Preparation And Properties Of Cellulose Separator For Lithium-ion Batteries

Lithium-ion batteries?LIBs?become one of the most promising and important energy storage technologies.The separator is a critical component of a LIB.Although the separator does not take part directly in electrochemical reaction of a LIB cell,its property and architecture play a key role in influencing the cell performance,including energy density,power density,safety,and service life.The separators used in modern LIBs are mainly polyolefin types,such as polyethylene?PE?and polypropylene?PP?.The shortcoming of polyolefin separators is obvious,and they can hardly meet the demands of future development.From a practical point of view,the two biggest drawbacks of polyolefin separators are poor thermal stability and limited electrolyte wettability,which affect the safety and performance of LIBs.Furthermore,polyolefin separators depend on the finite fossil oil,and thus are not renewable,sustainable or biodegradable.Cellulose materials are good alternatives to replace petroleum-based materials due to their abundant reserves,biodegradable and renewable nature,easy accessibility,and low cost.Furthermore,cellulose has gained extensive attention as an alternative material for LIB separators because of its good properties such as high porosity,non-toxic,light weight,and especially electrolyte excellent wettability and thermal stability.However,the preparation method for cellulose LIB separators needs to be improved to decrease the production cost and reduce the environmental impact.The tensile strength?machine direction?and puncture strength of cellulose membranes should also enhance.Herein,a series of studies have been carried out to promote utilization of cellulose membranes.The properties of membranes derived from various types of plant fibers commonly used in paper making were compared with those of Celgard2325.All membranes were prepared via a facile papermaking process,which are used in the large-scale production of cellulose LIB separators.It was demonstrated that the plant fiber-derived nanofibril membranes possessed excellent electrolyte wettability,thermal dimensional stability,and tensile strength,suitable for LIBs.The contact angle of the cellulose membranes?distilled water?was less than 45°,and the membranes had no obvious change when they were placed at 160? for 120 min.Meanwhile,the membranes made from softwood pulp and cotton pulp had the best tensile strength,exceeding 130 MPa,which were ideally used as the matrix material of separators.Softwood pulp cellulose nanofibril membranes were prepared via vacuum filtered.The effects of forming conditions such as membrane basis weight,dispersion medium and drying method on membranes'pore structure were studied.It was found that the porosity of the membranes decreased with the increase of basis weight.The addition of ethanol into the dispersion medium helped to improve the membranes'porosity.The effect of drying methods on the porosity was obvious,and the drum dryer was a good choice.Moreover,it was difficult to improve the membranes'pore structure by additive like lignin microparticles,chitosan or?NH₄?₂CO₃.Ultra-light and ultra-thin nanocellulose porous membranes were successfully prepared for LIBs.The ethanol-soaked cellulose nanofibril membrane?ECM12?had a thickness of 12?m and an areal density of 7.1 g/m²,which was half of Celgard2325.The ECMs had high porosity,a uniform nanopores structure,and excellent tensile strength.The ECMs had outstanding electrolyte affinity,the wetted area was 4 times that of Celgard2325,and electrolyte uptake was as high as 281%.More importantly,the ECMs not only had good electrochemical stability,with an electrochemical stability window up to 4.8 V,but also had better cycling performance and rate behavior than Celgard2325,especially in the case of high rate charge-discharge.Cellulose nanofibril membranes were prepared by dispersion medium replacement,using purchased commercial cellulose nanofibrils.The characteristics of the membranes and the charge-discharge performance of the cells were analyzed.It was found that the batteries using lower basis weight membranes had better discharge capacity at low C-rates.With the increase of the C-rate,the discharge capacity decreased rapidly.The immersion time and drying temperature of the membranes had little effect on the discharge capacity of the batteries during low C-rates,and had a great influence at high C-rates.The batteris using the membrane with4.5 h soaked had the best rate behavior.When membranes dried at 90?,the batteris had better performance at high C-rates.The cellulose nanofibril membrane/filter paper?CNFM/FP?composite separator was prepared by simulated papermaking multi-layer forming technology.Water was used as dispersion medium and composite medium.Compared with filter paper and Cegard2325,the batteries using CNFM/FP had better first charge-discharge performance.Its first discharge capacity reached 160 mAh/g,which was about 94%of the theoretical discharge capacity.At low current density of 0.2 C,the cycling performance and rate capacities of the batteries using CNFM/FP were comparable with those of Celgard2325.