In-Situ AFM Observation of Solid Layer Formation at a Superconcentrated Electrolyte/Electrode Interface

1. Introduction<br/>Lithium-ion batteries have high convenience and prevalence but have safety issues due to the volatility and flammability of organic electrolytes. Superconcentrated electrolytes have attracted considerable attention for their low volatility and flammability owing to extremely high electrolyte concentration in which all solvent molecules interact with Li⁺[1]. For reversible charge/discharge, solid layers on the electrode surfaces play important roles and are known to be produced by reductive decomposition of solvents for dilute electrolytes which limits the choice of available solvents [2]. On the other hand, reversible charge/discharge can be realized with various solvents for superconcentrated electrolytes [1], indicating different origin of the interfacial layers; however, details are still under discussion.<br/>To elucidate the origin and formation process of the interfacial layers, we performed <i>in-situ</i> chemical and mechanical analysis of the superconcentrated electrolyte/electrode interface. Here we focus on potential-dependent mechanical properties of the interface analyzed by atomic force microscopy (AFM) combined with electrochemical techniques.<br/><br/>2. Experimental<br/>Lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) was dissolved in acetonitrile at concentration of 4.2 M to obtain the superconcentrated electrolytes. Au thin film deposited on a cleaved mica substrate (Au/mica), Pt mesh, and Pt wire were used as the working, counter and quasi-reference electrodes, respectively. Mechanical properties of the interface were analyzed through force curve measurements with AFM at 1024 points at each potential (4.0 V to 1.0 V vs Li/Li⁺ where negligible Faradaic process was observed). All the measurements were carried out in an Ar atmosphere.<br/><br/>3. Results and Discussion<br/>The Young's modulus of the interface derived from a force curve measurements were ~58 GPa at the potential more positive than 1.5 V, corresponding to that measured for bare Au/mica in air, and decreased to 30 GPa at more negative potentials. On the other hand, simultaneously derived adhesion force shows an increase at 1.5 V, suggesting an increase in local viscosity at the interface, and decreased again at more negative potentials. These results are compared with our model for solid layer formation based on the insights from interface-selective spectroscopy. At 1.5 V, Li⁺ was attracted to the negatively charged electrode, and TFSA anions are also come to the vicinity of the electrode because of Li-TFSA interaction. Then, as a results of oversaturation, a solid salt layer of LiTFSA is deposited on the electrode surface. The change in the Young's modulus of the interface observed in this study is consistent with this model where bare Au surface is covered by solid salt layer at more negative potentials. Increase in local viscosity probably reflects the locally oversaturated solution on the electrode exists at the start of the solid layer deposition. Thus, the quantitative analysis of viscoelasticity using AFM combined with spectroscopic results enabled us to identify the formation process of the solid layer at the superconcentrated electrolyte/electrode interface.<br/><br/>References<br/>1. Y. Yamada et al., <i>J. Electrochem. Soc.</i>, 162, A2406 (2015)<br/>2. D. Aurbach et al., <i>J. Power. Sauces</i>, 68, 91 (1997)