… significant effort has been focused on developing anodes with high specific capacity, such as Si and Li metal. Alternative battery technologies, such as Li−S and Li−O₂ batteries have attracted considerable attentions because of their high energy densities, and although they consist of different chemistries, Li−S and Li−O₂ systems utilize the same Li metal anode due to its low redox potential (−3.04 V vs standard hydrogen electrode) and high theoretical gravimetric capacity of 3861 mAh/g. However, issues associated with Li metal anodes, including uncontrollable dendritic Li growth and low Coulombic efficiency upon electrochemical cycling, have impeded their use in practical application. With the urgent demand of high-energy-density batteries, developing safe and stable Li metal anodes has become very pivotal but remains a significant challenge to chemists and materials scientists.

In addition to the work on electrolyte additives, solid-state electrolytes, and interfacial materials, modifying the separator has also been studied as a method to achieve stable Li metal anodes. In previous research, different ceramic and organic materials were used to coat a commercial separator. In this study, we propose a novel boron nitride (BN) nanosheet coated separator for stable Li metal anodes. In a typical commercial separator, Li wires are grown on the current collector during initial Li deposition. The Li wires consume a large number of electrolytes due to their large surface area, resulting in low Coulombic efficiency and eventual growth of dendritic Li. As shown in Figure 1b, we hypothesize that a thermally conductive BN coating can result in a uniform deposition/striping of Li due to the smaller total surface area of the initial deposited Li wires and a more homogeneous thermal distribution, decreasing the risk of dendritic Li growth and improving cycling performance.

—Luo et al.

Schematic of Li deposition/striping process with pristine separator (a) and BN-coated separator (b): (a) repeated Li deposition/striping cycles lead to the growth of dendritic Li; (b) BN coating leads to a uniform deposition/striping of Li. Credit: ACS, Luo et al. Click to enlarge.

To investigate electrochemical performance, the team used pristine polypropylene/polyethylene and BN-coated separators employed in Li/Cu cells; Cu foil is used as the working electrode, Li foil as the counter/reference electrode, and 1.0 M LiPF₆ in ethylene carbonate/ diethyl carbonate (EC:DEC = 1:1 by volume) solution was the electrolyte.

They found that not only did the BN-coated Li metal anode work well at a small current density, but that the cycling performance of the anode at high current densities was significantly improved by using the BN-coated separator.

They attributed the improved performance to the formation of fewer nuclei around which Li wires are grown on the Cu working electrode when using the BN-coated separator. The diameter of the deposited Li wires is much larger than that of the cell using pristine separator due to the fewer nuclei; the larger diameter of Li wires leads to a smaller surface area, and thus fewer electrolytes are consumed.

Further, the BN-coating on the polypropylene/polyethylene separator has a high thermal conductivity, which enhances the temperature uniformity during Li deposition/ striping. This leads to a more uniform growth/dissolution of Li and thus a better Coulombic efficiency and cycling performance.

Considering that Li metal anodes are very promising for high-energy-density batteries, the concept of using a thermally conductive separator can shed light on further research. Moreover, by combining spray coating or other roll-to-roll technologies, the thermally conductive separator could be realized in practical applications.

—Luo et al.

Resources

• Wei Luo, Lihui Zhou, Kun Fu, Zhi Yang, Jiayu Wan, Michael Manno, Yonggang Yao, Hongli Zhu, Bao Yang, and Liangbing Hu (2015) “A Thermally Conductive Separator for Stable Li Metal Anodes” Nano Letters doi: 10.1021/acs.nanolett.5b02432