The Improvement of Shelf Life by Predischarging in Flexible Lithium Battery

Introduction Flexible Li/MnO2 polymer batteries comes many problems that were insignificant for Li/MnO2 batteries housed within metal containers. One of these is the swelling of the pouch cells during storage prior to use. Preliminary investigations suggest that the swelling of the pouch cells during storage is associated with gas generated by decomposition of the electrolyte solvent. The most commonly used solvent is propylene carbonate (PC). In this case, the gas most responsible for the swelling of the pouch cells during storage is CO2[1-3]. In this paper to prevent electrolyte decomposition and to improve shelf life by pre-discharging flexible Li battery. To accomplish this study, linear sweep voltammetry, cyclic voltammetry, and impedance analysis were introduced. Experimental The positive electrode plate was prepared as follows; To make binder solution, polyvinylidene fluoride (PVDF) powder was melted by N-methyl pyrrolidone (NMP). The binder solution was contained 8 wt% of PVDF. 53.1 wt% of electrolytic manganese dioxide (EMD, Mitsui mining), 2.4 wt% of acetylene black (AB) and 44.5 wt% of binder solution were mixed by planetary mixer for 5hrs. The homogenized slurry was obtained and then the slurry was coated on an Al-foil operated as a current collector. The resulting product was dried in a drying oven maintained at 120 for 30 minutes and it was dried again in a vacuum drying oven maintained at 120°C for 1 hr. The dried mixture was pressed by a roll press and then cut to a size of 2.2 × 3.35 cm to use as positive plate. A lithium foil was used as an anode. The gel electrolyte was synthesized from propylene carbonate (PC, Sigma-Aldrich), PMMA(Sigma-Aldrich, Mw=120K) and lithium bis(trifluoromethylsulfonyl) imide (LiTFSI, SigmaAldrich). The gel electrolyte prepared by 0.75M LiTFSI, PC solvent and melting 15 wt% of PMMA and finally, the obtained gel electrolyte was sieved by passing the stainless steel mesh(#325). The gel electrolyte was coated over the positive plate and then positioned the separator(a non-woven polypropylene fabric) on gel electrolyte. The negative plate was also positioned on the separator and the stacked electrodes were covered with a heat confusable laminated film(pouch), which constituted the exterior material for the cell. And then, the cell was sealed under vacuum condition to make a film-like flexible lithium battery. The surface area of the cell was about 14.1 cm. The thickest part of the cell was not more than 0.48 mm, and the average thickness was 0.45 mm. The test cell was assembled by sandwiching gel electrolyte between a composite cathode and lithium metal anode. Results and Discussion To control the cell potential in stable range, predischarging method was carried out after cell assembling. After cell assembling, the initial cell potential was about 3.55 V. To reduce the cell potential, the test cells were pre-discharged from 2.5% to 5% based on their theoretical capacity. And then, they were kept at 60°C drying oven. As time goes by, the OCV of no pre-discharged cells were decreased gradually from initial potential, but pre-discharged cells over 2.5% based on theoretical capacity of MnO2 were controlled under 3.3V until 40days at 60°C. To study the influence of pre-discharging current and method, we fixed the pre-discharging amount at 5% based on theoretical capacity of MnO2 and 3 types of discharging method was applied for this study. As the first approach, relatively the high rate of discharge current(C/4) is applied on the cell to pre-discharge. The secondly, relatively the low drain of discharge current(C/40) is applied on the cell to pre-discharge. Lastly, 2 step pre-discharging method was applied, the cells were pre-discharged by high rate current at the first step and then they were pre-discharged again by low drain current. The result is shown in Figure 3. The