Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
1999-03-09
2001-08-14
Dunn, Tom (Department: 1725)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S224000, C429S231100, C429S231950
Reexamination Certificate
active
06274278
ABSTRACT:
SPECIFICATION
The present invention concerns a lithium or lithium-ion rechargeable battery with improved cycling performance. More specifically, the invention relates to a kind of high energy density secondary cell which can be employed in several fields, in place of conventional rechargeable batteries such as lead acid and nickelcadmium batteries. Said secondary cell is based on the use of a lithium anode or of an anode capable of intercalating lithium ions, in combination with a non-aqueous electrolyte system, and of a cathode material whose nature is the specific subject of this invention.
Lithium batteries, which have been developed quite recently and are on the market since a relatively short time, have attracted much attention in view of the high levels of voltage and energy that they can offer in a quite reduced volume. This makes lithium batteries particularly promising for use in the field of consumer electronics (such as cellular telephones, camcorders, portable computers), in fixed appliances (such as telephone exchanges, alarm systems) or for various kinds of electrical vehicles.
All of the devices mentioned above require batteries endowed, in particular, with the following properties: high values of specific energy (normally measured in watt-hour/kg) and of specific power (watt/kg), a good cyclability (from about 200 to about 1500 cycles, depending on the kind of application), low cost, high safety. Moreover, it is highly desirable that, as cycling progresses, the energy supplied by the battery decrease in a limited way. In other words, the battery capacity (currently expressed in ampere-hour) should not show substantial losses with time.
According to the current state of the art, the anode element of a lithium secondary cell is made of lithium metal, alone or in alloy with other metals, while the anode of a lithium-ion secondary cell is made of an electrically conductive material, e.g. a carbonaceous material, wherein lithium is intercalated in ionic form. The latter type of cell is also called “rockingchair” cell or “swing system” cell, with reference to the oscillating rhythm with which lithium is removed from the anode intercalation compound for being intercalated in the cathode material in the discharge phase, and vice-versa in the charge phase.
As far as the cathode material is concerned, several compounds consisting of lithium oxides and transition metals have been studied so far. Among these materials LiCoO
2
and LiNiO
2
may be cited (originally disclosed in U.S. Pat. No. 4,302,518), as well as the manganese spinel LiMn
2
O
4
(identified in U.S. Pat. No. 4,312,930 as a lithium intercalation compound, from which lithium may be removed by acid treatment without altering the crystalline structure thereof). In many of these materials remarkable amounts of lithium may be reversibly intercalated. For instance, is has been reported that both LiCoO
2
and LiNiO
2
may deliver more than 140 Ah/kg by reversibly inserting lithium (Ohzuku et al., Chemistry Express, 7, 193 (1992)).
As a matter of fact, all of the cathode materials referred to above are characterised by capacities which decrease as the number of cycles undergone by the battery increases. Theoretical studies carried out in this respect appear to agree on the hypothesis that this behaviour is due to changes in the structure of the concerned materials when the lithium ions are insertedlextracted intfrom said structures.
The manganese spinel, LiMn
2
O
4
, is considered to be the most attractive cathode material for practical applications, in view of some advantageous features thereof, such as low cost, reduced pollution potential, high voltage and high power. However, said material is characterised by a limited specific capacity (110-120 Ah/kg), which, in addition, tends to decease with cycling. For instance, Tarascon et al. (J. M. Tarascon, E. Wang, F. K Shokoohi, W. R. McKinnon, and S. Colson, J. Electrochem. Soc., 18 No. 10, 2859-2864, 1991) have found that secondary lithium cells with LiMn
2
O
4
cathode working at an average voltage of 4.1 V with theoretical values of specific energy of 480 Wh/kg lose about 10% of their initial capacity over 50 cycles.
In order to limit the capacity loss with cycling, it has been proposed to replace part of the manganese with other metals such as Zn, Mg, Co, Ni, Cu and Fe. For instance, EP-A-0 390 185 (Matsushita) proposes a non-aqueous electrolyte secondary battery with lithium anode wherein the cathode material may be represented by the following formula:
Li
x
M
y
Mn
(2−y)
O
4
wherein
M=Co, Cr or Fe
0.02≦y≦0.3
0.85≦x≦1.15.
According to said document, materials of the kind of the manganese spinel wherein a portion of manganese is replaced by cobalt, chromium or iron have reduced lattice constants with respect to the starting spinel, and this would result in an increased stability of the resulting crystal structure. Said enhanced stability would cause a better ability to undergo a high number of charge and discharge cycles without great losses in the performance.
On the other hand, Tarascon et al. (J. Electrochem. Soc., loc. cit.) report, as a result of their studies on manganese spinels of the formula LiM
y
Mn
(2−y)
O
4
wherein small amounts of manganese are replaced by M=Ti, Ge, Zn, Ni or Fe, that the introduction of cations of valence 2 (such as zinc and nickel), or 3 (such as iron) reduces the capacity of the cells at 4.1 V, but does not result in any enhancement of their cycling performance.
According to Gummow et al. (R. J. Gummow, A. de Kock and M. M. Thackeray, Solid State Ionics, 69, 5967, 1994) the replacement of a portion of manganese as proposed by the cited EP-A-0 390 185 and by Tarascon et al. (loc. cit.) did not lead to any significant increase in the cell capacity at 4 V. In order to obtain batteries of this kind which offer a better constancy in the capacity with cyding, Gummow et al. propose to dope the LiMn
2
O
4
spinel with small amounts of monovalent or multivalent cations. Said cations are added in such proportions as to increase the average oxidation state of manganese in the spinel slightly above the normal value of +3.5 (in LiMn
2
O
4
one of the two Mn ions has oxidation state +3, while the other has oxidation state +4). Thus, the document proposes the use of a manganese spinel wherein a portion of manganese is substituted by lithium, according to the formula
Li
(1+&dgr;)
Mn
(2−&dgr;)
O
4
wherein 0≦5≦0.33
or it is substituted by bivalent metal cations, such as Mg
2+
or Zn
2+
(so as to result in a non-stoichiometric spinel, with a slight cation deficiency) according to the general formula
LiM
&dgr;/2
Mn
(2−&dgr;)
O
4
with 0≦5≦0.1.
Accordingly, the document U.S. Pat. No. 5,316,877, of the same research group, generalises the above concept and proposes cathode materials for lithium rechargeable batteries having a structure of the spinel type and the following general formula:
Li
1
D
x/b
Mn
2−x
O
4+&dgr;
wherein:
0≦x≦0.33 and 0≦5≦0.5
with x and 5 such that the oxidation state N of the manganese cation is 3.5≦N≦4;
D is a mono- or multivalent metal cation; and
b is the oxidation state of D.
As examples of cations different from the lithium ion, the document mentions the Mg
2+
ion for which the above formula reads Li
1
Mg
x/2
Mn
2−x
O
4+&dgr;
, and the Co
3+
ion, for which the above formula reads Li
1
Co
x/3
Mn
2−x
O
4+&dgr;
.
According to what set forth both in the latter US patent document and in the previously cited article, with the cathode materials based on modified manganese spinel according to the above formulae specific capacities have been obtained which remain constant for at least 20 cycles, differently from what happens with conventional manganese spinel. However, a reduction of the initial capacity has been obtained at the same time, from 110-120 Ah/kg (normal values for LiMn
2
O
4
) to about 100 Ah/kg average.
The foregoing is also confirmed in a later publication of one o
Antonini Alessandra
Bellitto Carlo
Pistoia Gianfranco
Consiglio Nazionale Delle Ricerche
Dunn Tom
Smith , Gambrell & Russell, LLP
Stoner Kiley
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