Phosphoric Acid Cross-linking Regulates Closed Pores for Scalable Hard Carbon Anodes

Abstract

The low-voltage capacity of hard carbon anodes, governed by closed-pore structures, directly determines the energy density of Na-ion batteries (NIBs), yet rational regulation of closed pores remains challenging. Herein, we report a phosphoric acid-induced cross-linking strategy to precisely tailor the closed pores of starch-derived hard carbon. H3PO4 promotes cross-linking with starch at low temperatures (150−400 °C), enhancing carbon conversion, while its removal by washing generates micropores through a templating effect that evolve into closed pores after high-temperature carbonization. Consequently, the carbon yield increases from 14.0% to 31.7%, and the resulting hard carbon delivers a reversible capacity of 395.8 mAh g−1 with an initial Coulombic efficiency of 89.4%. Additionally, the 3.5 Ah-level 26700 cylindrical cell based on the resulting anode demonstrates a high capacity retention of 96.5% after 500 cycles at the 1 C rate. This work provides a scalable and cost-effective pathway toward high-performance hard carbon anodes for practical NIB applications.