Understanding the Development and Properties of SEI in Concentrated Aqueous Electrolytes Via Scanning Electrochemical Microscopy
Authors: Gossage Zachary Tyson , Ito Nanako , Hosaka Tomooki , Tatara Ryoichi , Komaba Shinichi 
Published: February 2024

Solid-electrolyte interphases (SEI) are essential to the stability of high voltage lithium-ion batteries (LIBs) where they act as a protective barrier that prevents electrolyte decomposition during charge-discharge and during storage of the energy. Within emerging water-in-salt electrolytes (WISE), the SEI are thought to play a similar role in preventing electrolyte decomposition and expanding the potential window.(1, 2) The SEI reported in WISE are derived from the electrolyte ions, producing inorganic SEI (e.g. LiF) of similar thickness to non-aqueous batteries.(1) Others suggest the superconcentrated regimes promote anion reduction and shift its reduction potential at similar or more positive potentials to hydrogen evolution. However, our knowledge on the SEI found in concentrated aqueous electrolytes and their properties remains quite limited. Furthermore, WISE full cells can access >1000 cycles at high rates, but their capacities and retention are still heavily lacking compared with commercial LIBs.(2) Improving our understanding of the WISE-based SEI formation process, its stabilization, and prevention of gas evolution are key to achieving higher performing aqueous batteries.
Herein, we explore the use of advanced scanning electrochemical microscopy (SECM) methods(3) to characterize an SEI within a highly concentrated K(FSA)0.6(OTf)0.4 electrolyte. Focusing on a 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) composite electrode, we use both ex situ, approach curves as well as in situ single spot measurements to analyze SEI formation (Figure 1a). The approach curves were collected before and after galvanostatic cycling by using a three-electrode cell that could easily be converted between closed (cycling) and open-cell (SECM) measurements. After cycling, we observed passivating SEI structures with electron transfer rates comparable to those found in LIBs (Figure 1b). At the same time, our results indicated the SEI deposition was discontinuous with some regions showing reactivity comparable to an uncycled electrode. We noted an increase in roughness with cycling, which could produce some of the more reactive regions exposed at the electrode surface. Thereafter, we conducted in situ measurements at a constant distance from the PTCDI surface.(4) During the first cycle, we observed a reversible/transient decrease in the feedback current at ~ -0.7 V vs. Ag/AgCl, far positive to H2 evolution (Figure 1c,d). In addition, more stable passivation was observed when the PTCDI electrode reached more negative potentials accessing the second redox process of PTCDI (Figure 1c,e). As the electrode reached potentials more negative to -1.3 V, we observed significant hydrogen evolution. Our results were further confirmed with operando electrochemical mass spectrometry (OEMS). OEMS showed similar potentials for evolving hydrogen as well as the evolution of other gases indicative of SEI formation. In all, our interfacial SECM analyses combined with traditional battery measurements and OEMS provides direct quantification of the passivating properties of the SEI as well as identification of local and bulk gas evolution that can be expanded for other emerging aqueous systems.
References
1 L. Suo, et al., Science, 2015, 350, 938-943.
2 L. Droguet, et al., Adv. Energy Mater., 2020, 10, 2002440.
3 Gossage, Z.T., et al., "Application to Batteries and Fuel Cells." Scanning Electrochemical Microscopy. CRC Press, 2022. 481-512.
4 G. Zampardi, et al., RSC Advances, 2015, 5, 31166-31171.

Figure 1