Preparation And Properties Of High Performance Polypropylene Composite Separator

With the fast development of lithium-ion batteries(LIBs),they have become major power battery applied in communication,computers and consumer electronics,3C devices,electric vehicles(EV),storing energy products,etc.At present,the main commercially available materials for LIBs separators are polyethylene(PE)and polypropylene(PP)membranes.They have good chemical and electrochemical stability,but poor electrolyte wettability and poor thermal stability.The poor electrolyte wettability will hinder lithium ion(Li⁺)transport and limit the battery performance.Due to reaction or overcharge conditions,the membrane will crumple,which can result in battery short circuit.A modified PP composite separator was prepared through the coating with porous poly(ether ether ketone)(PEEK)layer in this study.Firstly,the surface of PP was modified by ozone to improve its surface activity and polarity.The PEEK solution was then coated on its surface,and a porous composite PP separator with honeycomb-like structure was thereafter prepared through phase inversion method.There was no obvious deformation for modified PP at 200°C for 0.5 h,demonstrating excellent dimensional thermo-stability,resulted by the high performance of PEEK.The electrolyte absorption was 181.4%,which was 54%higher than that of PP,illustrating a significantly improved electrolyte wettability because of the ether and ketone polarize groups of PEEK,which has high interaction with electrolyte.The initial capacity of the LIBs assembled with modified PP composite separator reached 160.7 m Ah/g at the rate of 0.2 C.After 150 cycles,the discharge specific capacity of the battery can be stable as 160 m Ah g^-1,and the coulomb efficiency was above95%.The electrochemical window was 4.52 V.Therefore,the composite separator with excellent dimensional thermo-stability,electrolyte wettability and electrochemical performance for LIBs was successfully prepared.In order to further reduce the bulk impedance of the composite separator and the interface impedance with the positive electrode while ensuring the dimensional thermal stability of the separator,and improving the battery discharge capacity and the coulomb efficiency,carbon nanotubes(CNT)were added into the PEEK coating solution to prepare the high safety and high performance Janus composite separator with single face conductivity based on the above studies to further improve the interface compatibility and electrochemical properties of the separator.The conductive side to the positive electrode can improve the conductivity of the cathode side,while promoting the uniform storage and transmission of Li⁺,and reducing the impedance,thus improving the electrochemical performance.Poly(ether ether ketone)(PEEK)and carbon nanotube(CNT)were compounded to coat on polypropylene(PP)to prepare Janus composite separator(PP@C)through phase inversion method.PP@C composite separator did not deform at 180 ^oC for0.5 h(only slight deformation),illustrating excellent thermal stability.The electrolyte absorption rate of PP@C2 was 193.8%.As a result,the specific discharge capacity of LIBs assembled with PP@C2 composite separator was 156.7 m A h/g at 0.2 C and 159.4 m Ah/g at 2 C,showing good rate performance,and the capacity recovery rate was more than 98%.This was attributed to the ultra-high thermal stability of PEEK and the good affinity of the electrolyte,as well as the high conductivity of CNT and the uniform dispersion of Li⁺,so that the separator can have excellent electrochemical performance while improving safety.