Enhancement of the cyclic stability of a Li-excess layered oxide through a simple electrode treatment for LiF-coating

Abstract

Lithium-rich layered cathodes (LLCs) are considered to be promising next-generation materials for lithium-ion batteries (LIBs) due to their high specific capacity and energy density. However, their poor cyclability poses a significant challenge for commercial applications. In this study, we introduce a straightforward one-step electrode heat-treatment method involving a lithium fluoride (LiF) coating on conventional LLCs, without the need for additional coating precursors, to enhance the cyclability. During the heat-treatment, lithium residues (LiOH and Li₂CO₃) and the PVDF used as a binder react to form an amorphous LiF coating layer (LiF-LLC). Although LiF-LLC initially exhibited a lower capacity compared to pristine LLC (220.2 vs. 246.6 mAh g⁻¹) due to its higher overpotential, it demonstrated superior performance after 100 cycles at 0.2 C. LiF-LLC maintained a discharge capacity of 219.4 mAh g⁻¹ with 95.1 % retention, while pristine LLC showed an outcome of only 164.8 mAh g⁻¹ with 83.4 % retention. A comprehensive analysis revealed that the LiF coating layer effectively passivated the cathode interface, preventing transition metal dissolution and a phase transformation caused by a HF attack. Additionally, LiF-LLC exhibited higher lithium-ion diffusivity, lower interfacial impedance, and enhanced Mn- and O-ion redox activities. These findings demonstrate that the simple electrode heat-treatment significantly improves the cyclic stability of LLCs.