Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-12-12
2004-02-03
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S223000, C429S057000, C429S059000
Reexamination Certificate
active
06686092
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a nonaqueous electrolyte type secondary battery including an electricity generating element accommodated in a film casing and more particularly to a nonaqueous electrolyte type secondary battery configured to swell little.
A secondary battery of the type described is conventional and includes an electricity generating element and a casing accommodating the electricity generating element. The casing is implemented by two aluminum laminate films each consisting or aluminum foil and a thermo-bonding resin film formed on the aluminum foil. Metallic leads protrude from the inside to the outside of the casing via the bonded portion of the casing. Today, there is an increasing demand for a thin, light battery configuration capable of implementing a thin, light electric apparatus. In this sense, the battery accommodated in the film casing is advantageous over a battery accommodated in a hard metallic casing.
The battery with the film casing needs more strict safety implementations than the battery with the hard metallic casing. For example, the battery should undergo a minimum of deformation in contour. Of course, the battery with the film casing should have an ability generally required of a battery, e.g., high energy density (high charge-discharge capacity), a high cycle characteristic, and a storage capacity characteristic despite self-discharge.
Lithium manganate is attracting increasing attention as one of positive electrode substances for a lithium ion secondary battery. Lithium manganate has a spinel structure represented by LiMn
2
O
4
and functions as a 4V class, positive electrode substance in relation to a &lgr;-MnO
2
composition. Lithium manganate with the spinel structure has a tridimensional host structure different from a layer structure particular to, e.g., LiCoO
2
, so that most of stoichiometric capacity available therewith can be used. Lithium manganate is therefore expected to have a desirable cycle characteristic.
Further, lithium manganate with the spinel structure allows lithium ions to be pulled out while maintaining its basic frame. This compound therefore starts releasing oxygen at a higher temperature than cobalt acid lithium having a layer, halite structure and is expected to be desirable from the safety standpoint. The safety feature is particularly important when it comes to the battery with the soft film casing- In practice, however, a lithium secondary battery including a positive electrode implemented by lithium manganate has a problem that its capacity decreases little by little due to repeated charging and discharging. This problem is serious in the aspect of practical use.
Various schemes have heretofore been proposed to improve the cycle characteristic of an organic electrolyte type secondary battery whose positive electrode is implemented by lithium manganate. For example, Japanese Patent Laid-Open Publication Nos. 3-67464, 3-119656, 3-127453, 7-245106 and 7-73883 teach improvements achievable by improving reactiveness at the time of production. Also, Japanese Patent Laid-Open Publication Nos. 4-198028, 5-28307, 6-295724 and 7-97216 teach improvements achievable by controlling a grain size. Further, Japanese Patent Laid-Open Publication No. 5-21063 teaches an improvement attainable by removing impurities. None of such schemes, however, achieves a satisfactory cycle characteristic.
Japanese Patent Laid-Open Publication No. 2-270268 proposes to improve the cycle characteristic by selecting an Li composition ratio sufficiently greater than a stoichiometric ratio. This kind of scheme is disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 4-123769, 4-147573, 5-205744 and 7-282798 also. Experiments actually indicated the improvement in cycle characteristic achievable with such a scheme.
Japanese Patent Laid-Open Publication Nos. 6-338320 and 7-262984, for example, each use LiMn
2
O
4
, which is a Mn spinel substance, and Li
2
Mn
2
O
4
, LiMnO
2
, Mi
2
MnO
3
or similar Li-Mn compound oxide, which is richer than the above Mn spinel substance, as a positive electrode active substance. However, adding excessive Li or mixing it with another Li-rich compound reduces the charge-discharge capacity and charge-discharge energy although improving the cycle characteristic. As a result, high energy density and long cycle life are not compatible with each other. By contrast, Japanese Patent Laid-Open Publication No. 6-275276 proposes to increase a specific surface area for achieving a high rate, charge-discharge characteristic (great current relative to capacity at the time of charging and discharging) and perfect reaction. This, however, obstructs an increase in cycle life.
On the other hand, it has been studied to improve the characteristics by adding another element to a Li-Mn-O compound. For example, Japanese Patent Laid-Open Publication Nos. 4-141954, 4-160758, 4-169076, 4-237970, 4-282560, 4-289662, 5-28991 and 7-14572 each propose to add or dope, e.g., Co, Ni, Fe, Cr or Al. However, the addition of such a metal element reduces the charge-discharge capacity and needs further studies to satisfy the total ability.
As for the addition of another element, boron is expected to improve the other characteristics, e.g., cycle characteristic and self-discharge characteristic while degrading the charge-discharge characteristic little. This described in, e.g., Japanese Patent Laid-Open Publication Nos. 2-253560, 3-297058, and 9-115515. In any case, a manganese dioxide or a lithium-manganese compound oxide is mixed with a boron compound (e.g. boric acid) in a solid phase or immersed in an aqueous solution of a boron compound and then heated, thereby producing a lithium-manganese compound oxide. The resulting compound powder of boron compound and manganese oxide decreases in surface activity and is expected to suppress reaction with an electrolyte and therefore to improve the storage characteristic.
The addition of boron, however, reduced the growth of particles and tap density and did not directly translate into high capacity required of a battery alone. Moreover, capacity decreased in the valid potential range when boron was combined with a carbon negative electrode, or the reaction of boron with an electrolyte could not be sufficiently suppressed, depending on the synthesizing conditions. Boron therefore did not satisfactorily improve the storage characteristic.
On the other hand, lithium manganate applied to the positive electrode of the battery with the film casing did not satisfy the expected degree of safety. Specifically, when the battery was repeatedly charged and discharged or left in a charging state at a high temperature, gases were generated in the battery and raised the pressure inside the battery, causing the battery to easily swell. The above gases are presumably ascribable to the decomposition of the electrolyte The swell of the contour of the battery is apt to exceed a space allocated thereto when mounted to an electric apparatus, exerting pressure on surrounding parts. In the worst case, the gases bring about the dangerous burst of the battery.
As stated above, although lithium manganate is a hopeful compound oxide capable of replacing LiCoO
2
, which is the predominant positive electrode active substance, the conventional battery using lithium manganate has the following problems (1) through (3) left unsolved.
(1) High energy density (high charge-discharge capacity) and high cycle life are not easily compatible.
(2) Storage capacity decreases due to self-discharge.
(3) When the battery with the film casing and using LiMn
2
O
4
is repeatedly charged and discharged or held in a charging state in a high temperature environment, gases presumably ascribable to the decomposition of the electrolyte are generated and cause the battery to swell.
Technical problems relating to the production of a battery and the compatibility of lithium manganate with an electrolyte have been pointed out as the causes of the above problems (1) through (3). Paying attention to the material of the positive e
Kanbe Chika
Kobayashi Akira
Numata Tatsuji
Shirakata Masato
Yageta Hiroshi
Dove Tracy
Michael & Best & Friedrich LLP
NEC Corporation
Ryan Patrick
LandOfFree
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