Investigation of the solid electrolyte interphase on hard carbon electrode for sodium ion batteries

The electrochemical performance of hard carbon (HC)/Na cells with NaPF6 in a mixture of EC/DEC (1:2, v/v) has been investigated in the voltage range of 0.05-2 V. An initial reversible capacity of 162 mAh g(-1) is observed. With continuous cycling, the reversible capacities fluctuate slightly and obtain a value of 183 mAh g(-1) on the 25th cycle. Ex-situ surface analysis of cycled HC electrodes has been conducted by a combination of scanning electron microscopy (SEM), infrared spectroscopy with attenuated total reflectance (IR-ATR) and X-ray photoelectron spectroscopy (XPS). The ex-situ surface analysis suggests that the major composition of solid electrolyte interphase (SEI) formed on the surface of HC electrode is sodium ethylene dicarbonate (SEDC) and NaF with lower concentrations of sodium alkyl carbonates and Na2CO3 which is similar with that reported in lithium ion batteries.