Nanostructured Surfaces, Coatings, and Films 2014View this Special Issue
Synthesis and Electrochemical Performance of Graphene Wrapped SnxTi1−xO2 Nanoparticles as an Anode Material for Li-Ion Batteries
Ever-growing development of Li-ion battery has urged the exploitation of new materials as electrodes. Here, solid-solution nanomaterials were prepared by aqueous solution method. The morphology, structures, and electrochemical performance of nanoparticles were systematically investigated. The results indicate that Ti atom can replace the Sn atom to enter the lattice of SnO2 to form substitutional solid-solution compounds. The capacity of the solid solution decreases while the stability is improved with the increasing of the Ti content. Solid solution with x of 0.7 exhibits the optimal electrochemical performance. The Sn0.7Ti0.3O2 was further modified by highly conductive graphene to enhance its relatively low electrical conductivity. The Sn0.7Ti0.3O2/graphene composite exhibits much improved rate performance, indicating that the solid solution can be used as a potential anode material for Li-ion batteries.
has long been considered as a potential substitute for the conventional graphite anode in lithium ion batteries, since the theoretical capacity of (783 mAh g−1) is much higher than that of commercial graphite anode (372 mAh g−1) . However, large volume change for occurs during the cycling process; thus it causes crumbling of the electrode and leads to destruction of the electrically conductive network. Therefore, the based anode materials suffer from rapid fading of capacity [2, 3]. There have been many strategies to improve the cycle stability of the based anode materials, including elaborate structure design and combination with conductive additives [3–7]. Unfortunately, the mitigation of the capacity fading is quite difficult because of the large volume change derived from alloying/dealloying reactions of Sn with Li, which is the fundamental electrochemical origin of the electrode [8, 9].
The /metal oxide hybrid electrode materials, such as SnO2-SiO2, SnO2-Fe2O3, and SnO2-TiO2 [10–13], have been reported to show improved electrochemical performance than pure SnO2 for lithium storage because metal oxide can act as a buffering matrix to accommodate the large change in volume as well as the conductive additive to improve the electrical conductivity. Among these metal oxides, TiO2 itself has also attracted great attention as an alternative anode material owing to its low cost, low toxicity, stable cyclability, and good safety. However, the practical application of the TiO2 anode is challenging due to its low specific capacity and low intrinsic electrical conductivity which leads to limited rate capability [14–16]. There have been some reports on the SnO2-TiO2 mixed oxides as the anode material for Li-ion batteries. The studies involve a fundamental investigation of SnO2-TiO2 mixed oxides and aim to improve the cyclic performance by manipulating the morphology and content of TiO2. Lin and coworkers  investigated the electrochemical performance of a TiO2-supported SnO2 nanocomposite formed of equimolar amounts of SnO2 and TiO2. The nanocomposite showed improved capacity and higher Coulombic efficiency. Du and coworkers  prepared three-dimensional SnO2/TiO2 composite by depositing SnO2 nanocrystals into TiO2 nanotube electrodes through solvothermal technique. The maximum reversible capacity of the composite could reach as high as ~300 μA h cm−2. Except the SnO2-TiO2 mixed oxides, Uchiyama and coworkers  first proposed that the solid solution could be used as the potential anode material for Li-ion batteries. They found that there was not major collapse of the structure in the solid solution during lithium insertion. However, the lithium insertion/extraction behavior of this rutile-type solid solution remains unclear, and the electrochemical performance such as cyclic performance, rate performance, and resistance of this solid solution should be further tested and improved.
In this paper, a series of () nanomaterials were successfully synthesized by an aqueous solution-based method. The morphology, structure, and the performance of solid solutions as anode materials were examined in detail. The results proved that the electrochemical performance of differed from either or TiO2. In order to further improve its electrochemical performance, the solid solution with a certain value was modified by graphene. The improved electrochemical performance indicates that /graphene composite can be used as a potential anode material for Li-ion battery.
2. Materials and Methods
All chemicals were of analytical grade and were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Graphene oxide (GO) nanosheets were prepared by a chemical exfoliation process using natural graphite flakes of 300 mesh size according to the method we reported previously .
2.2. Preparation of SnxTi1−xO2 Solid Solution
Precursors of solid solutions with different values were obtained by dissolving tetrabutyl titanate (TBT) and SnCl2·2H2O with a certain molar ratio into 50 mL of ethanol solution under stirring at room temperature. The total concentration of the metal ions (TBT + SnCl2·2H2O) was 0.1 M. The (TBT)/SnCl2·2H2O molar ratio in the precursor solutions was adjusted to 8 : 2, 7 : 3, 6 : 4, 5 : 5, 4 : 6, 3 : 7, and 2 : 8. After 30 min, 50 mL of deionized water was added into the solution. The resulting white slurry was stirred for another 1 h and additionally aged for 8 h. The precipitate was collected by centrifuge and repeatedly washed with ethanol and then air-dried at 80°C for 10 h. Different from the reported by Uchiyama et al. under the same calcination temperature , the obtained sample was further calcinated at 500°C−900°C for 5 h, resulting in the final solid solutions. In order to compare, we prepared TiO2 and particles with the precursor solutions only containing TBT or SnCl2·2H2O, respectively.
2.3. Preparation of SnxTi1−xO2/Graphene Composite
Sn0.7Ti0.3O2 was mixed with the GO solution to form a slurry with the weight ratio of Sn0.7Ti0.3O2 : GO = 4 : 1. The solid content of the slurry was adjusted to 10% by adding deionized water. After being ultrasonically exposed for 10 min, the slurry was spray-dried with an inlet air temperature of 200°C to form solid powders. The composite was then annealed at 500°C under Ar to form Sn0.7Ti0.3O2/G composite.
2.4. Structural Characterization
Powder X-ray diffraction (XRD) measurements were performed using AXS D8 Advance diffractometer (Cu Kα radiation; receiving slit, 0.2 mm; scintillation counter, 40 mA; 40 kV) from Bruker Inc. The morphology and structure were analyzed by a Hitachi S-4800 field emission scanning-electron microscope (SEM) and FEI Tecnai G2 F20 transmission-electron microscope (TEM) at an accelerating voltage of 200 kV.
2.5. Electrochemical Tests
The evaluation of electrochemical performance was carried out using CR2032-type coin cells. The working electrode contained 80 wt % of active materials, 10 wt% of Super P, and 10 wt % of polyvinylidene fluoride (PVDF). The Li metal foil served as the counter electrode. The electrolyte was composed of 1 M LiPF6 solution in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1 : 1 by volume). The coin cells were activated at a current density of 50 mA g−1 for the first cycle and then cycled under different current densities within the voltage range of 0.01–2 V using a LAND-CT2001A battery test system (Jinnuo Wuhan Corp., China). Cyclic voltammogram analysis was performed using Autolab 83710. Specific resistance of electrode materials was measured by four-point resistivity test system (NAPSON CRESBOX) after coating the anode slurry on an insulating substrate.
3. Results and Discussion
The XRD patterns of solid solutions with different were collected to determine the crystal structures, as shown in Figure 1. All samples exhibit similar XRD patterns as that of pure rutile and no peaks of other crystal phase are detected, indicating that Ti does not present in the form of TiO2 in the solid solution. When the value of decreases, all the peaks shift towards higher diffraction angles, demonstrating that the crystal structures remain the same whereas the lattice parameters of the solid solutions shrink gradually. It should be pointed out that the calcination temperature needs to be lowered to keep the pure solid solution structure at high Ti content. Though rutile TiO2 and rutile both belong to tetragonal systems and the lattice parameters of two oxides are close, phase separation of TiO2 along with solid solutions may occur especially at high Ti content. For example, at the calcination temperature of 650°C, TiO2 are detectable when the Ti content is above 0.5. Consequently, lower calcination temperatures were adopted in our experiments to prevent the formation of TiO2. Specifically, Sn0.9Ti0.1O2, Sn0.8Ti0.2O2, Sn0.7Ti0.3O2, Sn0.6Ti0.4O2, Sn0.5Ti0.5O2, Sn0.4Ti0.6O2, and Sn0.3Ti0.7O2 were obtained at 900°C, 900°C, 800°C, 700°C, 600°C, 600°C, and 500°C, respectively. As a result of lower calcination temperature, the width of the peaks in the XRD pattern decreases with increasing Ti content, indicating that the particle size or the crystallinity of the solid solution decreases.
The morphology of the precursors (before calcination) and the corresponding solid solutions were analyzed by SEM and TEM. As shown in Figures 2(a) and 2(b), both precursors of Sn0.9Ti0.1O2 and Sn0.4Ti0.6O2 present irregular sphere-like morphology. The particles of Sn0.4Ti0.6O2 precursor are more significantly aggregated than that of Sn0.9Ti0.1O2. After calcination at different temperatures, as shown in Figures 2(c) and 2(d), both Sn0.9Ti0.1O2 and Sn0.4Ti0.6O2 still maintain the irregular particulate morphology. The particle size of Sn0.4Ti0.6O2 is about 10–20 nm, which is much smaller than that of Sn0.9Ti0.1O2 (30–50 nm) (Figures 2(e) and 2(f)). The clearly visible set of lattice fringes observed in TEM images (Figures 2(g) and 2(h)) with a period of 0.334 nm and 0.328 nm is characteristic of the (101) lattice planes of rutile with of 0.9 and 0.4, respectively. The decrease of the particle size along with the increase of the Ti content is consistent with the XRD results, which is probably due to the lower calcination temperature. However, the aggregation of nanoparticles becomes more severe when the Ti content increases.
X-ray photoelectron spectroscopy (XPS) analysis of the Sn0.4Ti0.6O2 solid solution was performed in the range of 0–1200 eV, as shown in Figure 3(a). There are two symmetrical peaks at 487.15 eV and 495.60 eV in the Sn 3d spectrum (Figure 3(b)), which can be attributed to Sn 3d5/2 and Sn 3d3/2, respectively. The separation between these two peaks is 8.45 eV, slightly larger than the energy splitting reported for . Similarly, the Ti 2p spectrum (Figure 3(c)) is also composed of two symmetrical peaks with binding energies of 459.5 eV and 465.15 eV, which are derived from Ti 2p3/2 and Ti 2p1/2, respectively. The separation of 5.65 eV between these two signals indicates a normal state of Ti4+ in the rutile Sn0.4Ti0.6O2 solid solution . As for the O 1s spectrum shown in Figure 3(d), the O 1s peak can be deconvoluted into two peaks at 530.70 eV and 531.95 eV, corresponding to Ti–O–Ti and Sn–O–Ti bonds, respectively . It should be noted that the Ti–O–Ti bonds should be attributed to the trace of TiO2 on the surface of solid solution especially when Ti content is high. From the Sn–O–Ti bonds it can be inferred that Ti has been successfully incorporated into the matrix through Sn–O–Ti bonds.
Then the solid solutions were subjected to a systematic electrochemical analysis. Cyclic voltammetry (CV) was carried out at a scan rate of 0.2 mV s−1 to identify the characteristics of the redox reactions during charge/discharge cycles. As can be seen from Figures 4(a), 4(b), and 4(c), all the solid solutions with different present similar CV curves in the first cycle. The CV curves clearly indicate a reaction during the first discharge with a reduction peak around 1 V. The XRD results (Figure 4(d)) further confirm that, after the insertion, Sn as the only crystalline phase presents in both Sn0.9Ti0.1O2 and Sn0.7Ti0.3O2 anode material. Therefore, the peak around 1 V is speculated to represent the reaction of Li with to form Li2O and Sn, which is similar to the first discharge process of the based anode material as the reaction (1). For the based anode material, the reduction peak at 1 V disappears after the first discharge, indicating that the reaction of Li with to form Li2O and Sn is generally irreversible [5, 12, 22]. After the first cycle, electrochemically reduced Sn will react with to form a series of tin-lithium alloys during the following cycles as the reaction (2), which is reversible. On the other hand, it should be also mentioned that the insertion/extraction of Li+ in rutile TiO2 anode usually takes place in the potential range of 1.0–1.4 V in CV curves , and this process has a good reversibility as reaction (3):For the , there is no obvious redox peak at 1.0–1.4 V in the CV curves for the solid solution, suggesting that Ti in the solid solution does not undergo a similar electrochemical reaction as that in TiO2. Basically, the charge-discharge process of the solid solution is similar to that of .
The initial charge-discharge curves of Sn0.9Ti0.1O2, Sn0.7Ti0.3O2, and Sn0.4Ti0.6O2 solid solutions at a current density of 50 mA g−1 between 0.01 and 2 V are compared in Figure 5. Sn0.9Ti0.1O2 delivers a capacity of 888 mAh g−1 in the first charge and 1465 mAh g−1 in the first discharge, while Sn0.7Ti0.3O2 and Sn0.4Ti0.6O2 display 780, 1225, 600, and 1125 mAh g−1 in the first charge and discharge, respectively. The increasing capacity of with increasing indicates that the increasing of Sn content can enhance the capacity of the solid solution. Moreover, Sn0.9Ti0.1O2 exhibits lower irreversible capacity loss in the first cycles comparing to that of Sn0.4Ti0.6O2. Such phenomenon may be attributed to the smaller particle size of Sn0.4Ti0.6O2 and its lower crystallinity.
The cycling performance of bare TiO2, bare , and solid solution with of 0.9, 0.7, and 0.4 was evaluated at the current density of 100 mA g−1, as shown in Figure 6. The bare-TiO2 electrode presents a stable cyclability with the reversible capacity of 200 mAh g−1. The excellent cycle stability of the TiO2 anode material should be attributed to its negligible volume change (<4%) during the charge/discharge processes. The bare- electrode delivers a high initial capacity of 1100 mAh g−1, much higher than that of bare-TiO2. Unfortunately, there is a rapid fading of capacity due to the severe pulverization of during cycling. After 30 cycles, only a low reversible capacity of 94 mAh g−1 is remained, which was about 10% retention of the initial reversible capacity. The Sn0.9Ti0.1O2, Sn0.7Ti0.3O2, and Sn0.4Ti0.6O2 deliver the initial capacity of 887 mAh g−1, 743 mAh g−1, and 592 mAh g−1, respectively. It is obvious that the initial capacity of declines with the increasing of the Ti content but is still much higher than that of pure TiO2. As the content of Ti increases, the cycle stability is enhanced. After 30 cycles, Sn0.9Ti0.1O2 shows ~20% retention (174 mAh g−1) of the initial capacity, which is a little higher than that of pure anode materials. The capacity retention can be further significantly raised to 47% and 60% when is decreased to 0.7 (Sn0.7Ti0.3O2) and 0.4 (Sn0.4Ti0.6O2), respectively. With increasing Ti content, the resistivity of the solid solution also increases as displayed in Table 1. The above results thus demonstrate that the electrochemical performance of the solid solution found here is a combination of TiO2 and . A gradual transition of the electrochemical performance from to TiO2 can be observed along with the increase of Ti content in the solid solution, and the overall electrochemical performance of is strongly affected by the ratio of Ti/Sn. Consequently, with an optimal value, the characteristic high capacity of anode material and superior cycle stability of TiO2 can be optimally balanced in one solid solution anode material. Based on the above results, the optimal value is found to be 0.7.
Despite the fact that relative good electrochemical performance can be achieved, the solid solutions still suffer from low intrinsic conductivity (see Table 1). Therefore, modification of the solid solution with graphene by a spray drying method was carried out in our experiments in the case of Sn0.7Ti0.3O2 with the optimal performance. Graphene as the 2D carbon nanomaterial was recently widely used to improve the electrochemical performance of various electrode materials due to its superior electrical conductivity, large surface area, and excellent structural flexibility [24–29]. SEM image in Figure 7(a) shows an overview of the final Sn0.7Ti0.3O2/graphene nanocomposite. The sample consists of secondary quasispherical microparticles with diameters of 2~5 μm. SEM observation at a higher magnification (Figure 7(b)) reveals that each microsphere is actually a random aggregation of primary Sn0.7Ti0.3O2 nanoparticles that are covered by soft graphene sheets. The TEM image further demonstrates that the Sn0.7Ti0.3O2 nanoparticles are loosely wrapped by a thin graphene layer as presented in Figure 7(c). The high resolution TEM image (Figure 7(d)) reveals that the Sn0.7Ti0.3O2 nanoparticles are coated with graphene shells whose thickness is less than 3 nm. In addition, the SEM and EDS mapping images show that Sn, Ti, O, and C are dispersed uniformly in the microspheres.
To further study the effects of graphene wrapping on the rate capability and cyclic stability of the solid solution, the reversible capacity of Sn0.7Ti0.3O2/graphene nanocomposite and pure Sn0.7Ti0.3O2 were tested with variable current densities, as plotted in Figure 8. The reversible capacity of Sn0.7Ti0.3O2/graphene maintains 600 mAh g−1 at a charge rate of 100 mA g−1. Further increasing the charge rates to 200 mA g−1, 300 mA g−1, and 500 mA g−1, reversible capacities of 550 mAh g−1, 470 mAh g−1, and 330 mAh g can be obtained, respectively. Even at a high current density of 1000 mA g−1, graphene modified solid solution can still deliver a reversible capacity of ~200 mAh g−1. In contrast to Sn0.7Ti0.3O2/graphene, Sn0.7Ti0.3O2 electrode shows much poorer rate-performance. At the initial six cycles under the current density of 100 mA g−1, the charge capacity of Sn0.7Ti0.3O2 is slightly higher than that of Sn0.7Ti0.3O2/graphene. However, as the current density increases, the capacity loss of the Sn0.7Ti0.3O2 was much larger than that of the Sn0.7Ti0.3O2/graphene. Sn0.7Ti0.3O2 only delivers a capacity less than 100 mAh g−1 at a current density of 500 mA g−1. The results strongly confirm that graphene conductive network can effectively improve the rate performance of the solid solution. Comparing to our previous works on the graphene modified TiO2 and as anode materials for Li-ion battery, we could find that Sn0.7Ti0.3O2/graphene presents much higher capacity than TiO2/graphene [27, 30]. However, the cyclic life is not as high as /graphene, which can be attributed to the large particle size of solid solution. Although more studies are required to further improve the coulombic efficiency and cyclic stability of Sn0.7Ti0.3O2/graphene composite, we believe that it can be considered as a promising candidate for high-performance anode material in advanced LIBs.
A series of solid solutions were successfully prepared by sol-gel method. The electrochemical reaction of the anode material is similar to that of . The electrochemical performance of is strongly affected by the ratio of Ti/Sn, which can be regarded as a gradual transition from that of to TiO2 when the value increases. At a suitable value of 0.7, high specific capacity and good cycle stability can be achieved simultaneously, which make Sn0.7Ti0.3O2 a potential substitute anode material to or TiO2. Sn0.7Ti0.3O2 was further modified by graphene to enhance its electrical conductivity, which resulted in improved rate performance, indicating the potential application value of the solid solution as the anode material for Li-ion batteries.
Conflict of Interests
The authors declared that they have no conflict of interests to this work.
The authors are grateful for financial support from the Key Research Program of the Chinese Academy of Sciences (Grant no. KGZD-EW-202-4), Projects 21201173 and 21371176 supported by the National Natural Science Foundation of China, Science and Technology Innovation Team of Ningbo (Grant no. 2012B82001), and the 973 Program (Grant no. 2011CB935900).
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