Over-stoichiometric Metastabilization of Cation-Disordered Rock Salts.

Cation-disordered rock salts (DRXs) have been well known for their potential to realize the goal of achieving scalable Ni- and Co-free high-energy density Li-ion batteries. Unlike in most cathode materials, the disordered cation distribution may lead to more factors that control the electrochemistry of DRXs. An important variable that has not been emphasized by research community is regarding whether a DRX exists in a more thermodynamically stable form or a more metastable form. Moreover, within the scope of metastable DRXs, over-stoichiometric DRXs, which allow relaxation of the site balance constraint of a rock salt structure, are particularly under explored. In this work, we report our findings in locating a generally applicable approach to "metastabilize" thermodynamically stable Mn-based DRXs to metastable ones by introducing Li over-stoichiometry. The over-stoichiometric metastabilization greatly stimulates more redox activities, enables better reversibility of Li deintercalation/intercalation, and changes the energy storage mechanism. The metastabilized DRXs can be transformed back to the thermodynamically stable form, which also revert the electrochemical properties, further contrasting the two categories of DRXs. This work enriches the structural and compositional space of DRXs families and adds new pathways for rationally tuning the properties of DRX cathodes. This article is protected by copyright. All rights reserved.