Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-09-10
2003-04-29
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S217000, C429S231950
Reexamination Certificate
active
06555272
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a lithium secondary battery and an active material for negative electrode to be used therein. More particularly, this invention relates to a high performance lithium secondary battery having a high voltage and a large discharge capacity and showing only a small capacity loss during charging and discharging and a negative electrode-active material to be used in the lithium secondary battery.
This invention further relates to a negative electrode-active material which has a capacity exceeding the theoretical one of graphite, a high initial efficiency and an outstanding cycle property, and the secondary battery used thereof.
2. Description of the Related Art
The lithium secondary battery has a high energy density and, therefore, has been finding utility as power sources for mobile telecommunication terminals and portable information terminals. The market for the lithium secondary battery has been rapidly expanding in concert with the dissemination of such terminals. Concurrently, the graphite type carbonaceous material as the active material for terminal devices has a high discharge capacity exceeding 300 mAh/g and a high initial efficiency of not less than 90%. Further, it can retain a discharge capacity of not less than 50% even after 500 charge-discharge cycles because the expansion and contraction thereof in consequence of the insertion and withdrawal of lithium ions into and from the interlayer space of graphite during a cycle of charging and discharging is as low as about 10% and the deterioration of the amount of discharge thereof in consequence of the growth of the total of charge-discharge cycles is extremely small. The graphite type carbonaceous material, thus, is endowed with quality which is craved for by the negative electrode-active material in a lithium ion secondary battery.
The amount of lithium that can be reversibly inserted into and withdrawn from the material is one lithium atom at most per six carbon atoms. The theoretical limit of capacity of a carbonaceous material for charging and discharging is 372 mAh/g in terms of electric capacity. The existing secondary battery is used at a level approximating this threshold capacity, it cannot be expected to achieve a drastic improvement in performance. The existing secondary battery is nevertheless expected to achieve a further expansion of capacity. In the circumstance, the desirability of developing a negative electrode-active material having a high discharge capacity surpassing the theoretical capacity of graphite meanwhile retaining the high initial efficiency, the low expansion and contraction ratio during each charge-discharge cycle, and the high cycle property which are had by the graphite type carbonaceous material currently finding extensive utility in the practical cells has been finding enthusiastic recognition.
Under this circumstance, a search for a material other than carbon, i.e. an inorganic compound such as, for example, an alloy or an oxide having a capacity far surpassing 372 mAh/g has been gradually taking shape. It has been found that among other such inorganic compounds, crystalline and amorphous oxide materials containing tin and silicon manifest a discharge capacity approximating closely to 1000 mAh/g (as disclosed in JP-A-07-220,721 and JP-A-07-249,409, for example). It has been recently reported that a negative electrode-active material using elementary silicon exhibits an initial discharge capacity in the neighborhood of 3000 mAh/g;(Battery Symposium in Japan, 3A16 (1997)) and that a negative electrode-active material using silicon oxide exhibits an initial discharge capacity in the neighborhood of 1500 mAh/g (The 38th Panel Discussion on Cells, 3A17 (1997)), meaning a discharge capacity far surpassing the threshold capacity of the graphite type carbonaceous material. The fact that these materials invariably have a large initial charge capacity relative to their initial discharge capacity, namely the fact that the capacity loss during each charge—discharge cycle is very large (about 1000 mAh/g in both materials) and the cycle property is inferior (the capacity halved within first several charge—discharge cycles), has posed a serious problem.
Meanwhile, as materials of silicon containing boron, a silicon boride alloy structure of the general formula, SiB
n
, having n in the range of 0.1 to 3 (JP-A-53-136,630) and a and the powder of a silicon boride compound of the same general formula having n in the range of 3.2 to 6.6 and formed mainly of SiB
4
(JP-A-08-138,744 and U.S. Pat. No. 5,571,637) have been respectively disclosed.
The alloy structure mentioned above indeed exhibits a large discharge capacity proper for silicon which far surpasses that of the graphite type carbonaceous material but permits no easy quantity production as compared with an electrode which, as found in the recently feasibilized lithium ion cell, is fabricated by applying on a collector foil a slurry obtained by blending an active material with a binder because this structure is an electrode produced by immersing a collector matrix in an alloy bath. Further, since this alloy structure permits no easy decrease in the film thickness thereof, it encounters a big problem in reversibility as evinced by the fact that the electrode permits no thorough diffusion of lithium therein under a large current density and consequently induces a large polarization resistance and the fact that the capacity loss during each charge-discharge cycle is inevitably enlarged because the occluded lithium cannot be thoroughly extracted. In contrast, the powder of silicon boride compound having SiB
4
as a main component was not able to give a higher discharge capacity than the graphite type carbonaceous material.
WO98/24,135 discloses a material for a negative electrode which is obtained by subjecting silicon or a compound thereof to a heat treatment in the presence of an organic material or a carbonaceous material. Since this is a material formed of silicon and carbon substantially exclusively, the problem of a very large expansion-contraction which is attendant on the charging-discharging of silicon inducing an alloying reaction with lithium and the problem of an inferior cycle property remain yet to be solved.
SUMMARY OF THE INVENTION
An object of this invention is to provide a novel negative electrode-active material for use in a lithium secondary battery and a lithium secondary battery produced by using the material.
More specifically, an object of this invention is to provide a lithium secondary battery having a high initial efficiency and a high cycle property meanwhile realizing a high discharge capacity and a negative electrode-active material to be used in the secondary battery.
Still more specifically, an object of this invention is to provide a negative electrode-active material for use in a lithium secondary battery which is liberated from the aforementioned problems arising in the use of a boron-containing silicon material as a negative electrode-active material for a lithium secondary battery, namely the problem of unsuitability for quantity production, the problem of causing a serious capacity loss during each charge-discharge cycle and exhibiting only inferior reversibility, and the problem of obtaining only a lower discharge capacity than the graphite type carbonaceous material, and to provide a lithium secondary battery obtained by using the active material.
Yet another object of this invention is to provide a negative electrode-active material for a lithium secondary battery having a high initial efficiency and a high cycle property and offering a solution to the problem of expansion and contraction attendant on charging-discharging meanwhile realizing a discharge capacity exceeding 372 mAh/g which is the theoretical capacity of graphite, and to provide a lithium secondary battery obtained by using the active material.
The present inventors have pursued a diligent study on the powder of a boron-containing silicon material based on the electroc
Hamada Takeshi
Kono Taro
Shoji Hiromasa
Sugiura Tsutomu
Suzuki Kimihito
Chaney Carol
Nippon Steel Corporation
Wenderoth , Lind & Ponack, L.L.P.
Yuan Dah-Wei D.
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