Raman spectroscopic investigation on electrochemical energy storage for pseudocapacitors and Li-ion batteries
MetadataShow full item record
Electrochemical energy storage devices, such as batteries and pseudocapacitors, are the most promising power supply for many emerging technologies, from portable electronics to electrical vehicles and smart grids. While incremental progress in performance of these devices has been made in recent years, dramatic advancement is hindered by the lack of a profound understanding of the atomic level energy storage mechanism of Li-ion batteries and pseudocapacitors. This dissertation carries out a series of fundamental mechanism studies for a few important electrode materials of Li-ion batteries and supercapacitors using in operando Raman spectroscopy. This study revealed the detailed structural changes of electrode materials during energy storage from the evolution of vibrational structures as a function of electrochemical operations. To better explain this rationale, this dissertation discusses briefly the fundamental concepts and principles, including electrochemistry, Raman spectroscopy, and in operando configurations as well as basic experimental setups, prior to the chapters of detailed research results. The first material studied in this dissertation is layered manganese oxide (MnO2), the most characteristic pseudocapacitive material. The cation size effects observed in the in operando Raman evolution of MnO2 clearly proved the interlayer cation storage mechanism. Secondly, the dissertation also probed the energy storage of layered nickel hydroxide/oxo-hydroxide (NiO2Hx), which has a structure similar to that of layered MnO2 and features the transitional electrochemical behavior between the pseudocapacitor and the battery with very high energy density. Correlations between Raman spectroscopic evolution and electrochemical behavior proved that the break/formation of O-H bonds in NiO2Hx contribute to the electrochemical energy storage primarily while cation incorporation between NiO2Hx layers plays a minor role. Thirdly, this dissertation investigated the mechanism of energy storage of T-Nb2O5, which can store Li ions at an exceptionally fast rate similar to a capacitor. Through a comparison between in operando Raman spectroscopic evolution and a theoretical calculation of the vibrational structure of the proposed model, it is found that Li ions are preferably stored on the 2D voids of Nb-O bonding facets similar to the surface-bound capacitive behavior, which unravels the Li-ion incorporation mechanism responsible for fast energy storage. In addition to the research results, a few recommendations are provided about more aspects of energy storage/conversion mechanisms, the application of advanced Raman spectroscopy, and the advanced in operando mechanism analyses. The research work described in this dissertation has contributed significant new discoveries for fundamental chemistry and physics relevant to energy science. Moreover, information about mechanisms unraveled in this dissertation can be helpful for rational design of material structures and compositions for unique functionalities, which will ultimately contribute to engineering developments in the energy storage industry. Finally, the general methodology of this dissertation can be readily applied to other research fields to probe the correlation between external functionalities and intrinsic properties of materials.