High molecular weight crystalline PEO6 based polymer electrolytes for lithium-ion conduction– Effect of cellulose nano whiskers

We explore the crystal structure and ionic conductivity of unfilled and cellulose nanowhisker-filled polymer electrolytes, based on high molecular weight PEO and LiClO4 at a concentration EO: Li = 6:1 (EO=ether oxygen). At this concentration, the polymer electrolyte forms crystalline PEO6 [PEOy - the crystal structure co-crystallizes ‘y’ PEO ether oxygens to one Li-anion pair]. While the diffraction patterns for PEO3 and PEO6 with LiClO4 are available, their structures are not. Thus, we optimize these structures using density functional theory and show agreement between simulated and experimental diffraction patterns. The PEO6 crystal shows insufficient Li-ion conductivity due to a lack of percolated conduction paths, and the crystal structure is unstable following two months after annealing. Cellulose nanowhiskers, with their patterned surface -OH groups, extend the length of PEO6 tunnels, stabilize these tunnels for as long as a year, and result in the percolation of crystals. Although the EO: Li = 6:1 stoichiometry should not favor PEO3 formation, we observe this crystal phase in some unfilled samples. But, with the addition of nanowhiskers, the patterned surface templates PEO6 formation rather than PEO3. Despite these advantages, nanowhiskers do not improve ionic conductivity as hypothesized. The tools developed in this work allow further mechanistic exploration of conduction through crystalline domains.