The lithium-oxygen batteries have attracted much more attentions because of its high theoretical energy density, which is over 3500 Wh kg^-1. We have developed all-solid-state type Li-O₂ batteries using the inorganic solid electrolytes. The component of our cell is Li / Li_xAl_yGe_2-y(PO₄)₃ (LAGP) / LAGP-CNT. A sintering process is needed to make Li ion conduction paths within the LAGP-CNT air electrode. In our conventional sintering process, a heat treatment temperature of 700 oC, which is lower than the optimal sintering temperature of around 900 ^oC, is used to avoid the decomposition of CNT. Therefore, the Li ion conductivity of the air electrode prepared in the conventional process is low.

In this study, we have investigated a LAGP glass electrolyte to improve the Li ion conductivity of the air electrode calcined at 700 ^oC.

Experimental

A LAGP glass electrolyte powder was prepared by the melt quenching method. The starting materials of Li₂CO₃, Al₂O₃, GeO₂ and NH₄H₂PO₄ are mixed and heated at 1350 ^oC. To quench, the melt is pressed by two stainless steel plates. The resultant bulk glass was ground by a mortar and milled by a ball-milling method. The LAGP crystal powder used in the conventional process was synthesized by the conventional solid-phase method.

The air electrode composed of LAGP and CNT particles was fabricated on the LAGP pellet by sintering and Li anode was fused on another side of the LAGP pellet. Li anode was sealed by the plastic film. The cell was put into the bottles connecting to the oxygen gas flow channel.

Results and discussion

Figure 1 shows the conductivities of the LAGP crystal and glass pellets heated at several temperatures. The conductivity of the LAGP crystal increased with an increase in the heat treatment temperature. The conductivity of the LAGP crystal pellet heated at 700 ^oC was 5.7 x 10^-4 S cm^-1 at 30 ^oC, which was 10 times lower than that of LAGP crystal pellet heated at 900 ^oC. On the other hand, the LAGP glass pellet heated at 700 ^oC showed almost the same conductivity of 5.0 x 10^-4 S cm^-1 as the LAGP glass pellet heated at 900 ^oC. These results demonstrated that the glass electrolytes can construct the better Li ion conduction paths within the air electrode heated even at 700 ^oC.

Then the electrochemical performance of the all-solid-state Li-O₂ cells using the LAGP glass electrolyte was investigated and compared with the cell using the LAGP crystal electrolyte. The cell using the glass electrolyte showed the small capacity compared with the cell using the crystal electrolyte. From the SEM observation of the cross-section of the air electrode, the porosity of the air electrode using the glass electrolyte was very low owing to the large particle size and a shrink characteristic.

To improve the cell performance, the LAGP glass-crystal composite electrolyte was prepared. The composite electrolyte showed a relatively high conductivity of 2.3 x 10^-4 S cm^-1. The cell using composite electrolyte showed almost the same capacity as the cell using the crystal electrolyte at a lower current density.