Optimization On Electrochemical Operation Conditions Of Lithium Metal Anode For Lithium Metal Secondary Batteries

In this investigation, we obtained steady-state polarization curve 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 1mol?L-1 LiPF6, and used it as both fine description of polarization degree and performance overview of lithium anode. Steady-state polarization curve reflects the whole working region of current density of Li metal anode, so it can act as a criterion evaluating Li metal anode’s electrochemical performance and selecting the suitable current density scope that corresponding to optimal electrochemical performance. The size of micro-disc Pt electrode and scan rate of Linear Scanning Voltammetry test are two key parameters that affect outcome of steady-state polarization curve. We obtained perfect “S” type steady-state polarization curve on 100μm micro-disc Pt electrode by using 200mV·s-1 scan rate. High viscosity electrolytes, however, need larger scan rate or smaller size micro-disc Pt electrode to get available steady-state polarization curve, vice versa.The limiting mass transfer current density, jlims-s, was used as a feature parameter to optimize electrochemical redox performance of Li+/Li couple. Based on the steady state polarization curve and jlims-s, we found that performance of Li+/Li exhibited the best coulombic efficiency(from 65% to 80%) of the electrochemical redox process(i.e. electrodeposition/electro-dissolution process), as current densities were controlled in a moderate current density scope, 0.26jlims-s~0.72jlims-s, in the ternary organic electrolyte. In the moderate current density scope, lithium anode appeared the best cyclic performance and coulombic efficiency because Li metal deposition layer was compost and SEI was stable and uniform. However, in low current density and high current density conditions, lithium anode’s coulombic efficiencies were either good or durable. Self-corrosion rate of lithium anode in high current density condition was quit large because “moss-like” dendrite has larger specific area and higher chemical activity, which contribute to lithium loss through heavy side reaction, such as, forming SEI.The technique of in-situ optical microscopic analysis was used to observe the micropattern of lithium metal deposition layers formed in the whole current density scope. The observation 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, while impact lithium deposition was observed clearly at moderate current density condition. In order to figure out the mechanism of lithium dendrite generation and its morphology change, electro-crystallization behavior of lithium anode was analyzed, using Heerman’s electro-crystallization model to fit I-t curves. The fitting 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.As current density increased, both initial nucleation number N0 and nucleation rate constant A became larger, which means that the electro-crystallization behavior of lithium anode changed from progressive procedure to instantaneous procedure. At lower overpotential condition, initial nuclei N0 and nucleation rate constant A were both small, a great amount of “tree-like” dendrites generated due to the tip effect caused by high mass transfer rate at the end of single lithium nuclei’ fine tip. At higher over-potential, the numerous initial nuclei N0 and larger nucleation rate constant A contribute to larger space between lithium nuclei, therefore the “moss-like” dendrite generated easily and massively. While, in the middle over-potential condition, both initial nuclei N0 and nucleation rate constant A were suitable for compact lithium deposition layer, because the coulomb force nuclei acting on free lithium-ion was balanced and the interval between nuclei was even for lithium atom to insert.Based on aforesaid results, steady-state polarization curve and limiting mass transfer current density, jlims-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.