Fabrication And Properties Of PBA/PVDF As Solid Electrolytes In Lithium-ion Battery

Solid electrolytes have attracted lots of attention because of its easy to deform, no leaking and great cycle performance as a new type of electrolyte used in Li-ion batteries. But its intrisic conductivity and interfacial compatibility need improvement. Electrolyte solution absorption performance and dielectric properties have significant effect on the conductivity and interfacial compatibility.The polymer PBA is compatible with the electrolyte solution which can guarantee high solution absorption performance. PVDF has a high dielectric constant and can effectively separate the cathode and anode. It acts as the supporting material as well. In this work, we prepared different kind of PBA/PVDF copolymers to receive electrolyte with both high conductivity and good interfacial compatibility.LiFePO4 was used as the cathode and Lithium wafers was used as the anode during the assemble of the button batteries. The morphology and structure of polymer matrix were characterized by FT-IR, XRD, SEM and DSC, and the cycle stability was characterized by charging and discharging at a constant current. Electrochemical windows and AC impedances were tested using a electrochemical workstation. Conductivity of solid polymer electrolytes can be calculated from the AC impedance spectroscopy.Firstly, we grafted acrylic monomers(BA) on the molecule chains of polyvinylidene fluoride(PVDF) via free radical polymerization to get PBA-co-PVDF as IPNs. Hot pressing was used to make thin films with a thickness of dozens of micron. The film was soaked in electrolyte made of LiPF6, EC, DMC and DEC until saturated absorption to get the PBA/PVDF solid electrolytes. It’s found that with the molar ratio of PVDF increased, the crystallinity increased gradually. On the other hand, the crstallinity decreasedy firstly and then increased if percentage of the crosslinking agent increased. The electrochemical performance is affected both by crystallinity, uptake of electrolyte and so on. The polymer matrix made with 1.5% crosslinking in PBA/PVDF with a molar ratio of 1: 0.5 showed the best physical and electrochemical performance. In detail, its electrochemical window reaches 4.5 V at room temperature and the first discharge capacity is 90mAh/g. Good performance remains after 50 cycle of charge-discharge. Ac impedance testing shows that the interface impedance changed when more crosslink agent and more PVDF component were added.Then we treated the Celgard 2500 with C12H25SO3 Na, SnCl4, PbCl2 and HCl aqueous solution to active the membrane. Actived membrane was then put in the prepolymerization solution of PBA and PVDF. The three parts were mixed together to get a PBA /PVDF polymer matrix which is based on Celgard 2500. Its electrochemical window reaches 4.5 V at room temperature, a value higher than PBA /PVDF without Celgard 2500. Charging-discharging curves show the capacity increased to 118mAh/g. What’s more, after 200 cycle charge and discharge the discharge capacity can still reach 110 mAh/g. Ac impedance testing shows the interface impedance reduced to 70 Ohm, which means the interfacial compatibility in this part is much better PBA/PVDF. The conductivity was calculated to be 9.9×10-4 S/cm.At last, aceltylene black(ACET) was coated on one side of PBA /PVDF polymer matrix based on Celgard 2500. ACET was mixed with PVDF powders in N-methyl pyrrolidone solution by ball grinding. In this part, we assembled both Li-ion and Li-S batteries. When assembled Li-S batteries, sulfur was used as the anode and lithium wafers acted as the cathode. From the test results, we can clearly see that ACET make great effect on Li-S batteries. Both capacity and cycle performance were enhanced by coating ACET on one side. As for Li-ion batteries, ACET coated on polymer matrix showed limited electrochemical performance improvement. Their interfacial impedance is only 58 ohm, and the conductivity of solid polymer electrolyte coated by ACET is 1.9×10-2 S/cm.