Effect of nanofillers on the conductivity of solid polymer electrolytes
Solid polymer electrolytes (SPEs) consist of a polymer host with dissolved lithium salts. Li+ ions transport through the polymer matrix via a combination of hopping and segmental motion of PEO. These electrolytes can replace the volatile, toxic and flammable liquid electrolytes currently used in commercial Li-ion batteries. SPEs reduce dendrite formation and enable use of high energy density lithium metal anode. SPEs have applications in a wide range of devices including electronics, hybrid electric vehicles, medical implant devices, and for storing power generated from alternate energy devices such as solar cells.
Figure 1: The composition and surface chemistry dependence of conductivity. The y–axis % increase in conductivity is taken with respect to the unfilled electrolyte. Acidic filler: red and neutral filler: cyan. % Increase in conductivity with respect to the unfilled sample for both filler types as a function of EO/Li and temperature. 5 wt% filler gives maximum conductivity enhancement with both surface chemistries.
Despite their advantages they suffer from low room-temperature ionic conductivities. Addition of ceramic nanoparticles enhances ion conduction. The mechanism for this is not understood by the scientific community. The aim of this project is to understand this mechanism and determine ways that provide further conductivity enhancements. We tune the surface chemistry of the ceramic nanofillers and correlate the change in ionic conductivity to factors that affect lithium ion conduction in an unfilled electrolyte (thermal properties, PEO segmental dynamics, and morphology). We propose that lithium-transporting tunnels assemble on the surface of nanoparticles, enhancing conduction. To further exploit this mechanism we increased the aspect ratio of the particles by a factor of 100 and obtained a five-fold increase in ion conduction at room temperature. We are exploring the effect of surface chemistry and nanoparticle aspect ratio on this mechanism.
Single Ion Conductor Electrolytes
David Caldwell II and Keran Lu
Molecular dynamics is a powerful tool for probing properties of materials that are difficult to study through experimental techniques. One such phenomenon is ion conduction in these materials occurs through opportunistic hopping from one solvation site to another in the polymer host. We use atomistic molecular dynamics to probe ion behaviour in a PEO-based ionomer. In particular, we I working towards identifying the conductions upon which a new “super-ionic” conduction mechanism utilizing ion aggregations.We are using also coarse-grained technique to understand the behaviour of ion aggregation. In our approach polymer is implicitly represented, such that our simulation has only ions. There ions interact as if polymer were present.
Figure 2: String like ion aggregates observed in CG simulation demonstrate super ionic conductivity.
We find from our simulation that our aggregates are string-like random-walks of ions. In general, ions have two oppositely charged neighbours but there are instances of ions which have an extra, third neighbour. We find that these extra ions can “pinball”charge down an ion chain, transfering charge a greater distance than the motion of any particular ion. This may be the mechanism behind the superionic phenomenon found in atomistic simulations.