Research progress on freestanding carbon-based anodes for sodium energy storage

Sodium-ion batteries (SIBs) have received extensive research interest as an important alternative to lithium-ion batteries in the electrochemical energy storage field by virtue of the abundant reserves and low-cost of sodium. In the past few years, carbon and its composite materials used as anode materials have shown excellent sodium storage properties through structural design and composition regulation. The increasing popularity of wearable electronics has demanded higher requirements for electrode materials. A free-standing electrode is able to eliminate the massive use of electrochemical inactive binders and conductive additives, thereby increasing the overall energy density of the battery system. Research progress on carbon materials such as carbon nanofibers, carbon nanotubes and graphene and their composites (metallic compounds and alloy-type materials) is summarized. The preparation strategies and electrochemical properties of free-standing carbon-based anodes with and without substrates are categorized and reviewed. Finally, proposals are made for future research and developments for free-standing carbon-based anodes for SIBs.