Improved electrochemical properties of janus composite membranes obtained by modification of PEEK/nanocellulose on polyethylene for lithium-ion batteries

To solve the disadvantages of polyethylene (PE) separator for lithium-ion batteries (LIBs), such as low polarity, poor electrolyte wettability, and severe thermal dimensional shrinkage, the composite solution of poly(ether ketone) (PEEK) and carbon nanoparticles, which was obtained by the carbonization of nanocellulose (NCC) in the sulfuric acid solution, were applied to modify the PE separator to prepare Janus composite separator (PE/PC) with high porosity. PE/PC exhibited high dimensional thermal stability with a shrinkage rate of 53% after 0.5 h of treatment at 150 °C, and a higher electrolyte loading rate (196.3%) than PE. Accordingly, it also presented excellent battery performance. The discharge capacity of the cell using PE separator was 148 mAh g−1 after 100 cycles, while that with PE/PC separator were 155 mAh g−1 after 200 cycles. The initial efficiency of discharge-charge of PE/PC with 2% NCC (PE/PC2) was as high as 97.6%. This should be related to the high ion conductivity (1.21 × 10−3 S cm−1). Furthermore, the carbon nanoparticles effectively provided abundant secondary oxidation reaction sites for the active material between the separator and cathode, allowing the free diffusion of Li+, thus also reducing the charge transfer resistance and improving the battery performance.