Critical Role of Ultra-Microporous Tunnel Structure Within Hard Carbon in Boosting Sodium-Ion Storage

Accurately constructing pore structures and clarifying the relationship between pore structure and sodium storage performance to obtain high-quality hard carbon (HC) is crucial for the commercialization of sodium-ion batteries. However, it is still challenging to simultaneously obtain HC with high initial Coulombic efficiency (ICE), excellent rate performance, and satisfactory capacity in ester electrolytes by modifying the pore structure. Herein, the challenge is innovatively addressed by optimizing the dechlorination polymerization reaction to obtain the precursor, thereby modulating the carbon atom structure rearrangement and preoxidation process to design the HC with ultra-microporous tunnel structure (UMTS). The desolvation of UMTS (<1 nm) and the weak capillary effect at the surface can effectively reduce the decomposition of the electrolyte at the electrode surface and thus significantly enhance the ICE. The UMTS also facilitates the rapid transport of sodium ions and provides sites for the storage of sodium clusters, greatly enhancing the rate performance and reversible capacity. As a result, especially in ester electrolyte, the optimized material demonstrates an ultrahigh ICE of 90.9%, good rate performance (108.7 mAh g^-1 at 2 A g^-1) and an outstanding reversible capacity (357.2 mAh g^-1 with 98.3% of the capacity contribution below 1 V). This design concept of precisely constructing UMTS contributes significantly to the rational design of high-quality HC anode, and may be extended to other battery systems to promote the ongoing advancement of battery technology.