Recent progress on electrolyte functional additives for protection of nickel-rich layered oxide cathode materials

In advantages of their high capacity and high operating voltage, the nickel (Ni)-rich layered transition metal oxide cathode materials (LiNixCoyMnzO2 (NCMxyz, x + y + z = 1, x ≥ 0.5) and LiNi0.8Co0.15Al0.05O2 (NCA)) have been arousing great interests to improve the energy density of LIBs. However, these Ni-rich cathodes always suffer from rapid capacity degradation induced by unstable cathode-electrolyte interphase (CEI) layer and destruction of bulk crystal structure. Therefore, varied electrode/electrolyte interface engineering strategies (such as electrolyte formulation, material coating or doping) have been developed for Ni-rich cathodes protection. Among them, developing electrolyte functional additives has been proven to be a simple, effective, and economic method to improve the cycling stability of Ni-rich cathodes. This is achieved by removing unfavorable species (such as HF, H2O) or constructing a stable and protective CEI layer against unfavorable reactive species (such as HF, H2O). Herein, this review mainly introduces the varied classes of electrolyte functional additives and their working mechanism for interfacial engineering of Ni-rich cathodes. Especially, key favorable species for stabilizing CEI layer are summarized. More importantly, we put forward perspectives for screening and customizing ideal functional additives for high performance Ni-rich cathodes based LIBs.