HK1117647A1 - Alloyed zinc powders with pierced particles for alkaline batteries - Google Patents

Abstract

ALLOYED ZINC POWDERS FOR ALKALINE BATTERIES WITH PARTICLES PIERCED WITH AT LEAST ONE HOLE THE INVENTION CONCERNS ALLOVED ZINC OOWDERS FOR AL@.ALINE BATTERIES AND A METHOD TO MANUFACTURE SUCH POWDERS. THE POWDERS ARE CHARACTERISED BY THE PRESENCE OF PARTICLES PIERCED 10 WL@H AT LEAST ONE HOLE. THIS APPEARS TO BENEFIT THE HIGH DRAIN DISCHARGE CAPACITY WHILE PRESERVING THE PROCESS ABILITY OF THE POWDER, AND THE SHELF LIFE AND THE GASSING BEHAVIOUR OF THE BATTERIES. 15 THE INVENTED POWDERS CAN BE MANUFACTURED USING CENTRIFUGAL ATOMISATION IN A COOLED, OXYGEN-DEPLETED ATMOSPHERE. CM

Description

Alloyed zinc powders with pierced particles for alkaline cells

The invention relates to a key component for manufacturing an anode of an alkaline battery: alloyed zinc powder.

A significant problem facing cell manufacturers is the formation and accumulation of hydrogen gas in the cell, which is caused by the corrosion of zinc powder by alkaline electrolytes. The generation of gas increases the pressure inside the battery. Although a safety system is provided to prevent explosion of the battery, there is a risk of damage due to electrolyte leakage residues. Gassing can be carried out before the use of the battery, i.e. during the shelf life of the battery; gassing typically occurs after particle discharge, the phenomenon referred to previously as "PD gassing".

The capacity of a cell depends on the ability of the zinc powder to generate zinc ions and electrons according to the formula:

Zn+2(OH)^-→Zn(OH)₂+2e^- (1)。

however, if the reaction medium is in OH^-When the zinc oxide is used up, a reversible reaction is generated, ZnO on the surface of the zinc particles is precipitated, and the surface of the zinc particles is passivated. The net result is that some of the metallic zinc becomes ineffective for the electrochemical reaction (1), thus limiting the effective battery capacity to a fraction of the theoretical value of the battery capacity. This passivation phenomenon occurs mainly at high discharge rates, i.e. when the zinc particles release a large amount of zinc ions, while only a limited amount of hydroxide ions are effective at the reaction sites.

Several patents, such as US6284410, claim to obtain good high current discharge performance by using fine zinc powder as it is or in admixture with coarse zinc powder. The high specific surface area of the fine powder enhances high current discharge, but unfortunately this also limits shelf life and increases PD gassing.

As mentioned in US6022639, the use of large platelet particles also attempts to improve the high current service and impact resistance of alkaline cells. Other shapes such as spherical, drop-shaped, linear are investigated in WO 2004/012886. Unfortunately, these shapes greatly increase the viscosity of the gel, resulting in many problems during battery manufacturing.

Thus, there is a conflict between the requirements for low gassing, good processing capability and high current performance. The present invention aims to solve this conflict.

To this end, a new alloyed zinc powder for alkaline batteries is proposed, comprising particles pierced by at least one hole, said particles being greater in number than one or more of the following: 10% by number in the screening fraction of 250 to 425 μm, 3% by number in the screening fraction of 150 to 250 μm and 2% by number in the screening fraction of 105 to 150 μm. The zinc alloy powder of the 250 to 425 μm screening fraction preferably comprises more than 20% by number of particles pierced by the at least one hole, more preferably more than 30% by number, more preferably at least 31% by number, and even at least 36% by number. The "particle pierced by at least one hole" described above is hereinafter referred to as "pierced particle".

In a more specific example, the powder includes one or more of bismuth, indium, and aluminum as alloying elements. Preferred alloys include 0.001 to 0.05 wt% aluminum and 0.001 to 2 wt% indium; or 0.002-0.2 wt% bismuth and 0.001-2 wt% indium; or 0.002-0.2 wt% bismuth, 0.001-2 wt% indium and 0.001-0.05 wt% aluminum; or 0.002-0.2 wt% bismuth; and optionally up to 0.5 wt% of one or both of lead and calcium, the remainder being zinc. Furthermore, zinc powders comprising 0.005-0.05 wt% Pb, the remainder being zinc, are also suitable. In fact, said zinc also contains unavoidable impurities, such as those which are allowed to be present in the so-called ultra high grade (SHG) zinc.

As a further example, the invention relates to an alkaline cell comprising alloyed zinc powder according to any one of the above criteria.

Alloyed zinc powders suitable for alkaline cells can be manufactured by the centrifugal spray (CA) process described in EP1155464B 1. Liquid zinc is poured onto a disc which is rapidly rotated under an oxygen-free protective atmosphere. This procedure resulted in a standard CA powder. To obtain pierced particles according to the invention, it is necessary to cool the temperature of the ejection atmosphere to below 110 ℃ and preferably to a temperature below 100 ℃. This may be achieved by using forced air to cool the outer surface of the spray chamber. Advantageously, the sparging is performed under an atmosphere having an oxygen content of less than 8 vol%, and more preferably greater than 4 vol%.

The pierced particles allow the use of coarser powders without reducing the high current capacity of the battery while maintaining shelf life, limiting PD gassing, ensuring good processing capability. In contrast, the pierced particles enhance the high current behavior compared to classical powders with the same particle size distribution.

At the cathode of an alkaline cell, hydroxide ions are generated according to the following reaction:

MnO₂+H₂O+e^-→MnOOH+OH^-。

as explained above, these hydroxide ions are required in the anode zinc to avoid passivation of the zinc particle surface. It is hypothesized that the pierced particles improve the diffusion of hydroxide ions from the cathode to the anode, increasing the so-called chemical homogeneity of the alkaline cell. This important property can be evaluated by a fast paced intermittent discharge test, where the cell can rest and adjust to uniformity for a short period of time between high current discharge pulses.

The pierced particles can be detected and counted by direct visual observation under a microscope or by using a shape analyzer (e.g., the ALPAGA system of Occhio SA, belgium).

The zinc powder of the invention shows particles pierced by at least one hole. The usual shape of zinc powder particles is typically a microscopic flake, which can be curved, hollow or cup-like. Fig. 1 shows double-pierced particles of alloyed zinc powder according to the invention, and fig. 2 represents a large-area pierced particle. Fig. 3 shows conventional centrifugally sprayed powder particles. This zinc powder is more round and typically not pierced.

In connection with the problem of metal melting spraying, some authors, such as A.J. yule and J.J.Dunkley in pp.15-46(Clarendon Press, Oxford; 1994) of the Atomization of melts for Powder Production and spray deposition, indicate that spraying may occur in two steps. The primary spray is from liquid metal to droplet formation. The secondary ejection is a further ejection of these droplets by air friction: the high relative velocity breaks the droplet into smaller particles. Fig. 4 illustrates this phenomenon, showing the mechanism of liquid droplet breakup in the secondary spray.

The method of the invention comprises using suitable spraying conditions in the secondary spraying step to freeze the zinc particles early to obtain a major part of the pierced particles. This demonstrates the method of cooling the sparge atmosphere.

Example 1

Alloy zinc powder is produced under centrifugal spraying: liquid zinc alloy at 480 ℃ was poured into a disc of 170mm diameter rotating at 13000rpm under an oxygen-free protective atmosphere.

Table 1 compares the number percentage of pierced particles observed per sieve fraction in a standard CA zinc powder (i.e. without temperature control) with the powder produced according to the invention. Both powders were made using the same alloy of zinc containing 200ppm In, 100ppm Bi, 100ppm Al. Both were injected under an atmosphere containing 6.25 vol% oxygen. The standard CA powder was produced without a cooling spray atmosphere resulting in an ambient temperature of about 160 ℃, while the powder of the invention was produced at 100 ℃ by appropriate cooling. It is clear that the zinc powder of the invention has a higher number of pierced particles, especially in the coarser screening fraction.

Table 1: relationship between the number of punctured particles in Zn powder and fraction to be screened

Example 2

An electrochemical cell comprising a cathode, an anode, and a separator was prepared using the zinc powder sprayed in example 1, as shown in fig. 5. The tank (10) contains an annular cathode mix (11) consisting essentially of MnO₂C and KOH. The separator paper (12) prevents short-circuiting between the cathode and anode mix (13). A current collector (14) collects electrons from the anode mixture. Such a battery corresponds to a standard LR-6 battery.

The alkaline cell was tested in 2 high current discharge tests:

1A continuous consumption test, cut-off at 1.0V; and

1A intermittent consumption test, discharge 10 seconds after 50 seconds of each interruption, cut off at 0.9V.

The total number of discharges is shown in table 2, normalized to 100% using standard CA powder as a reference.

Table 2: o during injection₂Effect of content on the number of particles punctured at 425-250 μm and the Large Current cell Capacity of the selected cell

Fig. 6 represents the percentage of particles pierced as measured by the ALPAGA system versus the percentage of oxygen in the protective atmosphere of the ejection chamber. The upper curve represents the number of particles punctured in the range 425-250 μm and the lower curve represents the total percentage of particles punctured.

These experiments demonstrate that the use of pierced particles results in very good results in high current capacity compared to standard CA powder made using the same alloy. An oxygen content of more than 4 vol% is advantageous, however, a content below 8% is preferred, since otherwise gassing of the powder tends to rise to unacceptable levels.

Claims (12)

1. Alloyed zinc powder for alkaline batteries, comprising particles pierced by at least one hole, said particles satisfying in number one or more of the following conditions:

-more than 10% by number in the screening fraction 250 to 425 μm;

-more than 3% by number in the 150 to 250 μm screen fraction;

-more than 2% in the screened fraction of 105 to 150 μm.

2. Alloyed zinc powder according to claim 1, comprising particles pierced with at least one hole in a number of more than 20% in the screened fraction of 250 to 425 μm.

3. Alloyed zinc powder according to claim 2, comprising particles pierced with at least one hole in a quantity of more than 30% in the sifted portion of 250 to 425 μm.

4. Alloyed zinc powder according to claim 3, comprising particles pierced with at least one hole in an amount of at least 31% in the sifted portion of 250 to 425 μm.

5. Alloyed zinc powder according to claim 4, comprising particles pierced with at least one hole in an amount of at least 36% in the sifted portion of 250 to 425 μm.

6. Alloyed zinc powder according to any one of claims 1-5, comprising one or more of bismuth, indium, aluminium as alloying elements.

7. Alloyed zinc powder according to claim 6, comprising any one of the following:

-0.001-0.05 wt% of aluminium and 0.001-2 wt% of indium;

-0.002-0.2 wt% bismuth and 0.001-2 wt% indium;

-0.002-0.2 wt% bismuth, 0.001-2 wt% indium and 0.001-0.05 wt% aluminium; and

-0.002-0.2 wt% bismuth;

and optionally up to 0.5 wt% of one or both of lead and calcium, the remainder being zinc.

8. An alkaline battery comprising an alloyed zinc powder as claimed in any one of claims 1 to 7.

9. A method of making alloyed zinc powders for alkaline cells as claimed in any one of claims 1 to 7, comprising the step of centrifugally blasting a zinc alloy, wherein the atmosphere in the centrifugally blasted chamber is maintained at a temperature below 110 ℃.

10. A method of manufacturing alloyed zinc powder for alkaline batteries as defined in claim 9, wherein the atmosphere in the centrifugal spray chamber is maintained at a temperature of less than 100 ℃.

11. A production method of alloyed zinc powder for alkaline batteries as defined in claim 9, wherein said spraying is performed under a protective atmosphere having an oxygen content of less than 8 vol%.

12. A method of making alloyed zinc powder for alkaline cells as claimed in claim 11, wherein said oxygen content is higher than 4 vol%.