Potassium
(K) metal batteries have attracted great attention owing
to their low price, widespread distribution, and comparable energy
density. However, the arbitrary dendrite growth and side reactions
of K metal are attributed to high environmental sensitivity, which
is the Achilles’ heel of its commercial development. Interface
engineering between the current collector and K metal can tailor the
surface properties for K-ion flux accommodation, dendrite growth inhibition,
parasitic reaction suppression, etc. We have designed bifunctional
layers via prepassivation, which can be recognized as an O/F-rich
Sn–K alloy and a preformed solid-electrolyte interphase (SEI)
layer. This Sn–K alloy with high substrate-related binding
energy and Fermi level demonstrates strong potassiophilicity to homogeneously
guide K metal deposition. Simultaneously, the preformed SEI layer
can effectually eliminate side reactions initially, which is beneficial
for the spatially and temporally KF-rich SEI layer on K metal. K metal
deposition and protection can be implemented by the bifunctional layers,
delivering great performance with a low nucleation overpotential of
0.066 V, a high average Coulombic efficiency of 99.1%, and durable
stability of more than 900 h (1 mA cm–2, 1 mAh cm–2). Furthermore, the high-voltage platform, energy,
and power densities of K metal batteries can be realized with a conventional
Prussian blue analogue cathode. This work provides a paradigm to passivate
fragile interfaces for alkali metal anodes.