Ionic Liquid Electrolytes for Metal-Anode Batteries

在电动汽车、间歇性可再生能源和消费电子产品等应用中，对高能量电池的需求越来越大。在过去的几十年里，锂离子电池（LIB）占据了电池市场的巨大份额，但由于理论比能量限制在250 - 300 W h kg-1，显然需要更高能量的电池。由于理论比能量比LIB提高了10倍，锂金属电池（LMB）一直被认为是最终目标：用作阳极的Li金属具有高的比容量（3861 mA h g-1）和非常负的电势（相对于SHE为-3.04V）。与通常利用石墨作为阳极的LIB不同，LMB需要在充电和放电期间镀覆和剥离Li阳极表面。因此，Li金属表面上的钝化层或所谓的固体电解质界面（SEI）处于恒定的修复状态。SEI的形态和组成可能导致诸如Li枝晶形成的问题，这是一个巨大的安全问题，因为突起可能桥接“电极间空间”，从而使电池短路。如果使用挥发性电解质，这可能导致热失控和灾难性的电池故障。由于SEI由来自电解质分解产物的有机和无机物质组成，使用哪种电解质是电池成功的重要因素。为了抑制锂枝晶的问题，需要一种电解质，它能产生具有低电阻的弹性SEI，以实现均匀的锂沉积。通常用于LIB中的有机电解质（例如碳酸盐），由于其高挥发性、差的热稳定性、高易燃性和环境危害，导致了各种问题。一种可行的替代电解质越来越受到关注，是室温离子液体（RTIL）。使用RTIL作为LMB的电解质有许多优点，包括它们的高热稳定性、高电化学稳定性（大电化学窗口）和低蒸气压。RTIL还具有调节阴离子和阳离子的理化性质的能力，因此可以产生稳定的SEI。认为使用RTIL作为电解质的缺点是其相对高的粘度，因此低电导率，加上低Li+迁移数。本项目的目的是调查的性质，SEI的LMB，使用新的RTIL。此外，还将研究其他二次金属电池的SEI，包括钠电池和镁电池。研究SEI的重要性在于，它可以更好地为新型RTIL的设计提供信息，并最终在商业电池中使用离子液体。6进一步的目标包括研究增加迁移数的策略（例如纳米颗粒修饰的IL）7，并使用RTIL增加电导率所用的RTIL将基于已显示出前景的RTIL，包括四烷基铵阳离子和双8 RTIL的合成将遵循两阶段工艺：将采用表征SEI的技术，即X射线光电子能谱（XPS），傅里叶变换红外光谱（FTIR），电化学阻抗谱（EIS）、扫描电子显微镜（SEM）和X射线反射率。为了测量电化学性质，将使用循环伏安法（CV）和恒电流循环技术。另外，为了计算迁移数，可以使用脉冲场梯度回波-核磁共振光谱（PGSE-NMR）测量自扩散系数。该项目属于EPSRC物理科学研究领域的福尔斯。

The need for increasingly high-energy batteries is becoming realised in such applications as electric vehicles, intermittent renewable energy sources and consumer electronics. Over the past few decades, lithium-ion batteries (LIB) have taken a huge share of the battery market, but with a theoretical specific energy limitation of 250 - 300 W h kg-1, it's clear a higher energy battery is required.With a theoretical 10-fold increase in specific energy to LIBs, the lithium metal battery (LMB) has long been considered as the ultimate goal: Li metal used as the anode has a high specific capacity (3861 mA h g-1) and a very negative potential (-3.04 V vs. SHE).Unlike LIBs, which usually utilize graphite as the anode, LMBs require the plating and stripping of the Li anode surface during charge and discharge. The passivation layer, or the so-called solid electrolyte interface (SEI), on the surface of the Li metal, is therefore in a constant state of repair. The morphology and composition of the SEI can lead to such problems as Li dendrite formation, which is a huge safety concern, as the protrusion could bridge the "inter-electrode space" and thereby short-circuit the cell. If one uses a volatile electrolyte this could lead to thermal runaway and disastrous cell failure. With the SEI consisting of organic and inorganic species from the electrolyte decomposition products, which electrolyte one uses is a significant factor in the success of the cell. To curb the Li dendrite concerns, an electrolyte that leads to an elastic SEI with low resistance for uniform Li deposition is needed.Organic electrolytes (e.g. carbonates), which are typically used in LIBs, are responsible for a variety of concerns due to their high volatility, poor thermal stability, high flammability and environmental hazards. A viable alternative electrolyte gaining increasingly more attention, are room temperature ionic liquids (RTILs). There are many advantages to using RTILs as electrolytes for LMBs, including their high thermal stability, high electrochemical stability (large electrochemical window) and low vapour pressure. RTILs also cater the ability to adjust physiochemical properties with the anion and cation, which can thus lead to a stable SEI. Considered disadvantages to using RTILs as electrolytes are their relatively high viscosity, and therefore low conductivity, plus the low Li+ transference number. This project's aim is to investigate the nature of the SEI of LMBs, using novel RTILs. Additionally, SEIs of other secondary metal batteries will be studied, including sodium and magnesium batteries. The importance of studying the SEI is that it can better inform the design of new RTILs, and ultimately use ionic liquids in commercial batteries.6 Further aims include investigating strategies to increase the transference number (e.g. nanoparticle decorated ILs)7, and increase the conductivity using RTILs (e.g. organic-IL mixtures).RTILs used will be based on ones that have shown promise, including tetraalkylammonium cations and bis(fluoromethanesulfonyl)imide (FSI) anions.8 The synthesis of the RTILs will follow a 2-stage process: the preparation of the precursor salt and the subsequent metathesis reaction.Techniques to characterize the SEI will be employed, namely X-Ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared Spectroscopy (FTIR), Electrochemical Impedance Spectroscopy (EIS), Scanning Electron Microscopy (SEM) and X-Ray Reflectivity. To measure electrochemical properties, cyclic voltammetry (CV) and galvanostatic cycling techniques will be used. Additionally, to calculate transference numbers, self-diffusion coefficients can be measured using pulsed field gradient echo- nuclear magnetic resonance spectroscopy (PGSE-NMR). This project falls within the EPSRC physical sciences research area.