Diffraction anomalous fine structure pinpoints electrochemically active sites in robust heterojunctions

Heterojunctions composed of identical metallic elements have intrinsic lattice-matching, tunable compositions, and low interfacial resistance, positioning them as promising candidates for electrochemical applications. However, constructing robust heterojunctions and correlating site chemistry remain challenging. Here, we present a highly durable Nb₄CuC₃/Nb₄C₃T_x heterostructure that exhibits outstanding cycling stability with virtually no capacity degradation over 10,000 cycles (5.0 amperes per gram) in lithium-ion batteries. Synchrotron radiation core-level spectroscopy and scanning transmission electron microscopy resolved four distinct niobium (Nb) coordination environments within the heterojunction. Critically, by integrating a site-selective x-ray analysis (diffraction anomalous fine structure) with complementary synchrotron techniques, it was pinpointed that the Nb_I sites on the Nb₄C₃T_x surface represent the true electrochemically active centers, while the Nb₄CuC₃ component primarily serves as structural buffer during cycling processes. This work establishes a versatile methodological platform for precisely identifying active sites within complex multisite structures during the working process, thus guiding the rational design of targeted active sites in heterogeneous electrochemical systems for high-performance energy storage.