Probing the Mechanism of Improved Performance for Sodium-ion Batteries by Utilizing Three-electrode Cells: Effects of Sodium-ion Concentration in Ionic Liquid Electrolytes

We investigated the full-cell performance of sodium-ion batteries composed of a hard carbon (HC) negative electrode, a NaCrO2 positive electrode, and an ionic liquid electrolyte Na[FSA]-[C(3)C(1)pyrr][FSA] (FSA = bis(fluoro-sulfonyl) amide, C(3)C(1)pyrr = N-methyl-N-propylpyrrolidinium) at 333K. Before the full-cell tests, charge-discharge tests of the Na/HC and Na/NaCrO2 half cells were conducted, from which the practical capacities were determined to be ca. 250mAh (g-HC)(-1) and ca. 115mAh (g-NaCrO2)(-1), respectively. Using these capacities, the performance of HC/NaCrO2 full cells with practical loading masses was evaluated by three-electrode cells with a sodium metal reference electrode, and the energy density was calculated to be 177Wh (kg-(NaCrO2 + HC))(-1). In particular, we focused on the effect of the sodium-ion concentration on the performance by varying the molar fraction of Na[FSA] (x(Na[FSA])) from 0.20 to 0.50. The best rate capability was obtained at a composition of x(Na[FSA]) = 0.50. The effect of the sodium-ion concentration was discussed in terms of the potential profiles of the positive and negative electrodes. The results were explained by the sodium-ion supplying capability of the electrolyte inside the electrode, where the sodium insertion reaction occurs. (c) The Electrochemical Society of Japan, All rights reserved.