关键词: 锂金属二次电池, 金属锂负极, 枝晶, 异质微结构, LiC₆

Abstract:

Lithium metal has the highest theoretical specific energy density (3860 mAh∙g⁻¹) and the most negative redox potential (−3.04 V vs standard hydrogen electrode) among all alkali metals. These features have attracted the interest of battery researchers to put lithium metal into practical use in rechargeable batteries. However, lithium metal tends to deposit as dendritic or mossy morphology during the charging process, and such non-uniform deposition induces low Coulombic efficiency and poor cycling stability. In addition, dendritic metallic lithium can easily penetrate the separator, which causes internal short circuit and leads to severe safety issues. Thus it is important to control the electrodeposition process of lithium to inhibit the formation of Li dendrites. Surface modification of lithium is a widely adopted strategy that can induce uniform deposition of Li. In this paper, a LiC₆ heterogeneous interfacial layer is decorated on the surface of lithium metal anode. It is prepared in a simple manner by mechanically rolling graphitized carbon nanospheres on a Li foil. The increase in surface area by this LiC₆ layer can homogenize the current density on the surface of the lithium foil. Simultaneously, the electronegativity of LiC₆ can also homogenize the lithium ion flux. The effect of heterogeneous interface on the electrochemical plating and stripping behavior of lithium in carbonate electrolyte is also studied. Morphological characterization and electrochemical performance tests reveal that the LiC₆ heterogeneous interface can significantly improve the reversibility and uniformity of the electrochemical plating and stripping of Li, thereby inhibiting dendritic growth and maintaining the stability of the anode/electrolyte interface. Alternating current electrochemical impedance spectroscopy analysis determines that the solid electrolyte interface (SEI) impedance of bare lithium decreases from the initial 275 to 100 Ω as the deposition capacity increases, suggesting that severe rupture of the SEI is caused by the huge volume change after lithium deposition. On the contrary, the SEI impedance of the lithium foil with the LiC₆ heterogeneous interface layer remains nearly constant (from the initial 26 to 25 Ω after electrodeposition) indicates that the LiC₆ layer is able to inhibit dendrite growth and stabilize the interface. Thus, stable lithium plating/stripping over 300 h is achieved at a current density of 1 mA∙cm⁻² and at a fixed capacity of 1 mAh∙cm⁻² with a voltage hysteresis of less than 50 mV. The Li-LiFePO₄ full cell test demonstrates that the cycling stability of the modified lithium metal anode is superior to that of the bare one.

Key words: Lithium metal secondary battery, Lithium metal anode, Dendrite, Heterogeneous interface, LiC₆