Study On Vanadium Pentoxide-based Cathode For Zinc/magnesium Ion Batteries

Rechargeable multivalent ions batteries with low cost,high sustainability,and safety are expected to play an irreplaceable role in next-generation large-scale energy storage systems.Among them,zinc ion batteries and magnesium ion batteries have obtained increasing attention due to their relatively high energy density.As a typical layered transition metal oxide,V₂O₅ is featured with an adjustable layered structure,high operating voltage,and theoretical capacity,which is very promising in zinc and magnesium storage.Unfortunately,bivalent zinc ions and magnesium ions possess large charge to radius values,resulting in strong interaction with the host anion framework,large polarization,and structure failure during charge-discharge processes.The most commonly reported metal ions and water intercalation strategy to improve the zinc and magnesium storage capability of V₂O₅ is effective except for the low stability,reliability,and compatibility.Therefore,this thesis focuses on the improvement of zinc and magnesium storage capability of V₂O₅,proposing three strategies,such as conductive polymer intercalation on the surface,organic macromolecule intercalation,and aliovalent dopping to realize high-performance zinc ion batteries and magnesium ion batteries with V₂O₅ based cathodes.Below are the research contents:(1)The zinc storage capability of V₂O₅ was activated by the in-situ intercalation and polymerization of 3,4-ethylenedioxythiophene(EDOT)at the shallow layers of V₂O₅.The insertion of conductive polymer PEDOT could increase the conductivity of V₂O₅ remarkably and the hydrophobicity of PEDOT suppresses dissolution of active species.The integrity of V₂O₅ could be preserved,which could guarantee the stability of active species during cycling.The expansion of shallow layers of V₂O₅ is sufficient to guide Zn intercalation and activate the deeper interlayers successively via cascading effect.This synergic effect contributed to the fast and thorough redox process of bulk V₂O₅ with large monolithic grain.Based on theoretical computation results,the expansion of the surface interlayers could reduce the absorption energy and diffusion barrier of Zn²⁺in the host lattice effectively.Besides,the Zn²⁺diffusion coefficient was as high as 1.43x10^–9?1.81x10^–8 cm² s^–1.The above merits endow the PEDOT intercalated V₂O₅ composite with superior cycling and rate performance.At a current density of 5,8,and 10 A g^-1,the reversible capacities were as high as 388,367,and 351m A h g^-1,respectively.After 4500 cycles at 10 A g^-1,the retained capacity was 269 m Ah g^–1.Moreover,the energy density can reach 280.2 and 205.8 Wh kg^–1 with a power density of 700.5 and 5960 W kg^–1.This strategy can activate the electrochemical performance of cathode material with the grain integrity well preserved.(2)The PEDOT intercalated V₂O₅(VOP)nanowire cluster was synthesized by the intercalation and polymerization of EDOT in the deep interlayers of V₂O₅,which is compatible with magnesium metal anode and traditional APC electrolyte.On the one hand,the intercalation of PEDOT in the deep interlayers of V₂O₅ can increase the interlayer spacing,on the other hand,the intercalation processes can exfoliate the V₂O₅grains and facilitate the formation of hierarchical nanowire cluster,benefiting the reaction kinetics.The CTA⁺ions can insert into the VOP cathode irreversibly and improve the capacity and rate capability of VOP by the introduction of the CTAB additive.The CTAB can lower the interface resistance and smooth the Mg anode.In the APC-CTAB electrolyte,the Mg²⁺and Mg Cl⁺co-de/intercalation mechanism dominates the electrochemical processes of VOP after the initial irreversible insertion of CTA⁺ions.The results of the first-principles calculations indicate that the bandgap can be lowered remarkably(from 2.63 e V to 0.37 e V)by the intercalation of PEDOT and the absorption and diffusion barrier of Mg²⁺and Mg Cl⁺in the host can also be lowered by the charge shielding effect of PEDOT.Based on the above-mentioned merits,the constructed VOP/APC-CTAB/Mg full cell exhibits a reversible capacity of 288.7 m Ah g^–1 along with good rate performance(a capacity of 153.8 and 110.7 m Ah g^–1 at 300and 500 m A g^–1)and cyclic capability(a retained capacity of 144.2 m Ah g^–1 after 150cycles at 100 m A g^-1).(3)A large-size organic ion,tetrabutylammonium(TBA⁺)was used as intercalation species to expand the interlayers of V₂O₅ and improve its magnesium storage properties.The TBA⁺intercalated V₂O₅(TVO)possesses a large interlayer spacing of 12.48(?),which is beneficial to the intercalation of Mg²⁺.The TBA⁺with multi-orientation branch contributes to the charge shielding effect and can be anchored in V₂O₅.The curly nanobelts cluster structure of TVO benefits the charge transfer and provides a vital structure foundation to its superior electrochemical performance.Therefore,the TVO cathode exhibits large pseudocapacitance contribution(40.9%?68.1%)and diffusion coefficient(1.45×10^–13?1.75×10^–11 cm² s^–1)during charge-discharge processes,even better electrochemical performance compared with the VOP cathode in the last chapter:a reversible capacity of 393 m Ah g^–1;at 100,200,500 and1000 m A g^–1,the capacity were 317.6,274.4,201.1 and 132.7m Ah g^–1,respectively;the retained capacity was 90.4 m Ah g^–1 after 1000 cycles at 1000 m A g^-1;a coulombic efficiency of 100%was achieved during cycling.The CTAB in the APC electrolyte can increase the capacity of TVO,but not the way that the irreversible intercalation of CTA⁺accelerates the kinetics like the last chapter because the interlayer spacing of TVO is large enough.The function of CTAB is to modify the coordination environment of Mg²⁺and control the intercalated Mg species(Mg²⁺and Mg Cl⁺)during charge-discharge processes.(4)Given the shortcoming of ions and molecules intercalation strategies,the Br dopping was adopted to introduce defects in V₂O₅,improving the electrochemical performance.Only 2.3 wt‰dopping of Br in VOBr can expand the interlayer spacing remarkably(12.91(?)),larger than the V₂O₅·n H₂O without bromine.Because of weaker electronegativity of Br to O and less charge of Br^-to O^2-,the substitution of O with Br will generate V vacancy defects and weaken the Van der Waals'force between the interlayer,leading to the increasing of interlayer spacing.The expanded interlayer can provide enough space for ions intercalation.And the V vacancies can be insertion sites of Mg²⁺.The interaction between Mg²⁺and Br is weaker than the interaction between Mg²⁺and O,resulting in the improved diffusion kinetics.Different from the last two chapters,the Mg²⁺de/intercalation reaction mechanism dominates the VOBr cathode in the APC-MTAC electrolyte system,which can be attributed to the influence of the polarity of the additive ions on the coordination environment of ions in APC electrolyte.The VOBr with little Br dopping exhibits much better electrochemical performance than the V₂O₅·n H₂O synthesized under the same condition without Br dopping.The capacity of VOBr were 394.0,287.1,239.7,184.2,and 132.9 m Ah g^–1 at a current density of 50,100,200,500,and 1000 m A g^–1,respectively.This aliovalent dopping strategy provides a new approach for the structure designing of the oxide-based cathode.