Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-10-30
2004-11-23
Tsang-Foster, Susy (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S229000, C429S231950
Reexamination Certificate
active
06821675
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a non-aqueous electrolyte secondary battery (hereinafter, battery), and especially relates to batteries of which electrochemical properties such as the charge/discharge capacity and charge/discharge cycle life have been enhanced by improvements in negative electrode materials and non-aqueous electrolytes.
BACKGROUND OF THE INVENTION
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, 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.
The lithium ion secondary batteries which contain an organic electrolytic solution, and use carbon materials as negative electrode active materials and lithium-containing composite oxides as positive electrode active materials, have higher voltage and energy density, and superior low temperature properties compared with secondary batteries using aqueous solutions. As these lithium ion batteries do not use lithium metal for the negative electrode, they are superior in terms of cycle stability and safety, thus are now being commercialized rapidly. Lithium polymer batteries using macromolecular (polymer) gel electrolytes which contain an organic electrolytic solution, have been also under development as a new thin and light batteries.
As for polymer electrolyte batteries, various researches have been conducted on the subject since Armand et al. disclosed a polymer electrolyte battery comprising polyethylene oxides and electrolytic salts (Second International Meeting on Solid Electrolytes, Extended Abstracts, p20-22, 1978, U.S. Pat. No. 4,303,748). Although various polymer electrolytic materials have been mentioned, for example, in the conductive polymer edited by Naoya Ogata, Kodansha, 1990, and
Polymer Electrolyte Reviews
, Vol. 1 and 2, Elsevier, London (1987, 1989), ionic conductivity of these polymer electrolytic materials at room temperature is only about 10
−4
-10
−5
S/cm.
As another method to improve ionic conductivity, different types of electrolytes which can easily achieve the ionic conductivity of 10−
3
S/cm have been disclosed, for example, in J. Electrochem, Soc., 137, 1657 (1990), U.S. Pat. No. 5,085,952, U.S. Pat. No. 5,223,353 and U.S. Pat. No. 5,275,750. These electrolytes are called polymer gel electrolyte in which solvents of organic electrolytic solutions are added to polymers as plasticizers. Polymer batteries using these polymer gel electrolytes are expected to achieve the same performance as lithium ion batteries by improving ionic conductivity. However, in respect of capacity, both positive electrode and negative electrodes of these polymer batteries need to be made of composite materials containing polymers as well. Thus, the mass (density) of active materials in the casing of the battery is reduced. Therefore, when the same materials are used for both positive and negative electrodes, energy density of the lithium polymer secondary batteries becomes lower than that of the lithium ion batteries.
When a high-capacity lithium metal 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. The deposited dendrites have a large specific surface area, thus their reaction activity is high. Therefore, they react with plasticizers (solvents) of the polymer gel electrolytes, lowering charge/discharge efficiency. 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.
To suppress the formation of such dendrites, it has been disclosed that lithium alloys such as 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 de-intercalating lithium, they may undergo a phase change even when the crystal structure of the original skeletal alloy is maintained, or sometimes, the crystal structure of the skeletal alloy of elements changes. In this case, particles of the metal or alloy which are host materials of the lithium, an active material, swell and shrink. As the charge/discharge cycle proceeds, crystal grains are stressed and cracked, thus particles are pulverized and come off from the electrode plate. As the crystal grains are pulverized, 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 electricity charged in the battery. On the other hand, during discharging, the voltage drop occurs by the resistance polarization, reaching the discharge-termination voltage early. Thus, superior charge/discharge capacity and cycle properties can not be expected.
If currently used solvents such as ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, propylene carbonate, gamma-butyro lactone, and gamma-valero lactone are used in a system in which lithium metal or a lithium alloy is used in a negative electrode, the electrolyte may decompose and gas may be produced when the battery is fully charged and stored at high temperatures. Moreover, if the battery is repeatedly charged and discharged, parallel to the charge/discharge reaction of the negative electrode, the electrolyte is gasified, lowering the charge/discharge efficiency, resulting in decreased cycle properties.
Nowadays, lithium secondary batteries which use, as a negative electrode material, carbon materials capable of intercalating and de-intercalating lithium ions, are commercially available. In general, lithium metal does not deposit on carbon negative electrodes. Thus, short circuits are not caused by dendrite. However, the theoretical capacity of graphite which is one of currently used carbon materials is 372 mAh/g, only one tenth of that of pure Li metal.
If graphite-group carbon materials are used as a negative electrode material, and propylene carbonate is adopted for an electrolytic solution, the electrolytic solution is decomposed at potentials more precious than that of lithium metal. Consequently, lithium ions are not intercalated between layers of graphite, and the battery does not function. Considering these points, currently commercialized lithium secondary batteries with the graphite-group used for negative electrode materials frequently use electrolytic solution containing ethylene carbonate. However, the melting point of ethylene carbonate is 37° C. higher than room temperature. Therefore, at low temperatures, ionic conductivity of the electrolytic solution for lithium ions plummets, lowering charge/discharge priorities.
When inorganic compound materials such as TiS2 are used as a negative electrode active material, intercalation and de-intercalation of lithium occur at sufficiently more precious potentials compared with lithium metal and lithium alloys. Thus, even when the negative electrode active materials come in contact with the electrolytic solution, reductive decomposition does not occur. Moreover, even when propylene carbonate is used for the electrolytic solution, intercalation and de-intercalation are not impeded by decomposition as is the case with the graphite materials, therefore, a wider range of electrolytic solutions is applicable. However, potenti
Iwamoto Kazuya
Koshina Hizuru
Morigaki Kenichi
Nitta Yoshiaki
Shimamura Harunari
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