Converting spare electricity into heat for high-volume, long-term storage is under research at various companies, who claim that costs could be much lower than lithium-ion batteries. Graphite may be used as a storage medium, with molten tin as heat transfer, at temperatures around 2000°. See LaPotin, A., Schulte, K.L., Steiner, M.A. et al. Thermophotovoltaic efficiency of 40%. Nature 604, 287–291 (2022). Thermophotovoltaic efficiency of 40%.