Introduction

The metallic lithium as an anode in a rechargeable battery was first explored by Whittingham in 1970s at Exxon and realized commercialization by Moli Energy in the late 1980s[1-3], but frequent accidents, including fires caused by dendrite formation, brought serious safety issues to public attention, which leaded to the Moli Energy company ultimately recall all the cells. The first commercialization of lithium-metal anode rechargeable batteries ended in failure. Then Sony company developed the graphite anodes to replace metallic Li anode, matched with the LiCoO2 cathode, successfully built reliable Li-ion cells that have been widely used until now[2, 4-6]. However, with the booming growth in consumer electronic devices and electric vehicles, even the state-of-the-art lithium ion batteries (LIBs) using graphite anodes (with a theoretical specific capacity of 372 mA·h·g–1) have almost reached their theoretical energy density (350 Wh·kg–1), cannot provide the high energy density required for their demands[7-10].
Therefore, recently, particularly in recent 10 years, the rechargeable lithium metal batteries using the Li anode has acquired unprecedented attentions as it has the unsurpassably highest theoretical capacity (3860 mA·h·g−1, or 2061 mA·h·cm–3) and lowest electrochemical potential (–3.04 V versus the standard hydrogen electrode)[2, 11-15]. Additionally, with rapid development of cathodes alternative to conventional intercalation cathodes used in state-of-the-art Li-ion batteries, i.e., the sulfur (S) cathode (with energy density of ≈2600 Wh·kg–1) for the Li-S battery and the oxygen cathode (with energy density of ≈11140 Wh·kg–1) for the Li-air battery[16, 17], have undoubtedly hastened the arrival of rechargeable Li metal batteries.
However, the safety issues by using the Li anode as well as its poor cycle stability and low Coulombic efficiency are still not overcome and hinds its practical applications. Tremendous efforts have been devoted to solving the notorious lithium dendrite problem by employing advanced electrolytes, separators, and novel electrode materials/structures. In the aspect of electrolyte, increasing the concentrations of lithium salts and adding inorganic or organic additives in electrolyte could not only benefit to stabilize the spontaneous solid electrolyte interphase (SEI) films to reduce the side reactions happening, but also control the nucleation and growth of metallic lithium, thus enhancing the stabilities of lithium anodes during the stripping and plating processes[18-22]. Recently, our group employed octaphenyl polyoxyethylene as an electrolyte additive to enable a stable complex layer on the surface of the lithium anode. This surface layer not only promoted uniform lithium deposition, but also facilitated the formation of a robust SEI film[23]. While developed new type of modified separators are expected to physically suppress the growth of lithium dendrites. For instance, in situ fabricating a stable tissue-directed/reinforced bifunctional separator/protection film (TBF) on the surface of the Li anode effectively protected lithium from the corrosion of O2, discharge intermediates, H2O and electrolyte, and reduce morphology change of the surface of lithium anode[24]. In terms of novel electrode materials/structures, infusion the Li metal into a carbon framework or synthesis of lithium-based composites[25-27], enables to control the growth of lithium dendrites, but also solves the infinite volume change issue of lithium-based electrodes[18, 28]. For instance, Koratkar and co-workers described defect-induced plating of metallic lithium within the interior of a porous graphene network[27]. The network acted as seed points that initiated plating of lithium metal and prevented dendritic growth.
Based on the aforementioned considerations, stable operation of Li metal anodes, no matter inhibit the lithium dendrite growth or controllable side reaction and volume change, is critical for next-generation battery technologies. Thus, in this short perspective article, we briefly review the Li-containing alloys reported in the literatures to stabilize Li metal anodes and propose a few new suggestions for protecting the Li metal by reasonably combining the lithium-containing alloys with other strategies.