Heat storage materials preserving large amounts of thermal energy over long time frames enable the storage of waste energy from thermal energy generation or excess energy generated by intermittent and renewable sources. Trititanium pentoxide, Ti 3 O 5 , has emerged as a promising material with a high energy storage density and the capability of releasing the stored energy on demand, for example, by the application of pressure. During the release, the higher energy λ polymorph undergoes an isostructural phase transition to the lower energy β polymorph. Accurate predictions of the heat storage properties of the two polymorphs using first-principles methods are not yet available. Using hybrid density functionals and the recently proposed r 2 SCAN metageneralized gradient approximation functional augmented by the D3 dispersion correction, we offer a comprehensive description of the electronic ground state of this material. We find that r 2 SCAN-D3 provides a similar accuracy for structural and thermodynamic properties compared to hybrid functionals at an order of magnitude reduced computational cost. Inclusion of the dispersion interaction is essential to obtain correct energy differences between the two phases. The r 2 SCAN-D3 method is applied to the investigation of the reaction mechanism of the phase transition, resulting in full characterization of the transition state. We highlight the good performance of the r 2 SCAN-D3 functional by predicting the phase transition temperature and phase transition enthalpy of the heat storage system and recommend this method for high-throughput investigations of heat storage materials.

It is known that substitution of a small fraction of Ti atoms in Ti3O5 by aliovalent metal atoms alters its heat-storage properties, i.e., β → λ phase-transition enthalpy and temperature, and extends its range of application to, for example, the capture and utilization of waste heat. Heat-storage properties vary depending on the substituting element and its concentration. The exploration of the vast space of possible combinations of these quantities is challenging due to the small energy differences under consideration. Thus, computational approaches which reliably predict heat-storage properties of such defective systems can aid in the screening of potential material candidates for subsequent synthesis. Substituted compounds M x Ti3–x O5 with trivalent M = Sc, Al, and Mg have been reported in the literature. Here we present the first thorough study of the doping effect on the electronic structure and the heat-storage properties of Ti3O5 from first principles. Electronic ground states were calculated for all Ti–M substitution positions using the M06 hybrid functional. Doping leads to increased metallic character in both phases, which primarily results from a change in atomic positions and not from the substituting element itself. We applied the r2SCAN-D3 method to the study of heat-storage properties of those materials and found good agreement in phase-transition enthalpies with experimentally recorded data. Calculated phase-transition entropies show larger deviations from experiment. The phase-transition mechanism is studied as a function of the defect concentration by calculating the minimum-energy path. Doping primarily changes the relative energy of both phases and leaves the activation barrier virtually unchanged. Our results suggest that heat-storage systems of this kind are efficient only for M concentrations below 4 atom %.