Reduced positive-electrode material in an electrochemical cell

Details Reduced positive-electrode material in an electrochemical cell Reduced positive-electrode material in an electrochemical cell

2001-05-24

2004-04-27

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S232000, C252S182100, C423S618000

Reexamination Certificate

active

06727020

ABSTRACT:

The invention relates to novel materials based on modified oxidic materials, such as, for example, tin oxides, as active positive-electrode material in electrochemical cells.
Lithium ion batteries are amongst the most promising systems for mobile applications. The areas of application extend from high-quality electronic equipment (for example mobile telephones, camcorders) to batteries for electrically driven motor vehicles.
These batteries may include 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. Positive electrodes can consist of lithium metal, soft and hard carbons, graphite, graphitic carbons or other lithium intercalation and insertion compounds or alloy compounds. As electrolyte, use is made of 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 thereof in aprotic solvents.
In the lithium ion batteries currently commercially available, carbon is employed as positive electrode. However, this positive-electrode system has some problems. 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 the 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. A variety of efforts are being made here. The carbon materials are being replaced, for example, by oxidic materials or alloys. In Journal of Power Sources 75 (1998), Wolfenstine investigates the suitability of tin oxide/tin mixtures as positive-electrode material for lithium ion batteries. 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. All the systems investigated 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.
One feature of the present invention is to provide positive-electrode materials which offer better charging/discharging behaviour compared with carbon. The better behaviour should be characterized by higher capacities and good cycle stability.
This feature according to the invention can be achieved by the use of modified tin oxides, which are used as positive-electrode material in electrochemical cells.
It has been found that reduction of the tin oxide by means of reducing gases results in an improved positive-electrode system. This system consists of a mixture of tin oxide and tin particles which is finely distributed in the tin oxide grains and the conventional additives (for example PTFE, or PVDF; solvents, e.g. nonpolar solvents; and conductive black). Conductive black is provided in amounts of, generally, about 1-about 5% by weight; preferably about 3% by weight; PTFE or PVDF is provided in amounts of, generally, about 20-about 40% by weight, preferably about 35% by weight; and at least one solvent is provided in amounts of, generally, about 5-about 20% by weight, preferably about 12% by weight.
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 secondary particles having a diameter of less than 10 &mgr;m. These small particles result in an increase in the active surface area.
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 is 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) the gel is dried and heat-treated, and
f) the resultant SnO
2
is exposed to a reducing gas stream in an aeratable furnace.
The materials according to the invention are suitable for use in electrochemical cells, preferably in batteries, particularly preferably in secondary lithium ion batteries.
The positive-electrode material according to the invention can be employed in secondary lithium ion batteries with customary electrolytes. Examples of suitable electrolytes are those comprising conductive salts selected from the group consisting of 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 additive. Suitable are alkali metal borates of the general formula
Li
+
B
−
(OR
1
)
m
(OR
2
)
p
in which
m and p are 0, 1, 2, 3 or 4, where m+p=4, and
R
1
and R
2
are identical or different,
are optionally bonded directly to one another via a single or double bond,
are each, individually or together, an aromatic or aliphatic carboxylic, dicarboxylic or sulfonic acid radical, or
are each, individually or together, an aromatic ring from the group consisting of phenyl, naphthyl, anthracenyl and phenanthrenyl, which may be unsubstituted or mono- to tetrasubstituted by A or Hal, or
are each, individually or together, a heterocyclic aromatic ring from the group consisting of pyridyl, pyrazyl and bipyridyl, which may be unsubstituted or mono- to trisubstituted by A or Hal, or
are each, individually or together, an aromatic hydroxy acid from the group consisting of aromatic hydroxycarboxylic acids and aromatic hydroxysulfonic acids, which may be unsubstituted or mono- to tetrasubstituted by A or Hal, and
Hal is F, Cl or Br
and
A is alkyl having 1 to 6 carbon atoms, which may be mono- to trihalogenated.
Likewise suitable are alkali metal alkoxides of the general formula
Li
+
OR
−
in which R
is an aromatic or aliphatic carboxylic, dicarboxylic or sulfonic acid radical, or
is an aromatic ring from the group consisting of phenyl, naphthyl, anthracenyl and phenanthrenyl, which may be unsubstituted or mono- to tetrasubstituted by A or Hal, or
is a heterocyclic aromatic ring from the group consisting of pyridyl, pyrazyl and bipyridyl, which may be unsubstituted or mono- to trisubstituted by A or Hal, or
is an aromatic hydroxy acid from the group consisting of aromatic hydroxycarboxylic acids and aromatic hydroxysulfonic acids, which may be unsubstituted or mono- to tetrasubstituted by A or Hal,
and
Hal is F, Cl or Br
and
A is alkyl having 1 to 6 carbon atoms, which may be mono- to trihalogenated.
Lithium complex salts of the formula
 where
 R
1
and R
2
are identical or different, are optionally bonded directly to one another via a single or double bond, and are each, individually or together, an aromatic ring from the group consisting of 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 are each, individually or together, an aromatic heterocyclic ring from the group consisting of pyridyl, pyrazyl and pyrimidyl, which may be unsubstituted or mono- to tetrasubstitute

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