A “sand battery” is a heat storage that warms sand or sand-like materials to a high temperature to store energy.
To call it a “battery” might mislead some people into assuming it stores electricity. Nobody would call a hot water tank a “water battery”. A better name might be “sand heat store”. So far as we know, the heat has not yet been used to generate electricity, but only to heat a swimming pool in Finland.
Sand is a good insulator so it can store heat at 500C for months. Sand is also abundant so its use as a heat store could easily be extended.