Stabilized high-voltage operation of Co-free NMX cathode via CEI-controlling

A current emerging trend in materials research for Li-ion batteries (LIBs) is exploring cathode materials whose price is neutral to global supply chain issues. In this regard, removing cobalt in conventional high-Ni Li[Ni_xCo_yMn_z]O₂ (NCM) cathodes has been adopted as one of the most promising strategies. However, Co-free Li[Ni_xMn_y]O₂ (NMX, x 〈 0.8, y 〉 0.2) cathodes should require the high voltage (≈4.5 V vs. Li/Li⁺) operation to obtain the high energy density as the level of conventional high-Ni NCM cathodes, which is considered as the major barrier of their real application to the LIB industrial field. To address this issue, it is essential to develop the novel electrolyte system for optimizing the high voltage operation of NMX cathodes. In this study, a delicate consideration of the whole battery system's chemical ensemble and its target usage condition has been attempted, and, as an exemplifying example, a Co-free Li[Ni_0.75Mn_0.25]O₂ cathode has been paired to an conventional electrolyte with 1,4-butane sultone (BS) electrolyte additive. Since highest occupied molecular orbital (HOMO) energy of BS is lower than both other well-known electrolyte additives and similar with the carbonate-based solvent, BS enables formation of homogeneous and stable cathode-electrolyte interphase (CEI) layer especially at the high voltage (4.5 V vs. Li/Li⁺). Thus, 1wt% BS-added electrolyte can result in excellent cyclability of the full cell based on a Co-free Li[Ni_0.75Mn_0.25]O₂ cathode even in the 5.8 Ah prismatic cell with ∼78% capacity retention for 1000 cycles, which indicates that a Co-free Li[Ni_0.75Mn_0.25]O₂ cathode with BS-based electrolyte can be applied to the real LIB industry as the low-cost alternative to conventional high-Ni NCM cathodes.