Analysis of the Solid Electrolyte Interphase on Hard Carbon Electrodes in Sodium‐Ion Batteries

Abstract The composition, morphology, and evolution of the solid electrolyte interphase (SEI) formed on hard carbon (HC) electrodes upon cycling in sodium‐ion batteries are investigated. A microporous HC was prepared by pyrolysis of d ‐(+)‐glucose at 1000 °C followed by ball‐milling. HC electrodes were galvanostatically cycled at room temperature in sodium‐ion half‐cells using an aprotic electrolyte of 1 m sodium bis(trifluoromethanesulfonyl)imide dissolved in propylene carbonate with 3 wt % fluoroethylene carbonate additive. The evolution of the electrode/electrolyte interface was studied by impedance spectroscopy upon cycling and ex situ by spectroscopy and microscopy. The irreversible capacity displayed by the HC electrodes in the first galvanostatic cycle is probably due to the accumulation of redox inactive Na x C phases and the precipitation of a porous, organic‐inorganic hybrid SEI layer over the HC electrodes. This passivation film further evolves in morphology and composition upon cycling and stabilizes after approximately ten galvanostatic cycles at low current rates.