Revealing the Role of Poly(vinylidene fluoride) Binder in Si/Graphite Composite Anode for Li-Ion Batteries

Abstract

The conventional polyvinylidene fluoride (PVDF) binder works well with the graphite anode, but when combined with silicon in composites to increase the energy density of Li-ion batteries, it results in severe capacity fade. Herein, by using scanning electron microscopy and energy-dispersive X-ray spectroscopy analyses, we reveal that this failure stems from the loss of connectivity between the silicon (or its agglomerates), graphite, and PVDF binder because of the mechanical stresses experienced during battery cycling. More importantly, we reveal for the first time that the PVDF binder undergoes chemical decomposition during the cycling of not only the composite but also the Si-only or even graphite-only electrodes despite the excellent battery performance of the latter. Through X-ray photoemission electron microscopy and X-ray photoelectron spectroscopy techniques, LiF was identified as the predominant decomposition product. We show that the distribution of LiF in the electrodes due to the differences in the interactions between PVDF and either Si or graphite could correlate with the performance of the battery. This study shows that the most suitable binder for the composite electrode is a polymer with a good chemical interaction with both graphite and silicon.

On the Solubility and Stability of Polyvinylidene Fluoride

“…The longer-term effect of PVDF with these alkaline materials in the cell is still not known, and the degradation or cell failure mechanisms over the lifetime may be accelerated in these alkaline cathode-containing systems. A 2018 study [ 271 ] indicates that PVDF, while stable as part of a graphite electrode, suffers from degradation if silicon is also present. Therefore, PVDF should not be treated as a completely inert material, and its stability is an important subject of study.…”

Key Factors for Binders to Enhance the Electrochemical Performance of Silicon Anodes through Molecular Design

“…However, the weak van der Waals interaction with Si makes PVDF exhibit poor performance in Si anode. [22] Carboxymethyl cellulose (CMC) has a polar group, which can improve the cycle performance by forming a covalent bond with silicon. However, poor mechanical properties [23] make it impossible to be an ideal binder of Si anode.…”

Enhanced Electrochemical Properties of Na0.67MnO2 Cathode for Na-Ion Batteries Prepared with Novel Tetrabutylammonium Alginate Binder

“…The good electrochemical stability in the 0-5 V range, chemical resistance in electrolyte, durable adhesion to current collector, and good binding between PVDF and active electrode materials make polyvinylidene fluoride (PVDF) one of the most popular binder choices for LIBs, so PVDF is also often used in SIBs. However, PVDF-based electrodes cannot tolerate the large volume changes that some of the current electrode materials undergo upon charging and discharging [5]. Moreover, in SIBs, PVDF is shown to undergo defluorination if no additional source of fluorine is available [6], so alternative binders are starting to emerge.…”