Degradation Mechanism of Lithium-Oxygen Batteries with High Areal Capacity and Lean Electrolyte

Abstract

Because the nonaqueous Li–O2 batteries are still in their infancy due to numerous problems, there is no report, which actually attained the energy density of 500 Wh/kg in a complete cell level as a rechargeable battery. Except a few works[ref.1-3], almost all studies reported so far included too much excess weight of electrolytes, which results in much lower energy densities than those of LIBs. When the electrolyte amount is decreased to fulfill the energy density of 500 Wh/kg, the cell cannot be cycled. Therefore, it is especially important to quantify the decomposition reactions in Li–O2 batteries under more practical conditions of less electrolyte amount and high areal capacity[ref.4].In previous studies, less attention have been payed about the amounts of Li metal electrode and liquid electrolyte, when they determined cycle life of Li–O2 cells, even though these amounts are the important factors to determine not only energy density but also cycle life. Almost all studies so far have been carried out using a thick Li foil (100–500 μm thick). Because the equivalent weight of Li and graphite are 3860 and 372 mAh/g, respectively, the capacity ratio of negative to positive electrodes (N/P ratio) should be at least less than 3860/372 ≒ 10 to keep the advantage over graphite electrodes. When the areal capacity of the Li metal anode is set to 4 mAh/cm2, the thickness of the Li metal is calculated to be about 20 μm. On the other hand, the electrolyte amount has a more significant influence on the energy density than the Li metal thickness owing to the larger mass density of the electrolyte (1.16 g/cm3) than that of Li metal (0.53 g/cm3). The ratio of the electrolyte volume to the total pore volume of cell components is an important parameter, however, the ratio of electrolyte weight to cell capacity (E/C, gA/h) is empirically used to represent the electrolyte amount in LIBs.10 The E/C ratio in our experiments is 10 gA/h, which is much smaller than those of previous studies (E/C ≥ 50 gA/h).In the present study, we have examined reaction products in a porous carbon positive electrode by using a two-compartment cell design, where anode and cathode compartments were separated by a solid-state Li+ conductor to eliminate possible interference from the reactions at Li metal negative electrode. Because the monitoring the gas consumed/produced during the operation is nearly the only way to determine the CE of the O2 electrode, we used on-line gas analysis as well as pressure change measurements for understanding parasitic reactions coming from electrolyte decomposition. As a result, we clarified that the ratio of electrolyte weight to cell capacity is a good parameter to correlate with cycle life. Only 5 cycles were obtained at an areal capacity of 4 mAh/cm2. When the areal capacity was decreased to half, the cycle life was extended to 18 cycles. However, the total electric charge consumed for parasitic reactions was 35 and 59% at the first and the third cycle, respectively. This surprisingly large amount of parasitic reactions was suppressed by half using redox mediators while keeping a similar cycle life. Based on by-product distribution, we will propose possible mechanisms of TEGDME decomposition.