Rechargeable electrochemical cell

Details Rechargeable electrochemical cell Rechargeable electrochemical cell

2001-12-18

2004-05-04

Ryan, Patrick (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S199000, C429S231900, C429S231950

Reexamination Certificate

active

06730441

ABSTRACT:

The invention relates to a rechargeable electrochemical cell with a negative electrode, which contains—in the charged state—an active metal, an electrolyte based on sulfur dioxide, and a positive electrode which accumulates the active metal. During charging of the cell, ions of the active metal are released from the is electrode into the electrolyte solution and deposited onto the negative electrode. During the discharge process, the active metal deposited on the negative electrode dissolves and a corresponding amount of metal ions is accumulated in the positive electrode.
The term “SO
2
-based electrolyte” designates those electrolyte solutions which contain SO
2
not only as an additive in small concentration, but for which the movability of the ions of the conducting salt providing the charge transport in the electrolyte, is at least partially effected by the SO
2
.
Particularly important are cells in which the active metal of the negative electrode is an alkali metal, in particular lithium or sodium. In this case, the conducting salt is preferably a tetrachloraluminate of the alkaline metal, e.g. LiAlCl
4
. Active metals preferred in the scope of the invention are, apart from lithium and sodium, also calcium and zinc.
Rechargeable electrochemical cells with an SO
2
-based electrolyte have important advantages, whereby they seem advantageous for many applications for which rechargeable electrical energy sources are needed. In “The Handbook of Batteries” by David Linden, second edition 1994, Mc Graw Hill, on page 36.25, it is stated, for example, that cells with inorganic electrolytes based on SO
2
are attractive, as they can be operated with high charge and discharge currents due to the high ionic conductivity of the electrolyte. A high energy density, a low self-discharge rate, the possibility of a limited overcharge and exhaustive discharge, as well as a high cell voltage are mentioned as further advantages. Despite all these advantages they are, in the cited literature, considered as largely inappropriate for general use due to, inter alia, their potential safety risks.
Different types of the mentioned cells can be distinguished, the differences mainly relating to the positive electrode.
In a first group, carbon is used for the positive electrode, in form of a graphite material. The charge and discharge process in these cells involves a redox complex formation of the electrolyte salt (e.g. LiALCl
4
) with the carbon.
In a second group, the positive electrode is based on a metal halide, for example CuCl
2
, a simple electrode reaction taking place between the active metal and the electrode (see Handbook of Batteries, on the page mentioned above).
In a third group of cells, for which the invention is of particular interest, the positive electrode consists of a metal oxide, in particular in form of an intercalation compound. Such a cell with Lithium as active metal and a positive intercalation electrode on the basis of LiCoO
2
is the subject of the U.S. Pat. No. 5,213,914. Procedures for the production of appropriate intercalation electrodes are described in European patents 0357952 B1 and 0673552 B1. The content of these cited documents is incorporated herein by reference.
A problem which is common to the different types of cells with a SO
2
-based electrolyte is the fact that a self discharge reaction takes place at the negative electrode during storage periods. By this reaction the sulfur dioxide of the electrolyte solution reacts with the active metal of the negative electrode to form a poorly soluble compound. In case of a monovalent active metal A, for example, a dithionite of the metal is formed according to the reaction equation:
2A+2SO
2
→A
2
S
2
O
4
.
The poorly soluble product of this self discharge reaction is deposited onto the negative electrode as a covering layer.
This self discharge reaction consumes SO
2
, which is then no longer available as solvent for the conducting salt. On the other hand, a sufficient amount of SO
2
is mandatory for the operation of the cell. If the SO
2
-content would drop below a value sufficient for the movability of the conducting salt ions in the electrolyte, this would lead to an intolerable decrease of the electric conductivity. Therefore common electrochemical cells having an electrolyte which is based on sulfur dioxide, contain a large amount of SO
2
.
In order to provide an improved electrochemical cell with a negative electrode, which in charged state contains an active metal selected from the group consisting of the alkaline metals, the alkaline-earth metals and the metals of the second subgroup of the periodic table of elements, an electrolyte solution based on sulfur dioxide, and a positive electrode containing the active metal, from where ions are released into the electrolyte solution during the charging process, whereas a self discharge reaction takes place at the negative electrode, during which the sulfur dioxide of the electrolyte solution is reacts with the active metal of the negative electrode to form a poorly soluble compound it is proposed that the electrochemical charge quantity of the sulfur dioxide in the cell, calculated with one faraday per mole sulfur dioxide, is smaller than the charge quantity of the active metal which can be theoretically electrochemically accumulated in the positive electrode.
The capacity of a cell of the type described here is determined by the amount of the active metal which can be accumulated in the positive electrode. In the discharged state of the cell, the active metal is contained in maximum concentration in the positive electrode, and it is released into the electrolyte solution during the charge process. A corresponding amount of the active metal is deposited onto or incorporated into the negative electrode.
For the above mentioned cells, for example, having a positive electrode of a metal halide, the accumulation process is a chemical reaction. In case of a metal oxide intercalation compound, the active metal is accumulated in the positive electrode by storing its ions in the matrix lattice of the metal oxide, or releasing them from the matrix lattice.
The quantity of electrical charge corresponding to the maximum amount of the active metal which the positive electrode may theoretically contain on the basis of stoichiometric calculations is designated the charge quantity of the active metal which can theoretically be accumulated in the positive electrode. This value of the accumulateable charge quantity is always higher than the practically obtainable maximum capacity of the cell, since—for practical reasons—with no type of positive electrode known so far the theoretically accumulateable quantity can be accumulated completely into the electrode, nor can it be released from the electrode during the charge process.
The formation of the poorly soluble self discharge product on the negative electrode consumes active metal. For the preferred case, in which the active metal is lithium, the self discharge reaction is, for example:
2Li+2SO
2
→Li
2
S
2
O
4
.
In many cases, the cells are stored over a long period of time (several months or even years). Meanwhile the self discharge reaction continuously proceeds, whereby the active metal from the negative electrode reacts with the sulfur dioxide to form the poorly soluble product (in the exemplary case, lithium dithionit). The amount of SO
2
consumed thereby is equimolar to the amount of active metal converted at the negative electrode. For a completely charged cell, the maximum amount of the active metal on the negative electrode corresponds to the charge quantity theoretically accumulateable in the positive electrode. If the molar quantity of the sulfur dioxide in the cell is smaller than the electrochemically equivalent quantity of charge of the active metal theoretically accumulateable in the positive electrode, then finally, the entire amount of SO
2
of the electrolyte should be consumed in the course of the self discharge reaction. This would lead to the consequence that the electrolyte becomes solid,

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