-Particle Electrochemical Methods

Promoting interfacial kinetics of high-nickel LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) is a critical strategy for enhancing rate capability of lithium-ion batteries (LIBs). At the solid-liquid interface of positive electrode, charge transfer across the cathode-electrolyte interphase (CEI) is responsible for the interfacial redox reaction, which is dominated by desolvation process of Li⁺ and NMC811 CEI evolution. Since these two processes are significantly impacted by the solvation effect of electrolytes, herein single-particle Raman spectroscopy and single-particle probes are established to quantitatively study the influences of solvation effect on CEI evolution and desolvation process of Li⁺, respectively. Owing to oxidation of the unstable free cyclic carbonate ester from dissociation of solvation structure, the CEI is found to grow thicker during discharge process. Due to the decreased concentration of cyclic carbonate in the solvation structure, the desolvation process of Li⁺ in the ethyl methyl carbonate-ethylene carbonate electrolyte can be reduced by 11.8% compared to that in the dimethyl carbonate-ethylene carbonate electrolyte. With the presence of ethyl methyl carbonate-ethylene carbonate electrolyte, thinner CEI and easier desolvation process of Li⁺ can be simultaneously achieved on the NMC811 surface. Accordingly, its corresponding LIBs deliver the highest specific capacity ≈138.8 mAh g⁻¹ at 5C among the LIBs with the other commonly used electrolytes.