Aqueous Lithium-Ion Battery with Dual Electrolytes at Equilibrium across Cation-Exchange Membrane Separator

Abstract

Aqueous lithium-ion batteries (ALIBs) have been investigated as a promising safe and low-cost storage device for sustainable energy. For the stability of ALIBs, it is important to expand the electrochemical stability window of aqueous electrolytes. Although “Water-in-salt” electrolytes [1] containing significant amounts of lithium salts have been reported as an effective approach, cost reduction of the aqueous electrolyte remains a major challenge for the industrialization of such ALIBs.Controlling the pH of the aqueous electrolyte is another approach to widen the electrochemical window with a smaller use of lithium salts. The 2 V-class ALIB of Zn-coated Li4Ti5O12/LiMn2O4 was demonstrated using dual aqueous electrolytes having different pH separated by a lithium-ion conductive inorganic solid electrolyte.[2] However, lithium-ion conductive inorganic solid electrolytes may often be unstable in alkaline solutions. A perfluorinated sulfonic acid cation-exchange membrane (CEM) is one of the possible alternatives for separating two different aqueous electrolytes. In principle, the concentration of the entire electrolyte does not change during lithium-ion battery operation where lithium ions shuttle between the cathode and anode. Therefore, the specific compositions of the cathode and anode electrolytes are selected to achieve the Donnan equilibrium across the CEM, and stable battery operation is expected as long as the electrolyte equilibrium is maintained. In this study, the feasibility of this concept was investigated using ALIB cell of the anatase-TiO2/spinel-LiMn2O4.A stable combination of cathode and anode electrolytes was investigated experimentally. Several compositions of cathode and anode electrolytes were prepared, and equal volumes of the cathode and anode electrolytes were faced across the CEM, stored for 10 days at room temperature, and the composition of each electrolyte were determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES). After repeating this procedure, a stable combination of cathode and anode electrolytes was found as 27 g/L of Li and 8 g/L of K for an anode electrolyte, and 24 g/L of Li, 36 g/L of K, 41 g/L of P, and 68 g/L of S for a cathode electrolyte prepared using LiOH, KOH, Li2SO4, and KH2PO4. The pH was 4 and 11 for the cathode and anode electrolytes, respectively.The battery test was demonstrated for the TiO2/LiMn2O4 cells assembled with the cathode/anode electrolyte and the same 2 mol/kg Li2SO4 electrolyte in both sides for comparison. The initial charge/discharge profiles are shown in Fig. 1. The profiles were obtained by constant current of 15 mA/g regarding the weight of the cathode active material with the cutoff voltage of 2.8 V and 1.5 V in charge and discharge processes, respectively. The Coulombic efficiency was reached to 96% in the cells with electrolytes having the pH difference, whereas it was 69% in the cell with the 2 mol/kg Li2SO4. The alkaline anode electrolyte suppressed hydrogen generation sufficiently for the lithiation/delithiation of anatase TiO2, which leads to an excellent reversibility over 2 V operation with an improvement in the Coulombic efficiency of the cell with dual aqueous electrolytes.To further investigate the stability of the aqueous battery, a cycle test was conducted on the cell with the dual electrolytes. Figure 1(b) shows the discharge capacity during 100 charge-discharge cycles obtained by the constant current of 75 mA/g-cathode. The Coulombic efficiency reached >99%, and the capacity retention was calculated to be 75% at 100 cycles, which indicates that the suitable pH of the cathode and anode electrolyte was maintained during repeated charge/discharge cycles.In the presentation, we will discuss the details of the battery operation, including a role of perfluorosulfonated CEM and issues to improve the battery performance.