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, Ti4+-substituted Li2MnO3 was synthesized via a facile solution-gel method. Ti4+ 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 Li2Mn1–x Ti x O3 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 Li2Mn0.8Ti0.2O3, 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.

Ameliorating electrochemical performance of Li-rich Mn-based cathodes for Li-ion batteries by Fe substitution

“…The formation of a layered Li 2 MnO 3 structure consisting of Li and Li/TM slabs is conrmed by the dark regions of the particle interiors in the HAADF-mode images. [44][45][46][47] Comparing the images of LMFO-0 and LMFO-6, the interplanar distances of the (001) planes increased from 4.72 to 4.77 Å owing to Fe substitution (Fig. 2(c) and (g)).…”