Ionic liquid based Fluoropolymer solid electrolytes for Lithium-ion batteries
João P. Serra, Rafael S. Pinto, João C. Barbosa, Daniela M. Correia, Renato Gonçalves, M.M. Silva, S. Lanceros‐Méndez, Carlos M. Costa
Abstract
Composite materials based on ionic liquids (ILs) / poly(vinylidene fluoride) (PVDF) and their copolymers have emerged as an interesting approach to develop high ionic conductivity solid polymer electrolytes (SPEs) for lithium-ion battery application. This work reports the development of SPEs based on fluoropolymers, PVDF and poly(vinylidene fluoride co-hexafluoropropylene), PVDF-HFP, containing different ILs: 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide ([EMIM][TFSI]) and 1-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN]). The influence of IL type and content on the morphological, wettability, physical-chemical, thermal, mechanical and electrochemical properties were evaluated. It is shown that, independently of the polymer matrix, both ILs improve surface wettability and increase the polar β phase content and crystallinity degree of the polymers. The mechanical properties show that the incorporation of IL in the composites results in a plasticizing behaviour. Further, the [BMIM][SCN]/PVDF-HFP with 40 wt.% IL content shows the best room temperature ionic conductivity of 0.15 mS.cm-1 with excellent electrochemical stability in the 0.0 - 5.0 V potential window. The initial discharge capacity value at C/8-rate is 148 mAh.g-1 and 124 mAh.g-1 for the [BMIM][SCN]/PVDF and [BMIM][SCN]/PVDF-HFP composites, respectively, with high coulombic efficiency (98%). At C/8-rate, batteries with [BMIM][SCN]/PVDF-HFP show the lowest capacity fade (16 % after 50 cycles) of the prepared composites. Thus, it is demonstrated the suitability of developing SPEs based on IL and fluorinated polymers for the next generation of solid-state room temperature lithium-ion batteries.