AbstractReasonably elevating the working voltage (≥4.4 V vs. Li/Li⁺) of the cathode is one of the efficient approaches to maximize the energy density of lithium‐ion batteries (LIBs). As a preferred partner for high‐voltage LIB systems, localized high‐concentration electrolyte (LHCE), characterized by a stronger Li solvation structure, less free solvent, and robust electrode/electrolyte interphase has attracted much attention in academic circles. Herein, we systematically studied the role of the diluent in LHCE on the formation of the cathode electrolyte interphase (CEI) and elucidated that the existing anion‐diluent pairing in the inner Helmholtz plane (IHP) results in an uneven CEI and subsequent battery degradation under high voltage. A m‐fluorotoluene (mFT) diluent was further employed in the LHCE containing lithium difluoro(oxalato)borate (LiDFOB) to facilitate a uniform and rich‐anion‐derived CEI, since the weaker interaction of H_mFT−B_DFOB⁻, as compared to the H_{Hhydrofluoroether}−B_DFOB⁻, reduces the influence of mFT in IHP or initial CEI formation. Consequently, the mFT‐dominated LHCE propels the high‐voltage performance of LIBs one step forward, endowing a 4.6 V‐class 1.2‐Ah graphite||LiNi_0.8Co_0.1Mn_0.1O₂ pouch cells a 90.4 % capacity retention after 130 cycles. Our study thus describes a new index affecting the CEI formation and proposes novel strategies to deeply optimize the high‐voltage LIBs.