High-performance magnesium-ion thermal charging cell enabled by organic cation pre-intercalation

Abstract

Efficient recovery and conversion of ubiquitous low-grade heat are crucial for mitigating fossil fuel depletion and advancing sustainable energy technologies. Ionic thermoelectric (i-TE) systems, featuring high thermoelectric conversion efficiency and thermovoltage output capability, hold great promise in low-temperature energy harvesting. However, conventional i-TE cells based on monovalent ions (e.g., Li⁺) suffer from limited thermovoltage and power density, restricting their practical applications. Herein, we propose the concept of a magnesium-ion thermal charging cell (MTCC), leveraging the multivalent nature and Li⁺-comparable ionic radius of Mg²⁺. For such a purpose, a major challenge lies in the strong coulombic interactions between Mg²⁺ and host electrodes, which hinder ion diffusion and deteriorate cycling stability. To address this, we employ organic-cation pre-intercalation to tailor the V₂O₅ electrode, forming Pyr⁺–V₂O₅ (PVO) with expanded interlayer spacing, weakened electrostatic interactions, and enhanced lattice stability. This modification enables rapid Mg²⁺ transport, optimized interfacial kinetics, and excellent cycling durability. As a result, the MTCC delivers a record-high thermovoltage of 1.196 V, an ultrahigh Seebeck coefficient of 29.9 mV K⁻¹, and a power density of 6.076 W m⁻². Furthermore, a planar-integrated configuration significantly shortens the electrode spacing, reducing internal resistance from 516.7 Ω to 79.5 Ω, thereby boosting the power density to 15.8 W m⁻² while maintaining a high thermovoltage of 1.09 V at ΔT = 50 K. This study establishes an effective paradigm for multivalent-ion-based thermoelectric systems, offering a promising route for efficient low-grade heat utilization and sustainable energy conversion.