Research On Improving The Performance Of Paper-based Lithium Ion Battery Separators By Adding Nano-fibrillated Cellulose

Lithium ion battery(LIB)has been widely used in powering many portable electronic devices including laptop,digital camera,cellphone,and some other devices.LIB has high specific energy,efficiency and energy density,long cycle life,fast recharge rate and low environmental pollution.As a key element,LIB separator has crucial functions of physically separating the anode and cathode while permitting free flow of lithium ions,which can directly affect discharge rate,recycle times and safety characteristics.Polyolefin(polyethylene or polypropylene)based membranes have been widely used as the separators for LIBs because of their advantgaes on the electrochemical stability,proper thickness,and mechanical strength.However,with the popularization of high-performance LIBs,polyolefin-based separators show disadvantages such as poor thermal stability.However,the paper-based LIBs separators with renewable and good thermal stability have been favored.In this paper,polysulfonamide(PSA)and northern bleached softwood kraft(NBSK)fibers were the main raw materials to prepare the paper-based LIB separators.Firstly,the effect of beating degree of NBSK fiber and the ratio of NBSK to PSA on the basic performance of separators were studied.It was found that the beating degree of NBSK fiber promoted the improvement of the strength of paper-based separators.At the same time,the pore size of the paper-based separator became smaller and the air permeability increased.The electrolyte absorption of the paper-based separator given by NBSK fiber was still superior to the commercial separator.Furthermore,the addition of PSA fibers promoted the formation of a uniform pore structure in the paper-based separators,which further improved the air permeability,and substantially increased the electrolyte absorption rate.The optimum conditions were determined that the NBSK beating degree was 80 °SR,and the mass ratio of NBSK and PSA fibers was 3:1.Porous NBSK/PSA/NFC composite membranes were successfully fabricated using NBSK fibers,PSA fibers and NFC in a papermaking process for potential applications as the separators for LIBs.The composite membranes exhibited a higher electrolyte uptake(294%)and a higher electrolyte wetting rate,compared with a commercial LIB separator(Celgard 2350).More importantly,it was found that the NFC addition decreased the pore size and narrowed the pore size distribution markedly,which was highly desirable for LIB application.In addition,the NBSK/PSA/NFC composite membranes had superior thermal resistance.No dimensional change was observed after heat treatment at 180?for an hour.The outstanding ionic conductivity and cycle performance in cells of the composite membranes is desirable for LIB applications.The NBSK/S-PSA@NFC composite membrane was successfully prepared by pre-prepared S-PSA@NFC fibers and NBSK fibers.The superior results of composite membrane were obtained in cell application.The composite membrane formed a stable three-dimensional porous structure and showed good mechanical strength owing to NFC cross-linking NBSK and S-PSA to form lots of "bridge".Due to the special porous structure,the composite separator had superior properties such as air permeability,porosity,pore size distribution,and also exhibited excellent electrochemical performance.Compared with commercial separator,NBSK/S-PSA@NFC composite membranes showed higher ionic conductivity(2.15 mS·cm-1)and cycle performance.This excellent pore structure and electrochemical performance made the NBSK/S-PSA@NFC composite membrane a potential separator for LIB with higher capacitance.