Electrolyte for oxidation-reduction secondary battery comprising vanadium ions, and secondary battery comprising same

Abstract

The present invention relates to an electrolyte for an oxidation-reduction secondary battery comprising vanadium ions and a secondary battery comprising same and, more specifically, to an electrolyte comprising vanadium ions and a secondary battery comprising same, wherein the electrolyte has a controlled oxidation number of vanadium so as to improve a decrease phenomenon in charging energy (capacity) due to a concentration deviation phenomenon of the electrolyte comprising vanadium ions of a positive electrode and a negative electrode during charging and discharging of the secondary battery.

Description

Electrolyte for oxidation-reduction secondary battery containing vanadium ions and secondary battery containing the same

[0001] The present invention relates to an electrolyte for a redox secondary battery containing vanadium ions and a secondary battery containing the same, and more particularly, to an electrolyte containing vanadium ions in which the oxidation number of vanadium is controlled so as to improve the phenomenon of reduction in charging energy (capacity) due to the concentration difference phenomenon of the electrolyte containing vanadium ions of the positive and negative electrodes during charging and discharging of the secondary battery, and a secondary battery containing the same.

[0002] Redox Flow Battery (RFB), a type of oxidation-reduction secondary battery, is an electrochemical storage device that stores electrical energy as chemical energy of the electrolyte by charging and discharging it through oxidation and reduction of the active material in the electrolyte, unlike conventional secondary batteries.

[0003] Meanwhile, the electrolyte of a redox secondary battery uses a substance having a redox couple, and the electrolyte of the initial redox secondary battery used redox couples of different substances for the positive and negative electrodes. However, there may be a problem in that the capacity of the battery is reduced due to a phenomenon in which ionized substances cross over each other through the separator during charge and discharge. As an example, the crossover phenomenon may vary in the degree and speed of movement of vanadium ions depending on the type of separator, and this may affect the phenomenon of vanadium ion concentration deviation.

[0004] Accordingly, the use of vanadium in both the positive and negative electrodes was proposed as a method to alleviate the ion crossover phenomenon, and since the electrolyte containing vanadium ions is an aqueous electrolyte, it has been used until recently due to its advantages in that it not only has high safety against explosions when applied to secondary batteries, but also increases the lifespan of the batteries.

[0005] Vanadium ion electrolyte using vanadium ions as an active material has the longest lifespan when applied to redox secondary batteries and is advantageous in terms of large capacity. In addition, the electrolyte can be separated and reused through electrochemical reactions of charge and discharge, which is advantageous in terms of cost.

[0006] The electrolyte containing vanadium ions used in the positive and negative electrodes requires a vanadium compound having an oxidation state of 4+ for the positive electrode and 3+ for the negative electrode, respectively, based on a fully discharged state, and a vanadium compound having an oxidation state of 5+ for the positive electrode and 2+ for the negative electrode is required, based on a fully charged state. The charge and discharge reaction occurs as the oxidation state of the vanadium active material changes, and in order for an accurate electrochemical reversible reaction to occur, it is required to manufacture the positive and negative electrolytes by matching the oxidation states of the vanadium ions.

[0007] The anode and cathode are each anode oxidation-reduction couple (V 4+ / V 5+ ) and cathode oxidation-reduction couple (V 2+ / V 3+ ) is supplied with a dissolved electrolyte solution (electrolyte), which causes the following half-reactions.

[0008] Bipolar half-reaction: V 5+ + e- ←→ V 4+

[0009] Cathode half-reaction: V 2+ ←→ V 3+ + e-

[0010] Generally, as an electrolyte containing vanadium ions, an electrolyte with a vanadium oxidation number of 3.50 (3.5+) that can be commonly used for the electrolyte supplied to the cathode and the electrolyte supplied to the anode has been used.

[0011] However, when using an electrolyte with a vanadium oxidation number of 3.50 as in the past, it was discovered that as charging and discharging were repeated, the concentration of vanadium ions in the positive and negative electrodes varied, which became more severe and resulted in problems such as a decrease in charging energy (capacity).

[0012] Specifically, when charging a redox secondary battery using an electrolyte containing vanadium ions, a concentration difference phenomenon occurs in which the concentration of vanadium ions in the negative electrode decreases and the concentration of vanadium ions in the positive electrode increases. Accordingly, the charging energy decreases, the energy density also decreases, and as a result, the lifespan and efficiency of the redox secondary battery may decrease, resulting in a decline in performance.

[0013] The purpose of the present invention is to provide a vanadium ion electrolyte with an regulated oxidation number, which is an electrolyte for a redox secondary battery, and which improves the performance of the secondary battery by alleviating the problem of reduced performance of the secondary battery, particularly the charging energy (capacity).

[0014] Another object of the present invention is to provide a redox secondary battery capable of applying the vanadium ion electrolyte with the above-mentioned oxidation number control.

[0015] Another object of the present invention is to provide a method for producing an electrolyte containing vanadium ions having a controlled oxidation number.

[0016] The objects of the present invention are not limited to the purposes mentioned above, and other purposes not mentioned will be clearly understood by those skilled in the art from the entire description of this specification.

[0017] According to one aspect of the present invention, an electrolyte for an oxidation-reduction secondary battery can be provided, which includes vanadium ions and an acidic solution, and wherein the oxidation number of vanadium is greater than 3.50 and less than 4.00.

[0018] The oxidation number of the above vanadium may be greater than 3.50 and less than or equal to 3.80.

[0019] The oxidation number of the above vanadium may be 3.55 or more and 3.75 or less.

[0020] The above vanadium ions may be supplied by vanadium oxide.

[0021] The above vanadium oxide may include one or more of V2O5, VOSO4, V2O3, NH4VO3, and V2O4.

[0022] The above vanadium oxide may be in powder form.

[0023] The above acidic solution may contain one or more of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid.

[0024] According to another aspect of the present invention, a redox secondary battery can be provided, comprising: a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; a positive electrode electrolyte receiving portion formed between the positive electrode and the separator; and a negative electrode electrolyte receiving portion formed between the negative electrode and the separator; wherein an electrolyte for a redox secondary battery according to an aspect of the present invention is supplied to the positive electrode electrolyte receiving portion and the negative electrode electrolyte receiving portion.

[0025] The above separator may include an ion-conducting polymer.

[0026] The above ion-conducting polymer is a hydrocarbon-based polymer having an ion-conducting functional group introduced therein or a fluorine-based polymer having an ion-conducting functional group introduced therein, and the ion-conducting functional group may include at least one selected from a sulfonic acid group, a carboxyl group, and a phosphoric acid group.

[0027] The above ion conductive polymer may be a hydrocarbon polymer or a fluorine polymer having a sulfonic acid group introduced therein.

[0028] The hydrocarbon polymer or fluorine polymer having the sulfonic acid group introduced may include at least one of a perfluorosulfonic acid (PFSA) polymer, poly(styrene sulfonic acid) (PSSA), sulfonated poly(ether ether ketones) (SPEEK), sulfonated poly(ether sulfone) (SPES), sulfonated poly(aryl ether ketone) (SPAEK), sulfonated polybenzimidazole (SPBI), sulfonated poly(phenylen oxide) (SPPO), and sulfonated polyimide (SPI).

[0029] According to another aspect of the present invention, a method for producing an electrolyte for a redox secondary battery may be provided, comprising the steps of: preparing vanadium oxide having an oxidation number of 5.00; first reducing the vanadium oxide having an oxidation number of 5.00 into vanadium oxide having an oxidation number of 4.00 by a chemical reduction method; and second reducing the vanadium oxide having an oxidation number of 4.00 into vanadium oxide having an oxidation number of more than 3.50 and less than 4.00 by an electrochemical reduction method; and an electrolyte for a redox secondary battery produced thereby may be according to an aspect of the present invention.

[0030] The vanadium oxide having an oxidation number of 5.00 may include V2O5 powder.

[0031] The chemical reduction method of the first reducing step may be performed by adding vanadium oxide having an oxidation number of 5.00 to a solution containing an acidic solution and a reducing agent at a temperature of 50°C to 150°C.

[0032] The above acidic solution may contain one or more of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid.

[0033] The reducing agent may include one or more of oxalic acid, formic acid, formaldehyde, methanol, ethanol, and ammonium hydroxide.

[0034] The concentration of vanadium ions in the electrolyte for the above oxidation-reduction secondary battery may be 1.5 M to 3.0 M.

[0035] The electrolyte for a redox secondary battery of the present invention contains vanadium ions having an oxidation number of 3.5 or higher, and when applied to a secondary battery, can improve the phenomenon of reduction in charging energy (capacity) due to the phenomenon of deviation in the concentration of vanadium ions of the positive and negative electrodes that occurs during charging and discharging of an redox secondary battery.

[0036] When the electrolyte containing the vanadium ion of the present invention is applied, the energy density of a redox secondary battery can be significantly increased, and further, the efficiency and lifespan of the redox secondary battery can be improved. This is because a battery with a high energy density has a relatively low state of charge (SoC) even when the same amount of energy is charged, and thus the potential difference between the positive and negative electrodes becomes small, so the possibility of irreversible reactions such as hydrogen generation is reduced, and a battery with a high energy density uses a smaller depth of discharge (DoD) even when the same amount of energy is discharged, so the long-term lifespan of the battery is extended, and there is an advantage of being able to use a more efficient section.

[0037] Since the electrolyte containing vanadium ions of the present invention has a higher energy density than when using a conventional 3.50-valent vanadium electrolyte, there is an advantage in that the size of an aqueous secondary battery using the electrolyte can be further reduced.

[0038] The method for producing an electrolyte containing vanadium ions of the present invention has the advantage of being able to efficiently produce a vanadium ion electrolyte having a desired oxidation number and being suitable for mass production.

[0039] The effects of the present invention are not limited to the effects mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art from the description of the claims.

[0040] Figure 1 is a flowchart illustrating a method for manufacturing an electrolyte for an oxidation-reduction secondary battery according to one embodiment of the present invention.

[0041] Figure 2 shows a titration graph for confirming the oxidation number of an electrolyte containing vanadium ions according to one embodiment of the present invention.

[0042] The advantages and features of the present invention, and the methods for achieving them, will become clearer with reference to the embodiments described in detail below together with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below and may be implemented in various different forms. These embodiments are provided only to ensure that the disclosure of the present invention is complete and to fully inform those skilled in the art of the scope of the invention, and the present invention is defined only by the scope of the claims. Like reference numerals designate like elements throughout the specification.

[0043] In describing this specification, if it is determined that a detailed description of a related known technology may unnecessarily obscure the gist of this specification, the detailed description is omitted.

[0044] Although terms like "first" and "second" are used herein to describe various components, these components are not limited by these terms. These terms are used merely to distinguish one component from another, and unless otherwise specifically stated, a "first" component may also be a "second" component.

[0045] Throughout this specification, unless otherwise specifically stated, each component may be singular or plural.

[0046] In interpreting the components in this specification, even if there is no separate explicit description, it is interpreted to include the range of error.

[0047] In this specification, the phrase "any component is disposed "on (or below)" a component or "on (or below)" a component may mean not only that any component is disposed in contact with the upper surface (or lower surface) of said component, but also that another component may be interposed between said component and any component disposed on (or below) said component.

[0048] When it is described herein that any component is “connected,” “coupled,” or “connected” to another component, it should be understood that the components may be directly connected or connected to one another, but that other components may also be “interposed” between the components, or that each component may be “connected,” “coupled,” or “connected” through another component.

[0049] In this specification, when the terms "includes," "has," "consists of," "arranges," and "provides" are used for a component, other parts may be added, unless "only" is used. When a component is expressed in the singular, it includes the plural unless otherwise explicitly stated.

[0050] Throughout this specification, when we refer to "A and / or B", this means A, B, or A and B, unless otherwise stated, and when we refer to "C to D", this means C or more and D or less, unless otherwise stated.

[0051] Unless otherwise specified in this specification, any reference to a unit is interpreted to mean “weight.”

[0052] In this specification, unless otherwise specifically stated, “electrolyte containing vanadium ions” is interpreted to refer to the same thing as “vanadium ion electrolyte,” “electrolyte using (utilizing) vanadium ions,” “electrolyte containing vanadium oxide,” “vanadium electrolyte,” “electrolyte containing vanadium active material,” etc.

[0053] In this specification, the oxidation number of vanadium ions in the vanadium ion electrolyte may be the oxidation number of the electrolyte itself at the time of manufacture or supply, and may represent an average value of the oxidation numbers of the positive electrode electrolyte and the negative electrode electrolyte when the secondary battery is operated by charging and discharging.

[0054]

[0055] < Electrolyte for oxidation-reduction secondary batteries >

[0056] Hereinafter, an electrolyte containing vanadium ions as an electrolyte for an oxidation-reduction secondary battery according to one aspect of the present invention will be described in detail.

[0057] Conventionally, a vanadium electrolyte having an oxidation number of 3.50 is used in a redox secondary battery that uses an electrolyte containing vanadium ions. In particular, when the secondary battery is charged and discharged hundreds or thousands of times or more, the phenomenon of deviation in the concentration of vanadium ions in the vanadium electrolyte injected into the positive and negative electrodes becomes increasingly severe, and the charging energy (capacity) of the secondary battery decreases, resulting in a decline in performance. The inventors of the present invention have experimentally confirmed that when a vanadium electrolyte having an oxidation number exceeding 3.50 is used instead of the conventional vanadium electrolyte having an oxidation number of 3.50, the phenomenon of decreasing charging energy (capacity), which is a problem when repeating charging and discharging of a redox secondary battery multiple times, can be resolved despite the occurrence of deviation in the concentration of vanadium ions, and have devised the present invention.

[0058] Thus, the electrolyte containing vanadium ions of the present invention comprises vanadium ions and an acidic solution, and is characterized in that the oxidation number of the vanadium ions exceeds 3.50, and at this time, it can exhibit the effect of solving the problem of reduction in charging energy. In particular, the effect increases further as the number of charge / discharge cycles increases.

[0059] Specifically, the electrolyte in which the oxidation number of the vanadium ion exceeds 3.50 tends to have a somewhat lower charging energy after several charge / discharge cycles compared to the conventional electrolyte in which the oxidation number of the vanadium ion is 3.50. However, it was experimentally confirmed that the charging energy actually increases as the charging / discharging cycles are repeated tens or hundreds of times. Considering that redox secondary batteries are generally operated by charging / discharging hundreds or thousands of times or more, the experimental results of the present invention can be interpreted as very meaningful results. In particular, since the electrolyte including the vanadium ion of the present invention can have improved energy density, the size of a redox secondary battery using an aqueous electrolyte like the present invention can be significantly reduced, and the efficiency and lifespan of the redox secondary battery can be improved.

[0060] Meanwhile, when the oxidation number of the vanadium ion in the electrolyte becomes lower than 3.50, not only does the charging energy decrease after several charging and discharging cycles, but the charging energy also decreases further after tens or hundreds of charging and discharging cycles.

[0061] On the other hand, if the oxidation number of the vanadium ion increases too much, the effect of alleviating the decrease in the charging energy decrease will be lowered again. For example, the oxidation number of the vanadium ion may be less than 4.00, for example, the oxidation number of the vanadium ion may be 3.80 or less, for example, the oxidation number of the vanadium ion may be 3.75 or less, for example, the oxidation number of the vanadium ion may be 3.70 or less, for example, the oxidation number of the vanadium ion may be 3.65 or less, but this may vary depending on the path along which the vanadium ion moves. According to one embodiment of the present invention, the vanadium ion of the vanadium electrolyte of the present invention may be supplied by a vanadium oxide, and may include, for example, at least one selected from the group consisting of V2O5, VOSO4, V2O3, NH4VO3, and V2O4, but is not necessarily limited thereto, and may be selected according to electrolyte production conditions, production target amount, etc.

[0062] According to one embodiment of the present invention, the form in which the vanadium oxide is supplied is not particularly limited, but may be, for example, in powder form in terms of process efficiency.

[0063] According to one embodiment of the present invention, the acidic solution may include at least one selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid, but for example, a sulfuric acid solution may be used.

[0064] The concentration of the acidic solution may be, for example, 0.1 M to 10 M, for example, 1 M to 8 M, for example, 2 M to 5 M, but is not necessarily limited thereto, and may be appropriately adjusted depending on the type of acidic solution selected, etc.

[0065] The concentration of vanadium ions in the electrolyte containing the above vanadium may be 1.5 M to 3.0 M.

[0066]

[0067] <Oxidation-reduction secondary battery>

[0068] Below, a redox secondary battery according to another aspect of the present invention will be described in detail.

[0069] The oxidation-reduction secondary battery to which the electrolyte for the oxidation-reduction secondary battery of the present invention can be applied comprises: a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; a positive electrode electrolyte receiving portion formed between the positive electrode and the separator; and a negative electrode electrolyte receiving portion formed between the negative electrode and the separator; and any aqueous oxidation-reduction secondary battery in which an electrolyte containing vanadium ions can be used can be applied without limitation.

[0070] The electrolyte containing the above vanadium ion may be an electrolyte containing vanadium ion having an oxidation number exceeding 3.50 and less than 4.00 according to one aspect of the present invention.

[0071] The above-described positive and negative current collectors have low electrical resistance and can transmit current to the active material of the electrolyte during charging and discharging, or transmit current generated by an electrochemical reaction in the active material, and are used to manufacture thin film electrode plates. The materials for the positive and negative current collectors can be selected from those known in the art and are not particularly limited.

[0072] For example, the positive electrode current collector and the negative electrode current collector may be manufactured by laminating a metal current collector formed of an electrically connectable material such as aluminum, titanium, tantalum, nickel, stainless steel, etc., and a carbon current collector formed of a material such as graphite material, carbon fiber, carbon black, carbon nanotubes, activated carbon, graphene, carbon plastic, etc., which has high electrical conductivity and high acid resistance.

[0073] The separator, a key material in redox secondary batteries that use an aqueous electrolyte containing vanadium ions, functions as an ion exchange membrane to mediate reactions and separate the cathode and anode to prevent shunts. The separator requires high ion conductivity and selectivity. Furthermore, it must possess excellent mechanical properties to withstand the pressure of the electrolyte circulation, high chemical stability against strong acid environments, the oxidizing power of vanadium, and radicals generated within the aqueous electrolyte, as well as high process and cost efficiency. Furthermore, it must be able to effectively prevent the permeation of hydrogen gas. Such a separator can be formed of an ion-conductive polymer or manufactured by coating an ion-conductive polymer on a substrate.

[0074] According to one embodiment of the present invention, the ion conductive polymer may be a hydrocarbon-based polymer or a fluorine-based polymer.

[0075] The above hydrocarbon polymer is a polymer including a carbon and hydrogen skeleton, and the carbon and hydrogen skeleton may be aliphatic, alicyclic, aromatic, or a combination thereof, and may further include heteroatoms such as oxygen, sulfur, nitrogen, silicon, boron, and selenium. Non-limiting examples of the carbon and hydrogen skeleton include an alkyl group, an alkenyl group, an alkynyl group, a heteroalkyl group, an aryl group, a heteroaryl group, an aralkyl group, a heteroaralkyl group, an amide group, a sulfonyl group, an ether group, a ketone group, an ester group, and a combination thereof. The above hydrocarbon polymer may be substituted with a substituent such as a cyano group, a nitro group, an amine group, a hydroxyl group, an alkyl group, an alkoxy group, a ketone group, an ether group, an ester group, a silyl group, or the like.

[0076] For example, the hydrocarbon polymer may include a polyimide polymer, a polyetheretherketone polymer, a polyethersulfone polymer, a polybenzimidazole polymer, etc.

[0077] For example, fluorinated polymers may include polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), and ethylene-chlorotrifluoroethylene copolymer (ECTFE).

[0078] At this time, the ion-conducting polymer, such as a hydrocarbon polymer or a fluorine polymer, may include an ion-conducting functional group such as a sulfonic acid group, a carboxyl group, or a phosphoric acid group, and preferably may include a sulfonic acid group.

[0079] For example, an ion-conducting polymer containing a sulfonic acid group may be a polymer in which a sulfonic acid group is introduced into a specific type of hydrocarbon-based polymer or fluorine-based polymer described above. For example, the ion conductive polymer containing a sulfonic acid group may include, but is not limited to, one or more of a perfluorosulfonic acid (PFSA) polymer, poly(styrene sulfonic acid) (PSSA), sulfonated poly(ether ether ketones) (SPEEK), sulfonated poly(ether sulfone) (SPES), sulfonated poly(aryl ether ketone) (SPAEK), sulfonated polybenzimidazole (SPBI), sulfonated poly(phenylen oxide) (SPPO), and sulfonated polyimide (SPI).

[0080] Examples of the above perfluorosulfonic acid polymer separation membranes may include Nafion (Dupont), Flemion (Ashahi Kasei), Fumasep (Fumatech), Ballard advanced (Ballard), Gore-Select (Gore), Aciplex (Asahi Glass), Aquivion (Solvay), etc.

[0081]

[0082] <Method for manufacturing electrolyte for oxidation-reduction secondary batteries>

[0083] Referring to FIG. 1, a method for manufacturing an electrolyte for a redox secondary battery according to an embodiment of the present invention may include a step (S1) of preparing vanadium oxide having an oxidation number of 5.00; a step (S2) of first reducing the vanadium oxide having an oxidation number of 5.00 into vanadium oxide having an oxidation number of 4.00 using a chemical reduction method; and a step (S3) of secondarily reducing the vanadium oxide having an oxidation number of 4.00 into vanadium oxide having an oxidation number exceeding 3.50 and less than 4.00 using an electrochemical reduction method; and the resultant product of step S3 may be obtained as an electrolyte for a secondary battery.

[0084] The method for producing an electrolyte for an oxidation-reduction redox secondary battery of the present invention has the advantage of being able to produce a vanadium electrolyte having a desired oxidation number of more than 3.50 and less than 4.00, and of increasing process efficiency when mass-producing an electrolyte having a desired oxidation number.

[0085] The oxidation number of vanadium in the obtained secondary battery electrolyte may be less than 4.00, for example, the oxidation number may be 3.80 or less, for example, the oxidation number of vanadium may be 3.75 or less, for example, the oxidation number of vanadium may be 3.70 or less.

[0086] In general, for example, when manufacturing a vanadium electrolyte having an oxidation number of 3.50, it is known to use an oxide of V2O3 or V2O4, which is a type of vanadium precursor, to manufacture a vanadium electrolyte having an oxidation number of 3 and a vanadium electrolyte having an oxidation number of 4, respectively, and then mix them in a 1:1 ratio to manufacture vanadium having a oxidation number of 3.50. However, since the vanadium oxides such as the above are relatively expensive, there is a problem that the process efficiency is significantly reduced when mass-producing the electrolyte.

[0087] Therefore, the manufacturing method of the present invention uses vanadium oxide with an oxidation number of 5.00, which is relatively inexpensive and has excellent process efficiency as a vanadium precursor. From this perspective, according to one embodiment of the present invention, the vanadium oxide with an oxidation number of 5.00 may include V2O5 (vanadium pentoxide) powder.

[0088] The above 5.00-valent vanadium oxide V2O5 powder has the characteristic of dissolving only in very small amounts in acidic solutions, especially sulfuric acid solutions. Therefore, V is dissolved using a chemical reducing agent. 5+ → V 4+ The first chemical reduction (S2) is performed using a redox flow battery cell, and V 4+ → V X+ (At this time, X is greater than 3.50 and less than 4.00) By performing secondary electrochemical reduction (S3), the process efficiency of the manufacturing method can be increased.

[0089] According to one embodiment of the present invention, the chemical reduction method of the first reducing step (S2) can be carried out by adding a small amount of vanadium oxide having an oxidation number of 5.00 to a solution in which an acidic solution and a reducing agent are mixed, and can be carried out while heating at a temperature of, for example, 50°C to 150°C, for example, 60°C to 100°C, for example, 70°C to 90°C, and additionally, stirring can be performed together to ensure good mixing. The heating method is not particularly limited as long as it can provide a heat source capable of maintaining the desired temperature, and for example, a heating mantle can be used, and additionally, a stirring device such as a magnetic stirrer can be connected and used.

[0090] According to one embodiment of the present invention, the acidic solution may include at least one selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid. In this case, the acidic solution may be prepared by mixing the acid with deionized water (DI water) to produce a high-purity electrolyte.

[0091] The concentration of the acidic solution may be, for example, 0.1 M to 10 M, for example, 1 M to 8 M, for example, 2 M to 5 M, but is not necessarily limited thereto, and may be appropriately adjusted depending on the type of acidic solution selected, etc.

[0092] According to one embodiment of the present invention, the reducing agent may include at least one selected from the group consisting of oxalic acid, formic acid, formaldehyde, metonalo, ethanol, and ammonium hydroxide.

[0093] According to one embodiment of the present invention, the concentration of vanadium ions in the electrolyte for the oxidation-reduction redox secondary battery may be 1.5 M to 3.0 M.

[0094] The vanadium oxide having a valence of 4.00 manufactured by the first reduction step (S2) can be reduced to an oxidation number exceeding 3.50 and less than 4.00 by the second reduction step (S3). An electrochemical reduction method is selected for the second reduction step (S3) of the present invention. Conventionally, a method exists for chemical reduction using a metal such as Zn as a reducing agent in the second reduction step, but there is a limitation in that a large amount of impurities such as Zn ions remain in the electrolyte.

[0095] Therefore, in the present invention, an electrochemical reduction method was selected to solve the problem of residual impurities as described above while producing a vanadium electrolyte having a desired oxidation number. An example of the electrochemical reduction method may be a method of reduction at the cathode using an electrochemical cell having a structure similar to a redox flow battery (RFB), but the present invention is not necessarily limited thereto.

[0096]

[0097] In the second reduction step (S2) of the present invention, when an electrochemical cell is used to match the target electrolyte oxidation number, the amount of current required for electrolyte reduction can be calculated and applied. Specifically, vanadium oxide (V) having a valence of 4.00 4+ ) as vanadium oxide (V) of the x group x+ ) can be calculated using the following <Equation 1>, and is based on 1 L of the target electrolyte.

[0098] <Equation 1>ECx = { (4 - x) × C V × F} × v

[0099] In the above equation 1, ECx is 4.00 vanadium oxide (V 4+ ) as vanadium oxide (V) of the x group x+ ) is the total current per hour (unit: ampere (A)) flowing through the electrochemical cell as the amount of current to reduce to ), x is the vanadium oxidation number of the desired vanadium electrolyte, and C v is the molar concentration of the vanadium electrolyte (unit: mol / L), F is the Faraday constant, which is 96,485 (unit: coulomb / mole (C / mol)), and v is the amount of electrolyte flowing through the electrochemical cell per hour (unit: L / s).

[0100] According to one embodiment of the present invention, after the step (S2), a selective impurity removal process or a selective impurity capture process may be additionally performed to further increase the purity of the vanadium electrolyte.

[0101]

[0102] Hereinafter, the present invention will be described in more detail by describing manufacturing examples and experimental examples. However, these are only examples of the present invention, and the present invention is not limited thereto.

[0103]

[0104] Manufacturing Example 1: Manufacturing of an electrolyte containing vanadium ions

[0105] A stirrer was installed in a heating mantle heated to approximately 80°C, and then 3rd distilled water (DI water; DW) manufactured with a 3rd ultrapure water device, 98% sulfuric acid (H2SO4), and 99.5% oxalic acid as a reducing agent were first introduced, and then V2O5 powder was added in small amounts.

[0106] At this time, to prepare 1 L of vanadium electrolyte, 236 ml of 98% H2SO4, 108 g of 99.5% oxalic acid, and 780 ml of DW were mixed, and then 154 g of V2O5 powder was added to prepare a 1.7 M vanadium ion electrolyte. Then, heating and stirring were performed at approximately 80°C for approximately 12 hours.

[0107] Vanadium was sufficiently dissolved in sulfuric acid to prepare a vanadium electrolyte having an oxidation number of 4. Then, the vanadium electrolyte having an oxidation number of 4 was injected into a VRFB cell (manufactured by Standard Energy) for performing electrochemical reduction, and then reduced at the cathode to prepare vanadium ion electrolytes having oxidation numbers of 3.45, 3.50, 3.55, 3.60, 3.65, 3.70, 3.75, and 3.80, respectively.

[0108] At this time, the oxidation number of the vanadium ion electrolyte prepared above can be calculated by the following equation 2 according to a titration method using an oxidizing agent.

[0109] <Equation 2> Average oxidation number of vanadium ion electrolyte = 5 - {V b / (V b - V a )}

[0110] In the above equation 2, V a V means the amount (ml) of oxidizing agent consumed when the oxidation number of vanadium electrolyte is oxidized from more than 3.00 to 4.00. b It means the amount (ml) of oxidizing agent consumed when the oxidation number of vanadium electrolyte is oxidized from more than 3.00 to 5.00.

[0111] As shown in Fig. 2, when an oxidizing agent is added dropwise to a vanadium electrolyte having an oxidation number of 3.00, the relative potential value (V) to the reference electrode, Ag / AgCl, is measured, a vanadium electrolyte titration graph is plotted, and then a first derivative graph is plotted. There are two inflection points indicated by peaks, and the inflection points here represent equivalence points, which are points where the oxidation number of the vanadium electrolyte changes. Based on the inflection points, V is the amount (ml) of the oxidizing agent consumed when the oxidation number of the vanadium electrolyte is oxidized from more than 3.00 to 4.00. a And V is the amount (ml) of oxidizing agent consumed when the oxidation number of vanadium electrolyte is oxidized from more than 3.00 to 5.00. b You can check it, (V b - V a ) represents the amount of oxidant consumed (ml). In Fig. 2, Potential (V vs Ag / AgCl) represents the relative potential value (V) for the reference electrode, Ag / AgCl, and △V / △mL represents the differential of the change in potential value (△V) with respect to the volume (△mL) of the consumed oxidant.

[0112] In the present invention, 0.02M KMnO4 was used as the oxidizing agent, and as confirmed in Fig. 2, V a is 1.6 (ml), V b Since it was 4.8(ml), if calculated according to the above equation 2, it is calculated as 5 - {4.8 / (4.8 - 1.6)} = 3.5, so it can be seen that the oxidation number is 3.5.

[0113] According to this titration method, the oxidation number value of each vanadium electrolyte prepared above can be measured.

[0114]

[0115] Manufacturing Example 2: Manufacturing of a vanadium ion battery

[0116] Two current collectors were manufactured by stacking a carbon current collector (graphite composite, thickness: 0.2 mm) and a metal current collector (aluminum foil, thickness: 0.2 mm), and used as a positive current collector and a negative current collector, respectively. A vanadium ion battery including a positive electrolyte receiving portion and a negative electrolyte receiving portion was manufactured using Nafion 212 (Dupont) as an ion-conductive separator.

[0117]

[0118] Experimental example: Charge and discharge experiment of vanadium ion battery

[0119] In order to confirm the charging energy performance during charging and discharging according to each oxidation state of the vanadium ion electrolyte prepared in Manufacturing Example 1, the vanadium ion electrolyte with an oxidation state of 3.50 prepared in Manufacturing Example 1 was added to the positive and negative electrodes of the vanadium ion battery prepared in Manufacturing Example 2.

[0120] As for the charge / discharge operation conditions, the battery was charged with a constant current of 0.5 C until the voltage reached 1.70 V, and discharged with a constant current of 0.5 C until the voltage reached 1.10 V, which is the same as the charge.

[0121] Charging and discharging were performed up to 500 times (cycles) under the above charging and discharging conditions, and the charging energy (wh) was calculated for each of the 5, 100, 200, 300, 400, and 500 cycles. The charging energy is a value calculated by multiplying the charging voltage measured at each measurement time (sampling time) while the battery was being charged by the charging current (0.5 C).

[0122] Then, for vanadium ion electrolytes with oxidation numbers of 3.45, 3.50, 3.55, 3.60, 3.65, 3.70, and 3.75, charging and discharging were performed up to 500 cycles, and the charging energy was measured at 5, 100, 200, 300, 400, and 500 cycles, respectively.

[0123] Based on the vanadium ion electrolyte having an oxidation number of 3.50, the increase / decrease rate (percentage; %) of the charging energy of vanadium ion electrolytes having oxidation numbers of 3.45, 3.50, 3.55, 3.60, 3.65, 3.70, and 3.75 was calculated according to the following Equation 3, and is shown in Table 1 below.

[0124] <Equation 3> Increase / decrease rate of charging energy: { (E x - E 3.5 ) / E 3.5} × 100 (%)

[0125] In the above equation 3, E 3.5 is the charging energy of the vanadium ion electrolyte with an oxidation number of 3.50, and Ex is the charging energy of the vanadium ion electrolyte with an oxidation number other than 3.50.

[0126] [Table 1]

[0127]

[0128] As can be seen in Table 1 above, the increase / decrease rate of charge energy of the vanadium electrolyte exceeding the oxidation number 3.50 after 200 charge / discharge cycles was calculated as a positive value (+) compared to the vanadium electrolyte with the oxidation number 3.50, which confirmed that the charge energy was higher than that of the vanadium electrolyte with the oxidation number 3.50.

[0129] In this way, the vanadium electrolyte according to the present invention, despite the phenomenon of concentration deviation of the positive and negative electrolytes occurring during hundreds of charge and discharge cycles, solved the problem of reduced charging energy and rather showed the effect of increased charging energy compared to the conventional 3.50-valent electrolyte, which is generally used. This means that the vanadium electrolyte of the present invention can have significantly improved energy density, and thus can increase the lifespan and efficiency of a secondary battery of an aqueous electrolyte using the vanadium electrolyte, and can also have the advantage of overcoming limitations due to space constraints.

[0130]

[0131] Although the embodiments of the present specification have been described in more detail with reference to the attached drawings, the present specification is not necessarily limited to these embodiments, and various modifications may be implemented without departing from the technical spirit of the present specification. Therefore, the embodiments disclosed in this specification are not intended to limit the technical spirit of the present specification, but to explain, and the scope of the technical spirit of the present specification is not limited by these embodiments. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive. The scope of protection of this specification should be interpreted by the claims, and all technical ideas within a scope equivalent thereto should be interpreted as being included in the scope of rights of this specification.

Claims

1. Contains vanadium ions and acidic solutions, Vanadium whose oxidation number is greater than 3.50 and less than 4.00, Electrolyte for redox secondary batteries.

2. In paragraph 1, The oxidation number of the above vanadium is greater than 3.50 and less than or equal to 3.80, Electrolyte for redox secondary batteries.

3. In paragraph 1, The oxidation number of the above vanadium is 3.55 or more and 3.75 or less, Electrolyte for redox secondary batteries.

4. In paragraph 1, The above vanadium ions are supplied by vanadium oxide, Electrolyte for redox secondary batteries.

5. In paragraph 4, The above vanadium oxide comprises at least one of V2O5, VOSO4, V2O3, NH4VO3 and V2O4. Electrolyte for redox secondary batteries.

6. In paragraph 4, The above vanadium oxide is in powder form. Electrolyte for redox secondary batteries.

7. In paragraph 1, The above acid solution contains at least one of sulfuric acid, hydrochloric acid, nitric acid and phosphoric acid. Electrolyte for redox secondary batteries.

8. Bipolar; cathode; A separator disposed between the anode and cathode; A cathode electrolyte receiving portion formed between the cathode and the separator; and Including a cathode electrolyte receiving portion formed between the cathode and the separator; An electrolyte for an oxidation-reduction secondary battery according to any one of claims 1 to 7 is supplied to the positive electrode electrolyte receiving portion and the negative electrode electrolyte receiving portion. Oxidation-reduction secondary battery.

9. In paragraph 8, The above separator comprises an ion-conducting polymer, Oxidation-reduction secondary battery.

10. In paragraph 9, The above ion-conducting polymer is a hydrocarbon polymer with an ion-conducting functional group introduced or a fluorine polymer with an ion-conducting functional group introduced. The ion-conducting functional group includes at least one selected from a sulfonic acid group, a carboxyl group, and a phosphoric acid group. Oxidation-reduction secondary battery.

11. In paragraph 10, The above ion conductive polymer is a hydrocarbon polymer or a fluorine polymer having a sulfonic acid group introduced therein. Oxidation-reduction secondary battery.

12. In paragraph 11, The hydrocarbon polymer or fluorine polymer having the sulfonic acid group introduced therein comprises at least one of a perfluorosulfonic acid (PFSA) polymer, poly(styrene sulfonic acid) (PSSA), sulfonated poly(ether ether ketones) (SPEEK), sulfonated poly(ether sulfone) (SPES), sulfonated poly(aryl ether ketone) (SPAEK), sulfonated polybenzimidazole (SPBI), sulfonated polyphenylene oxide (SPPO), and sulfonated polyimide (SPI). Oxidation-reduction secondary battery.

13. A method for manufacturing an electrolyte for an oxidation-reduction secondary battery according to any one of paragraphs 1 to 7, A step for preparing vanadium oxide having a vanadium oxidation number of 5.00; A step of first reducing the vanadium oxide having an oxidation number of 5.00 to vanadium oxide having an oxidation number of 4.00 by a chemical reduction method; and A step of secondary reduction of vanadium oxide having an oxidation number of 4.00 to vanadium oxide having an oxidation number exceeding 3.50 and less than 4.00 by an electrochemical reduction method; comprising; Method for manufacturing an electrolyte for a redox secondary battery.

14. In paragraph 13, The vanadium oxide having the oxidation number of 5.00 above includes V2O5 powder. Method for manufacturing an electrolyte for a redox secondary battery.

15. In paragraph 13, The chemical reduction method of the first reduction step is as follows: The process is performed by adding vanadium oxide having an oxidation number of 5.00 to a solution containing an acidic solution and a reducing agent at a temperature of 50°C to 150°C. Method for manufacturing an electrolyte for a redox secondary battery.

16. In paragraph 15, The above acid solution contains at least one of sulfuric acid, hydrochloric acid, nitric acid and phosphoric acid. Method for manufacturing an electrolyte for a redox secondary battery.

17. In paragraph 15, The reducing agent comprises at least one of oxalic acid, formic acid, formaldehyde, methanol, ethanol and ammonium hydroxide. Method for manufacturing an electrolyte for a redox secondary battery.

18. In paragraph 13, The concentration of vanadium ions in the electrolyte for the above oxidation-reduction secondary battery is 1.5 M to 3.0 M. Method for manufacturing an electrolyte for a redox secondary battery.

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