Approaching the voltage and energy density limits of potassium–selenium battery chemistry in a concentrated ether-based electrolyte

Potassium–selenium (K–Se) batteries offer fairly high theoretical voltage (∼1.88 V) and energy density (∼1275 W h kg(Se)(−1)). However, in practice, their operation voltage is so far limited to ∼1.4 V, resulting in insufficient energy utilization and mechanistic understanding. Here, it is demonstrated for the first time that K–Se batteries operating in concentrated ether-based electrolytes follow distinctive reaction pathways involving reversible stepwise conversion reactions from Se to K(2)Se(x) (x = 5, 3, 2, 1). The presence of redox intermediates K(2)Se(5) at ∼2.3 V and K(2)Se(3) at ∼2.1 V, in contrast with previous reports, enables record-high average discharge plateau voltage (1.85 V) and energy density (998 W h kg(Se)(−1) or 502 W h kg(K2Se)(−1)), both approaching the theoretical limits and surpassing those of previously reported Na/K/Al–Se batteries. Moreover, experimental analysis and first-principles calculations reveal that the effective suppression of detrimental polyselenide dissolution/shuttling in concentrated electrolytes, together with high electron conductibility of Se/K(2)Se(x), enables fast reaction kinetics, efficient utilization of Se, and long-term cyclability of up to 350 cycles, which are impracticable in either K–S counterparts or K–Se batteries with low/moderate-concentration electrolytes. This work may pave the way for mechanistic understanding and full energy utilization of K–Se battery chemistry.