STUDY OF IONIC LIQUID AS NOVEL ELECTROLYTE FOR NEXT GENERATION LI BATTERIES

Kisoo Yoo

The main focus of this work is to develop mathematical and computational models for next generation lithium batteries. There are three major parts in this work. In the first part, a mathematical model was developed to study the performance of lithium-air batteries considering the significant volume changes at the anode and cathode sides. Here, moving boundary technique was used to obtain governing equations for transport of lithium ions and oxygen as well as for liquid phase potential. A numerical method was introduced to solve moving boundary problem using finite volume method. Using this model, the electric performance of lithium-air battery was obtained for various load conditions. Numerical results indicate that cell voltage drops faster with increase in load. The volume change significantly affects the electrical performance of lithium-air cell. Highly soluble lithium peroxide can reduce the passivation in the cathode, but it can also reduce the effective reaction area in the anode. However, the benefit of the former outweighs the detriment of later phenomenon. In the second part of the work, an electrochemical model was developed for transport of ionic components in a univalent ternary electrolyte frequently encountered in ionic liquid electrolytes of lithium batteries. Owing to the very high concentration of components in ionic liquid, the transport of lithium ions was described by the mutual diffusion phenomena using Maxwell-Stefan diffusivities, which are obtained from atomistic simulation. The model was verified with experimental studies for a lithium-ion battery where the electrolyte comprises ionic liquid with mppy+ (N-methyl-N-propyl pyrrolidinium) cation and TFSI- (bis(trifluoromethanesulfonyl) imide) anion. In the last part of the work, the mathematical model developed for ionic liquid electrolyte is used to study the electric performance of a Li-Air battery. We particularly studied the Li-air cell performance at elevated temperature where MPPY-TFSI is sued as electrolyte. Our simulation results showed that the battery performance can be improved significantly by increasing operating temperature. Simulation results also revealed that by increasing the operating temperature, the specific capacity can be improved significantly for high load current density, which is one of the most critical drawbacks in most Li-air battery.

STUDY OF IONIC LIQUID AS NOVEL ELECTROLYTE FOR NEXT GENERATION LI BATTERIES

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