The invention relates to an electrode for a lead-acid battery.

The invention also relates to a lead-acid battery comprising an electrode according to the invention.

The invention further relates to a method of manufacturing an electrode according to the invention.

Lead and its alloys have been used as a component in batteries since the mid-19th century. Lead-acid batteries, also known as acid lead-acid batteries, were the first rechargeable power source to be commercially available. It is the most widely used battery type in the world due to its low cost of production and the starter batteries of vehicles equipped with an internal combustion engine are almost exclusively lead-acid batteries.

Lead-acid batteries are relatively simple in design. The basic unit of a lead- acid battery is the battery cell, which contains at least two electrodes immersed in an electrolyte and a casing surrounding them. The electrolyte is usually an aqueous solution of sulphuric acid (H2SO4), in which the electrical charge is carried by positively charged hydrogen and negatively charged sulphate ions. When charged, the active material of the negative electrode (anode) is the metal lead (Pb), while the positive electrode (cathode) is lead dioxide (PbO2). When under load (discharge), the lead-acid battery acts as a source. The orderly movement of ions takes place in the electrolyte and electrons, i.e. current, appears on the electrodes and the load. On discharge, the active material of both electrodes is gradually converted into lead sulphate (PbSO4) and the electrolyte concentration is reduced. As the battery charges, the lead sulphate at the anode is converted back to lead and at the cathode to lead dioxide.

Since the volume of lead sulphate formed during discharge is larger than that of lead or lead dioxide, the volume of active material in the electrodes increases when loaded and decreases when charged. The charge-discharge cycles therefore mechanically stress the electrodes, weakening their structure. This results in a so- called mass shedding, which reduces the charge storage capacity of the lead-acid battery and can also cause a short circuit by accumulating at the bottom of the battery.

However, the biggest disadvantage of lead-acid batteries is their high mass relative to the amount of electrical energy stored in them, i.e. the specific capacity of current lead-acid batteries is low. This is because the electrodes of known lead- acid batteries are made entirely of lead (or its alloys) or lead dioxide, depending on their polarity. However, it is known that lead is a heavy metal with a high density. Its high mass is a clear disadvantage for use in cars and trucks, and is particularly disadvantageous for cheaper electric vehicles where lithium batteries are intended to be replaced by lead batteries for cost-efficiency reasons. In the latter case, the clear aim is to minimise weight and maximise capacity in order to achieve long range and efficient vehicle operation. It should be noted that the high mass of lead-acid batteries is a problem not only in their use but also in their manufacture and logistics. The high demand for lead in the production of lead-acid batteries also increases costs. It would therefore be necessary to reduce the weight of lead-acid batteries while maintaining their electrical capacity, and to improve the resistance of the electrodes to charge-discharge cycles.

Lach, Jakub et. al: “Applications of carbon in lead-acid batteries” reviews the applications of different forms of elemental carbon (e.g. graphene) in lead-acid batteries. It is pointed out that the addition of carbon to the material of each electrode can increase the battery lifetime, as the additive promotes the uniform distribution of the lead sulphate formed on the electrode (among other beneficial effects). Hu, Hai-Yan, et al: “Enhanced performance of e-bike motive power lead-acid batteries with graphene as additive to the active mass” describes the effect of graphene nanosheets and reduced graphene oxide additives added to the active mass of negative electrodes of lead-acid batteries. The article also highlights the beneficial effect on sulphate formation. In their experiments they added 0.3 wt% graphene additive to the negative electrode of the battery. US patent no. 3813300 describes a polymer grid used in a lead-acid battery, on which the electrode paste can be applied to form the individual electrodes. The objective of the invention is to replace the material of the grids commonly used in batteries (lead or lead alloy, which significantly increases the weight of the battery, thus reducing its specific capacity) with a lighter and cheaper solution. The material of the grid according to the paper is a polymer, the material qualities of which are only listed by the inventors in an exemplary manner (polyethylene, polypropylene, polystyrene, etc.). In Qaimkhani, Muhammad Atid, et al: “The Experimental Analysis od Lead Acid Battery by Introducing Graphene and Lead Composite”, the composition of lead-graphene grid electrodes is investigated, which are prepared by powder metallurgy. The process uses particles of 1 -100 micrometers, the compression pressure range is about 40- 1600 MPa and the sintering temperature is 229 °C. The percentage of graphene particles in the powder mixture is between 0.5-2% by weight. Other materials, such as binders, may be added to the powder mixture during the process.

The aim of the invention is to create an electrode for a lead-acid battery and a lead-acid battery containing such an electrode, which are free from the disadvantages of the state-of-the-art solutions. We realized that these goals can be achieved by using new materials and by modifying the structural elements of lead- acid batteries.

Graphene is a nanostructured allotropic modification of carbon, a single- atom-thick layer of carbon atoms arranged in a honeycomb lattice. In other views, it can also be seen as an infinitely extended aromatic giant molecule. It can be produced by micromechanical exfoliation of graphite, for example by rubbing it against a polished silicon surface or by tearing it off with adhesive tape. By reapplying adhesive tape to the torn-off layer of graphite and then peeling it off, a thinner and thinner layer, eventually one atom thick, can be obtained. This was the method originally used by the researchers who later won the Nobel Prize. Graphene has a number of extreme properties, for example, the mobility of electrons in ideal graphene can exceed 1 ,000,000 cm2A/s, which corresponds to lossless ballistic conduction at micrometre scale even at room temperature. This means that defect- free graphene does not produce any joule heat when conducting current and has very low resistance, as electrons hardly scatter when passing through it.

Polymers are high molecular weight compounds, without which it is difficult to imagine today's science and equipment manufacturing. Polymers whose macromolecules contain several monomer units are called copolymers. Note that hereafter the terms polymer and copolymer are used synonymously. Most polymers can be formed and deformed at sufficiently high temperatures, but the polymer itself does not change at the molecular level. Some polymers have a number of properties that make them suitable for a wide range of uses and even for replacing metals in certain circumstances. For example, some so-called heat-resistant polymers have a very high melting point of more than 180 or even 200 degrees Celsius. Ethylene tetrafluoroethylene copolymer (ETFE), for example, has high corrosion resistance and strength over a wide temperature range, as well as excellent electrical properties. Like thermoplastics, it is easy to process. ETFE's abrasion resistance, impact resistance and resistance to ionising radiation make it one of the best fluoropolymers. Its mechanical properties are similar to those of fully fluorinated polymers. ETFE achieves UL Class V-0 flame retardancy, - is odourless and nontoxic, - has excellent weathering and ageing resistance, exceptionally high UV transmittance, - has excellent dielectric properties. Its heat resistance is demonstrated by the fact that the wire, when connected to the mains, can be used up to 200°C when insulated with ETFE. The incorporation of a third monomer results in a chemically modified ETFE. Glass-fibre reinforced ETFE is harder, stiffer and has a higher tensile strength than pure ETFE. Poly tetrafluoroethylene (PTFE or Teflon), also a fluoropolymer, is found in a wide range of applications. The presence of a strong carbon-fluorine bond makes it chemically very resistant, but it also has good dielectric properties and an even higher melting point of + 330 degrees Celsius.

We recognized that by replacing the inside of the electrodes with a structural support grid made of a heat-resistant polymer and providing the active material in the coating layer of the support grid, the mass of the electrodes and thus the mass of the entire lead-acid battery can be significantly reduced.

At the same time, we recognised that the use of a polymeric structural support grid not only reduces the mass, but also the amount of active material in the electrode and thus the capacity of the lead-acid battery. Furthermore, we have found that by homogeneously embedding graphene grains in the active material (lead in the case of anode and lead dioxide in the case of cathode) covering the polymer support grid of the electrode, the capacity of the lead-acid battery can be increased, its internal resistance can be significantly reduced (by up to half) and the conductivity and mechanical resistance of the electrodes can be enhanced. The problem is, however, that the effective contact area between graphene and the active material of the electrode (lead or lead dioxide) is inherently small, which negatively affects the electrical capacitance, and the large density difference between graphene and lead makes it difficult to obtain a homogeneous mixture. It is recognised that by using the right size of graphene grains and the right size of lead (or lead oxide) grains in the right concentration, the filling of the grains and thus the contact area between the graphene and lead (or lead oxide) grains can be increased.

We also recognised that if a mixture of graphene and active material with the appropriate particle size is attached to the polymer support grid using a powder metallurgy process, the contact surface between the graphene and the lead (or lead oxide) grains will be even larger, and the capacity of the lead battery can be increased even further. We realized that with powder metallurgy processes it is also possible to mix materials that cannot be mixed with normal alloying and other processes, so for example graphene can be mixed with lead or lead dioxide. In addition, thanks to the powder metallurgy process, the mechanical properties of the graphene-containing coating layer attached to the polymer grid will also be exceptionally good, since graphene essentially prevents the volume changes of the electrodes, thus preventing the electrode from expanding so much that it breaks during the discharge. The electrodes according to the invention are thus able to withstand more charge-discharge cycles.

According to the invention, the task was solved with the help of the electrode according to claim 1 and the lead-acid battery according to claim 9. Furthermore, according to claim 10, the task was solved with a method of manufacturing such an electrode.

The essence of the invention is that the electrode contains a structural support grid made of heat-resistant polymer and a coating layer applied to its surface, which coating layer contains a mixture of graphene grains and lead grains in the case of the anode, and graphene grains and lead dioxide grains in the case of the cathode. In the coating layer according to the invention, the proportion of graphene particles is at least 1 .8 V/V% and the diameter of these graphene particles is between 20-40 pm. The lead (or lead dioxide) particles are made up of at least two size fractions, where the diameter of the lead particles of the first size fraction is between 20-40 pm, and the diameter of the lead particles of the second size fraction is between 60-80 pm.

A further aspect of the invention is that the coating layer is applied to a structural support grid made of a heat-resistant polymer by preparing a homogeneous powder mixture from graphene grains by adding lead or lead dioxide grains, applying at least one layer of the powder mixture to the structural support grid, and then fixing the powder mixture to the structural support grid by a powder metallurgy process, i.e. by applying appropriate pressure and temperature.

Preferred embodiments of the invention are defined in the dependent claims.

Further details of the invention will be explained with the help of a drawing using examples. In the drawing:

Figure 1 a is a schematic cross-sectional view of a negative electrode according to the invention;

Figure 1 b is a schematic cross-sectional view of a positive electrode according to the invention;

Figure 2 is a schematic front view of an exemplary embodiment of a structural support grid according to the invention;

Figure 3 is a schematic cross-sectional view of a lead-acid battery according to the invention.

Figure 1 a is a schematic cross-sectional view of a negative electrode 10a and Figure 1 b is a schematic cross-sectional view of a positive electrode 10b according to the invention. In the context of the present invention, the negative electrode 10a is understood to be the negative plate (cathode) of a lead-acid battery 100, the active material of which is lead (Pb), and the positive electrode 10b is understood to be the positive plate (anode) of the lead-acid battery 100, the active material of which is lead oxide (PbO2), as is known to the person skilled in the art. The electrodes 10a, 10b comprise a structural grid 20 made of a heat resistant polymer and a coating layer 30 applied to the grid 20. As used herein, a heat resistant polymer is understood to be a polymer having a high melting point, preferably having a melting temperature of at least 200 degrees Celsius. In one possible embodiment, the support grid 20 is made entirely of polytetrafluoroethylene, i.e. Teflon, which has excellent thermal resistance properties and mechanical stability equivalent to the lead-acid battery 100 plates. It is noted that the support grid 20 may be made of several different types of thermally resistant polymers, layered on top of each other. In the embodiment shown in Figure 1 a, for example, the support grid 20 comprises an inner portion 20a formed as a grid of ethylene tetrafluoroethylene and an outer portion 20b of - 1 - polytetrafluoroethylene. The advantage of this embodiment is that ethylene tetrafluoroethylene can be produced at a lower cost than Teflon. The lower heat resistance of ethylene tetrafluoroethylene is compensated by the Teflon coating. The thickness of the ethylene tetrafluoroethylene grid is preferably 1 .9-2.1 mm, and the thickness of the polytetrafluoroethylene coating is preferably 0.3-0.5 mm. The support grid 20 can be made of other heat resistant polymers or polymers such as other fluoropolymers. The structural stability and resistance to mechanical stresses of the support grid 20 may be enhanced by adding reinforcing fibres, such as glass fibre, to the polymeric material of the support grid 20. In an exemplary embodiment, therefore, the ethylene tetrafluoroethylene in the support grid 20 is reinforced with glass fibers.

A possible embodiment of the support grid 20 is shown in Figure 2, where the support grid 20 is configured as an openwork square grid with "holes". The openings in the square grid are preferably rectangles that are 3 mm high and 4 mm wide, but the openings may of course be of other sizes or shapes than rectangular. It is noted that in the context of the present description, the term "grid" is interpreted broadly and does not necessarily mean a square grid, and in fact, the grid 20 does not necessarily have to be openwork, i.e. , for example, a flat plate or a plate with a ribbed surface is also considered a grid in the sense of the invention.

The electrode 10a, 10b according to the invention has a support grid 20 made of a heat resistant polymer having a coating layer 30. Depending on the polarity of the electrodes 10a, 10b, the composition of the coating layer 30 is different as follows.

Coating layer 30a of the negative electrode 10a (cathode) comprises a mixture of graphene grains and lead grains, wherein the coating layer 30a having a graphene grain content of 1.8 V/V% or more and a graphene grain diameter of between 20 and 40 pm, and said lead grains are formed of at least two size fractions; a first size fraction and a second size fraction, wherein the lead grains of the first size fraction have a diameter between 20 and 40 pm and the lead grains of the second size fraction have a diameter between 60 and 80 pm. It is noted that the coating layer 30a may contain lead grains that do not fall into either the first or the second size fraction due to their size. In a preferred embodiment, the lead grains of the coating layer 30a comprise exactly two size fractions, wherein the lead grains of the first size fraction have a diameter of between 20-40 pm and the lead grains of the second size fraction have a diameter of between 60-80 pm. That is, in this embodiment, the lead grains of the coating layer 30a belong to either the first or the second size fraction. In a particularly preferred embodiment, the lead grains are substantially spherical.

Coating layer 30b of the positive electrode 10b (anode) comprises a mixture of graphene grains and lead dioxide grains, wherein the coating layer 30b having a graphene grain content of 1.8 V/V% or more and a graphene grain diameter of between 20 and 40 pm, and said lead dioxide grains are formed of at least two size fractions; a first size fraction and a second size fraction, wherein the lead dioxide grains of the first size fraction have a diameter between 20 and 40 pm and the lead dioxide grains of the second size fraction have a diameter between 60 and 80 pm. It is noted that the coating layer 30a may contain lead dioxide grains that do not fall into either the first or the second size fraction due to their size. In a preferred embodiment, the lead dioxide grains of the coating layer 30a comprise exactly two size fractions, wherein the lead dioxide grains of the first size fraction have a diameter of between 20-40 pm and the lead dioxide grains of the second size fraction have a diameter of between 60-80 pm. That is, in this embodiment, the lead dioxide grains of the coating layer 30a belong to either the first or the second size fraction. In a particularly preferred embodiment, the lead dioxide grains are substantially spherical.

The coating layer 30 according to the invention is applied and fixed to the surface of the support grid 20 in the manner described below, i.e. the support grid 20 is coated with the coating layer 30. The coating layer 30 preferably has a thickness of between 0.9 and 1.1 mm.

The invention also relates to a lead-acid battery 100 comprising electrodes 10a, 10b according to the invention. A schematic cross-sectional drawing of the lead-acid battery 100 is shown in Figure 3. The lead-acid battery 100 comprises, in addition to the electrodes 10a, 10b, conventional components such as electrolyte 110, for example dilute sulphuric acid and housing 120 comprising the electrodes 10a, 10b, and negative and positive poles 130a, 130b connected to the electrodes 10a, 10b, as is known to the person skilled in the art. It is noted that, as with conventional cathodes and anodes, a plurality of negative electrodes 10a according to the invention may be used to form a negative cell and a plurality of positive electrodes 10b may be used to form a positive cell (not shown in Figures). To increase energy density, the lead acid battery 100 according to the invention preferably comprises a plurality of such negative and positive cells.

The invention also relates to a method of manufacturing the electrodes 10a, 10b according to the invention. In a first step of the method, a structural support grid 20 made of the heat-resistant polymer described above is provided. In a preferred embodiment, the support grid 20 for both electrodes 10a, 10b is made of ETFE polymer coated with Teflon. The ETFE grid is formed as an openwork square grid with a thickness of 1.9-2.1 mm and a Teflon coating thickness of 0.3-0.5 mm. Depending on the polarity of the electrodes 10a, 10b to be produced, a coating layer 30a, 30b is applied to the surface of the support grid 20 as follows.

To produce the coating layer 30a of the negative electrode 10a, a homogeneous powder mixture of graphene grains is prepared by adding lead grains, in which powder mixture the graphene grain content is at least 1 .8 V/V% and in which powder mixture the graphene grains have a diameter of between 20 pm and 40 pm and in which powder mixture the lead grains are formed of at least two size fractions; a first size fraction and a second size fraction, wherein the lead grains of the first size fraction have a diameter between 20 and 40 pm and the lead grains of the second size fraction have a diameter between 60 and 80 pm. In a particularly preferred embodiment, the lead grains are produced by an atomisation process known per se, thereby producing substantially spherical lead grains. The spherical lead grains have excellent space filling, so that the contact area between the lead grains and the graphene grains is large, which can be further increased by using two different size fractions. It is noted that the lead grains can also be created by other processes such as mechanical comminution, chipping or grinding in an eddy current mill, as is known to the skilled person. The size fractions according to the invention are produced from the resulting lead particles of different diameters in a known manner, for example by classification with sieve lines or air separators, during which a neutral or reducing atmosphere is preferably used. The graphene grains and the lead grains of at least two of the above size fractions are, for example, mixed together in a mixing drum. To produce the coating layer 30b of the positive electrode 10b, a homogeneous powder mixture of graphene grains is prepared by adding lead dioxide grains, in which powder mixture the graphene grain content is at least 1 .8 V/V% and in which powder mixture the graphene grains have a diameter of between 20 pm and 40 pm and in which powder mixture the lead dioxide grains are formed of at least two size fractions; a first size fraction and a second size fraction, wherein the lead dioxide grains of the first size fraction have a diameter between 20 and 40 pm and the lead dioxide grains of the second size fraction have a diameter between 60 and 80 pm.

In the next step of the method, depending on the polarity of the electrodes 10a, 10b, the structural support grid 20 is coated with at least one layer of the powder mixture of the above-described composition. In a preferred embodiment, prior to the application, a binder, preferably glycerol, is added to the powder mixture to assist in adhering the powder mixture to the support grid 20. The powder mixture penetrates into and fills the openings or gaps in the support grid 20, thus creating a continuous surface on the support grid 20.

Subsequently, the powder mixture is fixed on the structural support grid 20 using a powder metallurgy process. Powder metallurgy involves the pressing and heat treatment (sintering) of the powder mixture as known to the skilled person. In a particularly preferred embodiment, the powder metallurgy is performed by pressing the powder mixture used for electrodes 10a, 10b onto the support grid 20 at a pressure of 345-500 MPa. The pressing is preferably performed by means of a mechanical or hydraulically operated press. When sizing the press tool, it should be taken into account that the dimensions of the electrodes 10a, 10b may be modified after pressing due to the elastic rebound of the particles after pressing, and therefore it is preferable to use allowances for the post-pressing calibration. In order to ensure even compression, the pressing is preferably carried out by double-sided cold pressing.

The electrodes 10a, 10b produced by powder metallurgy technology acquire their final properties during the heat treatment (sintering) following the pressing, therefore the support grid 20 coated with the powder mixture is heat treated after pressing. The temperature of the heat treatment must be below the melting point of the lowest melting component of the powder mixture, as will be apparent to the person skilled in the art. In a particularly preferred embodiment, the heat treatment temperature is 210-245 degrees Celsius for the powder mixture containing lead particles and 190-220 degrees Celsius for the powder mixture containing lead dioxide particles. The duration of the heat treatment is preferably between 0.5 and 0.7 hours. During sintering, some liquid phase may be formed and the grains may grow into each other by recrystallisation, diffusion or cohesive bonding. Sintering is carried out in a reducing atmosphere to avoid oxidation of the grains. The sintering operation increases the density of the electrodes 10a, 10b, reduces their dimensions and changes their physical and strength properties. The porous, brittle structure after pressing is transformed into a solid, shiny, metallic piece by heat treatment.

The lead-acid battery 100 of the invention is assembled in the usual, conventional way. The electrodes 10a, 10b according to the invention are immersed in the electrolyte 110, as can be seen in Figure 3. In order to increase the surface area, several electrodes 10a and 10b are used instead of one anode and one cathode, respectively, thus creating a much larger contact area between the active material of the electrodes 10a, 10b and the electrolyte 110. Various modifications will be apparent to a person skilled in the art without departing from the scope of protection determined by the attached claims.