Facile Preparation of Porous Rod-like Cu x Co3-x O4/C Composites via Bimetal-Organic Framework Derivation as Superior Anodes for Lithium-Ion Batteries.

To meet growing demand of energy, lithium-ion batteries (LIBs) are under enormous attention. The development of well-designed ternary transition metal oxides with high capacity and high stability is important and challengeable for using as electrode materials for LIBs. Herein, a new and highly reversible carbon-coated Cu-Co bimetal oxide composite material (Cu x Co3-x O4/C) with a one-dimensional (1D) porous rod-like structure was prepared through a bimetal-organic framework (BMOF) template strategy followed by a morphology-inherited annealing treatment. During the annealing process, carbon derived from organic frameworks in situ fully covered the synthesized bimetal oxide nanoparticles, and a large number of porous spaces were generated in the MOF-derived final samples, thus ensuring high electrical conductivity and fast ion diffusion. Benefiting from the synergetic effect of bimetals, the unique 1D porous structure, and conductive carbon network, the as-synthesized Cu x Co3-x O4/C delivers a high capacity retention up to 92.4% after 100 cycles, with a high reversible capacity still maintained at 900 mA h g-1, indicating an excellent cycling stability. Also, a good rate performance is demonstrated. These outstanding electrochemical properties show us a concept of synthesis of MOF-derived bimetal oxides combining both advantages of carbon incorporation and porous structure for progressive lithium-ion batteries.

Introduction

As one of the most promising technologies to solve the heavy consumption of fossil fuel resources and serious environmental problems, Li-ion batteries (LIBs) have been used as an advanced and portable source of electricity for various applications.[1−5] However, the specific energy of conventional LIBs is not sufficient for these applications due to the limited specific capacity of the conventional graphite anode (372 mA h g–1).[6−8] Many research studies have been carried out to find new electrode materials with high energy density or high capacity.[9,10] Among those materials, Co3O4, a transition metal oxide (TMO), owing to its complex electron transfer reaction upon cycling, exhibits excellent potential for its high specific capacities (nearly 1000 mA h g–1).[11,12]

Recently, in consideration of the nonenvironmentally friendliness, expensiveness, and high-lithium redox potential (2.2–2.4 V vs Li/Li+) of Co, some other eco-friendly and cheaper transition metal atoms, such as Zn,[13,14] Mn,[15−17] Ni,[18] and Cu, have been used to replace Co partially to form ternary metal oxides. By replacing Co with cheaper and more conductive Cu, the obtained Cu-Co bimetal oxides present an improved electronic conductivity and higher electrochemical activity than the single copper or cobalt oxide and thus proved to be excellent electrode materials for LIBs. Sharma et al. synthesized CuCo2O4 as an anode material for LIBs with a reversible capacity of 755 mA h g–1 at a current of 60 mA g–1.[19] Sun et al. reported a mesoporous CuCo2O4 structure; after six cycles, it still retained its capacity of 900 mA h g–1 at 60 mA g–1.[20] Although these reported CuCo2O4 electrodes present a much higher capacity than that of the conventional graphite, due to the large volume expansion with cycling and relatively poor electrical conductivity, the cycling stability, Coulombic efficiencies, and rate performance of Cu-Co bimetal oxides need to be further improved.

So far, researchers have done much effort to solve these problems. One effective method is to produce rational porous nanostructures, which can provide additional channels for fast diffusion of lithium ions. Meanwhile, it can alleviate the structural strain induced by the lithium insertion/extraction process.[21] Second, combining the transition metal oxides with more conductive carbon materials is another well-established strategy for enhancing the electrochemical performance of TMOs. Since carbon is in the composite, the volume expansion of TMOs could be restrained, and the cycling stability of the electrode material could be enhanced effectively. Moreover, the improved electrical conductivity of the electrode usually leads to an enhanced rate capability. Kang et al. indicated that the capacity of the CuCo2O4 nanocube encapsulated by reduced graphite oxide sheet was almost two times higher than that of pure CuCo2O4 nanocube.[22] Recently, Zhang et al. utilized an electrospinning method to synthesize CuCo2O4/C nanofibers, which present an excellent cycling stability with a high capacity of 865 mA h g–1 after 400 cycles.[23] Motivated by these progresses, combining the above two strategies to fabricate carbon incorporated Cu-Co bimetal oxide composite materials with a well-designed porous structure will be a promising method to improve the electrochemical performance. However, synthesis of such porous structured composite materials is still facing challenges.

In this work, we report a rational design and fabrication of CuCo3–O4/C composite with a porous rod-like structure by using bimetal–organic frameworks (BMOFs) as the precursors and then calcination at a certain temperature. Metal–organic frameworks (MOFs) are constructed from supramolecular assembly of metal ions or clusters and organic ligands, which are well known for their diverse porosity and architectural structures.[24−26] Inspired by their excellent configuration, MOFs are usually considered as the multifunctional templates and precursors for the preparation of new porous nanostructured materials with high surface area and graded pore distribution, basically through thermal decomposition. In addition, by accurately adjusting and controlling MOF precursors and calcination conditions, the structure and composition of MOF-derived nanomaterials can be easily adjusted and optimized to achieve excellent electrochemical properties as LIB anodes.[27−30] Specially, we made use of the organic frameworks to generate a microcrystal precursor with a well-designed rod-like structure, which then transformed to morphology-kept bimetal oxides during annealing in insert atmosphere. Attributed to the annealing process, a large number of porous spaces were generated in the MOF-derived final products because of the carbonization or graphitization of organic ligands in MOFs during annealing treatment. Most importantly, carbonized and graphitizated carbon originated from organic frameworks would in situ fully cover these metal oxide nanoparticles, which can not only improve the electronic conductivity of electrode materials but also protect the electrode materials against destruction during the charge/discharge process. The obtained CuCo3–O4/C particles connect each other and form a porous rod-like structure, ensuring fast diffusion of Li+/electrons. Owing to the unique structural and compositional features, the CuCo3–O4/C presents a high-lithium storage capacity (900 mA h g–1 after 100 cycles and a high capacity retention up to 92.4% with respect to the initial discharge capacity of 974.1 mA h g–1) and excellent rate capability when evaluated as anode materials.

Results and Discussion

Characterization of CuCo3–O4/C Nanocomposites

The overall strategy for the fabrication of porous rod-like CuCo3–O4/C is clarified in Scheme . First, a facile solvothermal method was used to synthesize Cu-Co-BTC precursors. Owing to the presence of a carboxyl functional group (H3BTC), Co2+ and Cu2+ ions can easily connect it by complexation or adsorption; thus, metal ions are uniformly distributed in these rod-like MOF precursors, providing the metal sources for further preparation of bimetal oxides. After annealing these MOF microcrystal precursors in an inert atmosphere, the morphology-kept bimetal oxide/C composites were obtained. The formation of porous rod-like structure is mainly due to carbon and oxygen in organic ligands reacting to generate gas to cause holes during the thermal decomposition of rod-like Cu-Co-BTC precursor. Meanwhile, these synthesized bimetal oxide nanocrystals are simultaneously covered by carbon derived from in situ carbonization of the organic ligands. Combining the advantages of Cu-Co bimetal oxide nanocrystals with a 1D rod-like structure, high porosity, and well-conductive carbon network, the as-synthesized CuCo3–O4/C has great potential to exhibit outstanding electrochemical lithium storage performance as an anode material for LIBs.

Scheme 1

Schematic Illustration of the Fabrication of Porous Rod-like CuCo3–O4/C Composites

The precursors of Cu and Co with different molar proportions were easily synthesized by a solvothermal reaction, which are denoted as Cu-Co-BTC-n (n represents the molar ratio of Cu to Co). Figure a shows a representative field-emission scanning electron microscopy (FESEM) image of Cu-Co-BTC-0.3. It can be seen that Cu-Co-BTC-0.3 presents a rod-like structure with a diameter around 1 μm and a length close to 10 μm. High-magnification SEM image in Figure b reveals the detailed structure of the rod, which actually consists of four prismatic planes and a relatively smooth surface. Transmission electron microscopy (TEM) investigation as shown in Figure c also reveals a solid structure for the Cu-Co-BTC-0.3 rods. The formation of bimetallic Cu-Co-BTC-0.3 is confirmed by elemental mapping images (Figure d–h), which present a uniform distribution of elements of Co, Cu, C, and O along the rods. Meanwhile, the amorphous nature of Cu-Co-BTC-0.3 rods is confirmed by powder X-ray diffraction (XRD) pattern, which is common for MOFs (Figure S1). We also took FESEM investigations for other Cu-Co-BTC-n samples with different Cu/Co molar ratios, and the results are shown in Figure S2. Notably, when the Cu/Co molar ratio is less than 0.1, the obtained Cu-Co-BTC still maintains a typical sphere-like morphology as the reported single-metal Co-BTC.[31,32] As the molar ratio increases, a rod-like structure for the Cu-Co-BTC appears, and the diameter size increases accordingly, from 1 μm for Cu-Co-BTC-0.3 to around 1.5–2 μm for Cu-Co-BTC-0.8. These results reveal that the amount of Cu source plays an important role in the formation of 1D rod-like Cu-Co-BTC-n.

Figure 1

(a, b) FESEM images, (c) TEM image, and (d–h) corresponding elemental mapping images of Cu-Co-BTC-0.3.

The thermal behavior of the Cu-Co-BTC-0.3 precursor was investigated with thermogravimetric analysis (TGA). As shown in Figure S3, below 200 °C, there is a weight loss of 20%, which may be attributed to the escape of DMF molecules and adsorbed water. The subsequent weight loss of 49% under 400–550 °C can be attributed to the oxidation of the BTC ligand and the decomposition of the skeleton. To ensure complete conversion of precursors to oxide products, 600 °C was selected as the calcination temperature in the next process.

The morphologies of CuCo3–O4/C samples obtained via calcination of Cu-Co-BTC precursors were observed with FESEM and TEM. Figure shows the morphology features of CuCo3–O4/C-0.3. It can be seen that the annealed products typically maintain almost the same size and rod-like morphology as that of the MOF precursors (Figure a). Careful morphology observation by high-magnification SEM (Figure a (inset),b) reveals that the CuCo3–O4/C rods are composed of particles with sizes varying from dozens to hundreds of nanometers. The surface of these rods is highly rough and porous, which can be attributed to the carbonization of organic ligands in the Cu-Co-BTC precursor during the annealing process. The structure features of these CuCo3–O4/C-0.3 rods were further examined by TEM. The microstructure displayed in Figure c shows that the particles interconnects each other and form a 1D rod-like structure, which corresponds to the SEM results (Figure a,b). Figure d shows a typical high-magnification TEM image taken from the edge of CuCo3–O4/C nanoparticle. A very thin carbon layer coated on the surface of the bimetal oxide nanocrystals could be found. Besides, a lattice fringe with an interplanar spacing of 0.28 nm could be clearly seen in the HRTEM image, which is corresponding to the (220) lattice plane of spinel CuCo2O4. The selected area electron diffraction (SAED) pattern (Figure S4) also demonstrates the crystalline feature of these particles. The elemental mapping shows the homogeneous coexistence and highly uniform distribution of Cu, Co, O, and C within one particle, confirming the composition of CuCo3–O4/C. On the basis of these observations, it can be safely concluded that the as-prepared Cu-Co MOF rods transformed into CuCo3–O4/C composites at 600 °C in a N2 atmosphere. The carbon shell is derived from carbonization of organic ligands, and the crystallized bimetal oxide particles are encapsulated by the carbon wall, thus forming a core–shell structure for each CuCo3–O4/C particle. Since the carbon framework still maintains a 1D structural characteristic, the CuCo3–O4/C particles interconnect each other and form a porous rod-like structure. Nitrogen adsorption–desorption measurement was used to investigate the specific porous nature of CuCo3–O4/C rods. As shown in Figure S5, the CuCo3–O4/C-0.3 exhibits a type IV isotherm with a distinct hysteresis loop in the relative pressure region of 0.5–1.0, indicating a mesoporous feature. Barrett–Joyner–Halenda (BJH) desorption analyses reveal a relatively wide pore size range from 1 to 50 nm (inset in Figure S5). The origination of pores may be attributed to the escape of gases via the calcination of organic ligands. In addition, the composite is assembled from small particles; these stacked particles may cause some pores on the surface of the material. A high Brunauer–Emmett–Teller (BET) surface area of 98.0 m2/g was also confirmed. As we know, such a mesoporous structure could ensure efficient diffusion of the electrolyte, and a high surface area may provide more electrochemical active sites. We also studied the influences of Cu/Co molar ratio on the structure of the product. These SEM results strongly support that idea that the morphology of CuCo3–O4/C-n keeps the same character as that of Cu-Co-BTC-n precursors (Figures S2 and S6). As for the obtained CuCo3–O4/C-0.8, it clearly displays a porous rod-like structure composed of nanoparticles. While it cannot be ignored that there are many disorderly packed CuCo3–O4/C particles, they appear due to the destruction of the rod-like framework. Actually, the thicker the rod, the easier it is to destroy the structure during calcination.

Figure 2

Structural characterization of CuCo3–O4/C-0.3. (a, b) FESEM images, (c) TEM image, (d) HRTEM image, and (e–i) EDS elemental mapping images.

XRD patterns of the obtained CuCo3–O4/C-n samples are shown in Figure . For all CuCo3–O4/C-n products, there is a main phase of spinel CuCo2O4, and a secondary phase CuO (JCPDS card no. 80-1917) could be observed. The relatively sharp reflections located at 18°, 31°, 36°, 38°, 44°, 55°, 58°, and 64° can be well indexed to the (111), (220), (311), (222), (400), (422), (511), and (440) planes of CuCo2O4 (JCPDS card no. 78-2177), respectively, revealing a high crystallization. The cubic lattice parameter, assessed by least-squares fitting of 2θ and (hkl), is a = 8.139(4) Å, which is in good agreement with the value of 8.133(2) Å. Since the atomic diameter of Cu is bigger than that of Co, the little higher diameter of the unit cell demonstrates that the Cu atoms were completely incorporated into the Co3O4 lattice. No reflection peaks originated from the graphitized carbon could be found, owing to the small amount of carbon. Meanwhile, no other redundant peaks are observed, suggesting a high purity of metal oxides.

Figure 3

XRD patterns of CuCo3–O4/C-n. (a) CuCo3–O4/C-0.1, (b) CuCo3–O4/C-0.3, and (c) CuCo3–O4/C-0.8.

X-ray photoelectron spectroscopy (XPS) measurements were applied to indicate the elemental composition and chemical bond of CuCo3–O4/C-0.3 (Figure ). The survey spectrum illustrated unique peaks of Cu, Co, O, and C (Figure a), demonstrating the successful doping of Cu in Co3O4, and no other peaks of elements could be found in the survey. The Co 2p3/2 peak of the CuCo3–O4/C-0.3 composite reveals two characteristic peaks at 779.2 and 780.1 eV (Figure b), corresponding to Co(III) and Co(II) species, respectively.[22]Figure c shows the Cu 2p XPS spectrum, which exhibits two major peaks at 934.5 and 954.6 eV for Cu 2p3/2 and Cu 2p1/2, respectively, and a small satellite peak between them, indicating the existence of copper in the oxidation state of Cu(II) in CuCo3–O4.[33] The peaks of O 1s present three characteristic locations at 529.3, 531.0, and 531.9 eV (Figure d).[34] All these results indicate the successful synthesis of copper cobalt oxides. The deconvolution of C 1s peak (Figure S7) indicates that carbon exists mainly in the form of graphitized carbon, which is dominant on the surface of CuCo3–O4/C.

Figure 4

XPS spectra of (a) survey spectrum, (b) Cu 2p, (c) Co 2p, and (d) O 1s for CuCo3–O4/C-0.3.

Electrochemical Performances of CuCo3–O4/C Nanocomposites

To explore the benefits of rational design for the porous rod-like CuCo3–O4/C composites as electrode materials of LIBs, the electrochemical behavior of CuCo3–O4/C as the anode was explored by the cyclic voltammetry (CV) technique. The CV curves for the first four cycles of the CuCo3–O4/C-0.3 electrode at the scan rate of 0.5 mV s–1 are shown in Figure a. Two obvious peaks can be found in the first cathodic cycle at 0.95 and 0.6 V. The intense peak located at around 0.95 V can be attributed to the decomposition of the CuCo3–O4/C-0.3 and the formation of Cu and Co nanoparticles in an amorphous matrix Li2O, as reported in earlier studies.[20] As stated in eq , during the initial discharge cycle, the irreversible structure destruction process of CuCo3–O4 consumes 8 mol of Li per mole of CuCo3–O4, and thus, electrochemically active Co and Cu nanoparticles are formed in the matrix of Li2O. The relatively weak peak located at around 0.6 V in the first cathodic scan and then disappearing in the subsequent cycles is due to the formation of the solid electrolyte interface (SEI) at the interface of the electrolyte and electrode surface.[35] In the first anodic process, the broad oxidation peak located at around 2.1 V can be assigned to the oxidation of Cu and Co to their corresponding metal oxides, as seen in eqs –4.[19] During the next cycling process, it is observed that the reduction peaks are found to be shifted to a higher potential of around 1.10 V with respect to the first cycle (0.95 V), indicating a different reaction mechanism with Li compared to the first discharge reaction. Meanwhile, there is a slight potential shift in the oxidation peaks in the second cycle, which can be attributed to the reorganization of the active materials.[36] After the second cycle, the intensity of the redox peaks changes slightly, indicating good cycling stability and high reversibility of the CuCo3–O4/C-0.3 electrode.

Figure 5

Electrochemical performances of the as-prepared CuCo3–O4/C-0.3 electrode. (a) CV curves at a scanning rate of 0.1 mV s–1 in the voltage range of 0.01–3.0 V. (b) Charge–discharge profiles at a current density of 100 mA g–1. (c) Comparison of rate capabilities at various rates of 0.1, 0.2, 0.5, 1, and 0.1 A g–1. (d) Cycling performance at a current rate of 100 mA g–1. (e) Nyquist plot of CuCo3–O4/C-0.3 electrode with the equivalent circuit used to model the impedance spectra (inset). (f) Z′ versus ω–0.5 plots in the low-frequency range.

Figure b shows the galvanostatic charge–discharge lines of electrode at a current density of 100 mA g–1 in a voltage window of 0.01–3.0 V. A large voltage plateau located at ∼1.2 V followed by a sloping profile is observed in the first discharge curve. The plateau can be attributed to the reduction of CuCo3–O4 to metal Co and Cu and the formation of Li2O at the same time. The slopping part is generally regarded as capacitive behavior of surface storage of lithium. The phenomenon has been widely observed in mesoporous materials with high BET surface area and some other nanostructured materials.[37] The first charge process presents a smoothly varying profile followed by a plateau at around 2.1 V, which is attributed to the oxidation of Cu and Co to corresponding metal oxides. These results are well consistent with the CV measurements. According to previous studies, the lithium storage mechanism of the CuCo3–O4/C could be considered as follows

As shown in Figure b, the first discharge and charge capacities are 974.1 mA h g–1 and 616.9 mA h g–1, respectively, corresponding to a Coulombic efficiency of 63.3%. The irreversible capacity loss can be attributed to the formation of a solid electrolyte interface (SEI) layer, decomposition of electrolyte, and some unresolved Li2O phases, which agree with most anode materials.[38] It is notable that the Coulombic efficiency increases gradually and maintains at above 96% after the first four cycles. For the 10th and 50th cycles, the curves demonstrate a high Coulombic efficiency of nearly 100%, indicating the increasing reversibility and good cycling stability.

The rate capability of the CuCo3–O4/C-0.3 electrode was measured with different current densities after 50 cycles. As shown in Figure c, the electrode exhibits a good rate capability with the average discharge capacities of 885, 800, 620, and 507 mA h g–1 at current densities of 100, 200, 500, and 1000 mA g–1, respectively. It is noteworthy that a high capacity of 866 mA h g–1 can be recovered when the specific current returns back to 100 mA g–1 after dozens of heavy-duty cycles, which still closes to the initial capacity at 100 mA g–1. This result shows that CuCo3–O4/C-0.3 has a great potential as high-rate anode materials for LIBs. Cycling stability is another important factor in LIB applications. Figure d shows the cycling performance of the CuCo3–O4/C-0.3 electrode at a current density of 100 mA g–1. Interestingly, the capacity of the electrodes decreases from the first discharge of 974.1 mA h g–1 to less than 600 mA h g–1 during the first few cycles, then increases gradually with cycling, and finally maintains stability at a certain value. There is a common phenomenon in other transition metal oxide electrodes, which is rationally attributed to the reversible formation of polymeric/gel-like films originating from kinetic activation in the electrode.[39] The porous structures and conductive carbon shell are other important factors for the increasing capacities since the electrolyte gradually penetrates into the inner part of the porous CuCo3–O4/C rods, thus leading to a slow electrochemical activation and reversible reactions of active materials.[22] Another suggestion is that the formed metal nanoparticles during the discharge processes can promote the reversible transformation of some SEI components. Also, it allows more accessible active sites available for Li+ insertion, thus leading to an increase of capacity.[33] Over the prolonged cycling, the CuCo3–O4/C-0.3 electrode shows outstanding capacity retention, and the reversible discharge capacity remains as high as 900 mA h g–1 after 100 cycles, indicating a high capacity retention of 92.4% with respect to the initial discharge capacity of 974.1 mA h g–1. In addition, the Columbic efficiency increases gradually and stabilizes at above 96% even at 100% after the first four cycles, indicating good cycling stability. For comparison, the cycling performance of other CuCo3–O4/C-n electrodes at a current density of 100 mA g–1 was also investigated. Owing to the synergetic effect of two types of metal elements, compared to the corresponding monometal oxides, the introduction of copper is an important factor for enhanced LIB performance. The conductivity of Cu is better than that of Co, while the theoretical specific capacity of CuO (670 mA h g–1) is much lower than that of Co3O4 (around 1000 mA h g–1). So, with the increase of Cu in CuCo3–O4/C-n, the conductivity will be enhanced, but the capacity may decrease. As shown in Figure S8, CuCo3–O4/C-0.1 shows a high initial capacity (1115.2 mA h g–1) but decays quickly with cycling. For CuCo3–O4/C-0.8, the capacity drops from the first discharge capacities of 836.7 mA h g–1 to less than 300 mA h g–1 after 100 cycles, which is much lower than that of CuCo3–O4/C-0.3. The bad cycling performance may also be attributed to the destruction of the rod-like structure, as revealed in Figure S6. Combining both capacity advantage and morphological integrity, CuCo3–O4/C-0.3 presents the best cycling performance.

Possible Reasons for High Performance

Electrochemical impedance spectroscopy (EIS) was carried out to explain the superior electrochemical performance of CuCo3–O4/C-0.3. The Nyquist plot is shown in Figure d. Obviously, a semicircle in the high-frequency region is closely related to the charge transfer resistance of the electrode/electrolyte interface. Meanwhile, an inclined line in the low-frequency region is associated with the lithium-ion diffusion process.[40] The intersection point of axis in the high-frequency region is correlated with electrolyte resistance. The classic equivalent circuit model of the investigated system is depicted in the inset of Figure d to represent the internal resistance of the battery. Re stands for the internal resistance of the battery; Rsl represents the resistance of migration, and Csl represents the capacity of the layer in the high-frequency semicircle; Cdl and Rct are related with the double-layer capacitance and charge transfer resistance, respectively; Zw stands for the diffusion-controlled Warburg impedance. The Zsimp Win computer program was used to calculate the main electrical parameters in the equivalent circuit. The Rct value of CuCo3–O4/C-0.3 electrode is 9.5 Ω, which is much smaller than that of the reported pristine CuCo2O4 electrode (≈31 Ω),[22] suggesting that the carbon coating on CuCo3–O4 nanoparticles derived from carbonization of organic ligands significantly lowers contact and charge transfer impedances.

Additionally, the Li-ion diffusion coefficients of CuCo3–O4/C-0.3 were determined with the low-frequency Warburg contribution of the impedance. The diffusion coefficient of Li ions (DLi+) can be calculated using the following equations[41]

As shown in Figure f, Z′ corresponds to the real impedance, ω(2πf) represents the angular frequency in the low-frequency region; meanwhile, Re + Rct and the Warburg coefficient (σ) can be calculated from the fitting line, and Z′ has a linear relationship with ω–0.5. In eq , T, R, A, and F represents the absolute temperature, gas constant, surface area of the electrode, and Faraday’s constant, respectively. CLi represents the molar concentration of Li ions. n is the number of electrons transfer per mole of the active material involved in the electrode reaction. According to the fitting linear equation in Figure f, the lithium-ion diffusion coefficients of CuCo3–O4/C-0.3 is calculated to be approximately 3.17 × 10–12 cm2 s–1, which is much better than another reported CuCo2O4/carbon composite (2.23 × 10–13 cm2 s–1).[22] These results further confirm that both the charge transfer and Li-ion diffusion kinetics of the porous rod-like CuCo3–O4/C are improved significantly, thus leading to superior electrochemical properties.

The high reversible capacity, superior rate performance, and excellent cycling stability of the porous rod-like CuCo3–O4/C-0.3 electrode may be attributed to several factors. First, the CuCo3–O4 particles are derived from Cu-Co-BTC precursors, accompanying with the carbonization of organic ligands to form graphitized carbon covered on the bimetal oxide particles. There is a strong and well-established interfacial connection with carbon, thus ensuring a high conductivity required. Second, the contact area between the active material and electrolyte could be enlarged for the mesoporous structure. Compared with the spherical morphology for CuCo3–O4/C-0.1, the unique rod-like porous structure for CuCo3–O4/C-0.3 can offer direct channels for efficient electron transport. However, 1D nanostructures still suffer from interfacial kinetic problems in contact areas. Once the 1D nanostructure combines with carbon, it can effectively solve interfacial kinetic problems. Particularly, the pores in the 1D rods allow a high utilization efficiency of active sites. Third, the typical one-dimensional structure and existence of carbon materials would mitigate the volume expansion, protect the active materials against pulverization during the charge/discharge process, and thus improve the stability of the electrode. The comparison of the electrochemical performance of the obtained CuCo3–O4/C-0.3 with earlier reports of other bimetal oxide for lithium-ion batteries is summarized in Table S1. Results confirm that the porous rod-like CuCo3–O4/C composite electrode with large reversible capacity and good cycling ability may accelerate the development of high-performance LIBs. Thus, the present research work demonstrated the important basis and wide applications in various self-assembled nanocomposites and functionalized nanostructures.[42−57]

Conclusions

In summary, the porous rod-like CuCo3–O4/C composite materials with diverse Cu/Co ratios have been synthesized with a solvothermal method to fabricate bimetal–organic frameworks followed by a morphology-inherited annealing treatment. The synthesized CuCo3–O4 nanoparticles were encapsulated in carbon coating, which may restrain the volume expansion and improve the electronic conductivity of electrode materials. The MOF-derived 1D porous structure provides the efficiently accessible lithium storage active sites for high capacity and the enhanced pathway for ion diffusion in the charge–discharge process. Combining the advantages of the well-designed structure and incorporation of carbon, the obtained CuCo3–O4/C-0.3 presents superior electrochemical performance, including high reversible capacity of 900 mA h g–1 even after 100 cycles and excellent cycling stability and rate capability. It is believed that the facile preparation process and high electrochemical performance of the porous rod-like CuCo3–O4/C composite would pave the way for its practical LIB application.

Experimental Section

Synthesis of Cu-Co-BTC MOFs

In a typical synthesis, 1 g of Cu(NO)3·6H2O and Co(NO)3·6H2O with different Cu/Co ratios, 1 g of polyvinylpyrrolidone (PVP), and 0.1 g of trimesic acid (H3BTC) were dissolved in 30 mL of DMF solution. After magnetically stirring for 5 min, the mixture was transferred into a Teflon-lined stainless steel autoclave and kept at 120 °C for 12 h. The obtained precipitate was washed several times with ethanol and deionized water and then dried at 70 °C for 12 h in vacuum. The final obtained powers are named Cu-Co-BTC-n (n represents the molar ratio of Cu to Co).

Preparation of CuCo3–O4/C Rods

The obtained Cu-Co-BTC-n powder was annealed in a nitrogen atmosphere at 600 °C for 2 h with a heating rate of 5 °C min–1. The powder samples were collected after cooling down to room temperature and named as CuCo3–O4-n (n = 0.1, 0.3 and 0.8).

Characterization

The structure, morphology, and composition of samples were characterized with X-ray diffraction spectroscopy (XRD, Rigaku 2550), X-ray photoelectron spectroscopy (XPS, Escalab 250Xi), scanning electron microscopy (SEM, Carl Zeiss Sigma HD), transmission electron microscopy (TEM, JEM-2100F), high-resolution TEM (HRTEM) imaging, and selected area electron diffraction (SAED). Thermogravimetric analysis (TGA) was carried out by a STA449C thermal analyzer with a heating rate of 10 °C min–1 under Ar.

Electrochemical Measurements

The active materials, carbon black, and poly(vinylidene fluoride) with a weight ratio of 80:10:10 were mixed in N-methyl-2-pyrrolidinone (NMP) solution to prepare the working electrode. The obtained slurry was coated onto the copper foil current collector and dried in vacuum at 70 °C for the whole night. The coin cells were assembled in a glove box filled with argon. The pure lithium foil, a Celgard 2400 membrane, and 1 M LiPF6 in a mixture of dimethyl carbonate (DMC) and ethylene carbonate (EC) (1:1 in volume) were used as the counter electrode, separator, and the electrolyte, respectively. Before electrochemical measurement, the cells were placed for 8 h. The electrochemical measurements were carried out in a CT-3008 battery testing system at room temperature. An electrochemical workstation (CHI660e) was used to test the cyclic voltammetry (CV) with a scan rate of 0.1 mV s–1 and a voltage window of 0.01–3.0 V. Electrochemical impedance spectroscopy (EIS) measurements were implemented at 5 mV amplitude ranging from 100 kHz to 0.01 Hz.