Copper nanowire/multi-walled CNT composites as all ... · Nano Res 1. Copper nanowire/multi-walled CNT composites as all-nanowire flexible electrode for fast charging/discharging

Nano Res 1

Copper nanowire/multi-walled CNT composites as all-nanowire flexible electrode for fast charging/discharging lithium-ion battery

Zhenxing Yin1, Sanghun Cho1, Duck-Jae You1, Yong-keon Ahn1, Jeeyoung Yoo1 (), and Youn Sang Kim1,2 () Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-017-1686-0 http://www.thenanoresearch.com on May. 19, 2017 © Tsinghua University Press 2017

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Nano Research DOI 10.1007/s12274-017-1686-0

64 Nano Res.

Copper nanowire/multi-walled CNT composites as

all-nanowire flexible electrode for fast charging/discharging

lithium-ion battery TABLE OF CONTENTS (TOC)

Copper nanowire/multi-walled CNT composites as

all-nanowire flexible electrode for fast

charging/discharging lithium-ion battery

Zhenxing Yin, Sanghun Cho, Duck-Jae You, Yong-keon Ahn, Jeeyoung Yoo *, and Youn Sang

Kim*

Seoul National University, Republic of Korea.

Advanced Institutes of Convergence Technology

(AICT), Republic of Korea.

Novel lightweight 3-D composite anode was fabricated by all 1-D nanomaterials of CuNWs and MWCNTs. Both half cell and full cell of LIBs exhibited high specific capacities and columbic efficiencies even at a high current density. More importantly, we firstly overcame the limitation of MWCNTs used as the anode materials for fast charging/discharging half-cell and full-cell LIBs with CuNWs adoption, further applied to flexible LIB.

Provide the authors’ webside if possible.

Youn Sang Kim, www.snunml.com

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

1 Nano Res.


Zhenxing Yin1, Sanghun Cho1, Duck-Jae You1, Yong-keon Ahn1, Jeeyoung Yoo1(), and Youn Sang Kim1,2() 1 Program in Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Seoul 08826, Republic of Korea. 2 Advanced Institutes of Convergence Technology, 145 Gwanggyo-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 16229, Republic of Korea.

Received: day month year Revised: day month year Accepted: day month year (automatically inserted by the publisher) © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2014 KEYWORDS 3-D composite, all-nanowire electrode, fast chargeable battery, full-cell LIBs, flexible battery

ABSTRACT Novel lightweight 3-D composite anode for fast charging/discharging lithium ion battery (LIB) was fabricated by all 1-D nanomaterials of copper nanowires (CuNWs) and multi-walled carbon nanotubes (MWCNTs). The 3-D composite with these two types of 1-D nanomaterials presents significant advantages to the transport pathway for both electrons and ions due to their excellent electrical conductivity, high aspect ratio structures and large surface areas. As an advanced binder-free anode, the CuNW-MWCNT composites (CNMC) film shows a considerably low sheet resistance and internal cell resistance with a controllable thickness (~600 µm). Furthermore, the randomly 3-D CuNWs network acting as a rigid framework not only prevents the MWCNT shrinkage and expansion by aggregation and swelling, but also minimize the effect of volume change during charge/discharge process. Both half cell and full cell of LIBs with CNMC anode exhibited high specific capacities and columbic efficiencies even operated at a high current. More importantly, we firstly overcame the limitation of MWCNTs used as the anode materials for fast charging/discharging LIBs (both half cell and full cell) with CuNWs adoption, further applied to flexible LIBs. This innovative anode structure led to development of ultrafast chargeable LIBs for electric vehicles.

1. Introduction

When internal combustion powered vehicles were first invented, no one predicted that they would

become part of regular transportation. However, it took only 13 years before carriages were replaced by

vehicles. Thereafter, the internal combustion engine (ICE) has been the dominant system for vehicles for almost 100 years. Thus, sometimes technology can alter the human lifestyle completely. A good example

is with autonomous vehicles (AVs). Many futurologists anticipate AVs to become generalized

Nano Research DOI (automatically inserted by the publisher)

Address correspondence to Youn Sang Kim, [email protected]; Jeeyoung Yoo, [email protected]

Research Article

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within 10 years [1, 2]. Due to the rapid development of AVs, the electric vehicle (EV) market has dramatically increased since AVs chose EVs as the most preferred system. Compared to conventional ICE vehicles, EVs not only provide many advantages like zero air pollutants, less noise, and vibrations, but are also operated by very simple electric motors with 80~90% energy conversion efficiency. In addition, EVs have huge energy resilience, because they can be charged from various energy sources like renewable energy and regenerative braking energy, which include typical power plant energy. However, EVs still have some limitations in mileage, cost, safety, restricted life time, lack of power grid for charging, and long charging time [3-5]. These issues mainly lead to the development of power systems with Li ion batteries (LIBs). To overcome mileage and cost problems, the energy density of LIBs must be increased. The best way to enhance energy density is to develop new active materials for both cathodes and anodes with high theoretical capacity [6-10].

Unfortunately, even if the high capacity materials are successfully applied to LIBs, it takes too much time to charge LIBs due to their low power densities. Although fast chargers with pulse power have been developed, these chargers cannot avoid the energy density fading at high current charge/discharge process due to the limitation in the energy conversion reaction of LIBs. Recently, Tesla constructed supercharger stations that can charge LIBs of EVs in 40 min to the 80% level [11]. The charging time of superchargers is shorter than that of public charging station by approximately 17 times. However, 40 min is too long when compared to gas charging times. To reduce the charging time, the total resistances of LIBs have to be decreased. Cell resistance is affected by the intrinsic resistance active materials and electrolytes. Furthermore, contact resistance between the current collector and active materials, particle interconnection resistance within active materials, and interfacial resistance between electrode and electrolyte are critical. Among them, the intrinsic resistance of materials is hard to be decreased. In addition, the rest of resistance factors can be tuned by cell design and electrode slurry composition, such as additional binders and conductive agents. However, the fast chargeable LIBs with high energy density still have a lot of obstacles

for conventional cell design due to the limitations in contact resistance decrease.

Herein, we propose a new all-nanowire electrode structure for fast charging/discharging LIBs (both half cell and full cell) using copper nanowires (CuNWs) and multi-walled carbon nanotubes (MWCNTs) without any binders and conductive agents. The MWCNTs are the representative carbon-based material of one-dimensional (1-D) nanostructure that efficiently transport pathways for both electrons and ions due to their unique structures like high aspect ratio and large surface area [12]. Although many researchers reported MWCNT as anode active materials for half-cell LIBs, the MWCNTs has not been applied for full-cell LIBs due to their highly irreversible lithium ion capacity and the lack of a stable voltage range (voltage plateau) caused by spatial non-uniformity derived from morphology [13, 14]. For these reasons, the MWCNTs were extensively explored as current collectors for composite materials in LIBs in recent years [12-15]. Interestingly, the MWCNTs with superior properties are still expected to be used for LIB anodes, because the super-lightweight flexible LIBs that have high energy density can be attained without conductive agents and current collecting metal foils. Therefore, we propose a new approach from cell design and use the MWCNTs as active materials to pursue the original objective, rather than the conventional role of simple current collectors.

2. Experimental

2.1 Synthesis of CuNWs. The CuNWs were synthetized by salt-assisted

polyol method. The 2 mmol of CuCl (95%, Junsei) as the precursor, 0.3 mmol of NH4Cl (99.5%, Daejung) as the capping agent and 6 mmol of oleylamine (70%, Sigma Aldrich) as the capping agent were putted into 30 ml of ethylene glycol (99%, Daejung) with the vigorous stirring in 100 mL of round-bottom flask. The temperature was heated up to 110 °C to form the Cu+-amine complex, which is the intermediate product. After another 20 min stirring for completely formation, the temperature was raised up to 198 °C with 9 °C min-1 of heating rate. When the temperature reached about 180 °C, the CuNWs were synthesized and changed to red-brown colour. For


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growth in length, the synthesized CuNWs then refluxed and kept at 198 °C for 20 min. After fully reaction, the products were quenched by cold water to room temperature. Finally, the CuNWs were washed by n-hexane (95%, Daejung) and centrifuged for 5 min in 8000 rpm. Repeating the 3 times of washing step, the CuNWs dispersed in Isopropyl alcohol (IPA) (99.5%, Daejung) to make the composites anode film. 2.2 Fabrication of CNMC anodes.

The MWCNT (95%, CM-95) was purchased from Hanwha chemical co., ltd. with 20 µm length and 10 nm width. The MWCNTs were also dispersed in IPA and mixed to CuNWs/IPA dispersion with various composite ratios (CuNW:MWCNT = 1:1, 3:1, 5:1 and 7:1, w/w). All of the MWCNT and CNMC samples were fabricated with the same amount of MWCNT. After 30 s sonicating, the composite dispersion was filtered on cellulose acetate (CA) membrane (0.2 µm pore, Advantec). Then, CuNW-MWCNT composites (CNMC) films were easily peeled from CA membrane and sintered in 180 °C for 30 min at glycerol reducing condition. Finally, the highly conductive CNMC electrodes were fabricated to use as the battery anodes. 2.3 Materials characterizations.

The CuNW, MWCNT and CNMC electrodes were characterized by field-emission scanning electron microscope (Hitachi, S-4800), high-resolution transmission electron microscope (JEOL, JEM-2100F) and x-ray diffraction measurement (Bruker, D8 DISCOVER). A conductivity meter with 4-point probe (AIT Co., Ltd CMT-SR2000N) was used to measure the sheet resistance of the electrode films. 2.4 Electrochemical measurements.

CR-2032 coin cells were assembled to investigate the electrochemical properties. The half cell was composed by CNMC electrode, separator (polypropylene, celgard 2300) with liquid electrolyte (1 M LiPF6 in EC/DMC/DEC, 1:1:1 of volume ratio, Panax E-Tec), and 150 µm thick Li foil. The full cell assembly was characterized under an Ar atmosphere with the LiFePO4 (99.9%, MTI Korea) and MWCNT (95%, CM-95, Hanwha chemical co., ltd.) composites cathode in a weight ratio of 1:2, which was also prepared by the above-mentioned filtration method. The full cell was assembled with the N/P ratio of 1:1.5. In case of bare MWCNT, the theoretical full-cell

capacity is 66.41 mAh g-1. In addition, the half cells were characterized from 0.02 V to 2.4 V and full cells were analysed at the potential range of 0.02 V and 2.5 V.

The cell resistance was evaluated via an AC impedance test conducted by electrochemical cells. The AC impedance spectroscopy was used (CHI660E, CH Instrument) from 0.1 HZ to 1 MHz at room temperature with 5 mV amplitude. The manufactured cells were characterized by capacity, cyclability, and rate capability with the charging-discharging characteristics in a battery cycler system (WBCS 3000L, WonATech).

3. Results and Discussion

To stabilize the irregular shaped MWCNTs and improve the anode resistance, we adopted CuNWs as a new type of current collector (Figure S-1 in the ESM). As a highly conductive 1-D metal nanostructure, CuNWs exhibited tremendous potential in developing of battery performances with strong contact to active materials, which is the same ingredients that of conventional current collector of LIBs anode. Furthermore, due to the excellent electrical conductivity, high aspect ratio, and large surface area of CuNWs, the three-dimensional (3-D) percolated network efficiently provides better conducting pathways and ion diffusion for fast charging/discharging LIBs [16, 17]. For traditional anode, the various compositions of active materials, binder and conductive agents was homogenously mixed to make the slurry, which was coated on copper foil. In addition to active materials containing significant cell capacity, other materials for slurry composition are required during this process. However, these extra materials and troublesome process further improve the overall battery price. In this work, the advanced CuNW-MWCNT composites (CNMC) film as a free-standing anode of LIBs was fabricated by a simple filtration method. The composite anodes were named CNMC 1, 3, 5 and 7 with different composition mass ratios (CuNW:MWCNT = 1:1, 3:1, 5:1 and 7:1, w/w). Figure 1a and 1b show the two types of 1-D structural nanomaterials; the CuNWs (80 nm in width and 30 µm in length) and the MWCNTs (10 nm in width and 20 µm in length) with high aspect ratio. Although the


4 Nano Res.

thinner CuNWs may lead to achieve a better result, taking into account the oxidation problem, structural stability and electron mean free path in NWs, we choose the 80 nm of stable Cu NWs in this work, which modified from reported method [16]. Generally, large surface areas and peripheral Cu atoms by nano-scale radius induce relatively active surface energy. To be a steady state, the CuNWs reduce their surface energy through surface passivation or mutual aggregation [16-18]. Therefore, the MWCNTs are strongly adsorbed on the surface of CuNWs by hydrophobic interaction and electrostatic attraction when the CNMC electrode was prepared (Figure 1c and Figure S-2 in the ESM) [19, 20]. Furthermore, randomly 3-D structural CuNWs network as a rigid framework to effectively prevent the shrinkage and expansion of active materials by aggregation and swelling. The X-ray diffraction (XRD) patterns of the CNMCs with different mixing ratios were shown in Figure 1d. In the presence of bare MWCNT, the broad C peak is only located near 26° due to diffraction of {002} plane. By increasing the CuNWs contents, the three representative Cu peaks are more clearly observed at 2θ = 43.4, 50.4, and 74.2°, which respectively corresponded to the diffractions of {111}, {200}, and {220} planes of Cu (JCPDS # 03-1018).

We prepared a 3-D structural anode with two types of 1-D nanomaterials and cell structures as described in Figure 2a and Figure S-3 in the ESM. This cell structure does not require current collecting metal foil, conductive agents, and binders. Additionally, the CNMC electrode thickness was freely controlled until 647 µm by a simple filtration method (Figure S-4 in the ESM). Therefore, the cell enables to become lighter and smaller by adopting suggested electrode system. Furthermore, the 3-D structural anode has a great advantage in terms of resistance. To demonstrate it, we observed the effect of resistance difference by various measurements when comparing CNMCs with MWCNT anodes. All of the MWCNT and CNMC samples were fabricated with a same amount of MWCNT, and the mass of bare CuNWs films is equal to CNMC7 for relative comparison. The loading mass of active materials (MWCNT) is fixed as 1.15 mg when applied to the 10 mm of coin-cell anodes. Furthermore, due to the difference in CuNW contents, the thickness of

MWCNT and CNMC1, 3, 5, 7 anodes were 61, 56, 69, 98 and 126 µm, respectively. Generally, the oxide layers on the CuNWs’ surface as the contact barrier avoid the electron transport and enhanced the resistance of the composite anodes [21]. In this study, the natural oxide layers were reduced and network was percolated in the glycerol condition at 200 ˚C, which provides a surface energy more than that can cause the Cu NWs welding. In Figure 2b, the sheet resistance of the CNMC films decreased dramatically, when the CuNWs were added and annealed. Although both CuNW and MWCNT are conductive 1-D nanomaterial, have a large contact resistance when the film is formed. Especially for a large area, these contact resistance are reflected more clearly [22]. However, the contact resistance of the CuNWs can be reduced by the above-mentioned welding process, which is not applicable to MWCNT. As a highly conductive framework for current pathways, the randomly 3-D structural CuNWs network was directly connected with MWCNTs, which effectively transport the electrons on the charge/discharge process. Therefore, CuNWs contents can easily affect the overall resistance of the composite films, that the resistance of bare MWCNT network is much higher than that of CuNWs network. The cell resistances also matched with sheet resistance of MWCNT and CNMCs, where employed polypropylene (PP) separator, LiPF6 of electrolyte and Li metal as the reference and counter electrode. Figure 2c shows the charge transport conductivities of CNMCs at various types of CNMC according to the half-cell test. The charge transport resistance was increased with the enhancement of CuNW ratio, which ranges from 167.85 Ω cm at the bare MWCNT to 103.94 Ω cm at high CuNWs contents. These tendencies are also confirmed in the full cell with the LiFePO4/MWCNT cathode as presented in Figure S-5 and S-6 in the ESM. To achieve full-cell and flexible batteries, the LiFePO4-MWCNT composites were used as the cathode without any metal foils. Due to the high resistivity of LiFePO4, the MWCNT as the current collector was added at 1:2 of weight ratio to improve the conductivity of cathode for better electrochemical performances (Figure S-7 in the ESM). In addition, the highly conductive CuNWs affect the internal resistance of the half cells described in Figure 2d, which shows that the Galvanostatic Intermittent


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Titration Technique (GITT) analysis examined at 1C rate [23, 24]. As a result, the CNMC7 exhibited the most excellent conductive characteristics according with the aforementioned tendencies. In addition, the Li cation diffusion coefficients are inversely proportional with the voltage change during a constant current, so the CNMC7 has higher Li cation diffusion rate, which is caused by the low resistance of CuNWs. Generally, most of the LIBs are not fully charged and discharged for their safety and durability, which are used from 20~30% to 70~80% based on the state of charge [25]. Resistance change of CNMC7 is smaller than MWCNT, which means the CNMC7 are more stable anode material due to the welded CuNWs network. As described in Figure S-8 in the ESM, the CuNWs network is percolated at the contact point, resulting in a rigid 3-D framework that finely controls MWCNT volume change by lthiation/delithiation during operation and effectively inhibits the lithium dendrite growth [26, 27].

These phenomena are also confirmed by the tap density and normalized Li ion diffusion coefficient as described in Figure 3. The tap density of the composite films is obviously increased by the CuNWs contents as seen in Figure 3a, because the relative atomic mass of copper is far greater than that of carbon, and the solid nanowire structure is much denser than the hollow nanotube structure [28, 29]. In addition, the biggest problem with pure MWCNT used in battery anode is expansion of anode film when the electrolytes are encountered. Generally, the MWCNT film shrinks during drying process due to the formation of a stable structure after agglomeration. Therefore, the porous MWCNT film obviously swells after absorbing the solvent. It increases the tap density of MWCNT film and enhances the contact resistance between MWCNT junctions, because of their weak Van der Waals forces [30]. This unstable structure is liable to cause short-circuit problems in the assembly process and the charge/discharge process of cells. During the volume expansion induced by Li cation intercalation, the weak Van der Waals forces cannot withstand the expansion pressure, resulting in the MWCNT breaking away from the film. However, the stable structure of the CNMC film is effective to prevent the shrinkage and expansion of MWCNT, because of

the rigid framework of the welded Cu NWs network. As a result, the CNMC film was no change and the bare MWCNT film was expanded about 16%, when wetted on electrolyte (Figure 3b). As well known, the highly packed electrodes normally interfere in ion transportations, so high tap density means low Li ion diffusion. However, CNMC7 has a high Li ion diffusion even though they are denser than MWCNT, because of their outstanding conductivity by welded CuNWs network (Figure 3c and 3d). Therefore, the additional binder and conductive agents are unnecessary in this system, which is easy to be applied for lightweight LIBs with high energy density and power density.

In this work, the superior characteristics of CNMCs are proved by chronopotentiometry. Every tested sample was examined after aging at 0.2C for 1 cycle. Figure 4a and 4b shows the capacity rate performance by the galvanostatic charge-discharge method, and we assumed that the theoretical capacity of MWCNT is 400 mAh g-1 [31, 32]. In Figure 4a, the 0.2C, 0.5C, 1C, 5C, 10C and 0.2C rate was applied for observation of capacity retention. The capacity was decreased at initial 3 cycles due to the aging effect, afterwards, maintained a stable performance at each C-rate. Furthermore, the capacity value was increased according to CuNWs addition, and the CNMC7 showed the highest value. Compared with other literatures, the CNMC7 anodes in this work exhibited an excellent electrochemical performance with high current operating (Table S-1 in the ESM). Although the CuNWs addition increases the overall weight of electrode, the reduction of internal resistance is very obvious as shown in Figure 2, which can effectively achieve rapid charging and discharging. In addition, the high volumetric capacity is also the main reason for select the CNMC7, which is another key factor for practical application (Figure S-9 in the ESM). These tendencies also corresponded with aforementioned results of anode resistance, which is the main reason for high capacity. As described in Figure 4c and Figure S-10 in the ESM, voltage difference of anode between the charging curve and discharging curve appears to get smaller with enhancement of CuNW contents. This voltage gap is also strongly related to the conductivity of working electrode, and the narrow voltage gap means that of a larger capacity


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[25]. Additionally, irreversible capacities were decreased by increments of CuNW contents as shown in Figure S-11 in the ESM. As a result, there are 8.31% of irreversible capacities gap between the MWCNT and CNMC7, even if the initial capacity of CNMC7 is higher than that of MWCNT.

The final capacities at 0.2C rate are obviously increased when compared to the initial value, depending on CuNW contents (Figure 4a). As a result, the pure MWCNT only recovered 76.62% of the capacity, and the CNMC7 was improved to 104.89% of the capacity. Although the CuNWs network was welded in the reducing condition before cell assembly, the contact resistance still existed in the junction parts. In the charging/discharging process, these contact resistance in electrode were efficiently decreased by the joule-heated welding with a current flow [33]. Therefore, the specific capacity was increased by the welding effect of CuNWs, and the increment was strongly depending on CuNW contents. The capacity increment was also confirmed with cycle stabilities in Figure 4b. The 0.5C rate was loaded for the initial and final 100 cycles, while the 5C rate was applied for 300 cycles after the initial 100 cycles. The capacity retained during cycling, and recovered 89.75% when comparing the initial value, even repeated charging and discharging with harsh electrical conditions of 5C rate. In particular, the CNMC7 maintained a high capacity of 215 mAh g-1 at 5C rate, which means that the LIBs prepared by the CNMC7 anode charged 55.78% of the capacity within 12 minutes, repeatedly. The excellent capacity properties of CNMC anodes were also confirmed with full-cell LIBs as shown in Figure 4d, when the 0.2C, 0.5C, 1C, 5C, and 0.2C rate was applied to test. In cathode part, we use the commercial LiFePO4 (EQ-Lib-LFPO, MTI Korea) to fabricate the LIBs without any treatment, which have a relative low conductivity. Through control the MWCNT contents and n/p ratio, the electrochemical performances are accurately responded in half-cell and full-cell batteries (Figure S-7 in the ESM and Figure 4d), even much higher than that of theoretical full-cell capacity (66.41 mAh g-1), which was mainly attributed to the highly conductive CNMC7 anode. Furthermore, the MWCNT shows the lowest capacity because the capacity was donated from double layer capacitance and not intercalation, as

shown in Figure 5a. These behaviours were attributed to the intrinsic property of MWCNT, such as the large voltage gap between charging and discharging, because of the inadequate conductivity for anode materials without additional conductive agent (Figure 5b). For this reason, the MWCNTs have not been used as anode materials in full-cell LIBs. However, CNMC7 shows a reasonable capacity with a narrow voltage gap, which roots from enhanced anode conductivity by highly conductive CuNWs framework. Hence, we firstly overcame the limitation of MWCNT used as the anode material for fast charging/discharging half-cell and full-cell LIBs with CuNWs adoption.

Since the MWCNTs and CuNWs are flexible 1-D nanomaterials, flexible LIBs are available for development by this CNMC electrode system without any conventional metal foil current collectors and binders. In order to fabricate all binder-free flexible LIBs, we choose the LiFePO4/MWCNT composite as the cathode. In difference from anode, the MWCNT acts as a current collector in cathode part; and the LiFePO4 used as the active materials. Figure 6a-d are the photographs of a flexible CNMC7 film and a red light-emitting-diode (LED) powered by a flexible LIB, which composed by a LiFePO4/MWCNT composite cathode, PVdF-HFP based gel polymer electrolyte and CNMC7 anode. In addition, the Ni coated fibre tape was used as current collecting leads in both electrodes, and the polyurethane was used as the substrate and sealant. To demonstrate the relevance of the discharge capacity and flexibility, the flexible LIB was measured after aging for 1 cycle, and bent by 10 mm of bending radius under 25 mm s-1 of motion speed from 1 to 1000 cycles by a lab designed bending machine. As a result, the flexible LIB retained capacity over 92.8 % after 1000 cycles of bending (Figure 6e). Compared with other flexible batteries, it still exhibited high retention performance with a tiny degradation [34-37].

4. Conclusions

In conclusion, we have demonstrated a new type of 3-D structural composite anode to be applied for fast charging/discharging LIBs without any binders and conductive agents. The CuNWs as the current


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collector and MWCNTs as the active material present significant advantages to improve the LIB performances, such as high electron transport, efficient ion diffusion, thick electrode formation, and lightweight flexible cell design. Furthermore, the randomly 3-D CuNWs network acts as a rigid framework connected closely with MWCNTs, which not only effectively prevents the shrinkage and expansion of MWCNTs by aggregation and swelling, but also minimizes the effect of volume change of MWCNTs during charging/discharging process. Also, the CNMC7 anode effectively decreased the voltage range between charging and discharging curves by reducing the anode resistance, and is successfully used for full-cell LIBs. As a result, the CNMC7 anode for LIBs (both half cell and full cell) exhibited high specific capacity and improved capacity retention with a high columbic efficiency even during high C-rate operation. In addition, the capacity of a flexible LIB also remained 92.8 % even after 1000 cycles of bending. We believe that the proposed 3-D conductive all-nanowire structure is a promising anode for LIBs in many electronic devices and electric vehicles.

Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP, Ministry of Science, ICT & Future Planning) (No. 2015R1A2A1A15053165), (No. 2016R1C1B2013145) and (No. 2016M3A7B4910458). Electronic Supplementary Material: Supplementary material (TEM and SAED patterns of CuNWs; SEM images of CNMC anodes and LiFePO4/MWCNT cathodes; Schematic illustration of fabrication of the CNMC anodes and applications for both half cell and full cell of LIBs; Electrochemical measurements of MWCNT and CNMC anodes based LIBs.) is available in the online version of this article at http://dx.doi.org/10.1007/s12274-***-****-* (automatically inserted by the publisher). References

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Figure 1.


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Figure 1. Scanning electron micrograph of two types of proposed 1-D nanomaterials and their composites. (a) CuNWs, (b) MWCNTs, and (c) CNMC7. (d) X-ray diffraction patterns of MWCNT and CNMCs. Figure 2.

Figure 2. (a) Schematic illustration of the conventional anode and proposed CNMCs anode. (b) Sheet resistance of CNMCs, MWCNT and CuNW films. (c) Electrochemical impedance spectroscopy of the half cells using CNMC7 and MWCNT as the anode in frequency from 1 MHz to 0.1 Hz. (d) Internal resistance of half cells with the change of state of charge and depth of discharge, and inset graphs show the GITT analysis, when the CNMC7 and MWCNT as the anode.

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Figure 3.

Figure 3. (a) The tap density of anodes with different composite ratios (w/w). (b) The photograph of swelling test of the CNMC7 and MWCNT electrodes. The CNMC7 film had no change and the area of MWCNT film was expanded about 16%, when wetted on electrolyte (1M LiPF6 in EC/DMC/DEC=1:1:1, v/v/v). (c-d) The Li+ diffusion coefficient of half cells using CNMC7 and MWCNT anodes with the change of the state of charge and depth of discharge, which calculated from GITT measurement. Figure 4.

Figure 4. (a) The rate capability of various CNMC and MWCNT anodes. (b) The cycling performance and Coulombic efficiency of various CNMC and MWCNT anodes (after 31st cycle of rate capability test). (c) A plot of voltage (vs. Li/Li+) versus state of charge and depth of discharge for CNMC7 and MWCNT anodes in half cells. (d) Rate capability of CNMC and MWCNT anodes in the full cell with PP separator, LiPF6

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electrolyte and LiFePO4/MWCNT cathode. Figure 5.

Figure 5. (a) Charge-discharge curves of the full cells at 0.2C rate in 1st cycle, which made by CNMC7 and MWCNT anodes, respectively. (b) Cyclic voltammogram curve of MWCNT electrode at a scan rate of 0.1 mV s-1 from -1.0 to 2.5 V (vs. Li/Li+). Figure 6.

Figure 6. (a-b) The photographs of a flexible CNMC7 anode film. (c-d) A red light-emitting-diode (LED) powered by a flexible LIB. (e) The relative discharge capacity (C/C1) of the flexible LIB under a number of bending.

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Electronic Supplementary Material


Zhenxing Yin1, Sanghun Cho1, Duck-Jae You1, Yong-keon Ahn1, Jeeyoung Yoo1(), and Youn Sang Kim1,2() 1 Program in Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Seoul 08826, Republic of Korea. 2 Advanced Institutes of Convergence Technology, 145 Gwanggyo-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 16229, Republic of Korea.

Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

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Table S-1. The comparison of electrochemical performances with other CNT-based anodes

Anode materials Half cell Full cell (LFP//CNMC) Reference

Current density [mA g-1]

Specific capacity

[mAh g-1]

Current density [mA g-1]

Specific capacity

[mAh g-1]

CNMC7

80 (0.2C) 466 34 (0.2C) 113

This work

200 (0.5C) 327 63 (0.5C) 104

400 (1C) 297 170 (1C) 93

2000 (5C) 215 850 (5C) 48

4000 (10C) 174 --- ---

3-D CNT-graphene 63 (0.17C) 250

--- S1 632 (1.7C) 59

Graphene wrapped CNT

100 373

--- S2 250 229

500 136

3-D CNT

74.4 (0.2C) 312

--- S3 186 (0.5C) 251

372 (1C) 211

1116 (3C) 155

Vertical-aligned MWNT array

100 350 --- S4

1200 ˚C heated CNT film

100 (0.5C) 446 --- S5

2000 (10C) 136

MWCNT Paper Anodes

74.4 340 --- S6

372 225

3D free-standing CNTs

37.2 (0.1C) 374

--- S7 372 (1C) 248

1116 (3C) 147

Reference S1. Kang, C.; Baskaran, R.; Hwang, J.; Ku, B.; Choi, W. Large scale patternable 3-dimensional carbon

nanotube-graphene structure for flexible Li-ion battery. Carbon 2014, 68, 493–500. S2. Sahoo, M.; Ramaprabhu, S. Effect of wrinkles on electrochemical performance of multiwalled carbon

nanotubes as anode material for Li ion battery. Electrochim. Acta 2015, 186, 142–150. S3. Kang, C.; Patel, M.; Rangasamy, B.; Jung, K.; Xia, C.; Shi, S.; Choi, W. Three-dimensional carbon

nanotubes for high capacity lithium-ion batteries. J. Power Sources 2015, 229, 465–471. S4. Bulusheva, L. G.; Arkhipov, V. E.; Fedorovskaya, E. O.; Zhang, S.; Kurenya, A. G.; Kanygin, M. A.;

Asanov, I. P.; Tsygankova, A. R.; Chen, X.; Song, H.; Okotrub, A. V. Fabrication of free-standing aligned multiwalled carbon nanotube array for Li-ion batteries. J. Power Sources 2016, 311, 42–48.

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S5. Yoon, S.; Lee, S.; Kim, S.; Park, K. W.; Cho, D.; Jeong, Y. Carbon nanotube film anodes for flexible lithium ion batteries. J. Power Sources 2015, 279, 495–501.

S6. Landi, B. J.; Dileo, R. A.; Schauerman, C. M.; Cress, C. D.; Ganter, M. J.; Raffaelle, R. P. J. Nanosci. Nanotechnol. 2009, 9, 3406–3410.

S7. Kang, C.; Cha, E.; Baskaran, R.; Choi, W. Three-Dimensional Free-Standing Carbon Nanotubes for a Flexible Lithium-Ion Battery Anode. Nanotechnology 2016, 27, 105402.

Figure S-1.

Figure S-1. (a-c) TEM images of CuNWs. (d) SAED patterns of CuNW correspond to (c) image.

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Figure S-2.

Figure S-2. SEM images of CNMC electrodes with different composite ratios (w/w). (a) CuNW:MWCNT = 1:1 (CNMC1), (b) CuNW:MWCNT = 3:1 (CNMC3) and (c) CuNW:MWCNT = 5:1 (CNMC5). Figure S-3.

Figure S-3. Schematic illustration of the CNMC anode film preparation and their application in battery cells (half cell and full cell).

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Figure S-4.

Figure S-4. The photographs of 647 µm of thick CNMC7 electrode, fabricated by a simple filtration method. Figure S-5.

Figure S-5. (a-b) The SEM images of the full-cell cathode of LiFePO4/MWCNT composites with various magnifications. The LiFePO4 and MWCNT were mixed in the IPA with the weight ratio of 1:2.

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Figure S-6.

Figure S-6. Electrochemical impedance spectroscopy of the full cells using CNMCs and MWCNT as the anodes in frequency from 1 MHz to 0.1 Hz, where employed PP separator, LiPF6 of electrolyte and LiFePO4/MWCNT of cathode, as described in Figure S3. Figure S-7.

Figure S-7. (a) Sheet resistance of LiFePO4-MWCNT films with various composite ratio. (b) Rate capability of LiFePO4-MWCNT cathodes in half cell with polypropylene (PP) separator, LiPF6 of electrolyte and Li metal as the reference and counter electrode. Charge-discharge curves of the half cells at (c) 0.1C, (d) 0.5C,

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(e) 1C, (f) 5C, (g) 10C and (h) 0.1C retention. Figure S-8.

Figure S-8. SEM image of CuNWs welding in CNMC7 anode. Figure S-9.

Figure S-9. Volumetric capacities of MWNCT and CNMC anodes with rate performance.

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Figure S-10.

Figure S-10. The charge-discharge curves of anodes made by CNMC7 and MWCNT at 0.2C rate in the 1st cycle. Figure S-11.

Figure S-11. (a-e) The discharge curves of half cells at 0.2C rate in initial cycle and 1st cycle, which made by CNMC and MWCNT anodes. (f) A diagram of relative reversible capacity (C1/C0) of half cells in 1st cycle, comparing the CNMC and MWCNT anodes.

Address correspondence to Youn Sang Kim, [email protected]; Jeeyoung Yoo, [email protected]

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