Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
pubs.acs.org/journal/ascecg
Using Orthogonal Ploughing/Extrusion to Fabricate ThreeDimensional On-Chip-Structured Current Collector for Lithium-Ion Batteries Wei Yuan,* Baoyou Pan, Zhiqiang Qiu, Ziming Peng, Yintong Ye, Yao Huang, Honglin Huang, and Yong Tang
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School of Mechanical & Automotive Engineering, South China University of Technology, Wushan Road 381, Guangzhou 510640, China S Supporting Information *
ABSTRACT: The surface microstructure of the current collectors significantly affects the electrochemical performance of lithium-ion batteries (LIBs). This study shows that an effective method of orthogonal ploughing/extrusion is to fabricate three-dimensional (3D) microstructures directly on a copper plate used as the current collector for LIBs. Such an onchip-structured current collector avoids introducing additional layers or components, dispenses with complex fabrication and integration, as well as provides abundant surface structures and morphologies. It is simply combined with mesocarbon microbeads graphite powders to form the anode electrode of CR2032 coin half-cells. Results indicate that the prepared current collector facilitates a high reversible discharge specific capacity of 341.8 mAh g−1 after 200 cycles at a current rate of 0.2 C with a capacity retention rate as high as 98.6%. This performance is significantly higher than the traditional bare copper current collector which maintains only 262.2 mAh g−1 with a capacity retention rate of 79.6% at 0.2 C after 100 cycles. It is proven that the combined microstructures of grooves, burrs, and reentrant cavities formed on the surface of the 3D on-chip-structured current collector effectively improve the electrochemical performance of LIBs in terms of reversible specific capacity, cycling stability, electrical conductivity, and Coulombic efficiency. KEYWORDS: lithium-ion battery, current collector, 3D structure, ploughing/extrusion
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INTRODUCTION With the rapid development and massive production of mobile phones, laptops, electric vehicles, and various electronic devices, it is highly urgent to explore high-efficiency energy harvesting and storage devices.1−3 As a competent candidate, lithium-ion batteries (LIBs) have been extensively used as industrial solutions to power commercial products because of their advantages such as high specific capacity, good cycle performance, slow self-discharge rate, and long lifespan.4−6 The LIB is an electrochemical system comprised of the battery shell, electrode active material, separator, current collector, and electrolyte.7−9 The specific capacity, energy density, and other key properties of the battery are closely related to the active materials and corresponding three-dimensional (3D) structures.10−12 At present, the anode active material for commercial LIBs is mostly made of graphite such as natural graphite, mesocarbon microbeads (MCMB), acetylene black, and pyrolysis carbon. Among these materials, the MCMB is widely used because it has good electrical conductivity, stable structure, and low cost and is easy to obtain.13−15 However, for practical applications under long-term cyclic discharge−charge operation, a serious volume change inevitably occurs to the © XXXX American Chemical Society
graphite electrode material, making the active material particles fall off the surface of the current collectors. This will destroy the electrode structure inside the LIBs, eventually leading to attenuated battery capacity.16−18 To this end, it is essential to understand and address how to improve the bonding quality between the active materials and current collector, restrain the expansion of the active material to prevent peel-off from the surface of current collector, as well as reduce the contact resistance between these components. The current collector acts not only as a carrier for the active material but also as a collector and transporter for the current through the materials in the battery. Its surface structures directly affect the performance-related properties of LIBs, e.g., conductivity, discharge−charge capacity, and cycle lifetime.19 However, the anode current collector of commercial LIBs mostly uses either rolled or electrolytic copper foil that has a smooth surface, simple structure, and single scale. As a result, the electrode material is likely to peel off from the surface of Received: April 4, 2019 Revised: June 3, 2019 Published: July 5, 2019 A
DOI: 10.1021/acssuschemeng.9b01899 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. Schematic diagram of machining for the 3D on-chip-structured current collector.
micro-/nanostructures on the current collector is very complicated and tedious because of hazardous chemical reactions and additional treatments, which leads to very low production efficiency.25 Even for the use of additional surface layers attached on the current collector, it is difficult to realize accurate assembly to reduce the contact resistance. Besides, because of the incompatibility of the traditional slurry during the deposition process, the amount of active material storage inside the electrode is still limited.31 Moreover, it is hard for the common method to construct an effective interface with a mutual connection between the current collector and active materials. This may cause destruction of the layer of active materials due to poor adhesion between the two-phase interfaces.24 In order to improve the structure of current collectors, our team has validated the use of different structured current collectors to overcome the aforementioned drawbacks.26,32,33 However, for these cases, the production processes still involve time-consuming and high-cost chemical methods, which hinders practical application of the previous methods. With this background, this study introduces a novel machining method, e.g., orthogonal ploughing coupled with extrusion forming, to create microstructures directly on the surface of a copper plate with no need for additional layers or components. This method shows high efficiency, diversity, and multiscale in structure formation on the current collector, which can be easily implemented on a planar. By altering the parameters of the machining tool and processing procedure, we can purposely control the formed structure and morphology of the targeted object. Thus, this method provides a simple, controllable, and cost-efficient way to form microstructures directly on the current collector so as to enrich its surface morphological elements. The new current collector has diverse microstructures such as grooves, burrs, and reentrant cavities, which combine to increase the specific surface area and help constrain the volume expansion of electrode materials. The effects of these surface structures on the performance
the current collector and thus increase the internal resistance and yield poor performance of the battery especially during long cycles.20,21 Therefore, it is of great significance to enhance the surface functions of the copper current collector through a structuralization strategy to gain abundant micro-/nanostructures, especially for high-capacity anode materials.22 This shall greatly improve the contact between the active materials and current collector, and even increase the anode loading, thereby enhancing the capacity, performance stability, and operational safety of LIBs.5 To reach the above goals, researchers have reported a variety of methods to create current collectors with modified surface structures in different scales. Chen et al.23 used the ultrasonic synthesis method to produce uniform nanopores on the surface of flat copper. Then, a silicon film was sputtered on its surface to form an integrated electrode with an area capacity of 2.2 mAh cm−2. Because of its nanoporous structure, the electrochemical capacity retention rate and cycle stability of the anode were both improved. Chu et al.24 fabricated CuNW foils by pressing Cu nanowire fabrics as the anode current collectors of LIBs. It has a rough surface and porous structure which allow the graphite slurry to have better wetting and adhesion properties on the CuNW foils. In the full battery test, the area capacity reached 3 mAh cm−2 with a capacity retention rate of 83.6% at 0.6 C. Such 3D nanostructured current collectors can enhance the electrolyte permeability, improve the electron transport efficiency during the lithiation/ delithiation processes, thereby delivering a higher cell performance.24 Following this idea, many studies focus more on creating complex features (e.g., 3D, graphical pattern, voiding, multiscale)21,22,25−27 to achieve special functions (e.g., improving integral conductivity, enhancing mechanical intensity, mitigating expansion and pulverization, increasing specific surface area, and reducing impedance)5,19,28−30of current collectors. However, most of the reported current collectors have to face the following issues. First, the previous fabrication process of B
DOI: 10.1021/acssuschemeng.9b01899 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 2. Schematic diagram of the measured device for the cutting force during manufacturing the 3D on-chip-structured current collector. solution, hydrochloric acid, and deionized water for 5 min each step during the ultrasound environment, respectively. For the sake of exploring the electrochemical properties of the as-prepared current collectors, CMC, SBR, Super-P, and MCMB graphite powders (in the weight ratio of 2:2:3:93) were mixed to make a graphite slurry. The particle size of the MCMB distributes from 5 to 20 μm (Figure S2a,b), and the diffraction peaks of the MCMB sample are marked by a purple crystal face index, which are related to the carbon crystal (JCPDS file no. 41-1487). A working electrode was prepared by uniformly coating the anode slurry onto the current collector, as shown in Figure 1. For comparison, the bare copper plate with smooth surfaces were also treated through the same coating procedure. All the electrodes were dried in a vacuum infrared oven at 60 °C to completely remove the residual water. A coin half-cell (CR2032) was assembled in an argon-filled glovebox (O2, H2O levels maintained at
