Novel Method of Fabricating Free-Standing and Nitrogen-Doped 3D Hierarchically Porous Carbon Monoliths as Anodes for High-Performance Sodium-Ion Batteries by Supercritical CO2 Foaming.

Sodium-ion batteries (SIBs), a promising candidate for large-scale energy storage systems, have recently attracted significant attention because of the low cost and high availability of the sodium resource. Hard carbon with a free-standing structure and plenty of active sites is considered to be the most potential anode material for SIBs. However, keeping a balance between the excellent performance and low cost for the large-scale commercial production of carbon anodes is still a great difficulty. Herein, a free-standing nitrogen-doped 3D hierarchically porous carbon monolith (denoted as 3DHPCM) anode for SIBs is successfully fabricated via a novel supercritical CO2 foaming technology and thermal treatment. Thanks to the tunable macro-meso-microporous and disordered structures, the 3DHPCM exhibits a high reversible specific capacity (281 mA h g-1 after 300 cycles at 50 mA g-1), superior rate performance (67 mA h g-1 at 10 A g-1), and excellent long-term cycling stability (175 mA h g-1 after 3000 cycles at 500 mA g-1). Remarkably, the 3DHPCM with such a high performance is fabricated via an environmentally friendly strategy from low-cost polyacrylonitrile and polymethyl methacrylate. Therefore, the strategy has great potential in practical application for fabricating high-performance hard carbon anodes and other composite electrodes for SIBs and more energy storage devices.