【作者】 李斌；

【导师】 李伟善；

【作者基本信息】 华南理工大学 ， 材料物理与化学， 2016， 博士

【摘要】 锂离子电池以其能量密度高、循环寿命长等优点而广泛应用于电子器件,并成为电动交通工具的主要动力电源之一。随着人们对锂离子电池性能的要求越来越高,电池的工作条件变得越来越苛刻,如高低温、高电压等。锂离子电池的电极/电解液界面性质对电池的性能影响巨大。本文提出了二种新型电解液添加剂,用于改善电极/电解液界面性质,通过设计和优化电极/电解液界面膜,开展了碳酸丙烯酯（PC）基电解液、高温LiMn₂O₄/石墨电池、高电位（4.7 V vs.Li+/Li）LiNi_0.5Mn_1.5O₄材料和高电压LiCoO₂/石墨电池体系相关的电极/电解液界面膜性质研究。主要得到以下结果:（1）为了提高宽液态温度范围（-48.8²42oC）溶剂PC与石墨负极的兼容性,通过理论计算,设计和优选出4-氟苯基醋酸酯（4-FPA）作为负极SEI成膜添加剂,并与醋酸苯酯（PA）进行比较。在石墨/Li电池和在LiFePO₄/石墨电池中,使用4-FPA的电池具有更好的电化学性能。用扫描电子显微镜（SEM）、能量色散光谱（EDS）和傅里叶转换红外光谱（FTIR）等表征手段,对4-FPA的成膜机理进行了分析。结果表明,氟原子的引入,令4-FPA具有更强的还原能力,能够在石墨表面形成比PA具有更好效果的SEI膜,有效抑制PC对石墨电极的破坏。（2）以理论计算为指导,设计和优选出有机分子丙烯基-1,3-磺酸内酯（PES）作为负极成膜添加剂,并与1,3-丙磺酸内酯（PS）进行比较。在LiCoO₂/石墨电池中,采用PES的电池表现出更好的电化学性能。用SEM、EDS、FTIR、X射线光电子能谱（XPS）等表征手段,探讨了添加剂PES的作用机理。结果表明,不饱和键的引入提高了PES的还原能力,使其能够在石墨电极上优先还原形成比PS更好的SEI膜,抑制PC对石墨的破坏,有效提高了电池的性能。（3）针对LiMn₂O₄/石墨电池高温循环性能差的问题,提出应用PES作电解液添加剂的解决方案,详细研究了PES对LiMn₂O₄/石墨电池高温性能的影响,并与添加剂VC作对比试验。结果表明含有PES电解液的电池能够保持良好的循环性能,并能较好地抑制电池气胀问题。用SEM、XPS、X-射线衍射（XRD）、热重分析（TGA）、分子轨道理论计算等方法,探讨了添加剂的作用原理,结果表明添加剂PES能够同时在电池的正极和负极表面形成界面膜,正极表面的界面膜能阻止尖晶石LiMn₂O₄材料锰的溶出,而负极表面的SEI膜能较好地抑制锰离子在负极的沉积,防止电解液的分解,提高了电池的高温循环性能。（4）为了解决高电位（4.7 V vs.Li+/Li）Li Ni_0.5Mn_1.5O₄材料在有机碳酸酯电解液中循环性能差的问题,研究了高电位下LiNi_0.5Mn_1.5O₄电极/电解液的界面性质,探讨了添加剂PES对界面性质的影响。结果表明PES能有效提高LiNi_0.5Mn_1.5O₄的循环性能。用SEM、透射电子显微镜（TEM）、XRD及XPS等手段,分析了添加剂的作用机理。结果表明,PES能够在LiNi_0.5Mn_1.5O₄上形成含硫的界面膜,钝化电极表面的催化活性,抑制电解液的分解,提高电池的循环性能。（5）为了解决LiCoO₂/石墨电池高电压下（4.5 V）性能差的问题,通过研究电极/电解液界面膜的性质,探讨了添加剂碳酸乙烯亚乙酯（VEC）对高电压LiCoO₂/石墨电池性能的影响。结果表明电池经过400次循环后,VEC的使用将电池容量保持率由38%提高到87%。用SEM、EDS、XPS等方法,分析了添加剂的作用机理。结果表明,VEC能够在电池的正负极表面形成优良界面膜,保护LiCoO₂晶型结构,阻止Co溶解和在负极上的沉积,有效抑制电解液的分解,明显提高了电池的循环性能。更多还原

【Abstract】 Due to its high energy density and long cycling life compared to other secondary batteries, lithium-ion battery has been successfully used in electronic devices and is regarded as a promising power source for electric vehicles. In order to meet the demands of advanced portable devices and achieve energy sustainability, considerable improvements and advancements of lithium ion battery（e. g., high performance at high voltage, elevated and low temperature, etc.） are still required. To meet these requirements, it is necessary to modify the interfacial properties of electrode/electrolyte. In this dissertation, two novel electrolyte additives were developed to improve the interfacial property improvements of several electrode/electrolyte systems, including graphite/propylene carbonate（PC） based electrolyte, LiMn₂O₄ and graphite at elevated temperature, LiNi_0.5Mn_1.5O₄, and LiCoO₂/graphite at high operation voltage, related mechanisms were understood. The obtained results are as follows:（1） To prevent the co-intercalation of PC, which has low melting point（about-49 oC）, 4-fluorophenyl acetate（FPA） was selected as an solid electrolyte interphase（SEI） formation additive for the graphite anode, based on density functional theory（DFT） calculations. Charge/discharge tests in graphite/Li and Li FePO₄/graphite cells show that the cells using 4-FPA exhibit better performance than that using phenyl acetate（PA） as the SEI formation additive. The results from scanning electron microscopy（SEM）, energy dispersive X-ray spectroscopy（EDS） and Fourier transform infrared spectrum（FTIR）, show that fluorine not only makes 4-FPA more reducible but also improves the stability of SEI on graphite, which inhibits the decomposition and co-intercalation of PC more effective than that of PA.（2） To improve the compatibility of PC and graphite anode, prop-1-ene-1, 3-sultone（PES） was selected as the anode SEI formation additive. The charge/discharge test in LiCoO₂/graphite cell shows that the cell with PES exhibits better performance than that with propane sultone（PS） as the SEI formation additive. The results from SEM, EDS, FTIR and X-ray photoelectron spectroscopy（XPS）, show that the C=C structure in PES increases the electro-withdrawing ability of the additive, resulting in PES has higher reduction activity and favors to build a more protective SEI than PS.（3） PES is applied as an electrolyte additive to improve the cycling stability of LiMn₂O₄/graphite cells at elevated temperatures, with a comparison of vinylene carbonate（VC）. Charge/discharge tests show that the cell using PES exhibits better performance than that using VC as the additive. The results from SEM, XPS, X-ray diffraction（XRD）, Thermogravimetric analysis（TGA） and DFT calculations, show that PES simultaneously forms protective interfacial films on the anode and cathode of the LiMn₂O₄/graphite battery. The cathodic interfacial film prevents the manganese dissolution from Li Mn₂O₄, while the SEI on the anode protects the graphite from the co-insertion of solvent, prevents the electrolyte decomposition and inhibits the deposition of the dissolved manganese from cathode.（4） To improve the cycling stability of LiNi_0.5Mn_1.5O₄ cathode, the cathode/electrolyte interface was modified by using PES as an electrolyte additive. Constant current charge/discharge test shows that the application of PES improves significantly the cycling stability of LiNi_0.5Mn_1.5O₄. The SEM, transmission electron microscopy（TEM）, XRD, and XPS analysis results show that PES decomposed preferentially to the electrolyte and the sulfur-containing compounds on the LiNi_0.5Mn_1.5O₄ cathode may poison the catalyst of the nickel manganese oxide. Thus suppressed the electrolyte decomposition at high voltage（>4.7 V vs. Li+/Li） and protects the cathode material, leading to the improved cycling performance of the LiNi_0.5Mn_1.5O₄/Li cell.（5） Vinyl ethylene carbonate（VEC） was used as an electrolyte additive to improve the cycling stability of Li CoO₂/graphite battery at high operation voltage（3.0-4.5 V）. Charge/discharge tests demonstrate that the battery with VEC exhibits significantly improved cycling stability. The capacity retention is increased from 38% to 87% after the using of VEC. The results from SEM, TEM, and XPS show that VEC simultaneously forms stable interfacial films on anode and cathode of the LiCoO₂/graphite battery, which protects the electrode particles from destruction, reduces the Co dissolution from cathode and deposition on anode, and suppresses the electrolyte decomposition on the electrodes.更多还原