Dehydrofluorination Process of Poly(vinylidene difluoride) PVdF-Based Gel Polymer Electrolytes and Its Effect on Lithium-Sulfur Batteries

Abstract

Gel polymer electrolytes (GPEs) are emerging as suitable candidates for high-performing lithium-sulfur batteries (LSBs) due to their excellent performance and improved safety. Within them, poly(vinylidene difluoride) (PVdF) and its derivatives have been widely used as polymer hosts due to their ideal mechanical and electrochemical properties. However, their poor stability with lithium metal (Li0) anode has been identified as their main drawback. Here, the stability of two PVdF-based GPEs with Li0 and their application in LSBs is studied. PVdF-based GPEs undergo a dehydrofluorination process upon contact with the Li0. This process results in the formation of a LiF-rich solid electrolyte interphase that provides high stability during galvanostatic cycling. Nevertheless, despite their outstanding initial discharge, both GPEs show an unsuitable battery performance characterized by a capacity drop, ascribed to the loss of the lithium polysulfides and their interaction with the dehydrofluorinated polymer host. Through the introduction of an intriguing lithium salt (lithium nitrate) in the electrolyte, a significant improvement is achieved delivering higher capacity retention. Apart from providing a detailed study of the hitherto poorly characterized interaction process between PVdF-based GPEs and the Li0, this study demonstrates the need for an anode protection process to use this type of electrolytes in LSBs.

Multifunctional Janus Separators for Safer and Dendrite‐Free Lithium‐Metal Batteries

“…Poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP) is widely investigated as polymer for both solid and gel electrolytes, due to several merits, such as good mechanical properties and thermal stability. Despite such copolymer can undergo to dehydrofluorination side reactions, [23] promoted by the reactivity of PVDF‐HFP with Li metal over cycling, some unexpected improvements of the cell stability have been observed in literature that were interpreted in terms of a LiF‐rich SEI formation on the surface of the Li metal anode [23,24] . This beneficial phenomenon can be further triggered by garnets‐like filler (e. g. LZTO) in case of PVDF‐HFP‐based composite used as separators or gel electrolytes [25] …”

“…Despite such copolymer can undergo to dehydrofluorination side reactions, [23] promoted by the reactivity of PVDF-HFP with Li metal over cycling, some unexpected improvements of the cell stability have been observed in literature that were interpreted in terms of a LiF-rich SEI formation on the surface of the Li metal anode. [23,24] This beneficial phenomenon can be further triggered by garnets-like filler (e. g. LZTO) in case of PVDF-HFP-based composite used as separators or gel electrolytes. [25] Three different Janus separators were obtained by changing the weight composition and the nature of the carbon-based material (e. g., Nitrogen-doped Carbon Quantum Dots, N-CQDs, or multi walled carbon nanotubes, MW-CNTs) in order to tune the electric resistivity of the Layer B (Table 1).…”

A Review of the Relationship between Gel Polymer Electrolytes and Solid Electrolyte Interfaces in Lithium Metal Batteries

“…Common polymer substrates include polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyacrylonitrile (PAN), polyethylene oxide (PEO), polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), etc. [39][40][41][42]. Castillo et al [43] introduced polyethylene glycol dimethyl ether (PEGDME) and LiTFSI into PVDF-HFP to prepare GPE (Figure 1a).…”