Method of producing positive active material for non-aqueous...

Details Method of producing positive active material for non-aqueous... Method of producing positive active material for non-aqueous...

1998-10-13

2001-06-05

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

42, 42, 42, 42, 42

Reexamination Certificate

active

06242134

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
This application is a U.S. National Phase application of the PCT International application PCT/JP97/04029.
The present invention relates to a method of producing positive active materials for secondary batteries employing a non-aqueous electrolyte such as organic electrolyte or polymer solid electrolyte.
BACKGROUND OF THE TECHNOLOGY
With the progress of electronics technologies in recent years, miniaturization, lighter weight, and lower power dissipation of electronic equipment have become possible along with sophistication of functions. As a result, a variety of cordless or portable consumer electronics equipment has been developed and commercialized and the market size is rapidly expanding. Typical examples include camcorders, lap top computers, and portable telephones. Further miniaturization and increasingly lighter weight as well as longer operating time of these equipment are always demanded. In association with this trend, there is a strong demand for continuing improvement in energy density and cycle life of small rechargeable batteries to be used in these equipment as a built-in power source. As built-in batteries, starting with lead-acid type and nickel-cadmium type batteries which were initially developed and commercialized, nickel-hydrogen (nickel-metal hydride system) storage batteries and lithium-ion secondary batteries which have higher capacity and higher energy density than these battery systems have subsequently been developed and commercialized. Among them, the lithium- ion secondary battery, which has a high energy density both per unit weight and per unit volume, is a battery system primarily using a complex oxide of lithium and a transition metal element as the positive electrode, a graphite-based carbon as the negative electrode, and a non-aqueous electrolyte such as organic electrolyte or polymer solid electrolyte as the electrolyte, and is recently enjoying a rapid growth in production. In this battery, during charge, lithium ions will desorb from the lithium-containing complex oxide of the positive electrode and transfer into the electrolyte, and at the same time, lithium ions of equal electrochemical equivalent will be fed from the electrolyte into the carbon of the negative electrode. Conversely, during discharge, lithium ions are fed to the positive electrode desorbing from the negative electrode. As this cycle is repeated, lithium-ion secondary battery is sometimes called a rocking-chair battery.
As the potential of the carbon negative electrode is close to the electrode potential of metallic lithium, a complex oxide of lithium which has a high electrode potential and a transition metal element is used as the positive electrode, for example, a complex oxide (LiCoO
2
of lithium and cobalt, a complex oxide (LiNiO
2
) of lithium and nickel, and a complex oxide (LiM
2
O
4
) of lithium and manganese. These complex oxides are often called as lithium cobaltate, lithium nickelate, and lithium manganate.
In the currently commercialized batteries, LiCOO
2
which has a high potential and a long cycle life is most generally used as the positive active material. Under this situation, a use of LiNiO
2
with which a higher capacity than tat of LiCoO
2
is expected is now being actively studied. The reason for a higher capacity is because, as the electrode potential of LiNiO
2
is lower than that of LiCoO
2
, it becomes possible to cause more quantity of lithium to desorb during charge before decomposition voltage of an aqueous electrolyte such as organic electrolyte is reached. As a result, the quantity of charged electricity is expected to increase, and hence the discharge capacity is also expected to increase.
Conversely, despite its large initial discharge capacity, LiNiO
2
suffers a problem of cycle deterioration by gradual decrease in the discharge capacity as charge and discharge are repeated.
As a result of disassembling a battery cell of which the discharge capacity has decreased due to repeated charge-discharge cycles and X-ray diffraction analysis of the positive active material by the inventors of this invention, a change of crystal structure was observed after charge and discharge cycles, which was confirmed to be the cause for deterioration.
Similar phenomenon has already been published by W. Li, J. N. Reimers and J. R. Dahn in Solid State Ionics, 67, 123 (1993). It is reported in this paper that, with the repetition of charge and discharge, lattice constant of LiNiO
2
changes while itself changes from a hexagonal to a mono-clinic system crystal, and further from a second hexagonal to a third hexagonal system crystal as lithium is desorbed. This type of change in crystal phase lacks reversibility and as charge-discharge reactions are repeated, the sites where insertion and desorption of lithium are possible are gradually lost. This phenomenon is considered to be the cause of decrease in the discharge capacity.
In contrast, with LiCoO
2
, such a change in the crystal phase as described above on LiNiO
2
will not occur in the region of normal voltage (a voltage at which an organic electrolyte oxidizes and decomposes), suggesting that a decrease in the discharge capacity due to charge-discharge cycles is not likely to take place.
For the purpose of addressing the problem of decrease in discharge capacity of LiNiO
2
due to charge-discharge cycles, many proposals have heretofore been made to substitute a part of the element Ni with transition metal elements, primarily Co.
As an example, in Japanese Laid-open Patent No. Sho 62-256,371, a method of synthesizing a Li-containing complex oxide of Co and Ni by firing at 900° C. a mixture of lithium carbonate (LiCO
3
, cobalt carbonate (CoCO
3
), and nickel carbonate (NiCO
3
).
Methods of synthesizing complex oxides are disclosed in Japanese Laid-open Patent No. Sho 62-299,056 in which a mixture of carbonates, hydroxides, and oxides of Li, Co, and Ni is used as the raw material, and in U.S. Pat. No. 4,980,080 in which a mixture of lithium hydroxide (LiOH), Ni oxides and Co oxides is used as the raw material, and both heated at 600° C. to 800° C.
In addition, in U.S. Pat. No. 5,264,201 and other patents, methods of synthesizing Li-containing complex oxides are disclosed in which lithium hydroxide (LiOH) is added to and mixed with a uniform mixture of oxides or hydroxides of Ni and oxides or hydroxides of Fe, Co, Cr, Ti, Mn, or V, followed by heat treatment at a temperature not lower than 600° C.
Furthermore, in Japanese Laid-open Patent No. Hei 1-294,364 and other patents, methods of synthesizing Li-containing complex oxides are disclosed in which carbonates, more precisely basic carbonates, of Ni and Co are co-precipitated from an aqueous solution containing Ni ions and Co ions, and a mixture of the co-precipitated carbonates and Li
2
CO
3
is fired.
There have also been proposed a method in which a Ni-containing oxide and a Co-containing oxide are mixed and fired after being further mixed with carbonates or oxides of Li, and a method in which a Li-containing complex oxide is synthesized by using oxides containing both Ni and Co such as NiCo
2
.
These inventions represent efforts to relax a change in crystal phase by substituting a part of Ni with Co or other transition metal elements. The reason why there are many proposals to substitute part of Ni with Co from among different transition metals is because substitution is easy as the ion radius of Co is approximately equal to that of Ni and that the bond strength of Co with oxygen is stronger than that of Ni, which suggest that the crystal structure may become more stable and that the decrease in discharge capacity due to charge-discharge cycles may be improved.
However, not all of the three battery characteristics, namely, discharge capacity, cycle life characteristic, and reliability as a battery, could be satisfied by the Li-containing complex oxides represented by a chemical formula Li
x
Ni
y
Co
z
O
2
(0.90≦×≦1.10, 0,7≦y≦0.95, y+z=1) as obtained by the methods of synthesis heretofore been

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