Design,Structure Engineering,and Electrochemical Performances Of Vanadium-Based Cathodes For Superior Sodium-Ion Batteries

With the massive consumption of fossil energy and the intensification of environmental pollution,the wide utilization of renewable energies has become increasingly important.However,considering the intermittent characteristic of renewable energy sources such as wind power and tidal energy,rechargeable batteries based on alkali-ion intercalation chemistry are promising technologies.Though lithium-ion batteries（LIBs）are dominating the global market in consumer electronics,the scarcity and high cost of metal Li and Co,two essential components in commercial LIBs limit their large-scale application.Sodium-ion batteries（SIBs）are emerging as a promising alternative based on their low cost,natural abundance of sodium,and similar working principles to lithium-ion batteries.Nevertheless,the drastic volume expansion and sluggish reaction kinetics caused by the large Na⁺radius（1.02（?））inhibit its development.Therefore,it is critical to customize cathode materials for SIBs to better accommodate the bulkier Na-ions.Vanadium-based materials have been intensively explored as one of the most promising cathodes due to their multiple valence states(V²⁺-V⁺⁵),appropriate operating voltages,high theoretical capacities,and facile synthetic chemistry,however,some drawbacks limited their applications.For example,vanadium-based nanostructure materials are easily agglomerated and restacked,resulting in inferior cycling stability and low volumetric energy density of the whole battery.What’s more,the phase transitions result in performance degradation.Further,it should also be admitted that the development of vanadium-oxide-based materials for SIBs in regard to reaction mechanism is still in the infancy stage compared to LIBs and even ZIBs.Moreover,numerous challenges still remain such as fast capacity fading,and short life.Therefore,this thesis targets on design and development of vanadium-oxide cathodes and their derivatives to overcome the issues and bring a deep insight into the energy storage mechanism.Step-by-step investigations of novel cathode materials were carried out to evaluate their physiochemical properties,electrochemical performance,and microstructures/chemical states evolutions upon the charge/discharge process.Moreover,Density functional theory（DFT）and 3D-tomography simulations were adopted to elucidate specific characteristics of as-prepared cathode materials and their corresponding electrodes.Finally,the as-fabricated sodium ion batteries exhibit competitive performance in terms of high energy/power density and long cycling stability compared with reported research in this area.The details of the work were summarized into three main aspects shown as follows:Sodium vanadate NaV₆O₁₅（NVO）is one of the most promising cathode materials for sodium-ion batteries because of its low cost and high theoretical capacity.Nevertheless,NVO suffers from fast capacity fading and poor rate capability.Here,free-standing Sodium vanadate NaV₆O₁₅（NVO）film was co-modified with multiwalled carbon nanotubes（MWCNTs）via a simple hydrothermal method followed by a dispersion technique with high safety and low cost.The kinetics analysis based on cyclic voltammetry measurements reveals that the intimate integration of the MWCNT 3D porous conductive network with the 3D pillaring tunnel structure of NVO nanorods enhances the Na⁺intercalation pseudocapacitive behavior,thus leading to inhibiting phase transitions.Further,a comprehensive ex-situ examination of their microstructures,chemical states,and electronic structure evolutions upon charge/discharge states,illustrated outstanding structural stability during the repeated sodiation/desodiation process.With these benefits,the NVO/MWCNTs cathode delivers a high discharge capacity of 217.2 mAh g^-1 at 0.1A g^-1 in a potential region of 1.5-4.0 V.It demonstrates the best rate capability of 123.7 mAh g^-1at 10 A g^-1 compared with the ever reported NVO as cathode for SIBs.More encouragingly,it displays an excellent long lifespan,impressively 96%of the initial capacity is retained at 5 A g^-1over 500 cycles.Therefore,this study illuminates prospects of two-pronged approaches to optimizing electrochemical properties by improving the electronic conductive by introducing carbon nanotube and stabilizing the structure via the freestanding method and 3D pillar design for advanced cathode materials in SIBs applications.Vanadium pentoxide（V₂O₅）has been the choice as cathode material for years,as it provides benefits in terms of abundance,low cost,appropriate operating voltages,high theoretical capacities,and facile synthetic chemistry.However,the main hindrances to its application in batteries are its poor capacity retention,low rate capability,and structural collapse.In this regard,this work successfully overcomes all of the aforementioned drawbacks by developing a general synthetic strategy that enables us to simultaneously control the phase,morphology,and composition of electrode materials via a one-pot solvothermal method followed by a single-calcination step.As result,a series of rational designs are obtained:Single crystalline V₂O₅ hollow shell（S-V₂O₅-HS）,Single crystalline V₂O₅ hollow core-shell（S-V₂O₅-HCS）,Polycrystalline carbon-coated V₂O₅ solid core-shell（P-V₂O₅/C-SCS）,amorphous carbon-coated V₂O₅ solid core-shell（A-V₂O₅/C-SCS）,and amorphous carbon-coated V₂O₅ well-defined porous core-shell heterostructure（A-V₂O₅/C-WCS）,and the performance differences between them are systematically studied based on the kinetics analysis（CVs,EIS,GITT,in/ex-situ XRD,XPS…）,DFT calculations,and electrochemical results.It has been found that the single crystalline V₂O₅core-shell cathode exhibits sluggish Na⁺diffusion and low electronic conductivity due to huge volume expansion during repeated cycles.In contrast,the polycrystalline V₂O₅ solid core with conductive carbon shell P-V₂O₅/C-SCS displays faster diffusion,better electrochemical performance,and higher structural stability than single crystalline V₂O₅ electrodes（S-V₂O₅-HS,S-V₂O₅-HCS）as confirmed by in situ XRD.Moreover,with similar size and morphology,the amorphous structure（A-V₂O₅/C-SCS）greatly accelerates ion diffusion rate,boosts electronic conductivity,and lowers binding ability compared to crystalline counterparts.Impressively,by tuning the core and shell,the amorphous V₂O₅ defect-rich porous core and well-defined conductive carbon shell structure（A-V₂O₅/C-WCS）enables large specific surface area,efficient Na⁺kinetic,highest ionic/electronic conductivity,and lowest energy barriers compared to its counterparts,resulting in an outstanding electrochemical performance for sodium-ion batteries,indicating that efficient tailoring strategies with the synergistic engineering of amorphization and deficiency are highly desired.Out of the ordinary,the A-V₂O₅/C-WCS applied as a symmetric electrode for SIBs displays outstanding cycling performance of 169 mAh g^-1 after 3000 cycles at 2.0 A g^-1 and a superior rate capability of 148 mAh g^-1 at 5.0 A g^-1,when applied as a cathode（1.5-4.0 V）and anode（0.01-3.0 V）for Na-ions batteries.The amorphous symmetric-electrode A-V₂O₅/C-WCS displays a fully pseudocapacitive manner over the whole voltage thus the electrochemical performance of the cathode and anode are well-matched.Impressively,the symmetric full cell offers a superior energy density of 381 Wh kg^-1 with an output voltage of 3.0 V,outperforming all reported A/Symmetric SIBs.Even at a high specific power of 6.0 KW kg-1 the cell still can provide a high energy density of 219 Wh kg^-1,demonstrating the great potential for practical application.What’s more,the battery can deliver 95%of the initial capacity over 1200 cycles at 1.0 A g^-1.This work comprehensively investigated the intercalation mechanisms of different phases of V₂O₅（single-poly/crystalline,amorphous）and addressed the effect of morphology,composition,and defects on Na storage.Importantly,this success at the A/Symmetric Na-ion battery level provides a novel general approach for designing advanced battery materials,and an open avenue for the development of room-temperature sodium-ion batteries with a view toward safety and low cost.Unlike other vanadium oxides,vanadium trioxide V₂O₃ with a low valence state(V³⁺)has been considered unusable as a cathode for Lithium/Sodium-ion batteries.Nevertheless,it demonstrates great potential as an anode for Li/Na storage,therefore novel approaches are still urgently demanded.Hence,a promising cathode design for sodium-ion batteries should balance high stability and fast kinetics by harmonizing the individual advantages of crystalline and amorphous phases.To achieve this,the construction of amorphous/crystalline（A/C）heterophase emerged as an advanced strategy to effectively modulate electron/ion behaviors and boost structural stability as they tend to have better physicochemical properties than their single counterparts.However,their different kinetics limit the synergistic effects.Further,its interaction functions and underlying mechanisms are yet to be fully understood.In this work,we develop a new and facile strategy for the fabrication of unique defect-rich V₂O₃ amorphous/crystalline heterophase.The unique engineered A/C-V₂O₃-x@C-HMCS heterophase composed of oxygen-deficient amorphous V₂O₃-x hollow mesoporous core（A-V₂O₃-x/HMC）and lattice distortion crystalline V₂O₃ shell（C-V₂O₃/S）encapsulated by carbon,which is successfully prepared via one-pot hydrothermal method followed by a reduction reaction and activation process.Based on DFT calculations,in pristine A/C-V₂O₃@C-HCS（defects free）,the adsorption energy is significantly reduced at the heterointerface sites due to the huge difference in kinetics between the crystalline and amorphous phases,suggesting that the sparse interfacial sites limited the optimization of adsorption energies,thus sluggish Na⁺kinetics.In contrast,the defective A/C-V₂O₃-x@C-HMCS hetero-phase demonstrates the strongest Na⁺adsorption in either crystalline phase,amorphous phase,or even at heterointerfaces sites,indicating that the defects induce the formation of highly dense A/C interfacial sites,which can not only modulate the electronic structure and enrich active sites,but also lowers the energy barriers,and promote adsorption energies,thus rapid Na⁺kinetics.In particular,tailoring the unsaturated active site is an effective strategy to promote charge transfer.In addition,the ex-HRTEM and ex-XRD results demonstrate that the dense A/C interfacial sites can be well maintained even after long-term cyclability,further confirming the robust stability of A/C-V₂O₃-x@C-HMCS.For the first time in the literature,V₂O₃ has been successfully investigated as a cathode for sodium storage in the range of 1.5-4.0 V.The resultant A/C-V₂O₃-x@C-HMCS exhibits superior electrochemical properties:a high capacity of 290 mAh g^-1 is achieved at a low rate of 0.1 A g^-1.Even at a high rate of 10 A g^-1,the A/C-V₂O₃-x@C-HMCS cathode still retains a high capacity of 192 mAh g^-1 over 6000 cycles,outperforming most of the reported vanadium-based cathodes for SIBs.These results evidenced that the electrochemical properties of electrode materials depend not only on the oxidation state but also largely on their crystal structure and morphology.Further,this work provides new insights into the fundamental understanding of heterophase material dynamics for next-generation sodium-ion batteries from both an experimental and theoretical perspective.Impact StatementThis Ph.D.project targets developing and investigating vanadium-based cathode materials to boost sodium-ions batteries performance.The thesis was divided into three perspectives of vanadium-based cathode materials in terms of novel freestanding sodium vanadate/MWCNTs,vanadium pentoxides,and vanadium trioxides,respectively.This work develops numerous novels,with simple,facile,controllable,and low-cost methods.We report diverse phase structures,including single,poly,amorphous,and amorphous/crystalline heterophase structures,and different crystals structure such as a monoclinic,orthorhombic,and rhombohedral,and various morphology such as nanorods and core shells nanostructured with several microstructures（porous,hollow,solid）.This diversity provides huge possibilities and choices for the investigation of NVO,V₂O₅,and V₂O₃ as cathode materials for emerging sodium-ion batteries.Therefore,we provide comprehensive characterizations of morphology,structural,electronic,kinetic reaction,and electrochemical performance via several techniques（in/ex situ XRD,XPS,TEM,HRTEM,CV,EIS,GITT….etc）combined with theoretical calculations,such as DFT calculations,theoretical reaction simulation,and massive atomic/electronic scale simulation,providing experimental evidence and theoretical guidance for future works.Meanwhile,universal strategies were also proposed in this thesis for improving the electrochemical performance in regard to specifically tuned micro-/electronic,phase,morphology compositions structures,vacancies,defects,and interfacial interactions,which contributed to high specific capacity,superior rate capability,long cycle life,rapid diffusion reaction kinetics,and robust structure.Interestingly,this Ph.D.project provides comprehensive research which started with novel cathode materials and finished with optimized properties for practical demonstrations for symmetric electrodes and full symmetric battery,that is,suggests an open avenue for the development of room-temperature sodium-ion batteries with a view toward safety and low-cost.We believe that these achievements will drive a step-change for next-generation high-energy SIBs and beyond and could draw broad interests not only from academics working in the field of materials science associated with energy storage techniques but from a wider community on advanced materials design and practical applications.