Enhancement of Electrochemical Performance by the Oxygen Vacancies in Hematite as Anode Material for Lithium-Ion Batteries
© The Author(s). 2017
Received: 12 October 2016
Accepted: 9 December 2016
Published: 5 January 2017
The application of hematite in lithium-ion batteries (LIBs) has been severely limited because of its poor cycling stability and rate performance. To solve this problem, hematite nanoparticles with oxygen vacancies have been rationally designed by a facile sol–gel method and a sequential carbon-thermic reduction process. Thanks to the existence of oxygen vacancies, the electrochemical performance of the as-obtained hematite nanoparticles is greatly enhancing. When used as the anode material in LIBs, it can deliver a reversible capacity of 1252 mAh g−1 at 2 C after 400 cycles. Meanwhile, the as-obtained hematite nanoparticles also exhibit excellent rate performance as compared to its counterparts. This method not only provides a new approach for the development of hematite with enhanced electrochemical performance but also sheds new light on the synthesis of other kinds of metal oxides with oxygen vacancies.
Because of its high theoretical capacity, natural abundance, and environmental friendliness, hematite (α-Fe2O3) has been regarded as a promising anode material for lithium-ion batteries (LIBs) [1–4]. However, the practical application of hematite is greatly limited because of its low conductivity, large volume variation, and easy aggregation during the discharge/charge process [5–9]. To overcome these drawbacks, two main methods are employed. The first method concerns on the synthesis of nano-sized iron oxides with different structures, which will shorten the transportation distances of electron and Li+. The second method focuses on elevating the conductivity of hematite, which is mainly realized by forming the composite between hematite and materials with high electronic conductivity [10–14]. Despite these progresses, a simpler method for the preparation of hematite with enhanced electrochemical performance is still needed when considering its practical uses.
The introduction of oxygen vacancies into metal oxides has been proved to be an effective method to modulate the intrinsic electrochemical properties of the metal oxides [15, 16]. The existence of oxygen vacancies could effectively change the electronic structure of the metal oxides, reduce the energy requirement for electron or ion diffusion, and lower the resistance, which could be beneficial to improve the electrochemical performances of the metal oxides . What is more, previous reports also clearly indicate that the existence of oxygen vacancies could facilitate the phase transition and reduce the stress during Li+ insertion/depletion, which will be helpful to improve the rate performance as well as the cycling stabilities of the electrode materials. Oxygen vacancies could also provide more physical space for Li+ storage thus improving the specific capacity of the materials [18, 19]. For this reason, a large number of efforts have been devoted to the synthesis of electrode material with oxygen vacancies, all of which have shown enhanced electrochemical performance when used in LIBs. For example, Li3VO4−δ was synthesized by annealing Li3VO4 powders in vacuum, and the introduction of oxygen vacancies lead to the enhanced initial coulombic efficiency, reversible capacity, and cycling stability . The as-synthesized Li3VO4−δ delivers a reversible capacity of 247 mAh g−1 after 400 cycles at 500 mA g−1, which is much higher than the corresponding value of pristine Li3VO4 (64 mAh g−1). MoO3−x nanosheets were synthesized by oxidizing Mo powers in the atmosphere containing H2O2 and absolute ethanol. The as-prepared materials exhibit fascinating reversible capacity and long-term cycling stability (179.3 mAh g−1 at 1 A g−1) when used as anode materials for sodium ion batteries . Anatase TiO2−δ–carbon nanotubes (CNTs) composites were prepared by a two-step CVD method. The CNT grown on TiO2 leads to the formation of oxygen vacancies under the reducing atmosphere, which greatly enhanced the electrochemical performance, especially the rate performance. The half cells cycled at 30 C can still deliver a capacity of more than 40 mAh g−1 . V2O5 nanosheets with oxygen vacancies were also prepared by a hydrothermal reaction . The as-prepared H-V2O5 electrode exhibits excellent cycling stability and improved rate capability, which could be mainly attributed to the introduction of oxygen vacancies. Tong and his co-workers proposed a facile method to generate oxygen vacancies into the materials by slight nitridation in NH3 atmosphere [23, 24]. Using this method, hematite and titanium dioxide with oxygen vacancies had been successfully synthesized and delivered enhanced cyclability and rate performance. Additionally, TiO2 heterostructured nanosheet was synthesized by hydrogenation process. This kind of heterostructured nanosheet delivered a fascinating electrochemical performance. When it was used as anode material in full battery, the full battery could achieve high energy and power density .
Thus, it is reasonable to believe that the electrochemical performance of hematite in LIBs could be effectively enhanced by the introduction of oxygen vacancies. However, few report concerning on the effect of oxygen vacancies in hematite has been published in the field of LIBs to date. Meanwhile, the reported method for the preparation of oxygen defect α-Fe2O3 are usually based on the thermal decomposition of FeOOH in the inert gas or in vacuum, which usually needs tedious procedure and complex equipment. In this work, we present a facile method for the synthesis of α-Fe2O3 with oxygen vacancies via a two-step process incorporating a sol–gel synthesis of the precursor and thermal annealing of the precursor in air. In this synthetic route, the precursor was synthesized by a sol–gel method and then calcined in air to yield α-Fe2O3 nanoparticles with oxygen vacancies. The partial reduction of Fe(III) during the carbon-thermic process leads to the formation of oxygen vacancies in the final product, which has also been reported for the synthesis of titanium dioxide with oxygen vacancies [21, 26]. Compared with the previous reports, the preparation of α-Fe2O3 nanoparticles with oxygen vacancies is more simple, which will lower the cost during the production process. What is more, this method can be easily scaled up by simply increasing the initial amount of the starting material. These two fascinating characteristics make this method suitable for the large-scale application in the future. Thanks to the oxygen vacancies, the electrochemical performance of α-Fe2O3 is greatly promoted. Remarkably, the as-prepared Fe2O3−δ still maintained a reversible capacity of 1252 mAh g−1 at 2 C after 400 cycles. Meanwhile, the as-obtained Fe2O3−δ also exhibit excellent rate performance. Even being cycled at 40 C, the as-prepared electrode material can still deliver a discharge capacity of 188 mAh g−1, which is much higher than the corresponding value than the reported α-Fe2O3 electrode material. This synthetic method not only provides a new method for the enhancement of hematite-based electrode materials but also sheds a new light for the preparation of metal oxides with oxygen vacancies.
Synthesis of Fe2O3−δ Nanoparticles
In a typical procedure, 2 mmol FeCl3·6H2O and 4 mmol urea were dissolved in 46 mL distilled water with continuous stirring. Then, 4 mL acrylic acid was added into the as-formed yellow solution. In the next step, the mixed solution was transferred into a 70-mL Teflon-lined stainless steel autoclave and maintained at 140 °C for 12 h. After cooling down to room temperature, the gel-like product was collected by centrifugation, washed with distilled water and absolute alcohol several times and then dried in an oven at 80 °C overnight. To obtain hematite with oxygen vacancies, the as-formed precursor was calcined at 350 °C for 1.5 h in air with a heating rate of 2 °C min−1.
The morphology and structure of the sample was investigated by transmission electron microscopy (TEM, Hitachi HT 7700) and high-resolution TEM (HRTEM, JEOL-2010). X-ray diffraction patterns were obtained using a Bruker D8 Advance with Cu-Kα radiation. X-ray photoelectron spectra (XPS) of the samples were recorded on an ESCALAB 250. The thermogravimetric analysis (TGA) was carried out on SDT 2960 with a heating rate of 10 °C min−1 from 20 to 600 °C. The BET surface area was determined on an ASAP 2460 sorption apparatus. All the as-prepared samples were degassed at 150 °C for 10 h prior to nitrogen adsorption measurements. Electron paramagnetic resonance (EPR) tests were carried out on a Bruker A300 spectrometer (X-band, frequency 9.43 GHz) equipped with Bruker ER4141VTM liquid nitrogen system. The microwave power was 0.595 mW and modulation amplitude 3.0 G. The samples were measured at 90 K with center field 3500 G and sweep width 5000 G.
The electrochemical measurements were performed on coin-type cells (CR2032). The electrode was prepared using active material, acetylene black (Super P), and polyvinylidene fluoride (PVDF) in a weight ratio of 6:2:2. The electrolyte was a solution of 1 M LiPF6 in a mixture of EC:DEC (1:1 by volume). The cells were assembled in in an argon-filled glovebox (Mikrouna, Super (1220/750/900)) with both moisture and oxygen concentrations below 0.1 ppm. The galvanostatic discharge/charge characteristics were tested in the potential window of 0.01 to 3.0 V using a Neware battery tester. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were tested on a CHI660E electrochemical workstation.
Results and Discussion
The hydrothermal process at 140 °C for 12 h will lead to the formation of the gel-like precursor, which will be used as the starting material for the preparation of hematite with oxygen vacancies. The corresponding XRD pattern of the precursor (Additional file 1: Figure S1a) clearly indicates that the precursor is mainly composed of FeOOH (JCPDS No. 29–0713), which is obtained by the hydrolysis of Fe3+ in the solution. The TEM observation of the precursor (Additional file 1: Figure S1b) further confirms that the as-formed nanoparticles are wrapped in a gel-like matrix. To get further insight into the composition of the precursor, FT-IR was employed and the corresponding result is shown in Additional file 1: Figure S2. The corresponding result clearly indicates the formation of polyacrylic acid (PAA). The absorption bands centering at 1634 and 984 cm−1 could be attributed to the C=C and =CH2, respectively. And the absorption band at 1705 cm−1 can be assigned to the C=O double bond vibration of COOH groups [27, 28]. And the formation of PAA could be proved according to the disappearance of the absorption bands centering at 1634 and 984 cm−1, which has been widely reported in the previous reports . Thus, the as-obtained precursor could be regarded as a nanocomposite forming by dispersing the FeOOH nanoparticles in the matrix of PAA.
Figure 5b is the initial three discharge/charge curves of α-Fe2O3−δ at a current density of 2 C. The initial discharge/charge capacities for the as-prepared α-Fe2O3−δ are 1863/1296 mAh g−1, respectively. According to previous reports, oxygen vacancies are easily re-oxidized over time and the high conductivity also gradually diminishes . Nevertheless, the as-prepared Fe2O3−δ sample in this work exhibits excellent cycling stability during the charge/discharge process (Fig. 5c). On the contrary, commercial Fe2O3 delivers a low initial coulombic efficiency and poor cyclability. Only about 250 mAh g−1 discharge capacity could be maintained after 20 cycles under identical condition. In initial several cycles, the electrodes of Fe2O3−δ show a slight decrease in capacity, which can be ascribed to the slow formation rate of complete SEI layer at high current density. Typical charge/discharge curves of the Fe2O3−δ sample during long-term cycles is shown in Fig. 5d. Only slight capacity decay could be found in the whole test process. And after 400 cycles at 2 C, the discharge capacity is about 1252 mAh g−1, which is higher than the theoretical value of hematite (1007 mAh g−1). The excessive capacity can be explained from several aspects. On one hand, the materials were obtained by calcination, which will lead to the formation of lattice defects in the typical nanostructure. These lattice defects will provide more active sites for Li+ insertion/extraction, which could improve the specific capacity of the materials . On the other hand, the decomposition and reformation of the SEI layer will also lead to the increase in capacity , but the central aspect is that the introduction of oxygen vacancies in the materials, which will provide more physical space for Li+ storage, changes the intrinsic property of the sample and leads to the higher specific capacity than theoretical value.
The as-prepared α-Fe2O3−δ also exhibits fascinating rate performance during the charge/discharge cycles when the current density increased from 0.5 to 40 C in a stepwise manner and then returned to 0.5 C (Fig. 5e). The average reversible capacities of α-Fe2O3−δ were 1549, 1389, 1258, 995, 848, and 556 mAh g−1 at the discharge rate of 0.5, 1, 2, 5, 10, and 20 C, respectively. It is worth noting that the as-obtained α-Fe2O3−δ can still deliver a reversible capacity of 198 mAh g−1 at a high current density of 40 C. As the current density increased, the discharge/charge plat becomes ambiguous, indicating the redox reaction mainly occurred on the surface of the electrode materials other than the inside of the material (Fig. 5f). An average discharge capacity as high as 1590 mAh g−1 can maintain when the current rate returned to 0.5 C. This result clearly demonstrates that the as-obtained α-Fe2O3−δ is a good candidate for the potential application as high-rate anode materials for LIBs.
where i is the peak current, v is the scan rate, and a and b are the adjustable parameters. The type of discharge/charge progresses can be determined by the value of b. When b = 1, the progresses mainly rely on pseudo-capacitive control, and when b = 0.5, the progresses are dependent on ionic diffusion. The linear relationship between log (i) and log (v) is shown in Fig. 6b. The b values (the slopes of the four lines) of the four redox peaks are 0.97, 0.86, 0.99, and 0.77, which means the electrochemical reactions of α-Fe2O3−δ are controlled by pseudo-capacitive behavior. The result is in good accordance with the cycling performance. And it can also be employed to explain the reason why α-Fe2O3−δ has a high reversible specific capacity even cycled at 2C.
The EIS of the electrodes were performed to illustrate the effect of oxygen vacancies in sample α-Fe2O3−δ. The Nyquist plots of the electrodes before cycling and after 400 cycles are shown in Additional file 1: Figure S4 with a frequency ranging from 100 to 0.01 Hz. The Nyquist plots are composed of semicircle in the high-to-middle frequency regions and a sloping long line in the low frequency region. The smaller diameter of the semicircle indicates lower contact resistance and charge transfer resistance. The more sloping long line indicates faster kinetics during cycles. Compared with the commercial hematite, Fe2O3−δ delivers a lower contact resistance and charge transfer resistance. This mainly ascribes to the introduction of oxygen vacancies, which could be regarded as electron donor, change the electronic structure, and facilitate the Li+ ion diffusion and electron transportation. After 400 cycles, the diameter of the semicircle became smaller and the long line became more sloping, which indicated the lower resistance and faster ion diffusion rate. This phenomenon may be ascribed to the irreversible reaction during discharge/charge progress, which will lead to the formation of metallic Fe or the activation of the electrode material and the formation of channels for the diffusion of lithium ions [55, 56]. Moreover, the existence of oxygen vacancies in the materials also could suppress the formation of insulated Li2O, which will lower the resistance.
In conclusion, α-Fe2O3−δ nanoparticles with oxygen vacancies were successfully synthesized by a two-step method incorporating a sol–gel process and following calcination of the precursor. The introduction of oxygen vacancies into hematite exerts positive impact on the electrochemical performance of the final product. The as-prepared α-Fe2O3−δ shows enhanced electrochemical performance and cycling stability when being used as anode materials for LIBs. The existence of oxygen vacancies not only provides more space for Li+ storage but also facilitates the transformation of electronic structure. Meanwhile, the introduction of oxygen vacancies could also lower the contact resistance and charge transfer resistance during the discharge/charge process, leading to the enhanced electrochemical performance of the sample.
The financial support of the Natural Science Foundation of China (NSFC 21101091, 21171007, 21671005) and the Programs for Science and Technology Development of Anhui Province (1501021019) is gratefully acknowledged.
PYZ prepared the manuscript and carried out the experiment. YWL, JWL, WWW, and ZF helped in the technical support for the characterizations. ZF designed the experiment. YYZ and XXW participated in the experiment. All the authors discussed the results and approved the final manuscript.
The authors declare that they have no competing interests.
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