Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-02-25
2002-10-01
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231100, C429S218100, C429S231600, C429S231950
Reexamination Certificate
active
06458487
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a positive electrode active material and non-aqueous secondary battery using the same which is improved in high-temperature characteristics so that a lowering of voltage due to high-temperature storage and an increase of impedance can be effectively prevented.
BACKGROUND ART
In recent years, developments of a relatively safe negative electrode material and a non-aqueous electrolyte having an increased decomposition voltage have been advanced, so that various non-aqueous secondary batteries having a high operating voltage have been practically used in many technical fields. In particular, a secondary battery using a lithium ion has excellent characteristics such as a high discharge voltage, light weight, and a high energy density or the like, so that the demand of the secondary battery has been rapidly increased as power sources of equipments such as portable telephone, note book type personal computer, camera-integrated video recorder, and as dispersed-type power sources, power sources of EV (electrical vehicle) and HEV (hybrid electrical vehicle), and a large-scaled battery.
The lithium ion secondary battery of this type comprises: positive and negative electrodes capable of reversibly deintercalate/intercalate lithium ion; and a non-aqueous electrolyte which is prepared by dissolving lithium salt into non-aqueous solvent.
As the positive electrode active material for the above lithium ion secondary battery, for example, lithium-cobalt composite oxides such as LiCoO
2
, lithium-nickel composite oxides such as LiNiO
2
, lithium-manganese composite oxides such as LiMn
2
O
4
and other metal oxides are generally used.
However, in case of the above secondary battery using metal oxides such as lithium-cobalt composite oxide or the like, a theoretical capacity of the battery can be increased while there is posed a disadvantage such that a discharge potential becomes large in comparison with the batteries using the aforementioned other two composite oxides. As a result, the discharge capacity in a range where the non-aqueous electrolyte is not decomposed is lowered to be about a half of the theoretical capacity. In addition, since cobalt as a rare resource was used as a material for constituting the battery, there was also posed a problem of disadvantageously increasing a manufacturing cost of the battery.
On the other hand, in case of the secondary battery using metal oxides such as lithium-nickel (composite) oxide, a large theoretical capacity and an appropriate discharge potential can be obtained. However, there were problems of changes in the discharge potential and lowering the charge/discharge capacity. The change of discharge potential is caused in relation to a change of crystalline structure during the charge/discharge process of the battery, while the lowering the charge/discharge capacity is resulted from collapse of the crystalline structure which is caused in accordance with progress of the charge/discharge cycles. With respect to the aforementioned problems, drastic measures to solve the problems have not been taken at all. As a result, there has been posed a problem such that a characteristic stability and reliability of the battery are insufficient.
In contrast, in case of the secondary battery using lithium-manganese composite oxide, though the theoretical capacity is slightly lowered in comparison with the batteries using the aforementioned other two composite oxides, the battery exhibits an appropriately higher charge/discharge potential. In addition, even in an over-charge state (&lgr; MnO
2
) where the lithium ions are completely extrated from (got out of) the positive electrode active material, it has been confirmed that the crystalline structure of the positive electrode active material can be maintained to be stable.
Further, this temperature at which the reaction of oxygen to get out of the active material in the over-charge state starts is a high temperature exceeding 400° C., which is greatly higher than operating temperature of the battery. Therefore, the secondary battery using lithium-manganese composite oxide can perform the charge/discharge operation with a high capacity close to the theoretical capacity, and is free from an accident of explosion due to ignition or spark in the battery, so that the battery exhibits an extremely high safety. Due to such remarkable advantages which cannot be easily obtained, the development of the secondary battery has been promoted so as to realize an actual use.
However, in case of the secondary battery using the positive electrode materials such as lithium-manganese composite oxide, there has been noted that the following unfavorable phenomena are liable to occur in comparison with the batteries using other type materials. That is, the lowering of the capacity becomes remarkable when the charge/discharge operations are repeated under a temperature higher than 40° C., an open-circuit voltage (OCV) after preserving or storing the battery under a high temperature of 40° C. or higher is lowered and the capacity of the battery is also decreased or the like. Accordingly, it has been recognized that the secondary batteries are not practically or commercially available if the aforementioned problems are not solved.
The present invention had achieved to solve the aforementioned problems and an object of the present invention is to provide a positive electrode active material and a non-aqueous secondary battery capable of suppressing the lowering of the capacity in accordance with the progress of charge/discharge cycles at a high temperature, and capable of suppressing the lowering of the open-circuit voltage (OCV) and capacity even after the battery is preserved or stored in a high-temperature situation for a long time.
DISCLOSURE OF THE INVENTION
In order to achieve the aforementioned object, the inventors of this invention had eagerly studied. As a result, the inventors had obtained the following findings. That is, when a non-aqueous secondary battery was prepared in such a manner that a positive electrode active material having a composition of Li
1+x
Mn
2−x−y
M
y
O
4
wherein 0≦x≦0.2, 0≦y≦0.3, and M denotes metal element other than Mn and M is at least one element other than alkaline metal and alkaline earth metal was prepared and then a cover layer composed of metal oxide having a predetermined composition is formed on at least part of surface of the positive electrode active material, there can be provided significant effects such that the lowering of the capacity in accordance with the progress of charge/discharge cycles at a high temperature can be effectively suppressed and the lowering of the open-circuit voltage (OCV) and capacity even after the battery is preserved or stored in a high-temperature situation can be also effectively suppressed.
In this regard, it is preferable that the cover layer contains at least Mn and Li, and the cover layer preferably has a composition in which the number of metal atoms other than Mn and Li is 0.01-20 times of the number of Mn atoms. In the present invention, the composition of the cover layer can be measured or identified by means of X-ray photoelectric spectroscopy (XPS) method. That is, in the XPS method, peak intensities of photoelectron at detecting angle of 45 degree are measured. Then, from relative sensitivity coefficient, we can know kinds of the atoms existing on the surface of the positive electrode active material and a concentration of the atom, whereby an atomic ratio of the respective atoms can be easily calculated.
Conventionally, for the purpose of improving the battery characteristics, the following countermeasures have been considered to be effective. That is, structures of coated electrodes are properly controlled such that pore distribution in the positive electrode is controlled or a contact area between the electrolyte and the positive electrode active material is controlled. As another example of the effective countermeasures, for the purpose of controlling electro-chemical rea
Kubo Koichi
Takeuchi Hajime
Foley & Lardner
Kabushiki Kaisha Toshiba
Weiner Laura
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