Critical role of polymeric binders on the electronic transport properties of composites electrode
Résumé
The role of the polymeric binder nature and composition on the electronic transport properties of composite electrodes based on Li1.2V3O8, carbon black CB, and poly-ethylene oxide PEO/poly-vinylidene difluoride-co-hexafluoropropylene/ethylene carbonate–propylene carbonate EC–PC/Li bis-trifluoromethansulfon-imide binders were examined. The variation of the electrical conductivity vs CB volume fraction is typical of tunneling-percolation systems. Lower percolation threshold c found for preplasticized binders is related to a more efficient CB dispersion due to the presence of EC–PC in the liquid suspension at the time of the composite processing. Above c the conductivity is a unique function of the PEO to CB concentration ratio in the suspension, log = logCB-aPEO/CB. This ratio controls the amount of polymer that adsorbs at the surface of the CB particles before the CB conducting network forms. The Li+/e− insertion behavior was studied at low current rate, for which ionic conductivity is not a limiting factor. The electrochemical capacity sharply increases at c. However, for CB content typical of practical composite electrodes, the electronic conductivity of the CB network is not the only parameter that governs the electrode performance. It depends also on the electronic wiring at the CB/Li1.2V3O8 interface, which is improved when adding EC–PC and lithium salt in the formulation.