Capacity fading of pulsed-laser deposited HT-LiCoO2 films cycled in LiClO4/PC

Well-crystallized HT-LiCoO2 films with preferential c-axis orientation were prepared by pulsed-laser deposition using stoichiometric LiCoO2 targets. The cyclicability of the deposited HT-LiCoO2 films was evaluated using constant current (CC) and pulse discharge (PD) cycling. X-ray diffraction, Raman spectroscopy and scanning electron microscopy were performed to characterize the changes in the structure and morphology of the HT-LiCoO2 films before and after cycling. The films delivered an initial discharge capacity of 40 μAh μm−1 cm−2 and the discharge capacity decreased to 22.4 after 100 constant current cycles, due to the following three effects which occurred simultaneously: (a) mechanical failure, (b) dissolution of cation ions and (c) transformation of HT-LiCoO2 to spinel phase (Co3O4 or LiCo2O4). Under PD cycling, the discharge capacity of the films decreased quickly from about 39 to 9 μAh μm−1 cm−2 after only five cycles. The faster capacity fading in PD cycling may be induced by significant increase in inhomogeneous lithium intercalation, while the underlying mechanisms under PD cycling may be the same with those under CC cycling.