Letter pubs.acs.org/NanoLett
Three-Dimensional Hierarchical Ternary Nanostructures for HighPerformance Li-Ion Battery Anodes Borui Liu,† Paulo Soares,† Constantine Checkles,† Yu Zhao,† and Guihua Yu*,† †
Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *
ABSTRACT: Silicon is considered one of the most promising anode materials for high-performance Li-ion batteries due to its 4200 mAh/g theoretical specific capacity, relative abundance, low cost, and environmental benignity. However, silicon experiences a dramatic volume change (∼300%) during full charge/discharge cycling, leading to severe capacity decay and poor cycling stability. Here, we report a three-dimensional (3D) ternary silicon nanoparticles/conducting polymer/carbon nanotubes hybrid anode material for Li-ion batteries. The hierarchical conductive hydrogel framework with carbon nanotubes as the electronic fortifier offers a continuous electron transport network and high porosity to accommodate the volume expansion of Si particles. By 3D wrapping of silicon nanoparticles/single-wall carbon nanotubes with conducting polymer nanostructures, a greatly improved cycling performance is achieved with reversible discharge capacity over 1600 mAh/g and 86% capacity retention over 1000 cycles at the current rate of 3.3 A/g. Our findings represent a new direction for fabricating robust, high-performance lithium-ion batteries and related energy storage applications with advanced nanostructured materials. KEYWORDS: Silicon anode, hybrid nanostructures, conductive polymer hydrogel, Li-ion battery, energy storage
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caused by the formation of an unstable solid-electrolyteinterphase (SEI) on the Si surface.5 The severe volume change results in the pulverization of Si and rapid capacity fading of the battery. Second, the electronic conductivity of elemental Si is poor, thus greatly impeding electron transfer and reducing rate capability of the electrode.6 To mitigate the above-mentioned challenges, major efforts have been centered on the development of novel nanostructures of silicon anodes, such as nanocrystals,7,8 nanowires,3,9,10 core−shell nanofibers,11 nanotubes,12−14 nanospheres,15,16 nanoporous materials17,18 and amorphous-Si/carbon nanocomposites,19,20 showing an improved cyclability over micrometer-sized Si. Nevertheless, high energy-consuming apparatuses and costly synthesis processes of the aforementioned Sibased nanostructured materials can hardly be avoided, resulting in their limited potential for scalable manufacturing.21 Silicon
here is an increasing demand for next-generation energy storage devices and systems with high energy and power densities, as well as cost-effective and facile strategies to provide sustainable power for modern lifestyles.1,2 Compared to traditional energy storage devices such as lead acid batteries, nickel metal hydroxide batteries, and a number of other wellknown batteries, rechargeable lithium-ion (Li-ion) batteries are considered one of the most promising candidates for a variety of electrical energy applications due to their relatively highoperational voltage, high-energy density, and long cycle life. Silicon (Si) is widely regarded as a very promising anode material for high-performance Li-ion batteries, possessing an ultrahigh specific capacity of ∼4200 mAh/g, which is theoretically ten times higher than that of commercialized graphite anodes.2−4 Moreover, Si has potential for low-cost exploration and production as it has large available reserves. However, two significant challenges of silicon anode have to be solved before practical application. First, the huge volumetric expansion and contraction associated with Si−Li alloying/ dealloying lead to continuous decomposition of the electrolyte © XXXX American Chemical Society
Received: May 22, 2013 Revised: June 14, 2013
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Figure 1. Ternary electrode structure design and fabrication. (a) Schematic illustration of the formation of 3D Si/PPy/CNT ternary electrode. Each Si nanoparticle is encapsulated within a thin polymeric coating layer and closely incorporated within the conductive PPy framework. The SWCNTs act both as the wrapping layer and conductive backbone, which further enhance the integration of SiNPs/conductive polymer framework and electric conductivity of the electrode. (b) Digital image shows that the Si/PPy/CNT slurry can be uniformly coated onto the copper foil in large scale. Left, the as-prepared homogeneous Si/PPy/CNT dark green slurry solution.
after 1000 deep cycles in the potential window of 0.01−1 V versus Li/Li+. This remarkable performance arises from the unique structures: the in situ polymerization forms a bifunctional conformal coating,34 binding to the Si particles and serving as continuous 3D pathways for electronic conduction incorporated with the highly conductive SWCNTs framework. Such a unique synthetic approach is also applicable to other alloy-type anode materials, offering 3D architecture to help improve the rate capability and cyclability of the whole battery. Furthermore, the all-solution-based process represents a potentially scalable preparation method toward practical industrial manufacturing. The detailed structure of Si/PPy/CNT ternary composite was schematically illustrated in Figure 1a. The formation of the 3D conductive framework is constructed by the nanostructured PPy hydrogel, offering both nanoscale pores (20−50 nm) to facilitate the penetration of electrolyte ions and micrometersized pores to accommodate the SiNPs and their volume changes in the charge/discharge processes. Simultaneously, a thin in situ polymeric layer (∼15 nm) was coated on the surface of SiNPs, which can link the SiNPs with the external conductive polymeric framework to provide the electrical connections of the interior material to further improve the electrochemical performance, especially high-rate performance. The affinity between the SiNPs and the in situ polymer coating could be attributed to the synergism of the electrostatic effect between the negatively charged −OH groups and positively charged polypyrrole polymer backbone and the hydrogen
nanoparticles (SiNPs) have been recently investigated as a practically scalable candidate for an active anode material. Most recent research efforts seeking novel electronically conductive battery binders other than the conventional polyvinylidene difluoride (PVDF)22−25 have been reported to help improve battery performance on account of the good adhesion between the functional groups on the polymers and the silicon oxide layer on SiNPs surface and/or the high elastic modulus of polymers. 26,27 For example, carboxy-methyl cellulose (CMC),28,29 poly(acrylic acid) (PAA),11,14 poly(9,9-dioctylfluorene-co-suorenone-co-methylbenzoic acid) (PFFOMB),30 alginate31,32 and mussel-inspired adhesive binders33 have been synthesized and applied in electrode materials to improve battery cycle life.30 Here, we report a novel silicon anode composed of SiNPs wrapped in a three-dimensional (3D) hierarchically porous nanostructured conductive polypyrrole (PPy) framework with single-walled carbon nanotubes (SWCNTs) as the electronic fortifier as shown in Figure 1a. Conducting polymer nanostructures offer unique benefits in Si-based battery electrodes, such as a 3D hierarchically porous framework and good electrical and electrochemical properties, which effectively improve the transport of both electrons and ions due to shortened diffusion pathways and 3D interconnectivity. Using the hierarchically designed 3D ternary nanostructured electrode, a reversible capacity of ∼1600 mAh/g and an average Coulombic efficiency of ∼99.5% over 1000 cycles have been achieved. Moreover, the capacity retention sustained over 85% B
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Figure 2. Structural characterization of 3D Si/PPy/CNT nanostructures. (a,b) SEM images of Si/PPy/CNT hierarchical nanostructured ternary electrode. (c,d) TEM image and elemental mapping of in situ formed polymeric coating on the surface of Si nanoparticles. The average diameter of a silicon nanoparticle is about 50 nm with a ∼15 nm PPy shell on average.
the hierarchically porous nanostructure from the hydrogel precursor shown in Supporting Information Figure S1a with SiNPs uniformly distributed inside the mesopores. The CNTs interlaced on and across the Si/PPy bulk. SWCNTs are about several micrometers long with an average diameter of ∼5 nm. The in situ formed polymeric layer on SiNPs surface was ∼15 nm thick, as shown in Supporting Information Figure S1b. The TEM image and elemental mapping of carbon (red) and silicon (green) revealed that the polymeric coating on SiNPs was uniform as present in Figure 2c,d. It is of noting that the PPy may not fulfill the capacity demand at very high rates due to still limited conductivity of PPy fibers, which could hinder fast and efficient electron transport and collection on current collectors. The highly conductive SWCNTs can effectively improve the electronic conductivity of the entire Si/PPy composite due to their excellent electrical conductivity and additional beneficial effect of physical “wrapping”. The 3D hierarchical ternary electrode exhibited excellent electrochemical characteristics. Electrochemical performance of SiNPs/PPy/CNT ternary electrodes was first examined using electrochemical impedance spectroscopy (EIS) to get insight into the stability of lithiation/delithiation of the ternary electrodes. Figure 3a shows the first ten Nyquist plots of the Si/PPy/CNT electrode in the frequency range of 0.1 Hz to 1 MHz, which was carried out at the lithiated state (10 mV vs Li/ Li+) at room temperature. The typical impedance spectrum was composed of a semicircle at high-to-middle frequency range and a straight line within the low frequency range which represents the diffusion-controlled process in the solid electrode. The impedance of the depressed semicircle was attributed to the resistances from the electrolyte, charge transfer and interfaces, with the resistance of charge transfer dominating the total resistance of the electrode.37 A decrease in the total resistance was observed between the first and subsequent plots, which could be attributed to the increasing conductivity of the ternary electrode during lithiation.38 No evident impedance increase was detected in the subsequent cycles, indicating the formation of a stable SEI layer owing to the unique hybrid electrode design and efficient electronic/ionic transport through the 3D hierarchically porous electrodes during cycling.39 Si/PPy/CNT ternary electrodes after 1,000 cycles
bonding between phosphoric acid groups in the phytic acid molecules and native SiO2 on the Si particle surfaces. Meanwhile, the SWCNTs are effectively intertwined with Si/ PPy framework and embedded throughout the whole electrode. The benefit of this unique ternary structure relies in the electrode integrality in long-term operation. Since part of the SiNPs may detach from the electronically conductive framework after a number of cycles due to the huge volume expansion/contraction, the 3D wrapping effect of SWCNTs can potentially help better confine the SiNPs in the polymer framework, thus maintaining the electrical connection within the entire electrode framework.35 The interconnected, conductive framework of the Si/PPy/CNT composite material ensures that the electrons can be effectively transported from the polymeric matrix and SWCNTs to the poorly conductive SiNPs. To achieve the 3D ternary electrode, pyrrole, which served as the monomer, and phytic acid, which served as both the crosslinker and dopant, were first added into the initiator solution to form an ivory white mixture. Then the as-prepared hydrogel precursor was mixed with SiNPs and SWCNTs dispersion to form the Si/PPy/CNT slurry. Afterward, the resultant viscous dark green Si/PPy/CNT ternary slurry was uniformly bladed onto a 5 cm × 25 cm sized copper (Cu) foil (Figure 1b). Finally, after 8 h steeping in deionized (DI) water and 12 h drying in vacuum oven, the electrode film was subjected to the calendar process to achieve better adhesion of the electrode material onto the Cu current collector. This solution-based synthesis method is potentially compatible with roll-to-roll coating methods, which makes the system readily scalable for large-area electrode films.36 Scanning electron microscope (SEM) studies show the structural information of the Si/PPy/CNT ternary electrode. SEM images (Figure 2a,b) demonstrated the hierarchically porous nanostructures of 3D ternary electrode material. The 20−50 nm mesopores can enhance the electrolyte penetration and Li-ion diffusion in the electrode, while the micrometersized pores can offer free space to spatially accommodate the volume expansion of the SiNPs during electrochemical cycling. After in situ polymerization with the coexistence of SiNPs and SWCNTs (Figure 2a,b), the formed PPy framework inherited C
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Figure 3. Electrochemical performance of Si/PPy/CNT ternary electrodes. (a) EIS spectra of the hybrid electrode in the frequency range between 0.1 Hz and 1 MHz. (b) A typical CV profile of the Si/PPy/CNT electrode at a scanning rate of 0.1 mV/s between 0.01 and 1.0 V (versus Li/Li+). (c) Voltage profiles of Si/PPy/CNT electrode at various current rates of 0.5, 1.0, 2.0, 4.0, and 8.3 A/g. (d) Rate performance demonstration of Si/ PPy/CNT electrode at 0.5, 1.0, 2.0, 4.0, and 8.3 A/g rates.
mg/cm2, SWCNTs only accounts for ∼0.2 wt %). The average discharge capacity at 0.5 A/g was ∼3100 mAh/g, which was ∼400 mAh/g higher than that of the Si/PPy electrode of the similar material loading at the same rate (Figure 3d). The higher discharge capacity was due to the enhanced electronic conductivity of the hybrid Si/PPy/CNT ternary electrode compared to the binary electrode without SWCNTs, of which the discharge capacity as well as the reversible capacity was much lower (Supporting Information Figure S2c,d). Overall, the capacity retention for Si/PPy/CNT electrode was observed to be ∼35% when the current rate was increased from 0.5 to 8.3 A/g, while only ∼20% capacity retention was observed for the case of Si/PPy electrode. The improved rate performance of the Si/PPy/CNT ternary anode compared to Si/PPy further confirms the advantageous feature of SWCNTs as the electronic fortifier. The 3D hierarchical ternary electrode also exhibited excellent cycling performance. The cycling performance of the Si/PPy/ CNT ternary electrode was measured using galvanostatic charge/discharge characterization. Figure 4a shows the discharge capacity of the ternary electrode along with those of binary Si/PPy and Si/PVDF electrodes at a current rate of 3.3 A/g. The first discharge capacity of the ternary electrode reached as high as ∼3600 mAh/g, approximately 10 times higher that of the commercial graphite anode (372 mAh/g). The ternary electrode achieved a stable discharge capacity of ∼1600 mAh/g over 1000 cycles with capacity retention of ∼86%. The Coulombic efficiency was 78.2% for the first cycle and readily increased to 99% in the second cycle and stabilized at ∼99.5% in the subsequent cycles. In contrast, the binary Si/ PPy electrode showed a discharge capacity of ∼1100 mAh/g after 1000 cycles with capacity retention of ∼78%, while the Si/ PVDF electrode dramatically decreased capacity in the first 100 cycles and showed negligible capacity at the end of 200 cycles. The capacity-controlled galvanostatic charge/discharge behavior of the ternary electrode was further examined at high current rate. Figure 4b shows the reversible Li-extraction capacity and Coulombic efficiency versus cycle number for the
confirmed a thin SEI layer stably formed on the electrode surface. Meanwhile, EIS plots for Si/PPy electrode shown in Supporting Information Figure S2a is similar with that of Si/ PPy/CNT electrode, indicating the advantage of the conductive PPy 3D framework. In contrast, only bulky, nonuniform, thick SEI layer was observed from the regular Si anode after 200 cycles. To further characterize the electrochemical performance, voltammetry measurements were carried out for the Si/PPy/ CNT electrodes. The cyclic voltammogram (CV) profile of the Si/PPy/CNT electrode at a scanning rate of 0.1 mV/s in the voltage range of 0.01−1.0 V versus Li/Li+ after 10 cycles is shown in Figure 3b. A strong redox peak at around 0.2 V and two well-defined redox peaks at 0.4−0.6 V could be clearly observed, which were consistent with previous reports.3,40 During the lithium insertion process, the beginning of the Si− Li alloying reaction can be observed at around 0.33 V, and the cathodic current reached the highest value at ∼0.2 V, corresponding to the formation of different phases of Li12Si7 and Li15Si4, respectively.41,42 The Si delithiation curve showed two typical redox peaks at ∼0.37 and 0.51 V, corresponding to the anodic process of amorphous (α) α-LixSi converted to αSi.19 The magnitude of all current peaks increases with cycle number in the first few cycles due to the gradually activation of SiNPs that reacted with lithium. The potential separation between the anodic peak and cathodic couple peaks was widened without the addition of SWCNTs as shown in Supporting Information Figure S2b. This alternation indicates that the electrochemical reversibility of the ternery electrode was improved by the addition of SWCNTs to Si/PPy binary electrode system. The 3D hierarchical ternary electrode exhibited the enhanced rate capability compared to Si/PPy binary electrode system. Figure 3c shows the charge/discharge profiles of the ternary electrode performed at various current densities ranging from 0.5 to 8.3 A/g. All the specific capacities were calculated based on the weight of SiNPs, which takes up about 70% mass fraction of the whole electrode (typical mass loading of 0.3−0.5 D
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Figure 4. Cycling performance and structural characterization of hybrid electrodes after long-term cycling. (a) Electrochemical cycling performance of three different electrodes Si/PPy/CNT (red), Si/PPy (green), and Si/PVDF (black) and Coulombic efficiency of Si/PPy/CNT electrode (blue) under deep charge/discharge cycles from 1 V to 0.01 V versus Li/Li+ at a current rate of 3.3 A/g. (b) Discharge capacity and Coulombic efficiency versus cycle number for Si/PPy/CNT ternary electrode were examined during the galvanostatic charge/discharge at constant charge capacity limited to 1000 mAh/g at a current rate of 8.6 A/g. (c) SEM image of the composite anode after cycling with SEI layer removed. (d) TEM image of the extensively cycled composite anode with SEI layer removed and elemental mapping of the designated area (marked in a rectangle).
changes of the Si NPs during lithiation and delithiation. Even though the surface morphology of the SiNPs deviated to a certain extent from the original spherical shape upon long-term cycling, most physical connections were well conserved, resulting in superior electrochemical and cycling stability of the Si/PPy/CNT electrodes. Second, based on the rate capability measurement, the acid-doped electronically conductive PPy framework has a limited electron transport/ conduction capability that can be greatly enhanced by introduction of SWCNTs additive to further enhance the physical connections and electrical contact of SiNPs with the external 3D conductive framework, thereby maximizing the effective electrochemical utilization of the active materials and ensuring a reversible lithium insertion/extraction process even at high current rates. Last, the in situ formed conductive polymer coating on the surface of the SiNPs not only prevents Si particles from total structural fracture but also helps maintain a stable SEI layer and keep the electrolyte from further decomposition while still allowing for Li+ ion transport within Si NPs. In conclusion, we present a unique 3D ternary Si/PPy/CNT nanostructured electrode as a high-performance Li-ion battery anode using a scalable, low-cost, and all-solution-processed method. The as-prepared ternary electrode exhibits long-term stability upon cycling, superior rate capability, and relatively high discharge capacity. The excellent electrochemical performance arises from the 3D hierarchically porous PPy fibers that effectively accommodate the volume change of SiNPs during charge/discharge and synergize with CNTs to maintain good electrical conductivity throughout the electrode. This promising material design and the concept of the scalable synthesis method are expected to be useful for other alloy-type anode such as germanium, tin, and tin oxides.
ternary electrode at the current rate of 8.6 A/g with discharge capacity limited to 1000 mAh/g. As clearly shown, stable, constant 1000 mAh/g capacity cycling was readily achieved for the measured 2000 cycles. The Coulombic efficiency is ∼76% for the first cycle and over 99% for the subsequent cycles. The greatly improved cycling performance and life-span of the electrode could be attributed to the stabilized SiNPs well confined within the 3D conductive framework composed of both 3D PPy hydrogels and SWCNTs. Figure 4c shows the SEM image of the cycled ternary electrode after the removal of SEI layer. The intrinsic 3D porous structure remained barely changed. Though the ternary electrode was extensively deformed upon deep cycling, the electrical connection between the SiNPs and PPy fibers/SWCNTs were relatively intact owing to the in situ formed polymeric layer on the surface of SiNPs as well as the 3D porous structure of the electrode. The polymeric coating on SiNPs surface was well maintained after the cycling test as confirmed in Figure 4d (Supporting Information Figure S3 also shows the 3D framework was still kept well), in which the SiNPs were found to be still covered by a homogeneous carbon coating. Overall, the cycling performance of the ternary electrode is significantly improved compared to previously reported Si anodes due to its desirable features, such as lessened internal strain of the electrode, enhanced electronic conductivity, and shortened pathway length for ion transport. The stable cycling performance of the Si/PPy/CNT ternary electrodes can be possibly attributed to the following reasons. First, the 3D hierarchically porous structure of the PPy framework can be well preserved during the electrochemical cycling process, which does not only help maintain efficient electron and ion transport between the electrolyte and the electrode, but also mechanically accommodate the vast volume E
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ASSOCIATED CONTENT
S Supporting Information *
Detailed experimental procedures and supplementary characterization methods including spectroscopic and electrochemical measurements and electrochemical performance of Si/PPy electrode control samples. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank Professor John Goodenough and Professor Arumugam Manthiram for the initial instrumental support and helpful discussions. G.Y. acknowledges the financial support of the startup grant from Cockrell School of Engineering at the University of Texas at Austin and of Ralph E. Junior Faculty Enhancement award grant.
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