Scanning Electrochemistry Microscopy Study of Electrochemical Reduction and Solid Electrolyte Interphase Coating of Indium Tin Oxide Thin-Film Electrode in LiPF6-Based Carbonate Electrolyte

Abstract

Cyclic voltammetry (CV) of indium tin oxide (ITO) electrodes in LiPF6-based carbonate electrolyte containing ferrocene reveals progressive electrochemical reduction of the ITO film, accompanied by the disappearance of ferrocene redox peaks upon continuous CV cycling. In situ scanning electrochemical microscopy (SECM) is employed to probe the synergistic relationship between tip feedback current and the irreversible reduction of ITO and formation of the solid electrolyte interphase (SEI). To understand the effect of CV reduction in both vertical and horizontal directions on tip feedback and potential SEI layer formation, the ITO surface is indexed with poly(3,4-ethylenedioxythiophene) and polystyrenesulfonate (PEDOT:PSS) spots for SECM imaging and optical characterization within the same sample region. SECM study reveals that initial cathodic polarization produces transiently more conductive regions on the ITO electrode due to partial reduction of In2O3 and SnO2. With continued cycling, the feedback signal progressively decreases, reflecting loss of interfacial conductivity due to indium and tin clustering together with growth of an insulating SEI layer. X-ray photoelectron spectroscopy and optical photothermal infrared spectroscopy of the same indexed region confirm the formation of a carbonate- and fluoride-rich SEI containing lithium alkyl carbonates, LiF, and phosphate species. SEM imaging of the indexed ITO surface also reveals that the SEI formation rates are higher at the edge of each PEDOT:PSS spot due to an enhanced electrolyte–solvent concentration gradient near that region with the PEDOT:PSS spot inactive toward SEI formation, while the bare ITO right next to the indexed spot reduces electrolyte and solvent accompanied by In and Sn clustering during electrochemical reduction, yielding a more severe coating of the SEI layer.