Selective Confinement by a COF‐Derived Sub‐Nanoporous Interface for High‐Performance CoF ₂ Thermal Battery Cathodes

The pervasive dissolution of transition metal fluoride (TMF) cathodes presents a fundamental barrier to their application in high-voltage thermal batteries and other Li⁺-conducting systems. Herein, we report a novel selective confinement strategy inspired by ion sieving to overcome this challenge by constructing a sub-nanoporous carbon interface in situ on CoF₂ particles. Derived from a covalent organic framework (COF), this interface features precisely defined 0.54 nm pores that exploit the size difference between Li⁺ ions (∼0.15 nm) and dissolved transition metal fluoride derived complex ions (∼0.8 nm), effectively confining the active material while enabling unimpeded ionic conduction. This tailored design successfully suppresses cathode shuttling effect, enabling a thermal battery that delivers an exceptional discharge plateau >2.5 V, a high specific capacity of 365 mAh g^-1, and a remarkable specific energy of 882 Wh kg^-1 at 100 mA cm^-2. Mechanism studies confirm the dissolved transition metal fluoride derived complex ions as CoCl₄ ^2- and efficient confinement of it. This work provides a general and effective interface engineering strategy for unlocking the full potential of metal fluoride cathodes in advanced energy storage.