Preparation Of New Titanium-based Phosphates And Vanadium Oxide And Their Application In Magnesium-based Batteries

Based on the new magnesium-based secondary ion battery system,this thesis focuses on the development and design of some new structures of titanium-based phosphate and vanadium oxide in order to solve the slow dynamics and structural instability of electrode materials in magnesium-based batteries.Meanwhile,the designed materials are applied to the new type of magnesium ion batteries and magnesium-based hybrid ion batteries.Through the advanced characterization methods to study its reaction mechanism,reveal its internal relationship between structure and performance,and further improve its electrochemical performance,which provides a theoretical basis for the development of new energy storage technology and the development of high-performance energy storage materials.The preparation,composition,structure and electrochemical properties of titanium phosphate lithium/carbon micron-flower composites,titanium phosphate sodium/carbon nanowire cluster composites,and magnesium ion pre-embedded hydrated vanadium oxide nanowire materials have been studied in this paper.Some meaningful research results have been achieved:?1?Titanium phosphate lithium micro-flower precursor was synthesized by a wet self-assembly process.After the late high-temperature carbonization and crystallization process,a carbon-coated titanium phosphate lithium micro-flower material with a size of about 2?m was successfully prepared?LTP-F/C?.The micron flower is assembled by interdigitating nanosheets with a thickness of about 10 nm,and its specific surface area is as high as 63.28 m² g^-1.When used as a cathode material for magnesium-lithium hybrid ion batteries,the LTP-F/C electrode material shows excellent rate performance and long cycle stability.It can provide a specific capacity of 94 mAh g^–1 at a high rate of 20 C.After 3000 cycles at 10 C rate,the initial specific capacity of 80%can be retained,and the Coulomb efficiency remains above 99.99%during the cycles.This hybrid ion battery provides an energy density of 220 Wh kg^-1,and the energy density is maintained at 147 Wh kg^-1 even at a power density of 2.94 kW kg^-1.This is obvious superior to the magnesium ion battery used Mo6S8 as the positive electrode and other advanced magnesium-lithium hybrid batteries electrode materials using stainless steel as the current collector.The titanium phosphate lithium micro-flower material has excellent electrochemical properties because it combines the advantages of NASICON type crystal structure and three-dimensional conductive carbon skeleton:unique three-dimensional hierarchical structure provides fast ion diffusion and electron conduction rate for LTP-F/C.Thanks to the support of the carbon skeleton,LTP-F/C can maintain"zero strain"structural stability during lithium ion intercalation/deintercalation.The design of three-dimensional open structure provides a new idea for the development of high-performance magnesium-based battery electrode materials.?2?The titanium phosphate sodium nanowire cluster precursor composed of self-assembly nanowire was successfully obtained by a solvothermal method.Through the late dopamine modification and heat treatment process,three-dimensional carbon coated titanium phosphate sodium nanowire cluster with a size of about 10?m was obtained?NTP-NW/C?.The nanowires in the three-dimensional nanowire cluster have a diameter of about 150 nm and are uniformly distributed in a radial shape.When used as a positive electrode material for sodium ion batteries,NTP-NW/C exhibits excellent rate performance and cycle stability.It provides a high reversible specific capacity of 92 mAh g^-1 at an ultra-high rate of 100 C.The reversible specific capacity at 20 C can reach 106 mAh g^-1,and after 20,000 cycles,the NTP-NW/C material can still provide a reversible capacity of70 mAh g^-1.At the same time,when used as a cathode material for magnesium-sodium mixed ion batteries,NTP-NW/C also exhibits excellent sodium storage performance.It provides a high reversible specific capacity of 124 mAh g^-1 at1 C rate.The reversible specific capacity at 10 C high rate can reach 60 mAh g^-1.After 100 cycles at 5 C rate,the capacity retention rate is still 97%.The in situ X-ray diffraction characterization reveals a typical two-phase reaction mechanism during the sodium storage process.The variation of the ion diffusion coefficient of NTP-NW/C in the reaction process is investigated by galvanostatic intermittent titration technique test.The calculation results show that the diffusion coefficient is relatively high compared with the common nanoparticle materials,which further proves its rapid charge diffusion kinetics.The excellent electrochemical performance of NTP-NW/C is due to its unique three-dimensional hierarchical structure combining the advantages of NTP nanowires and uniform carbon skeleton coating.Three-dimensional open structure facilitates electrolyte penetration.The uniform distribution of nanowires provides sufficient active sites for ion transport.The uniform carbon frame ensures fast and continuous electron transfer while improving structural stability.?3?A magnesium ion pre-embedded vanadium oxide nanowire electrode material(Mg_0.3V₂O₅·1.1H₂O)with bilayer structure was successfully prepared by chemical modification of?-V₂O₅.The Mg_0.3V₂O₅·1.1H₂O nanowire has a length of about 12?m,an average diameter of 100 nm,and a specific surface area of 38.23 m²g^-1.Thanks to the pillar effect of pre-embedded magnesium ions and the charge shielding effect of interlayer water,when used as the positive electrode of magnesium ion battery,the Mg0.3V2O5·1.1H2O exhibits excellent rate performance and unprecedented magnesium storage cycle stability.It can still obtain a discharge specific capacity of 50 mAh g^-1 at a high current density of 4 A g^-1.After 10,000cycles at high current density of 1 A g^-1 and 2 A g^-1,its capacity retention rates are both more than 80%.When assembled with MgNaTi₃O₇ anode material into a magnesium ion full cell,the full cell is able to achieve a discharge specific capacity of57 mAh g^-1 with a first coulombic efficiency of 88%.It is found by in situ X-ray diffraction characterization that the interlayer spacing of Mg_0.3V₂O₅·1.1H₂O only changed by 0.5?during the intercalation and deintercalation of Mg²⁺ions,which proves the pillar effect of pre-embedded magnesium ions,and provides an important basis of its long cycle life.It is found by X-ray absorption near-edge spectroscopy that when the magnesium ions are embedded,the K-edge and pre-edge of vanadium are transferred to lower energy positions,which proves that the valence state of vanadium is lowered.At the same time,the length of the V-O bond is increased and the length of the V-V bond is shortened,which proves that magnesium ions are intercalated into the Mg_0.3V₂O₅·1.1H₂O layer and coordinated with the lattice oxygen.The Solid-state magic angle spinning nuclear magnetic resonance technology directly proves the embedding of Mg²⁺ions and confirms the charge“shielding effect”of water,which greatly improves the diffusion kinetics of magnesium ions.The galvanostatic intermittent titration technique test and single nanowire device characterization shows that the ion diffusion coefficient and intrinsic electron conductivity of Mg_0.3V₂O₅·1.1H₂O are relatively high,which further proves its rapid charge diffusion kinetics.This new magnesium ion pre-embedded layered structure has made a significant contribution to the development of high-performance magnesium ion batteries.