High-Performance PVDF-HFP-Based Gel Electrolytes Design And Performance Study

Lithium metal batteries are promising high-energy-density batteries due to its high theoretical specific capacity and lowest redox potential.However,the use of conventional liquid electrolytes induces the haphazard growth of lithium dendrites during charge/discharge cycles,resulting in internal short circuit and thermal runaway,inhibiting the practical application of lithium metal batteries.Therefore,polymer electrolytes,including gel polymer electrolytes（GPEs）and solid polymer electrolytes（SPEs）,have started to receive much attention and research.The performance of polymer electrolytes is highly dependent on the properties of the polymer matrix.Among them,polyvinylidene fluoride-hexafluoropropylene（PVDF-HFP）is often used as polymer electrolyte matrix material due to its high dielectric constant,good electrochemical stability and easy processing.However,PVDF-HFP has poor mechanical properties,weak ability to inhibit lithium dendrites,and cannot achieve good cycling stability in lithium metal batteries.To this end,in this thesis,semi-interpenetrating networks of GPE and SPE were prepared by in situ copolymerization of neopentyl glycol diacrylate（NPGDA）and vinyl ethylene carbonate（VEC）monomers using in situ polymerization technology with electrostatically spun PVDF-HFP nanofiber membranes as the supporting backbone,thus improving the electrochemical stability and lithium dendrite inhibition of PVDF-HFP matrix.The research work of the paper is as follows:（1）NVPH,a composite gel polymer electrolyte with semi-interpenetrating networks,was prepared by dissolving NPGDA and VEC monomers and initiators in liquid electrolytes and polymerizing them in situ in electrospun PVDF-HFP nanofiber membranes（referred to as PH）.The GPE obtained by this method exhibited high ionic conductivity(2.89×10^-3 S·cm^-1)and high lithium ion mobility number（0.57）at room temperature,and the mechanical strength（13.82 MPa）and oxidation resistance voltage（5.2 V）were significantly increased.Tests on Li||Cu and Li||Li cells show that the lithium nucleation and growth overpotential of NVPH are significantly lower than that of PH,and the interface between electrolyte and electrode is more stable,which can effectively inhibit the growth of lithium dendrites.With these qualities,the LFP||NVPH||Li cell has excellent cycling stability with a capacity retention rate of 90.9%after 360 cycles at 2 C,and shows good high temperature resistance,flexibility and high safety performance.（2）The SPE were prepared by in situ polymerization method by adding NPGDA,VEC,initiator and lithium salts to the PH matrix.The nanofiber backbone improved the mechanical properties of the SPE,the in situ polymerization technique improved the interfacial contact problem between electrodes and electrolyte,and the added lithium borate dioxalate（Li BOB）suppressed the corrosion problem of lithium bis（trifluoromethanesulfonimide）（Li TFSI）on aluminum foil during the cycling process.The fabricated SPE has a lithium ion conductivity of 2.64×10^-4 S·cm^-1 at room temperature,a lithium ion mobility number of 0.4,and an electrochemical window of4.9 V.The Li||LFP cell assembled with this electrolyte can be stably cycled for 900cycles at 0.5 C with an average Coulomb efficiency of 99.99%.