Optimization for electrochemical redox performance of Li⁺/Li couple based on steady-state polarization curve

In this investigation, steady-state polarization curve was obtained on micro-disc Pt electrode in a kind of classical industrial ternary organic electrolyte, ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) (1:1:1 at volume ratio) containing 1 mol L^-1 LiPF₆, and used as both fine description of polarization degree and performance overview of lithium anode. The limiting mass transfer current density, j_lim^s-s, was used as a feature parameter to optimize electrochemical redox performance of Li⁺/Li couple. Based on the steady state polarization curve and j_lim^s-s, we found that performance of Li⁺/Li exhibited the best coulombic efficiency (from 65% to 80%) of the electrochemical redox process (i.e. electro-deposition/electro-dissolution process), as current densities were controlled in a moderate current density scope, 0.26j_lim^s-s∼0.72j_lim^s-s, in the ternary organic electrolyte. In the moderate current density scope, lithium anode appeared the best cyclic performance and coulombic efficiency due to two types of dendrites, "tree-like" dendrite and "moss-like" dendrite, could be suppressed effectively. The technique of in-situ optical microscopic analysis was used to observe the micro-pattern of lithium metal deposition layers formed in the whole current density scope. The results showed that the thicker dendrite ("tree-like" dendrite) and thinner dendrite ("moss-like" dendrite) were formed at lower and higher electro-deposition current density, respectively. Moreover, electro-crystallization mechanism of lithium anode was fitted by Heerman's model, and the results indicated that initial nucleation behavior was the main reason which influenced lithium anode redox performance with the impact of two parameters, initial nucleation number N₀ and nucleation rate constant A. Based on aforesaid results, steady-state polarization curve and limiting mass transfer current density, j_lim^s-s, were suggested as a universal and objective standard to normalize experimental parameters to confirm the most suitable polarization condition of lithium anode in used electrolytes.