Activation and degradation of electrospun LiFePO4 battery cathodes

Abstract

Electrodes prepared by electrospinning often exhibit an activation behavior, i. e. they are reaching their full capacity only after numerous charge and discharge cycles. The activation mechanism can be explained by the improvement of the accessibility of Li+ ions to the active particles of the cathode, which increases with the number of cycles. It is assumed that, as an effect of cycling, the dense, impermeable carbon layer which covers the active material due to the carbonization step during processing cracks and delaminates, allowing this way the Li+ ions to access the active material and to intercalate into it. This has been confirmed by scanning and transmission electron microscopy performed in correlation with the electrochemical performance of electrospun electrodes. However, with even further cycling a decrease in capacity is observed. The microscopic results suggest that this is partly caused by cracks at the carbon-LiFePO4 interfaces. Thus, the cracking responsible for the activation of the electrospun electrodes at the beginning of cycling seems also to cause a part of their degradation at the end of their life. Another slow degradation mechanism confirmed by scanning electron microscopy and by X-ray photoelectron spectroscopy is the ongoing formation of a cathode electrolyte interphase.