Background to the Invention

[0001] This invention relates to a battery for electrical energy storage.

[0002] The need for sustainable power generation technologies, which produce little to no emissions, and are efficient, cost effective and widely deployable, is increasingly evident. Many technologies already exist to meet these demands, such as wind turbines and photovoltaics. However, the widespread deployment of these technologies is hindered by their intermittent and often unpredictable power output, which arises due to passing clouds or infrequent gusts of wind. As a result, the supply of electrical power is not temporally aligned with the demand for electricity, limiting the value of these renewable energy sources. In order to mitigate this issue and improve the reliability of the electrical grid, large-scale energy storage is required. Such systems can effectively store energy when it is available, and shift it to periods when demand is highest.

[0003] Typical batteries utilize a difference of redox potentials of two or more redox couples to provide a cell voltage. For example, redox flow batteries (RFBs) are a technology being considered for large-scale electrical energy storage. These systems are easily deployed and require relatively little maintenance. They exhibit a long lifetime, and are tolerant of short, incomplete cycles. Accordingly, RFBs are appropriate for buffering renewable energy sources, such as wind and solar. See C. Ponce de Leon, A. Frias-Ferrer, J. Gonzalez-Garcia, D.A. Szanto, F.C. Walsh, Redox flow cells for energy conversion, Journal of Power Sources 160 (2006) 716-732. The system consists of two half-cells separated by an ion-permeable membrane, which permits ion transfer between the half-cells during charging and discharging. Each half-cell is connected to a circulatory system consisting of a pump and a tank. The pumps are used to transport the electrolytes in a continuous loop between the tanks and the cell. Many different types of redox flow batteries have been proposed: e.g. M. Bartolozzi, Development of redox flow batteries. A historical bibliography. Journal of Power Sources 27 (1989) 219-234; F. Pan, Q. Wang, Redox species of redox flow batteries: a review, Molecules, 20 (2015) 20499-20517; US-A-4,882,241; and US-A- 4,469,760.

[0004] During charging, the electrolytes are circulated through the cell, and a voltage is applied across the cell. In the negative half-cell, one redox couple is reduced, and in the positive half-cell the other redox couple is oxidized. During discharging, the opposite occurs: the redox species in the positive half-cell is reduced while the redox species in the negative half-cell is oxidized. For example, US-A-4,786,567 describes an all-vanadium RFB, in which the positive redox species are V(IV) and V(V), while the negative redox species are V(II) and V(III). Thus, the electrochemical reactions are as follows:

In the negative half -cell:

V(III) + e ^~ → V(II) [charge]

(la)

V(II)→ V(III) + e ^~ [discharge]

(lb)

In the positive half-cell:

V(V) + e ^~ → V(IV) [discharge]

(2a)

V(IV)→ V(V) + e ^~ [charge]

(2b)

Similar processes occur in RFBs based on other redox chemistries.

[0005] Contrary to RFBs, many conventional secondary batteries utilize solid compounds to store charge and energy. These materials often provide much higher energy density than soluble species, due to their intrinsically higher concentration of active redox species. See B. Dunn, H. Kamath, J.-M. Tarascon, Electrical energy storage for the grid: A battery of choices, Science, 334 (2011) 928-935.

[0006] Ion polarisation at a liquid-liquid interface is well known for those skilled with the art. Ion partition between two immiscible electrolyte solutions can generate a polarization between the two phases. See C. Gavach, Un type de piles a phases liquides - pile a double distribution, Journal de Chimie Physique et de Physico- Chimie Biologique, 64 (1967) 759-765; C. Gavach, Cells with liquid phases. I. Cells with dodecanol, Journal de Chimie Physique, 64 (1967) 799-809; C. Gavach, Cells with liquid phases. II. Cells with benzene, Journal de Chimie Physique, 64 (1967) 810-817 ;] H.H. Girault, D.J. Schiffrin, Electrochemistry of liquid-liquid interfaces, in: A.J. Bard (Ed.), Electroanalytical Chemistry: A Series of Advances, Marcel Dekker, New York, 1989: pp. 1-141; H.H. Girault, Electrochemistry at liquid-liquid interfaces, in: A.J. Bard, C.G. Zoski (Eds.), Electroanalytical Chemistry: A Series of Advances, Taylor & Francis, Boca Raton, FL, 2010: pp. 1-104 and P. Peljo, H.H. Girault, Liquid/ Liquid Interfaces: Electrochemistry at, in: R. A. Meyers (Ed.), Encyclopedia of Analytical Chemistry, John Wiley and Sons, Inc., Chichester, 2012.

Summary of the Invention

[0007] The present invention presents an additional method of generating a cell voltage based on the partition of ions, even non-redox ions, between immiscible phases.

[0008] The invention utilizes the polarisation between two immiscible phases to provide or enhance cell voltage.

[0009] The invention provides a battery system according to claims 1 and a redox flow battery according to claim 3. Optional features of the invention are set out in the dependent claims.

[0010] In one embodiment, the redox flow battery of the present invention includes a positive and negative non-aqueous electrolyte, each one containing at least a redox couple. The non-aqueous electrolytes are separated by a third aqueous electrolyte partially immiscible with the other two non-aqueous electrolytes. The electron transfer reactions of the redox couples at the respective electrodes are coupled to ion transfer reactions at the respective interfaces with the third aqueous electrolyte. The polarisation between the positive non-aqueous electrolyte and third aqueous electrolyte, and the polarisation between the third aqueous electrolyte and the negative non-aqueous electrolyte are utilized to provide or enhance the cell voltage.

[0011] Alternatively, the redox flow battery may include a positive and negative aqueous electrolyte, each one containing at least a redox couple. The aqueous electrolytes are separated by a third non-aqueous electrolyte partially immiscible with the other two aqueous electrolytes. The electron transfer reactions of the redox couples at the respective electrodes are coupled to ion transfer reactions at the respective interfaces with the third non-aqueous electrolyte. The polarisation between the positive aqueous electrolyte and third non-aqueous electrolyte and the polarisation between the third non-aqueous electrolyte and the negative aqueous electrolyte are utilized to provide or enhance the cell voltage.

[0012] The polarisation between the positive electrolyte and the third electrolyte results mainly from a partition of a common ion between both phases.

[0013] The polarisation between the negative electrolyte and the third electrolyte results mainly from a partition of a common ion between both phases.

[0014] In certain embodiments of the present invention when the third electrolyte is aqueous, the energy is stored by transferring a hydrophilic cation Cw ⁺ and a hydrophilic anion Aw- from the aqueous third electrolyte into the negative and positive non-aqueous electrolytes, respectively. The reaction upon charging of the battery composed of a redox couple Op/Rp dissolved in the non-aqueous positive and of a redox couple On/Rn dissolved in the non-aqueous negative electrolyte and a third electrolyte containing Cw ⁺ and Aw- ions are shown in reactions 3 and 4.

In the negative half -cell: On (negative electrolyte) + er→ Rn (negative electrolyte)

(3a)

Cw ⁺ (third electrolyte)→ Cw ⁺ (negative electrolyte)

(3b)

In the positive half-cell:

Rp (positive electrolyte)→ Op (positive electrolyte) + er

(4a)

Aw (third electrolyte)→ Aw- (positive electrolyte)

(4b)

The total reaction is:

Rp (positive electrolyte) + Cw ⁺ (third electrolyte) + Aw- (third electrolyte) + On (negative electrolyte)→ Op (positive electrolyte) + Cw ⁺ (negative electrolyte) + Aw (positive electrolyte) + Rn (negative electrolyte) (5)

The reactions are reversed upon discharge.

[0015] Alternatively, the negative half-cell reaction can include a transfer of a hydrophobic anion Ao ^_ from the non-aqueous negative electrolyte into the aqueous third electrolyte. In this case the negative half -cell upon charging of the battery is:

On (negative electrolyte) + er→ Rn (negative electrolyte)

(6a)

Ao ^" (negative electrolyte)→ Ao ^" (third electrolyte)

(6b)

The reverse reactions take place at the discharge. [0016] Alternatively, the positive half-cell reaction can include a transfer of a hydrophobic cation Co ⁺ from the non-aqueous positive electrolyte into the aqueous third electrolyte. In this case the positive half -cell upon charging of the battery is:

Rp (positive electrolyte)→ Op (positive electrolyte) + er

(7a)

Co ⁺ (positive electrolyte)→ Co ⁺ (third electrolyte)

(7b)

The reverse reactions take place at the discharge.

[0017] In certain embodiments of the present invention, any combinations of reactions 3, 4, 6 and 7 are possible.

[0018] Alternatively, in certain embodiments of the present invention when the third electrolyte is non-aqueous, the energy is stored by transferring a hydrophobic cation Co ⁺ and a hydrophobic anion Ao- from the non-aqueous third electrolyte into positive and negative aqueous electrolytes respectively. The reaction upon charging of the battery composed of a redox couple Op/Rp dissolved in the aqueous positive electrolyte and a second redox couple On/Rn dissolved in the aqueous negative electrolyte and an non-aqueous salt solution composed of Co ⁺ and Ao- are shown in reactions 8 and 9.

In the negative half -cell:

On (negative electrolyte) + er→ Rn (negative electrolyte)

(8a)

Co ⁺ (third electrolyte)→ Co ⁺ (negative electrolyte)

(8b)

In the positive half-cell: Rp (positive electrolyte)→ Op (positive electrolyte) + er

(9a)

Ao ^" (third electrolyte)→ Ao ^" (positive electrolyte)

(9b)

The reactions are reversed upon discharge.

[0019] Alternatively, the negative half -cell reaction can include a transfer of a hydrophilicc anion Aw- from the aqueous negative electrolyte into the non-aqueous third electrolyte. In this case the negative half -cell upon charging of the battery is:

On (negative electrolyte) + er→ Rn (negative electrolyte)

(10a)

Aw (negative electrolyte)→ Aw- (third electrolyte)

(10b)

The reverse reactions take place at the discharge.

[0020] Alternatively, the positive half-cell reaction can include a transfer of a hydrophilic cation Cw ⁺ from the aqueous positive electrolyte into the non-aqueous third electrolyte. In this case the positive half -cell upon charging of the battery is:

Rp (positive electrolyte)→ Op (positive electrolyte) + er

(11a)

Cw ⁺ (positive electrolyte)→ Cw ⁺ (third electrolyte)

(lib)

The reverse reactions take place at the discharge.

[0021] In certain embodiments of the present invention, any combinations of reactions 8, 9, 10 and 11 are possible. [0022] Alternatively, in certain embodiments of the present invention multiple redox couples are employed either in the negative electrolyte or in the positive electrolyte, or both.

Brief Description of the Drawings

[0023] The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:

[0024] Figure 1 schematically shows a battery constructed in an H-cell, according to an embodiment of the invention, with a third electrolyte acting as a polarisation enhancer between the positive and negative electrolytes.

[0025] Figure 2 schematically shows a battery according to another embodiment of the invention;

[0026] Figure 3 schematically shows a redox flow battery system according to another embodiment of the invention;

[0027] Figure 4 schematically shows a battery system according to yet another embodiment of the invention, in which the positive and negative electrolytes are dispersed in a partially immiscible third electrolyte;

[0028] Figure 5 shows a typical charge/ discharge cycling of the battery under vigorous stirring, measured with a H-cell.

[0029] Figure 6 shows the schematic of the experimental set-up for the cyclic voltammetry study of the redox reactions on a droplet modified electrode.

[0030] Figure 7 is a series of cyclic voltammetry measurements of a droplet of positive electrolyte containing decamethylferrocenium/ decamethylferrocene redox couple and 100 mM tetrahexylammonium perchl orate in 1,2-dichloroethane deposited on a carbon paste electrode, in contact with aqueous 1M L1CIO4 + 100 mM LiOH solution, where the interface is polarized by perchlorate ion. [0031] Figure 8 is a series of cyclic voltammetry measurements of a droplet of negative electrolyte containing decamethylferrocenium/ decamethylferrocene redox couple and 100 mM lithium tetrakispentafluorophenylborate in 1,2-dichloroethane deposited on a carbon paste electrode, in contact with aqueous 1M L1CIO4 + 100 mM LiOH solution, where the interface is polarized by lithium ion.

Detailed Description of Particular Embodiments

[0032] Different arrangements may be considered for this battery system. The following descriptions mention specific materials and configurations. However, the breadth of this disclosure is not limited by these specific examples.

[0033] There are several aspects of the invention, which may be modified or changed. Among these are the electrolytes, the redox couples, the solvents, the supporting electrolyte species and concentrations, electrode material and configuration, configuration of the cell, and number of redox couples.

[0034] Figure 1 shows an arrangement for a battery with ion polarisation enhancer in a so-called H-cell configuration. The electrochemical cell composed of two conductive electrodes, namely a negative electrode 1 and a positive electrode 2 in contact respectively with the negative and positive electrolyte solutions, separated by the partially immiscible third electrolyte acting as a polarization enhancer. The reactions shown in the figure are one of the possible reactions taking place upon charging of the battery.

[0035] Figure 2 shows an arrangement for a battery with ion polarisation enhancer. The electrochemical cell composed of two porous conductive electrodes, namely a porous negative electrode 1 and a porous positive electrode 2 in contact respectively with the negative and positive electrolyte solutions and in contact with a partially immiscible third electrolyte acting as a polarization enhancer. The third electrolyte can be either free-standing in a membrane-less configuration or physically supported in a porous material. The reactions shown in the figure are one of the possible reactions taking place upon charging of the battery.

[0036] Figure 3 shows a redox flow battery with ion polarisation enhancer. The electrochemical cell composed of two porous conductive electrodes, a negative porous electrode 1 and a positive porous electrode 2 in contact respectively with the negative and positive electrolyte solutions, separated by the partially immiscible third electrolyte acting as the polarisation enhancer. The negative electrolyte is stored in a first storage tank 3, the positive electrolyte is stored in a second storage tank 4 and the third electrolyte is stored in a third storage tank 5. The electrolytes are circulated through the electrochemical cell by pumps 6, 7 and 8. The reactions shown in the figure are one set of the possible reactions taking place upon charging of the battery.

[0037] In certain embodiments of the present invention, the redox active material in the negative half-cell belongs to or is a combination of a group of active materials previously known in the art, including those comprising of soluble, semi-solid, intercalation, capacitive or pseudo-capacitive, or plating type active materials.

[0038] Similarly in certain embodiments of the present invention, the redox active material in the positive half-cell belongs to or is a combination of a group of active material previously known in the art, including those comprising of soluble, semisolid, intercalation, capacitive or pseudo-capacitive, or plating type active materials.

[0039] In certain embodiments of the present invention, the redox active active material is the same in both negative and positive half-cells. In this case, most of the battery voltage is provided by the polarisation between the third electrolyte and the electrolytes in the negative and positive half -cells.

[0040] In certain embodiments of the present invention, the redox active material is different between both negative and positive half-cells. In this case, the battery voltage is enhanced by the polarisation between the third electrolyte and the electrolytes in negative and positive half -cells. [0041] Certain embodiments of the present invention provide flow batteries, each flow battery comprising a positive aqueous electrolyte containing a first redox active material; a third non-aqueous electrolyte containing a supporting electrolyte; a negative aqueous electrolyte containing a second redox active material; a positive electrode in contact with the said positive aqueous electrolyte; a negative electrode in contact with the said negative aqueous electrolyte; the third non-aqueous electrolyte in contact with both said positive and negative aqueous electrolytes.

[0042] Alternatively, the positive and negative electrolyte, containing respective first and second redox active materials, can be non-aqueous and the third electrolyte containing a supporting electrolyte can be aqueous.

[0043] In either case, the third electrolyte may be separated from the positive and negative electrolytes by a microporous separator, cation exchange membrane, anion exchange membrane, dialysis membrane or any other type of membrane, or combinations thereof.

[0044] The positive and third electrolytes contain a common ion establishing a polarisation across the interface between the phases. The negative third non-aqueous electrolytes contain another common ion establishing a polarisation across the interface between the negative electrolyte and the third electrolyte.

[0045] In certain embodiments of the system one or more of the non-aqueous solvents is 1,2-dichloroethane, 1,1-dichloroethane, 1,6-dichlorohexane, 1,2- dichlorobenzene, chlorobenzene, triflurotoluene, nitrobenzene, valeronitrile, caprylonitrile, 2-octanone, 2-decanone, 3-nonanone, 5-nonanone, 2-heptanone, methyl-isobutylketone, or any other non-aqueous solvent or mixture of solvents.

[0046] In certain embodiments of the system the common ion between the negative electrolyte and the third electrolyte is a proton, an alkali metal cation, an earth alkali metal cation, transition metal cation, a metal complex, a quaternary phosphonium or ammonium cation, or another type of cation. Or it can be an inorganic or organic anion, or another type of anion. Alternatively, multiple common ions can be employed.

[0047] Similarly in certain embodiments of the system the common ion between the positive electrolyte and the third electrolyte is a proton, an alkali metal cation, an earth alkali metal cation, transition metal cation, a metal complex, a quaternary phosphonium or ammonium cation, or another type of cation. In certain embodiments of the system the common ion between the positive electrolyte and the third electrolyte is an inorganic or organic anion, or another type of anion. Alternatively, multiple common ions can be employed.

[0048] In certain embodiments of the system there are no ion permeable membranes or separators separating the compartments.

[0049] In certain embodiments of the system one or more of the compartments are separated with an ion-permeable membrane, such as a microporous separator, cation exchange membrane, anion exchange membrane, a bipolar membrane, dialysis membrane or any other type of separator.

[0050] Figure 4 shows an arrangement for a battery with emulsion polarisation enhancer. The system includes a positive electrode 1 and a negative electrode 2. A negative electrolyte 3 and a positive electrolyte 4 are dispersed in a third partially immiscible electrolyte acting as a polarisation enhancer. The two sides of the cell are separated by an ion permeable membrane 5. The charge transfer reaction in the negative half-cell takes place when a droplet of negative electrolyte contacts the negative electrode, and the charge transfer reaction in the positive half-cell takes place when a droplet of positive electrolyte contacts the positive electrode. The reactions shown in the figure are one set of the possible reactions taking place upon charging of the battery.

Examples

Example 1 - Electrochemical performance of the battery [0051] The electrochemical performance of the battery was tested in a static H-cell configuration as illustrated in Figure 1 under anaerobic conditions, with porous reticulated vitreous carbon electrodes (or glassy carbon (GC) electrodes, 3 mm diameter) on both sides, in a two-electrode set-up, with 1,2-dichloroethame (DCE) as the non-aqueous phase, under vigorous stirring. The positive non-aqueous electrolyte containing 50 mM decamethylferrocenium tetrakispentafluorophenyborate (DMFcTB) and 100 mM tetrahexylammonium perchlorate (THxAC10 ₄ ) was separated from the negative non-aqueous electrolyte containing 50 mM decamethylferrocene (DMFc) and 100 mM lithium tetrakispentafluorophenylborate (LiTB) by an aqueous third electrolyte containing 1 M L1CIO4 and 100 mM LiOH. The ohmic resistance was calculated with a current step measurement, a current step from 0 to 100 μΑ being taken at the time of 10 ms after each charge and discharge cycle. All experiments were corrected for the IR drop to describe the theoretical maximum performance of the system. The charge- discharge curves are illustrated in Figure 5.

[0052] The IR compensated polarization measurement of the fully charged battery with two glassy carbon disc electrodes (radius 1.5 mm) in both electrolytes shows that the described battery is able to produce current densities up to 20 mA cm ^-2 , comparable to typical redox flow batteries, even at this low concentration of redox mediators.

[0053] The example of the cycling behaviour of the battery in Figure 5 shows that the coulombic efficiency of the system was close to 100%, when a current of 1 mA was used for both charging and discharging. The energy efficiency reached 83% at the charge/ discharge current of 0.3 mA when utilizing DCE as the organic solvent.

Example 2 - Cyclic voltammetry ofdecamethylferrocenium/decamethylferrocene redox couple on droplet modified electrodes polarized by different common ions

[0054] Cyclic voltammograms (CVs) were recorded under both ambient aerobic conditions and anaerobic conditions using a potentiostat. Three electrode experiments were performed with droplet-modified carbon paste electrodes (radius 2.5 mm) with Ag/AgCl/(3 M KC1) reference and a platinum counter electrode. A droplet of 5μΙ, of the non-aqueous solution was deposited onto the surface of the working electrode, which was immersed into the aqueous solution. In order to avoid the oxidation of DMFc by oxygen, the electrolytes were prepared with concentrations corresponding to the discharged battery. (DMFc can readily reduce oxygen in the presence of LiTB.) Additionally, 100 mM of LiOH was added into the aqueous solution to make oxygen reduction thermodynamically more unfavorable.

[0055] The reactions occurring at the positive and negative electrodes were investigated outside a glove box by cyclic voltammetry. The experimental set-up is shown in Figure 6, with a droplet of negative electrolyte added on the carbon paste disk electrode, immersed in an aqueous solution. Cyclic voltammograms obtained at different scan rates are displayed in Figure 7 for the negative electrolyte and in Figure 8 for the positive electrolyte, with half-wave potentials of -0.70 V and -0.12 V vs. Ag/AgCl/(3M KC1) for the DMFc ⁺ /DMFc couple with LiTB and with THxAC10 ₄ . The peak currents depend linearly on the square root of the scan rate, due to the mass transport of the DMFc/DMFc ⁺ couple. The solubility of DMFcTB (in 100 mM LiTB solution of DCE) could be estimated as ca. 40 mM considering that the diffusion coefficient of DMFc ⁺ is equal to the diffusion coefficient of DMFc. The reduction peak current is always slightly higher than the oxidation peak, indicating that some of the DMFc generated at the electrode during the reduction is oxidized by oxygen. These CVs demonstrate that a battery with a cell voltage of more than 0.8 V can be constructed.