Non-aqueous electrolyte secondary batteries

Details Non-aqueous electrolyte secondary batteries Non-aqueous electrolyte secondary batteries

2000-04-06

2002-09-17

Ryan, Patrick (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S231800

Reexamination Certificate

active

06451482

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a non-aqueous electrolyte secondary battery employing an organic electrolyte, a polymer solid electrolyte or the like. In particular, it relates to a novel constitution of positive and negative electrodes having a higher capacity without sacrificing the charge-discharge cycle life.
BACKGROUND OF THE INVENTION
A variety of secondary batteries including small-sized sealed lead-acid and nickel-cadmium systems have been developed as power sources for driving portable electronic equipment. To minimize the overall size and weight of such a driving secondary battery, newly introduced and marketed are nickel-metal hydride systems, lithium-ion systems, and other advanced secondary batteries, which are higher in the energy density. As their products are welcome widely in the market, lithium-ion batteries above all are focused which are substantially equal in the capacity density per unit volume (Wh/L) to nickel-metal hydride systems but almost two times higher in the capacity density per unit weight (Wh/kg) than the same. The lithium-ion second battery is now known as one of the lightest power sources and demanded for more improvement.
The lithium-ion secondary battery includes a negative active material of lithium and is thus regarded as a lithium secondary battery. Also, it uses a non-aqueous electrolyte such as organic electrolyte or polymer solid electrolyte and is regarded as a non-aqueous electrolyte secondary battery.
Essentially, the lithium-ion secondary battery comprises a positive electrode made of lithium contained cobalt oxide, LiCoO
2
, or a double oxide of lithium and cobalt, and a negative electrode made of a carbon material such as graphite or coke. The electrodes are separated by a separator, assembled to form an electrode group, and placed in an organic electrolyte. In use, when the secondary battery is initially charged, lithium ions are desorbed from the positive electrode of LiCoO
2
and dissolved into the electrolyte. Simultaneously, lithium ions in the electrolyte are absorbed in the carbon material of the negative electrode to form C
6
Li. During the initial discharge, lithium ions in the electrolyte are absorbed in the positive electrode and restored to a LiCoO
2
form. At the time, lithium ions are desorbed from the negative electrode of C
6
Li and dissolved into the electrolyte. Because the charge and the discharge reaction on the positive and negative electrodes are reversible, the battery system is called a rocking-chair battery. The rocking-chair battery may have a longer cycling life of over 1000 cycles provided that neither overcharge or over-discharge occurs.
It is, however, said that the reversible reaction on the positive and the negative electrodes in the charge and discharge are not uniform. As described above, the initial charge permits lithium ions to be desorbed from LiCoO
2
of the positive electrode, but not the whole amount of lithium ions is absorbed in the positive electrode and restored to LiCoO
2
in the initial discharge. In other words, it is common that a smaller amount of lithium ions than the amount desorbed in the initial charge is successfully absorbed in the positive electrode. Also, in the initial charge, an amount of lithium ions equivalent to the charge capacity of the positive electrode is absorbed in the negative electrode made of a carbon material to form to C
6
Li. The negative electrode however desorbs about 80% or more of the whole amount of absorbed lithium ions in the initial discharge. As the remaining 20% of lithium has been trapped in the negative electrode, it will not participate in the charge and discharge reactions of a succeeding cycle. Such an amount of lithium ions trapped in the negative electrode and isolated from the charge and discharge reactions is regarded as “dead lithium” and should be discriminated from the other active portion. Although the efficiency of reaction during the charge and discharge after the initial discharge is affected by the rates of charge and discharge and the ambient temperature at the site and may not reach 100%, its declination will is not compared to a difference between the initial charge capacity and the initial discharge capacity. It is hence essential to design constitution of the lithium-ion secondary battery to account a ratio of the initial discharge capacity to the initial charge capacity (referred to as an initial charge and discharge efficiency hereinafter) on the positive and negative electrodes in order to determine the theoretical capacity values of the positive and negative electrodes.
The capacity of the secondary battery will be increased when a material having a higher charge-discharge efficiency or, more specifically a higher initial charge-discharge efficiency, is used as the positive and negative electrodes. An example using LiCoO
2
as the positive electrode is disclosed in Japanese Patent Laid-open Application No. Sho63-59507. LiCoO
2
has a higher initial charge-discharge efficiency and also a higher electrode potential (thus producing a higher voltage output of the battery), hence being suited as a material for the positive electrode. Cobalt is however an expensive material that is produced in only a few regions of the earth (for example, Zambia in Africa). Hence, its supply and price largely depend on the political situation in the regions. It is thus proposed to substitute such a critical material as LiCoO
2
with lithium contained in nickel oxide, LiNiO
2
, which is favorable in both the price and availability, which may provide a higher capacity than that of LiCoO
2
, as disclosed in Goodenough, U.S. Pat. No. 4,302,518.
The electrode potential of LiNiO
2
is about 0.2 volt lower than that of LiCoO
2
and may thus promote the desorbing of lithium ions before the non-aqueous electrolyte such as organic electrolyte reaches its decomposition voltage in the charge. This results in increase of the charge capacity and thus improvement of the discharge capacity. However, the initial charge-discharge efficiency of LiNiO
2
is not high enough and causes declination of the capacity as the charge and discharge cycle is repeated again and again, whereby its practical use will be difficult.
To eliminate the drawback of LiNiO
2
, double oxide such as Li
x
Ni
x
Co
1−x
O
2
or Li
x
M
y
N
z
O
2
, including lithium and plural metals (where M is at least an element selected from a group of Fe, Co, and Ni, and N is at least an element selected from a group of Ti, V, Cr, and Mn) is provided as disclosed in Japanese Patent Laid-open Application Sho63-299056 or Publication Sho63-267053.
A. Rougiel et al,
Solid State Ionics,
90, 83-90 (1996) have reported that Li
x
Ni
x
Co
1−x
O
2
phases crystallize in the rhombohedral system with a layered structure. For small amounts of cobalt, i.e., (1−x) is less than 0.2, extra-divalent nickel ions are always present. However, cobalt substitution decreases the non-stoichiometric character of lithium nickelate. For compositions in which (1−x) is greater than 0.3, a pure 2D structure is observed.
To increase the capacity of the lithium-ion secondary battery, it is essential to use proper materials for the positive and negative electrodes to provide a high initial charge and discharge efficiency and as high a reversible capacity in the charge and discharge reactions as possible. It is also desired to minimize the amount of “dead lithium” and the irreversible capacity on the positive and negative electrodes so that the negative electrode is prevented from being overcharged and free from deposition of metallic lithium. If the positive electrode is higher in the initial charge and discharge efficiency than the negative electrode, it will extremely be difficult to design and fabricate an improved secondary battery that satisfies the above requirements.
The addition of metal oxide, such as FeO, FeO
2
, Fe
2
O
3
, SnO, SnO
2
, MoO
2
, V
2
O
5
, Bi
2
Sn
3
O
9
, WO
2
, WO
3
, Nb
2
O
5
, or MoO
3
, to a carbon material for the negative electrode is depicted in Japanese Patent

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