Micro/nano-structured Control Synthesis And Electrochemcial Performance Of Layered Vanadium-Based Oxides Electrode Materials

Li-ion batteries?LIBs?and Na-ion batteries?NIBs?are considered as the most promising energy storage technologies.LIBs have dominated the entire electronic appliance market.With the revolutionary electric vehicles?EVs?fast developing,LIBs are also considered as the best choice of their energy storage.NIBs possess the huge potential in the application of large-scale energy storage system?ESS?because of the abundance and low cost of the Na resource.However,with these novel applications?such as EVs and EES?rapid development,they put forward the higher requirements?such as higher energy and power density,longer cycling life,lower cost,etc.?for LIBs and NIBs.So,it is urgent to exploit and develop the more advanced electrode materials.Vanadium-based oxides?including vanadium oxides and vanadates?are considered as one of the most promising next-generation electrode materials duo to their high specific capacity,abundance resource and low cost.However,there are three huge challenges for the application of vanadium-based oxides in the energy storage,i.e.how to realized the controllable synthesis of vanadium-based oxides,how to in depth reveal their growth behavior and electrochemical reaction mechanism,and how to ensure that they exhibit excellent and stable electrochemical performance.So,based on the above problems,the controllable synthesis of low-dimensional nanostructured and 3D micro/nano-structured vanadium–based oxides are realized from the aspect of materials processing in microscopic scale.Moreover,their electrochemical performance are greatly improve roundly.Furthermore,the intrinsic nucleation behavior and electrochemical reaction mechanism of some important vanadium-based oxides are revealed for the first time by the Density Functional Theory?DFT?calculation,Rietveld refinement,ex-situ XRD and XPS technologies.The detailed research content are as follows.?1?Based on HTMM,different low-dimensional vanadium oxides nanomaterials?such as VO₂?A?nanorods,VO₂?B?nanorods and V₃O₇?H₂O nanoribbons?with good electrochemical performance were controllably synthesized by taking oxalic acid as reducing reagent.The reaction and crystal growth mechanisms were discussed in detail.The initial discharge capacities of V₃O₇?H₂O nanoribbons and VO₂?B?nanorods were 245 and 146 mAh/g at a current density of 100 mA/g,and their capacity retention after 100 cycles were both 83%.Moreover,the Rietveld refinement method was applied to analyze their crystal structures,and it was found that V₃O₇?H₂O possessed the open layered-type crystal structures with larger interlayer space,which was also the reason why V₃O₇?H₂O nanoribbons exhibited better electrochemical performance.?2?To further improve the electrochemical performance of V₃O₇?H₂O nanoribbons,a in situ carbon coating method was proposed by taking glucose as reducing reagent in HTMM,and V₃O₇?H₂O@C nanoribbons were successfully synthesized.By applying the ex situ XRD and XPS technologies,the electrochemical reaction mechanism and charge-discharge behavior were revealed for the first time.It was also found that V₃O₇?H₂O@C nanoribbons were a promising cathode materials for NIBs.When used as cathode materials of LIBs,V₃O₇?H₂O@C nanoribbons exhibited the greatly improved electrochemical performance:the energy density is up to 800 Wh/kg,the initial capacity is up to 319 mAh/g,and the capacity retention were 94%and 119%at 100 and 3000 mA/g,respectively.However,a gradual capacity fading can be found in the later period of cycling,which might be mainly caused by the self-aggregation of low-dimensional nanomaterials during cycling.?3?To simultaneously overcome the disadvantages of nanomaterials and develop their advantages,and improve the electrochemical performance evenly,a facile“hydrolysis-controllable crystallization”strategy was proposed to controllably synthesize different dimensional?i.e.1D,2D,and 3D?VO₂?B?nanostructures by a simple one-step HTMM.Especially,a unique 3D micro/nano-structured hierarchical porous sponge-like micro-bundles?SLMBs?self-assembled from 2D crumpled ultrathin VO₂?B?@C nanosheets with a thickness of only³.1 nm?denoted as VO₂?B?@C-SLMBs?was synthesized.Importantly,the intrinsic VO₂?B?crystallization behavior and controllable synthesis mechanism of VO₂?B?micro/nano-structures were revealed for the first time by the DFT calculation.The as-synthesized VO₂?B?@C-SLMBs possess the distinct structural advantages,which make them exhibit the excellent electrochemical performance in terms of long life,high rate,and large capacity as the cathode materials of LIBs.The discharge capacity is still up to 206 mAh/g after 160 cycles at100 mA/g,corresponding to 105%of the initial capacity.More importantly,the capacity can be kept stable in the whole cycling at a small or large current density.?4?To transplant the unique structural features of VO₂?B?@C-SLMBs to other valence-state vanadium oxides,the self-reducing and in situ oxalating methods were proposed to successfully synthesize V₂O₃@C?V₂O₅ micro/nano-structures with the similar morphology as VO₂?B?@C-SLMBs precursors.Importantly,it was found for the first time by the ex situ XRD technology that V₂O₃ was a new promising intercalation-type anode material for LIBs with a high capacity.V₂O₃@C micro/nano-structures can deliver a large capacity of 732 mAh/g without capacity loss at100 mA/g even after 136 cycles.Moreover,they also exhibited the excellent cycling and rate performance.Furthermore,it was also found for the first time that V₂O₃@C micro/nano-structures were also one new and promising anode material for NIBs.V₂O₅ micro/nano-structures also possessed unique structural advantages,can deliver a large reversible capacity of 233 mAh/g,and also exhibited the improved cycling and rate performance.?5?To further extend the strategy“Bulding 3D micro/nano-structural electrode materials improves the electrochemical performance.”to another important vanadium-based oxide,i.e.vanadate,a novel two-step hydrothermal method was proposed to controllably an interesting 3D NaV₂O₅ mesocrystal.The reaction and crystal growth mechanisms were discussed in detail.Moreover,by combining the advanced DFT calculation and electrochemical testing,the intrinsic electrochemical characters and potential applications of NaV₂O₅ were revealed.The Na-ions in the intrinsic NaV₂O₅ were hardly extracted,but external Na-ions can be intercalated into NaV₂O₅,which revealed that NaV₂O₅ may be a promising new anode material for NIBs.Electrochemical tests showed that the initial capacity of NaV₂O₅ mesocrystal was up to 473 mAh/g.