No abstract

Lithium‐oxygen batteries (LOBs) are promising next‐generation rechargeable battery candidates due to theoretical energy densities that exceed those of conventional lithium‐ion batteries. Although LOB with high cell level energy density has been demonstrated under lean electrolyte and high areal capacity conditions, their cycle life is still poor, and the cell degradation mechanism remains unclear. In the present study, by use of a three‐electrode electrochemical setup and in situ MS analytical techniques, it is revealed that the reaction efficiency of the negative lithium electrode largely decreases due to chemical crossover from the positive oxygen electrode side, such as H2O and CO2. Based on this mechanistic understanding, a LOB with an ultra‐lightweight flexible ceramic‐based solid‐state separator with 6 µm thickness that effectively protects the lithium electrode against chemical crossover without diminishing the energy density of LOBs is fabricated. Notably, a 400 Wh kg−1 class LOB exhibits a stable discharge/charged process for >20 cycles. The strategy demonstrated in this study sheds light on the direction for the practical implementation of LOBs with high energy densities and long cycle lives.

Lithium−oxygen batteries (LOBs) are promising nextgeneration rechargeable batteries due to their high theoretical energy densities. The optimization of the porous carbon-based positive electrode is a crucial challenge in the practical implementation of LOB technologies. Although numerous studies have been conducted regarding the relationships between LOB performance and the physicochemical properties of carbon electrodes, most of these studies evaluate the performances of the electrodes under unrealistic conditions with inappropriate technological parameters. In this study, we prepared carbon gel-based self-standing membranes as positive electrodes and evaluated their performances in LOBs under lean-electrolyte, high-areal-capacity conditions. We clarified the following three crucial points: (1) The nanometer-sized pores exhibited limited effects in improving the cycle performance, although they contributed in enhancing the discharge capacity. (2) The macro-sized pores displayed positive effects in enhancing the discharge capacity. (3) The crystallinity and/or surface functional groups influence the discharge potential and cycle life. The results of this study suggest the significance of controlling the physicochemical properties of a porous carbon-based positive electrode in preparing a LOB with a practically high energy density and an extended cycle life.

Lithium-air battery (LAB) is expected to have an extremely high energy density, 5-10 times higher than that of the current lithium-ion batteries. To extend the cycle life of LAB, the oxygen reduction/evolution reactions (ORR/OER) at the cathode must be close to the ideal 2Li+ + O2 + 2e- ↔ Li2O2. So far, these reactions have been quantitatively analyzed using pressure cells and/or differential electrochemical mass spectrometry (DEMS). However these methods are not suitable for long-term measurements such as long-term discharge/charge or cycle tests. We have developed a high-precision weight measurement system that automatically measures the weight of a coin cell during the discharge/charge process. Performing zero correction each time enables long-term monitoring of the cell weight with accuracy of the balance used. Cell weight measurement also provides a more accurate quantitative analysis of the oxygen consumed and the gases evolved than DEMS. Figure 1(a) shows the galvanostatic discharge/charge profile of a LAB cell under pure oxygen atmosphere. The cell was comprised with a carbon nanotube (CNT) sheet cathode (16 mm in diameter) and a Li metal anode filled with 1M LiTFSI/TEGDME electrolyte. Figure 1(b) presents the amount of cell weight change, which synchronously responded to the discharge/charge profile. The cell weight exhibited -5.63 mg/h weight loss during the first 50 h rest. This corresponds to slow evaporation of the electrolyte solvent and was subtracted as a background in the following analysis. The following 50 h discharge brought linear weight gain by ORR at the cathode equivalent of 1.955 (±0.005) e-/O2, which was slightly lower than the ideal number of 2 for two electron reduction. If we simply assume one-electron and two-electron reactions, this indicates that 4.5% of oxygen inhaled remained one electron reduction while the rest underwent two electron reduction. Then the cell experienced a linear weight loss at 2.109 (±0.006) e-/O2 during the first 30 h of charge, which corresponded to 94.8% weight of ideal oxygen evolution. The weight loss was gradually accelerated near the end of charge, which is attributed to the evolution of CO2 detected by the DEMS. Although the cell weight change gives a complementary profile of the DEMS, it provides a simple way to monitor the discharge/charge reactions of the LAB cells with high accuracy. Long-term cell weight changes under multiple discharge/charge cycles will also be discussed. Figure 1