Cobalt oxide thin films for high capacity and stable Li-ion battery anode

Here, we report reactive DC-sputter deposited CoO thin films as a promising and stable Li-ion battery anode. Thin films were deposited on stainless steel by reactive sputtering of cobalt target in O atmosphere. X-ray diffraction and X-ray photo electron spectroscopy confirm the formation of CoO crystal structure and absence of other impurities. The electron microscopy analysis shows a columnar growth morphology of the thin films while high resolution images reveal that the film is composed of ultra-small nanoparticles of average size of 5 nm. Fabricated half cells upon cycling between 3.0 and 0.01 V exhibit a stable capacity of 1125 mAh/g at a current density of 1 A/g for 100 cycles. Moreover, the electrode exhibited excellent rate capability and stability at higher rates; at current density of 10 A/g, a capacity close to 1000 mAh/g was observed. The excellent cycling stability of the cell was further confirmed by cycling at a high rate of 25 A/g (28 C) wherein the same was able to retain a capacity of 330 mAh/g even at the end of 1800 cycles. This enhanced performance could be related to the formation of 5-nm primary particles and columnar growth morphology, capable of reducing the lithium ion diffusion lengths and thus offered better kinetics even at high rates.