C to Modulate the Closed Pore Structures of Hard Carbon for High-Performance Sodium-Ion Batteries.

Hard carbon (HC) has emerged as a highly promising anode material for sodium-ion batteries (SIBs) attributed to its characteristic low-potential charge and discharge plateau. Recent studies have shown that the plateau capacity of HC mainly originates from the filling of the nanoscale closed pores by sodium. However, the precise design of the closed pore structure of HC remains a great challenge. Herein, C with a diameter of 0.7 nm is used to promote the formation of closed pores in phenolic resin-based HC. The spherical structure of C facilitates the oriented crystallization of graphitic microdomains within phenolic resin-based HC, thereby enhancing the uniformity of the closed pore structure of HC. Furthermore, during high-temperature carbonization, C undergoes fragmentation and structural reorganization, which increases the closed pore volume and introduces additional sodium storage sites. As a result, the optimal HC provides an excellent reversible capacity of 361 mA h g at 20 mA g and a high plateau capacity of 268 mA h g. This work provides deep insights into the mechanism of forming closed pores on the nanoscale, advancing the development of high-performance SIBs.