Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2001-04-25
2004-06-15
Ruthosky, Mark (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S221000, C429S224000, C429S231500, C429S219000
Reexamination Certificate
active
06749967
ABSTRACT:
RELATED APPLICATION DATA
The present application claims priority to Japanese Applications Nos. P2000-128999 filed Apr. 25, 2000 and P2000-129000 filed Apr. 25, 2000, which applications are incorporated herein by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
This invention relates to a positive electrode active material, capable of reversibly doping/undoping lithium, and to a non-aqueous electrolyte cell which uses this positive electrode active material.
Recently, with rapid progress in a variety of electronic equipment, investigations into a re-chargeable secondary cell, as a cell that can be used conveniently economically for prolonged time, are proceeding briskly. Representatives of the secondary cells are a lead accumulator, an alkali accumulator and a lithium secondary cell.
Among these secondary cells, a lithium secondary cell has various advantages, such as high output or high energy density. The lithium secondary cell is made up of a positive electrode and a negative electrode, each having an active material capable of reversibly doping/undoping lithium, and a non-aqueous electrolyte.
Among known positive electrode active materials of the lithium secondary cell, there are a metal oxide, a metal sulfide and a polymer. For example, there are known lithium-free compounds, such as, for example, TiS2, MoS2, MbSe2 or V2O5, and lithium compound oxides, such as LiMO2, where M is Co, Ni, Mn or Fe, or LiMn2O4.
As the positive electrode active material, having the potential of 4V with respect to lithium, LiCoO
2
is being put to extensive use. This LiCoO
2
is an ideal positive electrode in many respects in that it has a high energy density and a high voltage.
However, Co is a rare resource localized on the earth and hence it is expensive. Moreover, it cannot be furnished in stability without considerable difficulties. So, a demand is raised towards developing a positive electrode material which is based on inexpensive Ni or Mn and which is present in abundance as a resource.
The positive electrode containing LiNiO
2
has a large theoretical capacity and a high discharging potential. However, it has such a defect that, with the progress in the charging/discharging cycle, the crystal structure of LiNiO
2
is collapsed to lower the discharging capacity as well as the thermal stability.
As a Mn-based positive electrode material, LiMn
2
O
4
having a positive spinel structure and a spatial group Fd3m has been proposed. This LiMn
2
O
4
has a high potential of the 4V-grade potential with respect to lithium, which is equivalent to LiCoO
2
. Moreover, LiMn
2
O
4
is easy to synthesize and high in cell capacity so that it is a highly promising material and is being put to practical use.
However, the cell formed using LiMn
2
O
4
has a drawback that it undergoes serious deterioratiuon in capacity on storage at elevated temperatures, while it is insufficient in stability and cyclic characteristics, with Mn being dissolved in an electrolytic solution.
So, it has been proposed in Japanese Laying-Open Patent H-9-134724 to use a phosphoric acid compound of a transition metal M having an olivinic structure as a positive electrode of the lithium ion cell, where M is Fe, Mn, Co or Ni. It has also been proposed in Japanese Laying-Open Patent H-9-171827 to use e.g., LiFePO
4
, among the phosphoric acid compounds of the transition metal M having an olivinic structure.
It is noted that LiFePO
4
has a volumetric density as high as 3.6 g/cm
3
and develops a high potential of 3.4 V, with the theoretical capacity being as high as 170 mAh/g. Moreover, in the initial state, LiFePO
4
contains electrochemically dopable Li at a rate of one Li atom per Fe atom, and hence is a promising material as a positive electrode active material of the lithium ion cell.
However, as reported in the above patent publication, a real capacity only on the order of 60 to 70 mAh/g has been realized in an actual cell which uses LiFePO
4
as a positive electrode active material. Although the real capacity on the order of 120 mAh/g has been subsequently reported in the Journal of the Electrochemical Society, 144,1188 (1997), this capacity cannot be said to be sufficient in consideratiuon that the theoretical capacity is 170 mAh/g. There is also a problem that the discharging voltage of LiFePO
4
is 3.4V which is lower than that of the positive electrode active material used in the current lithium ion cell.
So, it has been proposed to use LiMnPO
4
, as a phosphoric acid having an olivinic structure, comprised mainly of Mn, which is an element having a redox potential highrer than that of Fe, as the positive electrode of the lithium ion cell.
However, in the Mn-based routine phosphoric acid compound, comprised basically of LiMnPO
4
, it is difficult to yield Mn by the redox reaction. It is reported in the aforementioned Journal of the Electrochemical Society, 144,1188 (1997) that, of the Mn-based phosphric compounds of the olivinic structure, only LiMn
x
Fe
1-x
PO
4
, in which Fe is substituted for part of Mn, is the sole phosphoric compound in which Mn can be generated by a redox reaction.
In the above treatise, there is a report that an actual cell constructed using LiMn
x
Fe
1-x
PO
4
as a positive electrode active material can develop a real capacity of the order of 80 mAh/g. However, this capacity may not be said to be sufficient in consideratiion that the theoretical capacity is 170 mAh/g.
In the above treatise, there is a report that, in an actual cell which uses LiMn
x
Fe
1-x
PO
4
as a positive electrode active material, the capacity is decreased when the proportion y of Mn exceeds 0.5. That is, according to the teaching in the above treatise, if the proportion Mn in LiMn
x
Fe
1-x
PO
4
is increased, the capacity is decreased, even though the high voltage is achieved, and hence the compound is not suitable as a material for practical use. If conversely the proportion of Mn in LiMn
x
Fe
1-x
PO
4
is lowered for realizing a high capacity, the proportion of Mn as a main reaction partner in the redox is lowered, with the result that the high redox potential proper to Mn cannot be sufficiently achieved. In addition, if the discharging voltage is lowered, the cell produced ceases to be compatible with the currently used lithium ion cell.
So, it is extremely difficult with LiMn
x
Fe
1-x
PO
4
to realize high capacity and high discharge voltage simultaneously.
On the other hand, in the Mn-based phosphoric acid compound, having the olivinic structure, M has a high redox potential and hence the compound is expected to manifest excellent properties. However, only a few of the compounds may be used in a cell. Thus, a demand is raised towards development of the phosphoric acid compound having the olivinic structure.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a positive electrode active material capable of realizing a high discharging capacity without lowering the capacity in order to manifest superior charging/discharging characteristics and a non-aqueous electrolyte cell which uses the positive electrode active material.
It is another object of the present invention to provide a positive electrode active material in which Mn generation by redox, not possible so far, is realized without lowering the capacity, and which exhibits a high discharging voltage and superior charging/discharging characteristics.
It is yet another object of the present invention to provide a non-aqueous electrolyte cell which uses such positive electrode active material.
In one aspect, the present invention provides a positive electrode active material containing a compound represented by the general formula Li
x
Mn
y
Fe
1-y
PO
4
, where 0<x≦2 and 0.5<y<0.95.
In this positive electrode active material, Fe is substituted for a portion of Mn of Li
x
Mn
y
Fe
1-y
PO
4
. Since this Fe is able to dilute the Yarn-Teller effect ascribable to Mn
3+
, it is possible to suppress distortion of the crystal structure of Li
x
Mn
y
Fe
1-y
PO
4
. Since the proportion y of Mn is in a range of
Li Guohua
Yamada Atsuo
Ruthosky Mark
Sonnenschein Nath & Rosenthal LLP
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