【作者】 陶然；

【导师】 张存中；

【作者基本信息】 北京理工大学 ， 环境工程（专业学位）， 2016， 硕士

【摘要】 随着人们对能源的迫切需求,理论上具有最高比容量(3861mAhg^-1)的锂金属二次电池进入研究视野,并逐渐成为一项热门研究。目前锂金属负极上的枝晶生长是制约锂金属二次电池安全性和循环寿命的关键因素之一。锂金属负极上枝晶生长刺穿隔膜,接触到正极,形成内部短路,轻则电池发热,严重时会发生爆炸,影响电池使用中的安全性;另外枝晶在生长过程中易断裂,减少了活性物质的量,降低电池的循环寿命。本研究从电解液着手,研究“刺穿”SEI膜与电解液接触的锂金属界面上电荷转移动力学行为与电解液中液相传质动力学行为,探究动力学行为对枝晶生长的影响,提出一个挑选电解液时的指标参考。研究中选取了四种支持盐（LiPF₆、LiAsF₆、LiBF₄和LiClO₄）和两种溶剂（PC和2MeTHF）,共配制八种电解液体系。使用Ni微电极（d=100μm）构成二电极体系测量电荷转移动力学参数,用Pt微电极（d=100μm）体系和Li（A=0.785cm²）对称电池体系结合测量液相传质动力学参数。结果证明,支持盐和溶剂这两个电解液的关键组分中,溶剂对电荷转移动力学行为和液相传质动力学行为的影响更明显,而在描述两种行为的四个参数里,不同溶剂的电解液得到的交换电流密度和迁移数具有一定的规律可循,而电荷转移系数和扩散系数则没有那么明显。结果得到,2MeTHF为溶剂时,交换电流密度小,迁移数大;PC为溶剂时,交换电流密度大,迁移数小。因此2MeTHF为溶剂时,电极表面不会形成明显的浓度差,锂的沉积致密平滑,循环寿命长;PC为溶剂时,电极表面浓度差较明显,锂沉积不均匀,锂枝晶长且易断,循环寿命短。在挑选电解液时,小的交换电流密度和大的迁移数可以作为挑选指标。此外,测量出的交换电流密度也是电池使用电流的参考值,使用电流密度大于交换电流密度30%-70%时,电极反应处于中极化状态,Li的沉积致密而均匀,这种状态下电池的循环寿命较长。为更加准确地得到交换电流密度和传递系数,本研究引入Hg微电极(9.922×10^-5cm²)做为工作电极,利用Hg微电极原子级平滑,电极表面Li原子不易堆积的特点,将锂金属/电解液界面上电荷转移步骤剥离出来,使测量更加准确。实验中使用LiPF₆+PC的体系作为电解液,Ni电极上测得的α=0.43±0.05,j₀=2.17±0.50mA/cm²;而Hg电极上测得的α=0.66,j₀=3.47mA/cm²。可以看出,固态电极上测得的电荷转移动力学参数会受到锂沉积步骤的影响,尤其是电荷转移系数。本研究的意义在于全面测量两种典型体系电解液（酯类和醚类）的电荷转移动力学参数和液相传质动力学参数,描述两种行为对枝晶生长的影响,提出可以通过交换电流密度和迁移数来作为挑选电解液时的参数。而随着技术的进步,当微汞电极能实现测量准确、重现性高时,可以通过测量交换电流密度来设计合适的使用电流密度。更多还原

【Abstract】 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=100μm）to constitute the two-electrode system to measure the kinetics parametersof surface reaction on anode.And using Pt microelectrode（d=100μm）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 resultα=0.43±0.05,j₀=2.17±0.50 mA/cm².And the Hg electrode’sresult areα=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.更多还原