Electrode electrolyte interphase (EEI) is a crucial determinant of battery performance and durability. Organic solvents and inorganic electrolytes decomposes to chemically active radicals under electrical potential near the electrode surface. A thin film consists of mosaic nano-scale structures, generated by organic and inorganic decomposed products is deposited on electrodes thus creates an interphase between electrode and electrolyte. Sandwiched in between the highly reducing anode (or oxidizing cathode) and active electrolytes, the physico-chemical properties of this thin layered EEI regulate the interfacial reactions, active ion transport and dictates the cycling stability. With growing demands of cost effective batteries, sodium ion batteries (SIB) come forth as an attractive alternative but often suffer from long cycle life owing to formation of unstable EEI. Determining the exact decomposed organic/inorganic composites in the EEI would shade light to the electrolyte decomposition mechanism as well as prevention strategies to stabilize EEI during prolonged electrochemical cycling.
Various analytical techniques such as XPS, EELS, EIS and NIR spectroscopy have been applied to precisely determine the building blocks of EEI. Being average surface/bulk measurement procedure, above mentioned techniques fail to capture the nanoscale inhomogeneity and only provide an overview of qualitative chemical information of the elements present in the EEI. Tip enhanced Raman spectroscopy (TERS) combines nano-scale spatial resolution of scanning tunneling microscope (STM) with Raman microscopy to provide near-field chemical fingerprint. We have studied the EEI in SIB deposited on cathode (Prussian blue as active material) and anode (hard carbon) during first few cycles using 1M NaClO4 in PC: FEC (98:2 V/V) as electrolyte.
We have used TERS to address nanoscale chemical heterogeneity and attempt to identify the fragmented electrolyte products present on the EEIs of SIB. The electrodes are prepared manually and the electrochemical cycles are done in half-cell arrangement using inorganic electrolyte dissolved in non-aqueous cyclic carbonate solvent. The topography of the EEI and the chemical composition at nanometer lateral resolution are correlated for hard carbon (HC) anode and Prussian blue (PB) cathode in SIB. The gradual evolution of the SEI and CEI are analyzed after first and fifth electrochemical cycle in these anode and cathode medium. Further, assisted by density functional theory (DFT) calculations, we report that organic/inorganic carbonates byproducts mixed with carboxylates are deposited on the electrode surface. Analyzing the TERS mapping we unveil that the EEIs resemble mosaic nano-island structure. This state of art Raman imaging provides a unique perspective both on nano-heterogeneous distribution and acute molecular information of the thin layer EEI surface.