Research On PVDF-HFP Based Gel Polymer Electrolyte Lithium-ion Battery

Gel polymer lithium-ion battery?GPLB?has characteristics of small size,high energy density,long life,no memory effect,no pollution which liquid lithium-ion battery shows.Meanw-hile,the electrolyte of GPLB is cured in the gel network,which greatly improved the safety of battery,and the shape of GPLB can be controlled.However,compared with the conductivity of the liquid electrolyte,which has a magnitude of 10-2 S·cm^-1,the room temperature ionic conductivity of the gel polymer electrolyte?GPE?can only reach the magnitude of 10-3 S·cm^-1,and there is still a lot of space for development.In addition,the main method of GPE preparation is through casting,that is,fabricate the GPE membrane through casting first,and then the film is combined with electrode chip to assemble the battery.The cost of process equipment is very high,and the operation is difficult.So it is not conducive for this method to the industrial production.Therefore,focusing on the preparation process of the GPLB,optimizing the component of the gel electrolyte,and doping nano-SiO₂ to the GPE system,the paper intends to improve the performance of the gel to obtain GPLB with more excellent electrochemical performance.Firstly,GPE for lithium ion battery was prepared via room temperature in-situ polymerization using poly?vinylidenefluoride-co-hexafluoro-propylene??PVDF-HFP?as polymer matrix,neopentylglycoldiacrylate?NPGDA?as crosslinking agent.2,2'-Azobis?2-methylpropionitrile??AIBN?as the initiator.The effects of the mass ratio of PVDF-HFP/NPGDA on the performances of GPE and LiNi_0.5Co_0.2Mn_0.3O₂ positive lithium ion batteries were investigated.The results showed that the GPE had a high ionic conductivity of 8.45 mS·cm^-1,lithium ion transport number of 0.78 and an electrochemical potential window of 4.5 V with the PVDF-HFP/NPGDA mass ratio of 1:1 and had a good compatibility with lithium anode.The initial discharge specific capacity was 143 mAh·g^-1 at a current density of 30 mA·g^-1,and the retention capacity was up to 135.3 mAh·g^-1 after 50 cycles.The discharge specific capacity of batteries was 100.2 mAh·g^-1 at a current density of 300 mA·g^-1.Secondly,monodisperse SiO₂ with a diameter of 200 nm were prepared by sol-gel method and doped it into the GPE.Designed of three factors four levels of orthogonal test and the effects of different ratios on the reaction time,ionic conductivity,electrochemical potential window and lithium ion transport number were investigated.The ionic conductivity was up to 8.95 mS·cm^-1,lithium ion transport number of 0.89,electrochemical potential window was 4.7 V.The LiNi_0.5Co_0.2Mn_0.3O₂ positive electrode lithium ion battery was prepared by in-situ polymerization at the best ratio.It was found that the cycle performance and rate capability of the battery were improved obviously.At the current density of 30 mA·g^-1,the initial discharge specific capacity was 150 mAh ·g^-1 and remained 149 mAh·g^-1 after 50 cycles.The discharge specific capacity of batteries was 108 mAh·g^-1 at a current density of 300 mA·g^-1.Finally,in order to improve the safety of lithium-ion batteries further,this paper prepared a bacterial cellulose?BC?separator,and found that the BC separator,with porous network structure and about 10-30 ?m thickness,can meet the requirements of lithium-ion battery.And then we assembled it as lithium-ion battery and the electrochemical performance of the battery was initially explored.