The porous structure-tuned cellulose nanofiber paper separators
(S-CNP separators) were developed via a facile fabrication
strategy based on colloidal SiO2 nanoparticle-assisted structural
control. The SiO2 nanoparticles were introduced as a CNFdisassembling
agent. In comparison to the CNP separator containing
no SiO2 nanoparticles, the S-CNP separators allowed the loose
packing of CNFs due to the presence of SiO2 nanoparticles (serving
as non-conductive spacer particles) dispersed between the CNFs,
thereby facilitating the evolution of more porous structure. This
structural uniqueness of the S-CNP separators, in combination with
their high polarity, brought significant improvements in the ionic
conductivity and electrolyte wettability. Notably, as the SiO2 content
was increased, the S-CNP separators showed a highlydeveloped
porous structure (i.e., high porosity and low Gurley
value) without impairing the OCV drop behavior. As a result, the
SiO2 content-directed morphological variation of S-CNP separators
exerted substantial influence on the cell performance. Among the
S-CNP separators explored herein, the S-CNP separator (fabricated
with 5 wt.% SiO2) provided the best cell performance. This excellent
cell performance of the S-CNP separator (SiO2 content ¼ 5 wt.%)
was attributed to its highest ionic conduction arising from the wellbalanced
properties between the porous structure and separator
thickness. These results underline that the S-CNP separator with
well-developed porous structure and optimized membrane properties
could be a promising alternative to outperform a commercial
PP/PE/PP separator used in mid to large-sized lithium-ion batteries
targeting (hybrid) electric vehicle and power grid applications.