Coreâshell structured hollow SnO2âpolypyrrole nanocomposite ...
Nano Energy (2014) 6, 73–81
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journal homepage: www.elsevier.com/locate/nanoenergy
RAPID COMMUNICATION
Core–shell structured hollow SnO2–polypyrrole nanocomposite anodes with enhanced cyclic performance for lithium-ion batteries Ruiqing Liua, Deyu Lia, Chen Wanga, Ning Lia,n, Qing Lib,n, Xujie Lüb, Jacob S. Spendelowb, Gang Wub,n a
School of Chemical Engineering & Technology, Harbin Institute of Technology, Harbin 150001, China Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM 87545, United States
b
Received 18 January 2014; received in revised form 6 March 2014; accepted 13 March 2014 Available online 27 March 2014
KEYWORDS
Abstract
SnO2 nanoparticles; Polypyrrole; Hollow morphology; Anodes; Lithium ion batteries
Core–shell structured hollow SnO2–polypyrrole (PPy) nanocomposites (SnO2@PPy) with excellent electrochemical performance were synthesized using a hydrothermal method followed by an in situ chemical-polymerization route. The thickness of the polymerized amorphous PPy coating covering on the hollow SnO2 microspheres is about 25 nm, demonstrated by microscopy images. As an anode in lithium ion batteries, the nanocomposite is capable of retaining a high capacity of 448.4 mAh g 1 after 100 cycles with a coulomb efficiency above 97%. Compared to other reported SnO2 anodes, the enhanced cycling stability is attributed to the unique core–shell structure and a possible synergistic effect between the PPy coating layer and the hollow SnO2 spheres. The PPy coating not only prevents the possible pulverization of the hollow SnO2 spheres, but also prevents the SnO2/Sn spheres from aggregating. Furthermore, the hollow space within the SnO2 nanoparticles effectively mitigates the enormous volume change during charge–discharge cycling. Meanwhile, the Li + diffusion coefficient in the hollow SnO2@PPy (21wt%) core–shell nanocomposite electrode is significantly improved compared to the hollow SnO2 microspheres electrode. Thus, these benefits from the PPy coating and the hollow SnO2 spheres are able to provide a robust architecture for lithium-ion battery anodes. & 2014 Elsevier Ltd. All rights reserved.
n
Corresponding authors. E-mail addresses: [email protected] (N. Li), [email protected] (Q. Li), [email protected] (G. Wu).
http://dx.doi.org/10.1016/j.nanoen.2014.03.010 2211-2855/& 2014 Elsevier Ltd. All rights reserved.
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Introduction Among currently studied anodes for Li-ion batteries (LIBs), SnO2-based materials have become one of the most promising materials. The theoretical specific capacity of SnO2 is 782 mAh g 1, substantially higher than currently used graphite anode materials (372 mAh g 1) [1–5]. In addition, its low potentials of lithium alloying/de-alloying would yield high energy density for LIBs [6]. However, the huge volume variation of SnO2 during charge–discharge cycles results in rapid capacity degradation [7], significantly limiting its practical applications. Recently, nanostructured SnO2 materials have shown improved cycling stability due to the reduced internal strain and shortened lithium diffusion length [8–10]. In addition, SnO2 composites containing a buffer layer, such as carbon materials, exhibited enhanced stability due to their good overall electronic conductivity and the capability to mitigate the internal stress of SnO2 [11–16]. Designing and synthesizing unique SnO2 morphology is another effective method to enhance the cyclic stability, such as nanorod arrays [2], nanotubes [17], and hollow nanostructures [18–21]. On the other hand, conducting polymers have been introduced to improve the anode performance in LIBs [22–24]. In addition to improving electron conductivity [25,26], the soft polymer matrix also can relax the internal stress of solid particle anodes that suffer from severe volume change during charge–discharge cycles [27]. Recently, it was found that SnO2 particles covered by polypyrrole (PPy) demonstrated improved cycling performance, compared to the standalone SnO2 nanoparticles [27– 29]. In these reports, several methods were developed to prepare various SnO2–polypyrrole composites for anode materials. In particular, a SnO2-polypyrrole composite material was prepared by a combined spray pyrolysis techniquechemical polymerization method. The composite was demonstrated significantly improved capacity and cycle durability compared to the bare SnO2 electrode. After 20 cycles, the SnO2–PPy composite and standalone SnO2 electrodes retained 70% (about 400 mAh g 1) and 40% of their initial capacities, respectively [28]. In another work, SnO2– polypyrrole hybrid nanowires were synthesized in a one-step process by a simple electrochemical method. Over 200 cycles, the hybrid nanowires showed superior cyclic performance and a charge capacity higher than 0.307 mAh cm 2, because the polypyrrole matrix effectively prevented agglomeration of the SnO2 nanoparticles and acted as an elastic buffer to mitigate the volumetric changes in the
Figure 1
nanoparticles [29]. Another synthesis of SnO2@polypyrrole nanocomposites was performed using a one-pot oxidative chemical polymerization method. After 20 cycles, the capacity was 430 mAh g 1, which corresponds to 40.8% retention of initial capacity [27]. However, in those works, the SnO2 in the composites consisted of solid particles, in which internal stress develops during cycling. Recent efforts in developing hollow structured SnO2 indicated that the unique morphology is able to reduce volume changes during cycling and to provide increased interfacial area for Li insertion–extraction [4,30,31]. Therefore, there remains much room for improvement in the electrochemical performances of SnO2 anodes through creation of hollow structures. In this work, hollow SnO2 microspheres coated by PPy layers were prepared to fabricate a new SnO2@PPy core– shell architecture. The Li + diffusion coefficient in the hollow SnO2@PPy core–shell nanocomposite is significantly improved compared to the hollow SnO2 microspheres. Thus, these advantages from the PPy coating and the hollow SnO2 microspheres are able to provide a robust architecture for lithium-ion battery anodes. The proposed synergistic effect between the PPy coating and the hollow structure can decrease the likelihood of pulverization of SnO2, and enhance electrochemical performance of SnO2 anode materials in LIBs.
Experimental section As shown in Figure 1, hollow SnO2 microspheres were prepared via a hydrothermal method. Briefly, CO(NH2)2 (1.92 g) and Na2SnO3 3H2O (1.28 g) were dissolved in 320 mL mixed solution of water and ethanol (EtOH/H2O=0.6, V/V). Then the solution was transferred into a Teflon-lined stainless steel autoclave at a temperature of 180 1C for 24 h. After cooling down to room temperature, the precipitate was filtered, washed with deionized water and ethanol, and dried at 90 1C overnight. The resulting hollow SnO2 particles were calcined at 500 1C for one hour. Next, the hollow SnO2 cores were covered by a PPy shell via an in-situ chemical-polymerization route. In a typical procedure, 0.1 g of hollow SnO2 microspheres were mixed with a 40 mL aqueous solution containing 4 mg of sodium lauryl sulfate (SDS) followed by sonication for 30 min and magnetic stirring for 3 h. After adding 25.8 μL pyrrole monomer, the solution was continuously stirred for another 1 h. Then, 11.2 mL of 0.1 M ammonium persulfate aqueous solution was dropwise added into the above solution. The gradually changing color from light gray to black indicated
Scheme of synthesis for the hollow SnO2@PPy core–shell nanocomposites.
SnO2-polypyrrole nanocomposite anodes with enhanced cyclic performance a formation of PPy. The polymerization process was conducted while stirring for 4 h at room temperature. The resulting black composites were centrifuged, washed with deionized water and ethanol at least three times, and then dried in a vacuum oven at 80 1C overnight. The prepared core–shell structured hollow SnO2@PPy nanocomposites were extensively characterized by using X-ray diffraction (XRD, Rigaku D/MAX-RB), field emission scanning electron microscopy (FESEM, Hitachi S4800), transmission electron microscopy (TEM, FEI TECNAI G2), FT-IR (EQUINOX55) and TGA (STA-400). The material was fabricated into working electrodes consisting of 80 wt% active samples, 10 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF). A piece of lithium foil was used for the combined counter and reference electrode. 1.0 M LiPF6 dissolved in a 1:1 volume ratio mixture of ethylenecarbonate (EC) and dimethyl carbonate (DMC) was used as the electrolyte. All cells were assembled in an argon-filled glove box. The fabricated coin cell (CR2032) underwent galvanostatic charge–discharge cycling and cyclic voltammetry (CV) testing using a battery testing system (Arbin BT-2000) and an electrochemical workstation (CHI660B), respectively. The specific capacity was calculated based on the total weight of SnO2 and PPy. The charge–discharge tests were conducted at a rate of 0.1 C (1 C = 782 mA g 1) in a voltage window of 0.04–2.00 V (vs. Li/Li + ). The CV curves were measured in a potential range of 0.02–2.70 V (vs. Li/Li + ) at a scan rate of 0.5 mV s 1.
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Figure 2(b) shows the FT-IR spectra of pristine SnO2 hollow microspheres, PPy, and SnO2@PPy composites with different PPy contents. The bands in the range of 537– 623 cm 1 observed with the SnO2 hollow microspheres and the core–shell structured hollow SnO2@PPy composites can be assigned to the anti-symmetric and symmetric vibrations of Sn–O–Sn [32,33]. The bands centered at 1707 and 1596 cm 1 for both the SnO2@PPy composites and the PPy correspond to typical C =C in plane vibration. In addition, the characteristic bands of PPy for C–C and C–H ring stretching were found at 1400 and 1258 cm 1, respectively. The sharp peak at 1049 cm 1 is attributed to C–H in-plane vibrations. The band at 929 cm 1 can be assigned to N–H inplane vibrations [34,35]. Thus, these characteristic bands of PPy for both SnO2@PPy composites and PPy are nearly identical, except for the lower intensity observed with lower PPy content in the SnO2@PPy composites. The FT-IR results provide direct evidence that PPy is present in the SnO2@PPy nanocomposites. As shown in Figure 3, thermogravimetric analysis was used to determine the amount of PPy in the SnO2@PPy composites. The composite samples were heated from 25 to 800 1C in air, displaying two weight loss regions. The weight loss in the range of 25–250 1C is caused by the loss of chemisorbed water, and the loss in the range of 250–800 1C 100
SnO 13 wt% PPy
Figure 2(a) shows the XRD patterns of hollow SnO2, standalone PPy, and the core–shell SnO2@PPy composites. All the diffraction peaks of SnO2 and SnO2@PPy can be well indexed to the tetragonal rutile structure of SnO2 (cassiterite, JCPDS no. 41-1445). The identified diffraction peaks at 26.51, 33.81, 37.81, 51.81, 54.61, 57.81, 61.71, 65.11, 71.31 and 78.71 can be well assigned to (110), (101), (200), (211), (220), (002), (310), (112), (301), (202), and (321) planes of tetragonal SnO2. The peak intensity of the SnO2@PPy composites decreases with increasing PPy content, compared to the SnO2 sample. As for the PPy, no diffraction peaks can be detected except for the broad peak around 21.5–26.41 corresponding to amorphous PPy.
Figure 2
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Results and discussion
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(a) XRD patterns and (b) FT-IR spectra of hollow SnO2@PPy core–shell nanocomposites with different PPy content.
76 is ascribed to the decomposition of PPy. The pure SnO2 hollow spheres show almost no weight change from 25 to 800 1C, while the PPy is nearly burnt out. The TGA analysis indicates that the PPy content in the nanocomposites is 13 wt%, 21 wt% and 29 wt%, respectively. Morphology of the composites was examined by SEM and TEM images, as shown in Figure 4 and Figure 5, respectively. A SEM image of a sample consisting of hollow SnO2 spheres is shown in Figure 4(a). The chemically polymerized PPy sample prepared from identical conditions exhibits granular morphology with diameters ranging from 300 to 500 nm. The morphology of samples containing hollow SnO2 microspheres covered by PPy layers, SnO2@PPy (21 wt%), is displayed in Figure 4(c and d). The PPy layers coated on the hollow SnO2 microspheres can be observed in these micrographs marked by red arrows. The particle diameters of the hollow SnO2@PPy composites with different PPy content are approximately equal (200 nm). Meanwhile, Figure 5(a, b) shows representative TEM images of hollow SnO2@PPy (21wt%) core–shell nanostructures, from which the outer shell of the hollow SnO2 microspheres is observed to be approximately 30 nm. A PPy layer coated on the SnO2 hollow spheres, with thickness around 25 nm, can also be observed. Moreover, a HR-TEM image in Figure 5(c) shows an interplanar spacing of about 0.335 nm, corresponding to the (110) planes of SnO2, which is in agreement with the XRD analysis. The cycling performance of the hollow SnO2@PPy core– shell nanocomposites is shown in Figure 6(a) at 0.1 C in the voltage window of 0.04–2.00 V, in comparison with those of the pure SnO2 hollow sphere anode and pure PPy anode. The PPy anode delivered a specific capacity of 29.7 mAh g 1 in the first discharge and retained only about 15 mAh g 1 in the subsequent cycles. Therefore, the low capacity of PPy
R. Liu et al. can be negligible in analysis of SnO2@PPy nanocomposites. As shown in Figure 6(a), the retained specific capacity for each sample is 406.5 mAh g 1 in the 65th cycle for the hollow SnO2 sphere anode, 281.1 mAh g 1 in the 96th cycle for the hollow SnO2@PPy (13 wt%) core–shell nanocomposite anode, 448.4 mAh g 1 in the 100th cycle for the hollow SnO2@PPy (21 wt%) nanocomposite anode, and 340.7 mAh g 1 in the 100th cycle for the hollow SnO2@PPy (29 wt%) core–shell nanocomposite anode. The measured performance with the core-shell structured SnO2–PPy, especially for the hollow SnO2@PPy (21 wt%) core–shell nanocomposite, is much higher than that of the standalone SnO2 hollow spheres, and those reported for SnO2 nanoparticles–PPy composites [27], SnO2 nano-single crystals [8], SnO2 hollow nanotubes [36], and mesoporous SnO2 on multiwalled carbon nanotubes (MWCNTs) [37]. Figure 6(b) shows the rate capabilities of SnO2@PPy nanocomposites from a current density of 78 mA g 1 (0.1 C) up to 3900 mA g 1 (5 C). During each high rate stage, the cell was cycled 10 times, followed by 5 cycles at low 0.1 C. The hollow SnO2@PPy core–shell nanocomposites display excellent rate capabilities. For example, the hollow SnO2@PPy (21 wt%) core–shell nanocomposites anode is still capable of delivering a substantial capacity above 1036.2, 614.0, 487.4, and 331.8 mAh g 1 at each current density of 78 (0.1 C), 390 (0.5 C), 780 (1.0 C), and 1560 (2.0 C) mA g 1, respectively. Even when the highest current density of 3900 mA g 1 (5.0 C) was applied, the composites exhibited a reversible capacity above 38.1 mAh g 1. It should be noted that, when the current rate was reversed back to low current (0.1 C) after 60 cycles, a specific discharge capacity of 614.9 mAh g 1 was recovered, indicating that 54.1% of the initial reversible capacity (1137.6 mAh g 1) was retained.
Figure 4 SEM images for (a) hollow SnO2 spheres (b) PPy, (c) and (d) hollow SnO2@PPy (21 wt%) core–shell nanocomposites (PPy layers marked by red arrows, covering onto SnO2 particles).
SnO2-polypyrrole nanocomposite anodes with enhanced cyclic performance
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Figure 5 Typical TEM images for the hollow SnO2@PPy (21 wt%) core–shell nanocomposites at different magnification.
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Figure 6 The cycling performance (a) and rate capability (b) for hollow SnO2@PPy core–shell nanocomposite anodes with different PPy contents.
Figure 7 EIS for hollow SnO2@PPy core–shell nanocomposite anodes with different PPy contents.
This result demonstrates that the hollow SnO2@PPy core–shell nanocomposites can tolerate varied discharge current densities, which is a desirable characteristic for high power application. It thus appears that PPy could work as a highly conductive matrix when anchored by hollow SnO2 microspheres. Meanwhile, the coulombic efficiency of a hollow SnO2@PPy (21 wt%) core–shell nanocomposite anode for the first cycle is only 54.3%, but it is raised to 90.7% for the 2nd cycle and up to 97% in the subsequent long time cycles. Compared to the hollow SnO2 microsphere anode, the coulombic efficiency of hollow SnO2@PPy (21 wt%) is more stable.
AC impedance measurements were performed on the hollow SnO2 microspheres and SnO2@PPy nanocomposite electrodes. Prior to the AC impedance testing, the electrodes were cycled galvanostatically for four cycles to ensure the stable formation of the SEI layers on the surface of the electroactive particles. The AC impedance experiments were then performed at 0.4 V in the 4th charge cycle. As shown in Figure 7, the Nyquist plot consists of a semicircle and a straight line, indicative of limitations in the charge transfer reaction (Rct) and the diffusion of Li + in the bulk electrode, respectively. The diameters of the semicircles for hollow SnO2 microspheres and SnO2@PPy nanocomposite electrodes are 162.8 Ω (0 wt% PPy), 148.5 Ω (13 wt% PPy), 107.5 Ω (21 wt% PPy) and 101.9 Ω (29 wt% PPy), respectively. The total resistance value is decreasing with an increase of PPy content, which is due to the good electronic conductivity of PPy. The result confirms that the incorporation of PPy coating is an effective method for enhancing the electron transport of hollow SnO2 microspheres, which leads to a significant improvement in the electrochemical performance. In the data discussed above, the hollow SnO2@PPy (21 wt%) core–shell nanocomposite anode exhibits the best cycling performance and rate capabilities, indicating that the ideal PPy content is around 21 wt%. The improved cycle stability and rate capabilities prompted us to further investigate the electrochemical reactivity and kinetics of SnO2@PPy (21 wt%) core–shell nanocomposite electrodes. At first, cyclic voltammetry was performed in a potential range of 0.02–2.70 V (vs. Li/Li + ) at a scan rate of 0.5 mV s 1.
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Figure 9 Cyclic voltammetry of hollow SnO2 microspheres (a) and hollow SnO2@PPy (21 wt%) core–shell nanocomposites anode (c). The scan rates are 0.1, 0.2, 0.3, 0.4 and 0.5 mV s 1, respectively. Plot of peak current (Ip) as a function of the square root of the scan rates (v1/2) for the hollow SnO2 microspheres (b) and the hollow SnO2@PPy (21 wt%) nanocomposites anode (d).
As shown in Figure 8(a), the CV curves of the hollow SnO2@PPy core–shell nanocomposites are in good agreement with the hollow SnO2 microsphere anodes [30], indicating that the PPy has no influence on the lithium insertion–extraction reaction. As described in Eq. (1), the peak at 0.71 V on the first cathodic scan is ascribed to the reduction of SnO2 to Sn and the formation of Li2O as well as the solid electrolyte interface (SEI) layer [20,33,38], accompanied with a large irreversible capacity loss in the initial cycle. The cathodic peak extending to 0.02 V and the anodic peak at 0.59 V are assigned to the alloying
and dealloying of Sn and Li, respectively [7], as described by Eq. (2). In the CV curves, an oxidation peak around 1.3 V and a reduction peak around 1.1 V are also clearly observed, which are most likely due to the partially reversible reaction of Eq. (1) [1,39], resulting in the formation and decomposition of nanosized Li2O during subsequent cycles. SnO2 + 4Li + + 4e -Sn+ 2Li2O
(1)
Sn + xLi + + xe 2LixSn (0 rx r 4.4)
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SnO2-polypyrrole nanocomposite anodes with enhanced cyclic performance
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Polypyrrole
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Figure 10 Scheme of the role of PPy in confining the volume change of the hollow SnO2 microspheres during the Li-ion insertion/ extraction reactions.
Figure 11 SEM images of (a) a hollow SnO2 spheres electrode after 100 cycles, and (b) a hollow SnO2@PPy (21 wt%) core–shell nanocomposite electrode after 100 cycles.
Figure 8(b) shows the 1st, 2nd, 10th, 20th, 40th, 80th and 100th charge–discharge voltage profiles of the hollow SnO2@PPy (21 wt%) nanocomposite at a current density of 78 mA g 1 with a voltage range of 0.04–2.00 V. The initial small plateau in the potential range of 1.0 to 0.8 V corresponds to a conversion reaction between SnO2 and Li + , resulting in the formation of Sn and Li2O in the first discharge process. The following long slope profiles of the SnO2–21 wt% PPy nanocomposite indicate the formation of Li–Sn alloys. The plateau (from 1.0 to 0.8 V) almost disappears at the second cycle, demonstrating that most Li2O is formed in the first cycle. The first discharge capacity of the composites is 1168.1 mAh g 1. The initial irreversible capacity is as high as 45.7%, which is likely caused by the formation of SEI layers, electrolyte decomposition, and the irreversible reaction of SnO2 +4Li + +4e -Sn+2Li2O during the discharge process. It is worth noting that, after 10 cycles, the reversible cycling capacity is well retained, suggesting an excellent reversibility of lithium insertion/extraction reactions in the nanocomposite. Figure 9(a and c) displays the CVs of the hollow SnO2 microsphere electrode and the hollow SnO2@PPy (21 wt%) nanocomposite electrode at different sweep rates (0.1, 0.2, 0.3, 0.4 and 0.5 mV s 1). The dependence of peak current (Ip) on scan rate (v) was determined from the CVs. The diffusion coefficient of Li + ions (DLi) can be calculated from a linear relationship between Ip and v1/2 according to the following Eqs. [40,41]: n 1/2 Ip = 2.69 105n3/2AD1/2 Li CLiv
(3)
In the above equation, n is the number of electrons per reaction species (it is 1 for Li + ), A is the electrode area
(cm2), and CnLi is the bulk concentration of the Li + ion in the electrode (mol cm 3). Figure 9(b, d) shows a good linear relationship between Ip and v1/2 for both electrodes. The Li + diffusion coefficient in the SnO2@PPy (21 wt%) nanocomposite electrode is calculated to be 7.4 10 9 cm2 s 1. In comparison, the diffusion coefficient for the hollow SnO2 microsphere electrode shows a lower value of 1.2 10 9 cm2 s 1. This enhancement could be attributed to the unique hollow core– shell structure and conductive PPy coating, which serves as an efficient diffusion channel for Li + . The enhanced cycling durability can be attributed to the synergistic effect between the PPy matrix and the SnO2 hollow spheres, which mitigates the enormous volume change of SnO2 during charge–discharge and keeps the nanoparticles from agglomerating. A possible mechanism of this interaction, in which the PPy acts as a buffer to mitigate the volume change of hollow SnO2 microspheres, is proposed in Figure 10. During the lithium insertion process, SnO2 nanoparticles are converted to a mixture of Li2O and LixSn. In turn, metallic Sn is generated from the LixSn dealloying process during lithium extraction. The effects of the enormous volume change of SnO2 can be mitigated by the PPy coating, which serves as a buffer between adjacent SnO2 microspheres. Meanwhile, the inner hollow space can also mitigate the enormous volume change to some degree. Thus, the proposed synergistic effect between the PPy coating and the hollow structure can decrease the likelihood of pulverization of SnO2. In addition, the PPy coating is able to prevent the Sn nanoparticles from aggregating and provides good electrical contact within the nanocomposite. To support the proposed mechanism, the morphology of standalone SnO2 hollow spheres and hollow SnO2@PPy (21 wt%) core–shell nanocomposite electrodes after cycling was studied
80 by SEM, as shown in Figure 11. Serious cracks were observed on the standalone SnO2 hollow spheres electrode after 100 charge–discharge cycles, suggesting enormous volume changes during charge–discharge cycling (Figure 11a). On the other hand, as shown in Figure 11b, the hollow SnO2@PPy (21 wt%) core–shell nanocomposite electrode shows mitigated cracking after 100 cycles compared to the SnO2 hollow spheres electrode. These results indicate that the PPy buffer on the SnO2 hollow spheres was able to mitigate the large volume expansion and maintain the relative integrity of the electrode. Structural integrity and high conductivity are critical characteristics of LIB anode materials, so the changes caused by the presence of PPy in the hollow SnO2@PPy core–shell nanocomposite electrode represent significant improvements.
Conclusions In this work, a hollow SnO2–polypyrrole (PPy) core–shell nanocomposite was synthesized via a hydrothermal method followed by an in situ chemical-polymerization to form PPy layers on SnO2 hollow spheres. The nanocomposite was successfully used as an anode material for lithium ion batteries, exhibiting significantly enhanced cycling performance (448.4 mAh g 1 after 100 cycles) and excellent coulomb efficiency (97%), compared to the standalone hollow SnO2 spheres. The battery performance achieved by using the unique core–shell structures is much higher than that reported for any other SnO2 based anode material so far. The enhanced cycling performance is attributed to the unique hollow structure and the PPy buffer layers, which are found to be beneficial for alleviating the volume changes of SnO2 and agglomeration of generated Sn particles during the lithium insertion–extraction process. Meanwhile, the Li + diffusion coefficient in the hollow SnO2–21 wt % PPy core–shell nanocomposite electrode is significantly improved compared to the hollow SnO2 microsphere electrode. This work may open a new path toward designing SnO2 based nanocomposite anode materials for Li-ion batteries with improved cyclic stability.
Acknowledgments Financial support from the Los Alamos National Laboratory Early Career Laboratory-Directed Research and Development (LDRD) Program (20110483ER) for this work is gratefully acknowledged.
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Ruiqing Liu is a Ph.D. student at Harbin Institute Technology in Prof. Ning Li’s group. He obtained his Bachelor and Master degrees in Chemical Engineering from the same school in 2007 and 2009, respectively. His research focuses on the anode materials for Li-ion batteries.
Xujie Lü is a postdoctoral research associate at LANL & UNLV. He received his Ph.D. from Chinese Academy of Sciences (CAS) in 2011. His research interests include Li batteries, solid electrolytes, nanostructured photovoltaic materials, thin film depositions and High P/T techniques.
Deyu Li is a lecturer in School of Chemical Engineering and Technology at Harbin Institute of Technology in China. He received his PhD in the Applied Chemistry from Harbin Institute of Technology in 2014. His research mainly covers electronic electroplating and Li-ion batteries.
Jacob S. Spendelow is a scientist at Los Alamos National Laboratory (LANL). He received his Ph.D. from the University of Illinois at Urbana-Champaign in 2006, where he was funded by a NSF graduate research fellowship. He joined LANL in August 2006 as a Director's Postdoctoral Fellow. His current research interests are in the field of battery materials and fuel cell electrocatalysts.
Chen Wang received his BSc and MSc from Heilongjiang University, China. He is currently pursuing his Ph.D. at Harbin Institute of Technology with Prof. Ning Li. His research interests are hydrothermal synthesis of nanostructured transition-metal oxides to control their morphological and structural for anode materials in lithium-ion batteries.
Gang Wu is a scientist at Los Alamos National Laboratory (LANL). He completed his Ph.D. studies on Electrochemical Engineering at the Harbin Institute of Technology in 2004. After postdoctoral trainings at Tsinghua University, the University of South Carolina, and LANL, he became a technical staff member at LANL in 2010. His research focuses on functional materials and catalysts for batteries, fuel cells, and electrochemical sensors.
Ning Li is a professor of Chemical Engineering in the School of Chemical Engineering and Technology at Harbin Institute of Technology. She received his PhD in Environmental Engineering in 2000 from Harbin Institute of Technology. Her major research interests are batteries, metal materials corrosion mechanism and control, and principles and applications of electrodeposition and electroless deposition. Qing Li is a postdoctoral research associate at Los Alamos National Laboratory. He received his BSc from Wuhan University in 2005 and his Ph.D. from Peking University in 2010, both in chemistry. His research interests include functional nanomaterials and their applications in PEM fuel cells, metal-air batteries, and biosensors.
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RAPID COMMUNICATION
Core–shell structured hollow SnO2–polypyrrole nanocomposite anodes with enhanced cyclic performance for lithium-ion batteries Ruiqing Liua, Deyu Lia, Chen Wanga, Ning Lia,n, Qing Lib,n, Xujie Lüb, Jacob S. Spendelowb, Gang Wub,n a
School of Chemical Engineering & Technology, Harbin Institute of Technology, Harbin 150001, China Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM 87545, United States
b
Received 18 January 2014; received in revised form 6 March 2014; accepted 13 March 2014 Available online 27 March 2014
KEYWORDS
Abstract
SnO2 nanoparticles; Polypyrrole; Hollow morphology; Anodes; Lithium ion batteries
Core–shell structured hollow SnO2–polypyrrole (PPy) nanocomposites (SnO2@PPy) with excellent electrochemical performance were synthesized using a hydrothermal method followed by an in situ chemical-polymerization route. The thickness of the polymerized amorphous PPy coating covering on the hollow SnO2 microspheres is about 25 nm, demonstrated by microscopy images. As an anode in lithium ion batteries, the nanocomposite is capable of retaining a high capacity of 448.4 mAh g 1 after 100 cycles with a coulomb efficiency above 97%. Compared to other reported SnO2 anodes, the enhanced cycling stability is attributed to the unique core–shell structure and a possible synergistic effect between the PPy coating layer and the hollow SnO2 spheres. The PPy coating not only prevents the possible pulverization of the hollow SnO2 spheres, but also prevents the SnO2/Sn spheres from aggregating. Furthermore, the hollow space within the SnO2 nanoparticles effectively mitigates the enormous volume change during charge–discharge cycling. Meanwhile, the Li + diffusion coefficient in the hollow SnO2@PPy (21wt%) core–shell nanocomposite electrode is significantly improved compared to the hollow SnO2 microspheres electrode. Thus, these benefits from the PPy coating and the hollow SnO2 spheres are able to provide a robust architecture for lithium-ion battery anodes. & 2014 Elsevier Ltd. All rights reserved.
n
Corresponding authors. E-mail addresses: [email protected] (N. Li), [email protected] (Q. Li), [email protected] (G. Wu).
http://dx.doi.org/10.1016/j.nanoen.2014.03.010 2211-2855/& 2014 Elsevier Ltd. All rights reserved.
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Introduction Among currently studied anodes for Li-ion batteries (LIBs), SnO2-based materials have become one of the most promising materials. The theoretical specific capacity of SnO2 is 782 mAh g 1, substantially higher than currently used graphite anode materials (372 mAh g 1) [1–5]. In addition, its low potentials of lithium alloying/de-alloying would yield high energy density for LIBs [6]. However, the huge volume variation of SnO2 during charge–discharge cycles results in rapid capacity degradation [7], significantly limiting its practical applications. Recently, nanostructured SnO2 materials have shown improved cycling stability due to the reduced internal strain and shortened lithium diffusion length [8–10]. In addition, SnO2 composites containing a buffer layer, such as carbon materials, exhibited enhanced stability due to their good overall electronic conductivity and the capability to mitigate the internal stress of SnO2 [11–16]. Designing and synthesizing unique SnO2 morphology is another effective method to enhance the cyclic stability, such as nanorod arrays [2], nanotubes [17], and hollow nanostructures [18–21]. On the other hand, conducting polymers have been introduced to improve the anode performance in LIBs [22–24]. In addition to improving electron conductivity [25,26], the soft polymer matrix also can relax the internal stress of solid particle anodes that suffer from severe volume change during charge–discharge cycles [27]. Recently, it was found that SnO2 particles covered by polypyrrole (PPy) demonstrated improved cycling performance, compared to the standalone SnO2 nanoparticles [27– 29]. In these reports, several methods were developed to prepare various SnO2–polypyrrole composites for anode materials. In particular, a SnO2-polypyrrole composite material was prepared by a combined spray pyrolysis techniquechemical polymerization method. The composite was demonstrated significantly improved capacity and cycle durability compared to the bare SnO2 electrode. After 20 cycles, the SnO2–PPy composite and standalone SnO2 electrodes retained 70% (about 400 mAh g 1) and 40% of their initial capacities, respectively [28]. In another work, SnO2– polypyrrole hybrid nanowires were synthesized in a one-step process by a simple electrochemical method. Over 200 cycles, the hybrid nanowires showed superior cyclic performance and a charge capacity higher than 0.307 mAh cm 2, because the polypyrrole matrix effectively prevented agglomeration of the SnO2 nanoparticles and acted as an elastic buffer to mitigate the volumetric changes in the
Figure 1
nanoparticles [29]. Another synthesis of SnO2@polypyrrole nanocomposites was performed using a one-pot oxidative chemical polymerization method. After 20 cycles, the capacity was 430 mAh g 1, which corresponds to 40.8% retention of initial capacity [27]. However, in those works, the SnO2 in the composites consisted of solid particles, in which internal stress develops during cycling. Recent efforts in developing hollow structured SnO2 indicated that the unique morphology is able to reduce volume changes during cycling and to provide increased interfacial area for Li insertion–extraction [4,30,31]. Therefore, there remains much room for improvement in the electrochemical performances of SnO2 anodes through creation of hollow structures. In this work, hollow SnO2 microspheres coated by PPy layers were prepared to fabricate a new SnO2@PPy core– shell architecture. The Li + diffusion coefficient in the hollow SnO2@PPy core–shell nanocomposite is significantly improved compared to the hollow SnO2 microspheres. Thus, these advantages from the PPy coating and the hollow SnO2 microspheres are able to provide a robust architecture for lithium-ion battery anodes. The proposed synergistic effect between the PPy coating and the hollow structure can decrease the likelihood of pulverization of SnO2, and enhance electrochemical performance of SnO2 anode materials in LIBs.
Experimental section As shown in Figure 1, hollow SnO2 microspheres were prepared via a hydrothermal method. Briefly, CO(NH2)2 (1.92 g) and Na2SnO3 3H2O (1.28 g) were dissolved in 320 mL mixed solution of water and ethanol (EtOH/H2O=0.6, V/V). Then the solution was transferred into a Teflon-lined stainless steel autoclave at a temperature of 180 1C for 24 h. After cooling down to room temperature, the precipitate was filtered, washed with deionized water and ethanol, and dried at 90 1C overnight. The resulting hollow SnO2 particles were calcined at 500 1C for one hour. Next, the hollow SnO2 cores were covered by a PPy shell via an in-situ chemical-polymerization route. In a typical procedure, 0.1 g of hollow SnO2 microspheres were mixed with a 40 mL aqueous solution containing 4 mg of sodium lauryl sulfate (SDS) followed by sonication for 30 min and magnetic stirring for 3 h. After adding 25.8 μL pyrrole monomer, the solution was continuously stirred for another 1 h. Then, 11.2 mL of 0.1 M ammonium persulfate aqueous solution was dropwise added into the above solution. The gradually changing color from light gray to black indicated
Scheme of synthesis for the hollow SnO2@PPy core–shell nanocomposites.
SnO2-polypyrrole nanocomposite anodes with enhanced cyclic performance a formation of PPy. The polymerization process was conducted while stirring for 4 h at room temperature. The resulting black composites were centrifuged, washed with deionized water and ethanol at least three times, and then dried in a vacuum oven at 80 1C overnight. The prepared core–shell structured hollow SnO2@PPy nanocomposites were extensively characterized by using X-ray diffraction (XRD, Rigaku D/MAX-RB), field emission scanning electron microscopy (FESEM, Hitachi S4800), transmission electron microscopy (TEM, FEI TECNAI G2), FT-IR (EQUINOX55) and TGA (STA-400). The material was fabricated into working electrodes consisting of 80 wt% active samples, 10 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF). A piece of lithium foil was used for the combined counter and reference electrode. 1.0 M LiPF6 dissolved in a 1:1 volume ratio mixture of ethylenecarbonate (EC) and dimethyl carbonate (DMC) was used as the electrolyte. All cells were assembled in an argon-filled glove box. The fabricated coin cell (CR2032) underwent galvanostatic charge–discharge cycling and cyclic voltammetry (CV) testing using a battery testing system (Arbin BT-2000) and an electrochemical workstation (CHI660B), respectively. The specific capacity was calculated based on the total weight of SnO2 and PPy. The charge–discharge tests were conducted at a rate of 0.1 C (1 C = 782 mA g 1) in a voltage window of 0.04–2.00 V (vs. Li/Li + ). The CV curves were measured in a potential range of 0.02–2.70 V (vs. Li/Li + ) at a scan rate of 0.5 mV s 1.
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Figure 2(b) shows the FT-IR spectra of pristine SnO2 hollow microspheres, PPy, and SnO2@PPy composites with different PPy contents. The bands in the range of 537– 623 cm 1 observed with the SnO2 hollow microspheres and the core–shell structured hollow SnO2@PPy composites can be assigned to the anti-symmetric and symmetric vibrations of Sn–O–Sn [32,33]. The bands centered at 1707 and 1596 cm 1 for both the SnO2@PPy composites and the PPy correspond to typical C =C in plane vibration. In addition, the characteristic bands of PPy for C–C and C–H ring stretching were found at 1400 and 1258 cm 1, respectively. The sharp peak at 1049 cm 1 is attributed to C–H in-plane vibrations. The band at 929 cm 1 can be assigned to N–H inplane vibrations [34,35]. Thus, these characteristic bands of PPy for both SnO2@PPy composites and PPy are nearly identical, except for the lower intensity observed with lower PPy content in the SnO2@PPy composites. The FT-IR results provide direct evidence that PPy is present in the SnO2@PPy nanocomposites. As shown in Figure 3, thermogravimetric analysis was used to determine the amount of PPy in the SnO2@PPy composites. The composite samples were heated from 25 to 800 1C in air, displaying two weight loss regions. The weight loss in the range of 25–250 1C is caused by the loss of chemisorbed water, and the loss in the range of 250–800 1C 100
SnO 13 wt% PPy
Figure 2(a) shows the XRD patterns of hollow SnO2, standalone PPy, and the core–shell SnO2@PPy composites. All the diffraction peaks of SnO2 and SnO2@PPy can be well indexed to the tetragonal rutile structure of SnO2 (cassiterite, JCPDS no. 41-1445). The identified diffraction peaks at 26.51, 33.81, 37.81, 51.81, 54.61, 57.81, 61.71, 65.11, 71.31 and 78.71 can be well assigned to (110), (101), (200), (211), (220), (002), (310), (112), (301), (202), and (321) planes of tetragonal SnO2. The peak intensity of the SnO2@PPy composites decreases with increasing PPy content, compared to the SnO2 sample. As for the PPy, no diffraction peaks can be detected except for the broad peak around 21.5–26.41 corresponding to amorphous PPy.
Figure 2
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Results and discussion
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76 is ascribed to the decomposition of PPy. The pure SnO2 hollow spheres show almost no weight change from 25 to 800 1C, while the PPy is nearly burnt out. The TGA analysis indicates that the PPy content in the nanocomposites is 13 wt%, 21 wt% and 29 wt%, respectively. Morphology of the composites was examined by SEM and TEM images, as shown in Figure 4 and Figure 5, respectively. A SEM image of a sample consisting of hollow SnO2 spheres is shown in Figure 4(a). The chemically polymerized PPy sample prepared from identical conditions exhibits granular morphology with diameters ranging from 300 to 500 nm. The morphology of samples containing hollow SnO2 microspheres covered by PPy layers, SnO2@PPy (21 wt%), is displayed in Figure 4(c and d). The PPy layers coated on the hollow SnO2 microspheres can be observed in these micrographs marked by red arrows. The particle diameters of the hollow SnO2@PPy composites with different PPy content are approximately equal (200 nm). Meanwhile, Figure 5(a, b) shows representative TEM images of hollow SnO2@PPy (21wt%) core–shell nanostructures, from which the outer shell of the hollow SnO2 microspheres is observed to be approximately 30 nm. A PPy layer coated on the SnO2 hollow spheres, with thickness around 25 nm, can also be observed. Moreover, a HR-TEM image in Figure 5(c) shows an interplanar spacing of about 0.335 nm, corresponding to the (110) planes of SnO2, which is in agreement with the XRD analysis. The cycling performance of the hollow SnO2@PPy core– shell nanocomposites is shown in Figure 6(a) at 0.1 C in the voltage window of 0.04–2.00 V, in comparison with those of the pure SnO2 hollow sphere anode and pure PPy anode. The PPy anode delivered a specific capacity of 29.7 mAh g 1 in the first discharge and retained only about 15 mAh g 1 in the subsequent cycles. Therefore, the low capacity of PPy
R. Liu et al. can be negligible in analysis of SnO2@PPy nanocomposites. As shown in Figure 6(a), the retained specific capacity for each sample is 406.5 mAh g 1 in the 65th cycle for the hollow SnO2 sphere anode, 281.1 mAh g 1 in the 96th cycle for the hollow SnO2@PPy (13 wt%) core–shell nanocomposite anode, 448.4 mAh g 1 in the 100th cycle for the hollow SnO2@PPy (21 wt%) nanocomposite anode, and 340.7 mAh g 1 in the 100th cycle for the hollow SnO2@PPy (29 wt%) core–shell nanocomposite anode. The measured performance with the core-shell structured SnO2–PPy, especially for the hollow SnO2@PPy (21 wt%) core–shell nanocomposite, is much higher than that of the standalone SnO2 hollow spheres, and those reported for SnO2 nanoparticles–PPy composites [27], SnO2 nano-single crystals [8], SnO2 hollow nanotubes [36], and mesoporous SnO2 on multiwalled carbon nanotubes (MWCNTs) [37]. Figure 6(b) shows the rate capabilities of SnO2@PPy nanocomposites from a current density of 78 mA g 1 (0.1 C) up to 3900 mA g 1 (5 C). During each high rate stage, the cell was cycled 10 times, followed by 5 cycles at low 0.1 C. The hollow SnO2@PPy core–shell nanocomposites display excellent rate capabilities. For example, the hollow SnO2@PPy (21 wt%) core–shell nanocomposites anode is still capable of delivering a substantial capacity above 1036.2, 614.0, 487.4, and 331.8 mAh g 1 at each current density of 78 (0.1 C), 390 (0.5 C), 780 (1.0 C), and 1560 (2.0 C) mA g 1, respectively. Even when the highest current density of 3900 mA g 1 (5.0 C) was applied, the composites exhibited a reversible capacity above 38.1 mAh g 1. It should be noted that, when the current rate was reversed back to low current (0.1 C) after 60 cycles, a specific discharge capacity of 614.9 mAh g 1 was recovered, indicating that 54.1% of the initial reversible capacity (1137.6 mAh g 1) was retained.
Figure 4 SEM images for (a) hollow SnO2 spheres (b) PPy, (c) and (d) hollow SnO2@PPy (21 wt%) core–shell nanocomposites (PPy layers marked by red arrows, covering onto SnO2 particles).
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Figure 5 Typical TEM images for the hollow SnO2@PPy (21 wt%) core–shell nanocomposites at different magnification.
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Figure 6 The cycling performance (a) and rate capability (b) for hollow SnO2@PPy core–shell nanocomposite anodes with different PPy contents.
Figure 7 EIS for hollow SnO2@PPy core–shell nanocomposite anodes with different PPy contents.
This result demonstrates that the hollow SnO2@PPy core–shell nanocomposites can tolerate varied discharge current densities, which is a desirable characteristic for high power application. It thus appears that PPy could work as a highly conductive matrix when anchored by hollow SnO2 microspheres. Meanwhile, the coulombic efficiency of a hollow SnO2@PPy (21 wt%) core–shell nanocomposite anode for the first cycle is only 54.3%, but it is raised to 90.7% for the 2nd cycle and up to 97% in the subsequent long time cycles. Compared to the hollow SnO2 microsphere anode, the coulombic efficiency of hollow SnO2@PPy (21 wt%) is more stable.
AC impedance measurements were performed on the hollow SnO2 microspheres and SnO2@PPy nanocomposite electrodes. Prior to the AC impedance testing, the electrodes were cycled galvanostatically for four cycles to ensure the stable formation of the SEI layers on the surface of the electroactive particles. The AC impedance experiments were then performed at 0.4 V in the 4th charge cycle. As shown in Figure 7, the Nyquist plot consists of a semicircle and a straight line, indicative of limitations in the charge transfer reaction (Rct) and the diffusion of Li + in the bulk electrode, respectively. The diameters of the semicircles for hollow SnO2 microspheres and SnO2@PPy nanocomposite electrodes are 162.8 Ω (0 wt% PPy), 148.5 Ω (13 wt% PPy), 107.5 Ω (21 wt% PPy) and 101.9 Ω (29 wt% PPy), respectively. The total resistance value is decreasing with an increase of PPy content, which is due to the good electronic conductivity of PPy. The result confirms that the incorporation of PPy coating is an effective method for enhancing the electron transport of hollow SnO2 microspheres, which leads to a significant improvement in the electrochemical performance. In the data discussed above, the hollow SnO2@PPy (21 wt%) core–shell nanocomposite anode exhibits the best cycling performance and rate capabilities, indicating that the ideal PPy content is around 21 wt%. The improved cycle stability and rate capabilities prompted us to further investigate the electrochemical reactivity and kinetics of SnO2@PPy (21 wt%) core–shell nanocomposite electrodes. At first, cyclic voltammetry was performed in a potential range of 0.02–2.70 V (vs. Li/Li + ) at a scan rate of 0.5 mV s 1.
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Figure 9 Cyclic voltammetry of hollow SnO2 microspheres (a) and hollow SnO2@PPy (21 wt%) core–shell nanocomposites anode (c). The scan rates are 0.1, 0.2, 0.3, 0.4 and 0.5 mV s 1, respectively. Plot of peak current (Ip) as a function of the square root of the scan rates (v1/2) for the hollow SnO2 microspheres (b) and the hollow SnO2@PPy (21 wt%) nanocomposites anode (d).
As shown in Figure 8(a), the CV curves of the hollow SnO2@PPy core–shell nanocomposites are in good agreement with the hollow SnO2 microsphere anodes [30], indicating that the PPy has no influence on the lithium insertion–extraction reaction. As described in Eq. (1), the peak at 0.71 V on the first cathodic scan is ascribed to the reduction of SnO2 to Sn and the formation of Li2O as well as the solid electrolyte interface (SEI) layer [20,33,38], accompanied with a large irreversible capacity loss in the initial cycle. The cathodic peak extending to 0.02 V and the anodic peak at 0.59 V are assigned to the alloying
and dealloying of Sn and Li, respectively [7], as described by Eq. (2). In the CV curves, an oxidation peak around 1.3 V and a reduction peak around 1.1 V are also clearly observed, which are most likely due to the partially reversible reaction of Eq. (1) [1,39], resulting in the formation and decomposition of nanosized Li2O during subsequent cycles. SnO2 + 4Li + + 4e -Sn+ 2Li2O
(1)
Sn + xLi + + xe 2LixSn (0 rx r 4.4)
(2)
SnO2-polypyrrole nanocomposite anodes with enhanced cyclic performance
Li+ insertion
Polypyrrole
79
Li+ extraction
SnO2
Li2O+LixSn
Li2O+Sn
Figure 10 Scheme of the role of PPy in confining the volume change of the hollow SnO2 microspheres during the Li-ion insertion/ extraction reactions.
Figure 11 SEM images of (a) a hollow SnO2 spheres electrode after 100 cycles, and (b) a hollow SnO2@PPy (21 wt%) core–shell nanocomposite electrode after 100 cycles.
Figure 8(b) shows the 1st, 2nd, 10th, 20th, 40th, 80th and 100th charge–discharge voltage profiles of the hollow SnO2@PPy (21 wt%) nanocomposite at a current density of 78 mA g 1 with a voltage range of 0.04–2.00 V. The initial small plateau in the potential range of 1.0 to 0.8 V corresponds to a conversion reaction between SnO2 and Li + , resulting in the formation of Sn and Li2O in the first discharge process. The following long slope profiles of the SnO2–21 wt% PPy nanocomposite indicate the formation of Li–Sn alloys. The plateau (from 1.0 to 0.8 V) almost disappears at the second cycle, demonstrating that most Li2O is formed in the first cycle. The first discharge capacity of the composites is 1168.1 mAh g 1. The initial irreversible capacity is as high as 45.7%, which is likely caused by the formation of SEI layers, electrolyte decomposition, and the irreversible reaction of SnO2 +4Li + +4e -Sn+2Li2O during the discharge process. It is worth noting that, after 10 cycles, the reversible cycling capacity is well retained, suggesting an excellent reversibility of lithium insertion/extraction reactions in the nanocomposite. Figure 9(a and c) displays the CVs of the hollow SnO2 microsphere electrode and the hollow SnO2@PPy (21 wt%) nanocomposite electrode at different sweep rates (0.1, 0.2, 0.3, 0.4 and 0.5 mV s 1). The dependence of peak current (Ip) on scan rate (v) was determined from the CVs. The diffusion coefficient of Li + ions (DLi) can be calculated from a linear relationship between Ip and v1/2 according to the following Eqs. [40,41]: n 1/2 Ip = 2.69 105n3/2AD1/2 Li CLiv
(3)
In the above equation, n is the number of electrons per reaction species (it is 1 for Li + ), A is the electrode area
(cm2), and CnLi is the bulk concentration of the Li + ion in the electrode (mol cm 3). Figure 9(b, d) shows a good linear relationship between Ip and v1/2 for both electrodes. The Li + diffusion coefficient in the SnO2@PPy (21 wt%) nanocomposite electrode is calculated to be 7.4 10 9 cm2 s 1. In comparison, the diffusion coefficient for the hollow SnO2 microsphere electrode shows a lower value of 1.2 10 9 cm2 s 1. This enhancement could be attributed to the unique hollow core– shell structure and conductive PPy coating, which serves as an efficient diffusion channel for Li + . The enhanced cycling durability can be attributed to the synergistic effect between the PPy matrix and the SnO2 hollow spheres, which mitigates the enormous volume change of SnO2 during charge–discharge and keeps the nanoparticles from agglomerating. A possible mechanism of this interaction, in which the PPy acts as a buffer to mitigate the volume change of hollow SnO2 microspheres, is proposed in Figure 10. During the lithium insertion process, SnO2 nanoparticles are converted to a mixture of Li2O and LixSn. In turn, metallic Sn is generated from the LixSn dealloying process during lithium extraction. The effects of the enormous volume change of SnO2 can be mitigated by the PPy coating, which serves as a buffer between adjacent SnO2 microspheres. Meanwhile, the inner hollow space can also mitigate the enormous volume change to some degree. Thus, the proposed synergistic effect between the PPy coating and the hollow structure can decrease the likelihood of pulverization of SnO2. In addition, the PPy coating is able to prevent the Sn nanoparticles from aggregating and provides good electrical contact within the nanocomposite. To support the proposed mechanism, the morphology of standalone SnO2 hollow spheres and hollow SnO2@PPy (21 wt%) core–shell nanocomposite electrodes after cycling was studied
80 by SEM, as shown in Figure 11. Serious cracks were observed on the standalone SnO2 hollow spheres electrode after 100 charge–discharge cycles, suggesting enormous volume changes during charge–discharge cycling (Figure 11a). On the other hand, as shown in Figure 11b, the hollow SnO2@PPy (21 wt%) core–shell nanocomposite electrode shows mitigated cracking after 100 cycles compared to the SnO2 hollow spheres electrode. These results indicate that the PPy buffer on the SnO2 hollow spheres was able to mitigate the large volume expansion and maintain the relative integrity of the electrode. Structural integrity and high conductivity are critical characteristics of LIB anode materials, so the changes caused by the presence of PPy in the hollow SnO2@PPy core–shell nanocomposite electrode represent significant improvements.
Conclusions In this work, a hollow SnO2–polypyrrole (PPy) core–shell nanocomposite was synthesized via a hydrothermal method followed by an in situ chemical-polymerization to form PPy layers on SnO2 hollow spheres. The nanocomposite was successfully used as an anode material for lithium ion batteries, exhibiting significantly enhanced cycling performance (448.4 mAh g 1 after 100 cycles) and excellent coulomb efficiency (97%), compared to the standalone hollow SnO2 spheres. The battery performance achieved by using the unique core–shell structures is much higher than that reported for any other SnO2 based anode material so far. The enhanced cycling performance is attributed to the unique hollow structure and the PPy buffer layers, which are found to be beneficial for alleviating the volume changes of SnO2 and agglomeration of generated Sn particles during the lithium insertion–extraction process. Meanwhile, the Li + diffusion coefficient in the hollow SnO2–21 wt % PPy core–shell nanocomposite electrode is significantly improved compared to the hollow SnO2 microsphere electrode. This work may open a new path toward designing SnO2 based nanocomposite anode materials for Li-ion batteries with improved cyclic stability.
Acknowledgments Financial support from the Los Alamos National Laboratory Early Career Laboratory-Directed Research and Development (LDRD) Program (20110483ER) for this work is gratefully acknowledged.
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Ruiqing Liu is a Ph.D. student at Harbin Institute Technology in Prof. Ning Li’s group. He obtained his Bachelor and Master degrees in Chemical Engineering from the same school in 2007 and 2009, respectively. His research focuses on the anode materials for Li-ion batteries.
Xujie Lü is a postdoctoral research associate at LANL & UNLV. He received his Ph.D. from Chinese Academy of Sciences (CAS) in 2011. His research interests include Li batteries, solid electrolytes, nanostructured photovoltaic materials, thin film depositions and High P/T techniques.
Deyu Li is a lecturer in School of Chemical Engineering and Technology at Harbin Institute of Technology in China. He received his PhD in the Applied Chemistry from Harbin Institute of Technology in 2014. His research mainly covers electronic electroplating and Li-ion batteries.
Jacob S. Spendelow is a scientist at Los Alamos National Laboratory (LANL). He received his Ph.D. from the University of Illinois at Urbana-Champaign in 2006, where he was funded by a NSF graduate research fellowship. He joined LANL in August 2006 as a Director's Postdoctoral Fellow. His current research interests are in the field of battery materials and fuel cell electrocatalysts.
Chen Wang received his BSc and MSc from Heilongjiang University, China. He is currently pursuing his Ph.D. at Harbin Institute of Technology with Prof. Ning Li. His research interests are hydrothermal synthesis of nanostructured transition-metal oxides to control their morphological and structural for anode materials in lithium-ion batteries.
Gang Wu is a scientist at Los Alamos National Laboratory (LANL). He completed his Ph.D. studies on Electrochemical Engineering at the Harbin Institute of Technology in 2004. After postdoctoral trainings at Tsinghua University, the University of South Carolina, and LANL, he became a technical staff member at LANL in 2010. His research focuses on functional materials and catalysts for batteries, fuel cells, and electrochemical sensors.
Ning Li is a professor of Chemical Engineering in the School of Chemical Engineering and Technology at Harbin Institute of Technology. She received his PhD in Environmental Engineering in 2000 from Harbin Institute of Technology. Her major research interests are batteries, metal materials corrosion mechanism and control, and principles and applications of electrodeposition and electroless deposition. Qing Li is a postdoctoral research associate at Los Alamos National Laboratory. He received his BSc from Wuhan University in 2005 and his Ph.D. from Peking University in 2010, both in chemistry. His research interests include functional nanomaterials and their applications in PEM fuel cells, metal-air batteries, and biosensors.
