Understanding the Mechanism of Two-Step Discharge Behavior in Lithium Air Batteries with Extremely High Capacity

Abstract

A rechargeable lithium air battery holds promise as a high theoretical energy density battery system for electric vehicles. However, it requires various air management components other than the batterycells, and this can cause a significant loss in volumetric energy density.1) Many researchers often report specific capacity in the range of 1,000 ~ 3,000 mAh/gcarbon without any consideration of areal and/or volumetric capacity. Based on our estimation of system level energy density including balance of plant, the areal capacity should be around 20 mAh/cm2 for 500-mile ranged vehicle applications. On the path to higher areal capacity, we found abnormally high capacity with two distinct discharge plateaus called two-step discharge behavior as shown in Fig. 1. The first and second discharge appeared at approximately 2.6 and 2.4 V, respectively. This unknown property was frequently accompanied by limited charging capacity, which was under 50% of its full discharge capacity. Several efforts such as controlling the amount of electrolyte and oxygen flow/pressure were performed in an attempt to explain this abnormal behavior at this high discharge capacity. In this presentation, we demonstrate reversible lithium air battery system with extremely high capacity over 20 mAh/cm2. The product analysis after each step of reaction confirmed that the main product is crystalline Li2O2 , and partially amorphous carbonates were formed as byproduct. From the result of in-situ DEMS analysis, the two-step discharge behavior had 70% oxygen efficiency (O2_OER/O2_ORR), and clearly exhibited two electron transfer reaction during charging/discharging cycle. Further results and detailed analysis will be discussed in this presentation. 1) K. G. Gallagher et. al., Energy Environ. Sci., 2014, 7 Figure 1