Effect of binders performance aproticLi-O2 cells

Abstract

theoretical density of gasoline, making it very attractive. However, actual efficiency is much lower than the theoretical value mainly due to problems with unwanted reactions generating overpotentials because of decomposition or accumulation of insoluble species on the cathode. To date, the most widely used binder for Li batteries is polyvinylidene fluoride, PVDF. It has recently been reported that this type of binder undergoes a decomposition reaction due to superoxide ion attack generated during the discharge process2. These unwanted reactions strongly affect the performance of the battery. On the other hand, it has been found that other binders such as Nafion are more stable2. In this work, the galvanostatic response obtained from a cathode under an unstable binder such as PVDF will be studied and their performance is compared with a cathode where Nafion was used. The cells were galvanostatically discharged by an Arbin BT-2000 battery tester at room temperature. During the tests, pure O2 was continuously circulated at the cathode (3.0 ml min−1). Li–O2 cells were tested after 6 h of rest at open circuit potential (OCV). Long time charge/discharge tests were also carried out by potential time controlled steps between 2.25 V and 4.4 V vs. Li+/Li at a current rate of 0.1 mA cm−2. In this study, 0,5M of LiClO4 /tetraglyme solution was used as electrolyte. The air cathode was prepared as a thin film over carbon paper GDL24BA based current collector. A N-methyl-2- pyrrolidone (NMP) slurry of previously prepared α-MnO2 or Co3O4 was mixed with CSW as electronic conductor and poly-(vinylidenefluoride) PVdF or Nafion as binder in the weight ratio of 10:70:20 respectively, was deposited over GDL using doctor blade technique. In this work the cell is used is ECCAir electrochemical cell (El-Cell, GmbH). 0 10 20 30 40 50 60 70 2,0 2,5 3,0 3,5 4,0 4,5 C SW : Nafion (80:20) C SW : PVDF (80:20) Potential vs Li+/Li (V) Time (hours) A 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 2,0 2,5 3,0 3,5 4,0 4,5 α-MnO2: C SW: Nafion α-MnO2: C SW: PVDF Potential vs Li+/Li (V) Time (hours) B 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 2,0 2,5 3,0 3,5 4,0 4,5 C o3O4: C SW: Nafion C o3O4: C SW: PVDF Potential vs Li+/Li (V) Time (hours) C Fig. 1. Comparison of first discharge/charge profile of the cells with Nafion and PVDF as binder in cathode (A) without catalyst and in presence of catalyst (B)α-MnO2 and (C) Co3O4. Figure 1A compares the 1st complete discharge/charge profiles of the cells assembled with two binders, Nafion and PVDF without catalyst. Both cells exhibited a high discharge capacity, however the cell with PVDF was completely unable to be recharged. In presence of catalyst (Fig1. b and c), in both cells with PVDF, the discharge is very similar and the recharge is better compared to uncatalyzed cathode but the times still are lower in comparison with cathodes with Nafion as a binder. Finally, based on these preliminary results, it could be established that the stability of the binder plays a very important role for the performance of Li-O2 cell.