Critical role of polymeric binders on the electronic transport properties of composites electrode

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 phi(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 phi(c) the conductivity is a unique function of the PEO to CB concentration ratio in the suspension, log sigma=log(sigma(CB))-a phi(PEO)/phi(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 phi(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.