Potassium (K)-based batteries hold great promise for cryogenic applications owing to the small Stokes radius and weak Lewis acidity of K+. Nevertheless, energy-dense (>200 W h kg-1cathode+anode) K batteries under subzero conditions have seldom been reported. Here, an over 400 W h kg-1cathode+anode K battery is realized at -40 °C via an anode-free and dual-ion strategy, surpassing these state-of-the-art K batteries and even most Li/Na batteries at low temperatures (LTs). By introduction of a strongly associating salt as an additive to this anode-free K battery, an anion-derived solid electrolyte interphase can be established for a highly reversible, zero-excess K plating/stripping behavior on a bare current collector. Meanwhile, a binary solvent is rationally designed for lowering the cation desolvation energy barrier, which ensures comparably facile cation and desolvation-free anion kinetics in this dual-ion structure even at LTs. Consequently, the K||Al half-cell delivers a high Coulombic efficiency of 99.98% at -40 °C. By pairing with a high-energy cathode, a proof-of-concept anode-free K dual-ion battery (N/P = 0) is fabricated, delivering a record-high energy density of 407 W h kg-1cathode+anode with stable cycling of 183 cycles (80% capacity retention) at -40 °C. This work paves a way toward energy-dense batteries at extreme scenarios.