Novolak‐Derived Hard Carbon Negative Electrodes for Lithium‐Ion Batteries: Closed Pore Engineering via Cross‐linking Density of Precursors

Abstract

Hard carbon (HC) has achieved huge success in sodium‐ion batteries (SIBs) with a plateau capacity in the low‐potential regime extending the working voltage of full cells, similar to graphite in lithium‐ion batteries (LIBs). However, this unique electrochemical signature is rarely observed in the application of HC in LIBs, due to the inherent differences in the HC microstructure and Li⁺ storage mechanism at the low‐potential regime. Herein, a novolak resin precursor with controllable cross–linking density (CLD) is used to fabricate HCs. By varying the catalysts, the binding position among oligomers shifts from random distributions to mostly ortho sites. Contemporary material analyses reveal that low‐CLD precursors tend to form pseudo‐graphitic layers at early stage, generating abundant closed pores under suitable carbonization condition. A well correlation between Li─ion plateau capacity and closed pore volume along with in situ XRD and Raman analyses confirms that low‐potential plateau results from Li+ filling into closed pores in these novolak‐derived HCs. Consequently, the HC with optimal synthesis conditions achieves a reversible capacity of 550 mAh g⁻1, including ≈ 50% plateau capacity (255 mAh g⁻1). This work provides comprehensive understanding in closed pore engineering and the origin of low‐potential plateau, offering a promising route for microstructural engineering toward high‐performance LIBs.