Effects Of Electrolytes On The Kinetic Performance Of Mass Transfer And Charge Transfer Of Li~+/Li System

With the urgent need for energy,metal lithium secondary battery is being researched and gradually become a hot research because of the highest theoretical specific energy(3861mAhg^-1).The dendrite growthing on lithium metal anode is one of the key factors which influence thesafety and cycle life of lithium metal secondary battery.The dendrites growth and pierce the separator,exposed to the positive electrode,forming an internal short circuit,causing fever and even explosion,threatening batteries'safety.In addition,dendrite is easy to break in its growthing process,reducing the amount of active materialand reduce the batteries'cycle life.This study started from the electrolyte,and researched the dynamic behavior on the electrode surface and the thermodynamic behavior in the electrolyte,explore the thermodynamic parameters and kinetic parameters on dendrite growth.The study selected four supporting salt?LiPF₆,LiAsF₆,LiBF₄ and LiClO₄?and two solvents?PC and 2MeTHF?,a total of eight kinds formulated electrolyte system.Using Ni microelectrodes?d=100?m?to constitute the two-electrode system to measure the kinetics parametersof surface reaction on anode.And using Pt microelectrode?d=100?m?system and Li?A=0.785cm²?symmetry battery system to measure thethermodynamics parameters in cell.The results demonstrated that in the key component ofelectrolyte,solvent and solute,the solvent made greater effect to kinetics behavior and thermodynamics behavior.And in the four parameters which to descrip the two behaviors,the exchange current density and transference number,of the different electrolytes,has a certain rules to follow,and transfer coefficient and diffusion coefficients are not so obvious.The result is,when 2MeTHF as solvent,the exchange current density is smaller than that PC as the solvent,and transport number is bigger than the PC as the solvent.Therefore when 2MeTHF as the solvent,the surface of electrode does not form a distinct concentration gradient of lithium ion,the Li deposition was dense and smooth,and the cell has a long cycle life;when the PC as solvent,the concentration gradient is obvious,Li deposition was uneven,lithium dendrites is long and easily broken,the cell has short cycle life.Therefore,when selectingan electrolyte,we want an electrolyte owns a small exchange current density and a large migration,it can be used as the indicators when selection.In addition,the exchange current density measured can used to design the using current density,if the current density is greater than the exchange current density of 30%-70%,the electrode reaction is in medium polarization the Li depositionis dense and smooth,the battery will have longer cycle life.To obtain a more accurate exchange current density and transfer coefficient,this study introduced Hg microelectrode(9.922×10^-5cm²)as the working electrode,because the Hg microelectrode has atomic scale's smooth,and Li atom is uneasy to accumulation in the electrode surfaces that means we can only research the kinetic behavior of the electrode's anode,make the measurements more accurate.Experiments using LiPF₆+PC system as the electrolyte,the conventional metal Ni electrode get the result?=0.43±0.05,j₀=2.17±0.50 mA/cm².And the Hg electrode'sresult are?=0.66,j₀=3.47mA/cm².It is obviously that the kinetic parameters measured on the conventional solid electrode were affected by lithium-deposition step,Especially the charge transfer coefficient.Significance of this study lies in the overall measurement of two typical systems of electrolyte?esters and ethers?in both kinetic parameters and thermodynamic parameters,describe the impact of these two acts to dendrite growth,made possible use exchange current density and transport number to selecte electrolyte.With advances in technology,if the Hg microelectrodes canachieved accuracy measurement and high reproducibility,it can be designed to measuring accurate exchange current density todecide the appropriate current density when actually use it.