Abstract:

The applications of lithium-ion batteries have been limited because their energy density can no longer meet the requirements of an emerging energy society. Lithium metal batteries (LMBs) are being considered as potential candidate for next-generation energy storage systems owing to the high theoretical specific capacity and low electrochemical potential of lithium metal. However, the commercialization of LMB is limited due to several challenges, such as uncontrollable formation of dendrites, unstable solid electrolyte interface, and infinite anode volume change, which can lead to grievous catastrophe. In this study, several typical mechanisms of lithium dendrite formation and growth are summarized. The results suggest that a smaller current density, greater Li⁺ transference number, higher mechanical strength of the electrolyte, and a more homogeneous distribution of Li⁺ on the substrate are conducive to the uniform deposition morphology of lithium metal. In view of these results, combined with the researches on LMBs conducted in recent years, composite anodes can be summarized into three level from internal to external. (ⅰ) Internal composite of lithium metal anode: the scaffolds composited with lithium metal are classified as non-conductive (NC), electron-conductive (EC), ion-conductive (IC), and mixed ion and electron-conductive (MIEC) scaffolds. Composited with NC scaffolds, the tip effect can be weakened through the interaction between polar functional groups and Li⁺. The composite of lithium metal and EC scaffolds can effectively reduce the local current density, while IC scaffolds can increase the ion flux. However, the performance of LMBs may be hindered by the insulation of electrons or Li⁺ at high rates. In comparison, MIEC scaffolds can provide fast ion/electron transfer channels for the deposition or dissolution of lithium metal, which is beneficial for the electrochemical performance of LMBs even at high rates. (ⅱ) Internal composite of LMB: Compared with liquid electrolytes, solid-state electrolytes (SSEs) and quasi-solid-state electrolytes are much safer. However, their interfacial contact with lithium metal anodes has been seriously criticized. Lithium metal anodes can be composited with SSEs or quasi-solid-state electrolytes to optimize the interface contact performance and reduce the interface resistance, thereby promoting the development of solid-state batteries. (ⅲ) Composite of internal environment and external operating conditions: Composited with external physical fields, such as electric fields, magnetic fields, and temperature fields, the distribution of Li⁺ can be homogeneous and the initial nucleation process can be regulated. Overall, this review summarizes several composite anodes that have greatly optimized the performance of LMBs and highlights the potential of multi-level composites for applications in lithium metal anodes.

Key words: Lithium metal battery, Lithium dendrite, Composite anode, Internal structure, External operating condition, Multi-level composite