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

Abstract

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