Non-aqueous electrolyte secondary batteries

The present invention relates to a non-aqueous electrolyte secondary battery employing organic electrolyte, polymer solid electrolyte or the like, and particularly to a novel constitution of positive and negative electrodes for having a higher capacity without sacrificing the charge-discharge cycle life.
A variety of secondary batteries including small-sized sealed lead-acid and nickel-cadmium systems have been developed as power sources for driving portable electronic apparatuses. For minimizing the overall size and weight of such a driving secondary battery, newly introduced and marketed are nickel-metal hydride systems, lithium-ion systems, and other advanced secondary batteries which are higher in the energy density. As their products are welcome widely in the market, lithium-ion batteries above all are focused which ire substantially equal in the capacity density per unit volume (Wh/1) to nickel-metal hydride systems but almost two times higher in the capacity density per unit weight (Wh/kg) than the same. The lithium-ion second battery is now known as one of the lightest power sources and demanded for more improvement.
The lithium-ion secondary battery includes a negative active material of lithium and is thus regarded as a lithium secondary battery. Also, it uses a non-aqueous electrolyte such as organic electrolyte or polymer solid electrolyte and is regarded as a non-aqueous electrolyte secondary battery.
In essential, the lithium-ion secondary battery comprises a positive electrode made of lithium contained cobalt oxide, LiCoO.sub.2, or a double oxide of lithium and cobalt, and a negative electrode made of a carbon material such as graphite or coke, which both electrodes are separated by a separator, assembled to form an electrode group, and placed in an organic electrolyte. In action, when this assembly of the secondary battery is initially charged, lithium ions are desorbed from the positive electrode of LiCoO.sub.2 and dissolved into the electrolyte. Simultaneously, lithium ions in the electrolyte are absorbed in the carbon material of the negative electrode to form C.sub.6 Li. During the initial discharge, lithium ions in the electrolyte are absorbed in the positive electrode and restored to a LiCoO.sub.2 form. At the time, lithium ions are desorbed from the negative electrode of C.sub.6 Li and dissolved into the electrolyte. Since the charge and the discharge reaction on the positive and negative electrodes are reversible, such battery system is called a rocking-chair battery. The rocking-chair battery may have a longer cycling life of over 1000 cycles provided that neither overcharge or over-discharge is involved.
It is however said that the reversible reaction on the positive and the negative electrodes in the charge and discharge are not uniform. As described above, the initial charge permits lithium ions to be desorbed from LiCoO.sub.2 of the positive electrode, but not the whole amount of lithium ions is absorbed in the positive electrode and restored to LiCoO.sub.2 in the initial discharge. In other words, it is common that a smaller amount of lithium ions than the amount desorbed in the initial charge is successfully absorbed in the positive electrode. Also, in the initial charge, an amount of lithium ions equivalent to the charge capacity of the positive electrode is absorbed in the negative electrode made of a carbon material to form to C.sub.6 Li. The negative electrode however desorbs about 80% or more of the whole amount of absorbed lithium ions in the initial discharge. As the remaining 20% of lithium has been trapped in the negative electrode, it will not participate in the charge and discharge reactions of a succeeding cycle. Such an amount of lithium ions trapped in the negative electrode and isolated from the charge and discharge reactions is regarded as "dead lithium" and should be discriminated from the other active portion. Although the efficiency of reaction during the charge and discharge after the initial discharge is affected by the rates of charge and discharge and the ambient temperature at the site and may not reach 100%, its declination will is not compared to a difference between the initial charge capacity and the initial discharge capacity. It is hence essential for designing a constitution of the lithium-ion secondary battery to account a ratio of the initial discharge capacity to the initial charge capacity (referred to as an initial charge and discharge efficiency hereinafter) on the positive and negative electrodes in order to determine the theoretical capacity values of the positive and negative electrodes.
The capacity of the secondary battery will be increased when a material having a higher charge-discharge efficiency or more specifically, a higher initial charge-discharge efficiency is used as the positive and negative electrodes. Such an example as using LiCoO.sub.2 as the positive electrode is disclosed in Japanese Patent Laid-open Application No. Sho63-59507. LiCoO.sub.2 has a higher initial charge-discharge efficiency and also a higher electrode potential (thus to produce a higher voltage output of the battery), hence being suited as a material for the positive electrode. Cobalt is however an expensive material as is produced only in particular regions of the earth (for example, Zambia in Africa). Hence, its supply and price largely depend on the political situation in the regions. It is thus proposed to substitute such a critical material as LiCoO.sub.2 with lithium contained nickel oxide, LiNiO.sub.2 which is favorable in both the price and the availability and may provide a higher capacity than that of LiCoO.sub.2, as disclosed in U.S. Pat. No. 4,302,518.
The electrode potential of LiNiO.sub.2 is about 0.2 volt lower than that of LiCoO.sub.2 and may thus promote the desorbing of lithium ions before the non-aqueous electrolyte such as organic electrolyte reaches its decomposition voltage in the charge. This results in increase of the charge capacity and thus improvement of the discharge capacity.
However, the initial charge-discharge efficiency of LiNiO.sub.2 is, not high enough and causes declination of the capacity as the charge and discharge cycle is repeated again and again, whereby its practical use will be difficult.
For eliminating the drawback of LiNiO2, double oxide such as LixNixCo.sub.1-x O.sub.2 or LixMyNzO.sub.2, including lithium and plural metals (where M is at least an element selected from a group of Fe, Co, and Ni and N is at least an element selected from a group of Ti, V, Cr, and Mn) is provided as disclosed in Japanese Patent Laid-open Application Sho63-299056 or Publication Sho63-267053.
It has been reported by A. Rougiel et al, "Solid state ionics", 90, 83(1996) that the lithium contained metal oxide expressed by LiNixM.sub.1-x O.sub.2 (M being at least a metallic element selected from a group of Co, Mn, Cr, Fe, V, and Al and 0.5.times.1.0) of the positive electrode is low in the initial discharge capacity (the absorption reaction quantity of lithium ions) as compared with the initial charge capacity (the desorption reaction quantity of lithium ions) over a potential range (of 4.3 to 2.0 V vs. Li/Li.sup.+) used in the non-aqueous electrolyte secondary battery, hence having a lower initial charge and discharge efficiency.
FIG. 1 schematically illustrates the potential behavior of the positive and negative electrodes in the initial charge and initial discharge of a secondary battery of which negative electrode is made of a graphite system material identical in the theoretical capacity to the positive electrode material.
In FIG. 1, (A-B) is an initial charge capacity at the positive electrode, (B-C) is an initial discharge capacity of the positive electrode, and (C-A) is a non-reversible capacity of the positive electrode.
Also, (A'-B') is an initial charge capacity at the negative electrode which is equivalent to the (A-B) at the positive electrode. (B'-C') is an initial discharge capable capacity of the negative electrode, and (C'-A') is a non-reversible capacity of the negative electrode. Since the initial discharge capable capacity of the negative electrode (B'-C') is greater by (C'-D) than the, initial discharge capacity of the positive electrode (B-C), the initial discharge capacity of the battery is limited by the initial discharge capacity of the positive electrode (B-C). In charge and discharge cycle after the initial discharge, the reversible reactions are proceeded between (B-C) at the positive electrode and between (B'-D) (equivalent between B-C) at the negative electrode. Accordingly, an amount of lithium of considerable capacity (C'- D) at the negative electrode traps as "dead lithium" in the negative electrode and will hardly act on the charge and discharge reactions thus not contributing to the increase of the battery capacity.
When the theoretical capacity of the both electrodes is adjusted by increasing of filling amount of the positive electrode so that the initial discharge capacity is equal between the positive electrode and the negative electrode, the negative electrode will be overcharged by (C'-D) determined by the amount of "dead lithium" at the negative electrode, which is equal to a difference between the non-reversible capacity (C-A) of the positive electrode and the non-reversible capacity (C'-A') of the negative electrode. As the result, a metallic lithium is deposited on the surface of the negative electrode, thus negative electrode is passivated by the reaction between deposited metal lithium and organic electrode solution and decreases the cycle life of the battery.
Accordingly, the adjustment of the theoretical capacity on both the positive and negative electrodes may be unsuitable.
For increasing the capacity of the lithium-ion secondary battery, it is essential to use proper materials for the positive and negative electrodes which can provide as a higher initial charge and discharge efficiency and a higher reversible capacity in the charge and discharge reactions as possible. It is also desired to minimize the amount of "dead lithium" and the irreversible capacity on the positive and negative electrodes so that the negative electrode is prevented from being overcharged and free from deposition of metallic lithium.
If the positive electrode is higher in the initial charge and discharge efficiency than the negative electrode, it will extremely be difficult to design and fabricate an improved secondary battery which satisfies the above requirements.
SUMMARY OF THE INVENTION
A non-aqueous electrolyte secondary battery according to the present invention comprises a positive electrode and a negative electrode separated by a separator impregnated with organic electrolyte solution or a solid electrolyte layer, the positive electrode consisting mainly of lithium contained metal oxide and the negative electrode consisting mainly of a mixture of a carbon material, as the primary material, and the metal oxide which can electrochemically be reduced to metal by charge. This allows the negative electrode to eliminate "dead lithium" which hardly contributes to the charge and discharge reactions and to be, prevented from being overcharged. Accordingly, the non-aqueous electrolyte secondary battery of the present invention will increase its cycle life and capacity.
The advantage of the present invention is enhanced with the positive electrode made of a lithium contained metal oxide which consists mainly of a lithium contained nickel oxide having 75 to 95% of initial charge and discharge efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration showing the potential behavior of the positive and negative electrodes of a conventional non-aqueous electrolyte secondary battery in the initial charge and initial discharge when both electrodes are set equal in the theoretical capacity;
FIG. 2 is a schematic illustration showing the potential behavior of the positive and negative electrodes of a non-aqueous electrolyte secondary battery according to the present invention in the initial charge and initial discharge; and
FIG. 3 is a longitudinal cross sectional view of a typical cylindrical non-aqueous electrolyte secondary battery.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of the present invention will be described in more detail referring to the accompanying drawings and tables.
Embodiment 1
FIG. 3 illustrates a longitudinal cross sectional view of a typical cylindrical non-aqueous electrolyte secondary battery employed in this embodiment. As shown in FIG. 3, an electrode group 4 which comprises a sheet of positive electrode plate 5 and a sheet of negative electrode plate 6 separated from each other by a separator 7 made of a poly-olefin microporous membrane and assembled in a roll form is installed in a stainless steel cell case 1 together with an upper insulating plate 8a and a lower insulating plate 8b mounted on the top and bottom thereof. The positive electrode plate 5 and the negative electrode plate 6 are provided with an aluminum foil positive electrode lead tab 5a and a copper foil negative electrode lead tab 6a respectively. The positive electrode lead tab 5a and the negative electrode lead tab 6a are joined by welding for electrical connection to the lower side of an assembled cell cover 2 of an explosion-proof structure and the inner side of the cell case 1 respectively.
An annular groove 1a is provided in an upper end region of the cell case 1 so that its wall extends inwardly and the cell case 1 is then poured with an appropriate quantity of organic electrolyte solution to impregnate into the electrode group 4. The assembled cell cover 2 equipped with a gasket 2 is fitted until its lower end engages with the groove 1a. When the upper end of the cell case 1 has been curled inwardly for sealing liquid-tightly and gas-tightly, the secondary battery is completed.
Lithium contained metal oxide used as a material of the positive electrode is prepared by the following manner. A mixture solution of nickel sulfate, NiSO.sub.4, and cobalt sulfate, CoSO.sub.4, is stirred and added with an excessive amount of sodium hydroxide, NaOH aqueous solution, to allow coprecipitation of nickel hydroxide, Ni(OH).sub.2, and cobalt hydroxide, Co(OH).sub.2. A coprecipitated sediment is rinsed by decantation, filtered, and dried. The sediment has a composition of Ni.sub.0.85 Co.sub.0.15 (OH).sub.2.
The sediment is then mixed with a specific amount of lithium hydroxide crystal, LiOHH.sub.2 O. The mixture is baked for 10 hours at 800.degree. C. under the oxidation atmosphere such as in the air, thus yielding a lithium contained metal oxide. It is proved by chemical analysis that the composition of the lithium contained metal oxide is LiNi.sub.0.85 Co.sub.0.15 O.sub.2 which is a mother material for the positive electrode.
100 parts by weight of LiNi.sub.0.85 Co.sub.0.15 O.sub.2 powder, 3 parts by weight of acetylene black, and 5 parts by weight of polyvinyliden fluoride (PVDF) resin binder are mixed together and added with N-methyl-pyrrolidone to prepare a positive electrode paste. The paste is applied to both sides of an aluminum foil of 0.020 mm thick, using a coater, dried, and roll pressed to a thickness of 0.130 mm. After trimmed to a size of 35 mm wide and 270 mm long, the positive electrode plate is completed. As described previously, the positive electrode plate is then provided with a positive electrode lead tab of the aluminum foil.
Then, the preparation for the negative electrode plate is explained.
Metal oxide is added and mixed with carbon material for the negative electrode in a ratio of 0.2 to 20%.
Graphite powder is used as a primary carbon material for the negative electrode. 100 parts by weight of graphite powder is mixed with each of fifteen different types of metal oxide including Ag.sub.2 O, PbO, NiO, Ni.sub.2 O.sub.3, CoO, CO.sub.2 O.sub.3, Co.sub.3 O.sub.4, TiO.sub.2, Bi.sub.2 O.sub.3, Sb.sub.2 O.sub.3, Cr.sub.2 O.sub.3, SeO.sub.2, TeO.sub.2, MnO.sub.2, and Fe.sub.3 O.sub.4 so that each corresponding mixture contains 9.06, 8.75, 3.10, 6.65, 3.11, 6.65. 10.31, 6.41, 16.67, 11.11, 6.12, 4.55, 6.41, 6.95, or 9.02% by weight of the metal oxide respectively. The mixture is further mixed with stylene-butadiene copolymer system binder and carboxy-methyl-cellulose aqueous solution to prepare a negative electrode paste.
The amounts of the 15 different types of metal oxide added to the carbon material are predetermined so that a sum of the electrical capacity (mAh) generated by electrochemical reduction of each metal oxide to a metal and the non-reversible capacity (mAh) of the carbon material of the negative electrode is equal to the non-reversible capacity of the lithium contained metal oxide of the positive electrode.
The negative electrode paste is applied to both sides of a copper foil of 0.015 mm thick, using a coater, dried, and roll pressed to a thickness of 0.20 mm. After trimmed to a size of 37 mm wide and 300 mm long, the negative electrode plate is completed. Similar to the positive electrode plate, the negative electrode plate is then provided with a negative electrode lead tab of the copper foil.
The organic electrolyte solution is prepared by dissolving lithium hexafluorophosphate LiPF.sub.6, into an equal volume mixed solvent of ethylene carbonate and ethyl-methyl carbonate to use a concentration of 1.0 mol/l.
It is assumed that the secondary batteries having the negative electrode made of the carbon material added with fifteen different types of metal oxide respectively are denoted by the numbers raging from 1 to 15 while a comparison cell in which the negative electrode is made of only the carbon material is denoted by 16.
A series of experimental charge and discharge were carried out using a constant current of 100 mA, an end-of-charge voltage of 4.2 V/cell, and an end-of-discharge voltage of 2.5 V/cell. Resultant measurements of the initial charge capacity, the initial discharge capacity, the initial charge and discharge efficiency, capacity, the initial discharge capacity, the initial charge and discharge efficiency, and the cycle life are listed in Table 1 as average values in five each calls. The cycle life is expressed by the number of cycles where the discharge capacity is kept 70% of that in the third cycle.
TABLE 1______________________________________ Initial Initial Initial charge/ Metal oxide charge discharge discharge for negative capacity capacity efficiency Cycle lifeCell No. electrode (mAh) (mAh) (%) (cycles)______________________________________1 Ag.sub.2 O 653 603 92.3 6892 PbO 648 589 90.9 6473 NiO 649 598 91.8 7024 Ni.sub.2 O.sub.3 645 597 92.6 7045 CoO 647 601 92.9 6096 Co.sub.2 O.sub.3 650 600 92.3 5977 Co.sub.3 O.sub.4 647 598 92.4 6058 TiO.sub.2 654 603 92.2 6089 Bi.sub.2 O.sub.3 656 601 91.6 65810 Sb.sub.2 O.sub.3 645 599 92.9 65411 Cr.sub.2 O.sub.3 649 598 92.6 65512 SeO.sub.2 648 604 93.2 69313 TeO.sub.2 644 609 94.6 63514 MnO.sub.2 647 602 93.0 59615 Fe.sub.3 O.sub.4 658 607 92.2 65416 -- 646 597 92.4 150______________________________________
As apparent from Table 1, the cells, No. 1 to No. 15, of which the negative electrodes are made of the carbon material added with the 15 different types of metal oxide is substantially similar in the initial charge and discharge efficiency but highly distinguished in the cycle life from the cell No. 16 added with no metal oxide. More particularly, the cycle life of the cell No. 16 added with no metal oxide is remarkably shorter than that of the cells No. 1 to No. 15.
It was found by disassembly and reviewing the cell No. 16 after the experimental charge and discharge cycle that a glossy deposition of metallic lithium was on the negative electrode. The deposition of metallic lithium partially created in the earlier cycle than of the other cells may be the reason of loss of the capacity due to the passivation of the negative electrode by the reaction between deposited metallic lithium and organic electrolyte solution.
In Embodiment 1, the amounts of the 15 different types of metal oxide added to the carbon material for the negative electrode are calculated from theoretical capacity determined by the following manner.
The metal oxide, for example, NiO, is mixed with about 30 wt % of acetylene black and pressed under a pressure of 250 kg/cm.sup.2 to a pellet firm which is then coupled integrally with a 18-8 stainless steel netting of 100 mesh to yield a working electrode. The counter electrode and reference electrode are made of metallic lithium. The three electrodes are placed in an organic electrolyte solution which is identical to that of the above mentioned secondary battery. The charge capacity is then measured until the voltage at the reference electrode is 0 V at a constant current of less than 0.1 mA/cm.sup.2. The electrochemical reduction caused by the charge is expressed by:
NiO+2Li++2e.sup.- .fwdarw.Ni+Li.sub.2 O
The generated metal such as Ni is stable chemically and electrochemically in the charge and discharge potential region of the carbon material of the negative electrode and is not oxidized but remains in metal phase during the discharge on the negative electrode as is irreversible. Since a quantity of metal is generated from the metal oxide in the initial charge, the negative electrode will increase its electronic conductivity and decrease its internal resistance and polarization.
FIG. 2 is a schematic illustration showing the potential behavior of the positive and negative electrodes in the initial charge and initial discharge of a non-aqueous electrolyte secondary battery of the present invention.
In FIG. 2, (A-B) is an initial charge capacity at the positive electrode, (B-C) is an initial discharge capacity of the positive electrode, and (C-A) is a irreversible capacity of the positive electrode.
Also, (A'-B') is an initial charge capacity at the negative electrode which is equivalent to the (A-B) at the positive electrode. At initial charge, the metal oxide added to the carbon material of the negative electrode is electrochemically reduced to charge a quantity of (A'-C') and then, lithium ions is absorbed in the carbon material of the negative electrode for charging the rest. The initial charge capacity of the carbon material is (C'-B'). The initial discharge capacity of the negative electrode is (B'-D) which is equivalent to (B-C) of the positive electrode. The initial discharge capacities of both the positive and negative electrodes are thus reversible capacities to each other. The irreversible capacity of the negative electrode is (C'-D) which is regarded as a irreversible capacity of the carbon material. As apparent from FIG. 2, the present invention permits the quantity of metal oxide added to the carbon material which is, a primary material of the negative electrode to correspond to a difference (A'-C') given by subtracting the irreversible capacity of the carbon material of the negative electrode from the irreversible capacity (C-A) of the positive electrode.
The metal oxide which is added to the negative electrode and in the charge, reduced electrochemically to generate a metal in irreversible reaction is higher in not only true specific weight but also bulk density than the carbon material such as graphite powder and its addition to the negative electrode will produce a negligible increase in the entire volume. Accordingly, since the negative electrode is added with a metal oxide which is electrochemically reduced in the charge or more specifically, in the initial charge to generate a quantity of metal in the irreversible reaction, the reversible capacities of both the positive and negative electrodes are utilized at optimum thus contributing to the higher capacity of the secondary battery. Also, the negative electrode is substantially prevented from being overcharged in the second and more charge after the initial cycle, hence avoiding declination of the cycle life.
Embodiment 2
The coprecipitated hydroxide, Ni.sub.0.85 CO.sub.0.15 (OH).sub.2, prepared in Embodiment 1 was mixed with a specific amount of LiOHH.sub.2 O and synthesized under the oxidation atmosphere at different temperatures to examine the effect of synthesizing temperature. The duration of the synthesization was constantly 10 hours and the temperature was varied from 800 to 700, 750, 850, 900, and 950.degree. C.
NiO was selected as the metal oxide to be mixed with graphite powder for the negative electrode and its content was 3.10 wt % equal to that in Embodiment 1.
A secondary battery was fabricated under the same conditions as of Embodiment 1 and tested for the charge and discharge cycle under the same conditions. Resultant measurements are shown in Table 2. The measurements of the cells No. 3 and No. 16 in Table 1 are appropriated for the positive electrode at the synthesizing temperature of 800.degree. C.
As notably apparent from Table 2, under the synthesizing temperature of 700.degree. C. or less than 750.degree. C. at No. 17 and No. 22, the cell No. 22 which is not added with NiO shows the cycle life as longer as the other cells No. 17 to No. 20 and No. 3 except for No. 21 of which negative electrode is added with metal oxide which can electrochemically be reduced by the initial charge to release a quantity of metal. This may be explained by the fact that when the synthesizing temperature for the positive electrode is 700.degree. C., the initial charge and discharge efficiency is high enough to reduce the irreversible capacity of the positive electrode to a small rate which can be offset by the irreversible capacity of graphite powder of the negative electrode.
TABLE 2__________________________________________________________________________ Initial Positive Initial Initial charge/ electrode Negative charge discharge discharge synthesizing electrode capacity capacity efficiency Cycle lifeCell No. temp. (.degree. C.) metal oxide (mAh) (mAh) (%) (cycles)__________________________________________________________________________17 700 NiO 712 650 91.3 65718 750 NiO 702 660 94.0 650 3 800 NiO 649 596 91.8 70219 850 NiO 623 570 91.5 68720 900 NiO 587 520 88.5 65821 950 NiO 413 220 53.3 63422 700 Non 712 690 96.9 65323 750 Non 702 660 94.0 30416 800 Non 646 597 92.4 15024 850 Non 623 572 91.8 26125 900 Non 587 515 87.7 15326 950 Non 413 224 54.2 120__________________________________________________________________________
Accordingly, the negative electrode is prevented from being overcharged thus not affecting the cycle life.
When the synthesizing temperature is as high as 950.degree. C., the crystalline structure of the positive electrode is separated into two phases, hexagonal system and rock salt structure. This decreases the initial charge and discharge efficiency to 50% more or less and impairs the discharge characteristics of the positive electrode.
It is thus clear that the present invention is eligible when the positive to electrode material is processed at a range of the synthesizing temperature from 750 to 900.degree. C. after the coprecipitated metal hydroxide is mixed with lithium hydroxide. In other words, the advantage of the present invention will be explicit when LiNixM.sub.1-x O.sub.2 (where M being at least a metallic element selected from a group of Al, V, Cr, Mn, Fe, and Co and 0.5.times.1.0) is used and its initial charge and discharge efficiency ranges from 75% to 95%.
Even if the synthesizing temperature for the positive electrode is 750 to 900.degree. C., the cycle life of the cells No. 23 to No. 25 in which the negative electrode is not added with such metal oxide as denoted by the present invention is found unfavorable. This may translate to the advantage of the present invention.
Although the metal oxide added to the negative electrode is used as a single material in Embodiments 1 and 2, it may be mixed with another material with equal success.
The primary material of the positive electrode is not limited to a particular type of lithium contained metal oxide, denoted by LiNixM1-xO.sub.2, where a part of Ni is substituted by Co but another metal oxide where the same is substituted by at least a metal element selected from a group of Mn, Cr, Fe, V, and Al may be employed as having been proved to exhibit the same effect in experiments.
Although the non-aqueous electrolyte described above is an organic electrolyte solution consisting mainly of LiPF6, ethylene carbonate and ethylmethyl carbonate mixed solvent solution, it may be any combination of lithium salt, such as lithium perchlorate, LiClO.sub.4, lithium tetrafluoroborate, LiBF.sub.4, lithium trifluoro-methansulfonate, LiCF.sub.3 SO.sub.3, or lithium hexafluoroarsenate, LiAsF.sub.6, and a binary or polytopic system organic solvent including circular ester such as propylene carbonate, circular ether such as tetrahydrofuran, chain ester such as dimetoxyethane, and chain ester such as methylpropionate.
It is understood that the organic electrolyte solution is not limited to the above listed items but may be polymer solid electrolyte or gel polymer electrolyte which comprises polymer solid electrolyte mixed with organic solvent provided as a plasticizer.
Although the secondary battery of the embodiment is of a cylindrical cell structure, the present invention does not depend on the cell shape. Any other shape, such as a rectangular or a sheet, of the cell may be used with equal success.
Similar to the present invention, the addition of metal oxide, such as FeO, FeO.sub.2, Fe.sub.2 O.sub.3, SnO, SnO.sub.2, MoO.sub.2, V.sub.2 O.sub.5, Bi.sub.2 Sn.sub.3 O.sub.9, WO.sub.2, WO.sub.3 O.sub.3, Nb.sub.2 O.sub.5, MoO.sub.3, to a carbon material for the negative electrode is depicted in Japanese Patent Laid-open Publication Hei7-192727. However, the metal oxide is selected from a compound which can absorb and desorb lithium in reverse relationship during the charge and discharge. Also, the metal oxide is intended for preventing the negative electrode potential to sharply rise in the end of the discharge and dissolve the copper foil as a core material of the electrode, hence decreasing the cycle life. The purpose and the effect of addition of the metal oxide are not equal to those of the present invention where metal oxide is carefully selected arid used for promoting electrochemical reduction in the initial charge to generate a quantity of metal in irreversible reaction.

Number	Name	Date	Kind
5576121	Yamada et al.	Nov 1996
5759719	Mao	Jun 1998

Number	Date	Country
56-54758	May 1981	JPX
7-192723	Jul 1995	JPX
8-213053	Aug 1996	JPX

Non-aqueous electrolyte secondary batteries

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Rougier et al., Solid State Ionics 90 (Sep. 1996) 83-90.
Caurant et al., "Synthesis by a soft chemistry route and characterization of LiNi(.times.)Co(1-.times.)O(2) (o<=.times.<=1) cathode materials," Solid State Ionics, vol. 91, pp. 45-54, Oct. 1996.