Doped positive-electrode material in electrochemical cells

Details Doped positive-electrode material in electrochemical cells Doped positive-electrode material in electrochemical cells

2001-05-25

2003-06-03

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

Reexamination Certificate

active

06573005

ABSTRACT:

The invention relates to novel materials for active positive electrode material in electrochemical cells which are based on modified oxidic materials, such as, for example, tin oxides.
Lithium ion batteries are among the most promising battery systems for mobile applications. The areas of application extend from high-quality electronic equipment (for example mobile telephones and camcorders) to batteries for electrically driven motor vehicles.
These batteries consist of a negative electrode, a positive electrode, a separator and a non-aqueous electrolyte. The negative electrode is typically Li(MnMe
z
)
2
O
4
, Li(CoMe
z
)O
2
, Li(CoNi
x
Me
z
)O
2
or other lithium intercalation and insertion compounds. The positive electrode is typically a lithium metal, soft and hard carbons, graphite, graphitic carbons, or other lithium intercalation and insertion compounds (i.e., compounds in which lithium has been inserted into the crystalline structure of an inorganic compound) or alloy compounds. The electrolyte is typically solutions containing lithium salts, such as LiPF
6
, LiBF
4
, LiClO
4
, LiAsF
6
, LiCF
3
SO
3
, LiN(CF
3
SO
2
)
2
or LiC(CF
3
SO
2
)
3
and mixtures there in aprotic solvents.
In the lithium ion batteries currently commercially available, carbon is employed as a positive electrode. However, this positive-electrode system has difficulties. A considerable drop in capacity occurs in this system during the 1st cycle due to irreversible incorporation of lithium into the carbon structure. In addition, the cycle stability of the carbons and graphites available is unsatisfactory. Furthermore, there is a safety aspect that kinetic limitations can cause critical parameters.
In order to improve the positive-electrode properties, new systems are being sought to replace, for example, the carbon positive electrode. The carbon materials are being replaced, for example, by oxidic materials or alloys. In
Journal of Power Sources
75 (1998), Wolfenstine investigated the suitability of tin oxide/tin mixtures as positive-electrode material for lithium ion batteries is investigated. The irreversible loss of Li due to the formation of Li
2
O is said to be minimized by the preferred use of SnO over SnO
2
. EP 0823742 describes tin mixed oxides doped with various metals. U.S. Pat. No. 5,654,114 also describes the use of tin oxides as positive-electrode material for secondary lithium ion batteries. These systems have the drawback that Li is converted into Li
2
O. This means that a large amount of Li is bound, and therefore is not available for the electrochemical processes in the battery.
The present invention provides positive-electrode materials which offer better charging/discharging behavior compared with carbon. The better behavior is characterized by higher capacities and good cycle stability. These are achieved by the use of modified tin oxides, which are used as positive-electrode material in electrochemical cells. It has been found that doping of the tin oxide SnO
2
on the positive-electrode side results in an improved anode system.
Surprisingly, it has been found that the modified tin oxide systems have excellent electrochemical properties. The irreversible loss of lithium during the 1st cycle is still observed. However, it is not as pronounced as before the treatment of the tin oxide SnO
2
.
The prior-art uses of tin oxides for positive-electrode materials have the problem of particle agglomeration. Surprisingly, it has been found that the process according to the invention enables the production of particles of defined diameter. The process according to the invention enables the production of primary particles in the nm range and larger secondary particles having a diameter of less than 10 &mgr;m. The smaller primary particles agglomerate to the larger secondary particles. These small particles result in an increase in the active surface area.
In a preferred embodiment, the process for the preparation of the positive-electrode material is characterized in that
a) urea is added to a tin chloride solution,
b) urotropin and a suitable doping compound are added to the solution,
c) the resultant sol is emulsified in petroleum ether,
d) the resultant gel is washed, and the solvent is removed by suction,
e) and the gel is dried and heat-treated.
These materials are suitable for use in electrochemical cells, preferably in batteries, and particularly in secondary lithium ion batteries.
The positive-electrode material are employed in secondary lithium ion batteries with customary electrolytes. Examples of suitable electrolytes are conductive salts such as LiPF
6
, LiBF
4
, LiClO
4
, LiAsF
6
, LiCF
3
SO
3
, LiN(CF
3
SO
2
)
2
and LiC(CF
3
SO
2
)
3
, and mixtures thereof. The electrolytes may also comprise organic isocyanates (DE 199 44 603) for reducing the water content. The electrolytes may also comprise organic alkali metal salts (DE 199 10 968) as additives.
Thus, suitable electrolytes contain alkali metal borates of the general formula I
Li
+
B
−
(OR
1
)
m
(OR
2
)
p
  I
in which
m and p are 0, 1, 2, 3 or 4,where m+p=4, and
R
1
and R
2
can be identical or different, R
1
and R
2
can be bonded directly to one another via a single or double bond, R
1
and R
2
can each be an aromatic or aliphatic carboxylic, dicarboxylic or sulfonic acid radical, or each can be an aromatic ring such as a phenyl, naphthyl, anthracenyl or phenanthrenyl ring which may be unsubstituted or mono- to tetrasubstituted by A or Hal, or each can be a heterocyclic aromatic ring such as pyridyl, pyrazyl or bipyridyl ring, which may be unsubstituted or mono- to trisubstituted by A or Hal, or each can be an aromatic hydroxy acid such as aromatic hydroxycarboxylic acids or aromatic hydroxysulfonic acids, which may be unsubstituted or mono- to tetrasubstituted by A or Hal.
Hal is halogen, e.g., F, Cl or Br and A is an alkyl group having 1 to 6 carbon atoms, which may be mono- to trihalogenated.
Likewise suitable electrolytes contain alkali metal alkoxides of the general formula II
Li
+
OR
−
  II
R can be an aromatic or aliphatic carboxylic, dicarboxylic or sulfonic acid radical, or can be an aromatic ring from the group such as a phenyl, naphthyl, anthracenyl or phenanthrenyl ring, which may be unsubstituted or mono- or tetrasubstituted by A or Hal, or can be a heterocyclic aromatic ring such as a pyridyl, pyrazyl or bipyridyl ring, which may be unsubstituted or mono- to trisubstituted by A or Hal, or can be an aromatic hydroxy acid such as aromatic hydroxycarboxylic acids or aromatic hydroxysulfonic acids, which may be unsubstituted or mono- to tetrasubstituted by A or Hal.
Hal is F, Cl or Br and A is an alkyl group having 1 to 6 carbon atoms, which may be mono- to trihalogenated.
Suitable electrolytes can also contain lithium complex salts of the formula III
R
1
and R
2
can be identical or different, are optionally bonded directly to one another via a single or double bond, and can each be, an aromatic ring such as phenyl, naphthyl, anthracenyl and phenanthrenyl, which may be unsubstituted or mono- to hexasubstituted by alkyl (C
1
to C
6
), alkoxy groups (C
1
to C
6
) or halogen (F, Cl or Br),
or can each be an aromatic heterocyclic ring pyridyl, pyrazyl and pyrimidyl ring, which may be unsubstituted or mono- to tetrasubstituted by alkyl (C
1
to C
6
), alkoxy groups (C
1
to C
6
) or halogen (F, Cl or Br),
or can each be an aromatic ring such as a hydroxybenzocarboxyl, hydroxynaphthalenecarboxyl, hydroxybenzosulfonyl or hydroxynaphthalenesulfonyl ring, which may be unsubstituted or mono- to tetrasubstituted by alkyl (C
1
to C
6
), alkoxy groups (C
1
to C
6
) or halogen (F, Cl or Br),
R
3-R
6
are each independently,
1. alkyl (C
1
to C
6
), alkoxy (C
1
to C
6
) or halogen (F, Cl or Br) or
2. an aromatic ring such as;
a phenyl, naphthyl, anthracenyl and phenanthrenyl ring, which may be unsubstituted or mono- to hexasubstituted by alkyl (C
1
to C
6
), alkoxy groups (C
1
to C
6
) or halogen (F, Cl or Br),
a pyridyl, pyrazyl and pyrimidyl ring, which ma

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