Reaction Transport Analysis and Parameter Identification for Designing Lithium–Sulfur Battery Electrodes
Authors: Ihara Minami , Mori Yuki , Yano Takeru , Inoue Gen 
Published: December 2024

Electrical energy storage systems for renewable energy sources and the popularization of next-generation vehicles with low CO2 emissions and high energy efficiencies are required to realize a sustainable society. However, current cathode materials such as lithium–containing oxides used in lithium-ion batteries (LiBs) have limited storage capacities, and their energy density peaks at approximately 150–250 Wh/kg. To improve this, lithium–sulfur batteries (LiSBs) with high theoretical energy densities are currently being developed and expected to be deployed as the next-generation battery technology. In LiSBs, the complex dissolution and precipitation reactions of sulfur are the driving forces. However, the complex operating mechanisms of LiSBs are yet to be investigated. Ren et al. (1) modeled the precipitation mechanism of lithium sulfide by focusing on the characteristics of a two-step discharge reaction to clarify the relationship between the complex reaction mechanism and the battery characteristics of LiSBs. In addition to considering multistep polysulfide dissolution and reduction, they describe the rate-dependent Li2S precipitation phenomenon using electro-crystallization kinetics based on nucleation and growth theory. Thus far, research has been actively conducted on clarifying the complicated reaction mechanisms of LiSBs. These studies only simplify the reaction mechanisms and calculations using estimates of many dynamic parameters of intermediate products that are otherwise difficult to measure. However, if such a numerical analysis is applied to a new electrode system, it can lead to incorrect conclusions regarding the reaction mechanisms. In our previous study, we established a technique to identify parameters that are difficult to measure after cell assembly by fitting the measured and calculated values of the fully discharged characteristics of LiBs using a complex nonlinear optimization method (2). For LiSBs, combining actual measurements and calculations is useful to estimate the unknown parameters of a new electrode system. In LiSBs, the volume change of sulfur during charging and discharging is problematic. During discharge, the volume expands by approximately 1.8 times when sulfur combines with lithium and changes to lithium sulfide (3). Although the microstructure of the electrode is expected to be destroyed by volume expansion, which affects the battery performance, only a few studies have verified the volume change by numerical analysis with complicated mechanisms. In contrast, in our study, reaction transport analysis models that consider volume changes were established for LiBs (4). Therefore, we aimed to construct a lithium–sulfur battery design support system that combines a numerical analysis method for lithium–sulfur batteries considering volume changes with an optimization method.

In this study, we examined the effect of sulfur-filled microporous activated carbon. Recently, it was reported that this microporous carbon with sulfur can be repeatedly cycled as a sulfur cathode exhibiting a high coulombic efficiency of almost 100% (5). Firstly, by using numerical simulation and optimization method, we evaluated mass transport and reaction properties with various parameters of pore structure. Next, we simulated the discharge performance with or without this activated pore. From this data, it was found that pore size strongly affects effective surface area and Li+ conduction and diffusion performance. As described above, it was suggested that an optimum pore structure exists from the viewpoint of the balance of reaction and mass transport, and the effect of different structures on transport properties was qualitatively evaluated. In the future, it is necessary to confirm the validity of the optimal structure through experiments and to quantitatively evaluate the effect of different CB.

Acknowledgment

This study was supported by JST GteX, Development of lithium-sulfur batteries with low environmental impact and high performance, Grant JPMJGX23S0, Japan.

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