Axially coordinated single-atom interface mitigates isolated K toward highly reversible anode-free K metal batteries

Potassium (K) metal anodes suffer from uncontrolled solid electrolyte interphase evolution and isolated K accumulation, greatly hindering the construction of practical anode-free batteries. To date, systematic investigations on K stripping behavior and isolated K formation, despite being fundamentally important, are still lacking. Here, we develop an axially coordinated single-atom iron (Fe) anchored on hollow carbon bowls to synergize promoted K desorption and stress-adaptive ion transport. Serving as current collector modification, the FeN₄O₂ moiety optimizes K adsorption/desorption strength, regulates FSI^- decomposition, and suppresses electronically isolated K. Meanwhile, the mechanically compliant carbon-bowl scaffold mitigates volumetric strain during cycling, preserving interfacial integrity and accelerating desorption at the stripping frontier. Multimodal evidence from cryo-transmission electron microscopy, x-ray photoelectron spectroscopy depth profile, and theoretical calculations collectively reveals a bidirectional regulation to enhance both deposition uniformity and stripping reversibility. The anode-free K metal full cell delivers nearly 100 milliampere-hours per gram over 200 cycles at 200 milliamperes per gram, readily rivaling the state-of-the-art counterparts.