Designing calcium-looping composites based on lattice energy and phase combination/separation for thermochemical energy storage

Abstract

The cyclic stability enhancement of the CaO/CaCO 3 thermochemical energy storage system is crucial for high-temperature thermal utilization of solar energy. In this study, we successfully enhance the long-term durability of calcium-looping composites by co-doping TiO 2 and MgCl 2 based on lattice energy and phase combination/separation mechanism. According to molecular dynamics simulations of metallic oxides, TiO 2 shows higher thermal stability and better doping effects in calcium-looping materials. Further experiments indicate that the phase combination mechanism acting on high-valance metallic oxides like TiO 2 can promote cyclic stability of calcium-looping materials. The calcium-based composite doped with 10 mol% TiO 2 prepared by wet-mixing method exhibits high energy storage performance in 15 cycles. MgCl 2 is also used as a dopant to improve cyclic stability of the calcium-based composite in long-term cycles further. The energy storage performance of the composite doped with 5 mol% TiO 2 and 5 mol% MgCl 2 is superior to single-doped composites. The average effective conversion ratio is 0.54 in 100 cycles and the energy storage densities are close to 1000 kJ/kg in most of the calcination/carbonation cycles. Microstructure characterizations indicate that the co-doped composite has abundant micropores and smaller particles with the more uniform size distribution. The modification mechanism at atomic scale will serve as guidelines for dopant selection preliminarily, and the novel TiO 2 /MgCl 2 co-doped calcium-based composite is promising for long-term industrial applications, such as high-temperature thermal utilization of solar energy. • TiO 2 /MgCl 2 co-doped calcium-looping composites were explored for high-temperature thermochemical energy storage. • Doping metallic oxides with higher absolute values of lattice energy show higher cyclic stabilities. • Co-doping TiO 2 and MgCl 2 increases the average energy storage density from 480.6 to 961.2 kJ/kg in 100 cycles. • Calcium-based precursors, types and ratios of dopants affect energy storage performances.