| Recently, rapid depletion of fossil fuels and growing environmental problems pose serious scientific and technological challenges to address the increasing global demand for energy. In view of these, energy will be the greatest challenge which the human faces in the 21st century. So the development of sustainable and clean energy technologies is needed to address this inevitable challenge. In this field, the lithium-ion battery and the water splitting are very important and promising energy storage and conversion technologies. As we know, cathode materials play a key role in the lithium-ion battery system, which has attracted much attention. For water splitting, the bottleneck is actually the development of materials for the catalysis of the electrochemical oxygen evolution reactions (OER) due to the fact that it requires the coupled transfer of four protons and four electrons from two water molecules to release one oxygen molecule. Therefore, it is urgent to design the electrode material with satisfactory electrochemical performance to improve the development of the energy conversion and storage and conversion.The thesis mainly focuses on designing and modifying olivine structure LiMPO4 (M= Fe, Ni, Mn) nanomaterials for lithium ion battery cathode and OER catalyst, subsequently improving their performance of the energy conversion and storage. The details of this thesis are summarized briefly as follows:The core-shell LiFePO4@C composites were synthesized by in-situ and ex-situ carbon coating methods respectively. The high resolution transmission electron microscopy (HRTEM) images show that both methods can coat-3 nm carbon layer on the surface of LiFePO4 nanoparticles. However, their electrochemical properties exhibit a distinguished difference. By means of Soft-X-ray-absorption spectra (XAS), it was demonstrated that carbon layer could be linked to LiFePO4 particle surface through C-O bond for the In-LFP@C sample that was synthesized by in-situ process, whereas carbon layer only loosely covered on the surface of LiFePO4 nanoparticle for the Ex-LFP@C sample which was synthesized through ex-situ carbon coating method. As we kown, the kinetics of carbon-coated LiFePO4 was limited by the electron transfer at the solid-solid (carbon-LixFePO4) interface. The different solid-solid interfacial structure in In-LFP@C and Ex-LFP@C sample may be the origin of their different electrochemical properties.The glycerol was used as the solvent, carbon source and surfactant to synthesize nanosized and spindle-shaped LiMnPO4/C. The high resolution transmission electron microscopy (HRTEM) displayed that LiMnPO4/C particles in different states could obtain a uniform and tight carbon coating layer forming a complete network of conductive carbon with a high degree of graphitization. Compared with pure phase LiMnPO4, the PO43- Raman peaks of LiMnPO4/C shifted to low-frequency about 10 cm-1, which indicated that the carbon layer had an effect on LiMnPO4. DFT calculations results show that LiMnPO4 may connect with C through C-O chemical bonding. The P-O bond length was increased and the bond energy was reduced, therefore the Raman peak shifted to low-frequency. The electrochemical test results showed that highly active LiMnPO4/C had the best electrochemical performance. The LiMnPO4/C synthesized in this method delivers specific discharge capacities of 164 mAhg-1,158 mAhg-1,156 mAhg-1,143 mAhg-1,139 mAhg-1 and 108 mAhg-1 at the rates of 0.05 C,0.1 C,0.5 C,1 C,2 C and 5 C (1C=171 mAg-1), respectively. Moreover, it showed excellent cycle performance, which keeps 97% capacity after 50 cycles at 0.05 C. Even at 5.0 C, the discharge capacity can be approximately maintained 80% after 50 cycles. It was indicated that the materials prepared by this method possesses excellent rate and cycle performance.The method of synthesizing LiMnPO4/C was expanded to the MnO/C anode material in lithium-ion batteries. Nano-sized MnO intimately embedded into porous carbon matrix has been synthesized by a facile method that the manganese-salts/glycerol sol was used as the precursor. The glycerol plays roles of the chelating agents, the carbon source and the solvent. The X-ray diffraction (XRD) and Raman results indicate that the carbon layer may have an obvious effect on the microstructure of MnO. The first-principles density functional theory (DFT) calculations further revealed a considerable charge transfer from MnO to the carbon, leading a decrease of the lattice parameters of MnO and the bond length of Mn-O in MnO/C composites. Modified microstructure could improve electrochemical performance and meanwhile may explain the phenomenon of exceeding the theoretical capacity as anode materials for Li-batteries. The prepared MnO/C nanocomposite as anode materials displays superior Li-battery performance with large reversible capacity, excellent cyclic performance and good rate capability.Splitting water to produce oxygen is the key technique in the development of various energy conversion including metal-air batteries, fuel cells and water splitting. Herein, for the first time, we report a mesoporous LiNi1-xFexPO4@C (0≤x≤1) nano-structure as highly effective catalyst for electrochemical oxygen evolution reaction (OER) through a spray dry method. In particular, the LiNi1-xFexPO4@C (3:1) shows superior activity to those state-of-the-art noble metal catalysts (e.g., RuO2 and IrO2), which only needs an overpotential of 311 mV to afford a current density of 10 mA·cm-2 and maintains its high catalytic activity after 1000 cycles. Importantly, our work offers an affordable strategy for the rational design and synthesis of functional materials for a variety of applications.In this thesis, olivine structured lithium mixed transition-metal phosphates material anchored on graphene sheets (LiNi0.75Fe0.25PO4/rGO) is reported as a highly effective OER catalyst. XANES results showed that the synergy between the LiNi0.75Fe0.25PO4 and graphene nanosheets made the valence of the surface of the transition metal tend to increase and speed up the rate-determining step of the reaction of OER. Moreover, the graphene can increase conductivity and surface area, and limit the growth of particles. Therefore, LiNi0.75Fe0.25PO4/rGO exhibited an extremely low overpotential of 295 mV to reach a current density of 10 mA·cm-2, fast kinetics with a small Tafel slope of 47 mV·dec-1 in an aqueous alkaline electrolyte and maintained its high catalytic activity after consecutive 2000 cycles, which was superior to the benchmark RuO2 and IrO2 catalyst under identical experimental conditions and comparable to the best noble metal-free OER electrocatalysts reported so far. This achievement provides a straightforward route to design cost-effective and efficient catalysts from commercially available materials to replace the state-of-art noble-metal electrocatalysts for large-scale water splitting. |
PDF Full Text Request