In-Situ TEM Observation of Li-Air Battery Charge/Discharge Reaction

We conducted in-situ transmission electron microscopy (TEM) experiments to observe Li-air battery charge/discharge process to understand the reaction mechanism. A liquid confining cell was developed to prevent the electrolyte from evaporating in the high vacuum environment inside TEM. The cell has a silicon nitride membrane with gold electrode patterned on it. The electron beam transparency of the membrane enabled us to view the reaction in nano-scale resolution using TEM. The growth of the lithium oxide particles was observed during discharge. When the bias potential was applied to charge the cell, the particles formed during discharge were swept away in to the electrolyte. This implies that the decomposition of the lithium oxide particles took place at the interface between the particles and the electrode weakening the bond between them, and as a result the particles were detached from the electrode. Once the bias was set to discharge, new particles started to form again on the electrode. These processes of particle growth and detachment repeated in the charge/discharge cycles.  A cyclic voltammetry data showed obvious peak for the discharge, but no peak was seen for the charging indicating the reaction was irreversible. This matches the observation of the detaching particles during charge. When the Li₂O₂ particles detach from the electrode, they lose electrical connection and cannot be decomposed. Therefore, Li is constantly consumed when the battery is cycled leading to the capacity loss. Our observation implies the detachment of discharge products from the electrode during charging may be responsible for poor cyclability of Li-air battery. The result also agrees with the improved cyclability of Li-air battery using porous electrodes which can confine the discharge product in the pore preventing it from flowing into the electrolyte.

Figure caption: TEM images showing a temporal evolution of discharge products during charge. Li₂O₂ particles formed on the gold electrode during discharge detached from the electrode and flowed away into the electrolyte during charge.  The particles (i)-(iii) are marked with arrowheads to track the flow directions.