电极界面浓差极化对锂金属沉积的影响

锂金属作为下一代高能量密度电池的理想负极材料受到研究人员广泛关注。然而,锂枝晶生长引起的安全隐患和循环寿命短等问题严重影响了锂金属电池的实用化进程。本文以电化学现象和理论为依据,从浓差极化角度详细分析锂金属电沉积过程中枝晶生长、死锂形成和全电池失效机制,并对目前研究较多的多孔宿主电极中的浓差极化及枝晶抑制进行分析,提出锂金属界面浓差电池现象。本文得到的结论为研究人员更深入地探究锂金属保护策略提供了理论依据。

Influence of Interfacial Concentration Polarization on Lithium Metal Electrodeposition

As an ideal negative electrode material for next-generation high-energy-density batteries, lithium (Li) metal has received extensive attention from the global research community. However, the safety hazards and short cycle life caused by the growth of Li dendrites have seriously hampered the application of Li metal batteries. Based on electrochemical phenomena and theory, this paper discusses the mechanism of dendritic growth, dead Li formation, and full battery failure from the perspective of concentration polarization. During the electrodeposition process, the consumption of Li ions on the surface induces concentration polarization. After the initial deposition, a relatively loose dendrite layer appears on the Li metal surface; the electrolyte can penetrate this dendrite layer to reach the dense Li metal surface. When the grown dendrites penetrate the concentration polarization layer, the interface concentration battery is short-circuited. In this case, the concentration difference battery tends to release all stored power and reach a potential balance between the high- and low-concentration regions, which causes the deposition of Li ions over the dendrites to reduce the ion concentration in the surrounding electrolyte. Meanwhile, the dissolution of Li ions that occurs at the roots of the dendrites increases the local ion concentration. This process accelerates the formation of a dead Li layer. A similar electrochemical process often occurs in columnar Li, as reported in other studies. When columnar Li is cycled several times, each Li column degenerates into a matchstick shape with a large head and thin neck. Therefore, eliminating concentration polarization is necessary for the application of columnar Li. Furthermore, in this work, concentration polarization and dendrite suppression in state-of-the-art porous host electrodes are analyzed. The larger specific surface area of the porous electrode greatly reduces the local current density on the electrode surface, which can reduce the interface concentration polarization and thus prevent dendrite growth. In charge-discharge cycling, a constant-voltage charging or shelving step is often inserted in each cycle in order to eliminate the influence of concentration polarization. However, if a dendritic layer has been formed on the Li metal surface after charging, in addition to the self-diffusion of ions, the self-discharge process of the interface concentration battery causes the detachment of the dendrite layer, thus resulting in the above-mentioned dead Li. Therefore, a larger amount of deposited Li yields a thicker Li dendritic layer, thus accelerating the capacity decay and failure of the battery, especially to those with high-capacity, high-voltage positive electrodes. The conclusions obtained in this paper can provide a theoretical basis for researchers to further explore Li metal protection strategies.