Study On The Nano-Manganse Dioxide Cathode In Nonaqucous Lithium-Oxygen Secondary Battery

This thesis focuses on the problems of lithium-oxygen battery that low specific power, huge gap between charge and discharge voltage and poor cycling performance.Dual-function catalyst of the MnO2was used in the cathode material. High specific power and energy lithium secondary battery nano electrode material preparation was chosen to studied.Sol-gel template synthesis method was used to produce the lithium titanate nanoarray electrode materials, atmospheric water solution precipitation was used to synthesize nanometer MnO2material. Take using nano-MnO2bifunctional catalyst for the cathode materials, making oxygen diffusion cathode piece, choosing nonaqueous electrolyte and other optimization conditions were used to investigate the nano-MnO2cathode performance in nonaqueous lithium-oxygen secondary battery.First, AAO film in this paper was prepared by electrochemical anodic oxide method; Li4Ti5O12nanoarrays were prepared with the method of Sol-filling AAO template. The morphology and structure of Li4Ti5O12nanoarrays were characterized by SEM, EDS et al. The experimental results show that spinel Li4Ti5O12nanoarrays with mean diameter of70nm were synthesized by immersing the porous AAO in sol of0.8mol/L under-0.1MP and drying at80℃then roasting at900℃for20h in the air after repeating steps above for four times.KMnO4and MnSO4were used to produce nano-MnO2through Chemical precipitation. Combined with the characterization methods such as XRD, SEM and BET to discover the influence of reactants molar ratio, reaction temperature and reaction time to produce manganese dioxide crystal form and morphology. Different crystal structure and morphology of nanoscale MnO2particles were produced by controlling appropriate reactant molar ratio, reaction temperature and reaction time. The reactant ratio was nKMnO4:nMnSO4=2:3, and reaction temperature control90℃for6h. A kind of diameter of about30nm, length about1000nm, specific surface area of77.241m2/g nano linear α-MnO2was obtained; The reactant ratio was nKMnO4:nMnso4=2:1.5, and reaction temperature controlled80℃for4h. A kind of diameter of about300nm, specific surface area of77.474m2/g nano layered δ-MnO2was obtained; The reactant ratio was nKMnO4:nMnSO4=2:12, and reaction temperature controlled80℃for4h. A kind of diameter of about100nm, specific surface area of33.303m2/g nano layered γ-MnO2was obtained.Rotating disk glassy carbon electrode as the working electrode, a three-electrode system was used to research catalytic oxygen reduction and oxidation of lithium oxide electrochemical performance of nano-α,γ,δ-MnO2in the three kinds of nonaqueous electrolyte:LiPF6(1mol/L)+EC/DEC/DMC (Vol.1:1:1)、LiPF6(1moI/L)+PC/DME(W.1:1) and LiPF6(1moI/L)+PC/DME (W.1:2). Al2O3suspension was used fully wipe the surface before the electrode using. lOmg MnO2particles weighted, plus5mL ultra-pure water were mixed,homogenized and dispersed for10minutes by ultrasonic. Pipetted pipette10.0μL of suspended droplets applied to the clean surface of the disc electrode, the temperature control105℃drying in a nitrogen atmosphere. Pipetted10.1μL concentration of0.2750g·L-1of Nafion (5wt.%) emulsion, applied it to the catalyst film layer and dried at105℃,there got the electrochemical experiments working electrode. Test the system of the three-electrode assembly in a vacuum glove box of the argon atmosphere. Glassy carbon electrode as the working electrode, the counter electrode was a platinum wire, a Ag/AgCl electrode as reference electrode, and controlled the scan speed of100mV/s, the scan range of-2to2V (vs. Ag/AgCl). Control testing system temperature of25℃, in order to maintain the electrolyte a certain concentration of oxygen, the oxygen was continuously to the electrolyte solution one hour before measurement. Experimental results showed that the order of nano-α,γ,δ-MnO2catalytic oxygen reduction peak current density in the three kinds of non-aqueous electrolyte. The former two kinds of nonaqueous electrolyte solution of the α-MnO2catalytic oxygen reduction peak current densities were-8.715、-18.54mA·mg-1, Two reduction peak appeared in the third electrolyte, a wide oxygen reduction peak formed at OV when the current density was-16.79mA·mg-1, and a reduction peak appeared at-0.71V when the current density was-21.71mA·mg-1. In the same test method, adding an excess of lithium peroxide obtained in the above three electrolyte saturated non-aqueous electrolyte lithium peroxide to test the catalytic oxidation electrochemical properties of α,γ,δ-MnO2to the lithium oxide. Experimental results showed that δ-MnO2had an obvious oxidation peak in the LiPF6(1mol/L)+EC/DEC/DMC(Vol.1:1:1) the peak current density was6.569mA·mg-1, no α-MnO2and γ-MnO2; γ-MnO2, α-MnO2and δ-MnO2oxidation peak were strong in the LiPF6(1mol/L)+PC/DME(W.1:1) electrolyte, The peak current density were27.93,24.44,19.13mA·mg-1; δ-MnO2, γ-MnO2and α-MnO2oxidation peak current density were33.712,32.075,1.272mA·mg-1in LiPF6(1mol/L)+PC/DME (W.1:2) electrolyte. Above all,α-MnO2and δ-MnO2had strong catalytic on oxygen reduction and lithium peroxide oxidation in LiPF6(1mol/L)+PC/DME (W.1:2) electrolyte. Therefore, in this paper LiPF6(1mol/L)+PC/DME (W.1:2)was selected as the electrolyte to study the lithium oxygen battery performance, α-MnO2+δ-Mn02(w.1:1) mixed crystal was chosen as cathode and the oxygen reduction and lithium peroxide oxidation of the dual-function catalyst.Weighed0.35g ketjen carbon black and0.15g α,δ-MnO2,15g poly vinylidene fluoride vinyl plastic(0.056g PVDF powder+15gNMP+10ml isopropanol), poured respectively into50ml beakers and sufficiently stirred and mixed to produce the slurry cathode bearing-material. Took two sets of diameter2.5cm nickel foam substrates to fill with bearing-material by15minutes sonication. After that took the substrates out and clear the excess pulp state cathode material, dried at80℃for2hours; Repeat the above steps once. One group suppressed under3MPa pressure, then put it into the oven for2hours thermostat heated at175℃,there got the test porous cathode. Weighed electrodes quality and calculated the quality of the cathode unit area or volume contained carbon/MnO2.With lithium foil anode, LiPF6(lmol/L)+PC/DME(w.1:2) electrolyte, Celgard2500polymer membrane for diaphragm and preparation of the cathode load cell mould constructed non water lithium-oxygen battery. In25℃-65℃condition, With pure oxygen at atmospheric pressure as cathode reactant tested lithium-oxygen battery first discharge and recharge cycles behavior studied the electrochemical performance.Studied temperature on lithium oxygen battery first discharge capacity at low current density (0.1mA/cm2). The first time constant current discharge performance results showed the battery capacity increased with testing temperature (25℃-65℃) increased, when the temperature reached to50℃the battery capacity grow up to maximum of3870mAh·g-1, However, when the temperature continue rising, the battery capacity declined, the reason for this is electrolyte unstable. In addition, the battery had the high discharge platform (2.81V) and low charging voltage platform (4.22V) in50℃; with the optimum conditions prepared cathode and assembled battery, In50℃, with the constant current density of0.1mA·cm-2, the lithium battery charging and discharging cycle to oxygen performance test results show that completed five times more complete cycle process.The first discharge and circulation charge and discharge experiments were conducted and discharge termination cathode was characterized by SEM and XRD. The results showed that:Test temperatures had a great influence on first discharging capacity and lithium-oxygen cell electrochemical performance;(3) Carbonic acid esters LiPF6nonaqueous electrolyte used in lithium-oxygen battery system existed two defects. One was after the charge and discharge termination electrolyte in battery anode zone had withered evidences, which showed that the solvent volatile was larger at50℃and the electrolyte loss was serious in the charge and discharge process. So the transfer channels of lithium ion between cathode and anode were lost, which was the main reason of charging and discharging cycle termination of lithium oxygen batteries. The another was electrolyte property was not stable and solvent and solute participated in reactions in cathode, which affected the formation of pure lithium peroxide in the process of the cathode discharge reaction. As a result, complex lithium compound formed among cathode discharge sediment, which was the key factor that influenced the charge and discharge process and cycle performance.In short, in nonaqueous lithium oxygen electrochemical system, temperature which had great influence on cathode catalyst activity and the stability of the electrolyte, was one of the key factors that affected the lithium oxygen cell electrochemical performance. The composition of electrolyte and stability of lithium oxygen system were key factors that influenced charging and discharging electrochemical reaction and cycle numbers, and also determined the battery specific capacity and charging and discharging current density. We believe that the primary task of the future research for lithium oxygen batteries and improving the electrochemical performance is to screen the electrolyte with more stable physical and chemical properties.