Organic materials for magnesium batteries

Abstract

As the world decarbonizes the energy systems, energy storage is an important pillar that cannot be ignored. Renewable energy sources are intermittent and non-flexible. Therefore, energy needs to be stored for shorter and longer periods. Since most renewable energy sources provide electricity, chemical battery storage is the most obvious first choice for storage.

And indeed, for short- and medium-term storage, chemical battery storage is economically feasible. However, the current reigning technology, the lithium-ion battery, is far too expensive for world-wide deployment for grid-scale storage. This comes down to material cost, which is connected to resource abundancy. Lithium, nickel, cobalt and other metals are too expensive. One should also not forget that their concentration in only a few places on Earth, makes for a weak supply chain.

New battery chemistries need to be found, that are solely based on abundant and/or renewable materials. Sodium, magnesium, iron, zinc and aluminum are metals that could replace lithium in batteries, while expensive transition metal oxides might be replaced by easily synthesized organic molecules. This thesis explores a few examples of materials that could be used in magnesium batteries.

In chapter 2 perylene diimides are introduced. These versatile and robust molecules have shown promise as battery electrode material. This chapter describes a new and mild synthesis route towards them, the mild imidization. Traditionally perylene diimide synthesis requires harsh conditions, high temperature and corrosive solvents. In this new approach, one only needs the biocompatible solvent DMSO and potassium carbonate at 100 oC. Also, a room temperature variant of the procedure has been developed that uses DBU as a base. The resulting products are also easily obtained through a simple precipitation in water, without the need for purification. The reaction kinetics and substrate scope are also investigated.

Perylene diamic acids, a sister compound to perylene diimides, are described in chapter 3. These are the intermediates in the imide synthesis. When the reaction is done with a secondary amine, the imide cannot be formed, and one is left with a water-soluble salt. The high solubility in water makes this class of molecules stand out in the perylene family. When diamic acids are exposed to acid, the reaction is undone and perylene dianhydride is formed. This compound is very insoluble in water. Chapter 3 shows that very controlled acidification can yield highly colored hydrogels. The gels are characterized by different techniques. The hydrogels might be of interest to material scientists, since they undergo uniform shrinkage over time. In their soluble form, perylene diamic acids are of interest as water-soluble compound in aqueous batteries.

The perylene family is known to be ‘redox tunable’. By changing the side groups of the perylene, one can change the redox potential at which it reacts, which determines the potential at which a battery operates. Chapter 4 looks into a new way to gain more control over the substitution of perylene tetraesters, by making a dibromo-dichloro perylene tetraester. In theory, this gives more control over the substitution pattern compared to tetrachloro and tetrabromo derivatives. This chapter investigates the kinetics of substitution reactions.

Chapter 5 presents a new aqueous polymer electrolyte for magnesium batteries, magnesium alginate. Alginate is a biopolymer that is harvested from algae. It is a polysaccharide of two monomers that both contain carboxylate groups. The difference between the monomers is only the spatial orientation of the carboxylate group. When an alginate solution is mixed with a solution that contains a multivalent cation, it will form a hydrogel. The carboxylate group from crosslinks with the cations, making a covalent-link hydrogel. Magnesium ions, two plus charged, are an exception, judged on the macroscale, they do not form a hydrogel. However, it is known that magnesium makes microscopic instable hydrogels that constantly form and break apart. From this, we can say that although magnesium is mobile in the solution, it is still heavily associated with the alginate backbone, and this is an indication that this electrolyte can act as a ‘water-in-salt’ electrolyte. Chapter 5 shows that magnesium alginate can have good ionic conductivity, even at very low water content. After some cycling, a black layer forms on the magnesium electrodes. This layer enhances the stability of magnesium in water.

Alginate hydrogels can also be used as matrices to encapsulate transition metal cations. Chapter 6 brings a new approach to make a ‘simple’ manganese-iron battery. By encapsulating manganese(II) on one electrode and iron(III) in alginate, we were able to cycle this simple ion pair multiple times. This material might have applications in semi-solid flow battery.