MWCNTs conductive paper collector improves the performance of SnO2 lithium-ion batteries

Accepted Manuscript Aramid Fiber/MWCNTs conductive paper collector improves the performance of SnO2 lithium-ion batteries Liang Guodong, Sun Xiaogang, Lai Jiamei, Wei Chengcheng, Huang Yapan, Hu Hao PII:

S0042-207X(19)30739-0

DOI:

https://doi.org/10.1016/j.vacuum.2019.04.052

Reference:

VAC 8693

To appear in:

Vacuum

Received Date: 9 April 2019 Revised Date:

25 April 2019

Accepted Date: 26 April 2019

Please cite this article as: Guodong L, Xiaogang S, Jiamei L, Chengcheng W, Yapan H, Hao H, Aramid Fiber/MWCNTs conductive paper collector improves the performance of SnO2 lithium-ion batteries, Vacuum (2019), doi: https://doi.org/10.1016/j.vacuum.2019.04.052. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Aramid Fiber/MWCNTs Conductive Paper Collector Improves the Performance of SnO2 Lithium-ion Batteries Liang Guodong1, Sun Xiaogang*12, Lai Jiamei1, Wei Chengcheng1, Huang Yapan1, Hu Hao1 (1. School of Mechantronics Engineering, Nanchang University, Nanchang 330031, China 2.NanoCarbon Co. Ltd, Nanchang 330052, China, e-mail: [email protected])

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Abstract: Aramid carbon nanotube conductive papers (ACP) were prepared by using multi-walled carbon nanotubes as conductive agent and aramid fiber as the matrix. The ACP were used as the anode current collector of SnO2 lithium ion batteries (LIBs) instead of copper foil. The structure was characterized by X-ray diffraction analysis (XRD), Raman spectroscopy, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The electrochemical performance was characterized by Galvanostatic charge-discharge test and cyclic voltammetry (CV) test. As a result, the ACP performed better than copper foil. After 60 cycles at a current density of 100 mA/g, the specific capacity can be maintained at 876 mAh/g, and the SnO2/ACP electrode exhibited excellent rate performance. Keywords: Aramid; Carbon Nanotube; SnO2; Lithium-ion batteries; Specific Capacity

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Introduction According to the current rate of exploitation and consumption, oil is still available for humans for another 41 years [1]. And the use of oil also brings air pollution and hazardous to human health, so it is essential to develop new clean energy. For now, new energy electric vehicles have become the research hotspot, and the key lies in the breakthrough of power battery technology [2]. Lithium-ion batteries (LIBs) are the best choice for power batteries due to their high energy density, long cycle life and strong safety performance [3-4]. However, LIBs also have shortcomings. Conventional lithium-ion batteries typically use graphite as anode electrode and have a lower specific capacity (372 mAh/g). And the cathode and anode current collectors basically adopt aluminum foil and copper foil, the mass of the metal current collector accounting for 15% to 50% of the total electrode quality [5-8]. Furthermore, the interface bonding problem also hinders the internal electronic transmission of the electrode. Therefore, developing new or modifying existing electrode current collectors has positive significance [9]. As one of the anode electrode materials of LIBs, SnO2 has been considered as the most likely candidate for the next generation of commercial LIBs due to its high theoretical specific capacity (782 mAh/g) and lower operating voltage. However, during the process of charge and discharge, SnO2 causes nearly 300% volume change result from the intercalation and deintercalation process of Li+, causing great internal stress, which led to aggregation, pulverization, and even fall off from the current collector [10-12]. In the latest report, Zhu et al. combined SnO2-Fe with graphite, the specific capacity can be maintained at 1045 mAh/g after 200 cycles at a current density of 100

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mA/g [13]. Shiratori et al. synthesized the self-standing carbon nanofiber and SnO2 nanorod composite, and its specific capacity was maintained at about 500 mAh/g after 900 cycles. However, the synthesis of these composite materials requires strict process and high cost [14]. Current collector also an important part of LIBs. And copper foil is usually used as current collector of anode to collect the current that generated by the active material. So that the current collector should be in full contact with the active material, and the internal resistance should be as small as possible. The copper foil has good electrical conductivity, flexibility and moderate potential, is resistant to winding and rolling, and has mature production technology, thus becoming the material of choice for the anode current collector of LIBs [15]. In the LIBs system, the copper foil is both a carrier for the active material and a conductor for the anode electrode electrons. Besides good conductivity, the current collector also needs to ensure that the surface can uniformly coat the active material without falling off. In order to make sure that the anode electrode material coated on the copper foil, some methods are usually used,

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such as adding a suitable binder, mechanically roughening or micro-treating the surface of the copper foil [16-17]. Para-aramid fiber is a new type of synthetic fiber material with high strength, high temperature resistance, low density, low heat shrinkage and good dimensional stability [18]. The para-aramid paper prepared by para-aramid fiber through papermaking process has the advantages of low density, high specific strength, impact resistance, outstanding corrosion resistance and good high temperature stability [19-20]. In addition, the one-dimensional carbon nanotubes (CNTs) have excellent properties such as high strength, good toughness, electrical conductivity, lightweight, and nanometer-diameter diameter [21-24]. Widely used in super capacitors, LIBs and composite materials. The macroscopic film or the composite functional paper formed by CNTs is also a novel application idea in the current collector of LIBs [25-28]. Different from previous research, we focus on the improvement of current collectors to maintain the specific capacity of SnO2. In this work, we prepared aramid carbon nanotubes conductive papers (ACP) by combining CNTs with aramid fiber to replace copper foil as the current collector of SnO2. SnO2 was pressed into the void of ACP by hot pressing technology to achieve the seamless connection of the active material and the current collector. The connection improves the interface performance and alleviates the volume effect of the SnO2. And the specific capacity can hold at 876 mAh/g after 60 cycles. This experiment also compared the performance of SnO2 coated on ACP and copper foil as anode of LIBs.

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Fig.1 Macroscopic map of ACP

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Experiment 1. Synthesis of SnO2 0.1M SnCl4 solution was placed in a magnetic stirring device, and 0.1M aqueous ammonia solution prepared in advance was dropped into the above solution until the PH was 11~13. With constant stirring for another 3 hours, the obtained solution was transferred to a stainless-steel reactor and kept at 180 °C for 24h, and after centrifuged and dried, SnO2 was obtained. 2. Synthesis of multiwalled carbon nanotubes (MWCNTs) MWCNTs were synthesized by our group by chemical vapor deposition (CVD). The reaction is carried out by using benzene as the raw material, organic-metallic compound (ferrocene) as the catalyst, thiophene as the growth promoter and hydrogen as the carrier gas at a temperature of about 1200 °C. Then the MWCNTs were heat-treated in a graphite electric resistance furnace under a vacuum atmosphere of 3000 ° C for 10h. 3. Preparation of aramid carbon nanotubes conductive papers (ACP) The para-aramid chopped fibers and the para-aramid pulp fibers were weighed in a mass ratio of 1:1, and the para-aramid chopped fibers were treated with 0.1M sodium dodecylbenzenesulfonate solution to dissipate for 30mins. At the same time, the pulp fiber was dissolved in the solution of ethanol and aqueous and added with 1% polyethylene oxide (PEO) for half an hour. Then the mixed solution of the chopped fiber and pulp fiber obtained above was beaten for 10 minutes by trough beater. MWCNTs were added at a mass ratio of w(fiber): w(CNT)=7:3, and sheared at high speed for 1 h. After vacuum filtered and dried at 80 °C, the original ACP was obtained. 4. The preparation of anode electrode SnO2 anode material was prepared with a mass ratio of w(SnO2): w(SP): w(PVDF)=8:1:1, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) solvent was added. After ball milling for 2h, the obtained uniformly dispersed slurry was coated on ACP. After vacuum drying at 60 °C, another 10mins for hot pressed at the temperature of 180 °C. The paper was cut into a disc of 14mm diameter to obtain the SnO2/ACP anode. For comparison, SnO2/copper foil anode was also prepared. 5. Characterizations X-ray diffraction (XRD) tests were performed on the prepared SnO2 and MWCNTs,

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and ACP was characterized by field-emission scanning electron microscope (SEM). The CR2025 button test cell was assembled in a glove box in an argon atmosphere, where the O2 and H2O contents were kept below 1 ppm. After 24 hours, the assembled battery was subjected to galvanostatic charge test, the voltage range was 0.01~3V. Cyclic voltammetry (CV) test was performed by electrochemical workstation, and the CV test was scanned at a scan rate of 0.1 mV/s with a voltage variation ranging from 0.01 to 3V.

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Result and discussion Fig. 2(a) and Fig. 2(b) show SEM and transmission electron microscope (TEM) images of MWCNTs, respectively. As we can see from Fig. 2(a), MWCNTs have a linear thin tubular structure with a relatively small lateral length and a large aspect ratio, which gives good sorption of Li+ performance to MWCNTs. Since the MWCNTs are mainly distributed in a straight line, MWCNTs would not be bent and entangled with each other and easy to disperse. Meanwhile, MWCNTs have excellent electrical conductivity, which makes MWCNTs have great advantages in improving the electrochemical performance of LIBs. As can be seen from Fig. 2(b), the MWCNTs used in this experiment are small hollow tubes with a diameter of about 80 nm and a wall thickness of 20~75 nm. (b)

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Fig.2 (a) SEM of MWCNTs and (b) TEM of MWCNTs

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Fig. 3(a) and Fig. 3(b) show the SEM and XRD of SnO2. As we can see SnO2 particles are irregular and the XRD shows that the peaks appear in the figure matches the JCPDS card No. 411445.

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Fig. 3 (a) SEM and (b) XRD of SnO2

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Fig. 4 (a) and Fig. 4 (b) (c) (d) are SEM images of the prepared aramid paper and ACP before hot pressing, respectively. Fig. 4 (a) show that the chopped fibers were used as matrix fibers and evenly dispersed in the paper, which determines the physical strength and mechanical structure of the paper. As a filling and bonding material, pulp fibers would been softened by heat during hot pressing, and the mechanical strength of the paper base was imparted by bonding the chopped fibers and generating the overall mechanical structure of the paper. Furthermore, it can be seen from Fig. 3(b)(c)(d) that MWCNTs were filled in the interstices between the fibers due to their nanostructures. Aramid fibers are insulators, but MWCNTs are excellent conductors. Combination of MWCNTs and aramid fibers makes ACP owns the advantages of the aramid paper and MWCNTs, and exhibits characteristics such as rich pores, flexible bendability, strong adsorption and excellent electrical conductivity. These characteristics make it possible for ACP to replace the copper foil as a current collector of SnO2 material. Furthermore, MWCNTs are uniformly distributed around the fiber-based skeleton structure to form a cross-linked porous network structure. Which facilitates the extraction and insertion of lithium ions and electrons. The active material is distributed in a staggered microporous network structure, and the flexible long straight MWCNTs can well buffer the stress generated by the volume expansion of SnO2 during charge and discharge. Prevents the active material from agglomerating, pulverizing, and even falling off from the current collector, maintaining the integrity of the electrode material. （a）

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Fig. 4 (a) SEM of aramid paper and (b) (c) (d) SEM of ACP

After hot pressing, the micrograph of SnO2 coated on ACP is shown in Fig. 5(a).

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Since the internal structure of ACP is uneven and porous. the active material can be easily penetrated into the interior of the conductive paper, and tightly bound with the ACP current collector. Which was further confirmed from the cross-sectional view of the ACP in Fig. 5(b). Since the copper foil is smooth and flat, SnO2 was contacted on the copper foil in a point-to-face manner. And there was a considerable gap between the two interfaces can be seen from the section of the SnO2/copper foil electrode in Fig. 5(c). Unlike copper foil, which can only be attached to the surface, ACP has abundant pores and strong adsorption for SnO2 to penetrate. After hot pressing, the pulp fibers soften and bond the internal materials. And the active material can be tightly combined with the ACP, there is almost no gap exists between SnO2 and ACP, showing good interface bonding effect. Therefore, owing to the advantages of interface properties, SnO2 would not easily detach from the ACP. SnO2/ACP electrodes could exhibit better electrochemical performance.

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Fig. 5 SEM of (a) front and (b) section of SnO2/ACP; (c) SEM of section of SnO2/copper foil

Fig.6 shows cycle performance comparison of SnO2/copper foil electrode and SnO2/ACP electrode at a current density of 100 mA/g. The specific capacity of the SnO2/ACP electrode held at 876 mAh/g after 60 cycles, which is higher than that of SnO2/copper foil electrode (88 mAh/g). And as showed in the figure, ACP provides reversible capacity of about 210 mAh/g for the SnO2/ACP electrode. The result demonstrated that when ACP was used as the current collector, the specific capacity of LIBs had been improved.

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Fig.6 Cycle performance of SnO2/ACP, SnO2/copper foil and ACP electrodes

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The rate performance of SnO2/ACP electrode also was tested, and the result was shown in Fig. 7. As the current density increased from 100 mA/g to 1600 mA/g, the specific capacity maintained at 931 mAh/g, 698 mAh/g, 530 mAh/g, 411 mAh/g, 227 mAh/g and 119 mAh/g, respectively. And when the current density was gradually returned to 100 mA/g, the specific capacity can gradually recover to 820 mAh/g. And we also investigated the effect of the hot-pressing process on the performance of the SnO2/ACP electrode. As the Fig.7 shows the unheated electrode had much lower specific capacity and when the current density reached above 1000 mA/g, the specific capacity was almost reduced to 0. Showing that the heat process helped to maintain a stable specific capacity. Analysis of the reason is probably because when the load pressure increases, the gap between the SnO2 particles become smaller, the contact between particles of MWCNTs and SnO2 become more closely. And the exchange channel between ions and electrons increases resulted in improving of conductivity. 2400

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Fig.8 is the comparison of the electrochemical impedance spectra (EIS) of the two electrodes. The intersection of the starting point and the axis of the high frequency region is the resistance Rs in the solution, the semicircle of the intermediate frequency region is the charge transfer resistance Rct between the SEI film and the solid electrode, and the straight line in the low frequency region is the diffusion resistance Zw of the lithium ion in the solid phase electrode material. It can be observed that the Rs of the two batteries are substantially the same. The Rct of SnO2/ACP electrode is 240Ω, and the Rct of SnO2/copper foil electrode is 510Ω. Showing that ACP replaces copper foil as current collector has a smaller charge transfer resistance, which is beneficial to increase the electron migration rate and reaction depth of the electrode during charge and discharge. 600

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Fig. 7 EIS of SnO2/ACP and SnO2/copper foil electrodes

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Conclusion In summary, aramid carbon nanotube (CNTs) paper (ACP) was prepared by combining aramid fiber with MWCNTs, and ACP was used as the current collector of SnO2 instead of copper foil. Owing to abundant pores and strong adsorption, after hot pressing, SnO2 can be easily penetrated into. SnO2 material and the current collector substrate can be more tightly combined, reduces the internal impedance of the battery, improves the specific capacity of the SnO2 material. Through the adjustment of MWCNTs distributed in the fiber voids, the volume expansion of SnO2 can be well suppressed. The SnO2/ACP electrode specific capacity was maintained at 876 mAh/g at the current density of 100 mA/g after 60 cycles, which was significantly higher than the specific capacity (88 mAh/g) of SnO2 coated on the copper foil. At a current density of 1600 mAh/g, the specific capacity can be kept at 119 mAh/g. And when the current density returns to 100 mAh/g, the specific capacity recovered to 820 mAh/g. Reference [1] Zhang Honghui, Shi Qingsheng. Study on critical materials of lithium-ion power battery in new energy vehicles[J]. Chinese Journal of Power Sources, 2013, 37 (5):877-879. [2] Kempton W, Letendre S E. Electric vehicles as a new power source for electric utilities[J]. Transportation Research Part D: Transport and Environment, 1997, 2(3):

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ACCEPTED MANUSCRIPT Highlight 1¡¢Preparation of aramid-carbon nanotube conductive paper as the current collector instead of copper foil of SnO2 material. 2¡¢The active material SnO2 is pressed into the aramid fiber and carbon tube structure by hot pressing process to improve the interfacial properties between them.

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3¡¢The prepared paper aramid-carbon nanotube conductive paper has both excellent mechanical properties of aramid paper and electrical conductivity of carbon tubes.

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4¡¢SnO2 lithium-ion battery exhibits excellent electrochemical performance and maintains good specific capacity.