The Research And Application Of PVDF-Based Composite Polymer Electrolyte

In this dissertation, we prepared composite polymer electrolytes (CPE) with high electrochemical performances by introducing a unique nano-sized inorganic material. The effects of inorganic fillers on the electrochemical performances of CPE and the mechanism were fully studied and analyzed, as well as the effects of the inorganic fillers and lithium salts on aluminum current collector's corrosion at both room and high temperature. The unique lithium salt-lithium bis(oxalate) borate (LiBOB) was applied as lithium salt additive, and its effect on aluminum corrosion, the compatibility with cathode active materials were thoroughly studied. We also combined the theoretical research and knowledge with practical application, applied the CPE preparation system to modify the separator used in commercial liquid Li-ion batteries. 383562-typed polymer Li-ion batteries used in airplane models were assembled. The electrochemical and safety performances of the battery were systematically studied.PC-401-a nano-sized fumed alumina with ultra-fine surface structure-was firstly introduced into the CPE system. CPE were prepared through phase inversion method. PVDF-HFP copolymer was used as matrix host, acetone and ethanol as solvent/non-solvent system and PC-401 as inorganic filler. The results revealed that the addition of inorganic fillers can significantly affect the physical and electrochemical performances of the composite polymer electrolyte, such as cystallinity, porosity, electrolyte uptake rate, as well as the ionic conductivity, Li ion transference number and the compatibility with metallic lithium. Through adjusting the content of inorganic fillers in the polymer electrolyte can optimize these properties. Series experiments showed that the inorganic fillers to copolymer ratio of 1:10 is optimization. With this ration, the crystallinity of the PE decreases from 23% (no filler is added) to 9%; in the meanwhile, the porosity and electrolyte uptake ration are also enhanced with similar extent. The ionic conductivity of the PE is 0.89 mS/cm, which can meet the practical application in commercial Li-ion batteries properly. The Li ion transference number is as high as 0.47, much higher than the liquid electrolytes with values in the level of 0.2-0.3. Therefore, the PE we prepared can be used in high power Li-ion batteries which ask for high rate charge and discharge.The aluminum corrosion behavior in Li-ion batteries was systematically studied for the first time. A novel equivalent circuit to analyze the aluminum corrosion behavior was proposed. The results obtained through various electrochemical methods show that the inorganic filler acts as a good inhibitor of aluminum corrosion behavior. And the higher of the working voltage, the better of the inhibiting effect. The research revealed that the addition of the inorganic filler can modify the composition, construction and the thickness of the passivation layer in the interface between the PE and aluminum. All these factors result in the protection of aluminum from corroding.We compared the aluminum corrosion behaviors of different lithium salts, the results showed that LiBOB exhibits the lowest corrosion current. Moreover, the high temperature stability of LiBOB towards aluminum foil is quite high: the corrosion current remains relatively low at temperatures as high as 70℃. The order of the stability of different lithium salts with aluminum is: LiBOB>LiBF4>LiPF6>LiClO4.The LiBOB was used as additive in commercial electrolytes and it's effect on the performance of the electrolyte was studied. It is shown that only 5% of LiBOB in LiPF6 electrolytes can greatly inhibit the aluminum corrosion behavior. This fact is beneficial to improve the cycle life and storage performance of the batteries. As additive, LiBOB can also enhance the initial cycle efficiency, as well as the stability and impedance of the interface layer of cathode material.The structural stability of LiMn2O4 is relatively low, especially at high temperatures. LiMn2O4 will dissolve in the LiPF6-based electrolyte and results in poor high temperature performances of LiMn2O4 Li-ion batteries. Storing at for 10 days, addition of 5% LiBOB can decrease the irreversible capacity loss of LiMn2O4 Li-ion batteries from 35% to 26%. It means that LiBOB can effectively decrease the self-discharge rate of LiMn2O4 Li-ion batteries. This fact is of certain practical importance to the application of polymer Li-ion batteries.We applied our preparation method and system of composite polymer electrolyte into modifying the separator used in commercial liquid Li-ion batteries. The SEM graphs clearly display the micrographic morphology of the surface and cross-section of the modified separator. 383562-typed polymer Li-ion batteries were assembled using the modified separator, LiFePO4 and complex graphite were used as cathode and anode active materials, respectively. The initial nominal capacity of the battery is over 500 mAh. The prepared battery performs well in high current rate charge and discharge. The discharge capacity at 10 C remains 90% of initial capacity. The high power cycle performance of the battery is very good, the capacity fading after 200 cycles at 10 C is about 5% of initial capacity. However, the discharge plateau at high rates and the high temperature performance of the battery is relatively poor. This is mainly limited by the cathode active material and the electrolyte based on LiPF6.The battery has high safety performance. It survives after various safety testings, including overdischarge to 0V, overcharge at 3 C to 10V and heat shock at 150℃for 1h and external short circuit. There was no electrolyte leakage, fire or explosion after all these tests.