Flexible Sodium-Ion Batteries with Reversible Multi-Electron Redox and Advanced Electrolyte-Electrode Interface

Abstract

In the past one-decade, outstanding efforts and significant advancement have been achieved on 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 owe to the structural features including hierarchical pore bulk with the large surface area of hard carbon materials as the cathode in Na+-ion cells. Robust structural stability for repeated bending and twisting stressed along with electronic/ionic conductivity retention for which nanofiber mesh with inter-networked structure offers flexible Na+-ion fuel 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 sodium-ion batteries. The interfacial compatible flexible materials pose major challenges including the high safety demand of electrolytes; however, with a major focus on designing next-generation hard carbon materials and installing flexibility. In this review, first, the significance of hard carbon materials in the context of reversible specific capacity, and the random orientation of HCs with a curved and defective non-graphitized turbostratic structure consisting of large inter-distance of sheets were discussed. Na+-ion insertion mechanism, Na+-ion energy density, and flexible free-standing electrode are three major directions in which research advancement has focused. The major focus resides around cell configurations, flexible battery cell, and sodiation mechanisms which are critically compared and systematically analyzed. Subsequently, beyond cell configurations, this article presents a broad and macro perspective from anode materials with critical features, the origin of flexibility, and cell configuration with a deep understanding of SIBs devices.