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]–[C₃C₁pyrr][FSA] (FSA = bis(fluorosulfonyl)amide, C₃C₁pyrr = N-methyl-N-propylpyrrolidinium) at 333 K. 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. 250 mAh (g-HC)⁻¹ and ca. 115 mAh (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 177 Wh (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