Oxygen Defects Modulation Of MnO₂ And Electrolyte Optimization For Zn Metal Batteries

Large-scale electrical energy storage（EES）technology with high-safety,low-cost and high-stability determines the future energy structural adjustment and smart grid construction.Aqueous rechargeable Zn batteries（ARZBs）would be an ideal candidate for EES device because of their intrinsic high energy,environment-friendly and cost-effective properties.However,up to now,high-voltage and stable cathodes are still in exploration stage due to the sluggish reaction kinetics,low capacity and poor cycling performance.Furthermore,there are still some challenges（e.g.water decomposition,corrosion and Zn dendrites）on electrolyte and anode sides because of the usage of weak acidic aqueous system.Therefore,electrolyte optimization is also in need for high-performance Zn battery.Take the above issues into consideration,defective MnO₂cathodes with different crystal structures are prepared,and the corresponding electrochemical performances and reaction mechanisms are studied in this work.Besides,eutectic electrolyte and aqueous/organic electrolytes are used to replace common aqueous system to solve the severe side reactions at electrolyte/anode interface.The electrochemical behaviors of Zn metal in these electrolyte systems are also investigated systematically.The main contents are in following:（1）β-MnO₂ nanorod with rich oxygen defects（D-β-MnO₂）is prepared through hydrothermal-calcination-reduction method.Firstly,the theoretical calculation results indicate that the H⁺is more likely than Zn²⁺to react with D-β-MnO₂because of its small radius,low charge and low atomic weight.Secondly,the binding energy of H⁺with D-β-MnO₂would be further reduced and a conversion reaction process would be speeded compared with perfect structuredβ-MnO₂.Various electrochemical tests confirm that D-β-MnO₂electrode performs more active with higher discharge capacity(300 m A h g^-1),faster reaction kinetics and more stable cycling performance（300 cycles）than that of perfect structuredβ-MnO₂.Further increasing the mass loading to 3 mg cm^-2,D-β-MnO₂electrode could deliver a capacity of 270 m A h g^-1.（2）λ-MnO₂ active material with spinel structure is prepared through high-temperature solid phase reaction.The contents of oxygen defects inλ-MnO₂are controlled through modulating different reaction times（12,24,36 hours）.Experimental results suggest that the contents of oxygen defects are not proportional to reaction times.Oxygen atoms would refill into the vacancy area inλ-MnO₂and structural collapse would take place when the reaction time reaches to 36 h.Finally,theλ-MnO₂electrode obtained after a 24-hour-reaction behaves best with the highest discharge capacity(280 m A h g^-1)and the most stable cycling performance（capacity retention of 80%after 500 cycles）.This work confirms that the initial spinel phase has transformed to layered structure in initial charging/discharging process,accompanying with Zn²⁺insertion/de-insertion in following cycles.（3）According to the Lewis acid-base theory and the Lewis acid-base reaction among zinc perchlorate（Zn（Cl O₄）₂.6H₂O）,methylsulfonylmethane（MSM）and water,a new kind of hydrated eutectic electrolyte（HEE）is prepared.Compared with the common aqueous electrolyte,the content of the free water molecules in HEE is decreased significantly.Furthermore,the Zn²⁺coordinates with H₂O,Cl O₄^-and MSM in primary shell forming Zn（MSM）_x（H₂O）_y（Cl O₄）_zcomplexes in HEE system.Because of the less free water and unique solvation structure of Zn²⁺in the HEE system,the dendrite growth,water reduction and interface side reactions are significantly suppressed.Zn//Zn symmetrical cell in this system shows an ultra-stable Zn platting/stripping process（2000 hours）.Zn//NVO batteries could deliver a high discharge capacity(350 m A h g^-1)and stable cycling performance(The capacity retention reached to 91%after 600 cycles at 500 m A g^-1).（4）Based on the Zn salt of Zn（Cl O₄）₂.6H₂O,a kind of concentrated aqueous/organic hybrid electrolyte is prepared.Raman,Infrared spectroscopy tests and electrochemical results demonstrate that the contents and activity of the free water molecules in the concentrated aqueous system are obviously decreased through increasing the concentration of Zn salt.However,the corrosive property of concentrated Cl O₄^-anion in aqueous solution leads to the bad cycling stability and the battery failure.Introducing the organic solvent（acetonitrile,AN）into the concentrated system further to prepare hybrid electrolyte.The Zn//Zn cell using this electrolyte could cycle for 2900 hours when the volume ratio between AN and water is 1:3,with the concentration of Zn（Cl O₄）₂.6H₂O of 5 M.The Coulombic Efficiency of Zn//Cu cell reaches to 99.2%and keeps it for 350 cycles.Molecular Dynamic（MD）results further confirm that the AN molecule could enter the solvation shell of Zn²⁺,which would contribute to the homogeneous Zn deposition and reversible stripping/platting procedures.There are 82 pictures,4 tables and 194 references...