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 andanionicredox chemistry being used as positive electrodes for Li-ion batteries,have raised interest due to their high specific discharge capacitiesexceeding 250 mAh/g. However, irreversible structural transformationstriggered by anionic redox chemistry result in pronounced voltagefade (i.e., lowering the specific energy by a gradual decay of dischargepotential) upon extended galvanostatic cycling. Activating or suppressingoxygen anionic redox through structural stabilization induced by redox-inactivecation substitution is a well-known strategy. However, less emphasishas been put on the correlation between substitution degree and theactivation/suppression of the anionic redox. In this work, Ti4+-substituted Li2MnO3 was synthesizedvia a facile solution-gel method. Ti4+ is selected as adopant as it contains no partially filled d-orbitals. Our study revealedthat the layered "honeycomb-ordered" C2/m structure is preserved when increasing the Ticontent to x = 0.2 in the Li2Mn1-x Ti (x) O-3 solidsolution, as shown by electron diffraction and aberration-correctedscanning transmission electron microscopy. Galvanostatic cycling hintsat a delayed oxygen release, due to an improved reversibility of theanionic redox, during the first 10 charge-discharge cyclesfor the x = 0.2 composition compared to the parentmaterial (x = 0), followed by pronounced oxygen redoxactivity afterward. The latter originates from a low activation energybarrier toward O-O dimer formation and Mn migration in Li2Mn0.8Ti0.2O3, as deducedfrom first-principles molecular dynamics (MD) simulations for the"charged" state. Upon lowering the Ti substitution to x = 0.05, the structural stability was drastically improvedbased on our MD analysis, stressing the importance of carefully optimizingthe substitution degree to achieve the best electrochemical performance.