Single-phase lithium ferrite based oxide

Details Single-phase lithium ferrite based oxide Single-phase lithium ferrite based oxide

2000-12-19

2004-04-13

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S224000, C252S518100, C252S520210, C252S521200, C252S182100

Reexamination Certificate

active

06720111

ABSTRACT:

The present invention relates to a single-phase lithium ferrite based oxide which is suitable as a cathode material (positive-electrode material) for lithium ion secondary batteries, a process for preparing the same and uses thereof.
In recent years, attention is directed to lithium ion secondary batteries for use as a secondary battery mounted on a portable device such as portable telephones, note-size personal computers or the like because of their high energy density. It is expected that this type of battery will be applied as a large-size battery for electric automobiles, power-load levelling systems and so on. In this situation, the importance of lithium ion secondary batteries is increasing.
The cathode material is closely related to battery performance such as working voltage of a battery (a difference between the redox potential of a transition metal in the cathode and the redox potential of anode element), and charge and discharge capacities (an amount of Li removable from or applicable to the cathode) so that presumably a demand for the cathode material will increase with an increase of a need for lithium ion secondary batteries.
Today lithium cobalt oxide (LiCoO
2
) is in use as a cathode material for lithium ion secondary batteries. However, LiCoO
2
which contains a rare metal cobalt is one of the factors raising the costs of raw materials for lithium ion secondary batteries.
For example, lithium manganese oxide (LiMn
2
O
4
) is receiving attention for use as a cathode material which is inexpensive and substantially free from the resource problem. Now the lithium manganese oxide has been partly put to practical use.
Further it is desired to commercially provide cathode materials prepared from iron, i.e. a cheap metal element, for abundance of resources and low toxicity compared with manganese. For example, lithium ferrite (LiFeO
2
) has been investigated for possible use as a material for electrodes. However, when lithium ferrite (LiFeO
2
) is prepared from an iron source such as iron oxide and a Li source such as lithium carbonate by calcining them at a high temperature or hydrothermally treating them, the lithium ferrite can scarcely function for charging and discharging and lacks an activity for lithium secondary batteries (K. Ado, M. Tabuchi, H. Kobayashi, H. Kageyama, O. Nakamura, Y. Inaba, R. Kanno, M. Takagi and Y. Takeda, J. Eelectrochem. Soc., 144, [7], L177, (1997)).
On the other hand, lithium ferrite (LiFeO
2
) prepared from &agr;-NaFeO
2
or FeOOH by H/Li or Na/Li ion exchange method has a flat charge potential in the vicinity of 4V, but has a discharge potential less than 3V. Thus LiFeO
2
is lower in discharge potential by about 1V or more than LiCoO
2
. Consequently it is difficult to use LiFeO
2
as a substitute for LiCoO
2
(R. Kanno, T. Shirane, Y. Kawamoto, Y. Takeda, M. Takano, M. Ohashi, and Y. Yamaguchi, J. Eelectrochem. Soc., 143, [8], 2435, (1996), Y. Sakurai, H. Arai, S. Okada, and J. Yamaki, J. Power Sources, 68, 711, (1997), L. Guenne, P. Deniard, A. Lecerf, P. Biensan, C. Siret, L. Fournes, and R. Brec, Ionics, 4, 220, (1998), and Japanese Unexamined Patent Publications 1998-120421 and 1996-295518).
On the other hand, Fe-doped LiNiO
2
and LiCoO
2
, which are iron-containing oxides, are reported to exhibit Fe
3+
/Fe
4+
redox behavior at about 4V (C. Delmas, M. Menetrier, L. Crogurnnec, I. Saadoune, A. Rougier, C. Pouillerie, G. Prado, M. Grune, L. Fournes, Electrochimica Acta. 45, 243, (1999) and H. Kobayashi, H. Shigemura, M. Tabuchi, H. Sakaebe, K. Ado, H. Kageyama, A. Hirano, R. Kanno, M. Wakita, S. Morimoto and S. Nasu, J. Electrochem. Soc., 147, [3], 960, (2000)).
However, the foregoing cathode materials contain Co or Ni, i.e., rare metals, so that the low-cost feature of iron is meaningless. Further the redox of iron may have been induced by the redox of Co or Ni, and it is unclear whether iron spontaneously undergoes Fe
3+
/Fe
4+
redox.
Whether the lithium ferrite based oxide can be put to practical use as a cathode material for lithium ion secondary batteries is determined by whether it has flat charge and discharge potentials in the vicinity of 4V which are attributed to Fe
3+
/Fe
4+
redox. As described above, a technique has been scarcely established for preparing inexpensive and resource-saving lithium ferrite based oxides having flat charge and discharge potentials in the vicinity of 4V due to Fe
3+
/Fe
4+
redox potential. Consequently the development of the technique is desired.
A principal object of the present invention is to provide a single-phase lithium ferrite based oxide which is suitable as a cathode material for lithium ion secondary batteries, a process for preparing the same and uses thereof.
The present inventors conducted extensive research to overcome the foregoing prior art problems and found the following. When lithium ferrite (LiFeO
2
) having a low discharge potential (3V or less) make a solid solution with a layered rock salt-type compound Li
2-x
MO
3-y
wherein M is at least one species selected from the group consisting of Mn, Ti and Sn, the solid solution has charge and discharge potentials in the region of 4V, and the charge and discharge potentials correspond to Fe
3+
/Fe
4+
redox potential. The present invention was completed based on this novel finding.
According to the present invention, there are provided single-phase lithium ferrite based oxides, cathode materials for a lithium ion secondary battery, processes for preparing the oxides (the solid solution) and a lithium ion secondary battery which are as follows.
1. A single-phase lithium ferrite based oxide having a layered rock salt-type structure, the oxide comprising lithium ferrite (LiFeO
2
) based solid solution with Li
2-x
MO
3-y
wherein M is at least one species selected from the group consisting of Mn, Ti and Sn, 0≦x<2, 0≦y≦1 such that the proportion of iron is 0.1≦Fe/(Fe+M)≦0.9 wherein M as defined above.
2. The single-phase lithium ferrite based oxide as defined in item 1, wherein the lithium ferrite (LiFeO
2
) make a solid solution with Li
2-x
MO
3-y
such that the proportion of iron is 0.21≦Fe/(Fe+M)≦0.75.
3. A cathode material for a lithium ion secondary battery, the material comprising a single-phase lithium ferrite based oxide having a layered rock salt-type structure, the oxide comprising lithium ferrite (LiFeO
2
) solid solution with Li
2-x
MO
3-y
wherein M is at least one species selected from the group consisting of Mn, Ti and Sn, 0≦x<2, 0≦y≦1 such that the proportion of iron is 0.1≦Fe/(Fe+M)≦0.9.
4. The cathode material for a lithium ion secondary battery as defined in item 3, wherein the lithium ferrite (LiFeO
2
) make a solid solution with Li
2-x
MO
3-y
such that the proportion of iron is 0.15≦Fe/(Fe+M)≦0.75.
5. A process for preparing a single-phase lithium ferrite based oxide having a layered rock salt-type structure, the oxide comprising lithium ferrite (LiFeO
2
) solid solution with Li
2-x
MO
3-y
wherein M is at least one species selected from the group consisting of Mn, Ti and Sn, 0≦x<2, 0≦y≦1 such that the proportion of iron is 0.1≦Fe/(Fe+M)≦0.9, the process comprising the steps of adding an aqueous solution of a lithium compound in a molar ratio (Li/(Fe+M)) of from 1 to 3 relative to the other metals to a mixed aqueous solution containing a water-soluble compound containing at least one species selected from the group consisting of Mn, Ti and Sn, and a water-soluble iron compound or to a water-alcohol mixed solution to give a precipitate, or adding a specified amount of lithium hydroxide to said mixed aqueous solution or said water-alcohol mixed solution, evaporating the aqueous solution and the precipitate to dryness, and calcining the residue in an oxidizing atmosphere or in a reducing atmosphere.
6. The process as defined in item 5, wherein the lithium ferrite (LiFeO
2
) is solid-dissolved in Li
2-x
MO
3-y
such that the proportion

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