Nanoporous nitrogen doped carbons with enhanced capacity for sodium ion battery anodes

Abstract

Hard carbons with a disordered graphitic structure show promise as anode materials in next generation Na-ion batteries with stable and high sodiation/desodiation capacities. Since the mechanism of adsorption is not stoichiometric, as opposed to the case of Li-intercalation into graphite (LiC6), the search for an upper limit for the reversible capacity is an important task. We herein present a highly nanoporous nitrogen doped carbon obtained from ionothermal carbonization of a Zn-imidazolium framework that shows a stable cycling capacity of 496 mA h g(-1) at 30 mA g(-1) and 280 mA h g(-1) at 5 A g(-1) thus demonstrating exceptionally high capacity and outstanding rate performance. Although the reversible capacity was obtained only after extensive SEI formation, our results reveal the potential for much higher reversible capacities than usually observed today using carbons with a tailored porosity in Na-ion batteries. The electrochemical behavior is explained by improved utilization through a nanoscopic transport pore system and large graphitic interlayer distances. Initial SEI formation is herein used to passivate the carbon surface and obtain an ion-sieving coating. The ion sieving can allow for stable cycling at high capacity without further SEI formation because of a formed physical barrier between solvent molecules and metallic sodium.