LiCoO2 Thin Film Deposition Using a Low-Temperature Electrochemical-Hydrothermal Route

The increasing miniaturization of electronic devices and the demand for stand-alone systems require adapted power sources in terms of dimensions and energy. All-solid-state microbatteries are an interesting solution for new applications (secured smart cards, medical implants, sensors, etc.). They are obtained by successive deposition of thin films of electrode active materials and solid electrolyte. However, these batteries have a rather low capacity (50-200 µAh/cm²) due to the difficulty to use electrodes thicker than 5µm (electronic conductivity, lithium diffusion and mechanical stress issues). Most of commercially available microbatteries use a LiCoO₂ positive electrode due to its interesting properties: a 4V/Li⁺/Li nominal voltage, an excellent cycle life and a high electronic conductivity [1] . Thin films of LiCoO₂ are mainly obtained by sputtering  [2, 3] , followed by an annealing (500-700°C) necessary to obtain the electrochemically active phase of LiCoO₂ (R-3m). This latter step then limits the possible substrates. Moreover the low deposition rate (10nm/mn) and the required equipments and facilities make this process quite costly.

To circumvent these drawbacks, thin films of LiCoO₂ were synthesized by a novel single-step route: the electrochemical-hydrothermal deposition. This technique combines the effects of the hydrothermal conditions (temperature and pressure) with a galvanostatic electrodeposition. Using ethanol as co-solvent in the aqueous media, we succeded in obtaining a pure and highly-crystalline LiCoO₂ films, without any Co₃O₄ impurity. Moreover, the surface roughness is greatly decreased (fig.1) compared with films obtained in pure aqueous media. The obtained thickness can be increased up to 30µm, equal to a deposition rate of 200nm/mn. Galvanostatic cycling and linear sweep voltammetry experiments were carried out on these electrodes using CR2032 coin cells. The electrochemical properties of these films are very promising and clearly evidence the influence of each deposition parameter (temperature, current density, water/ethanol ratio, etc.) (fig2).

[1] Reimers, J.N. and J.R. Dahn, Journal of the Electrochemical Society, 1992. 139(8): p. 2091-2097.

[2] Wei, G., T.E. Haas, and R.B. Goldner, Solid State Ionics, 1992. 58(1-2): p. 115-122.

[3] Wang, B., et al., Journal of the Electrochemical Society, 1996. 143(10): p. 3203-3213.