A High-Performance Thermal Charging Cell with High Power Density and Long Runtime Enabled by Zn2+ and NH4+ Co-insertion

• The hybrid ion system strategically combines the high-voltage characteristics of Zn2+ redox with the exceptionally fast kinetics of NH4+, significantly boosting thermoelectric performance for low-grade heat harvesting. • The Zn2+/NH4+ co-insertion/thermoextraction mechanism is elucidated, where NH4+ exhibits exceptionally fast migration due to its unique hydrogen bonding diffusion behavior. • The device achieves a record 19.6 mW m⁻2 K⁻2 normalized power density with 72 h continuous operation, demonstrating strong application potential. • The thermal charging cell achieves a high thermopower of 12.5 mV K−1 and a Carnot-relative efficiency of 12.74%, offering a simple and effective strategy for low-grade heat conversion and storage.