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.

Carrageenans as Sustainable Water-Processable Binders for High-Voltage NMC811 Cathodes

“…Furthermore, the sulfonate groups can be acting as lithium carriers during the charge and discharge of the battery. Therefore, by having a higher number of sulfonate groups in the structure, the lithium mobility may be boosted through ionic pathways . In addition, the larger number of sulfonate groups present in the 3SO 3 – Carr binder may contribute to the protection of the NMC811 particles against water, achieving a more stable cycling than the 1SO 3 – Carr and 2SO 3 – Carr .…”

“…Therefore, by having a higher number of sulfonate groups in the structure, the lithium mobility may be boosted through ionic pathways. 29 In addition, the larger number of sulfonate groups present in the 3SO 3 – Carr binder may contribute to the protection of the NMC811 particles against water, achieving a more stable cycling than the 1SO 3 – Carr and 2SO 3 – Carr . This effect could be similar to the one observed by Heidbüchel et al 30 that have recently reported the positive impact of the addition of Li 2 SO 4 during the aqueous processing of NMC811 cathodes since a protective coating around the active material was observed by XPS measurements.…”

Current Trends and Perspectives of Polymers in Batteries

“…However, PVDF has a high fluorine content and requires the usage of the toxic and expensive organic solvent N -methyl-2-pyrrolidone (NMP) for the electrode processing and elevated temperatures for the drying step, making the cathode manufacturing process environmentally unfriendly. As an alternative to PVDF, low-fluorine-content poly­(ionic liquids) or anionic single-ion lithium conducting polymers are demonstrating enhanced cycling stability and capacity values in Li–air and solid-state batteries. − As a recent example, pyrrolidinium-based poly­(ionic liquid)­s were reported as cathode binders in NMC cathodes which showed improved capacity values, rate performance, and cycling stability, In particular, PDADMA–CFSO showed a cell capacity increase of 26% at 5 C when compared to PVDF.…”

Fluorine‐Free Poly(ionic Liquid)s Binders for the Aqueous Processing of High‐Voltage NMC811 Cathodes

“…[14,15] An interesting approach to binder formulations is the use of poly (ionic liquids)s, which contain ionic moieties in the polymer structure and, if chosen carefully, may improve the lithium diffusion and the long-term stability. [16][17][18][19][20][21][22] This subclass of polyelectrolytes have aroused interest as binder components given the possibility of improving the ionic conduction pathways within the electrode, the possibility of forming strong bonds between the active and conductive material, [23] and the flexibility provided by the polymer backbone. [24] One of the most studied poly(ionic liquid)s in energy storage applications is based on poly(diallyldimethylammonium) (PDADMA) with different counteranions.…”