posted on 2024-01-31, 10:39authored byPratyusha Das, Charlene Z. Salamat, Billal Zayat, Rodrigo Elizalde-Segovia, Yunfei Wang, Xiaodan Gu, Sri R. Narayan, Sarah H. Tolbert, Barry C. Thompson
The
driving force behind the significant advancement of conductive
polymer binders for lithium-ion batteries (LIBs) stems from the poor
binding strength, limited mechanical properties, and absence of electronic
conductivity of the commonly used nonconjugated polymer binder, poly(vinylidene
fluoride) (PVDF). With a goal to induce stretchability and deformability
to the otherwise brittle conjugated backbone, we report here dihexyl-substituted
poly(3,4-propylenedioxythiophene)-based (PProDOT-Hx2-based)
conjugated polymers wherein conjugation break spacers (CBS, T-X-T) of varying alkyl spacer lengths (X = 6, 8, and 10) and varying contents (5, 10, and 20%) have been
randomly incorporated into the PProDOT backbone, generating a family
of nine random PProDOT-CBS copolymers. Electrochemical characterization
revealed that three out of the nine PProDOT-CBS polymers (5% T-6-T,
5% T-8-T, and 10% T-6-T) are electrochemically stable over long-term
cycling of 100 cycles. The electronic conductivity of the PProDOT-CBS
polymers is consistent with previous literature reports on CBS polymers,
where a decline in charge carrier mobility is observed with an increase
in CBS content and spacer length, although no significant difference
in ionic conductivity in these polymers was observed. This is supported
by GIWAXS studies indicating a decrease in lamellar peak intensity
with increasing CBS content and spacer length. Mechanical properties
of the three selected PProDOT-CBS polymers were investigated by using
the established “film-on-water” technique and a novel
“film-on-solvent” technique that we report here for
the first time, where the solvent used is the same as employed in
the battery electrolyte. Both techniques showcase a generally lower
tensile modulus (E) and higher crack onset strain
(COS) of the PProDOT-CBS polymers relative to fully conjugated PProDOT-Hx2. Furthermore, significant enhancement in mechanical properties
is observed with the “film-on-solvent” method, suggesting
that electrolyte-induced swelling has a plasticizing effect on the
polymers, accounting for their increased stretchability and deformability.
Finally, cell testing of the PProDOT-CBS polymers with NCA cathodes
aligned well with the electrochemical and mechanical studies, where
a higher crack onset strain was crucial for higher capacity retention
during long-term cycling. On the contrary, rate capability measurements
proved that higher electronic conductivity is favored over mechanical
properties during high rates of discharge. This work illustrates that
the strategic introduction of CBS units into conjugated polymer binders
is a viable method for the generation of stretchable conductive polymer
binders for emerging high-capacity electrodes in LIBs.