Sputtered Porous Li-Fe-P-O Film Cathodes Prepared by Radio Frequency Sputtering for Li-ion Microbatteries

The increasing demands from micro-power applications call for the development of the electrode materials for Li-ion microbatteries using thin-film technology. Porous Olivine-type LiFePO4 (LFP) and NASICON-type Li3Fe2(PO4)3 have been successfully fabricated by radio frequency (RF) sputtering and post-annealing treatments of LFP thin films. The microstructures of the LFP films were characterized by X-ray diffraction and scanning electron microscopy. The electrochemical performances of the LFP films were evaluated by cyclic voltammetry and galvanostatic charge-discharge measurements. The deposited and annealed thin film electrodes were tested as cathodes for Li-ion microbatteries. It was found that the electrochemical performance of the deposited films depends strongly on the annealing temperature. The films annealed at 500 °C showed an operating voltage of the porous LFP film about 3.45 V vs. Li/Li+ with an areal capacity of 17.9 µAh cm−2 µm−1 at C/5 rate after 100 cycles. Porous NASICON-type Li3Fe2(PO4)3 obtained after annealing at 700 °C delivers the most stable capacity of 22.1 µAh cm−2 µm−1 over 100 cycles at C/5 rate, with an operating voltage of 2.8 V vs. Li/Li+. The post-annealing treatment of sputtered LFP at 700 °C showed a drastic increase in the electrochemical reactivity of the thin film cathodes vs. Li+, leading to areal capacity ~9 times higher than as-deposited film (~27 vs. ~3 µAh cm−2 µm−1) at C/10 rate.

be the most versatile way to fabricate thin-film cathodes 16,28 because the sputtering parameters are easily controlled to obtain a good quality film, such as sputtering power, argon pressure, and substrate temperature during deposition.
More recently, RF sputtering have attracted substantial attention in the fabrication of the flexible and transparent thin-film batteries. For example, Lee et al. 29 reported a flexible battery applied on contact lens using LFP cathode material deposited on the flexible polyimide substrate by RF sputtering, the thin film reached 63.8% of the theoretical capacity without carbon coating and the cell showed stable performance under wet conditions. Also, Oukassi et al. 30 deposited transparent thin film batteries layers using RF sputtering, a discharge capacity of 0.15 mAh at C/2 rate with an average capacity loss of solely 0.15% per cycle can be obtained.
Due to a strong research interest focused on searching for lithium transition metal phosphates as cathode materials, NASICON type-Li 3 Fe 2 (PO 4 ) 3 has been reported of having a high lithium mobility and can intercalate up to two moles of lithium within the structure 31 . Besides a theoretical capacity of 128 mAh g −1 32 , Li 3 Fe 2 (PO 4 ) 3 cathode is currently attracting considerable interest due to several advantages such as low toxicity, low cost, good ionic conductivity, and large natural abundance 33 . Furthermore, the structure of the NASICON type-Li 3 Fe 2 (PO 4 ) 3 is more stable than that of olivine-type LiFePO 4 34 and its synthesis is easier as it can be prepared directly in air. NASICON type-Li 3 Fe 2 (PO 4 ) 3 have been commonly synthesized via various methods such as solid-state reactions 35 , hydrothermal 36 , sol-gel combustion 32 and ultrasonic spray pyrolysis 37 . However, there are only limited reports regarding the Li 3 Fe 2 (PO 4 ) 3 film deposited by RF Sputtering 12 . In the present work, we propose to study the effect of annealing temperature on RF sputtered LFP thin-films in terms of morphology, composition, structure, and electrochemical properties. We show that the thermal treatment performed at 700 °C in air leads to the formation of porous Li 3 Fe 2 (PO 4 ) 3 showing the best electrochemical performance and therefore being a good candidate as a cathode for Li-ion microbatteries.

Results and Discussion
Structural and chemical characterization. The schematic representation of the sputtering system is illustrated in Fig. 1. The sputtering process can be briefly described as follows: an inert argon gas is introduced in the vacuum chamber and the ionization process leads to the formation of a plasma. Then, the energized ions hitting the LFP target (cathode) eject out adatoms that are deposited on the substrate to form a thin film. Various parameters such as the nature of the substrate, the sputtering power, the argon pressure, the duration, and the deposition temperature have significant influences on the growth rate and features of the deposited film 14 .
In this work, a sputtering power of 3 W cm −2 and argon pressure of 7 mTorr during 12 hours of deposition was employed to obtain smooth and dense layers as well as a better adhesion between the films and the substrates. Owing to a low deposition rate, 12 hours of deposition is required to obtain LFP films with a sufficient thickness for the targeted application. The XRD pattern of the commercial LFP target and as-deposited film are plotted in Fig. 2a. Obviously, the initial target has well-defined peaks corresponding to the Olivine LiFePO 4 (JCPDS File No. 040-1499). However, after deposition at room temperature, only Ti peaks can be identified suggesting that the material is amorphous. The amorphous nature of the thin films deposited without heating treatment, either substrate heating during deposition or post-annealing have been reported in previous studies 12,14 . Thus, the thermal treatment step is essential because the crystalline materials are known for their better electrochemical reactivities versus Li ions 38 . Figure 2b,c display the SEM images of the thin films deposited at 3 W cm −2 W during 12 hours. As-deposited film shows a dense layer with a thickness of approximately 1.6 µm that corresponds to a growth rate of 2.2 nm min −1 . EDX analysis after deposition (Fig. 2d) revealed a Fe:P atomic ratio of 14.33(1): 14.58 (1), which is in agreement with the LFP formula. To further study in detail the effects of the annealing temperature, the as-deposited LFP films were annealed at 400 °C, 500 °C, 600 °C, and 700 °C, respectively in air atmosphere for one hour. In agreement with the previous studies, as-deposited films show poor crystallinity. As the films are annealed, the adsorbed atom on the surface of the films are activated and rearranged to form ordered crystal structure 39 . The XRD patterns for the LFP films are gathered in Fig. 3 for analyzing the effects of the thermal treatment at various temperatures on their crystal structure. At first glance, the typical peaks ascribable to crystalline LFP and Li 3 Fe 2 (PO 4 ) 3 phases can be detected at elevated temperatures. The amorphous nature of LFP is evidenced for the film annealed at 400 °C since only Ti peaks are visible. This result confirms that annealing temperature higher than 400 °C is required to crystallize LFP thin film 13 . As the temperature was increased to 500 °C, several characteristic peaks of 2θ suited at 17.21°, 20.82°, 25.65°, 29.87° and 32.36° marked as symbol (o) are attributed to the (0 2 0), (0 1 1), (0 2 1), (1 2 1) and (0 3 1) planes of LFP phase. Besides the presence of LFP peaks, two small Li 3 Fe 2 (PO 4 ) 3 peaks with low intensities marked as symbol (*) are also detected at 2θ of 24.4° and 35.8° suggesting that the decomposition of LiFePO 4 phase has also started. Nevertheless, crystalline LFP seems to be the most predominant phase at 500 °C. This is probably due to a slow heating rate that is applied during annealing treatment. As reported elsewhere 40 , the heating rate plays an important role in decelerating a second phase formation. It is important to note that in our work the films were heated at 2 °C min −1 to avoid the fast formation of Li 3 Fe 2 (PO 4 ) 3 phase due to the LFP sensibility to air atmosphere. Thus, the LFP phase can be clearly detected at 500 °C after annealing in air atmosphere. Presumably, Li 3 Fe 2 (PO 4 ) 3 phase might be quickly obtained when the heating rate is faster than 2 °C min −1 in air. When the annealing temperature further raised to 600 °C, more Li 3 Fe 2 (PO 4 ) 3 (JCPDS  file no. 047-0107) peaks have appeared. The annealed films at 600 °C are assumed to be composed of mixed LFP and Li 3 Fe 2 (PO 4 ) 3 phases. In a good agreement with the previous reports 17,18,24,40 , the optimum annealing temperature for the crystallization of LFP was 500 °C. As expected, by increasing the annealing temperature up to 700 °C, almost all phases are transformed to Li 3 Fe 2 (PO 4 ) 3 due to the oxidation of Fe 2+ by oxygen from air according to Eq. (1) 12 .
Another LFP phase with very low intensity at 2θ = 20.82° is detected as minor phase. However, the peak intensity appears to be weakening at this temperature. As previously reported 41 , encouraging results on the electrochemically active Li 3 Fe 2 (PO 4 ) 3 phase that obtained after annealing at 700 °C have been studied in the form of composite electrodes carrying polymer binder and carbon additive. NASICON-type Li 3 Fe 2 (PO 4 ) 3 can be considered as an interesting cathode material for Li-ion batteries by hosting about 2 additional Li + (Fe 3+ /Fe 2+ redox couple) with great reversibility at a voltage plateau of around 2.8 V [23][24][25][26][27] . However most reports on this material focus on composite Li 3 Fe 2 (PO 4 ) 3 cathodes prepared by slurry coatings instead of thin films.
In-depth structural studies have been also carried out by in-situ XRD experiments. The purpose of this particular structural analysis is to follow the evolution of the formed phases at elevated temperature after deposition time of 3 hours. The different XRD patterns given in Fig. 4 were acquired during annealing process between 400 °C and 700 °C using steps of 20 °C. Starting from 400 °C, a low intensity peak corresponding to LFP appeared at 2θ = 35.90°. In fact, Li 3 Fe 2 (PO 4 ) 3 starts to crystallize at 420 °C which is shown by the presence of a small peak at 2θ = 35.36° with a low crystallinity. Both Li 3 Fe 2 (PO 4 ) 3 and LFP peaks can be clearly observed in the temperature range between 560 °C and 640 °C, mainly the stronger peak intensity located at 2θ = 20.82° for LFP phase and 2θ = 24.14° for Li 3 Fe 2 (PO 4 ) 3 phase. Then, the intensities of both peaks are getting weaker and even disappeared when annealing temperatures are higher than 640 °C or lower than 560 °C. This temperature range is considered to be less preferred for the Li ion intercalation/de-intercalation in the cathode materials. At the temperature below 560 °C, the presence of LFP phase is more pronounced, on the contrary when the annealing temperature is higher than 640 °C the appearance of Li 3 Fe 2 (PO 4 ) 3 phase becomes predominant. Similar results on the LFP powders were observed by Hamelet et al. 41 and Delacourt et al. 42 , demonstrating the significant decomposition of the LFP phase occurred on thermal treatment at temperatures greater than ~ 500 °C and the progressive sharpening peaks for the Li 3 Fe 2 (PO 4 ) 3 can be observed afterwards. Bünting et al. 43 reported that the Ti interdiffusion occured during the crystallization process of the LFP thin film, leading to the LiTi 2 (PO 4 ) 3 (LTP) phase formation. From the XRD www.nature.com/scientificreports www.nature.com/scientificreports/ pattern, the reflection peaks at 2θ = 20.82° and 2θ = 24.4° which overlap with the reflection peaks of LFP and Li 3 Fe 2 (PO 4 ) 3 are characteristics for LTP phase 44 .
As the electrochemical performance of electrodes are often driven by their morphology, the examination of thin-films was performed by SEM. Figure 5 shows the surface of the annealed thin films. Apparently, the roughness increases with increasing of the annealing temperature.
According to the SEM examinations, LFP-400 is composed of small grains and exhibits a rough surface due to an inhomogeneous size distribution of particles (Fig. 5a,b). As the temperature is raised to 500 °C, the surface is characterized by the presence of large pores with various diameters (Fig. 5c,d), suggesting the formation www.nature.com/scientificreports www.nature.com/scientificreports/ of crystalline LFP phase 40,45 . As the temperature reaches 600 °C, the microstructured deposit is highlighted by appearance of grain boundaries with various grain sizes. After annealing at 700 °C, the surface becomes coarse and highly porous, which is consistent with the formation of the NASICON-type Li 3 Fe 2 (PO 4 ) 3 phase 12 . From the above results, it is interesting to note that the texture of LFP thin films shows some cracks except for the sample annealed at the highest temperature. electrochemical characterization. Cyclic voltammetry (CV) experiments were performed to evaluate the influence of the annealing process on the reactivity of LFP films with Li + . Figure 6 shows the voltammograms obtained from the first cycle. No redox peak can be identified for LFP-0 (Fig. 6a) confirming that amorphous LFP is not electrochemically active. A similar behavior is observed for LFP-400 (Fig. 6b) suggesting that amorphous LFP is predominant despite the detection of a small peak by in-situ XRD which is attributed to the crystalline LFP phase. The LFP-500 film showed a well-defined peak pair and reversible redox peaks at 3.65 V and 3.33 V vs. Li/ Li + corresponding to the anodic and cathodic peaks, respectively (Fig. 6c). These peaks are typical for the oxidation and reduction reactions of olivine-type LiFePO 4 material with Li + according to Eqs (2) and (3)  The low potential difference between the oxidation and reduction peaks (∆Ep) about 0.32 V vs. Li/Li + indicates that the film reveals a good electrochemical reversibility for Li + intercalation/de-intercalation. In addition, a broad peak pair with very low absolute current densities can also be observed at 2.79 V for the oxidation peak and 2.81 V for the reduction peak. These peaks correspond to the reaction of Li ions with the Li 3 Fe 2 (PO 4 ) 3 phase.
As the annealing temperature increases from 500 °C to 600 °C, a small oxidation peak at 3.48 V and a reduction peak at 3.42 V which are attributed to the two-phase transformation of the (Fe 3+ /Fe 2+ ) redox couple due to the reversible intercalation of Li ions in the LFP crystal structure (Fig. 6d) are progressively vanished. This result is in good agreement with the XRD data showing that the LFP phase is clearly observable at 600 °C while the complete transformation into Li 3 Fe 2 (PO 4 ) 3 phase has yet to be occurred. In addition, a pair of anodic and cathodic peaks at around 2.29 V and 2.26 V vs. Li/Li + could be attributed to the formation of the iron tavorite phase LiFePO 4 (OH) 12,47-49 according to Eq. (4) Thin film annealed at 700 °C (Fig. 6e) reveals a peak pair, a reduction peak at 2.77 V and an oxidation peak at 2.82 V which are attributed to the reversible intercalation reactions of Li + ions into Li 3 Fe 2 (PO 4 ) 3 as described by Eq. (5) 12 . The cathodic and anodic peaks are symmetric and have similar current density values, indicating the excellent degree of reversibility. In a good agreement with the XRD analysis, due to the interdiffusion between the Ti and the deposited LFP thin film, the additional cathodic peak at 2.38 V and anodic peak at 2.42 V are observable at 600 °C and 700 °C which are characteristics for the LiTi 2 (PO 4 ) 3 44 phase, according to Eq. (6) Preliminary CV results highlight that LFP thin films performances are strongly related to their structure and morphology. Indeed, the capacities of thin films prepared under different conditions varied significantly. Figure 7 shows the typical charge-discharge profiles of the five cathode thin films in the potential range 2-4 V at a current rate of C/10. In accordance with the CV curves, no voltage plateau can be observed for LFP-0 and LFP-400, delivering very low capacity values (<10 µAh cm −2 µm −1 ). More remarkably, a couple of defined charge-discharge plateaus at 3.3 and 3.5 V are clearly seen at 500 °C corresponding to the reversible intercalation of lithium ions in crystallized LiFePO 4 . These charge-discharge plateaus are fully consistent with the peaks of the CV curves. In many previous reports, crystalline LFP films can be obtained after annealing under inert atmosphere. In this work, the presence of the LFP phases can be detected after annealing treatment in air atmosphere. This result suggests that the phase transformation is slower, probably due to the low heating rate applied during the annealing treatment as previously mentioned. The capacities values for the 1 st , 2 nd , and 3 rd cycles are 39 µAh cm −2 µm −1 , 37 µAh cm −2 µm −1 and 37 µAh cm −2 µm −1 with the corresponding coulombic efficiencies of 88%, 90% and 88%, respectively. The slight decrease of the coulombic efficiency is probably due to the small amount of Li 3 Fe 2 (PO 4 ) 3 phase that has been evidenced by XRD analysis.
For LFP-600, the voltage plateau around 2.8 V is ascribed to the reaction implying the Li 3 Fe 2 (PO 4 ) 3 phase. The presence of a short plateau at 3.5 V can be attributed to the low amount of crystalline LFP. The capacities of the LFP film annealed at 600 °C for the 1 st , 2 nd , and 3 rd cycles are 27 µAh cm −2 µm −1 , 20 µAh cm −2 µm −1 and 18 µAh cm −2 µm −1 and the coulombic efficiencies are 57%, 80% and 84%, respectively. The low coulombic efficiency obtained in the 1 st cycle could be explained by the mixed phases. In addition, the LFP film annealed at 700 °C revealed a well-defined plateau around 2.4 V which indicates the Li ions insertion into the LiTi 2 (PO 4 ) 3 lattice structure and another plateau appeared at 2.8 Vcorresponds to redox reactions involving the Li 3 Fe 2 (PO 4 ) 3 phase [50][51][52] . The capacities of the film for the 1 st , 2 nd , and 3 rd cycles are 31 µAh cm −2 µm −1 , 28 µAh cm −2 µm −1 and 27 µAh cm −2 µm −1 , while the associated coulombic efficiencies can reach 83%, 90% and 92%, respectively. Beyond the influence of the chemical and structural composition, the enhanced electrochemical properties of LFP-500 and LFP-700 can be explained by the porosity of the both layers. Actually, it has been reported that the use of porous electrodes can improve the electrochemical performance of batteries owing to the larger electrode/ electrolyte interface 53,54 . In the present case, the porous nature of the LFP-500 and LFP-700 films is supposed to promote the penetration of the electrolyte providing more reaction electrochemical actives sites for the lithium ions intercalation/de-intercalation owing to larger accessible surface of the thin films. This is beneficial to improve the Li ion exchange between the LFP film electrode and electrolyte because the pores serve as channels for fast lithium supply 55 .
In order to have a better insight into the potential use of annealed LFP layer as cathode materials, rate capability and long-term cycling tests were carried out. Figure 8a confirms that LFP-400 film provides the lowest capacity values attaining only 2 µAh cm −2 µm −1 at the 50 th cycle. LFP-500 film exhibits the highest discharge capacities up to the 90 th cycle but this cathode is subjected to a continuous capacity loss with a drop after the 80 th cycle. The capacities obtained at the 1 st , 2 nd , and 100 th cycle are 36 µAh cm −2 µm −1 , 35 µAh cm −2 µm −1 , and 18 µAh cm −2 µm −1 , respectively. Thus, only half of the initial capacity is retained after 100 cycles. It can be noticed that similar behavior regarding the capacity fading has been reported for the annealed LiNi 1/3 Co 1/3 Mn 1/3 O 2 thin films at 500 °C 22 . Such a low cycling stability may be ascribed to the poor adhesion strength between the thin film and the substrate that are combined to the presence of cracks, leading to the detachment of the layer from the  www.nature.com/scientificreports www.nature.com/scientificreports/ current collector and the subsequent loss of electrical contact. For the LFP-600 film, the first discharge capacity is relatively high (29 µAh cm −2 µm −1 ) but severely drops to reach only 8 µAh cm −2 µm −1 after the 50 th cycle. This behavior could be attributed to the presence of cracks and mixed unstable LiFePO 4 and Li 3 Fe 2 (PO 4 ) 3 phases. Finally, LFP-700 film shows the best cycling stability. The capacities obtained for the LFP-700 were found to be 32 µAh cm −2 µm −1 for the first cycle and 22 µAh cm −2 µm −1 for the 100 th cycle corresponding to a capacity retention of around 70%. The good storage performance and the stability of the LFP-700 film could be mainly attributed to feature properties of the Li 3 Fe 2 (PO 4 ) 3 thin film. Compared to LFP-700 sample, the sharp capacity dropping of LFP-400, LFP-500 and LFP-600 in the first few cycles can be imputed to the cracks that are responsible for the poor mechanical properties. For more details, the areal capacity values and the capacity retention at the 50 th and 100 th cycles are summarized in Table 1.
Although the initial C/10 and C/5 discharge capacity are superior compared to LFP-700 film, the LFP-500 shows relatively larger degradation in capacity with further increasing discharge current. Indeed, significant capacity losses of 28% and 48% are observed for subsequent tests at C/5 and 1C rate, respectively. The LFP-700 film cathode delivers capacity values of 25 µAh cm −2 µm −1 at C/10 rate, 22 µAh cm −2 µm −1 at C/5 rate, and 19 µAh cm −2 µm −1 at 1C rate. The capacity slightly decreased of only 11% from C/10 to C/5 rate and decreased of about 15.5% from C/5 to 1C rate. More remarkably, the discharge capacity values of about 22 µAh cm −2 µm −1 and 25 µAh cm −2 µm −1 at C/5 and C/10 show that the full capacity is completely recovered even after fast cycling tests. Therefore, it is found that the highest annealing temperature treatment (700 °C) is able to strongly improve the electrochemical stability of LFP films electrodes, especially in the rapid charge-discharge regimes. After few initial cycles, LFP-700 film reveals high reversibility and prominent cycling stability. The better performance of LFP-700 film is also attributed to the presence of the LTP phase as a good ionic conductor 56 .
To sum up, the amorphous and partially crystalline LFP phases may not be stable during Li-ion intercalation/ de-intercalation, yielding in a fast capacity fading 22 . Thus, through this study, we highlight the important role of the post annealing treatment mainly for the intercalation cathodes. This treatment is responsible for the film texturation, thus it could help minimizing ionic limitation by enhancing the contact area between electrolyte and the film surface contact, as well as could enhance the electrochemical activity 54,57-59 . conclusion In summary, we have successfully prepared porous thin film cathodes by RF sputtering technique and subsequent annealing treatment in air atmosphere. The electrochemical performance of the thin films depends strongly on the annealing conditions. Different morphological and structural properties were obtained by varying the temperature treatment, leading to a significant influence on the cell performances. We report that annealing at 500 °C is required to obtain porous olivine type-LiFePO 4 film even if the electrochemical tests revealed a drop of capacities after the 80 th cycle. Complete transformation from porous olivine type-LiFePO 4 to porous NASICON-type Li 3 Fe 2 (PO 4 ) 3 occurring at 700 °C led to a remarkable stability of the electrode with good electrochemical performance even at fast kinetics.

Methods
Preparation of LFP thin films. Thin films were deposited on Ti foil substrate by radio frequency sputtering in a Plassys MP 300 apparatus at room temperature. In the present work, radio-frequency (RF) sputtering is utilized to deposit LFP layers from a commercial target. The target was LiFePO 4 (LFP) purchased from Neyco, purity 99.9%. Prior to their insertion in the deposition chamber, Ti foils were cleaned by ultrasonication in acetone, 2-propanol, and methanol for 10 minutes each. Pre-sputtering of the targets before deposition was employed to remove the native oxide layer. Before deposition, vacuum was applied into the sputtering chamber until the pressure was around 3 × 10 −6 mbarr. Sputtering was performed in a pure argon atmosphere. The argon pressure was set to 7 mTorr and the applied RF sputtering power was fixed to 75 W (3 W cm −2 ). Deposition time was adjusted for 12 hours. The post-annealing treatment was carried out at various temperatures (400 °C, 500 °C, 600 °C, and 700 °C) for one hour with a heating rate of 2 °C min −1 (Furnace: NABERTHERM Controller B 180). The as-deposited thin film was labelled as LFP-0 and post-annealed thin films at 400 °C, 500 °C, 600 °C and 700 °C were labelled as LFP-400, LFP-500, LFP-600, and LFP-700, respectively.