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 [3].

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 [4] and crystallisation of materials [5,6]; 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 [7]. Ab-initio and classical methods provide a powerful toolbox to quantify thermal transport [8-12], and identify interfacial heat-transport mechanisms [13] and phase coexistence [14]. 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  [15-17]. 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 [18], or ternary GeSbTe alloys [19]). Notwithstanding, recent works provide specific predictions [20] to test against experiments. 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 [21,22], barocalorics [23], or graphene-based nanofluids [24]. 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 [25].

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.