Electricity storage is needed on an unprecedented scale to sustain the ongoing transition of electricity generation from fossil fuels to intermittent renewable energy sources like wind and solar power. Today pumped hydro is the only commercially viable large-scale electricity storage technology, but unfortunately it is limited to mountainous regions and therefore difficult to expand. Emerging technologies like adiabatic compressed air energy storage (ACAES) or storage using conventional power-to-gas (P2G) technology combined with underground gas storage can be more widely deployed, but unfortunately for long-term to seasonal periods these technologies are either very expensive or provide a very low round-trip efficiency. Here we describe a novel storage method combining recent advances in reversible solid oxide electrochemical cells with sub-surface storage of CO 2 and CH 4 , thereby enabling large-scale electricity storage with a round-trip efficiency exceeding 70% and an estimated storage cost around 3 b kW À1 h À1 , i.e., comparable to pumped hydro and much better than previously proposed technologies. Broader contextToday about 2/3 of the global energy consumption is based on fossil fuels and only a minor fraction on renewable energy sources. With a growing consensus among countries that it is time to act and decrease greenhouse gas emissions to avoid uncontrollable climate changes, it is clear that the necessary transition towards renewable-based energy infrastructures has just begun. However, as intermittent wind and solar power displace fossil fuels, the need for storage to balance the gap between supply and demand increases. This is in particular the case for the electricity sector, where no widely available, energy efficient and cheap large-scale electricity storage technology exists. The present work analyzes the reversible electrochemical conversion of H 2 O and CO 2 to CH 4 inside novel pressurized solid oxide cells combined with subsurface storage of the produced gasses, showing that it should be possible to store about 3 months of electricity (500 GW h) with a round-trip efficiency greater than 70% and a storage cost around 3 b kW À1 h À1 . With the expected rise in arbitrage due to increasing balancing demands and consequent price fluctuations, the technology should eventually become economically viable. In summary, this disruptive new energy storage technology can facilitate a seamless transition towards a fossil-free future.

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