  The ability to manipulate rheological and electrical properties of colloidal
carbon black gels makes them attractive in composites for energy applications
such as batteries and fuel cells, where they conduct electricity and prevent
sedimentation of `granular' active components. While it is commonly assumed
that granular fillers have a simple additive effect on the composite
properties, new phenomena can emerge unexpectedly, with some composites
exhibiting a unique rheological bi-stability between solid-like and liquid-like
states. Here we report such bi-stability in suspensions of non-Brownian
graphite and colloidal carbon black in oil, a model system to mimic composite
suspensions for energy applications. Steady shear below a critical stress
elicits a transition to a persistent mechanically weak and poorly conducting
state, which must be `rejuvenated' using high-stress shear to recover a
stronger, high-conductivity state. Our findings highlight the highly tunable
nature of binary granular/gel composite suspensions, and present new
possibilities for optimizing mixing and processing conditions for Li-ion
battery slurries.
