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Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries
Journal of Alloys and Compounds xxx (xxxx) xxx
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Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries Chenying An a, 1, Caihui Li a, 1, Haoqing Tang a, *, Tao Liu b, Zhiyuan Tang b a b
College of Materials Science and Engineering, Hebei University of Engineering, Handan, Hebei, 056038, PR China Department of Applied Chemistry, School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 June 2019 Received in revised form 3 October 2019 Accepted 4 October 2019 Available online xxx
High performance flexible electrodes with ultrathin and ultralight characteristics are great potential as power supply for foldable and wearable electronic devices. One major problem is how to combine active material with conductive substrate firmly in the absence of polymeric binder. Here, sandwich structure of flexible electrode is successfully prepared which Li2ZnTi3O8@multi-walled carbon nanotubes (MWCNTs) composite as intermediate anode active ingredient and three-dimensional network MWCNTs as top and bottom conductive layers. The MWCNTs in active material and conductive layers cross-link each other to form a firm and stable three-dimensional network, which provides stereoscopic electron and ion transmission channels without metal copper substrate. This particular structure of electrode is effectively reduce the total weight and improve flexibility. Typically, the flexible Li2ZnTi3O8@MWCNTs electrode shows good cycling stability and achieved a high reversible capacity of 95.2 mAh g1 after 1000 cycles at current density of 7000 mA g1 with 89.0% capacity retention and 99% Coulombic efficiency. This study offers a new strategy for the development of high conductivity flexible electrode for wearable devices and deformable electronic devices. © 2019 Elsevier B.V. All rights reserved.
Keywords: Lithium-ion battery Lithium zinc titanate Flexible electrode Three-dimensional tunnel Electronic conductivity
1. Introduction Ultrathin and ultralight energy-storage devices are receiving extensive interest due to their promising applications in wearable devices, hand-held electronic devices, rollup displays and other portable electronics. Rechargeable lithium-ion batteries (LIBs) are widely used as energy input/output devices in the market of portable electronics, electronic vehicles (EVs) and hybrid electric vehicles (HEVs) because of high energy storage density [1e5]. A conventional lithium-ion battery electrode structure contains active material, non-active materials (conductive carbon and polymeric binder) and metal current collector. Inactive components (e.g. metal current collector) not only increase the total weight of battery, but also reduce the energy density. Moreover, high density and rigid structure of metal current collectors make them unsuitable for lightweight and flexible energy storage devices.
* Corresponding author. E-mail address: [email protected] (H. Tang). 1 These authors contributed equally to this work.
Furthermore, the active material is easily separated from the metal current collector after multiple bending [6e8]. To address these common problems, efforts have being devoted to finding more lighter metal current collector and designing brand new electrode preparation route to render the electrodes flexible. Up to now, replacing traditional Copper/Aluminium current collectors with lightweight and flexible current collectors (plastics [9,10] and textile [11,12]) is a simple and efficient method for increasing energy density. However, such flexible substrates are not conductive compared with metal substrates and electrode slurries still include inert components that inevitably compromise flexible energy storage devices’ volumetric and gravimetric density. As an alternative approach, carbon-based conductive materials, particularly two-dimensional material graphene and one-dimensional material carbon nanotubes, are selected as current collector for flexible devices [13e15]. In general, the design and fabricate of reliable flexible electrodes with high Liþ storage capacity, high rate capability and excellent long cycle stability remains challenging. In this study, we report the first high-performance flexible LIBs anode based on Li2ZnTi3O8/multi-walled carbon nanotubes (MWCNTs) composite active particles and MWCNTs conductive
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Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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network without using any other polymeric binder and metal current collector. Through this approach, the total weight of electrode is significantly reduced and the mass of active material is increased simultaneously, accordingly, a high-capacity battery can be obtained. Moreover, the network structure composed of MWCNTs can not only rapidly transport electrons, but also allow the electrolyte to penetrate deeply and more active Liþ sites can be reused efficiently. Li2ZnTi3O8 ((Li0.5Zn0.5)tet[Li0.5Ti1.5]octO4) was chosen as an anode active material due to inexpensive reaction materials, lower discharge voltage plateau, large lithium-intercalation capacity and good cycling performance [16e19]. However, an unsatisfactory problem is intrinsic low electrical conductivity derived from Ti which limits its practical application [20]. In our synthesis, Li2ZnTi3O8 electrode material was prepared via one-step solid-state high temperature calcination process and uniform mixing of Li2ZnTi3O8 and MWCNTs through cellular ultrasound device. The linear MWCNTs in Li2ZnTi3O8@MWCNTs composite not only provide spatial three-dimensional electron transport channels, but also can be well linked with the MWCNTs substrate material, thus avoiding use of binder (e.g. Polyvinylidene Fluoride, PVDF). Furthermore, the flexibility of MWCNTs provides a favorable guarantee for flexible battery devices. In summary, the challenge to effectively fabricate high performance flexible electrodes for electrochemical energy devices via simple industrialization method still remains.
cell disruptor. Then, the MWCNTs suspension was obtained. SP, Graphene, and Li2ZnTi3O8 suspension were using the same process without adding other reagents. The Li2ZnTi3O8@MWCNTs, Li2ZnTi3O8@SP, and Li2ZnTi3O8@Graphene mixed suspension were also prepared by ultrasonic cell crushing method. The Li2ZnTi3O8@Carbon suspension was prepared by mixing with 50 mg of Li2ZnTi3O8 sample and 5 mg of MWCNTs, SP, Graphene, respectively, which was added into a solution containing 50 mL anhydrous ethanol. After 2 h of low temperature ultrasonic, the suspension became uniform black mixture suspension. 2.4. Preparation of flexible anode First, 0.5 mL of MWCNTs suspension (1 mg/mL) was added in 20 mL absolute ethanol and ultrasound for 1 min at room temperature; Second, the resulting MWCNTs suspension was suction filtered onto the microporous polypropylene (PP, Celgard 2300) separator surface; Third, 3 mL of Li2ZnTi3O8@Carbon suspension (1 mg/mL) was added in 20 mL absolute ethanol and ultrasound for 1 min and suction filtered onto the MWCNTs substrate; Finally, 0.5 mL of MWCNTs suspension with 20 mL absolute ethanol was again covered on the surface of active material to prepare a flexible electrode. 2.5. Materials characterization
2. Experimental 2.1. Materials All the reagents in the experiment are analytical grade, and used without any purification. Chemical reagents such as titanium dioxide (TiO2), zinc acetate dihydrate (Zn(CH3COO)2$2H2O), lithium carbonate (Li2CO3), and anhydrous ethanol (CH3CH2OH) were purchased from Aladdin Reagent Shanghai Co., Ltd. (Shanghai, China). Ultrapure water was supplied with a HYP-QX Water System (18.25 MU/cm). Super P Conductive Carbon Black (SP) was purchased from Shenzhen Kejing Star Technology Co., Ltd (Shenzhen, China). Multi-walled carbon nanotubes (MWCNTs) was purchased from Chengdu Organic Chemistry Co., Ltd., Chinese Academy of Sciences. Graphene was prepared by reduction of Graphene oxide (GO, was prepared by a modified Hummers’ method as reported earlier [21].) 2.2. Synthesis of electrode active materials Irregular morphology of Li2ZnTi3O8 nano particles were prepared by a simple solid-state high temperature calcination process. Typically, titanium dioxide (TiO2), zinc acetate dihydrate (Zn(CH3COO)2$2H2O), lithium carbonate (Li2CO3), and anhydrous ethanol were mixed at a molar ratio of Ti:Zn:Li ¼ 3:1:2 and ballmilled for 4 h in planetary ball mill (Shanghai Jingxin Industrial Development Co., Ltd) with different agate balls size at 25 C. After full mixing, the white mixture was dried at 80 C for 12 h in electric drying oven. Last, the obtained white dry powder sample was ground in agate mortar and heated at 800 C for 5 h in muffle furnace. The final product was naturally cooled in air. 2.3. Preparation of samples suspension The MWCNTs, SP, Graphene, and Li2ZnTi3O8 suspension were prepared using the ultrasonic cell crushing method. In a typical procedure, 50 mg MWCNTs and 50 mL anhydrous ethanol were mixed in a 100 mL beaker placed in a low temperature bath (20 below zero) and ultrasound at a certain power for 2 h by ultrasonic
The crystal structure of Li2ZnTi3O8 sample was characterized by X-ray diffraction (XRD) (Rigaku D/max 2550) with Cu Ka radiation. The micro morphology of the prepared samples and compounds were characterized using field mission scanning electron microscopy (FESEM, Philips Quanta 200), transmission electron microscopy (TEM, JEOL JEM-2100F) and high-resolution transmission electron microscope (HRTEM, JEOL JEM-2100F). 2.6. Electrochemical measurements Electrochemical evaluations were evaluated using CR2032-type coin cell with Li metal was employed as the counter electrode. For pristine Li2ZnTi3O8 without other conductive additives, the working electrodes was prepared by spreading a slurry with a certain consistency composed of 80 wt% active materials, 10 wt% acetylene black and 10 wt% polyvinylidene difluoride (PVDF) dissolved in Nmethyl pyrrolidone (NMP). The resultant slurries were coated on Cu foil with a doctor blade and dried in a vacuum oven at 120 C. Afterwards, a diameter of 12 mm pole piece was punched in the form of disks. Then a 1 M LiPF6 solution contains ethylene carbonate and dimethyl carbonate (1:1 in volume) was used as an electrolyte and microporous polypropylene (PP, Celgard 2300) as separators. Finally, CR2032 coin cells were assembled in an argon filled glove box. As for Li2ZnTi3O8@MWCNTs, Li2ZnTi3O8@SP, and Li2ZnTi3O8@Graphene, the working electrodes was prepared by covering the active material on the microporous polypropylene (PP, Celgard 2300) separator surface without acetylene black and polyvinylidene difluoride (PVDF). Besides, the Cu foil was replaced by MWCNTs conductive network. Then a 1 M LiPF6 solution contains ethylene carbonate and dimethyl carbonate (1:1 in volume) was used as an electrolyte. Finally, CR2032 coin cells were assembled in an argon filled glove box. The electrochemical properties of the electrode active materials were investigated using n a multichannel battery testing system (LAND CT2001A) at different current densities between 0.05 and 3.0 V. Electrochemical impedance spectroscopy (EIS) tests were carried out on a GAMRY PC14-750 electrochemical workstation at
Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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constant temperature of 25 C. 3. Results and discussion The flexible electrode without copper current collector was fabricated through solvent ultrasonic and vacuum filtration approach. Scheme 1 illustrates the process of preparing flexible electrode using MWCNTs as conductive current collectors in place of traditional metal current collectors. First, ultrasonic treatment of Li2ZnTi3O8 and MWCNTs materials in anhydrous ethanol, respectively, which provides highly dispersible raw materials. Second, uniformly mixing Li2ZnTi3O8 and MWCNTs to improve the electronic conductivity of electrode material. Third, the Li2ZnTi3O8@MWCNTs composite is placed in the middle of the MWCNT conductive current collector to prepare a flexible electrode. The preparation process provides a network with three-dimensional space for electron and ion transmission, which is beneficial to the improvement of electrochemical performance. At the same time, it also improves the flexibility of flexible electrode and increase the mass energy density of lithium-ion battery. The crystalline structure of Li2ZnTi3O8 active material was affirmed by XRD spectrum and shown in Fig. 1. All the observed diffraction peaks of this sample can be assigned to cubic spinel Li2ZnTi3O8 (JCPDS 44e1037) in the range from 10 to 80 [22]. As can be clearly observed for the XRD of Li2ZnTi3O8, peaks at 15.1, 18.2 , 23.8 , 26.1, 30.2 , 34.0 , 35.6 , 43.3 , 49.9 , 53.7, 57.2 and 62.9 can be indexed to the (110), (111), (210), (211), (220), (310), (113), (004), (421), (422), (115) and (404) planes of Li2ZnTi3O8. No impurity phases such as TiO2, ZnO, and Li2O can be detected, indicating that raw materials are all decomposed to form target product when calcined at high temperature. Schematic diagram of Li2ZnTi3O8 crystal structure have been shown in Fig. 1b. It is clear that the Li2ZnTi3O8 consists of tetrahedron and octahedron, which zinc and titanium atoms are in the center of the tetrahedron and the octahedron, respectively. The crystal structure of Li2ZnTi3O8 can be described as (Li0.5Zn0.5)tet[(Li0.5)Ti1.5]octO4, which threedimensional tunnels are clearly visible after 90 and 180 rotation. The formation of this structure is beneficial to provides a stable frame during Liþ lithiation and de-lithiation process and improve long cycling performance. The particular micro morphology of Li2ZnTi3O8 is investigated by SEM. It can be seen from Fig. 2aec of that the shape of Li2ZnTi3O8 particles are irregular topography and the original particle size is not uniform. After high temperature calcination process, the original granules were easy to reunite and obtained larger size secondary particles which the diameter is between 1.0 and 3.0 mm.
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Moreover, many pile holes derive from original particles can be clearly seen, which is conducive to electrolyte penetration into the internal space and increasing ion transmission channels. This is reasonable since the high temperature calcination and decomposition of the precursor of Zn(CH3COO)2$2H2O and Li2CO3 resulted in released CO2 gas at the same time. In order to further investigate the detailed micro morphology and micro structure of pristine Li2ZnTi3O8, the TEM and HRTEM images were recorded, which also confirm the micro morphology acquired from the SEM results. As can be seen in Fig. 2d and e, the Li2ZnTi3O8 nano particles combine with each other, forming an aggregation structure. Its HRTEM image Fig. 2f shows lattice fringes with lattice spacing of 0.479 nm, which correspond to (111) crystallographic planes of Li2ZnTi3O8 crystal. In this work, SP with uniform sphere particles, threadlike MWCNTs and lamellar Graphene are selected as conductive additives. The Li2ZnTi3O8 particles were mixed with anhydrous ethanol for 1 h ultrasonic, then conductive additives (SP, MWCNTs and Graphene) were added into the mixed solution and continue ultrasound for 1 h, respectively. Finally, the solutions are filtered using vacuum pumping and dried at 80 C. The SEM images of composite materials are shown in Fig. 3 a-c. The high magnification SEM images (in Fig. 3 a-c) indicate that SP, MWCNTs and Graphene were mixed evenly with Li2ZnTi3O8 particles, respectively, which can prevent further agglomeration of particles, increase Liþ transmission channels and enlarger solid-liquid contact area. In short, it is conducive to exert of electrochemical properties. Besides, the EDS images and elemental mapping presented in Fig. 3 d-f indicate that C, Ti, Zn, and O are present in the composite. The preparation process of highly conductive flexible electrode is shown in Fig. S1 (Supporting Information). First, the surface of lithium-ion battery separator was covered by ultrasonically dispersed MWCNTs through vacuum filtration. Second, the mixture of Li2ZnTi3O8 active material and conductive agents (SP, MWCNTs, Graphene) were superimposed on the surface of MWCNTs. Third, overlay the ultrasonically dispersed MWCNTs once again. The formed sandwich-shaped three-dimensional conductive network facilitates the rapid transmission of electrons during chargedischarge process. Furthermore, eliminating the copper substrate not only reduces the weight of electrode, but also increases the available Liþ storage capacity of cell. Therefore, the reversible capacity and high rates cycle stability of flexible electrode can be improved. The nitrogen adsorption-desorption isotherms of LZTO, LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene are also presented in Fig. 4 a-d. As can be seen from the Figure, the curves of all the
Scheme 1. Schematic diagram of preparation of flexible electrodes using MWCNTs as conductive substrate.
Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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Fig. 1. (a) Typical XRD patterns of pristine Li2ZnTi3O8 particles. (b) Schematic representations of the crystal structure of Li2ZnTi3O8.
samples are similar in shape and present a typical type IV isotherm with a pronounced capillary condensation hysteresis loop at partial pressures P/P0 > 0.3, indicating the presence of textural
mesoporous (secondary particle stacked pore). The BET surface areas of LZTO, LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene samples were 3.0314, 3.9476, 7.5569 and 6.2596 m2 g1,
Fig. 2. SEM images of pristine Li2ZnTi3O8 at different magnification times (aec); TEM and HRTEM images of pristine Li2ZnTi3O8 material (def).
Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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Fig. 3. SEM images of Li2ZnTi3O8 composites with SP, MWCNTs, Graphene as the conductive additives (aec). EDS mapping of LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene (def).
respectively. Such a reticular formation in LZTO@MWCNTs ensured more active sites can be reused and rapid Liþ diffusion, and the high specific surface area confirmed the excellent contact between LZTO@MWCNTs and organic electrolyte, which are significantly attractive for fast Liþ intercalation and delithiation [23]. The pore size distribution based on the Barrett-Joyner-Halenda (BJH) method is shown in Fig. 4 a-d (inset). The average diameter of pores of LZTO, LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene are 8.94 nm, 9.90 nm, 17.43 nm and 9.92 nm, respectively. Based on the data obtained, there are more stacked pores between LZTO and MWCNTs, which means that the degree of agglomeration is reduced and more abundant pathways are provided for lithium ions conduction. One major concern of Cu foil current collector-free electrode is the insufficient mechanical flexibility/dimensional robustness upon external deformation. Fig. 4 shows that the separator with conductive additive and Li2ZnTi3O8 (LZTO) active sample coating was bendable after several times without any mechanical rupture. In addition, electrode active particles are firmly attached to the conductive additive MWCNTs layers. In summary, the mechanical flexibility of this excellent copper-free current collector electrode is beneficial for improving long-cycle stability and increasing charge and discharge capacities. In order to confirm the specific content of carbon, TG was measured in air and the weight loss curve is depicted in Fig. S2. It is found that there are two weight losses during heating from room
temperature to 650 C. One is in the range of room temperature to 260 C may be related to the evaporation of adsorbed water and crystal water. The other is related to the volatilization of carbon in composite from 260 to 615 C, and the specific carbon content is 0.381 mg (2.6%, sample weight: 14.652 mg). Interestingly, the layered configuration is observed on the crosssection of the flexible anodes (Fig. 5 a, b; Fig. S3, Supporting Information). The conductive carbon layers and the active material layer are stacked together by vacuum filtration. The thickness of the MWCNTs layers (top and bottom) is between 2 and 5 mm and the thickness of the intermediate layer (LZTO@MWCNTs) is between 10 and 19 mm (Fig. 5 b). Similarly, it can be seen from the cross-section SEM image that the other two flexible anodes have similar results (Fig. S3, Supporting Information). For each electrode, the mass ratio of composite material to conductive carbon is 3 : 2. Comprehensive analysis of the results, it is envisioned that such a layered configuration is beneficial to improve electrochemical performance. Fig. 5 c demonstrate the first discharge and charge curves obtained of LZTO/Li, LZTO@SP/Li, LZTO@MWCNTs/Li, and LZTO@Graphene/Li half cells in the potential window 0.05e3.0 V (vs. Li/Liþ) at 100 mA g1, respectively. The contour of all curves is basically the same, indicating that the additive has no effect on the electrode. There is a clear potential plateau near 1.39 V for charging and between 0.53 V and 0.57 V for discharging, corresponding to extraction and insertion of Liþ, respectively. The reason for this
Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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Fig. 4. The nitrogen adsorption-desorption isotherms of LZTO, LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene (aed) and corresponding pore size distributions of the LZTO, LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene (inset). Mechanical stability of fresh separator with MWCNTs and LZTO coating. Photograph (Fig. 4 e, left) showing the coated separator has been bended many times; Photograph (Fig. 4 e, right) showing the coated separator has not significant change after bending.
phenomenon can be attributed to the instinct characteristics of Li2ZnTi3O8 ¼ (Li0.5Zn0.5)tet[Ti1.5Li0.5]octO4, in which the tetrahedral sites would be occupied stable by Liþ at low potential [24]. Moreover, the voltage difference (V) (vs. Li/Liþ) between charge voltage platform and discharge voltage platform of LZTO@MWCNTs electrode is the minimum in all samples, indicating better electrochemical performance. The discharge capacities of LZTO, LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene are 211.9 mAh g1, 346.4
mAh g1, 561.3 mAh g1, and 366.6 mAh g1, and the charge capacities are 154.3 mAh g1, 177.1 mAh g1, 274.1 mAh g1, and 187.3 mAh g1, respectively. The irreversible capacity for the initial discharge and charge processes can be attributed to the formation of a solid electrolyte interface (SEI) on the interface between active materials and electrolyte due to the reduction of unstable species in electrolyte [24]. For further comparison and explanation, the cycling performance of pristine SP, MWCNTs and Graphene at
Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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Fig. 5. The cross-section SEM images of flexible anode with LZTO@MWCNTs as active material. (c) Initial discharge/charge curves of LZTO, LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene at 0.1 A g1, respectively.
1000 mA g1 is shown in Fig. S4. The discharge capacities are 108.3, 158.9, and 218.1 mAh g1 for SP, MWCNTs and Graphene after 100 discharge-charge cycles, respectively, and exhibits good cycle stability. Typical cyclic voltammograms (CVs) of LZTO@SP, LZTO@MWCNTs and LZTO@Graphene recorded at a scan rate of 0.5 mV s1 before electrochemical performance test are presented in Fig. S5. It is obvious that one pair of redox peaks appears between 1.60 V and 1.20 V for all the three samples. During the cathodic and anodic scan, the LZTO@MWCNTs electrode shows the highest peak current in all three samples, which suggests that it has better kinetic characteristics. Furthermore, the values of potential difference between oxidation peak and reduction peak are 0.28 V, 0.26 V and 0.28 V for LZTO@SP, LZTO@MWCNTs and LZTO@Graphene, respectively, which show LZTO@MWCNTs has lower electrochemical polarization. Based on the above results, the LZTO@MWCNTs may has better kinetic characteristics. The long-cycling performance of LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene samples at current densities of 1000 mA g1 and 5000 mA g1 in the voltage window of 0.05e3.0 V (vs. Li/Liþ) are displayed in Fig. 6. As can be seen, the initial and the 1000th discharge capacities of LZTO@MWCNTs sample is larger than that LZTO@SP and LZTO@Graphene at 1000 mA g1. While the current density increases to 5000 mA g1, the discharge capacity of LZTO@Graphene sample is superior to LZTO@SP and LZTO@MWCNTs before 200 cycles. Unexpectedly, from 200 to 1000 cycles, the reversible capacity of LZTO@Graphene presents severe capacity fading comparison with the others. Throughout the complete discharge-charge process, the LZTO@MWCNTs sample shows the optimal rate performance and cyclic stability. The reasons for these results include the following points: (1) The one-dimensional
structure and high electrical conductivity of MWCNTs can effectively improve electronic conductivity of the electrode material, thereby improving the rate performance [25,26]; (2) Large surface area of LZTO@MWCNTs provides sufficient contact between LZTO@MWCNTs and organic electrolyte and offers a large amount of reaction active sites during Liþ intercalation and delithiation; (3) Sandwich-shaped layered electrode is advantageous for increasing transmission paths of electrons and ions and expediting transmission process; (4) The diffusion distance of lithium ions between the electrode material and the metal lithium is significantly shortened due to the direct contact of the conductive layer with the separator. (5) Binder-free electrode could avoid the introduction of binder which will reduce inactive weight and increase energy density [27]. All these advantages above are beneficial to improving the specific capacity, long cycling stability and high rate capability in electrochemical energy storage. Electrochemical characteristics of fast charge and slow discharge or slow charge and fast discharge are more important in practical application. Fig. 7(aed) display cycling performance of LZTO@MWCNTs sample in the voltage window of 0.05e3.0 V (vs. Li/ Liþ). At 500 mA g1 charge and 2000 mA g1 discharge, and 2000 mA g1 charge and 500 mA g1 discharge, the LZTO@MWCNTs shows good cycling stability and electrochemical reversibility after 200 cycles. The 200th discharge capacities are 244.3 mAh g1 and 236.8 mAh g1 for LZTO@MWCNTs at 500 mA g1 charge and 2000 mA g1 discharge and 2000 mA g1 charge and 500 mA g1 discharge, respectively. Besides, the Coulombic efficiency of LZTO@MWCNTs more than 99% from second cycle to the 200th cycle. Furthermore, when the charge and discharge current densities increases to 1000 mA g1 and 5000 mA g1, the cycling stability of LZTO@MWCNTs has obviously
Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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Fig. 6. Long-cycling performance of LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene at 1000 mA g1 and 5000 mA g1 after first activating cycle at 100 mA g1, respectively.
attenuation trend. However, the LZTO@MWCNTs still displays high reversible discharge capacities which 151.0 mAh g1 and 142.0 mAh g1 after 500 cycles at 1000 mA g1 charge and 5000 mA g1 discharge, and 5000 mA g1 charge and 1000 mA g1 discharge, respectively. Based on the results obtained, it is found that the LZTO@MWCNTs sample has a good fast charging and discharging ability. Fig. 7e shows the cycling stability and capacity retention of the LZTO@MWCNTs electrode with the current density of 7000 mA g1 over 1000 discharge-charge cycles. The LZTO@MWCNTs electrode delivers a high reversible capacity of 95.2 mAh g1 after 1000 cycles, which is 89.0% retention of the second cycle. In addition, the Coulombic efficiency for LZTO@MWCNTs more than 99%, reflecting faster insertion/extraction kinetics [28]. To investigate the electrochemical characteristic of the LZTO@MWCNTs electrode further, a test of rate capability was conducted and the result is displayed in Fig. 7f. As the dischargecharge current density is increased (0.1, 0.5, 1.0, 2.0, and
3.0 A g1), the corresponding discharge capacities are 466.6, 267.6, 244.7, 226.3, and 212.6 mAh g1, respectively. Also, the charge capacities are 287.3, 259.4, 238.3, 221.3, and 208.4 mAh g1 at 0.1, 0.5, 1.0, 2.0, and 3.0 A g1 with the Columbic efficiency being 61.6%, 96.9%, 97.4%, 97.8%, and 98.0%, respectively. And when the current density is back to 0.1 A g1, the discharge capacity as high as 263.2 mAh g1, indicating the LZTO@MWCNTs electrode has a better capacity reversible. Fig. 7g displays the electrochemical impedance spectrum (EIS) of different electrodes before cycling. The EIS of all electrodes show a compressed semicircle from the high to medium frequency range of each spectrum, and a line inclined at z45 in the low-frequency range. On the basis of compressed semicircle and endpoints interaction with the X-axis, the LZTO@MWCNTs electrode has much lower ohmic resistance of solution and charge transfer resistance in all four electrodes. The addition of linear MWCNTs enables the LZTO to increase the electron conductivity, reduce the inner electronic transmission resistance in the electrode, and improve the
Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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Fig. 7. Cycling performance of LZTO@MWCNTs at various charge-discharge rates (aed); Long-term cycling performance for 1000 cycles at 7000 mA g1 of LZTO@MWCNTs. Rate capability under different current rates of LZTO@MWCNTs. Nyquist curves of LZTO, LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene electrodes for the frequency range from 0.05 to 1000 kHz.
Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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Table 1 Comparison on the electrochemical performances of Li2ZnTi3O8-based anode materials in this work and other previously reported literatures. (Second cycle discharge capacity: mAh g1). Anodes
Current density/A g1 0.1
1
2
3
5
7
LZTO@MWCNTs Li2Zn0.6Cu0.4Ti3O8 LZTO-M LZTO LZTO@GNS-CNT
312.2 210.5 200.5 183.3 e
244.7 124.5 165.0 104.0 226.0
222.9 e 153.0 73.0 187.6
208.4 e 140.0 e 175.0
163.9 e e 23.5 123.5
128.3 e e e e
Refs.
This work [28] [29] [30] [31]
charge transfer at the solid-liquid (electrode-electrolyte) interface, resulting in better electrochemical reaction kinetics [29,30]. Moreover, a more vertical straight line can be observed in the low frequency region for LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene compared with pure LZTO, implying that has better and faster ion diffusion path [31]. Conductive additives could prevent original particles agglomeration, increase available active sites for lithium ion storage, and expand the specific surface area between active materials and organic electrolyte, which can improve the electrochemical performance. Based on the obtained electrochemical data, the LZTO@MWCNTs electrode shows excellent lithiation and de-lithiation capacities, rates performance and cycle stability, which superior to other reported related materials [32e35]. The detailed results are compared and listed in Table 1 and Fig. 7h. To further confirm the flexible electrodes’ structure integrity, the electrode was taken out of the coin cell and dried in the oven at 50 C, then examined under SEM and the results are shown in Fig. 8. Based on the microtopography of electrode surface, lots of holes with different sizes can be clearly seen and no obviously collapse
(Fig. 8 a-b). Moreover, the LZTO active particles are uniform distributed in MWCNTs electric conduction network (Fig. 8 c-d), which contribute to electronic multichannel transmission. Looked from the overall, the as-prepared flexible electrode has good both structure stability and long cycle performance.
4. Conclusion In summary, we have demonstrated a simple ultrasonic and filtration process to synthesize flexible electrode with good stability without polymeric binder and metal current collector. The new design of flexible electrode structure can increase the proportion of active material in the electrode and enhance the discharge capacity, thus resulting in a further enhancement of energy density. The flexible electrode based on LZTO@MWCNTs composite active material showed excellent performance in high current densities and fast charge and slow discharge or slow charge and fast discharge cycling test, which demonstrated their cycling stability in sustaining large current and long cycle process. Moreover, the flexible electrodes’ structure still maintain integrity after multiple lithium de-insertion/insertion process. The results obtained can be interpreted as network of MWCNTs provide good mechanical properties, high electronic conductivity, facile ion transport, and three-dimensional network structure facilitates infiltration of electrolyte. This new method developed herein has great potential to promote the development of flexible electrode with ultrathin and ultralight energy-storage devices.
Declaration of competing interest There are no conflicts to declare.
Fig. 8. Morphology of the flexible electrode (LZTO@MWCNTs) after 1000 cycles. (a, b) microtopography of electrode surface; (c, d) microtopography of electrode cross section.
Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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Acknowledgment This work was supported by National Natural Science Foundation of China (Grant No. 51702081). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152580. References [1] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652e657. [2] B. Dunn, H. Kamath, J.M. Tarascon, Electrical energy storage for the grid: a battery of choices, Science 334 (2011) 928e935. [3] S. Chu, Y. Cui, N. Liu, The path towards sustainable energy, Nat. Mater. 16 (2017) 16e22. [4] J.B. Goodenough, Y. Kim, Challenges for rechargeable batteries, J. Power Sources 196 (2011) 6688e6694. [5] G.M. Zhou, F. Li, H.M. Cheng, Progress in flexible lithium batteries and future prospects, Energy Environ. Sci. 7 (2014) 1307e1338. [6] Z. Chen, J.W.F. To, C. Wang, Z.D. Lu, N. Liu, A. Chortos, L.J. Pan, F. Wei, Y. Cui, Z.N. Bao, A three-dimensionally interconnected carbon nanotube-conducting polymer hydrogel network for high performance flexible battery electrodes, Adv. Energy Mater. (2014) 1400207. [7] X.L. Jia, Y.F. Kan, X. Zhu, G.Q. Ning, Y.F. Lu, F. Wei, Building flexible Li4Ti5O12/ CNT lithium-ion battery anodes with superior rate performance and ultralong cycling stability, Nano Energy 10 (2014) 344e352. [8] X. Fang, M.Y. Ge, J.P. Rong, C.W. Zhou, Free-standing LiNi0.5Mn1.5O4/carbon nanofiber network film as lightweight and high-power cathode for lithium ion batteries, ACS Nano 8 (2014) 4876e4882. [9] M. Koo, K.-I. Park, S.H. Lee, M. Suh, D.Y. Jeon, J.W. Choi, K. Kang, K.J. Lee, Bendable inorganic thin-film battery for fully flexible electronic systems, Nano Lett. 12 (2012) 4810e4816. [10] K. Wang, W. Zou, B. Quan, A. Yu, H. Wu, P. Jiang, Z. Wei, An all-solid-state flexible micro-supercapacitor on a chip, Adv. Energy Mater. 1 (2011) 1068e1072. [11] A.M. Gaikwad, A.M. Zamarayeva, J. Rousseau, H. Chu, I. Derin, D.A. Steingart, Highly stretchable alkaline batteries based on an embedded conductive fabric, Adv. Mater. 24 (2012) 5071e5076. [12] L. Yuan, X.-H. Lu, X. Xiao, T. Zhai, J. Dai, F. Zhang, B. Hu, X. Wang, L. Gong, J. Chen, C. Hu, Y. Tong, J. Zhou, Z.L. Wang, Flexible solid-state supercapacitors based on carbon nanoparticles/MnO2 nanorods hybrid structure, ACS Nano 6 (2012) 656e661. [13] Y. Cheng, S. Lu, H. Zhang, C.V. Varanasi, J. Liu, Synergistic effects from graphene and carbon nanotubes enable flexible and robust electrodes for highperformance supercapacitors, Nano Lett. 12 (2012) 4206e4211. [14] A. Sumboja, C.Y. Foo, X. Wang, P.S. Lee, Large areal mass, flexible and freestanding reduced graphene oxide/manganese dioxide paper for asymmetric supercapacitor device, Adv. Mater. 25 (2013) 2809e2815. [15] Y. Meng, K. Wang, Y. Zhang, Z. Wei, Hierarchical porous graphene/polyaniline composite film with superior rate performance for flexible supercapacitors, Adv. Mater. 25 (2013) 6985e6990. [16] Z.S. Hong, X.Z. Zheng, X.K. Ding, L.L. Jiang, M.D. Wei, K.M. Wei, Complex spinel titanate nanowires for a high rate lithium-ion battery, Energy Environ. Sci. 4 (2011) 1886e1891.
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Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries Chenying An a, 1, Caihui Li a, 1, Haoqing Tang a, *, Tao Liu b, Zhiyuan Tang b a b
College of Materials Science and Engineering, Hebei University of Engineering, Handan, Hebei, 056038, PR China Department of Applied Chemistry, School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 June 2019 Received in revised form 3 October 2019 Accepted 4 October 2019 Available online xxx
High performance flexible electrodes with ultrathin and ultralight characteristics are great potential as power supply for foldable and wearable electronic devices. One major problem is how to combine active material with conductive substrate firmly in the absence of polymeric binder. Here, sandwich structure of flexible electrode is successfully prepared which Li2ZnTi3O8@multi-walled carbon nanotubes (MWCNTs) composite as intermediate anode active ingredient and three-dimensional network MWCNTs as top and bottom conductive layers. The MWCNTs in active material and conductive layers cross-link each other to form a firm and stable three-dimensional network, which provides stereoscopic electron and ion transmission channels without metal copper substrate. This particular structure of electrode is effectively reduce the total weight and improve flexibility. Typically, the flexible Li2ZnTi3O8@MWCNTs electrode shows good cycling stability and achieved a high reversible capacity of 95.2 mAh g1 after 1000 cycles at current density of 7000 mA g1 with 89.0% capacity retention and 99% Coulombic efficiency. This study offers a new strategy for the development of high conductivity flexible electrode for wearable devices and deformable electronic devices. © 2019 Elsevier B.V. All rights reserved.
Keywords: Lithium-ion battery Lithium zinc titanate Flexible electrode Three-dimensional tunnel Electronic conductivity
1. Introduction Ultrathin and ultralight energy-storage devices are receiving extensive interest due to their promising applications in wearable devices, hand-held electronic devices, rollup displays and other portable electronics. Rechargeable lithium-ion batteries (LIBs) are widely used as energy input/output devices in the market of portable electronics, electronic vehicles (EVs) and hybrid electric vehicles (HEVs) because of high energy storage density [1e5]. A conventional lithium-ion battery electrode structure contains active material, non-active materials (conductive carbon and polymeric binder) and metal current collector. Inactive components (e.g. metal current collector) not only increase the total weight of battery, but also reduce the energy density. Moreover, high density and rigid structure of metal current collectors make them unsuitable for lightweight and flexible energy storage devices.
* Corresponding author. E-mail address: [email protected] (H. Tang). 1 These authors contributed equally to this work.
Furthermore, the active material is easily separated from the metal current collector after multiple bending [6e8]. To address these common problems, efforts have being devoted to finding more lighter metal current collector and designing brand new electrode preparation route to render the electrodes flexible. Up to now, replacing traditional Copper/Aluminium current collectors with lightweight and flexible current collectors (plastics [9,10] and textile [11,12]) is a simple and efficient method for increasing energy density. However, such flexible substrates are not conductive compared with metal substrates and electrode slurries still include inert components that inevitably compromise flexible energy storage devices’ volumetric and gravimetric density. As an alternative approach, carbon-based conductive materials, particularly two-dimensional material graphene and one-dimensional material carbon nanotubes, are selected as current collector for flexible devices [13e15]. In general, the design and fabricate of reliable flexible electrodes with high Liþ storage capacity, high rate capability and excellent long cycle stability remains challenging. In this study, we report the first high-performance flexible LIBs anode based on Li2ZnTi3O8/multi-walled carbon nanotubes (MWCNTs) composite active particles and MWCNTs conductive
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Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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network without using any other polymeric binder and metal current collector. Through this approach, the total weight of electrode is significantly reduced and the mass of active material is increased simultaneously, accordingly, a high-capacity battery can be obtained. Moreover, the network structure composed of MWCNTs can not only rapidly transport electrons, but also allow the electrolyte to penetrate deeply and more active Liþ sites can be reused efficiently. Li2ZnTi3O8 ((Li0.5Zn0.5)tet[Li0.5Ti1.5]octO4) was chosen as an anode active material due to inexpensive reaction materials, lower discharge voltage plateau, large lithium-intercalation capacity and good cycling performance [16e19]. However, an unsatisfactory problem is intrinsic low electrical conductivity derived from Ti which limits its practical application [20]. In our synthesis, Li2ZnTi3O8 electrode material was prepared via one-step solid-state high temperature calcination process and uniform mixing of Li2ZnTi3O8 and MWCNTs through cellular ultrasound device. The linear MWCNTs in Li2ZnTi3O8@MWCNTs composite not only provide spatial three-dimensional electron transport channels, but also can be well linked with the MWCNTs substrate material, thus avoiding use of binder (e.g. Polyvinylidene Fluoride, PVDF). Furthermore, the flexibility of MWCNTs provides a favorable guarantee for flexible battery devices. In summary, the challenge to effectively fabricate high performance flexible electrodes for electrochemical energy devices via simple industrialization method still remains.
cell disruptor. Then, the MWCNTs suspension was obtained. SP, Graphene, and Li2ZnTi3O8 suspension were using the same process without adding other reagents. The Li2ZnTi3O8@MWCNTs, Li2ZnTi3O8@SP, and Li2ZnTi3O8@Graphene mixed suspension were also prepared by ultrasonic cell crushing method. The Li2ZnTi3O8@Carbon suspension was prepared by mixing with 50 mg of Li2ZnTi3O8 sample and 5 mg of MWCNTs, SP, Graphene, respectively, which was added into a solution containing 50 mL anhydrous ethanol. After 2 h of low temperature ultrasonic, the suspension became uniform black mixture suspension. 2.4. Preparation of flexible anode First, 0.5 mL of MWCNTs suspension (1 mg/mL) was added in 20 mL absolute ethanol and ultrasound for 1 min at room temperature; Second, the resulting MWCNTs suspension was suction filtered onto the microporous polypropylene (PP, Celgard 2300) separator surface; Third, 3 mL of Li2ZnTi3O8@Carbon suspension (1 mg/mL) was added in 20 mL absolute ethanol and ultrasound for 1 min and suction filtered onto the MWCNTs substrate; Finally, 0.5 mL of MWCNTs suspension with 20 mL absolute ethanol was again covered on the surface of active material to prepare a flexible electrode. 2.5. Materials characterization
2. Experimental 2.1. Materials All the reagents in the experiment are analytical grade, and used without any purification. Chemical reagents such as titanium dioxide (TiO2), zinc acetate dihydrate (Zn(CH3COO)2$2H2O), lithium carbonate (Li2CO3), and anhydrous ethanol (CH3CH2OH) were purchased from Aladdin Reagent Shanghai Co., Ltd. (Shanghai, China). Ultrapure water was supplied with a HYP-QX Water System (18.25 MU/cm). Super P Conductive Carbon Black (SP) was purchased from Shenzhen Kejing Star Technology Co., Ltd (Shenzhen, China). Multi-walled carbon nanotubes (MWCNTs) was purchased from Chengdu Organic Chemistry Co., Ltd., Chinese Academy of Sciences. Graphene was prepared by reduction of Graphene oxide (GO, was prepared by a modified Hummers’ method as reported earlier [21].) 2.2. Synthesis of electrode active materials Irregular morphology of Li2ZnTi3O8 nano particles were prepared by a simple solid-state high temperature calcination process. Typically, titanium dioxide (TiO2), zinc acetate dihydrate (Zn(CH3COO)2$2H2O), lithium carbonate (Li2CO3), and anhydrous ethanol were mixed at a molar ratio of Ti:Zn:Li ¼ 3:1:2 and ballmilled for 4 h in planetary ball mill (Shanghai Jingxin Industrial Development Co., Ltd) with different agate balls size at 25 C. After full mixing, the white mixture was dried at 80 C for 12 h in electric drying oven. Last, the obtained white dry powder sample was ground in agate mortar and heated at 800 C for 5 h in muffle furnace. The final product was naturally cooled in air. 2.3. Preparation of samples suspension The MWCNTs, SP, Graphene, and Li2ZnTi3O8 suspension were prepared using the ultrasonic cell crushing method. In a typical procedure, 50 mg MWCNTs and 50 mL anhydrous ethanol were mixed in a 100 mL beaker placed in a low temperature bath (20 below zero) and ultrasound at a certain power for 2 h by ultrasonic
The crystal structure of Li2ZnTi3O8 sample was characterized by X-ray diffraction (XRD) (Rigaku D/max 2550) with Cu Ka radiation. The micro morphology of the prepared samples and compounds were characterized using field mission scanning electron microscopy (FESEM, Philips Quanta 200), transmission electron microscopy (TEM, JEOL JEM-2100F) and high-resolution transmission electron microscope (HRTEM, JEOL JEM-2100F). 2.6. Electrochemical measurements Electrochemical evaluations were evaluated using CR2032-type coin cell with Li metal was employed as the counter electrode. For pristine Li2ZnTi3O8 without other conductive additives, the working electrodes was prepared by spreading a slurry with a certain consistency composed of 80 wt% active materials, 10 wt% acetylene black and 10 wt% polyvinylidene difluoride (PVDF) dissolved in Nmethyl pyrrolidone (NMP). The resultant slurries were coated on Cu foil with a doctor blade and dried in a vacuum oven at 120 C. Afterwards, a diameter of 12 mm pole piece was punched in the form of disks. Then a 1 M LiPF6 solution contains ethylene carbonate and dimethyl carbonate (1:1 in volume) was used as an electrolyte and microporous polypropylene (PP, Celgard 2300) as separators. Finally, CR2032 coin cells were assembled in an argon filled glove box. As for Li2ZnTi3O8@MWCNTs, Li2ZnTi3O8@SP, and Li2ZnTi3O8@Graphene, the working electrodes was prepared by covering the active material on the microporous polypropylene (PP, Celgard 2300) separator surface without acetylene black and polyvinylidene difluoride (PVDF). Besides, the Cu foil was replaced by MWCNTs conductive network. Then a 1 M LiPF6 solution contains ethylene carbonate and dimethyl carbonate (1:1 in volume) was used as an electrolyte. Finally, CR2032 coin cells were assembled in an argon filled glove box. The electrochemical properties of the electrode active materials were investigated using n a multichannel battery testing system (LAND CT2001A) at different current densities between 0.05 and 3.0 V. Electrochemical impedance spectroscopy (EIS) tests were carried out on a GAMRY PC14-750 electrochemical workstation at
Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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constant temperature of 25 C. 3. Results and discussion The flexible electrode without copper current collector was fabricated through solvent ultrasonic and vacuum filtration approach. Scheme 1 illustrates the process of preparing flexible electrode using MWCNTs as conductive current collectors in place of traditional metal current collectors. First, ultrasonic treatment of Li2ZnTi3O8 and MWCNTs materials in anhydrous ethanol, respectively, which provides highly dispersible raw materials. Second, uniformly mixing Li2ZnTi3O8 and MWCNTs to improve the electronic conductivity of electrode material. Third, the Li2ZnTi3O8@MWCNTs composite is placed in the middle of the MWCNT conductive current collector to prepare a flexible electrode. The preparation process provides a network with three-dimensional space for electron and ion transmission, which is beneficial to the improvement of electrochemical performance. At the same time, it also improves the flexibility of flexible electrode and increase the mass energy density of lithium-ion battery. The crystalline structure of Li2ZnTi3O8 active material was affirmed by XRD spectrum and shown in Fig. 1. All the observed diffraction peaks of this sample can be assigned to cubic spinel Li2ZnTi3O8 (JCPDS 44e1037) in the range from 10 to 80 [22]. As can be clearly observed for the XRD of Li2ZnTi3O8, peaks at 15.1, 18.2 , 23.8 , 26.1, 30.2 , 34.0 , 35.6 , 43.3 , 49.9 , 53.7, 57.2 and 62.9 can be indexed to the (110), (111), (210), (211), (220), (310), (113), (004), (421), (422), (115) and (404) planes of Li2ZnTi3O8. No impurity phases such as TiO2, ZnO, and Li2O can be detected, indicating that raw materials are all decomposed to form target product when calcined at high temperature. Schematic diagram of Li2ZnTi3O8 crystal structure have been shown in Fig. 1b. It is clear that the Li2ZnTi3O8 consists of tetrahedron and octahedron, which zinc and titanium atoms are in the center of the tetrahedron and the octahedron, respectively. The crystal structure of Li2ZnTi3O8 can be described as (Li0.5Zn0.5)tet[(Li0.5)Ti1.5]octO4, which threedimensional tunnels are clearly visible after 90 and 180 rotation. The formation of this structure is beneficial to provides a stable frame during Liþ lithiation and de-lithiation process and improve long cycling performance. The particular micro morphology of Li2ZnTi3O8 is investigated by SEM. It can be seen from Fig. 2aec of that the shape of Li2ZnTi3O8 particles are irregular topography and the original particle size is not uniform. After high temperature calcination process, the original granules were easy to reunite and obtained larger size secondary particles which the diameter is between 1.0 and 3.0 mm.
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Moreover, many pile holes derive from original particles can be clearly seen, which is conducive to electrolyte penetration into the internal space and increasing ion transmission channels. This is reasonable since the high temperature calcination and decomposition of the precursor of Zn(CH3COO)2$2H2O and Li2CO3 resulted in released CO2 gas at the same time. In order to further investigate the detailed micro morphology and micro structure of pristine Li2ZnTi3O8, the TEM and HRTEM images were recorded, which also confirm the micro morphology acquired from the SEM results. As can be seen in Fig. 2d and e, the Li2ZnTi3O8 nano particles combine with each other, forming an aggregation structure. Its HRTEM image Fig. 2f shows lattice fringes with lattice spacing of 0.479 nm, which correspond to (111) crystallographic planes of Li2ZnTi3O8 crystal. In this work, SP with uniform sphere particles, threadlike MWCNTs and lamellar Graphene are selected as conductive additives. The Li2ZnTi3O8 particles were mixed with anhydrous ethanol for 1 h ultrasonic, then conductive additives (SP, MWCNTs and Graphene) were added into the mixed solution and continue ultrasound for 1 h, respectively. Finally, the solutions are filtered using vacuum pumping and dried at 80 C. The SEM images of composite materials are shown in Fig. 3 a-c. The high magnification SEM images (in Fig. 3 a-c) indicate that SP, MWCNTs and Graphene were mixed evenly with Li2ZnTi3O8 particles, respectively, which can prevent further agglomeration of particles, increase Liþ transmission channels and enlarger solid-liquid contact area. In short, it is conducive to exert of electrochemical properties. Besides, the EDS images and elemental mapping presented in Fig. 3 d-f indicate that C, Ti, Zn, and O are present in the composite. The preparation process of highly conductive flexible electrode is shown in Fig. S1 (Supporting Information). First, the surface of lithium-ion battery separator was covered by ultrasonically dispersed MWCNTs through vacuum filtration. Second, the mixture of Li2ZnTi3O8 active material and conductive agents (SP, MWCNTs, Graphene) were superimposed on the surface of MWCNTs. Third, overlay the ultrasonically dispersed MWCNTs once again. The formed sandwich-shaped three-dimensional conductive network facilitates the rapid transmission of electrons during chargedischarge process. Furthermore, eliminating the copper substrate not only reduces the weight of electrode, but also increases the available Liþ storage capacity of cell. Therefore, the reversible capacity and high rates cycle stability of flexible electrode can be improved. The nitrogen adsorption-desorption isotherms of LZTO, LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene are also presented in Fig. 4 a-d. As can be seen from the Figure, the curves of all the
Scheme 1. Schematic diagram of preparation of flexible electrodes using MWCNTs as conductive substrate.
Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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Fig. 1. (a) Typical XRD patterns of pristine Li2ZnTi3O8 particles. (b) Schematic representations of the crystal structure of Li2ZnTi3O8.
samples are similar in shape and present a typical type IV isotherm with a pronounced capillary condensation hysteresis loop at partial pressures P/P0 > 0.3, indicating the presence of textural
mesoporous (secondary particle stacked pore). The BET surface areas of LZTO, LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene samples were 3.0314, 3.9476, 7.5569 and 6.2596 m2 g1,
Fig. 2. SEM images of pristine Li2ZnTi3O8 at different magnification times (aec); TEM and HRTEM images of pristine Li2ZnTi3O8 material (def).
Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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Fig. 3. SEM images of Li2ZnTi3O8 composites with SP, MWCNTs, Graphene as the conductive additives (aec). EDS mapping of LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene (def).
respectively. Such a reticular formation in LZTO@MWCNTs ensured more active sites can be reused and rapid Liþ diffusion, and the high specific surface area confirmed the excellent contact between LZTO@MWCNTs and organic electrolyte, which are significantly attractive for fast Liþ intercalation and delithiation [23]. The pore size distribution based on the Barrett-Joyner-Halenda (BJH) method is shown in Fig. 4 a-d (inset). The average diameter of pores of LZTO, LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene are 8.94 nm, 9.90 nm, 17.43 nm and 9.92 nm, respectively. Based on the data obtained, there are more stacked pores between LZTO and MWCNTs, which means that the degree of agglomeration is reduced and more abundant pathways are provided for lithium ions conduction. One major concern of Cu foil current collector-free electrode is the insufficient mechanical flexibility/dimensional robustness upon external deformation. Fig. 4 shows that the separator with conductive additive and Li2ZnTi3O8 (LZTO) active sample coating was bendable after several times without any mechanical rupture. In addition, electrode active particles are firmly attached to the conductive additive MWCNTs layers. In summary, the mechanical flexibility of this excellent copper-free current collector electrode is beneficial for improving long-cycle stability and increasing charge and discharge capacities. In order to confirm the specific content of carbon, TG was measured in air and the weight loss curve is depicted in Fig. S2. It is found that there are two weight losses during heating from room
temperature to 650 C. One is in the range of room temperature to 260 C may be related to the evaporation of adsorbed water and crystal water. The other is related to the volatilization of carbon in composite from 260 to 615 C, and the specific carbon content is 0.381 mg (2.6%, sample weight: 14.652 mg). Interestingly, the layered configuration is observed on the crosssection of the flexible anodes (Fig. 5 a, b; Fig. S3, Supporting Information). The conductive carbon layers and the active material layer are stacked together by vacuum filtration. The thickness of the MWCNTs layers (top and bottom) is between 2 and 5 mm and the thickness of the intermediate layer (LZTO@MWCNTs) is between 10 and 19 mm (Fig. 5 b). Similarly, it can be seen from the cross-section SEM image that the other two flexible anodes have similar results (Fig. S3, Supporting Information). For each electrode, the mass ratio of composite material to conductive carbon is 3 : 2. Comprehensive analysis of the results, it is envisioned that such a layered configuration is beneficial to improve electrochemical performance. Fig. 5 c demonstrate the first discharge and charge curves obtained of LZTO/Li, LZTO@SP/Li, LZTO@MWCNTs/Li, and LZTO@Graphene/Li half cells in the potential window 0.05e3.0 V (vs. Li/Liþ) at 100 mA g1, respectively. The contour of all curves is basically the same, indicating that the additive has no effect on the electrode. There is a clear potential plateau near 1.39 V for charging and between 0.53 V and 0.57 V for discharging, corresponding to extraction and insertion of Liþ, respectively. The reason for this
Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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Fig. 4. The nitrogen adsorption-desorption isotherms of LZTO, LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene (aed) and corresponding pore size distributions of the LZTO, LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene (inset). Mechanical stability of fresh separator with MWCNTs and LZTO coating. Photograph (Fig. 4 e, left) showing the coated separator has been bended many times; Photograph (Fig. 4 e, right) showing the coated separator has not significant change after bending.
phenomenon can be attributed to the instinct characteristics of Li2ZnTi3O8 ¼ (Li0.5Zn0.5)tet[Ti1.5Li0.5]octO4, in which the tetrahedral sites would be occupied stable by Liþ at low potential [24]. Moreover, the voltage difference (V) (vs. Li/Liþ) between charge voltage platform and discharge voltage platform of LZTO@MWCNTs electrode is the minimum in all samples, indicating better electrochemical performance. The discharge capacities of LZTO, LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene are 211.9 mAh g1, 346.4
mAh g1, 561.3 mAh g1, and 366.6 mAh g1, and the charge capacities are 154.3 mAh g1, 177.1 mAh g1, 274.1 mAh g1, and 187.3 mAh g1, respectively. The irreversible capacity for the initial discharge and charge processes can be attributed to the formation of a solid electrolyte interface (SEI) on the interface between active materials and electrolyte due to the reduction of unstable species in electrolyte [24]. For further comparison and explanation, the cycling performance of pristine SP, MWCNTs and Graphene at
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Fig. 5. The cross-section SEM images of flexible anode with LZTO@MWCNTs as active material. (c) Initial discharge/charge curves of LZTO, LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene at 0.1 A g1, respectively.
1000 mA g1 is shown in Fig. S4. The discharge capacities are 108.3, 158.9, and 218.1 mAh g1 for SP, MWCNTs and Graphene after 100 discharge-charge cycles, respectively, and exhibits good cycle stability. Typical cyclic voltammograms (CVs) of LZTO@SP, LZTO@MWCNTs and LZTO@Graphene recorded at a scan rate of 0.5 mV s1 before electrochemical performance test are presented in Fig. S5. It is obvious that one pair of redox peaks appears between 1.60 V and 1.20 V for all the three samples. During the cathodic and anodic scan, the LZTO@MWCNTs electrode shows the highest peak current in all three samples, which suggests that it has better kinetic characteristics. Furthermore, the values of potential difference between oxidation peak and reduction peak are 0.28 V, 0.26 V and 0.28 V for LZTO@SP, LZTO@MWCNTs and LZTO@Graphene, respectively, which show LZTO@MWCNTs has lower electrochemical polarization. Based on the above results, the LZTO@MWCNTs may has better kinetic characteristics. The long-cycling performance of LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene samples at current densities of 1000 mA g1 and 5000 mA g1 in the voltage window of 0.05e3.0 V (vs. Li/Liþ) are displayed in Fig. 6. As can be seen, the initial and the 1000th discharge capacities of LZTO@MWCNTs sample is larger than that LZTO@SP and LZTO@Graphene at 1000 mA g1. While the current density increases to 5000 mA g1, the discharge capacity of LZTO@Graphene sample is superior to LZTO@SP and LZTO@MWCNTs before 200 cycles. Unexpectedly, from 200 to 1000 cycles, the reversible capacity of LZTO@Graphene presents severe capacity fading comparison with the others. Throughout the complete discharge-charge process, the LZTO@MWCNTs sample shows the optimal rate performance and cyclic stability. The reasons for these results include the following points: (1) The one-dimensional
structure and high electrical conductivity of MWCNTs can effectively improve electronic conductivity of the electrode material, thereby improving the rate performance [25,26]; (2) Large surface area of LZTO@MWCNTs provides sufficient contact between LZTO@MWCNTs and organic electrolyte and offers a large amount of reaction active sites during Liþ intercalation and delithiation; (3) Sandwich-shaped layered electrode is advantageous for increasing transmission paths of electrons and ions and expediting transmission process; (4) The diffusion distance of lithium ions between the electrode material and the metal lithium is significantly shortened due to the direct contact of the conductive layer with the separator. (5) Binder-free electrode could avoid the introduction of binder which will reduce inactive weight and increase energy density [27]. All these advantages above are beneficial to improving the specific capacity, long cycling stability and high rate capability in electrochemical energy storage. Electrochemical characteristics of fast charge and slow discharge or slow charge and fast discharge are more important in practical application. Fig. 7(aed) display cycling performance of LZTO@MWCNTs sample in the voltage window of 0.05e3.0 V (vs. Li/ Liþ). At 500 mA g1 charge and 2000 mA g1 discharge, and 2000 mA g1 charge and 500 mA g1 discharge, the LZTO@MWCNTs shows good cycling stability and electrochemical reversibility after 200 cycles. The 200th discharge capacities are 244.3 mAh g1 and 236.8 mAh g1 for LZTO@MWCNTs at 500 mA g1 charge and 2000 mA g1 discharge and 2000 mA g1 charge and 500 mA g1 discharge, respectively. Besides, the Coulombic efficiency of LZTO@MWCNTs more than 99% from second cycle to the 200th cycle. Furthermore, when the charge and discharge current densities increases to 1000 mA g1 and 5000 mA g1, the cycling stability of LZTO@MWCNTs has obviously
Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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Fig. 6. Long-cycling performance of LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene at 1000 mA g1 and 5000 mA g1 after first activating cycle at 100 mA g1, respectively.
attenuation trend. However, the LZTO@MWCNTs still displays high reversible discharge capacities which 151.0 mAh g1 and 142.0 mAh g1 after 500 cycles at 1000 mA g1 charge and 5000 mA g1 discharge, and 5000 mA g1 charge and 1000 mA g1 discharge, respectively. Based on the results obtained, it is found that the LZTO@MWCNTs sample has a good fast charging and discharging ability. Fig. 7e shows the cycling stability and capacity retention of the LZTO@MWCNTs electrode with the current density of 7000 mA g1 over 1000 discharge-charge cycles. The LZTO@MWCNTs electrode delivers a high reversible capacity of 95.2 mAh g1 after 1000 cycles, which is 89.0% retention of the second cycle. In addition, the Coulombic efficiency for LZTO@MWCNTs more than 99%, reflecting faster insertion/extraction kinetics [28]. To investigate the electrochemical characteristic of the LZTO@MWCNTs electrode further, a test of rate capability was conducted and the result is displayed in Fig. 7f. As the dischargecharge current density is increased (0.1, 0.5, 1.0, 2.0, and
3.0 A g1), the corresponding discharge capacities are 466.6, 267.6, 244.7, 226.3, and 212.6 mAh g1, respectively. Also, the charge capacities are 287.3, 259.4, 238.3, 221.3, and 208.4 mAh g1 at 0.1, 0.5, 1.0, 2.0, and 3.0 A g1 with the Columbic efficiency being 61.6%, 96.9%, 97.4%, 97.8%, and 98.0%, respectively. And when the current density is back to 0.1 A g1, the discharge capacity as high as 263.2 mAh g1, indicating the LZTO@MWCNTs electrode has a better capacity reversible. Fig. 7g displays the electrochemical impedance spectrum (EIS) of different electrodes before cycling. The EIS of all electrodes show a compressed semicircle from the high to medium frequency range of each spectrum, and a line inclined at z45 in the low-frequency range. On the basis of compressed semicircle and endpoints interaction with the X-axis, the LZTO@MWCNTs electrode has much lower ohmic resistance of solution and charge transfer resistance in all four electrodes. The addition of linear MWCNTs enables the LZTO to increase the electron conductivity, reduce the inner electronic transmission resistance in the electrode, and improve the
Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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Fig. 7. Cycling performance of LZTO@MWCNTs at various charge-discharge rates (aed); Long-term cycling performance for 1000 cycles at 7000 mA g1 of LZTO@MWCNTs. Rate capability under different current rates of LZTO@MWCNTs. Nyquist curves of LZTO, LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene electrodes for the frequency range from 0.05 to 1000 kHz.
Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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Table 1 Comparison on the electrochemical performances of Li2ZnTi3O8-based anode materials in this work and other previously reported literatures. (Second cycle discharge capacity: mAh g1). Anodes
Current density/A g1 0.1
1
2
3
5
7
LZTO@MWCNTs Li2Zn0.6Cu0.4Ti3O8 LZTO-M LZTO LZTO@GNS-CNT
312.2 210.5 200.5 183.3 e
244.7 124.5 165.0 104.0 226.0
222.9 e 153.0 73.0 187.6
208.4 e 140.0 e 175.0
163.9 e e 23.5 123.5
128.3 e e e e
Refs.
This work [28] [29] [30] [31]
charge transfer at the solid-liquid (electrode-electrolyte) interface, resulting in better electrochemical reaction kinetics [29,30]. Moreover, a more vertical straight line can be observed in the low frequency region for LZTO@SP, LZTO@MWCNTs, and LZTO@Graphene compared with pure LZTO, implying that has better and faster ion diffusion path [31]. Conductive additives could prevent original particles agglomeration, increase available active sites for lithium ion storage, and expand the specific surface area between active materials and organic electrolyte, which can improve the electrochemical performance. Based on the obtained electrochemical data, the LZTO@MWCNTs electrode shows excellent lithiation and de-lithiation capacities, rates performance and cycle stability, which superior to other reported related materials [32e35]. The detailed results are compared and listed in Table 1 and Fig. 7h. To further confirm the flexible electrodes’ structure integrity, the electrode was taken out of the coin cell and dried in the oven at 50 C, then examined under SEM and the results are shown in Fig. 8. Based on the microtopography of electrode surface, lots of holes with different sizes can be clearly seen and no obviously collapse
(Fig. 8 a-b). Moreover, the LZTO active particles are uniform distributed in MWCNTs electric conduction network (Fig. 8 c-d), which contribute to electronic multichannel transmission. Looked from the overall, the as-prepared flexible electrode has good both structure stability and long cycle performance.
4. Conclusion In summary, we have demonstrated a simple ultrasonic and filtration process to synthesize flexible electrode with good stability without polymeric binder and metal current collector. The new design of flexible electrode structure can increase the proportion of active material in the electrode and enhance the discharge capacity, thus resulting in a further enhancement of energy density. The flexible electrode based on LZTO@MWCNTs composite active material showed excellent performance in high current densities and fast charge and slow discharge or slow charge and fast discharge cycling test, which demonstrated their cycling stability in sustaining large current and long cycle process. Moreover, the flexible electrodes’ structure still maintain integrity after multiple lithium de-insertion/insertion process. The results obtained can be interpreted as network of MWCNTs provide good mechanical properties, high electronic conductivity, facile ion transport, and three-dimensional network structure facilitates infiltration of electrolyte. This new method developed herein has great potential to promote the development of flexible electrode with ultrathin and ultralight energy-storage devices.
Declaration of competing interest There are no conflicts to declare.
Fig. 8. Morphology of the flexible electrode (LZTO@MWCNTs) after 1000 cycles. (a, b) microtopography of electrode surface; (c, d) microtopography of electrode cross section.
Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580
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Please cite this article as: C. An et al., Binder-free flexible Li2ZnTi3O8@MWCNTs stereoscopic network as lightweight and superior rate performance anode for lithium-ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152580