FR2750255A1 - Positive nickel electrode active material for alkaline secondary battery - Google Patents

Abstract

A positive nickel electrode active material consists of hydroxide (mainly nickel hydroxide) particles which are covered with a layer of a Ca(II) compound, preferably calcium hydroxide present in amount 0.5-5% (based on total particle weight). Also claimed are: (i) a process for making the above active material by (a) immersing the hydroxide particles in a solution containing calcium ions of valency <= 2; (b) separating and draining the particles; (c) immersing the particles in an alkaline solution; and (d) separating, water washing and drying the particles; and (ii) a nickel electrode comprising a porous three-dimensional conductive support and a paste containing a binder a conductive material powder and the above active material.

Description

Active ingredient Positive for nickel electrode
The present invention relates to a positive active material for the nickel electrode of an alkaline electrolyte battery. It also extends to the process for preparing this active material and to its use for the production of the electrode.

 Most recent applications of alkaline electrolyte batteries require the user to recharge these batteries at temperatures above ambient temperature. This is the case for example when these accumulators participate in the traction of a vehicle.

When the charging temperature is around 400l at 600C, the charging efficiency of the positive electrode is poor because the oxygen release overvoltage at the end of charging is low.

To increase the hot chargeability, US Pat. No. 3,826,684 proposes a method for manufacturing a nickel electrode which firstly consists in filling a conductive support with electrochemically active material comprising nickel hydroxide. The electrode obtained is then impregnated with a saline solution of an element chosen from zinc, mercury and alkali metals of group IIA. Then the metal salt is precipitated by an alkaline solution in the form of hydroxide. The patent
US-5,466,546 describes a nickel electrode for a nickel-metal hydride accumulator. By an analogous process of post-treatment by immersion, the electrode is coated with a compound containing calcium. Post-treatment improves the yield at high temperatures, but lowers it at ordinary temperature. In addition, this method has the drawback of leaving traces of the salt used in the electrode (nitrate, acetate, etc.).

 Japanese patent application JP-53-19086 proposes to increase the rate of use of the active material at high temperature by co-precipitating calcium hydroxide with nickel hydroxide in a conductive support sintered by chemical impregnation . Again it is practically impossible to remove the residual traces of the salt used for the impregnation.

 A nickel electrode produced by pasting consists of a porous conductive support into which a paste containing the active material is introduced. European patent application EP-0 587 973 describes an electrode for an alkaline accumulator of the pasted type capable of operating in a wide temperature range. The paste contains an active ingredient and at least one compound of yttrium, indium, antimony, barium, beryllium, calcium, or cobalt.

 US patent US-5,455,125 shows that the yield of the active material at 450C and 600C is higher when the additive is introduced into the dough, rather than syncrystallized. This patent suggests using these two modes of addition simultaneously in the positive electrode of a metal oxide-metal hydride accumulator.

 European patent application EP-0 634 804 mentions the fact that the addition of calcium hydroxide alone to the paste does not improve the efficiency of the electrode at ordinary or high temperature. This document proposes a paste comprising a mixture of active materials mainly composed of nickel hydroxide and containing at least one element chosen from cobalt, cobalt hydroxide, cobalt oxide, carbon or nickel, and a compound pulverulent of at least one element chosen from Ca, Sr, Ba, Cu, Ag and Y.

 The yields obtained with such an electrode are still insufficient for certain applications. These poor results are linked to an inadequate distribution of the compounds in the dough and to the formation of clumps.

 The present invention relates in particular to improving the hot chargeability of the nickel electrode while retaining high performance.

 The object of the present invention is a positive active material for a nickel electrode consisting of hydroxide particles containing mainly nickel, characterized in that the said particles are covered with a layer of a calcium compound of valence two Ca (II).

 The rate of additive used is minimized so that the electrode retains a high filling of electrochemically active material, therefore a high energy density. Consequently, the additive must be placed so that its influence on the active ingredient is maximum and that it is simultaneously distributed in a perfectly homogeneous manner within the dough.

 According to the present invention, the calcium compound is distributed homogeneously in the dough which never allows a simple mixture of powders to be achieved. By coating each of the hydroxide particles, the calcium compound thus exerts its influence on each particle as closely as possible.

 Advantageously, the proportion of said calcium compound is between 0.5 W and 5% by weight relative to the total weight of said particles. The yield of the active material, in particular at high temperature load, is significantly increased from a content of 0.5k by weight of calcium hydroxide. Above 5% by weight, this gain is canceled out by the decrease in the proportion of electrochemically active material, the calcium compound not participating in the electrochemical reactions.

 Preferably said calcium compound further occupies at least partially the surface microporosity of said hydroxide particle. This is the microporosity accessible to the electrolyte and which participates in the electrochemically active surface of the hydroxide.

 According to a preferred embodiment, said valence two calcium compound is calcium hydroxide Ca (OH) 2. However, other compounds can also be used, such as CaF2, CaO or CaS.

 The hydroxide particles can take any form ranging from a more or less spherical shape to a completely irregular shape.

 According to a variant, said hydroxide also contains in solid solution at least one hydroxide of a metal chosen from cobalt Co, aluminum Al, iron Fe and manganese Mn and at least one hydroxide of a metal chosen from cadmium Cd, zinc Zn and magnesium Mg. The hydroxide particles mainly contain nickel and most often a minor proportion, not exceeding 20%, of hydroxide of one or more other syncrystallized metals. The crystal structure of the hydroxide is not changed. It is understood that the term nickel hydroxide used in the present application means both a nickel hydroxide and a hydroxide containing mainly nickel but also at least one of these metals.

 The present invention also relates to a process for manufacturing the active material according to one of the preceding claims, comprising the following steps: - a solution is prepared containing calcium ions of valence less than or equal to two, - said particles are immersed of hydroxide in said solution, - the said particles are separated from the said solution and the said particles are drained - the said hydroxide particles are immersed in an alkaline solution, - the said particles are separated from the said alkaline solution, - the said particles are washed with water and we dry them.

Said solution containing calcium ions of valence less than or equal to two is preferably chosen from a solution of calcium nitrate Ca (NO3) 21, a solution of calcium sulphate CaSO4, a solution of calcium chloride CaCl2, and a solution of calcium acetate Ca (CH3CO2) 2
The alkaline solution is preferably chosen from a caustic soda solution (sodium hydroxide NaOH), a potassium solution (potassium hydroxide KOH) and a lithine solution (lithium hydroxide LiOH).

 A contact time with the alkaline solution of between 30 minutes and 5 hours is sufficient, but the duration of the immersion can reach 48 hours without inconvenience.

 The drying temperature is between 400C and 1000C for a period between 12 hours and 48 hours.

 Alkaline electrolyte accumulators until recently included a nickel electrode with sintered support in the pores of which the active material was deposited electrochemically, or chemically by several successive precipitation operations. This manufacturing process is long and expensive and the electrodes are now preferably made by pasting. A nickel electrode with an unsintered support consists of a porous three-dimensional conductive support, which can be a felt or a foam made of metal or carbonaceous material, into which a paste containing the active material is introduced.

The paste comprises a binder, a nickel-based hydroxide and a conductive powder which may be a compound of cobalt, nickel powder or carbon.

 The invention also relates to a nickel electrode comprising a porous three-dimensional conductive support and a paste containing a binder, a powdery conductive material and an active material, the particles of which are covered with a layer of a calcium compound of valence two. Ca (II).

 According to one embodiment, said support is a nickel Ni foam.

 According to another embodiment, said binder comprises at least one compound chosen from carboxymethylcellulose CMC, hydroxyethylcellulose and hydroxypropylmethylcellulose HPMC, and at least one compound containing polytetrafluoroethylene PTFE.

According to yet another embodiment, said conductive material is chosen from metallic cobalt
Co, a cobalt hydroxide Co (OH) 2, a cobalt oxide CoO and mixtures thereof.

 The invention also relates to an alkaline electrolyte accumulator comprising a negative electrode and a positive electrode containing an active material, the particles of which are covered with a layer of a calcium compound of valence two Ca (II).

 According to a first variant, said negative electrode contains a hydratable metal alloy MH.

 According to a second variant, said negative electrode contains cadmium Cd.

Other characteristics and advantages of the present invention will appear during the following examples of embodiment, given of course by way of non-limiting illustration, and in the appended drawing in which:
FIG. 1 represents a partial diagrammatic section view of a pasted type electrode comprising hydroxide particles coated with a valence two calcium compound according to the present invention,
- Figure 2 shows an enlarged schematic sectional view of a hydroxide particle coated with a valence two calcium compound according to the present invention.

EXAMPLE 1
A nickel electrode A is produced according to the prior art from a conductive support which is a nickel foam with a porosity of approximately 95% and from a paste. The weight composition of the dough is as follows:
- electrochemically active pulverulent material consisting of a hydroxide composed of 93% by weight of nickel hydroxide Ni, 4% by weight of zinc hydroxide Zn and 3% by weight of syncrystallized cobalt hydroxide Co: 66%,
- conductive material consisting of a mixture of powders of cobalt hydroxide Co (OH) 2 and metallic cobalt Co: 7%,
- binder consisting of a mixture of a cellulose binder and PTFE: 1.6%,
- water: 25.4%.

 Once the paste has been introduced into the support, the electrode is dried in order to remove the water therefrom.

EXAMPLE 2
An electrode B is manufactured according to the prior art analogous to that prepared in Example 1, with the exception of the fact that calcium hydroxide powder Ca (OH) 2 is added to the paste at the rate of 0 , 8t by weight relative to the active material.

EXAMPLE 3
An electrode C is manufactured according to the prior art analogous to that prepared in Example 1, except that the active material consists of a hydroxide composed of 92.2% by weight of nickel hydroxide Ni , 4% by weight of zinc hydroxide Zn, 3% by weight of cobalt hydroxide Co and 0.8W by weight of syncrystallized calcium hydroxide Ca.

EXAMPLE 4
A powder of electrochemically active material according to the present invention is prepared in the following manner.

A hydroxide powder composed of 93% by weight of nickel hydroxide Ni, 4% by weight of zinc hydroxide Zn and 3% by weight of syncrystallized cobalt hydroxide Co is suspended in a chloride solution calcium CaCl2 with a concentration of 2.5M and maintained at 800C. The powder is left in contact with the solution for approximately 2 hours.

The particles are then drained to remove the excess solution. Then the powder is immersed in a NaOH caustic solution of 2M concentration at room temperature for 1 hour. By this operation, the calcium salt is transformed into hydroxide. The powder is finally washed at room temperature with distilled water and then dried at a temperature of the order of 700C. By this method, particles of the starting hydroxide coated with a layer of calcium hydroxide representing 0.8W by weight of the total weight of the final particle are obtained.

 An electrode D similar to that prepared in Example 1 is manufactured, except that the active material previously prepared is used.

 Figure 1 shows a partial schematic sectional view of such an electrode. We can see the porous conductive support 1 filled with paste comprising the binder 2, the powder of conductive material 3 and the particles of electrochemically active material 4. In Figure 2 which is an enlarged schematic sectional view of a particle of active material 4 according to the invention, the starting hydroxide particle 5 is observed, which is coated with a substantially continuous layer 6 of a calcium compound of two valence Ca (II), the layer partially penetrating into the surface porosity 7 of hydroxide 5.

EXAMPLE 5
Sealed nickel-metal hydride Ni-MH accumulators are assembled, comprising respectively one of the electrodes A to D previously prepared. Each positive electrode is attached to a negative overcapacitive electrode with a hydrurable alloy of known type. The positive and negative electrodes are separated by a layer of non-woven polypropylene. The electrochemical bundle is then impregnated with an alkaline electrolyte which is an aqueous KOH 8.7M potassium solution, of lithine
0.6M LiOH and 0.5M NaOH caustic soda.

After a rest of 48 hours, the accumulators are tested in cycling at 200C under the following conditions, where
Ic represents the theoretical speed necessary to discharge the accumulator in one hour: cycle 1: charging at 0.1 lIc for 16 hours,
discharge at 0.2Ic up to a voltage of 0.9 volts; cycle 2: charge at 0.1 lc for 16 hours,
discharge at 0.2Ic (slow speed) up to
voltage of 1 volt; cycle 3: charge at Ic for 1.2 hours,
discharge at Ic (fast speed) up to a voltage
1 volt; then several cycles are performed at 450C: cycle 4 at 0.2Ic charge for 5 hours, cycle 14: then charge at 0.05Ic for 4 hours,
discharge at Ic up to a voltage of 0.8 volts;
The calculated yields are collated in the table below.

 The accumulator containing electrode A has a loss of capacity between 200C and 450C of 44%.

The yield at 450C of the accumulator containing the electrode B has a gain of 25% compared to the accumulator containing the electrode A. However, this method of addition leads to a poor distribution of the calcium within the dough. Calcium mapping performed by electron microscopy using a microprobe
EDS (Energy Dispersive Spectroscopy) shows the presence of numerous calcium agglomerates distributed in an inhomogeneous manner.

 The accumulator containing electrode C has a behavior similar to that containing electrode B.

 Compared to the accumulator containing the electrode A of the prior art, the efficiency of the accumulator containing the electrode D according to the invention has at room temperature a yield greater than 5%. The efficiency at 450C of the accumulator containing the electrode D according to the invention is 24% higher than that of the accumulators containing the electrodes B and C of the prior art and it is increased by 68% compared to the accumulator containing electrode A.

TABLE electrode yields in mAh / g ABCD cycle 2 239 241 243 251 cycle 3 239 241 243 244 cycle 14 134 181 182 225
Of course, the present invention is not limited to the embodiments described in the examples, but it is capable of numerous variants accessible to those skilled in the art without departing from the spirit of the invention. 'invention. In particular, without going beyond the scope of the invention, it is possible to modify the composition of the hydroxide and the nature of the syncrystallized elements. We can also consider the use of an electrode conductive support of a different nature and structure such as a metal support or a carbonaceous material, for example a felt, a foam, an expanded or a perforated strip. Finally the different ingredients used to make the dough, as well as their relative proportions, can be changed. In particular, additives intended to facilitate the shaping of the electrode, such as a thickener or a texture stabilizer, may be incorporated therein in minor proportions.

Claims (14)

REVENDI CATIONS 1. / Positive active material for nickel electrode consisting of hydroxide particles containing mainly nickel hydroxide, characterized in that said particles are covered with a layer of a calcium compound of valence two. 2. / active material according to claim 1, wherein the proportion of said calcium compound is between 0.58 and 5% by weight relative to the total weight of said particles. 3. / active material according to one of claims 1 and 2, wherein said calcium compound also occupies at least partially the surface microporosity of said hydroxide particle. 4. / active material according to one of the preceding claims, wherein said valence two calcium compound is calcium hydroxide. 5. / active material according to one of the preceding claims, wherein said hydroxide also contains in solid solution at least one hydroxide of a metal chosen from cobalt, aluminum, iron and manganese and at least one hydroxide of a metal chosen from cadmium, zinc and magnesium. 6. / A method of manufacturing the active material according to one of the preceding claims, comprising the following steps: - preparing a solution containing calcium ions of valence less than or equal to two, - immersing said hydroxide particles in said solution, - separating said particles from said solution and draining said particles - immersing said hydroxide particles in an alkaline solution, - separating said particles from said alkaline solution, - washing said particles with water and dries them. 7. / The method of claim 6, wherein said solution containing calcium ions of valence less than or equal to two is chosen from a solution of calcium nitrate, a solution of calcium sulfate, a solution of calcium chloride, and a calcium acetate solution. 8. / Method according to one of claims 6 and 7, wherein said alkaline solution is chosen from a caustic soda solution, a potash solution and a lithine solution. 9. Nickel electrode comprising a porous three-dimensional conductive support and a paste containing a binder, a powdery conductive material and the active material according to one of claims 1 to 5.  10./ An electrode according to claim 9, wherein said support is a nickel foam. 11./ An electrode according to one of claims 11 and 12, wherein said binder comprises at least one compound chosen from carboxymethylcellulose, hydroxyethylcellulose and hydroxypropylmethylcellulose, and at least one compound containing polytetrafluoroethylene. 12./ An electrode according to one of claims 11 to 13, in which said conductive material is chosen from metallic cobalt, a cobalt hydroxide, a cobalt oxide and their mixtures. 13./ An alkaline electrolyte accumulator comprising a negative electrode and a positive electrode containing the active material according to one of claims 1 to 5, wherein said negative electrode contains a metal alloy which can be hydrated. 14./ An alkaline electrolyte accumulator comprising a negative electrode and a positive electrode containing the active material according to one of claims 1 to 5, wherein said negative electrode contains cadmium.