Pre-construction Gas Nanochamber Strategy: Producing Hard Carbon Rich in Closed Pores and sp2-C for Sodium-ion Batteries

Abstract

Hard carbon still faces the challenge of incompatibility between high capacity and high initial Coulombic efficiency (ICE). Herein, a pre-construction gas nanochamber strategy is proposed to produce hard carbon with abundant closed pores and sp²-C, thereby simultaneously achieving high capacity and ICE. The gas nanochambers are engineered by balancing the self-generated pressure and external pressure during the carbon precursor pyrolysis. A combination of in-situ/ex-situ characterizations and molecular dynamics (MD) simulations reveal that the external pressure can restrict the intense release the pyrolyzed gases, thus forming abundant and uniform nanochamber-like close pores within macroparticles. Concurrently, the intervention of pressure facilitates the arrangement of carbon atoms into sp²-C with a few defects. Further insights from bulk phase/interface analysis and density functional theory (DFT) calculations demonstrate that this unique structure enables hard carbon to store more sodium ions and preferentially decompose NaPF₆ to form an inner inorganic-rich solid electrolyte interface (SEI) film. As a result, the optimized hard carbon can deliver high capacity of 370.4 mA h g⁻¹ with ICE of 89.3%, excellent rate capability and long-term cycling stability with a capacity of 136.1 mA h g⁻¹ after 10000 cycles at 5.0 A g⁻¹.