Flexible sodium-ion batteries with reversible multi-electron redox mechanism and an advanced electrolyte–electrode interface

Abstract

In the past decade, outstanding efforts and significant advancements have been achieved on the development of sodium-ion batteries (SIB) with flexible electrodes. Beyond the focus on the electrochemical performance of SIBs, free-standing flexible electrodes for large reversible capacities with superior rate and cycle performances are attributed to structural features, including hierarchical pore bulk that provides a large surface area in hard carbon (HC) materials. Robust structural stability for repeated bending and twisting stresses requires the nanofiber mesh with an inter-networked structure in a flexible sodium-ion full cell, which worked with a high working voltage. It is, therefore, an extensive research effort on flexibility and durability issues for the free-standing electrodes comprised of a range of materials for flexible SIBs. Interfacially compatible flexible materials pose major challenges, including the high safety demand of electrolytes; however, there is a major focus on next-generation HC materials and flexibility. In this review, first, the significance of HC materials are discussed in the context of reversible specific capacity and their random orientation with a curved and defective non-graphitized turbostratic structure with large inter-distance of sheets. Sodium-ion insertion mechanism, energy density, and flexible free-standing electrode are the three major directions of advancement discussed herein. We critically compared and systematically analyzed cell configurations, flexible battery cells, and sodiation/desodiation mechanisms. Subsequently, beyond cell configurations, this review presents a broad, macro perspective on anode materials, highlighting critical features such as redox at the electrode–electrolyte interface, the origin of flexibility, and cell configuration, with a deep understanding of SIB devices.