JP3591195B2 - Cathode active material for lithium ion secondary batteries - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a positive electrode active material for a secondary battery, and more particularly to a nonaqueous positive electrode active material for a lithium ion secondary battery capable of improving cycle characteristics and thermal stability, and a method for producing the same.
[0002]
[Prior art]
In recent years, new cordless electronic devices such as a camera-integrated VTR, an audio / video device, a notebook personal computer, and a mobile phone have appeared one after another, and have rapidly spread widely in a short time. In order to reduce the size and weight of these devices, it is essential to improve the performance of secondary batteries that are portable power supplies.
[0003]
Non-aqueous lithium ion secondary batteries have a high battery voltage, a high discharge capacity, and excellent cycle characteristics, and have been actively studied recently in conformity with such uses.
[0004]
Li metal oxide LiMO2 (M = Co, Ni, Fe, Mn, Cr, etc.) typified by lithium cobalt oxide (LiCoO2) is used as the positive electrode active material of this battery.
[0005]
Conventionally, in a non-aqueous secondary battery using LiMO2 as a positive electrode active material, there has been a problem of deterioration in cycle characteristics that the battery discharge capacity is gradually reduced by repeatedly performing charge and discharge cycles. This was thought to be due to the collapse of the LiMO2 crystal. In particular, by repeating charging and discharging, expansion and contraction of the fine particles constituting the positive electrode active material in the c-axis direction occur, and in the case of a polycrystal or the like, since there are many interfaces of crystallites, the crystal collapses therefrom, It has been considered that peeling of the positive electrode active material from the current collector of the positive electrode causes deterioration of cycle characteristics. On the other hand, Japanese Patent Application Laid-Open No. 9-22693 discloses a method for improving the cycle characteristics by monocrystallizing a crystal and growing flat particles oriented in a direction perpendicular to the c-axis ((003) plane). Proposed.
[0006]
JP-A-5-258755 discloses that the cycle characteristics can be improved by limiting the peak intensity ratio between the (003) plane and the (104) plane of the X-ray diffraction line of LiCoO2 as the positive electrode active material to a specific range. And JP-A-9-22692, JP-A-9-22693, and JP-A-8-55624.
[0007]
Furthermore, the positive electrode active material of a lithium ion secondary battery is stable in high-temperature air, but its thermal stability is reduced by being charged, and the organic solvent constituting the electrolyte of the secondary battery is oxidatively decomposed. In some cases, however, there is a serious problem of causing ignition.
[0008]
Until now, many studies have been conducted on the characteristics of the positive electrode active material for lithium ion secondary batteries to improve the charge / discharge cycle characteristics, but the thermal stability during charging was not mentioned much. . This is because it was common knowledge that if the thermal stability at the time of charging was improved, the cycle characteristics of charge and discharge would be contradictory.
[0009]
[Problems to be solved by the invention]
SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to provide a positive electrode active material that improves thermal stability during charging of a lithium ion secondary battery and achieves better charge / discharge cycle characteristics.
[0010]
[Means for Solving the Problems]
The present inventors have conducted intensive studies on the particle shape and structure of the positive electrode active material, and found that the positive electrode active material is composed of particles in which fine single crystals called crystallites are aggregated, and the size or shape of the crystallites and primary particles is The present inventors have found that they are closely related to the thermal stability and charge / discharge cycle characteristics of a lithium ion battery, and have completed the present invention.
[0011]
That is, the positive electrode active material for a lithium ion secondary battery of the present invention is a positive electrode active material for a lithium ion secondary battery represented by a general formula LiMO2,
The particle structure is composed of particles in which single crystals each having a fine crystallite as a unit are aggregated, and the crystallites and the shape of the particles are three-dimensionally substantially isotropic.
(Where M is at least one heavy metal element selected from the group consisting of Co, Ni, Fe, Mn, and Cr)
[0012]
The crystallite indicates a maximum set considered to be a single crystal, and can be calculated from XRD (X-ray diffraction) measurement by using the following Scherrer equation.
<Scherrer's formula>
Crystallite size D (angstrom) = Kλ / (β sin θ)
K: Scherrer's constant (K = 1.05 when β is calculated from the integral width)
λ: wavelength of the used X-ray tube (CuKα1 = 1.540562 Å) β: width of spread of diffraction line depending on the size of crystallite (radian)
θ: diffraction angle 2θ / 2 (degree)
[0013]
Here, the particle refers to the smallest particle that can be imaged by SEM. When the particle is formed of one single crystal, the crystallite diameter and the particle diameter are the same. When a single particle includes a plurality of single crystals, the sizes do not naturally match.
[0014]
The term “nearly isotropic in shape” refers to a shape in which particles have no orientation and grow isotropically in all directions of space, and are typically spherical, but are not necessarily limited to true spheres. Rather, it includes those that are almost spherical. An ordinary crystalline substance has a particle shape that reflects its crystal structure. On the other hand, it can be said that the positive electrode active material useful in the present invention has a special shape having no such orientation.
[0015]
When expressing the three-dimensional shape of the crystallites constituting the primary particles of the positive electrode active material in the direction in which the layers are superposed ((003) vector direction) and the direction perpendicular thereto ((110) vector direction), the (110) vector The ratio of the crystallite diameter in the (003) direction to the crystallite diameter in the direction is preferably in the range of 0.5 to 1.6. The direction in which the layers are stacked indicates that the basic lattice of LiMO2 as the positive electrode active material is hexagonal, and indicates the c-axis direction. Therefore, the direction perpendicular to the direction refers to the a-axis direction.
[0016]
The crystallite diameter of the crystallite of the positive electrode active material in the (003) vector direction is preferably in the range of 500 to 750 angstroms.
[0017]
The crystallite diameter of the crystallite of the positive electrode active material in the (110) vector direction is preferably in the range of 450 to 1000 Å.
[0018]
The average particle diameter of the positive electrode active material is obtained by measuring the specific surface area by an air permeation method and calculating the average value of the particle diameters of the primary particles. Specifically, the average particle diameter is determined by a Fischer sub-sieve sizer (FSSS). .)).
[0019]
It is preferable that the ratio of the minor axis particle diameter to the major axis particle diameter of the particles by SEM observation is in the range of 0.5 to 1.0. The ratio is calculated as follows. As shown in FIG. 1, the center of each of 20 randomly extracted particle images is determined from the SEM photograph, the longest diameter passing through the center is determined, and this is defined as the long-axis particle diameter. Next, the diameter in the direction perpendicular to the major axis passing through the center is defined as the minor axis particle diameter. The average of the ratio of the short axis particle diameter / the long axis particle diameter of each of the obtained particles is calculated.
[0020]
In the method for producing a positive electrode active material for a lithium ion secondary battery according to the present invention, the positive electrode is prepared by mixing raw material heavy metal oxides and lithium salts so that the Li / M ratio is in the range of 0.98 to 1.01 and firing. In the method for producing an active material,
The particle shape of the heavy metal oxide is composed of primary particles having a three-dimensionally almost isotropic shape or secondary particles in which the primary particles are aggregated, and has a center particle diameter of 0.1 to 10 μm.
(Where M is at least one heavy metal element selected from the group consisting of Co, Ni, Fe, Mn, and Cr)
[0021]
The center particle diameter of the heavy metal oxide as a raw material is a value measured by using a particle size distribution analyzer of an electric resistance method, and here is the center particle diameter measured by using a Coulter Multisizer 2. This can be said to be a particle size that includes knowledge of whether it is in a dispersed state or an aggregated state from the measurement principle.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
<Particle shape>
In the present invention, it has been found that the degree of growth of crystallites in the particles of the positive electrode active material affects battery characteristics, and that the degree of growth in a specific vector direction is correlated with individual characteristics. . LiMO2, which is a positive electrode active material of a lithium ion secondary battery, originally has a layered structure. The particle shape of the positive electrode active material of the present invention grows isotropically in the space as shown in FIG. 2A, and has no c-axis orientation as in the comparative example. If this is expressed using the ratio of the short axis particle diameter to the long axis particle diameter of the particles by SEM observation, it is in the range of 0.5 to 0.9. When the shape of the crystallites constituting the primary particles of the positive electrode active material of the present invention is expressed by using the crystallite diameter, it is in the range of 500 to 750 angstroms in the (003) vector direction and 450 to 1000 in the (110) vector direction. It can be said that it is in the range of Angstroms, (105) in the range of 500 to 900 Angstroms in the vector direction, and (113) in the range of 450 to 1000 Angstroms in the vector direction.
[0023]
In the present invention, the (105) and (113) vector directions are also referred to, but this is a means for expressing whether or not the crystal is growing three-dimensionally. If it is a plane, it may be represented by another plane direction such as the (104) or (108) vector direction. However, it is considered that if these crystallites are too large, diffusion of Li ions is hindered, and if they are too small, they cause crystal collapse.
[0024]
<Raw material of heavy metal oxide>
The positive electrode active material for a lithium ion secondary battery of the present invention is characterized by the particle shape as described above. In order to obtain such a crystallite and particle structure, a heavy metal oxide raw material in which the secondary particles are spherical and the crystallite diameter of the primary particles is small is selected as a raw material. As for the particle shape of the positive electrode active material, it is easy to take over the particle shape of the raw material as it is. For example, when the shape of the secondary particles of the heavy metal raw material is an octahedral structure, the obtained LiMO2 crystallites and particles are likely to have an octahedral structure, and when the shape of the heavy metal oxide is spherical, the obtained LiMO2 crystal Particles and particles tend to have a spherical structure. Further, when the secondary particles have a hexagonal plate-like structure, the obtained LiMO2 crystallites and particles tend to have a hexagonal plate-like structure. The present invention excludes a hexagonal plate-like structure. In particular, when the heavy metal element M is Co 3 O 4 where Co is Co, the crystallite size in the (222) vector direction is preferably in the range of 100 to 400 Å.
[0025]
The heavy metal oxide has a substantially spherical shape with primary particles having a particle size of 0.01 to 0.5 μm, and the secondary particles preferably have a particle size of 0.1 to 10.0 μm. For example, when M is Co, it can be obtained by thermally decomposing particles of cobalt carbonate CoCO 3 having a ratio of short axis particle diameter / long axis particle diameter in the range of 0.5 to 1.0. .
[0026]
As described above, the ratio of the ratio of the short axis particle diameter to the long axis particle diameter of the secondary particles of the heavy metal element oxide and the center particle diameter of the secondary particles of the heavy metal oxide is selected as described above. This is because the crystallite and particle structure are directly reflected on the LiMO2 crystallite and particle structure, so that the limitation of the parameters of the raw material particles is very important.
[0027]
<Lithium raw material>
As a result of various studies, as a raw material Li salt used for the lithium secondary battery in the present invention, Li2CO3, Li2 (COO) 2 or LiOH having a relatively high melting point can be preferably used.
[0028]
<Mixing of heavy metal materials and lithium>
In the present invention, M3O4 and a lithium salt are mixed so that the Li / M ratio is in the range of 0.98 to 1.01. This is because if the Li / M ratio deviates from this range, the excess acts as a flux, causing abnormal growth of the particles, making it difficult to control the particle diameter and particle shape.
[0029]
<Firing>
The obtained mixed raw material is fired at 750 to 1100 ° C. in an air atmosphere. When an alkali metal salt, B, Bi, Pb, or the like is added as a flux, or when granulation is performed, growth in the (110) vector direction is easily promoted, and the cycle characteristics are reduced. ) It is necessary to control the growth in the vector direction.
[0030]
[Action]
In a positive electrode active material of a lithium ion secondary battery, Li is desorbed from the crystal upon charging, and changes to a state of LixMO2 (x <1.0), and the hexagonal system LiMO2 changes to a monoclinic system. Transfer. Distortion of the crystal due to this transition is the largest cause of the decrease in thermal stability during charging of the positive electrode active material. This lowers the thermal stability by the following mechanism.
[0031]
If the crystal collapses during charging, oxygen is released from the positive electrode active material. The positive electrode active material constitutes the battery in a state of being in contact with an electrolyte such as EC (ethylene carbonate). The free oxygen oxidizes and decomposes the electrolyte to cause an oxidation reaction, thereby generating heat and, in some cases, igniting. Could lead to serious problems.
[0032]
<Thermal stability>
The thermal stability is improved in the present invention for the following reasons. LiMO2 of the cathode material of the present invention basically has a crystallite shape of a spherical shape isotropically grown in space, and has a large crystallite diameter. By making the crystallites have a spherical isotropic structure, the direction of crystal distortion due to the movement of Li due to charge and discharge becomes uniform in all spatial directions. In comparison, collapse can be minimized. In addition, by increasing the crystallite, it is possible to reduce the collapse of the crystal structure that occurs when Li ions are eliminated. For this reason, the crystallite in the (003) vector direction is preferably 500 Å or more, and the crystallite in the (110) vector direction is preferably 450 Å or more.
[0033]
The thermal stability of the positive electrode active material of the present invention was measured as follows.
The charged lithium ion secondary battery is disassembled in a dry box, the positive electrode plate is taken out, and about 10 mg is cut out to obtain a measurement sample. The obtained measurement sample was subjected to thermogravimetric analysis using a Thermal Gravimetric Analyzer (TGA). (Solid State Ionics vol.69, No.3 / 4 Page 265-270 (1994)) Basically, it is a method of measuring a limit temperature that causes a weight change of a sample while increasing the temperature of the sample from the outside. This weight change is mainly based on the weight loss due to liberation of oxygen from the positive electrode active material. An increase in the oxygen concentration in the battery electrolyte causes abnormal heat generation. Therefore, the higher the limit temperature, the lower the problem of abnormal heat generation, which is preferable.
[0034]
The relationship between the crystallite diameter and the critical temperature of the TGA apparatus was measured for a cathode active material having substantially spherical crystallites and plotted in FIG. Since the cathode active material of the present invention is almost spherical, the crystallite diameter is the same regardless of the vector direction, but here, the crystallite diameter in the (003) vector direction is calculated by the above-mentioned Scherrer equation. Calculated using FIG. 3 shows that the thermal stability during charging of LiMO2 is linearly related to the thermal stability. It can be understood that the larger the crystallite diameter, the higher the critical temperature, that is, the thermal stability is improved. This is because, as described above, when the crystallite diameter is large, the influence of crystal breakage due to the release of Li during charging is reduced, and the free oxygen concentration is reduced.
[0035]
<Charge / discharge cycle characteristics>
Regarding the charge / discharge cycle, it is important that the crystallite diameter in the (110) vector direction does not grow more than 1000 Å. This is because when the crystal grows too much in the (110) vector direction, as shown in FIG. 2B, when Li is inserted (discharged) into the positive electrode active material, Li ions are emitted only in the direction parallel to the layer. This is because the surface into which Li ions can be inserted is relatively reduced due to the inability to insert, and the diffusion of Li ions at the particle interface deteriorates. This tendency is particularly remarkable when discharging is performed at a high current density.
[0036]
FIG. 4 plots the relationship between the crystallite diameter of the positive electrode active material of the present invention and the 100-cycle deterioration rate. Here, since the product of the present invention is spherical, the crystallite diameter is almost the same regardless of the evaluation in any direction. Here, however, the crystallite diameter is plotted against the crystallite diameter in the (110) vector direction. From FIG. 4, it can be seen that the cycle characteristics hardly change when the crystallite diameter is in the range of 500 to about 1000 angstroms, but the capacity retention ratio defined later decreases when it exceeds about 1000 angstroms.
[0037]
The charge / discharge test of the lithium secondary battery is performed as follows. First, as a positive electrode, 70 parts by weight of LiMO2, 15 parts by weight of acetylene black, and 15 parts by weight of PTFT (polytetrafluoroethylene) were mixed with ethanol and kneaded to form a paste, which was then pressed on a SUS mesh and pressed. Dry to obtain a positive electrode plate. On the other hand, both electrodes are immersed in an electrolytic solution in which EC (ethylene carbonate), DEC (diethylene carbonate) and the electrolyte LiPCl4 are mixed, using Li metal as a negative electrode. The charging is set to a current load of 0.2 C (1 C is a current load at which charging or discharging is completed in one hour), and the charging upper limit voltage is set to 4.20 V. The discharge is set to a 0.6 C current load, the lower limit voltage is set to 2.75 V, and charge / discharge for 100 cycles is performed. The capacity retention ratio is calculated by the (%) formula of (100th discharge capacity) / (first discharge capacity) × 100.
[0038]
As described above, in the prior art, in order to improve the cycle characteristics, the particles are made into a plate-shaped single crystal, and the expansion and contraction of the particles in the c-axis direction are made in a certain direction, thereby preventing the particles from collapsing and from the current collector. It is said to prevent peeling. On the other hand, in the present invention, on the contrary, the crystal is formed into a shape close to a spherical shape without orientation. This increases the adhesion of the particles themselves to the current collector, prevents the positive electrode active material from peeling off from the current collector due to charge / discharge cycles, and further increases the surface on the particle interface where Li can be inserted. Thereby, the cycle characteristics are improved.
[0039]
【Example】
Examples of the present invention will be described with reference to LiCoO2 as a positive electrode active material, but the present invention is not limited to this composition, and LiMO2 (where M is at least one heavy metal element selected from the group consisting of Co, Ni, Fe, Mn, and Cr) The same is true for all possible compositions.
[0040]
[Example 1]
<Raw material preparation>
・ Cobalt tetroxide (Co3O4) ・ ・ ・ ・ ・ 3.000kg
・ Lithium carbonate (Li2CO3) ・ ・ ・ ・ ・ ・ 1.380kg
Cobalt tetroxide is a polycrystalline particle having a crystallite diameter in the (222) vector direction of 200 angstroms and a secondary particle having a substantially spherical shape. When the secondary particle size was measured using a Coulter Multisizer 2, the central particle size was 5.0 μm. The charge ratio of Li / Co of the above raw materials is 1.00. These raw materials are charged into a ceramic pot and ball milled to obtain a mixed raw material of the positive electrode active material.
[0041]
The obtained mixed raw material was calcined at 900 ° C. for 10 hours in the air and pulverized to synthesize a target LiCoO 2.
[0042]
The obtained LiCoO2 was subjected to powder X-ray diffraction measurement using CuKα as a radiation source, and the crystallite diameter in the (003) vector direction was 606 Å, and the crystallite size in the (110) vector direction was calculated using Scherrer's equation. The crystallite diameter was 879 angstroms, the crystallite diameter in the other (115) vector directions was 795 angstroms, and the crystallite diameter in the (113) vector directions was 848 angstroms.
[0043]
Regarding the particle size of the LiCoO2 cathode active material, F.I. S. S. S. The average particle diameter was 4.0 μm as measured by using the formula, and the average of the ratio of the short axis particle diameter / the long axis particle diameter of each particle was 0.9. When preparing a measurement sample to be subjected to the SEM, if the sample is prepared by applying pressure, it is difficult to distinguish it from the isotropically shaped particles of the present invention. Therefore, when preparing a SEM measurement sample, preparation was made so that pressure was not applied to the sample surface in order to make it less likely to receive the shape of the particles.
[0044]
<Cycle characteristics>
70 parts by weight of the obtained positive electrode active material LiCoO2, 15 parts by weight of acetylene black, and 15 parts by weight of PTFT (polytetrafluoroethylene) were mixed with ethanol and kneaded to form a paste, which was pressed on a SUS mesh. It is dried to obtain a positive electrode plate. On the other hand, both electrodes are immersed in an electrolytic solution in which EC (ethylene carbonate), DEC (diethylene carbonate) and the electrolyte LiPCl4 are mixed, using Li metal as a negative electrode. The charging is set to a current load of 0.2 C (1 C is a current load at which charging or discharging is completed in one hour), and the charging upper limit voltage is set to 4.20 V. The discharge is set to a 0.6 C current load, the lower limit voltage is set to 2.75 V, and charge / discharge for 100 cycles is performed. The capacity retention ratio was 95.2% as a result of an expression of (100th discharge capacity) / (first discharge capacity) × 100.
[0045]
<Thermal stability>
Using the obtained positive electrode active material LiCO2, a secondary battery was manufactured under the same conditions as in the cycle characteristic measurement, and charged under the conditions of a charging load of 0.2 C and a charging upper limit voltage of 4.20 V. The secondary battery is disassembled in a dry box, the positive electrode plate is taken out, and about 10 mg thereof is cut out to be used as a measurement sample. As a result of performing thermogravimetric analysis on the obtained measurement sample using a TGA apparatus, a weight change due to oxygen release was observed at 218.0 ° C., and the limit temperature was 218.0 ° C.
[0046]
[Comparative Example 1]
Cobalt tetroxide particles are hexagonal plate-like particles observed by SEM. The crystallite diameter in the (222) vector direction is 100 angstroms, and the secondary particles have a center particle diameter of 6.2 μm measured using a Coulter Multisizer 2. LiCoO2 was synthesized by mixing and firing the raw materials under the same conditions as in Example 1 except that a certain material was used.
[0047]
When the obtained LiCoO2 was calculated using Scherrer's equation in the same manner as in Example 1, the crystallite diameter in the (003) vector direction was 649 angstroms, and the crystallite diameter in the (110) vector direction was 1150 angstroms. The crystallite diameter in the (115) vector direction was 798 angstroms, and the crystallite diameter in the (113) vector direction was 868 angstroms.
[0048]
Regarding the particle size of the LiCoO2 cathode active material S. S. S. As a result, the average particle diameter was 3.5 μm.
[0049]
Further, the ratio of the minor axis particle diameter to the major axis particle diameter of the LiCoO2 particles by SEM observation was 0.3.
[0050]
<Cycle characteristics>
A secondary battery was fabricated in the same manner as in Example 1 except that the obtained positive electrode active material was used, and the battery was charged and discharged for 100 cycles. The capacity retention was 85.0%.
[0051]
<Thermal stability>
Using the obtained positive electrode active material LiCO2 and performing thermogravimetric analysis using TGA in the same manner as in Example 1, the critical temperature at which a weight change due to oxygen release was observed was 190.8 ° C. .
[0052]
【The invention's effect】
As described above, the LiMO2 of the cathode material of the present invention is basically a sphere in which the crystallite shape is isotropically grown in space, and the crystallite diameter is large. By making the crystallites have a spherical isotropic structure, the direction of crystal distortion due to the movement of Li due to charge and discharge becomes uniform in all spatial directions. In comparison, collapse can be minimized. Further, by further increasing the crystallite to a specific range, the collapse of the crystal structure caused when Li ions are eliminated can be reduced. In these respects, the product of the present invention was able to significantly improve the thermal stability and cycle characteristics of the positive electrode active material as compared with the conventional product.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a method for evaluating a long axis particle diameter and a short axis particle diameter based on an SEM photograph.
FIG. 2 is an enlarged schematic diagram showing a comparison of the particle shapes of a positive electrode active material of (a) the present invention and (b) a comparative product.
FIG. 3 is a characteristic diagram showing a relationship between a crystallite diameter and a limit temperature.
FIG. 4 is a characteristic diagram showing a relationship between a capacity retention ratio and a crystallite diameter.
[Explanation of symbols]
1 ····· The surface into which Li ions can be inserted

Claims (1)

A positive electrode active material for a lithium ion secondary battery represented by the general formula LiMO2,
The particle structure is composed of particles in which crystallites are united and they are aggregated,
When the three-dimensional shape of the crystallites constituting the primary particles of the positive electrode active material is expressed by a direction in which the layers are stacked ((003) vector direction) and a direction perpendicular to it ((110) vector direction), the (110) vector The ratio of the crystallite diameter in the (003) vector direction to the crystallite diameter in the direction is in the range of 0.5 to 1.6,
The crystallite diameter in the (003) vector direction is in the range of 500 to 750 angstroms,
The crystallite diameter in the (110) vector direction is in the range of 450 to 1000 Å,
The lithium ion secondary battery, wherein the average particle diameter of the particles is 0.1 to 10 μm, and the ratio of the minor axis particle diameter to the major axis particle diameter of the particles by SEM observation is in the range of 0.5 to 0.9. Positive electrode active material.
(Where M is at least one heavy metal element selected from the group consisting of Co, Ni, Fe, Mn, and Cr)

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