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

## TL;DR

This paper introduces a thermal charging cell that uses Zn2+ and NH4+ ions to efficiently convert low-grade heat into electricity with high power density and long runtime.

## Contribution

The novel contribution is the Zn2+/NH4+ hybrid ion system that combines high voltage and fast kinetics for improved thermoelectric performance.

## Key findings

- The device achieves a record 19.6 mW m⁻2 K⁻2 normalized power density.
- It operates continuously for over 72 hours with a 35 K temperature difference.
- The Carnot-relative efficiency reaches 12.74%.

## Abstract

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 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 online version contains supplementary material available at 10.1007/s40820-025-02011-9.

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.

The online version contains supplementary material available at 10.1007/s40820-025-02011-9.