Many gel polymer electrolytes comprising polymer matrices, plasticizing organic solvents and lithium salts have been intensively studied for applications in rechargeable lithium batteries and other electrochemical devices - . Gel polymer electrolytes in the form of very thin membranes act simultaneously as transport for lithium ions, separator, and binder between the anode and cathode electrodes . Although gel polymer electrolytes with high ionic conductivity can usually be achieved by adding large amounts of organic solvents, they do not have sufficient mechanical ruggedness to withstand winding and stacking during manufacturing -, which may cause internal short-circuiting or lead to thermal runaway . These drawbacks of gel polymer electrolytes hinder their application in large-scale applications.
Polyvinyl alcohol and its derivatives have been widely noted for their advantages in their excellent thin-film forming ability, good heat- and water-resistance and relatively stable chemical structure. In recent years, polyvinyl acetal based gel polymer electrolytes have aroused much interest, Japan SONY , Mitsui Chemicals  and other companies have applied for patents successively about the polyvinyl acetal based gel polymer lithium- ion batteries via in-situ polymerization process.
In this work, a novel trilayer membrane is prepared with a PVFM layer coated with PVA-Al2O3 solution onto both sides to obtain the synergistic effects from each layer. This unique approach to the manufacture of trilayer membrane overcomes the drawbacks of gel-type membranes, that include poor mechanical strength and thermal stability in the electrolyte saturation state.
2.1. Preparation of Al2O3/PVFM/Al2O3 Trilayer Membranes
Polyvinyl formal (PVFM, MW: 70,000, Aldrich) powder was dissolved in N-methyl pyrrolidone (NMP, SCRC) to obtain a homogenous solution with the aid of magnetic stirrer. Chemical cross-linking agent 4,4’-diphenyl- methane diisocyanate (MDI, Alfa Aesar) was subsequently added into the above solution under stirring for 30 min at 75˚C. The deinized water as the nonsolvent was finally added to precipitate a white micelles, which was continuously stirred until it became a clear and viscous solution. The resulting slurry was coated on the glass plate and immersed in the coagulation bath to precipitate the polymer membrane. The obtained membranes were washed with deinized water, and the residual solution on the surface was removed with filter paper and finally dried under vacuum at 25˚C for 24 h. Then, the PVFM polymer membrane was coated by PVA-Al2O3 solution and dried under vacuum at 25˚C for 24 h.
2.2. Preparation of the Gel Polymer Electrolytes
Al2O3/PVFM/Al2O3 trilayer membranes were punched into disks of 16 mm in diameter. Dropping a small amount of liquid electrolyte (1 M LiPF6 in EC/DMC = 3:7 in vol, Beijing Institute of Chemical Reagent, China) to the surface of the polymer membranes in the argon-filled glove box in which the content of water and oxygen are less than 0.5 ppm, and then the liquid electrolyte penetrated into pores of membranes and swelled the polymer chains to form the GPEs.
2.3. Test and Characterization
Surface morphology was observed using field emission scanning electron microscope (FESEM) (Carl Zeiss, SUPRA55, Germany). Mechanical strength measurements were carried out on a tensile testing apparatus (INSTRON 5567 USA) at a crosshead speed of 10 mm/min, using the size of samples of 1 cm × 3 cm at room temperature. To examine the thermal transition behaviour of the membrane, the samples were placed in an oven and heated at 150˚C for 30 min. The ionic conductivity of the GPE was determined by the symmetrical cell SS/ GPE/SS using EIS with potential amplitude of 5 mV from 100 kHz to 0.01 Hz. The charge-discharge test was carried out using Land Battery Test System (LAND, CT2001A, China).
3. Results and Discussion
3.1. Surface Morphology
A schematic diagram of the trilayer membrane is presented in Figure 1. The cross-section and surface morphologies of the trilayer membrane as observed by FESEM, are given in Figure 2. Al2O3/PVFM/Al2O3 trilayer membrane has apparent trilayer structure, as shown in Figure 2(a). The cross-section of PVFM based membrane presents sponge pores which promote the transportation of lithium ions, as shown in Figure 2(b). The inorganic particulate films have porous structures due to the distance between the Al2O3 particles, as shown in Figure 2(c). PVFM based membrane forms homogeneous pores, as shown in Figure 2(d).
3.2. Mechanical Strength
Considering the rigorous requirement in the battery manufacture, the mechanical property of a polymer should be considered for its use in GPE  . Typical stress-deformation curves of PVFM and Al2O3/PVFM/Al2O3 based membranes are shown in Figure 3. The mechanical strength of PVFM based porous membrane is increased from 1.29 MPa to 3.27 MPa through coating PVA-Al2O3 solution onto both sides of the PVFM based membranes. The reinforcement in mechanical performance should be related to the Al2O3 that acts as temporary
Figure 1. Schematic diagram of trilayer membrane
(a) (b)(c) (d)
Figure 2. SEM images of Al2O3/PVFM/Al2O3 trilayer membrane. (a) Cross-section SEM of Al2O3/PVFM/Al2O3 trilayer membrane; (b) Cross-section SEM of PVFM based membrane; (c) Surface morphology of Al2O3 coating layer; (d) Surface morphology of PVFM based membrane.
mechanical connection point and enhances the strength of the membrane.
3.3. Thermal Stability
In lithium-ion batteries, the essential role of the separator is to prevent electronic contact, while enabling ionic transport between the cathode and the anode. Thus, the separators should be stable in the battery for a long period of time at high temperature . In order to investigate the thermal-resistant characteristics of the trilayer membrane, thermal shrinkage behaviour is observed after storing the membranes at 150˚C for 30 min. Photographs of the pure PVFM membrane and trilayer membrane before and after storage at 150˚C for 30 min are given in Figure 4. It can be seen from Figure 4 that PVFM based porous membrane shows obvious curl and retention is 83%. Al2O3/PVFM/Al2O3 trilayer membrane shows no obvious curl and retention is 97%.
Figure 3. The stress-deformation curves of PVFM based membrane (a) and Al2O3/PVFM/Al2O3 based membrane (b).
Figure 4. Photographs of membranes (a) before and (b) after being stored at 150˚C for 30 min. (1. Al2O3/PVFM/Al2O3 trilayer membrane 2. PVFM based membrane).
3.4. The Ionic Conductivity Behavior
Figure 5 presents the Nyquist plot of the cells at room temperature. The ionic conductivity was calculated from the bulk electrolyte resistance using the following equation (Equation (1)) :
where L is the thickness of the GPE or separator, S is the contact area between GPE or separator and SS disc, the bulk electrolyte resistance was obtained from the complex impedance diagram. Based on Equation (1), the obtained ionic conductivity of PVFM based GPE is 1.25 × 10−3 S/cm. While the obtained ionic conductivity of Al2O3/PVFM/Al2O3 based GPE is 4.13 × 10−4 S/cm and the obtained ionic conductivity of liquid electrolyte is 2.37 × 10−4 S/cm. The decreased conductivity of Al2O3/PVFM/Al2O3 based GPE is that the coating layer of Al2O3 particles blocks the routes for the lithium ions transportation, however it has a comparative ionic conductivity with liquid electrolyte, which can meet in the application of lithium-ion batteries.
3.5. Battery Performance
Figure 6 shows the discharge performance of the Li/LiFePO4 half-cell using the Al2O3/PVFM/Al2O3 based GPE at 0.2C from 2.5 V to 4.25 V at constant temperature of 25˚C. The discharge plateau is stable, showing small electrode polarization. Figure 7 presents the discharge capacities as a function of cycle number in the cells prepared with the liquid electrolyte and the gel polymer electrolytes, respectively. The initial discharge capacity density of the cell using the Al2O3/PVFM/Al2O3 based GPE is 124.7 mAhg−1. The discharge capacity density of the cell after 15th cycle is 140.3 mAhg−1. Compared with liquid electrolyte, PVFM based GPEs including pure PVFM based GPE and Al2O3/PVFM/Al2O3 based GPE show comparative cycling performance matched with electrodes LiFePO4.
Figure 5. Nyquist plot of PVFM based GPE, Al2O3/PVFM/Al2O3 based GPE and liquid electrolyte.
Figure 6. Discharge curves of the coin cell Li/Al2O3/PVFM/Al2O3 based GPE/LiFePO4 at the 0.2C rate between 2.5 V and 4.25 V.
Figure 7. Cyclic stability of the coin cell (a) Li/liquid electrolyte/ LiFePO4, (b) Li/Al2O3/PVFM/Al2O3 based GPE/LiFePO4 and (c) Li/ PVFM based/LiFePO4.
An organic/inorganic trilayer membrane consisting of Al2O3/PVFM/Al2O3 has been prepared. The membrane shows not only exceptional thermal integrity, but also enhanced mechanical properties due to the inorganic layers. Lithium ion cell with the Al2O3/PVFM/Al2O3 based GPE matched with LiFePO4 shows excellent electrochemical performance. The results show that this unique multilayer polymer membrane is a promising candidate for separator applications in lithium ion batteries.
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