Surface Changes of LiNixMnyCo1-x-yO2 in Li-Ion Batteries Using in Situ Surface-Enhanced Raman Spectroscopy

Understanding and controlling parasitic reactions at the electrode-electrolyte interface (EEI) in Li-ion batteries is essential to enhance and predict the battery cycle life. Recent work has shown that increasing Ni in the positive electrode materials can promote the dehydrogenation of carbonate-based electrolytes and electrode impedance growth. Unfortunately, probing the interfacial reactions between positive electrodes and organic electrolytes is intrinsically difficult using in situ techniques. In this work, we used in situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) to examine the changes in the interface between lithium nickel manganese cobalt oxide like LiNi0.8Mn0.1Co0.1O2 (NMC811) or LiNi1/3Mn1/3Co1/3O2 (NMC111) and a carbonate-based electrolyte with 1 M LiPF6. The surface of charged NMC811 was changed to spinel and rock-salt structure at 4.6 V-Li. In addition, the formation and growth of surface lithium fluoride appeared at 3.8 V-Li and higher, which could be attributed to the hydrolysis of LiPF6 by protic species generated from dehydrogenation of carbonate solvents by charged NMC. On the other hand, minimal surface changes were found for charged NMC111, where lithium fluoride was found only at a very high voltage (4.8 V-Li). Based on these findings, mechanistic details of surface structural and chemical changes on charged NMC were discussed, providing insights to tackle the interfacial reactions between electrodes and electrolytes in Li-ion batteries.