Rechargeable lithium-ion-batteries (LIB) are widely used in customer electronics due to their excellent properties such as high energy density and low weight. The present-day research is mainly focused on the enhancement of their energy density and life-times by designing new electrode materials or stable electrolytes. However, the role of a binder material should not be disregarded as it holds the powdered electrochemically active species and conductive additives into a mechanically stable electrode and therefore can strongly influence the electrochemical performance of LIB.
A standard binder material used in LIB that features good electrochemical inertness is poly(vinylidene fluoride) (PVDF). Unfortunately, PVDF trends to the electrolyte swelling and has insufficient conductivity that leads to the deteriorating of its adhesion to the electrode material upon long-term operation and an increase of the contact resistance between the active material and carbon material.
To overcome these problems researchers propose several binders of different structures. Nevertheless, synthesis of binders in an industrial production scale und of reasonably low cost remains an open race.
We suggest the poly(ionic liquid) polymers (PILs) as a binder material for LIB cathodes, which currently remain as a bottleneck for battery design.
Fig. 1: The chemical structures of three PIL binders.
TFSI denotes the counter anion, bis(trifluoromethane sulfonyl)imide.
In contrast to other polymer binders, PIL binders can be easily fabricated in industrial scale by means of free radical polymerization method and exhibit longer life-times compared to PVDF. Due to their conductive properties at the interface, PIL binders serve additionally as a Li+ conductive matrix and therefore improve the overall electrochemical performance of the battery by their binding and Li+ conduction function.
Fig. 2: Hypothetic schematic illustration of effect on PIL/electrolyte heterophase for high Li+ conductivity (right).
On the other hand, PVDF blocks the Li+ transport (left), therefore, it shows a higher resistance (=low Li+ flux) compared to the cathode composed with the PIL binders.
For the simplification, carbon additives are not described in the figure.
The electrochemical performance of PIL compared to PVDF demonstrates an improved cycling stability and specific capacity.
Fig. 3: Electrochemical properties of LiFePO4 electrodes (A-D) using different PIL binder (compared with PVDF) and electrochemical characterization (E-F).
(A) Charge–discharge curves at a current density of 1C (=170 mA g−1).
(B) Cycling performance up to 100 cycles at 1C.
(C) Cycling performance up to 1000 cycles at 5C (=850 mA g−1).
(D) Rate capability for PIL binders and PVDF.
The rate capability data of PVDF (the black line in (A), (B) and (C)) was taken from J. Yuan et al. J. Mater. Chem. A, 2015, 3, 7229 (active materials loading 2.5 mg).
(E) Cyclic voltammograms, and (F) electrochemical impedance spectra at room temperature.
The results shown above highlight the advantages of the PILs as binder materials in LIBs.
Jung-Soo Lee, Ken Sakaushi, Markus Antonietti, Jiayin Yuan: "Poly(ionic liquid) binders as Li+ conducting mediators for enhanced electrochemical performance", RSC Adv., 2015,5, 85517-85522. DOI: 10.1039/C5RA16535K