Enhanced Electrochemical Stability of Solid‐State Electrolyte‐Coated High‐Voltage LiNi_0.5Mn_1.5O₄ Cathodes in Li‐Ion Batteries

Spinel‐structured LiNi_0.5Mn_1.5O₄ cathodes in lithium‐ion batteries have gained attention for their high operating voltage, which provides high energy density, and their cost advantages due to the absence of cobalt. However, issues such as low cycle and thermal stabilities have been identified, with side reactions occurring at the electrode/electrolyte interface during continuous charge/discharge cycles that degrade electrode performance. Herein, we first optimized LiNi_0.5Mn_1.5O₄ using the Pechini sol–gel method to achieve uniform particles and controlled calcination temperatures. We then employed density functional theory and electrochemical testing to identify the optimal conditions. Uniform coating of the electrode surface with the oxide solid electrolyte Li_6.28Al_0.24La₃Zr₂O₁₂ (LALZO) was confirmed, aiming to improve lithium‐ion conductivity and enhance cycle and thermal stability. As a result, the formation of a coating layer on the electrode surface suppressed side reactions with the electrolyte and blocked contact, leading to an increase in ion conductivity. This improvement resulted in an enhanced rate capability and a significant increase in retention over 100 cycles at 0.2 C. Additionally, the interface resistance significantly improved with the coating layer, demonstrating reduced voltage decay due to overvoltage and improved interface stability. Finally, thermal stability was enhanced, with retention improving after 100 cycles at 0.5 C.