CATHODE ACTIVE MATERIAL AND PROCESS FOR PRODUCING THE SAME

Abstract

It is an object of the present invention to provide a cathode active material capable of reducing degradation in an operation voltage and capacity as compared conventionally when used for a lithium ion secondary battery, and a method for manufacturing the same. The cathode active material contains a composite oxide of lithium and a transition metal (s), wherein a reduction loss of TLC in the composite oxide is 20 to 60%. Also, the composite oxide has a particle diameter of 0.5 to 100 μm, and is preferably fluorinated. The method for manufacturing the cathode active material includes the step of fluorinating the cathode active material. The composite oxide has a particle diameter of 0.5 to 100 μm. The fluorinating step is to fluorinate the composite oxide in a reaction vessel under conditions where fluorine gas partial pressure is 1 to 200 kPa, a reaction time is 10 minutes to 10 days, and a reaction temperature is −10 to 200° C.

Description

TECHNICAL FIELD

The present invention relates to a cathode active material suitably used for, for example, a lithium ion secondary battery, and a method for manufacturing the same.

BACKGROUND ART

Since a lithium ion secondary battery can be reduced in size and weight, and has a high operation voltage and high energy density, the lithium ion secondary battery has widely spread as a portable power supply or the like for electronic devices. As a conventional cathode active material used for this lithium ion secondary battery, currently LiCoO₂ has been mainly used. However, in order to enlarge the battery capacity and reduce material cost, the use of LiNiO₂, LiMn₂O₄, a compound obtained by substituting Co, Ni, and Mn sites of a lithium metal oxide thereof with Al, Mg, Ti and B or the like, or a composite or the like of the above compounds is being studied.

However, the independent use of LiNiO₂ of the above cathode active materials for a positive electrode increases the battery capacity, and LiNiO₂ is easily gelated in kneading LiNiO₂ with a PVDF binder in mass production of positive plates to cause problems in kneading or coatability. This is because pH of LiNiO₂ exhibits alkalinity at 12 or more as compared with other lithium metal oxides, and the dissolved PVDF is gelated when kneading using an NMP solution in which PVDF is dissolved, whereby the PVDF binder cannot be well dispersed. Even if the coating is performed by using this gelated slurry, the condensate of an active material is adhered on the coated surface, thereby generating a streak. Accordingly, the material becomes poor as an electrode plate for batteries. Therefore, pH has been reduced using a material obtained by substituting a part of Ni with a metal such as Co and Mn to prevent gelatification. Also, LiCoO₂ and Li₂MnO₄ have been merely mixed with LiNiO₂ to prevent gelatification.

Furthermore, even when the electrode plate can be manufactured, alkali components which is a residue of the cathode active materials such as Li₂CO₃ and LiOH as impurities exist on the surface of the cathode active material of LiNiO₂. When the electrode plate can be manufactured as a battery, these alkali components are reacted with an electrolytic solution to generate CO₂ gas. This generation increases the inner pressure of the battery, and has a negative influence on the swelling and discharge/charge cycle life of the battery. Also, in some cases, the generation has a negative influence on safety.

In order to suppress the above generation of gas, a technique for acting fluorine gas on a cathode active material has been reported (for example, the following Patent Reference 1). Also, the present applicants have proposed the following Patent Reference 2 as a technique for acting fluorine gas on a cathode active material.

• Patent Reference 1: Japanese Published Unexamined Patent Application No. 2004-192896
• Patent Reference 2: Japanese Published Unexamined Patent Application No. 2005-11688

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

However, although the techniques described in Patent References 1 and 2 reduce the generation of gas, the excessive reaction in the techniques generates insulating LiF on the surface of the active material, and may cause degradation in an operation voltage or capacity.

Therefore, it is an object of the present invention to provide a cathode active material which is particularly the above positive electrode material containing Ni; and which prevents gelatification, reduces the alkali components as a cathode active material residue and reduces the degradation in the capacity caused by resistance components such as LiF as compared conventionally when used for a lithium ion secondary battery, or which has little drop in operation voltage, and a method for manufacturing the same.

Means for Solving the Problems and Effect of the Invention

The present invention provides a cathode active material containing a composite oxide of lithium and a transition metal(s), wherein a reduction loss of TLC caused by fluorination in the composite oxide is 20 to 60%. The TLC (Total Lithium Carbonate) means an amount obtained by converting the unreacted (residue) Li amount remaining on the surface of the cathode active material as LiCO₃. Also, the composite oxide has a particle diameter of 0.5 to 100 μm, and is preferably fluorinated.

Fluorine gas can be reacted with an alkali content (Li₂CO₃, LiOH) on the surface of the active material by the above composition to provide a cathode active material in which the alkali content is removed. Therefore, for example, in assembling the lithium ion secondary battery using this cathode active material, a reaction between the cathode active material and an electrolytic solution is suppressed, and the generation of CO₂ gas can be remarkably reduced as compared with a conventional one.

Also, since the above composite oxide is fluorinated in the cathode active material of the present invention, the composite oxide itself is also reacted with the fluorine gas as in the alkali content on the surface of the cathode active material to produce a substance having a crystal structure where a part of oxygen atoms are replaced with fluoride atoms such as Li(Ni/Co/Mn)O_2-xF_x. The structure has little distortion in electronic distribution, does not produce oxygen desorption easily, and has high thermal stability and little collapse of a cathode active material in discharge and charge. Therefore, the cathode active material having excellent cycle characteristics and rate characteristics can be provided.

Also, although, for example, independent LiNiO₂ has the worst coatability among cathode active materials, the removal of the alkali content in the reaction with the fluorine gas can realize excellent coating to LiNiO₂.

The present invention provides a method for manufacturing a cathode active material including the step of fluorinating the cathode active material containing a composite oxide of lithium and a transition metal(s), wherein the composite oxide has a particle diameter of 0.5 to 100 μm; the fluorinating step is to fluorinate the composite oxide in a reaction vessel; and the fluorinating step is executed under conditions where fluorine gas partial pressure is 1 to 200 kPa, a reaction time is 10 minutes to 10 days, and a reaction temperature is −10 to 200° C. It is preferable that the fluorinating step further includes the step of rotating the reaction vessel itself to stir the composite oxide in the reaction vessel. However, even if a batch type sealed reaction vessel is used, the same cathode active material can be obtained. A fluorine gas concentration, time and temperature of the fluorinating condition, when being smaller than the above condition, reduce the effect as the object. The fluorine gas concentration, time and temperature, when being larger than the condition, generates insulating LiF on the surface of the active material to degrade the operation voltage and the capacity.

Since the fluorine gas is not reacted with the alkali content contained in the composite oxide in the set fluorinating condition range and does not cause an excessive reaction, a method can be provided for manufacturing a cathode active material which generates an extremely small insulating substance degrading the operation voltage and the capacity such as LiF.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, one example of a cathode active material and a method for manufacturing the same according to an embodiment of the present invention will be described. The cathode active material according to the embodiment of the present invention contains a composite oxide of lithium and a transition metal(s) Examples of the transition metals include Co, Ni and Mn. Examples of the composite oxides include LiCoO₂, LiNiO₂ and LiMn₂O₄. Other examples include an olivine lithium composite oxide of an iron phosphate compound, which provides the same effect. When the composite oxide which is particularly preferable as the cathode active material of the present invention is a lithium nickel cobalt manganese composite oxide represented by a general formula; LiNi_xCo_yMn_zO₂, wherein x is greater than or equal to 0.4 and is less than or equal to 1.0, preferably x is greater than or equal to 0.7 and is less than or equal to 1.0, y is greater than or equal to 0 and is less than or equal to 0.2, preferably y is greater than or equal to 0 and is less than or equal to 0.16, z is greater than or equal to 0 and is less than or equal to 0.4, preferably z is greater than or equal to 0 and is less than or equal to 0.2, provided that x+y+z=1. Particularly preferably, the lithium nickel cobalt manganese composite oxide can exhibit a high synergistic effect with a fluoride atom, and further can enhance battery performance such as cycle characteristics and load characteristics.

The above composite oxide has a reduction loss of TLC of 20 to 60% (more preferably 30 to 40%) and a particle diameter of 0.5 to 100 μm (more preferably 10 to 50 μm). The composite oxide has a surface on which fluorinated alkali content exists. Since the reduction loss of TLC, when being smaller than the above condition, removes the alkali content insufficiently, problems with coatability due to generation of CO₂ gas and slurry gel occur. The reduction loss of TLC, when being larger than the above condition, causes excessive progress of the fluorination of the active material to result in degradation of an operation voltage and capacity due to the increase in resistance components such as LiF. The deviation of the particle diameter from the above condition range causes the streak of an electrode coated surface and the inner short circuit of the battery, and results in remarkable defectives in the battery production.

The method for manufacturing the cathode active material according to the embodiment of the present invention includes the step of fluorinating the cathode active material containing the composite oxide (the particle diameter: 0.5 to 100 μm) of lithium and a transition metal(s).

In the reaction vessel, the fluorination of the composite oxide of lithium and a transition metal(s) is executed under conditions where fluorine gas partial pressure is 1 to 200 kPa (more preferably 5 to 50 kPa), a reaction time is 10 min to 10 days (more preferably 1 hour to 1 day), and a reaction temperature is −10 to 200° C. (more preferably 0 to 100° C.). As the reaction vessel, a batch-type sealed reaction vessel is used. Herein, it is preferable that the fluorination further includes the step of rotating the reaction vessel itself to stir the composite oxide in the reaction vessel.

For the fluorination as the above object, the composite oxide previously formed in an electrode plate shape (a metal foil to which a composite oxide is applied) may be produced and fluorinated to manufacture the cathode active material and the positive plate.

According to the embodiment, the cathode active material and the method for manufacturing the same capable of reducing degradation in the operation voltage and capacity as compared conventionally when used for, for example, the lithium ion secondary battery, can be provided.

EXAMPLES

Hereinafter, the present invention will be specifically described with reference to examples.

Examples 1 to 68

10 g each of cathode active materials LiNiMnCoO₂ (Ni:Mn:Co=8:1:1) and LiNiO₂ was separately placed into a reaction vessel made of stainless steel, and the reaction vessel was then evacuated. Fluorine gases of predetermined partial pressures respectively shown in the following Tables 1 and 2 were then introduced into the reaction vessel, and the cathode active materials were reacted while stirring the cathode active material. Comparative examples 1 to 17 and comparative examples 18 to 34 to be respectively described later are also shown in the following Tables 1 and 2.

TABLE 1
		Fluorinating Condition	Alkali Content	Coatability
	Particle	F2 Partial	Treatment	Treatment	TLC Reduction	of Positive
LiNiMnCoO2	Diameter (μm)	Pressure (kPa)	Temperature (° C.)	Time	Loss (%)	Electrode
Example 1	0.5	1	−10	10	Minutes	20	∘
Example 2	0.5	1	−10	10	Hours	24	∘
Example 3	0.5	1	−10	10	Days	30	∘
Example 4	0.5	50	50	1	Day	37	∘
Example 5	0.5	100	20	60	Minutes	34	∘
Example 6	0.5	200	−10	10	Minutes	28	∘
Example 7	0.5	200	−10	10	Days	36	∘
Example 8	0.5	200	100	10	Days	41	∘
Example 9	0.5	200	200	10	Days	48	∘
Example 10	0.5	1	200	10	Minutes	27	∘
Example 11	0.5	1	200	10	Days	38	∘
Example 12	0.5	100	200	10	Minutes	39	∘
Example 13	0.5	200	200	10	Minutes	42	∘
Example 14	10	1	−10	10	Minutes	23	∘
Example 15	10	50	50	1	Day	41	∘
Example 16	10	100	20	60	Minutes	37	∘
Example 17	10	200	200	10	Days	50	∘
Example 18	20	1	−10	10	Minutes	24	∘
Example 19	20	50	50	1	Day	41	∘
Example 20	20	100	20	60	Minutes	40	∘
Example 21	20	200	200	10	Days	54	∘
Example 22	100	1	−10	10	Minutes	31	∘
Example 23	100	1	−10	10	Hours	34	∘
Example 24	100	1	−10	10	Days	38	∘
Example 25	100	50	50	1	Day	46	∘
Example 26	100	100	20	60	Minutes	43	∘
Example 27	100	200	−10	10	Minutes	34	∘
Example 28	100	200	−10	10	Days	41	∘
Example 29	100	200	100	10	Days	55	∘
Example 30	100	200	200	10	Days	60	∘
Example 31	100	1	200	10	Minutes	31	∘
Example 32	100	1	200	10	Days	42	∘
Example 33	100	100	200	10	Minutes	42	∘
Example 34	100	200	200	10	Minutes	52	∘
Comparative	0.5	—	—	—	0	x
example 1
Comparative	0.2	1	−10	10	Minutes	8	x
example 2
Comparative	0.2	200	200	10	Days	39	x
example 3
Comparative	120	1	−10	10	Minutes	36	x
example 4
Comparative	120	200	200	10	Days	65	x
example 5
Comparative	0.5	0.5	−10	10	Minutes	12	x
example 6
Comparative	100	0.5	200	10	Days	18	x
example 7
Comparative	0.5	250	−10	10	Minutes	62	∘
example 8
Comparative	100	250	200	10	Days	72	∘
example 9
Comparative	0.5	1	−20	10	Minutes	15	x
example 10
Comparative	100	200	−20	10	Days	18	x
example 11
Comparative	0.5	1	250	10	Minutes	61	∘
example 12
Comparative	100	200	250	10	Days	75	∘
example 13
Comparative	0.5	1	−10	1	Minute	11	x
example 14
Comparative	100	200	200	1	Minute	18	x
example 15
Comparative	0.5	1	−10	12	Days	62	∘
example 16
Comparative	100	200	200	12	Days	77	∘
example 17
		Rate Characteristic	Cycle characteristics
		Discharge	Discharge	Rate	Discharge	Discharge
		Capacity at	Capacity at	Charac-	Capacity at	Capacity at	Cycle
		the First	the Fourth	teristic	the First	the Tenth	charac-
		Cycle (0.2C)	Cycle (2C)	(2C/0.2C)	Cycle (0.2C)	Cycle (0.2C)	teristics
	LiNiMnCoO2	(mAh/g)	(mAh/g)	(%)	(mAh/g)	(mAh/g)	(%)
	Example 1	191	120	63	191	176	92
	Example 2	193	122	63	193	178	92
	Example 3	200	134	67	200	188	94
	Example 4	206	146	71	206	198	98
	Example 5	210	153	73	210	202	96
	Example 6	198	137	69	198	186	94
	Example 7	205	144	70	205	197	96
	Example 8	204	143	70	204	194	95
	Example 9	202	137	68	202	190	94
	Example 10	193	122	63	193	176	91
	Example 11	198	137	69	198	186	94
	Example 12	199	135	68	199	185	93
	Example 13	187	134	68	197	181	92
	Example 14	193	124	64	193	178	92
	Example 15	205	144	70	205	195	95
	Example 16	213	160	75	213	204	96
	Example 17	199	135	68	199	183	92
	Example 18	200	136	68	200	184	92
	Example 19	204	143	70	204	192	94
	Example 20	212	157	74	212	201	95
	Example 21	203	140	69	203	189	93
	Example 22	190	118	62	190	173	91
	Example 23	192	121	63	192	175	91
	Example 24	193	122	63	193	178	92
	Example 25	206	145	71	206	194	94
	Example 26	209	150	72	209	196	94
	Example 27	193	120	62	193	178	92
	Example 28	195	123	63	195	179	92
	Example 29	197	126	64	197	183	93
	Example 30	196	123	63	196	180	92
	Example 31	194	122	63	194	178	92
	Example 32	199	135	68	199	187	94
	Example 33	200	138	69	200	190	95
	Example 34	198	135	68	198	184	93
	Comparative	186	108	58	188	166	89
	example 1
	Comparative	170	88	52	170	141	83
	example 2
	Comparative	172	91	53	172	144	84
	example 3
	Comparative	181	100	55	181	154	85
	example 4
	Comparative	179	97	54	179	150	84
	example 5
	Comparative	186	108	58	186	166	89
	example 6
	Comparative	185	107	58	185	165	89
	example 7
	Comparative	180	99	55	180	153	85
	example 8
	Comparative	173	93	54	173	144	83
	example 9
	Comparative	186	108	58	166	166	89
	example 10
	Comparative	185	107	58	185	165	89
	example 11
	Comparative	175	95	54	175	145	83
	example 12
	Comparative	161	69	43	161	134	83
	example 13
	Comparative	186	108	58	186	166	89
	example 14
	Comparative	185	107	58	185	165	89
	example 15
	Comparative	178	96	54	178	150	84
	example 16
	Comparative	160	69	43	160	133	83
	example 17

TABLE 2
		Fluorinating Condition	Alkali Content	Coatability
	Particle	F2 Partial	Treatment	Treatment	TLC Reduction	of Positive
LiNiO2	Diameter (μm)	Pressure (kPa)	Temperature (° C.)	Time	Loss (%)	Electrode
Example 35	0.5	1	−10	10	Minutes	20	∘
Example 36	0.5	1	−10	10	Hours	26	∘
Example 37	0.5	1	−10	10	Days	29	∘
Example 38	0.5	50	50	1	Day	37	∘
Example 39	0.5	100	20	60	Minutes	32	∘
Example 40	0.5	200	−10	10	Minutes	24	∘
Example 41	0.5	200	−10	10	Days	31	∘
Example 42	0.5	200	100	10	Days	40	∘
Example 43	0.5	200	200	10	Days	45	∘
Example 44	0.5	1	200	10	Minutes	25	∘
Example 45	0.5	1	200	10	Days	41	∘
Example 46	0.5	100	200	10	Minutes	40	∘
Example 47	0.5	200	200	10	Minutes	42	∘
Example 48	10	1	−10	10	Minutes	22	∘
Example 49	10	50	50	1	Day	39	∘
Example 50	10	100	20	60	Minutes	40	∘
Example 51	10	200	200	10	Days	51	∘
Example 52	20	1	−10	10	Minutes	22	∘
Example 53	20	50	50	1	Day	41	∘
Example 54	20	100	20	60	Minutes	44	∘
Example 55	20	200	200	10	Days	57	∘
Example 56	100	1	−10	10	Minutes	23	∘
Example 57	100	1	−10	10	Hours	30	∘
Example 58	100	1	−10	10	Days	41	∘
Example 59	100	50	50	1	Day	45	∘
Example 60	100	100	20	60	Minutes	38	∘
Example 61	100	200	−10	10	Minutes	35	∘
Example 62	100	200	−10	10	Days	44	∘
Example 63	100	200	100	10	Days	57	∘
Example 64	100	200	200	10	Days	60	∘
Example 65	100	1	200	10	Minutes	36	∘
Example 66	100	1	200	10	Days	45	∘
Example 67	100	100	200	10	Minutes	42	∘
Example 68	100	200	200	10	Minutes	50	∘
Comparative	0.5	—	—	—	0	x
example 18
Comparative	0.2	1	−10	10	Minutes	10	x
example 19
Comparative	0.2	200	200	10	Days	42	x
example 20
Comparative	120	1	−10	10	Minutes	35	x
example 21
Comparative	120	200	200	10	Days	70	x
example 22
Comparative	0.5	0.5	−10	10	Minutes	13	x
example 23
Comparative	100	0.5	200	10	Days	18	x
example 24
Comparative	0.5	250	−10	10	Minutes	63	∘
example 25
Comparative	100	250	200	10	Days	74	∘
example 26
Comparative	0.5	1	−20	10	Minutes	6	x
example 27
Comparative	100	200	−20	10	Days	11	x
example 28
Comparative	0.5	1	250	10	Minutes	62	∘
example 29
Comparative	100	200	250	10	Days	72	∘
example 30
Comparative	0.5	1	−10	1	Minute	12	x
example 31
Comparative	100	200	200	1	Minute	18	x
example 32
Comparative	0.5	1	−10	12	Days	64	∘
example 33
Comparative	100	200	200	12	Days	71	∘
example 34
		Rate Characteristic	Cycle characteristics
		Discharge	Discharge	Rate	Discharge	Discharge
		Capacity at	Capacity at	Charac-	Capacity at	Capacity at	Cycle
		the First	the Fourth	teristic	the First	the Tenth	charac-
		Cycle (0.2C)	Cycle (2C)	(2C/0.2C)	Cycle (0.2C)	Cycle (0.2C)	teristics
	LiNiO2	(mAh/g)	(mAh/g)	(%)	(mAh/g)	(mAh/g)	(%)
	Example 35	207	141	68	207	165	80
	Example 36	210	147	70	210	170	81
	Example 37	215	155	72	215	181	84
	Example 38	222	162	73	222	189	85
	Example 39	227	168	74	227	195	86
	Example 40	211	150	71	211	171	81
	Example 41	220	158	72	220	187	85
	Example 42	216	153	71	216	179	83
	Example 43	213	149	70	213	175	82
	Example 44	211	150	71	211	171	81
	Example 45	224	164	73	224	190	85
	Example 46	228	171	75	228	194	85
	Example 47	218	157	72	218	181	83
	Example 48	207	143	69	207	166	80
	Example 49	220	158	72	220	185	84
	Example 50	230	175	76	230	198	88
	Example 51	212	146	69	212	172	81
	Example 52	210	145	69	210	168	80
	Example 53	219	158	72	219	182	83
	Example 54	226	167	74	226	192	85
	Example 55	209	144	69	209	169	81
	Example 56	210	145	69	210	168	80
	Example 57	213	149	70	213	173	81
	Example 58	214	150	70	214	175	82
	Example 59	218	158	72	219	182	83
	Example 60	225	164	73	225	191	85
	Example 61	215	153	71	215	178	83
	Example 62	222	160	72	222	189	85
	Example 63	216	153	71	216	179	83
	Example 64	211	148	70	211	171	81
	Example 65	212	148	70	212	172	81
	Example 66	218	157	72	218	181	83
	Example 67	220	161	73	220	185	84
	Example 68	217	156	72	217	180	83
	Comparative	198	123	62	198	150	76
	example 18
	Comparative	181	105	58	181	132	73
	example 19
	Comparative	173	92	53	173	119	69
	example 20
	Comparative	190	112	59	190	137	72
	example 21
	Comparative	183	99	54	183	124	68
	example 22
	Comparative	198	123	62	198	150	76
	example 23
	Comparative	199	123	62	199	151	76
	example 24
	Comparative	190	112	59	190	137	72
	example 25
	Comparative	170	88	52	170	114	67
	example 26
	Comparative	198	121	61	198	150	76
	example 27
	Comparative	199	123	62	199	151	76
	example 28
	Comparative	191	113	59	191	138	72
	example 29
	Comparative	171	91	53	171	116	68
	example 30
	Comparative	199	123	62	198	150	76
	example 31
	Comparative	199	125	63	199	151	76
	example 32
	Comparative	193	116	60	193	143	74
	example 33
	Comparative	174	94	54	174	118	68
	example 34

<TLC (Alkali Content) Measuring Method>

TLC was calculated by a measuring method shown below.

(1) 5 to 20 g of a cathode active material is precisely weighed (Ag), and is placed into a beaker. 50 g (precisely weighed) of water is added thereto (Bg), and the cathode active material and water are stirred (washed) for 5 minutes. The resultant mixture is then left to stand, and the supernatant liquid is filtered.

(2) 50 g (precisely weighed) of water is added to the cathode active material washed in process (1) again (Cg), and the resultant is stirred for 5 minutes, and filtered.

(3) Filtrates collected by the processes (1) and (2) are gently mixed, and about 60 g (precisely weighed) of the filtrates for neutralization titration is then divided (Dg).

(4) TLC is calculated according to the following formula from 0.1 mol/l HCl reference solution input amount (E ml).

F(g)=E/1000×0.1/2×73.89 (Li₂CO₃ molecular weight)

G(g)=A×D/(B+C)

TLC (%)=F(g)/G(g)×100

<Preparation of Secondary Battery>

To 95 wt % of fluorinated LiNiMnCoO₂ or LiNiO₂, 2 wt % of acetylene black and 3 wt % of PVDF were sufficiently mixed, and the mixture was molded to produce a positive electrode of 40 mm×40 mm. Also, a lithium metal was used for a negative electrode, and a mixed solvent (mixed volume: 3:7) of ethylene carbonate and diethyl carbonate containing LiPF₆ of 1 mol/dm³ was used for an electrolytic solution. A separator formed of a polyethylene film was provided between the positive electrode and negative electrode thus prepared to produce nonaqueous electrolytic solution secondary batteries (lithium ion secondary batteries) of examples 1 to 68.

Comparative Examples 1 to 34

In producing positive electrodes, LiNiMnCoO₂ of comparative example 1 and LiNiO₂ of comparative example 18 were not fluorinated. LiNiMnCoO₂ of comparative examples 2 to 17 and LiNiO₂ of comparative examples 19 to 34 were fluorinated in conditions shown in Tables 1 and 2 to produce a nonaqueous electrolytic solution secondary battery in the same manner as in examples 1 to 68.

<Secondary Battery Performance Evaluation Test>

For the lithium ion secondary batteries produced as described above, the the cut off potential upper and lower limits, and current density were, respectively, set to 4.3V, 2.0V and 1.0 mA/cm²at 25° C., and discharge and charge tests were performed. In the discharge and charge tests, the lithium ion secondary batteries were discharged and charged in 0.2 C at the first to third cycles, charged in 0.2 C at the fourth cycle, and then discharged in 2 C. The ratio of the discharge capacity at the fourth cycle to the discharge capacity at the first cycle was used as the index of the rate characteristic. The ratio of the discharge capacity at the tenth cycle to the first cycle was used as the index of the cycle characteristics. The results are shown in Tables 1 and 2.

Tables 1 and 2 show that use of the positive electrodes of the examples reduced the alkali content and enhanced the rate characteristic and the cycle characteristics. Also, improvement in the coatability of the cathode active material in producing the positive plate was confirmed.

Therefore, it was found that the cathode active material capable of reducing degradation in the operation voltage and capacity as compared conventionally when used for a lithium ion secondary battery can be provided.

The present invention can be varied in design in the range which does not depart from the Claims, and is not limited to the above embodiments and examples.

Claims

1. A cathode active material comprising a composite oxide of lithium and a transition metal(s), wherein a reduction loss of TLC caused by fluorination in the composite oxide is 20 to 60%.

2. The cathode active material according to claim 1, wherein the composite oxide has a particle diameter of 0.5 to 100 μm.

3. The cathode active material according to claim 1 or 2, wherein the composite oxide is a lithium nickel cobalt manganese composite oxide represented by a general formula; LiNixCoyMnzO2, wherein x is greater than or equal to 0.4 and is less than or equal to 1.0, y is greater than or equal to 0 and is less than or equal to 0.2, and z is greater than or equal to 0 and is less than or equal to 0.4, provided that x+y+z=1.

4. A method for manufacturing a cathode active material comprising the step of fluorinating the cathode active material comprising a composite oxide of lithium and a transition metal(s), wherein the composite oxide has a particle diameter of 0.5 to 100 μm;the fluorinating step is to fluorinate the composite oxide in a reaction vessel; andthe fluorinating step is executed under the conditions where fluorine gas partial pressure is 1 to 200 kPa, a reaction time is 10 minutes to 10 days, and a reaction temperature is −10 to 200° C.