Design And Preparation Of High-performance PVDF-based Gel Polymer Electrolyte And Study On The Performances Of Lithium-ion Batteries

With the continuous development of electrochemical energy,designing and exploring electrolyte system with high performance of mechanics,safety and electrochemistry is becoming the development trend and the focusing of attention in world's energy.It is noteworthy that the organic liquid electrolytes suffer from a series of safety problems because of their highly volatile and flammable nature.In addition,the leakage or evaporation of solvent from the electrolyte solution weakens the long-term stability of the battery.Especially,lithium battery of high specific energy with lithium cathode has severe problem of lithium dendrites which causes the decline of cycle performance.Poly(vinylidene fluoride)(PVDF)could effectively improve the properties of safety and cycle stability of lithium battery because of its high ignition temperature and stable potential.Therefore,it shows a good application prospect.In this study,gel polymer electrolyte(GEP)with three-dimensional multihole network was constructed by the introduction of an inorganic layered clay with nanosheet structure and solution casting.And its structure,morphology,heat stability and mechanical property were all investigated deeply.At the same time,the Li//GPE//Li Fe PO₄lithium battery was prepared and its electrochemical properties were tested.And the reasons of long cycle stability and good rate performance were studied as well.The specific work is as follows:1.A novel gel polymer electrolyte(GPE)based on poly(vinylidene fluoride)(PVDF)has been fabricated by introducing Laponite(LAP)nanoplates followed by activation in liquid electrolyte.The optimal doping amount of LAP was optimized to be 3 wt%.The composite GPE presents the optimal electrolyte uptake of 352 wt%and a breaking strength of 21.5 MPa with a strain of 300%are obtained,indicating a high ion conductivity and mechanical strength.When applied to LIBs with Li Fe PO₄ as cathode,the battery exhibits a high capacity of 157 m A h g^-1 at 0.2 C with an excellent initial coulombic efficiency of95%.After 1000 cycles,the capacity retention is as high as 97%.Remarkably,a superior capacity of 111 m A h g^-1 can still be maintained even at a high rate of 10 C.In addition,after 20 times bending and 30 times violent hammer strikes on the soft-pack battery,the electronic watch remains lighting.The results suggest that this composite membrane may have potential application in energy storage and provides a new way for the design of high-performance electrolyte system.2.Based on the above,in order to obtain physical and electrochemical properties of the composite electrolyte membrane,LAP was modified to get further degree of dispersion.Taking the characteristics of LAP that is prone to cation exchange,the hydrophilic LAP is modified into organo-LAP and introduced into a highly hydrophobic PVDF polymer system.Thus,a three-dimensional carrier with low crystallinity and no nano-particles aggregation is obtained,which improves the cycle stability of the polymer electrolyte.It is worth mentioning that the high ion conductivity 3.29×10^–4 S cm^–1 was obtained after adding the modifie organo-LAP into PVDF.The ion conductivity was obstained by SS//polymer electrolyte//SS battery.After 300 cycles,the capacity retention is as high as 91%.