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近年来锂离子电池在电动汽车领域的应用日益广泛，与此同时，该领域对锂离子电池提出更高能量密度的需求，使用高压正极材料是提高能量密度的有效手段之一，然而，高电压下传统电解液与正极材料之间界面不稳定，电解液易氧化分解，增大锂离子电池的不可逆容量，进而降低了锂离子电池的循环稳定性，因此锂离子电池耐高压电解液的开发对进一步提高锂离子电池的能量密度至关重要。目前，提高高压下电解液与正极材料界面稳定性的有效方法之一是使用高压电解液添加剂。添加剂自我分解调控界面膜，使得该界面膜可以有效隔绝电解液与正极材料间接触，抑制正极材料的溶解和电解液的氧化分解。此外，添加剂用量少、效果显著，因此近年来备受科学界和产业界的研究和关注。本论文主要研究了无机锂盐氟锆酸锂和有机物氟代碳酸乙烯酯及多巴胺两类电解液添加剂对“Li/LiNi0.5Mn1.5O4”和“Li/LiNi0.6Co0.2Mn0.2O2”电池界面膜的调控，研究内容和结果如下：(1) 合成了无机添加剂氟锆酸锂，将其作为添加剂应用于“Li/LiNi0.5Mn1.5O4”电池中，研究其循环性能和倍率性能。实验结果表明在常规电解液中加入0.15 mol L-1氟锆酸锂后，Li/LiNi0.5Mn1.5O4半电池的首圈放电比容量为128 mAh g-1，200圈后的容量保持率为88.79 %；同样测试条件下，常规电解液首圈放电比容量为113.6 mAh g-1，200圈后电池的容量保持率为83.89 %。EIS分析表明氟锆酸锂参与形成的SEI膜的阻抗较小；SEM和TEM测试表明无添加剂时，循环200圈后LiNi0.5Mn1.5O4表面形成不均一且较厚的SEI膜，而有氟锆酸锂存在时，氟锆酸锂可以调控界面膜，使其均一稳定；XPS和XPS刻蚀分析表明，氟锆酸锂可以有效抑制电解液高压下的氧化分解且保护正极材料。(2) 采用Gaussian 09中B3LYP方法计算添加剂氟代碳酸乙烯酯、多巴胺以及传统溶剂EC、DMC、EMC的HOMO能级，并将其分别应用在“Li/LiNi0.6Co0.2Mn0.2O2”、“Li/LiNi0.5Mn1.5O4”半电池中，测试添加剂对电池性能的影响。恒电流充放电测试表明，在基础电解液中添加质量比为10 %的FEC后，可以显著提高Li/LiNi0.6Co0.2Mn0.2O2电池的循环稳定性。在基础电解液中添加质量比为0.1 %的多巴胺后，有利于Li/LiNi0.5Mn1.5O4容量的发挥。电化学分析方法表明，含有添加剂的电解液在正极表面分解形成的SEI膜的阻抗值较小且较稳定。;Nowadays, the use of lithium ion batteries in the field of electric vehicles have been increasingly extensive. Meanwhile, the demand for higher energy density of lithium-ion batteries has been raised. The use of high-voltage cathode materials is one of the effective means to increase the energy density. However, traditional electrolytes are prone to oxidative decomposition under high voltage, which results in increased irreversible capacity and inferior cycling stability of lithium-ion batteries. As a result, the development of high-voltage electrolytes is critical to the further development of lithium-ion batteries of high energy density.At present, one of the effective ways to improve the stability of the cathode/electrolyte interface under high voltage is to use additive. The additive can self-decompose and regulate the interfacial film which effectively blocks the contact between the electrolyte and the cathode, and inhibits the dissolution of the cathode and the oxidative decomposition of the electrolyte. In addition, the use of additives is small and the effect is remarkable, so high-voltage additives have been widely studied and applied.In this paper, we mainly studied the interface layer regulation of inorganic lithium salt lithium fluozirconate and organic fluoroethylene carbonate, dopamine for Li/LiNi0.5Mn1.5O4 and Li/LiNi0.6Co0.2Mn0.2O2. The main tasks and results include:(1) Lithium fluorozirconate was synthesized and used as an additive to study its effect on the cycle and rate performance of Li/LiNi0.5Mn1.5O4 half cells. The experimental results showed that the initial discharge capacity of Li/LiNi0.5Mn1.5O4 half cell with 0.15 M Li2ZrF6 was 128 mAh g-1; and the capacity retention after 200 cycles was 88.79 %. However, the initial discharge capacity of Li/LiNi0.5Mn1.5O4 cycled in traditional electrolyte was 113.6 mAh g-1, and the capacity retention after 200 cycles was 83.89 %. EIS analysis showed that lithium fluorozirconate involved in the formation of the SEI film which has less impedance; SEM and TEM tests showed that the surface of LiNi0.5Mn1.5O4 was not uniform and there was a thick SEI film after cycling in blank traditional electrolyte. As a contrast, the surface of LiNi0.5Mn1.5O4 cycled in the additive-containing electrolyte was still clean and sharp. And there was a uniform SEI film on the electrode surface due to the interface layer regulation of the additive; XPS and XPS depth analysis showed that lithium fluorozirconate could effectively protect the electrolyte from oxidative decomposition under high voltage and protect the positive electrode material.(2) The HOMO levels of fluoroethylene carbonate, dopamine, and EC/DMC/EMC were calculated theoretically using the B3LYP method in Gaussian 09. Fluoroethylene carbonate and dopamine were used as additive to study their effect on the cycle and rate performance of Li/LiNi0.6Co0.2Mn0.2O2 and Li/LiNi0.5Mn1.5O4, respectively. The galvanostatic charge/discharge test showed that the cycling performance of Li/LiNi0.6Co0.2Mn0.2O2 can be significantly increased with 10 wt.% fluoroethylene carbonate. In addition, the addition of 0.1wt.% dopamine to the blank electrolyte could facilitate the discharge capacity of the Li/LiNi0.5Mn1.5O4. The electrochemical analysis method showed that the SEI formed by decomposition of the electrolyte additive was stable and had low resistance.
|凡俊田. 基于界面膜调控锂离子电池高压电解液的研究[D]. 中国科学院研究生院,2018.|
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