Tanghong Yi

Asymmetric pathways in the electrochemical conversion reaction of NiO as battery electrode with high storage capacity

Electrochemical conversion reactions of transition metal compounds create opportunities for large energy storage capabilities exceeding modern Li-ion batteries. However, for practical electrodes to be envisaged, a detailed understanding of their mechanisms is needed, especially vis-à-vis the voltage hysteresis observed between reduction and oxidation. Here, we present such insight at scales from local atomic arrangements to whole electrodes. NiO was chosen as a simple model system. The most important finding is that the voltage hysteresis has its origin in the differing chemical pathways during reduction and oxidation. This asymmetry is enabled by the presence of small metallic clusters and, thus, is likely to apply to other transition metal oxide systems. The presence of nanoparticles also influences the electrochemical activity of the electrolyte and its degradation products and can create differences in transport properties within an electrode, resulting in localized reactions around converted domains that lead to compositional inhomogeneities at the microscale.

Phase-Controlled Electrochemical Activity of Epitaxial Mg-Spinel Thin Films.

We report an approach to control the reversible electrochemical activity (i.e., extraction/insertion) of Mg(2+) in a cathode host through the use of phase-pure epitaxially stabilized thin film structures. The epitaxially stabilized MgMn2O4 (MMO) thin films in the distinct tetragonal and cubic phases are shown to exhibit dramatically different properties (in a nonaqueous electrolyte, Mg(TFSI)2 in propylene carbonate): tetragonal MMO shows negligible activity while the cubic MMO (normally found as polymorph at high temperature or high pressure) exhibits reversible Mg(2+) activity with associated changes in film structure and Mn oxidation state. These results demonstrate a novel strategy for identifying the factors that control multivalent cation mobility in next-generation battery materials.

Direct characterization of the Li intercalation mechanism into α-V2O5 nanowires using in-situ transmission electron microscopy

The Li-V2O5 system has been well studied electrochemically, but there is a lack of systematic in-situ studies involving direct investigations of the structural changes that accompany the lithiation process. The open-cell battery setup inside a transmission electron microscope is ideal for studying the reaction pathway of intercalation of Li+ into nanowire cathodes. In this work, we utilize in-situ transmission electron microscopy to study the Li-V2O5 system. More specifically, we employ electron beam diffraction and electron energy-loss spectroscopy (EELS) in an open-cell battery setup to examine the phase changes within α-V2O5 nanowire cathodes upon in-situ lithiation. Our results suggest that the pristine α-V2O5 nanowire forms a Li oxide shell which then acts as a solid state electrolyte to conduct Li+ ions, and the bulk of the V2O5 nanowire undergoes transformation to the γ−Li2V2O5 phase.