Simulation of Lithium-Sulfur Batteries and Optimization of Cathode Structure

Abstract

Recently, the spread of the mobile devices and clean energy, the demand for high-power and high-capacity secondary batteries has been growing. However, the energy density of lithium-ion batteries (LiB) that is widely used nowadays is reaching its design limit. Instead of LiB, lithium-sulfur batteries (LiSB) are expected as one of the next-generation secondary batteries. Compared to LiB, LiSB has the advantages of higher energy density, lower cost, and easier weight saving. In LiB, the driving force is the insertion and desorption reaction of lithium ions between the cathode and anode, while in LiSB, is the complex sulfur dissolution and precipitation reaction. In LiSB, the electrolyte determines the dissolution and precipitation of sulfur, which is thought to have a significant impact on battery characteristics. Therefore, new electrolyte systems are being developed daily and techniques to elucidate the reaction mechanisms are needed. Furthermore, the effect of carbon black, a conductive material, on battery characteristics has also been considered. In this study, we investigated the optimal initial lithium ion concentration and calculation focused on the precipitation form of each carbon species for the design of optimal electrode structures.Kumaresan's one-dimensional model [1] based on porous electrode theory was used. Sulfur was used as the cathode active material and lithium metal as the anode active material. Electron and ionic potentials were calculated from the electro-neutrality condition, concentration distribution in the electrolyte was calculated from the non-stationary mass balance equation, and electrochemical reactions at the interface between active material and electrolyte were calculated using the Butler-Volmer equation. Furthermore, stepwise reactions were considered by incorporating the electrochemical reaction of sulfur and the dissolution and precipitation reactions into the concentration distribution and the interfacial reactions. The temperature in the electrode layer was assumed to be constant, and no heat loss or side reactions due to overvoltage were assumed to occur. In LiSB, sulfur dissolved in the electrolyte on the cathode surface is sequentially reduced as polysulfide ions as the discharge progresses. It then combines with lithium ions transferred from the anode and expands to about 1.8 times. Therefore, based on our previous research [2], the effect of the expansion and contraction of the electrode particles was reflected by dynamic changes in the structural properties of the electrode layer.Ionic conductivity and transport number are important parameters that affect the properties of the electrolyte. The higher the lithium ion concentration, the higher the transport number, but the lower the ionic conductivity [3]. In this study, we focused on sulfolane-based electrolyte and investigated the effect of initial lithium ion concentration on discharge capacity by incorporating the relationship between ionic conductivity and transport number into a model. As a result, the discharge capacity reached its maximum when the initial lithium ion concentration was 3.0 M. In the static state, the ionic conductivity was highest around 1.6 M, but considering the operating conditions during discharge, the concentration distribution became be more remarkable due to the low transport number, which may have resulted in excessive Li2S precipitation near the separator. As an extension to the microstructure of the electrode layer, we focused on carbon black (CB). Two types of CB (KB, MCND) were targeted. When MCND and KB were used as CB under the same conditions in the experiment, it was confirmed that the discharge characteristics of MCND were significantly better than those of KB. In LiSB, Li2S precipitation has a significant effect on discharge characteristics. Therefore, we focused on the pores of both CBs. We reconstructed the binarized structure from X-ray CT images of both CBs, hypothetically precipitated Li2S in the pores, and calculated the relative conductivity. The relative conductivity represents the transport property and is highly dependent on the structure of the electrode layer. As a calculation result, the relative conductivity of KB became significantly smaller as the porosity decreased. From the above, the transport properties in both CBs were confirmed by calculation.As described above, it was suggested that an optimum lithium ion concentration exists, and the effect of different CB structures on transport properties was qualitatively evaluated. In the future, it is necessary to confirm the validity of the optimal concentration through experiments and to quantitatively evaluate the effect of different CB.AcknowledgmentThis study was supported by JST ALCA-SPRING Grant JPMJAL1301, Japan.