Protection of High-Nickel Cathodes from H ₂ O-Triggered Electrochemical Degradation Enabled by Moisture-Responsible Dehumidifying Separators

High-Ni NCM (LiNi_<i>x</i>Co_<i>y</i>Mn_<i>z</i>O₂) has been widely adopted as a promising solution for enhancing the energy density of lithium-ion batteries (LIBs) owing to its high theoretical specific capacity and wide operating voltage. Because of the high oxidation stability, a carbonate electrolyte containing LiPF₆ salt is employed in such a battery system; however, LiPF₆ reacts with the residual moisture, which remains even after thorough drying processes, to produce detrimental hydrofluoric acid (HF) promoting the significant structural degradation of high-Ni NCM. Herein, a moisture-responsive dehumidifying separator incorporating an oxygen-deficient TiO_2-<i>x</i> nanolayer was developed to mitigate the water-induced electrochemical degradation of LIBs paired with high-Ni NCM cathodes, thereby stabilizing the electrochemical performance even under highly humid conditions. Owing to the advantageous characteristics of the oxygen-deficient TiO_2-<i>x</i>, which spontaneously transforms into TiO₂ by reacting with moisture, LIBs employing the TiO_2-<i>x</i>/PE separator achieved a stable electrochemical performance even under extreme conditions of high H₂O concentration and high cutoff voltage by suppressing the H₂O-induced electrochemical deterioration. This dehumidifying separator would expedite the commercialization of the LIBs using high-Ni NCM and LiPF₆-based electrolyte by mitigating the electrochemical degradation of electrodes caused by residual H₂O.