Electrochemical element Zn / MnO2

Abstract

An electrochemical element comprising: - an aqueous electrolyte with a pH less than 7 containing Zn2+ and Mn2+ ions, - a negative electrode comprising zinc or a zinc-based alloy, - a positive electrode comprising manganese dioxide, a first carbon black with a specific surface area S1, and a second carbon black with a specific surface area S2, with S2 / S1 ≥ 2. Abbreviated figure: Figure 1

Description

Title of the invention: Zn / MnO2 electrochemical element technical field

[0001] The present invention belongs to the technical field of Zn / MnO2 electrochemical elements comprising a positive electrode based on manganese dioxide MnO2, a negative electrode based on zinc or a zinc alloy and a weakly acidic aqueous electrolyte. Background

[0002] The Zn / MnO2 electrochemical elements, comprising a positive electrode based on manganese dioxide (MnO2), a negative electrode based on zinc or a zinc alloy, and a slightly acidic aqueous electrolyte (i.e., with a pH between 3 and 6), are known. They have the property of being rechargeable. The slightly acidic electrolyte preserves the reversibility of the electrochemical reactions at both the negative and positive electrodes and improves the element's durability by limiting zinc corrosion by the electrolyte. Thanks to this improvement, it is possible to cycle the element for approximately one hundred cycles. To explain the rechargeable operation of these elements, the reversible insertion of Zn2+ ions into the MnO2 structure is sometimes hypothesized, and these elements are sometimes referred to as "zinc ion elements."

[0003] During the manufacture of the positive electrode of a Zn / MnO2 element, it is common practice to mix carbon black with the MnO2 powder to compensate for the low electrical conductivity of MnO2. To the applicant's knowledge, there is no commercially available form of carbon black that allows for the production of an element exhibiting both high specific capacitance and a long lifetime. Achieving a high specific capacitance often comes at the expense of the element's lifetime.

[0004] We are therefore looking for an electrochemical element Zn / MnO2 with a weakly acidic electrolyte, which has a high specific capacitance and a long lifetime. An "element with a high specific capacitance and a long lifetime" is defined as an element with a specific capacitance greater than or equal to 150 mAh per gram of MnO2 for at least 300 cycles.

[0005] EP 4169096 describes a rechargeable electrochemical element comprising a negative zinc electrode, a positive electrode which may be MnO2-based, and an electrolyte having a pH between 3.5 and 6.5. The positive electrode includes a current collector on which a primer layer and an active material composition layer are applied in that order. The primer layer serves to protect the current collector of the positive electrode against corrosion by the electrolyte. This corrosion occurs when the positive electrode is subjected to a high voltage. The primer layer and the active material composition layer may each comprise a mixture of a particulate carbon filler and a fibrous carbon filler. No information is provided regarding the specific surface area of ​​these two fillers or the proportions in which they are used.

[0006] US 2022 / 376231 describes an electrochemical element comprising a negative zinc electrode, a positive electrode that may be MnO2-based, and an electrolyte consisting of an aqueous solution of zinc sulfate. The positive electrode includes a current collector on which a layer of an active material composition is deposited. This layer comprises composite particles, at least one conductive carbon, and a binder. The conductive carbon may be carbon black, acetylene black, carbon fibers, graphene, natural graphite, artificial graphite, fullerenes, hard carbon, mesocarbon microbeads, carbon nanofibers, or activated carbon. This document does not consider the mixing of two carbon blacks.

[0007] EP 3806219 describes an active material composition for a positive electrode of a Zn / MnO2 element, said composition comprising a compressed mixture of MnO2 particles, an electrically conductive additive, and a binder. The electrically conductive additive may be carbon black, expanded graphite, or a mixture thereof. This document does not consider the mixture of two carbon blacks.

[0008] None of the documents cited above describes a mixture of two carbon blacks such as that of the present invention. Summary

[0009] The invention relates to an electrochemical element comprising: - an aqueous electrolyte with a pH below 7 and containing Zn2+ and Mn2+ ions, - a negative electrode comprising zinc or a zinc-based alloy, - a positive electrode comprising manganese dioxide, a first carbon black having a specific surface area SI and a second carbon black having a specific surface area S2, with S2 / S1>2.

[0010] It has been discovered that the use of two carbon blacks having a specific surface area ratio S2 / S1 greater than or equal to 2 made it possible to obtain a Zn / MnO2 element having a high mass capacity and lifetime.

[0011] According to one embodiment, the mass proportion of the first carbon black ranges from 15 to 80%, the mass proportion of the second carbon black ranges from 20 to 85%, the mass proportions being expressed in relation to the total mass of the first and second carbon black.

[0012] According to one embodiment, the specific surface area SI ranges from 10 to 200 m2 / g and the specific surface area S2 ranges from 500 to 1400 m2 / g.

[0013] According to one embodiment, the specific surface area SI ranges from 50 to 100 m2 / g and the specific surface area S2 ranges from 700 to 900 m2 / g.

[0014] According to one embodiment, the mass proportion of the first carbon black ranges from 25 to 50% and the mass proportion of the second carbon black ranges from 50 to 75%.

[0015] According to one embodiment, the mass proportion of the first carbon black is 50%, the mass proportion of the second carbon black is 50%, the first carbon black has a specific surface area SI ranging from 50 to 100 m2 / g and the second carbon black has a specific surface area S2 ranging from 700 to 900 m2 / g.

[0016] According to one embodiment, the mass proportion of the first carbon black is 25%, the mass proportion of the second carbon black is 75%, the first carbon black has a specific surface area SI ranging from 50 to 100 m2 / g and the second carbon black has a specific surface area S2 ranging from 700 to 900 m2 / g.

[0017] According to one embodiment, the aqueous electrolyte has a pH ranging from 5 to 6.

[0018] According to one embodiment, the aqueous electrolyte further contains sulfate anions.

[0019] According to one embodiment, the concentration of Zn2+ ions ranges from 1 to 2 mol.L 1 and the concentration of Mn2+ ions ranges from 0.05 to 0.2 mol.L *.

[0020] According to one embodiment, the positive electrode comprises a current collector which is a stainless steel strip or a carbon fiber-based non-woven material.

[0021] According to one embodiment, the current collector is a stainless steel strip. Brief description of the figure

[0022] [Fig. 1] represents the variation of the discharged capacitance of the electrochemical elements whose positive electrode contains the carbon black compositions 1 to 5 during a cycling. Detailed description of the embodiments Positive electrode

[0023] The positive electrode of the element according to the invention comprises two carbon blacks. A carbon black is a partially crystalline carbonaceous product, resulting from the partial decomposition of hydrocarbons, and which has a structure in the form of primary particles, connected in aggregates, i.e., connected to each other by covalent bonds, the aggregates being optionally agglomerated together, i.e., connected to each other by Van der Waals-type bonds. The term "carbon black" encompasses acetylene black, furnace black, soot, lamp black, thermal black, and tunnel black. The following are not included in the definition of The term "carbon black" includes graphite, graphene, fullerene, carbon fibers, carbon nanotubes, and activated carbon.

[0024] The first carbon black preferably has a specific surface area SI ranging from 10 to 200 m² / g or from 30 to 150 m² / g or from 50 to 100 m² / g or from 60 to 80 m² / g. The second carbon black preferably has a specific surface area S2 ranging from 500 to 1400 m² / g or from 500 to 1000 m² / g or from 600 to 900 m² / g or from 700 to 800 m² / g. Preferably, the first carbon black has a specific surface area SI ranging from 50 to 100 m² / g and the second carbon black has a specific surface area S2 ranging from 700 to 900 m² / g. Preferably, the first carbon black has a specific surface area S1 of 70 m2 / g and the second carbon black has a specific surface area S2 of 800 m2 / g.

[0025] The specific surface area can be obtained by measuring the adsorption of a gas onto the surface of carbon black at a given temperature and within a certain relative pressure range. The resulting curve is called the adsorption isotherm. The most widely used technique is based on the Brunauer, Emmett, and Teller (BET) theory. This technique measures the amount of nitrogen required to form a monolayer of this gas on the surface of the carbon black. The procedure for measuring the specific surface area using the BET method is described in ASTM D6556-21, "Standard Test Method for Carbon Black—Total and Extreme Surface Area by Nitrogen Adsorption."

[0026] The ratio between the specific surface area S2 of the second carbon black and that of the first carbon black SI can be at least equal to 5 or at least equal to 10.

[0027] Carbon black particles can be in the form of primary particles having a median volume equivalent diameter Dv50 ranging from 10 to 100 nm, and more specifically from 35 to 50 nm. The term median means that 50% of the particle volume consists of particles with an equivalent diameter smaller than the value of diameter Dv50, and that 50% of the particle volume consists of particles with an equivalent diameter greater than or equal to the value of diameter Dv50. The parameter Dv50 can be measured by laser diffraction.

[0028] The mass proportion of the first carbon black can range from 15 to 80%, or from 15 to 60%, or from 20 to 50%, or from 25 to 50% relative to the total mass of the first and second carbon blacks. The mass proportion of the second carbon black can range from 20 to 85%, or from 40 to 85%, or from 50 to 80%, or from 50 to 75% relative to the total mass of the first and second carbon blacks. Preferably, the mass proportion of the first carbon black ranges from 25 to 50% and the mass proportion of the second carbon black ranges from 50 to 75%.

[0029] In a first preferred embodiment, the mass proportion of the first carbon black is 50%, the mass proportion of the second carbon black is 50%, the first carbon black has a specific surface area SI ranging from 50 to 100 m2 / g, for example 70 m2 / g and the second carbon black has a specific surface area S2 ranging from 700 to 900 m2 / g, for example 800 m2 / g.

[0030] In a second preferred embodiment, the mass proportion of the first carbon black is 25%, the mass proportion of the second carbon black is 75%, the first carbon black has a specific surface area SI ranging from 50 to 100 m2 / g, for example 70 m2 / g and the second carbon black has a specific surface area S2 ranging from 700 to 900 m2 / g, for example 800 m2 / g.

[0031] The two carbon blacks and the MnO2 particles can be mixed using a paddle mixer, a planetary mixer, or other mixing means. MnO2 can be obtained from an ore or synthesized chemically or electrolytically.

[0032] MnO2 and carbon black particles are generally mixed with one or more binders whose function is to improve the cohesion of the MnO2 particles with each other and to improve the adhesion of the MnO2 and carbon black particles to the current collector. A binder may be polyvinylidene fluoride (PVDF) or its copolymers.

[0033] An ink is prepared by dispersing a mixture comprising MnO2 particles, particles of the first and second carbon black, and the binder(s) in a solvent or a mixture of several organic solvents. The organic solvent may be n-methyl-2-pyrrolidone (NMP). By varying the amount of solvent incorporated into the mixture, the viscosity of the ink can be varied before it is deposited on at least one face of a current collector.

[0034] The current collector is a current-conducting support that can take, for example, the form of a grid, foam, or strip. The collector material can be stainless steel or a carbon fiber-based nonwoven. Preferably, it is stainless steel.

[0035] The ink-coated current collector is dried and can then be laminated to adjust its thickness. After evaporation of the solvent(s), a layer of active material composition is obtained, the proportions of whose various constituents are typically: - from 60 to 90%, for example 70% by mass of one or more beneficial active ingredients, - from 1 to 30%, for example 20% by mass of the two carbon blacks, - from 1 to 10% by mass of binder(s). Negative electrode

[0036] The negative electrode comprises a strip of zinc or a zinc-based alloy. Alloying elements may be chosen to reduce the corrosion reaction of the zinc by the electrolyte. A powder of zinc or a zinc alloy may also be used. Electrolyte

[0037] The electrolyte is obtained by dissolving a zinc salt and a manganese salt in water. Generally, the zinc salt and the manganese salt are zinc sulfate and manganese sulfate. Other zinc salts, such as zinc triflate, may be used. The concentration of Zn2+ ions can range from 1 to 2 mol / L*. Adding a manganese salt, such as manganese sulfate, to the electrolyte improves the lifetime of the element during cycling. The concentration of Mn2+ ions can range from 0.05 to 0.2 mol / L*.

[0038] The pH of the electrolyte can be adjusted within the desired range, for example from 3 to 6, 4 to 5, or 5 to 6, by adding sulfuric acid or potassium hydroxide. A pH that is too acidic accelerates the corrosion of the zinc electrode, which reduces the lifespan of the element. A pH that is too alkaline accelerates the passivation reaction of the zinc electrode, which is also undesirable. Separator:

[0039] The separator material can be selected from the following materials: a polyolefin, for example polypropylene, polyethylene, a polyester, glass fibers that can be bonded together by a polymer, polyimide, polyamide, polyaramid, polyamideimide, and cellulose. The polyester can be selected from polyethylene terephthalate (PET) and polybutylene terephthalate (PBT).

[0040] An electrochemical bundle is formed by interposing one or more separator layers between at least one positive electrode and at least one negative electrode. The electrochemical bundle is inserted into a container, which may be made of steel. The container for the element may be of one of the following shapes: parallelepiped, cylindrical, button, or pouch. In the case of the cylindrical shape, the electrochemical bundle is spiraled to form a cylindrical arrangement of the electrodes. The element may be manufactured in standard sizes AA, AAA, C, D, or other.

[0041] The element according to the invention is of interest in the field of large-scale energy storage due to its low cost and high safety of use. It is also of interest in applications requiring a large number of cycles. It can constitute an alternative to nickel-cadmium elements for applications in which the use of cadmium is prohibited. Examples

[0042] Different Zn / MnO2 elements have been manufactured. They differ in the composition of the carbon blacks of the positive electrode. The positive electrode consists of a A stainless steel strip coated with an ink containing the active ingredient MnO2, a binder, and carbon black in the form of either a single carbon black or two carbon blacks. The solvent for this ink is n-methyl-2-pyrrolidone (NMP). After evaporation of the solvent, the deposited layer consists of 70% by mass MnO2, 20% by mass carbon black(s), and 10% by mass binder. The binder is polyvinylidene fluoride (PVDF). The negative electrode is made of a metallic zinc strip.

[0043] The proportions of the two carbon blacks are as follows: [Tables 1] Mass proportion of the first carbon black with a specific surface area of ​​BET SI of 70 m² / g Mass proportion of the second carbon black with a specific surface area of ​​BET S2 of 800 m² / g Composition No. 1* 0% 100% Composition No. 2 25% 75% Composition No. 3 50% 50% Composition No. 4 75% 25% Composition No. 5* 100% 0% * outside invention

[0044] The electrolyte is an aqueous solution containing zinc sulfate and manganese sulfate. The concentration of ZnSO4 is 2 mol / L and the concentration of MnSO4 is 0.1 mol / L. A glass fiber separator is positioned between the positive and negative electrodes. This separator is impregnated with the electrolyte. The electrochemical elements are thus formed. In these elements, the capacitance of the negative electrode is greater than that of the positive electrode.

[0045] The elements are subjected to a cycling test at room temperature. Charging and discharging are carried out by applying a constant current. The charging and discharging current is IC, where C is the nominal capacitance of the positive electrode, considering a nominal capacitance of 308 mAh / g of MnO2. The end-of-charge voltage is set at 1.9 V. The end-of-discharge voltage is set at 0.8 V. The capacitances shown in [Fig. 1] correspond to the capacitance measured at the end of the discharge.

[0046] The elements exhibiting the highest discharge capacity during cycling are those whose positive electrode contains carbon blacks in the following mass proportions: 0 / 100, 25 / 75, 50 / 50, and 75 / 25 (compositions 1 to 4). However, it should be noted that the element whose positive electrode contains composition 1, which is not part of this invention, does not have a long lifespan. From the 150th cycle onward, its discharge capacity decreases rapidly.

[0047] The element whose positive electrode contains composition 5 has insufficient capacity. From the 75th cycle onwards, this capacity is less than 150 mAh / g.

[0048] Only the elements whose positive electrode contains carbon blacks in the following mass proportions: 25 / 75, 50 / 50 and 75 / 25 (compositions 2 to 4) exhibit a mass capacitance greater than or equal to 150 mAh per gram of MnO2 for at least 300 cycles.

[0049] Surprisingly, the elements whose positive electrode comprises compositions 2, 3, and 4 according to the invention exhibit, from the 182nd cycle onward, a discharged capacitance greater than that of the elements comprising compositions 1 and 5 outside the scope of the invention. These results show that the improved lifetime after prolonged cycling of the element of at least 300 cycles stems from the presence of the two carbon blacks having different specific surface areas. The presence of only one of the two carbon blacks does not allow for a long lifetime, as evidenced by the low capacitances of the elements comprising compositions 1 and 5 outside the scope of the invention.

Claims

Demands

1. Electrochemical element comprising: - an aqueous electrolyte having a pH less than 7 and containing Zn2+ and Mn2+ ions, - a negative electrode comprising zinc or a zinc-based alloy, - a positive electrode comprising manganese dioxide, a first carbon black having a specific surface area S1 and a second carbon black having a specific surface area S2, with S2 / S1>2.

2. Electrochemical element according to claim 1, wherein the mass proportion of the first carbon black ranges from 15 to 80%, the mass proportion of the second carbon black ranges from 20 to 85%, the mass proportions being expressed in relation to the total mass of the first and second carbon black.

3. Electrochemical element according to claim 1 or 2, wherein the specific surface area SI ranges from 10 to 200 m2 / g and the specific surface area S2 ranges from 500 to 1400 m2 / g.

4. Electrochemical element according to claim 3, wherein the specific surface area SI ranges from 50 to 100 m2 / g and the specific surface area S2 ranges from 700 to 900 m2 / g.

5. Electrochemical element according to any one of claims 2 to 4, wherein the mass proportion of the first carbon black is from 25 to 50% and the mass proportion of the second carbon black is from 50 to 75%.

6. Electrochemical element according to any one of claims 2 to 5, wherein the mass proportion of the first carbon black is 50%, the mass proportion of the second carbon black is 50%, the first carbon black has a specific surface area S1 ranging from 50 to 100 m2 / g and the second carbon black has a specific surface area S2 ranging from 700 to 900 m2 / g.

7. Electrochemical element according to any one of claims 2 to 5, wherein the mass proportion of the first carbon black is 25%, the mass proportion of the second carbon black is 75%, the first carbon black has a specific surface area S1 going

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12. from 50 to 100 m2 / g and the second carbon black has a specific surface area S2 ranging from 700 to 900 m2 / g. Electrochemical element according to any one of the preceding claims, wherein the aqueous electrolyte has a pH ranging from 5 to 6. An electrochemical element according to any one of the preceding claims, wherein the aqueous electrolyte further contains sulfate anions. An electrochemical element according to any one of the preceding claims, wherein the concentration of Zn2+ ions ranges from 1 to 2 mol.L⁻¹ and the concentration of Mn2+ ions ranges from 0.05 to 0.2 mol.L⁻¹. Electrochemical element according to any one of the preceding claims, wherein the positive electrode comprises a current collector which is a stainless steel strip or a carbon fiber-based nonwoven. Electrochemical element according to claim 11, wherein the current collector is a stainless steel strip.

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