Design of a Novel Apparatus for the in-situ Formulation and Characterization of Safer Electrolytes

LIB和SC都是可替代的能量存储装置，其分别通过轨道电子交换（化学）和通过静电（物理）分离正电荷和负电荷来存储能量。在这些商业电化学装置中使用的大多数液体电解质通过将盐如锂盐或四氟硼酸四乙基铵分别溶解在特定溶剂如碳酸烷基酯混合物或乙腈（ACN）中来获得。然而，通常与所选电解质的稳定性相关的许多问题限制了它们的寿命。例如，在文献中报道，当向电极施加高电势时，LIB电解质在高电势下的氧化反应导致形成如甲烷、乙烷、二氧化碳和一氧化碳的气体，这增加了密封电池内部的压力，然后改变了电解质的物理性质。而在SC的情况下，与四氟硼酸四乙基铵和基于ACN的电解质的使用相关的主要问题是其安全性，因为在某些特殊条件下，当燃烧时，ACN可以产生氰化氢（HCN）。事实上，HCN的生成量取决于ACN的体积、火灾温度和环境。然而，这些问题不能被评估，而不同时了解它们的热力学性质，流体相平衡和电化学性质的存在和不存在外部刺激（气体的存在，温度等）。本项目将首先开发一种基于等容饱和技术的新型原创设备，通过使用气相色谱分析气相组成，结合不同的电化学设备来解决这一问题（伏安计，恒电位仪或加伐诺司他）该方法允许在真实的条件下通过pVTxy测量同时测定流体相平衡和宽范围的电化学性质。为了了解限制电解质寿命和安全性的因素，我们对LIB和SC应用的电解质范围进行了分析。此外，在该项目期间，还将研究一种方法，该方法包括覆盖这些特性，目的是研究离子液体，IL，作为添加剂组分，而不是作为介质，也将在有用的技术溶剂中进行研究，如烷基碳酸酯（用于LIB），乙腈和其他二腈结构（用于SC）。这种选择的目的是保持每种溶剂的优点，并通过添加少量IL来改善它们的缺点（腐蚀性，挥发性）。为了澄清离子液体和熔盐之间的差异，还将使用四氟硼酸四乙基铵对其在SC中的使用进行类似的实验研究。为此，我们计划集中精力研究电解质配方对其独特性质的影响，如溶剂化，体积，热，电化学，通过使用本项目中提出的原始装置以及QUB已经开发的不同实验和理论技术，研究了热力学和传输特性。此外，每种电解质的电化学稳定性将通过应用不同的充电-放电循环并通过分析气相组成随时间和温度的变化来确定。此外，降解产物，如乙烷和二氧化碳的存在下，对电解质的性能的影响也将在原位进行研究。然后将确定乙烷和二氧化碳在所选电解质中的溶解度，以得出关于使用离子液体而不是传统熔融盐的主要优点和缺点的一些结论，以及提出关键参数以提高用于LIB和SC应用的电解质的稳定性。

LIB and SC are both alternative energy storage devices, which store energy via orbital electron exchange (chemically) and by electrostatically (physically) separating positive and negative charges, respectively. Most liquid electrolytes used in these commercial electrochemical devices are obtained by dissolving a salt like lithium salt, or tetraethylammonium tetrafluoroborate in a specific solvent like alkylcarbonate mixture or the acetonitrile (ACN), respectively. Nevertheless, many issues often linked to the stability of selected electrolytes limit their lifetime. For example, it is reported in the literature that the oxidation reaction of the LIB electrolyte at high potentials leads to the formation of gases like methane, ethane, carbon dioxide and carbon monoxide when high potentials are applied to the electrode, which increases the pressure inside the sealed cell and modify then the physical properties of the electrolytes. While in the case of SC, the main issue links to the use of the tetraethylammonium tetrafluoroborate and ACN-based electrolyte is its safety since under certain special conditions ACN can produce hydrogen cyanide (HCN) when is burned. In fact, amount of HCN produced depends on volume of ACN, temperature of fire and environment. Nevertheless, these issues cannot be evaluated without the simultaneous knowledge of their thermodynamic properties, fluid phase equilibria and electrochemical properties in presence and in absence of external stimulus (presence of gas, temperature, etc.). This project will address this perspective by developing first a new and original apparatus based on an isochoric saturation technique, supplied by an analysis of the gas phase composition using a Gas Chromatography, combined with different electrochemical devices (voltamperometer, potentiostat or gavalnostat) that allows the simultaneous determination in real conditions of the fluid phase equilibria through pVTxy measurements and the electrochemical properties of a wide range of electrolytes for LIB and SC applications in order to understand factors, which limit electrolytes lifetime and safety. Additionally, during this project, an approach which consists to cover these properties with the objectives to study ionic liquids, ILs, as an additive component, and not as a media, will be also investigated in useful technological solvents like alkylcarbonates (for their uses in LIB), acetonitrile and other dinitrile structures (for their uses in SC). This choice was made with the objective to keep the advantages of each solvent and improve their disadvantages (corrosive, volatile) by adding small quantity of IL. To clarify the differences between ILs and molten salts, similar experiences will be also investigated using the tetraethylammonium tetrafluoroborate for its use in SC.For that, we plan to focus then our effort on the effect of the electrolyte formulation on their unique properties like the solvation, volumetric, thermal, electrochemical, thermodynamic and transport properties by using the original apparatus proposed in this project and different experimental and theoretical techniques already available and developed at QUB. Additionally, the electrochemical stability of each electrolyte will be determined by applying different charge-discharge cycles and by analysing then the gas phase composition changes over the time and temperature. Furthermore, the effect of presence of degradation products, like ethane and carbon dioxide, on the properties of the electrolytes will be also investigated in situ. The determination of the ethane and carbon dioxide solubility in the selected electrolytes will be then determined to dress some conclusions on the main advantages and disadvantages to use ILs rather than classical molten salts, as well as, to propose key parameters to improve the stability of electrolytes for LIB and SC applications.