A Rational Design of a Porous Copper Current Collector for Alkali Metal Anodes with a High Areal Capacity
Authors: Song Hayong , Cho Ki-Yeop , Kim Subin , Eom KwangSup 
Published: December 2020

Alkali metals are the ideal anode materials for improving the specific energy of secondary ion batteries because of the lowest redox potentials (3.04 and 2.71 VSHE for lithium and sodium, respectively) and the highest capacities (3860 and 1166 mAh g-1 for lithium and sodium, respectively). Nevertheless, using alkali metal anode is still challenging, because the dendritic growth and large volume changes of alkali metal anode cause many problems such as cell shorting, dead lithium, excessive SEI formation, and so on. Among the previously suggested solutions, the strategy to accommodate the Li or Na metal in the porous electrode has a great potential to address the aforementioned issues. [1] Particularly, the volume changes in the deposition and stripping processes of alkali metal can be minimized by adopting a porous electrode, and therefore increase the areal capacity of the anode. Furthermore, it is expected to increase the nucleation sites and impede the ununiform growth of specific position on the alkali metal by increasing the exposed surface area (that is, reducing the current density).
In designing an alkali metal anode using a porous electrode, the required characteristics of the electrodes to improve the electrochemical performances such as coulombic efficiency and specific energy of full-cell are considered as follows. Firstly, the weight of the porous electrode should not be too heavy. In the case of porous Cu electrode, the porosity determines the specific capacity, as shown in Figure 1(a), thus the porous electrode having low porosity rather reduces a specific capacity. [2] Therefore, in the case of porous Cu electrode, the specific capacity of the electrode including the mass of current collector is improved when the porosity is more than 85%. Secondly, the conductivity of the thickened porous electrode should be high and uniform to achieve less voltage hysteresis, because the larger voltage hysteresis (high overpotential) of anode reduces the specific energy and energy efficiency of full-cell. [3]
In this context, we focused on the control of structural characteristics of the porous Cu electrode including porosity, thickness, and surface area. As shown in Figure 1(b), the self-standing porous Cu sheet was fabricated by calendaring of the Cu/SiO2 mixture, and then followed by sintering of Cu and etching of SiO2.. Namely, the amount of SiO2 in the Cu/SiO2 mixture determines the porosity of the porous Cu electrode, leading to a facile control of the porosity of Cu.
The fabricated porous Cu current collector with 15 mg cm-2 could deliver a high areal capacity of 40 mA h cm-2 at a current density of 10 mA cm-2 with high coulombic efficiency (99.98%) and low voltage hysteresis (~20 mV), as shown in Figure 1(c). The required weight of the porous Cu electrode (Cu:SiO2=1:2) for storing 4 mAh cm-2 was only 1.5 mg, whereas the porous Cu electrode (Cu:SiO2=1:0) required 3.6 mg. Furthermore, the number of cycle for reaching maximum coulombic efficiency was shorter in high porosity (90%) compared to the lower porosity (30%~80%). Therefore, the outstanding performance would be due to the high porosity and conductivity of the porous Cu electrode providing multiple nucleation sites, as well as enough space for the growth of alkali metal. This work will provide insights into the rational design of the alkali metal anode with high areal capacity and low voltage hysteresis, by providing a facile method to control of structural characteristics of the Cu current collector.

References

[1] R. Zhang, N.-W. Li, X.-B. Cheng, Y.-X. Y Tin, Q. Zhang, and Y.-G. Guo, Adv. Sci., 2017, 4, 1600445
[2] D. Zhang, A. Dai, M. Wu, K. Shen, T. Xiao, G. Hou, J. Lu, and Y. Tang, ACS Energy Lett., 2020, 5, 180-186
[3] X. Gao, X Yang, K. Adair, X. Li, J. Liang, Q. Sun, Y. Zhao, R. Li, T.-K. Sham, and X. Sun, Adv. Energy Mater., 2020, 10, 1903753

Figure 1