Gravimetric analysis of lithium-air batteries during discharge/charge cycles

Abstract

Because the mass of lithium-air battery (LAB) changes due to oxygen reduction/evolution reactions (ORR/OER), weighing of the battery is an indispensable method for determining its actual energy density and analyzing the battery reactions. To this end, we have newly developed a weight monitoring system which can trace the semi-permanent weight change of air-batteries (∼11 g including a holder) with a precision of 7.87 μg. The system reveals the weight decrease of LAB in each cycle before finally losing its capacity. The in-situ weighing during battery operation also provides first-hand evidence of double layer capacitance and electrolyte evaporation of LABs. Specifically, the e−/O2 ratio during discharge is determined to be 1.954 ± 0.004 based on the weight increase of an LAB cell using tetraethylene glycol-based electrolyte at a current density of 100 μA cm−2, confirming the slightly lower e−/O2 ratio than the ideal two electron ORR. During charge, the initial weight decrease is 5.4% less than the 2e−/O2 OER process, and the weight decreases sharply at the end of charge due to CO2 evolution. The main rates of weight increase/decrease remain the same throughout the cycle life, but side reactions are accelerated with cycling, causing the deterioration of LABs.