Letter pubs.acs.org/NanoLett
Quantum Confinement and Its Related Effects on the Critical Size of GeO2 Nanoparticles Anodes for Lithium Batteries Yoonkook Son, Mihee Park, Yeonguk Son, Jung-Soo Lee, Ji-Hyun Jang, Youngsik Kim,* and Jaephil Cho* Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 689-798, South Korea S Supporting Information *
ABSTRACT: This work has been performed to determine the critical size of the GeO2 nanoparticle for lithium battery anode applications and identify its quantum confinement and its related effects on the electrochemical performance. GeO2 nanoparticles with different sizes of ∼2, ∼6, ∼10, and ∼35 nm were prepared by adjusting the reaction rate, controlling the reaction temperature and reactant concentration, and using different solvents. Among the different sizes of the GeO2 nanoparticles, the ∼6 nm sized GeO2 showed the best electrochemical performance. Unexpectedly smaller particles of the ∼2 nm sized GeO2 showed the inferior electrochemical performances compared to those of the ∼6 nm sized one. This was due to the low electrical conductivity of the ∼2 nm sized GeO2 caused by its quantum confinement effect, which is also related to the increase in the charge transfer resistance. Those characteristics of the smaller nanoparticles led to poor electrochemical performances, and their relationships were discussed. KEYWORDS: quantum confinement, GeO2, critical size, electrochemical performance
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Accordingly, in spite of the importance of the size-dependent effect on semiconductor oxide anode materials, their size effect has been rarely reported due to difficulty in obtaining various sizes of nanoparticles below 10 nm. For instance, Cho et al. reported SnO2 nanoparticles using different sizes of ∼3, ∼4, and ∼8 nm obtained from a hydrothermal method.27 They reported that the smallest particle size of ∼3 nm exhibited a superior cycling stability as compared to the others. However, to the best of our knowledge, all of the previous studies have rarely mentioned the importance effect of quantum confinement on the electrochemical performance and more importantly have not shown a real nanosize effect stemmed from quantum confinement. It is believed that the quantum confinement effect on the electrochemical performance is critical so that it should be considered in few nanometer-sized active materials in terms of electron conductivity and lithium diffusivity. Herein, we prepared GeO2 nanoparticles with different sizes of ∼2, ∼6, ∼10, and ∼35 nm and revealed their effect of the quantum confinement and its related effects on the electrochemical performances to ensure a long-term electrochemical cycling. Size-controlled GeO2 nanoparticles with ∼2, ∼6, ∼10, and ∼35 nm were synthesized via different kinds of solvents, different concentrations of precursors, and different reaction
here has been a strong request to develop a large-capacity anode that replaces a graphite anode (372 mAh/g) at a voltage below 1.0 V vs lithium. Well-known several semiconductor elements, such as Si, Ge, and Sn, that form alloys with Li have been intensively studied as potential large-capacity anodes.1−6 Among those elements, Ge has a large specific capacity (1600 mAh/g) and a high electrical conductivity as well as fast lithium ion diffusivity.7 The oxide form of semiconductor elements such as SiOx, GeOx, and SnOx have been also intensively studied because they produced better cycle life performance by the formation of amorphous Li2O matrix that alleviates the catastrophic volume changes of the elements.1,8−16 However, the negative result of the above approach is to yield a low Coulombic efficiency by the formation of the Li2O phase. To understand the mechanism of alloying with lithium and minimize its effects on the electrochemical performance, the semiconductor oxides have been intensively studied.13,17−20 Among them, germanium oxide has been studied frequently due to its relatively good electrochemical performance in spite of the Li2O matrix formation.21−26 The theoretical reversible lithium storage of GeO2 is 1126 mAh/g, corresponding to Li4.4Ge. However, it shows a fast capacity fade during cycling due to the large volume change between Ge and Li4.4Ge that causes particle cracking and pulverization. Thus, to minimize the effects of the stress and pulverization on the electrochemical performances, the syntheses of nanosized materials such as the GeO2 nano films and the GeO2 nanocrystals have been attempted.21−27 © 2014 American Chemical Society
Received: December 3, 2013 Revised: January 2, 2014 Published: January 6, 2014 1005
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electron microscopy (TEM) data (Figure 1) and band gap data (Figure 2a).29 To the best of our knowledge, such a small size of the GeO2 particle has not been reported yet. Fast Fourier transform (FFT) image of the ∼2 nm sized sample shows the clear presence of crystalline phase (SI, Figure S1), and therefore the broadening of the XRD peaks is associated with the nanosize effect. As shown in the TEM images of Figure 1b−e, the average particle size of the each sample agrees with its XRD result. Figure 2b shows the voltage profiles of ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticles at a 0.05 C rate between 1.5 and 0.005 V during the first cycle which is obtained from a lithium half cell (2016R) under 24 °C (1C was set at 1130 mA/ g). The calculated theoretical reversible capacity of GeO2 is ∼1126 mAh/g, according to the following equations: GeO2 + 4Li+ → Ge + 2Li2O (irreversible) and Ge + 4.4Li+ ↔ Li4.4 Ge (reversible). The first discharge (lithiation) capacities of the ∼2, ∼6, ∼10, and ∼35 nm sized samples are 2904, 3078, 2605, and 2533 mAh/g, respectively. Their first charge (delithiation) capacities are 746, 928, 971, and 1033 mAh/g, corresponding to Coulombic efficiencies of 25.7%, 30.1%, 37.2%, and 40.7%, respectively. It is a well-known fact that a smaller nanoparticle has a larger irreversible capacity on the first cycle, but it lends a beneficial effect for relieving a high internal stress during the reaction with lithium. It should be noted that a smaller nanoparticle is not necessarily a good candidate for an anode material of lithium batteries, especially in metal oxides with a low electronic conductivity. Moreover, the quantum confinement effect should be considered to understand the size effect on the electrochemical performances. At first, to further investigate the quantum confinement effect, ultraviolet (UV) spectroscopy and electrochemical impedance spectroscopy (EIS) of the samples is carried out (Figure 2a and c−d). UV spectroscopies of the samples show that a smaller GeO2 nanoparticles have a larger band gap. The effective band gap, Eeff g , can be generally defined by
temperatures. Methanol, ethanol, and isopropanol were used as solvents. The precursor concentration was varied with a solvent content, and synthesis reaction temperatures were either 4 °C or room temperature. We observed that smaller GeO2 nanoparticles could be obtained in more nonpolar alcohol solvent, lower concentration, and lower temperature, because these experimental conditions lead to a slow reaction rate for the nucleation of the particles. The detailed experimental procedure is described in the Supporting Information (SI). Figure 1a shows the powder X-ray diffraction (XRD) patterns
Figure 1. (a) XRD patterns of the ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticles. HRTEM images for the as-synthesized GeO2 nanoparticles with the size of (b) ∼2 nm, (c) ∼6 nm, (d) ∼10 nm, and (e) ∼35 nm.
of the GeO2 nanoparticles with the different sizes. The sizes of the GeO2 nanoparticle estimated by the Scherrer equation28 were ∼2, ∼6, ∼10, and ∼35 nm, respectively. All XRD diffraction peaks of the ∼6, ∼10, and ∼35 nm sized GeO2 can be indexed with a hexagonal phase (JCPDS No. 85-0473) without showing any impurity peak. For the ∼2 nm sized sample, its XRD pattern shows the formation of amorphouslike phase. Hence, its size was determined from transmission
Figure 2. (a) UV spectra of the ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticles. Inset of part a shows the band gap calculated from the UV spectra results. (b) Voltage profiles of the ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticles during the first cycle at 0.05 C rate. Electrochemical impedance spectra (EIS) of the ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticle (c) before and (d) after the first cycle. 1006
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Figure 3. (a) HRTEM images for the as-synthesized GeO2-RGO composite, (b) EIS of the ∼2 nm sized GeO2 nanoparticle compared with that of the ∼2 nm sized GeO2-RGO composite before cycle, (c) voltage profiles of the ∼2 nm sized GeO2 nanoparticles compared with that of the ∼2 nm sized GeO2-RGO composite during the first cycle at 0.05 C rate, and (d) rate capability of the ∼2 nm sized GeO2 nanoparticle compared with that of the ∼2 nm sized GeO2-GO composite from 0.2 to 10 C rate in lithium half cells at 24 °C.
Egeff = Eg + Econfinement = Eg +
ℏ2π 2 ⎛ 1 1 ⎞ ⎜ ⎟ + 2 mh* ⎠ 2a ⎝ me*
interface. Larger GeO2 nanoparticles exhibit a smaller value of Rct, which should contribute to the faster transfer rate of the Li+ ion. Also note that this EIS data is consistent with the electrical conductivity data obtained from the band gap measurement. Accordingly, we believe that the first charge capacity is strongly related to active material conductivity, and especially a ∼2 nm sized GeO2 nanoparticle has a very low first charge capacity due to high Rct caused by its quantum confinement effect. After the first charge of the samples in lithium half cells, the EIS of each sample is measured again as shown in Figure 2d. The shape of the impedance spectra changes and additionally their sizes are sharply decreased compared to those observed in the initial cells. Since the phase change from the GeO2 to the Ge and Li2O occurs after the first charge, the observed impedance spectra may correspond to the Rct related to the Ge particles and the resistance (RSEI) of the solid−electrolyte interface between the electrode and the electrolyte. The electrical conductivity of Ge (∼1 S/m) is reported to be much higher than that of GeO2 (60%.33 In ref 33, their normalized conductivities were reported to be saturated near 60% volume fraction of the particles. Therefore, in the nanosized range, other effects, such as quantum confinement, could be more dominant. If interparticle resistance has influenced GeO2 nanoparticles seriously at any possibility, a resistance trend of our samples should be similar to ref 33. However, our result (Figure 2) shows that quantum confinement effect is more dominant. Additionally, although the ketjen black content of our sample was 15 wt %, its volume is enough to be abundant to cover with the GeO2 nanoparticles due to its bulky nature. Consequently, our case is different from the interparticle resistance model.33,34 Figure 2c exhibits electrochemical impedance spectroscopy (EIS) of the ∼2, ∼6, ∼10, and ∼35 nm sized GeO 2 nanoparticles before cycling. The diameter of semicircle in the higher frequency region is attributed to the Rct that is related to the reaction between active material and electrolyte 1007
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fringes are clearly observed after the first cycle. Unexpectedly, recrystallized Ge and GeO2 nanoparticles have been found as confirmed by FFT images (insets of Figure 4). Note that ex situ XRD data shows that all GeO2 nanoparticles have turned to amorphous phases even after the first cycle (SI, Figure S5). Pristine particle sizes of ∼2, ∼6, and ∼35 nm sized samples decreased to ∼2 nm (Figure 4a), ∼5 nm (Figure 4b), and ∼10 nm sized (Figure 4c) Ge and GeO2 particles, respectively, after the first cycle. It is obvious that GeO2 nanoparticles could be transformed into not only amorphous phase but also crystalline phase. Cho et al. reported reoxidation of germanium due to the catalytic role of Ge.40 As we suggested in Figure 2d, during cycling, smaller GeO2 nanoparticles could produce smaller Ge nanoparticles that are small enough to be influenced by the quantum confinement effect. FFT images of the nanoparticles confirmed the existence of the GeO2 and Ge nanocrystals. Similarly, it is expected that smaller GeO2 nanoparticles should yield smaller Li2O nanoparticles, and therefore the smaller Li2O nanoparticle matrix should have a lower conductivity. The HRTEM images (magnification of 150 k) of the ∼2, ∼6, and ∼35 nm sized GeO2 samples are taken again after 30 cycles as shown in Figure 4d−f, and their morphologies are quite different from those observed in HRTEM taken after the first cycle. Ge nanoparticles aggregate into larger ones, and the most severe particle aggregation is observed in the ∼35 nm sized GeO2 sample (Figure 4f). Particularly, it is worthwhile to note that the ∼2 nm sized GeO2 sample exhibits larger aggregated particles than 6 nm sized GeO2 one. It is well-known that small particles aggregated each other to reduce their high surface energy, and therefore much smaller Ge nanoparticles with ∼2 nm have an extreme high surface energy compared to those of larger nanoparticles (SI), thus aggregated into larger particles. Accordingly, the critical size of minimizing aggregation should possibly exist, as shown in Figure 4. Based on our results, the critical size would be ∼6 nm for the GeO2 anode material. In addition, Li+ diffusivity should be lower in more aggregated particles, and therefore, GeO2 nanoparticle size distribution is strongly related to battery cycling performance and rate capability. Figure 5 shows the cycling performances and rate capability of the ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticles and the voltage profiles of the 6 nm sized GeO2 sample between 1.5 and 0.005 V in lithium half cells (2016R) under 24 °C. The cycling performance of each sample at 0.2 C rate is shown in Figure 5a. The maximum charge capacities are 726 mAh/g for the ∼2 nm, 841 mAh/g for the ∼6 nm, ∼882 mAh/g for the ∼10 nm, and 919 mAh/g for the ∼35 nm sized GeO2, respectively. They decrease to 575, 816, 787, and 543 mAh/g, respectively, after 100 cycles. Thus the capacity retention is 79%, 97%, 89%, and 59%, respectively (the capacity retention is estimated from the maximum capacity). Under 1 C rate condition (Figure 5b), the maximum charge capacities of ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticles are 615, 701, 753, and 599 mAh/g, respectively, which gives the capacity retentions of 50%, 82%, 26%, and 6.2%, respectively, after 500 cycles. At 0.2 C rate, a 100 cycles test is enough to provide the size-dependent effect of the GeO2 nanoparticles, but at 1 C rate at least 200 cycles are required to observe distinguishable sizedependent effect due to the difference of the lithiation depth during the discharge. As shown in the insets of Figure 5a−b, when discharging, more unreacted parts of the Ge still remain in core area of the Ge particle at high C rate. The unreacted part of the Ge takes a role as buffer zone, not to be pulverized
graphene oxide.35 In spite of many reported metal oxide-RGO composites, however, this is the first report for GeO2-RGO composite anode. It has been often reported that metal oxideRGO composites have superior electrical properties than those of the bare metal oxides.13,35−39 Since the high conductivity of RGO lends large contact areas for anchoring GeO2 nanoparticles, the surface conductivity of the nanoparticles should be increased. As clearly observed in the comparative EIS of the ∼2 nm-sized GeO2 nanoparticle to the composite before cycle, it is obvious that the Rct is influenced by metal oxide’s conductivity (Figure 3b). The composite exhibited higher discharge and charge capacities of 3195 mAh/g and 864 mAh/g (Figure 3c) than those of the bare ∼2 nm GeO2 nanoparticle at a 0.05 C rate. The capacity contribution of 4 wt % of RGO in the composite is negligible (SI, Figure S3a). Accordingly, this result strongly supports the effect of quantum confinement on the electrochemical performance showing that a lower capacity and higher Rct of the nanoparticle is related to its intrinsic lower conductivity. As shown in Figure 3d, improved rate capability of the composite is due to its higher conductivity of RGO. The charge capacity for ∼2 nm sized GeO2-RGO composite is 360 mAh/g at 5 C rate, while the nanoparticle alone shows only 81 mAh/g. The same behavior is also observed in the ∼6 nm sized GeO2-RGO composite (SI, Figure S4). Figure 4a−c shows the HRTEM images of the ∼2, ∼6, and ∼35 nm sized samples after the first cycle. In contrast to the pristine sample that did not show the clear lattice fringes, lattice
Figure 4. HRTEM images of (a) ∼2 nm, (b) ∼6 nm, and (c) ∼35 nm sized GeO2 nanoparticles after the first cycle. Insets show FFT diffraction patterns of entire area and selected area (red square). HRTEM images of (d) ∼2 nm, (e) ∼6 nm, and (f) ∼35 nm sized GeO2 nanoparticles after the 30th cycle. 1008
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believe that both the low Li+ diffusivity and the poor electrical conductivity of the ∼2 nm sized sample could be the major causes for the poor rate capability, compared to ∼6 nm sized one. In general, amorphous nanoparticles could lead to a better cycle performance because its amorphous nature may be more beneficial for reducing stress from large volume changes during cycling than the crystalline one. In our case, nevertheless, ∼2 nm sized GeO2 nanoparticle shows worse performance than the ∼6 nm sized sample. In other words, if the ∼2 nm sized GeO2 nanoparticle has the amorphous-like phase by any possibility, it should been shown the improved the cycling performance. However, our data clearly shows that the ∼2 nm sized GeO2 nanoparticle has relatively poor electrochemical performance due to dominant effects from the quantum confinement effect and lithium diffusivity of aggregated particle. In conclusion, we synthesized different sizes of GeO2 nanoparticles via solvent, reaction concentration, and reaction temperature control to identify its size effect on the electrochemical performance. Contrary to popular concept, we found that smaller nanoparticles were not always better for the electrochemical performances. A few nanometer sizes of the GeO2 nanoparticle have a large band gap and poor conductivity due to the quantum confinement effect, which is also consistent with its large charge transfer resistance results. Additionally, a smaller GeO2 nanoparticle has higher aggregation characteristics during cycling due to its higher surface energy, which leads to the difficulty of Li mobility into the core side of the active material during the discharge. As a result, the nanoparticle aggregation has a negative influence on the electrochemical performances including the capacity, charge/ discharge cycling performance, and the rate capability. We found the critical size of the GeO2 particles that produced the best electrochemical performance, of ∼6 nm. “NANO” is not always good for the Li-ion battery performance due to its quantum confinement effect.
Figure 5. Cycle performances of the ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticles at rates of (a) 0.2 C and (b) 1 C. Insets are schematics of the depth of lithiation during cycling at rates of 0.2 and 1 C rates, (c) voltage profiles of the 6 nm sized GeO2 nanoparticle between 0.005 and 1.5 V at a 0.2 C rate, and (d) rate capability of ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticles from 0.2 to 10 C rate. All of the tests were performed in lithium half cells (2016R) under 24 °C.
much. This is the reason why the size-dependent effect can be more clearly observed in later cycles at a higher C rate. Overall, the ∼6 nm sized GeO2 nanoparticle shows the best cycling performance. The capacity retention of the ∼6 nm sized GeO2 nanoparticle is 89.7% at a 2 C rate after 200 cycles, which is a higher value than that measured at a 1 C rate (SI, Figure S6). This is consistent with the above results. Here, we need to consider the Li+ ion diffusivity into the Ge nanoparticles. As discussed above, the ∼6 nm sized GeO2 nanoparticles exhibited the smallest degree of particles aggregation after the 30th cycle. The extent of the particle aggregation should influence the Li+ diffusivity. To prove this hypothesis, we performed a galvanostatic intermittent titration technique (GITT) test for the ∼6 and ∼35 nm sized GeO2 nanoparticles after 30 cycles (SI, Figure S7). Calculated Li+ ion diffusion coefficients of the ∼6 and ∼35 nm sized GeO2 nanoparticles are 6.57 × 10−9 cm2/s and 4.82 × 10−9 cm2/s, respectively, indicating that the Li+ ion is more of a struggle to diffuse into the more aggregated Ge particle. Accordingly, the poor specific capacity during cycling can be caused by the difficulty of the Li ion diffusion. In addition, the larger aggregated Ge particle should also have a bigger internal strain during cycling, accompanying much pulverization with capacity fading. Figure 5c shows the voltage profiles of the ∼6 nm sized GeO2 nanoparticle at a 0.2 C rate. The characteristic thing is that the plateau observed at around 1.2 V gets vanished during cycling and completely disappears after 10 cycles (also see Figure 2a and SI, Figure S6). This plateau indicates reoxidation of Ge into GeO2, showing the reversibility of the conversion reaction.40 Rate capabilities of the ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticles were also tested up to 10 C rate (Figure 5d). The ∼35 nm sized GeO2 nanoparticle yield the worst rate capability, showing rapidly decreased capacities from 960 mAh/g to 11 mAh/g from 0.2 to 10 C rate. The rate capability of the ∼2 nm sized GeO2 nanoparticle is also poor, showing that its charge capacity drops from 756 mAh/g to 22 mAh/g when increasing C rate from 0.2 to 10 C rate. We
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ASSOCIATED CONTENT
S Supporting Information *
Experimental method, HRTEM images ex situ XRD analysis, and electrochemical cell performance data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Converging Research Center Program (2013K000210) and by the C-ITRC (Convergence Information Technology Research Center) support program (NIPA-2013-H0301-13-1009) supervised by the NIPA (National IT Industry Promotion Agency) through the Ministry of Science, ICT and Future Planning, Korea.
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Quantum Confinement and Its Related Effects on the Critical Size of GeO2 Nanoparticles Anodes for Lithium Batteries Yoonkook Son, Mihee Park, Yeonguk Son, Jung-Soo Lee, Ji-Hyun Jang, Youngsik Kim,* and Jaephil Cho* Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 689-798, South Korea S Supporting Information *
ABSTRACT: This work has been performed to determine the critical size of the GeO2 nanoparticle for lithium battery anode applications and identify its quantum confinement and its related effects on the electrochemical performance. GeO2 nanoparticles with different sizes of ∼2, ∼6, ∼10, and ∼35 nm were prepared by adjusting the reaction rate, controlling the reaction temperature and reactant concentration, and using different solvents. Among the different sizes of the GeO2 nanoparticles, the ∼6 nm sized GeO2 showed the best electrochemical performance. Unexpectedly smaller particles of the ∼2 nm sized GeO2 showed the inferior electrochemical performances compared to those of the ∼6 nm sized one. This was due to the low electrical conductivity of the ∼2 nm sized GeO2 caused by its quantum confinement effect, which is also related to the increase in the charge transfer resistance. Those characteristics of the smaller nanoparticles led to poor electrochemical performances, and their relationships were discussed. KEYWORDS: quantum confinement, GeO2, critical size, electrochemical performance
T
Accordingly, in spite of the importance of the size-dependent effect on semiconductor oxide anode materials, their size effect has been rarely reported due to difficulty in obtaining various sizes of nanoparticles below 10 nm. For instance, Cho et al. reported SnO2 nanoparticles using different sizes of ∼3, ∼4, and ∼8 nm obtained from a hydrothermal method.27 They reported that the smallest particle size of ∼3 nm exhibited a superior cycling stability as compared to the others. However, to the best of our knowledge, all of the previous studies have rarely mentioned the importance effect of quantum confinement on the electrochemical performance and more importantly have not shown a real nanosize effect stemmed from quantum confinement. It is believed that the quantum confinement effect on the electrochemical performance is critical so that it should be considered in few nanometer-sized active materials in terms of electron conductivity and lithium diffusivity. Herein, we prepared GeO2 nanoparticles with different sizes of ∼2, ∼6, ∼10, and ∼35 nm and revealed their effect of the quantum confinement and its related effects on the electrochemical performances to ensure a long-term electrochemical cycling. Size-controlled GeO2 nanoparticles with ∼2, ∼6, ∼10, and ∼35 nm were synthesized via different kinds of solvents, different concentrations of precursors, and different reaction
here has been a strong request to develop a large-capacity anode that replaces a graphite anode (372 mAh/g) at a voltage below 1.0 V vs lithium. Well-known several semiconductor elements, such as Si, Ge, and Sn, that form alloys with Li have been intensively studied as potential large-capacity anodes.1−6 Among those elements, Ge has a large specific capacity (1600 mAh/g) and a high electrical conductivity as well as fast lithium ion diffusivity.7 The oxide form of semiconductor elements such as SiOx, GeOx, and SnOx have been also intensively studied because they produced better cycle life performance by the formation of amorphous Li2O matrix that alleviates the catastrophic volume changes of the elements.1,8−16 However, the negative result of the above approach is to yield a low Coulombic efficiency by the formation of the Li2O phase. To understand the mechanism of alloying with lithium and minimize its effects on the electrochemical performance, the semiconductor oxides have been intensively studied.13,17−20 Among them, germanium oxide has been studied frequently due to its relatively good electrochemical performance in spite of the Li2O matrix formation.21−26 The theoretical reversible lithium storage of GeO2 is 1126 mAh/g, corresponding to Li4.4Ge. However, it shows a fast capacity fade during cycling due to the large volume change between Ge and Li4.4Ge that causes particle cracking and pulverization. Thus, to minimize the effects of the stress and pulverization on the electrochemical performances, the syntheses of nanosized materials such as the GeO2 nano films and the GeO2 nanocrystals have been attempted.21−27 © 2014 American Chemical Society
Received: December 3, 2013 Revised: January 2, 2014 Published: January 6, 2014 1005
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electron microscopy (TEM) data (Figure 1) and band gap data (Figure 2a).29 To the best of our knowledge, such a small size of the GeO2 particle has not been reported yet. Fast Fourier transform (FFT) image of the ∼2 nm sized sample shows the clear presence of crystalline phase (SI, Figure S1), and therefore the broadening of the XRD peaks is associated with the nanosize effect. As shown in the TEM images of Figure 1b−e, the average particle size of the each sample agrees with its XRD result. Figure 2b shows the voltage profiles of ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticles at a 0.05 C rate between 1.5 and 0.005 V during the first cycle which is obtained from a lithium half cell (2016R) under 24 °C (1C was set at 1130 mA/ g). The calculated theoretical reversible capacity of GeO2 is ∼1126 mAh/g, according to the following equations: GeO2 + 4Li+ → Ge + 2Li2O (irreversible) and Ge + 4.4Li+ ↔ Li4.4 Ge (reversible). The first discharge (lithiation) capacities of the ∼2, ∼6, ∼10, and ∼35 nm sized samples are 2904, 3078, 2605, and 2533 mAh/g, respectively. Their first charge (delithiation) capacities are 746, 928, 971, and 1033 mAh/g, corresponding to Coulombic efficiencies of 25.7%, 30.1%, 37.2%, and 40.7%, respectively. It is a well-known fact that a smaller nanoparticle has a larger irreversible capacity on the first cycle, but it lends a beneficial effect for relieving a high internal stress during the reaction with lithium. It should be noted that a smaller nanoparticle is not necessarily a good candidate for an anode material of lithium batteries, especially in metal oxides with a low electronic conductivity. Moreover, the quantum confinement effect should be considered to understand the size effect on the electrochemical performances. At first, to further investigate the quantum confinement effect, ultraviolet (UV) spectroscopy and electrochemical impedance spectroscopy (EIS) of the samples is carried out (Figure 2a and c−d). UV spectroscopies of the samples show that a smaller GeO2 nanoparticles have a larger band gap. The effective band gap, Eeff g , can be generally defined by
temperatures. Methanol, ethanol, and isopropanol were used as solvents. The precursor concentration was varied with a solvent content, and synthesis reaction temperatures were either 4 °C or room temperature. We observed that smaller GeO2 nanoparticles could be obtained in more nonpolar alcohol solvent, lower concentration, and lower temperature, because these experimental conditions lead to a slow reaction rate for the nucleation of the particles. The detailed experimental procedure is described in the Supporting Information (SI). Figure 1a shows the powder X-ray diffraction (XRD) patterns
Figure 1. (a) XRD patterns of the ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticles. HRTEM images for the as-synthesized GeO2 nanoparticles with the size of (b) ∼2 nm, (c) ∼6 nm, (d) ∼10 nm, and (e) ∼35 nm.
of the GeO2 nanoparticles with the different sizes. The sizes of the GeO2 nanoparticle estimated by the Scherrer equation28 were ∼2, ∼6, ∼10, and ∼35 nm, respectively. All XRD diffraction peaks of the ∼6, ∼10, and ∼35 nm sized GeO2 can be indexed with a hexagonal phase (JCPDS No. 85-0473) without showing any impurity peak. For the ∼2 nm sized sample, its XRD pattern shows the formation of amorphouslike phase. Hence, its size was determined from transmission
Figure 2. (a) UV spectra of the ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticles. Inset of part a shows the band gap calculated from the UV spectra results. (b) Voltage profiles of the ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticles during the first cycle at 0.05 C rate. Electrochemical impedance spectra (EIS) of the ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticle (c) before and (d) after the first cycle. 1006
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Figure 3. (a) HRTEM images for the as-synthesized GeO2-RGO composite, (b) EIS of the ∼2 nm sized GeO2 nanoparticle compared with that of the ∼2 nm sized GeO2-RGO composite before cycle, (c) voltage profiles of the ∼2 nm sized GeO2 nanoparticles compared with that of the ∼2 nm sized GeO2-RGO composite during the first cycle at 0.05 C rate, and (d) rate capability of the ∼2 nm sized GeO2 nanoparticle compared with that of the ∼2 nm sized GeO2-GO composite from 0.2 to 10 C rate in lithium half cells at 24 °C.
Egeff = Eg + Econfinement = Eg +
ℏ2π 2 ⎛ 1 1 ⎞ ⎜ ⎟ + 2 mh* ⎠ 2a ⎝ me*
interface. Larger GeO2 nanoparticles exhibit a smaller value of Rct, which should contribute to the faster transfer rate of the Li+ ion. Also note that this EIS data is consistent with the electrical conductivity data obtained from the band gap measurement. Accordingly, we believe that the first charge capacity is strongly related to active material conductivity, and especially a ∼2 nm sized GeO2 nanoparticle has a very low first charge capacity due to high Rct caused by its quantum confinement effect. After the first charge of the samples in lithium half cells, the EIS of each sample is measured again as shown in Figure 2d. The shape of the impedance spectra changes and additionally their sizes are sharply decreased compared to those observed in the initial cells. Since the phase change from the GeO2 to the Ge and Li2O occurs after the first charge, the observed impedance spectra may correspond to the Rct related to the Ge particles and the resistance (RSEI) of the solid−electrolyte interface between the electrode and the electrolyte. The electrical conductivity of Ge (∼1 S/m) is reported to be much higher than that of GeO2 (60%.33 In ref 33, their normalized conductivities were reported to be saturated near 60% volume fraction of the particles. Therefore, in the nanosized range, other effects, such as quantum confinement, could be more dominant. If interparticle resistance has influenced GeO2 nanoparticles seriously at any possibility, a resistance trend of our samples should be similar to ref 33. However, our result (Figure 2) shows that quantum confinement effect is more dominant. Additionally, although the ketjen black content of our sample was 15 wt %, its volume is enough to be abundant to cover with the GeO2 nanoparticles due to its bulky nature. Consequently, our case is different from the interparticle resistance model.33,34 Figure 2c exhibits electrochemical impedance spectroscopy (EIS) of the ∼2, ∼6, ∼10, and ∼35 nm sized GeO 2 nanoparticles before cycling. The diameter of semicircle in the higher frequency region is attributed to the Rct that is related to the reaction between active material and electrolyte 1007
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fringes are clearly observed after the first cycle. Unexpectedly, recrystallized Ge and GeO2 nanoparticles have been found as confirmed by FFT images (insets of Figure 4). Note that ex situ XRD data shows that all GeO2 nanoparticles have turned to amorphous phases even after the first cycle (SI, Figure S5). Pristine particle sizes of ∼2, ∼6, and ∼35 nm sized samples decreased to ∼2 nm (Figure 4a), ∼5 nm (Figure 4b), and ∼10 nm sized (Figure 4c) Ge and GeO2 particles, respectively, after the first cycle. It is obvious that GeO2 nanoparticles could be transformed into not only amorphous phase but also crystalline phase. Cho et al. reported reoxidation of germanium due to the catalytic role of Ge.40 As we suggested in Figure 2d, during cycling, smaller GeO2 nanoparticles could produce smaller Ge nanoparticles that are small enough to be influenced by the quantum confinement effect. FFT images of the nanoparticles confirmed the existence of the GeO2 and Ge nanocrystals. Similarly, it is expected that smaller GeO2 nanoparticles should yield smaller Li2O nanoparticles, and therefore the smaller Li2O nanoparticle matrix should have a lower conductivity. The HRTEM images (magnification of 150 k) of the ∼2, ∼6, and ∼35 nm sized GeO2 samples are taken again after 30 cycles as shown in Figure 4d−f, and their morphologies are quite different from those observed in HRTEM taken after the first cycle. Ge nanoparticles aggregate into larger ones, and the most severe particle aggregation is observed in the ∼35 nm sized GeO2 sample (Figure 4f). Particularly, it is worthwhile to note that the ∼2 nm sized GeO2 sample exhibits larger aggregated particles than 6 nm sized GeO2 one. It is well-known that small particles aggregated each other to reduce their high surface energy, and therefore much smaller Ge nanoparticles with ∼2 nm have an extreme high surface energy compared to those of larger nanoparticles (SI), thus aggregated into larger particles. Accordingly, the critical size of minimizing aggregation should possibly exist, as shown in Figure 4. Based on our results, the critical size would be ∼6 nm for the GeO2 anode material. In addition, Li+ diffusivity should be lower in more aggregated particles, and therefore, GeO2 nanoparticle size distribution is strongly related to battery cycling performance and rate capability. Figure 5 shows the cycling performances and rate capability of the ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticles and the voltage profiles of the 6 nm sized GeO2 sample between 1.5 and 0.005 V in lithium half cells (2016R) under 24 °C. The cycling performance of each sample at 0.2 C rate is shown in Figure 5a. The maximum charge capacities are 726 mAh/g for the ∼2 nm, 841 mAh/g for the ∼6 nm, ∼882 mAh/g for the ∼10 nm, and 919 mAh/g for the ∼35 nm sized GeO2, respectively. They decrease to 575, 816, 787, and 543 mAh/g, respectively, after 100 cycles. Thus the capacity retention is 79%, 97%, 89%, and 59%, respectively (the capacity retention is estimated from the maximum capacity). Under 1 C rate condition (Figure 5b), the maximum charge capacities of ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticles are 615, 701, 753, and 599 mAh/g, respectively, which gives the capacity retentions of 50%, 82%, 26%, and 6.2%, respectively, after 500 cycles. At 0.2 C rate, a 100 cycles test is enough to provide the size-dependent effect of the GeO2 nanoparticles, but at 1 C rate at least 200 cycles are required to observe distinguishable sizedependent effect due to the difference of the lithiation depth during the discharge. As shown in the insets of Figure 5a−b, when discharging, more unreacted parts of the Ge still remain in core area of the Ge particle at high C rate. The unreacted part of the Ge takes a role as buffer zone, not to be pulverized
graphene oxide.35 In spite of many reported metal oxide-RGO composites, however, this is the first report for GeO2-RGO composite anode. It has been often reported that metal oxideRGO composites have superior electrical properties than those of the bare metal oxides.13,35−39 Since the high conductivity of RGO lends large contact areas for anchoring GeO2 nanoparticles, the surface conductivity of the nanoparticles should be increased. As clearly observed in the comparative EIS of the ∼2 nm-sized GeO2 nanoparticle to the composite before cycle, it is obvious that the Rct is influenced by metal oxide’s conductivity (Figure 3b). The composite exhibited higher discharge and charge capacities of 3195 mAh/g and 864 mAh/g (Figure 3c) than those of the bare ∼2 nm GeO2 nanoparticle at a 0.05 C rate. The capacity contribution of 4 wt % of RGO in the composite is negligible (SI, Figure S3a). Accordingly, this result strongly supports the effect of quantum confinement on the electrochemical performance showing that a lower capacity and higher Rct of the nanoparticle is related to its intrinsic lower conductivity. As shown in Figure 3d, improved rate capability of the composite is due to its higher conductivity of RGO. The charge capacity for ∼2 nm sized GeO2-RGO composite is 360 mAh/g at 5 C rate, while the nanoparticle alone shows only 81 mAh/g. The same behavior is also observed in the ∼6 nm sized GeO2-RGO composite (SI, Figure S4). Figure 4a−c shows the HRTEM images of the ∼2, ∼6, and ∼35 nm sized samples after the first cycle. In contrast to the pristine sample that did not show the clear lattice fringes, lattice
Figure 4. HRTEM images of (a) ∼2 nm, (b) ∼6 nm, and (c) ∼35 nm sized GeO2 nanoparticles after the first cycle. Insets show FFT diffraction patterns of entire area and selected area (red square). HRTEM images of (d) ∼2 nm, (e) ∼6 nm, and (f) ∼35 nm sized GeO2 nanoparticles after the 30th cycle. 1008
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believe that both the low Li+ diffusivity and the poor electrical conductivity of the ∼2 nm sized sample could be the major causes for the poor rate capability, compared to ∼6 nm sized one. In general, amorphous nanoparticles could lead to a better cycle performance because its amorphous nature may be more beneficial for reducing stress from large volume changes during cycling than the crystalline one. In our case, nevertheless, ∼2 nm sized GeO2 nanoparticle shows worse performance than the ∼6 nm sized sample. In other words, if the ∼2 nm sized GeO2 nanoparticle has the amorphous-like phase by any possibility, it should been shown the improved the cycling performance. However, our data clearly shows that the ∼2 nm sized GeO2 nanoparticle has relatively poor electrochemical performance due to dominant effects from the quantum confinement effect and lithium diffusivity of aggregated particle. In conclusion, we synthesized different sizes of GeO2 nanoparticles via solvent, reaction concentration, and reaction temperature control to identify its size effect on the electrochemical performance. Contrary to popular concept, we found that smaller nanoparticles were not always better for the electrochemical performances. A few nanometer sizes of the GeO2 nanoparticle have a large band gap and poor conductivity due to the quantum confinement effect, which is also consistent with its large charge transfer resistance results. Additionally, a smaller GeO2 nanoparticle has higher aggregation characteristics during cycling due to its higher surface energy, which leads to the difficulty of Li mobility into the core side of the active material during the discharge. As a result, the nanoparticle aggregation has a negative influence on the electrochemical performances including the capacity, charge/ discharge cycling performance, and the rate capability. We found the critical size of the GeO2 particles that produced the best electrochemical performance, of ∼6 nm. “NANO” is not always good for the Li-ion battery performance due to its quantum confinement effect.
Figure 5. Cycle performances of the ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticles at rates of (a) 0.2 C and (b) 1 C. Insets are schematics of the depth of lithiation during cycling at rates of 0.2 and 1 C rates, (c) voltage profiles of the 6 nm sized GeO2 nanoparticle between 0.005 and 1.5 V at a 0.2 C rate, and (d) rate capability of ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticles from 0.2 to 10 C rate. All of the tests were performed in lithium half cells (2016R) under 24 °C.
much. This is the reason why the size-dependent effect can be more clearly observed in later cycles at a higher C rate. Overall, the ∼6 nm sized GeO2 nanoparticle shows the best cycling performance. The capacity retention of the ∼6 nm sized GeO2 nanoparticle is 89.7% at a 2 C rate after 200 cycles, which is a higher value than that measured at a 1 C rate (SI, Figure S6). This is consistent with the above results. Here, we need to consider the Li+ ion diffusivity into the Ge nanoparticles. As discussed above, the ∼6 nm sized GeO2 nanoparticles exhibited the smallest degree of particles aggregation after the 30th cycle. The extent of the particle aggregation should influence the Li+ diffusivity. To prove this hypothesis, we performed a galvanostatic intermittent titration technique (GITT) test for the ∼6 and ∼35 nm sized GeO2 nanoparticles after 30 cycles (SI, Figure S7). Calculated Li+ ion diffusion coefficients of the ∼6 and ∼35 nm sized GeO2 nanoparticles are 6.57 × 10−9 cm2/s and 4.82 × 10−9 cm2/s, respectively, indicating that the Li+ ion is more of a struggle to diffuse into the more aggregated Ge particle. Accordingly, the poor specific capacity during cycling can be caused by the difficulty of the Li ion diffusion. In addition, the larger aggregated Ge particle should also have a bigger internal strain during cycling, accompanying much pulverization with capacity fading. Figure 5c shows the voltage profiles of the ∼6 nm sized GeO2 nanoparticle at a 0.2 C rate. The characteristic thing is that the plateau observed at around 1.2 V gets vanished during cycling and completely disappears after 10 cycles (also see Figure 2a and SI, Figure S6). This plateau indicates reoxidation of Ge into GeO2, showing the reversibility of the conversion reaction.40 Rate capabilities of the ∼2, ∼6, ∼10, and ∼35 nm sized GeO2 nanoparticles were also tested up to 10 C rate (Figure 5d). The ∼35 nm sized GeO2 nanoparticle yield the worst rate capability, showing rapidly decreased capacities from 960 mAh/g to 11 mAh/g from 0.2 to 10 C rate. The rate capability of the ∼2 nm sized GeO2 nanoparticle is also poor, showing that its charge capacity drops from 756 mAh/g to 22 mAh/g when increasing C rate from 0.2 to 10 C rate. We
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ASSOCIATED CONTENT
S Supporting Information *
Experimental method, HRTEM images ex situ XRD analysis, and electrochemical cell performance data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Converging Research Center Program (2013K000210) and by the C-ITRC (Convergence Information Technology Research Center) support program (NIPA-2013-H0301-13-1009) supervised by the NIPA (National IT Industry Promotion Agency) through the Ministry of Science, ICT and Future Planning, Korea.
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