Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2001-11-15
2004-11-30
Kalfut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S221000, C429S224000, C429S225000, C429S229000, C429S231500, C429S231600, C423S409000
Reexamination Certificate
active
06824921
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a negative electrode material for a non-aqueous electrolyte secondary battery and a method for producing the same. Also, the present invention relates to a negative electrode containing the above negative electrode material, as well as to a non-aqueous electrolyte secondary battery having such negative electrode and exhibiting a high capacity and a long cycle life.
As the negative electrode for a non-aqueous electrolyte secondary battery, metallic lithium or lithium compounds have been intensively studied because they can realize a high energy density and high voltage. On the other hand, as the positive electrode, oxides and chalcogenides of transition metals such as LiMn
2
O
4
, LiCoO
2
, LiNiO
2
, V
2
O
5
, Cr
2
O
5
, MnO
2
, TiS
2
, MoS
2
and the like have been studied. These materials are known to have a layered or tunneled structure that allows free intercalation and deintercalation of lithium ions.
There is a drawback that, when metallic lithium is used in the negative electrode, a deposition of lithium dendrites occurs on the surface of the metallic lithium in the negative electrode during charging, which reduces the charge/discharge efficiency of the battery or causes internal short-circuiting due to contact between formed lithium dendrites and the positive electrode. For this reason, lithium ion batteries have recently been put into practical use that use in the negative electrode graphite type carbon materials which can reversibly absorb and desorb lithium, and have an excellent cycle life and good safety, though the carbon materials have a smaller capacity than metallic lithium.
However, when carbon materials are used in the negative electrode, the fact that the practical capacity thereof is as small as 350 mAh/g and that the theoretical density thereof is as low as 2.2 g/cc presents an obstacle for achieving batteries with a high capacity. Consequently, the use of metallic materials having a higher practical capacity is desired as the negative electrode material.
On the other hand, when metallic materials are used as the negative electrode active material, there is a problem that the active material repeatedly expands and contracts, thereby pulverizing the active material along with absorption and desorption of lithium. The pulverized active material particles lose contact with other particles of the active material or electrically conductive agent in the negative electrode to become inactive, which reduces the electrical conductivity of the negative electrode as well as the capacity.
For solving this problem, there has been proposed a method of allowing a phase absorbing lithium to coexist with a phase not absorbing lithium in one particle of the active material (Japanese Laid-Open Patent Publication No. Hei 11-86854). In this case, the phase not absorbing lithium relaxes the stress caused in the active material particle due to absorption of lithium, and thereby suppresses expansion and pulverization of the active material.
Further, there has been a suggestion in which two or more phases absorbing lithium are allowed to coexist in one particle of the active material, intending to relax the stress by the change in the structure during absorbing lithium of each phase (Japanese Laid-Open Patent Publication No. Hei 11-86853). In this case, it is considered that, since a plurality of minute phases exist in the active material particles, it is possible to let the stress go to the interface of the crystal grains at the time of absorption of lithium.
However, negative electrode active materials capable of sufficiently suppressing the expansion and pulverization of the active material by relaxing the stress caused by absorption of lithium, have not been achieved so far because an appropriate synthesizing method of the active material has not been found.
For example, by conventional methods such as atomizing method and roll quenching method, the crystal grain size of each phase is as large as several microns at minimum, and therefore effective stress relaxation is not possible.
Also, by conventional methods of mechanically applying a shearing force to raw materials comprising simple substances such as mechanical alloying method and mechanical grinding method under vacuum or argon atmosphere, crystal grains in the range of less than several microns to several nanometers can be formed; however, these methods are not practical since a variety of phases of substances having a variety of compositions are formed and the control of phase formation is difficult.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide a negative electrode material that sufficiently suppresses the expansion or pulverization thereof by relaxation of the stress caused during absorption of lithium in the negative electrode material. Also, the present invention has another object of providing a non-aqueous electrolyte secondary battery which is equipped with a negative electrode containing the above negative electrode material and which has a high capacity, a long cycle life and excellent high rate discharge characteristics.
The present invention relates to a method for producing a negative electrode material for a non-aqueous electrolyte secondary battery including a step of, applying a shearing force or a shearing stress to a raw material comprising an intermetallic compound under a nitrogen-containing atmosphere to make the intermetallic compound react with nitrogen. The raw material comprising an intermetallic compound is well known in the art.
The intermetallic compound comprises: at least one element(A) which reacts with nitrogen and forms a nitride, but is substantially non-reactive with lithium; and at least one element(B) that is substantially non-reactive with nitrogen, but reacts with lithium.
The intermetallic compound forms a mixture containing at least one nitride of element(A), and at least one substance of element(B) by the above reaction with nitrogen. It is preferable that 30% or more of element(A) in the intermetallic compound converts into a nitride of element(A) by the step of applying a shearing force to the raw material.
The present invention also relates to a method for producing a negative electrode material for a non-aqueous electrolyte secondary battery including the steps of: providing an intermetallic compound comprising at least one element(A) selected from the group A consisting of Ce, Co, Cr, Fe, La, Mn, Mo, Nb, P, Sc, Sr, Ta, Ti, V, Y, Yb, Zr, B, Ca, Mg, Na and Zn, and at least one element(B) selected from the group B consisting of Ge, Sn, Pb, Sb and Bi, and applying a shearing force to the intermetallic compound under a nitrogen-containing atmosphere to make the intermetallic compound react with nitrogen, thereby forming a mixture containing at least one nitride of element(A) and at least one substance of element(B).
The aforementioned nitrogen-containing atmosphere is preferably an atmosphere of a gas containing 50% by volume or more of nitrogen.
The pressure of the aforementioned gas is preferably 1.0×10
5
Pa or more.
The step of applying a shearing force to the raw material, as described above, is preferably performed by a mechanochemical method using for example a ball milling system. The mechanochemical method is obvious to one of ordinary skill in the art.
The present invention also relates to a method for producing a negative electrode material for a non-aqueous electrolyte secondary battery including the steps of: providing an intermetallic compound comprising at least one element(A) selected from the group A consisting of Ce, Co, Cr, Fe, La, Mn, Mo, Nb, P, Sc, Sr, Ta, Ti, V, Y, Yb, Zr, B, Ca, Mg, Na and Zn, and at least one element(B) selected from the group B consisting of Ge, Sn, Pb, Sb and Bi, mixing the intermetallic compound with a compound containing nitrogen to obtain a raw material mixture comprising the intermetallic compound and the compound containing nitrogen, and applying a shearing force to the resultant raw material mixture to make the intermetallic compo
Nakamoto Takayuki
Okamura Kazuhiro
Sato Toshitada
Shimamura Harunari
Akin Gump Strauss Hauer & Feld & LLP
Kalfut Stephen
Parsons Thomas H.
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