Effect of Crystal Orientation of Cu Current Collectors on Cycling Stability of Li Metal Anodes
Authors: Ishikawa Kohei , Ito Yasumasa , Harada Shunta , Tagawa Miho , Ujihara Toru 
Published: February 2020

Li metal anode is an attractive material for secondary batteries due to its high capacity of 3860 mAh/g. However, Li metal is well known for its hazardous behavior with air and moisture. Therefore, reducing the amount of Li metal in anodes is effective not only increasing energy density but also guarantee the safety of the batteries. For these reason, anode-free lithium batteries(AFLBs), in which Li electrodeposits onto Cu current collector, has been proposed by in-situ formation of Li metal1. Despite the potential increase in energy density of AFLBs comparing to conventional Li-ion batteries2, low coulombic efficiency due to non-uniform deposition nature of Li metal prevents AFLBs for practical applications. To address this issue, many methods such as high concentrated electrolyte, formation of protective layer, and electrolyte additives are developed, but AFLBs are still not realized. On the other hand, our previous work3 revealed that shape of Li precipitates is influenced by crystal orientation of Cu current collectors. This fact suggests that cycling stability of Li metal anode is also affected by the orientation of the Cu collector. Thus, in the present work, we performed cyclic voltammetry to investigate the transition of the characteristics.
The experiments of Li electrodeposition and stripping on single crystal Cu current collectors were conducted in a three-pole cell. The orientation of Cu are (111), (101), and (001). Counter and reference electrodes are both Li metal. Prior to experiments, the crystal orientation of collectors was analyzed by electron backscatter diffraction (EBSD). An electrolyte is consisted of 1 M LiPF6 in EC:EMC=5:5. Li was deposited at a current density of 0.5 mA/cm2 with charge capacity of 0.1 mA/cm2. Subsequently, the Li was stripped at same current density with 0.05 mA/cm2. Cyclic voltammetry was performed at a scan rate of 5.0 mV/s. SEM analysis was also conducted to examine Li morphology.
Figure 1 shows cyclic voltammograms of Li deposition and stripping on various Cu collectors. Negative and positive peaks represent Li deposition and stripping respectively. Each peak current decreases substantially as cycle proceeds, probably because of the development of the resistive SEI layer. The smallest overpotential for the first Li deposition was observed in Cu(111), followed by Cu(101) and Cu(001). Uniform formation of the resistive layer expectedly causes this since the SEM analysis revealed that Li morphology on Cu(111) is the most homogeneous. Moreover, Cu(111) delivered the lowest and the most stable overpotential over 10 cycle, while those in the other cases are larger and increase as the cycle progress. Therefore, it can we concluded that crystal orientation of collectors is an important factor for Li metal batteries and Cu(111) is effective for high cycle stability.

References

[1] B. J. Neudecker, et. al., J. Electrochem. Soc. 2000, 147(2), 517-523.
[2] J. Qian, et. al., Adv. Funct. Mater. 2016, 26, 7094-7102.
[3] K.Ishikawa, et. al., Cryst. Growth Des. 2017, 5, 2379-2385.

Figure 1