Understanding the Activation of Anionic Redox Chemistry in Ti4+-Substituted Li2MnO3 as a Cathode Material for Li-Ion Batteries

Abstract

Layered Li-rich oxides, demonstrating both cationic and anionic redox chemistry being used as positive electrodes for Li-ion batteries, have raised interest due to their high specific discharge capacities exceeding 250 mAh/g. However, irreversible structural transformations triggered by anionic redox chemistry result in pronounced voltage fade (i.e., lowering the specific energy by a gradual decay of discharge potential) upon extended galvanostatic cycling. Activating or suppressing oxygen anionic redox through structural stabilization induced by redox-inactive cation substitution is a well-known strategy. However, less emphasis has been put on the correlation between substitution degree and the activation/suppression of the anionic redox. In this work, Ti 4+-substituted Li 2 MnO 3 was synthesized via a facile solution-gel method. Ti 4+ is selected as a dopant as it contains no partially filled d-orbitals. Our study revealed that the layered "honeycomb-ordered" C2/m structure is preserved when increasing the Ti content to x = 0.2 in the Li 2 Mn 1−x Ti x O 3 solid solution, as shown by electron diffraction and aberration-corrected scanning transmission electron microscopy. Galvanostatic cycling hints at a delayed oxygen release, due to an improved reversibility of the anionic redox, during the first 10 charge−discharge cycles for the x = 0.2 composition compared to the parent material (x = 0), followed by pronounced oxygen redox activity afterward. The latter originates from a low activation energy barrier toward O−O dimer formation and Mn migration in Li 2 Mn 0.8 Ti 0.2 O 3 , as deduced from first-principles molecular dynamics (MD) simulations for the "charged" state. Upon lowering the Ti substitution to x = 0.05, the structural stability was drastically improved based on our MD analysis, stressing the importance of carefully optimizing the substitution degree to achieve the best electrochemical performance.