Detection and Influence of Residual Water Content on Electrochemical Performance of Solid State Polymer PEO-Litfsi Electrolytes

Abstract

All solid state lithium-ion batteries are one of the promising candidates to power future electric and hybrid electric vehicles due to their outstanding potential for safety improvement and power density. Typically all solid state battery concepts require a lithium-metal (Li-metal) anode to achieve higher energy density. For reasons of processability, polymer based solid electrolytes are often a preferred material class to form the separator and cathode. However, maximum attention has to be paid on the interface of solid polymer electrolyte (SPE) and Li-metal due to the extreme reactivity. Side reactions at the interface cause an increase of the contact resistance and a rapid drop of the cell performance. Generally solid polymer electrolytes consist of at least two components: a polymeric host and a lithium salt. A well-known combination is polyethylenoxide (PEO) and lithium bis(trifluoromethanesulfone)imide (LiTFSI), which can be prepared by dissolving the two components in a suitable organic solvent which is subsequently removed during a drying step. Although this process appears rather simple, the details of this procedure have a significant impact on the functionality and stability of the SPE. This impact comes from the residual water content, solvents and impurities which as a result react with the Li-metal and cause undesired aging effects. We studied the drying time of PEO and LiTFSI in-depth based on different variations applying the Karl Fischer titration technique. Commonly for the SPE preparation, solvents like n-methyl-2-pyrrolidion (NMP), acetonitrile, methanol etc. are used which also contain certain amounts of water depending on the purity grade. The interface between Li-metal and SPE was characterized by impedance spectroscopy in symmetrical cells (Li|SPE|Li) at 80°C. The work in our laboratory demonstrates a non-damaging approach to produce water-free PEO and LiTFSI. The results show that the Karl Fischer titration technique is a useful method to determine the residual water content of the PEO and LiTFSI independent of different drying time periods and methods. Moreover, it could be demonstrated, that there is an impact of the residual water on the interface resistance between the Li-metal and the SPE. Figure 1