Mechanism of lithium transport through carbonaceous materials by analysis of current transients

Abstract

Lithium transport through the carbonaceous electrode was investigated in a 1M solution of LiPF6 in ethylene carbonate/diethyl carbonate (1:1 by vol. %) using galvanostatic intermittent titration technique, ac-impedance spectroscopy and potentiostatic current transient technique. All the experimental current transients measured on the SFG6 graphite composite electrode did not follow Cottrell behavior but Ohmic behavior which means the relationship between the initial current level and the potential step obeys Ohm's law. These experimental findings can be reasonably simulated under the 'cell-impedance-controlled' constraint. Thus, it is strongly asserted that the flux of the lithium ion at the electrode/electrolyte interface during lithium transport through the SFG6 graphite electrode is purely governed by 'cell-impedance'. Moreover in the case of the disordered carbon electrode (MCMB 610), besides non-Cottrell behavior 'quasi-current plateaux' were observed in the anodic current transients. These experimental results were theoretically analyzed based upon the modified McNabb-Foster equation as a governing equation, and the 'cell-impedance-controlled' constraint as a boundary condition. This strongly indicates that the difference in the kinetics of lithium transport between through the normal-sites and through the extra-sites is responsible for the difference in the site energy between of the normal-sites and of the extra-sites within the disordered carbon.