Quantifying Phase Transformations in Alloying Anodes via in-situ Liquid Cell Hard X-ray Spectroscopy and Cryogenic Microscopy

Understanding electrochemical phenomena at complex liquid-solid interfaces requires linking real-time structural dynamics with atomic-scale interfacial chemistry. Here, we integrate synchrotron X-ray fluorescence and diffraction with high-resolution cryogenic electron and ion multimodal microscopy to provide a mechanistic understanding of Pt-based alloying anodes across length scales. We directly observe the initial lithiation-driven formation of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:msub> <a:mi>Li</a:mi> <a:mn>2</a:mn> </a:msub> <a:mi>Pt</a:mi> </a:math> and its evolution to a stable LiPt intermetallic phase during extended cycling via a solid-solution type reaction mechanism. Simultaneously, the solid-electrolyte interphase transitions from an unstable carbonate-rich to a stable LiF-dominated composition, confirmed by cryogenic scanning transmission electron microscopy-electron energy loss spectroscopy. Crucially, cryogenic atom probe tomography reveals spatially distinct compositional regimes within the alloy anode: a lithium-flux-limited, heterogeneous interfacial zone and a diffusion-controlled, homogeneous LiPt alloy bulk. This nanoscale compositional gradient rationalizes the emergent solid-solution reaction mechanism and highlights how kinetic limitations and interface dynamics govern alloy formation and electrochemical stability. Our findings establish a correlative experimental framework that directly links structural dynamics with preserved near-atomic resolution interfacial chemistry, advancing the rational design of durable alloy electrodes for next-generation energy storage.