【作者】 赵晓凤；

【导师】 唐永福； 韩宝航；

【作者基本信息】 燕山大学 ， 化学工程（专业学位）， 2025， 硕士

【摘要】 近年来,锂离子电池（LIBs）因其高能量密度、长循环寿命等优点,在便携式电子产品如手机、电动汽车等众多领域得到了广泛应用。然而,传统碳酸酯类电解液在低温下存在离子电导率下降、界面阻抗增大等问题,导致LIBs出现显著的容量衰减和循环稳定性劣化,严重限制了其在低温场景下的实际应用。氟化羧酸酯溶剂因其高氧化稳定性、稳定的界面形成和弱溶剂化能受到了极大关注。因此,本文探究了氟化羧酸酯溶剂对锂离子电池低温性能的影响。具体研究内容如下:一方面,系统探究了不同碳链长度（C3～C5）的氟化羧酸酯类电解液体系对LiNi_0.8Co_0.1Mn_0.1O₂（NCM811）//石墨（Gr）电池常温和低温的电化学性能影响,并研究了不同电解液的溶剂化结构。以氟化羧酸酯为主溶剂、氟代碳酸乙烯酯为添加剂,构建包含二氟乙酸甲酯（MDFA）、二氟乙酸乙酯（EDFA）、二氟丙酸甲酯（M（2,2）DFA）、二氟丙酸乙酯（E（2,2）DFA）的电解液体系,其中E（2,2）DFA基电解液展现出优异的综合性能。在25℃下,该电池以1 C倍率下循环200次后仍具有162 mAh g^-1放电比容量,容量保持率达91.25%;在-20℃下,该电池以0.2 C倍率下循环100次后仍能放出116.4 mAh g^-1的比容量。此外,通过红外光谱和拉曼光谱技术对电解液溶剂化结构进行表征分析,结果证实,在E（2,2）DFA基电解液中存在较多接触离子对（CIPs）。另一方面,为了适配更低温度要求（<-20℃）,针对六氟磷酸锂（LiPF₆）基电解液在-20℃下因界面副反应加剧导致的循环容量快速衰退问题。本研究基于溶剂化能调控策略,采用双氟磺酰亚胺锂与二氟草酸硼酸锂组成的双锂盐体系替代单一的LiPF₆体系,以E（2,2）DFA为主溶剂,引入小分子体积的氟乙腈（FAN）作为共溶剂,通过调整E（2,2）DFA与FAN的比例来对电解液进行优化。实验表明,使用EF11电解液（E（2,2）DFA/FAN,1:1 v/v）的电池展现出较为优异的电化学性能,在-40℃下,采用该电解液的电池在0.1 C倍率下经100次循环后,容量保持率达73.4%(初始放电容量为116.6 mAh g^-1)。并且通过扫描电子显微镜和X射线光电子能谱表征手段进一步证实了通过电解液组分的优化,可以实现SEI膜组分的调整,从而改善电池的低温性能。更多还原

【Abstract】 In recent years,lithium-ion batteries（LIBs）have been widely applied in numerous fields such as portable electronic products like mobile phones and electric vehicles due to their advantages such as high energy density and long cycle life.However,traditional carbonate-based electrolytes have issues such as a decrease in ionic conductivity and an increase in interfacial impedance at low temperatures,leading to significant capacity decay and deterioration of cycle stability in LIBs,which severely restricts their practical applications under extreme working conditions.Fluorinated carboxylate solvents have attracted great attention because of their high oxidation stability,stable interfacial formation,and weak solvation energy.Therefore,this paper explores the influence of fluorinated carboxylate solvents on the low-temperature performance of lithium-ion batteries.The specific research contents are as follows:On the one hand,the influence of fluorinated carboxylic acid ester electrolyte systems with different carbon chain lengths（C3～C5）on the electrochemical performance of LiNi_0.8Co_0.1Mn_0.1O₂（NCM811）//Gr batteries at both room temperature and low temperature was systematically investigated,and the solvation structures of different electrolytes were explored.An electrolyte system comprising methyl difluoroacetate（MDFA）,ethyl difluoroacetate（EDFA）,methyl 2,2-difluoropropionate（M（2,2）DFA）,and ethyl 2,2-difluoropropionate（E（2,2）DFA）was constructed using fluorinated carboxylate esters as the main solvent and fluoroethylene carbonate as an additive.Among them,the E（2,2）DFA-based electrolyte exhibited excellent comprehensive performance.At 25℃,after 200 cycles at a rate of 1 C,the battery still had a discharge specific capacity of 162mAh g^-1,with a capacity retention rate of 91.25%.At-20℃,after 100 cycles at a rate of0.2 C,the battery could still deliver a specific capacity of 116.4 mAh g^-1.Furthermore,the solvation structure of the electrolyte was characterized and analyzed by means of infrared spectroscopy and Raman spectroscopy techniques.The results confirmed that there are more contact ion pairs（CIPs）in the electrolyte system based on ethyl2,2-difluoropropionate（E（2,2）DFA）.On the other hand,to address the requirement for lower temperatures（<-20℃）and tackle the issue of rapid cyclic capacity degradation caused by intensified interfacial side reactions in lithium hexafluorophosphate（LiPF6）-based electrolytes at-20℃,this study employed a solvation energy regulation strategy.A dual lithium salt system composed of lithium bis（fluorosulfonyl）imide（LiFSI）and lithium difluoro（oxalato）borate（LiDFOB）was used to replace the single LiPF₆ system.Using E（2,2）DFA as the main solvent,fluoroacetonitrile（FAN）with a small molecular volume was introduced as a cosolvent,and the electrolyte was optimized by adjusting the ratio of E（2,2）DFA to FAN.experiments showed that the battery using the EF11 electrolyte（E（2,2）DFA/FAN,1:1 v/v）exhibited relatively excellent electrochemical performance.At-40℃,after 100 cycles at a rate of 0.1 C,the battery using this electrolyte had a capacity retention rate of 73.4%(with an initial discharge capacity of 116.6 mAh g^-1).Moreover,scanning electron microscopy（SEM）and X-ray photoelectron spectroscopy（XPS）characterization methods further confirmed that by optimizing the components of the electrolyte,the components of the solid electrolyte interphase（SEI）film could be adjusted,thus improving the low-temperature performance of the battery.更多还原