二次电池金属锂电极循环性能改进的基础研究

发布时间：2018-03-30 05:01

本文选题：金属锂电极　切入点：锂二次电池　出处：《武汉大学》2014年博士论文

【摘要】：随着对高比能二次电池要求的逐渐提高,金属锂作为理想的二次电池负极材料近些年来又重新得到了人们的重视。这是因为金属锂在目前研究的固态电池负极材料中具有最高的理论比容量和最低的标准电极电势等优点。但同时它又存在着在沉积—氧化循环过程中易生成锂枝晶的致命缺点,这是阻碍金属锂二次电池成功商业化的最重要的原因。一方面生成的锂枝晶容易脱离金属锂基体而形成“死锂”,造成电池库仑效率降低、容量衰减以及寿命缩短；另一方面锂枝晶可能会穿透隔膜引发电池内部短路,影响电池的安全性能。因此抑制锂枝晶的生长也成为发展金属锂电池的关键课题。针对金属锂负极在循环过程中容易形成锂枝晶的问题,本文从选择适合金属锂电极循环的基底材料和改进电解液体系这两方面入手,以提高金属锂电极在常用的有机电解液中的沉积—氧化循环性能为目的,开展了对金属锂电极的系统研究。本论文的研究工作主要包括以下四方面的内容： (1)利用超声辅助电沉积的方法得到一种金属锌基底材料Znuae。通过恒电流充放电循环、恒电流脉冲极化、循环伏安(CV)、交流阻抗(EIS)、扫描电子显微镜(SEM)等方法的测试,分析讨论了Znuae基底材料适合金属锂沉积—氧化循环的原因。采用恒电流充放电循环测试比较了锂在金属铂(Pt)、金属铜(Cus)、金属锌(Zns)、电沉积锌(Zne)和超声辅助电沉积锌(Znuae)这五种基底材料上的循环性能,证实Znuae基底最适合金属锂的沉积—氧化循环。并对几种常见的电解液体系进行了优化,发现金属锂在LiBOB/EC+DMC的电解液里的沉积—氧化循环特性最优。金属锂在Znuae基底材料上以0.1mA cm-2的电流密度进行沉积—氧化循环时,循环900周后的库仑效率还可以保持在95%以上；金属锂循环的电流密度增大到0.3mA cm-2时,在循环600周时的库仑效率还可以达到90%。金属锂在这种基底材料上的循环性能得到明显改进可能存在两方面的原因：一方面金属锌能够与沉积的金属锂形成合金,使金属锂更容易进行均匀沉积,同时也有利于减缓由金属锂与电解液反应形成的SEI膜的生长速度,从而得到致密均匀的SEI膜；另一方面的原因是利用超声辅助电沉积方法制备的基底材料表面具有纳米尺寸的沉积颗粒和纳米孔洞,使电极的真实比表面积增大,这样能够减小金属锂在沉积—氧化过程中的实际电流密度。 (2)在Znuae基底的基础上利用同位电沉积金属锂的方法制备了新型Li-Zn合金基底。通过恒电流脉冲沉积、CV、EIS和SEM测试,计算出在这种基底材料上金属锂沉积的交换电流密度相比于在金属锂和Zne上的结果均有明显提高,并且基底材料表面的界面阻抗随着循环的进行能保持良好的稳定性。金属锂以0.1mA cm-2的电流密度在这种基底上进行充放电循环时,在单周沉积30min、库仑效率设定为97%的情况下,其循环寿命可以达到400周。增大电流密度到0.2mAcm-2,单周沉积为30min,库仑效率设定在97%时循环寿命也可以达到250周,且在这种基底材料上金属锂首周库仑效率可以达到95%以上。产生这种结果的原因主要是由于Li-Zn合金自身的性质决定了金属锂在其表面的沉积容易进行；其次是在Li-Zn合金基底形成的过程中金属锂的沉积速度缓慢,这样更有利于在基底表面生成均匀致密的SEI膜,对后续金属锂的循环起到良好的保护作用。同时,在这种基底材料上电解液的影响被弱化,金属锂在LiClO4/EC+DMC电解液体系中的循环性能表现良好。 (3)以Li-Zn合金基底为基础,在LiClO4/EC+DMC电解液中添加不同量的NaC104配制成混合电解液体系,研究了金属锂在Li+:Na+不同比例的混合电解液中的循环性能,优化出1mol L-1LiClO4+0.5mol L-1NaClO4/EC+DMC是最适合金属锂循环的电解液体系。通过EDX测试证实在这种混合电解液中,金属锂的沉积过程为Li-Na共沉积。通过ICP方法对电解液的分析可知,在共沉积的Li-Na产物中的Na可以被再氧化。利用恒电流脉冲沉积、CV、EIS和SEM等测试手段证明,随着循环的进行,电解液中添加的Na+对SEI膜的性能具有改进作用,不仅提高了SEI膜的稳定性,而且使基底材料表面的界面阻抗减小,使金属Li-Na的共沉积—共氧化过程更容易进行。充放电循环测试的结果表明,在Li-ZAn合金基底上以0.2mA cm-2的电流密度进行Li-Na共沉积—共氧化循环时,其沉积电势升高而氧化电势降低,在沉积时间设为60min的条件下,循环的库仑效率可以达到95%以上,并且能够以90%以上的库仑效率循环500周；当库仑效率降为83.3%时循环寿命可以达到800周以上；当以同样的电流密度沉积120min时,金属锂仍然能够以90%的库仑效率循环200周。这表明利用Li-Na共沉积—共氧化的过程来代替纯金属锂在有机电解液中的循环有利于抑制锂枝晶的生长,从而提高金属锂负极的循环性能,同时Li-Zn合金基底对Li-Na共沉积—共氧化也具有促进作用。 (4).以Li-Zn合金基底为基础,初步探究了在LiClO4/EC+DMC电解液中添加少量的Mg2+添加物对金属锂循环的影响。利用CV、EIS和SEM等测试手段证明,电解液中添加ppm级的Mg2+能使金属锂的沉积—氧化过程更容易进行。充放电循环测试的结果表明,金属锂在1mol L-1LiClO4+400ppm Mg(C104)2/EC+DMC电解液中以0.2mA cm-2的电流密度沉积60mmin,当库仑效率降为83.3%时的循环寿命可以达到500周以上,与在未添加Mg2+的电解液中的金属锂循环性能相比得到了明显提高。
[Abstract]:Lithium dendrite has the highest theoretical specific capacity and the lowest standard electrode potential in the cathode material of solid - state battery , which is the most important reason for the successful commercialization of lithium secondary battery .
On the other hand , the lithium dendrite may penetrate the diaphragm to induce short - circuit in the battery and affect the safety performance of the battery . Therefore , it is a key task to develop the lithium dendrite in the process of cycling . In order to improve the deposition and oxidation cycle performance of the lithium metal electrode in the usual organic electrolyte , the system research on the metal lithium electrode is carried out . The research work of this paper mainly includes the following four aspects :

The cycling performance of metal - zinc - base material Znuae was studied by means of constant current charge - discharge cycle , constant current pulse polarization , cyclic voltammetry ( CV ) , alternating current impedance ( EIS ) and scanning electron microscope ( SEM ) .
when the current density of the lithium metal cycle is increased to 0.3 mA cm - 2 , the coulombic efficiency at the cycle of 600 weeks can reach 90 % .
On the other hand , the surface of the base material prepared by the ultrasonic auxiliary electrodeposition method has nano - sized deposition particles and nano pores , so that the real specific surface area of the electrode is increased , so that the actual current density of the lithium metal in the deposition - oxidation process can be reduced .

On the basis of Znuae substrate , a novel Li - Zn alloy substrate was prepared by the method of co - electrodeposition of metal lithium .
secondly , the deposition rate of the metal lithium is slow during the formation of the Li - Zn alloy substrate , which is beneficial to generating uniform and dense SEI film on the surface of the substrate , and has good protection effect on the cycle of the subsequent metal lithium ; meanwhile , the influence of the electrolyte on the base material is weakened , and the cycle performance of the lithium metal in the LiClO 4 / EC + DMC electrolyte system is good .

Based on Li - Zn alloy substrate , different amounts of NaC104 are added to the mixed electrolyte system in LiClO 4 / EC + DMC electrolyte to study the cycling performance of Li - Na in the mixed electrolyte of Li + : Na + . The results show that the deposition potential of Li - Na is higher than that of Li - Na . Under the condition that the deposition time is 60min , the coulombic efficiency can reach more than 95 % , and can be circulated for 500 weeks with coulombic efficiency over 90 % .
When the coulombic efficiency is reduced to 83.3 % , the cycle life can reach more than 800 weeks ;
When the same current density was deposited for 120 min , the lithium metal was able to circulate for 200 weeks at 90 % coulombic efficiency . This indicates that the use of Li - Na co - deposition - co - oxidation is beneficial to the inhibition of the growth of the lithium dendrite , thus improving the cycle performance of the lithium metal negative electrode , while the Li - Zn alloy substrate has a promoting effect on Li - Na co - deposition - co - oxidation .

( 4 ) Based on the Li - Zn alloy substrate , the effect of adding a small amount of Mg2 + on the cycle of lithium metal was investigated by means of CV , EIS and SEM . The results show that the cycle life of lithium metal lithium in 1 mol L - 1LiClO _ 4 + 400 ppm Mg ( C104 ) 2 / EC + DMC electrolyte can reach more than 500 weeks , which is obviously improved compared with the cycle performance of lithium metal in the electrolyte without Mg 2 + .

【学位授予单位】：武汉大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TM912

【参考文献】