Method of producing positive active material for non-aqueous electrolyte secondary batteries

The present invention relates to a method of producing a positive active material for a non-aqueous electrolyte secondary battery generally referred to as a lithium-ion secondary battery and is intended to provide a method of producing a new positive active material which is superior in both discharge capacity and cycle life through improvement of a complex oxide of lithium and nickel from which a high discharge capacity can be expected. In order to attain this object, a Li-containing complex oxide is synthesized in this method by firing a mixture of a nickel oxide, in which an element M (M being at least an element selected from the group consisting of Co, Mn, Cr, Fe, and Mg) is dissolved as a solid solution, and lithium hydroxide or its hydrate. By using this as an active material in the positive electrode, it is possible to realize a non-aqueous electrolyte secondary battery which has a high capacity and a long cycle life, and which will not suffer from storage at a high temperature in a charged state.

TECHNICAL FIELD OF THE INVENTION
 This application is a U.S. National Phase application of the PCT
 International application PCT/JP97/04029.
 The present invention relates to a method of producing positive active
 materials for secondary batteries employing a non-aqueous electrolyte such
 as organic electrolyte or polymer solid electrolyte.
 BACKGROUND OF THE TECHNOLOGY
 With the progress of electronics technologies in recent years,
 miniaturization, lighter weight, and lower power dissipation of electronic
 equipment have become possible along with sophistication of functions. As
 a result, a variety of cordless or portable consumer electronics equipment
 has been developed and commercialized and the market size is rapidly
 expanding. Typical examples include camcorders, lap top computers, and
 portable telephones. Further miniaturization and increasingly lighter
 weight as well as longer operating time of these equipment are always
 demanded. In association with this trend, there is a strong demand for
 continuing improvement in energy density and cycle life of small
 rechargeable batteries to be used in these equipment as a built-in power
 source. As built-in batteries, starting with lead-acid type and
 nickel-cadmium type batteries which were initially developed and
 commercialized, nickel-hydrogen (nickel-metal hydride system) storage
 batteries and lithium-ion secondary batteries which have higher capacity
 and higher energy density than these battery systems have subsequently
 been developed and commercialized. Among them, the lithium- ion secondary
 battery, which has a high energy density both per unit weight and per unit
 volume, is a battery system primarily using a complex oxide of lithium and
 a transition metal element as the positive electrode, a graphite-based
 carbon as the negative electrode, and a non-aqueous electrolyte such as
 organic electrolyte or polymer solid electrolyte as the electrolyte, and
 is recently enjoying a rapid growth in production. In this battery, during
 charge, lithium ions will desorb from the lithium-containing complex oxide
 of the positive electrode and transfer into the electrolyte, and at the
 same time, lithium ions of equal electrochemical equivalent will be fed
 from the electrolyte into the carbon of the negative electrode.
 Conversely, during discharge, lithium ions are fed to the positive
 electrode desorbing from the negative electrode. As this cycle is
 repeated, lithium-ion secondary battery is sometimes called a
 rocking-chair battery.
 As the potential of the carbon negative electrode is close to the electrode
 potential of metallic lithium, a complex oxide of lithium which has a high
 electrode potential and a transition metal element is used as the positive
 electrode, for example, a complex oxide (LiCoO.sub.2 of lithium and
 cobalt, a complex oxide (LiNiO.sub.2) of lithium and nickel, and a complex
 oxide (LiM.sub.2 O.sub.4) of lithium and manganese. These complex oxides
 are often called as lithium cobaltate, lithium nickelate, and lithium
 manganate.
 In the currently commercialized batteries, LiCOO.sub.2 which has a high
 potential and a long cycle life is most generally used as the positive
 active material. Under this situation, a use of LiNiO.sub.2 with which a
 higher capacity than tat of LiCoO.sub.2 is expected is now being actively
 studied. The reason for a higher capacity is because, as the electrode
 potential of LiNiO.sub.2 is lower than that of LiCoO.sub.2, it becomes
 possible to cause more quantity of lithium to desorb during charge before
 decomposition voltage of an aqueous electrolyte such as organic
 electrolyte is reached. As a result, the quantity of charged electricity
 is expected to increase, and hence the discharge capacity is also expected
 to increase.
 Conversely, despite its large initial discharge capacity, LiNiO.sub.2
 suffers a problem of cycle deterioration by gradual decrease in the
 discharge capacity as charge and discharge are repeated.
 As a result of disassembling a battery cell of which the discharge capacity
 has decreased due to repeated charge-discharge cycles and X-ray
 diffraction analysis of the positive active material by the inventors of
 this invention, a change of crystal structure was observed after charge
 and discharge cycles, which was confirmed to be the cause for
 deterioration.
 Similar phenomenon has already been published by W. Li, J. N. Reimers and
 J. R. Dahn in Solid State Ionics, 67, 123 (1993). It is reported in this
 paper that, with the repetition of charge and discharge, lattice constant
 of LiNiO.sub.2 changes while itself changes from a hexagonal to a
 mono-clinic system crystal, and further from a second hexagonal to a third
 hexagonal system crystal as lithium is desorbed. This type of change in
 crystal phase lacks reversibility and as charge-discharge reactions are
 repeated, the sites where insertion and desorption of lithium are possible
 are gradually lost. This phenomenon is considered to be the cause of
 decrease in the discharge capacity.
 In contrast, with LiCoO.sub.2, such a change in the crystal phase as
 described above on LiNiO.sub.2 will not occur in the region of normal
 voltage (a voltage at which an organic electrolyte oxidizes and
 decomposes), suggesting that a decrease in the discharge capacity due to
 charge-discharge cycles is not likely to take place.
 For the purpose of addressing the problem of decrease in discharge capacity
 of LiNiO.sub.2 due to charge-discharge cycles, many proposals have
 heretofore been made to substitute a part of the element Ni with
 transition metal elements, primarily Co.
 As an example, in Japanese Laid-open Patent No. Sho 62-256,371, a method of
 synthesizing a Li-containing complex oxide of Co and Ni by firing at
 900.degree. C. a mixture of lithium carbonate (LiCO.sub.3, cobalt
 carbonate (CoCO.sub.3), and nickel carbonate (NiCO.sub.3).
 Methods of synthesizing complex oxides are disclosed in Japanese Laid-open
 Patent No. Sho 62-299,056 in which a mixture of carbonates, hydroxides,
 and oxides of Li, Co, and Ni is used as the raw material, and in U.S. Pat.
 No. 4,980,080 in which a mixture of lithium hydroxide (LiOH), Ni oxides
 and Co oxides is used as the raw material, and both heated at 600.degree.
 C. to 800.degree. C.
 In addition, in U.S. Pat. No. 5,264,201 and other patents, methods of
 synthesizing Li-containing complex oxides are disclosed in which lithium
 hydroxide (LiOH) is added to and mixed with a uniform mixture of oxides or
 hydroxides of Ni and oxides or hydroxides of Fe, Co, Cr, Ti, Mn, or V,
 followed by heat treatment at a temperature not lower than 600.degree. C.
 Furthermore, in Japanese Laid-open Patent No. Hei 1-294,364 and other
 patents, methods of synthesizing Li-containing complex oxides are
 disclosed in which carbonates, more precisely basic carbonates, of Ni and
 Co are co-precipitated from an aqueous solution containing Ni ions and Co
 ions, and a mixture of the co-precipitated carbonates and Li.sub.2
 CO.sub.3 is fired.
 There have also been proposed a method in which a Ni-containing oxide and a
 Co-containing oxide are mixed and fired after being further mixed with
 carbonates or oxides of Li, and a method in which a Li-containing complex
 oxide is synthesized by using oxides containing both Ni and Co such as
 NiCo.sub.2.
 These inventions represent efforts to relax a change in crystal phase by
 substituting a part of Ni with Co or other transition metal elements. The
 reason why there are many proposals to substitute part of Ni with Co from
 among different transition metals is because substitution is easy as the
 ion radius of Co is approximately equal to that of Ni and that the bond
 strength of Co with oxygen is stronger than that of Ni, which suggest that
 the crystal structure may become more stable and that the decrease in
 discharge capacity due to charge-discharge cycles may be improved.
 However, not all of the three battery characteristics, namely, discharge
 capacity, cycle life characteristic, and reliability as a battery, could
 be satisfied by the Li-containing complex oxides represented by a chemical
 formula Li.sub.x Ni.sub.y Co.sub.z O.sub.2
 (0.90.ltoreq..times..ltoreq.1.10, 0,7.ltoreq.y.ltoreq.0.95, y+z=1) as
 obtained by the methods of synthesis heretofore been disclosed or
 proposed. For example, even though a positive electrode produced by using
 a Li-containing complex oxide synthesized from a mixture of each
 respective carbonates, hydroxides, or oxides of Li, Co, and Ni did have a
 certain large initial discharge capacity, the discharge capacity decreased
 with charge-discharge cycles though not as markedly as with LiNiO.sub.2 of
 which no Ni had been substituted with Co and it was not satisfactory as a
 positive active material.
 A close study of the causes of such performance deterioration by the
 inventors of the present invention has revealed that, in the conventional
 methods of production, as the quantity (value of z) of Co substituting Ni
 increases (z.gtoreq.0.1), in reality, Ni and Co are not uniformly
 dispersed in the obtained complex oxide, partially leaving LiNiO.sub.2 and
 LiCoO.sub.2 as a mixture. It has been found that although a positive
 electrode made from these active materials shows a somewhat large
 discharge capacity, the part not properly substituted with Co had caused a
 change in the crystal phase with repeated charge and discharge, thus
 damaging the crystal structure and loosing reversibility, resulting in the
 decrease of discharge capacity.
 To address the non-uniform dispersion of Ni and Co in such active
 materials, a method of producing a positive active material using as the
 raw material carbonates co-precipitated from an aqueous solution of a
 mixture of Ni ions and Co ions is proposed in Japanese Laid-pen Patent No.
 Hei 1-294,364. With this method, it is true that a carbonate in which Ni
 and Co are uniformly dispersed can be obtained. However, when this
 co-precipitated carbonate is mixed with Li.sub.2 CO.sub.3 and fired, the
 co-precipitated carbonate is first decomposed generating a large volume of
 carbon dioxide (CO.sub.2) causing an increase in the CO.sub.2 partial
 pressure of the firing atmosphere. It further causes a decrease in the
 partial pressure of oxygen (O.sub.2) which is necessary for producing a
 complex oxide, and blocks the progress of the synthesizing reaction.
 Consequently, unless some forcible means of increasing O.sub.2 partial
 pressure is adopted, it has been difficult to obtain a perfect complex
 oxide. As a result, when using a Li-containing complex oxide synthesized
 by conventional methods of production, although the deterioration of
 discharge capacity due to charge-discharge cycles may be low, the initial
 discharge capacity is not necessarily high and is considered to be
 insufficient. Furthermore, because of the presence of CO.sub.2 during
 synthesis, Li.sub.2 CO.sub.3 remains mixed as an excess lithium salt after
 synthesis has been completed. When a cell which uses an active material
 containing this Li.sub.2 CO.sub.3 is stored at a high temperature of
 80.degree. C. in a charged state, Li2CO.sub.3 in the positive electrode
 decomposes and releases CO.sub.2, thus causing an increase in the internal
 cell pressure. Therefore, this production method has not been put into
 practice.
 In addition to these methods, there is a method to use NiCoO as a
 synthesizing material and lithium oxide (Li.sub.2 O) as a source of supply
 of Li. However, as the melting point of Li.sub.2 O is not lower than
 1,700.degree. C., the reactivity is low and a perfect Li-containing
 complex oxide could not be synthesized. Consequently, the initial
 discharge capacity of a cell using a positive electrode made of this
 complex oxide was not satisfactory.
 As a means for uniformly dispersing Ni and Co, apart from the use of
 co-precipitated carbonates of Ni and Co or NiCoO.sub.2, there exists a
 method of co-precipitating them as hydroxides, or a method of production
 by mixing a Ni hydroxide in which Co is dissolved as a solid solution and
 a lithium salt and then firing. In this production method, too, the
 hydroxide as the raw material decomposes first and a large volume of 120
 is generated. Since the H.sub.2 O partial pressure increases, it is
 difficult to maintain the O.sub.2 partial pressure of the firing
 atmosphere at a level appropriate to synthesize a perfect Li-containing
 complex oxide as in the case of use of a co-precipitated carbonate as the
 raw material, and hence the progress of synthesizing reaction is blocked.
 Consequently, a co-precipitated hydroxide of Ni and Co is first thermally
 decomposed at 190.degree. C. to 250.degree. C. to prepare (Ni.sub.y
 Co.sub.1-y).sub.3 O.sub.4 or Ni.sub.y Co.sub.1-y O. There is a published
 report of mixing LiOH to these oxides and firing at 450.degree. C. (Ref.
 J. Solid State Chemistry, 113, 182-192(1994)). In this method, presumably
 because the firing temperature during synthesis was relatively low,
 nonreacted lithium compounds (Li2CO3 and LiOH) were observed by X-ray
 diffraction analysis when the quantity of substitution of Ni with Co was
 not greater than 60 mol %. Consequently, it was not possible to synthesize
 a perfect Li-containing complex oxide even with this method.
 The present invention aims at addressing various problems encountered in
 using Li-containing complex oxides synthesized by the heretofore proposed
 or reported methods of production as positive active materials. That is,
 as a result of studying conventional methods of production in detail, the
 present invention provides a new method of production of positive active
 material for a high-reliability non-aqueous electrolyte secondary battery
 of which the discharge capacity is high and the decrease of the discharge
 capacity due to charge-discharge cycles is controlled by using in the
 positive electrode a Li-containing complex oxide obtained by using nickel
 oxides in which a specific element has been dissolved as a solid solution
 as a synthesizing material, and firing it using LiOH or a mixture of LiOH
 and its hydrates as the source of supply of Li.
 DISCLOSURE OF THE INVENTION
 The present invention provides a method of production of a positive active
 material for a non-aqueous electrolyte secondary battery in which a
 Li-containing complex oxide as represented by a chemical formula Li.sub.x
 Ni.sub.y M.sub.z O.sub.2 (M being at least an element selected from the
 group consisting of Co, Mn, Cr, Fe and Mg;
 0.90.ltoreq..times..ltoreq.1.10, 0,7.ltoreq.y.ltoreq.0.95, y+z=1) is
 synthesized by firing a mixture of nickel oxide containing M dissolved as
 a solid solution as represented by a chemical formula Ni.sub.v M.sub.w O
 (0.7.ltoreq.v.ltoreq.0.95, v+w=1) and LiOH or its hydrates. By employing
 this active material in the positive electrode of a non-aqueous
 electrolyte secondary battery, it has become possible to realize a
 high-reliability battery with which the discharge capacity during the
 initial cycles is large and the decrease of the discharge capacity due to
 charge-discharge cycles is lowered.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Exemplary embodiments of the present invention will be described in the
 following referring to drawings and tables.
 First Exemplary Embodiment
 To begin with, aqueous solutions of mixed sulfates were prepared with
 various atomic ratios of Ni and Co by using nickel sulfate (NiSO.sub.4)
 and cobalt sulfate (CoSO.sub.4). By pouring an aqueous solution of sodium
 hydroxide (NaOH) to this and stirring, a precipitate in which Ni hydroxide
 and Co hydroxide are uniformly dispersed is obtained by co-precipitation.
 After fully washing this precipitate with water, it was filtered and dried
 at 80.degree. C. to obtain Ni.sub.v Co.sub.w (OH).sub.2. By thermally
 decomposing this hydroxide at 700.degree. C. for 5 hours, an oxide,
 Ni.sub.v Co.sub.w O, of Ni and Co was prepared.
 Next, by adding a monohydrate (LiOH.multidot.H.sub.2 O) of lithium
 hydroxide to Ni.sub.v Co.sub.w O of varying compositions at a ratio of
 1.05 to 1 so that the constituent Li atom is slightly in excess of each
 pair of (Ni+Co) atoms of the latter in number and fully mixing, the
 mixture was heated at 700.degree. C. for 10 hours in an oxidizing
 atmosphere to obtain a Li-containing complex oxide (LixNi.sub.y Co.sub.z
 O.sub.2) of Ni and Co.
 The synthesized Li.sub.x Ni.sub.y Co.sub.z O.sub.2 is a relatively fragile
 aggregate, which is easily pulverized in a mortar.
 A positive paste is then prepared by mixing 100 parts by weight of Li.sub.x
 Ni.sub.y Co.sub.z O.sub.2 powder, 3 parts by weight of acetylene black,
 and 5 parts by weight of fluorocarbon resin binder, and adding to the
 mixture N-methylpyrolidinone and blending. The paste is then coated on
 both sides of an aluminum foil having a thickness of 0.02 mm and rolled so
 that a thickness of 0.13 mm can be obtained after drying. It is then cut
 to a width of 35 mm and length of 270 mm to obtain a positive electrode.
 Using this positive electrode, a cylindrical test cell with a diameter of
 13.8 mm and a total height of 50 mm was produced as illustrated by the
 vertical cross sectional view in FIG. 1.
 In FIG. 1, an electrode group 4 is configured by spirally winding a
 positive electrode 5 and a negative electrode 6 with a separator 7
 interposed, and is housed inside a cell case 1 made of stainless steel
 plate with insulating plates 8 and 9 provided on top and bottom surfaces
 of the electrode group 4. An aluminum lead ribbon 5a provided in advance
 on the positive electrode 5 is connected by welding to the bottom surface
 of a seal plate 2 which has a built-in explosion proof mechanism. A nickel
 lead ribbon 6a provided in advance on the negative electrode 6 is welded
 to the inner bottom surface of the cell case 1. After pouring and
 impregnating an organic electrolyte in the electrode group 4, the seal
 plate 2 which has been fit with a gasket 3 in advance is forced into the
 upper periphery of the cell case 1, after which the upper periphery of the
 cell case 1 is sealed by burring inward to obtain a completed cell.
 The negative electrode 6 is made by first preparing a paste by adding to a
 mixture of 100 parts by weight of graphite powder and 10 parts by weight
 of fluorocarbon resin binder an aqueous solution of carboxymethyl
 cellulose and blending. The paste is then coated on both sides of a copper
 foil having a thickness of 0.015 mm and rolled to an after-dry thickness
 of 0.2 mm, and cut to a width of 37 mm and a length of 280 mm.
 As the organic electrolyte, a solution prepared by dissolving lithium
 hexafluorophosphate (LiPF.sub.6) in an equal-volume mixed solvent of
 ethylene carbonate and diethyl carbonate to a concentration of 1 mol/l.
 In addition to the above test cell, a comparative test cell (P-1) based on
 a conventional method was also produced by using a positive active
 material based on a complex oxide of Li and Ni prepared by using nickel
 hydroxide, Ni(OH).sub.2, no part of the Ni of which has been substituted
 with Co, as the starting material, with other conditions the same, and
 using the same materials and components as the above test cell with the
 exception of use of this material as the positive active material. Here
 the discharge capacity of the negative electrode of the test cell was made
 larger so that the discharge capacity of the test cell can be regulated by
 the discharge capacity of the positive electrode.
 Each of the above test cells was subjected to a charge-discharge test under
 the following conditions. After being charged at a constant current of 120
 mA at 20.degree. C. to a voltage of 4.2 V, the cells were left for 1 hour,
 and then discharged at a constant current of 120 mA until an end voltage
 of 3.0 V was reached, and this process was repeated.
 The discharge capacity of the third cycle was defined as the initial
 capacity and the number of charge-discharge cycles until the discharge
 capacity decreased to 50% of the initial capacity was defined as the life.
 Also, specific capacity (mAh/g) of the positive active material was
 obtained by the loaded amount of Li-containing complex oxide used as the
 positive active material in each cell and the initial capacity. Table 1
 summarizes the test results.
 It can be seen from Table 1 that though the initial capacity of the
 comparative test cell No. P-I in which no part of Ni has been substituted
 with Co is high, the life is extremely short. This is considered to be due
 to a poor reversibility of charge-discharge reaction causing a change of
 the crystal phase of the positive active material and to a gradual loss of
 the sites for insertion and desorption of Li as described earlier.
 TABLE 1
 (a) Cell No.
 (b) Ratio of Ni substituted with Co(atomic %)
 (c) Composition of oxide materials for synthesis of Li-containing
 complex oxides
 (d) Initial capacity (mAh)
 (e) Specific capacity of active material (mAh/g)
 (f) Life (cycles)
 (a) (b) (c) (d) (e) (f)
 P-1 0 Ni.sub.1.0 Co.sub.0 O 774 172 120
 1-1 5 Ni.sub.0.95 CO.sub.0.05 O 788 175 400
 1-2 10 Ni.sub.0.9 CO.sub.0.1 O 797 177 420
 1-3 20 Ni.sub.0.8 CO.sub.0.2 O 792 176 456
 1-4 30 Ni.sub.0.7 CO.sub.0.3 O 779 173 440
 1-5 40 Ni.sub.0.6 CO.sub.0.4 O 743 165 210
 In contrast, with the exception of the cell No. 1-5, in cells the positive
 active material of which is based on a Li-containing complex oxide
 synthesized by using nickel-cobalt oxide with a part of the Ni substituted
 with Co, both the specific capacity and life showed such a superior value
 as 170 mAh/g and 400 cycles, respectively. This is considered to be due to
 an effective control of change of the crystal phase due to
 charge-discharge cycles by the substitution of 5 to 30 atomic % of the Ni
 with Co.
 However, when the ratio of Co substitution reaches 40 atomic % as in the
 cell No. 1-5, the specific capacity of the active material decreases and
 the life characteristic lowers. It is considered that when the percentage
 of Co substitution is increased like this, open circuit voltage of the
 cell increased and the quantity of charged electricity until charging to
 4.2 V decreased, thus lowering the initial capacity and hence the specific
 capacity of the active material. It is also likely that, in the
 Li-containing complex oxide obtained, as the Ni and Co are not in a state
 of a uniform solid solution, a mixture of LiNiO.sub.2 and LiCoO.sub.2 is
 locally generated, and in the part where Ni is not fully substituted with
 Co, a change of crystal phase has occurred, thus damaging the crystal
 structure and lowering the cycle characteristic.
 On the cell Nos. 1-1, 1-2, 1-3, and 14 which showed a high initial
 capacity, a small decrease in discharge capacity due to charge-discharge
 cycles, and a long life, the products produced during the process of
 synthesis of the positive active material were identified by elementary
 analysis. The co-precipitated hydroxide used as the star ting material is
 represented by a chemical formula Ni.sub..alpha. Co.sub..beta. (OH).sub.2
 (0.7.ltoreq.a.ltoreq.0.95, a+b=1). The nickel-cobalt oxide obtained by
 thermally decomposing the co-precipitated hydroxide is represented by a
 chemical formula Ni.sub.v Co.sub.w O (0.7.ltoreq.v.ltoreq.0.95, v+w=1). By
 using the above nickel-cobalt oxide as the material for synthesis and
 LiOH.multidot.H.sub.2 O as the source of supply of Li and firing at
 700.degree. C. for 10 hours in an oxidizing atmosphere, the material
 produced was found to be represented by a chemical formula Li.sub.x
 Ni.sub.y CO.sub.z O.sub.2 (0.90.ltoreq..times..ltoreq.1.10,
 0.7.ltoreq.y.ltoreq.0.95, y+z=1).
 By using the Li-containing complex oxide thus prepared as the positive
 active material, a non-aqueous electrolyte secondary battery which is
 superior in discharge capacity and in life characteristic can be obtained.
 Second Exemplary Embodiment
 It was confirmed in the first exemplary embodiment that Ni.sub.v Co.sub.w O
 which is appropriate as a material for synthesizing a Li-containing
 complex oxide can be obtained by thermally decomposing at 700.degree. C. a
 hydroxide Ni.sub.v Co.sub.w (OH).sub.2 obtained by co-precipitation from
 an aqueous solution of mixed sulfates of Ni and Co and in which Ni and Co
 are uniformly dispersed at a predetermined ratio.
 In this exemplary embodiment, the influence of the thermal decomposition
 temperature on Ni.sub.0.8 Co.sub.0.2 (OH).sub.2 was studied over a wide
 range of temperature points as 200, 250, 300, 500, 700, 900, 1000, and
 1100.degree. C. with the quantity of Co substituting Ni fixed at 20 atomic
 %.
 With other conditions kept exactly the same as in the first embodiment,
 Li-containing complex oxides were synthesized, and positive electrodes
 using them as the active material and test cells using these positive
 electrodes were produced. Further, these test cells were subjected to
 charge-discharge tests under the same conditions as in the first exemplary
 embodiment, and the results are shown in Table 2.
 As is clear from Table 2, the initial capacity of the cell No. 2-1 of which
 the thermal decomposition temperature of Ni.sub.0.8 Co.sub.0.2 (O).sub.2
 was 200.degree. C. was slightly low and the life was also short.
 TABLE 2
 Specific
 Thermal Decomp. Initial Capacity of
 Temperature Capacity Active Material Life
 Cell No. (.degree. C.) (mAh) (mAh/g) (cycles)
 2-1 200 752 167 370
 2-2 250 783 174 435
 2-3 300 792 176 448
 2-4 500 797 177 452
 1-3 700 792 176 456
 2-5 900 788 175 439
 2-6 1000 774 172 432
 2-7 1100 723 161 360
 It is conceivable that, at a heating temperature of 200.degree. C., a
 perfect Ni.sub.0.8 Co.sub.0.2 O was not produced with the raw material
 Ni.sub.0.8 Co.sub.0.2 (OH).sub.2 locally remaining, and, as a result, when
 synthesized by firing after being added with LiOH.multidot.H.sub.2 O,
 H.sub.2 O is generated from the residual hydroxide thereby lowering the
 O.sub.2 partial pressure of the firing atmosphere and blocking
 synthesizing reaction. Also, with the cell No. 2-7 in which the thermal
 decomposition temperature was as high as 1100.degree. C., the initial
 capacity, the specific capacity of the active material, and the life were
 all low. This may be attributed to the fact that a nickel-cobalt oxide
 such as Ni.sub.0.8 Co.sub.0.2 has a characteristic of making a sudden
 crystal growth when the temperature exceeds 1000.degree. C., and that the
 nickel-cobalt oxide which has been in a polycrystalline state until then
 turns to single crystals by crystal growth thus grain size becoming large.
 Therefore, it is further conceivable that, when synthesizing a
 Li-containing complex oxide from nickel-cobalt oxide as such, it is
 difficult for Li to be inserted into the center of a grain and the
 progress of synthesizing reaction is blocked.
 When the thermal decomposition temperature of Ni.sub.0.8 Co.sub.0.2
 (OH).sub.2 was in the range of 250.degree. C.-1000.degree. C., both
 initial capacity and life were confirmed to show superior values.
 As has been described earlier, it became clear that a Li-containing complex
 oxide prepared by using Ni.sub.v Co.sub.w (OH).sub.2 as the starting
 material, Ni.sub.v Co.sub.w O obtained by thermally decomposing the
 starting material at 250.degree. C.-1000.degree. C. for 5 hours as the
 material for synthesis, and LiOH.multidot.H.sub.2 O as the source of
 supply of Li, can exhibit a superior discharge capacity and cycle life
 characteristic as a positive active material for a non-aqueous electrolyte
 secondary battery. Here, it is to be noted that, although generally
 available monohydrates of lithium hydroxide were used as the source of
 supply of Li when synthesizing Li-containing complex oxides in the first
 and second exemplary embodiments, an equal effect can be obtained by the
 use of anhydrous LiOH which has been dehydrated in advance.
 Next, with the quantity of Ni substituted with Co fixed at the same 20
 atomic % as in Table 2, conventional examples of various positive active
 materials were produced and compared with the exemplary embodiments of the
 present invention by using co-precipitated Ni.sub.0.8 Co.sub.0.2 CO.sub.3
 as the starting material in addition to Ni.sub.0.8 Co.sub.0.2 (OH).sub.2,
 the same co-precipitated Ni.sub.0.8 Co.sub.0.2 (OH).sub.2 as used as the
 starting material, a mixture of NiO and Co.sub.3 O.sub.4 and
 co-precipitated Ni.sub.0.8 Co.sub.0.2 CO.sub.3 as the synthesizing
 materials in addition to Ni.sub.0.8 Co.sub.0.2 (OH).sub.2, and Li.sub.2
 CO.sub.3 or lithium oxide (Li.sub.2 O) as the source of supply of Li in
 addition to LiOH.multidot.H.sub.2 O. Here, the atomic ratio (Ni +Co) : Li
 for synthesis of Li-containing complex oxide was chosen to be 1.0:1.05,
 and the firing condition of synthesis was fixed at 700.degree. C. for 10
 hours in an oxidizing atmosphere.
 Methods of synthesis of each conventional example are as follows.
 Second Conventional Example
 A Li-containing complex oxide was synthesized by using the starting
 material of the present invention as it is, namely, co-precipitated
 Nio.sub.0.8 CO.sub.0.2 (OH).sub.2 as the synthesizing material, and
 LiOH.multidot.H.sub.2 O as the source of supply of Li.
 Third Conventional Example
 A Li-containing complex oxide was synthesized by using mixed powders of NiO
 and Co.sub.3 O.sub.4 at a ratio in mole of 8:2/3 as the synthesizing
 material and LiOH.multidot.H.sub.2 O as the source of supply of Li.
 Fourth Conventional Example
 An excess amount of an aqueous solution of sodium bicarbonate (NaHCO.sub.3)
 was added to an aqueous solution prepared by dissolving nickel chloride
 (NiCl.sub.2.multidot.H.sub.2 O) and cobalt chloride
 (COCl.sub.2.multidot.H.sub.2 O) at a ratio in mole of 8:2 in water
 saturated with Co to obtain a co-precipitated Ni.sub.0.8 Co.sub.0.2
 CO.sub.3. By using the co-precipitated carbonate as the synthesizing
 material after it was washed and dried and LiOH.multidot.H.sub.2 O as the
 source of supply of Li, a Li-containing complex oxide was synthesized.
 Fifth Conventional Example
 Using the co-precipitated Ni.sub.0.8 Co.sub.0.2 CO.sub.3 prepared in the
 fourth conventional example as the starting material and thermally
 decomposing it at 700.degree. C. for 5 hours, Ni.sub.0.8 Co.sub.0.2 O was
 obtained. Using it as the synthesizing material and LiOH.multidot.H.sub.2
 O as the source of supply of Li, a Li-containing complex oxide was
 synthesized.
 Sixth Conventional Example
 Using the Ni.sub.0.8 Co.sub.0.2 O prepared in the first exemplary
 embodiment as the synthesizing material and Li.sub.2 CO.sub.3 as the
 source of supply of Li, a Li-containing complex oxide was synthesized.
 Seventh Conventional Example
 Using the Ni.sub.0.8 Co.sub.0.2 O prepared in the first exemplary
 embodiment as the synthesizing material as in the sixth conventional
 example and Li.sub.2 O as the source of supply of Li, a Li-containing
 complex oxide was synthesized.
 Positive electrodes that use Li-containing complex oxides synthesized by
 the second through seventh conventional examples were made, and test cells
 were produced by using these positive electrodes under the same conditions
 as the first exemplary embodiment. These test cells were subjected to a
 charge-discharge test under the same conditions as the first exemplary
 embodiment, and the results obtained are shown in Table 3.
 From Table 3, it can be seen that though the cell No. P-3 of the third
 conventional example had a relatively superior initial capacity of 170
 mAh/g in terms of specific capacity of the active material, the life was
 extremely short when compared with the exemplary embodiments of the
 present invention. This may be attributed to the use of a mixture of NiO
 and Co.sub.3 O.sub.4 as the synthesizing material, which might have caused
 synthesis of a Li-containing complex oxide in which LiNiO.sub.2 and
 LiCO.sub.2 are locally mixed, a situation different from the case of use
 of Ni.sub.0.8 Coo.sub.0.2 O in an exemplary embodiment of the present
 invention. Therefore, in the regions where Ni has not been fully
 substituted by Co, changes of the crystal phase might have occurred and
 the crystal structure might have been damaged, thus lowering the discharge
 capacity and shortening the life.
 The cells No. P-2 and P4 in accordance with the second and fourth
 conventional examples had a relatively long life exceeding 300 cycles, but
 the initial capacity was slightly lower than in exemplary embodiments of
 the present invention. This is considered to be due to the use of
 co-precipitated hydroxides and carbonates as they were as the synthesizing
 material, which has caused generation of a large volume of H.sub.2 O or
 CO.sub.2 during firing for synthesis. As a result, the partial pressure of
 O.sub.2 of the firing atmosphere was lowered, thus failing in preparing a
 perfect complex oxide.
 The cell No. P-5 in accordance with the fifth conventional example is
 superior in both the initial capacity and the life characteristic.
 However, when stored in a separate test at a high temperature of
 80.degree. C. in a charged state, the explosion-proof mechanism of the
 seal plate started to operate, allowing a gas inside the cell to be
 released and the electrolyte to leak out.
 TABLE 3
 (2)
 (5)
 (1) (3) (4) (6) (7) (8) (9)
 (10)
 1-3 Present Ni.sub.0.8 Co.sub.0.2 (OH).sub.2 Ni.sub.0.8 Co.sub.0.2 O
 LiOH.H.sub.2 O 792 176 456
 Invention
 P-2 Conv. Ex. 2 -- Ni.sub.0.8 Co.sub.0.2 (OH).sub.2
 LiOH.H.sub.2 O 747 166 362
 P-3 Conv. Ex. 3 -- NiO + CO.sub.3 O.sub.4 LiOH.H.sub.2 O 765
 170 153
 P-4 Conv. Ex. 4 -- Ni.sub.0.8 Co.sub.0.2 CO.sub.3
 LiOH.H.sub.2 O 711 158 310
 P-5 Conv. Ex. 5 Ni.sub.0.8 Co.sub.0.2 CO.sub.3 Ni.sub.0.8 Co.sub.0.2 O
 LiOH.H.sub.2 O 770 171 402
 P-6 Conv. Ex. 6 Ni.sub.0.8 Co.sub.0.2 (OH).sub.2 Ni.sub.0.8 Co.sub.0.2 O
 Li.sub.2 CO.sub.3 765 170 360
 P-7 Conv. Ex. 7 Ni.sub.0.8 Co.sub.0.2 (OH).sub.2 Ni.sub.0.8 Co.sub.0.2 O
 Li.sub.2 O 743 165 365
 (1) Cell No.
 (2) Positive Active Material
 (3) Type
 (4) Starting Material
 (5) Synthesis
 (6) Material
 (7) Source of Li
 (8) Initial Capacity (mAh)
 (9) Specific Capacity of Active Material (mAh/g)
 (10) Life (cycles)
 This is due to a small amount of residual carbonate caused when using a
 co-precipitated Ni.sub.0.8 Co.sub.0.2 CO.sub.3 as the starting material
 and from which Ni.sub.0.8 Co.sub.0.2 O was prepared by thermal
 decomposition. When synthesizing a Li-containing complex oxide, this
 residual carbonate and LiOH react with each other producing a small amount
 of LiCO.sub.3, which remains mixed in the complex oxide. It is conceivable
 that this Li.sub.2 CO.sub.3 is eventually contained in the positive
 electrode, and when a cell is left at a high temperature in a charged
 state, Li.sub.2 CO.sub.3 decomposes within the cell causing an increase in
 the internal cell pressure due to CO.sub.2 gas generated. Therefore, it is
 improper to use a co-precipitated carbonate as the starting material to
 prepare a Ni.multidot.Co oxide as a synthesizing material as in the fifth
 conventional example.
 With the cell No. P-6 in accordance with the sixth conventional example in
 which Li.sub.2 CO.sub.3 was used as the source of supply of Li, it was
 superior in both the initial capacity and life characteristic as in No.
 P-5 in accordance with the fifth conventional example, but it suffered a
 problem of an increase in the internal cell pressure when stored at a high
 temperature in a charged state. The reason for this problem is also
 considered to be due to non-reacted excess of Li.sub.2 CO.sub.3 used as
 the source of supply of Li remaining in the positive active material as in
 the case of the cell No. P-5. Consequently, it is improper to use Li.sub.2
 CO.sub.3 as a source of supply of Li.
 In the seventh conventional example, Ni.sub.0.8 Co.sub.0.2 O was used as
 the synthesizing material as in the exemplary embodiments of the present
 invention. Presumably because Li.sub.2 O was used as a source of supply of
 Li, the life characteristics of the cell No. P-7 was superior but the
 initial capacity was not satisfactory. This may be attributed to too high
 a melting point exceeding 1700.degree. C. of Li.sub.2 O used as the source
 of supply of Li that poor reactivity was caused leading to a failure in
 synthesizing a perfect Li-containing complex oxide.
 Shown in FIG. 2 and FIG. 3 are respective powder X-ray diffraction patterns
 of Ni.sub.0.8 Co.sub.0.2 O used in the present invention in which Ni and
 Co are considered to be in a state of a solid solution, and of a mixture
 of NiO and CO.sub.3 O.sub.4 of the third conventional example, both used
 in synthesizing the Li-containing complex oxides shown in Table 3. As the
 source of beam for X-ray diffraction, K.alpha. beam from a Cu target was
 employed.
 In FIG. 2, only 3 peaks are observed, namely, at 2q=37.1.degree..+-.10,
 43.1.degree..+-.1.degree., and 62.6.degree..+-.1.degree.. This indicates
 that Ni and Co atoms are uniformly dispersed and are in a state of a solid
 solution.
 In contrast, the diffraction pattern in FIG. 3 shows many low peaks in
 addition to the 3 peaks. This suggests that the peaks of NiO and C.sub.3
 O.sub.4 are mixing.
 When a nickel-cobalt oxide is prepared by thermally decomposing a
 co-precipitated Ni.sub.v Co.sub.w (OH).sub.2, Ni and Co will not always
 make a uniform solid solution. When a material in which Ni oxides and Co
 oxides are non-uniformly mixed without making a complete solid solution is
 used as a synthesizing material, it is not possible to obtain a positive
 active material having a high discharge capacity and a superior life
 characteristic as described before. It is therefore desirable to select as
 an adequate Ni.sub.v Co.sub.w O material for synthesis the one in which Ni
 and Co are in a state of a solid solution at the atomic level and only 3
 peaks are observed by powder X-ray diffraction analysis, i.e., at
 2q=37.1.degree..+-.1.degree., 43.1.degree..+-.1.degree., and
 62.6.degree..+-.1.degree..
 In the foregoing exemplary embodiments, Co has been described in detail as
 an example of an element to substitute a part of Ni. It is to be noted
 that similar effect can be obtained by using at least one type of element
 selected from the group consisting of Mn, Cr, Fe, and Mg, in addition to
 Co.
 Also, in the foregoing exemplary embodiments, although the firing
 temperature for synthesis of a Li-containing complex oxide was fixed at
 700.degree. C, in practice there was observed no particular problem when
 the temperature was within the range 700.degree. C. to 900.degree. C.
 In obtaining a nickel oxide represented by a molecular formula Ni.sub.v
 Co.sub.w O in which an element M is dissolved as a solid solution, a
 nickel hydroxide represented by a molecular formula Ni.sub.a M.sub.b
 (OH).sub.2 in which an element M has been uniformly dispersed by
 co-precipitation or other method was thermally decomposed at 250.degree.
 C. -1000.degree. C. in the foregoing exemplary embodiments. It can also be
 obtained by mixing a monohydrate of lithium hydroxide to nickel hydroxide
 in which an element M has been uniformly dispersed by co- precipitation or
 other method, and then heating at temperatures not lower than 250.degree.
 C. and not higher than the melting point of lithium hydroxide, i.e.,
 445.degree. C. Here, the reason for heating the lithium hydroxide at a
 temperature not higher than the melting point of lithium hydroxide was to
 dehydrate nickel hydroxide containing an element M without allowing
 synthesizing reaction of the nickel hydroxide containing M and lithium
 hydroxide to start.
 Furthermore, although an organic electrolyte was described as an example of
 a non-aqueous electrolyte of a test cell in the exemplary embodiments, a
 solid polymer electrolyte or a solid electrolyte comprising an inorganic
 salt of lithium such as lithium iodide (LiI) is also effective.
 It goes without saying that the present invention is not limited to the
 negative material, construction of the electrode group, the cell
 configuration, etc., which have been described in the exemplary
 embodiments.
 INDUSTRIAL APPLICATION
 The present invention provides a method of producing positive active
 materials for non-aqueous electrolyte secondary battery through synthesis
 of a Li-containing complex oxide obtained by firing in an oxidizing
 atmosphere a mixture of nickel oxide in which an element M (M representing
 at least one element from the group consisting of Co, Mn, Cr, Fe and Mg)
 is dissolved as a solid solution and LiOH or its hydrates. With this
 method of production, decrease of the O.sub.2 partial pressure in the
 firing atmosphere during synthesis and mixed presence of Li.sub.2 CO.sub.3
 can be controlled. As a result, a non-aqueous electrolyte secondary
 battery employing this Li-containing complex oxide as the positive active
 material not only has a large initial capacity and a long cycle life
 characteristic but also its reliability can be increased in that the
 internal cell pressure does not increase due to storage at a high
 temperature in a charged state.