Abstract: Fiber-shaped batteries that feature outstanding flexibility, light weight, and wovenability are extremely attractive for powering smart wearable electronic textiles, which further stimulates their demand in extreme environments. However, there are rare reports on ultralow-temperature fiber batteries to date. This is mainly attributed to the poor conductivity of electrodes and freezing of electrolytes that restrain their satisfactory flexible operation in cold environments. Herein, we propose a fiber cooper metal battery consisting of a conductive polyaniline cathode, an anti-freezing Cu(BF₄)₂ + H₃PO₄ electrolyte and an acid-resistant copper wire anode, which can withstand various deformations at ultralow temperatures. Impressively, enhanced capacity and cyclic stability can be achieved by cryoactivated abundant reactive sites in the polyaniline, while benefiting from redox reactions with rapid kinetics involving protons rather than copper ions. Consequently, this well-designed polyaniline/Cu fiber battery delivers excellent flexibility without obvious capacity decay after being bent at -30 °C, as well as a remarkable discharge capacity of 120.1 mA h g^-1 and a capacity retention of 96.8% after 2000 cycles at -50 °C. The fiber bat-teries integrated into wearable textiles can power various electronic devices. These performances greatly outperform those of most reported works. Overall, this work provides a promising strategy toward appli-cations of cryogenic wearable energy storage devices.

Key words: Cryoactivated sites, Proton-involved redox reactions, Fiber cooper metal battery, Polyaniline, Cyclic stability, Low temperature