Transition-Metal Triggered High-Efficiency Lithium Ion Storage via

Transition-Metal Triggered High-Efficiency Lithium. Ion Storage via Coordination Interactions with. Redox-Active Croconate in One-Dimensional Metal-. ...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 6398−6406

Transition-Metal-Triggered High-Efficiency Lithium Ion Storage via Coordination Interactions with Redox-Active Croconate in OneDimensional Metal−Organic Anode Materials Lin Zhang,† Fangyi Cheng,†,‡,§ Wei Shi,*,†,‡,§ Jun Chen,†,‡,§ and Peng Cheng*,†,‡,§ †

College of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (MOE), ‡State Key Laboratory of Elemento-Organic Chemistry, and §Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: Coordination polymers (CPs) have powerful competence as anode materials for lithium-ion batteries (LIBs) owing to their structural diversity, tunable functionality, and facile and mild synthetic conditions. Here, we show that two isostructural one-dimensional croconate-based CPs, namely, [M(C5O5)(H2O)3]n (M = Mn for 1 and Co for 2; C5O52− = croconate dianion), can work as high-performance electrode materials for rechargeable LIBs. By means of the coordination between the redox-active transition metal ion and the ligand, the anode materials were stable in the electrolyte and showed high capacities, impressive rate capabilities, and excellent cycling performance during the discharging/charging processes. The chain-based supramolecular structures of the CPs also make them stand out from a crowd of porous three-dimensional molecular materials due to their free channels between the chains for lithium ion diffusion. When tested in a voltage window of 0.01−2.4 V at 100 mA g−1, CPs 1 and 2 demonstrated high discharge specific capacities of 729 and 741 mA h g−1, respectively. The synergistical redox reactions on both metal centers and the organic moieties play a crucial role in the high electrochemical performance of CPs 1 and 2. After undergoing elevated discharging/charging rates to 2 A g−1, the electrodes could finally recover their capabilities as those in the initial stage when the current rate was back to 100 mA g−1, indicating excellent rate performance and outstanding cycling stabilities of the materials. KEYWORDS: coordination polymers, one-dimensional, redox active, croconate, transition metal ions, synergistical redox reactions

1. INTRODUCTION Electrochemical energy storage (EES) technology has considerably changed our life for its various applications in portable electronic products, electric vehicles, medical instruments, and aerospace equipments.1−3 Among the various EES devices, lithium-ion batteries (LIBs) are one of the advanced devices because they are endowed with high energy density and operation potential, light mass, and low environmental impact.4−6 Currently, traditional LIBs with a LiCoO2/graphite couple can hardly meet the increasing requirements of the new electronic devices. 7−9 Among the alternative advanced electrode materials, organic materials, especially organic carbonyl compounds, have been highlighted for their high capacities owing to the multielectron transfer processes during the electrochemical redox reactions.10−12 A formidable challenge for organic carbonyl electrode materials is their dissolution in aprotic electrolytes, which leads to capacity decay.13,14 To conquer this problem, a promising strategy is the introduction of organic carbonyl salts, which can maintain the high capacities from the organic components and alleviate © 2018 American Chemical Society

solubility in the electrolyte due to the formation of ionic bonds. Until now, a series of organic carbonyl salts processing high theoretic capacities have been studied for rechargeable LIBs.15,16 However, the solubility is still a target issue that needs to be resolved. In addition, the redox-inert metal ions in the organic carbonyl salts will restrict the lithium-ion insertion numbers and hence prevent the further enhancement of theoretical capacities. Meanwhile, the volume change during lithiation/delithiation also limits the power density and cycling stability of these electrode materials upon continuous discharging/charging processes. Metal−organic frameworks (MOFs), constructed by inorganic nodes and organic linkers, are new generation of crystalline materials.17,18 Owing to their hybrid compositions and variable structures, MOFs have shown promising applications in gas storage19 and separation,20 luminescence,21 Received: December 9, 2017 Accepted: January 31, 2018 Published: January 31, 2018 6398

DOI: 10.1021/acsami.7b18758 ACS Appl. Mater. Interfaces 2018, 10, 6398−6406

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Figure 1. (a) One-dimensional structure. The metal, C, and O, atoms are shown as turquoise, gray, and red balls, respectively. (b) Supramolecular networks formed via the connection of the chains by hydrogen bonds. The green dotted lines indicate O−H···O hydrogen bonds. (c) Dissolution tests of the CPs in the electrolyte for 10 days, indicative of the high stabilities of CPs 1 and 2 in the electrolyte.

magnetism,22 catalysis,23 and so on.24 Recently, MOFs have shown great potential as promising electrode materials.25 Except the ligand, the redox-active metal center can accommodate multiple electrons.26−29 Moreover, the structural flexibility can buffer the large volume change during repeated discharging/charging processes and hence overcoming the degradation problems caused by the volume change.30,31 The challenges of classic MOFs as electrode materials are their limited channel for lithium ion diffusion and obstruction of the pore window by organic solvents.32 To solve these issues, an alternative way is the application of one-dimensional coordination polymers (CPs) as electrode materials. The coordination chains are linked by weak hydrogen bonds or π···π stacking interactions rather than strong covalent bonds, which can provide adjustable space between the chains and benefit rapid diffusion of lithium ions, allowing the materials to approach their maximum potential capacities.32,33 Among the various organic carbonyl salts, Na2C5O5 has the potential to act as a high-performance material for LIBs due to the lithium enolization reaction at the polycarbonyl oxygens.16 However, in experiments, it displayed only a low specific capacity due to solubility in electrolytes. Chemically, C5O52− can act as a bridging ligand for the construction of versatile CPs,34−36 which have been studied for magnetism35 and ferroelectrics,36 but there is no report on the comprehensive investigation of croconate-based CPs as electrode materials for LIBs. To achieve a practically high capacity with C5O52−, the introduction of transition metal ions to coordinate with C5O52− is a promising way because the formation of coordination bonds can not only increase the stability of the material in the electrolyte but also construct specific channels for lithium ion diffusion. In addition, the incorporation of redox-active transition metal ions and C5O52− can also contribute to an enhanced electrochemical performance in comparison to that of Na2C5O5, in which the Na(I) ion is redox-inert.

In this contribution, two isostructural transition metal CPs of [M(C5O5)(H2O)3]n (M = Mn for 1 and Co for 2; C5O52− = croconate dianion) with chain structures were obtained by adopting a modified facile slow evaporation method at room temperature. CPs 1 and 2 were studied as anode materials of LIBs. The coordination of C5O52− with transition metal ions can greatly benefit the stability of the electrolyte and contribute to the cycle performance during the discharging/charging processes. CPs 1 and 2 with Mn(II) and Co(II) centers, respectively, demonstrated high discharge specific capacities of 729 and 741 mA h g−1 at 100 mA g−1 after 140 cycles. The Xray photoelectron spectroscopy (XPS) study demonstrated that the involvement of both metal centers and the organic moieties in lithium-ion storage is responsible for the high specific capacities of CPs 1 and 2. Importantly, both the CPs exhibited high reversibility, excellent rate performance, and long-term cycling stability in comparison to those of the conventional Na2C5O5 electrode, as expected when used as anode materials for LIBs.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. All raw reagents were commercial and used without further purification. Redetermination of the crystal structures of CPs 1 and 2 was performed by single-crystal X-ray diffraction with an Agilent Technologies Supernova diffractometer using graphite-monochromatized Mo Kα radiation. Elemental analyses of C, H, and N were performed by a Perkin Elmer 2400-II CHNS/O analyzer. Thermogravimetric analyses (TGA) were performed under a nitrogen atmosphere in the temperature range of 40−800 °C at the heating rate of 10 °C min−1 using a Labsys NETZSCH TG 209 Setaram apparatus. Powder X-ray diffraction (PXRD) patterns were measured at the scanning rate of 5° min−1 in the 2θ range of 3−50° on a Rigaku Ultima IV instrument using Cu Kα radiation. Fourier transform infrared spectra (FTIR) were recorded on a Bruker Alpha-T infrared spectrophotometer in the wavenumber range of 400−4000 cm−1. Morphologies were obtained by scanning electron microscopy (SEM) on a ZEISS MERLIN Compact (field emission) scanning electron microscope. X-ray photoelectron spectroscopy (XPS) was 6399

DOI: 10.1021/acsami.7b18758 ACS Appl. Mater. Interfaces 2018, 10, 6398−6406

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Figure 2. Charge−discharge curves at 100 mA g−1 (a, b), cycling performance and Coulombic efficiency over 140 cycles (c, d), and rate performance at different current densities (e, f) for CPs 1 and 2. conducted on a Kratos AXIS Ultra DLD spectrometer using an Al Kα X-ray source. 2.2. Electrochemical Measurements. 2032 coin cells were assembled to test the electrochemical performances of the CPs and Na2C5O5. 70 wt % electrode-active materials, 20 wt % Ketjen black, and 10 wt % poly(vinylidene fluoride) were dispersed in N-methyl-2pyrrolidone uniformly to form a slurry. The slurry was coated on copper foil and dried at 80 °C for 12 h before punching into the disks of 12 mm diameter and used as an electrode. The active material loading is 1.0 mg cm−2. During the fabrication of the coin cells, Li metal was used as counter and reference electrodes, 1 M LiPF6 in ethylene carbonate/diethylcarbonate (1:1 in volume) as the electrolyte, and a polypropylene film as a separator. The operation was carried out in an argon-filled glovebox where the water and oxygen levels were both below 0.1 ppm. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) of the cells were

carried out by a PARSTAT 4000 electrochemical workstation. Galvanostatic charge−discharge measurements were conducted on a LAND battery station at 298 K in the range of 0.01−2.4 V. 2.3. Syntheses of CPs 1 and 2. A mixture of MnCl2·4H2O (19.7 mg, 0.1 mmol) and disodium croconate (Na2C5O5) (9.3 mg, 0.05 mmol) in 3 mL of deionized water after slow evaporation at room temperature generated dark green crystals of CP 1. CP 2 were prepared by the same method as that used for 1 except that Co(NO3)2·6H2O (29.1 mg, 0.1 mmol) was used as the starting material. CP 2 formed as purple crystals. The products were obtained with yields of 70 and 75% for CPs 1 and 2, respectively, based on Na2C5O5. Anal. calcd for C5H6MnO8 1: C, 24.11; H, 2.43. Found: C, 24.15; H, 2.46. Calcd for C5H6CoO8 2: C, 23.73; H, 2.39. Found: C, 23.70; H, 2.41. 6400

DOI: 10.1021/acsami.7b18758 ACS Appl. Mater. Interfaces 2018, 10, 6398−6406

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ACS Applied Materials & Interfaces Table 1. Selected MOFs and CPs 1−2 As Anode Materials for LIBs MOFs and our CPs

structure

capacity (mA h g−1)/current rate (mA g−1)

cycle number

voltage (V)

refs

MOF-177 CoBTC-EtOH Co2(OH)2BDC Zn3(HCOO)6 Co3(HCOO)6 [Co(C8H4O4)4]n Mn-BTC Mn-BDC Mn-LCP Co-LCP Ni-MOF [Cd(HTCPPA)·2H2O]n Cu3(BTC)2 Mn(3,5-PDC)·2H2O Mn(2,5-FDC)·3H2O [Co3(L1)(N3)4] [Mn2(L1)(N3)2(H2O)2]·3H2O [Co4L2(N3)(H2O)2] [Mn4L2(N3)6(H2O)2] [Mn(C5O5)(H2O)3]n [Co(C5O5)(H2O)3]n

3D 3D 3D 2D 2D 2D 3D 3D 2D 2D 2D 3D 3D 2D 2D 3D 3D 3D 3D 1D 1D

110/50 856/100 650/100 560/60 410/60 700/60 694/100 974/100 390/50 545/50 620/100 302/100 474/383 554/100 503/300 580/100 358/100 595/100 595/100 729/100 741/100

2 100 50 60 60 100 100 100 50 50 100 100 50 240 326 200 200 200 200 140 140

0.1−1.6 0.01−3 0.02−3 0.01−3 0.005−3 0.005−2.8 0.01−2 0.01−3 0.1−3 0.1−3 0.01−3 0.01−3 0.05−3 0.05−3 0.05−3 0.01−3 0.01−3 0.01−3 0.01−3 0.01−2.4 0.01−2.4

47 48 49 50 50 51 43 52 53 54 55 56 57 58 58 59 59 59 59 this work this work

3. RESULTS AND DISCUSSION Isostructural CPs 1 and 2 were obtained by a facile slow evaporation method at room temperature instead of the original procedure initially introduced by West in 1963.37 Preliminary studies on the structures of the CPs were carried out by Fabretti and Fabre.38,39 Single-crystal X-ray diffraction showed that CPs 1 and 2 crystallized in orthorhombic space group Pbca (Table S1). The crystal structure of CP 1 is described as an example. Each unit cell of 1 has one crystallographically independent metal ion, one C5O52− anion, and three coordinated H2O molecules (Figure 1a). Each metal center is coordinated by three oxygen atoms from two C5O52− anions and three oxygen atoms from three H2O molecules to form a distorted octahedron, with Mn−O bond lengths ranging from 2.121(4) to 2.254(2) Å (Table S2). Each C5O52− anion is chelated through two oxygen atoms to one metal ion and singly coordinated to another metal ion, forming a chain structure and leaving two of its five oxygen atoms uncoordinated. In CP 1, the shortest intrachain Mn···Mn distance is 8.145(8) Å. Adjacent chains are connected through the interchain O−H··· O hydrogen bonds (Table S3) to form a three-dimensional (3D) supramolecular architecture with the closest interchain Mn···Mn distance of 5.436(1) Å (Figure 1b). The formation of the supramolecular networks based on the coordination chains connected via hydrogen bonds can increase the stability and make the materials unsolvable in the electrolyte. Dissolution tests (Figure 1c) and PXRD patterns (Figure S1) of the two CPs after being soaked in the electrolyte for 10 days suggest that they are highly stable in the electrolyte. PXRD patterns of isostructural CPs 1 and 2 are similar and nearly identical to those of the simulated one from the single crystal structure of CP 1, indicative of the pure solid-state phases (Figure S2). In TGA, the CPs exhibit an obvious weight loss of 21.59 and 21.85% from 120 to 280 °C for CPs 1 and 2, respectively, which correspond to the release of three coordinated water molecules (calcd 21.70 and 21.36%). Then, CPs 1 and 2 show no weight loss until the framework starts to collapse at 340 °C (Figure S3). In FTIR spectra

(Figure S4), the positions and intensities of the peaks are nearly identical for the two CPs. A broad and strong band from 3000 to 3400 cm−1 can be ascribed to the OH stretching vibrations of the water molecules, whereas the νHOH bending affords a strong absorption at 1627 cm−1.37 These OH bands disappear in the FTIR spectrum of anhydrous Na2C5O5. The band at 1723 cm−1 is attributed to the noncoordinated CO group. The IR peak of the coordinated C−O group is at 1677 cm−1. The broad and intense absorption located in the 1300−1600 cm−1 range is characteristic of the C5O52− anion, which can be assigned to vibrational modes of a mixture of C−O and C−C stretching motions.40 Both CPs exhibit several absorptions between 400 and 570 cm−1, which may be attributed to the νM−O stretching, suggesting the formation of the coordination bonds of transition metal ions and C5O52− anions.41 Isostructural CPs 1 and 2 allow us to study if different metal centers can influence the lithium-ion storage performance. Two-electrode 2032 coin-cell batteries with Li metal counter electrodes were assembled and galvanostatic charge−discharge measurements were performed. The voltage window of 0.01− 2.4 V and the current density of 100 mA g−1 were selected as the operation conditions.42,43 For CP 1, two plateaus are clearly presented in the discharge profile in the voltage ranges of 0.72− 1.21 and 0.25−0.72 V, indicating two distinct electron-uptake processes (Figure 2a). The initial discharge capacity of 1 is 2490 mA h g−1. It decreases to 800 mA h g−1 in the second cycle. The initial capacity loss is mainly from the formation of a solid electrolyte interphase (SEI) layer and interfacial lithium storage.44,45 In addition, the reaction of coordinated water molecules within CP 1 with lithium to form Li2O is also responsible for the first cycle irreversible capacity loss.46,47 From the second cycle, with the formation of SEI layers and the disappearance of side reactions, no apparent capacity loss is found and the Coulombic efficiency tends to rise and remains constant at almost 100% in the following cycles. The galvanostatic charge−discharge curves show rather symmetrical voltage profiles for charge and discharge processes in the subsequent cycles. In addition, the Ketjen black conducting 6401

DOI: 10.1021/acsami.7b18758 ACS Appl. Mater. Interfaces 2018, 10, 6398−6406

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Figure 3. Cyclic voltammetry curves (2−4 cycles) in the range of 0.01−2.4 V at a scan rate of 0.1 mV s−1 for 1 (a) and 2 (b).

charging processes. The galvanostatic charge−discharge profile exhibits a relatively low discharge capacity of 775 mA h g−1 in the first cycle. The capacity dives to 202 mA h g−1 in the second cycle owing to the low initial Coulombic efficiency of 19.6%. With respect to the cycle performance, the discharge capacity remains at 176 mA h g−1 and the Coulombic efficiency appears to be 96.2% after 50 cycles (Figure S7b). The reversible capacities measured are 172, 135, 105, 77, and 58 mA h g−1 with elevated current rates from 100 mA g−1 to 200, 500, 1000, and 2000 mA g−1, respectively (Figure S7c). The poor electrochemical performance of Na2C5O5 in this LIB, in comparison to that of CPs 1 and 2, suggests that the introduction of the coordination bond with the transition metal ion is of great importance for the enhanced electrochemical properties. To explore the reason for the high electrochemical performance of the CPs, electrochemical impedance spectroscopy (EIS) of the anodes in the fresh state and after 140 discharge−charge cycles is also presented (Figure S8). CP 1 has a much smaller semicircle diameter than that of CP 2 in the fresh state, suggesting lower contact and charge-transfer resistance than that for 2. The semicircle diameters of the two CPs significantly decrease after 140 cycles, indicating elevated electron transport and Li+ diffusion speed upon cycling. The EIS results are consistent with the lithium-storage behaviors of the CPs during the electrochemical lithiation and delithiation reactions. The cyclic voltammetry (CV) analyses were also performed to study the Li+ intercalation/deintercalation processes of the two CPs. As shown in Figure S9, the remarkable irreversibility in the first cycle can be assigned to the formation of solid electrolyte interphase (SEI) layers.44,45 The CV curves of 1 in the subsequent cycles are almost overlapping, indicating a good cyclic stability of 1 during cycling (Figure 3a). Two isolated reversible peaks at 0.32 and 1.10 V correspond to two distinct electron-uptake steps and lithium intercalations of the Mn2+ ion and carbonyl groups. Accordingly, the oxidation peaks at 1.28 and 1.58 V are ascribed to the associated electron removal and lithium extraction processes. In comparison, CP 2 undergoes an analogous redox behavior with CP 1. Two oxidation peaks appear at 1.04 and 1.34 V, and two reduction peaks located at 0.63 and 1.23 V for 2 are clearly observed after the first cycle, indicating a similar redox process as that in 1 (Figure 3b).

additive is about 20 wt % of the total loading mass and contributes a negligible capacity (Figure S5). Isostructural CP 2 was also studied as an anode material. Similar to that of CP 1, the discharge profile of CP 2 in the first cycle shows two obvious plateaus located at 1.10−1.38 and 0.51−1.10 V (Figure 2b). The initial discharge capacity of 2 is 2024 mA h g−1, and it decreases to 974 mA h g−1 in the second cycle and remains at 711 mA h g−1 at the 50th cycle. The cycling performance of the CPs was also evaluated at a current density of 100 mA g−1. CPs 1 and 2 retain the capacities of 729 and 741 mA h g−1 after repetitive discharging/charging to 140 cycles (Figure 2c,d). The higher capacities of CPs 1 and 2 suggest a synergistic contribution from the metal centers and organic ligands. In addition, both the CPs exhibit excellent cycle stabilities. The achieved 99.4 and 99.9% Coulombic efficiencies for CPs 1 and 2 upon cycling suggest that the SEI layers obtained in the first cycle are extremely stable and there are no apparent side reactions.44,45 To further investigate the cycling stabilities and rate capacities, we studied the rate performance of the CPs. Discharge and charge cycles at various current densities (from 100 to 2000 mA g−1) are shown in Figure 2e,f. CPs 1 and 2 show very close discharge capacities of 783 and 740 mA h g−1 at 100 mA g−1. With elevated current rates to 200, 500, and 1000 mA g−1, discharge capacities of 738, 608, 458 mA h g−1 for CP 1 and 677, 528, 409 mA h g−1 for CP 2 are achieved, respectively. Even at a high rate of 2 A g−1, the capacities can be maintained at 317 and 341 mA h g−1, which are comparable to the theoretical capacity of graphite (372 mA h g−1).46 When the current rate was back to 100 mA g−1, the electrodes can finally resume their capacities as those at the initial stage, indicating that the anode materials remain stable during the rate cycling process. More strikingly, when tested at 500 mA g−1, stable lithiation capacities of 680 and 476 mA h g−1 after 400 cycles are still obtained, along with a steady Coulombic efficiency near 100% (Figure S6). On the basis of the above results, it can be stated that the two CPs are highperformance anode materials in comparison with MOF-based anode materials (Table 1).43,47−59 The electrochemical properties of Na2C5O5 as an anode material were also investigated to further understand the electrochemical mechanism of CPs 1 and 2. As shown in Figure S7a, the single plateau at 0.75−0.95 V can be assigned to the electron-uptake step of carbonyl groups during the discharging/ 6402

DOI: 10.1021/acsami.7b18758 ACS Appl. Mater. Interfaces 2018, 10, 6398−6406

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Figure 4. XPS spectra of the two CPs before cycling (a, b) and after being fully discharged (c, d).

1000, and 2000 mA g−1, the reversible capacities of CP 1 are higher than those of 2. CP 1 showed a reversible capacity of 680 mA h g−1 at 500 mA g−1 after 400 galvanostatic discharging/charging cycles, whereas the reversible capacity of 2 under the same conditions is 476 mA h g−1 (Figure S6). Thus, the lithium-ion storage performance of CP 1 is higher than that of 2 at high rates, which may result from the different coordination affinity to ligands and the electrochemical nature of the metal centers. Stability is an important feature of CPs as electrode materials during the discharging/charging processes. To confirm the structural stabilities of the anode materials, the coin cells of the electrodes were disassembled after 10 cycles. The active materials were then peeled off from electrode disks and washed several times with dimethyl carbonate. Combining the measurements of PXRD, IR, and SEM analyses, we can conclude that the two CPs show great endurance upon insertion and extraction reactions of Li+ ions in multiple cycles. PXRD analyses and FTIR spectroscopy of the electrode materials at the fully charged (2.4 V) states are well consistent with the pristine CPs (Figures S12 and S13). Moreover, it is clearly demonstrated that the morphologies of the prepared CPs can still be well retained after 10 discharge/charge cycles (Figure 5), indicating that the chain-based supramolecular structures can accommodate the large volume change during delithiation/lithiation processes. We also studied the long-term cycling stabilities of the materials. The PXRD, FTIR, and SEM results after fully charged at the 40th cycle stay almost the same with the pristine CPs and the material at the 10th cycle

To further investigate the redox reaction mechanism, ex situ X-ray photoelectron spectroscopy (XPS) measurements of the electrodes before and after the fully discharged states were carried out to track the valence change of metal centers and the chemical bonding state of organic moieties. As shown in Figure 4a,b, the distinct peaks located at binding energies of 641.8 and 652.8 eV for Mn 2p and peaks at 781.5 and 798.3 eV for Co 2p are assigned to the characteristic peaks of Mn2+ and Co2+ ions, respectively. After the discharge process, the Mn 2p peaks shift to 640.5 and 651.0 eV and the Co 2p peaks shift to 778.3 and 793.5 eV, indicating the formation of Mn0 and Co0 (Figure 4c,d).60 The XPS spectra of C 1s and O 1s were further investigated. The C 1s spectra after the discharge process are fitted into three peaks at 284.6, 286.4, and 289.8 eV for 1 and 284.6, 285.8, and 289.6 eV for 2 (Figure S10), which are attributed to the characteristic peaks of the CC bonds of the five-membered ring, C−C single bonds of the carbonyl groups, and enol structures.61 The transformation of carbonyl into the enol structures after full discharge confirms the electron-uptake process of organic moieties. In the O 1s spectra, the two peaks at 529.6 and 531.7 eV for 1 and 530.8 and 528.8 eV for 2 correspond to the lattice oxygen and the Li−O bond during the discharge process (Figure S11).62 The above results suggest that the involvement of cobalt(II) and manganese(II) ions in the redox process for CPs 1 and 2 during the discharging− charging cycles can contribute to the improved capacities. The lithium-ion storage performances of CPs 1 and 2 are similar in terms of the reversible capacities at 100 mA g−1. However, when the current rates were elevated to 200, 500, 6403

DOI: 10.1021/acsami.7b18758 ACS Appl. Mater. Interfaces 2018, 10, 6398−6406

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Figure 5. SEM images of CPs 1 and 2 as electrodes before cycling (a, b) and after 10th cycle (c, d).

tinuous exploration of novel electrode materials based on lowdimensional CPs is under way in our laboratory.

(Figures S14−S16), indicating no apparent changes of crystalline structure and morphology of the samples during long-term cycling.

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4. CONCLUSIONS In summary, two isostructural one-dimensional transition metal croconate CPs were studied as anode materials for rechargeable LIBs. In comparison with the Na2C5O5 salt as anode, the CPs showed enhanced electrochemical performance, less solubility in electrolyte, and the tolerance of the volume change during repeated discharging/charging processes. When evaluated as anode materials for rechargeable LIBs, CPs 1 and 2 exhibit high reversible capacities of 729 and 741 mA h g−1 at the 140th cycle with Coulombic efficiencies up to 99.4 and 99.3%, respectively, at a current density of 100 mA g−1. The nearly 100% Coulombic efficiency implies the stabilities of the SEI layers and efficient transport of ions and electrons in these anodes. When tested at 500 mA g−1, stable lithiation capacities of 680 and 476 mA h g−1 after 400 cycles are still obtained, confirming endurance upon the insertion and extraction reactions of Li+ ions and long-term cycling stability of CPs 1 and 2 in multiple cycles. Importantly, all of the CPs exhibited high reversibility and excellent rate performance. The studies of the electrochemical mechanism demonstrate that synergistical redox reactions on both metal centers and organic moieties play a crucial role in the lithium insertion/ extraction process and enhance the electrochemical performance. In addition, the chain-based supramolecular structures free of channel size restrictions could also benefit the rapid diffusion of lithium ions within the materials, which can further contribute to the improvement of LIB performance. Con-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b18758. Additional figures (Figures S1−S16) of TGA, PXRD, FTIR, and XPS for different states of the CPs; cycling performances of CPs 1 and 2 at high current rates of 500 mA g−1; electrochemical performance of Na2C5O5; impedance plots of CPs 1 and 2 as anodes before and after 140 cycles; cyclic voltammetry curves of the 1st cycle in the range of 0.01−2.4 V at a scan rate of 0.1 mV s−1 (PDF) Crystallographic data (CIF) (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.S.). *E-mail: [email protected] (P.C.). ORCID

Fangyi Cheng: 0000-0002-9400-1500 Wei Shi: 0000-0001-6130-1227 Jun Chen: 0000-0001-8604-9689 Peng Cheng: 0000-0003-0396-1846 Notes

The authors declare no competing financial interest. 6404

DOI: 10.1021/acsami.7b18758 ACS Appl. Mater. Interfaces 2018, 10, 6398−6406

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant numbers 21622105 and 21421001) and the Ministry of Education of the People’s Republic of China (grant number B12015).

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