Active positive-electrode material in electrochemical cells,...

Details Active positive-electrode material in electrochemical cells,... Active positive-electrode material in electrochemical cells,...

2001-03-30

2004-06-15

Ruthkosky, Mark (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S231950, C029S623100

Reexamination Certificate

active

06749964

ABSTRACT:

The invention relates to novel materials based on coated metal cores, such as, for example, Sb, Bi, Cd, In, Pb, Ga or tin particles, or alloys thereof, for use 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(MrMe
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. The electrolyte used can be a solution containing lithium salts, such as LiPF
6
, LiBF
4
, LiClO
4
, LiAsF6, 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 1
st
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 reduce the irreversible loss in capacity in the 1
st
charging cycle, new systems are sought which 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 0823741 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.
SUMMARY OF THE INVENTION
A feature of the present invention is to provide positive-electrode materials which offer higher capacitance compared with carbon. In addition, the aim is to minimize the irreversible loss of lithium and to achieve good cycle stability.
This feature according to the invention maybe achieved by the use of coated metal cores, such as, for example, Sb, Bi, Cd, In, Pb, Ga or tin particles, and alloys thereof, particularly preferably tin particles, which are used as positive-electrode material in electrochemical cells. The alloys can include all combinations of Sb, Bi, Cd, In, Pb, or Ga.
It has been found that defined metal-oxide layers can be applied to the metal or alloy core.
Surprisingly, it has been found that the coated metal or, alloy particles, in particular tin particles, have excellent electrochemical properties. The irreversible loss of lithium over the 1
st
cycle is significantly less than in the case of conventional oxidic positive-electrode materials.
The prior-art uses of tin oxides for positive-electrode materials have the problem of particle agglomeration. Surprisingly, it has been found that particles of defined diameter can be produced by the process according to the invention. The process according to the invention enables primary particles in the nm range (generally less than about 1 &mgr;m) and secondary particles having a diameter of less than about 10 &mgr;m to be produced. Generally, the secondary particles can range in diameter from about 0.01 &mgr;m-about 1.0 &mgr;m, preferably from about 0.1 &mgr;m-about 10 &mgr;m, and optionally from about 1 &mgr;m-about 10 &mgr;m. These small particles result in an increase in the active surface area.
It has been found that the defined coating of the primary particles with an oxide layer results in a defined oxygen content. This enables the formation of Li
2
O to be regulated.
It is possible to carry out single or multiple coatings with metal hydroxides or metal oxyhydroxides, which are then converted into the oxides by heat treatment.
The process for the production of the positive-electrode material is characterized in that
a) a suspension or a sol of the metal or alloy core in urotropin is prepared,
b) the suspension is emulsified with C
5
-C
12
-hydrocarbons, preferably benzine or petroleum ether,
c) the emulsion is precipitated onto the metal or alloy cores, and
d) the metal hydroxides or oxyhydrides are converted into the corresponding oxide by heating the system.
Although not wishing to be bound by any particular theory, it is believed that a metal hydroxide or an oxyhydroxide is formed at a); see e.g. hereinafter, Solution 1 of Example 1.
It has been found that tin, molybdenum, cerium, tungsten and antimony hydroxides or oxyhydroxides, which are converted into the oxides by heat treatment, are suitable for the coating. In the case of multiple coatings, the same oxide or different oxides may be used for the coating.
On use of the coated metal or alloy particles, in particular tin particles, as positive-electrode ,material in electrochemical cells, improved cyclability due to the build-up of defined metal-oxide layers has been observed. The materials according to the invention are; preferably suitable for use in electrochemical cells and batteries, particularly preferably in secondary lithium ion batteries.
The positive-electrode material according to the invention can be employed in secondary lithium ion batteries containing customary electrolytes. Examples of suitable electrolytes comprising conductive salts are those 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 unsubsti

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