Theoretical and Experimental Study on Mass Transfer of Battery Organic Electrolyte in Ambient Air

A Lithium-ion (Li-ion) batteries are major energy storage resources for electronic devices and electric vehicles. Among the types of Li-ion batteries, the lithium-air (Li-air) battery is promising due to its theoretically high energy capacity which is comparable to that of gasoline. However, formation of lithium oxides from the oxygen reduction reaction (ORR) in the Li-air cathode during discharging operation can cause voltage loss by lithium oxides precipitation in the cathode electrode and eventually leads to a failure of operation. Therefore, understanding the ORR spatial variation is crucial in Li-air cathode design and development. On the other hand, electrolytes are key parts of battery, which provide a pathway for ion transport to the electrochemical reaction sites. Battery electrolyte organic solvents are volatile, flammable, and harmful to the human body, causing eye and skin irritation, soreness, and/or nausea. Battery surface damage in events of electric vehicle collisions or dropping portable devices may cause electrolyte leakage, exposure to air and diffusion in air, raising health concerns. In addition, operation of the Li-air cathodes, which involves direct contact with ambient air, has a major concern of electrolyte loss if no effective protection layer to prevent electrolyte evaporation is applied. In the first part of this study, the spatial variation of cathode reaction rates in a Li-air battery was investigated with 1-D reaction-diffusion transfer models for a single- and double-layer cathode structure. In order to find an optimal cathode design with minimum spatial variation, the nondimensionalized equation was analyzed with various Damköhler numbers (Da) and transfer coefficients (β) values. In the second and third parts of this study, evaporation-driven mass transport models were proposed to estimate diffusivity of battery organic solvents in ambient air and the impacts of adding a porous layer. The measured organic solvents are 1,2-dimethoxyethane (DME), dimethyl carbonate (DMC), diethyl carbonate (DEC), and propylene carbonate (PC) which are typical electrolyte materials. After the model of diffusivity in air was validated using liquid water and alcohol, diffusivities of DME, DMC, and DEC are measured and found to be 0.0925 cm2/s, 0.2116 cm2/s, and 0.0569 cm2/s, respectively. The PC testing that showed a liquid loss smaller than the measurement uncertainty due to its slow evaporation rate resulted in failure of measurement. Thus, the proposed method is valid for liquid solvents that have relatively fast evaporation under ambient conditions. The evaporation measurement for the case of a porous layer showed that adding a porous layer effectively depressed solvent evaporation and diffusion. However, the model prediction and the experimental results were inconsistent as they have 1,000 times difference in correction constant (ετ). One possible hypothesis to account for this large discrepancy is the solubility between the pore material and the liquid solvents