Mechanocaloric effects in superionic thin films from atomistic simulations

Abstract

Solid-state cooling is an energy-efficient and scalable refrigeration technology that exploits the adiabatic variation of a crystalline order parameter under an external field (electric, magnetic, or mechanic). The mechanocaloric effect bears one of the greatest cooling potentials in terms of energy efficiency owing to its large available latent heat. Here we show that giant mechanocaloric effects occur in thin films of well-known families of fast-ion conductors, namely Li-rich (Li₃OCl) and type-I (AgI), an abundant class of materials that routinely are employed in electrochemistry cells. Our simulations reveal that at room temperature AgI undergoes an adiabatic temperature shift of 38 K under a biaxial stress of 1 GPa. Likewise, Li₃OCl displays a cooling capacity of 9 K under similar mechanical conditions although at a considerably higher temperature. We also show that ionic vacancies have a detrimental effect on the cooling performance of superionic thin films. Our findings should motivate experimental mechanocaloric searches in a wide variety of already known superionic materials.Mechanocaloric effects are a promising path towards solid-state cooling. Here the authors perform atomistic simulations on the well-known fast-ion conductor silver iodide and computationally predict a sizeable mechanocaloric effect under biaxial strain.

Fig. 1

Effects of vacancies on the ionic conductivity of type-II thin films. a F⁻ diffusion coefficient in perfect and defective (c _v = 2.5%) calcium fluoride expressed as a function of temperature. The creation of a Frenkel pair defect, the fundamental atomistic mechanism for superionic transport in type-II FIC, is sketched. Red and green arrows indicate the critical superionic transition in the perfect and defective systems. b Critical superionic temperature expressed as a function of biaxial stress in perfect and defective CaF₂. The lines are guides to the eye and the error bars in b correspond to the resolution of our calculations

Fig. 2

Effects of vacancies on the mechanocaloric performance of type-II superionic thin films. Isothermal entropy a and adiabatic temperature b shifts calculated in defective CaF₂ (c _v = 2.5%) as a function of temperature and applied biaxial tensile stress. N and S denote normal and superionic states and the magenta dashed line depicts the boundary between them. c Comparison of the adiabatic temperature shifts calculated in perfect and defective CaF₂ thin films as a function of biaxial stress at a fixed temperature of 1100 K. Lines in c are guides to the eye

Fig. 3

Mechanocaloric effect in the Li₃OCl superionic conductor with a point-defect concentration of 2.5%. The Li⁺ diffusion coefficient a, in-plane strain b, isothermal entropy shift c, and adiabatic temperature change d estimated at T = 1000 K and expressed as a function of biaxial (tensile) stress. Lines in a, b are guides to the eye, and the error bars in b correspond to the standard deviation from 8,000 configurations generated during the simulations

Fig. 4

Mechanocaloric effect in the AgI superionic conductor without vacancies at T = 300 K. The Ag⁺ diffusion coefficient a, in-plane strain b, isothermal entropy shift c, and adiabatic temperature change d estimated at room temperature and expressed as a function of biaxial (compressive) stress. Lines in a, b are guides to the

Fig. 5

Mechanocaloric effect in the AgI superionic conductor without vacancies at T = 400 K. The Ag⁺ diffusion coefficient a, in-plane strain b, isothermal entropy shift c, and adiabatic temperature change d estimated at high temperature and expressed as a function of biaxial (compressive) stress. Lines in a, b are guides to the eye

Fig. 6

Ionic radial pair distribution functions in AgI thin films with the zincblende (γ) structure at T = 300 K. Results are expressed as a function of ionic pairs and biaxial compressive stress. a Ag-I and σ _xx = σ _yy = +1 GPa; b I-I and σ _xx = σ _yy = +1 GPa; c Ag-I and σ _xx = σ _yy = 0 GPa; d I-I and σ _xx = σ _yy = 0 GPa