Renewable sources of energy such as wind or photovoltaic are intermittent; production peaks do not necessarily follow the demand peaks. Storing green energy is therefore essential to moving away from fossil fuels. The energy produced by photovoltaic cells and wind power is stored to be used later on when needed.
source https://www.lifetechnology.com/blogs/life-technology-news-blog/a-new-material-for-the-battery-of-the-future