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
1999-11-17
2003-02-04
Weiner, Laura (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...
C429S231950, C429S231100, C429S231300, C429S221000, C429S223000, C429S224000, C029S623100
Reexamination Certificate
active
06514638
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery having a large capacity with improved charge/discharge characteristics.
TECHNICAL BACKGROUND OF THE INVENTION
Recent development of devices in the electronic field is remarkable, and various devices such as video cameras, liquid crystal cameras, portable telephones, laptop computers, and word processors are now being developed. In accordance therewith, there are increasing needs for reduction in size and weight and achievement of high energy density in batteries that are to be used as power sources for these electronic devices.
Conventionally, lead batteries or nickel-cadmium batteries have been used for these electronic devices. These conventional batteries cannot sufficiently meet the needs for reduction in size and weight and achievement of high energy density.
Accordingly, there are proposed non-aqueous electrolyte batteries using a non-aqueous electrolytic solution containing a lithium salt dissolved in a non-aqueous solvent. As these non-aqueous electrolyte batteries, batteries in which a metal lithium, a lithium alloy, or a carbon material capable of being doped and undoped with lithium ions is used as a negative electrode material and a lithium cobalt composite oxide is used as a positive electrode material are already in practical use.
Having a high operation voltage of 3 to 4 V, the non-aqueous electrolyte batteries of this type can be made to have a high energy density and excellent cycle characteristics with only a small amount of self-discharge.
Also, in order to attain further reduction in size and weight and achievement of high energy density in these non-aqueous electrolyte batteries, eager researches for development of active materials and the like are now being carried out. As positive electrode active materials, Ni-containing lithium composite oxides such as lithium-nickel composite oxides and lithium-nickel-cobalt composite oxides are also proposed.
Meanwhile, Japanese Laid-open Patent Publication No. 5-290847/1993, for example, discloses use of Li
1+x
CoO
2
as a positive electrode active material to provide lithium corresponding to the latent capacity thereof for precharging the negative electrode so as to increase the battery capacity. However, the battery capacity has not been sufficiently increased.
DISCLOSURE OF THE INVENTION
Objects of the Invention
On examining the capacity and the charge/discharge characteristics of a non-aqueous electrolyte secondary battery, the present inventors have found that it is not possible to improve the capacity and the charge/discharge characteristics of the battery as designed even if each of the capacities of the positive and negative electrodes is simply increased.
Accordingly, the object of the present invention is to solve the above-mentioned problems of the prior art and to provide a non-aqueous electrolyte secondary battery having a large capacity with improved charge/discharge characteristics.
Summary of the Invention
The present inventors have made an eager research and found out that a battery can have increased capacity and improved charge/discharge characteristics by observing each of the initial efficiencies of positive and negative electrodes and by combining the positive electrode and the negative electrode so that the initial efficiencies of the positive and negative electrode satisfy a specific relationship, thereby completing the present invention.
Accordingly, the present invention provides a non-aqueous electrolyte secondary battery including a positive electrode and a negative electrode each capable of being doped and undoped with lithium ions, wherein the positive electrode and the negative electrode are combined so that the relationship:
0.9≦Kp/Kn≦1.1
is satisfied, where an initial efficiency of the positive electrode is represented by Kp and an initial efficiency of the negative electrode is represented by Kn.
In the present invention, the initial efficiency Kp of the positive electrode is a ratio of a discharge capacity to a charge capacity when it is first charged to 4.2 V and then discharged to 3.0 V using lithium as a counter electrode. In other words,
Kp=(initial discharge capacity)/(initial charge capacity).
The initial efficiency Kn of the negative electrode is a ratio of a charge capacity to a discharge capacity when it is first discharged to +0.0 V and then charged to 2.0 V using lithium as a counter electrode. In other words,
Kn =(initial charge capacity)/(initial discharge capacity).
Although nickel-containing lithium composite oxide as an active material has a large capacity, its initial efficiency tends to be poorer than cobalt lithium oxide. Also, hard carbon and polymer carbon tend to have a larger capacity but with a poorer initial efficiency than an active material of graphite. In a conventional method, if the initial efficiencies of the positive electrode and the negative electrode are different, the one having the poorer initial efficiency is packed in a smaller amount to fabricate a battery, or alternatively the one having the better initial efficiency is loaded. If an attempt is made to increase the capacity, the initial charge/discharge capacities tend to be poorer. Conventionally, an improvement to increase the initial efficiency has been made. Also, in a conventional lead battery, for example, the concentration of an electrolyte in an electrolytic solution changes by charging and discharging. In a battery where electricity is produced by anions and cations in the electrolyte reacting with the positive electrode and the negative electrode, the capacity changes greatly according to the initial efficiency. Therefore, increased initial efficiency contributes to increase in the capacity.
However, in a lithium ion secondary battery, electricity is taken out by means of lithium ions moving between the positive electrode and the negative electrode, so that basically the concentration of the electrolyte in the electrolytic solution does not change. In other words, the electrolyte in the electrolytic solution is not consumed by charging or discharging. Therefore, it has been found out that it is not necessary to increase the initial efficiency with efforts but, rather, the initial efficiency ratio between the positive electrode and the negative electrode is important. Thus, the present invention provides a battery having a large capacity with improved charge/discharge characteristics by allowing the ratio of the initial efficiencies of the positive and negative electrodes to be within the above-mentioned specific range.
In the present invention, the active material of the positive electrode preferably contains a lithium composite oxide having a composition of Li
x
Ni
y
M
z
O
2
(where x satisfies 0.8<x<1.5, y+z satisfies 0.8<y+z<1.2, z satisfies 0≦z<0.35, and M is at least one element selected from Co, Mg, Ca, Sr, Al, Mn and Fe).
Detailed Description of the Invention
In the present invention, a lithium composite oxide is used as an active material of the positive electrode. Examples of the lithium composite oxides to be used include Li
x
CoO
2
(0<x≦1.0), Li
x
N
i
O
2
(0<x≦1.0), Li
1+X
Mn
2−X
O
4
(0≦x≦⅓), Li(M, Mn)
2
O
4
(M=Cr, Co, Al, B), and others.
In the present invention, it is particularly suitable in obtaining large capacity with low costs that the lithium composite oxide is Li
x
Ni
y
M
Z
O
2
(where x satisfies 0.8<x<1.5, y+z satisfies 0.8<y+z<1.2, z satisfies 0≦z<0.35, and M is at least one element selected from Co, Mg, Ca, Sr, Al, Mn and Fe). In this case, the metal M is more preferably Co, and may be two or more kinds of the metals.
An example of a method for producing such a lithium composite oxide is, for example, a process in which a basic metal salt and an alkaline water-soluble lithium compound containing respectively an anion that volatilizes at the time of
Iijima Tadayoshi
Kurose Shigeo
Takahashi Tetsuya
Rader & Fishman & Grauer, PLLC
TDK Corporation
Weiner Laura
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