Correlation Between Electrical Conductivity of Solid Electrolyte Interphase and Rechargeable Battery Performance Revealed by Cryo and In Situ TEM

The solid–electrolyte interphase (SEI) critically governs the performance of rechargeable batteries. An ideal SEI is expected to be electrically insulative to prevent persistently parasitic reactions between the electrode and the electrolyte and ionically conductive to facilitate Faradaic reactions of the electrode. Despite the critical importance of the electrical properties of the SEI, quantitative measurement of this parameter remains unsolved due to the lack of a proper and reliable method [1, 2]. Here, we integrated in situ TEM with scanning tunnelling microscopy (STM) technique to measure the electrical properties of SEIs on Cu and Li, as illustrated in Fig. 1A. A STM tungsten (W) nanoprobe with atomically clean surface was used as the counter electrode, which was manipulated by the piezo system with three-axis nanometer-scale control. As ion-blocking Cu and W electrodes are used, the measured current (I)–voltage (V) data directly reflect the electron transport behavior of the SEI. It should be noted that this measurement with two blocking electrodes does not exactly resemble a real Wagner–Hebb polarization measurement but rather provides an upper limit value of the electrical conductivity. As the SEI is very sensitive to electron beam, we performed the I–V measurements at very low magnification of electron dose rate of 1 e-Å-2 s-1 and characterized at low temperature (cryo TEM) to avoid electron-beam-induced damage to the SEI. The designed approach could directly measure the electrical properties of SEI layers grown on copper (Cu) and lithium (Li) substrates, revealing the electrical characteristics of the SEI in terms of I–V relationship, differential conductance, critical field strength and bandgap as shown in Figs. 1B-C. To systematically study different SEIs, Li bis(fluorosulfonyl)imide (LiFSI) and 1,2-dimethoxyethane (DME) were chosen to make four electrolytes with designed microscopic solvation structures [3, 4]: (1) a low-concentration electrolyte (LCE) comprised of 1 M LiFSI in DME with a molar ratio of 1:9; (2) a high-concentration electrolyte (HCE) of LiFSI and DME with a molar ratio of 1:1.2; (3) a localized high-concentration electrolyte (LHCE) formed by adding bis(2,2,2-trifluoroethyl) ether (BTFE) diluent into the HCE to yield LiFSI-DME-BTFE = 1:1.2:3 by mole (LHCE-BTFE); and (4) an LHCE with bis(2,2,2-trifluoroethyl) ether carbonate (BTFEC):LiFSI-DME-BTFEC = 1.0:1.2:3.0 by mole (PLHCE, as free DME molecules are not closely coordinated with Li+ and making it a pseudo-LHCE). We unveil that the I–V characteristics of SEIs resemble certain electrical conductance, rather than electrical insulators as assumed in most studies. Combined with cryo TEM and electrochemical test (Fig. D), we further established relationship between SEI electrical property, SEI microstructure, and battery performance. The SEI with a higher rate differential conductance tends to exhibit a greater thickness and more complex topographic features, consequently leading to an inferior electrochemical performance [5]. The work highlights the governing role of electrical properties of the SEI layer and their tuning towards the enhanced performance of an electrochemical cell. The method established here can be generally used for other types of electrochemical cell as well [6]. A. Schematic of experiment set-up. B. TEM image showing Li deposit with the surface SEI layer using Cu wire as electrode. C. I–V curves of the SEI formed on Li deposits in the different electrolytes. D. Atomic structure of SEI layers on the Li deposits formed in the different electrolytes.