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 nanostructured materials 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 materials. More importantly, it enables time-resolved in situ observations with high resolution. Based on preliminary works that we have done previously, I propose to do a series of studies to understand the phase selectivity of metals in nanoscale, and to use the findings to explore their applications in batteries.
For the preliminary studies, Cobalt (Co) was selected as the model system to investigate the nanoscale phase selectivity through in situ solvothermal and in situ electrodeposition method. It was found that despite the hexagonal close-packed (hcp) phase is the thermodynamically stable phase in bulk, at nanometer scale, face-centered cubic (fcc) Co phase is more favorable under neutral or acidic conditions (pH<8), while hcp Co is only more stable under basic condition (pH >8), mainly resulting from the change of surface energy under various pH conditions. First principles Density Functional Theory (DFT) computations were used to theoretically explain this phenomenon. From the in situ electrodeposition study, it was found that over-potential was another key factor, for that high over-potential expedites the kinetics of the reaction and yields both the metastable and stable phases concurrently under certain pH regions. Furthermore, it was also found that electrodeposition could easily tune the morphology of nanoparticles by using additives or changing the current density of deposition.
Based on the preliminary results, I propose to do the following work: 1. Further exploring the phase selectivity of alloys, we will target at alloys such as Co-Ni with potential novel mechanical or magnetic properties; 2. Further exploring the morphology tuning effect in electrodeposition, with using Copper (Cu) as the model system for that it can be used as the current collector in lithium ion batteries. Recent studies have indicated that Cu nanoparticles instead of Cu foil, can effectively suppress the growth of Li dendrites, which could potentially enable high-energy-density Li-metal batteries.