Locally Curved Surface with CoN₄ Sites Enables Hard Carbon with Superior Sodium-Ion Storage Performances at −40 °C

The impressive electrochemical performance of sodium-ion batteries at low temperatures has long been recognized as a promising technical advantage. However, the inadequate transport kinetics of Na⁺ ions and complex interfacial reactions at the hard carbon anode surface hinder the practical implementation of commercial sodium-ion batteries. Herein, a novel approach to address this issue by introducing a homogenized functional carbon coating layer with a locally curved configuration is proposed. This coating layer is designed to accommodate single CoN₄ sites on the surface of commercial hard carbon particles, resulting in enhanced sodium storage performance at low temperatures. The surface-modified hard carbon anode material (HC-Z1) demonstrates a commendable rate performance of 220.6 mAh g⁻¹ at 3 A g⁻¹@25 °C and a substantial reversible capacity of 288.7 mAh g⁻¹ with an 89% capacity retention at 0.06 A g⁻¹@-20 °C. Furthermore, even at a temperature as low as −40 °C, the reversible capacity remains at 270 mAh g⁻¹ at 0.06 A g⁻¹. Extensive characterizations and theoretical calculations provide evidence that the optimized interface between the electrode and electrolyte effectively enhances the desolvation and migration of Na⁺ ions, particularly at low temperatures.