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L-Leucine Templated Biomimetic Assembly of Sn[O.sub.2] Nanoparticles and Their Lithium Storage Properties.

1. Introduction

Lithium-ion batteries (LIBs) are the dominant power supply for portable electronics and also show promising applications for electric vehicles and power storage systems, due to their high specific energy, good cycling performance, high coulombic, and energy efficiency [1, 2]. In accordance with the increasing energy requirement for these industries, LIBs develop toward higher capacity, higher energy, and higher power. Therefore, it is of great significance to explore electrode materials with high capacity for the next generation LIBs. Graphite is the principal commercialized anode for LIBs since invented in 1991, but its explored capacity has been reached the theoretical limit (372mAh/g) [3-10]. Metal oxides ([Co.sub.3][O.sub.4], [Fe.sub.3][O.sub.4], and Sn[O.sub.2]) have been studied as anodes to enhance energy density of LIBs, for they can deliver higher capacity than graphite [11-13]. In this regard, Sn[O.sub.2] has been considered as an outstanding alternative to graphite because of its high theoretical capacity of 782 mAh/g and moderate lithiation potential (~0.6V vs. [Li.sup.+]/Li) [8, 14-17]. It is commonly recognized that Sn[O.sub.2] experiences a two-step lithiation process, namely,

Sn[O.sub.2] + 4[Li.sup.+] + [4e.sup.-] [right arrow] Sn + 2[Li.sub.2]O (1)

Sn + 4.4[Li.sup.+] + [4.4e.sup.-] [right arrow] [Li.sub.4.4]Sn (2)

The reaction shown in (1) is irreversible, which would induce low initial coulombic efficiency (CE) of ~50%. Additionally, Sn[O.sub.2] electrode would suffer a large volume variation (~260%) resulted by the reaction of (2) as well as Sn, which would cause crack and collapse. And then, the capacity fading of Sn[O.sub.2] is dramatical during cycling. To improve cycling performance for Sn[O.sub.2], tremendous investigations have indicated that Sn[O.sub.2]/C composite anode with nanosized Sn[O.sub.2]-coated carbon is the most effective strategy [15, 18-20]. In Sn[O.sub.2]/C electrode, nano-sized Sn[O.sub.2] could sustain large volume change of Sn during lithiation and delithiation and carbon can buffer the volume change and also maintain the conductivity network for the whole electrode. Therefore, the cycling life of Sn[O.sub.2]/C would greatly be extended. However, most previous studies presented that Sn[O.sub.2] was usually coated by amorphous carbon via the pyrolysis of carbonaceous organic material at 400~500[degrees]C, such as glucose. Unfortunately, amorphous carbon with a higher specific surface area might bring large amounts of side reactions with electrolyte, leading to a lower initial CE of Sn[O.sub.2]. Besides, amorphous carbon usually shows higher average lithiation/delithiation voltage and larger voltage hysteresis, which would contribute little improvement in terms of energy density for LIBs in fact. Now, even since it has been reported that amorphous carbon can be catalytically graphitized at a lower temperature of about 600~700[degrees]C [21-23], Sn[O.sub.2] particles would grow greatly large at this temperature and experience a rapid capacity fading as lithium-ion anodes. Accordingly, it is of importance to obtain nanosized Sn[O.sub.2] and suppress its growth at 600~700[degrees]C for the application implementation of Sn[O.sub.2]/C.

In this work, nanosized Sn[O.sub.2] were synthesized by a sol-gel method assisted with biomimetic assembly. Biomimetic synthesis is a novel route to fabricate nanosized inorganic particles with organic templates. Investigations have identified that specific molecular interactions at inorganicorganic interfaces could result in the controlled nucleation and growth of inorganic crystals [24-26]. During the biomimetic assembly process, the organic template could promote self-assembly, recognize the reactant substrate, guide the nucleation, and limit the growth of inorganic particles by utilizing biological adsorption, hydrogen bond, van der Waals force, and so on. Considering that L-leucine could regulate the synthesis and the growth of organic particles and even enzymes, while no researches related to the regulation of inorganic materials by L-leucine could be found [27], we would like to control Sn[O.sub.2] nucleation and growth by biomimetic assembly using L-leucine as a biomimetic template here. The regulated mechanism of Sn[O.sub.2] synthesis and growth is studied for the first time in this work. Moreover, the electrochemical performance of as-prepared Sn[O.sub.2] was also measured as lithium-ion anodes.

2. Experimental

2.1. Preparation of Materials. Sn[O.sub.2] nanoparticles were synthesized by a sol-gel method assisted with biomimetic assembly using leucine as a biotemplate. Firstly, 9% of dilute aqua ammonia was dripped slowly into Sn[Cl.sub.4] (0.4 M/40 ml) including 0.001 mol L-leucine with continuous stirring in a water bath under 65[degrees]C. Secondly, the pH of the as-produced solution was adjusted to 3 and then kept stirring for 1 h to get white sol. Finally, the resulting sol was centrifuged, washed with ethanol and deionized water three times after keeping stand under 80[degrees]C for 24 h, respectively, and then dried at 80[degrees]C. The resultant was heated to 450[degrees]C, 550[degrees]C, and 650[degrees]C at a ramp rate of 10[degrees]C-[min.sup.-1] in the air for 4h to get Sn[O.sub.2], named as L-Sn[O.sub.2]-450[degrees]C, L-Sn[O.sub.2] 550[degrees]C, and L-Sn[O.sub.2]-650[degrees]C, respectively. For comparison, Sn[O.sub.2] nanoparticles were also prepared using the same procedure without the L-leucine biotemplate, named as Sn[O.sub.2]450[degrees]C, Sn[O.sub.2]-550[degrees]C, and Sn[O.sub.2]-650[degrees]C.

2.2. Material Characterization. The microstructure of Sn[O.sub.2] particles was carried out by a Shimadzu X-ray 6000 diffractometer (XRD) with [CuK.sub.[alpha]] radiation at 40 kV, 30 mA and a Quanta 250 FEG scanning electron microscope (SEM). Fourier transform infrared (FTIR) spectra were recorded using a Bruker Alpha spectrometer. Brunner-Emmet-Teller (BET) measurements were recorded using a QUADRASORB SI analyzer.

2.3. Electrochemical Measurements. The electrode slurry was prepared by mixing 90 wt% active material, 2 wt% super-p, and 8 wt% carboxymethyl cellulose (CMC). And then, the slurry was spread on Cu foil and dried at 120[degrees]C for 1 h. The electrochemical measurements of the electrodes were tested using CR2032 coin cells assembled with Li foil as the counter and reference electrodes in an argon-filled glove box. The electrodes were separated by two layers of a Celgard separator. The electrolyte was 1M LiPF6 dissolved in a mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate (EC: EMC: DMC = 1: 1: 1 in volume).

Cycle test was conducted with a system of LAND CT2001A from 0.005 to 2.5 V at a 0.1 C rate and trickle discharged at 0.005 V to a C/40 rate in the first three cycles. For subsequent cycles, cells were cycled from 0.005 to 2.5V at a 0.2 Crate and trickle discharged at 0.005V to a C/20 rate. All electrochemical tests were carried out at ambient temperature.

3. Results and Discussion

Figure 1 compares the XRD patterns of as-prepared Sn[O.sub.2] nanoparticles divided into three groups of (a) L-Sn[O.sub.2]450[degrees]C and (b) Sn[O.sub.2]-450[degrees]C, (c) L-Sn[O.sub.2]-550[degrees]C and (d) Sn[O.sub.2]-550[degrees]C, and (e) L-Sn[O.sub.2]-650[degrees]C and (f) Sn[O.sub.2]-650[degrees]C. All patterns show obvious diffraction peaks at 2d of 26.61[degrees], 33.89[degrees], 37.95[degrees], 51.78[degrees], and 65.94[degrees], corresponding to (110), (101), (200), (211), and (301) planes of rutile Sn[O.sub.2] (JCDF no. 41-1445), respectively. No peak corresponding to crystallographic impurities was observed, indicating the high purity of Sn[O.sub.2]. As the increase of calcination temperature from 450[degrees]C to 650[degrees]C, the diffraction peaks become sharp and the full width at half maximum (FWHM) narrows significantly for both L-Sn[O.sub.2] and Sn[O.sub.2], which is an indication that the grain size of Sn[O.sub.2] increases with an increase of calcination temperature. What is more, it could be observed that the grain size of Sn[O.sub.2] is finer in L-Sn[O.sub.2] than that of Sn[O.sub.2] prepared without leucine templates under the same temperature. Moreover, such distinctions in Sn[O.sub.2] grain size get more evident as the calcination temperature increases.

Compared Figure 1(a) and (b), the FWHM of the Sn[O.sub.2] diffraction peak is very close for L-Sn[O.sub.2]-450[degrees]C and Sn[O.sub.2]450[degrees]C, while it is obviously wide in L-Sn[O.sub.2]-550[degrees]C than Sn[O.sub.2]-550[degrees]C as shown in Figure 1(c) and (d). Especially in Figure 1(e) and (f), the diffraction peaks become much sharper in Sn[O.sub.2]-650[degrees]C than L-Sn[O.sub.2]-650[degrees]C. Based on peak profile analysis using a Voigt function, it is confirmed that the grain size of Sn[O.sub.2] is calculated as 11.2, 16.7, and 20.5 nm for L-Sn[O.sub.2]-450[degrees]C, L-Sn[O.sub.2]-550[degrees]C, and L-Sn[O.sub.2]650[degrees]C, respectively, while 11.0, 23.0, and 39.2 nm for Sn[O.sub.2]450[degrees]C, Sn[O.sub.2]-550[degrees]C, and Sn[O.sub.2]-650[degrees]C, respectively. It could be concluded that the Sn[O.sub.2] growth can be suppressed when synthesized by a sol-gel method assisted with biomimetic assembly using leucine as a biotemplate.

Figure 2 shows the FT-IR spectra of L-Sn[O.sub.2]-650[degrees]C and Sn[O.sub.2]-650[degrees]C. The main peak of L-Sn[O.sub.2]-650[degrees]C and Sn[O.sub.2]650[degrees]C is at the same position of 658 [cm.sup.-1] attributed to the Sn-O-Sn asymmetric stretching mode of surface bridging oxide. It confirms Sn[O.sub.2] formation in L-Sn[O.sub.2]-650[degrees]C and Sn[O.sub.2]-650[degrees]C. The weak peak is around 3425 [cm.sup.1] in Sn[O.sub.2]650[degrees]C corresponding to the stretching vibration of O-H bond, which may be due to the presence of water molecule on the surface of Sn[O.sub.2] nanoparticles and the stretching vibrations of Sn-OH groups [28, 29]. However, the peak of 530 [cm.sup.1] assigned to Sn-O vibration of Sn-OH group is weak in Sn[O.sub.2]-650[degrees]C. Therefore, it could be considered that the peak around 3425 [cm.sup.1] is mainly due to the vibration of absorbed water molecules. A stronger peak of 3425 [cm.sup.1], together with 1387 [cm.sup.1] of -C[H.sub.3] and 1638 [cm.sup.1] of -N[H.sub.2] in L-Sn[O.sub.2]-650[degrees]C, is an indication of L-leucine residuum bonding with Sn[O.sub.2], though Sn[(OH).sub.4] precursor was washed by ethanol and deionized water before calcination.

It is predicted that the growth of Sn[O.sub.2] particles in L-Sn[O.sub.2] gets suppressed for its direction and rate of interfacial migration between individual grains is regulated by L-leucine. This should maintain a block structure accompanied with wrinkled morphology and retain a smooth and dense surface structure for L-Sn[O.sub.2], while a large number of scattered particles and flakes increase to the surface of the control Sn[O.sub.2] group. When calcined at 650[degrees]C as shown in Figures 3(c) and 3(f), the block structure of L-Sn[O.sub.2]-650[degrees]C and Sn[O.sub.2]-650[degrees]C has damaged at a certain degree, with Sn[O.sub.2] nanoparticles reuniting on the various surfaces and edges along the block. It should be noted that L-Sn[O.sub.2]-650[degrees]C shows highly porous foam-like morphology.

To observe the porous structure of L-Sn[O.sub.2]-650[degrees]C, Figure 4 shows the pore size distribution curves for L-Sn[O.sub.2]-650[degrees]C and Sn[O.sub.2]-650[degrees]C and Table 1 shows the surface area, pore volume, and pore diameter of L-Sn[O.sub.2]-650[degrees]C and Sn[O.sub.2]-650[degrees]C. Compared with Sn[O.sub.2]-650[degrees]C, L-Sn[O.sub.2]-650[degrees]C possesses more mesopores of about 30 nm, the pore volume of 0.15 [cm.sup.3]/g is much bigger than 0.07 [cm.sup.3]/g, and the pore diameter of 24.9 nm is eight times than 3.3 nm. Therefore, the surface area of L-Sn[O.sub.2]-650[degrees]C is nearly twice larger than that of Sn[O.sub.2]-650[degrees]C.

Based on above microstructure characterization, the mechanism of Sn[O.sub.2] synthesis by biomimetic assembly could be schematic in Figures 5 and 6. First, -N[H.sub.2] of L-leucine accelerates the self-assembled process of [Sn.sup.4]+ and OH- to form Sn[(OH).sub.4]. And then, Sn[(OH).sub.4] could be recognized and integrated with L-leucine. In addition, -COOH of L-leucine, an electron-withdrawing group, easily form a hydrogen bond with -N[H.sub.2] of the next L-leucine molecule. Therefore, a "nanocage" with L-leucine molecules enclosed with Sn[(OH).sub.4] would be created by the intermolecular hydrogen bonding of L-leucine. Lastly, a high-order nanocage group would be formed. When heating Sn[(OH).sub.4], Sn[O.sub.2] nucleates in the "nanocage" and its growth would be restricted by the "nanocage." Therefore, fine and high-order layered Sn[O.sub.2] particles could be obtained by biomimetic assembly using the Lleucine template. Figure 6 illustrates the formation mechanism of the porous foam-like surface for L-Sn[O.sub.2]. L-Leucine has integrated with Sn[(OH).sub.4] to form a "nanocage" group, which would not be washed. When calcined at a high temperature, L-leucine would decompose into gaseous product such C[O.sub.2], N[H.sub.2], and CO as reported in [30]. Then, the escapement of these gases and the pyrolysis removal of the L-leucine template would leave holes around Sn[O.sub.2] and promote the formation of porous morphology.

Figure 7 shows voltage-capacity curves of the 1st, 2nd, and 5th cycles for as-prepared Sn[O.sub.2] electrodes cycled versus lithium metal from 0.005 to 2.5 V. All the voltage curves are characteristic of Sn[O.sub.2] in appearance, having an initial discharge sloping above 1.0 V, a flat plateau about 0.8 V, and a sloping plateau during the subsequent lithiation and delithiation process. A large irreversible capacity at above 1.0 V during the first discharge could also be observed, corresponding to the irreversible reaction between Sn[O.sub.2] and Li (1). The flat plateau about 0.8 V is consistent with the reaction between [Li.sup.+] and Sn. The LSn[O.sub.2]-450[degrees]C, L-Sn[O.sub.2]-550[degrees]C, and L-Sn[O.sub.2]-650[degrees]C deliver the initial discharge capacity of 1488.3 mAh/g, 1616.1 mAh/g, and 1408.9 mAh/g, respectively, which is higher than 1441.3mAh/g, 1491.7mAh/g, and 1370.0mAh/g of Sn[O.sub.2]450[degrees]C, Sn[O.sub.2]-550[degrees]C, and Sn[O.sub.2]-650[degrees]C, respectively. And the initial charge capacity of L-Sn[O.sub.2] is also higher than the latter, with 704.0 mAh/g, 914.9 mAh/g, and 824.8 mAh/g for LSn[O.sub.2]-450[degrees]C, L-Sn[O.sub.2]-550[degrees]C, and L-Sn[O.sub.2]-650[degrees]C, respectively, while 655.1 mAh/g, 869.7 mAh/g, and 698.3 mAh/g for Sn[O.sub.2]-450[degrees]C, Sn[O.sub.2]-550[degrees]C, and Sn[O.sub.2]-650[degrees]C, respectively. Additionally, the compacted density is 3.63 g/[cm.sup.3], 3.74 g/ [cm.sup.3], and 3.38g/[cm.sup.3] for L-Sn[O.sub.2]-450[degrees]C, L-Sn[O.sub.2]-550[degrees]C, and L-Sn[O.sub.2]-650[degrees]C, respectively. So, the corresponding reversible volumetric capacity is 982.9 mAh/[cm.sup.3],1316.0 mAh/[cm.sup.3], and 1072.2 mAh/[cm.sup.3], respectively, which is higher than commercial graphite of 720 mAh/[cm.sup.3]. Moreover, the flat plateau of L-Sn[O.sub.2] is a little higher than single Sn[O.sub.2] calcined at the same temperature. This should be related to smaller Sn[O.sub.2] in L-Sn[O.sub.2], so they could provide more passageways for [Li.sup.+] diffusion, deliver more capacity, and react with [Li.sup.+] easily to form LizSn. Additionally, L-Sn[O.sub.2]-450[degrees]C and LSn[O.sub.2]-550[degrees]C perform a coulombic efficiency of 47.3% and 56.6%, respectively, which is equal to 45.5% and 58.3% for Sn[O.sub.2]-450[degrees]C and Sn[O.sub.2]-550[degrees]C, respectively. This would be concluded that the functional groups in the surface of Sn[O.sub.2] have no irreversible capacity contribution.

Figure 8 shows the differential capacity versus potential curves for the 1st, 2nd, 5th, and 25th cycles for as-prepared Sn[O.sub.2] electrodes. The differential capacity refers to the calculated value of two adjacent points on the voltage-time curve ([V.sub.(n)], [V.sub.(n+1)], [t.sub.(n)], [t.sub.(n+1))], and the known charge, discharge current I, and the value of the active material mass m in the electrode according to dQ/dV = (I[t(n +1 - t(n)])/(m [V(n +1 - V(n)]). Researchers have testified that the reversibility of lithiation and delithiation of Sn[O.sub.2] should be good while the differential capacity curve is broad. The peaks on the differential capacity curve correspond to the platform of the voltage-capacity curve. The change of the area is enclosed by the curve which reflects the attenuation degree of the capacity; the area changes larger, and capacity attenuates faster. As shown in Figure 8, L-Sn[O.sub.2]-450[degrees]C, L-Sn[O.sub.2]550[degrees]C, and L-Sn[O.sub.2]-650[degrees]C show a sharp peak around 0.80 V, 0.90 V, and 0.77 V during the first discharge, respectively, which is higher than 0.78 V, 0.81V, and 0.65 V of Sn[O.sub.2]-450[degrees]C, Sn[O.sub.2]-550[degrees]C, and Sn[O.sub.2]-650[degrees]C, respectively. This phenomenon would also exist during the 2nd and 5th discharges, with lithiation peaks at 0.20 V, 0.33 V, and 0.34 V for L-Sn[O.sub.2]-450[degrees]C, L-Sn[O.sub.2]-550[degrees]C, and L-Sn[O.sub.2] 650[degrees]C, respectively, while 0.20 V, 0.27 V, and 0.30 V for Sn[O.sub.2]-450[degrees]C, Sn[O.sub.2]-550[degrees]C, and Sn[O.sub.2]-650[degrees]C, respectively. This suggests that smaller Sn[O.sub.2] is easy to alloy with [Li.sup.+]. No sharp peak could be observed after the first charge curves, which manifests that no 2-phase district exists. The reductive electric potentials around 0.3 V are also a featured platform of Sn corresponding to Sn and Li formed an alloy. The differential capacity curve of the 25th discharge process was basically a smooth state, and it proves that there is some irreversible oxidation-reduction reaction happened. This phenomenon was associated with electrolyte consumption of SEI on the fresh surface of electrodes. Sn would reunite and easily collapse in charge and discharge processes. Thus, SEI should reform on the exposed fresh surface of Sn. As for the smoothness of dQ-dV curves, peak of L-Sn[O.sub.2] is broader than single Sn[O.sub.2], suggesting that L-Sn[O.sub.2] have preferable transmission property for lithiation and delithiation. Therefore, the reason that the capacity of L-Sn[O.sub.2]-450[degrees]C was less than Sn[O.sub.2]-450[degrees]C is due to the adsorption of functional groups on the surface of the material, thus blocked the passage of the electrons and led to a decrease in the lithium storage capacity of the material. As for L-Sn[O.sub.2]-550[degrees]C and L-Sn[O.sub.2]-650[degrees]C, their curves were also smoother as well as indicated advantageous performance than Sn[O.sub.2]-550[degrees]C and Sn[O.sub.2]-650[degrees]C.

Figure 9 shows the cycling performance for L-Sn[O.sub.2] electrodes and the control group of Sn[O.sub.2] electrodes. Compared L-Sn[O.sub.2]-450[degrees]C with Sn[O.sub.2]-450[degrees]C, the capacity retention of L-Sn[O.sub.2]-450[degrees]C is inferior to that of Sn[O.sub.2]-450[degrees]C. It should be mainly related to the larger specific surface area for L-Sn[O.sub.2]-450[degrees]C while the particle size of Sn[O.sub.2] is close. The cyclic performance of L-Sn[O.sub.2]-550[degrees]C and L-Sn[O.sub.2]-650[degrees]C is much better than the Sn[O.sub.2]-550[degrees]C and Sn[O.sub.2]-650[degrees]C, because the particle size of Sn[O.sub.2] is much smaller in the former, which would experience less inner stress. Moreover, the porous structure of L-Sn[O.sub.2] can provide more channels and placeholders for the embedding and deembedding of ions. This was helpful to reduce Sn[O.sub.2] crushing and improve the stability of electrode materials. Even so, all Sn[O.sub.2] electrodes do not show expected excellent cyclability as well as those reported previously. However, it is believed that nanosized Sn[O.sub.2] coated by graphitic carbon at 600~700[degrees]C would perform better cyclability, which would be further studied.

4. Conclusion

Sn[O.sub.2] nanoparticles have been prepared by biomimetic synthesis combined with a sol-gel method using L-leucine as a biotemplate for the first time. L-Leucine could form a "nanocage" by its intermolecular hydrogen bond and accelerate the assembly of Sn4+ and OH- in the nanocage during the preparation process. Therefore, Sn[O.sub.2] growth could be regulated at a high temperature calcination of 650[degrees]C. As-prepared L-Sn[O.sub.2] show a block and porous structure. As anodes for lithium-ion battery, L-Sn[O.sub.2] perform better electrochemical performance than Sn[O.sub.2]. This should give a promising route to produce enhanced Sn[O.sub.2]/C electrodes with nanosized Sn[O.sub.2] coated by graphitic carbon at high temperature for lithium-ion batteries.

https://doi.org/10.1155/2018/4314561

Data Availability

All data generated or analyzed during this study are included in this published article (and its supplementary information files).

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was supported by the NSFC under project no. 51701073 and the Youth Fund of Hunan Agricultural University no. 16QN13.

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Peng Yu, (1) Mili Liu, (1) Haixiong Gong, (1) Fangfang Wu, (1) Zili Yi, (2) and Hui Liu [ID] (1)

(1) College of Science, Hunan Agricultural University, Changsha (410128), China

(2) College of Bioscience and Biotechnology, Hunan Agricultural University, Changsha (410128), China

Correspondence should be addressed to Hui Liu; liu.hui@hunau.edu.cn

Received 27 May 2018; Accepted 30 July 2018; Published 19 August 2018

Academic Editor: Liqing He

Caption: Figure 1: XRD patterns of as-prepared Sn[O.sub.2] nanoparticles divided into three groups of (a) L-Sn[O.sub.2]-450[degrees]C and (b) Sn[O.sub.2]-450[degrees]C, (c) L-Sn[O.sub.2]-550[degrees]C and (d) Sn[O.sub.2]-550[degrees]C, and (e) L-Sn[O.sub.2]-650[degrees]C and (f) Sn[O.sub.2]-650[degrees]C.

Caption: Figure 2: FT-IR spectrum of synthesized L-Sn[O.sub.2]-650[degrees]C and Sn[O.sub.2]-650[degrees] C.

Caption: Figure 3: SEM images of (a) L-Sn[O.sub.2]-450[degrees]C, (b) L-Sn[O.sub.2]-550[degrees]C, (c) L-Sn[O.sub.2]-650[degrees]C, (d) Sn[O.sub.2]-450[degrees]C, (e) Sn[O.sub.2]-550[degrees]C, and (f) Sn[O.sub.2]-650[degrees]C.

Caption: Figure 4: Pore size distribution of L-Sn[O.sub.2]-650[degrees]C and Sn[O.sub.2]-650[degrees]C.

Caption: Figure 5: The formation of "nanocage" and intermolecular hydrogen bonds between L-leucines.

Caption: Figure 6: Schematic for the foam-like morphology of L-Sn[O.sub.2]-650[degrees]C.

Caption: Figure 7: The 1st, 2nd, and 5th voltage-capacity profiles of as-prepared Sn[O.sub.2] electrodes.

Caption: Figure 8: Differential capacity vs. potential curves of the 1st, 2nd, 5th, and 25th cycles for L-Sn[O.sub.2] electrodes and the control group of Sn[O.sub.2] electrodes.

Caption: Figure 9: Cyclic performance of L-Sn[O.sub.2] electrodes and the control group of Sn[O.sub.2] electrodes.

Table 1: The surface area, pore volume, and pore diameter of
L-SnO2-650[degrees]C and Sn[O.sub.2]-650[degrees]C.

Sample                Surface          Pore volume     Pore diameter
                 area ([m.sup.2]/g)   ([cm.sup.3]/g)        (nm)

L-Sn[O.sub.2]-          14.7               0.15             24.9
650[degrees]C

Sn[O.sub.2]-            7.8                0.07             3.3
650[degrees]C
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Title Annotation:Research Article
Author:Yu, Peng; Liu, Mili; Gong, Haixiong; Wu, Fangfang; Yi, Zili; Liu, Hui
Publication:Scanning
Date:Jan 1, 2018
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