The electrochemical dynamic behavior of the solid electrolyte interface (SEI) formed on LiCoO 2 (LCO) by lithium bis(oxalato)borate (LiBOB) is investigated at various cutoff voltages. Particularly, for layered cathode active materials, various cutoff voltages are used to control the delithiation states; however, systematic investigations of the voltage and SEI are lacking. To increase the practical energy density of the LCO, a high cutoff voltage is pursued to utilize a state of high delithiation. However, this high cutoff voltage causes the electrolyte to undergo side reactions and the crystalline structure changes irreversibly, limiting the cycle life. In a lowvoltage environment (<4.7 V), LiBOB improves the initial Coulombic efficiency and cycling performance by forming an effective SEI, which suppresses side reactions. At higher voltage levels (4.7−4.9 V), LiBOB no longer effectively protects the surface, causing the electrochemical performance to decrease rapidly. The main cause of this phenomenon is the decomposition of LiBOB-SEI at a high voltage, as shown by systematic surface and electrochemical analyses comprising linear sweep voltammetry, cyclic voltammetry, and electrochemical impedance spectroscopy. In conclusion, LiBOB can suppress side reactions of the electrolyte by SEI formation, but the SEI decomposes at voltage levels higher than 4.7 V.

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Ni‐rich layered oxide cathodes are an immediately applicable alternative for meeting the high energy density demand of lithium‐ion batteries. The most significant hurdle of Ni‐rich layered cathode materials is their poor cyclability because of their increasing surface resistance due to their electrochemically reactive surfaces causing side reactions and the occurrence of Li/Ni cation mixing. Surface coating has been extensively studied for safeguarding particles, thereby leading to increases in electrochemical performance; however, the synthesis conditions must be carefully controlled due to the fragile surface of a Ni‐rich layered cathode. Herein, an effective coating method with self‐assembled ZrO2 (SA–ZrO2) on the surface of a Ni‐rich layered cathode material, LiNi0.82Co0.09Mn0.09O2 (NCM82), through a low‐temperature self‐combustion reaction is proposed. SA–ZrO2 is built by a combustion reaction of Zr(SO4)2·4H2O and thioacetamide to improve the surface stability of the cathode. In addition, a very small content of SOx is retained from the precursor, which promotes high lithium diffusion on the surface. Systematic analyses by X‐ray photoelectron spectroscopy and transmission electron microscopy demonstrate that this highly homogeneous ZrO2 coating layer, which is prepared at a low temperature of 500 °C, largely enhances the electrochemical performance in the half‐cell and full‐cell.