Modeling Of The Discharge Process For The Positive Electrode Of Lithium-Sulfur Batteries

High energy density secondary batteries are of great significance for energy storage,transportation tools,mobile electronic devices,etc.And it is also an important part of the current strategic goal of carbon peaking and carbon neutrality.In the case of lithium-ion batteries,the room for energy density improvement is extremely limited,and the development of next-generation high energy density secondary batteries is imminent.Lithium-sulfur batteries using lithium metal negative electrodes and inexpensive sulfur with high theoretical specific capacity as the positive electrode are expected.The commerical application of lithium-sulfur batteries is limited by the low effective loading of cathode sulfur,large amount of electrolyte,poor power performance,polysulfide shuttle,charge and discharge volume changes,lithium dendrites and other problems.Except for lithium dendrites,it is a common problem in lithium batteries,the rest of the problems are related to the multi-step electrochemical reaction process of lithium and sulfur on the sulfur cathode.Otherwise,researchers have also conducted experimental studies from various aspects such as electrode materials,electrolyte,electrocatalyst and electrode construction,and have made a lot of useful progress.Due to the complexity of the reaction system of lithium-sulfur batteries,many influencing factors are intertwined to affect the battery performance,so it is necessary to model the macroscopic electrochemical reaction process in lithium-sulfur batteries on the basis of experimental research,quantitatively analyze the influence of kinetics,transfer and electrode structure on battery performance,and provide the necessary theoretical support for the corresponding experiments and provide theoretical support for electrode optimization and design.Based on the idea of Newman’s porous electrode model,combined with the typical characteristics of porous sulfur cathode,a one-dimensional quasi homogeneous model which can describe the discharge process of lithium-sulfur batteries and the influence of various parameters is established through reasonable simplification.The model is composed of electrode reaction kinetics,mass balance equation of each component participating in the battery reaction,charge balance equation and model parameter correlation equation.The relevant reaction kinetic parameters necessary for model calculation,the transfer property parameters and structural parameters are summarized.Finally the model is solved.The following conclusions are obtained:(1)A comparison of the literature experimental data and the constant-current discharge curves calculated by the present model shows that the simulation results can basically represent the trends and key features of the experimental discharge process,which verifies the validity of the model.(2)The simulation results show that the reaction of the first discharge platform is dominated by reaction S8(s)→S8(l)→S82-→S62-,and the reaction process of the first and second discharge platform conversion section is dominated by reduction reaction S82-→S62-→S42-,and the reaction S42-→S22-→S2process occurs on the second discharge platform.(3)The influence law of kinetic parameters is that increasing the lithium ion exchange current density is beneficial to improve performance,taking into account the lithium dendrite problem,the practical battery needs to be combined with performance and safety appropriate tries to control i0,Li;for the multi-step electrochemical sulfur reaction process(?)of sulfur on the cathode,the exchange current density i0,1 needs to be maintained at a high level,i0.2 and i0.3 is not the control step of the multi-step electrochemical reduction reaction,it needs to be combined with shuttle effect requirements to control.Increasingi0.4 and i0.5 is beneficial to improve the battery specific capacity performance and power performance,it needs to consider the introduction of electrocatalyst to improve the reaction rate.(4)For the dissolution process S8(s)→S8(l),the larger dissolution rate is conducive to the formation of a higher first discharge platform.When the dissolution rate is lower,the discharge potential will continue to fall until reach the second discharge platform especially in the larger discharge current density,the discharge potential will quickly fall to the cut-off voltage,resulting in the cut-off discharge when most of the sulfur is not used;Li2S2 and Li2S precipitation reaction rate increasing on the battery specific capacity is unfavorable.(5)Higher lithium ion diffusion coefficient is extremely critical to improve the battery discharge performance;the diffusion coefficient of each polysulfide ion does not have a significant effect on the battery discharge performance,but considering the need to control the shuttle effect in the actual battery,the diffusion coefficient of polysulfide ions can be controlled at a lower level in combination with the electrolyte selection;consideration needs to be given to improving the cathode solid phase conductivity,active specific surface area and pore volume through electrode material selection and electrode construction optimization.The results of this model study can help deepen the understanding of the process of multi-step sulfur electrochemical reduction reaction in batteries,and provide some guidance for lithium-sulfur battery-related experiments,electrode structure design and optimization.