Enhanced Interfacial Stability of a LiNi_0.9Co_0.05Mn_0.05O₂ Cathode by a Diboron Additive

Ni-rich LiNixCoyMnzO2 (NCM, xNi ≥ 0.9) layered materials are appealing cathode candidates for future-generation higher-energy-density lithium-ion batteries (LIBs). However, structural and interfacial instability of Ni-rich cathodes needs to be addressed prior to the large-scale deployment in electric vehicles. Herein, a diboron electrolyte additive 5,5,5′,5′-tetramethyl-2,2′-bi-1,3,2-dioxaborinane (TBDB) is proposed for constructing a stable LiNi0.9Co0.05Mn0.05O2 (NCM90)/electrolyte interface, thus improving the long-term cycle life and rate capability of an NCM90 cathode. Using an optimized 0.3% TBDB-containing electrolyte, the NCM90 cathode shows a capacity retention up to 84.3% after 200 cycles at 1C at a charge cutoff of 4.4 V, much superior over 70.1% when using the baseline electrolyte. The obvious performance improvement could be explained by the fact that TBDB with higher highest occupied molecular orbital (HOMO) energy is preferentially oxidized before electrolyte solvents, through boron–boron bond cleavage, generating a compact, thin, and protective cathode electrolyte interphase (CEI) layer on the cathode electrode. As a result, the electrolyte decomposition is effectively restrained with fewer poorly conducting byproducts being accumulated, which significantly enhances the interfacial stability and redox reaction kinetics of a Ni-rich NCM90 cathode.