Improving the Performance of PEO-Based Electrolytes to Enable Metallic Li As an Anode

Abstract

Replacing carbon-based anodes with metallic Li can drastically improve the performance of Li-ion batteries. However, stabilizing the Li metal/liquid electrolyte interface has been difficult and it is critical to replace them with solid state electrolytes. Solid polymer electrolytes (eg. PEO-LiTFSI) are of particular interest as they are easy to fabricate and have superior ductility, leading to suitable mechanical properties. This work addresses two challenges in developing viable PEO-based electrolytes – 1) factors affecting the SPE/Li interface are not well understood and 2) these electrolytes inherently have low room temperature conductivity. The first part of this work focuses on minimizing the interfacial impedance between PEO-LiTFSI (SPE) and lithium anode by controlling the external variables in a Li/PEO-LiTFSI symmetric cell. The stability and kinetics of the interface were correlated with temperature, stack pressure and electrode type using electrochemical impedance spectroscopy (EIS) and dc cycling in order minimize interfacial impedance and maximize the limiting current (j lim). Second, a critical stack pressure was established for 60oC and 80oC as 400 kPa and 200 kPa respectively by relating the stack pressure with interfacial impedance, below which interfacial impedance was shown to be at least 50 times higher. The effects of increasing temperature on the kinetics and mechanics of the interface manifest as variations in the EIS and cycling profiles. Lastly, the PEO-LiTFSI/Li interfacial impedance was reduced by 67%, from 4000 Ohms.cm2 to 1675 Ohm.cm2 at 30oC by using vapor deposited Li over Li foil electrode, increasing the limiting current from 0.5 mA/cm2 to 1mA/cm2. The second part of this work was dedicated towards increasing the room temperature conductivity of PEO-LiTFSI electrolytes by adding a filler material. LLZO (Lithium Lanthanum Zirconium Oxide) was chosen because of its high room temperature conductivity and good cycling capabilities. Low values of interfacial impedance calculated from EIS on a trilamilar PEO-LiTFSI and LLZO cell with blocking electrodes proved the viability of the composite electrolyte. A solvent casting method was developed for the fabrication of flexible composite electrolytes of 100-500 microns thickness. The flexibility of the composite electrolytes was comparable to that of nascent PEO-LiTFSI electrolyte membranes, while their ionic conductivity was an order of magnitude higher. EIS was also used to determine the temperature dependence of the composite electrolyte’s conductivity. Lastly, the variation in bulk impedance with increasing LLZO volume percent in the PEO-LiTFSI was studied to optimize conductivity, while maintaining its flexibility. Overall, this study provides effective guidelines for improving the performance of PEO-based electrolytes towards enabling viable all-solid state batteries.