A NONAQUEOUS ELECTROLYTE FOR BATTERIES AND A LITHIUM
SECONDARY BATTERY INCORPORATING SAID ELECTROLYTE
Technical Field
The present invention relates to a lithium secondary
battery with improvements in charge/discharge and cycle life
characteristics at ambient and high temperatures, and/or
storage characteristics and safety at high temperature, as
well as a nonaqueous electrolyte for use therein.
Background Art
With the recent advancement of electronic technology,
portable information devices, such as mobile phones, PDA and
laptop computers, are widely used. In such portable
information devices, there are strong demands for smaller
size, lighter weight, and continuous long-term driving. As a
driving power source for such portable information devices,
batteries are used. Thus, studies to develop batteries,
particularly lithium secondary batteries using nonaqueous
electrolytes, which have light weight while showing high
voltage, high capacity, high power, high energy density and
long cycle life, are being actively conducted.
Generally, lithium secondary batteries utilize lithium-
containing transition metal oxide as a positive active
material. Examples of the positive active material include
LiCoO2, LiNiO2, LiMn2O4, LiMnC2, LiNi1-xCoxMYO2 (M = Al, Ti, Mg
or Zr; 0 < X < 1; 0 < Y < 0.2) LiNixCoyMn1-x-yO2 (0 < X < 0.5; 0
< Y < 0.5), and a, mixture of two or more thereof.
Furthermore, the lithium secondary batteries utilize carbon,
lithium metal or alloy as a negative active material. Also,

metal oxides, such as TiO2 and SnO2, which can store and
release lithium ions and have a potential of less than 2V for
lithium, may be used as the negative active material.
When such lithium secondary batteries are stored at
high temperature or exposed to high temperature, gas will be
generated within the batteries by the side reaction of
electrodes with the electrolyte oxides, resulting in
deterioration in storage life characteristics and safety at
high temperature, as well as deterioration in battery
performance.
Meanwhile, regarding an improvement in the cycle life
of the lithium secondary batteries, Japanese Patent Laid-open
Publication No. 1996-138735 describes that if LiPF6 was used
as an electrolyte, ar effect on the improvement of cycle life
by the addition of metal halides would not be obtained.
Disclosure of the Invention
It is an object: of the present invention to provide a
lithium secondary oattery which has improvements in
charge/discharge efficiencies and cycle life characteristics
even when it operates at ambient or high temperature.
Another object of the present invention is to provide a
lithium secondary battery with high-temperature safety, in
which the generation of gas by the side reaction of
electrolyte oxides with electrodes is inhibited even when the
battery is stored at high temperature or exposed to high
temperature.
The present inventors have found that the use of metal
halide in a non-aqueous electrolyte has little or no effect
on the improvement of battery cycle life and shows a

reduction in battery cycle life, whereas the use of halogen,
such as iodine, chlorine or bromine, in the nonaqueous
electrolyte, has an effect on the improvement of battery
cycle life and shows improvements in. storage characteristics
and safety at high temperature, unlike the case of the metal
halide.
Moreover, the present inventors have found that the
addition of both a pyrrole or its derivative and halogen to
the nonaqueous electrolyte has a synergistic effect on the
improvement of battery cycle; life.
The present invention has been made based on these
findings.
The present invention provides:
(i) a nonaqueous electrolyte for batteries, which is
characterized by containing halogen; .
(ii) a nonaqueous electrolyte for batteries, which is
characterized by containing pyrrol or its derivative and
halogen; and
(iii) a lithium secondary battery which is
characterized by including the nonaqueous electrolyte (i) or
(ii).
The addition of halogen, such as iodine, chlorine or
bromine, into the nonaqueous electrolyte, results in an
improvement in the cycle life of the lithium secondary
battery.
Meanwhile, although an SEI insulator film having no
electron conductivity is formed on the negative electrode
surface of the lith.um secondary battery, the addition of
pyrrole or its derivative to the nonaqueous electrolyte leads
. to the formation cf polypyrrole, an electron-conducting

polymer, thus lowering resistance.
Furthermore, by a synergistic effect with halogen, the
pyrrole or its derivative in the nonaqueous electrolyte
provides an improvement in charge/discharge cycle
characteristics and an outstanding improvement in battery
cycle life.
Moreover, if halogen is used as an electrolyte additive
as described above, the high-temperature safety of the
battery will be secured. The reason thereof is as follows.
If the battery is stored at high temperature or exposed
to high temperature, the solvent in the nonaqueous
electrolyte will be partially oxidized to cause a side
reaction with the positive and negative electrodes of the
battery, thus generating gas. This will cause not only
deterioration in the battery performance but also
deterioration in the battery swelling leading to
deterioration in the battery safety.
Halogen, such as iodine, chlorine or bromine, which is
used as the electrolyte additive, is a material having strong
adsorption property. Thus, the halogen is adsorbed on the
electrodes upon initial charge, so that when the battery is
stored at high temperature or exposed to high temperature,
the halogen inhibits "he side reaction between the oxide of
the electrolyte and :he positive and negative electrodes,
thus inhibiting the generation of gas. For this reason, a
swelling phenomenon at high temperature occurs less
seriously. Thus, the use of the halogen can provide a battery
having excellent storage characteristics and safety at high
temperature.
Particularly, the use of iodine as the electrolyte

additive has the greatest effect on the inhibition of gas
generation.
The halogen is added to the nonaqueous electrolyte at
an amount ranging from 0.005% by weight to 1% by weight. If
the halogen is used at amounts out of this content range, it
will have a reduced effect on the improvement of battery
cycle life. The content of the halogen in the nonagueous
electrolyte is preferably in a range of 0.01-0.5% by weight.
At a content of less than 0.01% by weight, the halogen will
have an insignificant effect on the inhibition of gas
generation, and at a content of more than 0.5% by weight, it
will cause deterioration in the battery performance.
The pyrrole or its derivative is preferably added to
the nonaqueous electrolyte at the amount of 0.01-0.5% by
weight. At less than 0.01% by weight, the thickness of a film
formed from the pyrrole or its derivative will be
insufficient, and at more than 0.5% by weight, the charge
characteristic of the battery will be poor.
Examples of the halogen include, but are not limited
to, iodine, chlorine and bromine.
Examples of. the pyrrole derivative include, but are not
limited to, 2,5-dimethylpyrrole, 2,4-dimethylpyrrole, 2-
acetyl N-methylpyrrole, 2-acetylpyrrole, and N-methylpyrrole.
The inventive lithium secondary battery includes the
inventive nonaqueous electrolyte. Examples of the lithium
secondary batteries include lithium-metal secondary
batteries, lithium-ion secondary batteries, lithium polymer
secondary batteries, and lithium-ion polymer secondary
batteries.
The inventive lithium secondary battery includes:

a) a positive electrode capable of storing and
releasing lithium ions;
b) a negative electrode capable of storing and
releasing lithium ions;

c) a porous separator; and
d) a nonaqueous electrolyte containing:
i) a lithium salt; and
ii) a liquid electrolyte compound.
The inventive nonaqueous electrolyte preferably
contains cyclic carbonate and/or linear carbonate. Examples
of the cyclic carbonate include, are not limited to/ ethylene
carbonate (EC) , propylene carbonate (PC) and gamma-
butyrolactone (GBL) Examples of the linear carbonate
include, but are not limited to, diethyl carbonate (DEC),
dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), and
methylpropyl carbonate (MPC).
The inventive nonaqueous electrolyte contains lithium
salts which are preferably selected from the group consisting
of LiClO4, LiCF3SO3, LLPF6, LiBF4, LiAsF6, and LiN(CF3SO2)2.
In the present invention, lithium-containing transition
metal oxide is used as a positive active material. Examples
of the positive active material include, but are not limited
to, LiCoO2, LiNiO2, L.LMn2O4, LiMnO2, LiNi1-xCOxMyO2 (M = Al,. Ti,
Mg or Zr; 0 < X < 1; 0 < Y < 0.2), LiNixCoyMn1-x-yO2 (O < X <
0.5; 0