Zn / mno 2 electrochemical element

Abstract

The invention relates to an electrochemical element comprising: - an aqueous electrolyte having a pH of 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, where S2 / S1≥2.

Description

Description Title of the invention: Zn / MnCh electrochemical element technical field

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

[0002] The electrochemical elements Zn / MnCh, comprising a positive electrode based on manganese dioxide (MnCl₂), 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 well-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. Thanks to this improvement, the element can be cycled for approximately one hundred cycles. To explain the rechargeable behavior of these elements, the reversible insertion of Zn ions is sometimes hypothesized. 2+ in the structure of MnCl and these elements are sometimes referred to as "zinc ion elements".

[0003] During the fabrication of the positive electrode of a Zn / MnCl element, it is common practice to mix carbon black with MnCh powder to compensate for the low electrical conductivity of MnCl. To the applicant's knowledge, there is no commercially available form of carbon black that allows for the production of an element with 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 a Zn / MnCh electrochemical element with a weakly acidic electrolyte, which exhibits high specific capacitance and a long lifetime. An element with high specific capacitance and a long lifetime is defined as one with a specific capacitance greater than or equal to 150 mAh per gram of MnCh for at least 300 cycles.

[0005] EP 4169096 describes a rechargeable electrochemical element comprising a negative zinc electrode, a positive electrode that may be MnCh-based, and an electrolyte having a pH between 3.5 and 6.5. The positive electrode includes a current collector onto 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 from 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 indication is given as to the surface specific to these two charges or to the proportions in which these two charges are used.

[0006] US 2022 / 376231 describes an electrochemical element comprising a negative zinc electrode, a positive electrode that may be MnCl-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, graphite, 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 / MnCl element, said composition comprising a compressed mixture of MnCl 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 describe 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 Zn ions 2+ and Mn 2+ , - 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 was discovered that using two carbon blacks with a specific surface area ratio S2 / S1 greater than or equal to 2 made it possible to obtain a Zn / MnCL element with 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] In one embodiment, the specific surface area SI ranges from 10 to 200 m² 2 / g and the specific surface area S2 ranges from 500 to 1400 m 2 / g.

[0013] According to one embodiment, the specific surface area SI ranges from 50 to 100 m² 2 / g and the specific surface area S2 ranges from 700 to 900 m² 2 / 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 SI surface area ranging from 50 to 100 m 2 / g and the second carbon black has a specific surface area S2 ranging from 700 to 900 m 2 / 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%, and the first carbon black has a specific SI surface area ranging from 50 to 100 m².2 / g and the second carbon black has a specific surface area S2 ranging from 700 to 900 m 2 / 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 Zn ions 2+ ranges from 1 to 2 mol. 1 and the concentration of Mn ions 2+ ranges from 0.05 to 0.2 mol. 1 .

[0020] According to one embodiment, the positive electrode includes a current collector which is a stainless steel strip or a carbon fiber 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 in the discharged capacitance of electrochemical elements whose positive electrode contains carbon black compositions 1 to 5 during a cycle. Detailed description of the implementation methods Positive electrode

[0023] The positive electrode of the element according to the invention comprises two carbon blacks. 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, that is to say, connected to each other by covalent bonds, the aggregates being optionally agglomerated together, that is to say, connected to each other by Van der Waals forces. The term "carbon black" encompasses acetylene black, furnace black, soot, lamp black, thermal black, and tunnel black. Graphite, graphene, fullerene, carbon fibers, carbon nanotubes, and activated carbon are not included in the definition of the term "carbon black."

[0024] The first carbon black preferably has a specific surface area (SI) ranging from 10 to 200 m 2 / g or from 30 to 150 or from 50 to 100 m2 / g or 60 to 80 m 2 / g. The second carbon black preferably has a specific surface area S2 ranging from 500 to 1400 m 2 / g or from 500 to 1000 m 2 / g or from 600 to 900 m 2 / g or from 700 to 800 m 2 / g. Preferably, the first carbon black has a specific surface area SI ranging from 50 to 100 m 2 / g and the second carbon black has a specific surface area S2 ranging from 700 to 900 m 2 / g. Preferably, the first carbon black has a specific SI surface area of ​​70 m 2 / g and the second carbon black has a specific surface area S2 of 800 m 2 / g.

[0025] The specific surface area can be obtained by measuring the adsorption of a gas on the surface of carbon black at a given temperature and within a certain range of relative pressure. The resulting curve is called the adsorption isotherm. The most widely used technique is based on the Brunauer-Emmett-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 External 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 equivalent volume diameter D vso ranging from 10 to 100 nm and more specifically from 35 to 50 nm. The median term means that 50% of the particle volume consists of particles with an equivalent diameter smaller than the diameter value D v so that 50% of the particle volume consists of particles with an equivalent diameter greater than or equal to the diameter value D v So. Parameter D v so can be measured by the laser diffraction technique.

[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%, and the first carbon black has a specific SI surface area ranging from 50 to 100 m². 2 / g, for example 70 m 2 / g and the second carbon black has a specific surface area S2 ranging from 700 to 900 m 2 / g, for example 800 m 2 / 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%, and the first carbon black has a specific SI surface area ranging from 50 to 100 m². 2 / g, for example 70 m 2 / g and the second carbon black has a specific surface area S2 ranging from 700 to 900 m 2 / g, for example 800 m 2 / g.

[0031] Both carbon black and MnCl particles can be mixed using a paddle mixer, a planetary mixer, or other mixing methods. MnCl can be obtained from an ore or synthesized chemically or electrolytically.

[0032] MnCl and carbon black particles are typically mixed with one or more binders designed to improve the cohesion of the MnCl particles with each other and to enhance their adhesion to the current collector. A binder can be polyvinylidene fluoride (PVDF) or its copolymers.

[0033] An ink is prepared by dispersing a mixture comprising MnCl 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 the form of, for example, 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, - 1 to 10% by mass of binder(s). Negative electrode

[0036] The negative electrode consists of a strip of zinc or a zinc alloy. Alloying elements can be chosen to reduce the corrosion reaction of the zinc by the electrolyte. Alternatively, zinc powder or a zinc alloy can be used. Electrolyte

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

[0038] The electrolyte pH 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, thus reducing the element's lifespan. 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 chosen from the following: 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 chosen from polyethylene terephthalate (PET) and polybutylene terephthalate (PBT).

[0040] An electrochemical beam is formed by interposing one or more separator layers The electrochemical bundle is inserted into a container, which may be made of steel. The container can be rectangular, cylindrical, button-shaped, or pouch-shaped. In the cylindrical format, the electrochemical bundle is spirally wound to form a cylindrical electrode arrangement. The element can 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 serve as an alternative to nickel-cadmium elements for applications where the use of cadmium is prohibited. Examples

[0042] Different Zn / MnCl elements were manufactured. They differ in the composition of the carbon blacks in the positive electrode. The positive electrode consists of a stainless steel strip coated with an ink containing the active ingredient MnCl, 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 MnCl, 20% by mass carbon black(s), and 10% by mass binder. The binder is polyvinylidene fluoride (PVDF). The negative electrode consists of a metallic zinc strip.

[0043] The proportions of the two carbon blacks are as follows: Table 1 * outside invention

[0044] The electrolyte is an aqueous solution containing zinc sulfate and manganese sulfate. The concentration of ZnSCh is 2 mol. 1 and the concentration of MnSCU is 0.1 mol. 1 A fiberglass separator is positioned between the positive and negative electrodes. This separator is impregnated with the electrolyte. This process forms the electrochemical elements. In these elements, the capacitance of the negative electrode is greater than that of the positive electrode.

[0045] The components are subjected to a cycling test at room temperature. Charging and discharging are performed 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 MnCh. 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 Figure 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). It should be noted, however, that the element whose positive electrode contains composition 1, which is not part of this invention, does not have a long lifespan. From the 150 eme cycle, its discharged capacity decreases rapidly.

[0047] The element whose positive electrode contains composition 5 has insufficient capacitance. From the 75th eme cycle, this 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 MnCh for at least 300 cycles.

[0049] Surprisingly, the elements whose positive electrode comprises compositions 2, 3 and 4 according to the invention exhibit from 182 emecycle a discharged capacity greater than that of 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 two carbon blacks with 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 capacities of elements comprising compositions 1 and 5 outside the scope of the invention.

Claims

Demands

1. Electrochemical element comprising: - an aqueous electrolyte with a pH below 7 and containing Zn ions 2+ and Mn 2+ , - 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.

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 2 / g and the specific surface area S2 ranges from 500 to 1400 m 2 / g.

4. Electrochemical element according to claim 3, wherein the specific surface area SI ranges from 50 to 100 m² 2 / g and the specific surface area S2 ranges from 700 to 900 m² 2 / 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%, and the first carbon black has a specific SI surface area ranging from 50 to 100 m² 2 / g and the second carbon black has a specific surface area S2 ranging from 700 to 900 m 2 / 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%, and the first carbon black has a specific SI surface area ranging from 50 to 100 m² 2 / g and the second carbon black has a specific surface area S2 ranging from 700 to 900 m 2 / g.

8. Electrochemical element according to any one of the preceding claims, wherein the aqueous electrolyte has a pH ranging from 5 to 6.

9. Electrochemical element according to any one of the preceding claims, wherein the aqueous electrolyte further contains sulfate anions.

10. Electrochemical element according to any one of the preceding claims, wherein the concentration of Zn ions 2+ ranges from 1 to 2 mol. 1 and the concentration of Mn ions 2+ ranges from 0.05 to 0.2 mol.1 .

11. 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 fibre-based nonwoven.

12. Electrochemical element according to claim 11, wherein the current collector is a stainless steel strip.

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