Alkaline secondary electrochemical generator with zinc anode

Abstract

The invention relates to the field of alkaline electrochemical generators, in particular to the field of accumulators. More specifically, the invention relates to a secondary electrochemical generator with a zinc electrode, characterized in that it comprises: a) an electrolyte, which is an aqueous alkaline solution with a molar concentration of 4 M to 15 M of hydroxide anions, and a concentration of soluble silicates, expressed as silicon dioxide (SiO2), of 0.15 to 80 g / l; b) a zinc electrode comprising a conductive ceramic, which at least partially comprises nitrides and / or hafnium carbides and / or carbides and / or magnesium silicides and / or carbides and / or niobium nitrides and / or carbides and / or carbides and / or titanium nitrides and / or vanadium nitrides and / or carbides and / or azides of any two metals selected from the group consisting of hafnium, magnesium, niobium, titanium and vanadium.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of alkaline electrochemical generators, in particular to the field of accumulators.

[0002] More specifically, it relates to secondary generators with zinc anodes, such as nickel-zinc, zinc-manganese dioxide, silver-zinc, zinc-air, and generators with fully or partially soluble cathodes, such as zinc-iodine, zinc-bromine, zinc- ferricyanide, zinc-manganese oxide, aiming at obtaining high levels of cycling capacity with zinc electrodes. BACKGROUND

[0003] The energy characteristics of zinc (820 Ah / kg, 5845 Ah / l), electronegativity (1.65 V), low cost and ease of recycling make it a particularly suitable anode material for electrochemical generators: thus, the theoretical specific energy of nickel-zinc batteries and zinc-air batteries is 334 Wh / kg and 1320 Wh / kg, respectively. In practice, the specific energy of prismatic nickel-zinc accumulators can reach 80 Wh / kg, which is two to three times that of lead batteries.

[0004] Although zinc is widely used in alkaline batteries and carbon-zinc batteries , it is rarely found in industrial alkaline batteries, with the exception of silver-zinc batteries, which have a cycling capacity of only a few times and are mainly used for military purposes, and the first industrial nickel-zinc accumulators that have recently appeared.

[0005] Zinc is easily soluble in alkaline media in the form of zincate, and when zinc anode accumulators are charged, it is easy to form dendritic growth, which leads to short circuits between the electrodes of opposite polarity.

[0006] In addition, the negative electrode area where zinc deposition occurs also changes during the charge and discharge cycles: thus, densification phenomena are observed, which reduce the porosity of the electrode and thus the working capacity at the corresponding current density when the battery is actually used. Other factors adversely affect the zinc anode, such as the formation of a passivation layer by the precipitation of zinc oxide, which causes a reduction in the active area of the electrode.

[0007] Several studies have been carried out in the relevant field regarding the deposition and dissolution mechanisms of zinc in alkaline media, and several patents exist that offer various solutions:

[0008] • the subject of most patents is the use of additives, either by adding them to the electrolyte or by incorporating them into the anode active ingredient, to reduce the solubility of zincate;

[0009] • ensuring uniform zinc deposition by mechanical methods that make the electrolyte flow alone or by dispersing the zinc electrode in the electrolyte, thus avoiding dendritic growth;

[0010] • at least partially dissolve dendrites using pulsed current with or without reverse polarity;

[0011] • finally, limit the formation of zinc ions or inhibit their diffusion from the anode to the cathode with separators: for example, multi-layer microporous separators or exchange membranes.

[0012] These techniques can be used alone or in combination, but they only provide partial solutions and, in some cases, increase the internal resistance, increase the cost of the battery or are complex to implement. In addition, there are proposals to add lead or cadmium to the active material of the anode, which is difficult to accept due to the greater pollution it generates.

[0013] Progress has been made in this area by adding additives to the electrolyte, which is mainly composed of potassium salts, such as the small nickel-zinc (NiZn) cylindrical batteries sold on the market. However, their cycle count does not meet the needs of industrial batteries, which must guarantee at least 1000 deep charge and discharge cycles at a depth of discharge of 80% and above. Significant progress has also been made by adding conductive ceramic to the zinc electrode, preferably titanium nitride (TiN), the innovative technology described in French patent FR 2788887 (SCPS) published on January 27, 1999, which makes it possible to achieve more than 1000 cycles at a depth of discharge of 80% and above (as shown in the battery 1). The capacity loss of nickel-zinc batteries during cycling is mainly related, on the one hand, to the partial redistribution and densification of the active ingredients and, on the other hand, to dry-out and the resulting passivation of the zinc electrode. Figure 3

[0014] The present invention aims to break through the performance limits of zinc electrodes containing conductive ceramic using a new method, thus providing a large number of cycle weeks, i.e. by changing the oxygen recombination conditions on the zinc surface, significantly slowing down the passivation of the zinc on the anode surface and the dry-out of the generator, inhibiting dendrite formation and reducing the redistribution and densification of the zinc.

[0015] This is a significant breakthrough and, contrary to the aforementioned opinion, the authors of the present invention have demonstrated that the addition of silicates to the alkaline electrolyte, applied to a generator containing such an anode, greatly reduces the passivation of the zinc electrode and the dry-out of the generator, thus extending the cycle life of the battery.

[0016] By searching the state of the art of zinc anode batteries, it was found that several patents and studies mention the use of silicates. According to these documents, the addition of silicates presents very different results if the electrolyte is used in limited amounts and cannot move, or if the electrolyte is used in large amounts and / or moves / flows in the generator.

[0017] ​Methods of adding silicates have been around for a long time, such as US patent 858 862, filed on July 2, 1907 by Thomas Edison, which relates to the Larange cell (ZnCuO) and describes that an electrolyte containing silicate accelerates the dissolution of zinc. US patent US 3 466 195 A (ESB Inc.) issued on September 9, 1969 mentions that when a relatively small amount of 0.05% of silicate is added to the electrolyte or positive electrode, the initial capacity and stability of the capacity during the storage or calendar life of a ZnAgO or ZnMnO2 primary cell is increased. The authors of this patent point out that the presence of silicate increases the impedance, so the amount of silicate in the electrolyte should be controlled to the minimum necessary to achieve the related purpose. This patent also indicates the negative effects of adding 3% silicate.

[0018] Marshall et al. (Surface technology 5 (1977) 149-163) studied the effect of potassium silicate on the dissolution reaction of polycrystalline zinc in potassium hydroxide solution. The evidence provided confirms that the potential of potassium silicate is adsorbed in large amounts on the metal zinc near the dissolution potential. Since the interaction with the metal surface is limited OH - , thus inhibiting the dissolution of zinc. The adsorbed silicate layer also affects the movement of ionic species near the electrode surface. Thus, the amount of electricity required to cause passivation is reduced, indicating that silicate promotes the passivation of zinc.

[0019] In European patent EP 1 819 002 A2 (Powergenix System Inc.) of August 15, 2007, it is also mentioned that silicates can limit the solubility of zinc.

[0020] In Chinese patent CN 103794824 B B (Hunan Science and Technology Research and Development Institute) of January 20, 2016, it is mentioned that a 5.5-6.5 mol / l KOH electrolyte gel is prepared, characterized by the addition of silicate in a mass ratio of 0.3-0.5:1 to solve the cycling problem of nickel-zinc batteries. It is also mentioned that this gel reduces the formation of dendrites and the dissolution of zinc electrode plates, the latter point being consistent with the study by Marshall et al. The addition of silicate is carried out with lithium magnesium silicate or aluminum magnesium silicate, resulting in the formation of a gel characterized by silicate particles of 10-200 nm. Thus, the concentration of silicate is low and not specified, and the electrolyte is formed from the partial dissolution of silicate solid particles.

[0021] In French patent FR 2 630 862 (Sorapec SA) published on November 3, 1989, it is mentioned that 1% to 5% of silicate is added to the electrolyte in 8 M KOH. The quantity of electrolyte is large, and therefore, at the oxidized state (zinc completely discharged), zinc is mainly present in the form of zincate in the electrolyte.

[0022] The positive effect of silicate in certain energy storage applications is the greater solubility of zinc in the form of zincate, as in the French patent FR 2 630 862 mentioned above, and in the zinc-air flow cell below. It can be used in particular in a zinc-air flow electrolyte cell or in a flow cell with a zinc electrode dispersed in the electrolyte, to increase the amount of zinc dissolved in the alkaline electrolyte, which is necessary to increase the mass and volumetric energy density of the cell.

[0023] Indeed, A. Gordon Briggs et al. (J. Chem. Soc, Faraday Trans. 2, 1974, 70) showed that the addition of potassium silicate at a molar concentration of 0.15 mol / l (equivalent to 9 g / l of silica) allows the dissolution of zinc in the form of an electrochemical species in a 10 mol / l potassium salt solution, in an amount 30% greater than in the absence of silicate. They also observed that the stability of the supersaturated solution (i.e. without precipitation of zinc oxide) increases over time in the presence of silicate. P.C. Foller (J. Appl. Electrochem. 16, 1986, 527) describes a zinc-air cell using a dispersed zinc electrode. Silica is used at 25-28 g / l in a potassium salt solution at a molar concentration of 12 mol / l. In the presence of silicate, the discharge duration of zinc is doubled for the same volume of electrolyte, which again demonstrates an increase in the solubility of zinc. It is noted that Thomas Edison already proposed the same factor, i.e. 2, in 1907. The authors believe that the solution without silicate appears white with a precipitate of zinc oxide, while the solution with silicate is darker, which is related to the formation of a quasi-colloid. At 30 g / l of silica, the effect of silicate becomes detrimental due to the sharp increase in viscosity and the passivation of the zinc surface by deposition of silica, which reduces the discharge. The same electrolyte mixture is also used in US patent 5 006 424 (University of California) of April 9, 1991, for a zinc-air cell with a layer of zinc particles.

[0024] By the same token, French patent FR 2 214 977 (Shell Int Research) of 19 August 1974 mentions, to increase the solubility of zincate ions in a caustic electrolyte solution at a molar concentration of 5-7 mol / l, the addition of silicate ions to the electrolyte solution before the onset of zinc oxide precipitation. This patent claims an electrolyte with KOH of 2-4.75 mol / l and silicate ions of 9-36 g / l, i.e. 5.86-23.5 g / l of Si02. The authors also mention that this electrolyte can limit the passivation of the zinc electrode in primary batteries. For secondary batteries, in particular those using a microporous electrode, it is proposed to entrap zinc oxide in the pores of the electrode and then to inhibit dendrite growth and shape change during charging, thus improving the lifetime. If the battery uses metallic zinc as the electrode and is suspended in the electrolyte, passivation can be prevented or delayed. The authors mention that no beneficial effect is obtained when the silicate ion concentration is greater than 23 g / l or 15 g / l of Si02.

[0025] US patent US 4 147 839 (Diamond Shamrock Corp.) of 3 April 1979 protects the use of an electrolyte which is a 45% KOH electrolyte containing 54 g / l of potassium silicate, characterised by a K20 to Si02ratio of 1 :2.2, corresponding to 37 g / l of Si02. The field of application of this patent is very specific. The US 4 147 839 patent applies to a zinc negative electrode formed by a layer of active metal suspended in the electrolyte and a current collector, the sought principle being to make all the reaction products completely soluble during discharge and to reverse the reaction during charging. Again, the silicate is said to increase the solubility of the reaction products, as mentioned in French patent FR 2 214 977.

[0026] Finally, J.Y Huot studied the effect of silicates on the corrosion of zinc (J. Appl. Electrochem. 22, 1992, 443). The results show that the addition of 2 g / l of silica slows down the corrosion of zinc powder in alkaline medium, but this effect is attenuated when zincate is present in the solution.

[0027] In addition to the study of the reduction of zinc corrosion in alkaline medium described by J.Y. Huot, all the documents analysed relating to the use of silicates show that the effect of silicates is intended to increase the solubility of zinc, with the side effect of increasing passivation, in particular for high concentrations of silicates.

[0028] For electrolyte flow batteries or batteries in which the zinc electrode is dispersed in a flowing electrolyte, it is logical to seek to increase the solubility of zinc.

[0029] However, for secondary generators with more or less limited volume of electrolyte, and for secondary generators with solid anodes, the use of silicates logically appears inappropriate. Indeed, in this configuration, upon continuous charging, the zinc in discharge dissolves in large quantities, leading to a significant redistribution of zinc between the anodes or inside each anode, and to the formation of densification zones inside the anodes. This series of actions gradually reduces the available capacity of the anodes, thus accelerating the imbalance of the cathode charge state.

[0030] Z.P. Arkhangel'skaya et al. (Russ. J. Appl. Chem. 70, n° 1 1997, 68) in their study particularly highlighted these logical inconsistencies, especially in nickel-zinc accumulators.

[0031] On the contrary, the goal pursued by these configurations of generators is to reduce the solubility of zinc, in order to limit its redistribution during charging, as described in the claims of US 5 556 720 (Charkey) published on September 17, 1996.

[0032] Thus, after analyzing the state of the art, patents and studies on alkaline batteries using silicates, it was found that they all emphasize the possibility of using silicates to increase the solubility of zinc in alkaline medium. None of these documents mentions or seeks to improve the stability and cycle life of the battery, which is achieved by the combined action of silicates and electrically conductive ceramics, preferably titanium nitride, which limits the drying of the battery and the passivation of the zinc electrode. SUMMARY

[0033] To find a way to avoid the drying of the accumulator and the densification of the zinc electrode, various parameters affecting the stability of the zinc electrode during charging of the accumulator were studied.

[0034] It was found that two particular parameters had a beneficial effect: the addition of certain electrically conductive ceramics to the zinc anode, in particular TiN, and the addition of silicates to the electrolyte, see respectively Figure 3 batteries 1 and 2 in. The addition of these ingredients alone can give effective results, but the benefit of adding ceramics is greater than that of adding silicates alone to the electrolyte, as described in French patent FR 2788887. The two ingredients, electrically conductive ceramics and silicates, can respectively inhibit the formation of zinc dendrites and reduce the passivation of the zinc electrode. The capacity drop observed finally is related to different mechanisms, the drying of the accumulator being related to the electrically conductive ceramics and the passivation of the zinc electrode being related to the silicates, the silicates added here corresponding to 45 g / l of SiO2, much higher than the 30 g / l indicated by P.C. Foller.

[0035] The results show that the simultaneous use of both components produces a significant increase in the beneficial effect compared to the effect produced by each component used alone. This result is due to a new mechanism, since it is related to an increase in the oxygen complex on the conductive ceramic, which is not limited by the passivation of the zinc electrode: that is, a double inhibition is achieved, on the one hand, of the negative effects and, on the other hand, of the positive effects limited when each component is used alone.

[0036] Therefore, the subject of the present application is a zinc anode rechargeable alkaline electrochemical generator containing a conductive ceramic, whose alkaline aqueous electrolyte has a molar concentration of 4 M to 15 M of hydroxide anions, and a concentration of silicates, expressed as silicon dioxide (SiO2), of about 0.15 to 80 g / l.

[0037] More specifically, the present application relates to an electrochemical generator that complies with the following point 1 :

[0038] 1. - Secondary electrochemical generator with zinc electrode, characterized by containing:

[0039] a) an electrolyte, which is an alkaline aqueous solution, having a molar concentration of 4 M to 15 M of hydroxide anions, and a concentration of soluble silicates, expressed as silicon dioxide (SiO2), of 0.15 to 80 g / l;

[0040] b) a zinc electrode containing a conductive ceramic, which contains at least part of nitrides and / or hafnium carbides and / or carbides and / or nitrides and / or magnesium silicides and / or carbides and / or niobium nitrides and / or carbides and / or nitrides and / or titanium silicides and / or vanadium nitrides and / or carbides and / or azides of any two of the metals selected from hafnium, magnesium, niobium, titanium and vanadium.

[0041] The following points 2 to 8 will indicate the advantageous characteristics of the electrochemical generator of the above point 1 :

[0042] 2. - The electrochemical generator according to point 1, whose zinc electrode contains a conductive ceramic containing titanium nitride.

[0043] 3. - The secondary electrochemical generator according to point 1 or 2, whose alkaline solution has a molar concentration of between 7 and 13 M.

[0044] 4. - The secondary electrochemical generator according to any of points 1 to 3, whose electrolyte has a concentration of silicates, expressed as silicon dioxide, of 20 to 60 g / l.

[0045] 5. - The secondary electrochemical generator according to any of points 1 to 4, whose alkalinity of the electrolyte solution is provided by lithium hydroxide, sodium hydroxide or potassium hydroxide, alone or in mixture.

[0046] 6. - The secondary electrochemical generator according to any of points 1 to 5, whose silicates are provided by silicas, fumed silicas, fumed silicas, potassium or sodium silicate, potassium and sodium disilicate, potassium and sodium metasilicate, potassium and sodium tetrasilicate, which can be used individually or in mixture.

[0047] 7. - The secondary electrochemical generator according to any of points 1 to 6, whose electrolyte further comprises zincates.

[0048] 8. - The secondary electrochemical generator according to any of points 1 to 7, whose electrolyte further comprises borates, phosphates and / or fluorides. BRIEF DESCRIPTION OF DRAWINGS

[0049] Other characteristics and advantages of the present application will now be explained in detail in the following detailed description of the invention, with reference to the attached drawings, whose content is the following:

[0050] [ Figure 1 ] : 4.6 Ah nickel-zinc battery charge-discharge voltage as a function of the percentage of the battery A and B rated capacity C = 4.6 Ah;

[0051] [ Figure 2 ] : Voltage and pressure of 3 Ah nickel-zinc batteries C and D in 3 formation cycles, with or without silicates;

[0052] [ Figure 3 ] : Capacity curve measured at discharge of 8 Ah nickel-zinc batteries during cycling (8 A charge for 1 hour, discharge 8 A, 1 V, discharge depth 100%)

[0053] 1 : Anode with TiN and electrolyte without silicates,

[0054] 2 : Anode without TiN and electrolyte with silicates,

[0055] 3 : Anode with TiO2 and electrolyte with silicates,

[0056] 4 : Anode with TiN and electrolyte with silicates;

[0057] [ Figure 4 ] : Figure 3 Mass loss accumulated by 8 Ah nickel-zinc batteries during cycling,

[0058] 1 : Anode with TiN and electrolyte without silicates,

[0059] 2 : Anode without TiN and electrolyte with silicates,

[0060] 3 : Anode with TiO2 and electrolyte with silicates,

[0061] 4: TiN-containing anodes and silicate-containing electrolytes;

[0062] [ Figure 5 [The following is a capacities chart:] Capacity curve of an 8Ah nickel-zinc battery measured during discharge (8A charging for 1 hour, discharging at 8A and 1V, depth of discharge 100%).

[0063] 5: TiN-free anode and silicate-containing electrolyte,

[0064] 6: TiN-containing anode and silicate-free electrolyte,

[0065] 7: TiN-containing anodes and silicate-containing electrolytes;

[0066] [ Figure 6 ]: Figure 5 The accumulated mass loss of an 8Ah nickel-zinc battery during cycling.

[0067] 5: TiN-free anode and silicate-containing electrolyte,

[0068] 6: TiN-containing anode and silicate-free electrolyte,

[0069] 7: TiN-containing anodes and silicate-containing electrolytes;

[0070] [ Figure 7 [This refers to the capacity curve of an 8A nickel-zinc battery during discharge during the cycling process (8A charging for 1 hour, discharging at 8A and 1V, depth of discharge 100%).

[0071] 8: TiN-containing anode and silicate-free electrolyte,

[0072] 9: TiN-containing anode and 0.45M silicate-containing electrolyte.

[0073] 10: TiN-containing anode and 0.85M silicate-containing electrolyte. Detailed Implementation

[0074] Zinc anode batteries are manufactured according to methods known to those skilled in the art. The electrodes are plate-shaped and consist of a current collector and an active material. The active material may be doped with compounds that do not participate in the electrochemical reaction, which will provide, for example, electronic conduction or mechanical connection between the active material and the current collector, or even provide retention of electrochemical reaction products.

[0075] For zinc anodes, in addition to polymers such as polytetrafluoroethylene (PTFE), polyethylene glycol, polyvinyl alcohol, styrene butadiene rubber, carboxymethyl cellulose, etc. that ensure the binder function of the electrode composition, calcium hydroxide can be used to limit the formation of soluble zincates, as can conductive ceramics (as described in French patent 2 788 887).

[0076] A separator is used to separate the anode compartment from the cathode compartment: it can be a felt, a porous membrane or an ion exchange membrane, or a combination of felt and porous membrane can be used.

[0077] Depending on the manufacturing method, the zinc anode battery can be prismatic, cylindrical, or, if the battery is bipolar, in the form of a filter-press battery.

[0078] The present application is particularly suitable, but not exclusively, for the manufacture of nickel-zinc batteries designed according to the following main features.

[0079] According to one preferred embodiment, a plasticized nickel electrode and a zinc electrode, also containing an organic binder, are combined to make a nickel-zinc battery.

[0080] 1) Nickel electrode

[0081] The nickel electrode is preferably made using fine-pored nickel foam. Certain foams are designated as "battery grade". Suppliers include Sumitomo Electric (Japan) and Kelong New Energy (China). The thickness of the foam is chosen according to the surface capacity required of the nickel electrode: the thickness is usually between 1.2 and 2 mm, but it can be pressed to fine-tune the thickness to achieve the required surface capacity.

[0082] The active ingredient consists of nickel hydroxide, which preferably contains zinc and cobalt co-precipitated particles. The particles are preferably spherical or near-spherical to increase the volumetric capacity. The particles can be coated with oxides and cobalt hydroxide, which will be converted into electrically conductive cobalt oxyhydroxide (Oshitani et al., J. Electrochem. Soc. 1989 136, 6, 1590) during the formation of the battery.

[0083] Conductive additives (fibres, metal powders) can also be added to the nickel hydroxide powder.

[0084] The various ingredients described above are mixed together with deionized water to which carboxymethyl cellulose has been added to make a paste. A polymeric binder (e.g. PTFE) can be added at this stage in the form of a suspension, or the current collector, in particular the nickel foam, can be subsequently filled or covered by immersing the active paste in the suspension.

[0085] In the laboratory, the filling of the foam nickel is done by using a spatula to penetrate the paste into the thickness of the support, while in industry, the paste can be injected under pressure into the foam to complete the filling of the foam nickel.

[0086] After drying, the electrode is compressed to ensure cohesion between the current collector, the active material and the additives, and cut to the desired size.

[0087] 2) Zinc electrode The zinc electrode current collector can be a perforated metal strip, a woven cloth shape, an expanded strip or a metal foam. Copper, because of its electrical conductivity, can be the first choice, but must be covered with a protective metal: zinc, tin or an alloy.

[0088] Before making the zinc electrode, a paste composed of zinc oxide and various additives is prepared beforehand:

[0089] - electronic conductor: zinc metal, carbon, copper, conductive ceramics in powder or filament form, etc.

[0090] - corrosion inhibitor: indium, bismuth, etc.

[0091] - compounds that react with zincates: calcium hydroxide, barium hydroxide, etc.

[0092] The liquid phase is deionized water or an alcohol to which carboxymethylcellulose has been added as a binder and thickener. Other binders can be added, such as those mentioned in European patent EP 1 715 536.

[0093] Depending on the technique chosen, a high-viscosity paste can be prepared, which is applied under pressure on both faces of the metal support to form a "sandwich" structure; or a paste of medium viscosity can be prepared, the current collector is immersed in it, then removed and the excess paste is removed with a spatula to adjust the thickness of the electrode, which is then dried. Finally, a dry powder mixed with the binder can be used and pressed onto the metal support to constitute the electrode.

[0094] 3) electrolytes

[0095] The electrolyte in which the silicates are added is preferentially a concentrated alkali solution with a molarity of 4 to 12 M (4-12 m / l) of hydroxide anions. The alkalinity is provided by potassium hydroxide, sodium hydroxide and lithium hydroxide, alone or in mixture.

[0096] The amount of silicates added to the electrolyte is expressed in mass of silicon dioxide per liter, and is between 0.15 and 80 g / l, preferably between 20 and 60 g / l. The electrolyte can also contain different proportions of zincates.

[0097] Silicates are provided by silica, fumed silica, fumed silica, potassium and sodium silicates (such as potassium disilicate and sodium disilicate, potassium metasilicate and sodium metasilicate, potassium tetrasilicate, sodium orthosilicate). These silicates can be used alone or in mixture.

[0098] The electrolyte can also contain borates, phosphates and fluorides, used alone or in mixture, as described for example in US patent US 5215 836.

[0099] oxygen recombination process

[0100] To clarify the method followed to achieve the present application, the analysis of the mass loss and oxygen recombination of a nickel-zinc battery will be described first. The charging process of the nickel electrode involves an oxygen evolution reaction which occurs before the nickel electrode is fully charged. To limit the oxygen expulsion from the battery through the sealing valve, which causes the battery to dry up, we want to recombine the oxygen inside the battery, thus limiting the capacity drop. The oxygen recombination on the surface of the zinc electrode can occur according to two mechanisms:

[0101] (1) a chemical oxidation, with the reaction 2Zn + O2+ 2H2O → 2Zn(OH) 2,

[0102] (2) a catalytic reduction of the conductive areas (such as zinc metal or titanium nitride), with the reaction:

[0103] O2+ 2H2O + 4e → 4OH and / or O2+ H2O + 2e → HO2 - + OH - (only during charging).

[0104] To be effective, both reaction mechanisms require a three-phase contact point, i.e. solid-liquid-gas, where the gas is the oxygen on the surface of the zinc electrode. Therefore, the oxygen reaching the surface of the zinc electrode is a decisive parameter to ensure the completion of the oxygen recombination.

[0105] Various barriers exist which limit the oxygen access to the surface of the zinc anode. Classified according to the degree of limitation from high to low, they have different physical state characteristics: the liquid phase is the electrolyte on the surface; the solid phase is the porous separator placed on the surface of the zinc electrode; the gaseous phase is the hydrogen produced at the surface of the anode. When the level of the electrolyte is low, it increases the amount of electrolyte at the interface, which indirectly causes this evolution of hydrogen.

[0106] According to the general description given above, two nickel-zinc batteries A and B with a nominal capacity of 4.6 Ah were made using the same electrode and electrolyte composition. The electrolyte used was a KOH concentrated base solution with a molar concentration of 10 M of hydroxide anions, without the addition of silicates.

[0107] Titanium nitride is homogeneously introduced into the zinc anode active material of battery A. For battery B, the same amount of TiN is preferably deposited on the surface of the zinc electrode. These batteries are sealed and equipped with a valve that can be opened at a pressure of more than 2 bar.

[0108] Batteries A and B are charged at a C / 10 rate in the 100%-180% state of charge and then discharged at a C / 5 rate with a terminal voltage of 1.2 V, as shown in Table 1 and Figure 1 The example is for a charge of 180%. The mass loss (AM) of the batteries is measured and the gas recombination rate (TRG) is calculated by converting the difference in ampere-hour values between charge and discharge into the mass of water that can be lost, as shown in Table 1 below.

[0109] For battery A, the gas recombination rate decreases as the state of charge increases. For battery B, this has not yet been verified, which is characterised by a halving of the overall mass loss and an increase in the recombination rate of the gas at a state of charge of 180% compared to the value calculated at a state of charge of 140%.

[0110] According to process (2), this result shows that the catalytic activity of TiN on oxygen is enhanced, which we attempt to illustrate by comparison, i.e. this effect is exacerbated when TiN is mainly located on the surface of the anode compared to the same total amount of TiN distributed throughout the volume of the anode.

[0111] The present invention aims to enhance the catalytic activity of TiN on oxygen recombination in order to inhibit the passivation of the negative electrode and the dry-out mechanism of the zinc anode battery.

[0112] Table 1

[0113]

[0114] In order to determine new examples that can inhibit the dry-out and passivation of the negative electrode mechanism of the generator described in the present invention, the authors carried out comparative tests on identical nickel-zinc batteries C and D (without electrolyte) with a capacity of 3 Ah, according to the general description above. Their zinc anode contains TiN.

[0115] For battery C, the electrolyte used is a concentrated alkaline solution with a molar concentration of 10 M of hydroxide anions. For battery D, the electrolyte is modified by adding 0.82 M of silicate (provided by silicon dioxide).

[0116] The internal pressure of the batteries is measured using a 0-10 bar pressure sensor, the batteries being equipped with a safety valve that can be opened at a pressure of more than 0.85 bar.

[0117] The initialization of the two batteries was achieved by 3 cycles, charging at C / 10 for 12 hours and then discharging at C / 5 until reaching the end voltage of 1.2 V.

[0118] Figure 2 The voltage and pressure of the batteries C and D are given for 3 cycles.

[0119] The pressure measurement is characterized by the fact that the valve does not open for the battery D containing silicate in the electrolyte. On the contrary, the internal pressure of the battery C, which does not contain silicate in the electrolyte, is characterized by the opening of the valve, especially in the region where the charging begins, for which the hydrogen evolution at the zinc electrode is given priority.

[0120] This result shows that the presence of silicate in the electrolyte reduces the hydrogen formation and improves the oxygen recombination.

[0121] Without silicate, the low hydrogen overvoltage of TiN creates a limited hydrogen in the center of the porous anode. A part of the electrolyte in the pores of the zinc electrode moves towards the interface of the electrode. Then, the thickness of the electrolyte film at the interface of the electrode increases and limits the oxygen access to the catalytic recombination zone of TiN. Thus, the chemical recombination process (1) will preferentially recombine on the metallic zinc, which will lead to a drying out and passivation phenomenon due to the formation of Zn(OH)2, resulting in a gradual decrease in capacity.

[0122] With silicate, the pressure measurement shows a significant reduction in the amount of hydrogen evolution. By analyzing the phenomenon, a hypothesis can be made, which does not limit the scope of the invention, according to which the silicate, by depositing on the surface of the zinc electrode, separates TiN and the metallic zinc, thus creating an insulating interface between the two, which ultimately reduces the hydrogen evolution. Thus, the oxygen contact with TiN is improved and then recombination according to the catalytic process (2) is preferred, which does not lead to a drying out and passivation phenomenon.

[0123] In order to continue to explain and illustrate the function and definition of the invention by example, nickel-zinc batteries 1, 2, 3 and 4 with a nominal capacity of 8 Ah were made according to the general description above, in the same way.

[0124] These batteries are equipped with a low pressure valve of 0.2 bar. The nickel hydroxide used for the cathode of the batteries 1 to 4 contains 5% of cobalt. For the battery 1, the electrolyte used is a concentrated alkaline solution with a molar concentration of 10 M of hydroxide anions. For the batteries 2, 3, 4, the electrolyte is modified by the addition of silicate. The parameters that distinguish the batteries 1 to 4 are summarized in table 2 below.

[0125] Table 2: Characteristics of nickel-zinc batteries (nominal capacity 8 Ah, cycling charge 8 A 1 h, discharge 8 A 1 V 100%).

[0126]

[0127] In the battery 3, the titanium nitride is replaced by a titanium oxide compound.

[0128] The batteries were cycled at constant current 8 A (equivalent to C mode), charged for 1 hour and discharged until the voltage reached 1 V.

[0129] Figure 3 and Figure 4 The evolution of the discharge capacity and the cumulative mass loss of the batteries 1 to 4 as a function of the number of cycles was compared. It can be seen that the stability of the battery capacity increases in the following order:

[0130] • addition of silicate only, corresponding to batteries 2 and 3 (TiO2 does not provide the role of TiN),

[0131] • addition of TiN only, corresponding to battery 1,

[0132] • addition of TiN and silicate, corresponding to battery 4.

[0133] The results are given in Table 2. We observed that batteries 2, 3, 1 and 4 retained 90% of the initial capacity after 770, 770, 1120 and 1380 cycles respectively. After more than 90% of the remaining capacity, the capacity of the batteries other than battery 4 (containing TiN and silicate) decreased rapidly. Battery 4 (containing TiN and silicate) retained more than 75% of the initial capacity after 1885 cycles, which is an increase of 50% compared to battery 1 which decayed to 75% of the initial capacity after 1260 cycles.

[0134] The mass losses of batteries 2 (containing silicate but not TiN) and 3 (containing silicate and TiO2) were the greatest, indicating that oxygen complexing is not sufficient to limit the dry-out of the battery. From this, it can be seen that the passivation of the zinc electrode is in agreement with the article by P.C. Foller, which shows that when the amount of silica added is greater than 28-30 g / 1, the passivated silica deposits on the zinc surface. This deposit limits the complexing capacity of the zinc electrode.

[0135] Within the scope of the application, the authors also emphasize that the complex formed between silicate ions and zincate ions in alkaline medium, especially in the porous zinc anode, presents a stability which, compared to the smaller monomer of zincate ions, is measured with a lower mobility because, by complexing the zincate with the silicate, the steric effect of the zincate is increased. By complexing with silicate ions, the mobility of the zincate ions is limited, which constitutes an attribute which contributes to reducing the redistribution of zinc in the anode and the densification phenomenon.

[0136] The formation of complexes between silicate ions and zincate ions in alkaline media has been mentioned by several authors and was described by Michel R. Anseau et al. (Inorg. Chem. 44, 8023-8032, 2005) for the case of very strong alkaline media: zincate reacts with monomeric, dimeric and cyclic trimeric silicates in 14-15 mol / l potassium or sodium hydroxide solutions to form very stable compounds.

[0137] The mass loss of batteries 1 and 4 (containing TiN) is much lower than the mass loss of batteries 2 and 3. It is remarkable that the progression of batteries 1 (containing TiN) and 4 (containing TiN and silicate) is different:

[0138] • For battery 4, the mass loss is very moderate and linear from 0 to 900 cycles, while for battery 1 the mass loss accelerates from 300 cycles. As expected, this result is consistent with the conclusion that the complexation of zincate and silicate leads to an improvement in the stability of the zinc electrode by reducing its densification.

[0139] • Between 900 and 1260 cycles for battery 1 and between 1000 and 1300 cycles for battery 4, the mass loss increases sharply due to the increasing amount of oxygen evolution at the end of charge.

[0140] • For battery 1, the mass loss becomes excessive from 1200 cycles, which means that the capacity decays rapidly after the battery has dried out and the electrode has passivated.

[0141] • Battery 4 behaves strangely, characterized by a moderate overall mass loss, which becomes weak beyond 1300 cycles, which means that the effective oxygen recombination persists, limiting the drying out of the battery and the passivation of the electrode.

[0142] It can therefore be confirmed that the addition of titanium nitride in the anode, as for battery 1, or the addition of silicate in the electrolyte, as for battery 2, inhibits the formation of dendrites and reduces the densification of the zinc electrode, indeed with a satisfactory effect on the stability of the zinc electrode.

[0143] However, the addition of titanium nitride is significantly more effective than the addition of silicate in the electrolyte.

[0144] However, the simultaneous addition of titanium nitride to the zinc electrode and of silicate in the electrolyte (battery 4) leads to a very large functional improvement, which means that the oxygen recombination capacity at the zinc electrode is greatly improved, thus inhibiting the drying out of the battery and the passivation of the zinc electrode. The silicate enhances the oxygen catalytic reduction capacity of the conductive ceramic (such as titanium nitride) (not yet determined).

[0145] In the following comparisons, nickel-zinc accumulators 5, 6 and 7 of 8 Ah nominal capacity were prepared in the same way, according to the general description above.

[0146] These batteries were fitted with a low pressure valve at 0.2 bar.

[0147] The nickel hydroxide used for the cathode of batteries 5 to 7 contained 8% of cobalt. Increasing the amount of cobalt increases the electrical conductivity of the nickel hydroxide. Oxygen evolution is reduced, the mass accumulation loss is reduced and the charge performance of the nickel electrode is thus improved.

[0148] For battery 6, the electrolyte used was a concentrated alkaline solution with a molarity of 10 M of hydroxide anions. For batteries 5 and 7, the electrolyte was modified by adding a large amount of silicate. The anode of battery 5 did not contain TiN.

[0149] The parameters of batteries 5 to 7 are summarized in table 2 above.

[0150] Batteries 5 to 7 were cycled at a constant current of 8 A (equivalent to C mode), charged for 1 hour and discharged until the voltage reached 1 V.

[0151] Figure 5 and Figure 6 The evolution of the discharge capacity and the cumulative mass loss of batteries 5 to 7 as a function of the number of cycles was compared. The results are given in table 2 above.

[0152] First, without TiN but with silicate (battery 5), then with TiN but without silicate (battery 6), and finally with both TiN and silicate (battery 7), the stability of the capacity of the batteries improved in this order, the remaining capacity reaching 90% of the initial capacity after 500, 1200 and 1700 cycles respectively, see table 2. Compared to batteries 2 and 3 (0.75 M, i.e. 45 g / 1 Si02), battery 5 (0.45 M, i.e. 27 g / 1 Si02) has a lower cycle life, which shows that the addition of silicate at a concentration (expressed in Si02) greater than 30 g / 1 has a positive effect, contrary to the conclusion described in the state of the art which shows a decrease in capacity due to passivation by Si02 precipitation or by too high a viscosity of the electrolyte (article by P.C. Foller).

[0153] At 90% of the initial capacity, the capacity stability of battery 7 was improved by 42% compared to battery 6. The evolution of the mass loss of batteries 6 and 7 was similar to that of batteries 1 and 4, confirming that the batteries with a zinc electrode containing titanium nitride and an electrolyte containing silicate have a singular oxygen recombination capacity.

[0154] Finally, nickel-zinc batteries 8, 9 and 10 with a nominal capacity of 8 Ah were made in the same way as described above. These batteries were equipped with a low pressure valve at 0.2 bar. The nickel hydroxide used in the cathode of batteries 8 to 10 was the same as for batteries 5 to 7. The zinc electrode of batteries 8 to 10 contained titanium nitride.

[0155] In comparison with batteries 6 and 7, the preparation of the paste of the zinc electrode active material was improved in a way that can increase the homogeneity of the paste active material composition, without changing the chemical composition.

[0156] For battery 8, the electrolyte used was a concentrated alkali solution with a molar concentration of 10 M of hydroxide anions. For batteries 9 and 10, the electrolyte was modified by adding a large amount of silicate with a molar concentration of 0.45 M and 0.85 M, respectively. Batteries 8 to 10 were cycled at a constant current of 8 A (equivalent to C mode) with a charge of 1 hour and a discharge ended at a voltage of 1 V.

[0157] Figure 7 The evolution of the discharge capacity of batteries 8 to 10 as a function of the number of cycles was compared.

[0158] The results are shown in Table 2 above.

[0159] Battery 8 (containing TiN but no silicate) retained only 70% of the initial capacity after 1300 cycles, while battery 9 (containing both TiN and 0.45 M silicate) retained 70% of the initial capacity after 2640 cycles, which is an improvement of 103% over battery 8. Battery 10 (containing both TiN and 0.85 M silicate) retained 70% of the initial capacity after 2680 cycles, which is an improvement of 106% over battery 8.

[0160] These different results show that the simultaneous use of titanium nitride in the anode and the addition of silicate in the electrolyte can have a surprising effect, with an improvement of more than 100% in the number of cycles.

[0161] As expected by the authors of the present invention, these results also reflect a decrease in the densification of the zinc electrode and a change in the shape of the zinc electrode, which is related to the formation of zincate-silicate complexes, which limit the mean free path of zincate ions in the electrolyte (including inside the anode), thus reducing the redistribution of zinc.

[0162] In addition, the combination of a conductive ceramic (such as TiN) and silicate can promote the recombination of oxygen by the catalytic process (2) described above, thus modifying the ability of the zinc electrode to recombine oxygen.

[0163] Without TiN, the silicate no longer passivates the surface of the zinc electrode. These phenomena can limit the dry-out of the battery and significantly improve the cycle life.

[0164] Thus, in combination of the addition of electrically conductive ceramic (such as TiN) to the zinc anode and the addition of silicate to the electrolyte, we measured the above-mentioned multiple new effects, which are the subject of the present invention.

[0165] Of course, as mentioned above, the present invention is not limited to the specific embodiments which have been described by way of illustration and example. The present invention is not limited to the embodiments given, but covers all variant embodiments.

Claims

1. A secondary electrochemical monogenerator characterized by comprising: a) a single static electrolyte, which is an aqueous alkaline solution containing hydroxide anions at a molar concentration of 7 M - 13 M, containing soluble silicates at a concentration expressed as Si02 of 20 g / l - 60 g / l; b) a zinc electrode, said zinc electrode being a perforated metal strip, a woven cloth, an expanded metal strip or a metal foam, said zinc electrode having a paste composed of zinc oxide and additives, said paste being then dried, said zinc electrode containing titanium nitride throughout its volume; and c) a nickel electrode, said nickel electrode comprising nickel hydroxide.

2. The secondary electrochemical monogenerator according to claim 1, the alkalinity of the electrolyte solution being provided by lithium hydroxide, sodium hydroxide or potassium hydroxide, alone or in mixture.

3. The secondary electrochemical monogenerator according to claim 1 or 2, the silicates being provided by silicon dioxide, fumed silica, fumed silica, potassium or sodium silicate, potassium and sodium disilicate, potassium and sodium metasilicate, potassium and sodium tetrasilicate, these silicates being used alone or in mixture.

4. The secondary electrochemical monogenerator according to claim 1 or 2, the electrolyte further comprising zincate.

5. The secondary electrochemical monogenerator according to claim 1 or 2, the electrolyte further comprising one or more of borate, phosphate and fluoride.

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