Degradation Pathways in Lithium-Ion Batteries with Ethylene Carbonate-Free Electrolytes

Abstract

The development of stable electrolyte solutions is critical for improving the lifetime and performance of lithium-ion batteries (LIBs). Electrolyte instability is a prominent issue with many next-generation electrode materials, such as Ni-rich cathodes, with recent reports identifying ethylene carbonate (EC) as a bad actor at the cathode surface. Herein, electrochemical methods, operando pressure measurements, and post-mortem X-ray and solution NMR studies are combined to investigate electrolyte and interfacial degradation phenomena in Ni-rich LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811)/graphite full cells with EC-containing and EC-free electrolytes. One key finding is that the mechanism for improved performance in EC-free electrolyte–1.5 M LiPF₆ in ethyl methyl carbonate (EMC) with 10 wt.% fluoroethylene carbonate (FEC)–arises from improved stability at both electrode-electrolyte interfaces. The EC-free electrolyte is found to suppress lattice oxygen release and reduce surface layer formation at NMC, leading to improved ion transport into the cathode particles, suppressed electrolyte solvent oxidation reactions, less formation and crossover of species that disrupt the graphite solid electrolyte interphase (SEI), and ultimately improved capacity retention. These insights are helpful in understanding and mitigating degradation in LIBs with Ni-rich cathodes.