In‐Situ Constructed Core‐Shell Catalyst Enabling Subzero Capacity Unlocking of Cost‐Effective and Long‐Life Vanadium Flow Batteries

Vanadium flow battery (VFB) promises a route for achieving grid‐scale power storage by harnessing renewable energy sources. However, the sluggish reaction kinetics of vanadium redox couples and serious hydrogen evolution reaction (HER) still restrict the further development of VFB. Addressing these challenges requires not only effective solutions but also ones that are cost‐efficient and scalable to meet the demands of affordable energy storage. Here, we present an in situ constructed Cu@Cu 6 Sn 5 core‐shell catalyst by incorporating metal ions into the electrolyte. The Cu core, encapsulated by the Cu 6 Sn 5 shell, forms an excellent conductive pathway to the graphite felt electrode. Charge transfer between Cu and Sn within Cu 6 Sn 5 shell accelerates the reaction kinetics of V 2+ /V 3+ redox couple and selectively inhibits HER, as confirmed through in situ weak measurement imaging method. The Cu@Cu 6 Sn 5 battery achieves a peak power density of 1119.1 mW cm −2 at 1350 mA cm −2 , operates stably for 1200 cycles without catalyst failure, and is available over a wide‐temperature range. Furthermore, we identify a demand of subzero capacity unlocking. Achieving a 23.4 % theoretical capacity unlocking at −10 °C with a cut‐off voltage up to 1.75 V, bespeaking a crucial breakthrough toward cost‐effective VFB.