Improving the Electrochemical Long-Term Stability of a Si Dominant Anode Using a Porous Cu Current Collector Fabricated By the Chemical Methods
Authors: Kim Subin , Choi Hongsuk , Song Hayong , Eom KwangSup 
Published: December 2020

Growth of consumer electronics such as portable devices, and electronic vehicles(EVs) has brought a huge demand of energy storage system with high capacity and superior rate capability[1]. Also, renewable energy sources should be combined with effective energy storage systems in order to maintain a good conversion efficiency. Among many energy storage systems, lithium ion batteries (LIBs) are the most predominant due to its high energy density and power density[2].
A lot of researches are focusing on developing anode and cathode material, which are main components that determine energy and power density of batteries. For anode material, a graphite is predominantly used in the commercial area. However, its low theoretical capacity (372 mAh g-1) is insufficient to meet the demand of large capacity for the next-generation battery.
In particular, silicon is one of the most promising anode materials because of its large theoretical capacity (4200 mAh g-1). However, it’s a large volume change of 300-400% during cycling causes a severe degradation of electrode performance and detachment of active materials from current collector especially in Si dominant anodes (Si content > 70%) [3]. To alleviate the volume change of silicon, most of researchers have focused on the synthesis of silicon particles which can be resistant to a huge volumetric change, such as ant-nest like structure [4], and yolk-shell structure [5], and the use of modified binder, which can buffer the structure change (for instance, introducing binder consisted of soft and hard polymer to give a high mechanical strength and self-healing behavior to electrode [6]). Moreover, the strategy using porous 3D current collectors has been adopted. Specifically, the 3D current collector can strengthen attachment between slurry and current collector, since it has a large contact area. However, this strategy, despite its importance and effectiveness, has not been fully understood compared to other strategies, especially in Si anode materials.
In this work, we study that the effect of adopting the 3D-structured copper current collectors(3D Cu CC) on the electrochemical performances of Si anode for Li-ion battery. Figure 1 schematically shows a fabrication method of 3D Cu CC used in this work. It was prepared by a facile two step method, chemical oxidation and thermal reduction. By immersing copper foil into the solution gained by mixing 10 M NaOH and 1 M ammonium persulfate in distilled water, the 3D structured copper oxide(CuO) was formed at the surface of copper foil. Subsequently, the thermal reduction process was conducted in a H2/Ar (5 v% of H2) atmosphere. Figure 2 shows the SEM surface images of (a) pristine copper foil(2D Cu CC) and (b) 3D Cu CC, cross-section images of (c) 2D Cu CC and (d) 3D Cu CC. The thickness of 3D porous layer is about 10 µm and several porous Cu balls, of which size is 3-5 µm, consist of the 3D Cu surface. Figure 3 shows the electrochemical cyclic performances of the Si on 3D Cu CC(Si@3D Cu CC) electrode and Si on pristine copper foil(Si@2D Cu CC). The Si@3D Cu CC shows the discharge capacity of 0.9 mAh cm-2 at the 300th cycles with current density of 1.0 mA cm-2 (loading mass of Si of 1.0 mg cm-2). Compared with the Si@2D Cu CC, it shows the improved cyclic performance for 300 cycles (440% higher retention rate).
In summary, the 3D Cu CC with high porosity was prepared by a facile chemical method in order to use as a CC for Si dominant anode, which showed the enhanced electrochemical performances compared to 2D Cu CC, probably due to the strengthened attachment between Si and 3D Cu CC. However, to clarify the mechanism and contributions of 3D Cu CC, surface analysis as well as the optimization will be performed, and then presented additionally.

References

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[4] An, Weili, et al. Scalable synthesis of ant-nest-like bulk porous silicon for high-performance lithium-ion battery anodes, Nature communications, 10.1 (2019): 1-11.
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Figure 1