A Nanocrystalline Fe2O3 Film Anode Prepared by Pulsed Laser Deposition for Lithium-Ion Batteries

Nanocrystalline Fe2O3 thin films are deposited directly on the conduct substrates by pulsed laser deposition as anode materials for lithium-ion batteries. We demonstrate the well-designed Fe2O3 film electrodes are capable of excellent high-rate performance (510 mAh g− 1 at high current density of 15,000 mA g− 1) and superior cycling stability (905 mAh g− 1 at 100 mA g− 1 after 200 cycles), which are among the best reported state-of-the-art Fe2O3 anode materials. The outstanding lithium storage performances of the as-synthesized nanocrystalline Fe2O3 film are attributed to the advanced nanostructured architecture, which not only provides fast kinetics by the shortened lithium-ion diffusion lengths but also prolongs cycling life by preventing nanosized Fe2O3 particle agglomeration. The electrochemical performance results suggest that this novel Fe2O3 thin film is a promising anode material for all-solid-state thin film batteries.


Background
With the ever-increasing applications of lithium-ion batteries (LIBs) in portable electronics and electric vehicles, there has been extensive research on developing advanced electrode materials with higher energy and power densities [1][2][3][4][5][6][7]. Since the first report on reversible lithium storage in transition metal oxides (TMOs) by Poizot et al. [8], TMOs (Co 3 O 4 [9,10], NiO [11,12], Fe 2 O 3 [13][14][15], and CuO [16,17]) have been widely explored as anode materials due to their higher theoretical specific capacity and better safety in comparison with traditional carbon anode materials. Among all these TMOs, Fe 2 O 3 received much attention in recent years due to its high theoretical specific capacity (~1005 mAh g − 1 ), low cost, abundant resources, and environmental benignity. However, like other TMOs, the huge volume variations associated with Li-ion insertion/extraction often leads to the pulverization and subsequent falling off of the active materials from the electrode, which results in a significant capacity fade, poor cycling stability, and poor rate capability. To circumvent these problems, many nanostructures of Fe 2 O 3 have been synthesized for lithium-ion batteries, such as nanorods [18,19], nanoflakes [20,21], hollow sphere [22][23][24], coreshell arrays [25], and micro-flowers [26].
Besides all the above nanostructures, nanocrystalline thin film anodes (NiO [27], MnO [28], Cr 2 O 3 [29], CoFe 2 O 4 [30], Si [31], and Ni 2 N [32]) deposited directly on conducting substrates by pulsed laser deposition or sputtering can also exhibit an excellent electrochemical performance due to the enhanced electrical contact between the substrates and active materials, the shortened diffusion lengths for lithium-ion, and the structure stability. What is more important is that thin films of TMOs have potential applications in all-solid-state microbatteries as self-supported electrodes [33,34]. The TMOs' films can replace the lithium film anode which limits the integration of microbatteries with circuits due to the low melting point and strong reactivity with moisture and oxygen. However, up to now, there have been few reports on the Fe 2 O 3 film anodes deposited by pulsed laser deposition or sputtering, and the reported specific capacities were much lower than the theoretical specific capacity of Fe 2 O 3 [35,36].
In this work, we prepared nanocrystalline Fe 2 O 3 films by pulsed laser deposition (PLD) as an anode material for lithium-ion batteries. The Fe 2 O 3 thin film anodes with average grain size of several tens of nanometers showed high reversible capacity of 905 mAh g − 1 at 100 mA g − 1 and high rate capacity of 510 mAh g − 1 at 15000 mA g − 1 . The remarkable electrochemical performance demonstrates that nanocystalline Fe 2 O 3 thin film has potential applications in high performance LIBs, especially all-solid-state thin film batteries.

Synthesis of Nanocrystalline Fe 2 O 3 Films
The films of Fe 2 O 3 were deposited directly on copper foils or stainless steels by a PLD technique in oxygen ambient. A KrF excimer laser with a wavelength of 248 nm was focused on the rotatable target of metal Fe. The repetition rate was 5 Hz, and the laser energy was 500 mJ. The distance between the target and the substrate was 40 mm. In order to get nanocrystalline Fe 2 O 3 films, we grew samples at room temperature under oxygen pressure of 0.3 Pa on both copper foil and stainless steels. They showed the same electrochemical performance. The thickness of the nanocomposite film is approximately 200 nm as determined by atomic force microscope (AFM, Park systems XE7). The mass of 0.121 mg was obtained by measuring the difference of substrate before and after deposition via electrobalance (METTLER TOLEDO).

Material Characterization
The crystalline phase of the Fe 2 O 3 film was characterized by X-ray diffraction (XRD) on a Rigaku D/Max diffractometer with filtered Cu Kα radiation (λ = 1.5406 Å) at a voltage of 40 kV and a current of 40 mA. High-resolution transmission electron microscopy (TEM) and selected area electron diffraction (SEAD) were carried out by a JEOL 100CX instrument. For the TEM measurement, the Fe 2 O 3 film grown on NaCl substrate was put into water to dissolve the NaCl. After that, the suspension was dropped onto a holey carbon grid and dried. The morphology of the samples were observed by scanning electron microscopy (SEM) using a SU8010. X-ray photoelectron spectroscopy (XPS) measurement was performed on a Thermo Scientific ESCALAB 250XI photoelectron spectrometer.

Electrochemical Measurements
For the electrochemical measurements, conventional CR2032 type coin cells with the Fe 2 O 3 nanocrystalline film anodes were assembled inside an argon-filled glove box with the oxygen and moisture content below 0.1 ppm. The electrochemical cells were prepared using lithium metal as the counter electrode and a standard electrolyte of 1:1:1 ethylene carbonate (EC)/dimethyl carbonate (DMC)/LiPF 6 . Galvanostatic cycling measurements were processed at room temperature by a LAND-CT2001A battery system at various current rates between 0.01 and 3.0 V. Cyclic voltammetry (CV) and AC impedance measurements were performed with a CHI660E electrochemical workstation (CHI Instrument TN). The scanning rate was 0.1 mV s − 1 .

Results and Discussion
X-ray diffraction (XRD) patterns of the Fe 2 O 3 film are shown in Fig. 1a. It can be observed that there is no obvious peak except the peaks of cubic crystal Cu substrate, suggesting that the Fe 2 O 3 film is amorphous or crystallized with nanosized grains. Such phenomenon could be attributed to the deposition occurred at room temperature. In order to determine the chemical composition of the obtained film, XPS measurement was performed as shown in  the literature [37][38][39]. To further reveal the structure and composition of as-deposited thin films, TEM characterization was conducted as shown in Fig. 2. It revealed that the Fe 2 O 3 films were made of small nanograins with average size of several tens of nanometers. The HRTEM image clearly presents the lattice fringes of the (110) corresponding to d-spacing of 0.251 nm of α-Fe 2 O 3 . Meanwhile, the ring-like feature of the selected area electron diffraction (SAED) confirmed the polycrystalline nature of Fe 2 O 3 film. As shown by the SEM images in Fig. 2c, the Fe 2 O 3 film consists of particles in nanometer scale. Based on all these results, we can confirm that the film deposited at room temperature is composed of Fe 2 O 3 with ultrafine nanosized crystalline grains.
The electrochemical performance of the electrode made of Fe 2 O 3 nanocrystalline film was firstly evaluated by cyclic voltammetry (CV). Figure 3 shows the first three CV curves of Fe 2 O 3 nanocrystalline film anode. The CV curves are similar to the previous reports of Fe 2 O 3 anode [40][41][42][43][44][45][46]. In the first cathodic process, three peaks were observed at 1.38, 1.02, and 0.84 V, which could be related to a multi-step reaction. First, the very small peak at 1.38 V may be due to the lithium insertion into the crystal structure of Fe 2 O 3 film forming Li x Fe 2 O 3 without change in the structure [40,43]. Second, another peak at about 1.02 V could be ascribed to phase transition from hexagonal Li x Fe 2 O 3 to cubic LiFe 2 O 3 . The third sharp reduction peak at 0.84 V corresponds to the complete reduction of iron from Fe 2+ to Fe 0 and the formation of solid electrolyte interface (SEI). In the anodic process, two broad peaks observed at 1.57 and 1.85 V represent the oxidation of Fe 0 to Fe 2+ and further oxidization to Fe 3+ . In the subsequent cycles, the reduction peaks were replaced by two peaks locating around 0.88 V because of the irreversible phase transformation in the first cycle. The overlapping of the CV curves during the following 2 cycles demonstrated good reversibility of the electrochemical reactions, and this was further confirmed by the cycling performance. Figure 4a shows the discharge and charge profiles of the Fe 2 O 3 nanocrystalline film for different cycles at a specific current of 100 mA g − 1 with a voltage range of 0.01-3 V. Obvious voltage hysteresis are observed due to the conversion reaction during charge/discharge processes, and the voltage plateaus are in good agreement with the above CV results. The clear voltage slopes observed in each charge/discharge process indicate the oxidation of Fe to Fe 3+ and the reduction of Fe 3+ to Fe, respectively. The smooth slope from 1.5 to 2.0 V in the charge process represents the two oxidation peaks in the CV curves. Meanwhile, the plateau or slope around 0.9 V in the discharge process represents the reduction peak in the CV curves. The initial discharge and discharge capacity of the Fe 2 O 3 nanocrystalline film are 1183 and 840 mAh g − 1 , The irreversible capacity loss is mainly attributed to the formation of SEI layer on the surface of anode, which is commonly observed in most anode materials [44][45][46][47].
The cycling performance of the film electrode at a specific current of 100 mA g − 1 at room temperature is shown in Fig. 4b. It can be seen that the reversible capacity gradually increases to 951 mAh g − 1 after the 70 cycles and then keeps stable in the range of 900-950 mAh g − 1 with a Coulombic efficiency nearly 100% during the following cycles. Similar phenomenon of the capacity increasing during cycling has been found in many transition metal oxide electrodes in previous studies [13,[48][49][50][51][52]. The possible reason for this would be the electrode activation, which induces the reversible growth of polymer/gellike films to increase the capacity at low potentials [50]. Compared with the previous reports of Fe 2 O 3 film anode batteries deposited by pulsed laser deposition or sputtering [35,36], the capacity of Fe 2 O 3 in our work has a considerable improvement as summarized in Table 1.
Previous studies on the effect of particle size on lithium intercalation into Fe 2 O 3 shows that nanocrystalline Fe 2 O 3 exhibited better electrochemical performance    [53]. To confirm the role of particle size in the electrochemical performance, we annealed the as-prepared Fe 2 O 3 film on stainless steels at 400°. The prepared Fe 2 O 3 film anode at high temperature was deposited on stainless steels only due to the instability of copper foil. The morphology comparison in Fig. 5a and Fig. 2c confirms that the particle sizes of the samples annealed at high temperature are obviously larger. Figure 5b shows that the capacities was only about 263 mAh g − 1 after 100 circles, which was much lower than the specific capacity of as-prepared Fe 2 O 3. In addition, we also fabricated Fe 2 O 3 film anode with larger particle size on stainless steels under 400°C as shown in Fig. 6a. Figure 6b shows its discharge and charge profiles for different cycles at a specific current of 100 mA g − 1 . The capacities dropped to 361 mAh g − 1 after 50 circles. These results indicate that the enhanced reversible capacity of nanocrystalline Fe 2 O 3 film grown at room temperature can be attributed to the nanoscaled structure of the thin film electrode, which can sustain high lithium insertion strain because of the smaller number of atoms and large surface areas within nanoparticles [13,14,54].
To investigate the kinetics of lithium inserting/deinserting, electrochemical impedance spectra measurement was performed in Fig. 7a. The charge-transfer impedance on the electrode/electrolyte surface is about 50 Ω, which can be deduced from the single semicircle in the high-middle frequency. The superior conductivity of the film electrode without binder can be attributed to the nanocrystalline structure of the Fe 2 O 3 film and the enhanced electrical contact between active anode and substrate. The good conductivity of the nanocrystalline Fe 2 O 3 film anode led to excellent rate performance. Figure 7b shows the charge/discharge capacities at different current densities. The anode delivered capacities up to 855, 843 , 753, 646, and 510 mAh g − 1 at high current densities of 750, 1500, 3000, 7500, and 15,000 mA g − 1 , respectively, which is corresponding to 98.2, 96.7, 87.8, 75.3, and 59.5% retention of the capacity at 250 mA g − 1 (about 871 mAh g − 1 ). More importantly, when the specific current reduced to 250 mA g − 1 , the capacity could recover to 753 mAh g − 1 . The excellent rate performance benefits from both the good conductivity of the anode and the increase of capacity upon cycling.

Conclusions
In summary, nanocrystalline Fe 2 O 3 film anode has been deposited by pulsed laser deposition at room temperature. The results of structure and morphology characterization showed that the deposited films are composed of nanocrystalline Fe 2 O 3 with grain size of several tens of nanometers. The prepared Fe 2 O 3 exhibits an excellent electrochemical performance, such as superior cycling stability (905 mAh g − 1 at a specific current of 100 mA g − 1 after 200 cycles) and high rate capability (510 mAh g − 1 at 15000 mA g − 1 ). The outstanding electrochemical performance can be related to the nanocrystalline structure of Fe 2 O 3 which could sustain high strain, shorten diffusion lengths for lithium-ion, and keep the structure stable. The excellent electrochemical performance and room temperature growth suggest that nanocrystalline Fe 2 O 3 has potential application in high performance LIBs, especially in all-solid-state thin film batteries.