Metal single-atom catalysts for stable Li metal anode: Principles, progress and perspectives

The ever-increasing penetration scenarios of electronic devices as well as electric vehicles have spurred the development of rechargeable batteries with high energy density, long cycle life, and high security. Compared with several other types of rechargeable batteries, Li-ion batteries have emerged as the most widely employed electrochemical energy storage system, attributed to their long cycle life, high operating voltage, small self-discharge, and environmental friendliness. Currently, most mature and commercial Li-ion batteries rely on graphite anode. However, the theoretical specific capacity of graphite anode is approaching the critical limit after many years of research, falling short of the high-energy-density demands required for cutting-edge electronic devices. Consequently, developing new-generation Li-ion batteries with high theoretical specific capacity and high energy density has become an irresistible trend. In this context, Li metal anode has been considered as one of the most promising candidates for next-generation Li-ion batteries, due to the ultrahigh theoretical capacity (3860 mAh g−1) and lowest redox potential (−3.04 V vs the standard hydrogen electrode). Unfortunately, the practical application of Li metal anode is primarily hindered by severe dendritic Li deposition, which stems from uneven Li-ion flux during battery operation. On one hand, uncontrollable dendritic Li growth can easily penetrate the separator, leading to short circuits, thermal runaway, cell combustion, and even explosions. On the other hand, massive needle-like Li dendrites with high specific surface area constantly cause rupture and rebirth of the structurally inhomogeneous and mechanically fragile solid-electrolyte interphase (SEI), leading to sustained consumption of electrolyte and Li ions during cycling. The above issues result in low Coulombic efficiency, poor cycle performance and even serious safety risks, precluding the practical use of Li metal anode. Various strategies have been proposed to address the aforementioned issues and improve the performance of Li metal anode, such as constructing three-dimensional (3D) porous scaffold/host for decreasing the local deposition current density and promoting uniform deposition, introducing functional additives or using new solvents for forming an anion-aggregated solvation shell and enhancing the interfacial stability and desolvation kinetics, building artificial SEI layers for modulating the transport of Li ions at the interface and alleviating volume changes. Metal single-atom catalysts (SACs), with atomically dispersed metal on a solid support, have advantages of maximum atom utilization efficiency, unique local atomic environment and high surface free energy and they demonstrate promising applications in the field of energy storage and conversion. Recently, SACs have been applied in Li metal batteries to tailor the lithiophilicity of substrates, regulate Li metal nucleation, modulate Li kinetic behavior, and avoid uneven Li deposition, so as to meet the harsh requirements in practical operating conditions. Although SACs have attracted significant attention and progress over the past decades, a comprehensive summary of their uses in Li metal batteries is currently lacking. In this review, we begin with a brief introduction of SACs and then comprehensively summarize the applications of SACs in Li metal anode from the following five aspects: (1) tailoring the lithiophilicity of substrates, (2) modifying the stability of atomic structure, (3) homogenizing Li-ion flux, (4) accelerating Li-ion desolvation kinetics, (5) enhancing the surface migration. Finally, challenges and opportunities for SACs in achieving future high-stability and long-cycling Li metal anode are discussed.