Permanent underground storage where it will naturally decay. Are a couple of different methods available from what I understand. And the amount of material that actually needs to be stored is a fraction of what is instead released into the air, water & soil from fossil based fuel. Not to mention toxins like mercury etc.
Sand has what’s called a high thermal capacity. This means it takes a long time to heat up, but also a long time to cool down. Metals such as copper have a low thermal capacity relative to sand. You can quickly heat copper but it also cools just as fast.
The chimneys of fireplaces was often surrounded by pockets of sand or similar material. This meant when the fireplace had a fire going. The heat would travel through the chimney and slowly heat up the sand. Once it became night and you extinguished the fire the sand and stone of the fireplace would slowly release the stored heat. Making sure the house was heated even at night.
The relatively low cost of sand as well as it not having a significant reaction to heat (water would boil for example). Means it’s a pretty cost-effective material for storing thermal energy. Assuming the batteries are extremely well insulated the sand might stay warm for that long.
(It’s been a decade since I studied thermal energy though)
Shitty code