Current Challenges in Materials for Thermal Energy Storage
Human activities produce and require vast amounts of energy, with a significant fraction dispersed into heat. Being able to store this energy for further use would have enormous and far-reaching economic and societal impacts. This objective drives current efforts aimed at designing materials for energy storage, efficienly and reliably.
Thermal energy-storage materials (TESMs) [1,2] store/release heat via ´sensible heat’ or via the latent heat of a phase transition. The second approach ensures that the absorption/release of energy happens at well-defined temperatures. Current TESMs rely primarily on empirical knowledge, targetting a few observables - melting enthalpy/temperature or Thermal Conductivity (TC), albeit with limited physical insight. The TC influences the kinetics of heat transfer, and is a common shortcoming given its low value in TESMs. Interfacial heat transport (Kapitza resistance) imposes additional dynamical constraints .
Computer simulations can complement experimental approaches to design TESMs, by acquiring a sound understanding of the microscopic behaviour underpinning their properties. Classical and ab-initio simulations are used to study the composition  and crystallisation of materials [5,7]; they provide insight into barrierless growth kinetics, which challenges classical nucleation theory. Simulations of phase-change materials have revealed new kinetic scales, not accounted for by classical heat transfer theory . Ab-initio and classical methods provide a powerful toolbox to quantify thermal transport [9-12,15], and identify interfacial heat-transport mechanisms  and phase coexistence . However, most simulations have not focused on TESMs, making it difficult to benchmark simulations against experiments, a missed opportunity to accelerate materials discovery.
In addition, there are yet-to-be-tapped synergies in the area of TESMs, which can be realized by bringing together simulations and radiation-scattering techniques (neutrons and X-rays). The use of these experimental quantum probes has evolved significantly [16-18]. New facilities continue to emerge in the European landscape (European Spallation Source in Sweden; or X-ray free electron lasers in Germany, Italy, or Switzerland). The use of these experimental techniques to study TESMs has been quite sparse and limited to a few materials (GeTe , or ternary GeSbTe alloys ). The experimental information often goes beyond traditional measurements of thermophysical properties or time-averaged structural parameters. The dynamic structure factor and the vibrational density of states are amenable to study using both neutrons and X-rays, and also dictate thermal-transport characteristics. Moreover, they provide a fruitful and robust framework to link with simulations. This is a key ingredient for the rational design of TESMs, or to identify new classes of materials and paradigms: molecular media [22,23], barocalorics , or graphene-based nanofluids . Combined simulation and experimental studies can help to improve simulations, which might suffer from poor sampling or inaccurate models. Neural-network potentials offer promise in this direction as well .
There is strong potential for experimental and computational interactions in this area of energy research, since a range of techniques have not been fully deployed to study TESMs. A coordinated approach will accelerate the development of new methods and, hence, facilitate new discoveries.
Felix Fernandez-Alonso (Materials Physics Center) - Organiser
Fernando Bresme (Imperial College London) - Organiser