The initial process of Li-metal electrodeposition
on the negative
electrode surface determines the charging performance of Li-metal
secondary batteries. However, minute depositions or the early processes
of nucleation and growth of Li metal are generally difficult to detect
under operando conditions. In this study, we propose an optical diagnostic
approach to address these challenges. Surface plasmon resonance (SPR)
spectroscopy coupled with electrochemical operation is a promising
technique that enables the ultrasensitive detection of the initial
stage of Li-metal electrodeposition. The SPR is excited in a thin
copper film deposited on a glass substrate, which also serves as a
current collector enabling electrochemical Li-metal deposition. For
a propylene carbonate (PC)-based Li-ion battery electrolyte, under
both cyclic voltammetry and constant-current operation, Li-metal deposition
is readily detected by changes in the SPR absorption dip in the reflectance
spectrum. Electrochemical SPR is highly sensitive to metal deposition,
with a demonstrated capability of detecting an average thickness of
approximately 0.1 nm, corresponding to a few atomic layers of Li.
To identify the growth mechanism, the SPR reflectance spectra of various
possible Li-metal deposition processes were simulated. Comparison
of the simulated spectra with the experimental data found good agreement
with the well-known nucleation and growth model for Li-metal deposition
from PC-based electrolytes. The demonstrated operando electrochemical
SPR measurement should be a valuable tool for basic research on the
initial Li-metal deposition process.