Method of making a nickel hydroxide material

Details Method of making a nickel hydroxide material Method of making a nickel hydroxide material

2000-09-13

2002-08-13

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S049000

Reexamination Certificate

active

06432580

ABSTRACT:

BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention pertains to a method for making high density nickel hydroxide for alkali rechargeable batteries. More particularly, the present invention pertains to a method for making nickel hydroxide for a battery electrode from a secondary low cost nickel source.
II. Description of the Background Art
The demand for batteries has grown dramatically over the past decade and continues to grow at a phenomenal rate. Rechargeable batteries with high energy density and high capacity are particularly desirable. Two types of batteries that are widely used are the Ni—Cd (nickel cadmium) type and the more desirable Ni-MH (nickel metal hydride) type. These batteries have a positive and negative electrode. In both types of batteries the positive electrodes are made primarily of nickel hydroxide active material.
Ni-MH cells utilize a negative electrode that is capable of the reversible electrochemical storage of hydrogen. Ni-MH cells usually employ a positive electrode of nickel hydroxide material. The negative and positive electrodes are spaced apart in an alkaline electrolyte. Upon application of an electrical potential across a Ni-MH cell, the Ni-MH material of the negative electrode is charged by the electrochemical absorption of hydrogen and the electrochemical discharge of a hydroxyl ion, as shown in equation 1.
M+H
2
O+e
−
⇄M-H+OH
−
  (1):
The negative electrode reactions are reversible. Upon discharge, the stored hydrogen is released to form a water molecule and release an electron.
The reactions that take place at the nickel hydroxide positive electrode of a Ni-MH cell are shown in equation 2.
Ni(OH)
2
+OH
−
⇄NiOOH+H
2
O+e
−
  (2):
The use of nickel hydroxide, Ni(OH)
2
, as a positive electrode material for batteries is generally known. See for example, U.S. Pat. No. 5,523,182, issued Jun. 4, 1996 to Ovshinsky et al., entitled “Enhanced Nickel Hydroxide Positive Electrode Materials For Alkaline Rechargeable Electrochemical Cells”, the disclosure which is herein incorporated by reference.
Several forms of positive electrodes exist at the present and include sintered, foamed, and pasted electrode types. Processes for making positive electrodes are generally known in the art, see for example U.S. Pat. No. 5,344,728 issued to Ovshinsky et al., the disclosure of which is herein incorporated by reference, where capacity in excess of 560 mAh/cc was reported. The particular process used can have a significant impact on an electrode's performance. For example, conventional sintered electrodes normally have an energy density of around 480-500 mAh/cc. Sintered positive electrodes are constructed by applying nickel powder slurry to a nickel-plated, steel base followed by sintering at high temperature. This process causes the individual particles of nickel to weld at their points of contact, resulting in a porous material that is approximately 80% open volume and 20% solid metal. This sintered material is then impregnated with active material by soaking it in an acidic solution of nickel nitrate, followed by the conversion to nickel hydroxide by reaction with an alkali metal hydroxide. After impregnation, the material is subjected to electrochemical formation.
To achieve significantly higher loading, the current trend has been away from sintered positive electrodes and toward pasted electrodes. Pasted electrodes consist of nickel hydroxide particles in contact with a conductive network or substrate, most commonly foam nickel. Several variants of these electrodes exist and include plastic-bonded nickel electrodes, which utilize graphite as a microconductor, and pasted nickel fiber electrodes, which utilize spherical nickel hydroxide particles loaded onto a high porosity, conductive nickel fiber or nickel foam support.
The production of low cost, high capacity nickel hydroxide is critical to the future commercialization of Ni-MH batteries. As with electrode formation, the properties of nickel hydroxide also differ widely depending upon the production method used. Generally, nickel hydroxide is produced using a precipitation method in which a nickel salt and a hydroxide salt are mixed together followed by the precipitation of nickel hydroxide. Active, nickel hydroxide material preferably has high capacity and long cycle life, see U.S. Pat. No. 5,348,822 to Ovshinsky et al., the disclosure of which is herein incorporated by reference.
It has been discovered that nickel hydroxide suitable for use in a battery electrode should have an apparent density of 1.4-1.7 g/cm
3
, a tap density of about 1.8-2.3 g/cm
3
, and a size range of about 5-50&mgr;. Active, nickel hydroxide particles are preferably spherical in shape with a high packing density and a narrow size distribution Preferably, average particle size should be about 10 &mgr;m and tap density should be about 2.2 g/cc. Paste made with this kind of nickel hydroxide has good fluidity and uniformity, and thus it is possible to fabricate high capacity, uniformly loaded electrodes. The use of this kind of nickel hydroxide also improves the utilization of the active material and discharge capacity of the electrode. If the process is not carefully controlled, the precipitate will have an irregular shape and/or low tap density. For example, if the rate of reaction is too fast, the precipitate formed may be too fine and the density too low. A fine powder with low density requires longer filtering or washing times and increases the adsorption of water on the surface. Further, if the precipitated particles have too wide a size distribution (ranging from 1 to hundreds of microns), the nickel hydroxide may require pulverization to render it useful. Electrodes formed with low-density nickel hydroxide will lack high capacity and high energy density. For these reasons and others, an active powder having an irregular shape and/or low density is less than desirable for use as a high capacity battery electrode material.
In order to produce high density, substantially spherical nickel hydroxide, particles are gradually grown under carefully controlled process conditions. A nickel salt provided in solution is combined with an ammonium ion. The nickel salt forms complex ions with ammonia to which caustic is added. Nickel hydroxide is then gradually precipitated by decomposition of the nickel ammonium complex. The reaction rate is difficult to control, so methods have been introduced to separate critical steps in the production process to compensate for said difficulties. For example, U.S. Pat. No. 5,498,403, entitled “Method for Preparing High Density Nickel Hydroxide Used for Alkali Rechargeable Batteries”, issued to Shin on Mar. 12, 1996, the disclosure of which is herein incorporated by reference, discloses a method of preparing nickel hydroxide from a nickel sulfate solution using a separate or isolated amine reactor. Nickel sulfate is mixed with ammonium hydroxide in the isolated amine reactor to form a nickel ammonium complex. The nickel ammonium complex is removed from the reactor and sent to a second mixing vessel or reactor where it is combined with a solution of sodium hydroxide to obtain nickel hydroxide. Such a method relies heavily on a raw material source of very high purity or what is termed throughout the ensuing specification as primary nickel.
Thus, particular notice should be taken in the fact that all of present day processes for making positive electrode materials, such as those described above, have utilized expensive, high grade, and highly pure primary nickel for the production of nickel salt starter solutions. As modem process technology and automation have reduced the cost of labor in the production of battery electrode materials, the cost of primary nickel and its associated salts have become a significant factor in determining the cost of active electrode materials, battery electrodes, and the batteries the electrodes are placed within, making up as much as 60% of the direct manufacturing

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