Enhanced Lithium Ion Diffusivity of Nanosized Li1.2Mn0.8O2 Cathode Material Via Na and Ti Co-Doping

Abstract

Ionic doping is amongst the most effective modification methods capable of altering the local atomic environment, which in turn governs the redox reactions and electrochemical performance of Li-rich cathode materials. It has been demonstrated to improve bulk structural stability and suppress oxygen loss to some extent. Independently doping with Na and Ti has sparked interest in them however, very little is understood about their combined impact in a quest to address the structural complexities encountered during the operation of Li-rich cathodes. Herein, molecular dynamics simulations were carried out on both the bulk and nanospherical structures of Li1.2Mn0.8O2, Li1.2Mn0.72Ti0.08O2, and Li1.15Na0.05Mn0.72Ti0.08O2 whereNa+ doping in the Li-site and Ti in the Mn site. The temperature variations were between 300K and 2000 K. The findings of this study depict that bulk structures have diffusion coefficients that are orders of magnitude less than those of nanospherical structures. This strongly supports the unique capabilities of nanoparticles in reducing the Li-ion diffusion distance and increasing the contact area between the electrode and the electrolyte. In terms of variation in doped systems, it was observed that singly doping the layered structure with titanium slightly increases ionic diffusion in the Li-Rich structure whereas, co-doping the same structure yields even higher diffusion coefficients. These are possible results of enhanced interlayer spacing as Na generally has a larger ionic radius than Li. Furthermore, the co-doping may have contributed to structural stability wherein cationic mixing influenced by Mn migration in pure Li1.2Mn0.8O2 is largely reduced [1]. This places the two dopants at the pinnacle of capacity enhancement for the Li-rich electrode and sparks significant interest in further performance and structural stability studies.