Preparation And Performance Of Three-dimensional Carbon Cathode For Lithium-oxygen Batteries

Recently,the research and development of electrochemical power sources mainly focus on the next generation lithium batteries,because of the demand of portable electronics,electric vehicles?EVs?and smart grids for power sources with high energy densities and power densities.As a potential power source for future EVs,lithium-oxygen batteries?LOBs?have attracted extensive attention due to its ultrahigh energy density.However,the practical applications of LOBs are still hindered by several critical issues,including large polarization,low round-trip efficiency,poor rate capability and short cycle life.On one hand,these issues come from the sluggish kinetics of oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?.On the other hand,the insoluble discharge products Li2O2 will deposit inside the pores of cathode,which block the transport of electrolyte,oxygen and electrons and consequently result in larger polarization.Therefore,for enhancing the performance of oxygen electrodes,it is very important to rationally design the pore structures of materials and electrodes for maintaining the transport of ions and electrons in the solid/electrolyte/gas tri-phase region.In addition,rationally designed electrode structures are also important for the loading and utilization of catalysts.To this end,this work prepares porous carbon by scarified hard template,and prepares porous carbon electrode by self-assembling method,because of the excellent conductivity,low cost and abundant resource of carbon materials.The influences of the pore structures of carbon materials and electrodes and the catalysts on the performance of oxygen electrode are systematically studied.The main contents and results are as follows:?1?A series of porous carbon microspheres marked as CF-100,CF-80 and CF-50are prepared by the polydopamine coating with different thicknesses on ZnO microspheres template assembled with distorted nanosheets followed by the heat treatment at 800^oC under nitrogen gas.The SEM and TEM analyses show that the carbon microspheres?CF?inherit the morphology of ZnO microspheres template to form a hollow structure.The N₂ adsorption-desorption isotherm confirms the hollow structure.Among all the three samples,CF-50 shows the highest specific area of1007.8 m²·g^-1 and the highest pore volume of 2.66 cm³·g^-1,which are much higher than the commercially available Super P?SP?with the specific area of 60.0m²·g^-1and the pore volume of 0.12 cm³·g^-1.The XPS investigations indicate the N doping from dopamine precursors in the prepared carbon microspheres.When used as materials for oxygen electrodes,the charge overpotentials of CF are 900 mV lower than that of SP.CF-50 shows the highest capacity of 9154 mA·h·g^-1 at a current density of 100mA·g^-1 among all the three samples.The N doping provide the catalytic performance of CF,and the high specific area and appropriate pore structure of CF provide the channel for gas and electrolyte transport and the accommodation for discharge product Li2O2.This work propose a new method for the preparation of three-dimensional porous carbon based on the nanostructure of transition metal oxides.?2?Three-dimensional porous graphene electrodes?GMS?are prepared by a self-assembling method on a carbon paper with F127(EO₁₀₆PO₇₀EO₁₀₆)as a soft template and hydrazine hydrate as reduction reagent.When used as the oxygen electrode for LOBs,GMS shows a high specific capacity up to 10200 mA·h·g^-1 at a current density of 200 mA·g^-1.MnO₂ and RuO₂ catalysts are loaded on the prepared GMS by electrodeposition and impregnation methods respectively.The obtained electrodes are denoted as MnO2@GMS and RuO2@GMS.Electrochemical performance studies show that the charge plateau of MnO₂@GMS?3.99 V?is 70 mV lower than that of GMS?4.06 V?,and the charge plateau of RuO₂@GMS?3.62 V?is440 mV lower than that of GMS?4.06 V?.In addition,RuO₂@GMS show the best cycling performance,and can work for 50 cycles at the current density of 200 mA·g^-1and the curtailed capacity of 1000 mA·h·g^-1.This part of work propose a method to prepare binder-free three-dimensional porous carbon electrode,which could be used as oxygen electrode for LOBs after the loading of catalyst.