Trimetallic MOF-Derived Cu0.39Zn0.14Co2.47O4-CuO Interwoven

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Trimetallic MOF-Derived Cu Zn Co O-CuO Interwoven with Carbon Nanotubes on Copper Foam for Superior Lithium Storage with Boosted Kinetics Jia Lin, Chenghui Zeng, Xiao-Ming Lin, R. Chenna Krishna Reddy, Jiliang Niu, Jincheng Liu, and Yue-Peng Cai ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b03744 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 19, 2019

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ACS Sustainable Chemistry & Engineering

Trimetallic MOF-Derived Cu0.39Zn0.14Co2.47O4-CuO Interwoven with Carbon Nanotubes on Copper Foam for Superior Lithium Storage with Boosted Kinetics Jia Lin,† Chenghui Zeng,‡ Xiaoming Lin,*,†,ǁ Chenna Krishna Reddy,† Jiliang Niu,† Jincheng Liu,§ and Yuepeng Cai*,† †

Laboratory of Theoretical Chemistry of Environment, Ministry of Education, School of Chemistry, South China Normal University, No. 378, Outer Ring West Road, Guangzhou Higher Education Mega Center, Guangzhou 510006, P. R. China. Email: [email protected]; [email protected].

‡

College of Chemistry and Chemical Engineering, Key Laboratory of Functional Small Organic Molecule, Ministry of Education and Jiangxi’s Key Laboratory of Green Chemistry, Jiangxi Normal University, No.99, Ziyang Road, Nanchang 33002, P. R. China. §

EVE Energy Co. Ltd, No.38, Huifeng 7th Road, Huizhou 516006, P. R. China. State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, No. 2699, Qianjin Street, Changchun, 130012, P.R. China. ǁ

ABSTRACT: Transition metal oxides (TMOs), pivoted as potential candidate for high-energy anode materials for state-of-the-art lithium-ion batteries (LIBs), suffer from the inherent defects of low electronic conductivity and dramatic volume variation hindering their practical applications. It is still a great challenge to synthesize novel TMOs anodes with satisfactory lithium storage performance. Herein, trimetallic Zn-Co-Cu-ZIF is designed with carbon nanotubes (CNTs) and copper foam (CF) serving as multifunctional bridges by postsynthetic metal-ion exchange and in-situ solvothermal growth. After annealing, a novel tirmetallic MOF-derived polymetallic oxide Cu0.39Zn0.14Co2.47O4-CuO@CNTs/CF hybrid was successfully prepared. The introduction of conductive CNTs and 3D CF substrate effectively boosts the mechanical robustness and electronic conductivity of metal oxide composites, accelerates the lithium-ion diffusion, and reduces the impedance during lithiation/delithiation process. When directly testing as a conductive-agent- and binder-free electrode in LIBs, it presents distinguished long-cycling stability and high-rate capacity via the dominant mechanism of pseudocapacitive charge storage and the “electron-shared metal-Li+ double electric layer”. The as-prepared Cu0.39Zn0.14Co2.47O4-CuO@CNTs/CF electrode delivers a high specific capacity of 1649 mAh g−1 at 0.2 A g−1 together with 1282 mAh g−1 at 5 A g−1 over 1000 cycles, respectively. The novel 3D self-supported MOF-derived polymetallic oxide synthetic strategy proposed in this work sheds light on creation of potential anode materials for next generation LIBs. KEYWORDS: Trimetallic MOF, Polymetallic oxide, Carbon nanotubes, Copper foam, Lithium storage

INTRODUCTION

excellent rate preformance in order to keep pace with the demands of state-of-the-art LIBs.

With the ever-growing demands for electrochemical energy storage system in electric appliances such as electric vehicles, hybrid electrical vehicles and electronic devices,1 lithium-ion batteries (LIBs) have been recognized as one of the exuberant candidates for electrochemical energy storage devices owing to their superior cycling life, high energy density, and ideal operating voltage.2-4 Since the currently commercial graphite has been approached the theoretical capacity limits (372 mAh g-1) and exhibits inferior Li-ion transport rate capability,5 which is an obstacle to the superior electrochemical performance for LIBs. Therefore, great efforts have been conducted on design different kinds of distinguished anode materials with higher capacity and

Transition metal oxides (TMOs), as the optimal candidates for LIB anodes, possess a specific capacity more than twice of the commercial graphite with the merits of conversion electrochemical reaction mechanism, abundant sources and compacted density.6 In rencent years, Co- and Zn-based oxides have drawn numerous attention for their potential application as anodes because of their higher theoretical capacities.7 Among them, bimetallic oxides have become a hot research with their own synergetic effect by utilizing different metal ions and complex composition as precursors.8,9 However, it’s inavoidable that a majority of them suffer from drastic volume change, low electronic conductivity, poor rate capability and inferior 1

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electrochemical performance during the lithiation/delithiation process, which impends the implementation of TMOs in commercial applications.10 To tackle the above mentioned hurdles, quantities of efficient methods have been explicitely illustrated for their fabrication via hybridisation. By fabricating compounds with some electronic conductive buffer matrix, especially carbon materials (such as carbon cloth, carbon nanotubes, reduced graphene oxide, and carbon fibers), become widely employed as remarkable conductive matrix, such as sandwich-like RGO/ZnCo2O4-ZnO-C,11 hollow embedded carbon/RGO Co3O4,12 and hierarchical CNT/Co3O4 microtubes.13 On the other hand, constructing a core-shell structure to provide extra void space for the volumetric expansion has also been proposed,14 including multi-shelled mixed ZnMn2O4, ZnCo2O4, NiCo2O4,15 double-shelled hollow ZnO/CNTs,16 hierarchical ZnCo2O4/NiO core/shell nanowire arrays,17 porous ZnO/ZnCo2O4/C hybrids,18 hollow mesoporous ZnO/ZnMnO3 microspheres.19 Both of the strategies can not only efficiently favor the charge transfer, but also buffer the volume changes, giving rise to the notable improvement in lithium storage performance. Even many achievements have been made, the enhancement of electrochemical performance still confronts with great challenge. Metal-organic frameworks (MOFs), a hybrid porous composite consisting of organic linkers as struts and single metal ions/clusters as inorganic nodes and serving as the precursor for metal oxides or carbon/nitrogen- doped metal oxides by appropriate method, have drawn global attention for their applications in LIBs with their tunable porosities, large specific areas, as well as universe skeletal structures.20,21 So far, zeolitic imidazolate frameworks (ZIFs), as a unique subset of metal-organic framework, have been proved to be ideal sacrificial templates for C/N-doped TMOs. For instance, N-doped ZnO/ZnCo2O4/CuCo2O4,22 porous hollow polyhedral ZnxCo3-xO4,23 and ZnO@ZnO quantum dots/C,24 have been synthesized by utilizing suitable ZIF precursors and exhibited superior electrochemical performance. Nevertheless, there is scarcely reported progress in designing polymetallic oxides. It’s the potential challenge in further research on polymetallic oxides, which may benefit from the notable synergetic and complex composition. On the other hand, current collector, such as copper foam and carbon cloth, has been an attractive tendency of 3D electrodes in LIBs by in-situ growing active materials on

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them, which not only realizes close contact between active materials and substrate with outstanding mechanical robustness and flexibility, but also enlarges the 3D surface area and shortens lithium ion diffusion length owing to the directly electric pathway to the conductive substrates.25,26 Compared with carbon cloth, copper foam is considered as the promising substrate for 3D electrode with its lower expense. What’s more, the electrode with active material clamped on copper foam can directly apply as conductiveagent- as well as binder-free electrode, which simplifies the assembling process. Nonetheless, probably lacking of effective synthetic stragety, there haven’t been reported such a C/N-doped MOF-derived polymetallic oxides 3D selfsupported electrode interwoven with CNTs for LIBs.27,28 Inspired by this, herein, for the first time, we fabricated a unique C/N-doped Cu0.39Zn0.14Co2.47O4-CuO@CNTs/CF self-supported 3D anode for LIBs via in-situ growing postsynthetic exchanged Zn-Co-Cu-ZIF interwoven with CNTs on CF material and further annealing under nitrogen atmosphere. As a consequence, this electrode can be used as conductive-agent- and binder-free electrode. Owing to the conductive CNTs buffer, CF substrate and the synergistic effect from Cu0.39Zn0.14Co2.47O4-CuO polymetallic oxides, the fabricated anode exhibits boosted kinetics and high-rate performance. EXPERIMENTAL SECTION Material Characterization. Bruker-AXS D8 Advance system with a Cu Kα radiation was conducted on measuring the crystal phase with Powder X-ray diffraction (PXRD) in the different 2θ range. Renishaw inVia confocal Raman microscope provision with an argon ion laser beam was implemented to test Raman spectra. Netzsch Thermo Microbalance TG 209 F1 Libra was conducted to observe thermogravimetric analysis (TGA) from ambient temperature to 800 ºC with a heating rate of 5 °C min-1 under flowing N2 and air, respectively. Belsorp max gas sorption analyzer was used to analyze the sorption isotherms at 77 K. ESCALAB 250Xi XPS spectrometer was operated using Al Kα radiation to evaluate X-ray photoelectron spectroscopy (XPS) and the surface compositions. Furthermore, FESEM (TESCAN Maia 3, Czech) and TEM (FEI Talos F200X, USA) with high-angle annular dark-field (HAADF) STEM and energy dispersive X-ray spectrometer (EDS) were achieved to study the surface morphology and architecture. The four-point probe measurement was applied to evaluate the electronic conductivity (Keithley 6220/2182A, USA).

Scheme 1. Schematic Description of Synthetic Tactics C/N-Doped CZCOC@CNTs/CF Material.

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Figure 1. (a) PXRD spectra of the simulated ZIF67, precursors Zn-Co-Cu-ZIF@CNTs and Zn-Co-Cu-ZIF@CNTs/CF. (b) XRD pattern, (c) nitrogen adsorption-desorption isotherm with pore size distribution (inset) and (d) Raman spectra of CZCOC@CNTs/CF.

Material synthesis. Synthesis of Trimetallic Zn-Co-CuZIF@CNTs: Firstly, sodium hydroxide aqueous solution (NaOH, 100 mL, 1 mol/L) was added dropwise into 2methylimidazole aqueous solution (C4H6N2, 100 mL, 4 mol/L). Then, cetyltrimethylammonium (CTMAB, 0.1 g) and sodium dodecyl sulfate (SDS, 0.08 g) were sequentially injected to form a homogeneous solution by sonicating, which was denoted as solution A. Next, solution B consisted of Co(NO3)2 aqueous solution (5 mL, 1.6 mol/L), Zn(NO3)2 aqueous solution (5 mL, 0.1 mol/L) and the pretreated CNTs (0.02 g) with sonicating 2 h. Moreover, solution B was injected into solution A vigorously. The mixture reacted at 50 °C for 5 min via microwave-assisted method. The product of purple solid was obtained via centrifugation at 7000 rpm for 5 min, further washed with methanol three times and dried overnight at 60 ºC for further use. The above product (named as Zn-Co-ZIF@CNTs) was added into Cu(CH3COO)2 aqueous solution (5 mL, 0.4 mol/L) by ultrasonic dispersion to form homogeneous aqueous solution, which was further kept for aging for 24 h at room temperature. Then, the purple solid product was gathered by centrifugation, washed with methanol and dried overnight at 60 °C. Synthesis of Zn-Co-Cu-ZIF@CNTs/CF: The copper foam (CF) was cut into rounds with a diameter of 12 mm, and then ultrasonically pretreated with acetone, hydrochloric acid, and distilled water to eliminate all possible organic species and oxide films, which finally dried in 60 °C vacuum drying oven for further use. The resultant Zn-Co-CuZIF@CNTs products was dispersed into 50 mL distilled methanol by sonicating with one piece of CF and further

solvothermal reacted for 1000 min at 50 °C. Centrifugation with methanol washing was done to yield Zn-Co-CuZIF@CNTs/CF and kept in vacuum drying oven at 60 °C overnight. Synthesis of Cu0.39Zn0.14Co2.47O4-CuO@CNTs/CF (CZCOC@CNTs/CF) and Cu0.39Zn0.14Co2.47O4-CuO (CZCOC): The obtained Zn-Co-Cu-ZIF@CNTs/CF and Zn-Co-Cu-ZIF were placed into a temperature-programmed tube furnace and annealed in a nitrogen flow at 550 °C for 2 h with a heating rate of 2 °C min-1 to obtain the final products CZCOC@CNTs/CF and CZCOC. Electrochemical Measurements. The electrochemical performances were evaluated by using CR 2032 coin-type cells, in which the CF-based materials directly functioned as the working electrode (mass loading of active material, 3.23.4 mg cm-2; thickness, 1 mm), Celgard 2400 membrane as the separator, and the liquid electrolyte composed of 1 M LiPF6 in the mixture of diethyl carbonate and ethylene carbonate (DEC:EC, 1:1 by volume). After coin cell assembling in an Ar-filled glove box, Land CT 2001A battery tester (CT 2001A, China) was carried out to perform galvanostatic charge/discharge cycling tests between 0.01 and 3.0 V under 25 °C. The cyclic voltammetry (CV) measurements at different scan rates and electrochemical impedance spectroscopy (EIS) tests were implemented on electrochemical workstation (CHI-760E, China) with the frequency ranging from 100 kHz to 0.01 Hz and the amplitude of 5 mV. RESULTS AND DISCUSSION



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Figure 2. SEM images of (a-c) Zn-Co-Cu-ZIF@CNTs/CF material, (d-f) C/N-doped CZCOC@CNTs/CF sample. (g) TEM image, (h) HRTEM, (i) the corresponding SAED patterns and (j) EDS elemental mapping images of Zn, Co, Cu, C, N, and O for C/N-doped CZCOC@CNTs.

Scheme 1 depicts the preparation procedure of carbon/nitrogen-doped CZCOC@CNTs/CF material by annealing of Zn-Co-Cu-ZIF@CNTs/CF as precursor. Firstly, the zeolitic imidazolate framework analogue Zn-Co-CuZIF@CNTs sample was synthesized according to the “postsynthetic exchange” method,29 one kind of postfunctionalization strategy to exchange the metal cations without the initial framework structure change.30,31 Subsequently, Zn-Co-Cu-ZIF@CNTs powders were in-situ grown on the copper foam by solvothermal method and annealed under nitrogen flow to obtain CZCOC@CNTs/CF. For comparison, CZCOC was also synthesized by the same procedure except for CF and CNTs. Thermogravimetric analysis (TGA) of Zn-Co-Cu-ZIF@CNTs/CF in air flow reflects the abrupt weight loss between 300 and 450 °C, corresponding to the decomposition of the organic ligands, arrestingly followed by the increase weight beyond 450 °C indexed to the oxidation of copper foam (Figure S1a). As a result, we chose the nitrogen flow as the optimal atmosphere

to impede the oxide reaction of CF (Figure S1b). According to Figure 1a, Powder X-ray diffraction (PXRD) patterns of the obtained precursors Zn-Co-Cu-ZIF@CNTs and Zn-CoCu-ZIF@CNTs/CF demonstrate the successful preparation of the ZIF analogue similar to ZIF67. Additionally, the weak and broad peaks around 23-28° for both of the samples originate from the CNTs (Figure S2). The appearance of the copper diffraction peaks (JCPDS No. 07-3039) further illustrates the precursors ideally in-situ grows on the CF substrate. While the PXRD spectrum of the as-prepared C/N-doped CZCOC@CNTs/CF displays characteristic peaks of cubic Cu0.39Zn0.14Co2.47O4 (JCPDS No. 54-0845), monoclinic CuO (JCPDS No. 89-5899), as well as cubic Cu (JCPDS No. 07-3039) phases (Figure 1b). Nitrogen adsorption-desorption curve exhibits typical type IV adsorption/desorption isotherm with hysteresis loops (Figure 1c), designated for mesoporous character of the fabricated materials.32 Calculated by the Barrett-JoynerHalenda (BJH) method, the Brunauer-Emmett-Teller (BET)


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Figure 3. (a) XPS survey spectrum. (b-d) High-resolution XPS spectra of Zn 2p, Co 2p, and Cu 2p of C/N-doped CZCOC@CNTs/CF.

surface area is 195.6 m2 g-1 and the distribution of the corresponding pore size mainly centers at 4 nm and 23 nm. Moreover, Raman spectrum of C/N-doped CZCOC@CNTs/CF was performed in Figure 1d, two representative carbon Raman peaks at 1341 and 1597 cm-1 ascribed to the disorder degree for D band and graphitic degree for G band, respectively. The ID/IG value for the electrode material is 0.84, verifying the high degree of graphitization of the material, which further improves the electrochemical performance.33,34 The morphology and architecture of the obtained C/Ndoped CZCOC@CNTs/CF product were further observed via scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. As depicted in Figure 2a-b, the roughness of the copper foam surface indicates the successful attachment of precursors Zn-Co-CuZIF@CNTs on the conductive substrate. Particularly, the uniform polyhedron Zn-Co-Cu-ZIF particles with an average size of 250 nm interwove with the CNTs to form numerous strings-of-pearls-like structure (Figure 2c). After calcination, the pearl-necklace-like CZCOC@CNTs was still firmly attached on the copper foam without destroying the initial morphology and structure (Figure 2d-f). TEM image in Figure 2g reveals that the CZCOC particles are immobile on the CNTs. As the high-resolution TEM (HRTEM) shown in Figure 2h, lattice fringes with the d-spacing of 0.232 nm and 0.245 nm can be indexed to the (111) interplanar spacing of CuO along with the (311) lattice plane of Cu0.39Zn0.14Co2.47O4, respectively, evidencing mixed metal oxide formation.21 The

ring with dot-distributed selected-area electron diffraction pattern (SAED, Figure 2i) further confirms the crystallization of the existence of both Cu0.39Zn0.14Co2.47O4 and CuO, where the characteristic diffraction points can be distinctly observed, such as Cu0.39Zn0.14Co2.47O4 (400, 222, and 311) and CuO (311 and 002). The corresponding elemental mapping analysis shows the existence of Zn, Co, Cu, C, N, and O elements (Figure 2j), confirming the successful formation of the hybrid sample. The elemental compositions and chemical states of the assynthesized material were further performed by X-ray photoelectron spectroscopy (XPS). The survey scan spectrum indicates the coexistence of Zn, Co, Cu, C, N, and O elements in the product (Figure 3a). Evidently, the C 1s characteristic peak located at 284.5 eV, while the N 1s characteristic peak appears in 398.5 eV. The high-resolution XPS Zn 2p curve can be indexed to two distinct peaks at 1044.6 eV for Zn 2p1/2 and 1024.4 eV for Zn 2p3/2 (Figure 3b).18,34 For Co 2p spectrum, two deconvoluted characteristic peaks located at 796.1 eV for Co 2p1/2 as well as 780.8 eV for Co 2p3/2. Additionally, two distinguished shake-up satellite peaks were also observed, indicating the presence of Co2+ and Co3+ states (Figure 3c).35 In the Cu 2p spectrum, the peaks at 922.6 eV for the Cu 2p3/2 and 952.5 eV for Cu 2p1/2 can be assigned to Cu0 originated from copper foam, while the peaks at 955 eV in the Cu 2p1/2 and 935 eV in the Cu 2p3/2 along with the shake-up satellite peaks at 944 eV and 961 eV can be allocated to the Cu2+ (Figure 3d).32,36



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Figure 4. (a) CV curves of CZCOC@CNTs/CF electrode at the sweep rate of 0.3 mV s-1. (b) The galvanostatic charge/discharge plots of CZCOC@CNTs/CF electrode 0.1 A g-1. (c) Rate capabilities of CZCOC@CNTs/CF and CZCOC at different current densities. (d) Long-cycle stabilities at 0.2 A g-1 and (e) at 5 A g-1 for both CZCOC@CNTs/CF and CZCOC electrodes, respectively.

To better investigate the electrochemical performance, the as-synthesized CZCOC@CNTs/CF electrode material directly functioned as the binder- and conductive-agent-free working electrode, which was further assembled as CR2032 coin cell with Li foil as the counter electrode. The initial three cyclic voltammogram (CV) curves of both CZCOC@CNTs/CF and CZCOC were implemented at the scan rate of 0.3 mV s-1 (Figure 4a and Figure S3). Evidently, the shape of the CV curves is similar to each other, indicating the identical electrochemical reactions of two electrodes. During the first cathodic scan, three pronounce peaks locate at 1.13, 0.98, and 0.41 V attributed to the reduction reaction of CZCOC turn to metallic Zn, Cu, and Co, solid electrolyte interphase (SEI) layer formation, as well as the decomposition of ZnO into Zn along with the alloying generation of LiZn.18,35 As for the first anodic sweep, welldefined broad peaks appear at 1.38, 1.71, and 2.15 V corresponding with reversible re-oxidation of Co, Cu, Zn into metallic ions, respectively.36,37 In the subsequent cycles,

the fundamental reduction peaks shift to 1.07 V, demonstrating an irreversible phase transformation in the initial cycle, but the oxidation peaks agree well with that in the first scan cycle. The CV curves almost overlap each other since the second scan, implying distinguished reaction reversibility. It’s confirmed that the introduction of copper foam into the CZCOC@CNTs/CF haven’t bring about additional oxidation/reduction peaks in CV scans, revealing its electrochemical inertness during lithiation/delithiation procedure.38 Based on the above experimental analyses, the electrochemical reactions for lithium storage and the asprepared electrode can be provided as follows: Cu0.39Zn0.14Co2.47O4＋8Li+＋8e-→0.39Cu＋0.14Zn＋2.47Co ＋4Li2O (1) CuO＋2Li+＋2e-↔Cu＋Li2O

(2)

Zn＋Li ＋e ↔LiZn

(3)

+

-

Zn＋Li2O↔ZnO＋2Li ＋2e


+

-

(4)

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Figure 5. Ex-situ XRD patterns of CZCOC@CNTs/CF electrode after (a) discharge and (b) charge process for 1000th cycle.

Cu＋Li2O↔CuO＋2Li+＋2e-

(5)

Co＋Li2O ↔CoO＋2Li ＋2e +

-

(6)

(7) 3CoO＋Li2O ↔Co3O4＋2Li ＋2e To verify the above-mentioned electrochemical reaction mechanism, PXRD patterns after charge and dischange process were investigated after 1000 cycles, respectively. As shown in Figure 5, the strong diffraction peaks of Cu is ascribed to the copper foam. After discharge process, the characteristic peaks for the Li2O (JCPDS Card No. 12-0254), LiZn (JCPDS Card No. 03-0954), metallic Zn (JCPDS Card No. 04-0831), Co (JCPDS Card No. 15-0806), and Cu (JCPDS Card No. 07-3039) can be detected. Otherwise, the diffraction peaks of the PXRD after charge process can be indexed to ZnO (JCPDS Card No. 36-1451), Co3O4 (JCPDS Card No. 43-1003), CuO (JCPDS Card No.45-0937), and Li2O (JCPDS Card No. 12-0254), which further confirms the proposed mechanism. Concurrently, the peaks of Li2O is also noticed, indicating extra Li2O originated from CZCOC@CNTs/CF by initial irreversible reaction.39 +

-

The galvanostatic charge/discharge analyses were implemented to value the specific capacity of electrodes in

Figure 4b. The apparent potential plateaus at about 1.25, 0.91, and 0.41 V in the initial discharging process, related to the reduction reaction of Cu0.39Zn0.14Co2.47O4-CuO in to Zn0, Cu0, and Co0, along with the formation of Li2O and SEI layer, which consistents with the CV plots. The electrode possesses the initial discharge and charge capacities of 2736 and 2183 mAh g-1 at 0.1 A g-1, respectively, with an initial Coulombic Efficiency (CE) of 79.8%. The irreversible capacity loss can be associated with incomplete decomposition of Li2O,40 inevitable formation of a SEI film and decomposition of electrolyte.41 Although the inevitable intial capacity loss occured, there is little change in the charge/discharge specific capacity observed in the subsequent cycles. Simultaneously, the CE retains almost 100% after several initial cycles, further confirming the better cycling reversibility of lithium ion storage behavior. As for high-rate and long-cycle properties of CZCOC@CNTs/CF, Figure 4d and e evaluate the longperiod cycling stability at low current densities of 0.2 A g−1 and high current densities of 5 A g−1. CZCOC@CNTs/CF demonstrates a distinguished high-rate cyclability with stable cycle performance, which maintains high specific capacities

Figure 6. Nyquist plots for (a) CZCOC@CNTs/CF and (b) CZCOC electrodes.



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Figure 7. Kinetics analyses of the lithium-ion storage behavior and quantitative analyses for the pseudocapacitive contribution for CZCOC@CNTs/CF electrode. (a) CV profiles at the different sweep rates from 0.3 to 1.2 mV s-1. (b) Determinational b-value of the main anodic and cathodic peaks by the relationship between sweep rate to corresponding peak current. (c) CV curve separation of the pseudocapacitive current and total current contribution at 1.2 mV s−1. (d) Contribution percentage of the diffusion- and capacitive-controlled capacities at related sweep rates. (e) EIS plots measured at various depths of discharge of CZCOC@CNTs/CF. (f) Lithium ion diffusion coefficients (DLi) of CZCOC@CNTs/CF and CZCOC electrodes at the measured states of discharge, respectively.

of 1649 mAh g−1 at 0.2 A g−1 and 1282 mAh g−1 at 5 A g−1 over 1000 cycles, compared with capacity fading to 995 mAh g−1 over 600 cycles at 0.2 A g−1 and 584 mAh g−1 over 500 cycles at 5 A g−1 for CZCOC@CNTs/CF. Interestingly, a progressive growth trend in discharge capacities is perceived for CZCOC@CNTs/CF, which is in relation to the activation process during lithiation/delithiation procedure due to the generation of polymeric gel–like layers.40 Moreover, the Coulombic efficiency (CE) upswings rapidly from the second cycle onwards because that the relatively high lithiation potential shed light on reducing the decomposition of electrolyte and generation of SEI. Since the 6th cycle, the CE maintains about 100%, endowing the

remarkable cycling stability and excellent high reversibility of the CZCOC@CNTs/CF sample. When it comes to evaluating index for the power delivery capability of LIBs in practical application, the rate capabilities of both CZCOC@CNTs/CF and CZCOC electrodes are shown with gradually increased current densities (Figure 4c). CZCOC@CNTs/CF delivers a reversible capacity of 1909, 1649, 1445, 1247, 1021, and 798 mAh g-1 with the current density increasing from 0.1, 0.2, 0.5, 1, 5, to 10 A g-1, respectively. However, the CZCOC exhibits severely decrease with the current density increasing, which just possesses 434 mAh g-1 at the current of 10 A g-1, verifying the inferior rater performance. When the current returns to 0.1 A g-1, the CZCOC@CNTs/CF electrode has a high


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specific capacity of 1852 mAh g-1 with little diminution capacity. Noticeably, it’s crucial for practical electrode to provide superior high-rate performance and recovery capacity. Furthermore, to investigate the benefit of the structural features for reaction kinetics, electrochemical impedance spectroscopy (EIS) measurements were conducted to calculate the internal resistances of elecctrode after initial deep cycle with the complete formation of fresh SEI film, as well as the electrode after 500 cycles. Figure 6 shows the Nyquist plots of both CZCOC@CNTs/CF and CZCOC electrodes. Accordingly, the semi-circle in the high-middle frequency range is ascribed to the charge-transfer (Rct) resistance and the surface layer resistance (Rs), while the sloping line in the low-frequency region generally corresponds to ion diffusion process. The Nyquist plot of CZCOC@CNTs/CF (Figure 6a) shows that the semi-circle at high-medium frequency range changed into smaller after 500 cycles, while the slope of the straight line increased, manifesting its accelerated reaction kinetics, because of the electronic conductive CNTs and copper foam and the close contact between active materials and 3D substrate with outstanding mechanical robustness and flexibility. Neverthless, the Nyquist plots for CZCOC behave the opposite trend that the semicircle became larger semi-circle and decreased slope of the straight line after 500 cycles (Figure 6b). The kinetics analyses were conducted in a bid to further understand the superior electrochemical capacity. The higher rate performance of CZCOC@CNTs/CF than CZCOC may be ascribed to the enhanced pseudocapacitive behaviors, on account of much faster lithium ion storage of surface-controlled than of diffusion-controlled.41,42 Therefore, a quantitative analysis mean with the help of CV measurements at different sweep rates was performed to distinguish the electrochemical kinetics of electrode for lithium ion storage.43,44 As depicted in Figure 7a and S4a, CV curves with raising scan rates feature identical peak shapes during cathodic and anodic processes. Generally, the peak current (i) and the scan rate (ν) in CV curves obey the power law relationship as follows:45,46 i=avb (8) in which a is a constant and b is an estimated value of the lithium storage kinetics. From the obtained b values, b value of 0.5 shows the lithium storage in the electrode is a total diffusion-controlled process, b value of 1 represents for surface-controlled charge storage behavior, whereas the b value located among 0.5-1 means a mixed mechanism.47 The b values of CZCOC@CNTs/CF for the main current peaks (a), (b), and (c) calculated from the slope of log ip against log ν plot are 0.80, 0.77 and 0.89, respectively (Figure 7b), demonstrating the dominant pseudocapacitive contribution by surface-controlled kinetics, which was favorable for fast charge storage and long-term cyclability. As for CZCOC, the corresponding values are much lower for 0.53, 0.52, and 0.57 (Figure S4b), endowing that the lithium storage is predominantly based on diffusion-controlled mechanism. To intensively study the electrochemical kinetics of CZCOC@CNTs/CF, the capacitive contributions ratio is capable to quantitatively enumerated via separating the

diffusion-controlled current (k1ν1/2) and pseudocapacitive behavior (k2ν) according to the following equation:48,49

i(V) = k1ν1/2 + k2ν (9) in which k1 and k2 are adjustable parameters and can be quantified at fixed potentials by plotting iν-1/2 against ν1/2. It’s remarkably noticed that pseudocapacitive behavior contributes up to 82.6% to the total capacity of CZCOC@CNTs/CF electrode at a scan rate of 1.2 mV s-1 (Figure 7c), which is much higher than the 60.3% value of CZCOC (Figure S4c). Further with the scan rate increasing, the correlative values of CZCOC@CNTs/CF electrode are gradually boosted and much higher than that of CZCOC (Figure 7d and S4d). Such a prominent pseudocapacitive behavior of CZCOC@CNTs/CF can be conducive to the overall high-rate capacity and cycling stability. What’s more, the lithium ion diffusion coefficients (DLi) at different stages of the discharge process were further quantitatively obtained from the EIS results (Figure 7e and S5). It is well-known that the low-frequency Warburg contribution of the impedance response is affiliated with the determination of DLi. The relationship for DLi can be denoted as:

DLi=1/2{[Vm/(FAσw)]dE/dx}2 (10) where Vm, F, and A accordingly mean the molar volume, Faraday constant, and electrode surface, while the Warburg coefficient (σw) was gathered from the line of Z'~ω-1/2 (ω is the angular frequency) in the Warburg region (Figure S6). Z' against ω-1/2 in the low frequency Warburg region can be written as follows:

Z'= Rct+Re+σwω-1/2

(11) The dE/dx values were obtained from slopes of the discharge curve at 25%, 50%, 75%, and 100% of CZCOC@CNTs/CF and CZCOC electrodes, respectively. Attempted to gather such discharge curves, each battery cell was precycled for 5 cycles at 0.1 A g−1 prior testing, followed by stopping at the exceptional percentage of the total attained discharge capacity (Figure S7). The Rct exhibits an abrupt enlargement at 100% of the discharge depth in virtue of the formation of electronically insulating Li2O. This similar behavior can also be observed in other TMO anode materials for LIBs.50,51 The calculated DLi value at different discharge depths of both the two electrodes are shown in Figure 7f. The DLi in CZCOC@CNTs/CF electrode (7.85 × 10-11 ~ 3.34 × 10-10 cm2 s-1) is about one or even three orders of magnitude higher than that in CZCOC electrode (1.09 × 10-13 ~ 2.11 × 10-12 cm2 s-1). The superior lithium ion diffusion benefits from its intimate integration of CNTs and copper foam with polymetallic oxides CZCOC, which furnishes fast ion conductor and pathways as well as synergistic effect among the complex components and the doped C/N elements. The rate performance and cycle performance of the CZCOC@CNTs/CF are further in comparison with other recently reported multi-component metal oxide anodes for LIBs (Figure S8 and S9). Clearly, CZCOC@CNTs/CF has equipped with the superiority of both high-rate and longcycle capacity. In the view of the distinguished electrochemical performance, the particular structure and multi-composition advantages of the as-synthesized CZCOC@CNTs/CF material can be proposed as follows:



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Figure 8. Schematic illustration of the synergistic effect for CZCOC during lithiation/delithiation processes.

(i) Polymetallic oxides effectively enhance the excellent capacity and high-rate perfomance due to the synergistic effects between the different metal element and complex composition. As for the CZCOC@CNTs/CF electrode, metallic Zn, Co, and Cu endow the electrochemical activity toward lithium, effectively supplementing the lithium ion storage capacity because of the complementary behavior as well as synergistic effect.50 Associated with the mentioned reactions (1)-(3), this electrode experiences irreversible reactions between the lithium ions and CZCOC, further with the generation of nanodispersed metallic products (Cu, Zn, and Co), extra Li2O, as well as LiZn alloy since the initial discharge process (Figure 8). Moreover, the Cu, Zn, and Co metal products can achieve the extra Li2O reversibly convert into lithium ions followed by subsequent cycles, which is beneficial to enhance better reversible capacity and high-rate performance of the electrode.52 Interestingly, the reversible alloying/dealloying reactions of LiZn are able to supply extra storage capacity during the lithiation/delithiation process.53 Stimulatingly, to further investigate the ultrahigh electrochemical capacity of CZCOC polymetallic oxides, the “job-sharing” model was anticipated that a hybrid of both ionic and electric conductors can rearrange the ions and electrons into the corresponding conductors,54-56 which effectively improves the extra lithium storage performance. As for the discharge process, the metallic Zn, Co, Cu, alloy LiZn and the Li2O attract more electrons, among which the electron conductors Zn, Co, Cu and LiZn are more electronegative than metal Li and likely to attract a portion of electrons.57 However, the counter ions (Li+) are not only partially attached on Li2O surface but adsorb on the interface of the hybrids to balance the charge. By locating the electrons on the surface of the electron conductor, the electric double layer is formed by the multifunctional Li ions, among which the electrons contribute to the capacitive lithium storage by sharing (Figure 8). Herein, we propose an original electrochemical mechanism, denoted as “electronshared metal-Li+ capacitor”, which affirms the extraordinary lithium storage by the formation of the electric double layer capacitor, and manifests the exceeding specific capacity of the CZCOC polymetallic oxides. (ii) The introduction of electronic conductive buffer matrix (CNTs and copper foam) to fabricate a 3D selfsupported electrode can constructively buffer the drastic volume change and advance the electronic conductivity of

practical electrode, which not only provides numerous pathway for electron and lithium ion diffusion, but realizes more compact affinity between active material and the current collector reducing impedance during the lithiation/delithiation process without conductive-agent or binder, further improving electrochemical performances (Figure 9). As evaluated in Table S1, the electronic conductivity of CZCOC@CNTs/CF is effective improved by the combination of the electronic conductive CNTs and CF, together with the stable electrical contacts between active material and conductive matrix.58 For comparison, the electrochemical performances of the CZCOC@CNTs and CZCOC/CF are displayed in Figure S11. Unsurprisingly, the CZCOC@CNTs and CZCOC/CF electrodes achieve a reversible discharge capacity of 969 and 790 mAh g-1 after 500 cycles at 5 A g-1 (Figure S10), which is inferior to that of the CZCOC@CNTs/CF. Moreover, CZCOC@CNTs/CF and CZCOC electrodes were refolded for five times. However, the CZCOC on Cu foil simultaneously cracks and detaches due to the inferior mechanical Cu foil current collectors (Figure S11), implying the distinguished mechanical robustness of the as-synthesized CZCOC@CNTs/CF electrode ascribed to the flexible CF substrate.

Figure 9. Schematic illustration of the charge transfer through copper foam and carbon nanotubes.

(iii) The nano-scale structures, the doped C/N elements, high porosity as well as the large surface area of the material can offer shorter and faster diffusion pathway, mountainous volume space for alleviating the unavoidable volume variations, which is vital to preserve the structural stability and integrity during long cycle performance. As depicted in Figure S12, the morphology of CZCOC@CNTs/CF and the structures of 3D CF are expectedly maintained after 100


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Figure 10. (a) Schematic illustration, (b) cycling performance at 0.2 A g-1 of the CZCOC@CNTs/CF//NCM523 full cell.

cycles of discharge process, even if slight agglomerates among the nanoparticles are observed. The hollow interior structure is retained after the 100th discharge cycle, which was confirmed by TEM images (Figure S13). Furthermore, ex-situ Raman spectra of CZCOC@CNTs/CF after longterm cycling was investigated with the ID/IG value maintaining 0.86, indicating that the degree of graphitization is still preserved during the lithiation/delithiation process (Figure S14). Further motivated by the outstanding electrochemical performance of the CZCOC@CNTs/CF, a full cell to evaluate the practical feasibility of the proposed CZCOC@CNTs/CF. As schematically illustrated in Figure 10a, commercially available cathode LiNi0.5Co0.2Mn0.3O2 (NCM523) has been coupled with CZCOC@CNTs/CF anode, which the mass ratio of the cathode to anode is optimized to be 1.4:1. The CZCOC@CNTs/CF electrode was prelithiated by direct contact with Li foil in the electrolyte, and then assembled in full batteries.59,60 The NCM523 cathode in half cells achieves 150.6 mAh g−1 at a current density of 0.2 A g−1 after 100 cycles (Figure S15). The CZCOC@CNTs/CF//NCM523 full cells were tested at a current density of 0.2 A g-1 at the voltage window between 3.0 and 4.2 V under 25 °C. Figure S16a shows the initial charge/discharge capacities of 254.3 and 227 mAh g-1, respectively. The cycling stabilities of the as-prepared full cell is shown in Figure 10b, CZCOC@CNTs/CF//NCM523 full cell delivers an excellent reversible discharge capacity of 174 mAh g-1 (based on the cathode mass) after 100 cycles, with the capacity retention of 76.7%, suggesting distinguished cyclability performance during the lithiation/delithiation process. The full cell delivers an excellent capacity of 137.5 mAh g−1 even at a harsh current density of 2 A g−1. When the current density turn to 0.2 A g−1 (Figure S16b), it still maintains a reversible capacity of 186.2 mAh g−1, manifesting the superior rate capability of the assembled full cell. Although the cell assembly procedures are still supposed to be further optimized, the present electrochemical performance of the full cell is considerably promising, indicating CZCOC@CNTs/CF is a potential candidate for the next-generation advanced anode LIB applications. CONCLUSION In summary, a novel 3D trimetallic MOF-derived CZCOC@CNTs/CF was successfully designed and

prepared by postsynthetic exchange and in-situ solvothermal method and further annealing process. The introduction of conductive CNTs buffer and 3D CF substrate effectively boosts the mechanical robustness and electronic conductivity in the LIB anode material. What’s more, the high porosity, doped C/N elements and polymetallic oxides effectively accommodate the volume variation, enhance the reaction sites and exert synergistic effects, which contributes to the ultrahigh long-cycling reversible specific capacity and highrate performance via the dominant “electron-shared metalLi+ double electric layer” together with the favorable mechanism of pseudocapacitive charge storage. Herein, the conductive-agent- and binder-free CZCOC@CNTs/CF electrode delivers robust long-cycling stability and superior high-rate capacity with a high specific capacity of 1649 mAh g−1 at 0.2 A g−1 and 1282 mAh g−1 at 5 A g−1 over 1000 cycles, respectively. The novel 3D self-supported electrode synthesis strategy proposed in this work sheds light on the potential anode materials, which confront with low electrical conductivity and drastic volume variation.

ASSOCIATED CONTENT Supporting Information TGA curve, SEM images, TEM images, PXRD patterns, CV curves and EIS plots. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author *E-mail: [email protected]. *E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 21671071), China Postdoctoral Science Foundation Funding (2018M643069), Science and Technology Planning Project of Guangdong Province (2017A010104015, 2017B090917002 and 2015B010135009), Science and Technology Planning Project of Cuangzhou, China (201904010213), Great Scientific Research Project of Guangdong Ordinary University (No. 2016KZDXM023), Open Project of State Key Laboratory of Supramolecular Structure and Materials (sklssm2019018), and



Special Funds for the Cultivation of Guangdong College Students' Scientific and Technological Innovation (pdjh2019a0126).

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A multi-component Cu0.39Zn0.14Co2.47O4-CuO@CNTs/CF anode electrode via annealing of the trimetallic Zn-Co-Cu-ZIF precursor is fabricated and displays distinguished electrochemical performance for rechargeable LIBs.


Trimetallic MOF-Derived Cu0.39Zn0.14Co2.47O4-CuO Interwoven

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