Porosity Blocking in Highly Porous Carbon Black by PVdF Binder and Its Implications for the Li–S System

In this work, the influence of cathode binders on the porosity of composite electrodes for lithium–sulfur (Li–S) batteries employing high surface area carbon blacks has been closely scrutinized. This has been accomplished by comparison of PVdF with the related copolymer, PVdF-HFP. Analysis of carbon black porosity after addition of binder in NMP solution reveals that PVdF(-HFP) fills pores of almost any size in carbon black, which can effect a severe reduction in pore volume and surface area accessible to the electrolyte in a Li–S cell. Noting the different swelling behavior of both binders, the implications of pore filling by the binder on the electrochemistry of Li–S cells can be determined. Because of the low swellability of PVdF in dimethoxyethane:dioxolane (DME:DOL)-based electrolytes, access of the electrolyte to the carbon surface area and pore volume is restricted, with potentially severe detrimental effects on the available capacity of the cell. Furthermore, this effect is still clearly significant for common binder loadings and with preinfiltration of sulfur; this study is therefore a clear demonstration that PVdF is an unsuitable choice of binder for the lithium–sulfur system and that alternatives must be considered.