Advances in the chemistry and applications of alkali-metal–gas batteries

Rechargeable metal–gas batteries have the promise of exceeding the energy densities of Li-ion batteries. An archetypal metal–gas system is the nonaqueous lithium–oxygen (Li–O2) battery, which was developed with a view to deploying it in electric vehicles. However, operating this battery comes with substantial challenges that include parasitic chemical reactivity and degrees of electrochemical irreversibility, which contribute to poor charging and cycling. To address these challenges, researchers began exploring new nonaqueous metal–gas battery paradigms by manipulating the underlying O2 redox behaviour through electrolyte and materials design, using non-Li-metal anodes to change the nature of the solid discharge phase and improve reversibility, and using other gaseous reactants as the cathode. This Review presents the new understanding of nonaqueous gas-to-solid electrochemistry that has emerged from these concerted efforts, along with new hurdles that have been revealed as cells have gradually been reformulated. The ultimate impact of new metal–gas batteries needs to be re-examined for applications beyond electric vehicles that are more amenable to the individual chemistries and performance characteristics. Demand for energy-dense electrochemical storage systems has drawn increasing focus to metal–gas batteries. This Review describes the chemistry of these batteries and the underlying trade-offs between energy density and electrochemical reversibility.