Nanocomposites incorporating cellulose nanocrystals with potential applications in lithium ion batteries

Abstract

Cellulose nanocrystals (CNCs) have received considerable attention over the last ten years because CNCs can be produced from renewable materials such as straw, wood, cotton, and sea animals (tunicates). CNCs are slender rods with high aspect ratios (10-100) and the final dimensions of the CNCs depend on the starting material. CNCs are one of the strongest and stiffest organic materials, with an estimated tensile strength (TS) of 7.5 GPa and theoretical calculations estimate the tensile modulus (E) of CNCs at 168 GPa. In addition, composites incorporating CNCs have been fabricated from a variety of matrices and CNCs increase the E significantly and to a lesser extent the TS. Improvement is dependent on many variables, but good dispersion of the filler in the matrix is critical. A copolymer of poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDFHFP), has gained popularity over the years in the area of lithium ion battery technology, mainly, as an electrode binder, but also as a polymer electrolyte separator. However, the mechanical properties of neat PVDFHFP do not meet the standard requirement for commercial separators, namely the low E. In this work, novel PVDHFHFP/CNC nanocomposite films were fabricated and characterized. It was found that incorporation of CNCs improves the E and TS. The improvement in mechanical properties of PVDFHFP upon addition of CNCs makes PVDFHFP a more suitable candidate for polymer separators in lithium ion batteries. Separators for Li⁺ batteries are most commonly polyolefins (e.g., polyethylene or polypropylene). Thus, the second part of this project focused on incorporation of CNCs into high density polyethylene (HDPE).