Electrolyte Design to Control Sodium Electrode Potentials for High Energy Density Sodium-Ion Batteries

Introduction Sodium-ion batteries (NIBs) are attracting an attention because of their abundant raw material resources and low material costs. In particular, the recent surge in lithium prices has led to a demand for alternatives to lithium-ion batteries (LIBs), which accelerates moves toward the practical use of NIBs. The major issue for their practical application is its lower energy density than that of LIBs. One of the critical challenges for improving the energy density of NIBs is the difficulty to handle high-voltage positive electrodes. This difficulty is attributed to the higher operating potential of electrodes in NIBs than that in LIBs as the standard electrode potential of sodium is higher than that of lithium. Electrode potential of a metal ( E M ) is a function of chemical potential of metal ion in electrolytes (μ M n+ ) as shown in equation 1. Recently, our group revealed that the coordinating ability of solvents greatly affects both and E Li [1] . Here we apply this concept to sodium system, aiming to control E Na with Na + electrolyte design. The E Na may be shifted downward in proper electrolytes, which enable us to manage high-voltage positive electrodes. So far, the severe reactivity of sodium metal has hindered the accurate measurement of E Na in organic solvents. In this study, to prevent the side reactions, sodium bis(fluorosulfonyl)imide (NaFSI) was used to form stable solid electrolyte interphase (SEI) on sodium metal. The influences of both solvent species and salt concentration on E Na were systematically investigated to gain insights into the control of electrode potentials in NIBs. Experimental NaFSI or sodium hexafluorophosphate (NaPF 6 ) was used as a sodium salt and electrolytes were prepared by dissolving them in some different solvents. The stability of the electrolytes to sodium metal was examined by putting sodium metal into the electrolytes for three days. Cyclic voltammetry was performed to evaluate E Na by measuring the redox potential of ~1 mM ferrocene as an internal standard with reference to sodium metal in each electrolyte. A three-electrode cell was assembled using platinum plate as the working electrode and sodium metal as the counter and the reference electrodes. Results Figure 1 shows the picture of the electrolytes three days after adding sodium metal fragments. The color of the sodium metal or electrolytes was changed in the NaPF 6 -based electrolytes, suggesting that the electrolytes were reduced and decompo