LiF和LiCl对石墨电极电化学性能的影响
Influence of Electrochemical Performance of Lithium Ion Batteries with the Adding of LiF and LiCl
Received date: 2016-08-05
Online published: 2016-10-20
六氟磷酸锂是目前商品化锂离子电池中使用最广泛的电解质锂盐，LiF和LiCl是除水和酸之外六氟磷酸锂产品中最重要的杂质.运用扫描电子显微镜（SEM）、充放电、循环伏安法（CV）以及电化学阻抗谱测试（EIS）等研究了LiF和LiCl对石墨电极电化学性能的影响.充放电结果表明，在1 mol/L LiPF6-EC：DEC：DMC电解液中添加饱和的LiF，可以显著提高石墨电极的充放电可逆容量并改善其循环性能，而在1 mol/L LiPF6-EC：DEC：DMC电解液中添加饱和的LiCl，虽也可提高石墨电极的首次充电容量，但严重恶化石墨电极的充放电循环稳定性.CV结果表明，电解液中LiF、LiCl的存在对EC的还原分解过程影响较小.但SEM和EIS的结果指示，LiF、LiCl对石墨电极表面SEI膜的形成过程影响较大.在添加饱和LiF的电解液中石墨电极表面形成的SEI膜较薄且电阻较小，进而提高了石墨电极的可逆循环容量及改善了其循环稳定性；但在饱和的LiCl电解液中石墨电极表面形成的SEI膜较厚且电阻较大，严重恶化石墨电极的电化学循环稳定性.
任彤 , 庄全超 , 郝玉婉 , 崔永丽 . LiF和LiCl对石墨电极电化学性能的影响[J]. 化学学报, 2016 , 74(10) : 833 -838 . DOI: 10.6023/A16080394
In the past few decades, lithium hexafluorophosphate (LiPF6) is the most widely employed ionic component in organic electrolyte solutions for commercial lithium ion battery, which is manufactured using PCl5, LiF and HF as raw materials via the HF solvent method in the large scale production, and then it commonly contains LiF and LiCl impurities besides water and acid. However, the influence of LiF and LiCl on the performance of lithium ion battery is still not clear. Thus, in this paper, the influence of LiF and LiCl on the electrochemical performance of graphite electrode was investigated using charge-discharge test and cyclic voltammetry (CV) combining with scanning electron microscope (SEM) and electrochemical impedance spectrum (EIS). Charge-discharge test results showed that the electrochemical performance of graphite electrode such as reversible capacity and cycling stability were significantly improved in 1 mol/L LiPF6-EC:DEC:DMC electrolyte with the saturation of LiF. The initial charge capacity of graphite electrode in 1 mol/L LiPF6-EC:DEC:DMC electrolyte with the saturation of LiF is 331.0 mAh/g, which is higher than that in 1 mol/L LiPF6-EC:DEC:DMC electrolyte (307.9 mAh/g). After 65 charge-discharge cycles, the charge capacity of graphite electrode in 1 mol/L LiPF6-EC:DEC:DMC electrolyte with the saturation of LiF is 340.1 mAh/g, which is also higher than that in 1 mol/L LiPF6-EC:DEC:DMC electrolyte (297.0 mAh/g). However, although the first charging capacity of graphite electrode was enhanced in 1 mol/L LiPF6-EC:DEC:DMC electrolyte with the saturation of LiCl, the charge-discharge cycling stability was serious deteriorated. The initial charge capacity of graphite electrode in 1 mol/L LiPF6-EC:DEC:DMC electrolyte with the saturation of LiCl is 334.2 mAh/g, yet after 65 charge-discharge cycles, the charge capacity of graphite electrode in 1 mol/L LiPF6-EC:DEC:DMC electrolyte with the saturation of LiCl is 251.2 mAh/g. CV results showed that the influence of LiF and LiCl on the decomposition process of EC in electrolyte is small. SEM and EIS results stated that the SEI film which was formed on the graphite electrode is thinner and has a smaller resistance in 1 mol/L LiPF6-EC:DEC:DMC electrolyte with the saturation of LiF than that in 1 mol/L LiPF6-EC:DEC:DMC electrolyte. Thus the reversible cycle capacity of graphite electrode was increased and its cycle stability was improved. Nevertheless the SEI film which was formed on the graphite electrode is thicker and its resistance is higher in 1 mol/L LiPF6-EC:DEC:DMC electrolyte with the saturation of LiCl than that in 1 mol/L LiPF6-EC:DEC:DMC electrolyte, which leads to the deterioration of electrochemical performance of graphite electrode.
Key words： lithium ion battery; graphite electrode; LiPF6; LiF; LiCl
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