Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-07-07
2002-04-09
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S224000, C429S231500, C429S223000, C429S231600
Reexamination Certificate
active
06368750
ABSTRACT:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to, among secondary batteries to be used as an operational power source for portable electronic equipment, or as a motor driving battery for an electric vehicle or a hybrid electric vehicle, etc., a lithium secondary battery which has small internal resistance and has good charge-discharge cycle characteristics, with a lithium transition metal compound being used as a positive active material.
In recent years, miniaturization to go with lighter weight is being investigated in an accelerated fashion with respect to electronic equipment such as a personal handy phone system, a video tape recorder, a notebook-sized personal computer, etc., and a secondary battery comprising a lithium transition metal compound as a positive active material, with a carbon material as a negative material, and an electrolyte obtained by dissolving a Li ion electrolyte in an organic solvent, has become common as the power source battery.
Such a battery is generally called a lithium secondary battery or a lithium ion battery, and since they are provided with larger energy density as well as with higher unit cell voltage of approximately 4V, attention is being paid to these batteries not only for the aforementioned electronic equipment but also as a motor driving power source for an electric vehicle or a hybrid electric vehicle which is under consideration for positive proliferation to the general public as a low pollution vehicle, in view of the recent environmental problems.
In such a lithium secondary battery, its battery capacity as well as its charge-discharge cycle characteristics (hereinafter called “cycle characteristics”) heavily depends on the material characteristics of the positive active material to be used. The lithium transition metal compound to be used as the positive active material includes lithium cobalt oxide (LiCoO
2
), lithium nickel oxide (LiNiO
2
), or lithium manganese oxide (LiMn
2
O
4
), etc. in particular.
Here, LiCoO
2
as well as LiNiO
2
comprises such features as a large Li capacity, a simple configuration, and excellent reversibility, and is provided with a two dimensionally layered configuration that is excellent in ion diffusion. On the other hand, however, as concerns LiCoO
2
, producing areas of Co are limited and it hardly is true that output quantity is abundant. Accordingly, these materials are expensive, and thus there is a cost issue and a problem that its output density is smaller compared with LiMn
2
O
4
. In addition, as concerns LiNiO
2
, synthesis of compounds of stoichiometric composition is difficult since the trivalent status of Ni is comparatively unstable, and in the case where detachment of Li becomes abundant, Ni will become subject to transition to bivalent status, emitting oxygen to constitute NiO, which creates problems such that the battery will stop functioning as a battery and there is a risk of battery explosion due to oxygen detachment.
On the contrary, LiMn
2
O
4
has a feature that raw materials are inexpensive and larger output density as well as higher voltage is provided. However, in the case where LiMn
2
O
4
has been used as a positive active material, there is a problem that repetition of charging-discharging cycle gradually decreases discharge capacity and good cycle characteristics will not become obtainable. It is deemed that the major cause of this is reduction of the positive capacity since crystal configuration changes irreversibly due to insertion and detachment of Li
+
.
Thus, a lithium transition metal compound such as LiCoO
2
, etc. respectively has both advantages and disadvantages together as a positive active material, and therefore, there are no rules which substances must be used, and it is deemed advisable that a positive active material which can show an appropriate feature for a particular purpose should be suitably selected for use.
Incidentally, regardless of the kind of positive active material, it is preferred in terms of characteristics of a battery that the internal resistance of the battery is small, and it is a common problem to all the positive active materials to be solved that resistance in a positive active material (namely electronic conduction resistance) should be reduced, or in other words, electronic conductivity should be improved for this reduction of the internal resistance. Particularly, in a lithium secondary battery of large capacity used as a motor driving battery for an electric vehicle, etc., it is very important to obtain large current output necessary for acceleration and gradeability, etc. to improve charging-discharging efficiency.
Under the circumstances, conventionally, trials to improve electronic conductivity by adding to a positive active material conductive fine grains such as acetylene black, etc. to reduce internal resistance of a battery have been conducted. This is becaused the above-described lithium transition metal compound is a mixed conducting body comprising both lithium ion conductivity and electronic conductivity together, but its electronic conductivity is not always strong.
However, there is a problem that addition of acetylene black causes reduction of filling quantity of a positive active material to reduce battery capacity. In addition, it is deemed that improvement of electronic conductivity is not unlimited since acetylene black is a kind of carbon and is a semiconductor. Moreover, acetylene black is voluminous and presents such a problem that it is difficult to handle when an electrode plate is to be produced. Accordingly, the volume of its addition is to be limited to an appropriate quantity, comparing and considering the advantageous effect of reduction of internal resistance, the disadvantageous effect of reduction of battery capacity, and the simplicity in production, etc.
Now, as described above, in the case where acetylene black has been added, acetylene black exists only on surfaces of particles of a positive active material, resulting in contributing to improvement of electronic conductivity among particles of positive active material, but not resulting in contributing to improvement of electronic conductivity inside a particle of a positive active material. Thus, conventionally, for improving electronic conductivity of a positive active material, attention was only paid to electronic conductivity among particles of a positive active material, but the relationship between diffusion of Li
+
and electronic conductivity inside a particle of a positive active material at the time of battery reaction was not regarded as a problem.
In short, detachment of Li
+
from a particle of a positive active material as well as insertion of Li
+
to a particle of a positive active material is proceeded by diffusion of Li
+
inside a particle of a positive active material, simultaneously accompanied by transfer of electrons taking place inside a particle of a positive active material, and at this time, if electronic conductivity inside a particle of a positive active material is low, diffusion of Li
+
hardly is apt to take place and velocity of detachment and insertion of Li
+
, namely velocity of battery reaction, becomes slow, resulting in an increase in internal resistance, which was not taken into consideration at all.
The present inventors paid attention to this point, and considered in earnest to improve electronic conductivity of a positive active material itself so that diffusion of Li
+
inside a positive active material may proceed well, thus reducing resistance of the positive active material itself, and at the same time, when a battery has been assembled without increasing volume of acetylene black to be added, internal resistance of that battery may be reduced, and as a result the present invention has been achieved.
SUMMARY OF THE INVENTION
According to an aspect of the present invention, there is provided a lithium secondary battery, comprising a lithium transition metal compound LiMe
X
O
Y
, in which a portion of transition elemen
Kitoh Kenshin
Nemoto Hiroshi
Takahashi Michio
Alejandro R
Burr & Brown
Kalafut Stephen
NGK Insulators Ltd.
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