JP2000323143A - Positive electrode active material and lithium secondary battery using the positive electrode active material - Google Patents

Abstract

(57) Abstract: A positive electrode active material capable of suppressing heat generation due to an internal short-circuit current that causes battery rupture or ignition without increasing manufacturing cost or lowering battery performance, and an L using the same.
Provide iNiO ₂ -based secondary batteries. Kind Code: A1 In a composite oxide mainly composed of Li and Ni having a layered crystal structure, a part of Ni is converted to a predetermined ratio of A.
l, B, Y, Ce, Ti, Sn, etc.
o, Mn and the like to improve the thermal stability of the positive electrode active material, the compressed density of 4.0g / cm ³
The positive electrode active material has a conductivity at 1 ° C. within a range of 1 × 10 ^{−2 to} 5 × 10 ⁻⁴ S / cm to achieve safety at the time of internal short circuit. In addition, a lithium secondary battery using a positive electrode made of this active material is provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a positive electrode active material for a non-aqueous secondary battery and a non-aqueous secondary battery using the positive electrode active material.

[0002]

2. Description of the Related Art In recent years, miniaturization, high performance, and portability of electronic devices have been progressing, and the demand for high energy density batteries used in these portable electronic devices has increased. As a battery system that satisfies these requirements, lithium secondary batteries are expected to be lightweight and satisfy high energy density requirements.

As a positive electrode active material of such a battery, a layered compound capable of inserting and removing lithium, such as LiCoO ₂ and LiNiO ₂ , or a compound having a tunnel structure, such as LiMn ₂ O ₄ , is used. And composite oxides mainly composed of transition metals. Among such lithium-containing composite oxides, LiCoO ₂ has already been put to practical use. However, since rare and expensive cobalt is used as a resource, LiNiO ₂ is more inexpensive and has a higher energy density. Material development is being vigorously conducted.

[0004] The practical application of LiNiO _2, LiNiO ₂
Has problems in cycle deterioration, which is considered to be caused by the crystal structure, and in securing safety and reliability in a charged state. Regarding cycle deterioration, a part of Ni was converted to Co or Mn.
It is possible to suppress cycle deterioration by substituting with a dissimilar metal such as Al, and to improve the thermal stability by replacing a part of Ni with another element such as Al for high temperature storage and heating tests. It has been proposed and improved for practical use.

However, at present, LiNiO ₂ -based cathode active material has not been put to practical use. The reason is that the mechanical misuse test for battery safety evaluation
In a nail penetration test and a crush test, it is difficult to ensure both safety and a high-performance battery even with the means for improving the thermal stability of the positive electrode active material that has been proposed so far. Here, the nail penetration test and the crush test are test methods defined in the guidelines of the Japan Storage Battery Association, "Safety Evaluation Standard Guidelines for Lithium Secondary Battery SBAG1101".
An internal short-circuit situation due to battery damage is assumed. In the case of such an internal short circuit, the protection circuit does not function such as the operation of the PTC element or the shutdown due to the melting of the separator, and rapid heat generation inside the battery due to the short circuit current causes the battery to burst or ignite. It has been pointed out that.

From such a viewpoint, various safety measures from the viewpoint of battery design have been proposed as safety measures against an internal short circuit. For example, Japanese Patent Laid-Open No.
In order to suppress the generation of Joule heat during an internal short circuit,
It is disclosed that graphite having a volume resistivity of 5 × 10 ⁻³ Ω · cm or less is used as a negative electrode active material. Japanese Patent Application Laid-Open No. 10-199574 discloses that a large current discharge at the time of short circuit is suppressed by forming a resistor layer having a higher resistance value than the conductive substrate on the surface of the conductive substrate. . Further, in JP-A-10-116633, a metal portion electrically connected to a positive electrode and a metal portion electrically connected to a negative electrode are opposed to each other via a separator, and conductive powder is applied to one of the metal portions. It is disclosed that, when an internal short circuit occurs due to deformation of the battery, a short circuit current flows between the metal parts, thereby suppressing heat generation.

[0007]

[SUMMARY OF THE INVENTION However, the safety measures on cell design for these internal short-circuit, with increase in the manufacturing cost, a reduction in the drop-load rate characteristics of the active material loading, LiNiO ₂ based cathode It was not optimal for realizing a low-cost, high-performance battery originally expected for active materials. That is, in a high-capacity nonaqueous secondary battery using a composite oxide powder of an element mainly composed of lithium and nickel having a layered crystal structure as a positive electrode active material, improvement of safety during an internal short circuit is an issue. There is a need for improvement as a positive electrode active material that does not rely on safety measures in battery design that sacrifices manufacturing cost and battery performance.

Accordingly, an object of the present invention is to provide an internal short-circuit test.
With improved safety in storage _Two -Based positive electrode active material powder
And high-performance lithium secondary batteries using the same
It is to be.

[0009]

Means for Solving the Problems As a result of intensive studies on the above-mentioned problems, the present inventors have found in nail penetration tests and crush tests on non-aqueous secondary batteries using LiNiO ₂ -based positive electrode active materials. In order to ensure safety in such an internal short circuit situation, a part of Ni is replaced with a different element to improve thermal stability at the time of charging, and at the same time, reduce electrical conductivity as a powder property. It was found that it was necessary to achieve this.

The layered crystal structure compound LiCoO ₂ which has been used as a positive electrode active material is a nonstoichiometric compound and is known to be a P-type semiconductor. Therefore, the oxide has a relatively high electron conductivity of about 10 ⁻² to 10 ⁻³ (S / cm). In the charged state, Li
It is reported that the electron conductivity becomes higher by about one digit than that in the uncharged state because Co in the lattice is oxidized by desorbing from the crystal structure and the average valence becomes +3 or more. .

However, the absolute value of the electric conductivity of these layered compounds is lower than that of the current collector or the negative electrode active material, and since they are in the form of powder, in order to secure conduction between particles, they are not used in practical batteries. A highly conductive substance represented by a carbon material such as graphite or acetylene black is added to the positive electrode mixture as an auxiliary agent. The amount of such a conductive additive is generally about 3% by weight, which is about 5% in terms of a volume fraction, which is not suitable for forming a conductive circuit in a positive electrode mixture layer by a percolation model. That is enough. Furthermore, when the positive electrode active material is a semiconductive material, it is known that a small amount of the conductive auxiliary agent present in the voids or at the contact points of the particles also establishes a conductive path that penetrates the inside of the particles, thereby significantly reducing the electric resistance. ing. Therefore, it is considered that a path through which electrons flow from the current collector substrate through the active material particles exists in the positive electrode mixture layer.

On the other hand, the electron conductivity of LiNiO ₂ as a P-type semiconductor is LiCoO 2 having the same layered crystal structure.
LiMn _{2 having an} order of magnitude higher than ₂ and having a spinel structure
It is known that it is about three orders of magnitude higher than O ₄ . The present inventors have found that when an active material such as LiNiO ₂ having an electrical conductivity significantly higher than that of LiCoO ₂ or LiMn ₂ O ₄ is used, a large current flows through the active material particles when a battery is short-circuited. Flows, and the active material itself is rapidly self-heated by the Joule heat of the through current to cause thermal decomposition.
_It was found that this was a safety problem of the _secondary cathode active material.

The problem of self-heating of the active material generated at the time of short circuit is further accelerated by the fact that the temperature characteristic of the resistance of the LiNiO ₂ compound is NTC (Negative Temperature Coefficient). Further, the rate of heat generation due to the reaction resistance of Li ions at the interface of the active material in the electrochemical reaction is also proportional to the magnitude of the short-circuit current, that is, the conductivity of the active material. Therefore, the internal heat generation factor of the battery related to the conductivity of the positive electrode active material is complex, and it can be said that the effect of improving safety by reducing the conductivity is very large.

Further, as seen in the prior art, it is also essential to improve the thermal stability during charging by replacing a part of Ni with another element in order to ensure the safety of the battery.
Since these substitution elements decrease the discharge capacity of the active material in proportion to the addition amount, it is preferable to select an element that exhibits an improvement effect with a small substitution amount.

The technical idea of the present invention to improve the thermal stability of the positive electrode active material as well as to improve the safety in the event of an internal short circuit by reducing the electrical conductivity is a novel idea which has never been seen before. . In the prior art, it has been desired to increase the conductivity of the positive electrode active material. For example, in JP-A-10-241691, the electron conductivity of the positive electrode active material is increased by the presence of Mg at the Li position in the LiMeO ₂ structure. The reason why the technical idea of reducing the conductivity was not made was that LiNiO
The load factor characteristics of the _two-system active material differ from those of the conventional material LiCoO.
Inferior to ₂ . Therefore, it is generally considered that a higher electron conductivity relating to the load factor characteristic is more advantageous.

However, the present inventors have proposed that LiNiO
We have obtained the knowledge that the problem of the load factor characteristics of the _two- system active material is that the electron conductivity in the electrochemical reaction is not rate-determining but is due to the interface reaction (activation polarization) between the active material surface and the organic electrolyte. I have. Therefore, it has been found that the active material can be improved by optimizing its pore distribution with a porous structure, that is, by increasing the solid-liquid interface area and securing a sufficient ion diffusion path. Was. .

On the other hand, Proc. 2nd Japan-France Joint
Seminar on Lithium Batteries, _p . 38, Li _x N
The dependence of the electronic conductivity on the charged state and the temperature has been reported for the i _0.8 Co _0.2 O ₂ composition, and the electronic conductivity at room temperature in the uncharged state at X = 1 is about 1 × 10 ⁻³ S / cm. And a positive electrode active material having an electron conductivity lower by about one digit or more than the previous report example, but does not disclose any technical concept regarding electron conductivity and safety in the event of an internal short circuit. From the viewpoint of ensuring the safety of the above, improvement of the thermal stability is not sufficient if only 20 mol% of the Ni site is replaced with Co, and does not satisfy the requirements of the present invention.

[0018] Namely, the present invention is, firstly, in the composite oxide composed mainly of lithium and nickel having a layered crystal structure, the general _{formula: Li a Ni 1-bc M} 1 b M 2 c O 2 (1 0.95 ≦ a ≦ 1.05, 0.01 ≦ b ≦ 0.10, 0.10 ≦ c ≦ 0.20 (where M ¹ is Al, B, Y, Ce, Ti, Sn, V,
One or more elements selected from Ta, Nb, W, and Mo;
² is at least one element selected from the group consisting of Co, Mn and Fe), and a powder having a compression density of 4.0 g / cm ³ when the powder is pressed. Conductivity of the body at 25 ° C .: σ is 5 × 10 ⁻² ≧ σ ≧ 5 × 1
A positive electrode active material characterized by being within a range of 0 ^-4 [S / cm], and secondly, a lithium secondary battery using the positive electrode active material according to the first aspect. is there.

[0019]

The positive electrode active material of the embodiment of the present invention, in the composite oxide composed mainly of lithium and nickel having a layered structure, wherein (1) ^{_{Li a Ni 1-bc M 1}} b M 2 c O _Two
In the general formula represented by, Li is an element necessary for taking charge transfer in the battery, and a needs to be in the range of 0.95 to 1.05. If a is less than 0.95, the discharge capacity is significantly reduced. On the other hand, if a exceeds 1.05, an excessive amount of Li remains and gelation of the paste is apt to occur during the production of the electrode, which causes an adverse effect. Further, Ni is necessary to form a layered crystal structure, and the amount of Li to be added is determined substantially on the basis of Ni.

M ¹ in the composition formula is Al, B, Y, Ce,
1 selected from Ti, Sn, V, Ta, Nb, W, and Mo
Element consisting of more than one species, to improve the thermal stability of the crystal in the charged state, and to reduce the conductivity, 0.01
Add in the range of ~ 0.10. If it is less than 0.01, the effects of improving the thermal stability and reducing the electric conductivity are insufficient, and if it exceeds 0.10, the capacity is significantly reduced. Al, B, and Y have a large effect of improving thermal stability, and have a wide solid solution range in LiNiO _{2 in} the order of Al>B> Y in proportion to the ionic radius difference with Ni (+3 valence). Al and B also have the effect of reducing the conductivity. Y
Has a high effect of improving thermal stability even when a small amount of b = 0.005 is added. In addition, an element group that exhibits n-type semiconductivity as an oxide and is stable at a valence of +4 or more: Ce, Ti, Sn, V, T
When a, Nb, W, and Mo are added, the thermal stability can be improved and the conductivity can be reduced. M ² in the composition formula is Co, Mn,
Fe is one or more elements selected from Fe, has the effect of suppressing the capacity deterioration over time of the charge and discharge cycle, 0.10 ~
Add in the range of 0.20. If it is less than 0.10, the effect is insufficient, and if it exceeds 0.20, the initial capacity decreases.

By adding these elements in combination, the thermal stability and the effect of reducing the electrical conductivity may be improved, and a composite oxide as a more preferred embodiment of the invention is represented by the following formula: Li _a Ni _{1 -xyc} N ¹ _x N ² _y M ² _c O ₂ 0.95 ≦ a ≦ 1.05, 0.01 ≦ x + y ≦ 0.10, 0.10 ≦ c ≦ 0.20 (where N ¹ is one or more elements selected from Al, B and Y, N ² is Ce, Ti, Sn, V, Ta, Nb, W, M
at least one element selected from the group consisting of o, M ² is Co, Mn, F
e) at least one element selected from (e). The effect on thermal stability and electrical conductivity of the added element greatly depends on the synthesis conditions, synthesis method, starting materials, etc. of the active material. It is.

The conductivity of the positive electrode active material of the present invention: σ is 5 ×
It is necessary to be in the range of 10 ^{−2 to} 5 × 10 ⁻⁴ (S / cm). More preferably, 1 × 10 ^{−2 to} 5 × 10 ⁻⁴ (S /
cm). The reason will be described in detail below.
Naturally, even if the compositional requirements of the present invention are satisfied, it is difficult to ensure safety at the time of an internal short-circuit if the composition is outside the conductivity range.

First, the method for measuring the electrical conductivity according to the present invention will be described. FIG. 1 shows a conductivity measuring apparatus. That is, the insulating plate 3 such as PVC and the metal plate 4 are attached to the fixed platen 2 of the hydraulic press machine 1, and the insulating pressure-resistant container 6 for storing the sample, which also serves as a die, is set via the current collector 5 such as Al foil. The movable plate 7 also has an insulating plate 8, a metal plate 9, and a mold 10
Is attached so that the electrical conductivity between the mold 10 and the current collector 5 can be measured by the resistance measuring device 11.

Approximately 5 g of the positive electrode active material powder was weighed as a sample S, stored in an insulated pressure-resistant container made of PVC (inner diameter: 17.6 mm) 6, and the sample powder S was compressed while applying a constant compression load (P). The thickness (t) and the DC resistance (r) of the powder were measured, and the conductivity:
σ (P) and compressed density: ρ (P) were calculated. Further, the compression load was changed in the range of 0.8 to 3.2 ton / cm ² to perform multi-point measurement. From the obtained relationship between σ (P) and ρ (P), the compression The error due to the change in the filling state was corrected by converting the conductivity to a density of 4.0 g / cm ³ . This conductivity measurement was performed in an environment at a room temperature of 25 ° C. and a relative humidity of 40 ± 10%.

In the case of particles of a semiconductor substance such as LiNiO ₂ , the relationship among the particle specific resistance (Rs), contact resistance (Rc), and surface adsorption resistance (Rs) is generally Rs>Rb> Rc,
It is known that the green compact resistance R is substantially equal to Rb.
Thus the measurement error is considered and less susceptible to such filling state (contact resistance) and moisture adsorption amount, in order to confirm the validity of the measurement conditions, using LiNi _0.8 Co _0.2 O ₂ powder, by humidifying operation 0.05wt% ～ 1wt% moisture adsorption
And the average particle size is 5 to 15 μm by the pulverizing operation.
As a result of confirming the effect on the conductivity by changing within the range of m,
In each case, σ = (9 ± 1) × 10 ⁻² (S / cm), which was a degree of reproduction error.

Table 1 shows the results of evaluation of three types of commercially available LiCoO ₂ that are currently in practical use by the above-described measurement method. The conductivity was in the range of 1 × 10 ⁻² to 1 × 10 ⁻⁴ (S / cm). Compared with LiNi _0.8 Co _0.2 O ₂ , the result was that the electric conductivity was lower by about 10 to 1000 times. In addition, although a correlation was recognized between the conductivity and the load factor characteristics, it was confirmed that the effect was not so large.

[0027]

[Table 1]

Next, the conductivity is 3 × 10 ^{−1 to} 5 × 10 ⁻³ (S
/ cm), and using a conductivity measuring device shown in FIG. 1, a direct current of 4 V was applied to the green compact to measure a current value, and the conductivity was obtained. This test was performed for the purpose of confirming the relationship between the conductivity of the active material and Joule heating due to the short-circuit current, assuming an internal short-circuit state of the battery. Conductivity is 3 × 10 ^-1 (S
/ cm) of the active material sample D, immediately after the application of 4 V, the current deviated from the measurement range and the conductivity became unmeasurable (conductivity> 1 × 10 ⁺⁰ S / cm). Only a few seconds after the application of 4 V, the inner wall of the insulating container made of PVC was burned by heat. This phenomenon is caused by a chain reaction in which the temperature of the active material rises due to Joule heating and the electrical resistance decreases (electrical conductivity rises), so that a larger current is conducted and heating is accelerated. It is determined that it has become.

On the other hand, the conductivity is 1 × 10 ^-2 (S / cm) and 5
× 10 ⁻³ (S / cm) of the active material samples A and B were similarly
When the conductivity was measured by applying a constant voltage, the value almost coincided with the conductivity obtained from the DC resistance measurement. Also 4V
When the constant voltage application state was continued for 30 seconds or more, no remarkable increase in conductivity was observed, and thermal runaway due to Joule heating as in Sample D did not occur.

From the above results, the conductivity of the active material is important as a heating factor at the time of an internal short circuit, and it is assumed that a thermal runaway state due to Joule heat occurs above a certain conductivity value. Table 2 shows the conductivity of the active material assuming that the green density is 4 g / cm ³ , the active material layer thickness is 50 μm, and the specific heat of the active material is 0.2 cal / g, and the Joule heat when 4 V is applied. 9 is a simulation result of calculating a relationship between self-heating rates according to the present invention.

[0031]

[Table 2]

The general conductivity of a LiNiO ₂ -based material is 1
× 10 ^-1 (S / cm), the thermal decomposition temperature by heating in the charged state is more than two hundred and several tens of degrees Celsius, and LiNiO ₂
Considering the chain reaction due to the increase in conductivity caused by the electric resistance (NTC) characteristic of the LiNiO ₂ -based active material, it is considered that the LiNiO ₂ -based active material reaches the thermal decomposition temperature almost instantaneously due to Joule heating.

Therefore, the range of the electrical conductivity, which is a requirement of the present invention, is at least 5 × 10 ^-2 (S / cm) even when safety measures are taken together with the conventional battery formulation. It is necessary, and more preferably, it is 1 × 10 ⁻² (S / cm) or less at which the conductivity is equivalent to that of LiCoO ₂ . Even in the case of thin-film / large-area electrodes where it is difficult to ensure safety in the event of short-circuit, or in the severe case of an overcharge test, if the conductivity is 5 × 10 ^-4 (S / cm), the problem is due to Joule heating. It is considered that there is no lower limit, and this is set as the lower limit of the conductivity.

A method for evaluating the electrochemical characteristics and thermal stability of the active material will be described. Method for evaluating electrochemical properties of positive electrode active material The positive electrode plate was prepared by using positive electrode active material, acetylene black and P
TFE (polytetrafluoroethylene) is added at a weight ratio of 8
After mixing in a mortar at a ratio of 7: 8: 5, the mixture was kneaded with a roll rolling mill and formed on a sheet. Using a metal Li for the negative electrode, a polypropylene film for the separator, and an electrolytic solution obtained by dissolving LiPF ₆ at 1 mol / L as an electrolyte in a solvent obtained by mixing ethylene carbonate and diethylene carbonate at a volume ratio of 1: 1. A test battery as shown in FIG. 2 was produced. In this test battery 12, the positive electrode 13 and the negative electrode 14
Is inserted into the stainless steel case 16 with the sealing plate 1
7 and a gasket 18 are provided. The charge and discharge test, after the current density was constant current charged to 4.2V at 0.53 mA / cm ^2, current density was constant voltage charging until 0.13 mA / cm ^2. Thereafter, a constant current discharge was performed to 2.7 V at 0.53 mA / cm ² to determine a discharge capacity per weight of the active material.
Evaluation of the load rate characteristic, the discharge current measured at 5 mA / cm ² using a test battery described above, expressed in retention to the discharge capacity at 0.53 mA / cm ² Discharge (%).

Evaluation Method of Thermal Stability of Positive Electrode Active Material In a glove box in an Ar atmosphere, a positive electrode plate was taken out of a test battery after being charged at 4.2 V, and about 20 mg of a sample containing an electrolytic solution was subjected to aluminum. Calorimetry after enclosing in a sealed container made of stainless steel and heating up to 300 ° C at 5 ° C / min
(TG-DTA) was performed. At this time, the temperature at which the sample began to rapidly generate heat was defined as a thermal runaway temperature, which was used as an index of the thermal stability of the active material. An active material having high thermal stability shifts the thermal runaway temperature to a higher temperature side. Hereinafter, the positive electrode active material of the present invention will be described in detail with reference to examples, but the scope of the present invention is not limited thereto.

[0036]

Example 1 A solution obtained by mixing nickel, cobalt, and aluminum nitrates at a molar ratio of Ni: Co: Al = 70: 20: 10 was continuously placed in a reaction vessel whose liquid temperature was controlled at 80 ° C. The mixture was charged, neutralized with a 48% by weight sodium hydroxide solution, and the pH was controlled at 10.0 ± 0.2 to obtain a precipitate of a coprecipitated hydroxide. This hydroxide is converted to Li / (Ni
+ Co + Al) = 1.05 and mixed with lithium hydroxide, and pressed at 1 ton / cm ² to obtain a molded body. This molded body is fired at 800 ° C. for 10 hours in an oxygen stream, crushed by a mortar-type crusher, and subjected to Li _1.04 Ni _0.70 Co _0.20 Al _0.10 O.
Powders of the layered crystal structure compound having _two compositions were obtained.

This powder is irregular secondary particles having an average particle size of about 8 μm, and has a specific surface area of 2.3 m ² / BET method.
g, the conductivity was 2.2 × 10 ^-2 (S / cm). When this powder was used as an active material, the electrochemical characteristics were as follows: discharge capacity was 150 mAh / g, initial efficiency was 86%, and load factor characteristic was 56%.
%Met. The thermal stability of the active material is determined by the thermal runaway temperature: 27
4 ° C. Note that the Li / (Ni + Co + Al) of the molded body and the Li-containing composition of the layered crystal structure compound after firing do not match,
It is considered that Li is volatilized during firing.

[0038]

Example 2 The hydroxide used in Example 1 was Li / (Ni
+ Co + Al) = 1.05 and mixed with lithium hydroxide, and pressed at 1 ton / cm ² to obtain a molded body. The molded body is fired in an oxygen stream at 700 ° C. for 10 hours, crushed by a mortar-type crusher, and then Li _1.04 Ni _0.70 Co _0.20 Al _0.10 O
Powders of the layered crystal structure compound having _two compositions were obtained. This calcined product was suspended in a 1% by weight lithium nitrate solution so as to have a solid concentration of 50% by weight, and wet-pulverized by a wet bead mill until the average particle diameter became 1 μm or less to obtain a dispersion slurry. . This slurry was spray-dried and granulated into a sphere. After this in an oxygen stream for 2 hours calcination at 800 ° C, and then deagglomeration by mortar type pulverizer _{Li 1.04 Ni 0.70 Co 0. 20 Al} 0.10 O
_Two compositions of layered crystal compound powder were obtained.

This powder was spherical secondary particles having an average particle diameter of about 12 μm, and many pores penetrating from the particle surface to the inside of the particle were observed. Specific surface area by BET method is 2.1
m ² / g and conductivity was 3.2 × 10 ^-2 (S / cm). When this powder was used as an active material, the electrochemical characteristics were as follows: discharge capacity: 173 mAh / g; initial efficiency: 91%;
9%. The thermal stability of the active material is determined by the thermal runaway temperature: 2
57 ° C.

[0040]

Comparative Examples 1-3 Li / (Ni + Co + Al) of molded body
A layered crystal structure compound was prepared in the same manner as in Example 2 except that the composition ratio was changed to 0.95, 1.00, and 1.10.
evaluated. Table 3 shows the results.

[0041]

Comparative Examples 4 and 5 Except that the ratio of Ni: Co: Al of the coprecipitated hydroxide was 80: 20: 0 and the composition ratio of Li / (Ni + Co + Al) of the compact was changed to 1.01, 1.10. A layered crystal structure compound was prepared and evaluated in the same manner as in Example 2. Table 3 shows the results.

[0042]

[Table 3]

As can be seen from the results of Examples 1 and 2 and Comparative Examples 1 to 5, the stoichiometric composition ratio (L
With respect to iMO ₂ ), the higher the Li content, the lower the electrical conductivity, but the thermal stability becomes unstable. Also, it is clear that the thermal stability is remarkably deteriorated when there is no substitution element such as Al that improves the stability. Conversely, when the Li content is small, although the thermal stability is excellent, an increase in conductivity and a remarkable decrease in discharge capacity are observed. Therefore, the desirable range of the Li / M composition ratio is in the range of 0.95 to 1.05.

[0044]

[Comparative Examples 6 to 9] When the coprecipitated hydroxide Ni: Co: Al was 79:20:
1, 77: 20: 3, 74: 20: 6, 68:20:12, and the L
Except that the i / (Ni + Co + Al) composition ratio is 1.03,
A layered crystal structure compound was prepared and evaluated in the same manner as in Example 2. Table 4 shows the evaluation results. As shown in Table 4, although the addition of Al can reduce the conductivity and improve the thermal stability, the effect of the decrease in the discharge capacity is significant.

[0045]

[Table 4]

[0046]

Example 3 A solution in which nickel, cobalt, and aluminum nitrates were mixed at a molar ratio of Ni: Co: Al = 77: 20: 3 was continuously placed in a reaction vessel whose liquid temperature was controlled at 80 ° C. And neutralized with a 48% by weight sodium hydroxide solution to control the pH to 10.0 ± 0.2 to obtain a precipitate of coprecipitated hydroxide. This hydroxide is converted to Li / (Ni
+ Co + Al) = 1.03 and mixed with lithium hydroxide, and pressed at 1 ton / cm ² to obtain a molded article. This compact was fired in an oxygen stream at 700 ° C. for 10 hours, and crushed with a mortar-type crusher to obtain a powder of a layered crystal structure compound.

This calcined product is mixed with Nb / Li_1.03Ni_0.77Co_0.20
Al_0.03O_Two= Nb equivalent to an amount of 0.01_TwoO_FiveThe solid
1% by weight nitric acid so that the partial concentration becomes 50% by weight.
Suspend in lithium solution and average particle size with wet bead mill
To obtain a dispersed slurry by wet grinding until the particle size becomes 1 μm or less.
Was. This slurry was spray-dried and granulated into a sphere. this
After firing at 800 ° C for 2 hours in an oxygen stream,
Disintegrate Li_1.02Ni _0.76Co_0.20Al_0.03Nb_0.01O
_TwoA layered crystal compound powder having the composition was obtained. This powder is average grain
The particles were spherical secondary particles having a diameter of about 9 μm. According to the BET method
Specific surface area is 3.3m^Two/ g, conductivity 2.0 × 10^-2(S / c
m). Electricity when this powder is used as an active material
The chemical characteristics are as follows: discharge capacity is 189 mAh / g, initial efficiency is 92
%, And the load factor characteristics were 57%. Thermal stability of active material
The property was a thermal runaway temperature: 257 ° C.

[0048]

Embodiments 4 to 10 Instead of Nb ₂ O ₅ , WO ₃ , Mo
O ₃ , Ta ₂ O ₅ , V ₂ O ₅ , SnO ₄ , TiO ₄ , CeO ₂
A layered crystal structure compound was prepared and evaluated in the same manner as in Example 3 except that was used. Table 5 shows the results. The feature of the element group added in Examples 3 to 10 is that the oxide is an N-type semiconductor, and the oxide is stable at +4 or more valence. Compared with Comparative Example 7, unlike the case of adding Al alone, a small amount of 1 at% has the effect of improving the thermal stability and has the effect of reducing the conductivity to about 1/2 to 1/10. The effect of improving thermal stability is that the stable valence of the oxide is +
Significant when pentavalent. It is not clear why such an effect occurs, but according to ESCA analysis, it is considered that the binding energy of oxygen has shifted and has some influence on the oxygen site of the layered crystal structure. Also,
In EPMA, the distribution state of these elements was observed, but no segregation state was observed.

[0049]

[Table 5]

[0050]

Comparative Examples 10 to 16 Instead of Nb ₂ O ₅ , Co ₃ O ₄ ,
Mn ₂ O ₃ , Fe ₂ O ₃ , BaO, CaO, MgO, Al ₂
A layered crystal structure compound was prepared and evaluated in the same manner as in Example 3 except that O ₃ was used. Table 6 shows the results.
These elements have low conductivity as an oxide alone, but cannot be expected to have the effect of lowering the conductivity even if added to the layered crystal structure compound, and do not improve the thermal stability.

[0051]

[Table 6]

[0052]

[Example 11, Comparative Examples 17 and 18] in place of Nb ₂ O _5, except for using _{B 2 O 3, P 2 O} 5, Si 2 O 3 , the same procedure as in Example 3, a layered crystal structure Compounds were prepared and evaluated. Table 7 shows the results.

[0053]

[Table 7]

[0054]

Comparative Example 19 The hydroxide used in Example 3 and Nb / Li
Nb ₂ O ₅ corresponding to an amount of _1.03 Ni _0.77 Co _0.20 Al _0.03 O ₂ = 0.01 was converted to Li / (Ni + Co + Al + Nb) = 1.
03 and mixed with lithium hydroxide, 1 ton / cm
Pressing at ² gave a compact. The compact was fired at 700 ° C. for 10 hours in an oxygen stream, and crushed with a mortar-type crusher to obtain a powder of a layered crystal structure compound. This calcined product was suspended in a 1% by weight lithium nitrate solution so that the solid content concentration was 50% by weight, and the average particle size was 1% by a wet bead mill.
The dispersion slurry was obtained by wet pulverization until the particle diameter became not more than μm.
This slurry was spray-dried and granulated into a sphere. This was fired in an oxygen stream at 800 ° C. for 2 hours, and then pulverized with a mortar type pulverizer to obtain a layered crystal compound powder having a composition of Li _1.03 Ni _0.77 Co _0.20 Al _0.03 Nb _0.01 O ₂ . This powder was spherical secondary particles having an average particle diameter of about 10 μm. The specific surface area by the BET method is 2.6 m ² / g, and the conductivity is 5.8 × 10 ^-2 (S / cm).
Met. When this powder was used as an active material, the electrochemical characteristics were as follows: discharge capacity was 191 mAh / g, and initial efficiency was 93%.
Met. The thermal stability of the active material is determined by the thermal runaway temperature: 227.
゜ C. As compared with Example 3, the effect of reducing the conductivity and the effect of improving the thermal stability were inferior.

[0055]

Comparative Example 20 A solution obtained by mixing nickel, cobalt, and aluminum nitrates in a molar ratio of Ni: Co: Al = 77: 20: 3 was continuously placed in a reaction vessel in which the liquid temperature was controlled at 80 ° C. And neutralized with a 48% by weight sodium hydroxide solution to control the pH at 10.0 ± 0.2 to obtain a precipitate of coprecipitated hydroxide. This hydroxide is converted to Li / (Ni
+ Co + Al) = 1.03 and mixed with lithium hydroxide, and pressed at 1 ton / cm ² to obtain a molded article. The compact was fired at 700 ° C. for 10 hours in an oxygen stream, and crushed with a mortar-type crusher to obtain a powder of a layered crystal structure compound.

This calcined product and Nb / Li _1.03 Ni _0.77 Co _0.20
Nb ₂ O ₅ corresponding to an amount of Al _0.03 O ₂ = 0.01;
/ Li _1.03 Ni _0.77 Co _0.20 Al _0.03 O ₂ = 0.01 parts of lithium nitrate are mixed and mixed with an Ishikawa-type Raikai machine for 30 minutes, and the powder is fired at 800 ° C. for 2 hours in an oxygen stream. Li _1.02 Ni _0.77 Co _0.20 Al _0.03 Nb _0.01 O
_Two compositions of layered crystal compound powder were obtained. This powder was irregular secondary particles having an average particle size of about 5 μm. The specific surface area by the BET method is 4.2 m ² / g, and the conductivity is 9.8 × 10 ^-2 (S
/ cm). When this powder was used as an active material, the electrochemical characteristics were as follows: discharge capacity was 179 mAh / g, and initial efficiency was 8
At 4%, the thermal stability of the active material is the thermal runaway temperature: 232 ° C
Met.

[0057]

Example 12 A solution obtained by mixing nickel, cobalt, and aluminum nitrates with boric acid at a molar ratio of Ni: Co: Al: B = 76: 20: 3: 1 was controlled at a temperature of 80 ° C. The reaction mixture was continuously charged into the reaction vessel, neutralized with a 48% by weight sodium hydroxide solution, and the pH was controlled at 10.0 ± 0.2 to obtain a precipitate of a coprecipitated hydroxide. This hydroxide is represented by L
It was mixed with lithium hydroxide so that i / (Ni + Co + Al + B) = 1.03, and pressed at 1 ton / cm ² to obtain a molded body. The compact was fired at 700 ° C. for 10 hours in an oxygen stream, and crushed with a mortar-type crusher to obtain a powder of a layered crystal structure compound.

This calcined product is suspended in a 1% by weight lithium nitrate solution so as to have a solid concentration of 50% by weight, and dispersed by wet grinding using a wet bead mill until the average particle diameter becomes 1 μm or less. A slurry was obtained. This slurry was spray-dried and granulated into a sphere. 800 in an oxygen stream
After firing at ゜ C for 2 hours, pulverize with a mortar type crusher to obtain Li _1.02 Ni
A layered crystal compound powder having a composition of _0.76 Co _0.20 Al _0.03 B _0.01 O ₂ was obtained. This powder was spherical secondary particles having an average particle diameter of about 13 μm. The specific surface area by the BET method is 2.6.
m ² / g, and conductivity was 6.9 × 10 ⁻³ (S / cm). When this powder was used as an active material, the electrochemical characteristics were as follows: discharge capacity: 180 mAh / g; initial efficiency: 90%;
5%. The thermal stability of the active material is determined by the thermal runaway temperature: 2
57 ゜ C.

[0059]

Comparative Example 21 A mixture of hydroxide and lithium hydroxide was prepared by mixing Li
A layered crystal structure compound was prepared and evaluated in the same manner as in Example 12, except that /(Ni+Co+Al+B)=0.95. The composition of this powder is Li _0.96 Ni _0.76 Co _0.20 Al _0.03 B _0.01
O ₂ was spherical secondary particles having an average particle diameter of about 11 μm. The specific surface area by the BET method was 2.2 m ² / g, and the conductivity was 7.2 × 10 ^-2 (S / cm). When this powder was used as an active material, the electrochemical characteristics were as follows: discharge capacity was 192 mAh /
g, the initial efficiency was 90%. The thermal stability of the active material was thermal runaway temperature: 245 ° C.

[0060]

Example 13 The hydroxide used in Example 12 was replaced with Li /
It was mixed with lithium hydroxide so that (Ni + Co + Al + B) = 0.95 and pressed at 1 ton / cm ² to obtain a molded body. The compact was fired at 700 ° C. for 10 hours in an oxygen stream, and crushed with a mortar-type crusher to obtain a powder of a layered crystal structure compound. This calcined product is suspended in a 3% by weight lithium nitrate solution so that the solid content concentration becomes 50% by weight,
The dispersion slurry was obtained by wet grinding with a wet bead mill until the average particle diameter became 1 μm or less. This slurry was spray-dried and granulated into a sphere. This was fired at 800 ° C. for 2 hours in an oxygen stream, and then crushed with a mortar crusher to obtain Li _1.03 Ni _0.76
A layered crystal compound powder having a composition of Co _0.20 Al _0.03 B _0.01 O ₂ was obtained. This powder was spherical secondary particles having an average particle diameter of about 12 μm. The specific surface area by the BET method is 3.0 m ² / g,
The conductivity was 3.8 × 10 ^-2 (S / cm). When this powder was used as an active material, the electrochemical characteristics were as follows:
90 mAh / g, the initial efficiency was 89%. The thermal stability of the active material was thermal runaway temperature: 247 ° C. Example 12
13 and the results of Comparative Example 21 indicate that the decrease in conductivity due to the addition of boron is affected by the Li content as in the case of the addition of Al.

[0061]

Example 14 The hydroxide used in Example 12 was replaced with Li /
It was mixed with lithium hydroxide so that (Ni + Co + Al + B) = 0.95 and pressed at 1 ton / cm ² to obtain a molded body. The compact was fired at 700 ° C. for 10 hours in an oxygen stream, and crushed with a mortar-type crusher to obtain a powder of a layered crystal structure compound. This calcined product and Nb / Li _1.03 Ni _0.76 Co _0.20 Al _0.
Nb ₂ O ₅ equivalent to the amount of ₀₃ B _0.01 O ₂ = 0.01 was suspended in a 3% by weight lithium nitrate solution so as to have a solid concentration of 50% by weight, and the average particles were dispersed by a wet bead mill. Wet pulverization was performed until the diameter became 1 μm or less to obtain a dispersion slurry. This slurry was spray-dried and granulated into a sphere. This is fired at 800 ° C. for 2 hours in an oxygen stream, and then crushed by a mortar type crusher to obtain Li _1.03 Ni _0.76 Co _0.20 Al _0.03 B _0.01 O
_Two compositions of layered crystal compound powder were obtained. This powder was spherical secondary particles having an average particle diameter of about 10 μm. The specific surface area by the BET method is 4.6 m ² / g, and the conductivity is 6.8 × 10 ⁻³ (S
/ cm). When this powder was used as an active material, the electrochemical characteristics were as follows: discharge capacity: 186 mAh / g;
7%. The thermal stability of the active material is determined by the thermal runaway temperature: 2
It was 71 ° C.

[0062]

Example 15 Li _1.03 Ni _0.76 Co _0.20 Al _0.03 B in the same manner as in Example except that the atmosphere during firing was air.
A layered crystal compound powder having a composition of _0.01 O ₂ was obtained. This powder was spherical secondary particles having an average particle diameter of about 11 μm. BE
Specific surface area by T method is 2.4 m ² / g, conductivity is 9.4 × 1
It was 0 ^-4 (S / cm). As for the electrochemical characteristics when this powder was used as an active material, the discharge capacity was 189 mAh / g and the initial efficiency was 91%. The thermal stability of the active material was thermal runaway temperature: 258 ° C.

[0063]

Example 16 Nickel, cobalt, aluminum and yttrium nitrates and boric acid were mixed at a molar ratio of Ni: Co: A
The solution mixed at l: B: Y = 75.5: 20: 3: 1: 0.5 was adjusted to a solution temperature of 8
The reactor was continuously charged into a reaction vessel controlled at 0 ° C., neutralized with a 48% by weight sodium hydroxide solution to adjust the pH to 1%.
By controlling to 0.0 ± 0.2, a precipitate of coprecipitated hydroxide was obtained. This hydroxide is converted to Li / (Ni + Co + Al
+ B + Y) = 0.95 and mixed with lithium hydroxide, and pressed at 1 ton / cm ² to obtain a molded article. The compact was fired at 700 ° C. for 10 hours in an oxygen stream, and crushed with a mortar-type crusher to obtain a powder of a layered crystal structure compound.

This calcined product is suspended in a 3% by weight lithium nitrate solution so as to have a solid concentration of 50% by weight, and wet-pulverized by a wet bead mill until the average particle diameter becomes 1 μm or less, and dispersed. A slurry was obtained. This slurry was spray-dried and granulated into a sphere. 800 in an oxygen stream
After firing at ゜ C for 2 hours, pulverize with a mortar type crusher to obtain Li _1.02 Ni
A layered crystal compound powder having a composition of _0.755 Co _0.20 Al _0.03 B _0.01 Y _0.005 O ₂ was obtained. This powder has an average particle size of about 14μ.
m were spherical secondary particles. The specific surface area according to the BET method was 0.3 m ² / g, and the conductivity was 3.2 × 10 ^-2 (S / cm).
The electrochemical properties when this powder is used as an active material are:
The discharge capacity was 180 mAh / g, and the initial efficiency was 89%.
The thermal stability of the active material was thermal runaway temperature: 244 ° C.

[0065]

As described above, according to the positive electrode active material of the present invention, in a composite oxide mainly composed of lithium and nickel having a layered crystal structure, nickel is replaced with another transition metal and at least Al, B, Y, Ce, Ti, S
By containing at least one element selected from n, V, Ta, Nb, W, and Mo, the thermal stability in a charged state is improved, and the compact having a compressed density of 4.0 kg / cm ² . By setting the electrical conductivity of the body at 25 ° C. to 5 × 10 ⁻² (S / cm) or less, more preferably 1 × 10 ⁻² (S / cm) or less, Also in this case, there is an effect that Joule heat generation due to a short-circuit current is suppressed, and safety is easily ensured.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a conductivity measuring device used for measuring the conductivity of a positive electrode active material of the present invention.

FIG. 2 is a cross-sectional view of a test battery incorporating a positive electrode made of the positive electrode active material of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Press machine 2 Fixing plate 3, 8 Insulating plate 4, 9 Metal plate 5 Current collector 6 Pressure-resistant container 7 Movable plate 12 Battery 13 Positive electrode 14 Negative electrode 16 Stainless steel case 17 Sealing plate

Claims (2)

[Claims] 1. A composite oxide having a layered crystal structure and containing lithium and nickel as main components, having a general formula: Li _a Ni _1-bc M ¹ _b M ² _c O ₂ 0.95 ≦ a ≦ 1.05, 0.01 ≦ b ≦ 0.10, 0.10 ≦ c ≦ 0.20 (where M ¹ is Al, B, Y, Ce, Ti, Sn, V,
One or more elements selected from Ta, Nb, W, and Mo;
² is a powder having an elemental composition represented by: Co, Mn, and Fe) and a powder having a compression density of 4.0 g / cm ^{3 when} the powder is compacted. Body 2
Conductivity at 5 ° C .: σ is 5 × 10 ⁻² ≧ σ ≧ 5 × 10 ⁻⁴
A positive electrode active material characterized by being in the range of [S / cm]. 2. A lithium secondary battery using the positive electrode active material according to claim 1.