Mesoscale Simulation On Mass Transfer And Discharge Progress In Cathode Of Non-Aqueous Lithium Air Batteries

As a new type of rechargeable energy storage system,the theoretical discharge specific capacity of non-aqueous lithium-air batteries is very large,with a value about 10 times that of lithium-ion batteries.It has promising potential for application in electric vehicles,aerospace and other fields.However,its current cyclic and discharge performance are not available for the practical applications.One of the key reasons for their poor discharge performance is that the unreasonable microstructure of the cathode makes it difficult for oxygen as a reactant to diffuse to the depth of the cathode.To improve the actual discharge performance of non-aqueous lithium-air batteries,numerical simulations of the mass transport and discharge processes are conducted based on geometric models of materials with different scales used in the cathode,and the mechanism of mass transport and termination of the discharge process is analyzed.An improved partial bounce-back scheme applied to the lattice Boltzmann method（LBM）is proposed in this work based on the characteristics of the multi-scale structure of the cathode geometric model and the changes in the local porosity of the electrode during the discharge process.To reduce the difficulty of O₂ diffusion to seperator side of the cathode,a new cathode structure with microchannels is proposed,and its feasibility is verified through the analysis of its transport characteristics and discharge performance.The research content in this work can be divided into the following aspects:（1）Geometric Reconstruction and mass transport in carbon paperThe three-dimensional（3D）geometric models of carbon paper are reconstructed by X-ray tomography and three numerical methods A,B and C.The mass transport in carbon paper is simulated using LBM model.The effects of carbon fiber diameter,porosity,reconstruction method,and carbon fibers’orientation on the effective diffusion coefficient in different directions and anisotropy coefficient of carbon paper are studied.dφ=60° is considered the best reconstruction parameter considering the effective diffusion coefficient;the power exponents a of the Bruggeman correction applied to carbon paper with different microstructures are calculated;the values of α applied to perpendicular to the surface of the carbon paper using numerical methods A,B（dφ=0°）,B（dφ=1 80°）,C（dφ=0°）,and C（dφ=180°）are 1.997,2.389,1.990,2.374,and 2.101;the values of a applied to parallel to the surface of the carbon paper using numerical methods A,B（dφ=0°）,B（dφ=180°）,C（dφ=0°）,and C（dφ=180°）are 1.767,1.590,1.814,1.592.and 1.761.（2）Geometric reconstruction and reaction mass transfer characteristics of porous electrodeThe 3D geometric model of the porous electrode containing carbon particles（solid phase）,electrolyte（liquid phase）and O₂（gas phase）is reconstructed.A multitime-scale coupled model is established to simulate the multi-phase mass transport process within a porous electrode under non-discharge state,and the discharge process is simulated by coupling the electrochemical reaction model at the solidliquid interface.The influence of electrode morphology on the effective diffusion coefficient and electrode discharge utilization are investigated.The results indicate that the the range of power exponent α of the Bruggeman correction of porous electrodes with a secondary carbon particle range of 149～246 nm and a porosity range of 0.6～0.8 is 1.475～1.943;within the scope of this study,an increase in porosity and secondary carbon particle diameter,as well as a moderate decrease in electrolyte saturation increases the effective diffusion coefficient of the porous electrode,increase the product volume deep in the porous electrode,and volume of the discharge product in the depth of the porous electrode,and thereby improving discharge utilization of the electrode.（3）Mass transport in cathode of non-aqueous lithium air batteries under the non-discharge stateThe 3D geometric model of the mesoscale cathode including carbon paper（solid phase）,electrode material（porous media phase）and electrolyte（liquid phase）is reconstructed,and a LBM model with improved PBB boundary conditions is proposed to simulate the oxygen diffusion in the cathode under non-discharge state,and the influence of the volume distribution of porous electrode on the mass transport characteristics is analyzed.The simulation results indicate that the changes in the interface area between the electrode and electrolyte within the cathode and the total volume of electrode material are key factors affecting diffusion resistance;the increase in the dispersion of porous electrode,local electrode load,and total electrode load in the cathode lead to a decrease in the effective diffusion coefficient of the cathode;the local electrode load only affects the O₂ concentration distribution in this region and in the downstream region of the diffusion direction,but has no effect on the oxygen concentration distribution in the upstream region of the diffusion direction.（4）Discharge process in cathode of non-aqueous lithium air batteriesA multi-physic coupled LBM model is proposed to simulate the galvanostatic discharge process of a non-aqueous lithium-air battery based on the mesoscale cathode geometric model,and the effects of the spatial distribution of porous electrode and some electrolyte parameters on the discharge performance are studied.The results indicate that the discharge performance of the battery is improved by increasing in the degree of electrode dispersion,the local diffusion coefficient and the solubility of oxygen in the electrolyte;as the total electrode load increases,the discharge specific capacity decreases,and the discharge capacity first increases and then decreases,the maximum discharge capacity is obtained when the total electrode load is 2.669 mg cm^-2;the discharge capacity is almost unaffected by the local electrode load at the depth of the cathode,and the better discharge performance is achieved by adjusting the local electrode volume fraction on the O₂ inlet side between 0.4 and 0.5;for the high cathode porosity batteries,appropriately increasing V_np/V_mp increases the discharge performance,but excessive V_np/V_mp will damage the discharge capacity.（5）Construction of microchannels in the cathodeA novel cathode structure with microchannels constructed inside is proposed,and the galvanostatic discharge process and the O₂ diffusion process under nondischarge condition are numerically simulated.The effects of the number,radius,and distribution of microchannels on the mass transport characteristics and discharge performance are studied.The simulation results indicate that the construction of microchannels effectively reduces the O₂ diffusion resistance within the cathode and increases the O₂ concentration in the cathode during the discharge process;moreover,the increase in the O₂ concentration on the seperator side is much greater than that on the O₂ inlet side;within the scope of this study,the increase in the number and radius of microchannels improves the discharge performance;when the total cross-area of microchannels is fixed and the number of microchannels is 5,the maximum increase in discharge capacity and specific capacity is obtained with values of 6.336%and 14.234%.