| As the contradiction between the shortage of fossil fuels and the increase of energy demand intensifies,renewable energy sources such as wind,solar,and tidal energy account for an increasing proportion of all energy sources.Electrochemical energy storage devices provide a feasible approach to achieve the reliability and efficient utilization of the renewable energy sources.Lithium-ion batteries have become the mainstream of the current energy storage market due to their high energy density.However,the limited reserves of lithium resources and the safety,environmental and cost issues brought by organic electrolytes have brought great pressure on its large-scale applications.In contrast,aqueous rechargeable batteries have the advantages of low cost,high ionic conductivity and high safety.The high safety and environmental friendly were endowed by the high flame retardancy of water.Moreover,the conductivity of aqueous electrolyte is great higher than that of organic system,which is meaning a better rate performance.Therefore,lead-acid batteries,the one which have more than 160 years history,is still occupy a great share of the current markets.However,lead-acid batteries have some weakness,such as poor cycling stability,short service life and high environmental toxicity.Lead-free water secondary battery is regarded as one of the ideal energy storage batteries.Aquous rechargeable batteries based on alkali metal ions(Li+,Na+,K+),alkaline earth metal ions(Mg2+,Ca2+)and Zn2+are the potential candidates for it.However,its ionic radius is too large,which seriously affects its rate performance and specific capacity.Hydrogen possess higher natural abundance(～1400ppm)than lithium(～20 ppm)which is only lower than oxygen and silicon in terms of atomic percentage.The ubiquitous of H given the proton batteries with the advantages of high sustainability and low cost.The smallest size of proton is not only conducive to the de/insertion of protons in the host materials,but also ensure a better cycling life in virture of samller volumetric changes during de/insertion.In addition,the proton has the smallest molar mass,which gives it a higher capacity at the same mass of electrode materials.It is worth noting that protons could be transported in a hopping Grotthus mechanisim in aqueous solutions or some electrode materials,which makes protons have high ionic conductivity in aqueous electrolytes.Thus,it is predictable that the aqueous proton batteries possessing high specific capacity,excellent rate performance and power density.The main contents of this thesis include:(1)MoO3,as one of the promising anode materials in rechargeable proton batteries,suffers from the severe dissolution in acidic electrolytes upon cycling.Here,an ultrathin TiO2 shell is coated on MoO3 nanorods to suppress the detrimental dissolution during cycles.TiO2 also lowers the desolvation energy of hydrated protons,promoting the reaction kinetics.Therefore,MoO3@TiO2 displays outstanding electrochemical performances,especially at high rates(171.0 mAh g-1 at 30 A g-1)and at high mass loadings(17 mAh cm2 at 104 mg cm-2).The full cells constructed with MnO2 deliver an energy density up to 252.9 Wh kg-1 and a power density of 18.3 kW kg-1.Ex-situ XRD and XPS indicate that protons shuttle back and forth between different monoclinic phases.The results offer a simple way to achieve the high performances of MoO3 in a diluted acidic solution.(2)Low temperature battery is an essential power supply in special application scenarios such as extreme weather and aerospace.Most of current commercial batteries could not operate smoothly under such low temperature conditions.Protons,which can be transported rapidly through the Grotthus mechanisim,are the ideal charge carriers of low-temperature batteries.Here,the addition of ethylene glycol(EG)to the electrolyte can destroy the hydrogen-bond network between H2O molecules and lower the freezing point of the electrolyte.In addition,EG will be adsorbed to the surface of MoO3 nanorods,which hinders the erosion of the electrode material and inhibits the dissolution of MoO3.Benefited from these profits,MoO3 nanorods exhibited excellent electrochemical performance.They show a capacity rentention of 96.9%after 2000 cycles at a current density of 10 A g-1.After assembled with CuHCF,the full cell displays an excellent electrochemical stability at-40℃,i.e.negligible capacity loss after more than 1500 times at 1 A g-1.These results indicate that the mixed solution of EG and H2O shows the promising potential in aqueous batteries.(3)Manganese dioxide(MnO2)draw a widely attention because of its environmental friendliness,low cost,high natural abundance,high theoretical specific capacity and high reaction potential.Its ultra-high specific capacity in the acid electrolyte is rooting from the plating/stripping of Mn2+/MnO2 conversion reaction,from which the electrode potential could be reached 1.229 V with excellent rate performance.However,in the above work,we found that the performance of MoO3//MnO2 full cell was greatly affected by the Mn2+/MnO2 reaction,and we have observed the abnormal color of electrolyte(1.0 M H2SO4+1.0 M MnSO4),where a side reaction occurs during charge/discharge process.Therefore,we designed to investigate thc influence of different concentrations of sulfuric acid and Mn2+ ions on the plating/stripping reactions of Mn2+/MnO2 conversion reaction.We found a side reaction from Mn2+ to Mn3+ occur during the Mn2+/MnO2 reaction.And it is will aggravate the side reaction when increase the concentration of H2SO4.The color of electrolyte gradually changes from nearly colorless(Mn2+)to dark red Mn3+ after cycling,with the increase the concentration of H2SO4,the intensity of Mn3+ absorption peak in UV-VIS absorption spectrum increases significantly as well.The results of Fe3+ titration reveal that the concentration of Mn3+reached 12.2 mM after 100 cycles.In addition,we further study the influence of MnSO4 concentrations in the electrolyte on the Mn2+/MnO2 reaction.It was found that the concentration of MnSO4 not only affected the side reaction of Mn2+to Mn3+,but also hider the dissolution and stripping of MnO2 from the carbon cloth.Therefore,we designed a dilute acid electrolyte(0.5 M Na2SO4+0.1M MnSO4+0.1 M H2SO4)to achieve over 2500 h stable reversible plating/stripping of Mn2+/MnO2 with excellent rate performance.In addition,it is assembled with anthraquinone(loading on activated carbon(AC@AQ))to obtained AQ@AC//MnO2 full battery,in which a voltage platform is about 1.2 V,and can stably cycle for more than 2000 cycles. |
