  Development of new polymeric binders can help enable the use of silicon-rich
anodes in Li-ion batteries, by providing stronger adhesion to the active
material particles. The compositional features that improve interfacial
interactions and mechanical properties can often impart electronic conductivity
and redox activity to these polymers, which are generally seen as beneficial to
cell performance. Alternatively, it is also possible that the addition of
charge-transferring centers to the electrode can accelerate cell degradation.
Here, we use an aromatic polyimide (~320 mAh/g of reversible capacity) to
explore how a redox-active conductive polymer can affect cell performance. We
demonstrate that the lithiated polymer is less stable than the traditional
binders upon storage, leading to increased rates of calendar aging.
Furthermore, we show that the adhesion properties of the polymer deteriorate
upon repeated cycling, to an extent that is proportional to the degree of
delithiation of the binder. More critically, we show that progressive
degradation of the redox behavior of the polymer leads to the release of extra
Li+ into the cell, which can give the false perception of good performance even
under conditions of poor stability. Our work suggests that redox-active
conductive binders can sometimes be detrimental to cell performance, and that
works evaluating new polymers must include careful experimental validation
under realistic conditions.
