Unraveling the Critical Role of Ti Substitution in P₂-Na_xLi_yMn_{1- y}O₂ 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 P₂-Na_xLi_yMn_1-yO₂ remains far from satisfactory. We herein develop a facile Ti-substitution method to improve the cyclability by taking Na_0.72Li_0.24Mn_0.76O₂ (NLMO) as an example. As expected, the novel layered oxide cathode Na_0.72Li_0.24Ti_0.10Mn_0.66O₂ (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 ²³Na/⁷Li 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.