(An) enhancement of cyclic stability of a Li-excess layered oxide through a simple electrode treatment for LiF-coating

Abstract

Recently, as the significance of Li-ion batteries (LIBs) especially for large-scale devices has dramatically increased, Li-rich layered oxides have drawn massive interest in academic research due to their high practical capacities and energy densities. However, the superior performances suffer from severe degradations during their operation, mainly originating from the interfacial side reactions as well as the sluggish anionic redox reactions. Herein, we proposed a simple 1-step post-treatment for the NMC-based LLC electrode, enhancing the electrochemical performances of the electrode. During the treatment, a LiF layer was formed through the reaction between lithium-ions from the residue compounds on the surface and fluorine-ions from PVDF binder molecules around the particles. After the treatment, the prepared LiF-LLC 250 sample exhibited a higher discharge capacity of 219.4 mAhg-1 and capacity retention of 95.1% after 100 cycles at 0.2C, compared to the as-synthesized LLC sample, 164.8 mAhg-1, and 72.3%, respectively. Besides, the LiF-LLC 250 sample also achieved higher energy density and better rate capability characteristics. The improvements were attributed to the robust LiF layer preventing degradation of active material from corrosive HF-attack, electrolyte decompositions, and the phase transformations. The capacity loss of LLC was mostly due to the lattice oxygen-ion release and Mn dissolution, the loss of the redox reaction mediators, and the following Li-ion diffusion inside the active material. Whereas, the LiF-LLC 250 expressed Mn and O redox reactions and maintained Li-ion diffusivities well during the prolonged cycles. We demonstrated the formation of an artificial shell through HRTEM, STEM, and XPS measurements and analyzed redox behaviors inside the particle during cycles with dQ/dV and GITT measurements.