DE4143645C2 - Rechargeable alkaline sec. battery - Google Patents

Abstract

Rechargeable alkaline secondary battery has a positive electrode comprising an active material of mainly Ni (OH)2 to which Zn or a Zn cpd. can be added, a negative electrode and a alkaline electrolyte contg. KOH, LiOH, and NaOH. Pref. the negative electrode consists of an H2-absorbing alloy, pref. a rare-earth alloy, Ti-Ni, Ti-Mn, Ti-Fe, Ti-Zr, Mg-Ni or Zr-Mn alloy. The negative electrode also contains Cd. Zn or Zn cpd. is added in an amt. of 3-10 mol.%. NaOH has a density of 0.3-0.9, LiOH 1-2, and KOH at least 3.

Description

The invention relates to a rechargeable alkaline storage cell according to the Oberbe handle of claim 1. Such a memory cell is known from EP 0 353 837 A1.

Nickel electrodes in alkaline storage cells are known to show the unwanted egg property that with repeated charge / discharge cycles by changing the crystal lattice structure-induced growth, d. H. an increase in volume of the electrode occurs. This So-called swelling of the electrode affects the performance of the cell and can even lead to mechanical destruction of the cell. You tried through different Additions to the nickel electrode to suppress swelling. An addition of cadmium has have proven to be effective for this, but are undesirable for environmental reasons. From the ge called EP 0 353 837 A1 it is known that an addition of zinc or magnesium in solid solution to the nickel electrode that can effectively suppress the swelling of the electrode.

From the book "Gas-tight nickel-cadmium accumulators", VDI-Verlag GmbH, Düs seldorf 1988 , it can be seen that electrolyte for gas-tight nickel-cadmium cells usually contains potassium hydroxide solution, but also electrolyte mixtures for special applications the base of KOH, NaOH and LiOH can be used.

Based on the prior art mentioned, the invention is the following pro The underlying principle: the addition of zinc to the nickel electrode, which is used to suppress the Swelling the electrode is also beneficial, has undesirable effects. These show especially when the positive nickel electrode is made with a negative electrode a hydrogen-absorbing alloy to form a nickel-hydrogen cell should be combined. The main two undesirable effects were as follows  of the zinc additive to the nickel electrode: First, the charging capacity when charging under higher ambient temperatures significantly reduced, secondly There may be an accumulation of oxygen, especially at the beginning of each charging process the negative electrode, which makes their absorption capacity for hydrogen misleading versibly reduced, thereby drastically reducing the cycle life of the cell becomes.

The invention has for its object a rechargeable alkaline storage cell of the type mentioned above, which is designed as a nickel-hydrogen cell and the described undesirable effects of the swelling of the electrode preventive addition of Zn does not occur or occurs only to a small extent, so that a Nickel-hydrogen cell with high charging capacity, good chargeability even at higher ones Ambient temperatures, long charge / discharge life and lack of swelling supply of the positive electrode is obtained.

The object is achieved by the alkaline SpeI cher cell specified in claim 1 . The subclaims relate to advantageous further developments of the invention.

The occurrence of the effects achieved with the invention can be explained as follows: When a cell with a nickel hydroxide electrode is charged, the following two reactions occur competing:

Ni (OH) ₂ + OH ^- → Ni0OH + H ₂ O + e ^- (1)

4OH ^- → 2H ₂ O + O ₂ + 4e ^- (2)

Reaction ( 1 ) is effective for charge storage in the electrode. Reaction ( 2 ) means an undesirable release of oxygen. It has been found that the addition of zinc to the nickel electrode in order to suppress swelling has the undesirable effect of promoting reaction ( 2 ) by increasing the overvoltage and increasing the equilibrium potential of reaction ( 1 ). As a result, the charging capacity of the cell, in particular the charging capacity at higher ambient temperatures, is reduced.

In the context of the present invention, it has now been found that when using an electrolyte which is an aqueous potassium hydroxide solution with additions of lithium hydroxide and sodium hydroxide in the concentrations specified in claim 1, the undesired reaction ( 2 ) is suppressed and the reaction ( 1 ) is strongly promoted , This has the surprising effect that the reduction in charging capacity previously observed with the addition of zinc to the nickel electrode does not occur and that the cell has a very good charging capacity even when charged under a higher ambient temperature. It was also found that this effect is obtained only when lithium and sodium are used together, but not when using lithium hydroxide or sodium hydroxide. If the density of sodium hydroxide and lithium hydroxide is lower than that specified in claim 1, the desired effect is not achieved to a sufficient degree. If the density is too high, the cell capacity is reduced.

In a nickel-hydrogen storage cell, the following effect is also achieved by the invention: If of the two reactions ( 1 ) and ( 2 ) mentioned above, the reaction ( 2 ) is promoted by the addition of zinc to the nickel electrode, then one occurs Release of oxygen already at the beginning of the charging process at a point in time at which the absorbing surfaces of the hydrogen-absorbing negative electrode are still little occupied by absorbed hydrogen. The oxygen released by the reaction ( 2 ) can accumulate on the unoccupied absorbing surfaces, as a result of which these surfaces fail for hydrogen absorption and the cell output is drastically reduced. According to the invention, however, the reaction ( 2 ) is suppressed by the electrolyte mixture used. As a result, oxygen release occurs only to a lesser extent and at a much later time of the charging process, when the absorption surfaces of the negative electrode are already covered with absorbed hydrogen and are therefore protected against oxidation. A deterioration of the cell performance by oxidation of the hydrogen-absorbing alloy of the negative electrode is therefore prevented.

The effect of the zinc additive preventing the nickel electrode from swelling remains here of untouched. The improved charging capacity of the nickel electrode, the suppression the oxidation of the hydrogen-absorbing alloy of the negative electrode and the Suppression of swelling of the positive electrode together lead to improved Cycle characteristics of the memory cell. To suppress the swelling of the positive A relatively small addition of zinc is sufficient for the electrode, so that the amount of active material than the electrode is not significantly reduced. That is why the above are described realized partial effects without reducing the cell capacity.  

Embodiments of the present invention are described in the fol described in more detail with reference to the drawings:

Fig. 1 a cross section of an alkaline nickel-hydrogen storage cell as an embodiment according to the present invention;

Fig. 2 is a graphical representation of the relationship between the ambient temperature and the cell capacity, the cells A relating to the present inven tion and the cells X ₁ to X ₃ serve as comparative examples;

Fig. 3 is a graph showing the cycle characteristics of the cells A and X ₁ to X _3;

Fig. 4 is a graphical representation of the increase in thickness as the charge / discharge cycle test progressed to the electrodes (a) and (x);

Fig. 5 is an X-ray diffractogram of the electrodes (a) and (x);

Fig. 6 is a graph showing the relationship between the amount of zinc and the growing electrode thickness;

Fig. 7 is a graph showing the relationship between the charge time and the electrode voltage obtained by using electrolytes c ₁ to c ₃ ;

Fig. 8 is a graph showing the relationship between the density of NaOH in the electrolyte and the cell capacity;

Fig. 9 is a graph showing the relationship between the density of LiOH in the electrolyte and the cell capacity;

Fig. 10 is a graphical representation of the relationship between the density of KOH in the electrolyte and the cell capacity and

Fig. 11 is a graph showing the cycle characteristics of the cell B according to the present invention and the cell Y of Comparative Example.

Embodiment I Example according to the present invention

Fig. 1 is a cross section through a cylindrical alkaline nickel-hydrogen memory cell as an embodiment according to the present invention. The cell comprises an electrode arrangement 4 with a nickel-sintered positive electrode 1 , a negative electrode 2 with a hydrogen-absorbing alloy and a separator 3 arranged in between. The electrode arrangement 4 is provided in a cell housing 5 , which also acts as a negative pole, the cell housing 5 being connected to the negative electrode 2 by a conductive strip 10 to the negative electrode. An upper opening of the Zel lengehäuses 5 is covered with a closure member 7 , which is BEFE Stigt on the cell housing 5 through a sealing body 6 . A coil spring 8 is arranged within the closure part 7 . If the internal pressure of the cell becomes abnormally large, the coil spring 8 is pressed in the direction of the arrow, whereby the internal gas can escape to the outside. The closure member 7 is connected to the positive electrode 1 by a conductive strip 9 to the positive electrode.

The cylindrical alkaline nickel hydrogen cell with the The above construction was made in the following manner.

A nickel sintered plate with a porosity of 85% was in an aqueous solution of nickel nitrate, the 3 mol% Cobalt nitrate and 7 mol% zinc nitrate were added dips, and then the nickel plate continued with a active material mainly comprising nickel hydroxide impregnated by the chemical impregnation ver drive, creating a positive electrode. The The positive electrode obtained is referred to as electrode (a) net.

Commercially available Mm (metal mixture: a mixture of rare earth elements), Ni, Co, Mn and Al were mixed in an elementary ratio of 1: 3.2: 1: 0.6: 0.2, dissolved in a high frequency furnace, and then cooled down to obtain a block of an alloy expressed by MmNi _3.2 CoMn _0.6 Al _0.2 . The block is crushed into powder, with a grain size of 50 µm or less, and the powder is mixed with 5% by weight of pulverized PTFE (polytetrafluoroethylene) to form a paste. The paste was applied to both surfaces of a collector made of a stamped metal under pressure adhesion to produce the negative electrode 2 .

The electrode (a) and the negative electrode 2 were bent round with the separator 3 between them to obtain the electrode assembly 4 , which was then inserted into the cell housing 5 . An alkaline electrolyte with potassium hydroxide (KOH, density: 5 normal), lithium hydroxide (LiOH, density: 1.5 normal), and sodium hydroxide (NaOH, density: 0.6 normal) was injected into the cell case 5 , and then the cell case 5 tightly closed. The cylindrical alkaline nickel-hydrogen storage cell produced in this way has a theoretical capacity of 1,000 mAh and is referred to as cell A.

Comparative Example 1

A cell X ₁ was produced in the same manner, except that an alkaline electrolyte was used with KOH, which has a density of 6 normal and LiOH, which has a density of 1 normal.

Comparative Example 2

A nickel sintered plate with a porosity of 85% was in an aqueous solution of nickel nitrate, the 10 mol%  Cobalt nitrate is added (no zinc was added) immersed, and then the nickel plate was continued with an active material mainly nickel hydroxide includes, by the chemical impregnation process imprä gniert, whereby a positive electrode was generated. The The positive electrode obtained is used as the electrode (x) records.

A cell X ₂ was generated in the same manner as cell A, but using the electrode (x).

Comparative Example 3

A cell X ₃ was created in the same manner as cell A, but using the electrode (x) and the electrolyte of cell X ₁ .

Experiment 1

With respect to cells A and X ₁ through X ₃ , the relationship between the ambient temperature and the cell capacity was checked, and the results are shown in FIG. 2. The cells were each charged with a current of 0.1 C for 16 hours in different ambient temperatures and then discharged with a current of 0.2 C until the cell voltage reached 1.0 V. The cell capacity (%) was calculated by setting the discharge capacity obtained when the cell was charged in an ambient temperature of 20 ° C as 100.

As can be seen from Fig. 2, cell A according to the present invention showed the same tendency as cells X ₂ and X ₃ to which zinc has not been added, namely that the decrease in cell capacity which corresponds to the increase in the ambient temperature during the Charging process should have occurred, was suppressed. Cell X ₁ showed a sharp decrease in cell capacity with increasing ambient temperature. The results indicate that the active material of cell A is charged much more easily at high temperature than that of cell X ₁ .

Experiment 2

The charge / discharge cycle properties of cells A and X ₁ through X ₃ were checked and shown in FIG. 3. The cells were charged by a current of 1.2 C for one hour at room temperature and then discharged by a current of 1 C until the cell voltage reached 1.0 V. The cell capacity (%) was calculated by setting the discharge capacity obtained after the first cycle as 100.

As can be seen from FIG. 3, cell A according to the present invention, which had a lifespan of more than 800 cycles, had a very many better cycle characteristics than cells X ₁ to X ₃ , each of which only had a service life of approximately 400 cycles.

Summary of experiments 1 and 2

It was determined from the above experiments that cell A according to the present invention has a much better cycle characteristic than cells X ₁ through X ₃ . The cell A is also charged almost as easily at high temperature as the cells X ₂ and X ₃ , which have no zinc, although the cell A uses a nickel-hydrogen electrode with zinc or a zinc compound. This is attributed to the following.

In cell A, firstly, the zinc added in the positive electrode suppresses its expansion, and secondly, the NaOH and LiOh added to the electrolyte make the cell easy to charge (especially at high temperature) and thirdly, the generation of oxygen is prevented by point two and it is therefore the effect of the negative electrode to prevent oxidation is entirely sufficient. In the cells X ₁ to X ₃ causes the reduction of the electrolyte in the separator, the dryout phenomenon, which is described in an individual below. In cell X ₁ , the hydrogen-absorbing alloy of the negative electrode is oxidized, since the promotion of oxygen generation leads to a large amount of oxygen in the cell. As a result, the gas consumption is reduced and the internal cell pressure increases, which leads to the electrolyte leaking out of the cell. In the cells X ₂ and X ₃ without zinc, the positive electrode expands, which is slid from an outflow of the electrolyte from the separator into the positive electrode.

Experiment 3

The electrode (a) with zinc or the zinc compound and the electrode (x) without zinc or a zinc compound were each immersed in an abundant amount of KOH with a specific gravity of 1.23, using the electrodes using a charge / discharge cycle test were subjected to a nickel plate as the opposite electrode to obtain the rate of increase in electrode thickness ( Fig. 4). After the twentieth cycle, an X-ray diffraction test was performed using a powder of the active material of electrodes (a) and (x), the results of which are shown in FIG. 5. The electrodes were charged by a current of 1.5 C for one hour and then discharged by a current of 1 C until the electrode voltage reached 0.1 V (vs Hg / HgO).

As can be seen from Fig. 4, the electrode (a) shows a much lower rate of increase in thickness than the electrode (x).

This phenomenon is attributed to the fact that the generation of γ-NiOOH, which is an active material of low density, is suppressed more in electrode (a) than in electrode (x), as can be seen from FIG. 5.

Experiment 4

Positive electrodes were produced in the same way as electrode (a), but with different amounts of zinc: 0, 3, 5, 7 and 10 mol%. The thickness increase rate of each of these positive electrodes was checked after the third cycle in the same manner as in Experiment 3 , with the results shown in FIG. 6.

As can be seen from Fig. 6, the thickness increase rate was extremely low with an amount of zinc of 3 mol% or more. The desirable amount of zinc is between 3 and 10 mol% because the electrode capacity is reduced if zinc is supplied in more than 10 mol%.

Experiment 5

The effectiveness of NaOH and LiOh in raising the electrode potential was tested using electrode (a), the results being shown in FIG. 7. The electrode was charged by a current of 0.2 C for 8 hours. The following three electrolytes were used in abundance.
Electrolyte c ₁ : KOH ( 7 N)
Electrolyte c ₂ : KOH ( 6 N) + LiOH ( 1 , 5 N)
Electrolyte c ₃ : KOH ( 5 N) + LiOH ( 1 , 5 N) + NaOH (0.6N)

As can be seen from FIG. 7, with the electrolyte c ₁ only a small difference occurs between the initial potential and the potential after the electrode has been fully charged (O 2) has been generated. This means that the reactions expressed by equations (1) and (2) occur competing. The electrolyte c ₂ with LiOH causes a larger and difference electrolyte c ₃ with LiOH and NaOh, which means that the charging process was promoted by the addition of LiOH and even more by the further addition of NaOH.

Experiment 6

Figure 8 shows the relationship between the cell capacity, the density of NaOH and the charge temperature. The cells used for this experiment corresponded to cell type A. The densities of KOH and LiOH were set at 6 normal and 1.5 normal, respectively. The cells were each charged by a current of 0.1 C for 16 hours at ambient temperatures of 20 ° C and 40 ° C and then discharged with a current of 0.2 C until the cell voltage reached 1.0 V at room temperature.

The results in Fig. 8 show that the desirable density of NaOH is 0.3 to 0.9 normal, which becomes more important as the charge temperature increases.

Experiment 7

Fig. 9 shows the relationship between the cell capacity, the density of LiOH and the charge temperature. The cells used for this experiment corresponded to cell type A. The densities of KOH and NOH were set at 6 normal and 0.6 normal, respectively. The test conditions were the same as in Experiment 6 .

The results in Fig. 9 show that the desirable density of LiOH is 1.0 to 2.0 normal, which becomes increasingly important as the charge temperature increases.

Experiment 8

Fig. 10 shows the relationship between the cell capacity, the density of KOH and the charge temperature. The cells used in this experiment correspond to cell type A. The densities of LiOH and NaOH were set at 1.5 normal and 0.6 normal, respectively. The test conditions were the same as in Experiment 6 .

The results in Fig. 10 show that the desirable density of KOH 3 is normal or more.

Summary of experiments 6 to 8

In an alkaline storage cell with a positive electrode comprising an active material mainly with nickel hydroxide, to which zinc or a zinc compound is added, the following densities are desirable for the electrolyte.
KOH: 3 normal or more
LiOH: 1.0 to 2.0 normal
NaOH: 0.3 to 0.9 normal

Embodiment II Example according to the present invention

A cell B was manufactured in the same manner as the cell A, except that a negative electrode with a hydrogen absorbing alloy expressed by Ti _0.5 Zr _0.5 Ni _1.5 V _{0.5 was} used.

Comparative example

Cell Y was made in the same way as cell B manufactured, except that an alkaline electro lyt with KOH, which has a density of 6 normal, and with LiOH, which has a density of 1.5 normal, is used has been.

experiment

Fig. 11 shows the charge / discharge cycle characteristics of cells B and Y. The test conditions were the same as in Experiment 1 of Embodiment I.

As can be seen from FIG. 11, cell B has a considerably longer lifespan than cell Y.

Further comments

• 1. It was found through experiments that the same effects with a negative cadmium electrode can be obtained as with the negative Hydrogen absorbing alloy electrode be preserved.
• 2. Instead of the hydrogen-absorbing alloy, expressed by MmNi _3.2 CoAl _0.2 Mn _0.6 , other types of hydrogen-absorbing alloys with the following components can also be used: a rare earth element, such as, for example, LaNi ₂ Co ₃ , Ti -Ni, Ti-Fe, Ti-Mn, Ti-Zr, Mg-Ni and Zr-Mn.

Claims (3)

1. Rechargeable alkaline storage cell with a positive electrode which contains nickel hydroxide as active material, in which there is an addition of zinc in solid solution, a negative electrode and an alkaline electrolyte based on an aqueous solution of potassium hydroxide, characterized in that
that the memory cell is a nickel-hydrogen memory cell, the negative electrode of which contains an alloy with a high hydrogen absorption capacity as active material,
and that the electrolyte is an aqueous solution of potassium hydroxide in a concentration of at least 3 normal, lithium hydroxide in a concentration of 1.0 to 2.0 normal and sodium hydroxide in a concentration of 0.3 to 0.9 normal. 2. The cell of claim 1, wherein the hydrogen absorbing alloy is an alloy of rare earth elements, a Ti-Ni alloy, a Ti-Mn alloy, a Ti-Fe alloy tion, a Ti-Zr alloy, a Mg-Ni alloy or a Zr-Mn alloy. 3. Cell according to claim 1, in which the addition of zinc 3 to 10 mol% of the nickel hydro xids is.