Understanding formation of metals and alloys in solution with in situ characterizations
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Phase diagrams are the maps for the design and synthesis of functional materials. However, the bulk phase diagrams oftentimes become not predictive in the synthesis of nanoparticles owing to the significant contribution of surface energy to the total energy. Synchrotron X-ray diffraction (XRD) is a powerful tool to monitor nucleation and crystal growth process of nanostructured materials with time-resolved observations. A series of in situ studies were conducted to understand the phase formation of metals in nanoscale. To achieve a systematical understanding on the polymorphism of nanomaterials, metallic Cobalt (Co) was chosen as a model system, where the two polymorphs, fcc and hcp phases, can be tuned with 100% selectivity. Through both in situ solvothermal and in situ electrodeposition, combining with first-principles Density Functional Theory (DFT), we found out that the bulk metastable phase becomes stable at nanometer scale, mainly resulting from the change of surface energy under various pH conditions. Furthermore, in situ electrodeposition studies show that high over-potential expedites the kinetics of the reaction and yields both the metastable phase and stable phase concurrently under certain pH regions. Based on these works, we first explored the nanometric phase formation of Cobalt (Co)-Nickel (Ni) system through in situ electrodeposition. It was found that the polymorphs formation of Co-Ni is determined by a number of key factors, including the composition (elementary ratio), the pH of electrolyte and the overpotential. These factors together impact the surface energy and the thermodynamics of the nanometric alloy particles, as well as the kinetics of reaction, and thus alternate phase boundaries in the phase diagram. Inspired by the new findings in above fundamental research, we also conducted research closely related to applications. We developed a facile one-step electrodeposition process for copper current collectors with 3-D architected porous nano-structures. The Li anode hosted in the 3-D Cu current collectors demonstrates excellent cycling performance with little dendrites formation for both liquid or solid-state full cells. This extremely simple and scalable process could be easily incorporated into the roll-to-roll manufacturing processes of battery industries.