DESC:FIELD OF THE INVENTION
The present invention relates to a novel electrolyte for a static Zinc-based battery, and particularly to an electrolyte for a Zinc-Bromine redox static battery, where the electrolyte comprises zinc bromide and a specific bromine complexing agent along with other electrolyte ingredients, where the electrolyte is both cold and heat stable.
BACKGROUND OF THE INVENTION
Fossil fuel resources are not only inherently finite, but also their combustion results in environmental problems, making the research community focus more towards green, renewable, and sustainable energy resources for energy generation, utilization, and storage. In order to exploit the sustainable energy resources, metal halide electrochemical cells were introduced for their high efficiency, low cost, durability, and eco-friendly rechargeable energy storage devices. However, the metal halide electrochemical cells undergo degradation over time, leading to a decrease in performance and a limited cycle life. In addition, the metal halide electrochemical cells have a limited operating temperature range and may not be suitable for extreme conditions, which limits their practical use, which is not desirable.
In recent times, the tremendous improvement in cost and performance of lithium-ion batteries (LIBs) have made them market dominators in the rechargeable energy storage devices segment. The desire to lower costs and increase energy density, as well as growing concern over Li ion's natural resource, has pushed the development of the so-called "beyond Li-ion" technologies.
In recent years, flow batteries with multiple redox couples are approaching as one of the most promising alternatives for large-scale rechargeable energy storage devices. So far, a variety of energy storage device chemistries involving metal ions of sodium (Na), potassium (K), magnesium (Mg), and zinc (Zn) have been investigated as a possible alternative to the currently existing rechargeable energy storage devices. Out of this Zn-based energy storage devices are considered a promising alternative candidate for large-scale grid applications due to their low cost, eco-friendliness, high safety, material abundance, and ease of manufacturing.
Typically, electrochemical cells are metal halogen cells in which the anode material most commonly employed is zinc, and the most commonly employed cathodic halogen is bromine. Such metal halogen electrochemical cells are favoured due to their extremely high theoretical energy density. For example, a zinc-bromine cell has a theoretical energy density of 200 watts hours per pound (Wh/lb) and an electric potential of about 1.85 volts per cell. However, such electrochemical cells of the foregoing type are known to suffer from a number of technical issues. Most of the technical issues are associated with side reactions, which may occur in such electrochemical cells. For example, during the charging process, free bromine is produced in the electrochemical cells. The free bromine is available for electrochemical reaction with the zinc anode, thereby resulting in self-discharge of the electrochemical cells. Additionally, there is a tendency for hydrogen gas to be generated when considerable amounts of the free bromine are present in an aqueous phase. Zinc-bromine flow batteries are appealing mainly due to their long cycling life and are semi-deposition flow batteries. In a typical zinc-bromine battery (flow and static), zinc bromide acts as both an active material and an ionic conductor. The zinc-bromine liquid battery is used as a combination of a flow battery technology and an energy storage device and has a very high application prospect in the field of rechargeable energy storage devices. The electrochemical reactions can be depicted as follows:
Negative side: Zn ? Zn2+ + e-
Positive side: Br2 + 2e- ? 2Br-
Overall: Zn + Br2 ? ZnBr2
During charging, Br2 is generated at the positive electrode and further complexes with bromide ions (Br-) present in aqueous media to form highly soluble Br3- ions (tribromide ions), while zinc is deposited at the negative electrode simultaneously. Reverse reactions happen at the respective electrodes during the discharge process. Although the highly soluble Br3- species accelerates the redox kinetics and diffuse to a zinc electrode by cross-diffusion, the technical issue is that they directly react with the plated zinc and therefore, trigger self-discharge and low coulombic efficiency. Another existing issue for the zinc-based based energy storage devices is the dendritic Zn deposition, which potentially triggers an internal short circuit. Such technical challenges are alleviated with the multiple strategies of a standard zinc bromide flow battery. However, it is to be understood that all the existing approaches are compromised at the expense of cell resistance, energy efficiency (typically < 60%), increased system size, and complexity. Overall, such conventional approaches sharply raise the capital costs from $8 per kWh based on ZnBr2 electrolyte and carbon electrode to over $200 per kWh for a battery system, which is not superior at all when compared with the conventional Li-ion technology. Further, the flowable and corrosive Br2/ Br3- species reduce the reliability of the conventional battery systems, leading to cell failure by parts corrosion instead of the intrinsic redox chemistry. Secondary hydrolysis reactions are also problematic for such types of storage batteries when the electrolytes are formulated with excess free water, because bromate solids form, which in turn reduces the amount of available bromide/bromine that can under reduction or oxidation in the electrochemical cell.
Br2 + H2O HBrO + HBr
water
Bromate solid
The development of efficient electrolytes that can prevent cross-diffusion and provide high energy efficiency with long cycle life. Moreover, the zinc dendrite formation is difficult to solve and has been a longstanding problem for several decades.
U.S. Patent 3,640,771 discloses a metal bromide aqueous electrolyte battery having a limited concentration of the metal bromide in solution in the electrolyte. Moreover, when the battery is charged, the remaining dissolved metal bromide salt and the total free bromine in the electrolyte are in minor quantity, the remaining free bromine is retained in a bromine-adsorbent layer provided at the cathode. The patent further provides a secondary battery comprising an anode, and a cathode including a bromine-adsorbent layer capable of adsorbing at least half its weight of molecular bromine when the battery is charged. The secondary battery further includes an electrolyte comprising a divalent electroplatable metal bromide salt dissolved in an aqueous medium, wherein the bromine adsorbent is an activated carbon.
U.S. Patent 4,105,829 discloses a metal halogen cell, which employs a circulating electrolyte system containing a complexing agent to effectively remove cathodic halogen from the electrolyte during the charging of the metal halogen cell. The complexing substituent or complexing agent is one which, in the presence of halogen, forms a water-immiscible halogen complex. This complex is separated and stored external to the metal halogen cell during charging but is returned to the metal halogen cell during discharge. Despite of certain efficiency achieved with the above-mentioned zinc-bromine battery/cell, the coulombic inefficiencies still persist. This loss in coulombic efficiency is attributed to the reaction of un-complexed dissolved bromine present in the metal halogen cell with elemental zinc.
Chinese patent, CN108711633B discloses electrolyte for the zinc-bromine flow battery comprising a positive electrolyte and a negative electrolyte, which is respectively composed of zinc bromide, zinc chloride, a complexing agent, an anti-dendrite agent, a wetting agent, and water. This patent discloses use of 1-(carboxymethyl) pyridine-1-onium bromide as a complexing agent in large amounts in combination with an anti-dendrite agent and the wetting agent, which are added into the electrolyte for the zinc-bromine flow battery so that the current efficiency of the zinc-bromine flow battery can be increased. However, the use of the large amount of bromine complexing agents, such as pyridinium compounds, makes the device very expensive. Also, such complexes are not compatible with all Zinc-bromine batteries. Further, this has the issue of high self-discharge.
Another Chinese patent application, CN 110767927 A, discloses an electrolyte for a zinc-bromine liquid battery, which mainly comprises zinc bromide, a bromine complexing agent in extremely low amounts, a dendrite inhibitor, a conductive agent, a pH regulator, and distilled water. The electrolyte related to the prior art is mainly suitable for a flow battery system with a circulating device, and the flow battery system is complex in structure, high in manufacturing cost, and not suitable for large-scale production and use. Journal of Power Sources 384 (2018) 232–239, provides a solution for dendrite growth by using an electrolyte modified with methanesulfonic acid. However, highly acidic solutions reduce the pH of the electrolyte to a significant value. At lower pH, Zinc-Bromine batteries suffer hydrogen evolution reaction and further reduce battery efficiency. Further gas evolution during cycling is a safety concern on a commercial scale.
Chinese patent CN113991191A discloses an aqueous Zinc-Bromine battery, which is used to improve poor battery reversibility, cross-contamination, low Coulomb efficiency, and cycle instability. This patent application talks about use of Ketjen black® modified carbon felt positive electrode in combination with an electrolyte containing tetrapropylammonium bromide as a bromine complexing agent along with zinc dendrite inhibitor, where the pH of the electrolyte is maintained by using pH adjusting agents such as sulphuric acid solution and the like.
US patent application US 20190198881 A1, discloses electrolyte composition for Zinc-Bromine battery, and said electrolyte comprises zinc bromide and small amount of quaternary ammonium agents. This patent application suggests using N-methyl-N-ethylmorpholinium bromide (MEMBr) and related quaternary ammonium agents in an amount of less than 0.3 times of zinc bromide but has the issue of high self-discharge.
To solve the above-mentioned inherent problems of the conventional zinc-bromine flow batteries, the development of a static zinc-bromine battery that can be produced on a commercial scale is necessary. One of the major technical problem in the development of the static zinc-bromine battery is freezing of the electrolyte at low temperature (for example at temperature below 0 0C) and evaporation of the electrolyte at high operating temperature. Conventionally, in order to solve the problem of freezing and evaporation, additional thermal management systems are being implemented, which leads to an increase in cost of the static zinc-bromine battery. Thus, there exists a technical problem of how to maintain physical state of electrolyte at wide range of operating temperatures without any additional support for thermal management. In view of the existing technical problems and unsolved issues in prior known literatures, there is a need for a suitable electrolyte, which is thermally and chemically stable, with enhanced shelf life, and overcomes the problems present with existing electrolytes for zinc-based rechargeable energy storage devices.
SUMMARY OF THE INVENTION
The present invention provides an electrolyte for use in a static zinc-based battery, particularly for a Zinc-Bromine redox static battery. One non-limiting advantage of the electrolyte of the present invention is that the disclosed electrolyte maintains a corresponding physical state during the operation of the Zinc-Bromine redox static battery between a wide range of temperatures. In other words, the composition of the disclosed electrolyte prevents freezing of the electrolyte at low temperatures (i.e., up to -200C) as well as evaporation of the electrolyte at high temperatures (i.e., up to 500C). The electrolyte significantly minimizes self-discharge, which is one of the major technical problems of Zinc-Bromine static batteries. Furthermore, the electrolyte manifests a surprising effect which not only minimizes the self-discharge but also enables to use of more active ingredient, i.e., the Zinc Bromide, which is observed to directly enhance the energy density of the Zinc-Bromine static battery. Any Zinc-Bromine static battery, when uses the electrolyte, manifests improved performance in terms of increased energy capacity and energy efficiency, for example, more than 20%, just by only replacing convention electrolyte with the electrolyte of the present invention. In other words, improvements in the battery performance may be achieved even if no hardware or operational changes are done in a given Zinc-Bromine static battery when the electrolyte (may also referred to as an aqueous electrolytic composition) of the present invention is used.
Accordingly, one aspect of the present invention provides an electrolyte of a static zinc-based battery comprising:
a) zinc bromide in molar concentration ranging from 1.5M to 3.0M;
b) tetraethylammonium bromide as a bromine complexing agent in half (i.e. approximately half) of the molar concentration of the zinc bromide;
c) a glycol-based anti-freezing agent in molar concentration ranging from 1.0M to 2.5M;
d) zinc chloride as supporting ionic conducting agent in molar concentration ranging from 1 M to 3.0 M; and
e) an additional supporting ionic conducting agent in molar concentration ranging from 0.5M to 3.0M, wherein the additional supporting ionic conducting agent is selected from magnesium chloride or calcium chloride or lithium chloride or a combination thereof.
It is known that the main challenges of the Zinc Bromide based batteries include (a) low current densities during cycling, (b) dendrite formation, (c) freezing of the electrolyte at low temperatures up to -200C and evaporation of the electrolyte at high temperatures up to 500C, (d) high viscosity of the electrolyte polybromide phase in low states of charge, (e) self-discharge (f) low ionic conductivity of the electrolyte at low operating temperatures and (g) pH variations during operation. Present invention has dealt and solved the abovementioned problems, especially a surprising technical effect is achieved when the additional supporting ionic conducting agent, specifically the magnesium chloride or calcium chloride or lithium chloride or a combination thereof, in molar concentration ranging from 0.5M to 3.0M. Due to the colligative properties of the electrolyte after adding the magnesium chloride or calcium chloride or lithium chloride salts, the electrolyte does not freeze even at low temperatures such as up to -200C as well as does not evaporate at high temperatures such as up to 500C. Surprisingly, other supporting ionic conducting agent like potassium chloride or sodium chloride in conventional electrolytes prevents self-discharge and/or dendrite formation in the static zinc-based battery, but cannot sustain at low temperature and may freeze at sub-zero temperatures. However, the use of magnesium chloride, lithium chloride or calcium chloride instead of potassium chloride in the electrolyte not only prevents self-discharge and/or dendrite formation, but also maintains the physical state of the electrolyte at sub-zero temperatures as well as high operating temperature up to 50 0C.
Further, the disclosed electrolyte includes the tetraethylammonium bromide as the bromine complexing agent in half of the molar concentration of the zinc bromide. Surprisingly, during experimentation, when the Tetraethylammonium bromide (TEAB) was used as the complexing agent in about half of the molar concentration of the zinc bromide when zinc bromide in molar concentration from about 1.5M to 3.0M, a significant improvement in reduction in self-discharge was achieved. The presence of the glycol-based anti-freezing agent, the zinc chloride, and the additional supporting ionic conducting agent specified in corresponding molar concentration further manifests a synergistic effect to significantly contribute to solving the aforementioned problems, especially the problem of freezing and evaporation of electrolyte over a wide range of temperatures.
As envisaged herein, an electrolyte for the static zinc-based battery is provided, comprising: a) zinc bromide in a molar concentration ranging from 1.5M to 3.0M, b) tetraethylammonium bromide as a bromine complexing agent in half of the molar concentration of the zinc bromide to arrest Br2/Br3- species (tribromide ions) in a solid form; c) a glycol-based anti-freezing agent in molar concentration ranging from 1.0M to 2.5M; d) zinc chloride as supporting ionic conducting agent in molar concentration ranging from 1M to 3M; and e) one or more additional supporting ionic conducting agents selected from magnesium chloride or calcium chloride or lithium chloride or a combination thereof, wherein each of the magnesium chloride or the calcium chloride or lithium chloride or their combination is in a molar concentration ranging from 0.5M to 3.0M. In an implementation, the said electrolyte is free of pH-maintaining agents and buffering agents.
In an implementation, the electrolyte is prepared in absence of external heat energy.
The zinc bromide and the tetraethylammonium bromide is present in the ratio of 2:1. (i.e., the molar concentration of the tetraethylammonium bromide is half of the molar concentration of the zinc bromide). Beneficially, the optimized molar ratio of 2:1 between the zinc bromide and the complexing agent (specifically, TEAB) and selection of salts in specific concentration range allows the dissolution of >1.25M of TEAB (or even higher than that) in the presence of ZnBr2 (2.5 M) even at room temperature. The higher dissolution of the complexing agent (e.g., the TEAB) not only minimizes the self-discharge but also allows the use of more active ingredient (i.e., zinc bromide), which directly enhances the energy density of the Zinc-Bromine static battery. Furthermore, when the molar concentration of the tetraethylammonium bromide is half of the molar concentration of the zinc bromide and additionally, the supporting ionic conducting agent, specifically the magnesium chloride or calcium chloride or lithium chloride or a combination thereof, is used in molar concentration ranging from 0.5M to 3.0M, the solubility of the tetraethylammonium bromide is increased while maintaining the stability of the electrolyte in wider temperature range (-20 degree Celsius to 50 degree Celsius).
In an implementation, the glycol-based anti-freezing agent is selected from the group comprising of monoethylene glycol, propylene glycol, 1,3-butylene glycol, polyethylene glycol 200, polyethylene glycol 400, or a combination thereof.
In an implementation, the glycol-based anti-freezing agent is monoethylene glycol.
In another implementation, the electrolyte is free of pH maintaining agents and buffering agents.
In yet another implementation, the electrolyte comprises the glycol-based anti-freezing agent in the molar concentration of 2M.
In an implementation, the monoethylene glycol is present in molar concentration of 1.35M.
In an implementation, the electrolyte comprises the zinc chloride as a supporting ionic conducting agent in the molar concentration of 1.5M.
In another implementation, the electrolyte comprises the additional supporting ionic conducting agent in the molar concentration of 1M.
In yet another implementation, the electrolyte comprises the tetraethylammonium bromide in molar concentration ranging from 0.75M to 1.5M.
In an implementation, the zinc chloride is present in the molar concentration of 1.7M.
In an implementation, the additional supporting ionic conducting agent is present in the molar concentration of 3.0M.
In an implementation, the electrolyte is maintained at a pH value ranging from 3-5 in absence of pH maintaining agents and buffering agents.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present disclosure will now be described, by way of example only, with reference to the following diagrams wherein:
FIG. 1 is a diagram illustrating a graphical representation of performance of a static zinc-based battery having an electrolyte with a specific composition, in accordance with an embodiment of the present disclosure;
FIG. 2 is a diagram illustrating a graphical representation indicating a cyclic voltammogram of an electrolyte in a static zinc-based battery at operating temperature of 50C, in accordance with another embodiment of the present disclosure;
FIG. 3 is a diagram illustrating a graphical representation indicating a cyclic voltammogram of an electrolyte in a static zinc-based battery within a range of operating temperatures 5 0C to 25 0C, in accordance with another embodiment of the present disclosure; and

DETAILED DESCRIPTION OF THE INVENTION
DEFINITIONS
The term "static energy battery" shall mean an energy storage device with physically non-flowing or non-moving electrolyte or cathode electrode or anode electrode materials.
The term "electrolyte solution" shall mean electrolyte solution that comprises ions and uses water as the solvent. It contains aqueous solvent and ions, atoms or molecules that have lost or gained electrons, and is electrically conductive.
The term "ambient temperature" shall mean temperature falling in the range of 25 to 30°C.
When referring to the concentration of components or ingredients for eutectic electrolytes, Moles shall be based on the total volume of the aqueous electrolyte.
DESCRIPTION
Certain embodiments of the disclosure provide an electrolyte (i.e., an aqueous electrolyte composition) for use in a static zinc-based battery, particularly Zinc-Bromine redox static battery.
In a first example, typically, it is observed that in the presence of zinc bromide, dissolution of a complexing agent (at ambient temperature) at higher concentration (>0.3 M) is technically challenging and not yet achieved by conventional electrolytes or reported yet, as there is precipitation in the form of bromozincate, which is not desirable.
In a second example, the zinc dendrite formation problem is difficult to solve and has been a longstanding problem for several decades. It is known that the dendritic zinc growth can somehow be suppressed by adding a small amount of inorganic or organic additives, but performance in terms of dendritic zinc growth is still not satisfactory. Additionally, it was observed that addition of external additive significantly reduces the ionic conductivity, naturally affects the reaction kinetics and, as a consequence, overall cell performance of Zinc-Bromine redox static battery got affected.
In a third example, it is known that bromine is a very good electro-active species, with improved and reversible kinetics. Typically, the reduction of diatomic bromine to two bromide ions has been used in the electrolyte fluids for decades. Battery types with functions relying on the bromine half reaction include zinc bromide, hydrogen bromide, and vanadium bromide batteries. However, even at relatively low concentrations, diatomic bromine has a propensity to form a vapor phase, which separates out of the liquid electrolyte, interfering with the recharge of bromine-containing batteries. For this reason, it is necessary to keep the free diatomic bromine concentration in the electrolyte low enough, such that vapor phase formation does not occur, which is achieved via bromine complexing agents. Such bromine complexing agents reduce reactivity and vapor pressure without changing the electrochemical properties of bromine ions. Prior art discloses the use of certain morpholine and pyridinium-based quaternary ammonium salts as bromine complexing agents, however, the major drawback of their use as complexing agents for bromine is that they are not always compatible with different bromide chemistries. Moreover, currently, a proper concentration in which the complexing agent is to be used with respect to zinc bromide is not known in the existing art.
In a fourth example, due to the addition of zinc bromide, tetraethylammonium bromide, glycol-based anti-freezing agent and zinc chloride, the ionic conductivity of the electrolyte gets decreased and TEAB crystalizes due to its limited solubility and also freezing of the electrolyte at low temperatures (for example, at -20 0C). Due to the presence of the additional supporting ionic conducting agent specifically the magnesium chloride, calcium chloride and/or lithium chloride in the electrolyte, the colligative property of the electrolyte components lowers the freezing point as well as elevates the boiling point of the electrolyte. Further, the additional supporting ionic conducting agent increases the ionic conductivity of the electrolyte and prevents crystallization of the tetraethylammonium bromide at low operating temperatures of the Zinc-Bromine redox static battery. The combination of additional supporting ionic conducting agents and synergistic effects of corresponding colligative properties prevents the freezing of the electrolyte and also prevents the crystallization of tetraethylammonium bromide up to a temperature of -200C without significantly affecting the viscosity and ionic conductivity of the electrolyte.
To overcome at least the technical problems discussed in the above examples and increase the performance of the Zinc-Bromine redox static battery, in one aspect of the present invention, an improved electrolyte (i.e., an improved aqueous electrolyte composition) is provided for high-performance static Zinc-Bromine batteries.
Accordingly, in an aspect of the present invention, an electrolyte is provided for use in a static zinc-based battery. The electrolyte comprises: a) zinc bromide in molar concentration ranging from 1.5M to 3.0M; b) tetraethylammonium bromide (TEAB) as a bromine complexing agent in about half of the molar concentration of the zinc bromide; c) a glycol-based anti-freezing agent in molar concentration ranging from 1.0M to 2.5M; d) zinc chloride as supporting ionic conducting agent in molar concentration ranging from 1.0M to 3.0M; and e) an additional supporting ionic conducting agent in molar concentration from about 0.5M to 3.0M. . Moreover, the additional supporting ionic conducting agent is selected from magnesium chloride or calcium chloride or lithium chloride or a combination thereof.
The zinc bromide, which is an active ingredient, takes part in the redox reaction. The higher the zinc bromide concentration, the higher the energy density achieved. The quaternary ammonium salt that is used as the bromine complexing agent is specifically the tetraethylammonium bromide (TEAB) (i.e., C8H20NBr). Surprisingly, during experimentation, when the Tetraethylammonium bromide (TEAB) was used as the complexing agent in about half of the molar concentration of the zinc bromide when zinc bromide in molar concentration from about 1.5M to 3.0M, a significant improvement in reduction in self-discharge was achieved.
As self-discharge is one of the major concerns for Zinc-Bromine chemistry, effective utilization of a complexing agent plays a crucial role in improving performance. By use of the TEAB as the suitable complexing agent in a controlled manner, the bromine cross-over is locked, and the self-discharge is reduced significantly. Higher solubility of zinc bromide (ZnBr2) can enhance the capacity of the battery, but due to the lower solubility of complexing agents in electrolytes, it generally suffers from high self-discharge. It was observed that the important factor for achieving greater efficiency of zinc bromide electrolytes is the amount of complexing agent used for a practical application. The disclosed electrolyte having a specific bromine complexing agent in its optimized composition, as disclosed in the above aspect of the present invention, is capable to arrest the self-discharge (almost) completely.
The present invention provides a specific ratio and specific selection of a quaternary ammonium salt that allow the dissolution of 1.25M (or even higher than that) of quaternary ammonium salt, particularly, the tetraethyl ammonium bromide in the presence of ZnBr2 (2.5 M) even at room temperature. During experimentation, a surprising effect was found that works specifically at a specific and optimized molar ratio between the zinc bromide and the complexing agent of tetraethyl ammonium bromide, which is enough to capture almost the entire Br3- ion and prevent the self-discharge of the Zinc-Bromine static battery. The optimized molar ratio between the zinc bromide and the complexing agent (specifically tetraethylammonium bromide (TEAB)) to arrest the self-discharge is experimentally found to be 2:1. The optimized molar ratio of 2:1 between the zinc bromide and the complexing agent (specifically, TEAB) and selection of salts in specific concentration range allows the dissolution of >1.25M of TEAB (or even higher than that) in the presence of ZnBr2 (2.5 M) even at room temperature. The higher dissolution of the complexing agent (e.g., the TEAB) not only minimizes the self-discharge but also allows to use of more active ingredient (i.e., zinc bromide), which directly enhances the energy density of the Zinc-Bromine static battery.
Generally, the aqueous electrolyte solution, which circulates through the cathodic side during the cell charge contains the complexing agent, which is capable of forming a water-immiscible liquid phase upon complexing with Br2/Br3- species, mainly tribromide ions. Thus, the elemental bromine generated at the cathodic side during cell charge reacts almost instantaneously with the water-soluble complexing agent, to form a water-immiscible oily phase, thereby effectively avoiding the diffusion of the elemental bromine, the cross-contamination of the positive and negative electrodes, and the like. In the present invention, there is provided the electrolyte (i.e., an aqueous electrolyte composition) for high-performance static Zinc-Bromine batteries, where effective utilization of a complexing agent in a controlled amount arrests the Br3- ions in a solid form resulting in almost wholly diminished intrinsic self-discharge. Beneficially as compared to conventional approaches, the electrolyte of the present disclosure manifests synergetic effects that improves battery stability and works without the use of expensive ion exchange membranes.
As explained above, the bromine-complexing agent is added to the electrolyte of a zinc-bromine battery to minimize the vapour pressure of elemental bromine. Properties considered important for screening the bromine-complexing agents include stability against crystallization down to low ambient temperatures, high conductivity of the electrolyte, low to negligible viscosity of the complex-containing solid phase, and the ability to maintain a minimal, yet effective, amount of 'free bromine' in the aqueous phase. Herein, the bromine complexing agent, TEAB, combines with tribromide ions on the positive electrode side during charging to form a bromine complex compound (in a solid form) and plays an important role of retaining electric energy.
The aforementioned electrolyte composition is suitable for versatile weather conditions. Further, the electrolyte with the aforementioned composition remains stable and effective at a temperature range varying from -20? to 50?. (without freezing up to -20? to 50?. Furthermore, from experimental observations, it is found that the aforementioned composition of the electrolyte improves energy efficiency of the static zinc-based battery around 85%) can be achieved using this electrolyte, whereas, for zinc bromide batteries with the conventional composition of electrolyte, practically energy efficiency of 65-70% is generally achieved. The aforementioned electrolyte composition possesses high ionic conductivity and low viscosity, which makes it suitable for practical application. The experimental investigation of the operation of the static zinc-based battery is performed at Ladakh (a place in India having a temperature at sub-zero level), and it is observed that the electrolyte with the aforementioned composition maintains the corresponding liquid state even at such a low-temperature level without freezing or precipitation.
In accordance with an embodiment, the electrolyte comprises the tetraethylammonium bromide in molar concentration ranging from 0.75M to 1.5M, the use of the TEAB as the bromine complexing agent prevents dendrite formation due to its interaction chemistry with the free bromide ions. Further, the TEAB arrest the tribromide ions in solid form due to which the electrolyte possesses high ionic conductivity and low viscosity, which makes it suitable for practical application.
It is known that the dendritic zinc growth problem can somehow be suppressed by adding a small number of inorganic/organic additives, but the performance of Zinc Bromide based batteries while using the conventional electrolytes was still not satisfactory (e.g., low performance). Further, in conventional approaches, the addition of an external additive significantly reduces the ionic conductivity and naturally affects the reaction kinetics and as a consequence, overall cell performance gets affected. Beneficially, as compared to conventional electrolytes, the electrolyte of the present invention does not require the addition of external additives to control Zinc dendrite formation. The TEAB when used as the complexing agent, alters the electrodeposition of the zinc from a dendritic growth to a non-dendritic development as the polar TEA+ ion (tetraethyl ammonium ion) prefers to be adsorbed on the zinc nucleus, blocking the strong electric field and regulating the ion distribution on the interface. Hence, the present invention provides the electrolyte that significantly improves the performance of the static zinc-based battery where the composition of the electrolyte is free from anti-dendrite agents.
Some additional advantages of the electrolyte are: a) cost-effective, energy-efficient, and scalable process in electrolyte production; b) eco-friendly and non-toxic electrolyte; c) non-flammable electrolyte; d) suitable for zinc bromide energy storage applicable for a range varying from electric vehicles to grid storage; and e) the electrolyte is compatible with commercially available Absorbed Glass Mat (AGM) separator, polyethylene (PE), or polypropylene (PP) separators.
In accordance with an embodiment, the glycol-based anti-freezing agent is selected from the group comprising of monoethylene glycol, propylene glycol, 1,3-butylene glycol, polyethylene glycol 200, polyethylene glycol 400, or a combination thereof. The use of glycol-based anti-freezing agents in the electrolyte can prevent the electrolyte from freezing in low temperatures, thereby maintaining the performance and longevity of the static zinc-based battery.
In accordance with an embodiment, the glycol-based anti-freezing agent is monoethylene glycol. In accordance with another embodiment, the electrolyte includes the glycol-based anti-freezing agent in the molar concentration of 2M. It was observed during experimentation that monoethylene glycol in the molar concentration of 2.0M specially was more effective and also contributed to the thermal stability of the electrolyte suitable for versatile weather conditions. The electrolyte remained stable and effective at a temperature range varying from -15? to >50?.
In accordance with an embodiment, the electrolyte includes the zinc chloride as a supporting ionic conducting agent in the molar concentration of 1.5M. The additional supporting ionic conducting agent is selected from magnesium chloride or calcium chloride or lithium chloride or a combination thereof. Specifically, in a preferred embodiment, two supporting ionic conducting agents are used, which includes the zinc chloride in the molar concentration ranging from about 1.5M to 3.0M, along with magnesium chloride in the molar concentration ranging from about 0.5M to 3.0M. Alternatively, in another embodiment, calcium chloride in the molar concentration ranging from about 0.5M to 3.0M is used instead of the magnesium chloride along with the zinc chloride in the molar concentration ranging from about 1.5M to 3.0M. Alternatively, lithium chloride in the molar concentration ranging from about 0.5M to 3.0M is used instead of the magnesium chloride along with the zinc chloride in the molar concentration ranging from about 1.5M to 3.0M.
In accordance with an embodiment, the electrolyte includes the additional supporting ionic conducting agent in the molar concentration of 1M. From experimental data (shown in FIGs. 2 and 3), it is observed that 1M concentration of the additional supporting ionic conducting agent such as magnesium chloride or calcium chloride, the electrolyte shows excellent reversibility at low temperature and retain the ionic conductivity of the electrolyte with increasing temperature.
Magnesium chloride or Calcium chloride or Lithium Chloride or the combination of all the salts can be used in an optimized ratio for other variants of electrolyte. Different colligative properties of the above-mentioned chloride salts help to design electrolytes as per requirement like low temperature (-20°C) by tuning the composition.
Although the zinc bromide acts as a conducting agent, poor ionic conductivity is still an issue for the electrolyte. In the present invention, it is observed that using supporting electrolytes like magnesium chloride or calcium chloride or lithium chloride enhances the ionic conductivity but does not affect other properties significantly like pH. In known commercial batteries, to enhance the ionic conductivity, highly acidic solutions (in some cases mild acid) are used as one of the major components in zinc-bromine-based electrolytes, which reduces the pH of the electrolyte to a significant value. At lower pH, the zinc-bromine battery suffers from hydrogen evolution and further reduces battery efficiency. Further gas evolution during cycling is a safety concern on a commercial scale. Hence, in yet another embodiment, the present invention provides the electrolyte in which no such acid(s) is used, and the pH of the electrolyte is maintained at a particular value that is in the range of 3.5 to 5.0, preferably the pH of about 4, such that no such gas evolution occurs during cycling.
In accordance with an embodiment, the electrolyte is free of pH maintaining agents and buffering agents. Typically, according to previously reported articles, researchers preferred to use buffering agent and/or pH maintaining agent to maintain the pH of the electrolyte throughout the cycling for an extended period to avoid side reactions at relatively lower or higher pH. However, it is observed that the use of such buffering agent reduces the ionic conductivity of the electrolyte. Hence, in a preferred embodiment, the present invention provides the electrolyte that is free from the buffering agent and the pH maintaining agents, and the pH of the electrolyte during cycling is maintained at about 4, even without using such buffering agent and pH maintenance agents. In other words, in an implementation, the electrolyte is maintained at a pH of about 4 in absence of pH maintaining agents and buffering agents.
In an implementation, the electrolyte is prepared in the absence of external heat energy. As no external heat energy is needed to prepare the electrolyte, a scalable and cost-effective process of production is achieved. The production of the electrolyte in bulk quantity thus is cost-effective, energy-efficient, and scalable. In the in absence of external heat energy, thermal decomposition of the electrolyte components are avoided and the risk of thermal runaway reactions are reduced, which further results in a stable and homogeneous electrolyte and can help to ensure the performance and safety of the static zinc-based battery.
In another aspect of the present invention, an electrolyte for static zinc-based battery is provided, where the electrolyte comprises: a) zinc bromide in molar concentration from about 1.5M to 3.0M, b) tetraethylammonium bromide as a bromine complexing agent in about half of the molar concentration of the zinc bromide to arrest tribromide ions in a solid form, c) monoethylene glycol as an anti-freezing agent in molar concentration from about 1.0M to 2.5M, d) zinc chloride as supporting ionic conducting agent in molar concentration from about 1.0M to 3.0M, and e) one or more additional supporting ionic conducting agents selected from magnesium chloride or lithium chloride or calcium chloride or a combination thereof, wherein each of the magnesium chloride or lithium chloride or calcium chloride or their combination is in molar concentration from about 0.5M to 3.0M, where the electrolyte is free of pH maintaining agents and buffering agents. High energy efficiency (around 85%) can be achieved using this electrolyte, whereas, for zinc-bromide chemistry, practical energy efficiency of 65-70% is generally achieved with other conventional electrolytes. No such buffering agent is required to maintain the pH during the cycling for the electrolyte of the present invention. In the context of batteries, for example, cycling refers to the process of repeatedly charging and discharging a battery, for example, to test its performance, capacity, and durability. This composition of electrolyte further prevents dendrite formation, due to the interaction chemistry of the complexing agent, as the tetraethylammonium bromide in about half of the molar concentration of the zinc bromide arrests tribromide ions in the solid form. The tetraethylammonium bromide in about half of the molar concentration of the zinc bromide completely locks the bromine cross-over and reduces the self-discharge significantly. In other words, the addition of tetraethylammonium bromide (TEAB) in a concentration equal to half the molar concentration of zinc bromide effectively prevents the movement or "cross-over" of free bromine molecule and tribromide ions and reduces the self-discharge of the electrolyte solution. The self-discharge refers to a decrease in the loss of stored energy in the solution over time. By controlling the bromine cross-over and reducing self-discharge, the overall performance and efficiency of the electrolyte and, thereby the performance of the Zinc-bromine redox battery is improved.
In the above embodiment, the electrolyte is prepared in the absence of external heat energy, and where the zinc bromide and the tetraethylammonium bromide is present in the ratio of 2:1. Further, the monoethylene glycol is present in molar concentration of about 1.35M. Furthermore, the zinc chloride is present in the molar concentration of about 1.7M, and the additional supporting ionic conducting agent is present in the molar concentration of about 3.0M. Additionally, the electrolyte is maintained at a pH value of about 4.7 in the absence of pH maintaining agents and buffering agents.
In an implementation, the electrolyte comprises 2.0 molar concentration of zinc bromide and 1.0 M concentration of tetraethylammonium bromide along with other electrolyte ingredients. In certain other embodiment, the electrolyte comprises 2.5 molar concentration of zinc bromide and 1.25 molar concentration of tetraethylammonium bromide along with other electrolyte ingredients.
Many electrolyte compositions with various permutations and the combination of zinc bromide to tetraethylammonium bromide were tested, for example, a) 2M zinc bromide and 0.4M TEAB; b) 1.5M zinc bromide and 0.5M TEAB; c) 1.5M zinc bromide and 1M TEAB; d) 2.25M zinc bromide, 0.5M TEAB, and other electrolytic ingredients, like potassium chloride, zinc chloride in various concentration, but precipitation of bromozincate ions from solution was observed. Typically, in the presence of zinc bromide, dissolution of TEAB or any other complexing agent, at ambient temperature, at higher concentration (>0.3 M) was challenging and not yet reported as it precipitates out in the form of bromozincate. The precipitation of bromozincate ions from solution occurs when the solution becomes supersaturated with the ions, and they form an insoluble solid that separates from the electrolyte solution.
In an implementation, the precipitation of bromozincate ions from the solution can be detected using various known methods, which includes the following. a) Visual observation: The appearance of a solid precipitate can be observed with the naked eye; b) pH measurement: The pH of the solution can be measured using a pH meter or indicator paper. A change in pH, particularly an increase in alkalinity, can indicate the precipitation of bromozincate ions; c) Conductivity measurement: The conductivity of the solution can be measured using a conductivity meter. A decrease in conductivity can indicate the formation of an insulating precipitate; d) Spectrophotometry: The concentration of bromo-zincate ions in the solution can be determined using spectrophotometry, by measuring the absorbance of light at a specific wavelength; and e) X-ray diffraction: The crystal structure of the precipitate can be analyzed using X-ray diffraction, which can confirm the presence of bromo-zincate ions in the precipitate.
Certain other quaternary ammonium salts were tested with different ratios, such as Tetramethylammonium bromide, Trimethylphenylammonium bromide, Tetrapropylammonium bromide and Tetrabutylammonium bromide for their efficiency and performance and it was observed that none of them helps in reducing precipitation of bromozincate ions from solution. Although higher solubility of ZnBr2 enhances the capacity of the battery, but due to the lower solubility of complexing agents in electrolytes, the battery generally suffers from high self-discharge.
In a preferred embodiment, when the electrolyte composition comprises 1.5M to 3M concentration of zinc bromide, 0.75.M to 1.5M concentration of TEAB, 2M concentration of monoethylene glycol, 1.5M concentration of zinc chloride and 1.0M concentration of magnesium chloride, said TEAB in a surprising effect, arrests the free tribromide ion from aqueous solution of electrolyte in solid form, forming clear electrolyte solution even at a low temperature -15oC.
The present invention moreover provides high energy efficiency of greater than equal to 85%, which can be achieved by using the electrolyte as described above, whereas, for zinc bromide chemistry, practically energy efficiency of 65-70% is generally achieved with other prior art-reported electrolytes. Further, as the TEAB arrest the tribromide ions in solid form, the electrolyte composition possesses high ionic conductivity and low viscosity, which makes it suitable for practical application.
In an embodiment, the solvent of the electrolyte is water, and no specific acid or alkaline solution is used. In another embodiment, the structure of Zinc-Bromine battery containing said aqueous electrolyte of the present invention can be a button battery, a cylindrical battery, a soft pack battery, a battery pack of any shape and size, or a square shell battery.
In an embodiment, the aqueous electrolyte of the Zinc-Bromine battery of the present invention is stable at -15oC to 50oC, which do not require any external energy during its production. In other words, production of bulk quantity electrolytes requires only commercially available magnetic stirrers and some typical lab utensils with no external heat source.
In another embodiment, the Zinc-Bromine battery prepared as per the present invention is suitable for zinc bromide energy storage applicable for a range varying from electric three-wheeler to grid storage, wherein the electrolyte is compatible with commercially available AGM separator, PE and PP separator.
In another embodiment, the present invention provides Zinc-Bromine battery composed of carbon-based bipolar electrodes and an aqueous electrolyte as described above. The electrodes act as the first and second current collectors and are configured to facilitate the conversion of halide ions to a polyhalide phase at or near the carbon material of the electrochemical cell i.e., battery, and to form a layer of zinc metal on the second surface of the second current collector upon charging of the electrochemical cell.
In one another embodiment, the present invention provides Zinc-Bromine battery composed of carbon-based electrodes and electrolyte as described above, wherein said electrolyte provides steady coulombic efficiency of greater than equal to 95% throughout the cycles indicating efficient participation of the redox couples within the applied voltage range. The above said electrolyte further possesses long cycling stability of up to 250 cycles at a higher current rate.
The present invention will now be described in detail with reference to examples.
EXAMPLES
General Method of Preparation:
The electrolyte was prepared by uniform mixing of ethylene glycol, zinc chloride, one or more combinations of magnesium chloride, lithium chloride or calcium chloride, and water to form a solvent to which was added the required amount of zinc bromide and TEAB (bromine complexing agent) to obtain an electrolyte for the zinc-bromine static battery. The electrolyte was prepared by using mechanical stirring alone without the use of external heat energy.
Example 1: The electrolyte was prepared by uniform mixing of 1.5 M of ethylene glycol, 1.4 M of zinc chloride, 1.5 M of magnesium chloride, and water to form a solvent to which was added the 1.6 M of zinc bromide and 0.8M of TEAB (bromine complexing agent) under stirring at ambient temperature.
Example 2: The electrolyte was prepared by uniform mixing of 1.0 M of ethylene glycol, 1.4 M of zinc chloride, 1.0 M of magnesium chloride, and water to form a solvent to which was added 1.6 M of zinc bromide and 0.8 M of TEAB (bromine complexing agent) under stirring at ambient operating temperature of 5 0C.
Example 3: The electrolyte was prepared by uniform mixing of 2.0 M of ethylene glycol, 1.4 M of zinc chloride, 1.0 M of calcium chloride, 2.0 M of potassium chloride, and water to form a solvent to which was added the 1.6 M of zinc bromide and 0.8 M of TEAB (bromine complexing agent) under stirring with a temperature range from 5 0C to 25 0C.
Example 4: The electrolyte was prepared by uniform mixing of 2.5 M of ethylene glycol, 1.4 M of zinc chloride, 1.0 M of calcium chloride, 2.0 M of potassium chloride, and water to form a solvent to which was added the 3 M of zinc bromide and 1.5 M of TEAB (bromine complexing agent) under stirring with a temperature range from 5 0C to 25 0C.
RESULTS
Now, referring to FIG. 1, there is shown a graphical representation of performance of a static zinc-based battery having an electrolyte with a specific composition, in accordance with an embodiment of the present disclosure. With reference to FIG. 1, there is shown a graphical representation of a static zinc-based battery 104 having an electrolyte 102. The electrolyte 102 is made up of composition as mentioned in the example 1. The graphical representation illustrates a first curve 106 indicative of the variation of the charging/discharging efficiency or coulombic efficiency (Y axis) of the static zinc-based battery 104 and a second curve 108 indicative of the variation of the energy efficiency (Y axis) of the static zinc-based battery 104 with respect to the number of operating cycles (Cycle) (X-axis). From the graphical representation, it is observed that due to the composition of the electrolyte 102 as mentioned in the example 1, the static zinc-based battery 104 shows charging/discharging efficiency or coulombic efficiency above (or minimum) 90% for up to 150 number of operating cycles and the energy efficiency above (or minimum) 80% for up to 150 number of operating cycles. The electrolyte 102 is used for the static zinc-based battery 104 that is applicable for a range varying from electric vehicles, home appliances, to grid storage. The electrolyte 102 is compatible with commercially available Absorbed Glass Mat (AGM) separator, polyethylene (PE), or polypropylene (PP) separator and other applications apart from energy storage. The electrolyte 102 was used for a static zinc-based battery 104. The electrolyte 102 is used for the static zinc-based battery 104 that is applicable for a range varying from electric vehicles, home appliances, to grid storage. The electrolyte 102 is compatible with commercially available Absorbed Glass Mat (AGM) separator, polyethylene (PE), or polypropylene (PP) separator and other applications apart from energy storage. In this embodiment, the electrolyte manifested steady coulombic efficiency of greater than equal to 95% as described in FIG. 1, throughout the cycles indicating efficient participation of the redox couples within the applied voltage range. The electrolyte further possesses long cycling stability of up to 150 cycles at a higher current rate.
Referring to FIG. 2, there is shown a cyclic voltammogram of an electrolyte in a static zinc-based battery at operating temperature of 5 0C, in accordance with another embodiment of the present disclosure. With reference to FIG. 2, there is shown a cyclic voltammogram of a static zinc-based battery 204 having an electrolyte 202. The electrolyte 202 is made up of composition as mentioned in the example 2. The cyclic voltammogram illustrates a first curve 106 indicative of the variation of the current (in mA) (shown by the Y axis) of the static zinc-based battery 204 with respect to the potential difference (in V) (shown by X-axis). Herein, the electrolyte shows excellent reversibility at a low temperature of 5 0C. In this case, the composition of the electrolyte was 1.6 M concentration of zinc bromide, 0.8 M concentration of TEAB, 1.4 M concentration of zinc chloride, 1.0 M concentration of magnesium chloride and 1.0 M concentration of monoethylene glycol.
Referring to FIG. 3, there is shown a cyclic voltammogram of an electrolyte in a static zinc-based battery within a range of operating temperatures 5 0C to 25 0C. With reference to FIG. 3, there is shown a cyclic voltammogram of a static zinc-based battery 304 having an electrolyte 302. The cyclic voltammogram illustrates a first curve 306 indicative of the variation of the current (in mA) (shown by Y axis) of the static zinc-based battery 304 with respect to the potential difference (in V) (shown by X axis) at operating temperature of 25 0C. Further, the cyclic voltammogram illustrates a second curve 308 indicative of the variation of the current (in mA) ( shown by Y axis) of the static zinc-based battery 304 with respect to the potential difference (in V) (shown by X axis) at operating temperature of 15 0C. In addition, the cyclic voltammogram illustrates a third curve 310 indicative of the variation of the current (in mA) (shown by Y axis) of the static zinc-based battery 304 with respect to the potential difference (in V) (shown by X axis) at operating temperature of 5 0C. The electrolyte 302 is made up of composition as mentioned in the example 3. From the cyclic voltammogram, it is observed that the composition of the electrolyte 302 provides retention of ionic conductivity of the electrolyte 302 with increasing temperature (from 5 0C to 25 0C) and reversibility at a wide range of useable temperature. For same potential, the static zinc-based battery 304 provides maximum current around 28 mA at 25 0C, maximum current around 23 mA at 15 0C and maximum current around 21 mA at 5 0C. In other words, maximum current for same potential decreases very nominally with decrease in the operating temperature of the static zinc-based battery 304 with no significant performance loss due to temperature variations in even cold regions, like Ladakh region.
For the following molar concentration provided in "Table 1" below and specific ratio between the zinc bromide (ZB) and Tetraethylammonium bromide (TEAB) in the electrolyte, high energy efficiency of greater than equal to 85% was achieved, as shown in FIG. 1. It is to be understood that, for zinc bromide chemistry, practically energy efficiency of 65-70% is generally achieved currently with conventional electrolyte composition. Further, as shown in FIG. 1, a steady coulombic efficiency (> 95 %) throughout the cycles indicates efficient participation of the redox couples within the applied voltage range when the electrolyte of the present invention is used, in the specific molar concentration of its ingredients and ratio of 2: 1 maintained between the ZB and the TEAB respectively. Furthermore, higher energy density (average ~ 85%) of the static zinc-bromine battery (composed of carbon-based electrodes and the electrolyte of table 1) and long cycling stability (reported up to 150 cycles) at a higher current rate, was observed.
TABLE 1: Aqueous Electrolyte composition of present invention
Electrolyte component Concentration (M) Purpose
Zinc bromide (ZnBr2) 1.5M to 3M The active ingredient (takes part in the redox reaction). The higher the ZnBr2 concentration, the higher the energy density.
Tetraethylammonium bromide (C8H20NBr) Approximately ½ of ZnBr2 molar concentration a bromine complexing agent used to minimize self-discharge (the main issue of static Zinc-bromine batteries)

Monoethylene Glycol (C2H6O2) 1M to 2.5M, preferably 2M anti-freezing agent

Zinc Chloride (ZnCl2) 1M to 3M, preferably 1.5M supportive electrolyte to enhance the ionic conductivity
Magnesium Chloride (MgCl2) 0.5M to 3M, preferably 1M supportive electrolyte to enhance the ionic conductivity as well as depress the freezing point of the electrolyte
Calcium Chloride (CaCl2) 0.5M to 3M, preferably 1M supportive electrolyte to enhance the ionic conductivity as well as depress the freezing point of the electrolyte

Lithium Chloride (LiCl) 0.5M to 3M, preferably 1M supportive electrolyte to enhance the ionic conductivity as well as depress the freezing point of the electrolyte

TABLE 2:
EG ZC ZB Supporting electrolyte II Physical appearance at 30 0C Physical appearance at -20 0C
0 M 1.5 M 1.5 M KCl (3 M) Transparent liquid Solidified
0 M 1.5 M 1.5 M NaCl (3 M) Transparent liquid Solidified
0 M 1.5 M 1.5 M MgCl2 (1 M) Transparent liquid Transparent liquid
0 M 1.5 M 1.5 M CaCl2 (1 M) Transparent liquid Transparent liquid
0 M 1.5 M 1.5 M LiCl (1 M) Transparent liquid Transparent liquid
1 M 1.5 M 1.5 M MgCl2 (1 M) Transparent liquid Transparent liquid
1M 1.5 M 1.5 M MgCl2 (0.4M) Transparent liquid Liquid form only until the complexing agent is not added
1M 1.5M 1.5M MgCl2 (3.2 M) Transparent liquid Liquid form only until the complexing agent is not added

The physical appearance of the electrolyte with multiple compositions at operating temperatures of 30 0C and -20 0C is shown in the table 2. Here, ZB indicates zinc Bromide, ZC indicates zinc chloride, EG indicates monoethylene glycol. From the above experimental data, it is observed that the use of MgCl2, CaCl2, LiCl as a supporting electrolyte in the concentration range 0.5 to 3M can maintain liquid state the electrolyte even at sub-zero temperature, such as –20 0C. In the absence of the bromine complexing agent, the addition of MgCl2 above or below the concentration range of 0.5 to 3M can maintain the liquid nature of the electrolyte. However, in the presence of the bromine complexing agent, the electrolyte gets solidified in case the concentration of MgCl2 falls out of the range 0.5 to 3M. Therefore, it is observed that the concentration range of MgCl2 between 0.5 to 3M is an optimum range to maintain the liquid state at temperatures 30 0C and –20 0C.
TABLE 3:
EG ZC ZB BCA Supporting electrolyte II Physical appearance at 30 0C
1 M 1.5 M 1.5 M 0.75 M TMAB MgCl2 (1 M) Transparent liquid
1 0 1.5 0.75 M TEAB MgCl2 (1 M) Precipitation
0 0 1.5 0.75 M TEAB MgCl2 (1 M)
Precipitation
1 M 1.5 M 1.5 M 0.75 M TEAB MgCl2 (1 M) Transparent liquid
1 M 1.5 M 1.5 M 0.75 M TPAB MgCl2 (1 M) Precipitation
1 M 1.5 M 1.5 M 0.75 M TBAB MgCl2 (1 M) Precipitation
1 M 1.5 M 1.5 M 0.75M TEAB CaCl2 (1 M) Transparent liquid
1 M 1.5 M 1.5 M 0.75M TEAB LiCl (1 M) Transparent liquid
1 M 1 M 1.5 M 0.75M TEAB MgCl2 (1.5 M) Transparent liquid
2.5M 2.5M 3.0M 1.5M
TEAB MgCl2 (3.0M) Transparent liquid
1 M 1.5M 1.5M 0.75M
TEAB MgCl2 (0.5M) Transparent liquid
1.5M 2 M 2 M 1 M
TEAB MgCl2 (1M) Transparent liquid
0.5M 0.5M 1.5M 0.75M
TEAB MgCl2 (0.25M) Precipitation
2M 1.5M 1.5M 0.75M
TEAB MgCl2 (1M) Transparent liquid
3M 3.5M 3.5M 1.75M
TEAB MgCl2 (3.5M) Precipitation
1M 1M 1M 0.5M MgCl2 (1M) Transparent liquid
1M 0M 1M 0.5M MgCl2 (1M) Precipitation

Here EG is ethylene glycol, ZC is zinc chloride, ZB is zinc bromide and BCA is a bromine complexing agent. The experimental data related to the physical appearance or state of the electrolyte at an operating temperature of 30 0C is shown in the table 3. The physical appearance of the electrolyte for different derivatives of tetra alkyl ammonium salts or bromides is tested. The tetra alkyl ammonium salts are able to capture tribromide generated during the charging of the static zinc-based battery in solid form, which is beneficial to reduce self-discharge of the static zinc-based battery. The concentration of ZB at 1M (or below 1M) maintains the transparent liquid state of the electrolyte (in the presence of ZC). However, the use of ZB below 1.5M as a constituent for the electrolyte concentration of ZB is not effective for a static battery and may result in precipitation (in absence of ZC).
TABLE 4:
Electrolyte composition (molar ratio) Physical appearance at different temperature

30 0 C 5 0 C -5 0 C
ZC: ZB: TEAB: MC = 1.5:1.5:0.75:1 Transparent liquid
Transparent liquid Transparent liquid
ZC: ZB: TEAB: SC = 1.5:1.5:0.75:1.5
Transparent liquid
Precipitation
Solidified

ZC: ZB: TEAB: PC = 1.5:1.5:0.75:2 Transparent liquid
Precipitation Solidified

Here ZC is zinc chloride, ZB is zinc bromide and TEAB is tetraethylammonium bromide, MC is magnesium chloride, PC is potassium chloride and SC is sodium chloride.
The experimental data related to physical appearance or state of the electrolyte at operating temperatures of 30 0C to -5 0C is shown in the table 4 for multiple combinations of composition. From the table 4, it is observed that the magnesium chloride in 1M concentration maintains the physical appearance of the electrolyte even at – 5 0C (which is not possible in case of commonly used salt like potassium chloride or sodium chloride). Therefore, the combination ZC: ZB: TEAB: MC = 1.5:1.5:0.75:1 retains the physical state of the electrolyte even at low operating temperatures and the above-mentioned combination also maintains the high solubility of the TEAB in the electrolyte with the decrease in temperature and avoids precipitation of the TEAB.
Furthermore, with respect to example 4, it was observed that the results were similar to that of FIG. 3, and that the electrolyte 302 in the static zinc-based battery 304 was functioning effectively within a range of operating temperatures 50C to 250C. The electrolyte in this case was made up of composition as mentioned in the example 4. From the experimental results, a cyclic voltammogram similar to FIG. 3 was observed (now shown due to similarity for the sake of brevity). The composition of the electrolyte of example 4 provided retention of ionic conductivity of the electrolyte with increasing temperature (from 50C to 25 0C and beyond up to 35 0C) and reversibility at a wide range of useable temperature. For same potential, the static zinc-based battery 304 provides maximum current around 28 mA at 25 0C, maximum current around 23 mA at 15 0C and maximum current around 21 mA at 5 0C similar to FIG. 3. In other words, maximum current for same potential decreases nominally with decrease in the operating temperature of the static zinc-based battery 304, with no significant performance loss due to temperature variations and suited for even colder regions for operations having temperature fluctuations within -20 to 50 degree Celsius.
Exemplary embodiments of the invention have been disclosed. A person of ordinary skill in the art recognizes that modifications fall within the teachings of this application. Any numerical values recited in the above application include all values from the lower value to the upper value in increments of one unit provided that there is a separation of at least 2 units between any lower value and any higher value. Further, the term “X is half of the molar concentration of Y” shall be construed to include half or approximately half (i.e. 0.4, 0.5, or 0.6 of Y) of molar concentration of Y. All possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application. Unless otherwise stated, all ranges include both endpoints and all numbers between the endpoints. The use of "about' or "approximately” in connection with a range applies to both ends of the range. Thus, "about 20 to 30" is intended to cover "about 20 to about 30", inclusive of at least the specified endpoints.

,CLAIMS:CLAIMS
I/We claim:
1. An electrolyte of a static zinc-based battery, the electrolyte comprising:
zinc bromide in molar concentration ranging from 1.5M to 3.0M;
tetraethylammonium bromide as a bromine complexing agent in half of the molar concentration of the zinc bromide;
a glycol-based anti-freezing agent in molar concentration ranging from 1.0M to 2.5M;
zinc chloride as supporting ionic conducting agent in molar concentration ranging from 1M to 3.0M; and
an additional supporting ionic conducting agent in molar concentration ranging from 0.5M to 3.0M, wherein the additional supporting ionic conducting agent is selected from magnesium chloride or calcium chloride or lithium chloride or a combination thereof.
2. The electrolyte according to claim 1, wherein the glycol-based anti-freezing agent is selected from the group comprising of monoethylene glycol, propylene glycol, 1,3-butylene glycol, polyethylene glycol 200, polyethylene glycol 400, or a combination thereof.
3. The electrolyte according to claim 1 or 2, wherein the glycol-based anti-freezing agent is monoethylene glycol.
4. The electrolyte according to any one of the preceding claims, wherein the electrolyte is free of pH maintaining agents and buffering agents.
5. The electrolyte according to any one of the preceding claims, wherein the electrolyte comprises the glycol-based anti-freezing agent in the molar concentration of 2M.
6. The electrolyte according to any one of the preceding claims, wherein the electrolyte comprises the zinc chloride as supporting ionic conducting agent in the molar concentration of 1.5M.
7. The electrolyte according to any one of the preceding claims, wherein the electrolyte comprises the additional supporting ionic conducting agent in the molar concentration of 1M.
8. The electrolyte according to any one of the preceding claims, wherein the electrolyte comprises the tetraethylammonium bromide in molar concentration ranging from 0.75M to 1.5M.