Dual-effect pre-potassiation unlocks stable and high-energy potassium-ion batteries

Potassium-ion batteries (PIBs) hold great promise as low-cost and sustainable alternatives to lithium-ion batteries, yet their practical deployment is hindered by rapid capacity decay driven by irreversible potassium loss and an unstable solid electrolyte interphase (SEI). Here, we present a dual-effect pre-potassiation strategy that addresses both issues simultaneously. First, it is shown that the Prussian blue analogue K₂Mn[Fe(CN)₆] can be overpotassiated to K_xMn[Fe(CN)₆] (2 < x < 4) by inserting excess K⁺ into its large interstitial sites, accompanied by K⁺ off-centering and Mn(ii) → Mn(i) reduction, while preserving structural integrity. Subsequently, an unconventional overdischarge strategy is proposed for the K₂Mn[Fe(CN)₆]–graphite PIBs, which enables in situ formation of K_xMn[Fe(CN)₆] to compensate the SEI-related potassium losses while inducing controlled electrolyte oxidation on graphite to produce a robust SEI with high stability. This dual mechanism significantly extends the cycling lifespan of the K₂Mn[Fe(CN)₆]–graphite cells from less than 100 cycles to more than 2000 cycles with high specific energy. This work pioneers a scalable and effective pre-potassiation approach to redefine the energy storage mechanism of Prussian blue analogues, advancing PIBs toward lithium-ion-level performance and offering a generalizable route for other alkali-ion batteries.