Understanding the Electrode/Electrolyte interface Layer on Li-rich Nickel Manganese Cobalt Layered Oxide Cathode by XPS
Maral Hekmatfar, Arefeh Kazzazi, Gebrekidan Gebresilassie Eshetu, Ivana Hasa and Stefano Passerini; ACS applied materials & interfaces, October 2019, Pages 1-47. doi: 10.1021/acsami.9b14389
Layered lithium rich nickel manganese cobalt oxide (LR-NMC) represents one of the most promising cathode materials for application in high energy density lithium-ion batteries. The extraordinary capacity delivered derives from a combination of both cationic and anionic redox processes. However, the latter ones lead to oxygen evolution which triggers structural degradation and electrode/electrolyte interface (EEI) instability that hinders the use of LR-NMC in practical application. In this work, we investigate the surface chemistry of LR-NMC and its evolution upon different conditions to give further insights into the processes occurring at the EEI. X-ray photoelectron spectroscopy (XPS) studies reveals that once the organic component of the layer is formed, it remains stable independently on the higher cut-off voltage applied, while continuous growth of inorganics along with oxygen evolution occurs. The results performed on lithiated and de-lithiated LR-NMC surfaces indicate an instability of the EEI layer formed at high voltages, which undergoes a partial decomposition. Furthermore, the tris(pentafluorophenyl)borane (TPFPB) electrolyte additive simultaneously prevents excess LiF formation and changes the chemical composition of the EEI layer. This latter is characterized by a higher amount of poly(ethylene oxide) oligomer (PEO) species and LixPOyFz formation. In addition, the presence of boron containing compounds in the EEI layer cannot be excluded, which may be also responsible of the increased thickness of the EEI layer. Finally, fast kinetics at elevated temperatures exacerbate the salt decomposition which results in the formation of an EEI which is thicker and richer in LiF.