Abstract
LiNiO2 thin films were fabricated by RF magnetron sputtering. The microstructure of the films was determined by X-ray diffraction and field-emission scanning electron microscopy. The electrochemical properties were investigated with a battery cycler using coin-type half-cells. The LiNiO2 thin films annealed below 500°C had the surface carbonate. The results suggest that surface carbonate interrupted the Li intercalation and deintercalation during charge/discharge. Although the annealing process enhanced the crystallization of LiNiO2, the capacity did not increase. When the annealing temperature was increased to 600°C, the FeCrNiO4 oxide phase was generated and the discharge capacity decreased due to an oxygen deficiency in the LiNiO2 thin film. The ZrO2-coated LiNiO2 thin film provided an improved discharge capacity compared to bare LiNiO2 thin film suggesting that the improved electrochemical characteristic may be attributed to the inhibition of surface carbonate by ZrO2 coating layer.
1. Introduction
In an attempt to miniaturize high performance portable electronic equipment, batteries with high power and energy density are required. Thin film batteries have been developed in an attempt to satisfy this requirement [1–3]; however, improving the performance of the cathode films is critical for advancing the use of electrochemical thin film batteries. Among the possible materials that could be used for a cathode film, LiCoO2, owing to its high theoretical specific capacity and ease of preparation, is a promising candidate [4–6]. However, the high cost and toxicity of cobalt limit the use of LiCoO2 in thin film batteries. Therefore, it is necessary to develop less expensive cathode materials for thin film battery applications. LiNiO2 has emerged as a useful cathode material owing to its low cost and high energy density [7–10].
In the current study, LiNiO2 thin films were deposited by using RF magnetron sputtering. The microstructure of the films was measured by X-ray diffraction and field-emission scanning electron microscopy. Finally, the electrochemical properties were investigated with a battery cycler using coin-type half-cells, in the potential range of 3.0 V–4.2 V.
2. Experimental
Bare and ZrO2-coated LiNiO2 thin films have been deposited onto stainless steel (STS 304) foil substrates held at a distance of 6 cm away from the target. The STS304 substrate was ultrasonically cleaned with acetone, alcohol, and distilled water in that order. The LiNiO2 and ZrO2 targets were made by Pascal Co. (Japan). A base vacuum of 5 × 10−6 Torr was obtained with a cryopump. Gas flow rate of Ar/O2 was 4/1 with a total gas flow amount of 150 sccm. Deposition pressure was maintained at 2 × 10−3 Torr during deposition. The LiNiO2 target was presputtered for 30 min and the deposition time was 360 min at 100 W RF power. ZrO2 coating layer was deposited for 10 min at 100 W RF power. Figures 1(a) and 1(b) show the surface and cross-section images of bare LiNiO2 thin film deposited on Si wafer substrate. As seen in Figure 1(a), a uniform distribution of clusters of ~50 nm was seen. The calculated deposition rates of LiNiO2 were approximately 1.7 nm/min. The deposited thin films were annealed from 400°C to 600°C in air to obtain the crystalline film.
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The structure of the LiNiO2 thin films was investigated by X-ray diffractometry (XRD, Rigaku, Miniflex). The XRD measurements were performed using Cu Kα radiation ( Å) and phase identification was made by comparing the diffraction patterns with the JCPDS references. The morphology of the deposited films was studied by field-emission scanning electron microscopy (FE-SEM, Jeol, JSM-6701F).
In order to examine the electrochemical properties of cathode thin films, coin-type cells were assembled with lithium foils as the counter and reference electrode and 1 M LiPF6 in ethylene carbonate (EC) : diethyl carbonate (DEC) (1 : 1, vol.%) electrolytic solution. The charge-discharge test was carried out with a battery cycler (Won A Tech, WDCS3000s) at a constant current density of 5 μA/cm2 in the potential range of 3.0–4.2 V.
3. Results and Discussion
Figure 2 shows the XRD patterns of as-deposited and annealed LiNiO2 thin films at various temperatures at 10 min. Crystallization peaks of LiNiO2 were not seen in the as-deposited films. However, the XRD pattern of the 400°C annealed film exhibited broad (104) LiNiO2 and lithium carbonate (Li2CO3) reflection peaks. The degree of crystallization of LiNiO2 was enhanced at 500°C because the (104) reflection became stronger as the annealing temperature was increased. The intensity of the Li2CO3 peak also increased. However, after annealing at 600°C, the reflection peaks of LiNiO2 and Li2CO3 disappeared and impurity peaks of NiCrFeO4, which was considered an oxide layer of the STS304 substrate, were observed (Figure 2(d)).
Surface images of the annealed LiNiO2 thin films are shown in Figure 3. The 400°C annealed film had a rough surface with no cracks and some surface impurities (Figure 3(a)). When the annealing temperature was increased to 500°C, the size of the surface impurities grew and became angular. After annealing at 600°C, complete removal of the surface impurities was achieved and the surface had a smooth morphology compared with that of the 400°C annealed film. The XRD (Figure 2) and FE-SEM (Figure 3) results suggest that the angulated surface impurity, which disappeared at an annealing temperature of 600°C, was lithium carbonate (Li2CO3).
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Figure 4 shows the initial discharge curves of the as-deposited and annealed LiNiO2 thin films. All of these films were tested at a current density of 5 μA/cm2 between 3.0 V and 4.2 V and a plateau was observed in the 400°C annealed films. When the annealing temperature was increased to 500°C, the plateau was more clearly exhibited and indicated a phase transition of crystalline LiNiO2 [11, 12]; the discharge capacity, however, did not increase. These results suggest that the surface carbonate interrupts the lithium intercalation and deintercalation during charge/discharge, affecting the discharge capacity. The 600°C annealed films exhibited a lower initial discharge capacity compared with the other annealed films. This may be attributed to the formation of an intermediate oxide layer between the substrate and active material film. The intermediate oxide layer that formed may lead to an oxygen deficiency in the annealed LiNiO2 thin films. In order for the LiNiO2 film to obtain good electrochemical characteristics, the deposited LiNiO2 thin films should be annealed at a temperature that prevents the formation of an intermediate oxide layer. In addition, it is necessary to remove the surface carbonate. It has been previously reported that surface coatings enhance the electrochemical properties of cathode materials without sacrificing the specific capacity of the respective cathode [13].
Figure 5 shows the XRD patterns of bare and ZrO2-coated LiNiO2 thin films annealed at 500°C for 10 min. As seen in Figure 5(a), the diffraction peak of the surface carbonate disappeared in the coated thin film, suggesting that the ZrO2 coating prevents the formation of surface carbonate during the annealing process.
Figure 6 shows the first discharge curves and cycle stability of bare and ZrO2-coated LiNiO2 thin films. As seen in Figures 6(a) and 6(b), a single plateau was observed in both of the films, corresponding to the phase transition of crystalline LiNiO2. Therefore, this would indicate that both films are crystallized. However, the first discharge capacity is slightly different such that the first discharge capacity of ZrO2-coated film is higher than that of the bare LiNiO2 thin film. The capacity retention rate is similar in both thin films as seen in Figure 6(b).
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4. Summary
LiNiO2 thin films were fabricated by RF magnetron sputtering. Crystallization began at annealing temperatures above 400°C; however, the films that were annealed below 500°C exhibited surface carbonate (in the form of Li2CO3) identified by XRD. Surface carbonate interrupts the Li intercalation and deintercalation during charge/discharge and therefore the capacity did not increase, although the degree of LiNiO2 crystallization was enhanced. When the annealing temperature was increased to 600°C, the FeCrNiO4 oxide phase was generated and the discharge capacity decreased due to the oxygen deficiency in the LiNiO2 thin film. The ZrO2-coated LiNiO2 thin film provided an improved discharge capacity compared to the bare LiNiO2 thin film at an annealing temperature of 500°C. Therefore, the improvement in electrochemical characteristics can be attributed to the inhibition of surface carbonate by the ZrO2 coating layer.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgment
This research was supported by the Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012-R1A1A2008821 and 2012-R1A2A1A01006546).