Unraveling the performance enhancement mechanism of Ti doping for Li4SiO4-based solar energy storage: a combined experimental, kinetic, and DFT study†

The huge amount of CO₂ emitted by the combustion of fossil fuels has been recognized as the main culprit of global warming. Solar energy as an alternative energy will be a major path to reduce CO₂ emissions if efficient energy storage technology can be achieved. Lithium orthosilicate (Li₄SiO₄) has been proven to be an efficient solar thermochemical energy storage medium. However, the unsatisfactory energy storage performance has been a critical barrier to technological commercialization. Ti doping has been demonstrated to effectively improve the performance of Li₄SiO₄, but its enhancement mechanism remains unclear. In this work, an efficient and stable Ti-doped Li₄SiO₄ heat carrier was synthesized by the sol mixing method. More importantly, the enhancement mechanism of Ti doping was systematically revealed for the first time through reaction kinetic analysis and DFT calculation. The Ti-doped Li₄SiO₄ heat carrier exhibited high energy storage density (678 kJ kg⁻¹) and cumulative energy storage capacity (17 080 kJ kg⁻¹ over 30 cycles). Kinetic analysis and DFT calculation confirmed that Ti doping was beneficial to reducing the apparent activation energy in the chemical-controlled stage. The essential reason was that the introduced Ti⁴⁺ replaced the Si site in the Li₄SiO₄ lattice, which led to the formation of lattice defects and an improved ion migration rate. This ultimately accelerated the diffusion of reactants to the reaction interface to improve the reaction rate in the chemical process. The systematically unraveled mechanisms of Ti doping will promote the application of Li₄SiO₄-based solar energy storage.