Gravimetric Measurement of Lithium-Air Battery Cell during Discharge/Charge

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 + + O 2 + 2e - ↔ Li 2 O 2 . 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 - /O 2 , 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 - /O 2 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 CO 2 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. Fi