Organic materials for rechargeable sodium-ion batteries

Rechargeable sodium-ion batteries (SIBs) have attracted great attention for large-scale electric energy storage applications and smart grid owing to the abundance of Na resources and comparable performance with lithium-ion batteries. The use of organic electrode materials enables a sodium storage system with high energy/power density, metal-free, environmental friendliness, flexibility, lightweight, and cost-effectiveness. More importantly, the structural diversity and the ease of functionalizing organic molecules allows straightforward controllability on the redox properties and thus on the battery performances. Despite the development of organic SIBs is still in its infant state, they have drawn dramatically growing attentions and shown great promises throughout research works. In this review, we summarize the research efforts to push forward the electrochemical performance of the organic SIBs. We explore the efforts made on molecular design and electrode design, and the combination of both forms the basis to regulate the electrochemical properties. Moreover, we summarize the strategies for reducing the solubility of the organic electrode materials in light of the importance of this issue. The crucial differences between organic lithium-ion batteries and organic SIBs are presented. Finally, future challenges and opportunities of further developing organic electrode materials are discussed.