Understanding the Role of Lithium Borate as the Surface Coating on High Voltage Single Crystal LiNi_0.5Mn_1.5O₄

Researchers investigated the potential of using lithium borate (LBO) to enhance the performance of high-voltage spinel lithium nickel manganese oxide (LNMO) cathode material in lithium-ion batteries (LIBs). LNMO/graphite (LNMO/Gr) full cells commonly suffer from capacity fading, limiting their practical applications. To address this issue, a dry mixing method was employed to apply an LBO coating on the LNMO surface. The LBO-coated LNMO exhibited improved cycling stability compared to uncoated LNMO in full cells, achieving a practical 3 mAh/cm² areal capacity. Various characterizations were conducted to understand the coating's effect on the cathode, electrolyte, and anode. The LBO-coated LNMO demonstrated a 5 nm cathode electrolyte interphase (CEI) with reduced phase change after long-term cycling, while the uncoated LNMO showed negligible CEI with noticeable phase change. Surprisingly, no boron was detected on the surface of the coated sample. Electrolyte and anode analyses revealed that the coating acted as a reservoir, gradually dissolving into the electrolyte and generating BF₄^- species. Consequently, the coating mitigated the dissolution of nickel and manganese from LNMO, as well as the extensive formation of a solid electrolyte interphase (SEI) on the anode side, leading to significant improvements in the cycling stability of the full cells. The insights gained from this study may serve as a guide for surface modification techniques applied to other high-voltage cathode materials with related electrolyte additive designs.