Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-11-15
2003-04-01
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231800, C429S231400, C429S232000, C029S623500, C252S182100
Reexamination Certificate
active
06541156
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a negative electrode material for a non-aqueous lithium secondary battery suitable for a power supply of a small lightweight electrical equipment or an electric automobile, a method for manufacturing the negative electrode material, and a non-aqueous lithium secondary battery using the negative electrode material.
In recent years, a secondary battery with a high capacity is demanded as the minimization of an electrical equipment proceeds. Accordingly, a non-aqueous lithium battery with a high energy density attracts more attention than a nickel-cadmium battery or a nickel-hydrogen battery.
As a negative electrode material, the use of lithium metal has first been attempted. However, it has been found that arborescent lithium precipitates by the repetition of charge/discharge and reaches a positive electrode through a separator, thereby bringing about short circuit.
Furthermore, Japanese Provisional Patent Publication No.208079/1982 has proposed the use of lithium as a negative electrode material and graphite having a high degree of crystallinity as an electrode plate. In the case that graphite is used, however, intercalation into graphite crystals is utilized for lithium occlusion, so that there is a drawback that the battery in excess of 820 mAh/cc which is a theoretical volumetric capacity cannot be obtained at ordinary temperature and under atmospheric pressure.
It is known that a metal such as Al, Si or Sn which can be combined with lithium can suitably be used as the negative electrode material capable of realizing the higher capacity, but there is a disadvantage that the capacity of such a material noticeably deteriorates with the repetition of a charge and discharge cycle.
Japanese Provisional Patent Publication No.286763/1993 discloses that adding a plurality of kinds of carbonaceous materials in Al can suppress the reduction in the capacity with the charge and discharge cycle, but the use of Al limits the capacity per volume up to 2,839 mAh/cc. On the other hand, the capacity per volume in case of using Si is large since its maximum value is 4,684 mAh/cc, but a change in volume during charge and discharge is also large and the occurrence of the above-described problem avoids a practical application as the negative electrode material.
As a method for solving these problems, a technique using silicide powder as a negative electrode material for a lithium secondary battery is recently disclosed in Japanese Provisional Patent Publication No.240201/1995. It has been found that such a chemical compound has a relatively large volumetric capacity and a long cycle life. However, this material has a low electric conductivity. When this material is used as a simple substance, it cannot be sufficiently combined with Li and hence cannot exhibit the theoretical capacity. As a method for solving the problem, Japanese Provisional Patent Publication No.153517/1996 discloses a technique by which a conductive agent such as acetylene black is added to the powder of this material to form the negative electrode material.
However, the particle diameter of acetylene black is generally as very small as less than 1 &mgr;m, and hence, its specific surface area is large, which disadvantageously increases the irreversible capacity in initial charge.
In order to solve such problems, Japanese Provisional Patent Publication No.199527/1998 discloses the use of graphite particles having the high crystallinity together with silicide as a conductive material. As a result, both the capacity and the cycle life are improved and the irreversible capacity in the initial charge is reduced as compared with the case where the silicide is used as a simple substance. However, when the charge and discharge cycle is effected for a long period of time, the capacity is gradually reduced. It can be considered that this reduction occurs for the reason that a change in volume of silicide involved by the charge and discharge of lithium has an influence on the structure of the negative electrode given an electrical conductivity with graphite and destroys this structure with the charge and discharge cycle.
In order to suppress the above-described destruction of the structure, Japanese Provisional Patent Publication No.249407/1997 proposes a method by which both the negative electrode material powder and graphite are mechanically treated to obtain such a structure that a graphite material as a conductive agent approximates to the main negative electrode material which is not silicide but Si metal. It can be inferred that the application of this technique to silicide can extend the cycle life as similar to the case of Si. On the other hand, if the graphite particles are subjected to the mechanical treatment, the specific surface area of graphite can be increased, which may be disadvantageously led to increase. in the irreversible capacity in the initial charge.
As a result of intense examination, the present inventors have found that the use of a later-described material having a specific constitution as a negative electrode material for a lithium secondary battery or the use of a negative electrode material for a lithium secondary battery produced by a later-described method can create a negative electrode which has a high capacity, is hardly degraded in the capacity even if a long charge and discharge cycle is effected and suppresses an irreversible capacity generated in the initial charge and discharge.
That is, an object of the present invention is to provide a negative electrode material which has a high capacity, a long cycle life and a small irreversible capacity in the initial charge, namely which is electrolyte friendly, a manufacturing method thereof, and a lithium secondary battery using the negative electrode material.
SUMMARY OF THE INVENTION
To achieve this aim, a lithium secondary battery is a non-aqueous lithium secondary battery comprising at least a positive electrode, a negative electrode and an electrolytic solution in which an electrolyte is dissolved in a non-aqueous solvent; the negative electrode material contains a metal material M consisting of solid phases A and B, a graphite material and a carbonaceous material having a crystallinity lower than that of the graphite material; the metal material M has such a structure that a part or all of the surface of a core particle consisting of the solid phase A is covered with the solid phase B; the solid phase A includes at least silicon as a constitutive element; the solid phase B is a solid solution or an intermetallic compound of silicon and at least one element selected from the group consisting of elements in the group 2, transition metal elements, elements in the group 12, elements in the group 13 and elements excepting carbon and silicon in the group 14 of the periodic table. Furthermore, a method for manufacturing the negative electrode material is characterized in that the metal material M, the graphite material and a precursor of a carbonaceous material which is an organic material are mixed and baked in the inactive atmosphere; the metal material M consists of solid phases A and B and has such a structure that a part or all of the surface of a core particle consisting of the solid phase A is covered with the solid phase B; the solid phase A includes at least silicon as a constitutive element; the solid phase B is a solid solution or an intermetallic compound of silicon and at least one element selected from the group consisting of elements in the group 2, reduced metal elements, elements in the group 12, elements in the group 13 and elements excepting carbon and silicon in the group 14 of the periodic table.
Here, although the detail will be described later, the precursor of the carbonaceous material means a material which can be a carbonaceous material having the crystallinity lower than that of the graphite material after baking.
When the metal material M having a large volumetric capacity and graphite which is a conductive particle are integrally subjected to the contact treatment by
Fuse Tooru
Kasamatsu Shinji
Nishioka Keiko
Nitta Yoshiaki
Satou Hideharu
Alejandro Raymond
Jordan and Hamburg LLP
Kalafut Stephen
Mitsubishi Chemical Corporation
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