Stress-relieved Si anode on a porous Cu current collector for high-performance lithium-ion batteries
Graphical abstract
Introduction
In lithium-ion batteries (LIBs), graphite with a theoretical capacity of 372 mAh g−1 has been utilized as an anode material [[1], [2], [3], [4], [5]]. However, novel anode structures and materials need to be proposed to increase the capacity of LIBs. The electrochemical behaviors of Li+ ion in non-carbon anode materials can be categorized as follows: the insertion reaction of xLi+ + MyZ → LixMyZ e.g. in lithium titanate, alloying reaction of xLi+ + M → LixM e.g. in silicon (Si), and the conversion reaction of xLi+ + MaNb → aM + bLixM e.g. in copper oxide [[6], [7], [8], [9], [10], [11], [12], [13]]. In particular, the high-capacity anode materials such as Si and Ge with the alloying process have been typically used [[14], [15], [16], [17], [18], [19]]. However, during the alloying process, the anode material such as Si showed cracking of the electrode due to the volumetric expansion, resulting in the deteriorated LIB performance [[20], [21], [22], [23], [24]].
In particular, to overcome the mechanical stress of the Si anode generated during the cycling process in LIBs, a 3D network electrode structure with electronic conducting polymer-coated Si nanoparticles (NPs) was formed using a composite of Si-based material and conducting polymer hydrogel [25]. The composite anode exhibited a retention rate of >90% measured at 6 A g−1 for 5000 cycles due to the increased electrical conductivity and the relief of the volumetric expansion. Recently, the mechanical stress generated on an Si anode was relieved by controlling the structure of the current collector. Cho et al. fabricated a Si electrode with hole patterns using a sputtering method, representing the enhanced LIB performance due to the stress-relieved structure with the gap [26]. Herein, we studied the effect of a porous Cu current collector on the intercalation of Li+ ion into an Si electrode fabricated using sputtering deposition method. For comparison, the Si electrode on a flat Cu current collector was prepared. During the cycling process, the Si anode on the porous Cu showed the relief of the mechanical stress due to the structural effect of the current collector, whereas the Si anode formed on the flat Cu exhibited the cracking of the electrode due to the mechanical stress.
Section snippets
Fabrication of Si electrodes
The Si electrodes on the porous Cu substrate (Fukuda Corporation) and flat Cu substrate (Hohsen Corporation) used as current collectors were fabricated using a radio frequency magnetron sputtering deposition method with a Si sputtering target (99.999%, LTS chemical) at 25 °C. Porous Cu substrate has an aperture ratio of 15% and hole size of 0.35ϕ. The sputtering process was conducted at an Ar flow rate of 30 SCCM with an RF power of 60 W for 90 min under a working pressure of 1.1 × 10−2 torr.
Structural characterization
Results and discussion
Fig. 1 shows photographic, SEM, and EDX mapping images of the as-deposited Si electrodes on the flat and porous Cu substrates used as current collectors in LIBs (denoted as flat Cu/Si and porous Cu/Si, respectively). The flat Cu/Si and porous Cu/Si exhibited a well-deposition and homogeneous distribution of Si on the Cu current collectors. The thickness and average loading amount of flat Cu/Si and porous Cu/Si were 110–130 nm and 70–80 μg cm−2, respectively (Fig. 2(a) and (b)). The silicon
Conclusions
In summary, the Si anode for LIBs was fabricated on a porous Cu current collector having holes using a sputtering deposition method. The porous Cu/Si showed the superior high-rate and cycling performance compared to the Si anode on the flat Cu. The excellent high-rate performance and stable cyclability of the porous Cu/Si can be predominantly attributed to the relief of the stress due to the volumetric expansion of the Si anode on the porous Cu current collector.
Acknowledgments
This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation funded by the Ministry of Science, ICT (NRF-2017M1A2A2086648).
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These authors equally contributed to this work.