Realizing Hard Carbon Anodes with Dual‐High Slope and Plateau Capacities: From Precursor Design Principle to Sodium Storage Mechanism

Abstract Developing hard carbon (HC) anodes with dual‐high slope capacity (C _s ) and plateau capacity (C _p ) is one of the most efficient ways to realize high energy and power Sodium‐ion batteries (SIBs). Herein, three cellulose‐derived HCs are prepared to investigate the precursor effects of crystallinity, side chains, and the oxygen‐containing functional groups on carbon structures. It is revealed that the precursor factors play different roles in regulating the carbon structures (e.g., microcrystal size, defect density, interlayer spacing, and closed pore). The effects of carbon structures on C _s /C _p are further explored, guiding the correction of the structure‐performance relationship. Considering sodium ion diffusion and storage, C _s is found to relate with microcrystal size, carbon layer spacing, and defect density. A structural factor µ _HC that has a linear relationship with C _s is proposed. Moreover, the C _p is found to show a linear relationship with the closed pore content. High µ _HC and closed pore content also lead to high C _s /C _p retentions under high currents. Therefore, the hydroxyethyl cellulose‐derived HC with high µ _HC and closed pore content simultaneously delivers high C _s /C _p (177.3/216.7 mAh g ⁻¹ ), exhibiting good rate and cycling performance in half cells. Furthermore, the assembled Ah‐level pouch cell also demonstrates high energy density and long cycle life.