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A composite electrode of tin dioxide and carbon nanotubes and its role as negative electrode in lithium ion hybrid capacitor
Electrochimica Acta 209 (2016) 332–340
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
A composite electrode of tin dioxide and carbon nanotubes and its role as negative electrode in lithium ion hybrid capacitor Chung-Lun Hsieh, Dah-Shyang Tsai* , Wei-Wen Chiang, Yen-Heng Liu Department of Chemical Engineering, National Taiwan University of Science and Technology, 43, Keelung Road, Section 4, Taipei 10607, Taiwan
A R T I C L E I N F O
Article history: Received 29 January 2016 Received in revised form 12 May 2016 Accepted 12 May 2016 Available online 13 May 2016 Keywords: lithium ion capacitor hybrid capacitor tin dioxide negative electrode carbon nanotubes current dependence
A B S T R A C T
A nanostructured electrode, denoted as SnO2/Cu/CNT, is prepared with tin dioxide and copper coated on the carbon nanotubes (CNT) backbone. Before coating of tin and copper, the CNT surface has been electrochemically oxidized and examined with the C1s photoelectron spectrum to ensure a minimum oxidation level while sufficient for electroless plating. With the prelithiated SnO2/Cu/CNT electrode, matching with an activated carbon (AC) electrode, this lithium ion hybrid capacitor taps into the lithium storage aptitude of tin dioxide for reversible capacity. Thus three capacitors, of the same AC loading, are assembled and studied with the AC:SnO2/Cu/CNT mass ratio 1:1, 1:0.67, 1:0.33. Their energy storage capacities are similar in magnitude at 0.1 A g1, around 90 Wh kg1 in a 3.8 V window, but the 1:0.67 cell surpasses the other two cells when the charging current increases. Superiority of the 1:0.67 cell on energy and electric charge storage is discussed, considering the SnO2/Cu/CNT role in capacitor. The discussion reveals the prelithiated SnO2/Cu/CNT has kept the potential at 0% state of charge (U0%SOC) low to constrict the maximum positive potential. But U0%SOC should avoid forcing the negative potential descends lower than 0.15 V (vs. Li/Li+), where the kinetic barrier of SnO2/Cu/CNT becomes problematic at high rates. ã 2016 Published by Elsevier Ltd.
1. Introduction The lithium ion hybrid capacitor (LIHC) bridges the gap between electrochemical capacitors and rechargeable batteries by means of trading its electrode of double-layer capacitance for a more energetic lithium storage electrode. In those commercial devices, prelithiated graphite or other carbon electrodes serve the purpose of lithium storage, preventing the electrolyte from lithium shortage in the course of cell charging. The energetic battery electrode raises the cell capacity, yet increases the cell internal resistance as well, because of its associated kinetic barriers of faraday reactions and diffusion. This perception implies there is room for improvement on the power of LIHC through searching for a fast-response battery electrode [1–5]. In light of the literature on battery research, enhancement of the redox reaction rates has often been rewarded through optimization of the electrolyteelectrode interface. A proven approach to alter the interface is to modify the electrodes with nanometer-sized particles or nanostructured composites, which drive many advances of today’s
* Corresponding author. E-mail address: [email protected] (D.-S. Tsai). http://dx.doi.org/10.1016/j.electacta.2016.05.090 0013-4686/ã 2016 Published by Elsevier Ltd.
battery technology [6,7]. Hence attention of this work is focused on a nanostructured electrode based on tin dioxide. As an anode component, tin and its oxides attract a great deal of attention because of their large capacities and cost-effectiveness [8,9], especially in form of dioxide [10–16]. Lithiation of SnO2 is usually divided into two steps. The first step involves reduction to tin metal with four lithium ions and four electrons, SnO2 + 4 e + 4 Li+ ! Sn + 2 Li2O, which yields a theoretical capacity of 710 mAh g1. Unfortunately, a large fraction of this capacity is not reversible [17–19]. The second lithiation reaction further reduces tin metal into LixSn alloys, Sn + x e + x Li+ ! LixSn. Depending on the end product, the theoretical capacity of this reaction differs somewhat. When the end alloy composition is Li22Sn5 [20,21], the storage of 4.4 lithium atoms per tin atom gives a theoretical capacity of 781 mAh g1, based on the mass of tin dioxide, not tin metal. When the end alloy compound is thought to be Li17Sn4, x = 4.25 instead of 4.4, the magnitude of theoretical capacity is somewhat less [22,23]. According to the literature, capacity of the second step is more reversible than that of the first step [24]. On the other hand, the second step is also plagued with pulverization problems due to a large volume expansion during lithium alloying. Addition of carbon materials has been reported effective in relaxing the tin lithiation stresses [11,18,21].
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Another approach for upgrading capacity retention is to implement tin dioxide nanowires or nanotubes of long geometric feature [25–29]. In this work, we have taken considerable care to prepare porous electrodes made of multi-walled carbon nanotubes (CNT) coated with tin dioxide and a small amount of copper. Multiwalled CNT is chosen as the backbone of the composites for one dimensional feature and electrical connection improvement. A uniform tin layer is coated through electroless plating with a minimum level of CNT surface oxidation, which prevents oxidation from damaging the conductivity of CNT. The composite electrode is prelithiated and applied as negative for the hybrid capacitor. We discuss the capacity and structure features of this composite electrode when it performs independently, and emphasize the current-dependent capacities and its behavior when the electrode is in series with an activated carbon (AC) positive electrode in a LIHC. 2. Experimental 2.1. Preparation of positive and negative electrodes Our preparations began with two pastes of active materials for positive and negative electrodes. The paste for positive electrode contained 0.80 g AC powder (YP-80F, Kuraray Chemical), 0.10 g Super P (Timcal Carbon Black), 0.10 g polyvinylidene fluoride binder (PVdF, Sigma-Aldrich), and 10 mL N-methyl-2-pyrrolidone (NMP, ACROS). These components are vigorously blended and vacuumed to remove bubbles. A calculated amount of AC paste was then loaded on the aluminum current collector of 2 cm in diameter, and dried at 60 C in a vacuum chamber. Preparation of the negative electrode was more complicated. We first weighed 0.6 g multi-walled CNT (UR-NTM003, Nanostructured and Amorphous Materials) and immersed it in an oxidizing acid solution of 9.0 mL sulfuric acid (95%) and 3.0 mL nitric acid (65%) at 60 C for 5 min. The CNT oxidation level could be adjusted through varying the bath time of this electrochemical oxidation to understand its processing influences. After oxidization, the CNT powder was washed with a large amount of deionized water, and dried at 80 C. The following electroless plating of tin and copper was performed in two steps. First, a solution mixture of oxidized CNT was blended with two solutions at 60 C, a 200 mL solution containing 0.6 wt% CuSO4 and 0.6 wt% EDTA, another 200 mL solution containing 0.35 wt% SnCl2. Second, tin and copper plating took place when a 200 mL solution of 0.1 wt % NaBH4 reducing agent was added with magnetic stirring at 60 C. After the electroless plating, the powder was washed with sufficient amount of deionized water. The powder was collected and dried in air to oxidize tin at 100 C. The dried powder was denoted as SnO2/Cu/CNT, to be used in electrode preparation. Prior to adopting electroless plating Sn/Cu on CNT, we had tried electrochemical deposition. But it was very difficult to achieve uniform coating on a dense CNT forest. And if nanotubes were dispersed, it would be more difficult and expensive to establish electric contacts with many nanotubes and recollect them. The paste for negative electrode was prepared with 0.4 g SnO2/ Cu/CNT and 0.04 g PVdF binder in 40 mL NMP, mixed in a ultrasonic cleaner (DC150H, DELTA) and vacuumed in a chamber to remove bubbles. A cleansed copper plate of 2 cm in diameter was loaded with the SnO2/Cu/CNT paste and dried at 60 C in a vacuum chamber for 12 h. The mass of SnO2/Cu/CNT was recorded as the weight difference before and after drying, so was the AC mass of positive electrode. Before assembling the capacitor, the SnO2/Cu/ CNT electrode was lithiated from 2.0 to 0.01 V (vs. Li/Li+) in a threeelectrode setup. Then, the lithiated SnO2/Cu/CNT electrode, ended at 0.01 V (vs. Li/Li+) in prelithiation, was assembled as negative in a test cell of Swagelok type (ECC-COMBI, EL-CELL), matching with
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the positive AC electrode. The electrolyte was 1.0 M LiPF6 solution for lithium ion batteries (EC: DMC 50:50 by volume, LB-301). And a polypropylene porous film of 25 mm thick (Celgard 2500) was the separator. 2.2. Assembling capacitors Prelithiation of the SnO2/Cu/CNT electrode and its capacity measurements were performed using the three-electrode setup with a lithium wire as reference and a lithium plate as counter electrode. And two layers of Celgard 2500 were used in this halfcell configuration. The capacitance of the positive electrode was also evaluated with cycle voltammetry from 2.5 to 4.0 V (vs. Li/Li+) in this assembly. Before measurement, each porous electrode was soaked in the electrolyte for 10 h to wet their porous interfaces thoroughly. The assembling procedure and the measurements were conducted in a glove box (GB-100, SunRay Science) with an argon atmosphere of water and oxygen impurities less than 1 ppm. Three full cells were assembled with the AC:SnO2/Cu/CNT mass ratio 1:1, 1:0.67, 1:0.33. The AC loading of positive electrode was fixed at 0.85 mg cm2. And the SnO2/Cu/CNT loading was 0.85, 0.56, 0.28 mg cm2 for the negative electrodes of 1:1, 1:0.67, and 1:0.33 cells; individually. Since the SnO2/Cu/CNT electrode had been prelithiated, the open circuit voltage (OCV) of the assembled full cell was not zero. The OCV values of 1:1, 1:0.67, 1:0.33 cells were 0.86 V, 0.94 V, 1.10 V; individually. 2.3. Materials characterization Phase analysis was performed with an X-ray diffractometer (D2 Phaser, Bruker), equipped with the CuKa radiation source and nickel filter. Morphology of the porous electrode was examined using a scanning electron microscope (SEM, JSM-6500F JEOL), equipped with an energy dispersive X-ray microanalyzer (EDX, INCA, Oxford). X-ray photoelectron spectra (XPS) of CNT were collected using a Thermo VG Scientific Theta Probe system operated at 7 1010 mbar. The Al Ka 1486.6 eV was the X-ray source, and the Ag 3d5/2 line at 368.26 eV was the calibration reference. Values of binding energy were determined through curve fitting with 30% Lorentzian and 70% Gaussian curves and Shirley baselines using the Avantage v3.2 software. Surface area of activated carbon was characterized using nitrogen adsorption and desorption isotherms, measured with a surface-area and pore-size analyzer (ASAP2020, Micromeritics). 3. Results and discussion 3.1. Characterization of SnO2/Cu/CNT and its preparation Fig. 1 presents the X-ray diffraction pattern of SnO2/Cu/CNT powder, which is dominated by the distinct features of tin dioxide crystallized in the cassiterite structure. The pattern of SnO2/Cu/ CNT also contains two or three less pronounced diffraction peaks of CNT, which are made legible when contrasted with the diffraction result of as-received CNT sample, as shown in Fig. 1. The strongest (002) diffraction line of CNT is located near that of (110) diffraction of SnO2. A combination of the two lines results in the strongest and broad feature at 26.7. It is logical to infer drying in air oxidized the electroless-plated tin metal, and produced SnO2 of the cassiterite structure. The diffraction results do not show any phase related to the minor component, copper. We believe copper exists in the metallic state since appreciable oxidation of copper metal starts at 350 C [30], and our processing temperature did not exceed 100 C. In the preliminary attempts to prepare the SnO2/Cu/CNT powder with electroless plating, we quickly realize that oxidation of CNT is indispensable to tin coating. Without proper oxidation,
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Fig. 1. X-ray diffraction results of the SnO2/Cu/CNT electrode. The SnO2/Cu/CNT pattern is dominated with the SnO2 cassiterite diffraction lines, which are indexed to the rutile lattice and marked in red. Note that the CNT diffractions, marked in black, are also involved.
the as-received CNT surface is hydrophobic, not accessible to nucleation and growth of metallic tin film. Addition of reducing sodium borohydride generally yields detached tin metal precipitates, not a film on CNT. On the other hand, electrochemical oxidation degrades, unfortunately, the one dimensional conductivity of CNT [31], a crucial reason that CNT is chosen as the backbone of electrode. To keep the conductivity damage at a minimum, we evaluate the conducting ability of oxidized CNT using the p-p* contribution in its C1s spectrum, and search for a compromise between uniform coating and conducting ability. Later the morphology of CNT surface is inspected to assure a uniform tin film. Fig. 2a–c illustrate variations in the C1s spectrum of CNT surface after electrochemical oxidation in the mixture of sulfuric and nitric acids for 0, 5, and 15 min. Fig. 2a is the C1s spectrum of as-received CNT, showing a major feature at binding energy (BE) 284.4 eV with an elongated shoulder in the high BE range. Following the literature report, the 284.4 eV feature is assigned to graphitic carbon (sp2 hybridized carbon). The positive-shifted features, extending from BE 286.7 to 291.5 eV, are considered as the contributions of carbon impurities and functionalized carbon. The 285.0-285.5 eV contribution is allocated to CNT defects, for example, sp3 carbon. Beyond the defect peak, the extended feature is deconvoluted into the components of CO, O C¼O, CO3, and the p-p* shake-up [31–33]. Fig. 2b and c show the diminished p-p* shake-up features of properly and excessively oxidized surfaces. Deconvolution results reveal the percentage of p-p* component decreases with increasing oxidation time, 6.2% (0 min), 4.5% (5 min), and 2.2% (15 min). Since a high p-p* percentage symbolizes a superior CNT conductivity and we cannot achieve uniform tin coating if oxidization persists less than 5 min, therefore, 5 min is chosen as the CNT oxidation time in preparation of the SnO2/Cu/CNT electrode. Fig. 3 presents the morphological features of SnO2/Cu/CNT electrode at three processing stages. The diameter of as-received CNT ranges from 15 to 25 nm, and the length varies from 10 to 30 mm, as shown in Fig. 3a. Electroless plating of tin and copper increases the diameter of multi- walled CNT evenly, up to 25– 90 nm, while the tube length seems to be cut short due to processing, Fig. 3b. The tube diameter increases further, up to 35 100 nm after prelithiation, the tube length ranges between 1 and 5 mm, as shown in Fig. 3c. EDX analysis of tin-copper plated electrode, at four selected positions of SnO2/Cu/CNT, yields the
Fig. 2. XPS results of CNT with increasing levels of oxidation. C1s spectra of the CNT samples that have undergone electrochemical oxidation in the mixed sulfuric and nitric acids at 60 C for (a) 0, (b) 5, (c) 15 min. The oxidation level increases with increasing oxidation time. In addition to the graphitic carbon (main) peak and the defect (next to main) peak, BE values of the rest assignments are given as follows; 286.5–287.0 eV (C-O), 288.0–288.5 eV (O C¼O), 289.5–290.0 eV (CO3), 291.0– 291.5 eV(p-p*).
average compositions as follows, carbon 43.5 wt%, copper 14.7 wt%, tin 41.7 wt%. 3.2. Capacities of individual electrodes The individual storage capacities of SnO2/Cu/CNT and AC electrodes are measured in a three-electrode setup and illustrated
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Fig. 3. SEM micrographs of CNT at three processing stages. Morphology is shown for (a) as-received CNT; (b) CNT after electroless plating of tin and copper, then oxidation in air; (c) SnO2/Cu/CNT after prelithiation.
in Figs. 4 and 5; respectively. Fig. 4a shows the galvanostatic charge and discharge curves during prelithiation of the SnO2/Cu/CNT electrode. The first lithiation cycle contains a high percentage of irreversibility, because extra charge was consumed in reducing oxides and building up the solid electrolyte interface (SEI) as the potential went down to 0.01 V (vs. Li/Li+). The following cycles display much less irreversibility. Two consecutive lithiation capacities are 638 and 573 mAh g1, with their corresponding delithiation capacities 457 and 441 mAh g1. The coulombic efficiencies are 72% and 77%. These initial efficiencies are
considered high for the SnO2-based electrodes [34]. The irreversible capacity loss is attributed to either Li2O traps of lithium ions, or the increase of contact and SEI resistances during delithiation [35]. Fig. 4b shows the cyclic voltammograms of the prelithiated SnO2/ Cu/CNT electrode. Two anodic peaks at 0.5 and 1.25 V (vs. Li/Li+) are evident on the anodic curve, corresponding to two short plateaus (marked) on the delithiation curves of Fig. 4a. The 0.5 V anodic peak of Fig. 4b denotes delithiation of the LixSn alloys, the alloying reaction in reverse, discussed in the introduction section. After 1.0 V, the broad and sloping 1.25 V peak signifies the
Fig. 4. SnO2/Cu/CNT electrode individual capacity. (a) Charge and discharge curves at 0.1 A g1 are scanned from 2.0–0.01 V (vs. Li/Li+) during prelithiation; (b) CV curves of prelithiated SnO2/Cu/CNT electrode at sweep rate 0.2 mV s1 between 0.01 and 2.0 V, the dashed line marks the border between de-alloying and recombination reactions which peak at 0.5 and 1.25 V (vs. Li/Li+); (c) the rate capabilities of prelithiated SnO2/Cu/CNT electrode at 1, 2, 3, 4, 5, 6, 7, 8, 10, 13, 15C.
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recombination reaction between Li2O and Sn, which is the first step [36]. Fig. 4b also shows the 0.5 V peak height remains the same after 4 cycles, but the 1.25 V peak height decreases somewhat, implying a superior reversibility of the de-alloying step. Rate capability of the prelithiated SnO2/Cu/CNT electrode is shown in Fig. 4c. Since this composite electrode does not have a theoretical capacity, we assign the electrode capacity, 600 mAh g1, with some arbitrariness, and measure the electrode delithiation capacity at stepwise increasing C-rate. The C-rate capability diagram exhibits considerable decay at low rates, and quite steady at high rates. Higher capacity retention at a higher rate is attributed to the fact that capacity reduction with increasing rate has excluded those slower and less reversible steps from the storage mechanism. For example, the slower steps involve in the 1.25 V anodic peak. Operated at a higher specific current, only the fast and reversible steps are able to contribute in capacity. Similar data have been reported on the aerogel electrode of SnO2 and graphene [37]. The electrode capacity of activated carbon is evaluated with CV, as shown in Fig. 5a. These nearly rectangular voltammograms indicate the storage mechanism is double layer capacitance. The cathodic scan shows a flat line shape, while the anodic scan is a tilted line. For the anodic scan, the degree of tilting increases with increasing scan rate, because more carbon surface of YP-80F has been left out due to ion trapping in micropores [38]. Exclusion of more carbon surface means a lower CV capacitance being measured, which is shown in Fig. 5b. At a low scan rate, 1.0 mV s1, the specific capacitance reaches 107 F g1. But at 100 mV s1, the capacitance of AC is 50.2 F g1. A central issue in designing a hybrid capacitor is to resolve a proper mass ratio of the active material loadings on the positive and negative electrodes. A commonly accepted rule for the electrochemical capacitors is to set the mass ratio such that the positive and negative capacities equal each other [39,40]. We have applied the preceding individual capacity data, and calculate the cell capacity with various mass ratios of AC:SnO2/Cu/CNT at a selected current. The estimated cell capacity increases rapidly with increasing mass ratio from 0 to 1.0, and the trend moderates when the mass ratio increases from 1.0 to 1.5. Further increase in cell capacity is minor when the mass ratio is higher than 1.5. We do not present this particular piece of calculation results, since many significant factors have been ignored. One critical factor is the cell capacity involves two mechanisms with different specific-current dependences. In this article, we opt to emphasize on the experimental results of the hybrid cells with different mass ratios
and analyze the roles of positive and negative electrodes operated at various specific currents. 3.3. Full cells of three mass ratios Galvanostatic charge/discharge curves have been measured for the full cells of three mass ratios. Based on the discharge curves, their specific energy Ecell and power Pcell are calculated according to Ztf cell . I is the the following equations; Ecell ¼ I Udt and Pcell ¼ tEf t i ti
specific current based on the combined mass of positive and negative electrodes; tf and ti are the end time and the start time of cell discharge, and U is the cell voltage. In this work, the cell voltage window DU is set to 3.8 V, ensuring a reasonable cycle life. Fig. 6a presents the specific power and energy values of three cells when operated at various currents. Because specific current dictates the cell power, the discussion is focused on the cell energy. At low currents, for example 0.1 A g1, the specific energy values are practically the same for the 1:0.67 and 1:0.33 cells, 90 and 91 Wh kg1; while somewhat less for the 1:1 cell, 84 Wh kg1. When operated at high currents, the specific energy values differ significantly. At 3.0 A g1, the specific energy of 1:0.67 cell is 39 Wh kg1, high than that of 1:1 cell, 29 Wh kg1, much higher than that of 1:0.33 cell, 19 Wh kg1. In between, specific energy of the 1:0.67 cell is generally the highest among the three. The above comparison indicates that the mass ratio 1:0.67 is optimal for this LIHC, considering both power and energy performance, even though the 1:0.33 cell could store more energy at a specific current less than 0.1 A g1. Also presented in Fig. 6a are the specific energy and power levels of four other LIHCs in literature. Among them, the LIHC, C: SnO2/C, equipped with a carbon positive electrode and an optimized negative of mesoporous carbon filled with SnO2, bears a resemblance to our LIHC [41]. Since their emphasis is on the tin dioxide negative, detailed discussion seems worthwhile. The original data of C:SnO2/C cell was 62 Wh kg1 and 1.60 kW kg1, measured at 1.0 A g1 and DU = 4.0 V. For comparison purpose, these energy and power values are converted into 56 Wh kg1 and 1.44 kW kg1 in a 3.8 V window, assuming the cell energy and power values are proportional to (DU)2. Our 1:0.67 cell shows a higher power performance at a comparable energy level, 54.4 Wh kg1 and 1.75 kW kg1 at 1.0 A g1. The other LIHC cell,
Fig. 5. AC electrode individual capacity. (a) Voltammograms are scanned at 5, 10, 50, 100, 200, 500 mV s1 between 2.5 and 4.0 V (vs. Li/Li+); (b) capacitance values of various sweep rates, calculated with voltammograms of (a) and the equation Q=2ðmAC DV Þ where Q is the sum of anodic and cathodic charge, mAC is the mass of activated carbon, DV is the scanned potential range.
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Fig. 6. Values of specific energy, power, and capacitance for three hybrid cells. (a) Ragone plots of three cells with AC:SnO2/Cu/CNT mass ratios 1:1, 1:0.67, 1:0.33 which are operated between specific current 0.1 and 3.0 A g1. (b) The corresponding cell capacitance values are plotted versus various specific currents.
AC:H-Carbon, with AC positive and hard carbon negative, also performs similarly with DU = 3.9 V [42]. Meanwhile, the hybrid cell, AC:S-Carbon, with AC positive and soft carbon negative, is clearly superior to our 1:0.67 cell [43]. On the other hand, the cell, C:TiO2tube, with carbon positive and titania nanotubes composite negative, is evidently inferior [44], Fig. 6a. Fig. 6b shows the current dependence of cell capacitance C cell , which is calculated as I(tfti)/DU. The cell capacitance of three capacitors decreases with increasing specific current. At 0.1 A g1, the capacitance of the 1:0.67 cell is 52.8 F g1, higher than that of the 1:0.33 cell, 51.9 F g1, also higher than that of 1:1 cell, 49.6 F g1. The 1:0.67 cell stores most electric charges among the three cells. The specific energy of 1:0.67 cell is slightly less than that of 1:0.33 cell at 0.1 A g1, Fig. 6a, because the 1:0.33 cell acquires more character of the prelithiated SnO2/Cu/CNT electrode, staying in the high voltage period longer. 3.4. Electrode performance in LIHC We also calculate the positive capacitance C þ for AC electrode, and the negative capacitance C for SnO2/Cu/CNT electrode using the discharge curves and the following equations, Q cell =mþ DU þ and Q cell =m DU , where Q cell is the stored electric charge. The two
electrode capacitances, plotted in Fig. 7, are quite different in magnitude, and vary with the specific current I in different manners. As a whole, the magnitude of negative capacitance is 2–3 times that of positive capacitance, therefore, it is the positive capacitance that mainly controls the cell capacitance. Although AC loadings of the three cells are the same, the AC capacities being exploited in capacitor are different. Fig. 7a shows that the AC (positive) capacity is exploited most thoroughly at 1:1, least at 1:0.33, which is reasonable since a higher negative capacitance lifts the cell capacitance. But it is the 1:0.67 cell, in which the AC capacity being exploited in between, produces the highest cell capacitance, Fig. 6b. The above observation is explained through contrasting the electrode capacitances of 1:0.67 and 1:1 cells. At low current 0.1 A g1, the C þ values of 1:0.67 and 1:1 cells are similar in magnitude, 130 and 138 F g1, while the C values are much higher, 452 and 357 F g1. Therefore, the cell capacitance is dominated by C þ, since two electrodes are in-series. The cell capacitance of 1:0.67 cell, 52.8 F g1 surpasses that of 1:1 cell, 49.6 F g1, because of a higher C value. At high current 1.0 A g1, again, the C þ value of 1:1 cell, 91.6 F g1, is higher than that of 1:0.67 cell, 76.7 F g1. Yet, the 1:0.67 cell capacitance, 34.4 F g1, exceeds that of 1:1 cell, 28.1 F g1, because the negative capacitance of 1:0.67 cell (340 F g1) is far larger than that of 1:1 cell (147 F g1). Hence, we conclude that the SnO2/Cu/CNT capacity still
Fig. 7. Electrode capacitances in the three LIHCs. The positive (or AC) electrode capacitance values of 1:1, 1:0.67, 1:0.33 cells are plotted in (a), while the negative (or SnO2/Cu/ CNT) electrode capacitance values are plotted in (b). Note that the AC mass loadings of three LIHCs are the same.
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plays an important role in cell capacitance even under the C þ dominance. Fig. 8 shows the electrode potentials of the three full cells at 0.3 A g1. The potential curves of 1:0.67 and 1:1 cells sketch two triangles of nearly isosceles shape, with the negative isosceles triangle much smaller than the positive one. The negative potential difference DU of 1:0.67 cell is 1.01 V, less than 1.15 V of the 1:1 cell, suggesting that the 1:0.67 cell has a larger negative capacity than that of 1:1 cell. Because the activated carbon loadings are the same for three cells, the SnO2/Cu/CNT loading of 1:1 cell is higher than that of the 1:0.67 cell. In principle, the negative of 1:1 cell ought to occupy a small share of DU. Why is the negative electrode DU of 1:1 cell larger? The answer lies in the minimum potential position of U . The minimum negative potential of 1:0.67 cell is 0.18 V, while that of 1:1 cell is 0.11 V (vs. Li/Li+). The negative electrode has difficulty delithiating when its minimum potential comes close to the potential of lithium metal. The difficulty is made evident when we compare the charging and discharging curves of negative electrode in Fig. 8a, c. Comparison of the charging curve of the negative and the discharging curve near the minimum potential of 1:1 cell reveals a kinetic barrier. Fig. 8c shows the charging curve descends normally near minimum, but the discharging curve rises up abruptly (marked in the graph) then tilts into a straight line. The sudden rise indicates that delithiation rate of the negative electrode cannot keep up with the designated current, 0.3 A g1. In contrast, Fig. 8a shows a straight tilted line in the discharge potential of the negative electrode for 1:0.67 cell. The sudden rise in the negative discharge potential grows even more pronounced with increasing current. Fig. 9a and c show the corresponding electrode potentials at 1.0 A g1. The minimum potential of 1:0.67 cell remains at a similar position, 0.20 V (vs. Li/ Li+), Fig. 9a. Yet the minimum potential of 1:1 cell decreases to
0.03 V (vs. Li/Li+), Fig. 9c. More dramatic potential rise at a higher current suggests its origin is kinetics related. The difference in kinetics, between the 1:0.67 and 1:1 cells, originates from a too low position of the minimum negative potential, <0.15 V (vs. Li/Li+). Extra kinetic barrier between lithiation and delithiation is caused by SEI and slow reactions of SnO2/Cu/CNT with a higher loading in 1:1 cell. The suggestion of extra kinetic barrier in the region <0.15 V (vs. Li/Li+) is opposite to the common notion that the capacity is higher at low potentials. Such a claim is consistent with the SnO2/Cu/CNT individual behavior, as shown in Fig. 4a, indicating lithiation and delithiation behaviors are rather different near 0.0 V (vs. Li/Li+) for SnO2/Cu/CNT at 0.1 A g1. During lithiation, the potential of SnO2/Cu/CNT decreases gradually, while the potential increases quickly during delithiation at the same current 0.1 A g1. As a whole, the kinetic barrier forces the negative electrode of 1:1 cell to consume more voltage window to match with the AC electrode. As for the 1:0.33 cell, its SnO2/Cu/CNT loading is too low, the voltage window is almost equally partitioned between positive and negative at 0.3 A g1, DUþ = 1.93 V, DU = 1.86 V, Fig. 8b. The cell capacity is least among the three, because this mass ratio does not exploit the SnO2/Cu/CNT benefit, i.e., allowing a small DU and giving more room to DU þ . When the current is raised to 1.0 A g1, Fig. 9b, DU decreases to 1.48 V, but two other unfavorable signs emerge. First, the positive potential curve bulges up in charging, such a bulge implies the cell is approaching the limit of electrolyte decomposition. Second, the negative potential curve shows a pronounced IR drop, suggesting the negative capacity of 1:0.33 cell is insufficient to meet the electric current demand. Discussion of Figs. 8 and 9 also reveals the significance of U0%SOC position, where the positive and the negative electrode potentials equal at 0% state of charge. U0%SOC denotes the position where
Fig. 8. Cell voltage and electrode potentials of three cells being charge and discharge at 0.3 A g1. Potential variations with time of positive and negative electrodes in the (a) 1:0.67 cell; (b) 1:0.33 cell; (c) 1:1.00 cell. The U0%SOC value is marked for each full cell. The kinetic barrier of the negative electrode in 1:1 cell is also indicated with a dashed square.
Fig. 9. Cell voltage and electrode potentials of three cells being charge and discharge at 1.0 A g1. Potential variations with time of positive and negative electrodes in the (a) 1:0.67 cell; (b) 1:0.33 cell; (c) 1:1.00 cell. The U0%SOC value is marked for each full cell. The kinetic barrier of the negative electrode in 1:1 cell is also indicated with a dashed square.
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Table 1 Potential values at 0% state of charge U0%SOC (vs. Li/Li+) versus specific charging current for three hybrid capacitors with the AC:SnO2/Cu/CNT mass ratios, 1:1.00, 1:0.67, 1:0.33. mass ratio
0.3 A g1
0.4 A g1
0.5 A g1
0.7 A g1
0.9 A g1
1.0 A g1
3.0 A g1
1:1.00 1:0.67 1:0.33
1.26 1.19 2.13
1.30 1.18 2.08
1.34 1.18 2.04
1.39 1.18 1.97
1.45 1.18 1.92
1.48 1.17 1.88
1.74 1.17 1.79
All listed values are in the unit of V (vs. Li/Li+).
positive and negative electrodes begin to store electricity. A slight shift in U0%SOC can alter how the voltage window is partitioned and how the electrode capacity is exploited. Unfortunately, the U0%SOC position is influenced by more than one factor. The major factor is the prelithiated negative electrode pulls down the potential of negative electrode near the lithium metal potential, such that the maximum potential of LIHC is kept around 4.0 V (vs. Li/Li+) and the undesired side reactions are avoided at the positive electrode. But the pull-down effect is not the only factor. According to the above reasoning, the U0%SOC position of 1:1 cell ought to be the lowest among the three cells, since the 1:1 cell has the highest SnO2/Cu/ CNT loading among three cells. Table 1 lists the U0%SOC values of the 1:1, 1:0.67, and 1:0.33 cells operated at various specific currents. The 1:1 cell does not follow that rationale, its U0%SOC values are higher than those of the 1:0.67 cell, not lower. Discussion in the preceding paragraph has revealed the minimum potential is another factor of U0%SOC position. The minimum potential of 1:1 cell reaches the region of kinetic barrier, <0.15 V (vs. Li/Li+), which diminishes the negative electrode capacitance, consumes more voltage window, and lifts the U0%SOC position. Furthermore, the kinetic barrier effect increases with increasing specific current for the 1:1 cell. Therefore, the U0%SOC position increases with increasing specific current for the 1:1 cell. Yet the U0%SOC positions of 1:0.67 and 1:0.33 cells decrease with increasing current, Table 1. Cycle stability of the hybrid cell with a 1:0.67 mass ratio is plotted in Fig. 10. When operated at 0.3 A g1, this cell begins with a capacitance 44.3 F g1. The cell capacitance decreases with increasing cycle number, and ends at 35.8 F g1 at 200th cycles. The capacity retention is 81%. On the other hand, the coulombic efficiency increases gradually with increasing cycle number. The increasing coulombic efficiency suggests the capacity retention of SnO2/Cu/CNT negative is improving with the cycling number. The initial efficiency is 92%, increases to 95% in the 15th cycle, ends at 97% in the 200th cycle.
Fig. 10. Cycle stability of the 1:0.67 hybrid capacitor. Capacity retention and coulombic efficiency are plotted versus cycle number for the 1:0.67 capacitor operated at 0.3 A g1. The inset shows the voltage curves of charge and discharge at the 5th, 25th, 70th, 110th, 160th, 200th cycle.
4. Conclusions Three LIHCs with 1:1, 1:0.67, 1:0.33 mass ratios have been assembled and studied to understand the role of prelithiated SnO2/ Cu/CNT negative electrode. Although the positive electrodes of the three capacitors are loaded with the same amount of AC, the positive capacitances are not the same. The positive capacitance of the 1:1 cell is the highest, that of 1:0.33 cell is the lowest, and that of 1:0.67 is in between, indicating a large SnO2/Cu/CNT capacity bring forth more capacity out of the AC electrode. But a higher AC capacitance does not mean a higher cell capacity. The 1:0.67 cell excels the other two capacitors in cell capacitance and specific energy. We investigate the electrode potentials during charge and discharge and uncover two main reasons, which involve the potential position of U0%SOC and the slow discharge kinetics of SnO2/Cu/CNT near 0.0 V (vs. Li/Li+). We note the 1:0.67 cell is featured with a nearly constant U0%SOC value, in contrast to those of the two other cells. And the negative electrode potential of 1:0.67 cell stays away from 0.0 V (vs. Li/Li+). The cycle stability of 1:0.67 cell is also measured, its capacity retention is 81% in 200 cycles at 0.3 A g1. Acknowledgment The authors would like to express their gratitude to Ministry of Science and Technology of Taiwan for partial support through the project MOST-103-2221-E-011-153-MY3. The authors also thank National Taiwan University of Science and Technology for paying the miscellaneous fee through one of Top University Projects 105H45140. References [1] J.H. Lee, W.H. Shin, S.Y. Lim, B.G. Kim, J.W. Choi, Mater. Renew. Sustain. Energy 3 (2014) 22. [2] J.H. Lee, W.S. Shin, M.H. Ryou, J.K. Jin, J. Kim, J.W. Choi, ChemSusChem 5 (2012) 2328–2333. [3] D. Cericola, R. Kotz, Electrochim. Acta 72 (2012) 1–17. [4] W.J. Cao, J.P. Zheng, J. Electrochem. Soc. 160 (2013) A1572–A1576. [5] W. Cao, J. Zheng, D. Adams, T. Doung, J.P. Zheng, J. Electrochem. Soc. 161 (2014) A2087–A2092. [6] D.P. Dubal, O. Ayyad, V. Ruiz, P. Gomez-Romero, Chem. Soc. Rev. 44 (2015) 1777–1790. [7] F. Zhang, T. Zhang, X. Yang, L. Zhang, K. Leng, Y. Huang, Y. Chen, Energy Environ. Sci. 6 (2013) 1623–1632. [8] X. Chen, J. Guo, K. Gerasopoulos, A. Langrock, A. Brown, R. Ghodssi, J.N. Culver d, C. Wang, J. Power Sources 211 (2012) 129–132. [9] N. Nitta, G. Yushin, Part. Part. Syst. Charact. 31 (2014) 317–336. [10] Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Nature 276 (1997) 1395–1397. [11] J.Y. Huang, L. Zhong, C.M. Wang, J.P. Sullivan, W. Xu, L.Q. Zhang, S.X. Mao, N.S. Hudak, X.H. Liu, X.H. Liu, A. Subramanian, H. Fan, L. Qi, A. Kushima, J. Li, Science 330 (2010) 1515–1520. [12] C. Kim, M. Noh, M. Choi, J. Cho, B. Park, Chem. Mater. 17 (2005) 3297–3301. [13] P. Lian, X. Zhu, S. Liang, Z. Li, W. Yang, H. Wang, Electrochim. Acta 56 (2011) 4532–4539. [14] G. Du, C. Zhong, P. Zhang, Z. Guo, Z. Chen, H. Liu, Electrochim. Acta 55 (2010) 2582–2586. [15] X. Li, X. Meng, J. Liu, D. Geng, Y. Zhang, M.N. Banis, Y. Li, J. Yang, R. Li, X. Sun, M. Cai, M.W. Verbrugge, Adv. Funct. Mater. 22 (2012) 1647–1654. [16] J. Zhu, D. Lei, G. Zhang, Q. Li, B. Lu, T. Wang, Nanoscale 5 (2013) 5499–5505. [17] X.W. Lou, J.S. Chen, P. Chen, L.A. Archer, Chem. Mater. 21 (2009) 2868–2874. [18] X.W. Lou, C.M. Li, L.A. Archer, Adv. Mater. 21 (2009) 2536–2539. [19] I.A. Courtney, J.R. Dahn, J. Electrochem. Soc. 144 (1997) 2045–2051.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
A composite electrode of tin dioxide and carbon nanotubes and its role as negative electrode in lithium ion hybrid capacitor Chung-Lun Hsieh, Dah-Shyang Tsai* , Wei-Wen Chiang, Yen-Heng Liu Department of Chemical Engineering, National Taiwan University of Science and Technology, 43, Keelung Road, Section 4, Taipei 10607, Taiwan
A R T I C L E I N F O
Article history: Received 29 January 2016 Received in revised form 12 May 2016 Accepted 12 May 2016 Available online 13 May 2016 Keywords: lithium ion capacitor hybrid capacitor tin dioxide negative electrode carbon nanotubes current dependence
A B S T R A C T
A nanostructured electrode, denoted as SnO2/Cu/CNT, is prepared with tin dioxide and copper coated on the carbon nanotubes (CNT) backbone. Before coating of tin and copper, the CNT surface has been electrochemically oxidized and examined with the C1s photoelectron spectrum to ensure a minimum oxidation level while sufficient for electroless plating. With the prelithiated SnO2/Cu/CNT electrode, matching with an activated carbon (AC) electrode, this lithium ion hybrid capacitor taps into the lithium storage aptitude of tin dioxide for reversible capacity. Thus three capacitors, of the same AC loading, are assembled and studied with the AC:SnO2/Cu/CNT mass ratio 1:1, 1:0.67, 1:0.33. Their energy storage capacities are similar in magnitude at 0.1 A g1, around 90 Wh kg1 in a 3.8 V window, but the 1:0.67 cell surpasses the other two cells when the charging current increases. Superiority of the 1:0.67 cell on energy and electric charge storage is discussed, considering the SnO2/Cu/CNT role in capacitor. The discussion reveals the prelithiated SnO2/Cu/CNT has kept the potential at 0% state of charge (U0%SOC) low to constrict the maximum positive potential. But U0%SOC should avoid forcing the negative potential descends lower than 0.15 V (vs. Li/Li+), where the kinetic barrier of SnO2/Cu/CNT becomes problematic at high rates. ã 2016 Published by Elsevier Ltd.
1. Introduction The lithium ion hybrid capacitor (LIHC) bridges the gap between electrochemical capacitors and rechargeable batteries by means of trading its electrode of double-layer capacitance for a more energetic lithium storage electrode. In those commercial devices, prelithiated graphite or other carbon electrodes serve the purpose of lithium storage, preventing the electrolyte from lithium shortage in the course of cell charging. The energetic battery electrode raises the cell capacity, yet increases the cell internal resistance as well, because of its associated kinetic barriers of faraday reactions and diffusion. This perception implies there is room for improvement on the power of LIHC through searching for a fast-response battery electrode [1–5]. In light of the literature on battery research, enhancement of the redox reaction rates has often been rewarded through optimization of the electrolyteelectrode interface. A proven approach to alter the interface is to modify the electrodes with nanometer-sized particles or nanostructured composites, which drive many advances of today’s
* Corresponding author. E-mail address: [email protected] (D.-S. Tsai). http://dx.doi.org/10.1016/j.electacta.2016.05.090 0013-4686/ã 2016 Published by Elsevier Ltd.
battery technology [6,7]. Hence attention of this work is focused on a nanostructured electrode based on tin dioxide. As an anode component, tin and its oxides attract a great deal of attention because of their large capacities and cost-effectiveness [8,9], especially in form of dioxide [10–16]. Lithiation of SnO2 is usually divided into two steps. The first step involves reduction to tin metal with four lithium ions and four electrons, SnO2 + 4 e + 4 Li+ ! Sn + 2 Li2O, which yields a theoretical capacity of 710 mAh g1. Unfortunately, a large fraction of this capacity is not reversible [17–19]. The second lithiation reaction further reduces tin metal into LixSn alloys, Sn + x e + x Li+ ! LixSn. Depending on the end product, the theoretical capacity of this reaction differs somewhat. When the end alloy composition is Li22Sn5 [20,21], the storage of 4.4 lithium atoms per tin atom gives a theoretical capacity of 781 mAh g1, based on the mass of tin dioxide, not tin metal. When the end alloy compound is thought to be Li17Sn4, x = 4.25 instead of 4.4, the magnitude of theoretical capacity is somewhat less [22,23]. According to the literature, capacity of the second step is more reversible than that of the first step [24]. On the other hand, the second step is also plagued with pulverization problems due to a large volume expansion during lithium alloying. Addition of carbon materials has been reported effective in relaxing the tin lithiation stresses [11,18,21].
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Another approach for upgrading capacity retention is to implement tin dioxide nanowires or nanotubes of long geometric feature [25–29]. In this work, we have taken considerable care to prepare porous electrodes made of multi-walled carbon nanotubes (CNT) coated with tin dioxide and a small amount of copper. Multiwalled CNT is chosen as the backbone of the composites for one dimensional feature and electrical connection improvement. A uniform tin layer is coated through electroless plating with a minimum level of CNT surface oxidation, which prevents oxidation from damaging the conductivity of CNT. The composite electrode is prelithiated and applied as negative for the hybrid capacitor. We discuss the capacity and structure features of this composite electrode when it performs independently, and emphasize the current-dependent capacities and its behavior when the electrode is in series with an activated carbon (AC) positive electrode in a LIHC. 2. Experimental 2.1. Preparation of positive and negative electrodes Our preparations began with two pastes of active materials for positive and negative electrodes. The paste for positive electrode contained 0.80 g AC powder (YP-80F, Kuraray Chemical), 0.10 g Super P (Timcal Carbon Black), 0.10 g polyvinylidene fluoride binder (PVdF, Sigma-Aldrich), and 10 mL N-methyl-2-pyrrolidone (NMP, ACROS). These components are vigorously blended and vacuumed to remove bubbles. A calculated amount of AC paste was then loaded on the aluminum current collector of 2 cm in diameter, and dried at 60 C in a vacuum chamber. Preparation of the negative electrode was more complicated. We first weighed 0.6 g multi-walled CNT (UR-NTM003, Nanostructured and Amorphous Materials) and immersed it in an oxidizing acid solution of 9.0 mL sulfuric acid (95%) and 3.0 mL nitric acid (65%) at 60 C for 5 min. The CNT oxidation level could be adjusted through varying the bath time of this electrochemical oxidation to understand its processing influences. After oxidization, the CNT powder was washed with a large amount of deionized water, and dried at 80 C. The following electroless plating of tin and copper was performed in two steps. First, a solution mixture of oxidized CNT was blended with two solutions at 60 C, a 200 mL solution containing 0.6 wt% CuSO4 and 0.6 wt% EDTA, another 200 mL solution containing 0.35 wt% SnCl2. Second, tin and copper plating took place when a 200 mL solution of 0.1 wt % NaBH4 reducing agent was added with magnetic stirring at 60 C. After the electroless plating, the powder was washed with sufficient amount of deionized water. The powder was collected and dried in air to oxidize tin at 100 C. The dried powder was denoted as SnO2/Cu/CNT, to be used in electrode preparation. Prior to adopting electroless plating Sn/Cu on CNT, we had tried electrochemical deposition. But it was very difficult to achieve uniform coating on a dense CNT forest. And if nanotubes were dispersed, it would be more difficult and expensive to establish electric contacts with many nanotubes and recollect them. The paste for negative electrode was prepared with 0.4 g SnO2/ Cu/CNT and 0.04 g PVdF binder in 40 mL NMP, mixed in a ultrasonic cleaner (DC150H, DELTA) and vacuumed in a chamber to remove bubbles. A cleansed copper plate of 2 cm in diameter was loaded with the SnO2/Cu/CNT paste and dried at 60 C in a vacuum chamber for 12 h. The mass of SnO2/Cu/CNT was recorded as the weight difference before and after drying, so was the AC mass of positive electrode. Before assembling the capacitor, the SnO2/Cu/ CNT electrode was lithiated from 2.0 to 0.01 V (vs. Li/Li+) in a threeelectrode setup. Then, the lithiated SnO2/Cu/CNT electrode, ended at 0.01 V (vs. Li/Li+) in prelithiation, was assembled as negative in a test cell of Swagelok type (ECC-COMBI, EL-CELL), matching with
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the positive AC electrode. The electrolyte was 1.0 M LiPF6 solution for lithium ion batteries (EC: DMC 50:50 by volume, LB-301). And a polypropylene porous film of 25 mm thick (Celgard 2500) was the separator. 2.2. Assembling capacitors Prelithiation of the SnO2/Cu/CNT electrode and its capacity measurements were performed using the three-electrode setup with a lithium wire as reference and a lithium plate as counter electrode. And two layers of Celgard 2500 were used in this halfcell configuration. The capacitance of the positive electrode was also evaluated with cycle voltammetry from 2.5 to 4.0 V (vs. Li/Li+) in this assembly. Before measurement, each porous electrode was soaked in the electrolyte for 10 h to wet their porous interfaces thoroughly. The assembling procedure and the measurements were conducted in a glove box (GB-100, SunRay Science) with an argon atmosphere of water and oxygen impurities less than 1 ppm. Three full cells were assembled with the AC:SnO2/Cu/CNT mass ratio 1:1, 1:0.67, 1:0.33. The AC loading of positive electrode was fixed at 0.85 mg cm2. And the SnO2/Cu/CNT loading was 0.85, 0.56, 0.28 mg cm2 for the negative electrodes of 1:1, 1:0.67, and 1:0.33 cells; individually. Since the SnO2/Cu/CNT electrode had been prelithiated, the open circuit voltage (OCV) of the assembled full cell was not zero. The OCV values of 1:1, 1:0.67, 1:0.33 cells were 0.86 V, 0.94 V, 1.10 V; individually. 2.3. Materials characterization Phase analysis was performed with an X-ray diffractometer (D2 Phaser, Bruker), equipped with the CuKa radiation source and nickel filter. Morphology of the porous electrode was examined using a scanning electron microscope (SEM, JSM-6500F JEOL), equipped with an energy dispersive X-ray microanalyzer (EDX, INCA, Oxford). X-ray photoelectron spectra (XPS) of CNT were collected using a Thermo VG Scientific Theta Probe system operated at 7 1010 mbar. The Al Ka 1486.6 eV was the X-ray source, and the Ag 3d5/2 line at 368.26 eV was the calibration reference. Values of binding energy were determined through curve fitting with 30% Lorentzian and 70% Gaussian curves and Shirley baselines using the Avantage v3.2 software. Surface area of activated carbon was characterized using nitrogen adsorption and desorption isotherms, measured with a surface-area and pore-size analyzer (ASAP2020, Micromeritics). 3. Results and discussion 3.1. Characterization of SnO2/Cu/CNT and its preparation Fig. 1 presents the X-ray diffraction pattern of SnO2/Cu/CNT powder, which is dominated by the distinct features of tin dioxide crystallized in the cassiterite structure. The pattern of SnO2/Cu/ CNT also contains two or three less pronounced diffraction peaks of CNT, which are made legible when contrasted with the diffraction result of as-received CNT sample, as shown in Fig. 1. The strongest (002) diffraction line of CNT is located near that of (110) diffraction of SnO2. A combination of the two lines results in the strongest and broad feature at 26.7. It is logical to infer drying in air oxidized the electroless-plated tin metal, and produced SnO2 of the cassiterite structure. The diffraction results do not show any phase related to the minor component, copper. We believe copper exists in the metallic state since appreciable oxidation of copper metal starts at 350 C [30], and our processing temperature did not exceed 100 C. In the preliminary attempts to prepare the SnO2/Cu/CNT powder with electroless plating, we quickly realize that oxidation of CNT is indispensable to tin coating. Without proper oxidation,
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Fig. 1. X-ray diffraction results of the SnO2/Cu/CNT electrode. The SnO2/Cu/CNT pattern is dominated with the SnO2 cassiterite diffraction lines, which are indexed to the rutile lattice and marked in red. Note that the CNT diffractions, marked in black, are also involved.
the as-received CNT surface is hydrophobic, not accessible to nucleation and growth of metallic tin film. Addition of reducing sodium borohydride generally yields detached tin metal precipitates, not a film on CNT. On the other hand, electrochemical oxidation degrades, unfortunately, the one dimensional conductivity of CNT [31], a crucial reason that CNT is chosen as the backbone of electrode. To keep the conductivity damage at a minimum, we evaluate the conducting ability of oxidized CNT using the p-p* contribution in its C1s spectrum, and search for a compromise between uniform coating and conducting ability. Later the morphology of CNT surface is inspected to assure a uniform tin film. Fig. 2a–c illustrate variations in the C1s spectrum of CNT surface after electrochemical oxidation in the mixture of sulfuric and nitric acids for 0, 5, and 15 min. Fig. 2a is the C1s spectrum of as-received CNT, showing a major feature at binding energy (BE) 284.4 eV with an elongated shoulder in the high BE range. Following the literature report, the 284.4 eV feature is assigned to graphitic carbon (sp2 hybridized carbon). The positive-shifted features, extending from BE 286.7 to 291.5 eV, are considered as the contributions of carbon impurities and functionalized carbon. The 285.0-285.5 eV contribution is allocated to CNT defects, for example, sp3 carbon. Beyond the defect peak, the extended feature is deconvoluted into the components of CO, O C¼O, CO3, and the p-p* shake-up [31–33]. Fig. 2b and c show the diminished p-p* shake-up features of properly and excessively oxidized surfaces. Deconvolution results reveal the percentage of p-p* component decreases with increasing oxidation time, 6.2% (0 min), 4.5% (5 min), and 2.2% (15 min). Since a high p-p* percentage symbolizes a superior CNT conductivity and we cannot achieve uniform tin coating if oxidization persists less than 5 min, therefore, 5 min is chosen as the CNT oxidation time in preparation of the SnO2/Cu/CNT electrode. Fig. 3 presents the morphological features of SnO2/Cu/CNT electrode at three processing stages. The diameter of as-received CNT ranges from 15 to 25 nm, and the length varies from 10 to 30 mm, as shown in Fig. 3a. Electroless plating of tin and copper increases the diameter of multi- walled CNT evenly, up to 25– 90 nm, while the tube length seems to be cut short due to processing, Fig. 3b. The tube diameter increases further, up to 35 100 nm after prelithiation, the tube length ranges between 1 and 5 mm, as shown in Fig. 3c. EDX analysis of tin-copper plated electrode, at four selected positions of SnO2/Cu/CNT, yields the
Fig. 2. XPS results of CNT with increasing levels of oxidation. C1s spectra of the CNT samples that have undergone electrochemical oxidation in the mixed sulfuric and nitric acids at 60 C for (a) 0, (b) 5, (c) 15 min. The oxidation level increases with increasing oxidation time. In addition to the graphitic carbon (main) peak and the defect (next to main) peak, BE values of the rest assignments are given as follows; 286.5–287.0 eV (C-O), 288.0–288.5 eV (O C¼O), 289.5–290.0 eV (CO3), 291.0– 291.5 eV(p-p*).
average compositions as follows, carbon 43.5 wt%, copper 14.7 wt%, tin 41.7 wt%. 3.2. Capacities of individual electrodes The individual storage capacities of SnO2/Cu/CNT and AC electrodes are measured in a three-electrode setup and illustrated
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Fig. 3. SEM micrographs of CNT at three processing stages. Morphology is shown for (a) as-received CNT; (b) CNT after electroless plating of tin and copper, then oxidation in air; (c) SnO2/Cu/CNT after prelithiation.
in Figs. 4 and 5; respectively. Fig. 4a shows the galvanostatic charge and discharge curves during prelithiation of the SnO2/Cu/CNT electrode. The first lithiation cycle contains a high percentage of irreversibility, because extra charge was consumed in reducing oxides and building up the solid electrolyte interface (SEI) as the potential went down to 0.01 V (vs. Li/Li+). The following cycles display much less irreversibility. Two consecutive lithiation capacities are 638 and 573 mAh g1, with their corresponding delithiation capacities 457 and 441 mAh g1. The coulombic efficiencies are 72% and 77%. These initial efficiencies are
considered high for the SnO2-based electrodes [34]. The irreversible capacity loss is attributed to either Li2O traps of lithium ions, or the increase of contact and SEI resistances during delithiation [35]. Fig. 4b shows the cyclic voltammograms of the prelithiated SnO2/ Cu/CNT electrode. Two anodic peaks at 0.5 and 1.25 V (vs. Li/Li+) are evident on the anodic curve, corresponding to two short plateaus (marked) on the delithiation curves of Fig. 4a. The 0.5 V anodic peak of Fig. 4b denotes delithiation of the LixSn alloys, the alloying reaction in reverse, discussed in the introduction section. After 1.0 V, the broad and sloping 1.25 V peak signifies the
Fig. 4. SnO2/Cu/CNT electrode individual capacity. (a) Charge and discharge curves at 0.1 A g1 are scanned from 2.0–0.01 V (vs. Li/Li+) during prelithiation; (b) CV curves of prelithiated SnO2/Cu/CNT electrode at sweep rate 0.2 mV s1 between 0.01 and 2.0 V, the dashed line marks the border between de-alloying and recombination reactions which peak at 0.5 and 1.25 V (vs. Li/Li+); (c) the rate capabilities of prelithiated SnO2/Cu/CNT electrode at 1, 2, 3, 4, 5, 6, 7, 8, 10, 13, 15C.
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recombination reaction between Li2O and Sn, which is the first step [36]. Fig. 4b also shows the 0.5 V peak height remains the same after 4 cycles, but the 1.25 V peak height decreases somewhat, implying a superior reversibility of the de-alloying step. Rate capability of the prelithiated SnO2/Cu/CNT electrode is shown in Fig. 4c. Since this composite electrode does not have a theoretical capacity, we assign the electrode capacity, 600 mAh g1, with some arbitrariness, and measure the electrode delithiation capacity at stepwise increasing C-rate. The C-rate capability diagram exhibits considerable decay at low rates, and quite steady at high rates. Higher capacity retention at a higher rate is attributed to the fact that capacity reduction with increasing rate has excluded those slower and less reversible steps from the storage mechanism. For example, the slower steps involve in the 1.25 V anodic peak. Operated at a higher specific current, only the fast and reversible steps are able to contribute in capacity. Similar data have been reported on the aerogel electrode of SnO2 and graphene [37]. The electrode capacity of activated carbon is evaluated with CV, as shown in Fig. 5a. These nearly rectangular voltammograms indicate the storage mechanism is double layer capacitance. The cathodic scan shows a flat line shape, while the anodic scan is a tilted line. For the anodic scan, the degree of tilting increases with increasing scan rate, because more carbon surface of YP-80F has been left out due to ion trapping in micropores [38]. Exclusion of more carbon surface means a lower CV capacitance being measured, which is shown in Fig. 5b. At a low scan rate, 1.0 mV s1, the specific capacitance reaches 107 F g1. But at 100 mV s1, the capacitance of AC is 50.2 F g1. A central issue in designing a hybrid capacitor is to resolve a proper mass ratio of the active material loadings on the positive and negative electrodes. A commonly accepted rule for the electrochemical capacitors is to set the mass ratio such that the positive and negative capacities equal each other [39,40]. We have applied the preceding individual capacity data, and calculate the cell capacity with various mass ratios of AC:SnO2/Cu/CNT at a selected current. The estimated cell capacity increases rapidly with increasing mass ratio from 0 to 1.0, and the trend moderates when the mass ratio increases from 1.0 to 1.5. Further increase in cell capacity is minor when the mass ratio is higher than 1.5. We do not present this particular piece of calculation results, since many significant factors have been ignored. One critical factor is the cell capacity involves two mechanisms with different specific-current dependences. In this article, we opt to emphasize on the experimental results of the hybrid cells with different mass ratios
and analyze the roles of positive and negative electrodes operated at various specific currents. 3.3. Full cells of three mass ratios Galvanostatic charge/discharge curves have been measured for the full cells of three mass ratios. Based on the discharge curves, their specific energy Ecell and power Pcell are calculated according to Ztf cell . I is the the following equations; Ecell ¼ I Udt and Pcell ¼ tEf t i ti
specific current based on the combined mass of positive and negative electrodes; tf and ti are the end time and the start time of cell discharge, and U is the cell voltage. In this work, the cell voltage window DU is set to 3.8 V, ensuring a reasonable cycle life. Fig. 6a presents the specific power and energy values of three cells when operated at various currents. Because specific current dictates the cell power, the discussion is focused on the cell energy. At low currents, for example 0.1 A g1, the specific energy values are practically the same for the 1:0.67 and 1:0.33 cells, 90 and 91 Wh kg1; while somewhat less for the 1:1 cell, 84 Wh kg1. When operated at high currents, the specific energy values differ significantly. At 3.0 A g1, the specific energy of 1:0.67 cell is 39 Wh kg1, high than that of 1:1 cell, 29 Wh kg1, much higher than that of 1:0.33 cell, 19 Wh kg1. In between, specific energy of the 1:0.67 cell is generally the highest among the three. The above comparison indicates that the mass ratio 1:0.67 is optimal for this LIHC, considering both power and energy performance, even though the 1:0.33 cell could store more energy at a specific current less than 0.1 A g1. Also presented in Fig. 6a are the specific energy and power levels of four other LIHCs in literature. Among them, the LIHC, C: SnO2/C, equipped with a carbon positive electrode and an optimized negative of mesoporous carbon filled with SnO2, bears a resemblance to our LIHC [41]. Since their emphasis is on the tin dioxide negative, detailed discussion seems worthwhile. The original data of C:SnO2/C cell was 62 Wh kg1 and 1.60 kW kg1, measured at 1.0 A g1 and DU = 4.0 V. For comparison purpose, these energy and power values are converted into 56 Wh kg1 and 1.44 kW kg1 in a 3.8 V window, assuming the cell energy and power values are proportional to (DU)2. Our 1:0.67 cell shows a higher power performance at a comparable energy level, 54.4 Wh kg1 and 1.75 kW kg1 at 1.0 A g1. The other LIHC cell,
Fig. 5. AC electrode individual capacity. (a) Voltammograms are scanned at 5, 10, 50, 100, 200, 500 mV s1 between 2.5 and 4.0 V (vs. Li/Li+); (b) capacitance values of various sweep rates, calculated with voltammograms of (a) and the equation Q=2ðmAC DV Þ where Q is the sum of anodic and cathodic charge, mAC is the mass of activated carbon, DV is the scanned potential range.
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Fig. 6. Values of specific energy, power, and capacitance for three hybrid cells. (a) Ragone plots of three cells with AC:SnO2/Cu/CNT mass ratios 1:1, 1:0.67, 1:0.33 which are operated between specific current 0.1 and 3.0 A g1. (b) The corresponding cell capacitance values are plotted versus various specific currents.
AC:H-Carbon, with AC positive and hard carbon negative, also performs similarly with DU = 3.9 V [42]. Meanwhile, the hybrid cell, AC:S-Carbon, with AC positive and soft carbon negative, is clearly superior to our 1:0.67 cell [43]. On the other hand, the cell, C:TiO2tube, with carbon positive and titania nanotubes composite negative, is evidently inferior [44], Fig. 6a. Fig. 6b shows the current dependence of cell capacitance C cell , which is calculated as I(tfti)/DU. The cell capacitance of three capacitors decreases with increasing specific current. At 0.1 A g1, the capacitance of the 1:0.67 cell is 52.8 F g1, higher than that of the 1:0.33 cell, 51.9 F g1, also higher than that of 1:1 cell, 49.6 F g1. The 1:0.67 cell stores most electric charges among the three cells. The specific energy of 1:0.67 cell is slightly less than that of 1:0.33 cell at 0.1 A g1, Fig. 6a, because the 1:0.33 cell acquires more character of the prelithiated SnO2/Cu/CNT electrode, staying in the high voltage period longer. 3.4. Electrode performance in LIHC We also calculate the positive capacitance C þ for AC electrode, and the negative capacitance C for SnO2/Cu/CNT electrode using the discharge curves and the following equations, Q cell =mþ DU þ and Q cell =m DU , where Q cell is the stored electric charge. The two
electrode capacitances, plotted in Fig. 7, are quite different in magnitude, and vary with the specific current I in different manners. As a whole, the magnitude of negative capacitance is 2–3 times that of positive capacitance, therefore, it is the positive capacitance that mainly controls the cell capacitance. Although AC loadings of the three cells are the same, the AC capacities being exploited in capacitor are different. Fig. 7a shows that the AC (positive) capacity is exploited most thoroughly at 1:1, least at 1:0.33, which is reasonable since a higher negative capacitance lifts the cell capacitance. But it is the 1:0.67 cell, in which the AC capacity being exploited in between, produces the highest cell capacitance, Fig. 6b. The above observation is explained through contrasting the electrode capacitances of 1:0.67 and 1:1 cells. At low current 0.1 A g1, the C þ values of 1:0.67 and 1:1 cells are similar in magnitude, 130 and 138 F g1, while the C values are much higher, 452 and 357 F g1. Therefore, the cell capacitance is dominated by C þ, since two electrodes are in-series. The cell capacitance of 1:0.67 cell, 52.8 F g1 surpasses that of 1:1 cell, 49.6 F g1, because of a higher C value. At high current 1.0 A g1, again, the C þ value of 1:1 cell, 91.6 F g1, is higher than that of 1:0.67 cell, 76.7 F g1. Yet, the 1:0.67 cell capacitance, 34.4 F g1, exceeds that of 1:1 cell, 28.1 F g1, because the negative capacitance of 1:0.67 cell (340 F g1) is far larger than that of 1:1 cell (147 F g1). Hence, we conclude that the SnO2/Cu/CNT capacity still
Fig. 7. Electrode capacitances in the three LIHCs. The positive (or AC) electrode capacitance values of 1:1, 1:0.67, 1:0.33 cells are plotted in (a), while the negative (or SnO2/Cu/ CNT) electrode capacitance values are plotted in (b). Note that the AC mass loadings of three LIHCs are the same.
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plays an important role in cell capacitance even under the C þ dominance. Fig. 8 shows the electrode potentials of the three full cells at 0.3 A g1. The potential curves of 1:0.67 and 1:1 cells sketch two triangles of nearly isosceles shape, with the negative isosceles triangle much smaller than the positive one. The negative potential difference DU of 1:0.67 cell is 1.01 V, less than 1.15 V of the 1:1 cell, suggesting that the 1:0.67 cell has a larger negative capacity than that of 1:1 cell. Because the activated carbon loadings are the same for three cells, the SnO2/Cu/CNT loading of 1:1 cell is higher than that of the 1:0.67 cell. In principle, the negative of 1:1 cell ought to occupy a small share of DU. Why is the negative electrode DU of 1:1 cell larger? The answer lies in the minimum potential position of U . The minimum negative potential of 1:0.67 cell is 0.18 V, while that of 1:1 cell is 0.11 V (vs. Li/Li+). The negative electrode has difficulty delithiating when its minimum potential comes close to the potential of lithium metal. The difficulty is made evident when we compare the charging and discharging curves of negative electrode in Fig. 8a, c. Comparison of the charging curve of the negative and the discharging curve near the minimum potential of 1:1 cell reveals a kinetic barrier. Fig. 8c shows the charging curve descends normally near minimum, but the discharging curve rises up abruptly (marked in the graph) then tilts into a straight line. The sudden rise indicates that delithiation rate of the negative electrode cannot keep up with the designated current, 0.3 A g1. In contrast, Fig. 8a shows a straight tilted line in the discharge potential of the negative electrode for 1:0.67 cell. The sudden rise in the negative discharge potential grows even more pronounced with increasing current. Fig. 9a and c show the corresponding electrode potentials at 1.0 A g1. The minimum potential of 1:0.67 cell remains at a similar position, 0.20 V (vs. Li/ Li+), Fig. 9a. Yet the minimum potential of 1:1 cell decreases to
0.03 V (vs. Li/Li+), Fig. 9c. More dramatic potential rise at a higher current suggests its origin is kinetics related. The difference in kinetics, between the 1:0.67 and 1:1 cells, originates from a too low position of the minimum negative potential, <0.15 V (vs. Li/Li+). Extra kinetic barrier between lithiation and delithiation is caused by SEI and slow reactions of SnO2/Cu/CNT with a higher loading in 1:1 cell. The suggestion of extra kinetic barrier in the region <0.15 V (vs. Li/Li+) is opposite to the common notion that the capacity is higher at low potentials. Such a claim is consistent with the SnO2/Cu/CNT individual behavior, as shown in Fig. 4a, indicating lithiation and delithiation behaviors are rather different near 0.0 V (vs. Li/Li+) for SnO2/Cu/CNT at 0.1 A g1. During lithiation, the potential of SnO2/Cu/CNT decreases gradually, while the potential increases quickly during delithiation at the same current 0.1 A g1. As a whole, the kinetic barrier forces the negative electrode of 1:1 cell to consume more voltage window to match with the AC electrode. As for the 1:0.33 cell, its SnO2/Cu/CNT loading is too low, the voltage window is almost equally partitioned between positive and negative at 0.3 A g1, DUþ = 1.93 V, DU = 1.86 V, Fig. 8b. The cell capacity is least among the three, because this mass ratio does not exploit the SnO2/Cu/CNT benefit, i.e., allowing a small DU and giving more room to DU þ . When the current is raised to 1.0 A g1, Fig. 9b, DU decreases to 1.48 V, but two other unfavorable signs emerge. First, the positive potential curve bulges up in charging, such a bulge implies the cell is approaching the limit of electrolyte decomposition. Second, the negative potential curve shows a pronounced IR drop, suggesting the negative capacity of 1:0.33 cell is insufficient to meet the electric current demand. Discussion of Figs. 8 and 9 also reveals the significance of U0%SOC position, where the positive and the negative electrode potentials equal at 0% state of charge. U0%SOC denotes the position where
Fig. 8. Cell voltage and electrode potentials of three cells being charge and discharge at 0.3 A g1. Potential variations with time of positive and negative electrodes in the (a) 1:0.67 cell; (b) 1:0.33 cell; (c) 1:1.00 cell. The U0%SOC value is marked for each full cell. The kinetic barrier of the negative electrode in 1:1 cell is also indicated with a dashed square.
Fig. 9. Cell voltage and electrode potentials of three cells being charge and discharge at 1.0 A g1. Potential variations with time of positive and negative electrodes in the (a) 1:0.67 cell; (b) 1:0.33 cell; (c) 1:1.00 cell. The U0%SOC value is marked for each full cell. The kinetic barrier of the negative electrode in 1:1 cell is also indicated with a dashed square.
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Table 1 Potential values at 0% state of charge U0%SOC (vs. Li/Li+) versus specific charging current for three hybrid capacitors with the AC:SnO2/Cu/CNT mass ratios, 1:1.00, 1:0.67, 1:0.33. mass ratio
0.3 A g1
0.4 A g1
0.5 A g1
0.7 A g1
0.9 A g1
1.0 A g1
3.0 A g1
1:1.00 1:0.67 1:0.33
1.26 1.19 2.13
1.30 1.18 2.08
1.34 1.18 2.04
1.39 1.18 1.97
1.45 1.18 1.92
1.48 1.17 1.88
1.74 1.17 1.79
All listed values are in the unit of V (vs. Li/Li+).
positive and negative electrodes begin to store electricity. A slight shift in U0%SOC can alter how the voltage window is partitioned and how the electrode capacity is exploited. Unfortunately, the U0%SOC position is influenced by more than one factor. The major factor is the prelithiated negative electrode pulls down the potential of negative electrode near the lithium metal potential, such that the maximum potential of LIHC is kept around 4.0 V (vs. Li/Li+) and the undesired side reactions are avoided at the positive electrode. But the pull-down effect is not the only factor. According to the above reasoning, the U0%SOC position of 1:1 cell ought to be the lowest among the three cells, since the 1:1 cell has the highest SnO2/Cu/ CNT loading among three cells. Table 1 lists the U0%SOC values of the 1:1, 1:0.67, and 1:0.33 cells operated at various specific currents. The 1:1 cell does not follow that rationale, its U0%SOC values are higher than those of the 1:0.67 cell, not lower. Discussion in the preceding paragraph has revealed the minimum potential is another factor of U0%SOC position. The minimum potential of 1:1 cell reaches the region of kinetic barrier, <0.15 V (vs. Li/Li+), which diminishes the negative electrode capacitance, consumes more voltage window, and lifts the U0%SOC position. Furthermore, the kinetic barrier effect increases with increasing specific current for the 1:1 cell. Therefore, the U0%SOC position increases with increasing specific current for the 1:1 cell. Yet the U0%SOC positions of 1:0.67 and 1:0.33 cells decrease with increasing current, Table 1. Cycle stability of the hybrid cell with a 1:0.67 mass ratio is plotted in Fig. 10. When operated at 0.3 A g1, this cell begins with a capacitance 44.3 F g1. The cell capacitance decreases with increasing cycle number, and ends at 35.8 F g1 at 200th cycles. The capacity retention is 81%. On the other hand, the coulombic efficiency increases gradually with increasing cycle number. The increasing coulombic efficiency suggests the capacity retention of SnO2/Cu/CNT negative is improving with the cycling number. The initial efficiency is 92%, increases to 95% in the 15th cycle, ends at 97% in the 200th cycle.
Fig. 10. Cycle stability of the 1:0.67 hybrid capacitor. Capacity retention and coulombic efficiency are plotted versus cycle number for the 1:0.67 capacitor operated at 0.3 A g1. The inset shows the voltage curves of charge and discharge at the 5th, 25th, 70th, 110th, 160th, 200th cycle.
4. Conclusions Three LIHCs with 1:1, 1:0.67, 1:0.33 mass ratios have been assembled and studied to understand the role of prelithiated SnO2/ Cu/CNT negative electrode. Although the positive electrodes of the three capacitors are loaded with the same amount of AC, the positive capacitances are not the same. The positive capacitance of the 1:1 cell is the highest, that of 1:0.33 cell is the lowest, and that of 1:0.67 is in between, indicating a large SnO2/Cu/CNT capacity bring forth more capacity out of the AC electrode. But a higher AC capacitance does not mean a higher cell capacity. The 1:0.67 cell excels the other two capacitors in cell capacitance and specific energy. We investigate the electrode potentials during charge and discharge and uncover two main reasons, which involve the potential position of U0%SOC and the slow discharge kinetics of SnO2/Cu/CNT near 0.0 V (vs. Li/Li+). We note the 1:0.67 cell is featured with a nearly constant U0%SOC value, in contrast to those of the two other cells. And the negative electrode potential of 1:0.67 cell stays away from 0.0 V (vs. Li/Li+). The cycle stability of 1:0.67 cell is also measured, its capacity retention is 81% in 200 cycles at 0.3 A g1. Acknowledgment The authors would like to express their gratitude to Ministry of Science and Technology of Taiwan for partial support through the project MOST-103-2221-E-011-153-MY3. The authors also thank National Taiwan University of Science and Technology for paying the miscellaneous fee through one of Top University Projects 105H45140. References [1] J.H. Lee, W.H. Shin, S.Y. Lim, B.G. Kim, J.W. Choi, Mater. Renew. Sustain. Energy 3 (2014) 22. [2] J.H. Lee, W.S. Shin, M.H. Ryou, J.K. Jin, J. Kim, J.W. Choi, ChemSusChem 5 (2012) 2328–2333. [3] D. Cericola, R. Kotz, Electrochim. Acta 72 (2012) 1–17. [4] W.J. Cao, J.P. Zheng, J. Electrochem. Soc. 160 (2013) A1572–A1576. [5] W. Cao, J. Zheng, D. Adams, T. Doung, J.P. Zheng, J. Electrochem. Soc. 161 (2014) A2087–A2092. [6] D.P. Dubal, O. Ayyad, V. Ruiz, P. Gomez-Romero, Chem. Soc. Rev. 44 (2015) 1777–1790. [7] F. Zhang, T. Zhang, X. Yang, L. Zhang, K. Leng, Y. Huang, Y. Chen, Energy Environ. Sci. 6 (2013) 1623–1632. [8] X. Chen, J. Guo, K. Gerasopoulos, A. Langrock, A. Brown, R. Ghodssi, J.N. Culver d, C. Wang, J. Power Sources 211 (2012) 129–132. [9] N. Nitta, G. Yushin, Part. Part. Syst. Charact. 31 (2014) 317–336. [10] Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Nature 276 (1997) 1395–1397. [11] J.Y. Huang, L. Zhong, C.M. Wang, J.P. Sullivan, W. Xu, L.Q. Zhang, S.X. Mao, N.S. Hudak, X.H. Liu, X.H. Liu, A. Subramanian, H. Fan, L. Qi, A. Kushima, J. Li, Science 330 (2010) 1515–1520. [12] C. Kim, M. Noh, M. Choi, J. Cho, B. Park, Chem. Mater. 17 (2005) 3297–3301. [13] P. Lian, X. Zhu, S. Liang, Z. Li, W. Yang, H. Wang, Electrochim. Acta 56 (2011) 4532–4539. [14] G. Du, C. Zhong, P. Zhang, Z. Guo, Z. Chen, H. Liu, Electrochim. Acta 55 (2010) 2582–2586. [15] X. Li, X. Meng, J. Liu, D. Geng, Y. Zhang, M.N. Banis, Y. Li, J. Yang, R. Li, X. Sun, M. Cai, M.W. Verbrugge, Adv. Funct. Mater. 22 (2012) 1647–1654. [16] J. Zhu, D. Lei, G. Zhang, Q. Li, B. Lu, T. Wang, Nanoscale 5 (2013) 5499–5505. [17] X.W. Lou, J.S. Chen, P. Chen, L.A. Archer, Chem. Mater. 21 (2009) 2868–2874. [18] X.W. Lou, C.M. Li, L.A. Archer, Adv. Mater. 21 (2009) 2536–2539. [19] I.A. Courtney, J.R. Dahn, J. Electrochem. Soc. 144 (1997) 2045–2051.
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