Preparation And Performance Of Pre-irradiation Grafting Single Lithium-ion Conducting Polymer Electrolyte

In the 21st century,one of the most important social challenges facing mankind is electrochemical energy storage.The high-energy and environmentally friendly of lithium ion batteries have the advantages of long cycle life,high specific energy,high working voltage,quick charge/discharge and no memory effect,so they are widely used in notebook computers,mobile phones,automobile,medical equipment and aviation,etc.As the important component of battery,electrolyte directly affect the electrochemical properties of lithium ion batteries.Liquid electrolytes are composed of lithium salts and polar non-proton organic solvents.Although the liquid electrolytes have good electrochemical stability,high boiling point and a wide range of use temperature,but they have the potential safety issues that cannot be ignored,such as water sensitivity,flammability,leakage and even explosion.The separator in the battery is immersed in the electrolyte as an ionic transmission channel,and the pore on the separator will promote the formation and growth of the lithium dendrites during the charging process.It will cause the short circuit of battery.Polymer electrolytes have attracted great attention as an important substitute for liquid electrolytes.It have the advantages of inhibiting lithium dendrites growth and have a high safety performance.Ordinary polymer electrolytes are mostly dual-ion conductors,in which both anions and cations move simultaneously in electric field.When lithium ions are deposited or intercalated on cathode,the anions move towards the anode and accumulate there.A concentration gradient will set up in the electrolyte,which results in generate of concentration polarization and decrease of Li+ transference number.It means that the real ionic conductivity of lithium ion is lower than the calculated value from data of electrochemical impedance spectroscopy.Most of dual-ion conductor electrolytes display a low Li+ transference number(0.3?0.5).In order to increase the conductivity,improve the output power and reduce the polarization loss,it is of great significance to develop the single lithium-ion conducting polymer electrolytes(SIPE),which combines the anion of lithium salts with the polymer backbone and makes the Li+ transference number close to unit.Compared with the chemical copolymerization method,the radiation grafting technology is more convenient,simple and easy to control.Only to adjust the composition,properties and reaction parameters of the compound,we will get the polymer structure suitable for the specific application system,even if the highly incompatible monomers can still be copolymerized.The radiation grafting reaction is caused by high energy rays,so no additional initiator is needed in the reaction process,and the subsequent purification process is simplified to get a very pure copolymer.In this paper,two new kinds of pre-radiation graft copolymerization SIPE were prepared by pre-radiation grafting technology,and the anions were fixed on the polymer chain.(1)A poly(vinylidene fluoride)(PVDF)-based copolymer was synthesized by grafting two kinds of monomer,2,2,3,4,4,-hexafluorobutyl methacrylate(HFMA)and 2-acrylamido-2-methylpropanesulfonic acid(AMPSLi),onto the PVDF powder with pre-irradiation grafting copolymerization.PVDF is one of the most commonly used polymer matrix.It has the advantages of good film-forming property,high dielectric constant and low glass transition temperature.PVDF polymers have high crystallinity and low compatibility with lithium salts,and the ionic conductivity of PVDF base polymer electrolytes are low,which limit its application in lithium ion batteries.HFMA monomer contains the Lewis base functional groups of O and F,which are favorable for the dissolution and dissociation of lithium salts,and then it improved the transmission capacity of lithium ion.The AMPSLi monomer was grafted onto PVDF as the lithium source,and the sulfonic anions were fixed,then the lithium ions were the only free moving groups.The lithium-ion transference number of the prepared polymer electrolyte is close to 1.The chemical structure,crystal form,and membrane morphology of SIPE were characterized by nuclear magnetic resonance spectroscopy(NMR),Fourier transform infrared spectroscopy(FTIR),X ray diffraction(XRD)and scanning electron microscopy(SEM),etc.It was proved that the structure of the prepared polymer was in agreement with the ideal design.The polymer electrolyte membrane exhibits excellent comprehensive properties.The mechanical properties exhibit the elongation at break of 72%and the tensile strength of 28 MPa.Comparatively high values of ionic conductivity,up to 10-5 S cm-1 at 30 ?,were obtained,combined with lithium-ion transference number of 0.93 and stable electrochemically oxidation voltage as high as 4.4 V vs Li/Li+.Except for these promising features,the charge/discharge cycle performance of the SIPE coin cell with single-ion polymer as binder is stable.This is due to the establishment of lithium-ion transport network,which reduces the internal resistance of the cell.(2)The copolymer of six fluorine propylene(HFP)and PVDF,PVDF-HFP,keeps the heat resistance,toughness and dielectric constant of PVDF.While the crystallinity of PVDF-HFP is lower,which is beneficial to the enhancement of the movement of the polymer electrolyte chain and the increase of the ionic conductivity.In this paper,PVDF-HFP powder was used as the polymer matrix,and after high-energy ray pre-irradiated,AMPS was used as a lithium source graft onto the polymer chain,and a new type of SIPE based on PVDF-HFP was prepared.The structure of graft copolymer was characterized by FTIR analysis.Ion exchange capacity,SEM and energy dispersive spectroscopy(EDS)were used to qualitatively and quantitatively analyze the elements on the membrane.The ionic conductivity,electrochemical window and charge/discharge cycle performance of SIPE cell were also measured.