Transition Metal Vacancies in Cathodes of batteries: Benefit or Detriment?

Sodium-ion batteries are among the most promising alternatives to lithium-based technologies for grid and other energy storage applications due to their cost benefits and sustainable resource supply. Sodium storage cathodes and efficient Na-ion batteries are crucial for advancing sustainable energy solutions. Among various sodium storage cathodes, P2-Na2/3MnO2 layered transition metal oxide stands out as a highly promising candidate due to its straightforward synthesis, cost-effectiveness, high theoretical specific capacity (265 mAh g-1). Nevertheless, the Jahn−Teller effect of Mn3+ (t2g3-eg1) leads to localized distortion and introduces detrimental phase transitions, resulting in a tendency for deterioration in cycle stability. To address these issues, defect engineering has been proposed for P2-Na2/3MnO2. Some researchers suggest that Mn vacancies in the Mn layer could mitigate lattice volume changes during charge and discharge1. In contrast, others argue that Mn vacancies will accelerate the Mn migration during cycling, leading to unfavourable phase transitions and rapid capacity loss. Given the difficulty of avoiding manganese vacancies during the synthesis of layered transition metal oxides, and the largely undefined and controversial relationship between manganese vacancies and structural stability, this proposal seeks to perform a comparative and systematic study. Concerning the long-standing analytical challenge in characterising the Mn vacancies evolution which related the local structure, operando XAS would be critical to gaining insights into these materials crystallographic and electronic structure to establish their structure-property relationship for further performance enhancement.