Novel Conjugated Ladder-Structured Oligomer Anode with High

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A Novel Conjugated Ladder-Structured Oligomer Anode with High Lithium Storage and Long Cycling Capability Jian Xie, Xianhong Rui, Pei-Yang Gu, Jiansheng Wu, Zhichuan J. Xu, Qingyu Yan, and Qichun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04277 • Publication Date (Web): 13 Jun 2016 Downloaded from http://pubs.acs.org on June 14, 2016

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ACS Applied Materials & Interfaces

A Novel Conjugated Ladder-Structured Oligomer Anode with High Lithium Storage and Long Cycling Capability Jian Xie,‡a Xianhong Rui,‡a Peiyang Gu,a Jiansheng Wu,c Zhichuan J. Xu,a Qingyu Yan*a and Qichun Zhang*a,b a

School of Materials Science and Engineering, Nanyang Technological University, Singapore,

639798, Singapore. b

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical

Sciences, Nanyang Technological University, Singapore 637371, Singapore c

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM),

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China

KEYWORDS: Organic Electrode; Ladder-like Oligomer; Rechargeable Battery; Lithium ion; Anode; High Performance; Long Cycling

ACS Paragon Plus Environment

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ABSTRACT

Herein

we

report

the

development

of

nanostructured

poly(1,4-dihydro-11H-

pyrazino[2',3':3,4]cyclopenta[1,2-b]quinoxalin-11-one) (PPCQ), a novel conjugated ladder-like oligomer with the presence of a rich amount of heteroatoms, as the anode material. Beyond its remarkable lithium storage of 972 mAh g-1 after 120 cycles, the superior cycle life and stable capacity performance of 489 mAh g-1 revealed by ultra-long testing of 1000 cycles (with an average Coulombic efficiency 99.8%) at a high current density of 2.5 A g-1 indicate its excellent electrochemical stability to be promisingly applied for high performance lithium-ion batteries (LIBs).

INTRODUCTION Developing high performance rechargeable LIBs is of great interest to achieve the everincreasing energy storage demands for electronic devices and electric vehicles. For a long time, investigations were mainly focused on inorganic materials due to their great achievements in practical applications.1 However, with the growing anxieties about environmental and resource issues regarding the metal-based inorganic electrodes, more and more efforts have been input on organic materials.2, 3 Although the research on organic electrode materials have been conducted as early as inorganic materials, it was still very challenging to design both efficient and ecofriendly organic electrode materials with promising performance. In the past two decades, organic π-conjugated compounds, especially, small molecules and polymers have been extensively investigated as the promising electrode materials for the applications of LIBs.4-15 Sun et al. reported that attaching carbonyl groups onto the backbone of


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polycyclic aromatic compounds could significantly decrease the free energy during the lithiation process, and as a result, each C6 ring in these materials could reversibly lithiate and delithiate up to six lithium ions to form a Li6/C6 additive complex, where 1,4,5,8-naphthalenetetracarboxylic di-anhydride (NTCDA) displays a high specific capacity of 1804 mAh g-1 initially, almost equal to its theoretical capacity.14 Ji et al. noticed that sodium ions could also be incorporated into the unsaturated carbon centers of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), providing an initial specific capacity of 1017 mAh g-1 (at the first discharge), which corresponds to the acceptance of fifteen sodium ions.7 Although PTCDA and NTCDA displayed excellent initial discharge capacities, their cycling performance were far away from the practical application since either the material could only bear small current densities during the cycling process or there was a large decay of capacity due to that the electrode materials were easily dissolved during cycling. To address this serious dissolution problem of organic small molecules, many researches have been switched to insoluble organic conjugated polymers because their poor solubility could provide much better cycling stability.16, 17 Especially, conjugated polymers including polyaniline (PANi), polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) have been widely employed as the matrices or coatings to improve the electronic conductivity of the composite electrodes.18-21 Nevertheless, the contribution of the capacity performance from these conducting polymers was still much lower than that from the inorganic materials. Therefore, it is very urgent to develop novel conjugated polymers that could be employed as the active electrodes with promising energy storage performance. One class of polymers that could offer superiority is the π-conjugated ladder polymers, which show great potential to be employed as the active electrode materials due to their extended π-


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conjugations, good electronic conductivities and high chemical stabilities.22-24 In addition, the double-stranded structure endows ladder polymers with rigid backbone properties (inherent stability) and poor solubility that can offset the serious dissolution issue, which is the main problem of most organic electrodes. Moreover, redox-active π-conjugated polymers with multielectron reactions can deliver excellent specific capacities as well as energy densities.25,

26

Because of these advantages, π-conjugated redox-active ladder polymers are being considered as the promising electrode materials for rechargeable LIBs. It was revealed that the electronic conductivity and electrochemical activity of polymer electrodes can be greatly improved with the presence of nitrogen heteroatoms,27 resulting in the increment of lithium storage. Taking over the merits of hetero-atoms and utilizing ladder polymers’ rigid backbone structure, our recent studies showed that conjugated ladder polymers with a rich number of oxygen and nitrogen heteroatoms could be promising redox-active anode materials for LIBs.28, 29

Scheme 1. Synthetic route to the ladder-structured polymer PPCQ. Building on our experience on conjugated ladder polymers, here we demonstrate the development of a novel nanostructured conjugated ladder-like oligomer poly(1,4-dihydro-11Hpyrazino[2',3':3,4]cyclopenta[1,2-b]quinoxalin-11-one) (PPCQ), which shows an excellent electrochemical performance in LIBs as an anode material. It is expected that the electrodes with nano-sized dimensions could result in a shorter diffusion length and therefore a reduction of the distance that both lithium ions and electrons need to travel during the cycling process could be


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achieved.30 Importantly, benefiting from the small diameters of nanostructures, the contact area between the active reaction sites of the electrode and the electrolyte would be increased and would in turn improve the energy efficiency.30 Our previous research has already shown that the introduction of heteroatoms (e.g., O, N atoms) into the conjugated systems is favorable to redox reactions due to their lone pair electrons.29 This phenomenon has also been investigated in PPCQ: the electrochemical performance of PPCQ in terms of initial reversible capacity (1678 mAh g-1), cycling capability (972 mAh g-1 after 120 cycles) and rate stability (0.2 A g-1 to 10 A g-1), indicates the superiority of PPCQ nanoparticles over all the previously reported organic small-molecule anodes and most of polymer anodes. More importantly, under a high charge and discharge rate of 2.5 A g-1, PPCQ still delivers a high specific capacity of 489 mAh g-1 after 1000 cycles, considerably better than the commercialized anode material (graphite, 372 mAh g-1). The ultra-long and fast cycling capability also reveals the potential of PPCQ to be possibly applied in fast cycling LIBs. RESULTS AND DISCUSSION PPCQ was obtained from a simple one-step polymerization reaction between 4,5dihydroxycyclopentenetrione (croconic acid) and 1,2,4,5-tetraaminobenzene tetrahydrochloride (TAB·4HCl) in 115% polyphosphoric acid (PPA) at 190 °C for 10 hours (scheme 1). The finally obtained black powders (pristine PPCQ) were insoluble in almost all commonly-used organic solvents including methanol, N,N-Dimethylformamide (DMF), dimethyl sulfoxide (DMSO), NMethyl-2-pyrrolidone (NMP) and etc. To fabricate PPCQ nanoparticles, pristine PPCQ was dissolved into strong protonic methanesulfonic acid (MSA). The resulted dark color solution was then dropwisely added into DI water under a rapid stirring. The as-obtained precipitates were


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ultrasonicated, centrifuged and washed with DI water and methanol respectively and later dried. The finally obtained PPCQ nanoparticles were employed as an anode material in lithium batteries.

(a)

(b)

1 µm

400 nm

Figure 1. Field emission scanning electron microscopy (FESEM) characterization of PPCQ nanoparticles. a) Low-magnification (×10000); b) High-magnification (×30000). Figure 1 shows the morphology characterization of the as-prepared PPCQ nanoparticles. The sizes of the PPCQ nanoparticles are larger than 100 nm. Although PPCQ nanoparticles showed a certain extent of aggregation, the much smaller diameters of PPCQ nanoparticles could be clearly observed as compared with the largely aggregated micro-structured pristine PPCQ blocks (Figure S1). In addition, the FESEM images of the composed PPCQ anode were also obtained (Figure S2). The XRD spectrum of PPCQ nanoparticles was displayed in Figure S3, and in agreement with the studies in conjugated polymers,31, 32 the distinctive diffraction peak observed at around 26° is explained by the π-π stacking between the conjugated main chains. To get a deeper understanding of the chemical structure of PPCQ, the elemental analysis (EA) has been performed. From the obtained EA results (Table S1), the measured contents of C, H and N in


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PPCQ are respectively 52.69%, 2.79% and 19.11%, indicating that the molar ratio of C/N is 2.76. Based on this result, we could possibly infer that the obtained PPCQ is an oligomer with an average degree of polymerization equaling to approximately four, and the terminal groups of PPCQ are mainly consisted of OH groups. The proposed chemical structures of PPCQ oligomers are presented in Figure S4, which contain might different structural possibilities (with monomeric units n = 3,4,5 and etc.) with variable contents of water molecules. Because of the strong hydrogen bond (O-H···O) between H2O and PPCQ, each carbonyl group (C=O) could be associated with one H2O to form the proposed mixtures of the oligomer structures of PPCQ (Figure S4).

To confirm and verify the successful polymerization of the targeted compound, the chemical structure of PPCQ was carefully characterized by Fourier transform infrared spectroscopy (FTIR) (Figure 2). The absorption peak of the carbonyl groups C=O is observed at 1696 cm-1.26, 33

The weak peak at around 3300 cm-1 and the peak at 1220 cm-1 could be attributed to the

stretching vibration of the N-H bonds and C-N bonds respectively.34 The peaks observed between 1628 and 1446 cm-1 are ascribable to the C=N stretchings,34 indicating the successful reaction between the two reactants to form the ladder-like oligomer. It is worthy to note that the enhanced intensity of PPCQ nanoparticles at around 856 cm-1 as compared with pristine PPCQ (Figure S5), which is associated with the benzene structure, suggests an enhancement of the conjugation of the PPCQ main chain.28 A large and broad peak between 3400 and 2000 cm-1 could be observed and should be resulted from the O-H stretching and water,35,

36

further

indicating the existence of O-H groups and water molecules in the product. This result is tally with the EA results and could reasonably explain why there is a large and broad peak between


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3400 and 2000 cm-1. The similar FTIR patterns between PPCQ nanoparticles and pristine PPCQ reveal that the organic structure of PPCQ was not destroyed during the nano-engineering process. To further confirm this, thermal stability of pristine PPCQ and PPCQ nanoparticles were investigated by thermogravimetric analysis (TGA) (Figure S6). The curves of pristine PPCQ and PPCQ nanoparticles were quite similar. TGA result shows that PPCQ nanoparticles loss approximately 5% of its weight below 200 °C caused by the water loss. This 5 wt% of water is close to the percentage of water content in the proposed chemical structures, further confirming the formation of the proposed oligomer structures in PPCQ (Figure S4). Owing to the existence of the mixtures of oligomers, PPCQ sample shows continuous degradation upon heating up to 700 °C, nevertheless, approximately 70% of PPCQ could still be maintained, revealing a moderate stability at high temperatures.

Figure 2. FTIR characterization of PPCQ nanoparticles. To understand the electrochemical properties of PPCQ nanoparticles, various investigations were performed through the fabrication of half cells, where lithium sheets were applied as the


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counter electrodes. The galvanostatic charge and discharge capability was evaluated at 100 mAh g-1 between 0.0 and 3.0 V. As shown in Figure 3a, the curves of the first two cycles were presented. There are two small plateaus observed at about 2.7 and 1.6 V, followed by a sloping plateau below 1.0 V, which are ascribed to the acceptance of lithium ions onto the carbonyl group,37 nitrogen atoms28, 29 and C6 aromatic rings,14 respectively. We could notice that PPCQ displayed a high capacity of 4098 mAh g-1 at the first discharge with a reversible capacity of 1678 mAh g-1 in the following charge process. The initial reversible capacity corresponded to a Coulombic efficiency (CE) of 41%. The large capacity loss was ascribed to the formation of a solid electrolyte interface (SEI) layer associated with a series of irreversible reactions. The formation of SEI is a very common phenomenon in the first cycle of anode materials for LIBs.38, 39

Although the CE of the first cycle was not high enough, the efficiency could be further

improved through some pre-processing operations. For example, a common method to address the low CE issue is through anode pre-lithiation,40,

41

which could effectively improve the

efficiency of anode materials at the initial cycles, therefore, avoiding the concern of low efficiency in full cell applications. Nevertheless, note that the Coulombic efficiency rapidly increased after the second cycle and stabilized at around 97% over 120 cycles (Figure 3c).


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Figure 3. Electrochemical characterization of the PPCQ anode. (a) Charge and discharge profiles for the initial two cycles; (b) Cyclic voltammetry (CV) curves; (c) Cycling performance and Coulombic efficiency for 120 cycles; (d) Rate performance (each rate consists of ten cycles). To verify and confirm this irreversible lithiation in PPCQ, CV was carried out in the range of 0.0-3.0 V at a scan rate of 0.2 mV s-1. The initial three cycles were examined as shown in Figure 3b. A large and broad tail below 1.1 V was observed at the 1st cathodic sweep, revealing the large contribution of the irreversible capacity resulted from the SEI film during the first discharge process. The completely disappeared peak at 1.1 V and the decreased intensity of the current in the 2nd and 3rd cycles confirmed the irreversible reactions caused by the SEI during the 1st cycle. The SEI film,39 which is a multi-component interface that contains both inorganic compounds and organic species, was formed due to the decomposition of electrolytes occurred


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on the PPCQ anode surface. The electrons generated from the reactions flowed through the surface of the PPCQ anode materials during the initial discharging process and caused the reduction of the electrolyte, resulting in a SEI nucleation. To step into the details of the redox reactions, in the first cathodic sweep, the small peak in the range of 1.4-1.6 V was associated with the addition of lithium ions onto the unsaturated nitrogen atoms.28, 29 The lithiation process accomplished with a sharp declination of the current from 1.0-0.0 V, which was due to the addition of lithium ions onto the C6 aromatic rings along with the formation of the SEI.14 On the reverse anodic sweep, the peak started at 1.4 to 1.75 V corresponded to the delithiation of lithium ions from nitrogen atoms. The oxidation peak centered at about 2.5 V was related to the occurrence of the enolation process, where the carbonyl group formed due to the reoxidation of the lithium enolating.37 The CV curves were almost overlapped from the 2nd cycle onwards, indicating that PPCQ nanoparticles possess good stability and high reversibility when cycling. Additionally, the peaks were in good agreement with the potential plateaus observed in the charge and discharge profiles. The excellent electrochemical stability of PPCQ can also be realized from the cycling test. Figure 3c shows that even after 120 cycles, PPCQ can still deliver a high reversible capacity of 972 mAh g-1. From our understanding, most of the organic materials have been preferred to be the cathode materials. Up to present, seldom pure organic anode materials have been reported, especially redox-active organic anodes with high lithium storage. As far as we know, this capacity performance of PPCQ is better than that of most reported organic anodes, including lithium salts8, 15 and carbonyl compounds.42, 43 It is worthy to note that such a good result is comparable to or even better than the metal-based inorganic anodes that reported with high


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capacity performances so far, for examples, SnO2 (580 mAh g-1 after 100 cycles),44 α-Fe2O3 (945 mAh g-1 after 30 cycles)45 and Co3O4 (970 mAh g-1 after 30 cycles).46 According to the theoretical capacity calculation,3 if we consider that one lithium ion per formula unit of PPCQ contributes to 129 mAh g-1, the reversible capacity of 1678 mAh g-1, referring to a utilization of 93.1% of its theoretical capacity (~1802 mAh g-1), indicates that PPCQ is capable of accepting nearly thirteen lithium ions. The capacity contribution of the 20% carbon nanotubes (CNTs) was also investigated in our present study, even though CNTs were only performed as the conductive additives for the PPCQ anode. The result showed that CNTs contributed a very small and negligible amount of capacity comparing to the PPCQ anode system (Figure S7). Confirmed by our previous investigations on conjugated ladder polymers, the existence of nitrogen and oxygen heteroatoms on the large conjugated ring structure could promote the occurrence of the redox reactions.28, 29 In this research, PPCQ also delivered a high practical capacity, which is closed to its theoretical capacity. Therefore, along with the CV result explained above, we could reasonably propose a two-step mechanism of the lithiation process as described in scheme 2. The first step is the enolation process occurs on the carbonyl group along with the lithiation process on the nitrogen atoms.28, 29, 47-49 The second step is ascribed to the addition of lithium ions onto the carbon centers to form a Li6C6 or Li5C5 complex, which is consistent with the investigations on polycyclic aromatic structures.14,

28, 29, 42

This novel

mechanism challenges the traditional lithiation method that each C6 ring could only receive a lithium ion to form LiC6, and it could provide possibilities to develop high performance organic anode materials through molecular modification and nano-engineering.


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Scheme 2. Schematic diagram of the proposed electrochemical reactions of PPCQ nanoparticles. The small sized nanoparticles, which are closely packed to carbon additives, provide large surface areas for electron transfer among PPCQ nanoparticles. Thus, a good rate performance should be expected. Figure 3d shows that under various current densities of 0.2, 0.5, 1.0, 2.0, 5.0 A g-1, PPCQ could achieve reversible capacities of 963, 667, 509, 419, and 329 mAh g-1, respectively. Even when cycled at an extremely high rate of 10 A g-1, a specific capacity of 269 mAh g-1 could still be achieved. It is noted that a moderate capacity of 396 mAh g-1 (calculated from the average of the 10 cycles at 0.2 A g-1) was attained when the current rate was reset to 0.2 A g-1. Such a capacity was relatively low as compared with its reversible capacity at 0.2 A g-1 during the initial ten cycles. The decay of capacity might be probably because (1) the structure of PPCQ was not stable or it was dissolved to some extent during the cycling process at such high rates (up to 10 A g-1), since most of the reported organic compounds including polymers could only endure low current rates.50 One of the examples was poly(1,5-anthraqui-none) P15AQ, which also encountered this issue when it undergoes various current rates up to 20 C and further returned back to its initial rate of 0.2 C, the retention was only about 67% of its initial reversible capacity; and (2) the ring-size effect could be another reason that affected the rate performance of PPCQ. Comparing with our earlier investigations on SBBL,29 we found that the organic


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material with five-membered ring structure might not be stable in terms of capacity storage and rate

stability

as

compared

with

its

analogue

compound

poly(benzobisimidazob-

enzophenanthroline) (BBL) with six-membered ring structure. In addition, Yoshida et al. also observed this effect by examining organic compounds with different ring sizes according to nucleus-independent chemical shifts (NICS) studies and density functional theory (DFT) calculations.51 They concludde that the compound with six-membered rings displays better stability during the redox reactions compared with the compound with five-membered rings.

Figure 4. Electrochemical performance of PPCQ nanoparticles at 2.5 A g-1. (a) Charge and discharge profiles at different cycles; (b) Coulombic efficiency for 1000 cycles; (c) Corresponding ultra-long cycling performance in terms of charge and discharge capacities.


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Nevertheless, to further understand the stability of PPCQ during the cycling process, PPCQ was tested at a consistent high current rate of 2.5 A g-1 for 1000 cycles. Notice that in the charge and discharge profiles of PPCQ nanoparticles (Figure 4a), even at such a high rate, PPCQ could still maintain at 489 mAh g-1 after 1000 cycles, exhibiting a high capacity retention of 87.8% with respect to its initial reversible capacity (the 2nd discharge, 557 mAh g-1). Figure 4b shows that the average Coulombic efficiency (exclude the 1st cycle) of PPCQ is as high as 99.8%, which is among the best efficiencies of the reported organic electrodes. The overall ultra-long cycling profile is also presented (Figure 4c), where the performance is quite stable and consistent during the 1000 cycles. This extraordinary excellent performance confirms the superiority of PPCQ and indicates that PPCQ is stable enough to be used at high current densities. Moreover, we also tested PPCQ to a low cutoff voltage (from 0.0 to 2.5 V) at 1 A g-1, the result shows that PPCQ is stable (with CE 99.5%) and still retains 407 mAh g-1 after 200 cycles (Figure S8). CONCLUSIONS In conclusion, we prepared a novel ladder-like conjugated oligomer through a simple one-step polymerization process. Through nano-engineering, we obtained the nanostructured PPCQ and investigated its feasibility as the anode material for rechargeable lithium batteries. This novel organic material delivers a high initial reversible capacity of 1678 mAh g-1, corresponding to the acceptance of thirteen lithium ions. Moreover, the capacity retentions of 972 mAh g-1 (at 100 mA g-1) and 489 mAh g-1 (at 2.5 A g-1) after 100 cycles and 1000 cycles, respectively, suggest an excellent cycling capability and electrochemical stability of PPCQ. These superior results could bring the battery performance of organic materials to a new level. Our investigations present an efficient method to develop multi-electron redox active materials with high and stable electrochemical performance. Conjugated ladder polymers (especially their nanostructures)


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containing heteroatoms (e.g. N and O) are superior anode materials for rechargeable lithium batteries. Considering their easy synthesis, low temperature reactions and non-metal containing structures, they are potential green resources for future eco-friendly and sustainable energy storage devices. EXPERIMENTAL Materials Synthesis: Poly(1,4-dihydro-11H-pyrazino[2',3':3,4]cyclopenta[1,2-b]quinoxalin-11one) (PPCQ) was synthesized through the following procedures: 50 g 115% polyphosphoric acid (PPA) was filled into a two-necked flask (100 ml) fitted with a magnetic stirrer and nitrogen inlet and outlet. Next, PPA was heated to 110 ºC with nitrogen bubbling through the stirred acid overnight to remove away the oxygen inside of the solvent. Under a nitrogen atmosphere, 1 mmol 1,2,4,5-tetraaminobenzene tetrahydrochloride (TAB·4HCl) was added into the deoxygenated PPA at 50 ºC. At the same time, the temperature was increased to 110 ºC, and by keeping TAB·4HCl at 110 ºC overnight, all HCl were thermally removed away. In the following process, 1 mmol 4,5-dihydroxycyclopentenetrione (croconic acid) was added into the solution at 70 ºC. Then, the temperature was slowly increased to 190 ºC at a heating rate of 2 ºC min-1 and maintained at 190 ºC for 10 hours with magnetic stirring. The resulted polymer solution was kept to be cooled down to room temperature and later to be precipitated in DI water. After the precipitation process, the black precipitates were ultrasonicated, centrifuged and washed several times by DI water and methanol separately. The precipitates were finally washed by methanol with soxhlet extractor, and the obtained black powders (pristine PPCQ) were placed in a vacuum oven to be dried. PPCQ nanoparticles were obtained by dissolving pristine PPCQ into the highpurity methanesulfonic acid (MSA). In details, 20 mg PPCQ was added into 100 ml MSA to be dissolved with ultrasonication and magnetic stirring. The solution was then dropwisely added


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into a rapid stirring 250 ml DI water and black precipitates were formed immediately. The precipitates were washed by DI water and ethanol separately until the MSA solvent was removed away. The finally obtained PPCQ nanoparticles were placed in vacuum oven at 100 ºC for 12 hours and the as-obtained dried samples were to be characterized and tested. Electrochemical Measurement: The coin-type batteries were fabricated in an argon filled glovebox with oxygen and moisture contents below 1 ppm. The PPCQ electrodes were obtained by mixing of 70 wt% PPCQ with 20 wt% CNTs and 10 wt% polyvinylidene fluoride (PVDF) in NMP solvent. The viscous slurry was then pasted onto the copper foils and dried in a vacuum oven at 60 ºC for 12 hrs. Lithium sheets were applied as the counter electrodes. For the electrolyte solution, 1 mol/L (M) LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1/1, w/w) were used. The cells were assembled and placed for approximately 6 hours and then tested by a multi-channel battery tester (NEWARE) in the voltage range of 3.0-0.0 V and 2.5-0.0 V. CV was performed on an electrochemical analyzer (CH Instrument CHI604e) between 3.0-0.0 V. Materials Characterization: FESEM characterizations were measured by JEOL/JSM-6340F with an accelerating voltage of 5 kV. FTIR spectra were performed on PerkinElmer Spectrum Frontier FTIR spectrometer. TGA curves were obtained by TA Instruments TGA Q500 at 10 ºC min-1 under a nitrogen atmosphere. XRD characterizations were performed on Shimadzu powder with Cu α-radiation (λ = 1.5418). EA was obtained by PerkinElmer 2400 elemental analyzer. ASSOCIATED CONTENT Supporting Information. The supporting information includes: FESEM characterization of pristine PPCQ before dissolving into MSA, FESEM characterizations of the composed electrode,


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XRD pattern of PPCQ nanoparticles, elemental analysis of PPCQ, proposed oligomer structures of PPCQ, FTIR spectrum of pristine PPCQ, TGA curves of PPCQ nanoparticles and pristine PPCQ, capacity contribution from the added CNTs, cycling performance of PPCQ between 0.0 and 2.5 V. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected] Author Contributions ‡These authors contributed equally to this work. ACKNOWLEDGMENT Q.Z. acknowledges financial support from AcRF Tier 1 (RG133/14 and RG 13/15) and Tier 2 (ARC 2/13) from MOE, and the CREATE program (Nanomaterials for Energy and Water Management) from NRF, Singapore. Q.Z. also thanks the support from Open Project of State Key Laboratory of Supramolecular Structure and Materials (Grant number: sklssm2015027), Jilin University, China. Q.Y. gratefully acknowledges Singapore MOE AcRF Tier 1 grants RG 2/13 and RG 113/15. X.R. acknowledges Technology Foundation for Selected Overseas Chinese Scholar (Ministry of Personnel of China), and Anhui Provincial Natural Science Foundation (No. 1608085QB35). REFERENCES


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Table of Contents: A novel nanostructured conjugated ladder-structured oligomer has been prepared and demonstrated to show high lithium storage and long cycling capability as the promising anode material for lithium-ion batteries.


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Novel Conjugated Ladder-Structured Oligomer Anode with High

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