Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2001-01-22
2004-01-06
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231100, C429S221000, C429S223000, C429S231600, C429S231950, C423S599000
Reexamination Certificate
active
06673491
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a lithium-ion secondary cell comprising an Mn cathode electroactive material, the cathode electroactive material, and a process for producing the active substance. The cell has high initial discharge capacity, and the discharge capacity barely reduces even when charging and discharging are repeated at 60° C.
BACKGROUND OF THE INVENTION
As a cathode electroactive material which is employed in a non-aqueous electrolyte secondary cell (hereinafter may be referred to as “non-aqueous secondary cell”), lithium manganate of spinel structure (hereinafter may be referred to as “spinel LiMn
2
O
4
”) has currently been studied due to its advantages: it has an abundant resource base, can be produced at low cost, and is very safe. However, the electric capacity of the cells is drastically reduced at 60° C. or higher, and thus the aforementioned materials are unsatisfactory as cathode electroactive materials employable in a non-aqueous secondary cell. For example, in a non-aqueous secondary cell comprising such a material serving as a cathode electroactive material, when charging and discharging are repeated 500 times, the discharge capacity of the cell is at most 90 mAh/g. Therefore, there has been a demand for further improvements to non-aqueous secondary cells.
Japanese Patent Application Laid-Open (kokai) No. 7-262984 discloses a cell comprising, as a cathode electroactive material, lithium manganate coated with an Li
2
MnO
3
layer which is produced through heat treatment of a mixture of a lithium compound and LiMn
2
O
4
at 400-1,325° C. In addition, Japanese Patent Application Laid-Open (kokal) No. 10-172571 discloses a cell containing lithium manganate having a two-layer structure which is produced by soaking lithium manganate having a spinel structure in a solution containing an Li ion or Mn ion, and then heating the resultant lithium manganate at 300-1, 200° C.
In view of the foregoing, the present invention provides a non-aqueous secondary cell exhibiting excellent cycle characteristics, which comprises a negative electrode containing lithium as an active substance, a non-aqueous electrolyte, and a positive electrode comprising, as an active substance, a spinel composite oxide comprising lithium, manganese, and oxygen, wherein the discharge capacity is maintained at 95 mAh/g or more after charging and discharging of the cell are repeated 500 times at 60° C. The present invention also provides a cathode electroactive material which enables production of a cell exhibiting excellent cycle characteristics, and a process for producing the active substance.
SUMMARY OF THE INVENTION
The present invention provides:
(1) a cathode electroactive material comprising a composite oxide comprising &bgr;-MnO
2
and a spinel oxide predominantly comprising lithium, manganese, and oxygen;
(2) a cathode electroactive material comprising a composite oxide according to (1), wherein the &bgr;-MnO
2
is comprised on the surface of the composite oxide;
(3) a cathode electroactive material comprising a composite oxide according to (1) or (2), wherein the &bgr;-MnO
2
is comprised in the composite oxide in an amount of about 1-13 mol %;
(4) a cathode electroactive material comprising a composite oxide according to any one of (1) through (3), wherein the spinel oxide predominantly comprising lithium, manganese, and oxygen is LiMn
2
O
4
formed of lithium, manganese, and oxygen; or a composite oxide Li
1+x
Mn
2−x−y
M
y
O
4
(wherein −0.1≦x≦0.2, and 0≦y≦0.2) in which a portion of Li or Mn in LiMn
2
O
4
is replaced by a different element such as chromium, cobalt, aluminum, nickel, iron, or magnesium;
(5) a cathode electroactive material comprising a composite oxide according to any one of (1) through (4), wherein the composite oxide is a granule having a particle size of about 3-50 &mgr;m;
(6) a cathode electroactive material comprising a composite oxide according to any one of (1) through (5), which has pores of about 30-400 Å;
(7) a paste for producing an electrode, which comprises a cathode electroactive material comprising a composite oxide as recited in any one of (1) through (6);
(8) a positive electrode comprising a cathode electroactive material comprising a composite oxide as recited in any one of (1) through (6);
(9) a process for producing a cathode electroactive material comprising a composite oxide as recited in any one of (1) through (6), which process comprises acid treatment of a composite oxide comprising a spinel oxide predominantly comprising lithium, manganese, and oxygen, and heat treatment of the resultant composite oxide at a temperature of about 200° C. or higher and lower than about 400° C.;
(10) a non-aqueous secondary cell comprising a negative electrode comprising lithium as an active substance, a non-aqueous electrolyte, and a positive electrode comprising, as an active substance, a composite oxide comprising lithium, manganese, and oxygen, wherein the composite oxide is a cathode electroactive material comprising a composite oxide as recited in any one of (1) through (6).
DETAILED DESCRIPTION OF THE INVENTION
The present invention will next be described in detail.
The process for producing a cathode electroactive material of the present invention comprises acid treatment of a composite oxide having a spinel structure predominantly comprising lithium, manganese, and oxygen; and heat treatment of the composite oxide at a specific temperature; i.e., about 200° C. or higher but lower than about 400° C. By means of the process, a composite oxide comprising &bgr;-MnO
2
and an oxide having a spinel structure predominantly comprising lithium, manganese, and oxygen can be produced. Preferably, the surface layer of the composite oxide compresses substantially of &bgr;-MnO
2
. As used herein, the term “a composite oxide having a spinel structure predominantly comprising lithium, manganese, and oxygen” refers to LiMn
2
O
4
formed of lithium, manganese, and oxygen; and a composite oxide Li
1+x
Mn
2−x−y
M
y
O
4
(wherein −0.1≦x≦0.2, and 0≦y≦0.2) in which a portion of Li or Mn of LiMn
2
O
4
is replaced by a different element such as chromium, cobalt, aluminum, nickel, iron, or magnesium. The lattice constant of the composite oxide is preferably 8.240 Å or less.
As used herein, the phrase “comprises substantially of &bgr;-MnO
2
” refers to the case in which the surface layer of a composite oxide having a spinel structure predominantly comprising lithium, manganese, and oxygen comprises at least one lattice of &bgr;-MnO
2.
In the present invention, no particular limitation is imposed either on the process for producing a composite oxide having a spinel structure predominantly comprising lithium, manganese, and oxygen, or the starting material of the composite oxide. For example, such a composite oxide may be produced through the following process: a mixture of a manganese compound and a lithium compound, or a mixture of a manganese compound, a lithium compound, and a compound comprising an element which can replace to manganese may be calcined at a temperature of about 300-850° C. for at least about one hour in air or under oxygen-gas flow.
No particular limitation is imposed on the crystallinity of a composite oxide having a spinel structure predominantly comprising lithium, manganese, and oxygen. The composite oxide may contain a non-reacted lithium compound or manganese oxide. Examples of manganese sources which may be employed as a starting material include electrolytic manganese dioxide (EMD), chemically synthesized manganese dioxide (CMD), manganese sesquioxide, trimanganese tetroxide, manganese oxyhydroxide, manganese carbonate, and manganese nitrate. Examples of lithium sources which may be employed include lithium hydroxide, lithium carbonate, and lithium nitrate. Of the aforementioned manganese sources, manganese carbonate is preferable due to its high reactivity with lithium.
In the present invention, no particular limitation is imposed on the s
Noda Takao
Shirakawa Akihiko
Showa Denko Kabushiki Kaisha
Weiner Laura
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