| For improving the electrochemical property and structure stability of cathode materials in magnesium ion battery,this thesis aims to solve the problem of poor structural stability of vanadium oxide cathode materials during the cycling process and the slow magnesium ion diffusion kinetics.Therefore,the optimization to the crystal structure and magnesium ions storage performance of layered vanadium oxide is carried out in this thesis.Through methods such as pre-intercalation of metal ions and introducing water molecules between layers,the bottleneck problems of vanadium oxides materials such as poor cycle stability and low specific capacity are effectively alleviated.In order to explore and reveal the internal relationship between magnesium storage performance and crystal structure of materials,various advanced testing methods are applied to characterize the reaction mechanism and structural changes in the magnesium ions storage process of materials,which provides the theoretical guidance and research basis for further improvement of the electrochemical performance of magnesium batteries cathode materials in the future.In this thesis,the synthesis,preparation,structure characterization,magnesium ions storage performance and electrochemical reaction mechanism of ultra-long H2V3O8 nanowires,Li/Na/KV3O8 nano-materials,three-dimensional flexible Fe5V15O39（OH）9·9H2O/CC（Fe VO/CC）composite materials and sea urchin-like H11Al2V6O23.2 electrode materials have been studied.Moreover,a series of significant research results have been obtained in this thesis.（1）The ultra-long H2V3O8 nanowires were synthesized by a simple,mild and efficient hydrothermal method and they were successfully applied in various magnesium-based batteries.When H2V3O8 nanowires are used as cathode materials for magnesium storage,they can exhibit a high working voltage of～2.0 V and a high specific capacity of 304.2 m Ah g-1 at a current density of 50 m A g-1.Their energy density are 6 times higher as that of traditional magnesium ions storage material Mo6S8.Moreover,H2V3O8 nanowires also exhibit excellent electrochemical performance in lithium-magnesium hybrid batteries,which can possess a high specific capacity of305.4 m Ah g-1 at a current density of 25 m A g-1.At the same time,the assembled lithium-magnesium hybrid battery also shows good working temperature adaptability,and can exhibit considerable specific capacity at high temperature of 55°C and low temperature of-20°C.The single-phase change of H2V3O8 nanowires during the process of energy storage has also been observed through different ex-situ tests.The high-capacity H2V3O8 nanowires have been proved to be a potential high-performance magnesium storage cathode material.（2）In order to explore the optimization effect of different metal ions pre-intercalation on the layered vanadium oxide,different metal ions pre-intercalated layered vanadium oxide Li/Na/KV3O8 were synthesized by a simple and efficient hydrothermal method.Through the comparison of electrochemical performance and the calculation of DFT,it is concluded that as the radius of pre-intercalated ions increases,the magnesium storage capacity of corresponding vanadium oxides will gradually decrease,but its cycle stability will gradually increase.Based on the results of electrochemical performance comparison in different dimensions,it is found that the pre-intercalation of sodium ions can maintain the structural stability and high magnesium storage capacity of vanadium oxides.During the migration of magnesium ions in the Na V3O8 crystal structure,the pre-intercalated sodium ions can maintain the free diffusion of magnesium ions and stabilize the layered structure.As the"pillar"between the layers,sodium ions effectively stabilize the structural changes of materials and buffer the"respiration effect"between the layers.Appropriate ions pre-intercalation in the crystal structure can improve the electrochemical stability and specific capacity of cathode materials in the magnesium batteries.This method provides a new idea for the optimization of magnesium batteries in the future.（3）In order to optimize the magnesium storage performance of Fe VO nanosheets with large interlayer spacing,a new three-dimensional flexible Fe VO/CC composite material is synthesized through a green and mass-synthesized water bath method.Compared with the bare Fe VO,the new three-dimensional array structure of Fe VO/CC composite material effectively improves its magnesium storage performance.At a low current density of 100 m A g-1,the Fe VO/CC composite exhibits a high specific capacity of 270 m Ah g-1.Under the high current density of 2000 m A g-1,it can still maintain more than 60%capacity after 5000 cycles.Based on different electrochemical analysis and test characterization,an ion/electron synergistic optimization mechanism is proposed to explain the excellent electrochemical performance of Fe VO/CC composites.The single-phase reaction mechanism of Fe VO crystal in the magnesium ions storage process was firstly explored by the in-situ XRD and in-situ Raman.The diffusion process of magnesium ions in Fe VO crystal has also been widely studied.In addition,a flexible magnesium ion soft-pack battery（Fe VO/CC|Mg Na Ti3O7）has been successfully assembled,which lit the lamp and demonstrated good electrochemical performance.This work embraces certain guiding significance for the optimization and practical application of cathode materials in magnesium ion batteries.（4）The aluminum vanadium oxide cathode material H11Al2V6O23.2 with interlayer water molecules was successfully prepared by a simple hydrothermal method.Its morphology is similar to the sea urchin with a solid sphere in the middle,and the surface is covered by bent nanowires.When used as a cathode material for magnesium ion batteries,sea urchin-like H11Al2V6O23.2 exhibits a specific capacity of about 165m Ah g-1 at a current density of 100 m A g-1.After 3000 cycles at a high current density of 1 A g-1,about 87%initial capacity can be maintained.The reaction kinetics of H11Al2V6O23.2 and Al V3O9 electrode materials were tested and compared.The results show that the ion diffusion coefficient of H11Al2V6O23.2 is larger,which is caused by the larger interlayer spacing and the charge shielding effect of water molecules.Advanced characterization methods such as in-situ XRD,in-situ Raman,ex-situ FTIR and XPS were used to reveal the changes in the crystal structure of H11Al2V6O23.2during the insertion and extraction of magnesium ions.It is found that the crystal structure of H11Al2V6O23.2 shows good crystal stability and reversibility.Through high-resolution TEM,it is clearly observed that when magnesium ions are embedded and released in the H11Al2V6O23.2 crystal,there is only a slight change in the lattice spacing.The sea urchin-like H11Al2V6O23.2,which has only slightly structural strain during magnesium storage,has good cycle stability and is a potential cathode material for magnesium ion batteries. |
