Recent Progress on Carbon-based Anode Materials for Na-ion Batteries

Na-ion batteries are currently an emerging and low-cost storage technology, which have attracted enormous attention and to its promising potentiality for large-scale energy storage applications. As the key electrode materials for Na-ion batteries, non-graphite carbonaceous materials have been regarded as the best choice for practical application due to its high sodium storage activity, low-cost and non-toxicity. According to the current research, graphite materials are not suitable to be anode materials of Na- ion batteries for practical application due to its low sodium storage capacity in carbonate electrolytes. Hard carbons have a high capacity of similar to 300 mAh.g(-1) with low sodium storage potential and thus are suitable for practical applications. Soft carbons have a sodium storage capacity about 200 mAh.g(-1) with sodium storage potential below 1 V vs. Na+/Na. Soft carbons usually exhibit excellent rate performances and thus are suitable to be used as anode materials for power Na-ion batteries. Reduced graphene oxide (rGO) has a sodium storage capacity of about 220 mAh.g(-1) and excellent rate performances. A high sodium storage capacity can be obtained by doping heteroatoms and introducing defect sites in rGO. However, the low material density, high sodium storage potential and large irreversible capacity of rGO will restrict its practical application. Porous carbons have high capacities of 300-450 mAh.g(-1) with excellent rate performances because their developed porous structure can provide more defects as the active sites for sodium storage and shorten the diffusion path of Na' to improve rate performances. Carbon nanowires/fibers have good flexibility due to their unique one-dimensional feature and stable sodium storage reversible capacity with good rate performance. These materials have advantages to be flexible electrodes for sodium-based flexible energy storage devices. By introducing N, S and other heteroatoms, heteroatom-doped carbons have more active sites for sodium storage and thus achieve higher sodium storage capacity. In summary, carbon materials with low graphitization degree are important development directions for anode materials of low cost Na-ion batteries. New carbon materials with unique microstructure and morphology have higher sodium storage capacity and rate capability, so they can be used as high power anode materials for sodium storage. Considering many factors, such as cycle life, energy density, power density and manufacturing cost, of practical application, hard carbon anodes is currently the best choice for practical application of Na-ion batteries. In the future, improving SEI stability, increasing Coulombic efficiency and improving electrical conductivity of hard carbon are urgent problems to be solved for practical application. Herein, the recent progress of carbonaceous anode materials is reviewed. The sodium storage mechanism and characteristics of carbon materials are summarized and discussed. Furthermore, the relationship between micro-structures and electrochemical performances, and remained problems of carbon anodes are discussed. This review will promote the development and understanding of carbon anode materials for sodium storage.