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J. Phys. Chem. B 2006, 110, 14754-14760
Hydrothermal Synthesis of Zn2SnO4 as Anode Materials for Li-Ion Battery A. Rong,† X. P. Gao,*,† G. R. Li,† T. Y. Yan,† H. Y. Zhu,*,‡ J. Q. Qu,† and D. Y. Song† Institute of New Energy Material Chemistry, Department of Materials Chemistry, Nankai UniVersity, Tianjin 300071, China, and School of Physical and Chemical Sciences, Queensland UniVersity of Technology, Brisbane, Queensland 4001, Australia ReceiVed: May 10, 2006; In Final Form: June 12, 2006
Spinel Zn2SnO4 particles with the cubic shape are prepared via a hydrothermal reaction under mild conditions. The hydrothermal conditions, such as alkaline concentration, reaction temperature, and duration time, have an important influence on the product structure and the performance of the electrode prepared with the product. The optimized product is cube-shaped Zn2SnO4 crystalline, which is prepared with 0.4 M of NaOH solution at 200 °C for 24 h. These cube-shaped Zn2SnO4 particles with the spinel structure exhibit a large electrochemical capacity of 988 mA h/g and a relatively good capacity retention as anode materials for Li-ion battery. The structures of the as-prepared product and specimens taken from the electrodes after charging-discharging cycles are analyzed by X-ray diffraction, scanning electron microscopy, and transition electron microscopy techniques. In particular, it is found for the first time that the spinel Zn2SnO4 structure exists to a great extent after the first cycle and contributes to the extremely high reversible capacity during the following cycles.
I. Introduction Recently, tin-based oxide materials have attracted a considerable amount of attention because of their potential application in Li-ion batteries. The theoretical capacity of tin oxide is as high as 1494 mA h/g, with a reversible capacity of 782 mA h/g, so that tin-based oxides are likely to be good candidates of anode materials for Li-ion batteries.1-5 It was also found that nanosized tin-based oxides exhibit especially good electrochemical performance on account of the large specific area and high Li-ion conductivity.6-10 However, nanosized tin-based oxides have a disadvantage of the large irreversible capacity loss because of the formation of Li2O in the first cycle. Besides, the cracking of tin-based oxide nanocrystals and the aggregation of metal components during charge and discharge process affects the cycle stability of anode materials. To solve these problems, some nonmetal oxides, metal oxides, and graphite were added to improve the electrochemical properties.11-15 Martos et al.12 prepared Sn1-xMoxO2 through ball-milling, which showed higher discharge capacity and better cycle stability in the potential range of 0.0-1.0 V as anode materials than pure SnO2. ZnO follows a similar lithium insertion mechanism to that of SnO2, but it’s lithium-storage capacity is low and fades rapidly during cycling.16,17 Belliard et al.18 investigated the electrochemical performance of ball-milled ZnO-SnO2 and found that the cycle performance of the electrode was greatly improved when a smaller amount of ZnO was added into the composites. In recent years, some compounds such as stannates of M2SnO4 (M ) Zn, Mg, Mn, Co), CaSnO3, CoSnO3,18-22 K(M,Sn)8O16 (M ) Li, Mg, Fe, or Mn),23 and ZnCo2O424 have been investigated as anode materials for Li-ion batteries because of their high lithium storage capacity. The compound materials of CoSnO3 are obviously superior to the ball-milled mixtures of CoO/SnO2 in electrochemical performance.22 In particular, the initial * To whom correspondence should be addressed. E-mail: xpgao@ nankai.edu.cn (X.P.G.) and
[email protected] (H.Y.Z.). † Nankai University. ‡ Queensland University of Technology.
irreversible capacity loss is markedly reduced for CoSnO3 material. The CaSnO3 compound also shows a negligible capacity fading despite its lower capacity. 20 Zn2SnO4 is an important material for potential applications as anode materials in photoelectrochemical cells and Li-ion batteries, photocatalysts to decompose benzene in water solution, sensors for gas humidity, and combustible gases.19,25-28 Zn2SnO4 has been prepared via thermal evaporation,29 hightemperature calcination,26,30 sol-gel synthesis,31 ball-milling,16 and hydrothermal reaction.32,33 However, it is difficult to obtain a single phase of Zn2SnO4 by conventional solid-state reaction, which always contains impurities. The hydrothermal method has proved to be an easy and ideal way to gain pure, uniform, and well-crystallized products. 34 Therefore, it can be a viable approach to synthesize Zn2SnO4 materials and optimize their structures by manipulating the synthesis conditions to achieve superior anode materials for Li-ion batteries. In the present study, we prepared spinel Zn2SnO4 particles via a hydrothermal reaction under different conditions, and the electrochemical properties of as-prepared products as anode material were tested. Alkaline solution concentration plays an important role on control over the size, morphology, crystallinity, and, thus, electrochemical performance of the final products. II. Experimental Section 1. Sample Preparation and Characterization. SnCl4‚5H2O (g99.0%) and ZnSO4‚7H2O (g99.5%) were dissolved into distilled water to form two transparent solutions. The two solutions were mixed together, and then 1.0 M NaOH was added dropwise into the mixed solution of SnCl4 and ZnSO4 under magnetic stirring. The calculated concentrations of zinc and tin in the reaction system were 0.05 and 0.025 M, respectively. The white precipitate obtained was transferred into a PTFElined stainless autoclave and filled two-thirds of the internal volume of the autoclave. Next, the autoclave was kept at a temperature between 130 and 200 °C for a designed period of time, so that we could examine the influence of the reaction
10.1021/jp062875r CCC: $33.50 © 2006 American Chemical Society Published on Web 07/11/2006
Zn2SnO4 as Anode Materials for Li-Ion Battery
Figure 1. XRD patterns of Zn2SnO4 particles synthesized at (a) 0.2 M NaOH at 200 °C for 24 h, (b) 0.3 M NaOH at 200 °C for 24 h, (c) 0.4 M NaOH at 200 °C for 24 h, and (d) 0.4 M NaOH at 150 °C for 24 h.
temperature and reaction time on the product structure and morphology. After the autoclave was cooled naturally to room temperature, the solid was separated by filtration, rinsed with distilled water and ethanol alternately, and then dried in an oven at 100 °C for 1 day. Scanning electron microscopy (SEM,
J. Phys. Chem. B, Vol. 110, No. 30, 2006 14755 Hitachi S-3500N), transmission electron microscopy (TEM, FEI Tecnai 20), and X-ray diffractometry (XRD, Rigaku D/ max-2500) were employed to characterize the as-prepared samples. 2. Electrochemical Performance. The working electrode was prepared by pressing a mixture of the active materials, conductive material (acetylene black), and binder (poly(tetrafluoroethylene), PTFE) in a weight ratio of 80/15/5. Lithium metal was used as the counter and reference electrodes. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC). The volume ratio of EC:PC:DMC in the mixture was 6:3:1. The cells were assembled in an atmosphere of high-purity argon in a glovebox (ZKX-2B). The galvanostatic method at a dischargecharge current density of 100 mA/g and between fixed potential limits (0.05-3.0V) was used to measure the electrochemical capacity and cycle stability of the electrodes at room temperature using a LAND-CT2001A instrument. The cyclic voltammetry (CV) experiment was carried out at a scan range of 0.05-3.0V (vs Li+/Li) and a scan rate of 0.1 mV/s using a CHI 600A potentiostat at room temperature. After cycling to the required states, electrode pieces were removed from the cell in the glovebox, washed with DEC, dried in a vacuum, and then kept in a glass bottle filled with Ar prior to the TEM and XRD tests.
Figure 2. SEM images of the as-prepared Zn2SnO4 particles synthesized at different hydrothermal conditions: (a) 0.2 M NaOH at 200 °C for 24 h, (b) 0.3 M NaOH at 200 °C for 24 h, (c) 0.4 M NaOH at 200 °C for 24 h, (d) 0.2 M NaOH at 150 °C for 24 h, (e) 0.4 M NaOH at 150 °C for 24 h, and (f) 0.4 M NaOH at 200 °C for 48 h.
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Figure 3. TEM and HRTEM images of the as-prepared Zn2SnO4 particles synthesized in 0.4 M NaOH solution (a, c) and 0.2 M NaOH solution (b, d) at 200 °C for 24 h. (c and d) HRTEM images.
III. Results and Discussion XRD patterns of the as-prepared samples at different conditions are shown in Figure 1. All diffraction peaks of the four samples are consistent with the JCPDS (24-1470) data of the pure spinel Zn2SnO4 with a lattice parameter of 8.657 Å. On the basis of the broadening of diffraction peaks, the as-prepared product obtained under hydrothermal conditions at 200 °C and an alkaline concentration of 0.4 M possesses much better crystallinity than the other samples, implying that hydrothermal treatment at low temperature and low alkaline concentration led to the poor crystallinity and smaller crystals. The alkaline concentration in the hydrothermal treatment is demonstrated to be a critical factor influencing the crystallinity, morphology, and size of as-prepared products. In the present study, pure SnO2 formed when the alkaline concentration was below 0.2 M, and ZnO was the dominant phase in the final products when alkaline concentration was larger than 0.4 M. The feasible NaOH concentration in the mixture for the expecting product is in the range of 0.2-0.4 M when [Sn] and [Zn] concentrations were fixed at 0.025 and 0.05 M, respectively. Besides, pure Zn2SnO4 particles cannot be obtained at hydrothermal temperatures below 150 °C. Figure 2 displays SEM images of the products synthesized at different hydrothermal reaction conditions. It is clear that the low NaOH concentration used in the experiment results in the reduction in the particle size with irregular shape, demonstrating that the alkaline concentration is the key parameter influencing the size and morphology of resulting Zn2SnO4 particles. The product obtained by the hydrothermal treatment at 200 °C for
24 h with a NaOH concentration of 0.4 M is uniform cubeshaped Zn2SnO4 with a particle size of 30-400 nm (Figure 2c). When a NaOH concentration of 0.2 M was used in the hydrothermal reaction, Zn2SnO4 nanosized particles are formed with irregular shapes (Figure 2a). The hydrothermal reaction temperature and duration time are also observed to have a considerable impact on the morphological features of the products. TEM and HRTEM images of the as-prepared Zn2SnO4 particles typically synthesized in 0.4 and 0.2 M NaOH solution at 200 °C are illustrated in Figure 3. It can be seen that Zn2SnO4 particles, obtained from the hydrothermal treatment in the 0.2 M NaOH solution, have irregular shapes and consist of a number of small crystallites with a diameter between 10 and 15 nm. When the NaOH concentration was increased to 0.4 M, small irregular crystallites grew into large regular cube-shaped crystallites with an edge size of 250-350 nm. It is highly possible that the irregular nanoparticles transform to large regular crystallites by the Ostwald ripening process, in which larger crystallites grow at the expense of smaller crystallites being dissolved.35 The calculated interference fringe spacing of Zn2SnO4 is about 0.51 (Figure 3c) and 0.26 nm (Figure 3d), which is almost consistent with the interplanar distance of (111) and (311) planes of the spinel fcc structure in the XRD results. The initial discharge-charge curves of as-prepared Zn2SnO4 electrodes, measured at a current density of 100 mA/g and room temperature in a potential range between 0.05 and 3.0 V (vs Li+/Li), are shown in Figure 4. There is a wide steady discharging plateau around 0.5 V (vs Li+/Li) for the lithium
Zn2SnO4 as Anode Materials for Li-Ion Battery
J. Phys. Chem. B, Vol. 110, No. 30, 2006 14757
Figure 5. Cyclic life of the electrodes of the as-prepared Zn2SnO4 particles synthesized in 0.4 M NaOH solution (a) and 0.2 M NaOH solution (b) at 200 °C for 24 h
Figure 4. Discharge-charge curves of the electrodes of the as-prepared Zn2SnO4 particles synthesized at 200 °C for 24 h. (a) Sample prepared in 0.2 M NaOH solution, and (b) sample prepared in 0.4M NaOH solution.
reaction with cube-shaped Zn2SnO4 particles and subsequent metallic Sn or Zn. The potential plateau shifts upward to near 1.0 V (vs Li+/Li) and displays more inclined shape during the second discharge, accompanied by large capacity losses. However, the discharge potential declines monotonically for irregular Zn2SnO4 particles with a small size due to more dispersed active sites for lithium reaction. The fine crystallinity of cube-shaped Zn2SnO4 particles may contribute to the extended potential plateau around 0.5 V. This discharge behavior is similar to that of pure Zn2SnO4, which had a flatter plateau and higher reversible capacity than the ball-milled mixture of ZnO and SnO2.16 The cycle performance of the as-prepared Zn2SnO4 electrodes at a current density of 100 mA/g and room temperature is indicated in Figure 5. It can be seen that a discharge capacity of as high as 1384 mA h/g is achieved in the first cycle and 988 mA h/g is retained in the following cycle for uniform cubeshaped Zn2SnO4 particles (Figure 5a). The initial discharge capacity of 1509 mA h/g in the first cycle and reversible discharge capacity of 667 mA h/g in the second cycle are achieved for irregular Zn2SnO4 particles. In general, small particles with a large specific area can easily react with lithium in the first cycle and then trigger a phase transition and structure change. The irreversible discharge capacities of cube-shaped and irregular Zn2SnO4 particles are 398 and 842 mA h/g after the first cycle, respectively. The large irreversible discharge capacity after the first cycle is probably due to the severe side reaction of the larger surface area with the electrolyte to form
Li2O and solid electrolyte interphase (SEI) film,8,36 especially for the irregular Zn2SnO4 particles with the small size. In addition, the large irreversible discharge capacity for irregular Zn2SnO4 particles is also attributed to the poor crystallinity, which can accelerate the phase transition and structure change in equations 1a and 1b as discussed in the next section. The reversible capacity of the cube-shaped Zn2SnO4 particle electrode decreases gradually to 580 mA h/g after 50 cycles. The tendency of the discharge capacity decay is almost identical for both as-prepared Zn2SnO4 electrodes. The reversible discharge capacity fade rates of the cube-shaped and irregular Zn2SnO4 particles are 0.8% and 1.0% per cycle, respectively, as calculated from Figure 5. To gain insight into the electrochemical mechanism of lithium insertion and extraction, the cube-shaped Zn2SnO4 particle electrode is investigated further in detail in the following section. Zn2SnO4 specimens were taken from the electrode prepared with the cube-shaped Zn2SnO4 after the 1st (Figure 6a and b), 8th (Figure 6c), and 30th (Figure 6d) cycles, and typical TEM images of these specimens are presented in Figure 6. The Zn2SnO4 particles retain the cube-shaped morphology after the first cycle, but the cleavage fracture morphology can be seen on the particle surface. In the HRTEM image (Figure 6b) the interference fringe (0.51 nm, corresponding to the interplanar distance of the (111) plane of the spinel Zn2SnO4 structure) through the dark areas, which are metallic nanocrystallites, can be clearly observed. Meanwhile, there are bright areas without fringes among the nanocrystals that are amorphous regions. These results confirm that the spinel Zn2SnO4 structure still exists to a great extent after the first cycle. After eight cycles we can still see cubic-shaped Zn2SnO4 particles with deteriorated crystallinity (Figure 6c) because the lithium insertion and extraction process is accompanied with a volume expansion of the Zn2SnO4 structure, which inevitably causes crystallinity deterioration. The deterioration is enhanced by more chargedischarge cycles as can be seen in Figure 6c. Some metal or alloy precipitates of about 20 nm around the amorphous Li2O matrix can be found from the inserted HRTEM image according to the mass contrast. To further confirm the existence of the spinel Zn2SnO4 structure, the XRD pattern of the Zn2SnO4 specimen taken from the electrode at the charged state and discharged state after the first cycle and prepared with cubeshaped Zn2SnO4 is presented in Figure 7. Three phases, spinel Zn2SnO4, metallic Sn, and Li2CO3, are detected at the charged state (Figure 7a). Li2CO3 was also found in FTIR spectra of Li
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Figure 6. TEM images of the Zn2SnO4 specimen taken from the electrode of the cube-shaped Zn2SnO4 after the 1st (a, b), 8th (c), and 30th (d) cycles.
we propose that the lithium insertion process on as-prepared Zn2SnO4 particles is as follows
Figure 7. XRD patterns of the as-prepared cube-shaped Zn2SnO4 electrode after the first cycles at charged state (a) and discharged state (b).
electrodes38 and XRD patterns of LiCoO2 electrodes37 and yielded by the interaction between electrolyte and electrode. A trace amount of metallic Zn crystallites can also be found to coexist, as demonstrated previously in the ZnO electrode after the first charge.17 In the case of the discharged state (Figure 7b), the spinel Zn2SnO4 phase almost disappeared. The metallic Sn and Zn phases are dominant. Typical cyclic voltammograms (CVs) of the electrode prepared with cube-shaped Zn2SnO4 are depicted in Figure 8. On the basis of the lithium storage mechanism of ZnO18 and SnO2,38
4Li+ + Zn2SnO4 + 4e f Sn + 2Li2O + 2ZnO
(1a)
8Li+ + Zn2SnO4 + 8e f 2Zn + Sn + 4 Li2O
(1b)
xLi+ + Sn + xe- T LixSn, x e 4.4
(2)
yLi+ + Zn + ye- T LiyZn, y e 1
(3)
Only one main cathodic peak is located near 0.4 V (vs Li+/ Li) in the first cycle, corresponding to the multistep electrochemical lithium reaction process and the wide steady discharging plateau in the first discharge curve. The irreversible capacity loss, caused by the formation of amorphous Li2O, is responsible for the substantial change in the peak potentials and the peak current intensity in the subsequent cycles. In the second and subsequent cycles the small cathodic peak is located around 0.29 V (vs Li+/Li) and the main cathodic peak shifts to 0.85 V (vs Li+/Li) after the first cycle, illustrating the different lithium reaction process. The redox couple indexed as b/b′ corresponds to the reversible reaction to some extent in eqs 1a and 1b, and this was previously confirmed in SnO2 electrodes.39,40 Voltages higher than 1.0 V may result in the formation and deformation of Li2O.38 There is another redox couple c/c′ at 0.29/0.50 V which is related to the alloying/dealloying process of LixSn and LiyZn isolated inside Zn2SnO4 particles in eqs 2 and 3. The theoretical reversible capacity is derived from the maximum uptake of 6.4 Li/Zn2SnO4 based on eqs 2 and 3, about
Zn2SnO4 as Anode Materials for Li-Ion Battery
J. Phys. Chem. B, Vol. 110, No. 30, 2006 14759 product. The product prepared with 0.4 M NaOH solution at 200 °C for 24 h is uniform cube-shaped Zn2SnO4 crystalline and exhibits the best performance as electrode materials. The discharge capacity of the solid is 1384 mA h/g in the first lithium insertion process and 988 mA h/g in the following cycle. After 50 cycles a relatively large discharge capacity of 580 mA h/g is still retained. TEM observation, XRD analysis, and CV curves in our experiment confirm that spinel Zn2SnO4 structure exists to a great extent after the first cycle, contributing to the extremely high reversible capacity. The discharge capacity decreases gradually owing to the isolated metallic nanocrystallite (Sn or Zn) aggregation during cycling. Acknowledgment. This work was supported by the 973 Program (2002CB211800), the NCET (040219), and NSFC (90206043), China. Financial support from the Australian Research Council (ARC) is also gratefully acknowledged.
Figure 8. Cyclic voltammograms of the as-prepared cube-shaped Zn2SnO4 electrode.
547 mA/g. The theoretical value from eq 1 to eq 3 is calculated to be about 1231 mA h/g. The decomposition of the electrolyte and subsequent formation of an organic layer on the surface of the particles as well as a trace amount water in the assembled cells accounts mostly for the excess capacity of the first cycle. The factors that result in the high reversible capacity beyond the theoretical value are not completely clear thus far for metal oxides or ternary oxides as anode materials. A high reversible capacity of more than 1190 mA h/g in CoSnO3 materials between 0 and 3.0 V vs Li+/Li was reported previously.22 In addition, SnO2 film,39,40 SnO2 nanorods,10 and SnO2 nanotubes41 were also reported to have high discharge capacities of over the theoretical value of SnO2 electrodes cycled in the high potential range because of the reversible reaction of Li4.4Sn alloys. Recently, Sandu et al. demonstrated the presence of Sn(II) and/or Sn(IV) along with lithium-poorer alloy (s) and/or Sn (0) from in-situ Mo¨ssbauer investigation in the SnO2 electrode upon reoxidation.42 Mohamedi et al. also observed a XPS signal for tin oxides even after charging the SnO2 film electrode up to 2.5 V,40 suggesting the lithium reaction with SnO2 was reversible to a certain extent. Therefore, the high potential for charging may be considered as a driving force for the reoxidation of oxides or ternary oxides as anode materials for Li-ion batteries. The TEM observation and XRD analysis in our experiment confirm that the spinel Zn2SnO4 structure exists to a great extent after the first cycle. It is also demonstrated that the redox couple in the higher potential region of CV curves corresponds to the reversible reaction of ternary oxides. This evidence may indicate that eqs 1a and 1b are reversible to some extent for the cube-shaped Zn2SnO4 particle with high crystallinity, contributing to the extremely high reversible capacity of 988 mA h/g (lower than the theoretical value of 1231 mA h/g). This result is helpful to explain the high discharge capacity of Sn-based oxides over the theoretical value in the higher potential region published recently. The discharge capacity decreases gradually owing to the isolated metallic nanocrystallite (Sn or Zn) aggregation in the case of cube-shaped Zn2SnO4 materials, as shown in the TEM images. IV. Conclusion In summary, spinel Zn2SnO4 particles are synthesized successfully via the hydrothermal reaction at different temperatures and NaOH concentrations. The alkaline concentration is the major factor determining particle size, morphology, and crystallinity, which influence the electrochemical performance of the
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