Structural Design And Performance Optimization Of Vanadium-Based Cathode Materials For Aqueous Zinc-ion Batteries

The energy crisis and environmental pollution have become a leading factor that restricts the process of globalization.A consensus has been reached by developing and taking full advantage of those sustainable and clean energy resources such as wind,solar,and tidal power,as well as establishing grid scale energy storage systems to protect the commn home of mankind.We have witnessed an enormous success in the applications of lithium-ion batteries that serve as a typical representative of electrochemical storage technique in the last few decades,which have completely changed our lifestyle by powering mobile and portable computers.However,scarce lithium and cobalt sources and increasing cost,as well as the severe security risks stemming from the flammable nature of the organic electrolytes,have become a sort of disadvantages that should not be neglected,making them uncompetitive to cope with the upcoming challenges for the demand of scalable energy devices especially the booming electric vehicle propulsion.Accordingly,advanced battery techniques with high safety,environmental benigh,low cost,and high performance have developed to be the subject of intensive investigations.Aqueous zinc-ion batteries(AZIBs)have drawn extensive attention in the last few years owing to their high safety and impressive electrochemical performance,benefiting from the low redox potential(-0.76 V vs.standard hydrogen electrode)and excellent electrochemical stability in water of Zn metal that make it become few of the candidates that could be able to directly serve as a safe anode in a metal-ion battery.In addition to a Zn metal anode,this technology is composed of aqueous electrolyte and a cathode material capable of reversible Zn2+insertion/extraction.Though massive efforts have been dedicated to the explorations of cathode materials,the scalable applications of AZIBs are still plagued by some intractable issues such as elusive charge storage mechanism,cathode dissolution,inadequate energy density and undesirable life span.The structural optimization and continuous exploition of cathode materials should be focused on since it can largely determine the specific capacity and energy density of the cell,particularly considering that the development of this technology is still in the incipient stage.Among various candidates,vanadium-based oxide cathodes deliver impressive capacities and relatively high cycle stability,but their development are still troubled in the limited energy density and cycle life.Herein,we focus on the exploration of novel vanadium-based cathode materials aimed at achieving high energy density and extended cycle life and provide important experimental assistance and theoretical basis by unraveling their respective charge storage mechanisms as well as the origins that cause the vanadium dissolution.The main contents are as follows:(1)Layered(NH4)2V6O16·1.5H2O single crystal nanobelts with V3O8-type construction stabled by hydrated ammonium ions were synthesized via a facile one-step hydrothermal method and served as a cathode material for AZIBs.In the constrction,the edge-sharing VO5 square pyramids and VO6 octahedral chains construct the V3O8 layers along the z axis by sharing their corners.NH4+ in the interstitial sites stabilize the layered structure by acting as the "pillars",thereby ensuring long-term cycling stability,while the structural water molecules facilitate the Zn2+diffusion kinetics by serving as the charge screening.The assembled Zn cell delivers a superior reversible specific capacity of 479.4 mA h g-1 at 0.1 A g-1 with a desirable energy density of 371.5 W h kg 1 and exhibits an impressive cycling stability of more than 3000 cycles(152.1 mA h g-1 retained after 3000 cycles at 5 A g-1).The electrochemical kinetics analyses reveal that this active material exhibits large pseudocapacitance charge storage behavior and Zn2+solid-state diffusion coefficient because of the fast ion migration and electron transfer provided by the expanded interlayer distance and nanobelt morphology.The investigation of charge storage mechanism reveals that the electrode exhibits highly reversible H+/Zn2+co-insertion behavior,companied by formation and disapprearance of a new phase Zn3(OH)2V2O7·2H2O during the discharge and charge process.(2)Based on the above results,I wonder whether the morphology and crystal structures of vanadium-based oxide cathode materials have a large effect on the electrochemical performance and Zn-ion strorage mechanism.Herein,a layer-by-layer stacked(NH4)2V4O9·0.5H2O nanosheet assemblies with V4O9-type construction stabled by hydrated ammonium ions were engineered and prepared by a mild hydrothermal protocol.A considerable interlayer spacing of 8.98 A favors the ingress/egress of Zn2+and the pillared hydrate NH4+stabilizes the layered structure.Stacked nanosheets provide abundant active sites,which enable them to deliver high rate capability(101 mA h g-1 at 15 A g-1)and long-term stability(89%and 84%capacity retention after 400 cycles at 1 A g-1 and 1000 cycles at 5 A g-1,respectively),befeniting from dominated pseudocapacitance charge storage and facialiated redox reaction kinetics.The investigation of charge storage mechanism reveals that the electrode exhibits highly reversible H+/Zn2+co-insertion behavior companied by formation and disapprearance of a new phase Zn4SO4(OH)6·xH2O during the discharge and charge process.In addition,the Zn cell could power a high energy density of 309 Wh kg-1 and a power density of 9324 W kg-1,pushing the potential in practical application to a high level.(3)The vanadium dissolution in vanadium-based compounds when subjected to an aqueous electrolyte environment not only reduces the utilization of active materials but also lowers the cycle stability.To address this issue,we reported several barium vanadate nanobelt cathodes constructed of two sorts of architectures,i.e.,Ba1.2V6O16·3H2O and BaV6O16·3H2O(V3O8-type)and BaxV2O5·nH2O(V2O5-type),which are controllably synthesized by tuning the amount of barium precursor.The three active materials efficiently achieve reversible zinc storage but yet exhibit remarkable differences in performance.The Ba1.2V6O16·3H2O electrode exhibits deliver superior rate capability(108.8 mAh g-1 at 10 A g-1)and long-term cyclability(95.6%capacity retention over 2000 cycles)as a typical consequence of fast and stable zinc-ion kinetics provided by the intrinsically roust layered architecture,which could efficiently suppress the cathode dissolution as well as greatly eliminate the generation of byproduct Zn4SO4(OH)6·xH2O during cycling.This work reveals the difference between the vanadium oxides constructed with different crystal structures and the vanadium dissolution behaviors in AZIBs.(4)According to previous results,another vanadium-based oxide cathode based on MgV2O6·1.7H2O nanobelts with V2O6-type and intrinsically robust constructions,which delivers a high capacity(425.7 mAh g-1 at 0.2 A g-1),a robust rate capability(182.1 mAh g-1 at 10 A g-1),and an ultra-stable cycle up to 1500 cycles without any visiable deterioration,as well as an adequate energy density(331.6 Wh kg-1),was developed.Such excellent electrochemical Zn-ion storage performance is believed to result from the fast ion migration and electron transfer boosted by a stable layered structure and an ultra-high intercalation pseudocapacitance reaction(88.4%capacity contribution ratio at 0.6 mV s-1),which are also benefited by a typical H+/Zn2+co-insertion mechanism,accompanied by an atypical Zn2+ intercalation chemistry with a partial but irreversible Mg2+-Zn2+ion-exchange reaction during the initial discharge.DFT calculations unravel that a much lower Zn2+diffusion energy barrier could be realized in the MgV2O6·1.7H2O crystal structure,thereby leading to the fast reaction kinetics.This work could provide new insights to better understand the charge storage mechanism in vanadium-based oxide cathodes.