The present invention relates to a rechargeable battery (also known as accumulator or secondary battery), and the related negative electrode, having very low internal resistance, high energy density and hence capacity, high number of life cycles and long life time, and that is reliable, efficient, safe, easily recyclable, inexpensive and simple to manufacture and transport, and allowing to be charged in short charge times and to be discharged with high discharge currents.

In general, in the present description and in the claims, it is meant that, during the charge phase, an external charger is connected to the positive and negative electrodes of a rechargeable battery and a current circulates so as to have an oxidation on the positive electrode and a reduction on the negative electrode, and that, during the discharge phase, an external load is connected to the positive and negative electrodes of the rechargeable battery and a current circulates so as to have a reduction on the positive electrode and a oxidation on the negative electrode.

Consequently, in the present description and in the claims, the negative electrode of the rechargeable battery means the electrode in which a reduction occurs during the charge phase and an oxidation during the discharge phase, and the positive electrode of the rechargeable battery means the electrode in which oxidation occurs during the charge phase and a reduction during the discharge phase.

It is known that development and diffusion of portable devices and electrically powered machines, which require significant amounts of energy, such as electric cars (and more generally vehicles), has increased the demand for efficient and inexpensive rechargeable batteries. To this end, research is currently also focused on identifying low-cost active materials.

By way of example, copper is one of the first elements to have been exploited to create experimental galvanic cells, such as the famous battery by Alessandro Volta that had copper and zinc disks stacked and separated by a saline solution to give a potential of a few volts capable to make the inventor carry out various experiments.

Copper and zinc technology was developed by numerous inventors, such as John Frederic Daniell, Felix Lalande, Georges Chaperon and Thomas Edison who, thanks to the excellent conductive properties of copper oxide, used them in galvanic cells sufficiently efficient to be used to power the first telegraph plants and the first prototypes of marine submarines. These first galvanic cells were capable to have an energy density of a few Watt-hour (Wh) per kilogram (where 1 Wh = 3.6 kJ), since the chemical reactions on which they were based were limited to oxidise the active materials only partially and in the early stages.

In this regard, the potentiality of zinc as a material for accumulation of energy has always been in competition with lithium, because zinc is capable to transfer two electrons, while lithium only one. This poses, albeit with a higher voltage, an intrinsic limit to the capacity that a normal lithium battery can have.

A further drawback of lithium is due to the inability to deposit the lithium in a metallic form in an aqueous basis and to the instability of the lithium that, should it come into contact with water or air, ignites spontaneously. This makes lithium batteries dangerous because lithium is used in cells in flammable organic solvents, the only materials capable of containing lithium instability. However, its light weight has made lithium batteries one of the best and most popular current technologies in terms of weight-to-power ratio.

In the prior art, the use of zinc in an aqueous electrolyte has been experimented. However, the diffusion of the use of zinc in rechargeable batteries has been made difficult, if not impossible, by the problem of the formation of zinc dendrites on the plates that make up the electrodes. In fact, with the progressive increase in the number of recharge cycles, the consistency of dendrites increases until they short-circuit the plates of the prior art zinc rechargeable batteries, making the cell unusable. In particular, the best prior art zinc batteries are composed of zinc oxide pressed on plates, opposed to nickel or copper hydroxide electrodes and immersed in a potassium hydroxide electrolyte. Initially, a prior art zinc battery works properly, but after a few discharge cycles, since zinc oxide is soluble in potassium hydroxide, the electrode disintegrates by dissolving in the electrolyte and, during charging, zinc dissolved in the electrolyte deposits again in metallic form on the plates forming the dendrites which, over time, short-circuit the electrodes making the battery life end.

In the prior art, an attempt was made to solve this problem by introducing a compound in the electrolyte that would minimise the solubility of zinc oxide. However, such solutions have not been effective and, although more slowly, the dendrites have always been formed due to the solubilisation, even if minimized, of the zinc, that is impossible to be completely eliminated.

Some prior art rechargeable batteries using zinc are disclosed in documents US 5196275 A, WO 95/31011 Al, WO 2007/059687 Al, WO 2009/123888 Al, WO 2012/012558 A2, US 2014/0248532 Al. Also, the prior art rechargeable batteries are polluting, since the materials of which they are made cannot be easily recycled or disposed of. In this regard, in the prior art rechargeable batteries, it is usual to use mercury in amalgam with zinc, to improve its performance and reduce its self-discharge.

It is an object of this invention, therefore, to allow to have a rechargeable battery that is reliable, efficient, safe, easily recyclable, inexpensive and simple to manufacture and transport, and that has a high energy density and thus capacity, and a high number of life cycles and long life time.

It is specific subject matter of the present invention a negative electrode, of rechargeable battery cell, on which an oxidation occurs during a rechargeable battery discharge process and a reduction occurs during a rechargeable battery charge process, formed by at least one negative plate comprising a support metal mesh structure on which a layer of steel wool consisting of a plurality of steel filaments is applied, wherein said steel filaments are coated with tin.

According to another aspect of the invention, said steel filaments can have diameter ranging from 10 to 100 micrometres, optionally ranging from 15 to 50 micrometres, more optionally ranging from 20 a 35 micrometres, still more optionally equal to 25 micrometres, and wherein said layer of steel wool can have a porosity comprising void spaces in a volume portion ranging from 60% to 98%, optionally ranging from 75% to 95%, more optionally ranging from 80% to 90%.

According to a further aspect of the invention, said layer of steel wool can have a thickness ranging from 2 millimetres to 10 millimetres, optionally ranging from 4 millimetres to 7 millimetres, more optionally equal to 5 mm.

According to an additional aspect of the invention, said support metal mesh structure can be made of nickel plated iron and/or iron and/or copper and/or stainless steel.

It is also specific subject matter of the present invention a process for manufacturing a negative electrode of rechargeable battery cell as just described, comprising the following steps: having a tin electrode,

applying a layer of steel wool consisting of a plurality of steel filaments on a support metal mesh structure,

immersing said tin electrode and said support metal mesh structure on which said layer of steel wool is applied in an electroplating bath containing an aqueous solution of stannous chloride (SnC ) and hydrochloric acid (HCI), wherein said tin electrode acts as an anode and said support metal mesh structure on which said layer of steel wool is applied acts as a cathode, and

applying in the electroplating bath a current from the anode to the cathode having density ranging from 80 mA/cm ² to 120 mA/cm ² for a time interval ranging from 15 to 25 minutes.

According to another aspect of the invention, said density of current can be equal to 100 mA/cm ² and said time interval can be equal to 20 minutes.

It is still specific subject matter of the present invention a rechargeable battery formed by one or more cells, each one of which includes a container (100; 100H) in which a negative electrode of rechargeable battery cell as previously described and a positive electrode, on which a reduction occurs during a rechargeable battery discharge process and an oxidation occurs during a rechargeable battery charge process, are contained, wherein said positive electrode is formed by at least one positive plate, wherein said negative electrode and said positive electrode are immersed in an electrolyte comprising or consisting of potassium tetrahydroxyzincate (K ₂ Zn(OH) ₄ ).

According to a further aspect of the invention, the electrolyte can comprise or consist of potassium tetrahydroxyzincate, potassium hexahydroxystannate (K2Sn(OH)6), potassium hydroxide (KOH) and water, wherein optionally a weight of potassium tetrahydroxyzincate ranges from 7% to 10%, a weight of potassium hexahydroxystannate ranges from 1% to 5%, a weight of potassium hydroxide ranges from 15% to 25%, and a weight of water ranges from 58% to 78%.

According to an additional aspect of the invention, the electrolyte can comprise or consist of potassium tetrahydroxyzincate, sodium tetrahydroxy-zincate (Na2Zn(OH)4), lithium tetrahydroxyzincate (Li2Zn(OH)4), potassium hexahydroxystannate (K2Sn(OH)6), potassium hydroxide, potassium silicate (foSiOs), ethylene glycol (C2H6O2) and water, wherein the electrolyte optionally further comprises sodium hexahydroxystannate (Na2Sn(OH)6), lithium hexahydroxystannate (Li2Sn(OH)6), wherein more optionally a weight of potassium tetrahydroxyzincate ranges from 7% to 10%, a weight of sodium tetrahydroxy-zincate ranges from 10% to 30%, a weight of lithium tetrahydroxyzincate ranges from 1% to 5%, a weight of potassium hexahydroxystannate ranges from 1% to 5%, a weight of potassium hydroxide ranges from 5% to 15%, a weight of potassium silicate ranges from 0,1% to 1%, a weight of ethylene glycol ranges from 0,1% to 1% and a weight of water ranges from 44% to 66%.

According to another aspect of the invention, said at least one positive plate forming said positive electrode can be made of one or more metal oxides selected from the group comprising or consisting of:

copper oxides, optionally mixed with metal powder of nickel and/or graphite, more optionally with a weight of metal powder of nickel and/or graphite ranging from 5% to 40% silver oxides, optionally mixed with metal powder of nickel and/or graphite, more optionally with a weight of metal powder of nickel and/or graphite ranging from 5% to 40%;

nickel hydroxide, optionally mixed with metal powder of nickel and/or graphite, more optionally with a weight of metal powder of nickel and/or graphite ranging from 30% to 50%; cobalt oxide and/or cobalt hydroxide, optionally mixed with metal powder of nickel and/or graphite, more optionally with a weight of metal powder of nickel and/or graphite ranging from 30% to 50%; and

manganese dioxide, optionally mixed with metal powder of nickel and/or graphite, more optionally with a weight of metal powder of nickel and/or graphite ranging from 30% to 70%.

According to a further aspect of the invention, the container can have a base configured to rest on a supporting plane, said negative electrode can be formed by a single negative plate resting on the base of the container, and said positive electrode can be formed by a single positive plate arranged in the container at a greater height from the base with respect to the negative plate.

According to an additional aspect of the invention, the container of each cell can be closed by a cap, that is optionally removable, wherein the cap is made of porous ceramic (thereby the cap is configured to let any gas generated inside the container flow away and to prevent air outside the container from coming into contact with the electrolyte).

According to another aspect of the invention, the container of each cell can be closed by a cap, that is optionally removable, wherein the cap is a cap comprising two graphite membranes coated with platinum or nickel separated from each other by a porous septum of ceramic or sintered polyethylene, wherein said porous septum is soaked in the electrolyte, and wherein one graphite membrane is placed in contact with outside air and the other graphite membrane is placed in contact with gases inside the cell, wherein:

the two graphite membranes are short-circuited to each other, or

the two graphite membranes are configured to be connected to a signalling device and/or to a safety circuit configured to interrupt a rechargeable battery charge process. According to a further aspect of the invention, the container of each cell can be provided with a circulator configured to stir the electrolyte during a rechargeable battery charge process, wherein the circulator optionally comprises an electric motor configured to rotate propeller blades.

According to an additional aspect of the invention, the circulator can be configured to be powered by the cell.

The rechargeable battery according to the invention is an innovative accumulator with very low internal resistance that exploits, for its operation, low cost active materials, such as copper, zinc, nickel, tin, carbon (possibly in the form of graphene) and lithium. In particular, contrary to the approaches adopted in the prior art solutions, the rechargeable battery according to the invention exploits the solubilisation of zinc in the electrolyte, according to specific chemical reactions, and the deposit of metallic zinc on a steel (or iron) sponge, coated with tin, with which the specific negative electrode receiving the zinc is provided.

The rechargeable battery according to the invention offers numerous advantages.

First of all, even if the zinc weight is higher than the lithium one, the possibility of being capable to be used in an aqueous electrolyte and the capability to be able to transfer twice the electrons of the lithium cause the rechargeable battery according to the invention to have energy densities higher than those of lithium batteries and with much lower costs. In other words, the energy density of the rechargeable battery according to the invention is very high, higher than that of the prior art lithium ion batteries, allowing to achieve high capacities.

Moreover, the rechargeable battery according to the invention has a very low internal resistance and a high number of life cycles and a long life time.

Furthermore, the rechargeable battery according to the invention is reliable, efficient, safe, easily recyclable, inexpensive and simple to manufacture and transport.

Also, the rechargeable battery according to the invention can be charged in short charging times and can be discharged with high discharge currents.

The present invention will be now described, by way of illustration and not by way of limitation, according to its preferred embodiments, by particularly referring to the Figures of the annexed drawings, in which:

Figure 1 shows a side view, in which the internal components are visible, of a cell of a first embodiment of the rechargeable battery according to the invention;

Figure 2 shows a front view of a first portion of the cell of Figure 1; Figure 3 shows a front view of a second portion of the cell of Figure 1;

Figures 4 and 5 show the charge and discharge graphs, respectively, of the first embodiment of the rechargeable battery according to the invention;

Figures 6 and 7 respectively show the combined charge and discharge voltage curves and the absorbed and delivered power curve of the first embodiment of the rechargeable battery according to the invention;

Figure 8 shows the graph representing the change in the capacity of the plates as a function of the number of charge and discharge cycles carried out for the first embodiment of the rechargeable battery according to the invention; and

Figure 9 shows a front view, in which the internal components are visible, of a cell of a further embodiment of the rechargeable battery according to the invention; and

Figure 10 shows a top plan view of a portion of the cell of Figure 9.

In the Figures identical reference numerals will be used for alike elements.

With reference to the Figures of the attached drawings, it can be observed that a first embodiment of the rechargeable battery according to the invention is formed by one or more cells, each of which comprises a container 100 in which a negative electrode and a positive electrode immersed in an electrolyte are housed. The negative electrode is formed by three (negative) plates, having a specific configuration and (vertically) arranged in the container 100 so that two negative plates 200S are lateral and a negative plate 200C is central. The positive electrode is formed by two (positive) plates 300 each interposed between a lateral negative plate 200S and the central negative plate 200C.

As stated, the negative electrode of the rechargeable battery is the electrode on which, in the charge phase, a reduction occurs, more particularly the reduction from potassium tetrahydroxyzincate (more commonly called potassium zincate - K2Zn (OH) 4) and potassium hexahydroxystannate (more commonly called potassium stannate - K2Sn (OH) 6) to metallic zinc and metallic tin, respectively, and, in the discharge phase, an oxidation occurs, more particularly the oxidation from metallic zinc and metallic tin to potassium tetrahydroxyzincate (more commonly called potassium zincate) and potassium hexahydroxystannate (more commonly called potassium stannate), respectively. The positive electrode of the rechargeable battery is the electrode on which, in the charge phase, an oxidation occurs during the charge phase, more particularly the oxidation from one or more among metallic copper, nickel hydroxide, cobalt hydroxide, manganese oxyhydroxide, metallic silver, to one or more among copper oxide, nickel oxyhydroxide, cobalt oxyhydroxide, manganese dioxide, and silver oxide, respectively, and, in the discharge phase, a reduction, more particularly from one or more among copper oxide, nickel oxyhydroxide, cobalt oxyhydroxide, manganese dioxide, silver oxide to one or more among metallic copper, nickel hydroxide, cobalt hydroxide, manganese oxyhydroxide, metallic silver, respectively.

In this regard, the rechargeable battery is configured to receive, and then store, a current in the charge phase, and to supply a current to an external load during the discharge phase.

In general, a cell of other embodiments of the rechargeable battery according to the invention has the negative electrode that is formed by one or more negative plates and the positive electrode that is formed by one or more positive plates, in which each negative plate is facing at least one positive plate (and similarly each positive plate is facing at least one negative plate).

In particular, the negative electrode, that is coated with a layer of tin, has a specific geometry, illustrated in greater detail later, preventing the formation of zinc dendrites. The positive electrode and the negative electrode are provided with two respective collectors 350 and 250 configured to be connected to the collectors of a negative electrode and a positive electrode, respectively, of other adjacent cells and/or to operate as terminals of the rechargeable battery. In particular, with reference to the collector 250 of the negative electrode shown in Figure 3, the collectors are connected to a metal end 260 at the top of the negative electrode (or positive electrode), for instance by means of a coupling screw interacting with a hole 265 of the metal end 260. The cell container 100 is closed by an optionally removable cap 400.

The electrolyte is an alkaline salt composed of potassium tetrahydroxyzincate (K2Zn(OH)4) (more commonly called potassium zincate). Also, the electrolyte of the first embodiment of the rechargeable battery according to the invention further includes potassium hexahydroxystannate (K2Sn(OH)6) (more commonly called potassium stannate) and potassium hydroxide (KOH). Furthermore, the electrolyte of a cell of the first embodiment of the rechargeable battery according to the invention comprises sodium tetrahydroxy-zincate (Na2Zn(OH)4) (more commonly called sodium zincate), potassium silicate (K2SiC>3), ethylene glycol (C2H6O2) and lithium tetrahydroxyzincate ((Li2Zn(OH)4) (more commonly called lithium zincate).

During the manufacturing phase, an example of electrolyte containing the aforementioned compounds, starting from raw materials which are easily available on the market, can be obtained by mixing lOOg of potassium hydroxide (KOH), 400 g of hydroxide of sodium (NaOH) and lOOg of lithium hydroxide (LiOH) in one litre of distilled water, then adding (keeping the mixture under stirring) in precise order, 150g of zinc oxide (ZnO), 5g of tin oxide (SnCh), 2g of potassium silicate (foSiOs) and finally 2g of ethylene glycol (C2H6O2). The electrolyte must be formed by respecting precise reaction times, first potassium hydroxide, sodium hydroxide and lithium hydroxide must be dissolved in water, when all the three compounds are dissolved then it is possible to proceed by adding zinc oxide, when this is completely dissolved as well, tin oxide is added, and when finally this is also dissolved, it is possible to proceed by adding in order first the potassium silicate and then the ethylene glycol.

This electrolyte production method will have in solution (with completely discharged battery) as final compounds, potassium tetrahydroxyzincate (K2Zn(OH)4), sodium tetrahydroxy zincate (Na2Zn(OH)4), lithium tetrahydroxyzincate (Li2Zn(OH)4), potassium hexahydroxystannate (K2Sn(OH)6), sodium hexahydroxystannate (Na2Sn(OH)6), lithium hexahydroxystannate (Li ₂ Sn(OH) ₆ ), potassium silicate (K ₂ S1O ₃ ), ethylene glycol (C ₂ H ₆ O ₂ ), free potassium hydroxide (KOH) and water (H ₂ O).

However, it must be noted that other embodiments of the rechargeable battery according to the invention can have the electrolyte that comprises sodium hydroxide (NaOH) as an alternative to or in combination with potassium hydroxide, still remaining within the scope of protection of the present invention.

Also, it must be noted that other embodiments of the rechargeable battery according to the invention may have the electrolyte consisting only of potassium tetrahydroxyzincate, or only of sodium tetrahydroxy-zincate, potassium tetrahydroxyzincate, potassium hexahydroxystannate (wherein optionally the weight of potassium tetrahydroxyzincate ranges from 7% to 10%, the weight of sodium tetrahydroxy-zincate ranges from 10% to 30%, the weight of potassium hexahydroxystannate ranges from 1% to 5%, and the weight of water ranges from 58% a 78%) and at least one among potassium hydroxide, sodium hydroxide and lithium hydroxide (wherein optionally, in the case where there is only potassium hydroxide, its weight ranges from 15% to 25%, while in the case where there are also sodium hydroxide and lithium hydroxide, the total weight ranges from 15% to 25%), still remaining within the scope of the present invention.

Potassium tetrahydroxyzincate, sodium tetrahydroxy-zincate and lithium tetrahydroxyzincate are the salts which contain all the dissolved zinc constituting the negative active material during recharge, i.e. metallic zinc.

Potassium hexahydroxystannate is capable to control the crystallization of zinc crystals, i.e. the arrangement of zinc atoms in the formation of metallic zinc crystals during the deposit in the charge phase of the rechargeable battery. In particular, potassium hexahydroxystannate causes the metallic zinc crystals to grow compact and in nanometric form contributing, along with the specific geometry of the negative electrode, to avoid the formation of dendrites. In fact, zinc, thanks to the stannate, grows without ramifications even with high charge currents. It must be noted that in the illustrated example of electrolyte composition, sodium hexahydroxystannate and lithium hexahydroxystannate, which behave in the same way as potassium hexahydroxystannate, are also present, wherein a weight of the assembly of potassium hexahydroxystannate, sodium hexahydroxystannate and lithium hexahydroxystannate ranges from 1% to 5%.

During the rest of the rechargeable battery according to the invention, the metallic tin, present together with the metallic zinc on the negative electrode, considerably slows down the dissolution of the metallic zinc, that would spontaneously tend to dissolve in potassium hydroxide. Also, the tin, also depositing in the zinc crystals during the charge phase of the rechargeable battery, creates an "amalgam" similarly to the addition of mercury used in the prior art solutions, that is less attackable and therefore more resistant to the action of alkali metal hydroxides, such as potassium hydroxide, and it further creates a protective film on zinc, protecting it from the corrosive action of potassium hydroxide.

In this regard, thanks to the excellent conductive properties of tin that is present in the metallic zinc crystals, the latter remains electrochemically active and it is sufficient to start discharging the rechargeable battery according to the invention, even with a low discharge current, to break the metallic tin fragments (present in the zinc crystals) and to dissolve them by returning to solution.

Tin, that is a catalyst that promotes oxidations, once the discharge process has started, promotes the solubilisation of zinc in the electrolyte, favouring the formation of potassium tetrahydroxyzincate.

Sodium tetrahydroxy-zincate allows to increase the energy density of the electrolyte, significantly increasing the amount of zinc that can be introduced in the electrolyte. Hence, in an initial state of completely discharged rechargeable battery, the zinc is dissolved in three salts: potassium tetrahydroxyzincate, sodium tetrahydroxy-zincate and lithium tetrahydroxyzincate.

Potassium silicate also contributes to minimizing the solubilisation of zinc during the rest phase of the rechargeable battery, creating a protective zinc silicate film ((ZnSiOs)) on the zinc metal crystals in "amalgam" with tin protecting crystals from the corrosive action of potassium hydroxide. This thin film of zinc silicate, as for tin, reacts by dissolving again in potassium silicate during the discharge phase, freeing the metallic zinc and thus returning to solution. Potassium silicate can be used in combination with tin to obtain a low self-discharge during long rest periods of the rechargeable battery.

Ethylene glycol also contributes to slowing down the dissolution of metallic zinc by promoting the formation of the protective zinc silicate film by the potassium silicate.

Potassium hydroxide, abundantly present, is the alkaline base permitting the solubilisation of zinc and the consequent formation of potassium tetrahydroxyzincate, besides permitting, with its high concentration, a very low electrical resistance of the electrolyte and thus a very low internal resistance of the rechargeable battery.

Lithium tetrahydroxyzincate contributes to increasing the energy density of the rechargeable battery, playing an important (though not essential) role on the cell voltage and its stability. Lithium tetrahydroxyzincate is formed during the discharge of the rechargeable battery when the metallic zinc, giving away electrons, dissolves in the electrolyte containing lithium hydroxide (LiOH) forming a third salt: lithium tetrahydroxyzincate. In fact, it must be noted that, differently from what happens for almost all the prior art rechargeable batteries in which, during the discharge, the voltage significantly drops, the cells of the rechargeable battery according to the invention, also thanks to the use of lithium tetrahydroxyzincate, succeed in maintaining the voltage almost constant during the discharge phase. This leads to a nominal voltage higher than normal, and thus a higher energy density. During the charge of the rechargeable battery, lithium tetrahydroxyzincate becomes lithium hydroxide and metallic zinc, thus leaving lithium always in aqueous solution. In other words, when the rechargeable battery is discharging it contains potassium tetrahydroxyzincate, sodium tetrahydroxy-zincate and lithium tetrahydroxyzincate; when the rechargeable battery is charging it contains metallic zinc, potassium hydroxide, sodium hydroxide and lithium hydroxide.

It is obvious that in the intermediate phases of charging/discharging all the species are present. In other embodiments of the rechargeable battery according to the invention, potassium hydroxide is replaced with sodium hydroxide (NaOH). In such embodiments, the solubility of zinc turns out to be much higher than in the case where the electrolyte comprises potassium hydroxide. In particular, the solubility of zinc oxide (that, as stated above, is a primary compound usable in the electrolyte preparation phase) increases by about four times, but the spontaneous corrosion of metallic zinc decreases. This leads to a better energy density of the electrolyte and a lower self-discharge of the cell. On the other hand, sodium hydroxide creates deposits both in the electrolyte and on the internal walls of the cell container 100 over time, and with the increase of the number of cycles metallic powders could accumulate which could short-circuit the positive electrode and the negative electrode. To overcome this, it is possible to mix sodium hydroxide and potassium hydroxide in a percentage of 65% and 35%, respectively.

In this way, a series of advantageous technical effects are obtained. First, fouling is avoided. Also, the energy density of the electrolyte is increased, creating two active salts, namely potassium tetrahydroxyzincate (K2Zn(OH)4) and sodium tetrahydroxy-zincate (Na2Zn(OH)4). Furthermore, the self-discharge of the cell is reduced, creating a correct balance between such various factors. These factors can then be changed according to the needs and the different balances which are desired to obtain.

In a second embodiment of the battery according to the invention, the electrolyte comprises:

potassium tetrahydroxyzincate ranging from 7% to 10%, optionally equal to about 8,5%, sodium tetrahydroxy-zincate ranging from 10% to 30%, optionally equal to about 20%, lithium tetrahydroxyzincate ranging from 1% to 5%, optionally equal to about 3%, potassium hexahydroxystannate ranging from 1% to 5%, optionally equal to about 3%, potassium silicate ranging from 0,1% to 1%, optionally equal to about 0,5%,

ethylene glycol ranging from 0,1 to 1%, optionally equal to about 0,5%,

potassium hydroxide ranging from 5% to 15%, optionally equal to about 10%, and water ranging from 44% to 64%, optionally equal to about 54,5%.

The specific configuration of each negative plate (200S and 200C) forming the negative electrode has been appropriately developed to permit a correct deposit of the zinc ions avoiding the formation of dendrites and considerably contributing to the lightweight of the rechargeable battery according to the invention. In this regard, each negative plate forming the negative electrode comprises a support wire mesh structure, advantageously very tightly meshed, - IB - optionally made of iron, and/or nickel plated iron, and/or copper, and/or stainless steel, on which a layer of steel wool consisting of a plurality of filaments pressed together having a diameter in the order of hundredths of a millimetre, optionally ranging from 10 to 100 micrometres, more optionally ranging from 15 to 50 micrometres, still more optionally ranging from 20 to 35 micrometres, even more optionally equal to 25 micrometres, and having a porosity comprising empty spaces in a portion optionally ranging from 60% to 98% of the volume, more optionally ranging from 75% to 95% of the volume, still more optionally ranging from 80% to 90% of the volume, is applied. This steel wool is similar to that used for other purposes, such as for wood sanding in restoration. The layer of steel wool applied to the support metal mesh structure optionally has a thickness ranging from 2mm to 10mm, more optionally ranging from 4mm to 7mm, even more optionally equal to about 5mm.

In particular, the steel wool (as well as the support metal mesh structure) of the negative electrode is advantageously coated with tin. This allows to deposit a large amount of metallic zinc on the negative electrode; moreover, the tin of the coating present on the steel wool, having a potential close to zero (differently from the bare steel that is at about -0.88 Volts), immediately drops the voltage to 0 Volts as soon as all the zinc has dissolved in the electrolyte, thus preserving the steel of the wool. To coat the steel wool (as well as the support wire mesh structure) of the negative electrode with tin, the support wire mesh structure on which the layer of steel wool is applied is immersed in an electroplating bath (i.e. a galvanic bath) containing an aqueous solution of stannous chloride (SnC ) and hydrochloric acid (HCI), where it constitutes the negative electrode of the galvanic formation cell, while a pure tin electrode constitutes the positive electrode of the bath, and a current having a density ranging from 80 mA/cm ² a 120 mA/cm ² , optionally equal to circa 100 mA/cm ² , is applied. After a time interval ranging from 15 minutes to 25 minutes, optionally equal to 20 minutes, the slender steel filaments of the wool are coated with the pure tin layer and the formation of the negative electrode of the rechargeable battery cell is complete. In particular, the indicated value of the current density applied in the electroplating bath causes the tin crystals not to grow compact but needle-shaped, increasing the tin surface exposed on the negative electrode making the deposit of zinc during the recharge of the battery according to the invention regular and uniform.

A typical composition of the electroplating bath can be considered as follows:

water (H2O) ranging from 60% to 70%, typically equal to 65%,

stannous chloride (SnC ) ranging from 25% to 30%, typically equal to 27,5%, and hydrochloric acid (HCI) ranging from 5% to 10%, typically equal to 7,5%.

It must be noted that other embodiments of the rechargeable battery according to the invention may have the steel wool (as well as the support metal mesh structure) of the negative electrode devoid of the tin coating; in this case, the steel used is advantageously 316 stainless steel.

In other words, the rechargeable battery according to the invention exploits, for its operation, the ions of zinc, tin and lithium which participate in the electrochemical reactions of the cell of the rechargeable battery according to the invention. The zinc and tin ions are deposited in metallic form on the tinned steel wool, although the tin, having a potential close to zero, does not contribute to the final electromotive force (e.m.f.) of the cell of the rechargeable battery according to the invention. As stated, the lithium ions always remain in liquid form, passing from lithium hydroxide to lithium tetrahydroxyzincate during the discharge process.

Thanks to the innovative electrolyte used in the cell, the two plates 300 forming the positive electrode can be made of various metal oxides, obtaining different results in terms of cell voltage and capacity. Optionally, the metals of the oxides of which the two plates 300 of the positive electrode can be made are selected from the group comprising or consisting of copper, nickel, manganese, cobalt and silver.

By using copper oxide (namely cupric oxide CuO and cuprous oxide CU2O) as material of the two plates 300 of the positive electrode, the best battery in terms of low costs and low internal resistance is obtained. Copper oxide is a natural semiconductor that does not need doping agents to increase its conductivity, which can be increased by mixing copper oxide with a weight percentage of metal powder of nickel and/or graphite ranging from 5% to 40%, which also allows to increase its porosity (in particular, with a low percentage of graphite the capacity and thus the energy density of the rechargeable battery is increased, while a high percentage of graphite favours discharges with high currents) . With this combination, a rechargeable battery cell according to the invention, comprising copper, zinc, tin and lithium, has a voltage of 0.8 Volt and a very high capacity, equal to about 619 ampere-hours per kilogram (Ah/Kg, where 1 Ah = 3600 Coulomb) for the positive electrode formed by the two plates 300 of copper oxide and 800 Ah/Kg for the negative electrode. By using copper as positive electrode, the most particular discharge curve is obtained, during which the voltage, instead of dropping or remaining constant, tends to rise even by several tenths of a Volt. This is possible thanks to the reduction in metallic copper of the copper oxide that, during the discharge, by transforming into metallic copper, further improves the conductivity of the positive electrode, further reducing the internal resistance of the cell with a consequent increase in voltage during the discharge. In particular, during the charge and discharge phases, copper and copper oxide participate to the following reactions:

By using silver oxide (namely in the two forms AgO and Ag20) as material of the two plates 300 of the positive electrode, the best battery in terms of energy density is obtained, far exceeding the current best lithium batteries by about 30-40%. Optionally, the silver oxide is mixed with a weight percentage of metal powder of nickel and/or graphite ranging from 5% to 40%. The behaviour of the cell is practically the same as the one of the cell having the positive electrode of copper oxide, but with a double voltage, equal to 1,6 Volt nominal. The capacity is still equal to about 619 Ah/Kg for the positive electrode formed by the two plates 300 of copper oxide and 800 Ah/Kg for the negative electrode. Also in this case, the cell shows the trend of rising voltage under load, due to the reduction of metallic silver to silver oxide. In particular, during the charge and discharge phases, silver and silver oxide participate to the following reactions:

Ag Ag 0 AgO

Copper oxide and silver oxide allow a high capacity for the positive electrode thanks to the double bond that copper and silver have with oxygen, providing a double oxidation stage (+1 and +2).

In this regard, copper and silver have not been used widespread throughout the history of accumulators, due to their solubility in alkaline compounds. This has always limited their use to non-rechargeable primary batteries, as their solubility only occurs during the charge phases but not in the discharge ones. Differently, in the rechargeable battery according to the invention, it is possible to use these metals without incurring the problem of solubility, thanks to the special electrolyte, that passives metals such as copper and silver when they tend to solubilise by passing in solution, and thanks to different catalysts which are added to the oxides of the positive electrode.

By using nickel hydroxide (Ni(OH)2) as material of the two plates 300 of the positive electrode, the cell acquires the best balance between performance, energy density and low cost of raw materials. In this case, the cell has 1,65 volts nominal and a capacity of approximately 250 Ah/kg for the positive electrode of nickel hydroxide and 800 Ah/kg for the negative electrode. In this arrangement, a single nickel oxidation jump from +2 to +3 is exploited, where during the recharge of the battery nickel hydroxide is transformed into nickel oxyhydroxide (NiO (OH)):

2Ni(OH) + K ₂ Zn(OH) ₄ <- 2NiO(OH) + Zn + 2KOH + 2H ₂ 0

Since nickel hydroxide is not reduced to metallic nickel, this configuration has not the voltage rise and for this reason it is advantageous to enrich the nickel hydroxide with a percentage ranging from 30% to 50% by weight of metal powder of nickel and/or graphite, neither of which takes part in chemical reactions, but both of which will exclusively act as conductors, greatly improving the internal resistance of the positive electrode.

By using cobalt oxide (C03O4) and/or cobalt hydroxide Co(OH) ₂ as material of the two plates 300 of the positive electrode, the cell has performances very similar to those of the cell in which the positive electrode is made of nickel hydroxide, although with a higher raw material cost. However, the slight solubility of cobalt oxide in the electrolyte makes it slightly more reactive in terms of fast discharges. Advantageously, the addition of 5-10% of cobalt oxide to nickel hydroxide as material of the two plates of the positive electrode favours and catalyses, in the charge phase, the passage from nickel hydroxide to nickel oxyhydroxide thus improving cell performance; in this case, the plates 300 of the positive electrode are very similar to the plates used in the conventional prior art nickel-cadmium batteries. Optionally, cobalt oxide and/or cobalt hydroxide is mixed with a percentage ranging from 30% to 50% by weight of metal powder of nickel and/or graphite.

By using manganese dioxide (Mn0 ₂ ) as material of the two plates 300 of the positive electrode, a cell with low costs of active material, but with an internal resistance higher than copper, is obtained, as for copper oxide. Also, the cell has a higher voltage equal to about 1,5 volts. Optionally, manganese dioxide is mixed with a percentage ranging from 30% to 70% by weight of metal powder of nickel and/or graphite. The capacity of the positive electrode of manganese dioxide is equal to 250 Ah/Kg, while the one of the negative electrode is still equal to 800 Ah/kg. The use of manganese dioxide in a rechargeable battery is an important innovation, as its use in secondary batteries has always been hampered by its strong solubility in aqueous electrolytes during charging. In the rechargeable battery according to the invention this phenomenon is resolved thanks to the particular composition of the electrolyte which is enriched by some catalysts. Manganese dioxide, although not presenting the same life as nickel hydroxide, in terms of the number of cycles, has a very low cost that allows various uses thereof, such as the stationary backup accumulation, or even the seasonal stationary accumulation . As with nickel hydroxide, even manganese dioxide, during discharge, does not transform into the metallic phase, stopping at an intermediate phase, manganese oxyhydroxide:

2Mn0 + Zn + 2KOH + 2H ₂ 0 2MnO(OH) + K ₂ Zn(OH) ₄

Based on the coupling of the negative electrode with these various metal oxides, different combinations of voltage, amperage and therefore energy density are obtained. In the best performance version, wherein the positive electrode is made of silver oxide, it is possible to have an energy density of up to about 200 Wh/Kg, while with the other oxides the energy density is variable from about 70 to about 110 Wh/kg .

Other embodiments of the rechargeable battery according to the invention may have the positive electrode made of a combination of two or more of the oxides illustrated above, obtaining hybrid versions of the positive electrode.

Thanks to the specific electrolyte, the rechargeable battery according to the invention can be recharged and discharged fast with a current up to 5C (i.e. with a current equal to 5 times the cell capacity: in the case of a rechargeable battery having a capacity of 10 amperes -hour, the 5C charging or discharging current is equal to 50 ampere). In this way, the rechargeable battery according to the invention can be recharged in 15-20 minutes, thus allowing fast recharging in case, for example, of applications on electric cars (and, more generally, on electric vehicles). By considerably concentrating potassium hydroxide (and/or sodium hydroxide), it is possible to reach even higher discharge current values.

Advantageously, the cell comprises a number of positive plates forming the positive electrode lower than the number of negative plates forming the negative electrode to obtain a better energy density. In the case where the number of positive plates forming the positive electrode is greater than the number of negative plates forming the negative electrode, the instantaneous power of the rechargeable battery is increased. In the first embodiment shown in the Figures, there are two positive plates and there are three negative plates forming the negative electrode.

The cap 400 of the cell is advantageously a cap of porous ceramic. In particular, thanks to such material, the cap is configured to let any gases generated during the overload to flow out, and at the same time to avoid any contact between air and electrolyte preventing the component of carbon dioxide (C0 ₂ ) of the air from contaminating the electrolyte by transforming the hydroxides into carbonates. The recharge of the cell (and consequently of the rechargeable battery according to the invention) is very simple because, thanks to the zinc ions dissolved in the electrolyte, the voltage remains practically constant as long as the ions in solution are present. When all the zinc ions have been deposited in metallic form, the voltage rises rapidly, permitting to easily identify the state of end of charge. If energy is still supplied to the cell, electrolysis is generated with production of gases, that in any case does not damage the active materials, simply leaving them deposited in the final state of charge.

The inventor has conducted a series of experimental tests on a rechargeable battery cell as shown in Figures 1-3 with 50 ampere-hour nominal and 1,65V of nominal voltage. The cycling protocol used during the tests has been the following:

full battery charge for a duration of 10 hours at constant current of 8 amperes,

complete battery discharge (reaching a minimum voltage of 1,2V) with constant current of 8 amperes.

The sensors used comprise hall effect sensors (LAS 50TP) for current detection and analog-digital converter for reading voltage.

Figures 4-8 show the graphs with a sampling of 1000 measurements per second. The battery capacity and the energy supplied to the battery or supplied by the battery is calculated by integrating the measured data over time.

Figure 4 shows the charge graph, in which it is noted that after about 6 hours and 30 minutes, after the battery charge process is completed, voltage remains constant (the current is also kept constant). In this phase the battery is no longer absorbing energy and the energy supplied is dissipated in the form of electrolysis. The battery can continue to dissipate energy in this mode indefinitely without any risk.

Figure 5 shows the discharge chart.

Figures 6 and 7 respectively show the combined charge voltage and discharge voltage curves and the absorbed and delivered power curve.

Based on the data obtained from the tests, the energy and capacity absorbed during the charge phase (excluding the overcharge) have been:

E _abs = -104.25 ± 0.04 Wh

Cabs = -54.10 ± 0.03Ah

while the energy and capacity delivered have been:

E _er = 82.5 ± 0.5 Wh C _er = 51.0 ± 0.3 Ah

The energy efficiency over the complete cycle is thus:

while the efficiency on capacity is:

Figure 8 shows the graph representing the change in capacity of the plates as a function of the number of charge and discharge cycles carried out. It is noted that the battery capacity remains higher than the nominal capacity even after 260 cycles.

In other words, based on the results obtained from the tests carried out, the tested rechargeable battery has the following characteristics:

an energy efficiency of about 80% over a complete cycle (with over-charging at 8A), an efficiency in capacity of about 95% over a complete cycle, and

overload resistance (does not require BMS as it can be fully charged or discharged).

Also, the efficiency in capacity is definitely higher than the energy efficiency, thereby there still exists a large margin of improvement on energy efficiency.

Thanks to these properties, the battery according to the invention does not require any battery pack management system, also known as BMS (Battery Management System) that is used in the prior art lithium cells to perform monitoring and consequent management of the pack batteries, ensuring safety and longer life of the battery pack and to balance it at the end of the charge, avoiding voltage rises and thereby fires and explosions. It is thus sufficient to connect, in the bank of the rechargeable battery according to the invention, all the cells in series and simply supply energy. The simplest mode is with constant current, wherein charging stops when sudden voltage rise occurs. The saving of the BMS circuit (indispensable in lithium cells) places the rechargeable battery according to the invention in an even safer and inexpensive condition.

Optionally, it is also possible to install, as additional protection, a ceramic cap containing platinum, so that, in case of a charger breakage, the possible electrolysis containing hydrogen and oxygen can be recombined and transformed back into water inside the cell. The cap looks like a sort of fuel cell comprising two graphite membranes coated with platinum or nickel separated from each other by a porous septum of ceramic (or sintered polyethylene). The porous septum of ceramic (or sintered polyethylene) is soaked in the electrolyte itself of the cell. The two graphite membranes are placed one in contact with the air and the other in contact with the gases inside the cell. This ceramic cap allows to solve the problem of generation of hydrogen inside the cell due to overcharging. The first solution is to short-circuit the two graphite membranes during construction by combining hydrogen with atmospheric oxygen to form water that will be sent back to the cell. The second solution is to exploit the voltage produced by the membranes to make a warning light turn on or to command a safety circuit to interrupt the supply of current by the charger.

In this regard, such ceramic cap could also be used as a protective device for other electrochemical cells other than those of the rechargeable battery according to the invention, including the prior art electrochemical cells.

Safety of the rechargeable battery according to the invention, greater than the one of the most modern prior art lithium battery technologies, makes it particularly usable in domestic and automotive applications. Thanks to its aqueous content and the presence of lithium, zinc and tin, performances in terms of charges and discharges are extremely high, and safety is equally high because, thanks to its aqueous content, the rechargeable battery according to the invention cannot explode or ignite, and is therefore intrinsically safe.

Also, the rechargeable battery according to the invention is non-polluting and much more inexpensive than the prior art lithium batteries both for the use of low-cost active materials and for its easier construction. In this regard, its recyclability makes it ecological and respectful of the environment, given that its components are neither toxic nor harmful, and are always well separable from each other. In fact, zinc and tin are found in the liquid, iron in the negative plates (200S and 200C) forming the negative electrode, and metal oxides in the positive plates (300) forming the positive electrode. During the recycling phase it is therefore easy to separate the components and recycle them.

Furthermore, the rechargeable battery according to the invention is easy to transport. In this regard, common prior art lithium batteries have many problems for air transport because of the danger of fire and explosions, since they cannot be transported dry or inactive. Differently, the rechargeable battery according to the invention has the possibility of being built and transported dry and inactive, not representing a problem for air transport. In fact, the rechargeable battery according to the invention can be activated on site simply by filling it with the electrolyte transported in sealed tanks, that is also inactive. Then, it will be sufficient to charge the batteries to make them active and ready for use. Advantageously, as shown in the Figures for the first embodiment, the container 100 of the cell of the rechargeable battery according to the invention is advantageously provided with a circulator 500. In particular, since the electrolyte is composed of metals, which are heavier than water, it tends to easily stratify and could compromise the correct deposit of zinc. The circulator 500 is configured to stir the electrolyte, counteracting its stratification. The circulator 500 is configured to operate advantageously in the recharge phase, to promote the correct deposit of zinc, while it is not fundamental in the discharge phase. The circulator 500 is composed of an electromagnetic device, e.g. comprising an electric motor, completely isolated and immersed in the cell liquid, that is coupled to propeller blades: when the electric motor is powered, it rotates the propeller blades which mix the electrolyte. The operating voltage can be equal to about 1 Volt, thereby the electric motor can be powered by the same voltage of the cell in which it is installed.

With reference to Figures 9 and 10, it can be observed that a third embodiment of the rechargeable battery according to the invention has the plates forming the positive and negative electrodes arranged horizontally, instead of vertically. In particular, the container 100H has a base 110 configured to rest on a supporting plane (not shown) and the negative electrode is formed by a single negative plate 200H resting on the base 110 of the container 100 (i.e. on the bottom of the container 100); the positive electrode is formed by a single positive plate 300H arranged in the container 100H at a height from the base 110 (i.e. from the bottom of the container 100) higher with respect to the single negative plate 200H. Such arrangement does not suffer from electrolytic stratification and, thus, does not require any circulator: the zincates, heavier than water, will stratify on the bottom where the negative plate 200H is, ready to completely collect all the zinc. The positive plate 300H, arranged at a higher height, does not need the zincates to operate, thereby this arrangement does not need the circulator, making the rechargeable battery particularly advantageous for automotive applications due to the limited height of the container 100H.

Other embodiments of the rechargeable battery can comprise two or more negative plates and/or two or more positive plates arranged horizontally (as shown in Figures 9 and 10), and in this case the container is provided with a circulator.

The preferred embodiments of this invention have been described and a number of variations have been suggested hereinbefore, but it should be understood that those skilled in the art can make other variations and changes without so departing from the scope of protection thereof, as defined by the attached claims.