(266h) Influence of Sodium Ion Doping on the Performance of Crystalline PEO Based Solid Polymer Electrolyte for Lithium-Ion Batteries

Polyethylene oxide (PEO) based solid polymer electrolytes (SPEs) are an attractive alternative to the flammable liquid/gel electrolytes currently used in rechargeable lithium ion batteries. In addition to improving safety, SPEs could allow the use of Lithium metal anode (3860 mAh/g) which has higher specific energy than commercially used lithium graphite anode (372 mAh/g). This increase in specific energy would greatly improve the range that an electric car can travel before recharging. However, SPEs suffer from low Li+ ion conductivity. In an amorphous polymer, the conductivity is linked to PEO segmental motion; In order to increase the segmental motion, we must reduce the glass transition temperature (T_g). Unfortunately, this increase in polymer dynamics reduces the mechanical strength of SPE. PEO₆-LiClO₄ (PEO₆) complex is a tunnel-like PEO/salt co-crystal which conducts Li+ based on a mechanism that decouples conductivity and segmental motion of the polymer. The studies conducted on PEO₆ used very low molecular weight PEO. At this molecular weight, the polymer has very low mechanical stability. In SPEs with high molecular weight PEO, conduction through PEO₆ is unfavorable as the tunnels fold to form lamellar structures and increase the conduction pathway. Our group has used cellulose Nano whiskers to stabilize the PEO6 structure for ultra-high molecular weight PEO. Despite this stabilization, conductivity (~10^-6 S/cm) is still below what is required for practical application.

Inspired from ceramics, we dope PEO₆ with small amounts of sodium cations to increase the conductivity of the SPE. We observe up to an order of magnitude increase in the conductivity of doped samples compared to the undoped ones. Interestingly, the increase in conductivity is not correlated with the decrease in Tg of the SPEs. To understand the impact of doping on crystalline structure, we perform Wide-angle X-ray scattering and correlate the data with density functional theory modelling.