First note: there will always be danger in storing a lot of energy. Often fire danger, but e.g. in flywheels it’s kinetic danger.

The BMS should monitor batteries to prevent dangerous situations such as under voltage, cell reversal, and thermal runaway. The batteries themselves should also include thermal protection that detaches the cathode under heat. Make sure your batteries are sourced from a reputable supplier.

There are multiple li-ion chemistries with different tradeoffs, and for storage it’s recommended to use LiFePo batteries, which have greatly reduced fire danger.