Compared with other commercial cathode materials, the LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathode (NCM811) has high specific capacity and a relatively low cost. Nevertheless, the higher nickel content in NCM811 leads to an extremely unstable interface between the electrode and the electrolyte, resulting in inferior cyclic stability of the corresponding cell. Use of film-forming additives is regarded as the most feasible and economic approach to construct a stable interface on the NCM811 cathode. However, less effective electrolyte additives have been reported to date. Herein, we propose a valid film-forming electrolyte additive, 2,4,6triphenyl boroxine (TPBX), for application in a high-voltage NCM811 cathode. Experimental and computational results reveal that the TPBX additive can be preferentially oxidized to generate a highly stable and conductive cathode electrolyte interface (CEI) layer on the NCM811 cathode, which efficiently suppresses the detrimental side reaction and improves the electrochemical performance eventually. In detail, the cyclic stability of the Li/NCM811 half-cell is enhanced from 57% (without additive) to 78% (with 5% TPBX) after 200 cycles at 1C between 3.0 and 4.35 V. At a high current rate of 15C, the TPBX-containing electrode delivers a capacity of about 135 mAh g −1 , which is much higher than that of the electrode without the additive (80 mAh g −1 ). Interestingly, the TPBX is also reduced earlier than the ethylene carbonate (EC) solvent to form an ionically conductive solid electrolyte interface (SEI) film on the graphite anode. Due to the CEI layer on the cathode and the SEI film on the anode simultaneously formed by the TPBX additive, the cyclic performance of the graphite/ LiNi 0.8 Co 0.1 Mn 0.1 O 2 full cell is enhanced. Therefore, the incorporation of the TPBX additive into the electrolyte provides a convenient method for the commercial application of the high-energy-density NCM811 cathode in high-voltage lithium-ion batteries. KEYWORDS: LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811), electrolyte additive, 2,4,6-triphenyl boroxine, cathode electrolyte interface, high-energy-density lithium-ion battery

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Electrolyte additives have been successfully applied for the performance amelioration of lithium-ion batteries, especially under high voltage, which are based on the protective interphases on anode and cathode. Many additives have been proposed but less knowledge is available on the relationship between additive molecule structure and the interphase stability. In this work, we uncover the significance of the additive molecule structure in constructing a stable and robust interphase by comparing the effects of two similar additives, trimethyl borate (TMB) and tripropyl borate (TPB), on the performance of a layered lithiumrich oxide cathode (LRO) under a high voltage (4.8 V). Electrochemical measurements combined with physical characterizations and theoretical calculations demonstrate that TMB and TPB exhibit similar oxidative activity and both can build protective cathode interphases on LRO but they yield different cyclic stability improvement for LRO. The B-containing species derived from the TMB oxidation are more stable, yielding a more robust interphase than those from the TPB oxidation. This established relationship paves a road to design electrolyte additives more efficiently for high-voltage batteries.

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