carbon nanotube network as high power cathode for rechargeable hybrid aqueous battery

Journal of Power Sources 326 (2016) 498e504

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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Binder-free flexible LiMn2O4/carbon nanotube network as high power cathode for rechargeable hybrid aqueous battery Xiao Zhu a, Xianwen Wu b, The Nam Long Doan a, Ye Tian a, Hongbin Zhao c, P. Chen a, * a

Department of Chemical Engineering and Waterloo Institute of Nanotechnology, University of Waterloo, Waterloo N2L 3G1, Ontario, Canada College of Chemistry and Chemical Engineering, Jishou University, 120 People South Road, Jishou, Hunan Province 416000, PR China c College of Science, Shanghai University, 99 Shangda Road, BaoShan District, Shanghai 200444, PR China b

h i g h l i g h t s
LiMn2O4 particles are entangled into CNT networks forming highly flexible electrode.
The LiMn2O4/CNT electrode can be used in aqueous battery with no binders.
The LiMn2O4/CNT electrode exhibits high capacity.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 May 2016 Received in revised form 4 July 2016 Accepted 8 July 2016 Available online 15 July 2016

Highly flexible LiMn2O4/carbon nanotube (CNT) electrodes are developed and used as a high power cathode for the Rechargeable Hybrid Aqueous Battery (ReHAB). LiMn2O4 particles are entangled into CNT networks, forming a self-supported free-standing hybrid films. Such hybrid films can be used as electrodes of ARLB without using any additional binders. The binder-free LiMn2O4/CNT electrode exhibits good mechanical properties, high conductivity, and effective charge transport. High-rate capability and high cycling stability are obtained. Typically, the LiMn2O4/CNT electrode achieves a discharge capacity of 72 mAh g1 at the large-current of 20 C (1 C ¼ 120 mAh g1), and exhibits good cycling performance and high reversibility: Coulombic efficiency of almost 100% over 300 charge-discharge cycles at 4 C. By reducing the weight, and improving the large-current rate capability simultaneously, the LiMn2O4/CNT electrode can highly enhance the energy/power density of ARLB and hold potential to be used in ultrathin, lightweight electronic devices. © 2016 Elsevier B.V. All rights reserved.

Keywords: Binder-free electrode LiMn2O4/CNT hybrid High power cathode Rechargeable Hybrid Aqueous Battery

1. Introduction Flexible energy storage devices have attracted increasing attention due to their potential applications in smart electronics, rollup displays, and wearable devices [1,2]. The key lies in the fabrication of flexible electrodes that can maintain the performance under deformations. Traditionally, the common approach to make battery electrodes is to mix electrode materials with carbon blacks and polymeric binders, and press them onto metal substrates. However, such electrodes have low flexibility due to their intrinsic rigid structure. As an alternative method, the binders and metal substrates can be eliminated, and binder-free electrodes are prepared using a conductive and robust matrix such as carbon

* Corresponding author. E-mail address: [email protected] (P. Chen). http://dx.doi.org/10.1016/j.jpowsour.2016.07.029 0378-7753/© 2016 Elsevier B.V. All rights reserved.

nanotube (CNT) and graphene networks [3e6]. But there are few researchers who have explored the use of such electrodes in aqueous batteries. Aqueous batteries have lower cost, higher level of safety, as well as better ionic conductivity than their organic counterparts [7e12]. But aqueous batteries usually have limited energy densities. Flexible electrodes built in this work are used for aqueous batteries, for better overall performance, including higher energy and power performance and cycling stability. To make an excellent battery electrode, it is critically important to design it from the operation principle. In essence, polymeric binders are not necessary in the electrodes. Battery electrodes can be made from direct growth of active particles on CNT surfaces for binder-free electrodes [13]. Due to the intimate contacts between active particles and CNTs, such electrodes can show good reversible capacities. However, CNTs have highly hydrophobic interfaces, which unavoidably bring on phase separation when in contact with the aqueous electrolyte, and lead to difficult fabrication of such

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electrodes. Similarly, flexible electrodes were fabricated by direct deposition methods [14], which also form free-standing electrodes. Such electrodes, however, may exhibit low active material loading. Simple mixing of active nanoparticles with surfactant-dispersed CNTs seems to be an effective solution [15]. If properly engineered, flexible electrodes can be built from surfactant-assisted assembly of active materials and CNTs. However, the introduced surfactants may result in unfavorable reactions in the battery. Recently, it is reported that high-aspect-ratio CNTs display high capacity to capture nanoparticles [16,17]. This phenomenon has been explored for air filtration [18]. We hope to utilize long CNT networks to capture active electrode materials for fabrication of binder-free battery electrodes, which can encounter none of the above mentioned issues. Overall, the dispersion of long CNTs is important but difficult. Despite the great efforts [19], direct construction of binder-free electrodes using CNTs remains challenging. Herein, a highly robust binder-free electrode composed of highaspect-ratio CNTs and LiMn2O4 nanoparticles is constructed, and developed for a new secondary aqueous Zn/LiMn2O4 battery system, i.e., Rechargeable Hybrid Aqueous Battery (ReHAB). A schematic illustration of the formation process of a binder-free flexible LiMn2O4/CNT electrode is shown in Fig. 1. First, we disperse CNTs into a sol-like dispersion using a high-speed fluid shearing method, where CNTs maintains their high aspect ratios, and steadily disperses in the solvent. Owing to the high aspect ratios, CNTs form a network structure, which can capture nanoparticles, just like “fishes on nets”. After simple mixing and a vacuum filtration process, binder-free flexible LiMn2O4/CNT electrodes for ReHAB are available. Note that LiMn2O4 nanoparticles are tightly entangled into the composite electrode without binders, forming a porous yet integrated structure. Through our design of electrode structure, the following benefits can be achieved: (1) the total weight of the electrode is highly reduced, since CNTs simultaneously act as the binder, conductive additive, and current collector. No other electrochemically inactive additives are introduced in the electrode, increasing the total storage density; (2) the conductivity of the electrode is greatly enhanced due to the good conductivity of CNT networks compared with the traditional conductors and binders, and additives used in surfactant- and/or polymer-assisted synthesis that are usually insulating; (3) the continuous and porous CNT networks facilitate the electrolyte infiltration in addition to electron transfer, and this way the LiMn2O4 particles can access Li ions more efficiently; (4) The resilient network structure also alleviates the volume changes of the active material during battery charge/ discharge processes, enhancing cycling stability. It is noted that such a synthesis process does not require expensive facilities and extra chemicals, since the CNT dispersion can tightly capture

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nanoparticles, thereby enabling the fabrication of a structurally stable composite. 2. Experimental details 2.1. Fabrication of binder-free flexible electrodes A vacuum filtration method was used to fabricate the electrodes with controllable thickness [20]. First, aligned CNTs with length approaching 100 mm and diameter of ~11 nm were produced from a chemical vapor deposition (CVD) method [13]. Pristine CNTs were purified to remove the catalysts. Then, they were washed using deionized water and dried before use. The purified CNTs were then dispersed in N-methyl-2-pyrrolidone (NMP, Sigma-Aldrich, 99.5% purity) using fluid shearing dispersion for 30 min to form 2.0 mg mL1 solution. 10 mL of the CNT dispersion was mixed with 180 mg of the LiMn2O4 (MTI Co. Ltd) particles; and then filtered and dried under vacuum at 90  C for 12 h to obtain the binder-free flexible films. The contents of CNT and LiMn2O4 are 10 and 90 wt %, respectively. 2.2. Characterization The crystal structure of the cathodes was characterized using Xray diffraction techniques (XRD, Advance D8, Bruker) with Cu-Ka radiation. The cathode morphology was observed by scanning electron microscopy (SEM, JEOL JSM-6700, JEOL). The internal morphology was studied using transmission electron microscopy (TEM, FEI T12, FEI) operated at 120 kV. Nitrogen sorption isotherms were measured at 77 K with a Micromeritics ASAP 2020 analyzer. The electrode films were cut into sheets for mechanical tests, which were conducted on an INSTRON 5564 with a speed of 2.0 mm min1 at room temperature. The volume conductivity of the composite electrodes under bending were measured using a 266 Clamp Meter. 2.3. Electrode preparation and electrochemical characterization For electrochemical test, the flexible LiMn2O4/CNT films were cut into disks of 12 mm diameter and soaked in the electrolyte under reduced pressure before assembling of batteries. The traditional electrodes were prepared by casting a slurry of 86 wt% LiMn2O4 (MTI Co. Ltd, ~0.2 mm), 7 wt% carbon conductor consisting of CNTs or acetylene black (AB, MTI), and 7 wt% polyvinylidene fluoride (PVDF, Kynar, HSV900) in 1-methyl-2-pyrrolidinone (NMP, Sigma-Aldrich, 99.5% purity) on graphite foil (Alfa Aesar), and vacuum drying at 60  C for 24 h. 12 mm in diameter zinc disks (Rotometals, thickness: 0.2 mm) were used as the anode. The

Fig. 1. Schematic fabrication of binder-free flexible LiMn2O4/CNT network electrodes through the dispersion and vacuum filtration processes.

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Fig. 2. a) Digital photographs of CNT dispersion of 2.0 mg ml1 after standing one week. b) TEM image of dispersed CNTs, showing interconnected networks. c) Resistance changes of 2.0 mg ml1 CNT dispersion varying time.

solution of 2 mol dm3 Li2SO4 (Sigma Aldrich, 99% purity) and 1 mol dm3 ZnSO4 (Alfa Aesar, 98% purity) in deionized water was used as electrolyte. pH was adjusted to 4 by titration with 0.1 mol dm3 LiOH. Absorbed glass mat (AGM, NSG Corporation) was used as separator. Finally, the electrodes were assembled into Two-electrode Swagelok cells before electrochemical test. The mass loading was 6 mg cm2 on each current collector. Cyclic voltammetry (CV) measurement was carried out on a Biologic-VMP3 electrochemical workstation at a scan rate of 0.1 mV s1. Electrochemical impedance spectroscopy (EIS) of the cells was conducted on an CS350 electrochemical workstation system, with the frequency range from 0.01 Hz to 100 kHz.

Galvanostatic charge-discharge measurement was carried out by Neware battery tester at various C-rates, 1e20 C, calculated based on the nominal specific capacity of LiMn2O4 (1 C ¼ 120 mA g1), between 1.4 and 2.1 V. 3. Results and discussion To make the hybrid electrode, CNTs were dispersed into the networks using high-speed fluid shearing dispersion, where the high-aspect ratio nanotubes effectively maintain their lengths. Note that making the CNT dispersion and maintaining the lengths have been difficult. For example, the common methods using ultra-

Fig. 3. Optical photographs of a) the produced hybrid film, and the states under various deformations such as b) bending and c) twisting. d) Stress-strain curve of the flexible electrodes consisting 90 wt% LiMn2O4 particles. e) Volume conductivity changes of the flexible electrode under bending.

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sonication or strong acid oxidation usually destroy CNTs [21]. The dispersion could be stably placed more than one week (Fig. 2a), suggesting the excellent stability of the dispersion. The reason was because that the CNTs were dispersed into network structure, as confirmed by TEM observation (Fig. 2b). It displays that the CNTs are in single-dispersion state, and more importantly, form interconnected networks due to the high aspect ratios. Fig. 2c shows that the dispersion with a CNT concentration of 2.0 mg mL1 has a stable conductivity varying time, further suggesting that the CNTs in the dispersion did not aggregate. Such network structure proves to have high capacity to capture nanoparticles, while mostly reported CNT dispersions with small lengths show very limited such capacity. Fig. 3a shows that the prepared electrode maintains their integrity at large sizes (e.g., 30 mm in diameter). Such hybrid electrode sheets are very flexible, and can be bent under force (Fig. 3b), or even twisted (Fig. 3c). Even at a high nanoparticle loading of ca. 90 wt%, the prepared electrodes retain the high flexibility. Fig. 3d displays the stress-strain curve of an electrode. Typically, a tensile strength of 2.7 MPa is achieved at a strain of 1.1%. The strength ensured the electrode structure stability, making them promising in actual device applications. The robust structure was further confirmed through electrical conductivity measurements under bending (Fig. 3e). Indeed, the flexible electrodes of 90 wt% LiMn2O4 had a constant conductivity of 170 S m1 under different degrees of bending. Such flexibility was usually not observed in binder-bound electrodes. The structure of binder-free flexible LiMn2O4/CNT hybrid is observed under scanning electron microscopy (SEM, Fig. 4a). Obviously, CNTs and LiMn2O4 nanoparticles are highly entangled, forming robust composite networks. Such structure endowed the electrodes with mentioned mechanical strength. Close TEM observation of the composite shows that their texture is uniform, with threading CNTs interweaved through nanoparticles (Fig. 4b). The nanocomposites exhibit direct interfacial contact between the CNT and LiMn2O4 components, key for the high performance of electrodes. Moreover, nitrogen sorption isotherms of the binderfree flexible LiMn2O4/CNT hybrid conforms the interconnected porous channels of this material (Fig. 4c). The surface area of the electrodes was measured to be 42.7 m2 g1 with broad pore size distributions. Such pore structure facilitates electrolyte transport, a key factor affecting electrodes’ rate performance. The electrochemical performance of LiMn2O4/CNT film electrode was used as cathode materials for ReHAB [9,22,23]. The mechanism of ReHAB differs from that of the “rocking-chair” type batteries. At the anode, zinc is dissolved and electroplated reversibly in a mild aqueous solution containing zinc ions during discharge and charge process. At the cathode, lithium ions can be reversibly intercalated into and deintercalated from the tunnels of LiMn2O4 in the same electrolyte system. The electrolyte in this battery is more than an ionic conductive medium. It is also the source of anode, which is formed by zinc deposition upon charge, being consumed and regenerated during battery cycling. The robust structure of LiMn2O4/CNT hybrid ensures binder-free electrodes with excellent ion-storage performance. Cyclic voltammetry (CV) studies were first conducted to investigate their lithium storage behaviors (Fig. 5a). There are two pairs of well-defined redox peaks located at 1.85/1.73 V (1st cycle), 1.84/1.75 V (2nd and 3rd cycle) and 1.98/1.89 V(1st cycle), 1.96/1.89 V (2nd and 3rd cycle) vs. Zn2þ/Zn, respectively. They are corresponding to the extraction and insertion of Li ions from/into the host spinel structure of LiMn2O4 in the aqueous electrolyte. More specifically, such electrochemical Li ions extraction/insertion occurring can be expressed as LiMn2O4 4 Li1þ  xMn2O4 þ x Li þ x e , where x is the mole fraction of inserted lithium ions, x < 1. The lithium extraction during the first cycle is

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Fig. 4. (a) SEM and (b) TEM images of the binder-free flexible LiMn2O4/CNT electrodes. (c) N2 sorption isotherms of the hybrid electrodes.

slightly different from those observed in subsequent cycles due to the initial activation process of the electrode. From the second cycle, however, the extraction/insertion process becomes stable. The potential differences between oxidation peaks and reduction peaks are contributed to the polarization of the electrode during CV testing. The potential difference between oxidation peaks and

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Fig. 5. Electrochemical performance of binder-free LiMn2O4/CNT network electrode. a) CV curves of binder-free LiMn2O4/CNT electrode collected at a scan rate of 0.1 mV s1. b) Charge-discharge curves of binder-free LiMn2O4/CNT electrode at 4 C in the potential window of 1.4e2.1 V. c) Comparison of rate capability of binder-free LiMn2O4/CNT, CNT þ LiMn2O4 and AB þ LiMn2O4 electrodes. d) Cycling stability of binder-free LiMn2O4/CNT electrode at a rate of 4 C in comparison with that of CNT þ LiMn2O4 and AB þ LiMn2O4 electrodes.

reduction peaks is as small as 0.07 V, resulting partially from the high ionic conductivity of the aqueous electrolyte. The symmetrical peaks reveal that the Li ions extraction/insertion process can be regarded as highly reversible. In addition, the peaks retain the welldefined shapes and almost the same redox potential over the cycles of the scanning process, demonstrating the stability of the LiMn2O4/CNT electrode. To further quantify the lithium storage capability, galvanostatic charge/discharge was conducted. Fig. 5b shows the charge/ discharge profiles of the binder-free flexible LiMn2O4/CNT electrode

at 4 C. There are two distinguished plateaus that reflect two-stage Li ions extraction/insertion behavior and are consistent with the obtained CV data. During the cycles, the Coulombic efficiency is close to 100% and remains stable, indicating good reversibility. This further confirms the stability of the binder-free flexible LiMn2O4/ CNT electrode. It should be noted that the specific discharge capacity of 116 mAh g1 can be comparable to other excellent aqueous battery systems with LiMn2O4 cathodes [24]. Moreover, the charge/discharge curves at different current densities were also performed to evaluate the charge-storage behaviors. It shows that

Fig. 6. a) EIS spectra of the pristine binder-free LiMn2O4/CNT network electrode at fresh half-coin cell, 15th and 300th cycles over the frequency range of 105e0.01 Hz (Inset is the enlarged EIS spectra of the composite electrode at 15th and 300th cycles). b) EIS spectra of the binder-free LiMn2O4/CNT network electrode compared to that of CNT þ LiMn2O4 and AB þ LiMn2O4 electrodes after 15 cycles at 4 C.

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the charge/discharge processes of the binder-free flexible LiMn2O4/ CNT electrode are highly reversible, suggesting the excellent reaction kinetics. The excellent ion-storage performance contributes to the improved transport in the porous structure of the electrodes and, more importantly, increased conductivity due to ultra-long CNT networks. The rate capability of the binder-free flexible LiMn2O4/CNT, CNT þ LiMn2O4 and AB þ LiMn2O4 electrodes were studied and compared (Fig. 5c). The specific discharge capacity of the binderfree flexible LiMn2O4/CNT electrode is significantly larger than that of the AB þ LiMn2O4 and CNT þ LiMn2O4 electrode at C-rates from 1 to 20 C. At 20 C, the binder-free flexible LiMn2O4/CNT electrode still exhibits a specific discharge capacity of 72 mAh g1, which is much higher than the CNT þ LiMn2O4 (45 mAh g1) and AB þ LiMn2O4 (42 mAh g1) electrode. Note that such excellent rate performance is comparable or even better than that of LiMn2O4 cathodes in other aqueous battery systems [10,11,25e36]. Besides the excellent rate capability, the binder-free flexible LiMn2O4/CNT electrode also exhibits remarkable cycling stability. As shown in Fig. 5d, the composite electrode was cycled at the rate of 4 C. After 300 cycles, the specific discharge capacity of 92 mAh g1 is still retained. Moreover, the Coulombic efficiency is close to 100% during the cycling. In contrast, control electrodes fabricated by mixing LiMn2O4 with CNTs or AB, and PVDF showed significantly lower stability. The results indicate that the LiMn2O4/ CNT hybrid electrode does significantly improve the cycling stability due to its robust network structure. Since the mechanical test, conductivity test, and electrochemical impedance spectroscopy (EIS) measurements all suggest that the binder-free LiMn2O4/CNT electrode has a stable structure, the capacity fading during cycling may be related to other factors, such as the stability of the Zn anode. Systematic studies on protection of a Zn anode are underway [37]. It is expected to improve the cycling stability of the battery with this binder-free electrode [8,38,39]. To further investigate the structure and interface stability of the hybrid electrodes during cycling, EIS was conducted along with galvanostaic charge/discharge (Fig. 6a). The Nyquist plots exhibit a single semicircle at high frequency, which is due to a combination of a resistance, capacitance, and a constant phase element of the electrode [2]. The ohmic resistance of the electrode is nearly constant during cycling, indicating good contacts of the ReHAB as well as structure integrity of the electrode. Very interestingly, EIS spectra of the following cycles almost maintain the same (inset of Fig. 6a, 15th and 300th), thereby suggesting that the resistances are relatively stable. This suggests that the LiMn2O4/CNT network structure forms a robust and stable electrode and accordingly, electrode capacity was well maintained. In addition, the EISs of the CNT þ LiMn2O4 and AB þ LiMn2O4 electrodes are measured, and compared with that of the binder-free LiMn2O4/CNT electrode. As shown in Fig. 6b, the binder-free LiMn2O4/CNT electrode displayed a little smaller resistances, thereby offering a better rate performance.

4. Conclusions In summary, we have demonstrated an efficient synthesis of highly robust, flexible, binder-free electrodes by using long CNTs to capture active nanoparticles. Such a robust network architecture possesses abundant porous structures and excellent electrical conductivity, which enables effective charge transport; at the same time, the electrode can be bent and twisted under high mechanical strength, endowing the electrode with high capacity, high ratecapability and excellent cycling stability for high-performance flexible device applications.

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Acknowledgements This research was financially supported by Positec (No. 2100500-105-2974-121701), the Natural Sciences and Engineering Research Council of Canada (NSERC, No. 216990), Canadian Foundation for Innovation (CFI, No. 202335), the Canada Research Chairs (CRC, No. 211464) program and Mitacs (No. IT04444). One of the authors (Xiao Zhu) thanks the China Scholarship Council for Study Abroad Scholarship (No. 201306440001). References [1] X. Wang, G. Shi, Flexible graphene devices related to energy conversion and storage, Energ. Environ. Sci. 8 (2015) 790e823. [2] X. Jia, Z. Chen, A. Suwarnasarn, L. Rice, X. Wang, H. Sohn, Q. Zhang, B.M. Wu, F. Wei, Y. Lu, High-performance flexible lithium-ion electrodes based on robust network architecture, Energ. Environ. Sci. 5 (2012) 6845e6849. [3] X. Jia, Y. Kan, X. Zhu, G. Ning, Y. Lu, F. 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