Atomic Structure of 2 nm Size Metallic Cobalt Prepared by Electrochemical Conversion: An in Situ Pair Distribution Function Study

Conversion reactions, offering high capacity in batteries, imply a restructuration of the pristine atomic structure down to the nanoscale. The associated large broadening in the Bragg peaks of the restructured electrode renders the description of atomic structure, almost impossible using conventional X-ray diffraction. On the other hand, atomic pair distribution function (PDF) is well suited to probe the structure of few nanometer-sized particles. Here, we investigated the formation mechanism and structure of 2 nm particle size of metallic Co obtained by electrochemical conversion of CoF₂. We found that the conversion process stabilized a disordered structure consisting of a hexagonal-close-pack-face-centered-pack intergrowth. The reaction was followed by in situ PDF, providing unique insight into the growth of highly defective Co nanoparticles.