On the surface modifications of high-voltage oxide cathodes for lithium-ion batteries: new insight and significant safety improvement

Abstract

A layered oxide solid solution of LiMeO₂ and Li₂MnO₃ (Me = Ni, Co, Fe, etc.), represented by the general formula Li[Li_1−x−yMe_xMn_y]O₂, is being considered a promising cathode for advanced lithium-ion batteries owing to its high capacity. Here, the surface of high-voltage oxide cathode, specifically Li[Li_0.167Ni_0.233Co_0.100Mn_0.467Mo_0.033]O₂, is modified with Al₂O₃ and AlPO₄ to address critical issues relating to safety and stability during cycling. Using in situ measurement of internal cell pressure, we demonstrate that the oxygen gas evolution upon high voltage charging is serious enough to raise a safety issue in practical batteries, but the problem can be greatly mitigated by surface modification. Furthermore, surface modification effectively prevents transition metal ions from leaching out of the electrode during cycling at elevated temperatures. The in-depth structural and electrochemical analyses indicate that in addition to the coated Al₂O₃ and AlPO₄ species, Al³⁺ ions incorporated into the transition metal layers modify the surface structure and play a crucial role in improving safety and stability. Our experimental findings provide a direction to future developments of high capacity electrodes and batteries that are tolerant to high-voltage charging and high-temperature environments.