Nonaqueous electrolyte secondary cell

Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer – spacer or materials for use therewith

Reexamination Certificate

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C429S145000, C429S231950, C429S129000

Reexamination Certificate

active

06818354

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary cell using a nonaqueous electrolyte and positive and negative electrode active materials that are capable of reversibly intercalating lithium ions, and more particularly, to a nonaqueous electrolyte secondary cell having improved safety.
2. Description of the Prior Art
In recent years, rapid advancements in size reduction and weight reduction of mobile information terminals, such as mobile telephones and notebook computers, have created an increasing demand for nonaqueous electrolyte secondary cells, which are lightweight and have high capacity.
Nonaqueous electrolyte secondary cells perform charge and discharge by migration of lithium between the positive electrode and the negative electrode. Generally, nonaqueous electrolyte secondary cells use a carbon-based material that is capable of reversibly intercalating lithium ions (for the negative electrode active material), a transition metal oxide such as lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, and the like (for the positive electrode active material), and a nonaqueous electrolyte containing a lithium salt. Such nonaqueous electrolyte secondary cells exhibit excellent charge-discharge characteristics insofar as charge and discharge are performed in an appropriate range.
However, when the cells are overcharged, the lithium ions that cannot be stored in the negative electrode deposit on the negative electrode in the form of lithium metal, and the deposit develops into dendrites. The developed dendrites pierce through the separator and reach the positive electrode, causing an internal short circuit. In conventional nonaqueous electrolyte secondary cells, the dendrites fully grow and quickly pierce through the separator. This causes great damage to the separator and the resulting internal short circuit cause the cell temperature to rise to such a degree that cell performance is degraded.
Moreover, overcharge causes the positive electrode potential to increase (for example, to exceed more than 5 V), and as a result, decomposition of the electrolyte solution occurs on the positive electrode. Decomposition of the electrolyte solution induces a shortage of electrolyte solution and an increase of cell internal pressure, and when the cell temperature increase mentioned above occurs in addition to these, the electrode active materials and the electrolyte solution react violently.
In view of this problem, conventional nonaqueous electrolyte secondary cells incorporate, in order to ensure safety of the cells, separately-produced protective circuits such that, for example, electric current is cut off when cell voltage excessively increases. Such incorporation of protective circuits, however, increases the cost of the cells and moreover impedes reductions in size and weight of the cells.
SUMMARY OF THE INVENTION
In view of the foregoing and other problems in the prior art, it is an object of the present invention to improve safety of a cell, without incorporating a separately-produced protective circuit therein, by effectively utilizing an internal short circuit induced by dendrites of lithium metal, which is a mechanism inherent to the cell.
It is another object of the invention to improve safety of a cell while achieving reductions in size, weight, and cost of the cell, by self-containedly suppressing an increase in cell temperature and gas formation that is caused by overcharge without using special components.
These and other objects are accomplished in accordance with the present invention by providing a nonaqueous electrolyte secondary cell comprising a positive electrode, a negative electrode, a nonaqueous electrolyte, a separator interposed between the positive electrode and the negative electrode, the positive electrode having a positive electrode active material comprising a chemical compound capable of reversibly intercalating lithium and the negative electrode having a negative electrode active material comprising a material capable of reversibly intercalating lithium, wherein the separator has through holes for passing lithium dendrites therethrough.
In cases where a cell is overcharged and thereby lithium is released from the positive electrode in an amount exceeding the capacity of the negative electrode or where charge is performed in a low temperature condition in which the reactivity of the negative electrode is decreased, lithium dendrites deposit on the negative electrode. When the lithium dendrites pass through the separator and connect the positive electrode and the negative electrode to allow electrical contact therebetween at an initial stage of the lithium dendrite formation (before the dendrites fully grow), a short circuit is caused and thereby charge reaction does not further proceed. In addition, since the lithium dendrites are small in diameter at this stage, safety problems caused by, for example, increases in cell voltage and cell temperature, are suppressed.
Thus, as described above, when the separator has through holes for passing lithium dendrites therethrough, electrical contact between the positive electrode and the negative electrode is established at the initial stage of lithium dendrite formation, and consequently, safety of the cell is maintained.
In the above-described nonaqueous electrolyte secondary cell, the through holes may have a substantially straight line-shape and the positive electrode and the negative electrode may be connected thereby.
When the through holes have a substantially straight line-shape and the positive electrode and the negative electrode are connected thereby, lithium dendrites can smoothly grow and thereby electrical contact between the positive electrode and the negative electrode is formed at an earlier stage of lithium dendrite formation. Thus, safety of the cell is further improved.
In the above-described nonaqueous electrolyte secondary cell, the through holes may be such that the positive electrode and the negative electrode are connected in the shortest possible distance.
This configuration makes it possible to form electrical contact between the positive electrode and the negative electrode at an even earlier stage of lithium dendrite formation, and accordingly, safety of the cell is even further improved.
In the above-described nonaqueous electrolyte secondary cell, the through holes may have a diameter of 5 &mgr;m or greater
When the diameter of the through holes is 5 &mgr;m or greater, the positive electrode and the negative electrode are easily connected even when the lithium dendrites grow significantly in a transverse direction (in a direction parallel to the substrate).
In the above-described nonaqueous electrolyte secondary cell, the through holes may have a diameter of 100 &mgr;m or less, and preferably 70 &mgr;m or less.
When the diameter of the through holes is 100 &mgr;m or less, the possibility of occurrences of internal short circuits is reduced under normal conditions of use (not in overcharge conditions or the like). When the diameter of the through holes is 70 &mgr;m or less, it is ensured that internal short circuits are prevented under normal conditions of use.
In the above-described nonaqueous electrolyte secondary cell, the through holes may have a diameter of 50 &mgr;m or less.
When the diameter of the through holes is 50 &mgr;m or less, a shutdown mechanism smoothly works in which the separator, which is composed of a microporous film made of polyethylene or polypropylene, melts in cases where a cell temperature increase occurs and thereby prevents current flow between the positive electrode and the negative electrode. As a consequence, safety of the cell is further improved.
In the above-described nonaqueous electrolyte secondary cell, the through holes may have a diameter of 30 &mgr;m or less.
When the through holes have a diameter of 30 &mgr;m or less, cell degradation due to self-discharge can be suppressed, and accordingly, when the cell is stored at high temperature, cell voltage variation

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