Impact of binder content on particle fracture and microstructure of solvent-free electrodes for Li-ion batteries

Abstract

The fraction of polytetrafluoroethylene (PTFE) binder in solvent-free Li-ion battery electrodes is shown to have a dramatic impact on their processability, microstructural evolution and electrochemical performance. We show experimentally that increasing binder fraction from 0.5 to 4 wt.% transformed the electrode microstructure from an efficient, open structure containing PTFE nano-fibrils to a compact morphology with fragmented active material and porosity blocked by PTFE agglomerates. The solvent-free electrodes showed a classical visco-elastic response during compression, comprising three distinct regions of deformation. The electrode stiffness and yield/flow strength increased non-linearly with binder fraction such that for higher binder contents (>2wt.%), there was extensive LiNi0.6Co0.2Mn0.2O2 (NMC) particle fracture during the calendering process, with cracks propagating along the grains of polycrystalline NMC particles. Conversely at lower binder fraction (<2wt.%), the PTFE readily fibrillated into highly textured (100) crystalline nano-fibrils and NMC particles remained largely intact. These electrodes showed superior electrochemical performance due to higher ionic mobility through the open nano-fibrillar microstructure and intact NMC particles.