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Electrochemical reactivity of ilmenite FeTiO3, its nanostructures and oxide-carbon nanocomposites with lithium
Electrochimica Acta 108 (2013) 127–134
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical reactivity of ilmenite FeTiO3 , its nanostructures and oxide-carbon nanocomposites with lithium Tao Tao ∗ , Alexey M. Glushenkov ∗ , Md Mokhlesur Rahman, Ying Chen Institute for Frontier Materials, Deakin University, Waurn Ponds, Victoria 3216, Australia
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Article history: Received 26 January 2013 Received in revised form 13 May 2013 Accepted 15 June 2013 Available online xxx Keywords: Ilmenite Lithium ion batteries Anode Ball milling Nanostructures
a b s t r a c t The electrochemical reactivity of the ball-milled ilmenite FeTiO3 and ilmenite nanoflowers with lithium has been investigated. The electrode assembled with the ilmenite nanoflowers delivers better electrochemical performance than that of the milled material during charging and discharging in the potential range of 0.01 and 3 V vs. Li/Li+ . The ilmenite nanoflowers demonstrate the capacity of ca. 650 mAh g−1 during the first discharge, and a reversible capacity of approximately 200 mAh g−1 in the course of the first 50 cycles. The possible reaction mechanism between ilmenite and lithium was studied using cyclic voltammetry and transmission electron microscopy. The first discharge involves the formation of an irreversible phase, which is either LiTiO2 or LiFeO2 . Subsequently, the extraction–insertion of lithium happens in a reversible manner. It was also observed that the lithium storage might be significantly improved if the electrode was prepared in the form of a nanocomposite of FeTiO3 with carbon. Crown Copyright © 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction Lithium-ion batteries have become the dominant power sources for portable electronic devices and power tools due to their high energy density and long cycle life [1,2]. They are also being currently considered as one of the most promising battery technologies for electric and hybrid vehicles [3]. The most important characteristics of the batteries, energy density and power density, are defined, to a significant extent, by their electrode materials used in anodes and cathodes. The conventional lithium-ion battery anode is graphite [4,5], which has the theoretical capacity of 372 mAh g−1 , approximately 10% volume change upon charge and discharge and experiences irreversible formation of the solid electrolyte interphase (SEI) in the first cycle. In order to improve some of these characteristics, novel anode materials are being actively researched. Specifically, tin (theoretical capacity of 959.5 mAh g−1 ) [6], silicon (theoretical capacity of 4198 mAh g−1 ) [7] and germanium (theoretical capacity of 1600 mAh g−1 ) [8], are under intensive investigation due to their enviable capacities, significantly exceeding that of graphite. At the same time, various titanium oxides (rutile and anatase TiO2 , TiO2 (B) and Li4 Ti5 O12 ) [9–12] have emerged as alternative electrode materials due to their negligible volume change or the possibility of avoiding undesired reactions with organic solvents in the first cycle.
∗ Corresponding authors. Tel.: +61 3 52479135/3 52272931; fax: +61 3 52271103. E-mail addresses: [email protected] (T. Tao), [email protected] (A.M. Glushenkov).
The electrode materials are generally not found in nature in their required form. For example, the main industrial source of tin is the mineral cassiterite [13] and the typical mineral precursors for silicon are silica sands and silicate minerals [14]. In order to produce the alternative electrode materials, significant processing of the raw precursors (i.e., natural minerals and sands) is normally required. To minimise the effort and costs of obtaining the electrode materials, an interesting option is to investigate if some natural minerals and sands (precursors for tin, germanium, silicon or titanium oxides) can be used as electrode materials directly or after minimal processing treatment. This idea was examined to some extent for quartz SiO2 (the main constituent of beach sand) by Chang et al. [15]. These researchers have demonstrated that the mechanically milled silica SiO2 material has significant reactivity with lithium and is able to deliver a stable capacity of about 800 mAh g−1 . For obvious reasons, it is important to investigate if other natural materials can emerge as possible electrode materials for batteries. Ilmenite (FeTiO3 ) is one of the main raw precursors for the industrial production of titanium dioxide via either the sulphate or chloride route [16,17]. Ilmenite rock and sands are very abundant on Earth (with worldwide reserves of 680 million tonnes [18]) and, importantly, are found on all major continents [19]. It is worthwhile to investigate if ilmenite FeTiO3 can have reactivity with lithium as it is or after minimal processing. This paper reports our results in investigating reactivity of ilmenite FeTiO3 with lithium as a possible anode material. We have performed the preliminary analysis of the electrochemical behaviour of ball milled ilmenite and its nanostructures. It has been found that the
0013-4686/$ – see front matter. Crown Copyright © 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.06.103
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performance of the nanostructured FeTiO3 is different from the ball milled material, and a stable capacity of 200 mAh g−1 can be measured by charge–discharge experiments. If the ilmenite is ball milled together with carbon to form a nanocomposite (50 wt.%, C), stable capacity of about 380–400 mAh g−1 is measured. Possible explanations of the electrochemical behaviour are discussed on the basis of electrochemical tests and transmission electron microscopy analysis. 2. Experimental 2.1. Materials and preparation Ilmenite FeTiO3 (99% purity, supplied by Consolidated Rutile Ltd, Australia) was used as a starting material. The chemical composition of the ilmenite used in this work can be expressed as TiO2 (dry basis) 49.6%, iron (total) 35.1% (FeO 32.8%, Fe2 O3 13.7%), Al2 O3 0.47%, Cr2 O3 0.25%, SiO2 0.45%. Graphite (Johnson Matthey, no. 00641) was used for preparation of ilmenite–carbon composites. 2.1.1. Preparation of ball milled ilmenite and its nanocomposites A magneto-ball mill [20–22] consisting of a stainless steel cell rotating around a horizontal axis and an external magnet was used in the mechanical treatment (10 grams) of pure ilmenite, a mixture of ilmenite and graphite or pure graphite with four hardened steel balls (diameter of 25.4 mm). The milling was conducted for 150 h at room temperature under argon atmosphere of 100 kPa. The magnet was located at the bottom of the mill at a 45◦ position in relation to the vertical direction, and the rotation speed was 160 rpm. The weight ratio of ilmenite to carbon in the various samples of composites was 9:1, 8:2, 7:3 and 1:1, respectively. 2.1.2. Preparation of ilmenite nanostructures The milled ilmenite, a stirring rod, and NaOH aqueous solution were loaded into a Schött bottle immersed in a paraffin bath setup on top of a hot plate. The solution with the powder was kept at 120 ◦ C for 2 h under magnetic stirring. The suspension was subsequently filtered, and the obtained samples were again washed and dried at 90 ◦ C for 4 h. More details of the synthesis route are reported in a previous paper [23]. 2.2. Structural characterisation The materials were characterised by X-ray diffraction (XRD, Panalytical X’Pert PRO diffraction system, Cu X-ray source, ˚ scanning electron microscopy (SEM, Carl Zeiss SUPRA = 1.5418 A), 55VP electron microscope) and transmission electron microscopy (TEM, JEOL JEM-2100F instrument operating at 200 kV). Energy dispersive X-ray (EDX) maps of a nanocomposite were acquired on the same transmission electron microscope in scanning TEM (STEM) mode with a probe size of 1 nm. Surface area was measured by low temperature N2 adsorption using a Micromeritics Tristar 3000 system. 2.3. Electrochemical measurements For electrochemical studies, the milled ilmenite electrodes and nanoflowers electrodes were fabricated by preparing slurries of the active material, carbon black and polyvinylidene difluoride (PVDF) in the weight ratio of 80:10:10 using N-methylpyrrolidone (NMP) as solvent. The ilmenite–carbon nanocomposites electrodes were prepared by mixing 90 wt.% of the active materials with 10 wt.% PVDF binder in NMP. The slurry in each case was uniformly pasted on Cu foil and dried overnight in a vacuum oven at 100 ◦ C for over 12 h. Coin cells (CR2032 type) were assembled in an Ar-filled glove box (Innovative Technology, USA). Li foil was used as the
Fig. 1. SEM images of the original ilmenite powder (a), milled ilmenite powder (b), and ilmenite nanoflowers (c), inset: an image of the nanoflower morphology at a higher magnification).
counter/reference electrode and a porous polyethene film was used as a separator. The electrolyte was 1 M LiPF6 in a 1:1:1 (by volume) mixture of ethylene carbonate (EC), diethylene carbonate (DEC) and dimethyl carbonate (DMC). The electrochemical tests were performed using Solartron 1470E and Ivium-n-stat instruments. The cells were galvanostatically discharged and charged over a voltage range of 0.01–3 V vs. Li/Li+ at various current rates, and cyclic voltammetry experiments were performed over the same voltage range. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range between 100 kHz and 0.01 Hz. The rate capability test of FeTiO3 nanoflowers depicted in Fig. 4d was conducted as follows. A number of coin cells were independently cycled at different charge and discharge rates (charge and discharge currents were equal for the same cell). The discharge
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Fig. 2. Energy dispersive X-ray elemental maps of FeTiO3 –carbon nanocomposite (weight ratio of 1:1): (a) carbon map; (b) oxygen map; (c) titanium map; (d) iron map.
capacities were then taken from the performance of each cell in the 20th cycle and plotted vs. current rate. The rate capability test of FeTiO3 –carbon nanocomposite (Fig. 8b) was conducted in the following way. A fixed value of current (0.05 A g−1 ) was applied for both charge and discharge of the same cell for five cycles. The value of the charge and discharge currents was then increased and set to a higher value of current for the next five cycles. This procedure is repeated upon increasing the current up to the value of 5 A g−1 . Finally, the value of charge and discharge current is returned to the initial value of 0.05 A g−1 for five cycles.
The maps of oxygen, titanium and oxygen (Fig. 2b–d) indicate that FeTiO3 is distributed within the carbon material. The maps are consistent with the idea that the carbon component contains nanoparticles of FeTiO3 with a non-uniform size distribution. Larger particles of FeTiO3 (up to hundreds of nm) are also present. XRD patterns of the ilmenite nanoflowers, the milled ilmenite, and ilmenite nanocomposites are presented in Fig. 3. All of these XRD patterns agree well with the standard XRD pattern of ilmenite FeTiO3 (JCPDS 01-075-1211), indicating that no new crystalline phases are formed in the nanoflowers, and nanocomposites. The broadened diffraction peaks in the patterns are due to the small crystallite size. The 0 0 2 peak of graphite disappears completely in the XRD pattern of the nanocomposite after 150 h of milling,
3. Results and discussion Fig. 1a shows an SEM image of the original mineral ilmenite (FeTiO3 ) powder consisting of large particles with a typical size of 100–300 m. After ball milling treatment, the large particles have been reduced to small particles of sub-micron size and the aggregation of the small particles of ilmenite can be seen in Fig. 1b. Fig. 1c shows the morphology of the ilmenite nanoflowers formed after NaOH leaching of the milled ilmenite for 2 h. The diameter of the uniform flower-like architectures is about 1–2 m, and they consist of petals with the thickness of 5–20 nm. The morphological transformation and the formation mechanism of these FeTiO3 nanoflowers have been previously discussed in detail elsewhere [23]. The milled ilmenite (Fig. 1b) and its nanostructures (Fig. 1c) were electrochemically analysed in this work. The structure of FeTiO3 –carbon nanocomposite (1:1 by weight) was evaluated by EDX mapping in TEM. Fig. 2 shows the elemental maps of carbon, oxygen, titanium and iron. The carbon signal in Fig. 2a repeats the shape of the particles in the nanocomposite.
Fig. 3. XRD patterns for the ilmenite nanoflowers, milled ilmenite, and ilmenite nanocomposites.
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Fig. 4. Selected galvanostatic charge/discharge curves at 50 mA g−1 for (a) the milled ilmenite and (b) the ilmenite nanoflowers; (c) variation of discharge versus cycle number for the milled ilmenite and ilmenite nanoflowers at 50 mA g−1 ; (d) current density dependence of the reversible 20th cycle discharge capacity for the ilmenite nanoflowers.
which can be attributed to the amorphisation of carbon induced by ball milling (as it was reported, for example, in Ref. [24]). The surface areas of the three powders are 2.7 m2 g−1 (ball milled ilmenite), 26 m2 g−1 (nanoflowers), and 76 m2 g−1 (nanocomposite with 50 wt.% carbon). The selected charge–discharge curves from the first 50 cycles of an electrode assembled with milled ilmenite are shown in Fig. 4a. The testing current density was 50 mA g−1 and the voltage range was 0.01–3.0 V (vs. Li/Li+ ). The initial discharge has pronounced plateaus initiating at about 1.15 and 0.4 V, resulting from the initial reactions of lithium with FeTiO3 . The first discharge capacity is 534.4 mAh g−1 and close to the theoretical capacity of the following reaction FeTiO3 + 3Li+ + e− → Fe + LiTiO2 + Li2 O,
530 mAh g−1 .
It is apparent that the electrochemical performance of the electrode assembled from the ball milled ilmenite starts to degrade after the first cycle and capacities lower than 100 mAh g−1 are measured from the fifth cycle and beyond. The lithium storage capacity of nanostructured ilmenite (in the form of nanoflowers) is significantly better. Fig. 4b shows the selected charge–discharge curves from the first 50 cycles of the electrode assembled with ilmenite nanoflowers. Importantly, the
shape of the charge–discharge profiles of FeTiO3 nanoflowers is different from that of the milled ilmenite and higher capacities can be observed. Fig. 4c demonstrates the cycling stability of both milled ilmenite and FeTiO3 nanoflowers. The capacity of ball milled material quickly fades and stabilises at the level of about 50 mAh g−1 . In contrast, modest capacity of 200 mAh g−1 is maintained by FeTiO3 nanoflowers after 50 cycles. This indicates that lithium storage in FeTiO3 nanostructures is more efficient, and the nanoflowers deliver both higher capacities and better capacity retention after multiple charge–discharge cycles. The dependency of the discharge capacity of FeTiO3 nanoflowers on the current rate is illustrated in Fig. 4d. The capacities recorded in the 20th cycle at each current rate are shown. The electrode is capable of operating at the current rate of 1 A g−1 . The capacities of 190, 135, 115, 75 and 56 mAh g−1 were measured at the current rates of 0.05, 0.1, 0.2, 0.5 and 1 A g−1 , respectively, in the 20th cycle. In order to shed light onto the possible mechanism of the electrochemical reactivity of ilmenite FeTiO3 with lithium, cyclic voltammetry and transmission electron microscopy studies were conducted. The electrodes assembled using both milled ilmenite and ilmenite nanoflowers were analysed. Selected cycles from the cyclic voltammetry experiments are shown in Fig. 5. The first sweep was cathodic (reaction with Li+ ) and started from the open-circuit voltage. Two obvious reduction peaks can be observed in the first
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Fig. 5. Typical cyclic voltammograms: the first and subsequent cycles of the milled ilmenite electrode (a and b, respectively), and the first and subsequent cycles of the electrode assembled with the ilmenite nanoflowers (c and d, respectively). The CV measurements were performed at a scan rate of 0.2 mV s−1 in the voltage range of 0.01–3 V vs. Li/Li+ .
cathodic scan for the milled ilmenite (Fig. 5a). The first peak is located at about 0.86 V, and the second peak is located at about 0.56 V vs. Li/Li+ . The subsequent anodic peaks were observed at about 1.17 V and 1.99 V vs. Li/Li+ , indicating the processes associated with the extraction of Li+ from the electrode materials. The second cathodic sweep (Fig. 5b) differs from the first one, indicating the altered redox behaviour. A pronounced cathodic peak can be seen at approximately 1.41 V vs. Li/Li+ . The peaks located at 0.86 and 0.56 V vs. Li/Li+ , visible in the first cathodic scan, disappear. After the first 5–10 cycles, the shape of the CV cure becomes relatively stationary, indicating the reversible electrochemical reaction occurring after the first few discharge–charge cycles. This corresponds well with the stabilisation of the charge/discharge capacity observed in the charge–discharge experiments after the first 10 cycles. The differences can be seen between the CV curves of the ilmenite nanoflowers (Fig. 5c and d) and those of the milled ilmenite. Three obvious reduction peaks can be observed in the first cathodic scan for the ilmenite nanoflowers (Fig. 5c). The first two peaks are located around 1.11 V and 0.86 V vs. Li/Li+ and can be attributed to the electrochemical reaction of FeTiO3 with Li+ ions. Another cathodic peak observed at 0.34 V vs. Li/Li+ can be assigned to the formation of the solid electrolyte interphase (SEI) layer on the interface of a high surface area materials (the measured surface area of FeTiO3 nanoflowers is 26 m2 g−1 ). The subsequent anodic
peaks, corresponding to the extraction of Li+ from the electrode materials, are observed at about 1.61 V and 2.53 V vs. Li/Li+ . During the second cycle (Fig. 5d), the peaks at 1.11 and 0.86 V detected in the first cycle are missing but a new reduction peak located at about 1.25 V vs. Li/Li+ can be detected. The electrochemical behaviour after the 5th cycle is repeatable (similar in the subsequent cycles), indicating a reversible electrochemical process in the electrode. Again, this correlates well with the results of the galvanostatic charge–discharge (Fig. 4c), demonstrating a stable capacity of about 200 mAh g−1 after the 10th cycle. Comparing the results of the cyclic voltammetry measurements for the ball milled material and the FeTiO3 nanoflowers, the CV curves for these two types of electrodes have both shared and distinctly different features. Specifically, it is worthwhile to note that the same peak at about 0.86 V vs. Li/Li+ is present in all cases in the first discharge while additional cathodic peaks are different. It indicates that, although a similar reaction step is present in the first discharge of both milled FeTiO3 and its nanostructures, the electrochemical reactivity of ilmenite nanostructures with lithium is generally different and results in higher capacity retained after the first few cycles. An improved electrochemical behaviour of the flower-like FeTiO3 nanostructure can be attributed to its special structural characteristics. The large surface area of nanoflowers provides a large electrochemical interface that facilitates highly efficient Li+ ion insertion, and the open and low density morphology in the discrete hierarchical nanostructures
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Fig. 6. Bright-field TEM images and electron diffraction patterns taken from the electrodes assembled with the ball milled FeTiO3 powder: (a and b) after the first discharge of the cell to 0.01 V vs. Li/Li+ ; (c and d) after the subsequent charge to 3 V vs. Li/Li+ .
Fig. 7. Bright-field TEM images and electron diffraction patterns taken from the electrodes assembled with the FeTiO3 nanoflowers: (a and b) after the first discharge of the cell to 0.01 V vs. Li/Li+ ; (c and d) after the subsequent charge to 3 V vs. Li/Li+ .
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facilitates efficient Li+ ion diffusion during discharge/charge cycling. In addition, the petals of nanoflowers are dominated by (2¯I0)-type planes of FeTiO3 [23], which may have an influence on the way lithium reacts with ilmenite. The TEM data for the electrode materials after the first discharge and the subsequent first charge of coin cells are shown in Fig. 6 (the ball milled material) and Fig. 7 (FeTiO3 nanoflowers). Bothbright field images and electron diffraction patterns are presented. The electron diffraction is a valuable tool because it allows us to evaluate the phase composition of the electrodes after each step. The bright-field TEM images of the electrode assembled with the ball milled FeTiO3 powders after the first discharge of the cell to 0.01 V vs. Li/Li+ and after the subsequent charge to 3 V vs. Li/Li+ are shown in Fig. 6a and c, respectively. The darker clusters in the image correspond to the active component of the electrode, and the brighter areas contain carbon black particles. The corresponding selected area electron diffraction patterns (Fig. 6b and d) consist of families of rings, indicating the nanocrystalline structure of the active component of the electrode after the discharge and charge. The lattice spacings can be derived from the diffraction patterns and used for the phase identification. The spacings of 1.19, 1.45 and 2.04 A˚ were calculated from the pattern of the discharged electrode. The pattern of the material after the first charge ˚ and contains a similar set of three rings (1.19, 1.45 and 2.03 A), ˚ are also present. We contwo additional rings (1.63 and 2.37 A) servatively estimate that the error in the measurement of lattice distances from electron diffraction patterns is 5% although the real accuracy was found to be better for the measurements of standard crystals with known lattice spacings. On the basis of a simple comparison, the two diffraction patterns contain the same three rings, ˚ indicating that one phase is likely to 1.19, 1.45 and 2.03 (2.04) A, remain stable upon cycling in a lithium cell. The three distances are in a reasonable agreement with either LiTiO2 (JCPDS file No. 00-016-0223), LiFeO2 (JCPDS file No. 01-074-2283) and Fe (JCPDS file No. 01-087-0721). The ring corresponding to the spacing of 2.37 A˚ also matches a diffraction peak in the standard diffraction patterns of LiTiO2 or LiFeO2 . The additional ring appearing in the diffraction pattern of the charged electrode and corresponding to the lattice spacing of 1.63 A˚ may belong to Li2 O (JCPDS file No. 01-073-0593). The same set of measurements for the electrode assembled with FeTiO3 nanoflowers is shown in Fig. 7. The bright-field TEM images for the discharged and charged materials are depicted in Fig. 7a and c, respectively, while the electron diffraction patterns are shown in Fig. 7b and d. Again, the same common diffraction ˚ 1.45 A˚ (1.41 A˚ for the discharged rings corresponding to 1.18 A, electrode) and 2.45 A˚ (2.00 A˚ for the discharged electrode) are observed. As indicated above, these rings match LiTiO2 , LiFeO2 or Fe phases. The interpretation is difficult since some lattice spacings of these phases are quite similar. Additional rings in ˚ the discharged material fit the distances of 1.60, 2.30 and 2.72 A, which are close to the lattice spacings in the standard diffraction file of Li2 O. It should be noted, in respect to the above analysis, that some diffraction rings with a very weak intensity could not be detected by the available camera on our TEM, and additional diffraction rings may, in fact, exist in addition to those depicted in Figs. 6 and 7. On the basis of the available data, it is concluded that an electrochemically inert phase forms after the first discharge. The diffraction intensities matching either LiTiO2 or LiFeO2 are visible. We propose that one of these two phases forms upon the initial discharge of the cell and stays inactive in further cycles. The formation of this phase is correlated with a peak at 0.86 V vs. Li/Li+ detected in the CV curves of both milled materials and FeTiO3 nanoflowers. Depending on whether LiTiO2 or LiFeO2 phase forms, one of the two mechanisms may be operational. Namely, the
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Fig. 8. Electrochemical performance of ilmenite–carbon nanocomposites: (a) cyclic stability of nanocomposites with different compositions at a current density of 50 mA g−1 , and (b) rate capability of the best performing nanocomposite (50 wt.% of carbon).
reversible reactivity with lithium may involve iron or iron oxides (formed after the initial discharge) in the first scenario or titanium oxides in the second scenario. The performance of ilmenite FeTiO3 was also assessed in the form of nanocomposites with carbon. Fig. 8a shows the performance of samples with varied carbon content as well as the electrochemical performance of ball milled graphite and ball milled FeTiO3 . It can be seen that the addition of 20 wt.% and 30 wt.% of carbon delivers improvements in respect to the performance of ball milled ilmenite, although capacity is still low. The dramatic improvement in the capacity and cyclic stability was observed, however, when a composite incorporating 50 wt.% was evaluated. After the initial few cycles capacity of about 400 mAh g−1 was observed, and it deteriorated only slightly to 380 mAh g−1 after 50 cycles. The measured capacity exceeds the capacity of FeTiO3 observed in the ball milled sample and the sample of FeTiO3 nanoflowers as well as the theoretical capacity of graphite. We believe that ilmenite FeTiO3 and carbon demonstrate synergistic behaviour, i.e. the mixture of individual components is capable of delivering higher capacity than that of individual components. The response of the best performing FeTiO3 –carbon composite (50 wt.% of carbon) to varied current rates of charge–discharge is shown in Fig. 8b. The initial reversible capacity was about 450 mAh g−1 . When the current rate was switched to 0.1, 0.2, 0.5, 1, 2, and 5 Ag−1 , the specific capacity was slowly reduced to approximately 300, 240,
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observed that the electrochemical performance of ilmenite might be improved if the electrode material was prepared in the form of a nanocomposite of FeTiO3 with carbon. Acknowledgements Financial support from the Australian Research Council is acknowledged. The authors also thank Dr. Pavel Cizek and technical officers at Deakin University, Robert Lovett, Rosey van Driel and Dr. Andrew Sullivan, for their help in this work. References
Fig. 9. Electrochemical impedance spectra for the fresh cells and their corresponding equivalent circuit model (inset).
160, 100, 66, and 33 mAh g−1 , respectively. When the current rate was decreased back to 50 mA g−1 after 40 cycles, the electrode still delivered a high reversible capacity of 400 mAh g−1 . The electrochemical impedance spectra for the coin cells assembled with the electrodes incorporating the two ilmenite samples are compared in Fig. 9. The Nyquist plots are semicircular in the range of high to medium frequency, which reflects the charge-transfer resistance (Rct ) of the electrodes. The inclined line (∼45◦ ) represents the Warburg impedance (Zw ) at low frequency, which indicates the diffusion of Li+ in the solid matrix. The intercepts on the real axis Z could be considered as representing the combined resistance (Rs ), including the ionic resistance of the electrolyte, the intrinsic resistance of the active materials and the contact resistance at the active material/current collector interface. The corresponding equivalent circuit for the Nyquist plots of the electrodes is shown in the inset, where CDL represents the double layer capacitance. The values of Rct for the cells with electrodes incorporating ilmenite nanocomposites, ilmenite nanoflowers and milled ilmenite were calculated to be approximately 126, 220 and 417 , respectively. This indicates that electrodes incorporating nanocomposites have easier charge transfer at the electrode/electrolyte interface and provide a lower overall battery internal resistance. 4. Conclusions We have demonstrated that the ball milled ilmenite FeTiO3 and the ilmenite nanostructures exhibit lithium storage ability as anode materials with a good retention of capacity for at least 50 initial cycles. The electrochemical performance of the ilmenite nanoflowers was found to be superior to that of the ball milled material. A capacity of ca. 650 mAh g−1 is obtained for these ilmenite nanostructures during the first discharge of the cell, and a reversible capacity of ca. 200 mAh g−1 is maintained after 50 cycles. Transmission electron microscopy analysis shows that during the first discharge, new phases such as LiTiO2 , LiFeO2 or Fe are produced by the electrochemical reaction of FeTiO3 with lithium. It is proposed that this initial reaction is irreversible, and an inactive phase (LiTiO2 or LiFeO2 ) forms after the initial discharge. Depending on whether LiTiO2 or LiFeO2 stabilises after the initial discharge, two possible scenarios of reactivity with lithium may be operational in the subsequent cycles. In the first scenario the reversible lithium storage happens due to iron oxides formed upon the initial discharge, while an alternative scenario assumes the reversible lithium storage in titanium oxides. It was
[1] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Challenges in the development of advanced Li-ion batteries: a review, Energy & Environmental Science 4 (2011) 3243. [2] A. Manthiram, Materials challenges and opportunities of lithium ion batteries, Journal of Physical Chemistry Letters 2 (2011) 176. [3] M.M. Thackeray, C. Wolverton, E.N.D. Isaacs, Electrical energy storage for transportation-approaching the limits of, and going beyond, lithium-ion batteries, Energy & Environmental Science 5 (2012) 7854. [4] J.R. Dahn, T. Zheng, Y.H. Liu, J.S. Xue, Mechanisms for lithium insertion in carbonaceous materials, Science 270 (1995) 590. [5] E. Peled, C. Menachem, D. Bar-Tow, A. Melman, Improved graphite anode for lithium-ion batteries chemically: bonded solid electrolyte interface and nanochannel formation, Journal of the Electrochemical Society 143 (1996) L4. [6] I.A. Courtney, J.R. Dahn, Key factors controlling the reversibility of the reaction of lithium with SnO2 and Sn2 BPO6 glass, Journal of the Electrochemical Society 144 (1997) 2943. [7] B.A. Boukamp, G.C. Lesh, R.A. Huggins, All-solid lithium electrodes with mixedconductor matrix, Journal of the Electrochemical Society 128 (1981) 725. [8] J. Graetz, C.C. Ahn, R. Yazami, B. Fultz, Nanocrystalline, Thin film germanium electrodes with high lithium capacity and high rate capabilities, Journal of the Electrochemical Society 151 (2004) A698. [9] Y.S. Hu, L. Kienle, Y.G. Guo, J. Maier, High lithium electroactivity of nanometersized rutile TiO2 , Advanced Materials 18 (2006) 1421. [10] Y. Ren, Z. Liu, F. Pourpoint, A.R. Armstrong, C.P. Grey, P.G. Bruce, Nanoparticulate TiO2 (B). An anode for lithium-ion batteries, Angewandte Chemie International Edition 51 (2012) 2164. [11] V. Subramanian, A. Karki, K.I. Gnanasekar, F.P. Eddy, B. Rambabu, Nanocrystalline TiO2 (anatase) for Li-ion batteries, Journal of Power Sources 159 (2006) 186. [12] T.F. Yi, L.J. Jiang, J. Shu, C.B. Yue, R.S. Zhu, H.B. Qiao, Recent developmentandapplicationofLi4 Ti5 O12 as anode material of lithium ion battery, Journal of Physics and Chemistry of Solids 71 (2010) 1236. [13] O. Gerald, U. Priscilla, O. Martin, E. Emmanuel, N. Chukwuka, J.Y. Hwang, Model for prediction of the concentration of extracted tin during leaching of cassiterite in potassium hydroxide solution, Journal of Minerals and Materials Characterization and Engineering 11 (2012) 730. [14] B.R. Edwards, J.K. Russell, A review and analysis of silicate mineral dissolution experiments in natural silicate melts, Chemical Geology 130 (1996) 233. [15] W.S. Chang, C.M. Park, J.H. Kim, Y.U. Kim, G.J. Jeong, H.J. Sohn, Quartz (SiO2 ): a new energy storage anode material for Li-ion batteries, Energy & Environmental Science 5 (2012) 6895. [16] T. Chernet, Applied mineralogical studies on Australian sand ilmenite concentrate with special reference to its behaviour in the sulphate process, Minerals Engineering 12 (1999) 485. [17] T.K. Pong, J. Besida, T.A. O’Donnell, D.G. Wood, A novel fluoride process for producing TiO2 from titaniferous ore, Industrial & Engineering Chemistry Research 34 (1995) 308. [18] U.S. Department of the Interior, U.S. Geological Survey, Mineral Commodity Summaries, 2009 http://minerals.usgs.gov/minerals/pubs/commodity/ titanium/mcs-2009-timin.pdf [19] Web page of Sheffield Resources Ltd., http://www. sheffieldresources.com.au/ market-hm.php [20] A. Calka, A.P. Radlinski, Universal high performance ball-milling device and its application for mechanical alloying, Materials Science and Engineering A 134 (1991) 1350. [21] Y. Chen, T.H. Hwang, J.S. Williams, Ball milling induced low-temperature carbothermic reduction of ilmenite, Materials Letters 28 (1996) 55. [22] Y. Chen, T. Hwang, M. Marsh, J.S. Williams, Mechanically activated carbothermic reduction of Ilmenite, Metallurgical and Materials Transactions A 28 (1997) 1115. [23] T. Tao, A.M. Glushenkov, H.W. Liu, Z.W. Liu, X.J. Dai, H. Chen, S.P. Ringer, Y. Chen, Ilmenite FeTiO3 nanoflowers and their pseudocapacitance, The Journal of Physical Chemistry C 115 (2011) 17297. [24] Y. Chen, J.F. Gerald, L.T. Chadderton, L. Chaffron, Nanoporous carbon produced by ball milling, Applied Physics Letters 74 (1999) 2782.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical reactivity of ilmenite FeTiO3 , its nanostructures and oxide-carbon nanocomposites with lithium Tao Tao ∗ , Alexey M. Glushenkov ∗ , Md Mokhlesur Rahman, Ying Chen Institute for Frontier Materials, Deakin University, Waurn Ponds, Victoria 3216, Australia
a r t i c l e
i n f o
Article history: Received 26 January 2013 Received in revised form 13 May 2013 Accepted 15 June 2013 Available online xxx Keywords: Ilmenite Lithium ion batteries Anode Ball milling Nanostructures
a b s t r a c t The electrochemical reactivity of the ball-milled ilmenite FeTiO3 and ilmenite nanoflowers with lithium has been investigated. The electrode assembled with the ilmenite nanoflowers delivers better electrochemical performance than that of the milled material during charging and discharging in the potential range of 0.01 and 3 V vs. Li/Li+ . The ilmenite nanoflowers demonstrate the capacity of ca. 650 mAh g−1 during the first discharge, and a reversible capacity of approximately 200 mAh g−1 in the course of the first 50 cycles. The possible reaction mechanism between ilmenite and lithium was studied using cyclic voltammetry and transmission electron microscopy. The first discharge involves the formation of an irreversible phase, which is either LiTiO2 or LiFeO2 . Subsequently, the extraction–insertion of lithium happens in a reversible manner. It was also observed that the lithium storage might be significantly improved if the electrode was prepared in the form of a nanocomposite of FeTiO3 with carbon. Crown Copyright © 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction Lithium-ion batteries have become the dominant power sources for portable electronic devices and power tools due to their high energy density and long cycle life [1,2]. They are also being currently considered as one of the most promising battery technologies for electric and hybrid vehicles [3]. The most important characteristics of the batteries, energy density and power density, are defined, to a significant extent, by their electrode materials used in anodes and cathodes. The conventional lithium-ion battery anode is graphite [4,5], which has the theoretical capacity of 372 mAh g−1 , approximately 10% volume change upon charge and discharge and experiences irreversible formation of the solid electrolyte interphase (SEI) in the first cycle. In order to improve some of these characteristics, novel anode materials are being actively researched. Specifically, tin (theoretical capacity of 959.5 mAh g−1 ) [6], silicon (theoretical capacity of 4198 mAh g−1 ) [7] and germanium (theoretical capacity of 1600 mAh g−1 ) [8], are under intensive investigation due to their enviable capacities, significantly exceeding that of graphite. At the same time, various titanium oxides (rutile and anatase TiO2 , TiO2 (B) and Li4 Ti5 O12 ) [9–12] have emerged as alternative electrode materials due to their negligible volume change or the possibility of avoiding undesired reactions with organic solvents in the first cycle.
∗ Corresponding authors. Tel.: +61 3 52479135/3 52272931; fax: +61 3 52271103. E-mail addresses: [email protected] (T. Tao), [email protected] (A.M. Glushenkov).
The electrode materials are generally not found in nature in their required form. For example, the main industrial source of tin is the mineral cassiterite [13] and the typical mineral precursors for silicon are silica sands and silicate minerals [14]. In order to produce the alternative electrode materials, significant processing of the raw precursors (i.e., natural minerals and sands) is normally required. To minimise the effort and costs of obtaining the electrode materials, an interesting option is to investigate if some natural minerals and sands (precursors for tin, germanium, silicon or titanium oxides) can be used as electrode materials directly or after minimal processing treatment. This idea was examined to some extent for quartz SiO2 (the main constituent of beach sand) by Chang et al. [15]. These researchers have demonstrated that the mechanically milled silica SiO2 material has significant reactivity with lithium and is able to deliver a stable capacity of about 800 mAh g−1 . For obvious reasons, it is important to investigate if other natural materials can emerge as possible electrode materials for batteries. Ilmenite (FeTiO3 ) is one of the main raw precursors for the industrial production of titanium dioxide via either the sulphate or chloride route [16,17]. Ilmenite rock and sands are very abundant on Earth (with worldwide reserves of 680 million tonnes [18]) and, importantly, are found on all major continents [19]. It is worthwhile to investigate if ilmenite FeTiO3 can have reactivity with lithium as it is or after minimal processing. This paper reports our results in investigating reactivity of ilmenite FeTiO3 with lithium as a possible anode material. We have performed the preliminary analysis of the electrochemical behaviour of ball milled ilmenite and its nanostructures. It has been found that the
0013-4686/$ – see front matter. Crown Copyright © 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.06.103
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performance of the nanostructured FeTiO3 is different from the ball milled material, and a stable capacity of 200 mAh g−1 can be measured by charge–discharge experiments. If the ilmenite is ball milled together with carbon to form a nanocomposite (50 wt.%, C), stable capacity of about 380–400 mAh g−1 is measured. Possible explanations of the electrochemical behaviour are discussed on the basis of electrochemical tests and transmission electron microscopy analysis. 2. Experimental 2.1. Materials and preparation Ilmenite FeTiO3 (99% purity, supplied by Consolidated Rutile Ltd, Australia) was used as a starting material. The chemical composition of the ilmenite used in this work can be expressed as TiO2 (dry basis) 49.6%, iron (total) 35.1% (FeO 32.8%, Fe2 O3 13.7%), Al2 O3 0.47%, Cr2 O3 0.25%, SiO2 0.45%. Graphite (Johnson Matthey, no. 00641) was used for preparation of ilmenite–carbon composites. 2.1.1. Preparation of ball milled ilmenite and its nanocomposites A magneto-ball mill [20–22] consisting of a stainless steel cell rotating around a horizontal axis and an external magnet was used in the mechanical treatment (10 grams) of pure ilmenite, a mixture of ilmenite and graphite or pure graphite with four hardened steel balls (diameter of 25.4 mm). The milling was conducted for 150 h at room temperature under argon atmosphere of 100 kPa. The magnet was located at the bottom of the mill at a 45◦ position in relation to the vertical direction, and the rotation speed was 160 rpm. The weight ratio of ilmenite to carbon in the various samples of composites was 9:1, 8:2, 7:3 and 1:1, respectively. 2.1.2. Preparation of ilmenite nanostructures The milled ilmenite, a stirring rod, and NaOH aqueous solution were loaded into a Schött bottle immersed in a paraffin bath setup on top of a hot plate. The solution with the powder was kept at 120 ◦ C for 2 h under magnetic stirring. The suspension was subsequently filtered, and the obtained samples were again washed and dried at 90 ◦ C for 4 h. More details of the synthesis route are reported in a previous paper [23]. 2.2. Structural characterisation The materials were characterised by X-ray diffraction (XRD, Panalytical X’Pert PRO diffraction system, Cu X-ray source, ˚ scanning electron microscopy (SEM, Carl Zeiss SUPRA = 1.5418 A), 55VP electron microscope) and transmission electron microscopy (TEM, JEOL JEM-2100F instrument operating at 200 kV). Energy dispersive X-ray (EDX) maps of a nanocomposite were acquired on the same transmission electron microscope in scanning TEM (STEM) mode with a probe size of 1 nm. Surface area was measured by low temperature N2 adsorption using a Micromeritics Tristar 3000 system. 2.3. Electrochemical measurements For electrochemical studies, the milled ilmenite electrodes and nanoflowers electrodes were fabricated by preparing slurries of the active material, carbon black and polyvinylidene difluoride (PVDF) in the weight ratio of 80:10:10 using N-methylpyrrolidone (NMP) as solvent. The ilmenite–carbon nanocomposites electrodes were prepared by mixing 90 wt.% of the active materials with 10 wt.% PVDF binder in NMP. The slurry in each case was uniformly pasted on Cu foil and dried overnight in a vacuum oven at 100 ◦ C for over 12 h. Coin cells (CR2032 type) were assembled in an Ar-filled glove box (Innovative Technology, USA). Li foil was used as the
Fig. 1. SEM images of the original ilmenite powder (a), milled ilmenite powder (b), and ilmenite nanoflowers (c), inset: an image of the nanoflower morphology at a higher magnification).
counter/reference electrode and a porous polyethene film was used as a separator. The electrolyte was 1 M LiPF6 in a 1:1:1 (by volume) mixture of ethylene carbonate (EC), diethylene carbonate (DEC) and dimethyl carbonate (DMC). The electrochemical tests were performed using Solartron 1470E and Ivium-n-stat instruments. The cells were galvanostatically discharged and charged over a voltage range of 0.01–3 V vs. Li/Li+ at various current rates, and cyclic voltammetry experiments were performed over the same voltage range. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range between 100 kHz and 0.01 Hz. The rate capability test of FeTiO3 nanoflowers depicted in Fig. 4d was conducted as follows. A number of coin cells were independently cycled at different charge and discharge rates (charge and discharge currents were equal for the same cell). The discharge
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Fig. 2. Energy dispersive X-ray elemental maps of FeTiO3 –carbon nanocomposite (weight ratio of 1:1): (a) carbon map; (b) oxygen map; (c) titanium map; (d) iron map.
capacities were then taken from the performance of each cell in the 20th cycle and plotted vs. current rate. The rate capability test of FeTiO3 –carbon nanocomposite (Fig. 8b) was conducted in the following way. A fixed value of current (0.05 A g−1 ) was applied for both charge and discharge of the same cell for five cycles. The value of the charge and discharge currents was then increased and set to a higher value of current for the next five cycles. This procedure is repeated upon increasing the current up to the value of 5 A g−1 . Finally, the value of charge and discharge current is returned to the initial value of 0.05 A g−1 for five cycles.
The maps of oxygen, titanium and oxygen (Fig. 2b–d) indicate that FeTiO3 is distributed within the carbon material. The maps are consistent with the idea that the carbon component contains nanoparticles of FeTiO3 with a non-uniform size distribution. Larger particles of FeTiO3 (up to hundreds of nm) are also present. XRD patterns of the ilmenite nanoflowers, the milled ilmenite, and ilmenite nanocomposites are presented in Fig. 3. All of these XRD patterns agree well with the standard XRD pattern of ilmenite FeTiO3 (JCPDS 01-075-1211), indicating that no new crystalline phases are formed in the nanoflowers, and nanocomposites. The broadened diffraction peaks in the patterns are due to the small crystallite size. The 0 0 2 peak of graphite disappears completely in the XRD pattern of the nanocomposite after 150 h of milling,
3. Results and discussion Fig. 1a shows an SEM image of the original mineral ilmenite (FeTiO3 ) powder consisting of large particles with a typical size of 100–300 m. After ball milling treatment, the large particles have been reduced to small particles of sub-micron size and the aggregation of the small particles of ilmenite can be seen in Fig. 1b. Fig. 1c shows the morphology of the ilmenite nanoflowers formed after NaOH leaching of the milled ilmenite for 2 h. The diameter of the uniform flower-like architectures is about 1–2 m, and they consist of petals with the thickness of 5–20 nm. The morphological transformation and the formation mechanism of these FeTiO3 nanoflowers have been previously discussed in detail elsewhere [23]. The milled ilmenite (Fig. 1b) and its nanostructures (Fig. 1c) were electrochemically analysed in this work. The structure of FeTiO3 –carbon nanocomposite (1:1 by weight) was evaluated by EDX mapping in TEM. Fig. 2 shows the elemental maps of carbon, oxygen, titanium and iron. The carbon signal in Fig. 2a repeats the shape of the particles in the nanocomposite.
Fig. 3. XRD patterns for the ilmenite nanoflowers, milled ilmenite, and ilmenite nanocomposites.
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Fig. 4. Selected galvanostatic charge/discharge curves at 50 mA g−1 for (a) the milled ilmenite and (b) the ilmenite nanoflowers; (c) variation of discharge versus cycle number for the milled ilmenite and ilmenite nanoflowers at 50 mA g−1 ; (d) current density dependence of the reversible 20th cycle discharge capacity for the ilmenite nanoflowers.
which can be attributed to the amorphisation of carbon induced by ball milling (as it was reported, for example, in Ref. [24]). The surface areas of the three powders are 2.7 m2 g−1 (ball milled ilmenite), 26 m2 g−1 (nanoflowers), and 76 m2 g−1 (nanocomposite with 50 wt.% carbon). The selected charge–discharge curves from the first 50 cycles of an electrode assembled with milled ilmenite are shown in Fig. 4a. The testing current density was 50 mA g−1 and the voltage range was 0.01–3.0 V (vs. Li/Li+ ). The initial discharge has pronounced plateaus initiating at about 1.15 and 0.4 V, resulting from the initial reactions of lithium with FeTiO3 . The first discharge capacity is 534.4 mAh g−1 and close to the theoretical capacity of the following reaction FeTiO3 + 3Li+ + e− → Fe + LiTiO2 + Li2 O,
530 mAh g−1 .
It is apparent that the electrochemical performance of the electrode assembled from the ball milled ilmenite starts to degrade after the first cycle and capacities lower than 100 mAh g−1 are measured from the fifth cycle and beyond. The lithium storage capacity of nanostructured ilmenite (in the form of nanoflowers) is significantly better. Fig. 4b shows the selected charge–discharge curves from the first 50 cycles of the electrode assembled with ilmenite nanoflowers. Importantly, the
shape of the charge–discharge profiles of FeTiO3 nanoflowers is different from that of the milled ilmenite and higher capacities can be observed. Fig. 4c demonstrates the cycling stability of both milled ilmenite and FeTiO3 nanoflowers. The capacity of ball milled material quickly fades and stabilises at the level of about 50 mAh g−1 . In contrast, modest capacity of 200 mAh g−1 is maintained by FeTiO3 nanoflowers after 50 cycles. This indicates that lithium storage in FeTiO3 nanostructures is more efficient, and the nanoflowers deliver both higher capacities and better capacity retention after multiple charge–discharge cycles. The dependency of the discharge capacity of FeTiO3 nanoflowers on the current rate is illustrated in Fig. 4d. The capacities recorded in the 20th cycle at each current rate are shown. The electrode is capable of operating at the current rate of 1 A g−1 . The capacities of 190, 135, 115, 75 and 56 mAh g−1 were measured at the current rates of 0.05, 0.1, 0.2, 0.5 and 1 A g−1 , respectively, in the 20th cycle. In order to shed light onto the possible mechanism of the electrochemical reactivity of ilmenite FeTiO3 with lithium, cyclic voltammetry and transmission electron microscopy studies were conducted. The electrodes assembled using both milled ilmenite and ilmenite nanoflowers were analysed. Selected cycles from the cyclic voltammetry experiments are shown in Fig. 5. The first sweep was cathodic (reaction with Li+ ) and started from the open-circuit voltage. Two obvious reduction peaks can be observed in the first
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Fig. 5. Typical cyclic voltammograms: the first and subsequent cycles of the milled ilmenite electrode (a and b, respectively), and the first and subsequent cycles of the electrode assembled with the ilmenite nanoflowers (c and d, respectively). The CV measurements were performed at a scan rate of 0.2 mV s−1 in the voltage range of 0.01–3 V vs. Li/Li+ .
cathodic scan for the milled ilmenite (Fig. 5a). The first peak is located at about 0.86 V, and the second peak is located at about 0.56 V vs. Li/Li+ . The subsequent anodic peaks were observed at about 1.17 V and 1.99 V vs. Li/Li+ , indicating the processes associated with the extraction of Li+ from the electrode materials. The second cathodic sweep (Fig. 5b) differs from the first one, indicating the altered redox behaviour. A pronounced cathodic peak can be seen at approximately 1.41 V vs. Li/Li+ . The peaks located at 0.86 and 0.56 V vs. Li/Li+ , visible in the first cathodic scan, disappear. After the first 5–10 cycles, the shape of the CV cure becomes relatively stationary, indicating the reversible electrochemical reaction occurring after the first few discharge–charge cycles. This corresponds well with the stabilisation of the charge/discharge capacity observed in the charge–discharge experiments after the first 10 cycles. The differences can be seen between the CV curves of the ilmenite nanoflowers (Fig. 5c and d) and those of the milled ilmenite. Three obvious reduction peaks can be observed in the first cathodic scan for the ilmenite nanoflowers (Fig. 5c). The first two peaks are located around 1.11 V and 0.86 V vs. Li/Li+ and can be attributed to the electrochemical reaction of FeTiO3 with Li+ ions. Another cathodic peak observed at 0.34 V vs. Li/Li+ can be assigned to the formation of the solid electrolyte interphase (SEI) layer on the interface of a high surface area materials (the measured surface area of FeTiO3 nanoflowers is 26 m2 g−1 ). The subsequent anodic
peaks, corresponding to the extraction of Li+ from the electrode materials, are observed at about 1.61 V and 2.53 V vs. Li/Li+ . During the second cycle (Fig. 5d), the peaks at 1.11 and 0.86 V detected in the first cycle are missing but a new reduction peak located at about 1.25 V vs. Li/Li+ can be detected. The electrochemical behaviour after the 5th cycle is repeatable (similar in the subsequent cycles), indicating a reversible electrochemical process in the electrode. Again, this correlates well with the results of the galvanostatic charge–discharge (Fig. 4c), demonstrating a stable capacity of about 200 mAh g−1 after the 10th cycle. Comparing the results of the cyclic voltammetry measurements for the ball milled material and the FeTiO3 nanoflowers, the CV curves for these two types of electrodes have both shared and distinctly different features. Specifically, it is worthwhile to note that the same peak at about 0.86 V vs. Li/Li+ is present in all cases in the first discharge while additional cathodic peaks are different. It indicates that, although a similar reaction step is present in the first discharge of both milled FeTiO3 and its nanostructures, the electrochemical reactivity of ilmenite nanostructures with lithium is generally different and results in higher capacity retained after the first few cycles. An improved electrochemical behaviour of the flower-like FeTiO3 nanostructure can be attributed to its special structural characteristics. The large surface area of nanoflowers provides a large electrochemical interface that facilitates highly efficient Li+ ion insertion, and the open and low density morphology in the discrete hierarchical nanostructures
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Fig. 6. Bright-field TEM images and electron diffraction patterns taken from the electrodes assembled with the ball milled FeTiO3 powder: (a and b) after the first discharge of the cell to 0.01 V vs. Li/Li+ ; (c and d) after the subsequent charge to 3 V vs. Li/Li+ .
Fig. 7. Bright-field TEM images and electron diffraction patterns taken from the electrodes assembled with the FeTiO3 nanoflowers: (a and b) after the first discharge of the cell to 0.01 V vs. Li/Li+ ; (c and d) after the subsequent charge to 3 V vs. Li/Li+ .
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facilitates efficient Li+ ion diffusion during discharge/charge cycling. In addition, the petals of nanoflowers are dominated by (2¯I0)-type planes of FeTiO3 [23], which may have an influence on the way lithium reacts with ilmenite. The TEM data for the electrode materials after the first discharge and the subsequent first charge of coin cells are shown in Fig. 6 (the ball milled material) and Fig. 7 (FeTiO3 nanoflowers). Bothbright field images and electron diffraction patterns are presented. The electron diffraction is a valuable tool because it allows us to evaluate the phase composition of the electrodes after each step. The bright-field TEM images of the electrode assembled with the ball milled FeTiO3 powders after the first discharge of the cell to 0.01 V vs. Li/Li+ and after the subsequent charge to 3 V vs. Li/Li+ are shown in Fig. 6a and c, respectively. The darker clusters in the image correspond to the active component of the electrode, and the brighter areas contain carbon black particles. The corresponding selected area electron diffraction patterns (Fig. 6b and d) consist of families of rings, indicating the nanocrystalline structure of the active component of the electrode after the discharge and charge. The lattice spacings can be derived from the diffraction patterns and used for the phase identification. The spacings of 1.19, 1.45 and 2.04 A˚ were calculated from the pattern of the discharged electrode. The pattern of the material after the first charge ˚ and contains a similar set of three rings (1.19, 1.45 and 2.03 A), ˚ are also present. We contwo additional rings (1.63 and 2.37 A) servatively estimate that the error in the measurement of lattice distances from electron diffraction patterns is 5% although the real accuracy was found to be better for the measurements of standard crystals with known lattice spacings. On the basis of a simple comparison, the two diffraction patterns contain the same three rings, ˚ indicating that one phase is likely to 1.19, 1.45 and 2.03 (2.04) A, remain stable upon cycling in a lithium cell. The three distances are in a reasonable agreement with either LiTiO2 (JCPDS file No. 00-016-0223), LiFeO2 (JCPDS file No. 01-074-2283) and Fe (JCPDS file No. 01-087-0721). The ring corresponding to the spacing of 2.37 A˚ also matches a diffraction peak in the standard diffraction patterns of LiTiO2 or LiFeO2 . The additional ring appearing in the diffraction pattern of the charged electrode and corresponding to the lattice spacing of 1.63 A˚ may belong to Li2 O (JCPDS file No. 01-073-0593). The same set of measurements for the electrode assembled with FeTiO3 nanoflowers is shown in Fig. 7. The bright-field TEM images for the discharged and charged materials are depicted in Fig. 7a and c, respectively, while the electron diffraction patterns are shown in Fig. 7b and d. Again, the same common diffraction ˚ 1.45 A˚ (1.41 A˚ for the discharged rings corresponding to 1.18 A, electrode) and 2.45 A˚ (2.00 A˚ for the discharged electrode) are observed. As indicated above, these rings match LiTiO2 , LiFeO2 or Fe phases. The interpretation is difficult since some lattice spacings of these phases are quite similar. Additional rings in ˚ the discharged material fit the distances of 1.60, 2.30 and 2.72 A, which are close to the lattice spacings in the standard diffraction file of Li2 O. It should be noted, in respect to the above analysis, that some diffraction rings with a very weak intensity could not be detected by the available camera on our TEM, and additional diffraction rings may, in fact, exist in addition to those depicted in Figs. 6 and 7. On the basis of the available data, it is concluded that an electrochemically inert phase forms after the first discharge. The diffraction intensities matching either LiTiO2 or LiFeO2 are visible. We propose that one of these two phases forms upon the initial discharge of the cell and stays inactive in further cycles. The formation of this phase is correlated with a peak at 0.86 V vs. Li/Li+ detected in the CV curves of both milled materials and FeTiO3 nanoflowers. Depending on whether LiTiO2 or LiFeO2 phase forms, one of the two mechanisms may be operational. Namely, the
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Fig. 8. Electrochemical performance of ilmenite–carbon nanocomposites: (a) cyclic stability of nanocomposites with different compositions at a current density of 50 mA g−1 , and (b) rate capability of the best performing nanocomposite (50 wt.% of carbon).
reversible reactivity with lithium may involve iron or iron oxides (formed after the initial discharge) in the first scenario or titanium oxides in the second scenario. The performance of ilmenite FeTiO3 was also assessed in the form of nanocomposites with carbon. Fig. 8a shows the performance of samples with varied carbon content as well as the electrochemical performance of ball milled graphite and ball milled FeTiO3 . It can be seen that the addition of 20 wt.% and 30 wt.% of carbon delivers improvements in respect to the performance of ball milled ilmenite, although capacity is still low. The dramatic improvement in the capacity and cyclic stability was observed, however, when a composite incorporating 50 wt.% was evaluated. After the initial few cycles capacity of about 400 mAh g−1 was observed, and it deteriorated only slightly to 380 mAh g−1 after 50 cycles. The measured capacity exceeds the capacity of FeTiO3 observed in the ball milled sample and the sample of FeTiO3 nanoflowers as well as the theoretical capacity of graphite. We believe that ilmenite FeTiO3 and carbon demonstrate synergistic behaviour, i.e. the mixture of individual components is capable of delivering higher capacity than that of individual components. The response of the best performing FeTiO3 –carbon composite (50 wt.% of carbon) to varied current rates of charge–discharge is shown in Fig. 8b. The initial reversible capacity was about 450 mAh g−1 . When the current rate was switched to 0.1, 0.2, 0.5, 1, 2, and 5 Ag−1 , the specific capacity was slowly reduced to approximately 300, 240,
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observed that the electrochemical performance of ilmenite might be improved if the electrode material was prepared in the form of a nanocomposite of FeTiO3 with carbon. Acknowledgements Financial support from the Australian Research Council is acknowledged. The authors also thank Dr. Pavel Cizek and technical officers at Deakin University, Robert Lovett, Rosey van Driel and Dr. Andrew Sullivan, for their help in this work. References
Fig. 9. Electrochemical impedance spectra for the fresh cells and their corresponding equivalent circuit model (inset).
160, 100, 66, and 33 mAh g−1 , respectively. When the current rate was decreased back to 50 mA g−1 after 40 cycles, the electrode still delivered a high reversible capacity of 400 mAh g−1 . The electrochemical impedance spectra for the coin cells assembled with the electrodes incorporating the two ilmenite samples are compared in Fig. 9. The Nyquist plots are semicircular in the range of high to medium frequency, which reflects the charge-transfer resistance (Rct ) of the electrodes. The inclined line (∼45◦ ) represents the Warburg impedance (Zw ) at low frequency, which indicates the diffusion of Li+ in the solid matrix. The intercepts on the real axis Z could be considered as representing the combined resistance (Rs ), including the ionic resistance of the electrolyte, the intrinsic resistance of the active materials and the contact resistance at the active material/current collector interface. The corresponding equivalent circuit for the Nyquist plots of the electrodes is shown in the inset, where CDL represents the double layer capacitance. The values of Rct for the cells with electrodes incorporating ilmenite nanocomposites, ilmenite nanoflowers and milled ilmenite were calculated to be approximately 126, 220 and 417 , respectively. This indicates that electrodes incorporating nanocomposites have easier charge transfer at the electrode/electrolyte interface and provide a lower overall battery internal resistance. 4. Conclusions We have demonstrated that the ball milled ilmenite FeTiO3 and the ilmenite nanostructures exhibit lithium storage ability as anode materials with a good retention of capacity for at least 50 initial cycles. The electrochemical performance of the ilmenite nanoflowers was found to be superior to that of the ball milled material. A capacity of ca. 650 mAh g−1 is obtained for these ilmenite nanostructures during the first discharge of the cell, and a reversible capacity of ca. 200 mAh g−1 is maintained after 50 cycles. Transmission electron microscopy analysis shows that during the first discharge, new phases such as LiTiO2 , LiFeO2 or Fe are produced by the electrochemical reaction of FeTiO3 with lithium. It is proposed that this initial reaction is irreversible, and an inactive phase (LiTiO2 or LiFeO2 ) forms after the initial discharge. Depending on whether LiTiO2 or LiFeO2 stabilises after the initial discharge, two possible scenarios of reactivity with lithium may be operational in the subsequent cycles. In the first scenario the reversible lithium storage happens due to iron oxides formed upon the initial discharge, while an alternative scenario assumes the reversible lithium storage in titanium oxides. It was
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