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

Abstract

We investigated the full-cell performance of sodium-ion batteries composed of a hard carbon (HC) negative electrode, a NaCrO 2 positive electrode, and an ionic liquid electrolyte Na[FSA]-[C 3 C 1 pyrr][FSA] (FSA = bis(fluorosulfonyl)amide, C 3 C 1 pyrr = N-methyl-N-propylpyrrolidinium) at 333 K. Before the full-cell tests, charge-discharge tests of the Na/HC and Na/NaCrO 2 half cells were conducted, from which the practical capacities were determined to be ca. 250 mAh (g-HC) −1 and ca. 115 mAh (g-NaCrO 2) −1 , respectively. Using these capacities, the performance of HC/NaCrO 2 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-(NaCrO 2 + 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.

Manipulating Electrode/Electrolyte Interphases of Sodium-Ion Batteries: Strategies and Perspectives

“…The performance of ILs largely depends on the operation temperature − of the batteries and the concentration ,− of added sodium salts, which has been validated by previous works identifying their strengths in terms of electrochemical capacity enhancement and maintenance, ,, as well as reduction of the charge transfer resistance, , compared to conventional organic electrolytes. Here, we will focus on the behavior of ILs during electrolyte decomposition to form interphase layers.…”

Amide‐Based Ionic Liquid Electrolytes for Alkali‐Metal‐Ion Rechargeable Batteries

“…We confirmed the improved rate capability of Na/NaCrO 2 half‐cells at 363 K when the molar fraction of Na salt ( x (Na[FSA])) in Na[FSA]−[C 3 C 1 pyrr][FSA] was increased from 0.20 to 0.40 [36a] . We further investigated the performance of HC/NaCrO 2 full‐cells with a Na metal reference electrode and evaluated the charge−discharge behaviors of positive and negative electrodes separately [36d] . As the charge−discharge rate increased, larger potential polarizations were observed at the positive and negative electrodes during discharging and charging, respectively.…”

Next-generation Rechargeable Batteries Utilizing Ionic Liquids and Various Charge Carriers

“…10 with highly concentrated IL electrolytes at 333 K using three-electrode HC/NaCrO2 full-cells with a Na metal reference electrode. 49 Figure 4a shows the charge-discharge curves of the HC/NaCrO2 full-cell using the Na[FSA]-[C3C1pyrr][FSA] IL electrolyte at a composition of x(Na[FSA]) = 0.20 at a chargedischarge rate of 0.5C (1C = 100 mA (g-NaCrO2) −1 ). The charge-discharge profiles of HC and NaCrO2 were clearly separated, enabling the evaluation of the rate-determining process in the full-cell.…”