The high-voltage spinel lithium nickel manganese oxide (LNMO) with an operating voltage of 4.8 V is a promising cathode material for next-generation lithium-ion batteries (LIBs). However, LNMO/graphite (LNMO/Gr) full cells suffer capacity fading, which limits their practical applications. In this study, lithium metaborate (LBO) is applied on the LNMO surface to improve the full cell performance via a dry mixing method. The LBO-coated LNMO delivers much better cycling stability than the uncoated LNMO in full cells with a practical 3 mAh cm−2 areal capacity. Different characterizations are performed to understand the coating effect to track the boron and its impact on the cathode, electrolyte, and anode. The LBO-coated LNMO owns a 5 nm cathode electrolyte interphase (CEI) with mitigated phase change after long-term cycling. The uncoated LNMO has negligible CEI with obvious phase change. However, no boron can be detected on the surface of the coated sample. Electrolyte and anode analyses indicate that the coating acts as an additive reservoir, gradually dissolves into the electrolyte, and generates BF4− species. As a result, Nickel/Manganese (Ni/Mn) dissolution from LNMO and the extensive generation of solid electrolyte interphase (SEI) on the anode side is mitigated, thus improving the full-cell cycling stability to a great extent.