Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-09-18
2003-08-12
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S229000, C429S231950, C429S304000, C429S231100, C429S231700
Reexamination Certificate
active
06605386
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a non-aqueous electrolyte secondary battery (hereinafter, battery). The present invention especially relates to batteries of which electrochemical properties such as charge/discharge capacity and charge/discharge cycle life have been enhanced by improvements in the negative electrode materials, separators and the amounts of electrolyte. The present invention further relates to batteries wherein the electrochemical properties mentioned above, as well as shelf stability, have been improved by designing a better balance between the positive electrode and the negative electrode materials, as well as the positive electrode and the negative electrode plates.
BACKGROUND OF THE INVENTION
Lithium secondary batteries with non-aqueous electrolytes, which are used in such areas as mobile communications devices, including portable information terminals and portable electronic devices, as power sources of portable electronic devices, domestic portable electricity storing devices, motor cycles using an electric motor as a driving source, electric cars and hybrid electric cars, have characteristics of a high electromotive force and a high energy density. Although the energy density of the lithium secondary batteries using lithium metal as a negative electrode material is high, there is a possibility that dendrite deposits form on the negative electrode during charging. By repeated charging and discharging, the dendrite breaks through separators to the positive electrode side, thereby causing an internal short circuit. The deposited dendrite has a large specific surface area, thus its reaction activity is high. Therefore, it reacts with solvents in the electrolyte solution on its surface and forms a surface layer which acts like a solid electrolyte having no electronic conduction. This raises the internal resistance of the batteries or causes some particles to be excluded from the network of electronic conduction, lowering the charge/discharge efficiency of the battery. Due to these reasons, the lithium secondary batteries using lithium metal as a negative electrode material have a low reliability and a short cycle life.
Nowadays, lithium secondary batteries which use carbon materials capable of intercalating and de-intercalating lithium ions as a negative electrode material are commercially available. In general, lithium metal does not deposit on carbon negative electrodes. Thus, in such batteries short circuits do not occur due to dendrite formation. However, the theoretical capacity of graphite, which is one of the currently available carbon materials, is 372 mAh/g, only one tenth of that of pure lithium (Li) metal.
Other known negative electrode materials include pure metallic materials and pure non-metallic materials which form composites with lithium. For example, composition formulae of compounds of tin (Sn), silicon (Si) and zinc (Zn) with the maximum amount of lithium are Li
22
Sn
5
, Li
22
Si
5
, and LiZn respectively. Within the range of these composition formulae, metallic lithium does not normally deposit to form dendrites. Thus, an internal short circuit due to dendrite formation does not occur. Furthermore, electrochemical capacities between these compounds and each element in pure form mentioned above is respectively 993 mAh/g, 4199 mAh/g and 410 mAh/g; all larger than the theoretical capacity of graphite.
As an example of other compound negative electrode materials, the Japanese Patent Laid-Open Publication No. H07-240201 discloses a non-metallic siliside comprising transition elements. The Japanese Patent Laid-Open Publication No. H09-63651 discloses negative electrode materials which are made of inter-metallic compounds comprising at least one of group 4B elements, phosphorus (P) and antimony (Sb), and have a crystal structure of one of the CaF
2
type, the ZnS type and the AlLiSi type.
However, the foregoing high-capacity negative electrode materials have the following problems. Negative electrode materials of pure metallic materials and pure non-metallic materials which form compounds with lithium have inferior charge/discharge cycle properties compared with carbon negative electrode materials. The reason for this is assumed to be destruction of the negative electrode materials caused by their increase and decrease in volume.
On the other hand, unlike the foregoing materials in pure form, the Japanese Patent Laid-Open Publication No. H07-240201 and the Japanese Patent Laid-Open Publication No. H09-63651 disclose negative electrode materials which comprise non-metallic silisides composed of transition elements and inter-metallic compounds including at least one of group 4B elements, P and Sb, and have a crystal structure of one of the CaF
2
type, the ZnS type and the AlLiSi type, as negative electrode materials with an improved cycle life property.
Batteries using the negative electrode materials of the non-metallic silisides composed of transition elements disclosed in the Japanese Patent Laid-Open Publication No. H07-240201 have an improved charge/discharge cycle property when compared with lithium metal negative electrode materials (considering the capacity of the batteries according to an embodiment and a comparative example at the first cycle, the fiftieth cycle and the hundredth cycle). However, when compared with a natural graphite negative electrode material, the increase in the capacity of the battery is only about 12%.
The materials disclosed in the Japanese Patent Laid-Open Publication No. H09-63651 have a better charge/discharge cycle property than a Li—Pb alloy negative electrode material (as shown in tests between an embodiment and a comparative example), and have a larger capacity compared with a graphite negative electrode material. However, the discharge capacity decreases significantly, up to the 10~20th charge/discharge cycles. Even with Mg
2
Sn, which is considered to be better than any of the other materials, the discharge capacity decreases to approximately 70% of the initial capacity after about the 20th cycle.
Examples of positive electrode active materials for the non-aqueous electrolyte secondary batteries, which are capable of intercalating and de-intercalating lithium ions, include a lithium transition metal composite oxide with high charge/discharge voltage such as LiCoO
2
, disclosed in the Japanese Patent Laid-Open Publication No. Other materials such as S55-136131, and LiNiO
2
, disclosed in the U.S. Pat. No. 4,302,518, aim at even a higher capacity. Examples of such positive electrode active materials further include composite oxides comprising a plurality of metallic elements and lithium such as Li
y
Ni
x
Co
1−x
O
2
, disclosed in the Japanese Patent Laid-Open Publication No. S63-299056, and Li
x
M
y
N
z
O
z
(M is one of Fe, Co and Ni, and N is one of Ti, V, Cr and Mn) disclosed in the Japanese Patent Laid-Open Publication No. H04-267053.
Active research has been conducted on LiNiO
2
since the supply of Ni, its raw material, is stable and inexpensive, and it is expected to achieve a high capacity.
It has been known that with the thus far disclosed positive electrode active materials, especially Li
y
Ni
x
M
1−x
O
2
(M is at least one material selected from a group consisting of cobalt (Co), manganese (Mn), chromium (Cr), iron (Fe), vanadium (V), and aluminum (Al); and x is 1≧x≧0.5) there are significant differences in charge/discharge capacity between the initial charging (de-intercalation reaction of lithium) and discharging (intercalation reaction of lithium) in the voltage region usually used as a battery (4.3V-2V against Li)( see, for example, A. Rougier et al., Solid State Ionics 90, 83 (1996)).
FIG. 2
shows a schematic view of the electric potential behavior at the initial charge and discharge of the positive electrode and the negative electrode of a battery in which composite particle materials with the same theoretical capacity as the foregoing positive electrode materials are used in the negative electrode.
In
FIG. 2
, (A-B) is the amount of electricity
Kasamatsu Shinji
Koshina Hizuru
Nitta Yoshiaki
Okamura Kazuhiro
Shimamura Harunari
Matsushita Electric - Industrial Co., Ltd.
RatnerPrestia
Weiner Laura
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