Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2001-09-28
2004-11-09
Bos, Steven (Department: 1754)
Metal working
Method of mechanical manufacture
Electrical device making
C423S306000, C423S277000, C429S224000, C429S231500, C429S220000, C429S223000, C429S229000, C429S231950, C429S231600, C429S231800, C429S221000
Reexamination Certificate
active
06814764
ABSTRACT:
RELATED APPLICATION DATA
The present application claims priority to Japanese Application(s) No(s). P2000-308300 filed Oct. 6, 2000, and P2000-308313 filed Oct. 6, 2000, which application(s) is/are incorporated herein by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a cathode active material capable of reversibly doping/dedoping lithium and to a non-aqueous electrolyte cell employing this cathode active material.
2. Description of Related Art
Recently, with drastic progress in the art of electronic equipment, investigations into a rechargeable secondary cell, as a power source that may be used conveniently and economically for prolonged time, are proceeding briskly. Among typical secondary cells, there are a lead storage cell, an alkali storage cell and a non-aqueous electrolyte secondary cell.
Among the aforementioned secondary cells, a lithium ion secondary cell, as a non-aqueous electrolyte secondary cell, has advantages such as high output or high energy density.
A lithium ion secondary cell is made up of a cathode and an anode, each having an active material capable of reversibly doping/dedoping at least lithium ions, and a non-aqueous electrolyte. The charging reaction of the lithium ion secondary cell proceeds as lithium ions are deintercalated into an electrolyte solution at the cathode and are intercalated into the anode active material. In discharging, reaction opposite to that of the charging reaction proceeds, such that lithium ions are intercalated at the cathode. That is, charging/discharging is repeated as the reaction of entrance/exit of lithium ions from the cathode into and from the anode active material occurs repeatedly.
As the cathode active material of the lithium ion secondary cell, LiCoO
2
, LiNiO
2
or LiMn
2
O
4
is used because these materials have a high energy density and a high voltage. However, these cathode active materials, containing metal elements of low Clark number in their composition, suffer from high cost and supply instability. Moreover, these cathode active materials are higher in toxicity and affect the environment significantly. So, there is presented a demand for a novel substitution material usable as a cathode active material.
Proposals have been made for use of LiFePO
4
having an olivinic structure, as a cathode active material for a lithium ion secondary cell. LiFePO
4
has a volumetric density as high as 3.6 g/m
3
and generates a high potential of 3.4V, with its theoretical capacity also being as high as 170 mAh/g. Additionally, LiFePO
4
contains an electrochemically dedopable Li at a rate of one atom per Fe atom, in its initial state, and therefore is promising as a cathode active material for a lithium ion secondary cell. Moreover, LiFePO
4
includes iron, as an inexpensive material plentiful in supply, in its composition, and therefore is less costly than any of the aforementioned materials, that is LiCoO
2
, LiNiO
2
or LiMn
2
O
4
.
However, since LiFePO
4
has only low electronic conductivity, the internal resistance of the cell may occasionally be increased if LiFePO
4
is used as a cathode active material. If the internal resistance of the cell is increased, the polarization potential on cell circuit closure is increased to decrease the cell capacity. Additionally, since the true density of LiFePO
4
is lower than that of the conventional cathode material, the active material charging ratio cannot be increased if LiFePO
4
is used as the cathode active material, such that the call cannot be increased sufficiently in energy density.
So, a proposal has been made of employing, as a cathode active material, a composite material of a compound represented by the general formula Li
x
FePO
4
, where 0<x≦1, having an olivinic structure, and a carbon material for its superiority in electronic conductivity. This composite material is referred to below as an LiFePO
4
composite material.
Meanwhile, if an impurity is left over in the Li
x
FePO
4
carbon composite material, as a cathode active material, the cell characteristics are lowered, because the impurity fails to contribute to the cell reaction. For improving the cell characteristics, it is necessary to prepare the Li
x
FePO
4
carbon composite material not containing residual impurity, that is to synthesize the Li
x
FePO
4
carbon composite material in a single phase.
For preparing the Li
x
FePO
4
carbon composite material, such a method has been proposed which consists in mixing starting materials for synthesis of Li
x
FePO
4
, milling the resulting mixture, sintering the milled product and adding a carbon material at an optional time point to the starting materials for synthesis.
It is however difficult to realize a smooth reaction for synthesis in the sintering process, such that there lacks at present a technique of synthesizing the Li
x
FePO
4
carbon composite material in a single phase and therefore a non-aqueous electrolyte cell employing the Li
x
FePO
4
carbon composite material synthesized in a single phase has not been realized.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method for the preparation of a cathode active material having a superior cell capacity through reliable single-phase synthesis of a compound represented by the general formula Li
x
Fe
1-y
M
y
PO
4
and a carbon material and a method for the preparation of a non-aqueous electrolyte cell having a high cell capacity.
In one aspect, the present invention provided a method for preparing a cathode active material including a mixing step of mixing starting materials for synthesis of a compound represented by a general formula Li
x
Fe
1-y
M
y
PO
4
, where M is at least one selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B and Nb, with 0.05≦x≦1.2 and 0≦y≦0.8, a milling step of milling a mixture obtained in the mixing step, a compressing step of compressing the milled mixture obtained in the milling step to a preset density and a sintering step of sintering the mixture compressed in the compressing step. A carbon material is added in any of the above steps previous to the sintering step and the density of the mixture is set in the compressing step to not less than 1.71 g/cm
3
and not larger than 2.45 g/cm
3
.
In the method for preparing the cathode active material, described above, there is provided the compressing step between the milling and sintering steps of compressing the milled mixture, that is the milled starting materials for synthesis of the cathode active material, to a preset density, that is to not less than 1.71 g/cm
3
and not larger than 2.45 g/cm
3
. This diminishes the gap between the particles of the mixture, that is the starting materials for synthesis of the cathode active material, charged into the sintering step, thereby assuring a sufficient area of contact of the particles of the starting materials for synthesis. By carrying out the sintering step as a sufficient contact area is maintained between the starting materials for synthesis, the synthesis reaction is improved in reaction efficiency to realize single-phase synthesis of the cathode active material, that is the composite material composed of Li
x
Fe
1-y
M
y
PO
4
and carbon. So, with the manufacturing method for the cathode active material, it is possible to produce a cathode active material which may assure a high cell capacity.
That is, with the method for preparing the cathode active material, according to the present invention, there is provided, between the milling step and the sintering step, a step of compressing the milled mixture, that is milled starting materials for synthesis of the cathode active material, to a preset density, that is to not less than 1.71 g/cm
3
and not larger than 2.45 g/cm
3
, thus realizing single-phase synthesis of the cathode active material, that is LiFePO
4
carbon composite material.
So, with the present manufacturing method for the cathode active material, there may be provided a manufacturing method for the
Fukushima Yuzuru
Hosoya Mamoru
Kuyama Junji
Sakai Hideki
Bos Steven
Sonnenschein Nath & Rosenthal LLP
Sony Corporation
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