  It is shown that surface conduction can stabilize electrodeposition in
random, charged porous media at high rates, above the diffusion-limited
current. After linear sweep voltammetry and impedance spectroscopy, copper
electrodeposits are visualized by scanning electron microscopy and energy
dispersive spectroscopy in two different porous separators (cellulose nitrate,
polyethylene), whose surfaces are modified by layer-by-layer deposition of
positive or negative charged polyelectrolytes. Above the limiting current,
surface conduction inhibits growth in the positive separators and produces
irregular dendrites, while it enhances growth and suppresses dendrites behind a
deionization shock in the negative separators, also leading to improved cycle
life. The discovery of stable uniform growth in the random media differs from
the non-uniform growth observed in parallel nanopores and cannot be explained
by classical quasi-steady leaky membrane models, which always predict
instability and dendritic growth. Instead, the experimental results suggest
that transient electro-diffusion in random porous media imparts the stability
of a deionization shock to the growing metal interface behind it. Shock
electrodeposition could be exploited to enhance the cycle life and recharging
rate of metal batteries or to accelerate the fabrication of metal matrix
composite coatings.
