Ti Substitution Facilitating Oxygen Oxidation in Na2/3Mg1/3Ti1/6Mn1/2O2 Cathode

Understanding structure-property chemistry of electrodes will be greatly helpful for developing advanced Na-ion batteries (NIBs). To search for promising electrodes, we employ chemical substitution in Na2/3TMO2 (TM = transition metal) layered compounds with different stacking modes: O3-Na2/3Mg1/3Ti2/3O2 and P2-Na2/3Mg1/3Mn2/3O2. Partial Ti-substituted Na2/3Mg1/3Ti1/6Mn1/2O2 enables significant improvements on electrochemical properties with respect to the stability of high-voltage plateau and total reversible capacity (∼230 mAh g−1). More than twice as much capacity is reserved in the high-voltage plateau area for the Ti-substituted cathode against non-Ti-substituted counterparts after the same cycles. The present work demonstrates that Ti substitution will decrease Mg and Mn ordering distribution in the pristine structure due to the intermediate size of Ti4+, which further increases the structural stability during Na+ deintercalculation and intercalculation. Additionally, Ti substitution leads to more localized electrons around oxygen ions in Mg–, Ti–, and Mn–O bonding, which can facilitate the charge-transfer reaction of oxygen redox. Our work provides new insights into structural chemistry on developing high-capacity, low-cost layered electrodes for NIBs.