Organic Li4C8H2O6 Nanosheets for Lithium-Ion Batteries - Nano

Aug 26, 2013 - Organic tetralithium salts of 2,5-dihydroxyterephthalic acid (Li4C8H2O6) with the morphologies of bulk, nanoparticles, and nanosheets h...
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Organic Li4C8H2O6 Nanosheets for Lithium-Ion Batteries Shiwen Wang, Lijiang Wang, Kai Zhang, Zhiqiang Zhu, Zhanliang Tao,* and Jun Chen* Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Synergetic Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: Organic tetralithium salts of 2,5-dihydroxyterephthalic acid (Li4C8H2O6) with the morphologies of bulk, nanoparticles, and nanosheets have been investigated as the active materials of either positive or negative electrode of rechargeable lithium-ion batteries. It is demonstrated that, in the electrolyte of LiPF6 dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC), reversible two-Liion electrochemical reactions are taking place with redox Li4C8H2O6/ Li2C8H2O6 at ∼2.6 V for a positive electrode and Li4C8H2O6/ Li6C8H2O6 at ∼0.8 V for a negative electrode, respectively. In the observed system, the electrochemical performance of high to low order is nanosheets > nanoparticles > bulk. Remarkably, Li4C8H2O6 nanosheets show the discharge capacities of 223 and 145 mAh g−1 at 0.1 and 5 C rates, respectively. A capacity retention of 95% is sustained after 50 cycles at 0.1 C rate charge/discharge and room temperature. Moreover, charging the symmetrical cells with Li4C8H2O6 nanosheets as the initial active materials of both positive and negative electrodes produces all-organic LIBs with an average operation voltage of 1.8 V and an energy density of about 130 Wh kg−1, enlightening the design and application of organic Li-reservoir compounds with nanostructures for all organic LIBs. KEYWORDS: Lithium-ion batteries, organic electrode materials, nanosheets, spectroscopy, energy level diagram

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lithiated naphthazarin unit and 3,6-dihydroxy-2,5-dimethoxyp-benzoquinone dilithium (Li2DHDMQ) reacted at a ∼3 V potential range but exhibited relatively low capacity (∼125 mAh g−1) and high polarization.21,22 Thus, there is plenty of room to develop organic Li-reservoir compounds for all-organic LIBs with improved performance such as high-output voltage, high capacity, and long cycle life. Recently, nanostructures and nanocomposites of organic conducting polymers have been investigated in energy storage fields such as batteries and supercapacitors because of their numerous active sites and facile electronic/ionic transfer and diffusion.23−25 Shifting from bulk to nanostructured electrode materials could offer the opportunity to develop advanced green batteries with large capacity, high energy and power density, high efficiency, and long cycle life.26 Therefore, the combination of lithiated organic molecules and nanostructures will be of importance for developing all-organic LIBs with high performance. In this Letter, we systematically studied the electrochemical performance of organic tetralithium salts of 2,5-dihydroxyterephthalic acid (Li4C8H2O6, Li4DHTPA) with the morphologies of bulk, nanoparticles, and nanosheets. It is demonstrated that with normal LiPF6-ethylene carbonate-dimethyl carbonate electrolyte, reversible two-Li-ion electrochemical reactions

rganic molecules as electrode materials of rechargeable lithium-ion batteries (LIBs) have captured worldwide attention because of their high capacities, molecular controllability, structural diversity, and resource renewability.1−3 Exciting progress has been made on organic electrode materials for LIBs, including organic free radical compounds,4−6 organosulfur compounds,7,8 and organic carbonyl compounds.9−16 However, one of the most critical issues related to the utilization of organic electrode materials is their high solubility in aprotic electrolytes commonly used in the LIBs. Among various strategies to address the problem, using lithium salts of organic molecules is very effective for inhibiting the dissolution of organic electrode materials in aprotic electrolytes.17−20 It is found that lithium dicarboxylate (Li2C8H4O4) delivered 234 mAh g−1 after 50 cycles with a potential plateau of 0.8 V at the current density of 15 mA g−1, which is about 78% of the theoretical capacity (301 mAh g−1).18 Meanwhile, ethoxycarbonyl-based lithium salt (Li2C18H12O8) showed the capacity of 125 mAh g−1 in the first cycle and ∼110 mAh g−1 in the 50th cycle over two voltage plateaus at ∼1.96 and 1.65 V.19 More interestingly, a tetralithium salt of tetrahydroxybenzoquinone (Li4C6O6) was reduced to Li6C6O6 at 1.8 V and oxidized to Li2C6O6 at 2.7 V with a reversible capacity of ∼200 mAh g−1.17,20 This battery system however shows low output voltage (∼1 V) but clearly reveals that using the lithiated (or Lireservoir) oxocarbon salt is feasible to construct all-organic symmetrical rechargeable LIBs. Following a similar concept, a few particularly designed lithiated compounds such as a © XXXX American Chemical Society

Received: June 19, 2013 Revised: August 10, 2013

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Scheme 1. Structures and Electrochemical Redox Reactions between Li4C8H2O6 (Li4DHTPA) and Li2C8H2O6 (Li2DHTPA)/ Li6C8H2O6 (Li6DHTPA)

occur with redox couples of Li4C8H2O6/Li2C8H2O6 in the positive electrode and Li4C8H2O6/Li6C8H2O6 in the negative electrode, respectively (Scheme 1). With three morphologies of Li4C8H2O6 as the active materials of either positive or negative electrode, nanosheets showed the best electrochemical performance with an initial discharge capacity of 223 mAh g−1 and a capacity retention exceeding 95% after 50 cycles at 0.1 C rate. Furthermore, two redox couples of Li4C8H2O6/ Li2C8H2O6 and Li4C8H2O6/Li6C8H2O6 with the starting Li4C8H2O6 nanosheets as both the positive and negative electrodes enable the obtaining of all-organic LIBs with remarkable performance such as high working voltage (1.8 V) and high capacity retention (91% capacity retention after 20 cycles). Details of the preparation of Li4C8H2O6 bulk, nanoparticles, and nanosheets are described in Supporting Information (SI). In brief, Li4C8H2O6 bulk was synthesized by a simple reaction of 2,5-dihydroxyterephthalic acid (DHTPA) with lithium methoxide in methanol solution (Scheme S1).27 Li4C8H2O6 nanoparticles with the size of ∼50−200 nm were prepared by ball-milling the mixture of bulk Li4C8H2O6 and carbon black in argon atmosphere. Li4C8H2O6 nanosheets with the thickness of several nanometers were obtained with the ultrasonic exfoliation of the corresponding bulk. Elemental analysis, IR, NMR, MS, and XRD (Table S1 and Figure S1−S4) clearly illustrate the formation of tetralithium salt (Li4DHTPA, Li4C8H2O6) from DHTPA. The tetralithium salt couples conjugated enolate and carboxylate groups at two para positions of one benzene ring. The electron donation effect of enolate sites and the electron delocalization capability of carboxylate groups in Li4C8H2O6 contribute synergistically to the positive shift of delithiation potential and the negative shift of lithiation potential, respectively. Thermogravimetric analysis reveals the thermal stability of Li4C8H2O6 up to 300 °C in Ar atmosphere (Figure S5), which is believed to be a key factor concerning the safety of LIBs. The as-synthesized bulk Li4C8H2O6 is in the form of pale yellow powders with lamellar morphology, and the average particle size is in the micrometer range (Figure 1a). After ball milling, Li4C8H2O6 exhibits the agglomeration of irregular nanoparticles with diameters of ∼50−200 nm (Figure 1b). After ultrasonication exfoliation, the lamellar bulk Li4C8H2O6 is exfoliated to highly dispersed nanosheets (Figure 1c), which is further confirmed by TEM image (Figure 1d). The nanosheets are characteristic of interweave structure with several nanometers thickness of each sheet (inset of Figure 1d). It is found that the exfoliation of Li4C8H2O6 bulk in higher protic solvents such as in methanol rather than in ethanol or diethyl ether is better to obtain the nanosheets. Both the ultrasonic dispersion effect and the interaction (e.g., solvent molecule coordination to lithium-ion) between tetralithium salt and solvent molecule methanol play the important role in the formation of nanosheets. The specific surface areas of Li4C8H2O6 bulk,

Figure 1. SEM images of Li4C8H2O6 (a) bulk (the inset shows the optical photograph of the powders in glass vial), (b) nanoparticles, and (c) nanosheets. (d) TEM image of Li4C8H2O6 nanosheets with the inset of a single nanosheet.

nanoparticles, and nanosheets were measured to be 20.4, 109.2, and 118.7 m2 g−1, respectively (Figure S6). Electrochemical performance of Li4C8H2O6 bulk, nanoparticles, and nanosheets was investigated using coin-type cells assembled with Li counter electrode and LiPF6-EC-DMC electrolyte (electrochemical measurements, SI). The opencircuit voltages of all fabricated cells are close to 2.4 V. To illustrate the two conjugated enolate and carboxylate functional groups in Li4C8H2O6, the cells were tested in two voltage ranges of 1.8−3.2 V (for positive electrode) and 0.4−2.0 V (for negative electrode). The electrochemical performance of the cells with Li4C8H2O6 bulk, nanoparticles, and nanosheets is shown in Figure S7, Figure S8, and Figure 2, respectively. The electrochemical test demonstrates that Li4C8H2O6 bulk, nanoparticles and nanosheets can reversibly delithiate at ∼2.6 V and lithiate at ∼0.8 V with two Li per formula unit, respectively. The overall performance from high to low follows the order of nanosheets > nanoparticles > bulk. Figure 2a shows the typical galvanostatic charge and discharge profiles of the cells with Li4C8H2O6 nanosheets cycled between 1.8 and 3.2 V. The first charge (Li4C8H2O6 to Li2C8H2O6) and discharge (Li2C8H 2O6 to Li4C8H2O 6) capacities were 226 and 223 mAh g−1, respectively. The measured capacities are about 93% of the theoretical value (241 mAh g−1), of which 95% was kept after 50 cycles. In addition to high capacity retention, the Coulombic efficiency is close to 99.9% (Figure 2b). In comparison, the first charge and discharge capacities for Li4C8H2O6 bulk were 148 and 145 B

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Figure 2. Electrochemical performance of the cells with Li4C8H2O6 nanosheets in the voltage range of (a, b, c) 1.8−3.2 V and (d, e, f) 0.4−2.0 V. (a, d) Charge and discharge curves at 0.1 C rate (24.1 mA g−1) with the inset showing the corresponding CVs; (b, e) Cycling and Coulombic efficiency at 0.1 C rate; (c, f) Various rate capability (nC corresponds to charging or discharging the full capacity in 1/n h).

mAh g−1, with capacity retention of 94% after 50 cycles (Figure S7). The nanoparticles outperformed the bulk sample but were slightly inferior to the nanosheets in reversible capacities (Figure S8). The rate capability of Li4C8H2O6 bulk and nanosheets was measured (Figure 2c). At a high rate of 5 C, Li4C8H2O6 nanosheets display the discharge capacity of 145 mAh g−1, which is about 7 times of that for Li4C8H2O6 bulk (20 mAh g−1). Cyclic voltammograms (CV) (inset of Figure 2a) show that, in the initial anodic and cathodic sweeping, only one sharp oxidation peak at 2.7 V and a pair of reduction peaks at 2.6 and 2.35 V were observed with a nearly equivalent integral area of redox peaks. Meanwhile, in the following cycle, two pairs of redox peaks emerge, which is in good agreement with the charge/discharge profiles. The initial anodic peak can be regarded as an activation process for Li4C8H2O6 nanosheets. The subsequent two redox pairs illustrate 2 × 1e reactions, as shown in the right part of Scheme 1. The observed small transition peak can be viewed as an overlay of the two distinctive peaks, suggesting that the two stepwise redox reactions proceed simultaneously to some extent. Figure 2d shows the typical discharge and charge curves of the cells with Li4C8H2O6 nanosheets cycled in the voltage range of 0.4 and 2.0 V. In this case, the cells were first discharged and then charged. The average discharge and charge plateaus of the cells are around 0.8 and 1.0 V, respectively. The first discharge capacity was 358 mAh g−1, while the subsequent charge capacity was 254 mAh g−1. This initial low Coulombic efficiency (∼71%) is due to the irreversible capacity arising from the formation of solid electrolyte interphase (SEI), which is a common character for anode materials of LIBs at the first cycle.28 In the low potential range (inset of Figure 2d), there is one reduction peak centered at 0.7 V and one oxidation peak at 1.0 V. In the following cycle, the reduction peak becomes more symmetrical and shifts to a slightly positive potential (0.8 V) with a lower integral area, while the oxidation peak is essentially preserved. The difference between the first and the second

cycle can be ascribed to the formation of SEI. The CVs in the low potential range of 0.4 to 2.0 V show only one redox pair (inset of Figure 2d), which is dissimilar to that in the high potential region of 1.8−3.2 V (inset of Figure 2a). This implies a fast transformation from molecular Li4C8H2O6 to radical anion (Li+[Li4C8H2O6]−•) and dianion (Li22+[Li4C8H2O6]2−) or a direct 2e-transfer redox reaction. A direct 2e process is more feasible as can be thermodynamically predicted from the density functional theory (DFT) calculations presented later. After 50 cycles, the capacity of 232 mAh g−1 was retained for Li4C8H2O6 nanosheets (Figure 2e). The common reasons for such fading include structural/morphological material changes on cycling or material solubility issue, although we did not observe any discoloration on dismantling the cell (Figure S9). It is noted that the conducting carbon additive (20 wt %) contributes partially to the total capacity of the electrode cycled below 1.5 V but negligibly above 1.8 V (Figure S10). By subtracting this contribution, the reversible capacity between Li4C8H2O6 and Li6C8H2O6 approaches the theoretical data of 241 mAh g−1, corresponding to two Li+ reactions as shown in the left part of Scheme 1. The rate capability of Li4C8H2O6 bulk and nanosheets was also measured (Figure 2f). At a high rate of 5C, Li4C8H2O6 nanosheets show the discharge capacity of ∼175 mAh g−1, which is much higher than that of Li4C8H2O6 bulk (80 mAh g−1). Thus, either as the positive electrode or as the negative electrode, Li4C8H2O6 nanosheets exhibit a greatly improved rate capability than that of the bulk. The superior performance can be understood from the beneficial factors that 2D nanosheets provide shorter Li+ diffusion paths and large contact areas both for the conducting agent and for the electrolyte. Notably, any morphology of Li4C8H2O6 shows a better rate capability in the negative electrode than that in the positive electrode. Furthermore, since only 20% carbon additive is added in the negative electrode while 30% carbon additive is used in the positive electrode, such a difference is not because of the carbon additive but because of the electronic states of the C

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Figure 3. Structural evolution on the cycling of the cells with Li4C8H2O6 nanosheets: (a) A voltage−composition profile at 0.1 C rate. (b) Ex situ IR for the samples taken at different states as marked by A, B, C, D, and E in (a). Selected in situ Raman spectra of the electrode cycled (c) in high potential range (from 2.4 to 3.2 V for charge and from 3.1 to 2.3 V for discharge) and (d) in low potential range (from 2.3 to 0.4 V for discharge and from 0.7 to 2.0 V for charge).

Figure 4. (a) HOMO plots of C8H2O62−, C8H2O64−, and C8H2O6− anions. (b) Energy level diagram of Li2C8H2O6, Li4C8H2O6, and Li6C8H2O6 relative to energy in vacuum (Evac).

phenolic-oxygen signal disappears, while the vibration at 1630 cm−1 indicates the presence of quinonoid carbonyl group. At fully discharged 0.4 V (point D), the carboxylate carbonyl vibration is not distinguishable while the enolate vibration remains. Inspecting the disparity in representative infrared data reveals redox reactions of enolate/quinonoid carbonyl couple at high potentials and carboxylate/enolate couple at low potentials. An in situ Raman technique was carried out to monitor the cycling process of Li4C8H2O6 (Scheme S2). Figure 3c−d shows a series of selected in situ data collected at high potential range (from 2.4 to 3.2 V for charge and from 3.1 to 2.3 V for discharge) and at low potential range (from 2.3 to 0.4 V for discharge and from 0.7 to 2.0 V for charge), respectively. Strong peaks due to electrolyte and carbon additive are also detected in the spectra, masking some of the characteristic signals. Fortunately, it is easy to discern Raman shifts at 634 and 1216 cm−1, which correspond to the out-plane bending of

organic molecules during the cathodic and anodic processes, as illustrated by theoretical modeling calculations later. To gain insight into the structural evolution of Li4C8H2O6, we performed infrared (IR) spectra and Raman spectroscopic analysis of the electrode on cycling within the full potential window of 0.4−3.2 V (spectroscopic study, SI). Well-defined plateaus can be observed both above 2.3 V and below 1 V with a low potential gap around 0.2 V (Figure 3a). Ex situ IR spectra were recorded for the fully charged and discharged samples that are recovered from the dismantled cells (Figure 3b). The IR spectra for the electrode at initial 2.3 V (point A), discharged 1.8 V (point C), and charged 2.0 V (point E) are reminiscent of that for the as-made Li4C8H2O6 (Figure S1). The strong vibrations centered at around 1231 and 1585 cm−1 are assigned to C−O stretching mode of phenolic groups and CO stretching mode of carboxylate groups, respectively. The peak at low frequency of ∼565 cm−1 is attributed to COO−Li outplane bending vibration. At fully charged 3.2 V (point B), the D

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Figure 5. (a) Typically galvanostatic charge/discharge profiles and (b) cyclability and Coulombic efficiency of symmetrical LIBs with organic Li4C8H2O6 nanosheets at 48.2 mA g−1. The loading amount of the active material for positive electrode and negative electrode is about 1.5 and 1.0 mg cm−2, respectively.

COO−Li and the symmetrical stretching of C−O, respectively. At 2.3−3.2 V, the carboxylate group vibrations are preserved, while the enolate group peaks are gradually weakened on charging, but this is enhanced on discharging. In comparison, the carboxylate intensity gradually declines within the low potential region of 0.4−2.3 V. It is noted that all Raman intensities decrease at the plateau region of 0.4−0.9 V. The weakened vibrations at low potentials are ascribed to an increase in structural symmetry and electrical conductivity of the Li-rich materials.29 Thus, the combination of IR and Raman spectral changes during the charge and discharge confirms the material evolution of Li4C8H2O6: the formation of Li2C8H2O6 on full charging and the generation of Li6C8H2O6 upon full discharging. DFT calculations were further carried out to understand the energy level of Li2C 8H2 O6, Li4C 8H2O 6, and Li 6C8 H2O6 (computational details, SI). The highest occupied molecular orbitals (HOMO) plots (Figure 4a) show that the anions of Li4C8H2O6 and the corresponding oxidized and reduced compounds are structurally stable with effective electron delocalization and conjugation. This should result in the high utilization of the active sites.14 The anions could be further stabilized with Li coordination to form the corresponding salts.12 Moreover, the calculated lowest unoccupied molecular orbital (LUMO) and HOMO energy levels reflect the difference in vertical electron affinity (VEA) and vertical ionization potential (VIP) of Li2C8H2O6, Li4C8H2O6, and Li6C8H2O6 (Figure 4b). The energy level of the molecules indicates their oxidizability and reducibility, which coincides with the experimentally determined redox potentials (i.e., 2.6 vs 2.73 V and 0.8 vs 0.61 V from Tables S2 and S3).10 The flexible electronic structure along with the insolubility (or extremely low solubility) of tetrelithium salt (Li4C8H2O6) in organic electrolyte (Figure S9) accounts for the prominent reversibility of Li4C8H2O6-based electrode. It should be noted that, in the low potential region of 0.4− 2.0 V, there is a large difference between the redox energies of Li4DHTPA/Li5DHTPA and Li5DHTPA/Li6DHTPA, while in the high potential region of 1.8−3.2 V, the redox energies of Li2DHTPA/Li3DHTPA and Li3DHTPA/Li4DHTPA are very close (Table S4). These results illustrate that one two-electron redox process within 0.4−2.0 V and two one-electron redox processes within 1.8−3.2 V are occurring, which are consistent with the CVs in the insets of Figure 2a and d. Furthermore, Li2C8H2O6 has a higher Eg value (LUMO−HOMO gap, a higher Eg implies a lower electric conductivity) and lower structure symmetry than that of Li6C8H2O6, which is feasibly the cause of the weaker spectroscopic signal of Li6C8H2O6. This

lower electric conductivity of Li2C8H2O6 also explains the need of more conducting carbon additive for the positive electrode cycled at a high potential window. The considerable electrode capacity and cycleability of reversible electrochemical reactions of Li4C8H2O6/Li2C8H2O6 and Li4C8H2O6/Li6C8H2O6 stimulates the assembly of cointype symmetrical cells. Figure 5 shows the typical cell performance with Li4C8H2O6 nanosheets as the initial active materials of both the positive and negative electrodes. As expected, the fabricated cells without any charging or discharging display open-circuit voltages near zero. However, the first charging to 2.7 V and the subsequent discharging to 1 V at 0.2 C (48.2 mA g−1) exhibit the capacities of 225 and 208 mAh g−1, respectively, which are based on the active mass of the negative electrode. The following galvanostatic charge− discharge curves show a similar sloping shape (Figure 5a). After 20 cycles of charge/discharge at 48.2 mA g−1, 91% of the cell capacity was sustained. The Coulombic efficiency is around 99.7% (Figure 5b). Notably, the average discharge voltage of the cell is about 1.8 V. Although this voltage is still lower than that of inorganic LiCoO2/C or LiFePO4/C LIBs, Li4C8H2O6 features the merits of resource abundance, high reversible capacity (at 200 mAh g−1 level) and structural robustness.30 The energy density of the as-assembled cell is about 130 Wh kg−1 based on the total weight of both positive and negative electrodes with that the performance improvement is still ongoing. In this regard, Li4C8H2O6 is promising for the construction of all organic rechargeable LIBs. In conclusion, we have systematically studied the electrochemical performance of organic tetralithium salts of 2,5dihydroxyterephthalic acid (Li4C8H2O6) with the morphologies of bulk, nanoparticles and nanosheets. The dissolution of Li4C8H2O6 in the EC/DEC electrolyte is extremely low. A combination of electrochemical investigation, DFT modeling, ex situ infrared study, and in situ Raman analysis reveals that Li4C8H2O6 can be reversibly cycled at both potential windows of 1.8−3.2 V and 0.4−2.0 V, corresponding to the redox reactions of Li 4 C 8 H 2 O 6 /Li 2 C 8 H 2 O 6 and Li 4 C 8 H 2 O 6 / Li6C8H2O6, respectively. Among the examined morphologies, nanosheets show the best electrochemical performance with the first discharge capacity of 223 mAh g−1 at 0.1 C (about 93% of the theoretical capacity 241 mAh g−1), the discharge capacity of 145 mAh g−1 at 5 C, and a capacity retention of 95% after 50 cycles at 0.1 C. All-organic LIBs constructed with Li4C8H2O6 nanosheets as the initial active materials of both positive and negative electrodes display high performance with an average operation voltage of 1.8 V and an energy density of about 130 Wh kg−1. The results show that Li4C8H2O6 nanosheets with a E

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(23) Nyström, G.; Razaq, A.; Stromme, M.; Nyholm, L.; Mihranyan, A. Nano Lett. 2009, 9, 3635−3639. (24) Pan, L. J.; Yu, G. H.; Zhai, D. Y.; Lee, H. R.; Zhao, W. T.; Liu, N.; Wang, H. L.; Tee, B. C. K.; Shi, Y.; Cui, Y.; Bao, Z. N. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 9287−9292. (25) Wu, H.; Yu, G.; Pan, L.; Liu, N.; McDowell, M. T.; Bao, Z.; Cui, Y. Nat. Commun. 2013, 4, 1943. (26) Chen, J.; Cheng, F. Y. Acc. Chem. Res. 2009, 42, 713−723. (27) Trukhan, N.; Müller, U.; Panchenko, A.; Malkowsky, I. M.; Fischer, A. U.S. Patent 2011/0260100 A1, 2011. (28) Wu, H.; Chan, G.; Choi, J. W.; Ryu, I.; Yao, Y.; McDowell, M. T.; Lee, S. W.; Jackson, A.; Yang, Y.; Hu, L.; Cui, Y. Nat. Nanotechnol. 2012, 7, 310−315. (29) Baddour-Hadjean, R.; Pereira-Ramos, J. P. Chem. Rev. 2010, 110, 1278−1319. (30) Cabana, J.; Monconduit, L.; Larcher, D.; Palacin, M. R. Adv. Mater. 2010, 22, E170−E192.

high-specific surface area and multifunctional groups are promising in the applications of all organic LIBs.

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ASSOCIATED CONTENT

* Supporting Information S

Details of the synthesis, characterization, and electrochemical measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Programs of National 973 (2011CB935900), NSFC (21231005 and 51231003), and 111 Project (B12015).

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