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Preparation and Characteristics of LiFePO4 Thin Film by Radio Frequency Magnetron Sputtering for Lithium Microbatteries Xian-Jun Zhu,†,‡ Long-Bin Cheng,† Cheng-Gang Wang,† Zai-Ping Guo,*,‡ Peng Zhang,‡ Guo-Dong Du,‡ and Hua-Kun Liu‡ College of Chemistry, Central China Normal UniVersity, Wuhan, Hubei 430079, China, and Institute for Superconducting & Electronic Materials, and School of Mechanical, Materials & Mechatronic Engineering, UniVersity of Wollongong, NSW 2522, Australia ReceiVed: March 20, 2009; ReVised Manuscript ReceiVed: June 13, 2009
LiFePO4 thin films were deposited by radio frequency (rf) magnetron sputtering. The effect of substrate temperature during the rf magnetron sputtering on the morphology and characteristics of the LiFePO4 thin films has been investigated. Different substrate temperatures, from 25 to 500 °C, were applied during deposition. When the substrate temperature increased, the film structure changed from amorphous to crystalline, as characterized by X-ray diffraction. At the high substrate temperature of 500 °C, an impurity phase of Li3Fe2(PO4)3 was developed, and carbon particles in the film tended to aggregate in clusters on the substrate surface. The surface and cross-sectional morphology of the thin film was observed by using scanning electron microscopy. Electrochemical tests showed that the different characteristics of the as-deposited films could be attributed to the crystallography and morphology. The film deposited at the 400 °C substrate temperature exhibited an initial discharge capacity of 56 µAh/(cm2 · µm) at a current density of 10 µA/cm2 between 3.0 and 4.3 V, with good cyclability, suggesting that LiFePO4 thin film can be used as the cathode film for lithium microbatteries. Introduction All-solid-state thin film lithium microbatteries with high energy density are of great interest because they are promising candidates for micropower sources used in microelectromechanical systems, complementary metal-oxide semiconductor backup, smart cards, implantable medical devices, intelligent labels, microsensors, etc.1-4 For the thin film cathodes, many different material systems have been considered and proved to be feasible. So far, lithium transition metal oxides, such as LiCoO2, LiMn2O4, and V2O5 thin films, are the most popular cathode materials and have been intensively studied in the past decade due to the rapid development of lithium ion batteries.5-15 Recently, the use of polyanionic framework materials was proposed by Padhi et al.16 as a route toward high-performance cathode materials. Of these, LiFePO4, with an ordered olivine structure, exhibits reversible lithium insertion/extraction reactions and has a theoretical capacity of 170 mAh/g. It also has better thermal stability under fully charged conditions than the conventional transition metal oxides.17-20 Because of these advantages, LiFePO4 thin film has been attracting more attention as a promising thin film cathode for lithium microbatteries. LiFePO4 thin films have been prepared by pulsed laser deposition;21-24 however, there have been few papers published regarding radio frequency (rf) magnetron sputtering deposition of LiFePO4 thin films up to now.25-27 In this paper, we report the effects of the substrate temperature during rf magnetron sputtering on the morphology, the crystal structure, and the electrochemical performance of LiFePO4 thin films. * To whom correspondence should be addressed. E-mail:
[email protected]. † Central China Normal University. ‡ University of Wollongong.
Figure 1. XRD patterns of the films deposited on Pt/Ti/quartz glass at different substrate temperatures of (a) 25, (b) 300, (c) 400, and (d) 500 °C, and (e) LiFePO4 target. S denotes the diffraction peaks from the Pt film.
Experimental Section LiFePO4 films were deposited on Pt or Pt/Ti/quartz glass substrates by rf magnetron sputtering, using a synthesized 2 in. target. LiFePO4/C powder was prepared by a solid state reaction with LiOH · H2O, FeC2O4 · 4H2O, and NH4H2PO4, together with 20 wt % sugar, which was used as an electronic-conductor precursor. The mixture was first calcined at 300 °C for 3 h, then ground, and this was followed by a 650 °C sintering under Ar/H2 (92/8 by vol %) atmosphere for 24 h. The LiFePO4/C target was made by cold-pressing the powder, which had been mixed with 10 mol % of Li2O, into a pellet, and then sintering at 650 °C for 2 h in the same atmosphere. During rf magnetron sputtering, the distance between the target and the substrate was 5 cm. The sputtering gas was high
10.1021/jp902560q CCC: $40.75 2009 American Chemical Society Published on Web 07/16/2009
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Figure 2. Top surface and cross-section morphology of LiFePO4 films deposited on Pt/Ti/quartz glass at different substrate temperatures.
purity Ar (purity 99.999%), the sputtering gas pressure was set at 2.7 Pa, and the gas total flow was 20 sccm. An rf magnetron sputtering power of 100 W was applied to the target. A titanium underlayer was deposited to improve the adhesion of the Pt current collector to the quartz glass substrate. Before deposition of the films, the target was presputtered for 30 min, and the substrates were sputtered for 3 min under identical conditions in order to remove any contaminant atoms from the surfaces of the target and the substrates. During deposition, the substrates were kept at various temperatures: room temperature and 300, 400, or 500 °C. Film morphology and thickness were characterized by fieldemission scanning electron microscopy (FE-SEM, JEOL, JSM6700F). The texture and crystallography of the as-deposited films were characterized by X-ray diffraction (XRD, PHILIPS, X’pert). A total of 2016 type coin cells, with Li wafers as counter and reference electrodes and the LiFePO4 thin film as the
working electrode, were assembled to study the electrochemical performance of the as-deposited films on Pt substrate. The LiFePO4 thin film electrode was 1.77 cm2 in area and ∼400 nm in thickness. Celgard 2300 was used as the separator, and 1 M LiPF6 in ethylene carbonate/diethyl carbonate/dimethyl carbonate (EC/DEC/DMC, 1:1:1 by vol %) was used as the electrolyte. Charge-discharge tests were carried out on a Land electrochemical measurement system (Land, China) over the voltage range of 3.0-4.3 V at a current density of 10 µA/cm2. Cyclic voltammograms were collected on a CHI650A electrochemical analyzer (Shanghai, China) at a scan rate of 0.05 mV/s over the voltage range of 3.0-4.3 V. Results and Discussion The XRD patterns of the films deposited on Pt/Ti/quartz glass substrates over 3 h at substrate temperatures from 25 to 500 °C are shown in Figure 1a-d. For comparison, the XRD pattern of the LiFePO4/C composite target is also shown in Figure 1e,
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which exhibits pure crystallinity with the orthorhombic (Pmnb) structure of LiFePO4, in which (1 3 1) is the strongest reflection peak.16 The carbon contents in the films are around 3-4 wt % as analyzed by energy dispersive spectroscopy (EDS). No carbon-related signals can be detected in the XRD measurements. As can be seen from the XRD patterns of Figure 1a,b, the 25 and 300 °C films are amorphous. Some of the peaks overlap with the peaks (marked as S) from the Pt film substrate, which has the (1 1 1) orientation. As the temperature was increased to 400 °C (Figure 1c), some characteristic peaks appeared, which can be assigned to the LiFePO4. On further increasing the temperature to 500 °C (Figure 1d), the film exhibited enhanced crystallinity in the LiFePO4 peaks, as the intensities of the characteristic peaks increased and the peaks became sharp. At 400 or 500 °C, the as-deposited films show the (0 1 1) orientation. With increasing substrate temperature, the angular positions of the diffraction peaks, such as (0 1 1), (0 2 0), and (1 3 1), shift to higher angles. However, at the 500 °C substrate temperature, there was a small amount of Li3(Fe2(PO4)3 impurity phase,24 as indicated by the asterisked peaks at 23.2°, 33.2°, and 58.5°, which are in accordance with JCPDS file no. 78-1106 of Li3Fe2(PO4)3. Because there were no impurity phases in the target, the formation of Li3Fe2(PO4)3 may be attributed to the decomposition of the as-prepared LiFePO4 film at high substrate temperature during rf magnetron sputtering due to reaction with the residual O2 and H2O vapor from the chamber and other vacuum trace components.28 The results suggest that an optimum substrate temperature is essential to obtain well-crystallized LiFePO4 thin film, as the Li-Fe-P-O compound is sensitive to the substrate temperature and other deposition conditions. Figure 2 shows the surface and cross-sectional morphology of the films deposited on Pt/Ti/quartz glass substrates at different substrate temperatures. The image of the film prepared at the 25 °C substrate temperature shows a film with particle sizes of 100-150 nm and an average thickness of ∼400 nm. Another film, deposited at the 300 °C substrate temperature, also has an average thickness of ∼400 nm with a widely varying particle size of 100-200 nm. For the film prepared at the higher substrate temperature of 400 °C, the film surface is composed of significantly larger square-shaped particles (150-250 nm), possibly resulting from aggregation during the higher substrate temperature deposition. On further increasing the substrate temperature to 500 °C, the particle size becomes less uniform, ranging from 200 to 300 nm. This may be due to the fact that the higher substrate temperature promotes continuous grain growth. Of particular interest is that the surface morphology shows 1-10 nm particles lying over the film. Even though exact identification of the particle phase is not possible at this moment, the particles seem to consist of carbon particles, which come from the LiFePO4/C target at higher substrate temperature, or Li3Fe2(PO4)3 particles, which are produced during rf magnetron sputtering at the high substrate temperature of 500 °C. Because rf sputtered thin films contain great amounts of defects, viewed as high free energy sites, especially at higher substrate temperature, these high free energy sites can serve as seeds for particle aggregation, resulting in smaller grains. On the other hand, high free energy sites are unstable and may trigger phase transformation. Note that in the XRD pattern in Figure 1d, an impurity phase of Li3Fe2(PO4)3 is indeed developed at 500 °C. The initial charge-discharge and cycling curves of LiFePO4 cathode films prepared at 25, 300, 400, and 500 °C are shown in Figure 3. The full cells were cycled between 3.0 and 4.3 V at a constant current density of 10 µA/cm2. It can be seen in
Zhu et al.
Figure 3. The first charge-discharge curves of the as-deposited films at different Pt substrate temperatures: (1) 25, (2) 300, (3) 400, and (4) 500 °C.
Figure 3 that the 25 °C film had a low initial charge capacity of 4.3 µAh/(cm2 · µm) and an initial discharge capacity of 1.3 µAh/(cm2 · µm), respectively. The low capacity can be attributed to the amorphous nature of the thin film. This is because a low substrate temperature cannot provide enough energy to cause the sputtered particles on the substrate surface to recrystallize, resulting in low electrochemical reactivity. For the 300 °C film, the voltage profile in Figure 3 shows a plateau around 3.52 V on charge and 3.43 V on discharge, corresponding to the twophase reaction between LiFePO4 and FePO4. Its first charge and discharge capacities were 25.1 and 16.1 µAh/(cm2 · µm), respectively. The irreversible capacity loss was 9 µAh/(cm2 · µm) at the initial cycle. Figure 3 also shows that the 400 °C film had 56 µAh/(cm2 · µm) of initial discharge capacity, which is the highest among the as-prepared films. Its charging plateau is at 3.50 V, and the discharging plateau is at 3.45 V. For the 500 °C film, the initial voltage profile exhibited a relatively higher charging plateau of 3.55 V and almost the same discharging plateau (3.45 V) as compared with the 400 °C film. The first charge and discharge capacities were 78.3 and 45.3 µAh/(cm2 · µm), respectively, corresponding to ∼60% columbic efficiency. The great irreversible capacity loss of the first cycle may be associated with surface impurities such as Li3Fe2(PO4)3 and/or an inhomogeneous carbon distribution in the film, as well as difficulty in Li diffusion due to its large particles. The causes of the surface impurities include the lithium source, which contributes to the large charge capacity, but seems not to contribute to the discharge capacity. In other words, the lithium insertion/deinsertion must be irreversible. Moreover, large particles have large polarization resistance resulting from the difficulty in Li diffusion during Li insertion and extraction. Figure 4 shows the cycling performance and Coulombic efficiency of the four LiFePO4 films prepared at 25, 300, 400, and 500 °C. The 25 °C film has very low capacity, ∼2 µAh/ (cm2 · µm) from the first to the 50th cycle (Figure 4a), and its efficiency increases gradually from 30.5% of the first cycle to 85% of the 50th cycle as shown in Figure 4b. The capacity of the 300 °C film changes from 16.1 to 14.3 µAh/(cm2 · µm), and its Coulombic efficiency runs from 64% to 95% through 50 cycles. Among all four films, the 400 °C film has the highest capacity and the highest Coulombic efficiency. From the third cycle on, the lithium insertion/deinsertion became reversible with an efficiency of 100%, delivering a capacity of about 54 µAh/ (cm2 · µm). Although the 500 °C film also has relatively high capacity and Coulombic efficiency compared with those of the 25 and 300 °C films, it has a relatively low initial efficiency compared with that of the 400 °C film, as shown in Figure 4b. After several cycles, the discharge capacity was stable at 41
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J. Phys. Chem. C, Vol. 113, No. 32, 2009 14521 reference electrode. No obvious redox peaks were recorded for the films deposited at 25 and 300 °C. However, an intense peak at 3.56 V is observed for the 400 °C film during the first anodic scan (lithium deintercalation from the film), and during the first cathodic scan (lithium intercalation into the film), a peak around 3.39 V can be seen (Figure 5c). In the second cycle, the CV curves show an anodic peak occurring at ∼3.53 V, which shifts to a lower voltage value, and an unchanged cathodic peak at around 3.39 V. However, the CV of the 500 °C film shows an anodic peak at around 3.60 V in the first cycle, which broadens and shifts to a higher value, and a cathodic peak at 3.38 V. In the following cycles, the anodic and cathodic peak currents of the 500 °C film decreased slightly with cycling, whereas the peak currents of the 400 °C film were quite stable, indicating that the 400 °C film has better capacity retention than the 500 °C film. Conclusions
Figure 4. Cycling performance (a) and Coulombic efficiency (b) of LiFePO4 thin films deposited on Pt substrates at different temperatures: (1) 25, (2) 300, (3) 400, and (4) 500 °C.
LiFePO4 thin films were deposited on Pt or Pt/Ti/quartz glass by rf magnetron sputtering at different substrate temperatures. At the 400 °C substrate temperature, the as-deposited film contains pure LiFePO4 phase. Charge-discharge cycling results show that the 400 °C LiFePO4 film has superior electrochemical performance. It has 56 µAh/(cm2 · µm) of initial discharge capacity and small, but noticeable, capacity fading. The 400 °C film exhibits better electrochemical performance than the 25, 300, and 500 °C films. The CVs exhibit the characteristic redox peaks of LiFePO4 for the 400 and 500 °C films. Preliminary studies on the sputtered thin films show that the substrate temperature has a strong effect on the morphology and the electrochemical characteristics of LiFePO4 thin films. It is demonstrated that LiFePO4 thin film can be used as a promising cathode film for lithium microbatteries. Acknowledgment. This research was sponsored by the Natural Science Foundation of Hubei Province (No. 2006ABA317) and an Australian Research Council (ARC) Discovery Project grant (DP0878611). References and Notes
Figure 5. CV curves of the films deposited on Pt at different substrate temperatures at a scan rate of 0.05 mV/s: (a) 25, (b) 300, (c) 400, and (d) 500 °C.
µAh/(cm2 · µm), with a Coulombic efficiency close to 100%. The good charge/discharge features of the 400 °C film should be attributed to the pure phase of LiFePO4 as well as the relatively smaller crystallize size compared to the 500 °C film, which provides a shorter path for Li+ diffusion. Cyclic voltammograms (CVs) of the thin films prepared at 25, 300, 400, and 500 °C are shown in Figure 5a-d. CVs were recorded at a scan rate of 0.05 mV/s, using a three-electrode system in which lithium was used as both the counter and the
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JP902560Q
