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Magnetic field regulating the graphite electrode for excellent lithium-ion batteries performance Li Zhang, Mingyang Zeng, Dandan Wu, and Xingbin Yan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06358 • Publication Date (Web): 18 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019
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Magnetic field regulating the graphite electrode for excellent lithium-ion batteries performance Li Zhang a, b*, Mingyang Zeng a, b, Dandan Wu a, b, Xingbin Yan b, c, d * a
Department of Physics, School of Science, Lanzhou University of Technology, 287
Langongping Avenue, Lanzhou, Gansu, 730050, P. R. China. b
Laboratory of Clean Energy Chemistry and Materials, State Key Laboratory of Solid
Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, 18 Tianshui Avenue, Lanzhou, Gansu, 730000, P. R. China. c Center
of Materials Science and Optoelectronics Engineering, University of Chinese
Academy of Sciences, 19 Yuquan Avenue, Beijing, 100080, P. R. China. d
Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, 457 Zhongshan Avenue, Dalian, Liaoning, 116000, P. R. China.
* Corresponding
Authors.
E-mail:
[email protected] (L. zhang) and
[email protected] (X. B. Yan).
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Abstract: Low power density limits the prospects of lithium ion batteries in practical application. In order to improve the power density, it is very important to optimize the structural alignment of electrode materials. Here, we study the alignment of the graphite flakes by using magnetic field and investigate the impact of the preparation conditions on the degree of alignment. It is found that the higher degree of alignment brings about the shorter Li+ transmission paths and facilitates Li+ diffusion in path, which leads to the greater rate performance of lithium ion batteries. The specific capacity of verticallyaligned electrodes with loading of 8.9 mg cm-2 can reach 59.1 mAh g-1 at 2C, which is 4.5 times higher than that of reference electrode. In addition, we obtain a find that quantitative relationship between the specific capacity at 2C and the angle of alignment, i.e the specific capacity increases by 0.58 mAh g-1 when the angle increases by 1°. This study not only can help us understand the mechanism of magnetic alignment technology, but also provides a reliable experimental basis for the application of magnetic alignment technology as an universal approach for constructing structure of various electrode materials.
Keywords: magnetically aligned graphite flakes, electrode, rate performance, lithium ion batteries
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Introduction As the global environmental pollution and energy shortage become more serious, it is urgent to develop new chemical energy storage devices with sustainable development. In many chemical energy storage devices, lithium ion batteries (LIBs) have the unparalleled advantage in portable equipment and vehicles due to their high energy density, high operating voltage, and long cycle life.1-5 The energy storage of LIBs is achieved by the intercalation and de-intercalation of Li+ in electrode materials during the charging and discharging process.6 However, due to slow diffusion rate of Li+, especially at high rate and in highly loaded electrode, LIBs usually exhibit the poor rate capability which seriously restricts the development and application of LIBs.7-9 Thus, improving the diffusion rate of Li+ in electrodes has been a significant amount of efforts made in the field. Up to now, most researches are focused on the development of new materials to compete with commercial materials, such as two-dimensional materials, which provide large space for Li+ to diffuse between layers. However, new materials not only require a lot of manpower and material resources in the process of industrialization, but also cannot exhibit similar performance to commercial electrode materials in a short time. In fact, the design of the electrode structure, which builds the shorter ion path for Li+ moving across the electrodes, is an effective approach to increase the energy density and rate capability of LIBs without changing the chemical composition. Traditionally, the electrode materials are aligned along the plane of electrodes due to the tableting method used in the electrode preparation process, which is not beneficial for ion transport (as shown in the Fig.1a). In comparison with the in-plane alignment, the electrode materials with vertically-aligned structure for electrodes have the following advantages which improves the 3
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efficiency and circulation stability of electrodes (as shown in the Fig.1b).10-12 The advantages of this structure: (1) it is favorable for rapid diffusion of Li+, especially at high current rate and in highly loading of electrode materials;13-14 (2) the structure can accommodate large volume change during the charging and discharging process;15-16 (3) the direct contact between electrode materials and current collector is more sufficient, which improving the conductivity of the whole electrode and electronic collection efficiency of the battery on account of shortening the transport path for electrons in the vertical direction;17 (4) it lead to sufficient contact between electrolyte and electrode, which increases the amount of the electrochemical activity sites. Thus a variety of strategies have been developed to prepare the electrode materials with the vertically-aligned nanostructures, such as mechanical shear stress,18 electrical fields,19 or chemical vapor deposition (CVD)20, 21 et al. However, these methods are difficult to achieve large-scale application due to the complicated production process, security issues or restrictions on materials.18,
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Magnetic alignment
technology, which has the advantage of simplicity for the manufacture and universality for various materials, such as graphene,25 carbon nanotubes,26-27 and nanoparticles chains,28 nickel nanowires29 and liquid crystalline epoxy thermosets30 etc., has gradually attracted people’s attention. In addition, as the magnetic field is used as a kind of driving force for the self-assemble of materials, it can avoid direct contact with materials and associated issues, such as electrode contamination. Furthermore, it is also facile to tune the experimental parameters related to magnetic field, so magnetic field is capable of self-assembling material and realizing large scale of production. Therefore, the magnetic alignment is an effective technique to obtain the vertical-aligned electrode. The advantages of the magnetic alignment technology have been demonstrated in recently literature. For example, Villevieille group prepared the graphite negative electrode with vertically4
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aligned structures by magnetic field.31 The electrochemical performance of aligned graphite electrode with a thickness of 200 µm and high loading of 10 mg cm-2 has the specific charge up to three times higher than that of non-architecture electrodes at a rate of 1C. In addition, Y. M. Chiang and co-workers prepared LiCoO2 electrode with vertically-aligned structures by magnetic templating method, for which the area specific capacity >12 mAh cm-2 is over 3 times higher than conventional electrode materials (
