Chemistry: electrical current producing apparatus – product – and – Sealed cell having gas prevention or elimation means – Prevention or elimination means is one of the cell...
Reexamination Certificate
2000-07-25
2002-09-03
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Sealed cell having gas prevention or elimation means
Prevention or elimination means is one of the cell...
C429S050000, C429S053000, C429S218200, C429S223000
Reexamination Certificate
active
06444349
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a sealed nickel-metal hydride (Ni-MH) storage cell with improved capacity per unit mass and per unit volume and increased service life. It relates more precisely to a storage cell of high capacity (at least 10 Ah) for aeronautical, stationary or rail applications, or for electric vehicle propulsion. Storage cells of this kind are usually combined to form batteries.
2. Description of the Prior Art
A storage cell is formed by the association at least one positive electrode whose electrochemically active material is principally nickel hydroxide Ni(OH)
2
and at least one negative electrode whose electrochemically active material is an intermetallic compound hydride M. The electrolyte is a concentrated alkaline solution including a plurality of hydroxides (KOH, NaOH, LiOH). A separator, generally made of polyolefin or polyamide, is placed between the positive electrode and the negative electrode.
The storage cell is charged by the following electrochemical reaction: the nickel hydroxide Ni(OH)
2
is oxidized until the valency of the nickel is greater than 2 and the metal hydride MH is formed at the same time by proton transfer:
discharged form
charged form
The positive electrode contains the electrochemically active material and other substances, in particular a conductive material intended to increase the conductivity of the electrode and certain additives required by the shaping process. These substances are also oxidized during charging, causing the parallel reduction of some of the negative active material.
The conductive material usually comprises metallic cobalt (CO
0
) and cobalt hydroxide (Co
2+
) which is transformed into Co
3+
to assure electrical conduction in the electrode; this reaction is not reversible. The organic additives added to facilitate shaping of the electrode are oxidized irreversibly during the first few cycles.
At the beginning of the service life of the storage cell these irreversible reactions in the positive electrode induce a charged capacity in the negative electrode which is referred to as the “precharge” and which cannot be discharged in a sealed storage cell. The precharge mobilizes a high proportion of the alloy to no useful effect and reduces the potential capacities per unit mass and per unit volume of the negative electrode and therefore of the storage cell.
When the positive electrode is charged oxygen is released and this oxygen is reduced (recombined) at the negative electrode. It is essential to avoid the release of hydrogen at the negative electrode at the end of charging because the resulting increase in the internal gas pressure would open the relief valve. Once the positive electrode has been fully charged, the negative electrode must therefore still contain a “surplus” of uncharged active material.
During cycling, the surplus negative capacity is consumed primarily by corrosion of the alloy in accordance with the reactions:
(1+d)M+d H
2
O→M(OH)d+d MH (I)
(1+d)M+d/2H
2
O→MO
d/2
+d MH (II)
where d is the average degree of oxidation of the alloy M.
The active material constituting the surplus is charged progressively and increases the unusable precharge commensurately. The service life of the storage cell is therefore closely dependent on the surplus quantity of uncharged active material.
During overcharging or accidental overdischarging the internal pressure of an Ni-MH storage cell can increase greatly. To maintain a reversible equilibrium in the storage cell under all conditions, U.S. Pat. No. 4,214,043 teaches that the quantity of electrochemically active material in the negative electrode must be greater than that of the material in the positive electrode and that the electrochemically active mass of the negative electrode must be partially present in the charged state when the positive electrode is in the completely discharged state. To achieve this aim the total electrochemical capacity of the negative electrode is preferably at least 15% greater than that of the positive electrode and the part of the negative capacity which cannot be discharged preferably represents at least 10% of this additional capacity.
The object of the present invention is to propose an Ni-MH storage cell whose service life is increased without reducing the capacities per unit mass and per unit volume of the storage cell.
SUMMARY OF THE INVENTION
The present invention provides a sealed nickel-metal hydride storage cell comprising a positive electrode whose electrochemically reactive material is a hydroxide containing principally nickel and a negative electrode whose electrochemically active material is an intermetallic compound capable of forming a hydride when charged, wherein the total quantity of electrochemically active material in the negative electrode exceeds the total quantity of electrochemically active material in the positive electrode so that the total negative capacity exceeds the total positive capacity by an amount of at least 15% referred to as the overcapacity, part of the overcapacity, referred to as the precharge, is partly in the charged state when the positive electrode is completely discharged and the remaining part, referred to as the surplus, is in the discharged state when the positive electrode is completely charged, and the precharge is less than 10% of the negative overcapacity.
The precharge must represent the smallest possible proportion of the negative overcapacity in order to increase the surplus negative capacity without increasing the overcapacity. The surplus capacity therefore represents more than 90% of the negative overcapacity. The consumption of the surplus reduces the service life of the storage cell. Thus the greater the quantity of uncharged active material constituting the surplus, the longer the service life of the storage cell. The precharge is preferably less than 6% and even more preferably less than 2% of the negative overcapacity.
The negative overcapacity is preferably from 20% to 60%, i.e. the quantities of electrochemically active material in the electrodes are chosen so that the total negative capacity is from 1.20 to 1.60 times the total positive capacity. The overcapacity is preferably from 38% to 46% of the positive capacity.
The expression “nickel hydroxide” refers to a hydroxide containing principally nickel but also at least one syncrystallized hydroxide of an element chosen from zinc, cadmium, magnesium and aluminum; it can also contain at least one syncrystallized hydroxide of an element chosen from cobalt, manganese, yttrium, calcium and zirconium.
The advantage of the present invention is that it increases the service life of the storage cell without penalizing its performance. When the storage cell enters service, the negative overcapacity is for the most part made up of an uncharged surplus capacity on which the service life depends. The surplus capacity can be regenerated in use, increasing the service life of the storage cell commensurately.
The present invention also provides a method of reducing the precharge of a nickel-metal hydride storage cell as previously described, the method including a treatment consisting of discharging at least some of the precharge.
In a first embodiment the treatment consists of discharging said storage cell at a moderate current. Said storage cell is preferably discharged at a current not greater than the current needed to discharge the capacity of said storage cell in one hour.
The precharge reduction treatment consists of overdischarging the positive electrode, which reverses it and releases hydrogen at the positive electrode. The negative precharge is discharged in parallel with this.
The over discharged capacity must of course not exceed the value of the negative precharge at the time the precharge reduction treatment begins. The charged electrochemically active material constituting the precharge before the reduction treatment increases the negative surplus when discharged.
In a first variant the trea
Berlureau Thierry
Liska Jean-Louis
Alcatel
Crepeau Jonathan
Ryan Patrick
Sughrue & Mion, PLLC
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