Ionic Liquid Electrolytes for Metal-Anode Batteries

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.