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

Sodium storage cathodes and efficient Na-ion batteries are crucial for advancing energy solutions. P2-type Na2/3MnO2 is a promising candidate due to its 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 . 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. Concerning the long-standing analytical challenge in characterising the subtle structure evolution which related the local structure, operando XAS would be critical to gaining insights into these materials crystallographic and electronic structure.