The Application Of X-ray Absorption Spectroscopy On The Design&Synthesis Of New Electrode Materials For Energy Storage Systems

Large chemical energy storage systems, such as lithium/sodium ion batteries, attracts more and more attention all around the world due to their environmental compatibility and rich resources. However, the simultaneous increasing demand of both large energy and power density for the new storage systems, require an intense effort to improve electrode materials, which is still one of the most important limiting factors of any further development. And the search for new high performance electrode materials remains a challenging issue.Nowadays, thanks to the unique advantages of high brilliance, broad spectrum, which covers from THz to hard X-rays together with its high time&space resolution, the characterization methods based on synchrotron radiation sources represent one of the most powerful techniques to investigate materials structures. Considering the sensitivity of many spectroscopic methods to the local chemical environments, we have successfully applied these new techniques to investigate electrode materials to study:compositions, oxidation state, local geometrical structures and coordination atoms. Furthermore, following the development of third generation synchrotron radiation sources, the associated new technologies allowed to achieve in situ characterization of a lithium-ion battery during the entire charge and discharge process. In this way, in-situ information that accurately show the battery reaction, including the structural evolution of the electrode material, the oxidation state of transition metal ions and local structural changes can be obtained. This work present and discuss local geometrical and electronic structures of commercial electrode materials systematically investigated from the atomic-molecular level, by combining synchrotron radiation based technologies and first-principles calculations. We attempted to establish in this way a new structure-performance relationship. In addition, based on this structure-performance relationship we discuss how to find and develop new electrode materials with improved performance. We also developed new and more efficient batteries thanks to in-situ observations of electrode materials during the whole charge and discharge reaction process. This unique synchrotron radiation based approach open the way to further studies to identify new better electrode materials.The main contents include the following: 1. The manganese is doped into the conventional lithium ion battery electrode material (olivine LiFePO4). A new composite electrode material with a good specific capacity was successfully prepared. Thanks to the sensitivity to the local structure of synchrotron radiation based X-ray absorption spectroscopy, experimental data show that manganese and iron are present with different crystal structures in this complex system. Moreover, ultra-high resolution electron microscopy images confirm that the crystalline size of LiFePO4and LiMnPO4are limited to small regions of only few nanometers. Among the various Mn doping concentrations we investigated, the electrochemical data show that the LiFe0.75Mn0.25P04exhibits the best electrochemical performance. Other studies revealed that the different composition and microstructure have a big influence on the performance of lithium-ion batteries in these composite electrode materials. This original structure-performance relationship of traditional electrode materials provides a new experimental basis and characterization methods for further foreseen development of high-performance electrode materials.2. A new type of hybrid polyanionic Na3FePO4CO3nanoplate, a kind of sodium ion battery cathode material, was successfully synthesized by a modified hydrothermal method. Electrochemical test showed that the NasFePO4CO3nanoplates not only provide a higher energy density than the conventional LiFePO4cathode materials, but increases the safety performance of commercial applications. In-situ and ex-situ X-ray absorption spectroscopy experiments confirmed that the higher capacity is associated to two pairs of redox reactions, including Fe2+/3+and Fe3+/4+. These results point out another new direction of future development for low-cost Fe-based electrode materials.3. We synthesized and investigated, for the first time, a novel fluffy alluaudite NaFePO4nanocactus and some nanocomposites with CNTs. We characterized them by XRD, SEM and TEM techniques, in order to use them as sodium ion batteries (SIBs) electrodes via a modified solvothermal method. This unique nanoparticle consists of nanorods of-20nm within an open three dimensional framework. Electrochemical tests showed that the NaFePO4/CNT nanocactus exhibits a high initial discharge capacity of-143mAh/g, approaching its theoretical maximum capacity and an excellent cyclability, up to0.9Na+. It can be reversibly used for50cycles. This outstanding electrochemical performance is due to its unique morphology, which not only ensures a successful migration of electron and Na+ions with short diffusion pathways, but also provides an efficient contact between electrode and electrolyte. Furthermore, we studied this enhanced mechanism with x-ray absorption measurements and ex-situ XAFS characterizations. The results point out the possibility to synthesize, starting from natural minerals new cathodes, candidates for SIBs with really high performance. Moreover, data clarify the relationship between morphology, crystalline structure and electrochemical performance in this novel alluaudite NaFePO4. Our work also points out the possibility to identify other safe and low cost polyanion type electrodes.4. On the basis of above synthesis work (recently published on Small) we further synthesized several other alluaudite mineral materials. For this series of alluaudite minerals, the differences are not only presenting in their morphology, but also in other physical properties, such as the electrochemical activity and the magnetic performance. Therefore, we took advantage of both soft and hard X-ray absorption spectroscopy to investigate electronic and geometrical information, combined with other advanced characterization techniques to analyze both their atomic structures and physical properties. This study shows an unexpected structure-performance relationship, completely different from that characteristic of olivine cathode materials. The result points out a completely new way to consider the existing mineral structures and the related materials for future researches.