Electrochemical stability of non-aqueous electrolytes for sodium-ion batteries and their compatibility with Na0.7CoO2

Abstract

The present study compares the physico-chemical properties of non-aqueous liquid electrolytes based on NaPF₆, NaClO₄ and NaCF₃SO₃ salts in the binary mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The ionic conductivity of the electrolytes is determined as a function of salt concentration and temperature. It is found that the electrolytes containing NaClO₄ and NaPF₆ exhibit ionic conductivities ranging from 5 mS cm⁻¹ to 7 mS cm⁻¹ at ambient temperature. The electrochemical stability window of the different electrolytes is studied by linear sweep voltammetry (LSV) and cyclic voltammetry (CV) measurements with respect to a variety of working electrodes (WE) such as glassy carbon (GC), graphite and a carbon gas diffusion layer (GDL). Electrolytes containing NaPF₆ and NaClO₄ are found to be electrochemically stable with respect to GC and GDL electrodes up to 4.5 V vs. Na/Na⁺, with some side reactions starting from around 3.0 V for the latter salt. The results further show that aluminium is preferred over different steels as a cathode current collector. Copper is stable up to a potential of 3.5 V vs. Na/Na⁺. In view of practical Na-ion battery systems, the electrolytes are electrochemically tested with Na_0.7CoO₂ as a positive electrode. It is inferred that the electrolyte NaPF₆–EC : DMC is favorable for the formation of a stable surface film and the reversibility of the above cathode material.