南方科技大学知识苑(SUSTech KC): 应用于磷酸铁锂电池的低温电解液设计与性能研究

题名	应用于磷酸铁锂电池的低温电解液设计与性能研究
其他题名	DESIGN AND PERFORMANCE STUDY OF LOW TEMPERATURE ELECTROLYTES FOR LiFePO4- BASED LITHIUM-ION BATTERIES
姓名	刘作成
姓名拼音	LIU Zuocheng
学号	12132056
学位类型	硕士
学位专业	0856 材料与化工
学科门类/专业学位类别	0856 材料与化工
导师	徐保民
导师单位	材料科学与工程系
论文答辩日期	2023-05-19
论文提交日期	2023-06-26
学位授予单位	南方科技大学
学位授予地点	深圳
摘要	锂离子电池由于能量密度高，循环寿命长，被大量应用在电动汽车和储能域。然而，在低温条件下，锂离子电池中的电解液由于离子电导率下降和Li+脱溶剂化过程困难等因素，导致充放电性能差，使用寿命降低，限制了其应用场景。因此，本论文基于新型锂盐和三元溶剂设计电解液，制备了低温条件下充放电性能优异的磷酸铁锂（LiFePO₄）电池。本论文选用碳酸二甲酯（DMC）、碳酸甲乙酯（EMC）、氟代碳酸乙烯酯（FEC）、双氟磺酰亚胺锂（LiFSI）作为电解液组分，设计了低温电解液配方（DMC : EMC=3 : 7 (vol)+8 vol% FEC，1.2 M LiFSI）。该电解液在−20 °C 的离子电导率为 2.82 mS/cm，高于商用电解液的 2.27 mS/cm。通过扫描电镜，透射电镜及电化学阻抗分析得知，该电解液能在正/负极表面上生成结构稳定且阻抗低的固体电解质界面（SEI）膜。该 SEI 膜均匀致密，有效地保护了石墨层的结构完整性，从而提升了低温性能。在−20 °C 下，使用该电解液的 Li‖LiFePO₄ 电池以 0.1 C 放电的比容量达到 107.66 mAh/g，与商用电解液（80 mAh/g）相比提升了 34.58%。以 0.2 C 循环 200 圈，容量保持率为 96.14%。 Li⁺在正负极界面处脱出/嵌入过程取决于 Li⁺与溶剂的结合能。因此，本文选用低溶剂化能的三氟乙酸乙酯（ETFA）设计了电解液（ETFA : EMC=3 : 7 (vol)+5 vol% FEC，1.2 M LiFSI）。在−20 °C 下，搭配该电解液的 Li‖C 电池放电容量达到 1.5 mAh @0.2 C，是常温放电容量的 49.5%。同时，使用ETFA 和 LiPF₆ 设计了电解液（ETFA : DMC : EMC=1 : 2~3 : 6~7 (vol)+5 vol% FEC，1.2 M LiPF₆）。在−20 °C 下，应用该电解液的 Li‖LiFePO₄ 电池以 0.1C 放电的比容量表现出 127 mAh/g，即使以 2C 高倍率充放电，放比容量仍然有 50 mAh/g。该电解液显著改善了 Li‖LiFePO₄ 电池的低温倍率性能，展现了低温快速充电的能力。通过透射电镜和 X 射线光电子能谱表征发现，LiFSI 和 LiPF₆均能在石墨上形成 SEI 膜，但只有 LiFSI 才能形成以无机盐为主的致密 SEI 膜。该无机盐 SEI 膜具有较高的离子导电率和结构稳定性，因此能显著提高低温性能。
其他摘要	Due to their high energy density and long lifespan, lithium-ion batteries have been extensively used in electric vehicles and the energy storage. However, the ionic conductivity of the electrolyte decreases and the desolvation process of Li⁺ is difficult at low temperatures. These problems lead to poor charge and discharge performance and reduced cycling life of lithium-ion batteries, which limit their application scenarios. Therefore, in this thesis, based on a new lithium salt and ternary solvent electrolytes, lithium iron phosphate (LiFePO₄) batteries with excellent charge and discharge performance under low-temperature conditions were obtained. We used dimethyl carbonate (DMC), methyl ethyl carbonate (EMC), fluoroethylene carbonate (FEC), and lithium bis(fluorosulfonyl)imide (LiFSI) as electrolyte components, and designed a low-temperature electrolyte formulation (DMC: EMC = 3:7 (vol) + 8 vol% FEC, 1.2 M LiFSI). The ionic conductivity of this electrolyte is 2.82 mS/cm at −20 °C, which is higher than the commercial electrolyte of 2.27 mS/cm. Scanning electron microscope (SEM), transmission electron microscope (TEM) and electrochemical impedance analyses showed that the electrolyte can generate structurally stable solid electrolyte interface (SEI) films with low impedance on the cathode/anode surfaces. The SEI films are dense and uniform, effectively protecting the structural integrity of the graphite layer and thus enhancing the low-temperature performance of lithium-ion batteries. At −20 °C, the specific capacity of Li‖LiFePO₄ batteries discharged at 0.1 C with this electrolyte reached 107.66 mAh/g, which is 34.58% higher than that of commercial electrolyte (80 mAh/g). After 200 cycles at 0.2 C rate, the Li‖ LiFePO₄ batteries capacity retention ratio is 96.14%. The process of Li+ detachment/embedding at the cathode and anode interfaces depends on the binding energy of Li+ to the solvent. Therefore, in this study, ethyl trifluoroacetate (ETFA) with low solvation energy was chosen to design the electrolyte (ETFA : EMC = 3 : 7 (vol) + 5 vol% FEC, 1.2 M LiFSI).The Li‖C batteries with this electrolyte had a discharge capacity of 1.5 mAh@0.2 C at −20 °C, which is 49.5% of the discharge capacity at room temperature.Also, the electrolyte was designed using ETFA and LiPF₆ (ETFA : DMC : EMC= 1 : 2~3 : 6~7 (vol) + 5 vol% FEC, 1.2 M LiPF₆). The Li‖LiFePO₄ cells with this electrolyte applied at −20 °C exhibited a specific capacity of 127 mAh/g discharged at 0.1 C. Even when charged and discharged at a high rate of 2 C, the discharge specific capacity was still 50 mAh/g. This electrolyte greatly improved the low-temperature rate performance of the Li ‖ LiFePO₄ batteries and demonstrated the ability for fast charging at low temperatures. The results of TEM and x-ray photoelectron spectroscopy (XPS) revealed that both LiFSI and LiPF₆ could form SEI films on graphite, but only LiFSI could form a dense inorganic salt-based SEI film. This inorganic salt-based SEI films has high ion conductivity and structural stability, which can significantly improve low-temperature performance of lithium-ion batteries.
关键词	磷酸铁锂电池低温电解液双氟磺酰亚胺锂三氟乙酸乙酯
其他关键词	LiFePO4 Batteries Low Temperature Electrolyte Lithium Bis(Fluorosulfonyl)Imide Ethyl Trifluoroacetate
语种	中文
培养类别	独立培养
入学年份	2021
学位授予年份	2023-06
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所在学位评定分委会	材料科学与工程
国内图书分类号	TM912.9
来源库	人工提交
成果类型	学位论文
条目标识符	http://sustech.caswiz.com/handle/2SGJ60CL/544001
专题	工学院_材料科学与工程系
推荐引用方式 GB/T 7714	刘作成. 应用于磷酸铁锂电池的低温电解液设计与性能研究[D]. 深圳. 南方科技大学,2023.

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