Electrochemical energy storage systems, such as batteries, supercapacitors and the like, have been widely proposed for large-scale energy storage applications. Various battery designs, including flow batteries, have been considered for this purpose. Compared to other types of electrochemical energy storage systems, flow batteries can be advantageous, particularly for large-scale applications, due to their ability to decouple the parameters of power density and energy density from one another.
Flow batteries generally include negative and positive active materials in corresponding electrolyte solutions, which are flowed separately across opposing faces of a membrane or separator in an electrochemical cell containing negative and positive electrodes. The flow battery is charged or discharged through electrochemical reactions of the active materials that occur inside the two half-cells. As used herein, the terms “active material,” “electroactive material,” “redox-active material” or variants thereof synonymously refer to materials that undergo a change in oxidation state during operation of a flow battery or like electrochemical energy storage system (i.e., during charging or discharging). Although flow batteries hold significant promise for large-scale energy storage applications, they have often been plagued by sub-optimal energy storage performance (e.g., round trip energy efficiency) and limited cycle life, among other factors. Despite significant investigational efforts, no commercially viable flow battery technologies have yet been developed.
Metal-based active materials can often be desirable for use in flow batteries and other electrochemical energy storage systems. Although non-ligated metal ions (e.g., dissolved salts of a redox-active metal) can be used as an active material, it can often be more desirable to utilize coordination complexes for this purpose. As used herein, the terms “coordination complex, “coordination compound,” “metal-ligand complex,” or simply “complex” synonymously refer to a compound having at least one covalent bond formed between a metal center and a donor ligand. The metal center can cycle between an oxidized form and a reduced form in an electrolyte solution, where the oxidized and reduced forms of the metal center represent states of full charge or full discharge depending upon the particular half-cell in which the coordination compound is present. In certain instances, additional electrons can be transferred through the oxidation or reduction of one or more of the molecules constituting the ligands.
Titanium coordination compounds can be particularly desirable active materials for use in flow batteries and other electrochemical energy storage systems, since such metal complexes can provide good half-cell potentials (e.g., less than −0.3 V) and current efficiencies exceeding 85% at high current density values (e.g., greater than 100 mA/cm2). Various catecholate complexes of titanium can be particularly desirable active materials, since they are relatively stable complexes and can exhibit a significant degree of solubility in aqueous solvents. Titanium catecholate coordination compounds having a coordination number of 6 are commonly used for this purpose, wherein the titanium center can cycle between an oxidation state of +3 and +4 during operation of a flow battery. As used herein, the term “coordination number” refers to the number of covalent bonds formed to a titanium center in a coordination compound. Although titanium (IV) is capable of having a coordination number of up to 8, a coordination number of 6 is far more common for this metal. As such, titanium catecholate coordination compounds having a coordination number of 6 are referred to herein as being “coordinatively saturated,” and those having a coordination number of 5 or less, particularly a coordination number of 3-5 and more particularly a coordination number of 4, are referred to herein as being “coordinatively unsaturated.”
Presently available methods for synthesizing coordinatively saturated titanium catecholate coordination compounds can be problematic from a number of standpoints. In some instances, it can be difficult to separate non-ligated catechol compounds from the coordination compounds after their synthesis. Titanium nanoparticles, which can be problematic when formulated into an electrolyte solution, can also form in some instances. Non-ligated catechol compounds can make it difficult to adjust the counterion content of an electrolyte solution to a sufficiently precise degree. Excessive counterions, for example, can decrease solubility of the active material due to a common ion effect, while insufficient counterions can result in incomplete formation of a desired salt form. It can also be problematic during typical syntheses of coordinatively saturated titanium (IV) coordination compounds to introduce different catecholate ligands onto a titanium center while maintaining compositional homogeneity, since typical syntheses rely upon a statistical reaction of mixed catechol compounds with titanium (IV). Mixtures of coordinatively saturated titanium (IV) coordination compounds bearing different groupings of catecholate ligands can be very difficult to separate from one another, and can be unsuitable for use within flow batteries in some instances.
In view of the foregoing, improved methods for synthesizing coordinatively saturated titanium coordination compounds containing catecholate ligands would be highly desirable to facilitate their use as active materials in energy storage applications. The present disclosure satisfies the foregoing needs and provides related advantages as well.