【作者】 王伟；

【导师】 胡源； Kim Meow Liew；

【作者基本信息】 中国科学技术大学 ， 安全科学与工程， 2019， 博士

【摘要】 近二十年来,锂离子电池由于具有突出的性能,例如高能量密度、长循环寿命、无记忆效应和可控的形状设计,被广泛应用于便携式设备(如电话、笔记本电脑和便携式电源),其作用在人们的生活中已经变得极为重要。不仅如此,在过去五年中,锂离子电池也被认为是混合动力电动汽车(HEV)、插电式混合动力电动汽车和电动汽车的最佳动力源。然而,由锂离子电池的燃烧和爆炸而引起的事故却时常发生,例如三星Note7手机的爆炸和2015年厦门HEV公共汽车的自燃事件。一般而言,大多数锂离子电池组成包括聚烯烃类隔膜、液体有机电解液(包括:碳酸乙烯酯、碳酸二乙酯和碳酸二甲酯)、锂盐和正负极。其中隔膜和电解液的低热稳定性和易燃性通常被认为是导致锂离子电池燃烧和爆炸的主要原因。因此,从隔膜和电解液角度来提高锂离子电池的安全性具有重要意义。一方面,通过掺入磷基液体阻燃剂,可以实现电解液可燃性的显著降低以及锂电池更优的电化学性能。在高温下产生的磷-氧自由基可以主动捕获由燃烧产生的自由基以终止燃烧。因此,含磷电解液体系是电解液易燃问题的最有前景的解决方法之一。另一方面,无机物基隔膜被广泛用于改善隔膜的热稳定性和孔隙率,可赋予锂离子电池更高的安全性能和优异的电化学性能。本篇论文基于以上研究思路,做了以下几部分研究工作:1.选择了三价和五价磷基阻燃剂以研究磷价态对锂离子电池安全性能和电化学性能的影响。为了消除其他非价态的影响因素,选择了两组具有类似结构含有三价磷和五价磷的含磷阻燃剂(磷酸三乙酯、亚磷酸三乙酯、和磷酸三甲酯、亚磷酸三甲酯)来作为研究对象。实验结果表明,五价磷基电解液相比三价磷基电解液而言,具有更宽的电化学窗口,这表明五价磷基阻燃剂更适合作为正极材料的电池体系的添加剂。有趣的是,三价磷基阻燃剂即使在添加量较低的情况下也可被分解并促进在石墨负极材料的表面上形成稳定的SEI膜,可以用作固态电解质界面膜促进剂。此外,值得注意的是,三价和五价磷基电解液均表现出令人满意的高电流充放电性能,这表明它在电动汽车和电动移动设备领域具有广阔的应用前景。这项工作可以为研究锂离子电池含磷阻燃剂的科学研究或工业应用研究提供帮助。基于上述结论,此外还设计了不燃的五价磷基电解液,并用于LiFePO4|Li锂金属电池。相应的结果表明,五价磷基阻燃剂可赋予锂金属电池高安全性、优异的循环稳定性和令人满意的倍率性能,可以被认为能够成为锂金属电池较好的电解液选择之一。2.锂金属电池由于其独特的优点,例如重量轻、最低的阳极电位和最高的理论比容量,被认为是下一代电能存储材料中最有潜力的选项之一。然而,锂金属电池的发展和实际应用受到由于锂枝晶引起的安全问题的严重阻碍。这里引入原子层沉积(ALD)技术在PVDF-HFP膜表面沉积A1203膜,赋予隔膜更高的热稳定性、电解液相容性、离子电导率和高杨氏模量等优异性能。采用LiFePO4|Li电池,ALD100/PH隔膜赋予电池最佳的循环性能和倍率性能。通过观察循环100圈后的锂枝晶,表明拥有极高杨氏模量和离子电导率的ALD100/PH隔膜能够有效地抑制锂枝晶的生长。此外,ALD100/PH隔膜在0.5 mA cm-2的电流密度下显示出超过1300小时的稳定运行,具有抵抗锂枝晶的能力和在锂金属电池领域的应用潜力。因此,拥有着可控和易制备等优点,ALD技术使得目前已有聚合物隔膜具有用于下一代高能量密度的可再充电电池系统的可能,如Li-S、Li-O2电池以及其他金属电池。3.如前所述,ALD技术主要通过覆盖基体表面的无机材料来抑制锂枝晶。与此想法不同,嵌入聚合物材料中的无机材料也能抑制树枝状锂并提高锂金属电池库仑效率。在第四章中,介绍了一种环保型纳米复合材料隔膜,由聚乙烯醇和羟基磷灰石组成,具有出色的杨氏模量和高机械柔韧性。结果表明,所制备的隔膜可有效抑制锂枝晶的生长,并使固态电解质界面均匀稳定。相比使用市售聚烯烃基隔膜的锂金属电池,使用纳米复合物隔膜的锂金属电池在2 mA cm-2高电流密度下的充放电循环过程中表现出更高的库仑效率和更优异的电化学性能。这项工作表明,纳米复合材料科学可以成为开发稳定、安全和可持续的锂金属电池有前景的方案之一。更多还原

【Abstract】 In recent twenty years,lithium-ion batteries have obtained an extremely importance as the energy storage of portable devices such as phones,laptops computers and portable power sources due to its outstanding properties,for instance,high energy density,rechargeable properties,excellent cycling performances and controllable shape designs.Therefore,during the past five years,lithium ion batteries have been also considered as the optimal power sources for hybrid electric vehicles(HEVs),plug-in hybrid electric vehicles(PHEVs)and electric vehicles(EVs).However,accidents caused by the explosion and combustion of lithium ion batteries are usually reported,such as the explosion of phone Samsung Note7 and the spontaneous combustion of HEVs buses in Xiamen in 2015.It is well-known that most lithium ion batteries comprising of separators such as polyethylene(PE)or/and polypropylene(PP)and liquid organic electrolytes containing ethylene carbonate,diethyl carbonate and dimethyl carbonate have potential safety problems due to their low thermal stabilities and flammability,which are usually regarded as the main reasons resulting the explosion and combustion of lithium ion batteries.Hence,it is of great significance to enhance the safety of lithium ion batteries.On one side,there a considerable reduction of flammability always can be obtained by the incorporation of phosphorus-based liquid flame retardants,as well as higher electrochemical properties.It is well recognized that phosphorus-oxygen radicals generated at high temperature could actively capture other free radicals emitted by the burning electrolytes to retard the combustion.So,electrolytes systems comprised of phosphorus are expected to be a promising method to solve the issue of flammability of electrolytes.On the other side,inorganic based separators are extensively employed to improve the thermal stability and porosity of separators,as well as endowing lithium ion batteries with safety performance and outstanding electrochemical properties.Atom Layer Deposition technique is used to fabricate inorganic material on the framework surface of polyvinylidene fluoride hexa-fluoropropylene(PVDF-HFP)for excellent thermal shrinkage performance and outstanding compatibility with electrolytes.Furthermore,incorporating two-dimensional boron nitride sheets can efficiently improve the porosity and thermal stability of cellulose porous separator.In addition,for the purpose of obtaining separators with high mechanical strength,polyvinyl alcohol(PVA)/hydroxyapatite(HAP)hybrid porous membrane are prepared to be used as separators for safer lithium ion batteries.Research work of this dissertation is composed of the following parts:1.Trivalent and pentavalent phosphorus-based flame retardants are purposely selected to comparably investigate the influence on safety and electrochemical performance of Li-ion batteries.In order to eliminate the influencing factors of non-valence state,we selected two groups of phosphorus-containing flame retardants(triethyl phosphate,triethyl phosphite,and trimethyl phosphate,trimethyl phosphite)with similar structure but containing trivalent phosphorus and pentavalent phosphorus.The experimental results show that pentavalent phosphorus-based electrolytes display wider electrochemical windows and could endure higher voltage compared to trivalent phosphorus-based electrolytes,revealing that pentavalent phosphorus-based flame retardants are more suitable as efficient additives for cathode materials systems.Interestingly,trivalent phosphorus flame retardants may serve as solid electrolyte interphase layer formers,which can be decomposed and meanwhile promote the formation of stable solid electrolyte interphase layer on the surface of graphitic anode materials even with a low loading.Additionally,it is worth noting that both trivalent and pentavalent phosphorus flame retardants show satisfying high current rate performance indicating the promising application in the field of EV and electrical mobile devices.This work can provide help for scientific research or industry in the study of phosphorus-containing flame retardants for batteries.Based on the conclusion above,to meet the requirement of electrolytes for lithium metal batteries,nonflammable pentavalent phosphorus-based flame retardants electrolytes are designed and used in LiFePO4|Li full cells.The corresponding results demonstrate that pentavalent phosphorus-based liquid flame retardants can endow lithium metal batteries with high safety,steady cycling stability and satisfying current rate capacity,thereby be promising candidates for lithium metal batteries.2.Lithium metal batteries,due to its unique advantages such as light weight,the lowest anode potential and the highest theoretical specific capacity,have been regarded as the promising candidate for next-generation electrical energy storage.Nevertheless,the development and practical application of lithium metal batteries has been seriously hindered by the safety issue induced by lithium dendrites.Here the atom layer deposition(ALD)technique is introduced to fabricate the Al2O3 on the surface of PVDF-HFP membrane to endow separator with advanced properties including higher thermal stability,great affinity with electrolytes,and improved ion conductivity.Employing LiFePO4|Li cells,ALD100/PVDF-HFP separator imparts batteries with the best cycling performances and rate capacity among various separators.Moreover,the ALD100/PVDF-HFP separator shows more than 1300 h of stable operation at current density of 0.5 mA cm-2,exhibiting the capability against metallic lithium and the potential for application in the field of lithium metal batteries.Thus,this interesting ALD technique capable of feasible fabrication procedure and desirable advantages maybe make commercial or as-prepared separators more advanced and potential candidates for the next-generation high-energy-density rechargeable battery systems such as Li-S and Li-O2 batteries,as well as other metal-based batteries.3.As introduced in previous paragraph,ALD technology mainly inhibits lithium dendrites by covering inorganic materials on the surface of the substrate.Different from this idea,inorganic materials embedded in polymer materix are also anticipated to suppress the dendritic lithium and enhance the Coulombic efficiency.In chapter 4,we report an environmentally friendly nanocomposites separator,comprising of polyvinyl alcohol and hydroxyapatite,which exhibits outstanding Young’s modulus and high mechanical flexibility.The results show that the as-prepared separator can efficiently suppress the growth of dendritic lithium and enable a homogeneous stable solid electrolyte interphase.Compared to lithium metal batteries using a commercial polyolefin-based separator,cells using the reported nanocomposite separator demonstrate a superior electrochemical performance with a much higher Coulombic efficiency during the charge/discharge cycling at a practical current density of 2 mA cm-2.This work indicates that nanocomposite engineering could be a promising strategy to develop stable,safe and sustainable Li-metal batteries.更多还原