HK1069017B - Electrochemical cell - Google Patents

Description

Electrochemical cell

Technical Field

The present invention relates to electrochemical cells, and more particularly to cells having a positive electrode comprising Electrolytic Manganese Dioxide (EMD), Chemical Manganese Dioxide (CMD), or lithiated manganates, cobaltates, or nickelates. The invention relates particularly to alkaline zinc manganese dioxide cells and more particularly to improvements in cathode rings including electrolytic manganese dioxide as the electroactive component and graphite as the conductive additive.

Background

Known alkaline zinc manganese dioxide cells typically employ a mixture of manganese dioxide in small particle form and graphite material as a conductive additive as the positive electrode (cathode). The graphite material improves the electrical conductivity of the positive electrode because the manganese dioxide particles have a relatively low electrical conductivity. It is therefore important to optimise the ratio of manganese dioxide to graphite in a given volume of cell. An increase in graphite volume decreases the battery capacity and thus the energy density of the battery, but decreases the internal resistance of the battery, and vice versa, a decrease in graphite volume increases the battery capacity and the energy density of the battery, but increases the internal resistance of the battery.

In order to increase the energy density and power density of alkaline batteries, improvements in the quality of electrolytic manganese dioxide and increasing the content of electrolytic manganese dioxide in the battery cathode have been proposed. However, in order to obtain more space for the electroactive cathode material in a given cathode volume, the amount of graphite acting as a conductive additive needs to be reduced, which leads to an increase in the internal resistance of the cell.

In EP0675556, expanded graphite having a specific particle size distribution in the range of 0.5 to 15 μm (micrometer) is proposed as a conductive additive instead of conventional carbon particles. The expanded graphite allows for the use of a greater amount of manganese dioxide in a given volume, thereby resulting in a more optimal ratio of manganese dioxide to carbon. Expanded graphite provides better electrical conductivity than conventional artificial or natural graphite for the same graphite content, especially at graphite contents below 7% in the cathode mixture. However, EP0675556 does not mention any expansion ratio for the manufacture of expanded graphite or the presence of expanded graphite in granular form (e.g. as worms).

A process for the manufacture of expanded graphite from flake graphite is disclosed in WO 99/46437. The method comprises the following steps: particles of flake graphite are provided, the treated graphite is expanded at elevated temperature using an expandable intercalation compound such as a high concentration of sulfuric or nitric acid at a level of at least 2 wt.%, preferably up to 3 wt.%, and finally air milling the expanded graphite. The expanded graphite has an initial degree of expansion (i.e., prior to grinding) that is 125 times greater than its initial volume.

WO99/34673 discloses an electrochemical cell having a cathode comprising expanded graphite as an electrically conductive material. It is essential that the expanded graphite is produced by treating flake graphite with an expandable intercalation compound, thereby employing the intercalation compound in an amount of at least 2 wt% (preferably up to 3 wt%), expanding the treated graphite at an elevated temperature, and finally grinding and crushing the expanded graphite to separate the particles of thermally expanded graphite to obtain expanded graphite crystals having a cup-shaped or baseball glove-shaped structure. This cup-shaped or baseball glove-shaped structure is a feature of the invention described in WO 99/34673.

The expanded graphite is an existing material. To form the expanded graphite, it is preferred to treat the naturally purified graphite flakes at elevated temperatures, optionally by vacuum impregnation, for example with sulfuric acid (H)₂SO₄) And hydrogen peroxide (H)₂O₂) Until these compounds penetrate between the graphite layers and are correspondingly incorporated into the graphite flakes of the graphite crystal structure. After filtering and washing the intercalated graphite, the acid treated graphite is heated under an inert gas atmosphere at a temperature above the decomposition temperature of the intercalation compound, typically above 700 c, preferably above about 1000 c, to obtain an expanded or exfoliated graphite material. The expanded graphite product is then milled to obtain its final particle size distribution.

The expanded graphite has a disadvantage in its difficult workability and processability, compared to conventional high crystalline artificial and natural graphite in conductive materials, and in particular, when it is mixed with an electroactive component of a cathode, it has reduced lubricity and oxidation resistance. The reduced lubrication performance leads to increased tool wear during the cathode production process. Oxidation of the expanded graphite, and hence the manganese dioxide in the cathode, results in self-discharge and reduces the shelf life of the battery containing the expanded graphite. To overcome these problems and at the same time to exploit the advantages of expanded graphite, particularly in terms of electrical conductivity, a potential solution is to use a mixture of expanded graphite and conventional graphite as an electrically conductive additive.

The replacement of a portion of the expanded graphite by the unexpanded graphite results in a decrease in the conductivity of the conductive additive. We have now found that this disadvantage in the mixture can be significantly reduced or even eliminated if a particular form of expanded graphite, i.e. the vermicular form of thermally expanded graphite, is used. The expression "thermally expanded graphite in vermicular form" or "vermicular expanded graphite" as used herein refers to the form of the vermicular corresponding morphology of expanded graphite obtained directly after thermal expansion. In particular, it shows that the natural form of the vermicular expanded graphite obtained directly after thermal expansion is not or has not been further treated by any mechanical forces (e.g. shear forces) as this would destroy the natural vermicular morphology. This means that, for example, to reduce the scott density, the vermicular form of natural exfoliated graphite may be milled using shear forces which do not alter or disrupt the vermicular morphology, for example using autogenous milling methods. The thermally expanded graphite, which is fully expanded in its crystalline c-axis in its original z-dimension, has a vermicular morphology, i.e., an accordion or vermicular structure. The expanded graphite in its vermicular form employed in the present invention may have different average particle sizes. If graphite flakes having a small particle size expand, the expanded graphite will have a small particle size; if the graphite flake having the larger particle size is expanded, the expanded graphite will have the larger particle size. But both particle sizes have good properties in the application according to the invention. However, the preferred values given herein are preferred.

It is worth mentioning that neither particle size nor particle shape indicates the presence of graphite in a vermicular morphology. The structure of the expanded graphite clearly indicates a vermicular morphology. In the case of highly anisotropic materials such as expanded vermicular graphite, there is a high deviation of the particle size distribution measured by laser diffraction from the true particle size, since this method is based on spherical particles. The performance enhancement of the vermicular expanded graphite is only obtained when the expanded graphite exhibits a structure typical for the vermicular morphology, as compared to other forms of expanded graphite. The expanded graphite in its vermicular form may be determined by the degree of expansion of the graphite starting material in the direction perpendicular to the crystalline c-direction of the graphite layers. The thermal expansion results in a significant increase in the z-dimension of the graphite particles perpendicular to the plane of the graphite particles. In general, this expansion of the vermicular form giving the folded morphology in the crystalline c-direction leads to a significant reduction in bulk density measured in terms of scott density and a significant increase in BET specific surface area.

The key features for the vermicular form of the expanded graphite in the present invention are: (i) the initial expansion ratio of the expanded graphite; and (ii) the expanded graphite in its vermicular form is not destroyed by post-processing, for example, grinding and/or crushing with shear forces can destroy the vermicular form.

It has been found that the initial degree of particle expansion required for the formation of the vermicular morphology is at least 80 times the z-dimension of the non-expanded graphite flakes. Preferably, the expanded graphite sheet is initially expanded in the z-direction to a degree in the range of 200 to 500 times its initial z-dimension.

Expanded graphite in its vermicular form is known per se and is described, for example, in U.S. patent 3323869, U.S. patent 3398964, U.S. patent 3404061 and U.S. patent 3494382, the contents of which are incorporated herein by reference.

Disclosure of Invention

The invention is defined in the claims. The invention relates to an electrochemical cell having a positive electrode comprising Electrolytic Manganese Dioxide (EMD), Chemical Manganese Dioxide (CMD) or lithiated cobaltates, manganates or nickelates as electroactive component and graphite as conductive additive, characterized in that the conductive additive comprises at least thermally expanded graphite in its vermicular form, wherein the initial particle expansion of the expanded graphite in the z-direction is more than 80 times its initial z-dimension, preferably in the range of 200 to 500 times the z-dimension of the initial non-expanded graphite particles.

The electrochemical cell is preferably an alkaline zinc manganese dioxide cell having a positive electrode comprising electrolytic manganese dioxide and/or chemical manganese dioxide, preferably electrolytic manganese dioxide.

Preferably, the expanded graphite has an initial degree of particle expansion in the z-direction which is greater than 300 times the z-dimension of the initial non-exfoliated graphite flakes, preferably in the range of from 300 to 500 times its initial z-dimension, preferably greater than 400 times its initial z-dimension, preferably in the range of from 400 to 500 times its initial z-dimension.

For graphite particle expansion in the z-direction of 200 to 500 times the z-dimension of the original graphite flake, graphite particles having a particle size in the range of 0.04 and 0.002g/cm are obtained³A Scott density of between 25 and 55m²A BET specific surface area of between/g of a vermicular expanded graphite material. For graphite particle expansion in the z-direction of 300 to 500 times the z-dimension of the original graphite flake, graphite particles having a particle size in the range of 0.02 and 0.002g/cm are obtained³And a Scott density between 35 and 55m²A BET specific surface area of between/g of a vermicular expanded graphite material. For graphite particle expansion between 200 and 400 times the initial z-dimension, 0.04 and 0.005g/cm are observed³Scott density in between and between 25 and 45m²BET specific surface area between/g. For graphite particle expansion at 80 times the original z-dimension, an expansion at 0.05g/cm is obtained³The following Scott Density and 20m²BET value of > g.

The conductive additive may include expanded graphite in its vermicular form, either as a 100% conductive material or as a binding additive in a graphite conductive material. If expanded graphite is used as the graphite additive in the conductive mass such that the graphite component comprises a graphite/vermicular expanded graphite mixture, the graphite is preferably an artificial or natural flake graphite powder having a highly anisometric particle shape, as is known for use as the graphite binder component in alkaline zinc manganese dioxide cells.

The invention also relates to a composition comprising electrolytic manganese dioxide as an electroactive component and graphite as an electrically conductive additive, wherein the electrically conductive additive comprises at least a thermally expanded graphite in its vermicular form having an initial degree of particle expansion in the z-direction of the particles of greater than 80 times its initial z-dimension, preferably in the range of 200 to 500 times its initial z-dimension. Preferably, the expanded particles in this composition have a scott density as described above.

The invention also relates to a method for manufacturing said composition.

Drawings

Fig. 1 shows that the resistivity of the cathode material (rings) decreases linearly with increasing content of the expanded graphite mixed in the graphite conductive additive.

Figure 2 shows the essentially linear relationship between the flexural strength of the cathode ring and the fraction of expanded graphite incorporated into the graphite conductive additive as a function of the manganese dioxide/graphite cathode mixture.

Fig. 3 and 3A show scanning electron micrographs of the worm-like modified expanded graphite.

Figure 4 shows the flexural strength of a cathode material comprising a conductive mixture containing 20 wt% expanded graphite. These mixtures were obtained using two different mixing conditions.

Figure 5 shows the resistivity of a cathode material containing EMD and a conductive additive.

Figure 6 depicts the flexural strength of a cathode ring comprising EMD and a conductive additive.

Fig. 7 depicts the resistivity of a cathode material comprising EMD and a conductive additive.

Fig. 8 schematically shows three basic possibilities for mixing expanded graphite and graphite or expanded graphite, graphite and electrolytic manganese dioxide.

Detailed Description

The expanded graphite in its vermicular form preferably has a density of 0.05g/cm³The following Scott density is preferably 0.002g/cm³-0.04g/cm³In the range of (1), preferably 0.005g/cm³-0.04g/cm³In the range of (1), preferably 0.002g/cm³-0.02g/cm³Within the range of (1). Preferably, the expanded graphite in its vermicular form is composed of coarse vermicular particles to be equally effective as a reinforcing material.

Scott density measurement is a standard method for characterizing the apparent density of powdered materials (reference: ASTM B329). The scott density was determined by subjecting the dried carbon powder to a scott volume meter. The powder was collected at 1 inch³(equivalent to 16.39 cm)³) To an accuracy of 0.1 mg. The ratio of weight to volume corresponds to the scott density. For characterizing small-grained graphite, the scott density is a parameter that implicitly describes the particle size and the degree of particle anisotropy. The particle size distribution determined by laser diffraction as described above cannot be employed as a method for characterizing expanded graphite, and is therefore not given here. To characterize the use of expanded natural graphite in cathode materials, the more relevant material parameters are the scott density and the BET specific surface area.

The vermicular graphite is an expanded graphite that has been expanded in the z-direction of the graphite particles by at least about 80 times, preferably more than 200 times, its original z-dimension. More preferred values are given above.

The expanded vermicular graphite used according to the invention preferably has a BET value of at least 20m²A/g or higher, preferably higher than 25m²In g, preferably above 35m²In g, preferably above 40m²A/g, preferably higher than 45m²/g。

The Scott density of the vermicular expanded graphite used according to the invention is 0.05g/cm³Preferably less than 0.04g/cm³Preferably less than 0.02g/cm³Preferably less than 0.005g/cm³Especially at 0.002g/cm³And 0.04g/cm³And preferably between 0.005 and 0.04g/cm³Preferably from 0.002g/cm³-0.02g/cm³Within the range of (1).

It has been shown that conductivity measurements, if only a portion of the expanded (non-vermicular) graphite is used in the conductive additive consisting essentially of conventional graphite, the resistivity of the cathode ring decreases linearly with increasing content of the expanded graphite mixed in the graphite conductive additive, as shown in figure 1.

The flexural strength of the cathode rings was measured in Newton N for the mechanical stability of graphite and expanded (non-vermicular) graphite. The inventors have discovered a relationship between the flexural strength of the manganese dioxide/graphite cathode mixture and the fraction of expanded graphite incorporated with the graphite conductive additive, as shown in figure 2.

We have found that surprisingly low resistivity and surprisingly high mechanical stability can be obtained if expanded graphite in its vermicular form is used instead of conventional expanded and milled graphite, compared to the values given in fig. 1 and 2. The vermicular form can be used both as a 100% conductive additive in the cathode and as an additive to the conductive mass of graphite, i.e., a conductive additive including conventional artificial or natural graphite as well as expanded graphite having a vermicular morphology.

A surprising improvement in the above properties can be obtained if the expanded graphite in its vermicular form is able to remain stable in the cathode ring. Expanded graphite in its vermicular form is known per se. It is an extremely two-dimensional form of expanded graphite, exhibiting a typical folded structure, as represented in the SEM picture of fig. 3.

We have further found that when graphite is employed in its vermicular form, and the vermicular graphite particles expand in the z-direction to at least about 80 times (preferably more than 200 times) their original size and correspondingly have a density of less than 0.05g/cm³At scott density values of (a), a dramatic change in the linear trend of resistivity towards lower values and mechanical stability towards higher values occurredThe change in linear trend. Figure 4 shows the non-linear increase in flexural strength of the cathode rings when the scott density of the expanded graphite in the graphite/expanded graphite conductive mixture is decreased (increasing the scott density ratio in figure 4).

Figure 5 shows the non-linear decrease in cathode resistivity when the scott density of the expanded graphite in the graphite/expanded graphite conductive mixture is decreased (increasing the scott density ratio in figure 5). During the scott density reduction, the expanded graphite converts to its vermicular morphology, resulting in these improvements in cathode performance.

When the conventional graphite in the cathode is continuously replaced by vermicular expanded graphite with a scott density within the defined values (up to 100%), the flexural strength of the cathode ring is increased to be stronger (fig. 6). The resistivity of the cathode rings decreased more significantly when the conventional graphite in the cathode was continuously replaced by vermicular expanded graphite having a scott density within the defined values (up to 100%) (fig. 7).

In the production of expanded graphite, expanded graphite is subjected regularly to a grinding treatment with high shear force. The product obtained after the grinding treatment has almost no worm-like morphology. We have found that if the expanded graphite is mechanically treated in a milling process, the expanded graphite in its vermicular form is retained provided that no high shear and/or vibrational forces (corresponding to sufficiently low forces) are applied to the expanded natural graphite.

The content of the vermicular expanded graphite added as the conductive graphite substance or as part of the conductive graphite substance is preferably in the range of 5 to 100 wt.%, preferably in the range of 10 to 50 wt.%. The most preferred range is 10-30 wt.%, i.e., the electrically conductive graphite material is comprised of conventional graphite and vermicular expanded graphite, wherein the weight ratio of conventional graphite to vermicular expanded graphite is 95: 0 to 5: 100, preferably 90: 50 to 10: 50, more preferably 90: 70 to 10: 30. This preferred ratio combines the advantages of both graphite and expanded graphite having its vermicular morphology, which improves the mechanical stability and conductivity of the cathode ring comprising a high ratio of electrolytic manganese dioxide to graphite, with a graphite content of 7 wt% or less, in the battery cathode, especially in batteries with high energy density. In addition to this performance advantage, it also provides a cost-effective system since only small amounts of vermicular expanded graphite are required to achieve these advantages.

The content of the conductive additive, including at least expanded graphite in its vermicular form, is preferably below 7 wt%, preferably in the range of 1-6 wt%, preferably in the range of 2-5 wt%, calculated on the total weight of the cathode components, i.e. on the total weight of electrolytic manganese dioxide as electroactive component and graphite material as conductive additive. Electrolytic manganese dioxide as the electroactive component in alkaline zinc manganese dioxide cells is known per se and these prior forms are also used in the present invention.

A significant effect in flexural strength is seen with the incorporation of greater than 30 wt% of the expanded (non-vermicular) graphite component in conventional graphite conductive additives, as compared to the use of vermicular expanded graphite. The beneficial effects of vermicular expanded graphite in terms of cathode ring flexural strength are clearly seen in conductive materials of graphite having a vermicular expanded graphite content of less than 30 wt%.

The vermicular expanded graphite may be prepared by existing methods, for example, by treating natural or artificial graphite flakes, coke or anthracite-based carbon having an average particle size between 10 μm (microns) and 10mm with concentrated sulfuric acid at a temperature between room temperature and 200 ℃. Perchloric acid, hydrogen peroxide, ammonium peroxodisulfate or fuming nitric acid can also be used as the oxidizing agent. This treatment forms a graphite oxide salt with intercalated molecules (e.g., sulfate ions) between the graphite layers of the graphite crystal structure. Other intercalating agents, such as fuming nitric acid, nitric oxide or bromine, may also be employed. The graphite salt was filtered off, the intercalation liquid was washed thoroughly with water to remove traces of intercalating agent, and dried. The graphite salt is then subjected to a thermal shock treatment at a temperature between 400 ℃ and 1200 ℃ to obtain exfoliated graphite.

The intercalation and exfoliation conditions of the manufacturing process were optimized for the electrochemical performance and mechanical stability of alkaline cell cathode rings containing vermicular expanded graphite. The optimum conditions were to treat natural graphite flakes having an average particle size of between 100 microns and 1mm with fuming nitric acid (100%), nitric oxide gas (NOx) or sulfuric acid mixed with fuming nitric acid (5-30%), hydrogen peroxide (30% aqueous solution, 5-40 wt%) or an equivalent amount of ammonium peroxodisulfate.

The amount of intercalant in the graphite flake prior to expansion is preferably at least 5 wt.%, preferably at least 8 wt.%, and most preferably 10 wt.%, calculated with respect to the graphite flake. Most preferably, the content is in the range of 10-20 wt.%, calculated with respect to the graphite flakes.

The insertion temperature of the insertion process is room temperature, optionally with vacuum. The use of high temperatures between 50-120 c can accelerate the insertion process. After isolating the intercalated graphite salts by filtration and subsequent washing and drying, the graphite is exfoliated by thermal shock treatment at a temperature of at least 900 ℃, preferably about 1000 ℃. During this heat treatment, the desired results are achieved with short treatment times of the exfoliation treatment of less than one second, particularly with respect to the electrical conductivity of the electrolytic manganese dioxide/expanded vermicular graphite/graphite mixture.

The invention also relates to a process for the manufacture of thermally expandable graphite in its vermicular form having an initial degree of expansion of the graphite particles in the z-direction of the particles which is greater than 80 times its initial z-dimension, preferably in the range of 200-fold 500 times its initial z-dimension, optionally as a mixture with non-expanded graphite, for the production of a positive electrode for a battery having a positive electrode containing Electrolytic Manganese Dioxide (EMD), Chemical Manganese Dioxide (CMD) or lithiated cobaltates, manganates or nickelates, in particular for an alkaline zinc manganese dioxide battery, characterized in that: (i) natural graphite flakes having an average particle size between 100 microns and 1mm are treated with an intercalant and the content of the intercalant in the graphite flakes before expansion is preferably at least 5 wt%, preferably at least 8 wt%, more preferably 10 wt%, most preferably in the range of 10-20 wt%, calculated on the graphite flakes; (ii) isolating and then washing and drying the intercalated graphite; (iii) the thermal vibration treatment is carried out at a temperature of at least 900 ℃, preferably at a temperature of about 1000 ℃, to exfoliate the graphite, wherein the treatment time for the exfoliation treatment is below one second. Natural graphite flakes having an average particle size in the range of about 150-250 microns are preferably used as the starting material.

As the intercalating agent, fuming nitric acid (100%), nitrogen oxide gas (NOx) or sulfuric acid mixed with fuming nitric acid (5-30%), hydrogen peroxide (30% aqueous solution, 5-40 wt%) or an equivalent amount of ammonium peroxodisulfate is preferably used.

The thermally expanded graphite thus obtained in its vermicular form generally has a mass fraction of between 0.05g/cm³The following Scott density, in particular at 0.002g/cm³And 0.04g/cm³Preferably between 0.005 and 0.04g/cm³Wherein is 0.05g/cm³The following scott densities correspond to particle expansion degrees of 80 times in the z-dimension; at 0.002g/cm³And 0.04g/cm³The scott density in between corresponds to a particle expansion degree of 500-; at 0.005g/cm³And 0.04g/cm³The scott density in between corresponds to a particle expansion of 400-200 times the z-dimension.

After heat treatment, the raw exfoliated graphite material is preferably employed in its natural state. It is also permissible to grind natural exfoliated graphite in the following manner: the shear or vibration forces applied do not alter or disrupt the vermicular, pleated, or thread-like structure. Under such conditions, the vermicular natural graphite may be preferably milled using a self-milling (autogeneous milling) method, thereby improving the handling capacity of the cotton-like material. Self-grinding is performed in order to avoid in this way high shear and vibration forces, which are mainly exerted when mechanical grinding methods are used. Mechanical milling methods tend to disrupt the worm-like morphology. Therefore, to avoid damaging the materialThe proper milling conditions for the exfoliated graphite are critical. Preferably, the self-milling is carried out to obtain less than 0.05g/cm³The scott density of (a).

Also, the type of mixing method used to mix expanded vermicular graphite in a graphite conductive material must stabilize the vermicular morphology. Only by providing an optimized grinding and mixing process will the expanded graphite in its vermicular form be stabilized. A problem with mixing expanded graphite with graphite or electrolytic manganese dioxide is that the differences in scott densities of the components make it difficult to form a homogeneous mixture. To overcome this problem of the prior art methods, high energy is used in the cathode ring manufacturing process to mix the components together. These high mixing energies lead to a reduction in the properties of the expanded graphite, especially in terms of the mechanical stability of the cathode rings, especially when high shear forces are involved.

Figure 4 shows the flexural strength of a cathode material comprising a conductive mixture containing 20 wt% expanded graphite. These mixtures were obtained using two different mixing conditions. Method 1 mixes graphite with expanded graphite primarily using gravity (i.e., mix type 3). Method 2 (i.e., mixing type 1 or 2) applies mainly shear force. It is apparent from the curves that the increase in flexural strength after the transition of expanded natural graphite in vermicular form can only be obtained by method 1. Method 2 appears to disrupt the vermicular form of the expanded graphite so that the flexural strength of the cathode ring stays within the range obtained for non-vermicular forms of expanded graphite, even at the low scott densities of the graphite/expanded graphite mixture. It is apparent that high shear force or vibration force tends to destroy the folded structure of the vermicular expanded graphite during the mixing of the vermicular expanded graphite with conventional graphite and with electrolytic manganese dioxide in the manufacturing process of the cathode material for alkaline batteries.

Fig. 8 schematically shows three basic possibilities for mixing expanded graphite with graphite or expanded graphite, graphite and electrolytic manganese dioxide:

type 1: blenders that employ shear stress as a mixing principle (e.g., blade blenders, propeller blenders with single or multiple blades/propellers); an example is given of a single propeller mixer.

Type 2: a blender that combines shear stress and gravity; an example is given of an inclined drum mixer with a twin propeller system rotating in opposite drum rotations.

Type 3: a stirrer which uses gravity as a rotation motion of a mixing chamber of a mixing principle; an example is given of a single-shaft rotary drum mixer. These blender types also include a more complex action cylindrical mixing chamber.

The use of a type 1 blender is not recommended. Type 1 blenders do not readily allow for uniform mixing of the vermicular expanded graphite and graphite due to the different apparent densities of the powders. Conductive materials prepared by such mixing methods often do not achieve reproducible results due to the non-uniformity of the mixture. In addition, the accordion-like structure of the vermicular expanded graphite is destroyed after the mixing step.

Good results were obtained with a type 2 blender that employed a combination of shear and gravity to mix the expanded graphite and graphite. Improved flexural strength values are obtained, especially with graphite components having a low apparent density. It was observed that the lower the apparent density of the graphite component, the greater its ability to be mixed with the expanded graphite, and the less damage in the accordion-like structure of the expanded graphite.

The best results were obtained with a type 3 blender that uses only gravity to mix expanded graphite with other graphite. In this process, the absence of shear forces is complete, minimizing damage to the vermicular expanded natural graphite, and achieving higher mechanical strength of the cathode ring. In order to effectively mix the vermicular expanded graphite with the graphite, the mixing chamber of this system should not be filled to more than 50% of its volume.

Fig. 1 shows that the resistivity of the cathode material (rings) decreases linearly with increasing content of the expanded graphite mixed in the graphite conductive additive. Utensil for cleaning buttockIn terms of volume, fig. 1 shows that the graphite particles contain expanded graphite (scott density: 0.037 g/cm)³BET specific surface area of 25m²/g) anisometric, non-exfoliated graphite [ d ] mixed in different proportions₅₀9 μm (micrometers), scott density 0.063g/cm³BET specific surface area of 8m²/g]The resistivity of the cathode material. It should be understood that the BET values and scott densities used herein are related as follows:

expansion ratio	BET value, m2/g	Scott Density, g/cm3
			80	20	0.05
200	25	0.04
			300	35	0.02
400	45	0.005
			500	55	0.002

Figure 2 shows the essentially linear relationship between the flexural strength of the cathode ring and the fraction of expanded graphite incorporated into the graphite conductive additive as a function of the manganese dioxide/graphite cathode mixture. Specifically, fig. 2 shows a composition containing expanded graphite (scott density of 0.037 g/cm)³BET specific surface area of 25m²/g) anisometric, non-exfoliated graphite [ d ] mixed in different proportions₅₀9 μm (micrometers), scott density 0.063g/cm³BET specific surface area of 8m²/g]The flexural strength of the cathode ring of (1).

Fig. 3 and 3A show scanning electron micrographs of the worm-like modified expanded graphite.

Figure 4 shows the flexural strength of a cathode material comprising a conductive mixture containing 20 wt% expanded graphite. These mixtures were obtained using two different mixing conditions. Method 1 (i.e., mixing type 3) mixes graphite with expanded graphite mainly using gravity, resulting in a mixture of graphite and expanded graphite according to the present invention. In the case of method 2 (i.e., blend type 1 or 2), the flexural strength was not improved with a decrease in the Scott density of the expanded graphite (increase in the Scott density ratio) as in method 1Is obvious. This indicates that the vermicular morphology of the expanded graphite is disrupted. Method 2 primarily applies shear forces to obtain a mixture of graphite and expanded (non-vermicular) graphite. Specifically, fig. 4 shows the flexural strength of a cathode ring containing EMD and a conductive additive having the following composition: 80% non-exfoliated graphite [ d ]₅₀9 μm (micrometers), scott density 0.063g/cm³BET specific surface area of 8m²/g]And 20% of different expanded graphite distinguished by scott density. The X-axis corresponds to the ratio: graphite scott density/expanded graphite scott density. The graphite components in the mixture are generally the same, but different expanded graphite is used to prepare the conductive mixture. Two mixing methods were used. Shear forces are avoided in process 1 and process 2 primarily employs shear forces to mix the graphite and expanded graphite components.

Fig. 5 shows the resistivity of a cathode material containing EMD and a conductive additive having the following composition: 80% anisotropic non-exfoliated graphite [ d₅₀9 μm (micrometers), scott density 0.063g/cm³BET specific surface area of 8m²/g]And 20% of different expanded graphite distinguished by scott density. The X-axis corresponds to the ratio: graphite scott density/scott density of expanded graphite. The graphite components in the mixture are generally the same, but different expanded graphites are used to prepare the conductive mixture. The scott density of the expanded graphite decreases along the x-axis. Two mixing methods were used. Shear forces are avoided in process 1 and process 2 primarily employs shear forces to mix the graphite and expanded graphite components.

Figure 6 depicts the flexural strength of a cathode ring comprising EMD and a conductive additive. The conductive additive is prepared from expanded graphite (Scott density 0.0037 g/cm) in different ratios³BET specific surface area of 25m²(scott density ═ 0.009 g/cm) or vermicular expanded graphite³BET specific surface area of 56m²Per g) with conventional highly crystalline graphite (d)₅₀9 μm, scott density 0.063g/cm³BET specific surface area of 8m²,/g) is formed.

Fig. 7 depicts the resistivity of a cathode material comprising EMD and a conductive additive. The conductive additive is prepared from expanded graphite (Scott density 0.0037 g/cm) in different ratios³BET specific surface area of 25m²(scott density ═ 0.009 g/cm) or vermicular expanded graphite³BET specific surface area of 56m²Per g) with conventional highly crystalline graphite (d)₅₀9 μm, scott density 0.063g/cm³BET specific surface area of 8m²,/g) is formed.

Fig. 8 schematically shows three basic possibilities for mixing expanded graphite and graphite or expanded graphite, graphite and electrolytic manganese dioxide. In particular, fig. 8 shows a schematic diagram of the 3 basic mixing principles employed in the mixing method. In order to avoid damage to the expanded graphite in its vermicular form during the mixing process with graphite and electrolytic manganese dioxide, type 2 or type 3 should be used.

Test section

Measurement of flexural Strength

94% Electrolytic Manganese Dioxide (EMD) (TOSOHMK97, stored in an atmosphere with constant humidity of 65% r.h.) and 6% graphite component were mixed in a turbo blender. 3 rings having an outer diameter of 24.3mm, an inner diameter of 16.0mm and a length of 1cm were pressurized, each graphite sample being at a pressure of 3t/cm². These rings were broken in ERICHSEN PA010 in Newton N]The flexural strength of the rings was measured. Measurements of other similar compositions were made in the same manner.

Measurement of resistivity

94% Electrolytic Manganese Dioxide (EMD) (TOSOHMK97, stored in an atmosphere with a constant humidity of 65%) and 6% graphite component were mixed in a turbo blender. Using 3t/cm²A sample in the form of a rectangle (10 cm. times.1 cm) was pressurized. The resistivity, m Ω cm, was measured using a 4-point measurement technique.

Preparation of mixtures of expanded graphite with graphite and Electrolytic Manganese Dioxide (EMD)

The structure and particle configuration of the expanded graphite is maintained by mixing different types of expanded graphite into different artificial and natural graphites via type 3 (otherwise indicated). The mixture of expanded graphite and graphite was then mixed with EMD to form a cathode material, which was pressed into an alkaline cell ring.

Graphite

The artificial graphite is produced by graphitizing a carbon precursor under graphitization conditions, followed by grinding into a suitable particle size distribution. The obtained artificial graphite has ash content of less than 0.1%, high crystallinity (c/2 is 0.3354-0.3356nm, Lc is 50-1000nm, and xylolene density is 2.25-2.27 g/cm)³). The particle size distribution of the material considered has a d between 3 and 50 microns₅₀The value (MALVERN), BET specific surface area, is between 1 and 20m²Between/g.

Natural graphite ore is purified by flotation and subsequent thermal or chemical purification to an ash content of less than 0.1%, thereby producing natural graphite. The raw graphite is ground to obtain a suitable particle size distribution. The material properties are the same as those of artificial graphite.

Electrolytic Manganese Dioxide (EMD)

The EMD used throughout the experiment showed a mean particle size distribution of 30-40 microns and 4.5g/cm³The bulk density of (a).

Claims (37)

1. An electrochemical cell having a positive electrode comprising electrolytic manganese dioxide, chemical manganese dioxide, lithiated manganates, cobaltates or nickelates as electroactive components and graphite as a conductive additive, characterized in that said conductive additive comprises at least a thermally expanded graphite in its vermicular form, wherein said expanded graphite has an initial particle expansion in the z-direction of the particles of greater than 80 times its initial z-dimension.

2. The electrochemical cell according to claim 1, wherein the expanded graphite has an initial particle expansion in the z-direction of the particles in the range of 200 to 500 times its initial z-dimension.

3. An electrochemical cell according to claim 1, characterized in that the cell is an alkaline zinc manganese dioxide cell having a positive electrode comprising electrolytic manganese dioxide and/or chemical manganese dioxide.

4. The electrochemical cell according to claim 1 or 3, characterized in that the expanded graphite in its vermicular form, in which the raw graphite particles are expanded in their z-direction to more than 300 times their original size.

5. The electrochemical cell according to claim 4, wherein the raw graphite particles expand in their z-direction to greater than 400 times their original size.

6. An electrochemical cell according to claim 4, wherein the raw graphite particles expand in their z-direction to within 300 to 500 times their original z-dimension.

7. An electrochemical cell according to claim 6, wherein the raw graphite particles expand in their z-direction to within 400 to 500 times their original z-dimension.

8. Electrochemical cell according to claim 1 or 3, characterized in that the expanded graphite in vermicular form exhibits a density in the range of 0.002g/cm³To 0.04g/cm³A scott density in the range of (a).

9. The electrochemical cell according to claim 8, wherein the expanded graphite in its vermicular form exhibits a density in the range of 0.005 to 0.04g/cm³A scott density in the range of (a).

10. The electrochemical cell according to claim 8, wherein the vermicular formThe expanded graphite of (2) shows a particle size of from 0.002g/cm³-0.02g/cm³A scott density in the range of (a).

11. An electrochemical cell according to claim 1 or 3, characterized in that the conductive additive comprises expanded graphite in vermicular form as 100% of the conductive substance.

12. An electrochemical cell according to claim 1 or 3, wherein the conductive additive comprises expanded graphite in vermicular form as a binding additive in the graphite conductive mass.

13. An electrochemical cell according to claim 1 or 3, characterized in that the expanded vermicular graphite has a BET value of at least 20m²(ii) a/g or higher.

14. An electrochemical cell according to claim 13, wherein the expanded vermicular graphite has a BET value of greater than 25m²/g。

15. An electrochemical cell according to claim 14, wherein the expanded vermicular graphite has a BET value of greater than 40m²/g。

16. An electrochemical cell according to claim 15, wherein the expanded vermicular graphite has a BET value of greater than 45m²/g。

17. Electrochemical cell according to claim 1 or 3, characterized in that the content of the vermicular expanded graphite added as conductive graphite substance or as part of conductive graphite substance is in the range of 5% to 100% by weight.

18. The electrochemical cell according to claim 17, wherein the content of the vermicular expanded graphite is in the range of 10 to 50% by weight.

19. The electrochemical cell according to claim 18, wherein the content of the vermicular expanded graphite is in the range of 10 to 30% by weight.

20. An electrochemical cell according to claim 1 or 3, characterized in that the content of the conductive additive comprising at least expanded graphite in its vermicular form is below 7% by weight, calculated with respect to the total weight of electrolytic manganese dioxide as the electroactive component and graphite material as the conductive additive component.

21. The electrochemical cell according to claim 20, wherein the content of the electrical additive is in the range of 1-6% by weight.

22. The electrochemical cell as recited in claim 21, wherein the content of the electrical additive is in the range of 2-5% by weight.

23. Electrochemical cell according to claim 1 or 3, characterized in that the natural exfoliated graphite is ground with shear forces that do not alter or destroy the vermicular morphology, thereby obtaining less than 0.05g/cm³The scott density of (a).

24. The electrochemical cell according to claim 23, wherein the natural exfoliated graphite is milled using a self-milling process.

25. A method of manufacturing a thermally expanded graphite in its vermicular form having an initial degree of expansion of the graphite particles in the z-direction of the particles greater than 80 times its initial z-dimension for use in manufacturing a positive electrode for a battery according to any one of claims 1 to 24, characterized in that: (i) natural graphite flakes having an average particle size of between 100 microns and 1mm are treated with an intercalant and the intercalant content in the graphite flakes prior to expansion is at least 5% by weight, calculated with respect to the weight of the graphite flakes; (ii) isolating and then washing and drying the intercalated graphite; (iii) performing a thermal vibration treatment at a temperature of at least 900 ℃ to exfoliate the graphite, wherein a treatment time for the exfoliation treatment is below one second.

26. The method as set forth in claim 25 wherein the initial degree of expansion of the thermally expanded graphite particles in the z-direction of the particles in the vermicular form is in the range of 200-500 times its initial z-dimension.

27. The method of claim 25, wherein the thermally expandable graphite is in a mixture with non-expandable graphite.

28. The method according to claim 25, wherein said intercalating agent is present in an amount of at least 8% by weight.

29. The method according to claim 28, wherein said intercalating agent is present in an amount of at least 10% by weight.

30. The method according to claim 29, wherein the content of the intercalating agent is in the range of 10-20% by weight.

31. The method of claim 25, wherein the vibrational heat treatment is performed at a temperature of about 1000 ℃.

32. A method according to claim 25, characterized in that the intercalant is 100% fuming nitric acid, nitric oxide gas, NOx.

33. A method according to claim 25, wherein the intercalating agent is sulfuric acid mixed with 5-30% fuming nitric acid, hydrogen peroxide, which is a 30% aqueous solution, 5-40% by weight, or an equivalent amount of ammonium peroxodisulfate.

34. The method according to any one of claims 25 to 33, characterized in that the obtained heat expanded natural graphite in vermicular form has a density of between 0.05g/cm³The following scott densities.

35. The method of claim 34, wherein the thermally expanded natural graphite has a particle size of 0.002g/cm³And 0.04g/cm³Scott density in between.

36. The method according to claim 35, wherein the thermally expanded natural graphite has a density of between 0.005 and 0.04g/cm³Scott density in between.

37. A process according to any one of claims 25 to 33, characterised in that the expanded graphite in vermicular form is milled by a self-milling process until less than 0.05g/cm is obtained³Scott density values of.