Rate Limiting Effect of Charge Transfer during Lithiation from Ionic Liquids

Tuesday, 30 May 2017
Grand Ballroom (Hilton New Orleans Riverside)
M. T. F. Rodrigues (Rice University), X. Lin (Boston University), H. Gullapalli (Rice University), M. W. Grinstaff (Boston University), and P. M. Ajayan (Rice University)
Room temperature ionic liquids have long been investigated as potential solvents to improve the safety of electrolytes used in Li-ion batteries. Despite the benefits conferred by their non-flammability and negligible volatility, these compounds often present limited electrochemical performance at high cycling rates, reducing the power output of the cell. Although the rate capability can benefit from an increase in ionic conductivity, the overall reaction kinetics also depend on the charge transfer at the electrode/electrolyte interface and on the solid-state diffusion of lithium ions in the host lattice. The contribution of each of these three processes to the lithiation kinetics of lithium titanate spinel are described, using an ionic liquid containing an alkoxy-modified phosphonium cation and the TFSI anion in the electrolyte. Based on the analysis of activation energies, the charge transfer was found to be the rate-limiting step for cell operation. This finding was further corroborated by the observation that a 50x decrease in charge-transfer resistance at high temperatures led to a dramatic performance improvement. Moreover, analysis of typical activation energy values reported in the literature for each of these processes indicates that charge transfer will very frequently determine the cycling rate when ionic liquids are used as electrolyte. We propose that charge-transfer resistance and electrolyte wetting on the electrode surface need to be explicitly considered when designing new ionic liquids electrolytes in order to equal or exceed the power density obtained with the use of current carbonate-based electrolytes.