Highly safe lithium-ion batteries: High strength separator from polyformaldehyde/cellulose nanofibers blend

As a pivotal part of lithium-ion batteries, separator is supposed to have high strength , thermal stability and excellent wettability . In this study, polyformaldehyde/cellulose nanofibers blend separators are firstly prepared via thermally induced phase separation to improve the performance, especially the safety of LIBs. The polyformaldehyde/cellulose blend separators show considerable tensile strength (116 MPa) and Young's modulus (6.07 GPa) owing to the high crystallinity and high performance of polyformaldehyde and cellulose . In particular, benefiting from their abundant polar groups and highly porous structure , the polyformaldehyde/cellulose blend separators possess high electrolyte uptake (412%) and small contact angle (19°) as compare to the commercial polyethylene separator (115%, 58°). Moreover, the thermal treatment tests indicate that the polyformaldehyde/cellulose blend separators are thermally stable at as high temperature as 180 °C. And the polyformaldehyde/cellulose blend separators show higher ionic conductivity (1.39 mS cm −1 ) and lower interface resistance (49.5 Ω) than polyethylene separator (0.76 mS cm −1 , 136.4Ω). As expected, the LiFePO 4 /Li cell with polyformaldehyde-20-cellulose blend separators exhibit the best stable cycling performance and improved rate performance than PE separator, especially at high rate. In summary, the polyformaldehyde/cellulose blend separators are promising new kind of separator for LIBs with high safety and enhanced performance.