Enhanced electrochemical properties of fluoride-coated LiCoO2 thin films
© Lee et al; licensee Springer. 2012
Received: 8 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
The electrochemical properties of fluoride-coated lithium cobalt oxide [LiCoO2] thin films were characterized. Aluminum fluoride [AlF3] and lanthanum fluoride [LaF3] coating layers were fabricated on a pristine LiCoO2 thin film by using a spin-coating process. The AlF3- and LaF3-coated films exhibited a higher rate capability, cyclic performance, and stability at high temperature than the pristine film. This indicates that the AlF3 and LaF3 layers effectively protected the surface of the pristine LiCoO2 film from the reactive electrolyte.
Lithium-ion batteries are used as power sources for a wide range of applications such as cellular phones, personal digital assistants [PDAs], laptop computers, and electric vehicles. The cathode is one of the critical components of a lithium-ion battery, and it determines the capacity, cyclic performance, and thermal stability of the battery. In order to improve the electrochemical properties of the cathode material, researchers have attempted to modify the cathode surface by using stable materials [1–5]. The coated cathode exhibits an enhanced rate capability, thermal stability, and cyclic performance. However, the coating effect is highly dependent on the material and shape of the coating layer [4, 5]. Therefore, the identification of a suitable coating layer is a key factor in obtaining a highly improved cathode material by using the coating process. In this work, a fluoride-coated lithium cobalt oxide [LiCoO2] thin film was characterized. The surface of a LiCoO2 thin film cathode is much wider and smoother than that of a bulk-type electrode, which may enable careful observation of the interface reaction of a coating layer. Fluorides such as aluminum fluoride [AlF3] and lanthanum fluoride [LaF3] are promising coating materials for surface modification of the cathode [6–8]. Myung et al. proposed that a stable coating layer such as metal oxide transformed into a metal-fluoride layer during cycling, thereby leading to a greater resistance to HF attack . This implies that the fluoride layer can be effectively used to protect the cathode surface from unwanted reactions with the electrolyte. Fluorine [F] has also been investigated for use as a doping material for enhanced structural and thermal stability [9–11]. In this study, we focused on the discharge capacity, rate capability, and cyclic performance of the pristine fluoride-coated LiCoO2 thin films to characterize the coating effect.
The pristine LiCoO2 thin film was supplied by GS NanoTech Co., Ltd (Gangdong-gu, Seoul, South Korea). In order to prepare the AlF3 coating solution, aluminum nitrate nonahydrate (Al(NO3)39H2O; Sigma-Aldrich, St. Louis, MO, USA) and ammonium fluoride [NH4F] (Sigma-Aldrich, St. Louis, MO, USA) were dissolved in 10 ml of a mixed solvent consisting of distilled water, 1-butanol, and acetic acid. The LaF3 coating solution was also prepared by dissolving lanthanum nitrate hexahydrate [La(NO3)36H20] and NH4F in a mixture of distilled water, 1-butanol, and acetic acid. The resultant solution was applied as a coating to the LiCoO2 thin film substrate by using a spin-coater (K-359 S-1, Kyowa Riken Co., Ltd., Tokyo, Japan). The coated LiCoO2 thin films were then heat-treated in a rapid thermal annealing [RTA] system at 400°C for 30 min. The microstructures of the films were observed by field emission - scanning electron microscopy [FE-SEM] (JEOL JSM-6500F, JEOL Ltd., Akishima, Tokyo, Japan). The electrochemical characterization of the coated LiCoO2 films was performed in non-aqueous half-cells. The cells were subjected to galvanostatic cycling using a galvanostatic system (WonATech, Seocho-gu, Seoul, South Korea).
Results and discussion
The cyclic performances of the pristine and the AlF3- and LaF3-coated films were also investigated in the voltage range of 4.5 to 3.0 V (at a current density of 0.4 mA· cm-2). These are considered to be severe measurement conditions because LiCoO2 undergoes structural instability in the high voltage range (i.e., above 4.25 V) . As shown in Figure 3b, all the samples showed a sharp drop in discharge capacities during several cycles. However, the AlF3- and LaF3-coated films showed a relatively moderate capacity fading. It is important to note that the AlF3 coating is more effective than the LaF3 coating in suppressing capacity fading in the high cutoff voltage range. This result indicates that AlF3 is a more effective coating material than LaF3 for increasing the structural stability of LiCoO2 in the high voltage range.
Stable AlF3 and LaF3 coating layers were fabricated on a pristine LiCoO2 thin film electrode. The rate capability of the film electrode was evidently improved by the AlF3 and LaF3 coating layers. In particular, the coated film showed a greatly enhanced cyclic performance under severe cycling conditions. This indicates that the AlF3 and LaF3 coating layers were successful in preventing the surface of the LiCoO2 film from reacting with acidic electrolyte.
energy dispersive spectroscopy
field emission - scanning electron microscopy
personal digital assistants
rapid thermal annealing
scanning electron microscopy.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0071073).
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