Preparation Of PVDF-HFP Based Microporous Membrane And The Application In Li-ion Battery

The microporous polymer electrolyte has high electricalconductivity at room temperature, the better thermodynamic stabilityand mechanical properties, while reducing battery assembly process forthe dry environment. It is the most promising polymer electrolyte to beused in lithium ion batteries. Especially the matrix of PVDF-HFP (alsoknown as 2801) for the polymer electrolyte, owns both HFP's highabsorbing ability and PVDF's high mechanical characteristics. It hasbroad prospects for development in the polymer lithium ion battery field.The paper prepared two kinds of composite polymer membrane——2801/PVP and 2801/SiO₂, by adding PVP and SiO₂ into PVDF-HFP(2801) on the basis of the condition's optimization of phase inversionmethod and natural evaporation method. Related properties of themembranes were tested and analyzed. At the same time, electrochemicalproperties of assembled lithium battery based on the two polymermembranes were characterized systematically.Firstly, optimized the related technology of phase transfer method,determined DMF as solvent, deionized water as non-solvent, earlyforming film 2h, coagulation bath temperature at room temperature(about 23℃), under above conditions, the best properties of polymermembrane was prepared.On this basis, When 2801: PVP as 1:0.4 (mass ratio), the membraneowns good morphology and swelling ability, the ionic conductivity is upto 5.85×10^-3S/cm at room temperature, the lithium ion transferencenumber is 0.74, the electrochemical stable window is 5.5V, theseproperties meet the requirement of lithium ion battery. The type of2801/PVP battery exhibits an initial discharge capacity of 136mAh/g anddischarge plateau of about 3.88V at 0.1 current rate. After 10 cycles, itsdischarge capacity goes up to 139mAh/g, then it keeps stable and thecoulombic efficiency exceeds 98% during the cycling. When dischargingat 0.5 and 1 current rate, it can retain 97%,94% of the dischargecapacity of 0.1 current rate, respectively. The interfacial impedance ofthe polymer battery keeps stable with the voltage while theelectrochemical impedance is increased with the reducing of voltage. On the other hand, assembled the polymer electrolyte acquired bynatural evaporation method into 2801/PVP battery, it exhibits an initialdischarge capacity of 142.37mAh/g and discharge plateau of about3.88V at 0.1 current rate. After 30 cycles, its discharge capacity goesdown to 136.68mAh/g, the coulombic efficiency keeps stable at 96% orso. When discharging at 0.5 and 1 current rate, it can retain 95.4%,92.2% of the discharge capacity of 0.1 current rate, respectively.Secondly, the composite polymer membrane of 2801/SiO₂ wasprepared by the natural evaporation method. When 2801: SiO₂ as 1:0.1(mass ratio), the membrane owns good morphology, the ionicconductivity is up to 4.24×10^-3S/cm at room temperature, the lithium iontransference number is 0.65, the electrochemical stable window is 5.5V.The type of 2801/SiO₂ battery exhibits an initial discharge capacity of138mAh/g and discharge plateau of about 3.8V at 0.1 current rate. Theinitial coulombic efficiency is 88 %, then after 3 cycles, its coulombicefficiency keeps stable at 95 % during the cycling. The dischargecapacity falls down when the cycle increases, it is only 119mAh/g at thethe 30th cycle.Finally, the paper compared the acquired excellent 2801/PVPbattery with traditional liquid lithium ion battery, the results show thatthe polymer battery's performance come close to the liquid battery, thecapacity retains well, the discharge plateau is high.