Negative-Thermal-Expansion Strategy Enables Hard Carbon With Tuned Heat/Strain Tolerances for Sodium Storage

The thermal/stress runaways pose a risk to the safety and industrialization of sodium-ion battery (SIB), which hence requires external management equipment. Actually, the thermal/stress generation originates from the repetitive Na⁺ intercalation within carbon interlayers and pore-filling in the initial stage. Herein, we propose a regulation strategy of "in situ thermal strain relief" to achieve internal thermal and stress managements of SIB hard carbon (HC) anode by incorporating negative thermal expansion oxide ZrW₂O₈. The results indicate that the heat and strain were reduced obviously after cycling, leading to rapid diffusion kinetics and enhanced sodium storage capacity. Moreover, the microstructure and efficient electron/ion transport within HC were preserved, demonstrating excellent capacity stability even after high-rate and long-term cycling over wide operating temperature ranges (-20°C to 60°C). This study provides a new approach for thermal and stress management inside the battery, which may play a joint effort with the conventional management equipment outside the battery and beyond.