Research Notes
Thermally stable solid membranes
conduct ions like a liquid
Rui Qiao
Developing materials for
batteries is a tough job. Con-
sider the membrane electro-
lyte separators in batteries.
Ideally, they must be highly
ion conducting, mechanically
strong, and thermally stable.
Materials that can meet these
requirements remain elu-
sive despite decades of intensive research. In 2016,
Virginia Tech Chemistry Prof. Lou Madsen and his
PhD student Ying Wang invented a new ”molecular
ionic composite” (or MIC) with just such a combina-
tion of properties: it offers the conductivity of liquid
electrolytes but remains rigid and stable at tempera-
tures up to 300°C. However, the mechanisms behind
the superior properties of these materials remain
unclear, which is slowing their further development.
Shortly after the invention of MICs, Prof. Rui Qiao
(Mechanical Engineering) and his students started
working with the Madsen group to examine this ma-
terial using computer simulations. Their joint work,
the very
first study
of the ion
transport
mecha-
nism in this
material, was
published in the
journal Lang-
muir early this
year.
In this work, Zhou
Yu and Yadong He,
both PhD students in the Qiao group, built the
first molecularly resolved model for the molecular
ionic composites developed by the Madsen group.
These composites are essentially rigid polyanion rods
aligned within room-temperature ionic liquids. The
polyanion rod models retrieved from their extensive
molecular simulations exhibit the same double heli-
cal structure inferred from Ying Wang’s experimental
measurements. They then performed molecular
simulations to probe the equilibrium structure and
dynamics of ionic liquids in the composite. It was
discovered that the ion distribution in the interstitial
space between polyanion rods exhibits the hallmarks
of the ionic liquids structure near charged surfac-
es, i.e., cations and anions form alternating layers
around the polyanion rods. The distinct ordering of
ions suggests the formation of a long range “electro-
static network” in the composite, which gives insight
into its mechanical cohesion and high modulus (up
to 3 GPa). It was found that while some cations
become associated with the sulfonate groups of the
polyanions, such association only exist on nanosec-
ond time scales, thus explaining the composite’s high
ionic conductivity. These
insights are currently
being exploited to engi-
neering molecular ionic
composites with both
high mechanical modulus
and fast ion conduction
by the Madsen group. The
Qiao and Madsen groups
are further collaborating
with battery experts to engineer
safer and higher energy density lithium
and sodium batteries.