Design Of Vanadium-Based Cathodes For Aqueous Zinc-Ion Batteries And Their Energy Storage Mechanisms

Recently,aqueous batteries have attracted much attention,especially in the field of large-scale energy storage,because of their high safety,low cost,and environment friendliness.Among various aqueous batteries,zinc ion batteries are considered as one of the promising energy storage devices since the zinc as anode is cheap and abundant.However,aqueous zinc ion batteries are still in their infancy and there are still some issues.For example,cathode materials are limited due to the large ion radius,high mass,high charge and serious solvation effect of zinc ions.In addition,their energy storage mechanism is complex and unclear,and their electrochemical performance needs to be further improved.In order to solve the above problems,different layered vanadium-based compounds（Na V₃O₈?1.5H₂O,KV₃O₈?0.75H₂O,and VOPO₄）were developed to serve as the cathode materials of aqueous zinc ion batteries.Their electrochemical properties and energy storage mechanisms were systematically investigated by combining the optimization of electrolyte and the design of electrode structure.The main contents are listed as follows:1.NaV₃O₈?1.5H₂O nanobelts were prepared by a simple dissolution-recrystallization method and used as the cathode materials of aqueous zinc ion batteries.The unique structure of Na V₃O₈?1.5H₂O nanoribbons effectively shortens the ion transport path during the charging and discharging process.In addition,we optimize the electrolyte by using sodium sulfate as additive in zincsulfate electrolyte.The sodium sulfate additive can not only limit the dissolution of NaV₃O₈?1.5H₂O,but also restrain the growth of zinc dendrites.More importantly,different from the traditional Zn²⁺ion insertion/extraction mechanism,the Zn/Na V₃O₈?1.5H₂O battery system exhibits the continuous co-insertion/extraction of H⁺and Zn²⁺ions.Therefore,the aqueous Zn/Na V₃O₈?1.5H₂O battery system exhibits a high capacity up to 380 m Ah g^-1,high rate capability and excellent cycle stability(high capacity retention of 82%after 1000 cycles at a current density of 4 A g^-1).In addition,owing to the nanoribbon structure of Na V₃O₈?1.5H₂O,the corresponding electrodes can remain the structure integrity during the bending process.Thus,soft-packed batteries based on Na V₃O₈?1.5H₂O nanobelts display stable electrochemical performance under different bending states.2.In order to further improve the cycling stability of vanadium-based zinc ion batteries,we prepared KV₃O₈?0.75H₂O nanobelts by a simple hydrothermal method.KV₃O₈?0.75H₂O is more stable than the case of NaV₃O₈?1.5H₂O in aqueous electrolyte.And then,flexible and free-standing KV₃O₈?0.75H₂O/single-walled carbon nanotubes（SWCNTs）composite films were fabricated by a spray printing strategy.The three-dimensional conductive network of SWCNTs can not only promote the transfer of ions and electrons,but also ensure the intimate contact between KV₃O₈?0.75H₂O and conductive network.In addition,the dissolution of KV₃O₈?0.75H₂O in electrolyte is further suppressed by using high concentrated Zn（CF₃SO₃）₂ electrolyte.Moreover,the solvation effect of zinc ion is also limited by high concentrated Zn（CF₃SO₃）₂ electrolyte.Therefore,the Zn/KV₃O₈?0.75H₂O battery shows a high capacity of 379 m Ah g^-1 and excellent rate performance.More impressively,the Zn/KV₃O₈?0.75H₂O battery also displays high capacity retention rate of 91%even after 10000 cycles at a current density of 5 A g^-1.In addition,owing to the high conductivity and excellent flexibility of KV₃O₈?0.75H₂O/SWCNT composite films,flexible packaged batteries were assembled based on them.The soft-packed batteries possess stable electrochemical performance under bending and even folding states.3.Although above vanadium-based zinc ion batteries show excellent electrochemical performance,their average charge/discharge voltage is only～0.8 V,which limits their energy density and practical applications in the future.In order to improve the average charge/discharge voltage of vanadium-based compounds,layered VOPO₄ was prepared by a stirring-reflux method.Zn/VOPO₄ batteries were assembled based on VOPO₄ and 21M Li TFSI/1MZn（CF₃SO₃）₂ electrolyte.The dissolution of VOPO₄ and the corrosion of zinc anode were inhibited by the high concentrated 21M Li TFSI/1M Zn（CF₃SO₃）₂ electrolyte,which displays wide stable voltage window.Different ex-situ and in-situ characterizations confirm that the Zn/VOPO₄ battery not only exhibits the traditional redox mechanism of vanadium at low voltage range（0.8-1.8 V）,but also displays the redox process of oxygen at high voltage range（1.8-2.1 V）.This oxygen redox provides an additional specific capacity of 30 m Ah g^-1,which is 27%of that(109 m Ah g^-1)based on the vanadium redox.Furthermore,such oxygen redox increases the average charge/discharge voltage of Zn/VOPO₄ batteries from 1.47 to 1.56 V,which is about twice that of traditional vanadium-based zinc ion batteries.Therefore,the energy density of Zn/VOPO₄battery increases from 160 to 217 Wh kg^-1.In addition,the oxygen redox process also promotes the reversible crystal structure evolution of VOPO₄ during charge/discharge process.As a result,the Zn/VOPO₄ battery also shows excellent rate performance(a high capacity of 50 m Ah g^-1 at a high current density of 5 A g^-1)and excellent cycle performance(93%capacity retention after 1000 cycles at a current density of 1 A g^-1).