Meanwhile, as a new approach to overcome these stringent
drawbacks of conventional polyolefin separators, our group has
recently developed eco-friendly cellulose nanofiber paper-derived
separators (referred to as “CNP separators”) [26] as a promising
alternative to synthetic polymer-based separators. The CNP separators
are featured with an electrolyte-philic, nanoscale labyrinth
structure established between closely piled cellulose nanofibers
(CNFs), in contrast to typical papers comprising macro/microscopic
cellulose fibers. Owing to the unusual thermal stability and polarity
of the CNFs, the CNP separators provide substantial improvement
in the thermal shrinkage and electrolyte wettability, as compared
to commercial tri-layer (polypropylene (PP)/polyethylene (PE)/
polypropylene (PP)) separators. Despite these advantageous characteristics,
however, the CNP separators suffer from some limitations
in providing high-porous structure. This drawback of the CNP
separators may stagger ionic transport via the liquid electrolyte-
filled CNP separator. This structural shortcoming may arise from
the dense packing [27e32] of CNFs composed of close-packed
polysaccharide chains with b-(1 / 4)-D-glucopyranose repeat
units (i.e., cellulobiose unit).