Enhancing the Interfacial Stability of Li6PS5Cl through Adsorption of Trimethylsilyl Compound-Based Solid Electrolyte Additives
Authors: Kim Kanghyeon , Lee Kyu Tae 
Published: December 2024

The demand for lithium-ion batteries (LIBs) is increasing due to the rapid expansion of energy storage systems (ESS) and electric vehicles (EVs). However, conventional LIBs raise safety concerns due to the presence of flammable organic solvents in their electrolytes. All-solid-state batteries (ASSBs) are considered state-of-the-art solutions capable of achieving high-energy density while mitigating safety risks. Among these, sulfide-based ASSBs have garnered significant attention owing to exceptional ionic conductivity and mechanical flexibility of sulfide-based solid electrolyte.[1] Nonetheless, the paradigm shift to solid-state battery technology introduces complex solid-solid interfacial side reaction that must be addressed to facilitate the commercialization of sulfide-based ASSBs. This challenge arises from the narrow electrochemical stability window of the sulfide-based solid electrolytes compared to oxide cathode materials, leading to chemical and electrochemical decomposition at the interface.[2] To overcome these inherent obstacles, the development of a chemically and electrochemically stable passivation layer at the interface is imperative.[3]

For LIBs containing liquid electrolytes, various electrolyte additives have been commonly utilized to form stable interphases, such as cathode-electrolyte interphase (CEI) on the cathode surface.[4] In particular, electrolyte additives composed of trimethylsilyl functional groups are known to significantly enhance the electrochemical performance of high-voltage cathode materials. Drawing inspiration from this, we have introduced solid electrolyte additives based on trimethylsilyl compounds to improve the interfacial stability between Li6PS5Cl (LPSCl) and LiCoO2. Since solid electrolyte additives cannot dissolve in solid electrolytes, achieving a uniform and thin distribution of the additives is crucial. Additionally, LPSCl undergoes electrochemical decomposition at the surface of carbon additives, underscoring the necessity for a stable passivation layer at the interface between LPSCl and carbon additives.[5] To address these challenges, we aimed to achieve uniform adsorption of a trimethylsilyl-based additive molecule with a thiol functional group, such as 2-(trimethylsilyl)ethanethiol, onto the LPSCl surface through hydrogen bonding and chalcogen-chalcogen interactions.
A straightforward solution-based additive adsorption process successfully yields a thin and uniformly coated layer of LPSCl with the additives. The amorphous 2-(trimethylsilyl)ethanethiol (TMS-SH) adsorption layer was approximately a few nanometers thick. TMS-SH underwent electrochemical decomposition at 4.2 V (vs. Li/Li+) during charging, forming a stable silicate-based CEI layer. This stable passivation layer enhanced the electrochemical and chemical stability of the LPSCl surface. Consequently, electrodes utilizing LPSCl adsorbed with TMS-SH demonstrated exceptional cycle life, exceeding 2000 cycles with 85.0% capacity retention after 2000 cycles, and exhibited superior rate capability compared to those utilizing bare LPSCl. This outstanding electrochemical performance stemmed from the stable passivation layer derived from the TMS-SH, which mitigated the interfacial side reactions and effectively suppressed the accumulation of the by-products, such as sulfate, phosphate, and cobalt sulfide. Additionally, TMS-SH enhanced the chemical stability of LPSCl in a dry oxygen atmosphere. Thus, the TMS-SH additive holds promise for sulfide-based solid electrolytes, offering not only in terms of electrochemical performance but also practicality in industry.

References

[1] J. Janek, W. G. Zeier, Nat. Energy 2023, 8, 230.
[2] Y. Zhu, X. He, Y. Mo, ACS Appl. Mater. Interfaces 2015, 7, 23685.
[3] N. Ohta, K. Takada, I. Sakaguchi, L. Zhang, R. Ma, K. Fukuda, M. Osada, T. Sasaki, Electrochem. Commun. 2007, 9, 1486.
[4] H. Rong, M. Xu, B. Xie, W. Huang, X. Liao, L. Xing, W. Li, J. Power Sources 2015, 274, 1155.
[5] W. Zhang, T. Leichtweiß, S. P. Culver, R. Koerver, D. Das, D. A. Weber, W. G. Zeier, J. Janek, ACS Appl. Mater. Interfaces 2017, 9, 35888.

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