Understanding the improved electrochemical performance of Ti substituted Li2MnO3

Abstract

In the layered Li2MnO3 cathode material for Li-ion batteries manganese is exclusively present in a 4+ oxidation state. The high theoretical capacity of 460 mAh/g can be explained by the contribution of anionic redox chemistry from the oxygen sublattice as the Mn5+/Mn4+ redox couple is situated below the top of the O-2p band. The gradual layered to spinel structural transition upon galvanostatic cycling causing voltage fade is originates from a lack of Mn-O covalency, resulting in irreversible O_h→T_h migration of Mn3+ and oxygen release upon charging.1 Here, we present the partial substitution of Mn4+ for Ti4+ as a promising way to enhance the reversible discharge capacity upon extended galvanostatic cycling. Ti substituted Li2MnO3 has been synthesized via a facile solution-gel method. The structure of pristine Li2Mn1-xTixO3 (x=0-0.2) has been comparatively investigated by PXRD, Raman spectroscopy, 7Li NMR and advanced TEM. Galvanostatic charge/discharge measurements performed at C/10 show a significant improvement of discharge capacities up to 30 galvanostatic charge/discharge cycles as compared to unsubstituted Li2MnO3. The ameliorated reversible discharge capacities are correlated with the structure of the pristine samples and the structure of post mortem samples analyzed by ex situ advanced TEM.