The Polypropylene Membrane Modified By An Atmospheric Pressure Plasma Jet As A Separator For Lithium-ion Button Battery

Rechargeable lithium-ion button battery?LIB?owns a high energy and power density,high working voltage,long cycle life,good safety performance and fast-charging capability,etc.thus it has become the research hot points in the field of new power source technologies in recent years.LIB consists of a cathode,an anode,and an electric separator saturated with liquid electrolyte,and the separator takes an important role for assembling the LIB because the structure and performance of the separator determine the interface structure and internal resistance of the LIB,greatly affecting the performances of the battery,such as the specific capacity,cycle life and safety of the battery.Currently,the polyolefin membranes dominant the separator market,and the advantages of these polyolefin separators are their superior mechanical strength and chemical stability,as well as their relatively low cost of raw materials and the mature fabrication technology.However,the polyolefin separators show hydrophobic property,and it has a poor compatibility with the electrolyte,resulting in a worse liquid retention and a high interface resistance.Separators obtained with a single preparation technique have many drawbacks,and they cannot meet the need for assembling LIB with a high performance.For example,the separator fabricated with melt-drawn protocol shows poor mechanical properties,low porosity,and poor affinity with the electrolyte;The separator prepared with the non-woven fabric technique show the shortage of big porosity and a poor mechanical property,affecting the performance of the battery.Therefore,it is very necessary to modify the compatibility of polyolefin separators.In this paper,an atmospheric pressure plasma jet?APPJ?with the acrylic acid?AA?monomers is used to modify the polypropylene?PP?membrane because plasma modification acts only on the surface layer of the material without changing the nature of the matrix itself.Although the thickness of the layer is very thin,it has a stronger adhesion with the pp substrate,and it also can enhance the chemical and thermal stability of the PP separator.The surface morphology,hydrophilicity,mechanical properties,pore size distribution and electrochemical properties of PP membrane before and after modification were analyzed by several instruments.The experimental results show that the AA polymer?PAA?was uniformly coated on the surface of PP membrane,and the average diameter of PP separator modified by Ar/O₂/AA and He/O₂/AA plasma was about 27.5 nm and 20.1 nm.Especially,the contact angle of PP modified by using He/O₂/AA plasma decreased from 112°to 39°,and the electrolyte uptake of the modified PP separator increased 6 times than that of the unmodified one.The interface impedance spectroscopy shows that the interfacial impedance of PP membrane coated with PAA was only one third of that of unmodified PP membrane.The batteries assembled with PP membranes modified with Ar/O₂/AA or He/O₂/AA plasma exhibited a higher specific capacity.In addition,we found that the batteries with modified membranes showed a better charge-discharge rate performance.