Approaching the voltage and energy density limits of potassium-selenium battery chemistry in a concentrated...

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 K₂Se (x = 5, 3, 2, 1). The presence of redox intermediates K₂Se₅ at ∼2.3 V and K₂Se₃ 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/K₂Se , 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.