A significant interest in developing high volumetric energy density and high power energy storage systems (ESSs) has facilitated development of ideal cathode material in Lithium-ion batteries. Among various cathode materials, LiCoO₂ (LCO) is one of the promising candidates because of single crystalline morphology with relatively high electrode density (>3.9g cm^-3) and superior electrochemical performance. However, Increased cut-off voltage, higher than the conventional operating voltage of LCO (~4.3 V vs. Li/Li⁺), to maximize energy storage capacity gives rise to severe electrochemical degradation at the electrode-electrolyte interface and irreversible structural deformation at highly delithiated states of cathode material, which result in a rapid decrease of capacity and nominal voltage in a continuous cycle.

Herein, for an enhanced cycle life of LCO particularly in high cut-off voltage, we proposed LiCo_0.95Ni_0.05O₂ (LCNO) which not only enabled to demonstrate remarkable cycling performance and but also to be synthesized as a single crystalline with layered structure(R-3m) by the solid-state method in air condition. For comparison, the electrochemical behaviors of active materials, LCO and LCNO, charged up to 4.45V, were investigated. Even though LCO showed an initial reversible capacity of ∼182 mAh/g, higher than that of LCNO (∼176 mAh/g), LCNO exhibited much better capacity retention of 93.6% than that of LCO (74.3%) under continuous 100 cycles. We identified the less formation of irreversible byproducts on the surface of LCNO, compared to that of LCO, indicating the reactivity of interface between LCNO-electrolyte was highly stabilized with the assistance of Ni-ions. Furthermore, the evolution of interfacial change between LCNO-electrolyte after electrochemical cycling has been also investigated by high-resolution scanning transmission electron microscopy (HR-TEM). Based on electron energy-loss spectroscopy measurements (EELS), we confirmed that Ni-ions in LCNO may have a potential ability to initiate the oxidation state of Co ions around the surface to be partially reduced to Co²⁺/Co³⁺ mixed-valence state during cycles with a change of surface structure from layered structure to spinel-like phase, thus reducing surface side reaction simultaneously.