Abstract
AbstarctThe thin film Li-ion batteries have been extensively used in micro-electronic devices due to their miniaturization, high capacity density and environmental friendliness, etc. In order to further prolong the lifetime of the film batteries, one of important tasks is to explore the aging mechanisms of the cathode films. In this paper, we especially focused on the multi-characterization of the LiCoO2 film in nanoscale, which is carried out by combining advanced AFM-based techniques with capacity measurement. The surface morphology, contact stiffness as well as surface potential were measured by amplitude modulation-frequency modulation (AM-FM) and kelvin probe force microscope (KPFM), respectively. Remarkable changes after different numbers of charge/discharge cycling were observed and the intrinsic reasons of them were discussed in detail. To acknowledge the relationship with these microscopic changes, the macro-capacity of the thin films was also measured by the galvanostatic charge/discharge method. These comprehensive results would provide a deep insight into the fading mechanism of the cathode film, being helpful for the design and selection of the cathode film materials for high performance batteries.
Highlights
Lithium ion batteries are important energy storage devices and have been widely applied in lots of fields, such as the electric grid, cellphone and electric vehicles[1,2,3]
Since f2 is sensitive to the elastic tip-sample interaction[29], it is applied to obtain the information of the surface elasticity by the method of frequency modulation
The surface morphology, contact stiffness and surface potential distribution of the LiCoO2 thin film were investigated by the amplitude modulation-frequency modulation (AM-FM) and kelvin probe force microscope (KPFM) methods, respectively
Summary
Lithium ion batteries are important energy storage devices and have been widely applied in lots of fields, such as the electric grid, cellphone and electric vehicles[1,2,3]. Numerous studies indicated that the thin film batteries aging depends on the changes in structure and properties of the electrode film, which had been successfully characterized by lots of techniques, such as scanning electron microscope (SEM)[6,7], transmission electron microscope (TEM)[8,9], X-ray diffraction (XRD)[10,11,12,13], X-ray photoelectron spectroscopy (XPS)[14,15,16], electrochemical impedance spectroscopy (EIS)[17,18] and so on. Several techniques, including nano-indentation, contact resonance atomic force microscope (CRAFM) and amplitude modulation-frequency modulation (AM-FM), had been used to study the mechanical properties of the film electrode, such as stiffness and elastic modulus. Since the aging mechanism of the thin film Li-ion batteries is complicated, and both the surface morphology and the corresponding properties depend on the electrochemical history, the intermediate processes of charging/discharging are worthy to be traced
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