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Abstract The solid-electrolyte interphase (SEI) plays crucial roles for the reversible operation of lithium metal batteries. However, fundamental understanding of the mechanisms of SEI formation and evolution is still limited. Herein, we develop a depth-sensitive plasmon-enhanced Raman spectroscopy (DS-PERS) method to enable in-situ and nondestructive characterization of the nanostructure and chemistry of SEI, based on synergistic enhancements of localized surface plasmons from nanostructured Cu, shell-isolated Au nanoparticles and Li deposits at different depths. We monitor the sequential formation of SEI in both ether-based and carbonate-based dual-salt electrolytes on a Cu current collector and then on freshly deposited Li, with dramatic chemical reconstruction. The molecular-level insights from the DS-PERS study unravel the profound influences of Li in modifying SEI formation and in turn the roles of SEI in regulating the Li-ion desolvation and the subsequent Li deposition at SEI-coupled interfaces. Last, we develop a cycling protocol that promotes a favorable direct SEI formation route, which significantly enhances the performance of anode-free Li metal batteries.

Electrochemical reactions on nanostructured noble electrodes have received much attention, however, the reaction mechanism and reaction kinetics are still difficult to be studied. Probe molecule can give an insight to the investigation of electrochemical reactions on noble electrodes with nanostructures. In this paper, the electrochemical process of p-aminothiophenol (PATP) adsorbed on the gold electrode was studied by electrochemical cyclic voltammetry and surface-enhanced Raman spectroscopy (SERS). Here, we used one-step sodium citrate reduction method (Frens method) to synthesize gold nanoparticles, which are used to construct the nanostructured gold electrode. The Raman electrolytic cell used was based on the traditional three-electrode electrolytic cell. The gold electrode was used as the working electrode (WE), the saturated calomel electrode (SCE) as the reference electrode (RE), and the platinum wire (Pt) as the counter electrode (CE). After the careful pretreatment of the gold electrode surface, the cell was assembled and placed on the platform of the XploRa instrument to get started. With the assistance of potentiostat, the SERS spectra at different potentials were acquired and combined together, a so-call electrochemical surface-enhanced Raman spectroscopic (EC-SERS) experiment. In a 0.05 mol⋅-1 sulfuric acid solution (pH = 1), an irreversible oxidation peak was found in the cyclic voltammogram, which is considered to correspond to the oxidation of the PATP molecule. The oxidation mechanism is proposed by combination of previous work in literature, and it is pointed out that the PATP molecule was initially transformed into cationic radical. Then, this cationic radical coupled with the PATP molecule to an intermediate NPQDH2, and finally electrochemically oxidized to 4'-mercapto-N-phenylquinone diamine (NPQD). On the basis of this mechanism, the surface coverage of PATP on the electrode surface was calculated and the coverage value was found to be larger at the nanostructured electrode due to the modification of gold nanoparticles than that of general gold electrodes. In the following, the electrochemical oxidation product was characterized by the EC-SERS spectra. Finally, we experimentally and theoretically studied the electrochemical oxidation kinetics of PATP on the gold nanoparticle-modified gold electrode (Au NPs@Au). The apparent reaction rate constant k and transfer coefficient α of PATP were calculated by electrochemical linear sweeping voltammetry and theoretical simulation, respectively, finding that the cationic radical formation step is the rate-limiting step. We believe that this work will no doubt stimulate the basic research of PATP on gold electrodes consisting of nanostructures and provide a guide to electrochemical kinetic research in other metal-adsorbate systems. [ABSTRACT FROM AUTHOR]