Cu composite current collectors with array-pattern porous structures for lithium-ion batteries

Electrochimica Acta 226 (2017) 89–97

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

High-performance CuO/Cu composite current collectors with array-pattern porous structures for lithium-ion batteries Wei Yuan* , Jian Luo, Zhiguo Yan, Zhenhao Tan, Yong Tang School of Mechanical and Automotive Engineering, South China University of Technology, Wushan Road 381, Guangzhou 510640, PR China

A R T I C L E I N F O

Article history: Received 31 August 2016 Received in revised form 14 December 2016 Accepted 22 December 2016 Available online 30 December 2016 Keywords: lithium-ion battery composite current collector porous structure copper oxide equivalent circuit

A B S T R A C T

The surface structure and material composition of current collectors significantly affect the electrochemical performances of lithium-ion batteries. This study forms array-pattern blind holes and creates a layer of copper oxide (CuO) on the surface of thin copper plates using the chemical etching method. This copper plate is made into a CuO/Cu composite current collector with array-pattern porous structures for lithium-ion batteries. Using mesocarbon microbead graphite powders as the anode material, this new composite current collector is assembled into CR2032 coin half-cells for electrochemical tests. Batteries with this porous current collector exhibit high reversible discharge capacities of 383.9 mAh g
1 at 0.5 mA and 374 mAh g
1 even after 0.2C and 0.5C rate cycles, whereas batteries with a complanate current collector deliver only 309.6 mAh g
1 and 296.7 mAh g
1. It is believed that the array-pattern blind holes coupled with the morphological effects of the oatmeal-like CuO significantly enhance the electrochemical performances of batteries in terms of reversible capacity, cycling stability, electrical conductivity and coulombic efficiency. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction As one of the most successful applications in the electrochemical field, the lithium-ion battery (LIB) is considered to be a promising versatile power source and has been widely used in various fields because of its advantages, such as its high energy density, electrical potential and stability, low self-discharge, no memory effect, and so on [1–6]. Currently, with the rapid popularization of electric vehicles and portable electronic devices, it is of great urgency to enhance the electrochemical performance of LIBs in terms of reversible capacity, cycling stability, electrical conductivity, and so on [7–10]. However, the anode based on commercial graphitic carbon mostly shows a low theoretical capacity of 372 mAh g
1, which cannot satisfy the requirement of future energy-storage devices with high power densities and energy densities [11–14]. Therefore, it is essential to explore new high-capacity anode materials for LIBs. In this regard, some researchers focus on the use of transition metal oxides (TMOs) based on a different conversion mechanism

* Corresponding author. E-mail addresses: [email protected], [email protected] (W. Yuan). http://dx.doi.org/10.1016/j.electacta.2016.12.139 0013-4686/© 2016 Elsevier Ltd. All rights reserved.

from graphite materials. As first reported by Poizot et al. [15], TMOs have been proven to have extremely high reversible capacities. Among all developed transition metal oxides, CuO is considered to be a promising candidate material for the anode of LIBs because of its high theoretical capacity of 674 mAh g
1 and ease of preparation [16–21]. Zhang et al. [18] prepared a relatively pure CuO as the anode material for LIBs using a hydrothermal synthesis method, and the battery showed good capacity retention during high-rate cycles. However, the pure CuO in their studies inevitably decreases other battery performances because of its poor ion transport kinetics, inferior intrinsic electrical conductivity and large volume variation during the discharge-charge processes. To address this issue, some have proposed various optimization strategies [22– 28]. For example, Lamberti and coworkers [22] presented an easy and effective strategy to prepare nanostructured cuprous oxide thin films directly on copper current collectors by a rapid thermal oxidation process. As a result, the batteries with these nanocomposite films exhibited excellent cycling stability and capacity retention due to the high surface area, short diffusion n path and good conduction of the nanocomposite films. Similarly, Wang et al. [27] fabricated porous CuO microspheres with a dandelion-like hollow structure using a hydrothermal synthesis method. Because these CuO microspheres, which have a large surface area and

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Nomenclature

CMC CNT EIS LIB MCMB PVD SEI TMO

carboxymethylcellulose carbon nanotube electrochemical impedance spectroscopy lithium-ion battery mesocarbon microbead physical vapor deposition solid electrolyte interface transition metal oxide

porosity, were used as the anode material, the battery exhibited stable capacity retention and high reversible capacities. Xiang et al. [28] reported a simple self-assembled synthesis of hierarchical CuO particles with various morphologies and concluded that the special structural features of CuO particles dramatically improved the discharge capacities and cycling performances of the battery. Obviously, the aforementioned optimization strategy mainly enhanced the electrochemical performance of LIBs by synthesizing CuO with various unique nanostructures and porous morphologies. Another popular method to optimize the anode structure is to fabricate a hybrid nanocomposite with conductive matrixes, e.g. carbon, carbon nanotubes and graphene nanosheets, and so on, to overcome the drawbacks of the inferior intrinsic electrical conductivity of CuO [29–34]. For example, Liu et al. [30] prepared CuO/C microspheres and applied them to the anode of LIBs. They found that this method yielded a better rate capability than pure CuO because of a series of positive effects from the carbon microspheres. Likewise, Ko et al. [34] introduced mesoporous CuO

particles that were threaded with CNTs for LIBs and significantly improved the reversible capacity and rate capability of the batteries. In addition, it is noteworthy that the current collector is considered to be a key component in LIBs, which helps carry electrode materials and collect the current. Therefore, the diverse surface structures of a current collector also play an important role in enhancing the electrochemical performances of LIBs. However, most commercially used current collectors for LIBs are made of electrolytic copper foils at the anode or aluminum foils at the cathode. According to their different morphological properties, copper- and aluminum-based foils either have smooth or rough surfaces on both sides, which significantly limits their application in future LIBs. In this regard, Poetz et al. [35] prepared a new threedimensional cathode current collector for LIBs by physical vapor deposition (PVD) to increase the contact surface between the cathode material and current collector, which significantly improved the electrical conductivity of the electrode. Zhong et al. [36] and Choi et al. [37] presented a new method to prepare the cathode current collectors with excellent interface features by using carbon nanotubes (CNTs). They claimed that the structure of CNTs and the porous surface of CNT films improved the electrode conductivity. For the anode current collector, copper foams with a high porosity were fabricated by Yue et al. [38] and Li et al. [39], who successfully validated that the special structure of the copper foams effectively restrained the volume change of the anode materials. Moreover, for the higher performance of LIBs, Fan and coworkers [40,41] prepared porous copper plates as the anode current collectors by electrodepositing. Following this method, Jiang et al. [42] developed a valley-ridge copper architecture and systematically evaluated the structural advantages of the special copper framework in restricting severe volume changes of the anode materials.

Fig. 1. (a) Procedure to prepare the CuO/Cu composite current collector with array-pattern porous structures; digital microscope images of (b) the porous current collector and (c) complanate current collector.

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The historical background indicates that the electrode materials and surface structure of current collectors closely relate to the electrochemical performances of LIBs. Therefore, modifying the morphologies of electrode materials and structures of current collectors is an effective method to enhance the electrochemical performances of LIBs. However, most studies have only reported the unilateral effect of electrode materials or the structures of current collectors on the performance of LIBs without considering the combined effects of these two aspects. Therefore, in this study, we successfully prepared a high-performance CuO/Cu composite current collector with an array-pattern porous structure using a special chemical etching method. With this method, we can easily form blind holes with special patterns on the current collectors and synchronously coat their surfaces with a layer of oatmeal-like CuO, which is significantly different from the traditional methods based on chemical depositing or etching. The prepared samples inherited the properties of porous current collectors and benefited from the morphological advantages of CuO with nanostructures. Because of the new porous structure of current collectors and the synergistic lithium storage effect between the electrode materials, MCMB/ CuO/Cu batteries with this porous current collector exhibited a remarkable performance improvement with higher electrical conductivity, reversible capacity and cycling stability. Because this new structure has a better electrochemical performance and is easy to prepare, it enables the current collector to be used for nextgeneration LIBs. 2. Experimental 2.1. Sample preparation The procedure to prepare the CuO/Cu composite current collector with array-pattern porous structures is illustrated in Fig. 1(a). First, two surfaces of thin copper plates (diam  ply = 15 mm  0.2 mm) were polished with a piece of fine sandpaper to remove the contaminants and covered with corrosion-resistant photosensitive films. Second, the array patterns of blind holes (square: 1 mm  1 mm) were exposed to one surface (surface A) of the copper plates with an ultraviolet exposure machine, and the counter surface (surface B) was entirely exposed. The copper plates were developed in the developer solution for 10 min. Then, the copper plates were placed in a (NH4)2S2O8 solution at a concentration of 0.2 g mL
1 and were etched for 15 min under ultrasonic conditions. After the etching process completed, the copper plates were quickly transferred from the (NH4)2S2O8 solution directly to the autostrip solution (autostrip powder: deionized water = 1:50 wt) and were maintained for 20 min with their surface A upward under the ultrasonic condition, which simultaneously stripped the corrosion-resistant photosensitive films from the copper plates and formed a layer of CuO in the blind holes. Finally, the surfaces of the copper plates were cleaned with deionized water and dried in air at 60  C overnight to cover a layer of CuO onto the entire surface of the current collectors. For comparison, CuO/Cu composite current collectors with complanate structures were also prepared using the identical method without being exposed and developed. The digital microscope images of the porous current collector and complanate current collector are shown in Fig. 1(b) and Fig. 1(c), respectively. The electrode material slurry was prepared using four types of materials: CMC, SBR, Super P carbon and mesocarbon microbead (MCMB) graphite powder with a weight ratio of 2:2:3:93. To prepare MCMB/CuO/Cu composite electrodes, the electrode material slurry was evenly coated on the surface of both current collectors; then, the samples were dried at 60  C for 6 h. Finally, the electrodes were cold-pressed at 25 Mpa for 5 min and dried at 60  C

91

Fig. 2. Images of (a) the MCMB/CuO/Cu composite electrode, (b) CR2032 coin halfcell and (c) structure of the CR2032 coin half-cell.

for another 6 h. The image of the MCMB/CuO/Cu composite electrode is shown in Fig. 2(a). 2.2. Electrochemical testing and characterization The electrical conductivity of the composite electrodes was tested using voltammetry, and the electrochemical performances of both current collectors were tested using CR2032 coin half-cells (see Fig. 2(b)). The coin half-cells were assembled in an Ar-filled glove box with lithium foil as the reference electrode and Celgard 2325 as the separator. The electrolyte consists of LiPF6 (Organic solvent: EC + DMC + DEC, 1:1:1 by volume). The cyclic dischargecharge test for the batteries was conducted on the LANHE CT2001A battery test system, and the electrochemical impedance spectroscopy (EIS) test was performed on the CHI650D electrochemistry work station. The surface topographies of the porous and complanate current collectors were obtained with scanning electron microscopy (SEM). The images were collected using a field emission SEM (Merlin, Hitachi, Japan).

Fig. 3. Curves of the relationship between the electrical conductivity and external pressure of the composite electrodes.

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3. Results and discussion To improve the tightness of the CR2032 coin half-cells, we usually apply a large pressure to their shells while assembling composite electrodes in the batteries. Therefore, it is necessary to disclose the relationship between the electrical conductivity of the electrodes and the external pressure [37]. Fig. 3 shows that the resistance of the electrodes with porous current collectors and complanate current collectors decreases to a relatively stable level with the increase in external pressure. In addition, they have approximately equal resistance values with no external pressure, which enables us to make a convincing comparison of their electrical conductivity with identical criteria. However, the curves gradually separate from each other with the increase in external pressure. More specifically, the downward trend of the curve for composite electrodes with porous current collectors is increasingly faster than that of the complanate samples. The electrode material and current collector become increasingly closer with the increase in external pressure, and this trend contributes to reducing their interfacial contact resistance [37,43,44]. Furthermore, for the porous current collector, the array-pattern blind holes and nanostructure of CuO on its surface significantly increase the quantity of effective contact points between the electrode material and current collector. As a result, electrodes with this porous current collector have better electrical conductivity than those with a complanate collector. This result highlights the superiority of applying this porous current collector in LIBs. Fig. 4(a) and (b) show the discharge-charge curves for the MCMB/CuO/Cu batteries with these two types of current collectors

under different current conditions. It is worth noting that the maximum discharge capacity of the battery with a porous current collector was 383.9 mAh g
1 at a constant current of 0.5 mA. Obviously, this capacity is much larger than that of the complanate setup (309.6 mAh g
1) and even the theoretical capacity of pure graphite (372 mAh g
1) because of the effect of the CuO layers (674 mAh g
1) [45]. In contrast, the specific capacity of the battery with a complanate current collector is much lower than the theoretical capacity of pure graphite, even under the effect of the CuO layer. To explain this phenomenon, the microscale structures of the porous and complanate current collectors were characterized with scanning electron microscopy (SEM) (see Fig. 5(a)–(c)). Fig. 5(a) displays an oatmeal-like structure of the CuO layer in the blind holes, which is believed to significantly improve the reversible capacity of the batteries [45,46]. Fig. 5(c) indicates that the microstructure and CuO content on the current collectors significantly affect the discharge-charge capacity of the batteries. Specifically, for the complanate current collector, the oatmeal-like CuO tends to detach from their surface because of their poor CuOstorage capability. Hence, the battery with a complanate current collector exhibits a low reversible capacity because of its simple structure on the surface and low CuO content on its current collectors. In several previous cycles, the capacities of both batteries gradually decrease with the increase in cycle numbers because a thick solid electrolyte interface (SEI) layer forms on the electrode surface. Consequently, the SEI layer delays the electron transport, extends the diffusion length for lithium ions, and eventually decreases the capacities of the batteries [46]. Subsequently, because of the lithiation-induced reactivation and lithium intercalation, the SEI layer may fracture and a new thin SEI layer may form. With time, the new SEI layer becomes more stable to resist the drastic volume variation and fracture [47], for which the capacities of the batteries gradually increase to a relatively stable level in subsequent cycles. In addition, Fig. 4(a) and (b) also show the discharge-charge capacities of both batteries at various current rates, where the discharge-charge capacity decreases with the increase in current rate. Furthermore, after the rate cycles at 0.2C and 0.5C, the stable capacity of the batteries with a porous current collector is 374 mAh g
1, which is 97.42% of the original capacity before the rate cycles at a constant current of 0.5 mA. By contrast, a battery with a complanate current collector has a capacity of only 296.7 mAh g
1, which is 95.83% of the original capacity before the rate cycles. These results suggest that the array-pattern blind holes and oatmeal-like CuO can significantly enhance the rate capability and capacity retention of the batteries. Fig. 6(a) displays the initial discharge-charge voltage profiles of the batteries with a porous current collector and a complanate current collector at a constant current of 0.5 mA. Interestingly, the voltage profiles of the initial discharge reaction for both batteries show a voltage plateau at approximately 0.1 V, which corresponds to the reduction of MCMB to LiC6. However, in the subsequent cycles, the voltage profiles of both batteries show a short voltage plateau at 0.18 V, a long voltage plateau at approximately 0.1 V, and a downward-sloping profile to the cutoff voltage of 0.002 V as shown in Fig. 6(b) and (c). These plateaus can be indexed to the reduction of MCMB to LiC6 and CuO to Cu, as well as the formation of amorphous Li2O and solid electrolyte interphase. The plateaus for the reduction of CuO to Cu mainly correspond to the following multi-step electrochemical reaction process [48,49]. CuO

Fig. 4. Discharge-charge curves for the MCMB/CuO/Cu batteries with (a) porous current collectors and (b) complanate current collectors under different current conditions.

þ

þ xLi þxe
! ½CuII 1
x CuI x Oð1
xÞ=2 þx=2Li2 O ð0  x  0:4Þ ð1Þ

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93

Fig. 5. SEM images of the (a) array-pattern blind holes, (b) surface outside the blind holes for porous current collectors and (c) surface of the complanate current collectors.

½CuII 1
x CuI x Oð1
xÞ=2 ! Cu2 Oð0:4  x  1Þ

þ

Cu2 O þ 2Li þ 2e
! 2Cu þ Li2 O

ð2Þ

ð3Þ

Moreover, the initial discharge-charge capacities are 294.4 mAh g
1 and 272.1 mAh g
1 for the battery with a porous current collector, but only 194.7 mAh g
1 and 174.8 mAh g
1 for the battery with a complanate current collector. Hence, the formation of an SEI film on the electrode surface causes an irreversible capacity loss of 22.3 mAh g
1 and 19.9 mAh g
1 during the first cycle and simultaneously reduces the coulombic efficiency of the battery, as reported by Lamberti et al. [22]. Fig. 6(b) and (c) illustrate the discharge-charge voltage profiles of the batteries during the 5th, 15th and 20th cycle at a current of 0.5 mA. The curves of both batteries move rightward and become narrow during the following discharge-charge process, which indicates that the electrode reactions become more reversible. In addition, except for the first cycle, the coulombic efficiency of the battery increases to almost 100% in subsequent cycles, which manifests the high dischargecharge reversibility of the batteries. More importantly, batteries with porous current collectors exhibit their reversible dischargecharge capacities of 274.8 mAh g
1 and 273.9 mAh g
1 in the 5th cycle, 383.8 mAh g
1 and 383.6 mAh g
1 in the 15th cycle, and

383.9 mAh g
1 and 383.7 mAh g
1 in the 20th cycle. This result clearly identifies a small capacity loss in subsequent cycles. By contrast, the batteries with complanate current collectors retain only 107.1 mAh g
1 and 106.6 mAh g
1 in the 5th cycle, 308.6 mAh g
1 and 308.1 mAh g
1 in the 15th cycle, and 309.7 mAh g
1 and 309.6 mAh g
1 in the 20th cycle. Thus, we confirmed that the oatmeal-like CuO preserve their morphologies and integrity during the insertion-deinsertion processes of lithium and ensure the efficient particle-to-particle contact. As a result, the electrochemical performances of the batteries can be significantly improved. Fig. 6(d) and (e) compare the cycling performances of the batteries with porous and complanate current collectors at a constant current rate of 0.2C and 0.5C. The battery with a porous current collector yields a rate-discharge capacity of 329.8 mAh g
1 after the 5th 0.2 C-rate cycle and 101.8 mAh g
1 after the 5th 0.5 Crate cycle. These values correspond to 94.9% and 77.8% of the capacities in the 1 st 0.2 C-rate cycle (347.7 mAh g
1) and 0.5 C-rate cycle (130.9 mAh g
1), respectively (see Fig. 6(d)). However, the battery with a complanate current collector retains only 218.7 mAh g
1 after the 5th 0.2 C-rate cycle and 38 mAh g
1 after the 5th 0.5 Crate cycle, which is 70.6% and 55.6% of the capacities in the 1 st 0.2 C-rate cycle (309.7 mAh g
1) and 0.5 C-rate cycle (68.4 mAh g
1), respectively (see Fig. 6(e)). These results demonstrate that the battery with a porous current collector has better rate performance than the complanate pattern. This improvement may be closely

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Fig. 6. (a) Initial discharge-charge voltage profiles of the batteries with porous current collectors and complanate current collectors at a constant current of 0.5 mA; fifth, 15th and 20th discharge-charge voltage profiles of batteries with (b) porous current collectors and (c) complanate current collectors; the rate-cycle voltage profiles of the batteries with (d) porous current collectors and (e) complanate current collectors.

related to the morphological structure and electrochemical behaviors of the oatmeal-like CuO. To clarify the essential differences of the electrochemical behaviors of CuO between the two current collectors, an electrochemical impedance spectroscopy (EIS) test was performed on the batteries. Fig. 7(a) and (b) depict the Nyquist plots of the batteries and show that the AC impedance of both batteries decreases with the increase in cycle number during the first 20 cycles at a constant current of 0.5 mA because of the improved contacts among the particles [50]. Moreover, the Nyquist curves for

the batteries with porous current collectors evidently dramatically change under different current conditions, whereas the curves of the batteries with complanate current collectors are relatively stable. This phenomenon shows that the electrochemical behavior of the oatmeal-like CuO is more sensitive to the discharge-charge current condition, which significantly depends on the morphologies of the oatmeal-like CuO. For a more in-depth investigation, Fig. 7(c) provides the representative Nyquist plots of the batteries with a porous current collector and a complanate current collector after 0.2C and 0.5C

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95

Fig. 7. Nyquist plots of the batteries with (a) porous current collectors and (b) complanate current collectors under different current conditions; (c) Nyquist plots of the batteries with porous current collectors and complanate current collectors after 0.2 C and 0.5 C rate cycles; (d) calculated situation of the Nyquist curves; (e) equivalent circuit of the batteries with porous current collectors and complanate current collectors.

Table 1 Resistances of the batteries with porous current collectors and complanate current collectors after 0.2 C and 0.5 C rate cycles. Current collectors

Rs(V)

RSEI1(V)

Rct1(V)

RSEI2(V)

Rct2(V)

Rb(V)

Zw1(V)

Zw2(V)

RP(V)

Porous current collectors Complanate current collectors

8.697 8.682

2.642 2.739

5.183 5.171

2.504 1.802

20.493 25.392

2.461 3.791

2.386 2.289

0.082 0.021

44.448 49.887

“1”: MCMB electrode material; “2”: CuO electrode material.

rate cycles. We observe that the battery with a porous current collector has a much smaller AC impedance than that with a complanate current collector. This result may also be attributed to the morphologies of CuO on current collectors. To support this hypothesis, the curves in Fig. 7(c) were calculated using an equivalent circuit [51] (see Fig. 7(e)). Here, Rs is the ohmic resistance of the electrolyte, RSEI is the resistance of the SEI film, Rct is the charge-transfer resistance between the electrolyte and electrode materials, and Zw is the solid-state diffusion resistance in electrode materials. The resistance Rb characterizes the structure

change of the electrode material particles. The calculated situation of the Nyquist curves is shown in Fig. 7(d), which validates the correctness of the equivalent circuit. The calculated results of the curves are summarized in Table 1. As shown in Table 1, batteries using porous current collectors have an approximately equal Rs, RSEI1, Rct1 and Zw1 with those using complanate patterns, which indicates that both batteries have identical ohmic resistance values. In addition, the electrochemical behavior of MCMB can be considered to be an identical factor. Nevertheless, we can find a great difference in RSEI2, Rct2, Rb and

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Fig. 8. Schematic diagrams of the electrochemical behaviors for CuO (a) outside the blind holes and (b) in the blind holes.

Zw2 between the two batteries. To clarify this phenomenon, the electrochemical behaviors of CuO outside and inside the blind holes are shown in Fig. 8(a) and Fig. 8(b), respectively [51]. For porous current collectors, CuO on their surface consists of the oatmeal-like CuO in the blind holes and the CuO that formed outside the holes. The oatmeal-like CuO is more porous than the CuO outside the blind holes, which makes it easier to form an SEI film on the surface of the oatmeal-like CuO. However, it is difficult for the oatmeal-like CuO to shed from the blind holes. This trend implies that the porous current collector has a higher total CuO content than the complanate one. Considering the effects of these factors, the SEI resistance of CuO (RSEI2) on a porous current collector is larger than that on a complanate current collector. Obviously, the blind holes and nanostructure of the oatmeal-like CuO significantly increase the surface area of the porous current collectors. As a result, the quantity of effective contact points between Li+ and electrons should be significantly increased, which improves the electrical conductivity of the electrodes. Furthermore, it is easier for the conductive additive to diffuse through the microchannels of the oatmeal-like CuO (see Fig. 8(b)). Therefore, the presence of CuO on the surface of a porous current collector results in a smaller charge-transfer resistance (Rct2) than the use of a complanate current collector. In addition, the battery with a porous current collector has a larger solid-state diffusion resistance (Zw2) than that based on a complanate current collector because of the large crystal size of the oatmeal-like CuO, which extends the diffusion length for lithium ions. Furthermore, the smaller Rb of the battery with a porous current collector indicates that the oatmeal-like CuO particles can better resist their phase change and severe pulverization [52,53]. Considering all aspects, the total resistance (RP) of the battery with a porous current collector significantly decreases, which indicates that the electrode

with a porous current collector has better electrical conductivity and electrochemical performance. 4. Conclusions In summary, this study presents a high-performance CuO/Cu composite current collector with array-pattern porous structures for LIBs. This composite current collector inherits the structural superiorities of the porous current collector and benefits from the morphological advantages of CuO with nanostructures. The battery based on this composite current collector exhibits enhanced electrochemical performances, such as high electrical conductivity, excellent cycling stability, high reversible capacity, and so on. The array-pattern blind holes and nanostructure of oatmeal-like CuO help increase the contact area between the electrode material and current collector, which increases the electrical conductivity. Because of the high theoretical capacity and morphological advantages of the oatmeal-like CuO, the battery with this new structure displays a better reversible capacity and cycling stability. The new CuO/Cu composite current collector with array-pattern porous structures has an outstanding electrochemical performance and preparation convenience, so it is a potential candidate for next-generation LIBs. Acknowledgements This work is supported by the following funding: (1) National Natural Science Foundation of China (No. 51475172); (2) Guangdong Science Fund for Distinguished Young Scholars (No. 2015A030306013); (3) Guangzhou Zhujiang Science and Technology Star Program (No. 201506010026); and (4) Fundamental Research Funds for the Central Universities (No. 2015ZJ001 and 2015PT029).

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