(Battery Division Research Award) Aqueous Lithium-Air Batteries with Protected Lithium Electrode
Authors: Imanishi Nobuyuki 
Published: February 2020

Lithium-air battery system consists of a lithium metal anode, an electrolyte, and a carbon-air cathode. Two types of lithium-air batteries have been developed based on the electrolyte type; non-aqueous and aqueous. The aqueous system was firstly proposed by Visco et al in 2004. This system consists of a lithium metal protected by a water-stable lithium conducting solid electrolyte, aqueous electrolyte, and a carbon air electrode. The cell reaction is

4 Li + 6H2O + O2 = 4(LiOH•H2O)

Water molecules are involved in the reaction and the theoretical cell voltage in an alkaline electrolyte is ca. 3.0 V, which is dependent on the pH value of the electrolyte. The energy density of the aqueous system is 1910 Wh kg-1 including oxygen.

Materials development is critical to accomplish aqueous lithium-air rechargeable batteries. One requirement is to develop a protected lithium electrode, which can perform a lithium stripping/deposition with a long lifespan in aqueous electrolyte. The key point of the electrode is in adopting a NASICON-type lithium conducting solid electrolyte, Li1+x+yAlxTi2-xP3-ySiyO12, (LATP), as the protect layer which covers and isolates lithium metal from contacting directly with aqueous electrolytes. LATP is unstable in a direct contact with lithium metal, so that a buffer layer should be placed between lithium metal and LATP.

In the early stage of our study, polymer electrolyte was adopted for the buffer layer. The interfacial resistance between lithium metal and polymer electrolytes has been demonstrated a key factor in initiating lithium dendrite formation by addition of ionic liquids, oligomer ethers and/or nanofillers. All these additives can reduce the interfacial resistance substantially and prolong the onset time of lithium dendrite initiation. One main approach to enhance electrode kinetics is to use ether-oligomer solvent without polymeric counterpart. A novel electrolyte prepared by the complexation of lithium cations supplied by lithium bis(fluorosulfonyl)imide and tetraethylene glycol dimethyl ether with the co-solvent of 1,3-dioxolane give metallic lithium anodes with high specific areal capacity in the absence of lithium dendrite formation.

Along with this buffer material, lithium ion conducting solid electrolyte that is stable in aqueous solution is another key component of aqueous lithium-air batteries. Two types of solid electrolytes, NASICON-type LATP and garnet-type Li7La3Zr2O12, have been a candidate for the protecting material, since they show high lithium ion conductivity at room temperature and chemical stability in a highly concentrated aqueous solution of lithium ions. For its practical applications the solid electrolyte film thickness must be minimized to obtain a high gravimetric and volumetric energy density, while higher ionic conductivity is required to reduce ohmic resistance. We have developed a solid electrolyte thin film using a tape casting method.

In addition to the detailed studies of those materials, other important materials about carbon air cathode, mechanisms, and the direction of future development will be mentioned in this presentation.