Structure of the Electric Double Layer in Non-Aqueous Electrolytes for Lithium Metal Batteries

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The electric double layer (EDL) plays a crucial role in electrochemistry as it controls the orientation and movement of ions and solvent molecules at the electrode-electrolyte interface. Ever since the concept of the EDL was first introduced by Helmholtz in 1879, there has been a proliferation of research studies in electrocatalysis and electrochemical energy storage dedicated to the structure of the EDL in aqueous electrolyte solutions [1, 2]. In recent years, probing the structure of the EDL in non-aqueous electrolyte solutions has gained more attention, predominantly in EDL capacitors (EDLCs) [3, 4]. Meanwhile, despite its contribution to the formation of solid electrolyte interphase (SEI) in lithium-metal batteries (LMBs), the comprehension of the EDL in LMBs is still very limited [5, 6]. Compared to EDLCs, it is more challenging to accurately capture the structure of the EDL in LMBs due to the complexity of Faradaic processes and chemical reactions at the interface. Furthermore, with the formation of SEI in LMBs after charge/discharge cycling, it becomes a formidable task to analyze the structure of the EDL because the length scales of SEI and EDL are comparable (approximately 10 nm), coupling with the complicated structure of the SEI. For these reasons, it is important to thoroughly examine the structure of the EDL in non-aqueous electrolytes with the presence of SEI in LMBs, which will provide insight to the interfacial chemistry as well as help design electrolytes for better battery performance.In this work, the structure of the EDL in non-aqueous electrolyte solutions in LMBs was experimentally investigated. Specifically, by using electrochemical impedance spectroscopy (EIS), we performed the differential capacitance measurements for varied concentrations of LiPF6 in varied ratios of EC:EMC solvent. Due to the complexity of interfacial chemistry in LMBs, we developed a rigorous equivalent circuit to precisely extract the differential capacitance from EIS measurements. We then applied thin crosslinked polymer and inorganic layers to the electrode to study how the EDL capacitance changes in the presence of these model SEIs. This work serves as an important step towards understanding the evolution of the EDL structure in LMBs after charge/discharge cycling with the interference of SEI at the electrode-electrolyte interface.