Metal- and covalent-organic frameworks as solid-state electrolytes for metal-ion batteries

Society’s long-standing energy demands have fuelled for centuries the quest for power-dense, portable and economically viable energy carriers. Since the birth of the first rechargeable battery in 1860 [1], emerging battery technologies have provided both answers to these demands as well as additional obstacles. One ubiquitous energy storage device, the metal or metalion battery, offers quintessential examples of both. The strongly reducing nature of Group 1 and 2 metal ions qualifies these elements as viable energy-dense anode materials: standard reduction potentials several volts below that of the standard hydrogen electrode (SHE) allow a thermodynamically favourable oxidation of these metals to readily release electrons that shuttle through an external circuit, generating the electric current that serves as the power supply during battery discharge. Integration of energy-dense materials into devices allows power sources to be compact and portable, by maximizing energy output per unit mass of material. Further, the reversibility of these oxidation events makes possible extensive battery cycling, thus providing a rechargeable power source. Indeed, current Li-ion batteries boast an energy density of 265 Wh kg−1, with the potential of a 20% improvement, and are operable for over 1000 charge–discharge cycles [2]. Although the chemical properties of metal-ion batteries offer impressive performance and exciting possibilities, harnessing the power of such reactive workhorses in a controlled manner comes with its own challenges. In Li-ion batteries, formation of Li dendrites during charging can puncture the battery