Current Challenges in Materials for Thermal Energy Storage
*** EXPLANATORY NOTES FOR THIS RE-SUBMISSION ***
This proposal was submitted to the 2021 call and subsequently approved by CECAM. The meeting was initially scheduled for September 2021. However, with the uncertainty around the COVID pandemia, we had to consider whether to go ahead with the conference online or reschedule it for 2022 to have a face-to-face meeting. Upon discussing this possibility with the CECAM (Prof. Pagonabarraga) and the CECAM-ES Directors (Prof. Velazquez-Campoy), we have decided to reschedule the workshop to take place in June 2022. All speakers have already agreed to this change of our original plans.
Prof. Velazquez-Campoy has advised us to resubmit the proposal to ensure that the funds can be used in 2022. The project is essentially the same one we submitted last year, but we have modified the session descriptions to reflect the face-to-face component (all changes have been highlighted in bold yellow). Also, we note that the meeting has attracted ten additional participants who have already registered to attend the meeting, which underlines the timeliness of the workshop.
*** PROPOSAL DESCRIPTION ATTACHED BELOW ***
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 [5,6]; 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 . Ab-initio and classical methods provide a powerful toolbox to quantify thermal transport [8-12], 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 [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 , or ternary GeSbTe alloys ). Notwithstanding, recent works provide specific predictions  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 , 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