New Discoveries About Sulfur Cathodes For Lithium Sulfur Battery
- Theoretical model of sulfur cathode conversion
- Application of theoretical models in lithium sulfur battery
- Summary and outlook
Lithium sulfur battery has an ultra-high theoretical energy density (2600 Wh kg−1), and sulfur electrodes are abundant and inexpensive, and have received extensive attention in the field of rechargeable battery research. And the energy density of lithium ion battery is not the same as lithium sulfur battery’s.
However, a series of thorny problems caused by sulfur cathode materials have seriously hindered the practical application of lithium sulfur battery. First of all, sulfur and its discharge products (Li2S or Li2S2) are difficult to directly act on the positive electrode material due to low conductivity and insulation, resulting in a decrease in the utilization rate of sulfur active materials.
Secondly, due to the difference in bulk density between elemental sulfur and discharge product Li2S, the positive electrode volume expansion (~80%) is severe during cycling. In addition, the shuttling effect and self-discharge behavior of soluble polysulfides are severe, leading to rapid capacity fading of the battery.
Although great progress has been made by addressing these challenges in regulating and designing sulfur cathodes, the underlying working mechanisms are still lacking in-depth understanding. Theoretical calculations have unique advantages in explaining the reaction mechanism and providing design strategies.
Sorting out the theoretical models, basic principles and application scenarios that can be used in the research of sulfur cathodes is of great significance for broadening the understanding of lithium sulfur battery and promoting their development.
Based on this purpose, the scientists gave the following three models: adsorption model and lithium bond chemistry, lithium ion diffusion and Li2S decomposition model, sulfur reduction reaction model, d-band and p-band center model, and electrocatalytic model. Based on the research progress of theoretical models in lithium sulfur battery, the application of computer technology in lithium sulfur battery in the future is prospected.
Theoretical model of sulfur cathode conversion
In recent years, with the in-depth research on sulfur host materials and sulfur cathode conversion, the corresponding theoretical research has experienced three stages of adsorption-diffusion-transformation, and the material descriptors constructed based on theoretical calculations have also evolved with the progress of research.
According to the evolution of descriptors, the existing theoretical models are mainly divided into five categories: adsorption models and lithium bond chemistry, lithium ion diffusion and Li2S decomposition models, sulfur reduction reaction models, d-band and p-band center models, and electrocatalytic models.
Adsorption model and lithium bond chemistry
The adsorption model quantitatively calculates the binding energy between polysulfides and sulfur hosts, and uses the binding energy as a descriptor to reflect the strength of the interaction between polysulfides and sulfur hosts, measure the adsorption capacity of different host materials, and overcome experimental difficulties in quantitatively describing the adsorption properties of host materials.
In addition, lithium bond chemistry explains in detail the essential origin of strong Lewis acid-base interactions between polysulfides and adsorption sites from the perspective of dipole-dipole interactions, showing great advantages in mechanistic interpretation.
Li-ion diffusion and Li2S decomposition model
The diffusion rate of polysulfides on the substrate interface and the oxidative decomposition kinetics of Li2S are important factors in the sulfur conversion reaction, which are crucial to achieve high reversible capacity and long cycle life of batteries. Here is a lithium ion battery cycle life article and it contains more contents of battery life.
The Li-ion diffusion model and the Li2S decomposition model, which use the Li-ion diffusion energy barrier and the Li2S decomposition energy barrier as descriptors, can quickly evaluate the diffusion properties of polysulfides on the surface of different substrate materials, as well as the ability to promote Li2S oxidation kinetics, and then screen Substrate materials that can be used in lithium sulfur battery.
Sulfur reduction reaction model
The sulfur reduction reaction model is used to evaluate the catalytic performance of the electrocatalyst during the discharge process. Taking each reaction step in the sulfur reduction process as the research object, calculate and compare the Gibbs free energy change value of each reaction step to determine the sulfur reduction reaction.
According to the influence of different catalysts on the Gibbs free energy value of the speed-determining step, high-activity catalyst materials were quickly screened and designed to promote the kinetics of the polysulfide reduction reaction and suppress the shuttle effect during the discharge process.
D-band center and p-band center models
According to the d-band center and p-band center models, the position of the d-orbital or p-orbital of different ions relative to the Fermi level in a compound has a great influence on its electrochemical properties. Therefore, changing the anion or cation in the base compound can effectively regulate the corresponding adsorption performance and catalytic performance.
The d-band center and p-band center models start from the perspective of electronic structure, and deeply excavate the essential reasons for the different properties of various substrate materials, which play an important role in understanding the adsorption capacity and catalytic ability in the sulfur conversion process.
The electrocatalytic model starts from the solid-solid conversion reaction of Li2S2 reduction to Li2S, focuses the complex multi-electron reduction reaction on the simple two-electron electrochemical reaction, and deeply explores the internal mechanism of the sulfur reduction reaction. Based on theoretical calculations, a volcano-shaped relationship diagram between the adsorption Gibbs free energy of LiS radicals and the reaction overpotential was obtained.
Application of theoretical models in lithium sulfur battery
The application scenarios of theoretical models in lithium sulfur battery are becoming more and more extensive. The author mainly summarizes the research progress of theoretical models in mechanism research, high-performance cathode material design, high-throughput screening and machine learning.
Compared with common experiments such as adsorption experiments, cyclic voltammetry experiments, and microstructure characterization (FESEM, XPS, etc.), theoretical models have unique advantages in understanding the intrinsic mechanism of electrocatalyst conversion kinetics.
For example, the essential reason for the improvement of the electrocatalytic performance of defected molybdenum disulfide (MoS2−x) can be explained, that is, the charge density of sulfur atoms in the system induced by sulfur defects increases, thereby enhancing the energy absorption capacity of polysulfides.
In addition, the theoretical model also elucidated the mechanism of the synergistic effect induced by the composite sulfur cathode and the mechanism of action in promoting the conversion of polysulfides.
Providing reasonable improvement and design strategies for sulfur cathode materials is another important application scenario of theoretical models in lithium sulfur battery. For example, theoretical models provide general atomic surface modification strategies for materials such as 2D transition metal carbides and nitrides, which can tune the adsorption performance and electrocatalytic behavior of the material surface.
The heteroatom doping strategy designed using theoretical models can change the polarity and active sites of the material surface, thereby regulating the surface properties of the material. In addition, bandgap tuning strategies are also widely used in the design of catalyst materials.
With the rapid development of computer technology, combining the above theoretical models with high-throughput computing and machine learning, a series of descriptors can be constructed to quickly search for potential electrocatalyst materials. The theoretical simulation based on a large amount of material data significantly reduces the cost of trial and error in the early stage.
Summary and outlook
Experimental characterization methods can reflect the overall performance of lithium sulfur battery, while theoretical simulations can provide deep mechanistic insights at the atomic scale, and have become an indispensable tool in lithium-sulfur battery research.
Based on the application of theoretical models in sulfur cathode materials, the author summarizes the important role of different models in understanding the correlation between battery materials and electrochemical performance, and summarizes the application of high-throughput screening and machine learning in lithium-sulfur battery research prospect.
At the same time, based on the existing theoretical model research, the future research paradigm of lithium sulfur battery is prospected, hoping to inspire and guide the further development of lithium sulfur battery.