Lagp-Based Composite Membrane for Rechargeable Aqueous Li-Air Battery

Novel high efficiency grid-scale high energy density storage systems, such as rechargeable Li-air batteries (LABs), are urgently needed to enable and manage a more wide-spread utilization of renewable energy sources. Among these organic electrolyte LABs offer the highest gravimetric energy density, but they suffer from limited cyclability and poor energy efficiency due to the high over-potential during charge and discharge. LABs relying on aqueous catholytes promise somewhat lower theoretical capacity but a higher potential to reach theoretical limits in practical devices. In these systems catholytes with high solubility for the discharge product enhance the cycle efficiency and power performance.

The fast ion-conducting electrolyte membrane is the key component of an aqueous Li-air battery, as it protects the lithium anode from reacting chemically with the catholyte solution. Here, an inorganic-organic hybrid membrane based on NASICON-type Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) ceramics and PEO:PVDF:LiBF4 with a thickness of 100-150 μm is designed and tested inside an aqueous LAB. By varying the mass fraction of the ceramic and polymer constituents mechanical properties and ionic conductivity of the membrane were optimized yielding conductivities of 5×10^-4 S cm^-1.

The membrane in an Li-air cell also has to be stable in contact with the lithium anode. Cyclic voltammetry of the membrane up to 4 V vs. Li/Li⁺shows only two distinct peaks of cathodic deposition and anodic dissolution of the anode, confirming the wide electrochemical window of the membrane. To identify a suitable solid electrolyte:catholye couple, the membrane was immersed in an aqueous solution of LiCl (10M) and the changes in the structure and conductivity were monitored. Geometry and XRD pattern of the membrane are almost identical before and after immersion for 20 days, while the conductivity of the membrane increases with immersion time.

Figure 1 demonstrates the schematic design of the cell. Hybrid inorganic-organic membrane is placed between lithium anode and catholyte in the reaction chamber (cell). The catholyte is circulated between the cell and a reservoir. The air cathode consists of finely dispersed Pt on multi-walled carbon nanotube arrays on a carbon fiber cloth, so that oxygen reduction and formation are effectively catalyzed.

Room temperature charge/discharge profiles of a cell cycled in the voltage range of 1.5 to 4.5 V for the first 10 cycles are shown in Figure 2. The cell is operated with a constant current density of 0.5 mAcm^-2. Both charge and discharge cycles are limited to one hour. During the first cycle, the cell shows charge and discharge plateaus at 4.0 V and 2.6 V, respectively. The equilibrium potential is considerably increased compared to cells without a continuous supply of the catholyte; all other components remaining the same.

To further enhance the energy efficiency of the LAB we explore ways to reduce the internal cell resistance by optimizing membrane thickness and patterning, as well as by tailoring charge transfer processes at the electrode:electrolyte:catholyte interfaces.