Colossal reversible barocaloric effects in a plastic crystal mediated by lattice vibrations and ion diffusion

TL;DR

This study demonstrates colossal, reversible barocaloric effects in LiCB$_{11}$H$_{12}$ near its phase transition, driven by lattice vibrations and ion diffusion, offering promising solid-state cooling solutions with large temperature and entropy changes.

Contribution

The paper reports the discovery of colossal and reversible barocaloric effects in a solid electrolyte, highlighting the roles of lattice vibrations and ion diffusion, which is a novel insight in the field.

Findings

Achieved reversible entropy change of 280 JK$^{-1}$kg$^{-1}$ with 0.23 GPa pressure shift.

Observed reversible temperature change of 32 K under similar pressure conditions.

Demonstrated large reversible barocaloric strength of approximately 2 JK$^{-1}$kg$^{-1}$MPa$^{-1}$.

Abstract

Solid-state methods for cooling and heating promise a more sustainable alternative to current compression cycles of greenhouse gases and inefficient fuel-burning heaters. Barocaloric effects (BCE) driven by hydrostatic pressure ($p$) are especially encouraging in terms of large adiabatic temperature changes ($|\Delta T| \sim 10$ K) and colossal isothermal entropy changes ($|\Delta S| \sim 100$ JK$^{-1}$kg$^{-1}$). However, BCE typically require large pressure shifts due to irreversibility issues, and sizeable $|\Delta T|$ and $|\Delta S|$ seldom are realized in a same material. Here, we demonstrate the existence of colossal and reversible BCE in LiCB$_{11}$H$_{12}$, a well-known solid electrolyte, near its order-disorder phase transition at $\approx 380$ K. Specifically, for $\Delta p \approx 0.23$ $(0.10)$ GPa we measured $|\Delta S_{\rm rev}| = 280$ $(200)$ JK$^{-1}$kg$^{-1}$ and $|\Delta T_{\rm rev}| = 32$ $(10)$ K, which individually rival with state-of-the-art barocaloric shifts obtained under similar pressure conditions. Furthermore, over a wide temperature range, pressure shifts of the order of $0.1$ GPa yield huge reversible barocaloric strengths of $\approx 2$ JK$^{-1}$kg$^{-1}$MPa$^{-1}$. Molecular dynamics simulations were carried out to quantify the role of lattice vibrations, molecular reorientations and ion diffusion on the disclosed colossal BCE. Interestingly, lattice vibrations were found to contribute the most to $|\Delta S|$ while the diffusion of lithium ions, despite adding up only slightly to the accompanying entropy change, was crucial in enabling the molecular order-disorder phase transition. Our work expands the knowledge on plastic crystals and should motivate the investigation of BCE in a variety of solid electrolytes displaying ion diffusion and concomitant molecular orientational disorder.