Non-Woven rGO Fibers Attached to Non-Woven Aramid Separator for High-Speed Charging and Discharging of Li Metal Anode

Abstract

Li metal is one of the best candidate anode materials for next-generation Li ion batteries (LIBs) and the most promising anode material that can replace the carbonaceous anode currently in use because of its high theoretical specific capacity and low redox potential (vs. SHE). Nevertheless, in order to apply Li metal to LIBs as an anode, issues such as those related to the cycle characteristics such as the life span and C-rate capability need to be resolved. These issues arise owing to the solid electrolyte interphase (SEI) formed on the surface of the Li metal anode (LMA). Unlike the conventional graphite anode, which exhibits a stable intercalation/de-intercalation mechanism, the Li metal anode, which utilizes a lithiation-delithiation mechanism known as hostless electrochemical plating/stripping to charge/discharge, undergoes repeated formation/collapse of an unstable SEI layer on its surface. Thus, the LMA is continuously exposed to undesirable interfacial reactions with the liquid electrolyte. The continuous occurrence of these side-reactions deteriorates the cycle characteristics of the LMA. In particular, this phenomenon gradually intensifies as the applied current increases, leaving the LMA operative within limited cycles at a high current density. Thus, the surface of the LMA needs to be subjected to additional treatments, based on studies of interfacial phenomena, for introducing it to LIBs, which many research groups attempted. Of the many approaches, studies that stabilization of the LMA surface by the introduction of a conductive interlayer between the LMA and separator or by applying a functional separator provided excellent results and the advantage using Li metal without structural modification. Similarly, a good interlayer or functional separator should be able to control undesirable interfacial reactions by the formation of the stable SEI layer, which prevents accumulation of an inactive layer and liquid electrolyte depletion, maintaining the cycle characteristics of LMA. In particular, when there is a conductive interlayer above the LMA, the conductive interlayer at the top structurally meets the Li+ ion flux first during the plating step. Therefore, the SEI layer formed is more stable on the conductive interlayer than on the LMA surface and could thus ensure surface stability of the LMA, preventing repeated SEI formation/collapse, dendritic growth, and liquid electrolyte depletion. In consideration of this point, in this work, we proposed non-woven type reduced graphene oxide fibers attached to aramid paper (rGOF-A) as an advanced separator to solve the aforementioned issues presented by an unstable SEI layer. When the rGOF side of rGOF-A contacts the Li metal anode, it functions effectively as a conductive frame, so the electrons can migrate from the underlying the LMA to the rGOF as the current is applied. Thus, the rGOF first meets the Li+ ion flux rather than the LMA, and the SEI layer, which has different chemical characteristics than those of the LMA surface, forms more stably mainly on rGOF in strong reductive conditions. In other words, rGOF can act a conductive layer and induces formation of the SEI layer in rGOF, not the LMA, helping toward stable operation of the LMA. In addition, this formed stable SEI layer can be effectively confined within the rGOF frame to have a high modulus. Moreover, as the electrolyte is consumed to form the SEI layer on the surface of rGOF, chemically reactive C–F bonds are generated at the surface of rGOF and the partially fluorinated rGOF surface induces the formation of LiF known as the component of the stable SEI during the Li+ ion plating process. LiF is a key component in a stable SEI layer on the LMA because it has a wide electrochemical stability window and improves the surface diffusion of ions, which can lead to smooth Li plating. Thus, LiF protects the LMA from further repeated SEI layer formation/collapse processes and helps the LMA to operate reliably. This passivation effect of rGOF on the LMA surface allows the LMA to maintain its cycle characteristics for a rapid charging/discharging condition (20 mA cm-2, 1 mAh cm-2) as well as at a higher areal capacity (20 mA cm-2,20 mAh cm-2) than practical application condition. Thus, the rGOF-A functional separator ensures cycling stability of the LMA without any deteriorating factor, which contributes to large Li source irreversibility, and this does not involve a structural change for the LMA or additional treatments such as those involving additives to maintain to the cycle characteristics or lifespan. Figure 1