Unraveling the Influence of Li+-cation and TFSI−-anion in Poly(ionic liquid) Binders for Lithium-Metal Batteries

Abstract

Ion transport in composite electrodes plays a key role in the electrochemical performance of lithium-metal batteries (LMBs), particularly at high current densities, and hence, some works have suggested the use of ionic conducting polymers as binders. Herein, in order to assess the importance of the type of ion conduction in binders, two poly(ionic liquid) polymers were analyzed as binders in LiFePO4 (LFP) cathodes: poly(lithium 1-[3-(methacryloyloxy)propylsulfonyl]-1-(trifluoromethane sulfonyl) imide) (PMTFSI−Li), and poly(diallyldimethylammonium bis(trifluoromethane sulfonyl)imide) (PDADMA−TFSI). Their functionalities allow modulating the individual transport of their counter-ions, Li+ and TFSI−, respectively; in comparison with conventional PVDF binder. Thus, LFP−C-Binder cathodes, namely C−PVDF, C−PDADMA−TFSI and C−PMTFSI−Li, were evaluated in LMBs. C−PMTFSI−Li exhibited the best performance reaching the theoretical specific capacity (170.3±0.8 mAh g−1) at C/10, an outstanding capacity at 10 C (100.6±0.5 mAh g−1), and long lifespan (>500 cycles at 1 C). C−PDADMA−TFSI showed good long-term cycling and high performance at high C-rate, while C−PVDF ended up fading before reaching 500 cycles. Surprisingly, it was observed that the presence of ionic binders into the cathode formulation influenced on Li0 metal deposition morphology, leading to a more homogeneous plating (specially PMTFSI−Li) in comparison with PVDF; and therefore, exhibiting a mitigation of mossy lithium growth.