Morphology Progression of Electrodeposited Li2O2 during Charging of Li/O2 Batteries

Non-aqueous Li/O₂ batteries could achieve much higher specific energies than lithium-ion (3-5x)¹, making battery powered electric vehicles more competitive with traditional combustion vehicles.² Realizing this promise however requires significant improvement in cycle life and cycling efficiency. Understanding the charge mechanism is crucial to lowering the charging overpotential, which in turn would increase efficiency. Observing the process of deposition and dissolution of electrodeposited Li₂O₂ in Li/O₂ cells can shed light on the charging mechanism.

During charge, the amount of energy necessary to decompose Li₂O₂ likely depends on the morphology of the Li₂O₂ particles deposited^3-5: higher surface energy morphologies should require less energy. The morphology formed on discharge depends heavily on cell potential with higher energy particles formed at higher overpotentials.^4,5 Observing how Li₂O₂  is consumed during charge could further explain the relationship between cell overpotentials and Li₂O₂ morphology, as well as probe any differences in the discharge and charge mechanisms.

In this talk we will show how the morphology of electrochemically deposited Li₂O₂ changes at intermediate states of charge in a Li/O₂ cell. Cells are discharged and charged at constant rate of 0.5 mAcm^–2 as described in a previous report.⁵ After discharge is complete, various intermediate states of charge are sampled by stopping the charge at cell potentials of 3, 3.375, 3.75, 4.125, and 4.5 V. Partially cycled electrodes are harvested from the cell and imaged with scanning electron microscopy.

 Figure 1 shows a typical discharge charge curve, with an image of a control electrode and the morphologies observed for a fully discharged cell and a cell charged to 4.5 V (coincidentally corresponding to about half of the discharge current). From this it is clear that Li₂O₂ is not consumed uniformly throughout the electrode during charge. The implications of the non-uniform consumption of Li₂O₂ for electron and O₂ transport in the electrode during charge will be discussed.