Multi-characterization of LiCoO2 cathode films using advanced AFM-based techniques with high resolution

Abstract

The 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.

Figure 1

The schematic of AM-FM and KPFM techniques.

Figure 2

The LiCoO₂ cathode thin film images and statistic data by AM-FM: (a–d) surface morphology; (e–h) contact stiffness after different cycles, N _c stands for the charge/discharge cycle number, the scan size is 1 μm × 1 μm. (i) Grain size and stiffness; (j) RMS surface roughness and stiffness with different cycle number.

Figure 3

The LiCoO₂ cathode thin film images and statistic data by KPFM: (a–d) surface morphology; (e–h) surface potential after different cycles, N _c stands for the charge/discharge cycle number, the scan size is 1 μm × 1 μm. (i) Grain size and surface potential; (j) RMS surface roughness and surface potential; (k) surface potential distribution with different cycle number.

Figure 4

Charge/discharge capacity curves of the LiCoO₂ film measured by the galvanostatic charge/discharge method.