Quantifying Multiphase SEI Growth Kinetics in Sulfide Solid Electrolytes

In solid-state batteries (SSBs), the liquid organic electrolyte used in lithisum-ion batteries (LIBs) is replaced with the goal of eventually surpassing their performance. Sulfide solid electrolytes (thio-SEs) have garnered considerable attention from both academia and industry due to their high ionic conductivities and ease of processing. Thus, thio-SEs, such as lithium argyrodite Li₆PS₅Cl, are considered well-suited for implementation in SSBs. The successful incorporation of lithium metal anodes could allow SSBs to achieve higher energy and power densities than LIBs, while also simplifying cell designs [1]. However, the application at the device level is prevented by the insufficient reduction stability of thio-SEs when in contact with lithium metal anodes, which necessitates the development of electrolyte materials with optimized properties. [2]
Due to its thermodynamic instability, Li₆PS₅Cl is reduced upon contact with highly reactive lithium metal, forming a multiphase solid electrolyte interphase (SEI). This comprises a mixture of compounds, including Li₂S, Li₃P, and LiCl. The growth mode, composition, and microstructure of a few model-type SEIs are slowly being unveiled. Consequently, the electronically insulating nature of the evolving SEI is believed to restrict further formation and cause a significant slowdown in growth [3]. Unfortunately, appropriate methods for investigating multiphase SEI layers are limited and the determination of SEI properties requires complex approaches.
The direct reaction of Li₆PS₅Cl with lithium metal powder is employed to gain insight into the transport properties of typical multiphase SEIs in this study. The synthesized bulk-scale SEI-type material is analyzed with respect to its composition and conduction properties. In order to estimate the SEI growth rate, using a Wagner-type diffusion model, both the partial ionic and electronic conductivity of the SEI-type material are measured using dc polarization (i.e., Wagner-Hebb configurations). The kinetic predictions obtained are consistent with recent electrochemical studies on cell-level multiphase SEIs. These findings enhance our understanding of SEI growth (specifically for Li₆PS₅Cl), leading to more accurate modeling of SEI transport parameters. Consequently, this improves predictions of SEI growth, its implication, and kinetics in SSB cells. Ultimately, these results highlight the importance of stabilizing the Li|SE interface through proper material design to control the partial conductivities of the resulting SEI. [4]


References
[1] a) J. Janek, W. Zeier, Nat Energy 2016, 1, 16141 b) J. Janek, W. Zeier, Nat Energy 2023, 8, 230–240
[2] S. Wenzel et al., Solid State Ionics 2018, 318, 102–112
[3] a) C. Lee et al., ACS Energy Letters 2021, 6, 3261-3269 b) S. Wenzel et al., Solid State Ionics 2015, 278, 98–105
[4] Alt et al., Joule 2024, 8,1–22