Figure 6 Schematic of the Li plating process on the (a) 3D Cu foil and (b) 3D Cu@Al foil. (Reproduced from ref.[104], with permission from Copyright © 2019 Wiley-VCH.) (c) Schematic illustration of the proposed lithium nucleation and deposition processes on a bare Cu foil (left) and a Zn modified Cu foil (right), (d) Cycling performance of Li-S full cells tested at 0.2 C for the first 3 cycles and at 0.5 C for the subsequent cycles with Li pre-deposited on a pristine Cu foil or a Zn coated Cu foil as the anode. (Reproduced from ref.[105], with permission from Copyright © 2019 Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature.) (e) Schematic illustration of the preparation process of PW@3D-Li, SEM images of (f) Au-G/Cu foam, (g) 3D Li and (h) PW@3D-Li, Lower-left inserts in (f), (g) and (h) represent optical photographs of Cu foam, 3D-Li and PW@3D-Li, respectively. (Reproduced from ref.[25], with permission from Copyright © 2019 Elsevier B.V.)
Trapping Li into three-dimensional (3D) conductive host to construct 3D-Li is an effective strategy to suppress the growth of Li dendrites. However, the increased contact area between 3D-Li and electrolyte unavoidably induces more side reactions to further deteriorate the electrochemical performance of lithium metal batteries. In order to keep the advantages of 3D lithium-alloys matrix as well as reduce the side effect, recently, Qu’s group construct a paraffin wax (PW) coating Au-graphene/Cu foam current collector[25]. Such a unique structure was able to effectively avoid the growth of Li dendrites and formation of “dead Li” during the Li plating process, which could be attributed to these merits: i) Au could react with Li to form Lix Au alloys that lowered Li nucleation overpotential and interfacial energy to effectively inhibit the formation of dendritic Li; ii) 3D lithiophilic graphene decreased the local current density and enabled the homogeneous growth of 3D Li; iii) Cu skeleton not only afforded interconnected pores to accommodate the volume variation of 3D-Li, but also served as a robust support to avoid the collapse of overall electrode especially during fast plating/stripping processes; iv) the PW protection layer confined Li during the plating and stripping to mitigate the corrosion of electrolyte and depress the formation of Li dendrites and “dead Li”.

3.3 | Artificial protective layers for lithium alloys anodes

Even using the 3D matrix to host the Li-containing alloys, unfortunately, these materials still suffer from poor SEI stability, resulting in unsatisfied electrochemical performances[48]. Therefore, constructing an artificial protective layer for lithium alloys anodes has been proposed. For example, Cui’s groups reported two methods to construct artificial-SEI layer of LiF to protect Lix Si alloy nanoparticles via reducing 1-fluorodecane and fluoropolymer CYTOP, respectively[107, 108]. Ci et al., reported a Li-O2 coin cell with the LiAlx anode experienced a high-current pretreatment[109], as a result, the SEI film (including Al2O3, LiF, ROCO2Li, LiOH, and Li2CO3) formed after the pretreatment process facilitated the uniform Li+shuttling during the following Li plating/stripping process and stabilizes the LiAlx anode interface even after hundreds of cycles. The LiAlxanode in lithium oxygen batteries could increase cycling to 667 cycles under a fixed capacity of 1000 mA·h·g−1 compared to 17 cycles of LiAlx anode without pre-treatment. Recently, Zhang’s group used a Li-Na alloy and 1,3-dioxolane (DOL) as anode and additive, respectively, to control dendrite growth and buffer the volume expansion of the alloy anode[85]. The 1,3-dioxolane additive could in situ react with Li-Na alloy to form a robust and flexible passivation film that suppress dendrite growth, buffer alloy anode volume expansion, prevent cracking. As shown in Figure 7a-7d, only the Li-Na alloy electrode with DOL additive existence can effectively suppress dendrite growth and wouldn’t crack after cycling.