Development of a Three-Dimensional Porous Scaffold Adopting Lithiophilic Silver for a High-Performance Lithium Metal Anode

Abstract

Lithium metal is the most promising candidate for a high-capacity anode in Li-ion batteries due to the highest theoretical specific capacity (3,860 mAh g-1) and the lowest redox potential (-3.04 V vs S.H.E.) [1]. However, the Li metal anode (LMA) is still challenging to be used practically due to mossy-like dendrite formation, dead Li growth, and infinite volume expansion resulting in safety hazards. Various approaches have been proposed to suppress Li dendrite growth, such as constructing an artificial solid electrolyte layer (ASEI) [2], modifying organic electrolyte [3], preparing lithiophilic substrates [4], and using a 3D scaffold [5].Among them, introducing a 3D scaffold is one effective method to prevent Li dendrite growth due to its high surface area to greatly reduce low local current density, based on Sand’s formula [6]. Furthermore, the 3D scaffold can accommodate Li metals inside a porous structure, reducing the cell volume expansion. In fact, owing to the advantages of a 3D scaffold, a variety of 3D scaffolds based on Cu [5], Ni [7], and carbon [8] have been reported over the past several years. However, the lithium metal is preferentially deposited on the top of the 3D scaffold since the top of the bare 3D scaffolds has low diffusion resistance than the bottom of the scaffold due to the short diffusion length. As one of the solutions, introducing lithiophilic materials to the bottom of the 3D scaffold can be regarded as an effective way to induce Li nucleation inside the porous scaffold [9]. However, since morphological properties like porosity, pore size, and thickness also affect the Li deposition behaviors inside the 3D scaffold [9, 10], both factors should be considered simultaneously.Here, we propose a rational design of a 3D porous scaffold for dendrite-free LMA, adopting lithiophilic silver on a current collector. Specifically, we suggest optimal structural properties of 3D scaffolds, such as porosity, pore size, and thickness to induce the Li nucleation inside the bottom of the current collector, thereby achieving inner space Li deposition. We deposited Ag nanoparticles with 20 nm evaporation thickness by a thermal evaporation method on the commercial copper foils. (thickness ≈ 25 µm). The 3D scaffold was obtained by simple mixing and casting processes. The Cu nanoparticles (Cu NPs), silica particles, and PVdF binder were added to PP bottles at a weight ratio of 1:1:0.1, and the mixture was mixed by a high-energy ball mill. A mixed sample was cast on the copper foil and Ag coated copper foil. Cu NPs were welded by heating at 350 ℃ in a H2/Ar flow (1:1=v:v) for 2h. Afterward, silica particles were etched with 5% hydrofluoric acid (aq). As a result, porous copper scaffold (pCu) and Ag coated porous copper scaffold (AgpCu) were fabricated for Li metal anodes.The SEM images of pCu are shown in Fig. 1a, b, and c from which we can see the uniformly distributed sphere-like pore generated through the etching of silica particles. The average diameter of the formed pore is around 1 µm, which is the same as the size of silica used in the experiment. Fig. 1c presents a cross-sectional SEM image of a pCu with a thickness of ~80 µm.The deposited Li morphology is investigated by optical and SEM images in Fig. 2. When Li is deposited at 0.5 mA cm-2 for 4 mAh cm-2, shiny Li is observed on the top of the pCu in optical images (Fig. 2a). In contrast with pCu, AgpCu shows a clean brown optical image on the top without Li metals in Fig. 2c. In addition, the SEM images of Fig. 2b and d shows the morphologies of Li deposits Li on the pCu and AgpCu, respectively, indicating that a silver inside layer in AgpCu effectively induces inner deposition of Li.