Silicon has been recognized as one of the most appealing alloying anode materials for lithium/sodium-ion
storage. However, the K-Si alloying reaction is still missing in potassium-ion batteries produced so far, even
though the theoretical phase diagram allows the existence of corresponding products. Herein, we synthesize
crystalline silicene from Zintl phase CaSi2 and demonstrate that potassium-ions can be inserted/extracted efficiently
in it with enhanced cycle stability (without capacity fading over 3000 cycles in silicene//K battery with
Coulombic efficiency remains above 99.4 %). Contrary to established perceptions of “inert silicon”, the reversible
kinetics-controlled K-Si phase transition occurring in silicene is illustrated by in situ synchrotron X-ray diffraction,
and enables the formation of KSi as the dominant discharged product realizing a reliable reversible
potassiation storage capacity of 180.1 mA h g􀀀 1. The versatile alloying electrochemistry of silicon observed here
is expected to spur the development of durable potassium-ion batteries.