Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-09-28
2004-11-30
Tsang-Foster, Susy (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S229000, C429S231950
Reexamination Certificate
active
06824920
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous electrolyte secondary battery (hereinafter, battery), and especially relates to batteries whose electrochemical properties such as the charge/discharge capacity and charge/discharge cycle life have been enhanced by improvements in negative electrode materials, and solvents used for the non-aqueous electrolytes.
In recent years, lithium secondary batteries with non-aqueous electrolytes, which are used in such fields as mobile communications devices including portable information terminals and portable electronic devices, main power sources of portable electronic devices, small size 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.
When lithium metal with a high capacity is used as a negative electrode material, dendritic deposits are formed on the negative electrode during charging. Over repeated charging and discharging, these dendritic deposits penetrate through separators and polymer gel electrolytes to the positive electrode side, causing an internal short circuit. During discharging, these dendritic deposits break, falling from the surface of the bulk lithium-metal negative electrode, thus forming “dead” lithium which does not contribute to charge/discharge reaction. Furthermore, reaction activity of the deposited lithium is high since they have a large specific surface area. Due to this, the lithium reacts with solvents in the electrolytic solution on their surfaces, and form a surface film similar to a solid electrolyte which has no electronic conductivity. This increases the internal resistance of the batteries, causing some particles to be excluded from the network of the electronic conduction, thereby lowering the charge/discharge efficiency of the battery. For these reasons, the lithium secondary batteries using lithium metal as a negative electrode material have a low reliability and a short cycle life.
To suppress the formation of such dendrites, it has been proposed that lithium alloys such as a lithium-aluminum alloy and a wood's alloy are used instead of lithium metal. Metals capable of forming alloys with lithium and alloys containing at least one such metal can be used as a negative electrode material with a relatively high electrochemical capacity in the initial charge/discharge cycle. However, by repeatedly alloying with and deintercalating lithium, they may undergo a phase change even when the crystal structure of the original alloy is maintained, or sometimes, the crystal structure of the alloy itself changes.
In this case, particles of the metal or an alloy which are host materials of the lithium (active material), swell and shrink. As the charge/discharge cycle proceeds, crystal grains are stressed and cracked, thus particles are pulverized and leave off from the electrode plate. As the particles are pulverized, grain boundary resistance and contact resistance of the grain boundaries increase. As a result, resistance polarization during charging and discharging increases. Thus, when charging is conducted at a controlled voltage level, charging depth becomes shallow, limiting the amount of charged electricity in the battery. On the other hand, during discharging, the voltage level is decreased by the resistance polarization, reaching the discharge-termination voltage early. Thus, superior charge/discharge capacity and cycle properties are difficult to achieve.
Nowadays, lithium secondary batteries which use, as a negative electrode material, carbon materials capable of intercalating and deintercalating lithium ions, are commercially available. In general, lithium metal does not deposit on carbon negative electrodes. Thus, short circuits are not caused by dendrites. However, the theoretical capacity of graphite which is one of the currently used carbon materials is 372 mAh/g, only one tenth of that of pure Li metal.
Other active material compounds include diniobium pentaoxide (Nb
2
O
5
), titanium disulfide (TiS
2
), molybdenum dioxide (MoO
2
), lithium titanate (Li
4
/
3
Ti
5
/
3
O
4
). In the case of these materials, lithium is ionized and maintained among the host substances. Due to this, compared with lithium metal whose chemical activity is high, these materials are chemically stable, do not form dendritic deposits, and contribute to higher cycle properties. Among them, some carbon materials are already commercialized.
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 respectively Li
22
Sn
5
, Li
22
Si
5
, and LiZn. Within the range of these composition formulae, metallic lithium does not normally deposit. Thus, an internal short circuit is not caused by dendrites. Furthermore, the electrochemical capacities between these compounds and each element mentioned above are respectively 993 mAh/g, 4199 mAh/g and 410 mAh/g; all are larger than the theoretical capacity of graphite.
As other compound negative electrode materials, the Japanese Patent Laid-Open Publication No. H07-240201 discloses a non-metallic silicide 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, P and Sb, and have a crystal structure of one of the CaF2 type, the ZnS type and the AlLiSi type.
As a solvent of the electrolyte of the battery, cyclic carbonates such as propylene carbonate and ethylene carbonate, acyclic carbonates such as diethyl carbonate, and dimethyl carbonate, cyclic carboxylate such as gamma-butyrolactone and gamma-valerolactone, acyclic ethers such as dimethoxy ethane and 1,3-dimethoxy propane, and cyclic ethers such as tetrahydrofuran and 1,3-dioxolane are widely used.
It is desirable to adopt electrolyte with high electrical conductivity to the batteries. Due to this, solvents with a high dielectric constant and a low viscosity are preferably used. However, being high in the dielectric constant simply means high in polarity, in other words, high in viscosity. Therefore, among the electrolytes mentioned above, solvents with high dielectric constant such as propylene carbonate (dielectric constant ∈=65) and solvents with low dielectric constant such as 1,2-dimethoxy ethane (∈=7.2) are often mixed and used.
The electrolyte used in the non-aqueous electrolyte batteries also contain supporting electrolytes dissolved in the solvents mentioned above at a concentration of about 1 mol. The supporting electrolytes include anion lithium salts of inorganic acid such as lithium perchlorate, lithium borofluorides and lithium phosphofluoride, and anion lithium salts of organic acid such as trifluoromethane sulfonic acid lithium and bis-trifluoromethane sulfonic acid imido lithium.
But, the above high capacity negative electrode materials include 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 the destruction of the negative electrode materials caused by their volume expansion and shrinkage.
On the other hand, as negative electrode materials with an improved cycle life property unlike the foregoing pure materials, the Japanese Patent Laid-Open Publication No. H07-240201 and the Japanese Patent Laid-Open Publication No. H09-63651 respectively disclose non-metallic silicides composed of transition elements and intermetallic compounds which are composed of at least one of group 4B elements, P and Sb, and have a crystal structure of one of the CaF2 type, the ZnS type and the AlLiSi type.
Batteries using the negati
Iwamoto Kazuya
Koshina Hizuru
Nitta Yoshiaki
Shimamura Harunari
Matsushita Electric - Industrial Co., Ltd.
RatnerPrestia
Tsang-Foster Susy
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