NASICON vs. Na metal: a new counter electrode to evaluate electrodes for Na secondary batteries

Abstract

Na metal has been used as a counter electrode in a half-cell configuration to test positive and negative electrode materials for Na secondary batteries. However, there are significant obstacles including high reactivity, which generates a resistive passivation layer along with electrolyte decomposition, dendrite formation that results in poor cyclability, dead Na metal accumulation, which impedes Na⁺ transport, and a low melting point, which limits its use to below 98 °C. Herein, an alternative counter electrode is devised using NASICON-type Na₃V₂(PO₄)₃ to measure the accurate electrochemical behaviour of working electrode materials and to use for measurements above the melting point of Na metal. The novel counter electrode is prepared by mixing Na₃V₂(PO₄)₃ and NaV₂(PO₄)₃, and the latter is prepared by the desodiation of Na₃V₂(PO₄)₃ using Cl₂ gas. The resulting Na₃V₂(PO₄)₃–NaV₂(PO₄)₃ electrode exhibits a flat plateau at 3.4 V vs. Na⁺/Na and lower polarization than the Na metal. The electrochemical behaviours of the Na₂FeP₂O₇, Na₃V₂(PO₄)₃, and NaCrO₂ electrodes tested with the new counter electrode match the known curves recorded with the Na metal at low current densities and show better cyclability and rate performance. Moreover, the electrochemical properties of these electrode materials are verified at a temperature above the melting point of Na metal for the first time.