Patent Application
20240136600
The present invention relates to the field of accumulators and more particularly that of alkaline electrochemical generators. It relates especially to zinc-manganese dioxide secondary generators with a high number of charge-discharge cycles.
Zn/MnO2 primary batteries are commonly used, while secondary batteries of this Zn/MnO2 system remain a challenge. The reversibility of this system is considered very limited. On the positive pole side, cathode MnO2, irreversible compounds are formed during discharge, such as hausmannite (Mn3O4) and hetaerolite (ZnMn2O4), leading to a rapid and significant loss of capacity. On the negative pole side, the Zn anode, the phenomena of shape changes, passivation, and dendrite formation limit the reversibility particularly when the zinc anodes are charged-discharged at high current densities and the utilization rate of the theoretical capacity is greater than 10%2′3.
The reduction of MnO2 in the alkaline medium takes place in two stages: First stage of reduction: MnO2+H2O+e−->MnOOH+OH− (1) Second stage of reduction: MnOOH+H2O+e−->Mn(OH)2+OH− (2) The first stage of reduction is characterized by a drop in voltage from 1.5 V to 0.9 V. The potential of the homogeneous solid-state insertion of electrons-protons into MnO2, reducing the oxidation state of the Mn atoms from 4+ to 3+, resulting in the formation of MnOOH. However, this process is also associated with a partial amorphization mechanism of the host crystallographic structure as a result of steric strains related to the insertion of protons (H+) and the increase in ionic radiation of the Mn cation (Mn3+ vs. Mn4+). This insertion can be reversible in a limited range of H+ insertion in accordance with a limited amorphization process, up to H0.7MnO2 (1.1-1 V) according to Patrice5 or H0.7MnO2 according to Gallaway6. MnOOH can further be reduced below 0.9 V, to Mn(OH)2, in what is called the second stage electron and via a heterogeneous dissolution (MnOOH to Mn3+)-reduction (Mn3+ to Mn2+)-precipitation (Mn2+ to Mn(OH)2) process. According to several studies, the second stage can take place before the completion of the first stage (the insertion of the first electron) due to the instability of MnOOH and the dissolution of Mn3+ in the electrolyte. Mn(OH)2 forming in the reaction (2) is considered a reversible discharge product, but it is not the only product that is formed in practical cases and in exchange for a Zn electrode. MnOOH is susceptible to reacting with the ions of the electrolyte according to a mechanism described by the chemical reactions (3) and (4) involving the formation of electrochemically inert reaction products, hetaerolite (ZnMn2O4) and hausmannite (Mn3O4):
2MnOOH+Zn(OH)42−->ZnMn2O4+2OH−+2H2O (3)
2MnOOH+Mn(OH)42−->Mn3O4+2OH−+2H2O (4)
The irreversibility of the reduction of MnO2 is therefore strongly correlated with the formation of the compounds Mn3O4 and ZnMn2O4. Once Mn3O4 is formed during the first discharge, this compound can only be partially reduced to Mn(OH)2 while ZnMn2O4 is not reduced to Mn(OH)2 at all. To increase the number of charges-discharges of MnO2 electrodes, different approaches were tried. Kordesch et al.1,7,8,9 showed that γ-MnO2 is rechargeable with a greater number of cycles only if the discharge is limited to less than 35% of the first reduction stage. More recently, Ingale et al.10 followed this strategy aimed at improving cyclability with a limited capacity of 5% to 20% DoD (depth of discharge). With inexpensive battery materials, they show a stable capacity during 3,000 cycles for 10% DoD and 500 cycles for 20% DoD. Other studies have shown an improvement in the life of the MnO2 electrode by using additives such as CeO2, MgO, TiS2, Bi2O3 or Ba- and Sr-based compounds allowing a limitation of the inert phases of manganese oxide and a maintenance of the γ-MuO2 phase during the cycling11′12′13′14,15,16.
Kannan et al.13 show that MnaO4 with Bi2O3 has a capacity that increases over 100 cycles to more than 400 mAh/g, demonstrating that the addition of Bi2O3 is capable of reactivating the normally inactive Mn3O4 compound. However, it should be noted that these cathodes cannot be charged-discharged for more than 20 or 30 cycles in the presence of zinc anodes, as the dissolved zincate ions combine with the manganese ions to produce inactive ZnMn2O4 during the process of discharging the second electron from the MnO2 cathode.
As other examples, a flat plate Zn/MnO2 cell was developed by Stani et al.17 with MnO2 modified by the addition of BaSO4, which minimizes capacity loss, but only 25 to 30 cycles were obtained.
Recently, Yadav et al.18 performed 1,000-6,000 cycles for the complete process of the two discharge stages. This result is correlated with the stabilization of the δ-MnO2 birnessite structure by the insertion of bismuth and copper additives as well as the addition of carbon nanotubes. This result is, however, obtained opposite a nickel electrode, meaning without the presence of zincate ions. Although a remarkable achievement, the MnO2 electrodes are still sensitive to Zn poisoning (irreversibly forming ZnMn2O4) and also required an initial procedure in exchange for a Ni(OH)2 electrode in order to “form” the electrodes before they could be paired with a Zn anode to form the Zn/MnO2 battery. In comparison, only 900 cycles are obtained in the presence of a zinc electrode19. In addition, the latter result is obtained with the addition of a Ca(OH)2 membrane to trap zincate ions and limit the formation of ZnMn2O4.
Maria Kelly et al.21 showed that triethanolamine (TEA) as an additive in Zn/MnO2 with a discharge limited to 10% significantly increases stability with 191 cycles without TEA and 568 cycles with TEA. The purpose of the present invention is to provide a novel response to the limits affecting the ability of MnO2 electrode-based accumulators to provide a large number of cycles. To do this, tests concern the addition of Ni(OH)2 compounds to the composition of the positive MnO2-based electrode, this electrode becoming a de facto hybrid, MnO2 and Ni(OH)2 together being electrochemically active. The idea is to provide, by the presence of stable Ni(OH)2/NiOOH crystallites in the alkaline electrolyte, a crystal growth substrate for the MnOOH/MnO2 compounds. This support of crystal growth is susceptible to increasing the reversibility of the electrochemical couple MnOOH/MnO2 by an increase in the coherence domains of these compounds which limits the reactivity of MnOOH with the electrolyte ions and de facto the formation of parasitic reactions in the (3) formation of hetaerolite (ZnMn2O4) and the (4) formation of hausmannite (Mn3O4).
Examination of the state-of-the-art of Zn/MnO2 systems shows that there are several patents and studies mentioning the use of Ni(OH)2 in the MnO2 electrode. A portion of these patents citing an electrochemical cell with a cathode characterized by a mixture of MnO2 and NiOOH relate to primary, and therefore non-rechargeable, systems. It is evident that, with the addition of NiOOH, the Ni(OH)2 charged state, to the MnO2 electrode, the ZnMnO2 battery can be discharged at a higher rate with a higher voltage, providing more energy for devices requiring more power or operating with a higher voltage. For example, the international patent application published under no. WO2006/026232 indicates, for an alkaline battery with a zinc anode, a cathode containing both NiOOH and manganese dioxide. It mentions better capacity maintenance for higher discharge densities with an MnO2/NiOOH mixture. However, this patent concerns primary batteries and not the rechargeable system. There are no reported effects on a possible improvement in the cycling ability of Zn/MnO2 batteries.
Contrary to the results concerning this invention, T. N. Ramesh et al. report that a mixture of MnO2 and Ni(OH)2 materials is not ready for use in an electrochemical storage application. However, an NiO(OH)/MnO2 composite phase obtained by common coprecipitation of the Ni2+ and Mn2+ ions is mentioned as being capable of greatly increasing the capacity by up to 2.25 electrons per metal atom, equivalent to 650 mAh/g. Only 20 cycles have been demonstrated.
Another portion of the patents citing the addition of Ni(OH)2 to the MnO2 electrode relates to rechargeable ZnMnO2 batteries. In these patents, it is mentioned that the addition of Ni(OH)2, which must remain low, makes it possible to reduce the problems of recharging the MnO2 electrodes by limiting the formation of oxygen at the end of the charge. For patent U.S. Pat. No. 5,011,752, the MnO2 used is preconditioned to lower the degree of oxidation of the manganese atom to be compatible with the MnOx formulation, where x is between 1.70 and 1.90. It is reported that 1.68% Ni(OH)2 is added to the composition of the MnO2 cathode to limit the overcharge of manganese dioxide and the formation of oxygen, which are the result of premature failures of the manganese dioxide cathode. The addition of Ni(OH)2 is a reserve of materials capable of reducing the consequences of MnO2 cathode overcharges. No value table is provided to support the beneficial action of adding Ni(OH)2. The Ni(OH)2 content is very low, less than 2%. The number of cycles demonstrated is only 5. In patent CN 104,701,521 A, it is mentioned that 5% by mass Ni(OH)2 is added to the composition of the positive MnO2 electrode, the percentage by mass being relative to the mass of the active material of the positive MnO2 electrode. The charge is capped at a constant voltage of 1.65 V, which limits the formation of oxygen. The voltage of 1.65 V is the quiescent voltage of the NiZn system. As a result, the Ni(OH)2 compound in the MnO2 electrode composition must not participate in the electrochemical reaction because a maximum voltage of 1.65 V does not allow the charge of the compound Ni(OH)2 to NiOOH. The action of Ni(OH)2 is not described, but it is reasonable to think that the desired action is similar to U.S. Pat. No. 5,011,752, with a limitation on the overcharge of manganese dioxide and the formation of oxygen.
In patent JP H10 74511 A, the addition of nickel hydroxide and nickel oxide to a rechargeable MnO2 electrode is mentioned. The composition of the MnO2 electrode is such that the atomic ratio of the Ni atom to the Mn atom corresponds to a range of 2% to 35%. Once again, this invention relates to recharging the MnO2 electrodes in alkaline solution by limiting the generation of oxygen. In this patent, a figure is given making it possible to compare the voltages during charging with and without the addition of Ni(OH)2. Following the addition of Ni(OH)2, the charge curve is characterized by a double plateau before a final increase correlated with the development of oxygen. The second plateau, above 1.85 V, corresponds to the charge of Ni(OH)2. The very pronounced voltage difference between the first and second plateau allows better end-of-charge detection and a reduction in oxygen development.
Without the addition of Ni(OH)2 to the MnO2 electrode, the number of cycles is 18. The addition of Ni(OH)2 makes it possible to increase this number of cycles to 54 and 62, respectively, for 9.1% and 10% atomic for the Ni/(Mn+Ni) ratio. For ratios greater than 10%, the number of cycles decreases to 37 when the Ni/(Mn+Ni) ratio is 45%. It is also indicated that the amount of nickel must remain low in order to preserve the discharge amount of the MnO2 compound.
State-of-the-art research mentioning the addition of Ni(OH)2 to the rechargeable MnO2 electrode is all related to a reduction in the formation of oxygen at the end of charging. In addition, it is mentioned that the amount of nickel must remain low in order to preserve the discharge amount of the MnO2 compound. None of these documents mentions or seeks out an improvement in the stability and cycling life of the battery that would result from a mixture of Ni(OH)2 and MnO2 compounds, making it possible to increase the reversibility of the electrochemical couple MnOOH/MnO2 linked to a substrate action of the Ni(OH)2/NiOOH compounds allowing better coherence of the MnOOH/MnO2 structures.
Patents and studies mentioning the addition of Ni(OH)2 to an MnO2 electrode cannot therefore anticipate the invention described in the present patent, which is based on seeking out this particular and novel effect that makes it possible to stabilize the discharge capacity of an electrode composed of a mixture of MnO2 and Ni(OH)2.
The goal of the present invention is to increase the cycling life of secondary electrochemical generators. It therefore aims to provide a novel response to the limits affecting the ability of the MnO2 electrode to provide a large number of cycles, with a discharge greater than 35% of the first reduction stage, a response provided by modifying the conditions of capacity loss of the MnO2 electrode by limiting the formation of an electrochemically inert crystal structure.
More specifically, the invention relates to a zinc-manganese dioxide secondary electrochemical generator according to the following statement 1:
1. A zinc-manganese dioxide hybrid secondary electrochemical generator, this generator being distinguished in that it comprises:
2. A secondary electrochemical generator according to statement 1, in which the mass of Ni(OH)2 is greater than 20% of the sum of the masses of Ni(OH)2 and MnO2.
3. A secondary electrochemical generator according to statement 1 or 2, in which the zinc negative electrode contains conductive ceramics.
4. A secondary electrochemical generator according to statement 3, in which the zinc negative electrode contains titanium nitride.
5. A secondary electrochemical generator according to one of statements 1 to 4, the molarity of the alkaline solution is between 7 and 13 M.
6. A secondary electrochemical generator according to one of statements 1 to 5, in which the alkalinity of the electrolyte solution is provided by lithium hydroxides and/or sodium hydroxides and/or potassium hydroxides.
7. A secondary electrochemical generator according to one of statements 1 to 6, in which the electrolyte further contains zincate ions (Zn(OH)42−).
8. A secondary electrochemical generator according to one of statements 1 to 7, in which the electrolyte further contains borates, silicates, and/or aluminates. Other characteristics and advantages of the invention will now be described in detail in the following description which is given with reference to the appended figures, which schematically show:
The authors of the present invention have shown that adding Ni(OH)2 to the active material of the cathode makes it possible to reduce the capacity losses linked to the MnO2 phase and to significantly increase the capacity of the MnO2/Ni(OH)2 hybrid electrode.
According to the present invention, and as illustrated by the non-exhaustive examples of demonstration and embodiment that follow, the active material of the positive electrode comprises a mixture of manganese dioxide and nickel hydroxide, the mass of Ni(OH)2 being greater than 5% of the sum of the masses of Ni(OH)2 and MnO2 and, preferably, greater than 20% of the sum of the masses of Ni(OH)2 and MnO2.
The manganese dioxide can be supplied to the preparation of the cathode by all chemical and electrochemical varieties of MnO2. Similarly, nickel hydroxide can be added by all varieties of Ni(OH)2, including all usual crystallographies and additives. The electrolyte used according to the present invention is an alkaline aqueous solution with a molarity of between 4 and 15 M, and preferably between 7 and 13 M, of hydroxyl anions. It can be composed of lithium, sodium, or potassium hydroxides taken alone or in mixtures.
The electrolyte may also contain zincates, borates, silicates, and/or aluminates, in varied proportions.
The zinc negative electrode preferably incorporates conductive ceramic powders, which can be selected from the borides, carbides, nitrides, and silicides of various metals such as: hafnium, magnesium, niobium, titanium, and vanadium. It may thus advantageously be hafnium nitride and/or carbide, and/or magnesium carbide and/or nitride and/or silicide, and/or niobium carbide and/or nitride, and/or titanium carbide and/or nitride and/or silicide, and/or vanadium nitride. It is also possible to use ceramic materials such as titanium suboxides of a general TinO2n-1 formula, where n is between 4 and 10. For both, these ceramics can be retained for use in the context of the present invention, insofar as they are conductive, chemically inert in the battery, and have a high hydrogen overvoltage. In addition, in order for them to fulfill their role optimally, these conductive powders used should be fine and dispersed as homogeneously as possible in the active mass.
An electrochemical cell of prismatic format consists of two zinc electrodes framing an MnO2 electrode. Each zinc electrode has an active surface of 26 cm2 for a capacity of 1.5 Ah. The MnO2 electrode with an active surface area of 24 cm2 and a capacity of 0.588 Ah calculated for the equivalent of an exchanged electron corresponding to the first reduction stage between MnO2 and MnOOH or 308 mAh/g, therefore has a capacity limiting the capacity of the Zn/MnO2 electrochemical cell.
Each zinc and MnO2 electrode is enveloped by a membrane in order to confine soluble Zn and Mn ions to the electrode surface. A piece of felt is also positioned between each electrode as a separator and electrolyte reservoir.
The composition of the active material of the zinc electrode includes TiN conductive ceramics, as described in patent FR 2,788,887, to eliminate the cyclability problems of the zinc electrode. The composition of the active material of the MnO2 electrode consists of, expressed by mass, 60% MnO2-EMD (electrolytic manganese dioxide), 6% Bi2O3, 30% carbon, and 4% binder-plasticizer (PTFE). Alcohol is added to the mixture of the preceding constituents to obtain a compact paste that is then prepared in the form of strips, which are deposited on each side of a current collector, the whole being compacted to obtain an MnO2 electrode.
The electrolyte is an alkaline aqueous solution with a molarity of 10 M, obtained from a mixture of NaOH and KOH. The electrolyte also contains 0.25 M of zincate ions obtained by the addition of ZnO.
The first two cycles at the C/20 rate are used as the training stage. The ZnMnO2 battery thus made is first discharged at a rate of C/20, up to 1 V or the equivalent of 20% of the capacity. Charging is done at C/20 with no voltage limit up to an equivalent of 21% of the capacity. After 2 cycles, the element of example 1 is cycled under the same conditions as the first two cycles but with a current rate equivalent to C/10. The capacity of example 1 as a function of the number of cycles is shown in
The cell of example 2 is identical to that of example 1. The capacity of the MnO2 electrode is 0.644 Ah. The utilization rate of the MnO2 electrode is increased to have a surface capacity greater than 10 mAh/cm2.
The example 2 battery thus made is first discharged at a rate of C/20, up to 1 V or the equivalent of 43% of the capacity calculated on the basis of one exchanged electron or 308 mAh/g of MnO2. Charging is done at C/20 with no voltage limit up to an equivalent of 45% of the capacity. After 2 cycles, the element of example 2 is cycled under the same conditions as the first two cycles but with a current rate equivalent to C/10.
The capacity of example 2 is shown in
The capacities of examples 1 and 2 as a function of the number of cycles can be seen in
Example 3 is similar to example 2 with a positive electrode modified by the addition of 3.75% Ni(OH)2 to the MnO2 electrode. The composition of the active material of the positive electrode modified by the addition of nickel hydroxide to replace part of the MnO2 is, by mass, 56.25% MnO2-EMD, 3.75% Ni(OH)2, 6% Bi2O3, 30% carbon, and 4% binder-plasticizer.
The positive electrode becomes hybrid with 2 electrochemically active materials in the voltage range between 1 V and 2.3 V. The capacity is calculated by considering for this voltage range an electron exchanged for each active material, or 308 mAh/g for MnO2 and 289 mAh/g for Ni(OH)2. The capacity of the hybrid electrode of example 3 is 0.690 Ah.
Example 3 is formed and cycled under the same conditions as example 2 with a 43% discharge. With this 43% discharge, the minimum discharge of MnO2 is 40% based on a 100% discharge for nickel hydroxide. The capacity of example 3 is shown in
Example 4 is identical to example 2 with a positive electrode modified by the addition of 7.5% Ni(OH)2 to the MnO2 electrode. The composition of the positive MnO2 electrode becomes, by mass, 52.5% MnO2-EMD, 7.5% Ni(OH)2, 6% Bi2O3, 30% carbon, and 4% binder-plasticizer. The capacity of the electrode of example 4 is 0.654 Ah.
Example 4 is formed and cycled under the same conditions as example 2 but with a greater discharge (47%), allowing a minimum 40% MnO2 discharge, identical to that of examples 2 and 3. The discharge of nickel hydroxide is considered to be 100%. The capacity of example 4 is shown in
Example 5 is similar to example 2 with a positive electrode modified by the addition of 15% Ni(OH)2 to the MnO2 electrode. The composition of the positive MnO2 electrode becomes, by mass, 45% MnO2-EMD, 15% Ni (OH)2 6% Bi2O3, 30% carbon, and 4% binder-plasticizer. The capacity of the electrode of example 5 is 0.616 Ah.
Example 5 is cycled under the same conditions as example 2. With this same 43% discharge, the minimum discharge of MnO2 is 26% for example 5, based on a 100% discharge for nickel hydroxide. The capacity of example 5 is shown in
Example 6 is similar to example 2 with a positive electrode modified by the addition of Ni(OH)2, fully replacing the MnO2 compound with Ni(OH)2. The composition of the positive electrode becomes, by mass, 60% Ni(OH)2, 6% Bi2O3, 30% carbon, and 4% binder-plasticizer. The capacity of the electrode of example 6 is 0.533 Ah. Example 6 is cycled at the same current rate as example 2, with a charge at C/10 102% of the capacity with no voltage limit and a discharge at C/10 up to 1 V. These cycling conditions reproduce a charge and a complete discharge of nickel hydroxide identical to that of examples 3, 4, and 5. The discharge capacity obtained from example 6 is characterized by an average value over the first 150 cycles of 95%, less than the assumed 100% of examples 3, 4, and 5. The average surface capacity over the first 150 cycles is 22.1 mAh/cm2. The capacity of example 6 is shown in
Example 7 is similar to example 2 457 cycles, greater than with a positive electrode modified by the addition of 30% Ni(OH)2 to the MnO2 electrode. The composition of the positive MnO2 electrode becomes, by mass, 30% MnO2-EMD, 30% Ni(OH)2, 6% Bi2O3, 30% carbon, and 4% binder-plasticizer. The capacity of the hybrid electrode of example 7 is 0.529 Ah. Example 7 is formed and cycled with the same conditions as example 2 with a 66% discharge. With this 66% discharge, the minimum discharge of MnO2 is 37% based on a 100% discharge for nickel hydroxide. However, example 6 demonstrates that for a 100% discharge of Ni(OH)2, the capacity obtained is not 100% but 95%, implying a minimum discharge for MnO2 of 41%, very close to the 43% of example 2. The capacity of example 7 is shown in
The discharge voltages of cycle no. 5 as a function of the discharged capacity, for examples 2 to 7, are shown in
The additions of Ni(OH)2 of 3.75% and 7.5%, respectively, for examples 3 and 4 do not modify the discharge profile of example 2.
With an addition of 15% of Ni(OH)2 for example 5, a slight shoulder at the beginning of the discharge is visible, but the discharge profile remains very close to example 2, purely MnO2. This result suggests, for cycle no. 5 of examples 3, 4, and 5, that Ni(OH)2 contributes very little to the discharged capacity. In fact, considering that the discharge of Ni(OH)2 under these test conditions is 95% (demonstrated in example 6), the 43% discharge of example 5 should be distributed at 21% over Ni(OH)2 and 22% over MnO2. The shoulder of example 5 is in accordance with a contribution to the discharge of Ni(OH)2 of less than 3%. This difference between a maximum calculated contribution (21%) and what could be deducted from the discharge curves (3%) can be explained by the fact that Ni(OH)2 is not fully charged under the applied cycling conditions.
In fact, the voltage does not increase sufficiently in the charge curves to reach the voltages corresponding to the charge of Ni(OH)2. With example 7, characterized by an identical amount between the two active electrochemical phases MnO2 and Ni(OH)2, the discharge profile is hybrid with a wide shoulder of up to 30%, characteristic of the discharge voltage of Ni(OH)2 then a fairly slow drop in the voltage corresponding to the discharge of MnO2. For this example 7, considering a 95% discharge for Ni(OH)2, the 66% discharge of example 7 should be distributed at 44% over Ni(OH)2 and 22% over MnO2. The shoulder of the discharge of cycle no. 5 of example 7 corresponds to a contribution of the discharge of Ni(OH)2 of less than 44%, implying that, in this example, Ni(OH)2 participates not maximally, but more than in examples 3, 4, and 5, with its discharge remaining partial, as can be seen in
Here again, the difference between a maximum calculated contribution (44%) and the deduction from the discharge curve is explained as mentioned above.
The discharge voltages of cycle no. 50, as a function of the discharged capacity, for examples 2 to 7, are shown in figure.
The discharge voltage profiles of examples 3, 4, 5 and 7 are strongly modified with the presence of a shoulder increasing with the Ni(OH)2 content. This shoulder is correlated with the discharge of Ni(OH)2. The shoulders of examples 5 and 7 are respectively around 25% and 45%, values close to the calculation of a maximum contribution of Ni(OH) 2, respectively 21% and 44%. The reason for a better agreement here between the maximum calculated contribution of Ni(OH)2 and the values deducted from the curve could be explained by the fact that Ni(OH)2 charges more than at the beginning of cycling, because MnO2 gradually loses its cycling performance. After the shoulder, the continuation of the discharge is correlated with the contribution of the discharge of MnO2, but the voltage is higher than for cycles no. 5, demonstrating a significant change in the discharge behavior of this MnO2 phase.
This result reveals in situ modifications during the first 50 cycles concerning the MnO2 and Ni(OH)2 phases. Ultimately, the increase in the cyclability of the manganese dioxide suggests that the capacity loss associated with the formation of the parasitic Mn3O4 and ZnMn2O4 phases is limited.
The X-ray diffraction diagram of preparation 1 is characterized by the diffraction peaks of the β-Ni(OH)2 and Bi2O3 phases. The diffraction peaks of the MnO2 phase are not identified, indicating that this MnO2 phase is insufficiently crystallized to be visible in the presence of the other β-Ni(OH)2 and Bi2O3 phases. For examples 2, without Ni(OH)2 in the active material, preparations 4 and 5, the diagrams are similar, with very few diffraction peaks, attesting to the low crystallization in accordance with the initial MnO2 phase. The parasitic ZnMn2O4 phase is not identified in these diagrams. For examples 7, the X-ray diffraction diagrams demonstrate, compared to preparation 1, a significant crystal transformation with the presence of peaks at low angles around 11-13 degrees, suggesting lamellar crystal phases with insertion. Diagram 3 differs from diagram 2 by the formation of a shoulder in the first peak, which indicates a structural change. This physical-chemical analysis associates the capacity stabilization with an in-situ crystal transformation between the initial active MnO2/β-Ni(OH)2 phases.
In contrast to patent JP H10 74511 A, significant additions of Ni(OH)2, such as 50% relative to MnO2, example 7, stabilizes the capacity of the hybrid electrode. The crystal result and the remarkable change in the discharge voltage, which increases between cycles 5 and 50, demonstrate an in situ and progressive transformation with the number of cycles of the compounds MnOOH/MnO2 and Ni(OH)2/NiOOH. This in-situ transformation demonstrates greater stabilization of the discharge capacity for this new MnO2/Ni(OH)2 hybrid electrode. The presence of stable Ni(OH)2/NiOOH crystallites in the alkaline electrolyte, providing a crystal growth substrate for the MnOOH/MnO2 compounds, a substrate susceptible to increasing the reversibility of the electrochemical couple MnOOH/MnO2 by an increase in the coherence domains of these compounds, has not been completely demonstrated. But the stabilization of the capacity is correlated with a new phenomenon suggesting an in-situ modification between the Ni(OH)2/NiOOH and MnOOH/MnO2 compounds.
| Number | Date | Country | Kind |
|---|---|---|---|
| FR2102083 | Mar 2021 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2022/051880 | 3/3/2022 | WO |
