Unraveling the Critical Role of Ti Substitution in P2-NaxLiyMn1-yO2 Cathodes for Highly Reversible Oxygen Redox Chemistry

Monovalent Li-substitution has been proven to be an effective strategy to resolve the pivotal problems confronted with P2-type layered Mn oxides, such as cooperative Jahn-Teller distortions of Mn3+ ions and drastic P2-(OP4)-O2 phase transformations occurring during desodiation. However, the cycling stability of most Li+-substituted P-2-NaxLiyMn1-yO2 remains far from satisfactory. We herein develop a facile Ti-substitution method to improve the cyclability by taking Na0.72Li0.24Mn0.76O2 (NLMO) as an example. As expected, the novel layered oxide cathode Na0.72Li0.24Ti0.10Mn0.66O2 (NLMTO-0.1) is able to deliver a very high reversible capacity of 165 mA h g(-1) for over 80 cycles within the voltage range of 1.5-4.5 V (vs Na metal), which is among the best for the reported Na-storage cathode materials. Moreover, the structure property relationship of Ti4+ substitution is scrutinized by an arsenal of Na-23/Li-7 solid-state nuclear magnetic resonance, dual-mode electron paramagnetic resonance, and synchrotron X-ray diffraction techniques. The results unequivocally substantiate that Ti substitution can effectively reduce the Li+/Mn4+ ordering in TMO2 slabs, assist the reversible migration of Li+ during Na+ extraction/intercalation, and ultimately enhance the reversibility of the oxygen redox process. This work provides a comprehensive insight into the structure chemistry in developing high-capacity and high-stability layered oxide cathodes.