Direct in situ measurements of electrical properties of solid-electrolyte interphase on lithium metal anodes

Abstract

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. However, the true nature of the electrical properties of the SEI remains hitherto unclear due to the lack of a direct characterization method. Here we use in situ bias transmission electron microscopy to directly measure the electrical properties of SEIs formed on copper and lithium substrates. We reveal that SEIs show a voltage-dependent differential conductance. A higher rate of differential conductance induces a thicker SEI with an intricate topographic feature, leading to an inferior Coulombic efficiency and cycling stability in Li∣∣Cu and Li∣∣LiNi_0.8Mn_0.1Co_0.1O₂ cells. Our work provides insight into the targeted design of the SEI with desired characteristics towards better battery performance.

Fig. 1 ∣. In situ bias TEM measurement of electrical properties of the SEI.

a, Schematic of experiment set-up. b. Low-magnification TEM image showing in situ bias set-up of W tip and Cu wire inside TEM. c, High-magnification TEM image showing contact between W tip and Cu wire with the SEI on the Cu. d, TEM image showing Li deposit with the surface SEI layer using Cu wire as electrode. e, Typical $I-V$ curves showing the critical voltage. Scale bars, 50 μm in (c) and 100 nm in (b,d).

Fig. 2 ∣. Simulation of the SEI structure and subsequent calculation of I−V curve.

a, Simulation cell with different E/A ratio. b, Final states of the SEI formed by reaction between electrolyte and Li metal with different E/A ratio and simulation time via AIMD and ReaxFF method. c, Schematic of $I-V$ curve calculation set-up of simulated SEI via ab initio DFT with a Green’s function approach. $\epsilon$, voltage.

Fig. 3 ∣. Electrical properties of SEI and electrochemical cell performances.

a, $I-V$ curves of SiO₂ insulator and TiO₂ semiconductor. Error bars are s.d.; n = 10. b, $I-V$ curves of the SEI formed on Cu. c, $I-V$ curves of the SEI formed on Li deposits. Error bars are s.d.; n = 10. d, Calculated $I-V$ curve based on sample cell (E/A ratio is 2.79, simulation time is 253 ps). e–h, Differential conductance, $\mathrm{d}I∕\mathrm{d}V$ as function of $V$, derived from the $I-V$ curves, with the critical voltage indicated, for SiO₂ and TiO₂ (e), for SEI on Cu (f), for SEI on Li (g) and for calculated SEI on Li (h). The slope of the $\mathrm{d}I∕\mathrm{d}V$ against $V$ in d–f is termed as rate of differential conductance. i, CE of Li∣∣Cu cells. Left inset: CE curve at higher magnification of the initial 20 h. Right inset: CE curve at higher magnification from 40 h to 50 h. Average CEs are from ten cycles. j, Long-term cycling stability of Li∣∣NMC811 cells in LCE, PLHCE, HCE and LHCE electrolytes. Error bars in (a–c) show the reproducibility of measured $I-V$ curves.

Fig. 4 ∣. Dependence of microstructure of Li deposits on rate of differential conductance.

a, Low-magnification cryo-STEM-HAADF images of Li deposits formed in LCE, PLHCE, HCE and LHCE; grey and blue bars indicate the area fraction of Li and SEI, respectively. b, SEI-layer configuration maps derived from the STEM-HAADF images. Insets: high-resolution TEM images of Li deposits. c, Three-dimensional reconstruction of Li deposits. d, $\mathrm{d}I∕\mathrm{d}V-V$ curves of the SEI on Li formed in those four electrolytes, where the slope of $\mathrm{d}I∕\mathrm{d}V$ as a function of $V$ is termed as rate of differential conductance. Scale bars, 5 μm in (a,c) and 5 nm (inset in b).

Fig. 5 ∣. Correlation between SEI structure and its electrical property.

a, Atomic structure of SEI layers on the Li deposits formed in LCE, PLHCE, HCE and LHCE. Scale bars, 5 nm. b, SEI thickness as a function of the critical field strength of the SEI on Cu and Li, indicating the SEI layer thickness decreases with increasing critical field strength. c, Measured bandgap of the SEI layer on Li for different electrolytes, demonstrating bandgap decrease from the SEI surface towards Li interface. The stars represent individual data points, and the circle signifies the mean. d, Snapshots of samples for four electrolytes reacting with Li metal in $I-V$ curve calculations (E/A ratio is 2.79; simulation time is 253 ps).