Molecular Mechanisms in Nano-filled Lithium Solid Polymer Electrolytes

技术摘要：固体聚合物电解质是锂离子电池中取代液体电解质的理想候选者，因为无毒的固体聚合物消除了对坚硬外壳的需要，从而增加了设计灵活性并减少了处置问题。阻碍有效应用的特点是室温电导率低，因此人们尝试了许多改进措施，并研究了它们对电导率的影响。一种这样的改性是在锂掺杂的聚氧乙烷[PEO]中添加纳米颗粒填料，这特别是在低温下提高了导电性。尽管这种效应已经确立，但它的作用机制却不是这样。聚合物主体的迁移率增加是一种常见的解释：然而最近的实验证明，即使在完全结晶的PEO中也可以获得高电导率，模拟研究表明，氧化物纳米颗粒减缓了PEO的动力学。对测量迁移率或电导率的实验的解释需要纳米颗粒聚集、结晶和湿度的知识，但这些变量往往没有报道。聚集很重要，因为分散体系会受到聚合物主体限制的影响。聚合物主体结晶动力学受锂盐和纳米粒子的影响。由于所需的工作温度[室温]低于熔点，结晶时间是一个需要监测的重要变量。同样重要的是，消除不同的湿度水平作为纳米颗粒增加传导性的原因，因为实际设备不能在有水的情况下运行。该项目结合了多种技术：准弹性中子散射、宽带介电光谱和小角中子散射，在相同的条件下对相同的样品执行。聚合物(PEO)、锂盐(LiClO4)和纳米颗粒(Al_2O_3)的身份将是固定的，而纳米颗粒的尺寸和浓度将围绕提供最佳导电性的既定尺寸和浓度而变化。纳米粒子对电导率和PEO迁移率的影响将在干燥和环境条件下以及PEO结晶所需的时间前后进行测试。这项研究的实际贡献将是分离纳米颗粒填料对电导率和PEO迁移率的影响，作为结晶、颗粒聚集和水分含量的函数。非技术综述：环保高效的新能源包括燃料电池、太阳能电池和长寿命充电电池。锂离子电池是这个项目的重点，可以在商业上买到，这种材料以液体或凝胶的形式将电池的两侧[电解液]分开。这需要一个外壳，添加的溶剂可以改善锂在电解液中的移动，这是一个寿命结束的处置问题。使用固体聚合物作为电解液缓解了这些困难，但如果没有溶剂，锂的移动不足以为设备供电。使用最先进的方法来表征各种组件的迁移率，该项目将确定添加纳米级填料提高器件性能的原因。这将允许轻便、灵活和安全的电池为计算机、手机和其他设备供电。该项目使用中子散射，这是一种在用户设施(如美国国家标准与技术研究所的中子研究中心)进行的实验技术。美国在中子散射设施方面投入了大量资金，其中包括橡树岭国家实验室的散裂中子源，该中子源目前正在开始运行，将使全国可容纳的中子用户数量增加两倍。该项目的一部分是对美国科学家进行这项技术的教育，并开始形成这个新的用户池。

TECHNICAL SUMMARY:Solid polymer electrolytes are ideal candidates to replace liquid electrolytes in lithium-ion batteries, because the non-toxic solid polymer eliminates the need for a rigid casing resulting in increased design flexibility and decreased disposal problems. The characteristic that precludes effective application is low room-temperature conductivity, and thus many modifications have been attempted and their effects on conductivity investigated. One such modification is addition of nanoparticle fillers to lithium-doped poly(ethylene-oxide) [PEO], which increases conductivity in particular at low temperatures. Although this effect is well established, the mechanism through which it acts is not. Increased mobility of the polymer host is a frequently offered explanation: yet recent experiments prove that high conductivity may be obtained even in completely crystalline PEO, and simulation studies suggest that oxide nanoparticles slow dynamics of PEO. Interpretation of experiments measuring mobility or conductivity requires knowledge of nanoparticle aggregation, crystallization and humidity, yet these variables are frequently not reported. Aggregation is important because a dispersed system will be influenced by confinement of the polymer host. The kinetics of polymer host crystallization are influenced both by the lithium salt, and the nanoparticles. Since the desired operating temperature [room temperature] is below the melting point, time to crystallization is an important variable to monitor. It is equally important to eliminate differing humidity levels as the cause of increased conductivity with nanoparticle addition, because practical devices cannot operate in the presence of water. This project combines a variety of techniques: quasielastic neutron scattering, broadband dielectric spectroscopy, and small-angle neutron scattering, performed on the same samples, under the same conditions. The identities of the polymer (PEO), lithium salt (LiClO4), and nanoparticle (Al2O3) will be fixed, while the nanoparticle size and concentration will be varied around that established to provide optimum conductivity. The influence of nanoparticles on conductivity and PEO mobility will be tested in dry and ambient conditions, and before and after the time required to crystallize the PEO. The practical contribution of this study will be to isolate the effects of nanoparticle fillers on conductivity and PEO mobility, as a function of crystallization, particle aggregation, and water content. NON-TECHNICAL SUMMARY:New environmentally friendly and efficient energy sources include fuel cells, solar cells and long life rechargeable batteries. Lithium ion batteries, the focus of this project, are available commercially, with the material separating the two sides of the battery [the electrolyte] in the form of a liquid or gel. This requires a casing, and the solvents added to improve movement of lithium across the electrolyte are an end of life disposal problem. Using a solid polymer as the electrolyte alleviates these difficulties, but without solvents lithium movement is not sufficient to power a device. Using state of the art methods to characterize mobility of the various components, this project will determine the reasons that the addition of nano-sized fillers improves device performance. This will allow for lightweight, flexible and safe batteries to power computers, cell phones, and other devices. This project uses neutron scattering, an experimental technique performed at user facilities, such as the Center for Neutron Research at the National Institute of Standards and Technology. The US has invested considerably in neutron scattering facilities, including the Spallation Neutron Source at Oakridge National Laboratory, which is currently starting operation and will triple the number of neutron users that may be accommodated nationally. Part of this project is to educate US scientists in this technique, and begin to form this new user pool.