Structural codes of organic electrode materials for rechargeable multivalent metal batteries

Rechargeable multivalent metal batteries (MMBs) are considered as promising alternatives to Li-ion and Pb-acid batteries for grid-scale energy storage applications due to the multi-electron redox capability of metal anodes. However, the conventional inorganic cathodes used in MMBs face challenges with the sluggish diffusivity and poor storage of charge-dense multivalent cations in their crystal lattice. Organic electrode materials (OEMs), on the other hand, offer several advantages as MMB cathodes, including flexible structural designability, high resource availability, sustainability, and a unique ion-coordination storage mechanism. This review explores the intrinsic connection between the structural features of OEMs and their charge storage performance, aiming to unveil key design principles for organic molecules used in various MMB applications. We begin with an overview of the fundamental aspects of different MMBs (i.e., Zn/Mg/Ca/Al batteries), covering electrolyte selection, metal stripping/plating electrochemistry, and the fundamentals of cathode operation. From a theoretical understanding of redox activities, we summarize the properties of different redox sites and correlate the electrochemical properties of OEMs with various structural factors. This analysis further leads to the introduction of critical design considerations for different types of OEMs. We then critically review a wide range of organic compounds for MMBs, from small organic molecules to redox-active polymers and covalent-organic frameworks, focusing on their structure-property relationships, key electrochemical parameters, and strengths and shortcomings for multivalent ion storage. Finally, we discuss the existing challenges and propose potential solutions for further advancing OEMs in MMBs.