Nanocrystalline Fe O thin films are deposited directly on the conduct substrates by pulsed laser deposition as 2 3 anode materials for lithium-ion batteries. We demonstrate the well-designed Fe O film electrodes are capable of 2 3 − 1 − 1 excellent high-rate performance (510 mAh g at high current density of 15,000 mA g ) and superior cycling − 1 − 1 stability (905 mAh g at 100 mA g after 200 cycles), which are among the best reported state-of-the-art Fe O 2 3 anode materials. The outstanding lithium storage performances of the as-synthesized nanocrystalline Fe O film 2 3 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 Fe O particle agglomeration. 2 3 The electrochemical performance results suggest that this novel Fe O thin film is a promising anode material for 2 3 all-solid-state thin film batteries. Keywords: Lithium-ion batteries, Nanocrystalline Fe O , Anode material 2 3 Background problems, many nanostructures of Fe O have been 2 3 With the ever-increasing applications of lithium-ion bat- synthesized for lithium-ion batteries, such as nanorods teries (LIBs) in portable electronics and electric vehicles, [18, 19], nanoflakes [20, 21], hollow sphere [22–24], core- there has been extensive research on developing advanced shell arrays , and micro-flowers . electrode materials with higher energy and power densities Besides all the above nanostructures, nanocrystalline [1–7]. Since the first report on reversible lithium storage in thin film anodes (NiO , MnO , Cr O , 2 3 transition metal oxides (TMOs) by Poizot et al. , TMOs CoFe O , Si , and Ni N) deposited directly 2 4 2 (Co O [9, 10], NiO [11, 12], Fe O [13–15], and CuO on conducting substrates by pulsed laser deposition or 3 4 2 3 [16, 17]) have been widely explored as anode materials sputtering can also exhibit an excellent electrochemical due to their higher theoretical specific capacity and better performance due to the enhanced electrical contact safety in comparison with traditional carbon anode between the substrates and active materials, the short- materials. Among all these TMOs, Fe O received much ened diffusion lengths for lithium-ion, and the structure 2 3 attention in recent years due to its high theoretical specific stability. What is more important is that thin films of − 1 capacity (~ 1005 mAh g ), low cost, abundant resources, TMOs have potential applications in all-solid-state and environmental benignity. However, like other TMOs, microbatteries as self-supported electrodes [33, 34]. The the huge volume variations associated with Li-ion inser- TMOs’ films can replace the lithium film anode which tion/extraction often leads to the pulverization and subse- limits the integration of microbatteries with circuits due quent falling off of the active materials from the electrode, to the low melting point and strong reactivity with which results in a significant capacity fade, poor cycling moisture and oxygen. However, up to now, there have stability, and poor rate capability. To circumvent these been few reports on the Fe O film anodes deposited by 2 3 pulsed laser deposition or sputtering, and the reported * Correspondence: firstname.lastname@example.org; email@example.com specific capacities were much lower than the theoretical College of Physics Science, Qingdao University, No.308 Ningxia Road, specific capacity of Fe O [35, 36]. 2 3 Qingdao 266071, China © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Teng et al. Nanoscale Research Letters (2018) 13:60 Page 2 of 7 In this work, we prepared nanocrystalline Fe O films processed at room temperature by a LAND-CT2001A 2 3 by pulsed laser deposition (PLD) as an anode material battery system at various current rates between 0.01 and for lithium-ion batteries. The Fe O thin film anodes 3.0 V. Cyclic voltammetry (CV) and AC impedance mea- 2 3 with average grain size of several tens of nanometers surements were performed with a CHI660E electrochem- − 1 showed high reversible capacity of 905 mAh g at ical workstation (CHI Instrument TN). The scanning rate − 1 − 1 − 1 100 mA g and high rate capacity of 510 mAh g at was 0.1 mV s . − 1 15000 mA g . The remarkable electrochemical per- formance demonstrates that nanocystalline Fe O thin Results and Discussion 2 3 film has potential applications in high performance LIBs, X-ray diffraction (XRD) patterns of the Fe O film are 2 3 especially all-solid-state thin film batteries. shown in Fig. 1a. It can be observed that there is no obvious peak except the peaks of cubic crystal Cu substrate, sug- Experimental gesting that the Fe O film is amorphous or crystallized 2 3 Synthesis of Nanocrystalline Fe O Films with nanosized grains. Such phenomenon could be attrib- 2 3 The films of Fe O were deposited directly on copper uted to the deposition occurred at room temperature. In 2 3 foils or stainless steels by a PLD technique in oxygen order to determine the chemical composition of the ambient. A KrF excimer laser with a wavelength of obtained film, XPS measurement was performed as shown 248 nm was focused on the rotatable target of metal Fe. in Fig. 1b.The Fe 2p and Fe 2p main peaks are clearly 3/2 1/2 The repetition rate was 5 Hz, and the laser energy was accompanied by satellite structures on their high binding- 500 mJ. The distance between the target and the sub- energy side, with a relative shift of about 8 eV. The peaks of strate was 40 mm. In order to get nanocrystalline Fe O Fe 2p locating at 710.9 eV and Fe 2p locating at 2 3 3/2 1/2 films, we grew samples at room temperature under oxygen 724.5 eV are similar with XPS spectra of Fe O reported in 2 3 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 O film was characterized 2 3 by X-ray diffraction (XRD) on a Rigaku D/Max diffract- ometer 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 O film grown on NaCl substrate was put into water to 2 3 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 micros- copy (SEM) using a SU8010. X-ray photoelectron spec- troscopy (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 O nanocrystalline film 2 3 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 Fig. 1 Structure and composition characterization of Fe O film 2 3 metal as the counter electrode and a standard electrolyte deposited at room temperature. a XRD patterns of Fe O film. b XPS 2 3 of 1:1:1 ethylene carbonate (EC)/dimethyl carbonate spectrum of Fe O film 2 3 (DMC)/LiPF . Galvanostatic cycling measurements were 6 Teng et al. Nanoscale Research Letters (2018) 13:60 Page 3 of 7 theliterature[37–39]. 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 O filmsweremadeofsmall nano- 2 3 grains 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 O . 2 3 Meanwhile, the ring-like feature of the selected area electron diffraction (SAED) confirmed the polycrystalline nature of Fe O film. As shown by the SEM images in 2 3 Fig. 2c,the Fe O film consists of particles in nanometer 2 3 scale. Based on all these results, we can confirm that the film deposited at room temperature is composed of Fe O 2 3 with ultrafine nanosized crystalline grains. The electrochemical performance of the electrode made of Fe O nanocrystalline film was firstly evaluated 2 3 by cyclic voltammetry (CV). Figure 3 shows the first three CV curves of Fe O nanocrystalline film anode. 2 3 The CV curves are similar to the previous reports of Fe O anode [40–46]. In the first cathodic process, three 2 3 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 O film forming Li Fe O 2 3 x 2 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 Fe O to cubic LiFe O . x 2 3 2 3 The third sharp reduction peak at 0.84 V corresponds to 2+ 0 the complete reduction of iron from Fe to Fe and the formation of solid electrolyte interface (SEI). In the anodic process, two broad peaks observed at 1.57 and 0 2+ 1.85 V represent the oxidation of Fe to Fe and further 3+ oxidization to Fe . In the subsequent cycles, the reduc- tion 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 revers- ibility of the electrochemical reactions, and this was further confirmed by the cycling performance. Figure 4a shows the discharge and charge profiles of the Fe O nanocrystalline film for different cycles at a specific 2 3 − 1 current of 100 mA g with a voltage range of 0.01–3V. Obvious voltage hysteresis are observed due to the con- version 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 3+ 3+ Fe and the reduction of Fe 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 Fig. 2 a TEM image. b HRTEM image with inset showing SAED discharge process represents the reduction peak in the CV patterns. c SEM image of the Fe O film prepared at room temperature 2 3 curves. The initial discharge and discharge capacity of the − 1 Fe O nanocrystalline film are 1183 and 840 mAh g , 2 3 Teng et al. Nanoscale Research Letters (2018) 13:60 Page 4 of 7 Table 1 The capacity comparison of our work with reported Fe O film anode 2 3 Fe O -based thin Current Cycle Capacity Ref. 2 3 −1 film anode density number (mAh g ) −1 Pulsed laser deposition 100 mA g 200 905 This − 1 100 mA g 50 361 work − 2 Pulsed laser deposition 100 μAcm 50 280  −1 Sputter deposition 165 mA g 120 330  − 1 gradually increases to 951 mAh g after the 70 cycles − 1 and then keeps stable in the range of 900–950 mAh g with a Coulombic efficiency nearly 100% during the following cycles. Similar phenomenon of the capacity in- creasing during cycling has been found in many transition metal oxide electrodes in previous studies [13, 48–52]. Fig. 3 Cyclic voltammetry curves of the nanocrystalline Fe O film. The 2 3 − 1 curves were measured at a scan rate of 0.1 mV s from 0.01 to 3 V The possible reason for this would be the electrode activa- tion, which induces the reversible growth of polymer/gel- like films to increase the capacity at low potentials . respectively, resulting in a Coulombic efficiency of 71%. Compared with the previous reports of Fe O film anode 2 3 The irreversible capacity loss is mainly attributed to the batteries deposited by pulsed laser deposition or sputter- formation of SEI layer on the surface of anode, which is ing [35, 36], the capacity of Fe O in our work has a con- 2 3 commonly observed in most anode materials [44–47]. siderable improvement as summarized in Table 1. The cycling performance of the film electrode at a spe- Previous studies on the effect of particle size on lith- − 1 cific current of 100 mA g at room temperature is shown ium intercalation into Fe O shows that nanocrystalline 2 3 in Fig. 4b. It can be seen that the reversible capacity Fe O exhibited better electrochemical performance 2 3 Fig. 4 a Discharge-charge profiles of the nanocrystalline Fe O film 2 3 − 1 anode cycled between 0.01–3 V at a specific current of 100 mA g . b Cycling performance of the nanocrystalline Fe O film anode and Fig. 5 a SEM image and b cycling performance of the Fe O film 2 3 2 3 − 1 − 1 corresponding Coulombic efficiencies at a specific current of 100 mA g anode annealed at 400 °C at a specific current of 100 mA g Teng et al. Nanoscale Research Letters (2018) 13:60 Page 5 of 7 To investigate the kinetics of lithium inserting/dein- serting, electrochemical impedance spectra measure- ment 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 O film and the 2 3 enhanced electrical contact between active anode and substrate. The good conductivity of the nanocrystalline Fe O film anode led to excellent rate performance. 2 3 Figure 7b shows the charge/discharge capacities at different current densities. The anode delivered capacities − 1 up to 855, 843 753, 646, and 510 mAh g at high current − 1 densities of 750, 1500, 3000, 7500, and 15,000 mA g , respectively, which is corresponding to 98.2, 96.7, 87.8, − 1 75.3, and 59.5% retention of the capacity at 250 mA g − 1 (about 871 mAh g ). More importantly, when the spe- − 1 cific current reduced to 250 mA g , the capacity could − 1 recover to 753 mAh g . The excellent rate performance benefits from both the good conductivity of the anode and the increase of capacity upon cycling. Fig. 6 a SEM image and b cycling performance of the Fe O film 2 3 − 1 anode grown at 400 °C at a specific current of 100 mA g than macro-sized (> 100 nm) Fe O . To confirm the 2 3 role of particle size in the electrochemical performance, we annealed the as-prepared Fe O film on stainless 2 3 steels at 400°. The prepared Fe O film anode at high 2 3 temperature was deposited on stainless steels only due to the instability of copper foil. The morphology com- parison 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 − 1 only about 263 mAh g after 100 circles, which was much lower than the specific capacity of as-prepared Fe O In addition, we also fabricated Fe O film anode 2 3. 2 3 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 − 1 − 1 of 100 mA g . The capacities dropped to 361 mAh g after 50 circles. These results indicate that the enhanced reversible capacity of nanocrystalline Fe O film grown 2 3 at room temperature can be attributed to the nanoscaled structure of the thin film electrode, which can sustain Fig. 7 a Electrochemical impedance spectra of the nanocrystalline high lithium insertion strain because of the smaller Fe O film. b Rate capabilities of the nanocrystalline Fe O film at 2 3 2 3 number of atoms and large surface areas within nano- different specific currents particles [13, 14, 54]. Teng et al. Nanoscale Research Letters (2018) 13:60 Page 6 of 7 Conclusions 3. Tang Y, Zhang Y, Li W, Ma B, Chen XD (2015) Rational material design for ultrafast rechargeable lithium-ion batteries. Chem Soc Rev 44:5926–5940 In summary, nanocrystalline Fe O film anode has been 2 3 4. 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Tang P, Han L, Genc A, He Y, Zhang X, Zhang L, Galan-Mascaros JR, Morante JR, Arbiol J (2016) Synergistic effects in 3D honeycomb-like hematite Competing Interests nanoflakes/branched polypyrrole nanoleaves heterostructures as high- The authors declare that they have no competing interests. performance negative electrodes for asymmetric supercapacitors. Nano Energy 22:189–201 Publisher’sNote 22. Wang B, Chen J, Wu H, Wang Z, Lou X (2011) Quasiemulsion-templated Springer Nature remains neutral with regard to jurisdictional claims in formation of alpha-Fe O hollow spheres with enhanced lithium storage 2 3 published maps and institutional affiliations. properties. J Am Chem Soc 133:17146–17148 23. Li C, Hu Q, Li Y, Zhou H, Lv Z, Yang X, Liu L, Guo H (2016) Hierarchical Received: 27 November 2017 Accepted: 12 February 2018 hollow Fe O @MIL-101(Fe)/C derived from metal-organic frameworks for 2 3 superior sodium storage. Sci Rep 6:25556 24. 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Published: Feb 23, 2018
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