Sub-micron-resolution temperature mapping of Zn negative electrode for flow batteries

Zinc-based flow batteries are gaining attention as safe, cost-effective, and sustainable energy storage solutions amid global energy transition challenges. However, their practical application is hindered by poor reversibility and dendrite formation of Zn negative electrode, particularly under high state-of-charge conditions. Despite extensive research on Zn side, the relationship between localized temperature distribution and dendrites remains underexplored, primarily due to limited microscopic observation techniques. Here, we present a non-invasive optically detected magnetic resonance with nanodiamond quantum sensors to monitor temperature variations during Zn deposition, achieving a sub-micron spatial resolution (~ 300 nm) and a temperature sensitivity of ~2 K/Hz^0.5. Our findings suggest that spatial temperature non-uniformity may play a critical role in accelerating dendrite growth and potentially leading to more severe short circuits. Simulations revealed that higher substrate thermal conductivity improves Zn deposition uniformity. Herein, we introduced a flowable gallium-indium liquid metal electrode, which disperses localized heat and lowers interfacial temperature gradients, thereby suppressing hotspot-driven dendrite growth and enabling in situ formation of a liquid Zn alloy. The zinc-bromine flow battery with the liquid metal electrode demonstrated enhanced cycling stability over 2400 hours at a high state-of-charge of 90%, achieving a cumulative discharge capacity of 46.2 Ah cm⁻² at 40 mA cm⁻².