Nonaqueous electrolytes for sustainable electrochemical applications

The ever-increasing demand of energy, which has a very negative ecological impact on our planet and is one of the main causes of global warming, has created the necessity for a transition to a sustainable and more responsible energy and resource policy. Electrochemical energy storage systems (i.e. secondary batteries) that are capable of grid-scale storage are a valuable tool to improve the efficiency of our existing power plants, as well as promote the integration of intermittent renewables. Moreover, the application of secondary batteries in electric and hybrid vehicles with high energy efficiency can also help to achieve this goal.

Nonaqueous electrolytes such as ionic liquids and organic electrolytes are being explored more and more for a variety of electrochemical applications, mainly because their electrochemical stability window is much wider than aqueous electrolytes. In battery applications, this creates the possibility to achieve significantly higher voltages, and therefore also higher energy densities than aqueous batteries. The commercial success of nonaqueous batteries is nicely illustrated by the lithium-ion battery. The Nobel prize of 2019 was awarded to the inventors of this technology because of the considerable impact that it had on our current society by enabling the development of the portable electronic devices that are now a permanent part of our lives. The wide electrochemical window of nonaqueous electrolytes also enables the possibility for the electrodeposition of reactive metals, which can be applied in batteries as metal anodes with high specific capacity, or for electrowinning of valuable metals.

In this PhD thesis, new nonaqueous electrolytes were developed for application in a variety of electrochemical technologies that can help to meet these sustainability goals. The focus was primarily on post-lithium-ion secondary batteries for large-scale energy storage. The investigated technologies were all-organic redox flow batteries, sodium-ion batteries, and rechargeable magnesium batteries.

The work on redox flow batteries involved the chemical modification of 1,4-diaminoanthraquinones, a promising class of redox-active compounds, with the goal of increasing the solubility in organic electrolytes, so high energy densities can be achieved. A variety of derivatives with alkyl, cationic and oligoethylene glycol ether groups was synthesized and thoroughly characterized with NMR, FT-IR, and DSC. The influence of solvent polarity and supporting electrolyte concentration on their solubility was studied with quantitative UV-Vis absorption spectroscopy. The viability of these new compounds in battery applications was evaluated with cyclic voltammetry. The most promising compound with the optimal combination of high solubility and electrochemical reversibility was selected for an in-depth electrochemical characterization, involving the determination of the electron-transfer kinetics and galvanostatic cycling experiments in a symmetric battery cell.

The work on sodium-ion batteries and rechargeable magnesium batteries involved the synthesis and characterization of new solvate ionic liquids, or liquid metal salts. This is a type of ionic liquid that consists of a solvated metal cation and weakly coordinating anion. Because the metal cation is part of the structure of the ionic liquid, very high concentrations can be achieved, which translates to high current densities. The used salts were sodium bis(fluorosulfonyl)imide and magnesium bis(trifluoromethylsulfonyl)imide. The solvents (ligands) were oligoethylene glycol dimethyl ethers (monoglyme, diglyme, triglyme), which exhibit excellent solvating properties for these metal cations and are exceptionally stable against strong reducing conditions. The relationships between their solvate structures, physicochemical properties, and electrochemical performance were revealed using several methods, including single crystal XRD, multinuclear NMR, and FT-Raman spectroscopy. The sodium-containing SILs were used as electrolytes for cycling of Na-NMC electrodes. The magnesium-containing SILs were used for reversible electrodeposition of magnesium metal, and the influence of a chloride source on this process was investigated.

The final part of this work involved the development of nonaqueous electrolytes for electrodeposition, not in batteries, but for the recovery of the valuable metals neodymium and dysprosium, which are present amongst other things in spent nickel metal-hydride batteries and permanent rare-earth magnets. A new type of organic electrolyte consisting of a lanthanide salt and a borohydride salt dissolved in an ether solvent was developed as a more sustainable alternative for high-temperature molten salts and expensive ionic liquids. Electrodeposition of both lanthanides was studied with cyclic voltammetry and the deposits were characterized with SEM, EDX, and XPS.