A rechargeable Ca/Cl₂ battery

Rechargeable calcium (Ca) metal batteries are promising candidates for sustainable energy storage due to the abundance of Ca in Earth's crust and the advantageous theoretical capacity and voltage of these batteries. However, the development of practical Ca metal batteries has been severely hampered by the current cathode chemistries, which limit the available energy and power densities, as well as their insufficient capacity retention and low-temperature capability. Here, we describe the rechargeable Ca/Cl₂ battery based on a reversible cathode redox reaction between CaCl₂ and Cl₂, which is enabled by the use of lithium difluoro(oxalate)borate as a key electrolyte mediator to facilitate the dissociation and distribution of Cl-based species and Ca²⁺. Our rechargeable Ca/Cl₂ battery can deliver discharge voltages of 3 V and exhibits remarkable specific capacity (1000 mAh g^-1) and rate capability (500 mA g^-1). In addition, the excellent capacity retention (96.5% after 30 days) and low-temperature capability (down to 0 °C) allow us to overcome the long-standing bottleneck of rechargeable Ca metal batteries.