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

Zn-based thermal charging devices, utilizing the synergistic effect of ion thermoextraction and thermodiffusion, are able to efficiently convert thermal energy into electrical energy and storage in the devices, making them a highly promising technology for low-grade heat recovery and utilization. However, the low output power density and energy conversion efficiency resulted by the slow diffusion kinetics of Zn²⁺ hinder their development. Herein, we present a high-performance thermal charging cell design using Zn²⁺/NH₄⁺ hybrid ion electrolyte, which not only maintains the high output voltage of the Zn-based thermoelectric system, but also significantly enhances the output power density due to the fast diffusion kinetics of NH₄⁺. Based on this strategy, the thermal charging cell displays a high thermopower of 12.5 mV K^-1 and an excellent normalized power density of 19.6 mW m^-2 K^-2 at a temperature difference of 35 K. The Carnot-relative efficiency is as high as 12.74%. Moreover, it can operate continuously for over 72 h when the temperature difference persists, achieving a balance between thermoelectric conversion and output. This work provides a simple and effective strategy for the design of high-performance thermal charging cells for low-grade heat conversion and utilization.