Lithium-air batteries are expected as next-generation secondary batteries for electric vehicles because of their high energy density. In particular, an aqueous lithium-air battery that uses an aqueous electrolyte has advantages such as a high power density and availability of operation under an air atmosphere. Here, we show the feasibility of an acidic aqueous lithium-air battery that consists of a lithium anode, a lithium-ion conducting liquid interlayer, a solid lithium-ion conductor separator of Li1.4Al0.4Ge0.2Ti1.4(PO4)3, an acetic acid catholyte, and a fuel cell-analogous air electrode. The theoretical energy density of this system based on the masses of the lithium anode, oxygen, and acetic acid is 1340 Wh kg−1. This system was successfully cycled at 0.2 mA cm−2 for 5 h polarization and room temperature under an air atmosphere for 30 cycles.

Aqueous lithium-air batteries are one of the most promising batteries for electric vehicles because of its high energy and power density. The battery system consists of a lithium anode and an aqueous solution catholyte, which are separated by a water-stable lithiumion-conducting solid electrolyte, and an air electrode. The theoretical energy density of this system is 1,910 W h kg −1 , which is around five times higher than that of conventional lithium-ion batteries. A key component of this system is the water-stable lithium-ion-conducting solid electrolyte. In this work, we have developed a water-stable and water-impermeable solid electrolyte with a high lithium-ion conductivity of around 10 −3 S cm −1 at room temperature by the addition of epoxy resin and LiCl into a tapecast NASICON-type Li 1. 4 Al 0. 4 Ge 0. 2 Ti 1. 4 (PO 4) 3 film. The aqueous lithium-air battery with the solid electrolyte separator was successfully cycled at 0.5 mA cm −2 and 25 • C in an air atmosphere.

NASICON-type Li 1.4 Al 0.4 Ge 0.2 Ti 1.4 (PO 4 ) 3 solid electrolyte (AG-SE) was synthesized using a rheological phase method precursor. The AG-SE powders prepared by this method had a low crystalline temperature of 595.5°C and much fewer impurity phases than that prepared by the conventional solid-state method. An AG-SE pellet sintered at 900°C for 14 h had a high relative density of 95.3%, with total and grain boundary room temperature conductivities of 1.21 and 4.35 mS cm −1 individually. The total activation energy for the AG-SE was as low as 0.29 eV. The 3-point bending strength was determined to be 102 N/mm 2 , which was higher than that of AG-SE prepared by solid-state and liquid-phase method precursors.

The electrical vehicles (EVs) are considered to reduce CO2 emissions and the consumption of fossil fuels, because the total energy conversion efficiency of batteries is higher than that of internal conversion (IC) engines (1). However, the driving range of the commercialized EVs with the current lithium-ion batteries is considerably lower than that of the vehicles with the IC engines, because the energy density for the lithium-ion battery is low compared with those of the IC engine. In this report, a new type high specific energy density battery is proposed. The proposed battery consists of a water- soluble organic redox couple of hydroquinonesulfonic acid potassium salt (HQSK) and benzoquinonsulfonic acid potassium salt (BQSK) and lithium metal anode. The water soluble redox couple was used for aqueous redox flow batteries (2). The catholyte and lithium metal anode were separated by a water-stable lithium-ion conducting solid electrolyte of Li1.4Al0.4Ge0.2Ti1.4(PO4)3 (LAGTP). The cell reaction is described as shown below in Figure A. The calculated specific energy density of the couple is 740 Wh/kg if lithium metal is used as the anode, which is around two times higher than that of a conventional lithium-ion battery. Figure 1 shows a schematic diagram of the proposed cell, where an equivolume of teragalyme (G4) dissolving (Li(FSO2)2N (LiFSI) in 2:1 molar ratio and 1,3 dioxolane (DOL) (2) was used as an electrolyte between lithium and LAGTP, because LAGTP is unstable in contact with lithium. The LAGTP films were prepared by a tape-casting method (3). LAGTP powder prepared by the sol-gel method was dispersed in a solution of ethanol and toluene, a binder and a plasticizer. The slurry was tape-casted and sintered at 900 oC for 7 h. LAGTP-epoxy resin composite film was prepared by soaking in a mixed solution of 1 M 1,3-phenylenediamine and 2 M 2,2-bis (4-glycidyloxy-phenyl) propane in tetrahydrofuran. It was heated at 170 oC for 24 h. Kinetics of the HQSK/BQSK redox reaction on the Ketjen black (KB) electrode in three type aqueous solutions of 1M H2SO4 (pH 0.2), 5.5 M CH3COOH (AcOH) (pH 1.9), and 1M AcOH-1M CH3COOLi (AcOLi) (pH 4.5) were examined using a beaker-type cell with a Ag/AgCl reference electrode. Figure 2 shows over-potential (η) vs current density (j) curves at 25 ℃ for the HQSK/BQSK redox reaction in different solutions, where the discharging over-potentials (BQSK + 2H+ +2e-→ HQSK) were measured after charged about 20 % of HQSK (HQSK → BQSK +2H+ + 2e-). The charging over-potentials were low and showed no significant dependence on the electrolytes. On the other hand, the discharge over-potentials are dependent on the electrolytes. The lowest discharge one was observed in 5.5 M AcOH aqueous solution. A Swagelok-type full cell of Li / (LiFSI-2G4)-50 vol% DOL / LAGTP / HQSK-BQSK in 5.5 M AcOH / KB showed open circuit voltage of 3.6 V at 25 ℃, which is comparable to the calculated value. The cell was charged successfully at 0.5 mA cm-2 for 30 h, but in the discharge at 0.5 mA cm-2 the voltages were decreased significantly with time. Steady discharge voltages were observed at a lower current density. This result suggests that a catalyst for BQSK reduction should be developed to obtain a high power density battery. References Ginshkumar et al. J. Phys. Chem. Lett. 1, 2193 (2010) Yang et al. J. Electrochem. Soc. 161, A1371 (2014) Morita et al. ACS Omega, 3, 5558 (2018) Figure 1