JP2012009151A - Positive electrode of lithium secondary battery, and lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode of a lithium secondary battery and the lithium secondary battery comprising such positive electrode with a structure capable of attaining high rate characteristics and high capacity.SOLUTION: A positive electrode includes positive electrode active material particles which are particles of lithium composite oxide having a laminar rock salt structure. These positive electrode active material particles include plate particles and isometric particles. The plate particles have a peak intensity ratio [003]/[104] of 1.6 or less in X-ray diffraction, and an aspect ratio of 2.1 to 20. The isometric particles have an aspect ratio equal to or less than 2.

Description

  The present invention relates to a positive electrode of a lithium secondary battery including positive electrode active material particles having a layered rock salt structure. Furthermore, this invention relates to the lithium secondary battery provided with this positive electrode.

  A lithium secondary battery (sometimes referred to as a lithium ion secondary battery) includes a positive electrode, a negative electrode, and an electrolyte interposed therebetween. As a positive electrode active material constituting this positive electrode, particles of lithium composite oxide (lithium transition metal oxide) having a layered rock salt structure are widely known (see, for example, JP-A-5-226004). And the positive electrode is equipped with the positive electrode active material layer (positive electrode active material film | membrane) comprised by disperse | distributing this particle | grain and conductive support agents, such as carbon, in a predetermined binder.

Here, the “layered rock salt structure” is a crystal structure in which a lithium layer and a transition metal layer other than lithium are alternately stacked with an oxygen layer interposed therebetween, that is, a transition metal ion layer and a lithium through an oxide ion. It refers to a crystal structure in which single layers are alternately stacked (typically an α-NaFeO ₂ type structure: a structure in which transition metals and lithium are regularly arranged in the [111] axis direction of a cubic rock salt type structure).

In such positive electrode active material particles, lithium ions (Li ⁺ ) enter and exit on crystal planes other than the (003) plane (lithium ion entrance / exit surfaces: for example, (101) plane and (104) plane). The charging / discharging operation | movement in a lithium secondary battery is performed by the entrance / exit of the lithium ion in such positive electrode active material particles.

  In the positive electrode of this type of battery, rate characteristics are enhanced by increasing the exposure of the lithium ion entrance / exit surfaces of the positive electrode active material particles to the electrolyte. On the other hand, the capacity can be increased by increasing the filling rate of the positive electrode active material particles in the binder in the positive electrode.

  In this regard, in the conventional positive electrode active material particles, when the particle diameter is reduced, the specific surface area is increased and the rate characteristics are increased. On the other hand, the filling rate is decreased and the capacity is also decreased. Thus, in the conventional positive electrode structure, the rate characteristic and the capacity have a trade-off relationship.

  The present invention has been made to solve such problems. That is, an object of the present invention is to provide a positive electrode of a lithium secondary battery and a lithium secondary battery including such a positive electrode having a structure capable of achieving both high rate characteristics and high capacity.

  The lithium secondary battery of the present invention includes a positive electrode including positive electrode active material particles that are particles of a lithium composite oxide having a layered rock salt structure, a negative electrode including a carbonaceous material or a lithium storage material as a negative electrode active material, and the positive electrode. And an electrolyte provided so as to be interposed between the negative electrode and the negative electrode. Specifically, the positive electrode includes a positive electrode active material film formed by dispersing the positive electrode active material particles (and a conductive additive) in a binder. A feature of the present invention is that the positive electrode active material particles include plate-like particles and equiaxed particles having the following characteristics.

  In the plate-like particles, the peak intensity ratio [003] / [104] is 1.6 or less (more preferably 1.0 or less), and the aspect ratio is 2.1 to 20. Here, the peak intensity ratio [003] / [104] is the ratio of the diffraction intensity (peak intensity) of the (003) plane to the diffraction intensity (peak intensity) of the (104) plane in X-ray diffraction. The aspect ratio is a value (d / t) obtained by dividing the particle diameter (d) by the thickness (t). The thickness of the plate-like particle is preferably 5 to 100 μm.

  The equiaxed particles have an aspect ratio of 2 or less. Moreover, it is preferable that the particle diameter of the equiaxed particle is equal to or less than the thickness of the plate-like particle.

  It is preferable that the plate-like particle mixture ratio, which is the volume% of the plate-like particles with respect to the sum of the plate-like particles and the equiaxed particles, is 5 to 80%.

  Here, the “plate-like particle” means a particle whose outer shape is plate-like. The concept of “plate-like” is clear from a social wisdom without any special explanation in the present specification, and as described above, the “aspect ratio is 2.1 to 20” is more defined. Although it has become clearer, if it dares to add, it can be explained as follows, for example.

  In other words, “plate-like” means that particles are stably placed on a horizontal plane (a plane perpendicular to the vertical direction where gravity acts) (external impact (the particles will fly from the horizontal plane). The first plane and the second plane orthogonal to the horizontal plane (the first plane and the first plane) in a state of being placed (in a manner that will not fall down even if subjected to a strong impact) When the cross section of the particle is observed by crossing two planes (typically perpendicular to each other), the cross section along the horizontal plane (parallel to the horizontal plane or formed with the horizontal plane) is observed in any cross section. The thickness direction (the direction corresponding to the “particle diameter”) is the direction perpendicular to the width direction, and the dimension in the width direction is an α degree (0 <α <45) direction. In the thickness direction) It is called “thickness”.) The above-mentioned “thickness” does not include a void portion between the horizontal plane and the particle.

  The plate-like particles are usually formed in a flat plate shape. Here, “flat plate” refers to a state in which the particle is stably placed on a horizontal plane, and the height of the void formed between the horizontal plane and the particle is smaller than the thickness of the particle. It shall be said. Since what is bent more than this usually does not occur in this kind of plate-like particle, the above definition is appropriate for the plate-like particle.

  In a state where particles are stably placed on a horizontal plane, the thickness direction is not necessarily parallel to the vertical direction. For example, when the particles are stably placed on a horizontal plane, the cross-sectional shape of the particles by the first plane or the second plane is (1) rectangle, (2) rhombus, (3) oval A case is assumed in which the most approximate shape is classified. When the particle cross-sectional shape approximates (1) a rectangle, the width direction is a direction parallel to the horizontal plane in the above state, and the thickness direction is a direction parallel to the vertical direction in the above state.

  On the other hand, in the case of (2) rhombus or (3) ellipse, the width direction makes a slight angle (45 degrees or less: typically about several to 20 degrees) with the horizontal plane in the above state. . In this case, the width direction is a direction connecting two points on the outline by the cross section and having the longest distance between them (this definition is a diagonal line in the case of the above-mentioned (1) rectangle) Is not appropriate to end up).

  In addition, the “plate surface” of the particle is a surface facing the horizontal plane in a state where the particle is stably placed on the horizontal plane, or is positioned above the particle and parallel to the horizontal plane when viewed from the horizontal plane. A surface that faces a virtual plane. Since the “plate surface” is the widest surface of the plate-like particle, it may be referred to as a “principal surface”. The plane intersecting (typically orthogonal) the plate surface (main surface), that is, the surface intersecting the plate surface direction (or in-plane direction) that is perpendicular to the thickness direction is a particle. From the edge of the particle in a state where the particle is stably placed on the horizontal plane (when the particle is stably placed on the horizontal surface and viewed from above in the vertical direction). , Referred to as “end face”.

  However, the plate-like particles often have a cross-sectional shape that approximates the above-mentioned (1) rectangle. For this reason, in the plate-like particles, the thickness direction may be a direction parallel to the vertical direction in a state where the particles are stably placed on a horizontal plane. Similarly, in the plate-like particle, the “plate surface” may be said to be a surface orthogonal to the thickness direction of the particle.

  The thickness t of the plate-like particles can be obtained, for example, by measuring the distance between the plate surfaces observed substantially in parallel when the cross section is observed with an SEM (scanning electron microscope). Further, the particle diameter d of the plate-like particles is defined by the minimum dimension in the plate surface direction orthogonal to the thickness direction. For example, the particle diameter d is obtained by measuring the diameter when an inscribed circle of the outer shape is drawn when the outer shape of the plate-like particle in plan view is observed by SEM.

  In the structure of this invention, it becomes possible to reduce the clearance gap between particle | grains by mixing the said plate-shaped particle | grain with a large size with respect to the said equiaxed particle | grain. Further, by dispersing the plate-like particles and the equiaxed particles in the positive electrode (the positive electrode active material film) at an appropriate mixing ratio, the equiaxed particles are formed so as to fill the gaps between the plate-like particles. Be placed. Thereby, the filling rate of the positive electrode active material particles in the positive electrode is increased.

  Moreover, in the said plate-shaped particle | grain, the lithium ion entrance / exit surface is exposed to the surface (the said plate surface) satisfactorily. Furthermore, the lithium ion entrance / exit surface exposed on the surface of the equiaxed particles also contributes to charge / discharge. Therefore, according to the structure of this invention, it can be made larger than the exposure amount of a lithium ion entrance / exit surface. Thereby, good rate characteristics can be obtained.

  Therefore, according to the present invention, it is possible to achieve both high rate characteristics and high capacity, which are trade-offs in the prior art.

It is sectional drawing which shows schematic structure of the lithium secondary battery which is one Embodiment of this invention. It is an expanded sectional view of the positive electrode shown by FIG. 1A. It is an expansion perspective view of the positive electrode active material plate-like particle | grains shown by FIG. 1B. It is an expansion perspective view of the positive electrode active material equiaxed particle | grains shown by FIG. 1B. It is an expansion perspective view of the positive electrode active material plate-like particle of a comparative example.

  Embodiments of the present invention will be described below with reference to the drawings. It should be noted that various modifications that can be made to the present embodiment are described at the end of the description because if they are inserted during the description of the embodiment, the understanding of the consistent description of the embodiment is hindered. ing.

<Configuration of lithium secondary battery>
FIG. 1A is a cross-sectional view showing a schematic configuration of a lithium secondary battery 10 according to an embodiment of the present invention.

  Referring to FIG. 1A, the lithium secondary battery 10 of the present embodiment is a so-called liquid type, and includes a battery case 11, a separator 12, an electrolyte 13, a negative electrode 14, and a positive electrode 15.

  The separator 12 is provided so as to bisect the inside of the battery case 11. In the battery case 11, a liquid electrolyte 13 is accommodated, and a negative electrode 14 and a positive electrode 15 are provided so as to face each other with the separator 12 therebetween.

  As the electrolyte 13, for example, a non-aqueous solvent based electrolyte in which an electrolyte salt such as a lithium salt is dissolved in a non-aqueous solvent such as an organic solvent is preferably used because of its electrical characteristics and ease of handling. The solvent for the non-aqueous electrolyte is not particularly limited, but for example, chain esters such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and methyl propion carbonate; dielectric constants such as ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate High cyclic esters; mixed solvents of chain esters and cyclic esters; and the like, and mixed solvents with cyclic esters having chain esters as the main solvent are particularly suitable.

As the electrolyte salt to be dissolved in the solvent described above when the preparation of the non-aqueous electrolyte, for _{example, LiClO 4, LiPF 6, LiBF} 4, LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiCF 3 CO ₂ , Li ₂ C ₂ F ₄ (SO ₃ ) ₂ , LiN (RfSO ₂ ) (Rf′SO ₂ ), LiC (RfSO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2), LiN (RfOSO ₂ ) ₂ [wherein Rf and Rf ′ are fluoroalkyl groups], etc. can be used. These may be used alone or in combination of two or more. Among the above electrolyte salts, a fluorine-containing organic lithium salt having 2 or more carbon atoms is particularly preferable. This is because this fluorine-containing organolithium salt has a large anionic property and is easily ion-separated, so that it is easily dissolved in the above-mentioned solvent. The concentration of the electrolyte salt in the nonaqueous electrolytic solution is not particularly limited, but is, for example, 0.3 mol / l or more, more preferably 0.4 mol / l or more, and 1.7 mol / l or less, more preferably 1. It is desirable that it is 5 mol / l or less.

The negative electrode active material according to the negative electrode 14 may be any material as long as it can occlude and release lithium ions. For example, graphite, pyrolytic carbons, cokes, glassy carbons, a fired body of an organic polymer compound, mesocarbon micro Carbonaceous materials such as beads, carbon fibers, activated carbon and the like are used. In addition, metal lithium, alloys containing silicon, tin, indium, etc., oxides of silicon, tin, etc. that can be charged and discharged at a low potential close to lithium, and nitriding of lithium such as Li _2.6 Co _0.4 N and cobalt Lithium storage materials such as materials can also be used as the negative electrode active material. Furthermore, a part of graphite can be replaced with a metal, oxide, or the like that can be alloyed with lithium. When graphite is used as the negative electrode active material, the voltage at the time of full charge can be regarded as about 0.1 V on the basis of lithium, so the potential of the positive electrode 15 is calculated for convenience by adding 0.1 V to the battery voltage. Therefore, it is preferable that the charging potential of the positive electrode 15 is easy to control.

  FIG. 1B is an enlarged cross-sectional view of the positive electrode 15 shown in FIG. 1A. Referring to FIG. 1B, the positive electrode 15 includes a positive electrode current collector 15a and a positive electrode active material layer 15b. The positive electrode active material layer 15b includes a binder 15b1, positive electrode active material plate-like particles 15b2, and positive electrode active material equiaxed particles 15b3.

  The positive electrode active material layer 15b is formed by dispersing positive electrode active material plate-like particles 15b2, positive electrode active material equiaxed particles 15b3, and a conductive aid (not shown) in a binder 15b1. Specifically, in the positive electrode active material layer 15b, the positive electrode active material plate particles 15b2 and the positive electrode active material equiaxed particles 15b3 are dispersed so that the mixing ratio of the plate particles is 5 to 80%. Here, the “plate-like particle blending ratio” is the volume% of the positive electrode active material plate particles 15b2 with respect to the total of the positive electrode active material plate particles 15b2 and the positive electrode active material equiaxed particles 15b3.

  The basic configuration of the lithium secondary battery 10 and the positive electrode 15 shown in FIGS. 1A and 1B (the battery case 11, the separator 12, the electrolyte 13, the negative electrode 14, the positive electrode current collector 15a, and the binder 15b1 are configured. In this specification, the detailed description thereof is omitted.

  FIG. 2A is an enlarged perspective view of the positive electrode active material plate-like particle 15b2 shown in FIG. FIG. 2B is an enlarged perspective view of the positive electrode active material equiaxed particles 15b3 shown in FIG. FIG. 2C is an enlarged perspective view of positive electrode active material particles of a comparative example.

  As shown in FIG. 2A, the positive electrode active material plate-like particles 15b2 are formed in a plate shape (that is, the aspect ratio is 2.1 to 20). The positive electrode active material plate-like particles 15b2 are formed to have a thickness of 5 to 100 μm.

  The positive electrode active material plate-like particles 15b2 are plate surfaces (upper surface A and lower surface B: hereinafter referred to as “upper surface A” and “lower surface B”) that are surfaces orthogonal to the thickness direction (vertical direction in the drawing). Surfaces other than (003) (for example, (101) surface and (104) surface) are exposed at “plate surface A” and “plate surface B”, respectively.

  That is, the positive electrode active material plate-like particles 15b2 are formed so that the surfaces other than (003) (for example, the (104) surface) are oriented so as to be parallel to the plate surfaces A and B. On the other hand, the particles of the comparative example shown in FIG. 2C are plate-like, but are formed such that (003) is exposed on both surfaces (plate surfaces A and B) in the thickness direction of the particles. (See the surface painted in black in the figure). Note that the (003) plane may be exposed at the end face C intersecting the plate surface direction (in-plane direction) of the positive electrode active material plate-like particle 15b2.

  On the other hand, as shown in FIG. 2B, the positive electrode active material equiaxed particles 15b3 are formed in an isotropic shape (an aspect ratio of 2 or less). The positive electrode active material equiaxed particles 15b3 are formed so that the particle diameter thereof is approximately equal to or less than the thickness of the positive electrode active material plate-like particles 15b2.

<Specific example of manufacturing method of positive electrode active material particles>
The specific example of the manufacturing method of positive electrode active material plate-like particle 15b2 and positive electrode active material equiaxed particle 15b3 in this embodiment is as follows.

<< Plate-like particles >>
First, a slurry was prepared by the following method: Co ₃ O ₄ raw material particles (particle size: 0.3 μm) prepared by pulverizing Co ₃ O ₄ powder (particle size: 1-5 μm, manufactured by Shodo Chemical Industry Co., Ltd.) 100 parts by weight of Bi ₂ O ₃ (particle size 0.3 μm, manufactured by Taiyo Mining Co., Ltd.) at a rate of 20 wt%, and 100 parts by weight of a dispersion medium (toluene: isopropanol = 1: 1), 10 parts by weight of a binder (polyvinyl butyral: product number BM-2, manufactured by Sekisui Chemical Co., Ltd.), 4 parts by weight of a plasticizer (DOP: Di (2-ethylhexyl) phthalate, manufactured by Kurokin Kasei Co., Ltd.) and a dispersant ( 2 parts by weight of a product name Rheodor SP-O30, manufactured by Kao Corporation) was mixed. The mixture was defoamed by stirring under reduced pressure and adjusted to a viscosity of 500 to 700 cP. The viscosity was measured with an LVT viscometer manufactured by Brookfield.

  The slurry prepared as described above was formed into a sheet shape on a PET film by a doctor blade method so that the thickness after drying was 10 μm.

  The sheet-like molded body peeled off from the PET film was cut into a 70 mm square with a cutter, and placed in the center of a zirconia setter (dimension 90 mm square, height 1 mm) with an embossed projection of 300 μm in size. After baking at 1500 ° C. for 5 h, the temperature was lowered at a temperature lowering rate of 50 ° C./h, and the portion not welded to the setter was taken out.

The fired Co ₃ O ₄ ceramic sheet was placed on a sieve (mesh) having an opening diameter of 20 μm, and crushed by passing through the mesh while lightly pressing with a spatula.

Co ₃ O ₄ powder obtained by pulverizing the ceramic sheet and Li ₂ CO ₃ powder (manufactured by Kanto Chemical Co., Inc.) were mixed so that Li / Co = 1.0, and the mixture was put into a crucible. The positive electrode active material plate-like particles 15b2 (LiCoO ₂ plate-like particles) were obtained by heat treatment at 760 ° C. for 20 hours.

<< Equiaxed particle >>
A positive electrode active material having a diameter of 10 μm was produced by the same manufacturing method as that for the plate-like particles, except that the fired Co ₃ O ₄ ceramic sheet was pulverized in a 1 L polypropylene pot for 10 hours using a nylon ball having a diameter of 10 mm. Equiaxial particles 15b3 (LiCoO ₂ equiaxed particles) were obtained.

<Evaluation method>
Mixing the obtained plate-like and equiaxed LiCoO ₂ particles in a predetermined ratio, acetylene black, and polyvinylidene fluoride (PVDF) in a mass ratio of 90: 5: 5 Thus, a positive electrode material was prepared.

The prepared positive electrode material is coated on an aluminum foil having a thickness of 21 μm so as to have a thickness of 100 μm, punched into a disk shape having a diameter of 20 mm, and then uniaxially pressed at 40 ° C. with a pressure of 500 kg / cm ^2. Thus, a positive electrode active material layer was produced. The active material filling rate in the positive electrode active material layer was determined as follows. First, after peeling off the coated portion, cross-section polishing was performed in the thickness direction by Ar ion milling (product name: Cross Section Polisher SM-09020CP JEOL). Next, the reflected electron image of the polished surface was taken using a scanning electron microscope (product name: JSM-6390, manufactured by JEOL). This captured image was analyzed using image processing software (product name: Photoshop Adobe), and the area ratio of the positive electrode active material layer to the active material ((active material cross-sectional area / positive electrode active material layer cross-sectional area) × 100). This was determined as the active material filling rate.

The produced positive electrode active material layer, the negative electrode made of a lithium metal plate, the stainless steel current collector plate, and the separator are arranged in the order of current collector plate-positive electrode active material layer-separator-negative electrode-current collector plate, and this aggregate is used as an electrolyte solution. The coin cell was produced by filling with. The electrolytic solution was prepared by dissolving LiPF ₆ in an organic solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at an equal volume ratio to a concentration of 1 mol / L. Using the battery thus manufactured (coin cell), the battery characteristics were evaluated as follows.

  After charging at a constant current until the battery voltage reaches 4.2 V at a current value of 0.1 C rate, and then charging at a constant voltage until the current value decreases to 1/20 while maintaining the battery voltage at 4.2 V, 10 Charging / discharging operation of stopping for 10 minutes and then stopping at constant current until the battery voltage reaches 3.0 V at a current value of 0.1 C rate for 10 minutes is defined as one cycle, and a total of 2 at 25 ° C. The cycle was repeated and the discharge capacity at the second cycle was measured (discharge capacity at 0.1 C rate).

  Subsequently, the current value at the time of charging was fixed at the 0.1 C rate, the discharge current value was 1 C rate, 5 C rate was repeated for each two cycles, and the discharge capacity at the second cycle was defined as the discharge capacity at each rate. As an index of the rate characteristic, a value obtained by dividing the discharge capacity at the 5C rate by the discharge capacity at the 0.1C rate (this is referred to as “discharge capacity maintenance rate (%)”) was used.

<Evaluation results>
Experimental example 1 using conventional LiCoO ₂ plate-like particles (see FIG. 2C) instead of the plate-like particles prepared by the production method of the above-described specific example, Bi ₂ O ₃ addition amount, firing / cooling conditions, and The evaluation results of five experimental examples (experimental examples 2 to 6: those corresponding to the above-described manufacturing method are “experimental example 5”) in which the degree of orientation was changed by changing the lithium introduction conditions are shown in the following table. It is shown in 1. In addition, the evaluation results of nine experimental examples (experimental examples 7 to 15) in which the degree of orientation is constant (same as experimental example 5) and the plate-like particle mixture ratio is changed are shown in Table 2 below (experimental example 12 is It is the same as Experimental Example 5).

  As shown in Table 1, in Experimental Example 1 in which the degree of orientation is low (the peak intensity ratio [003] / [104] is large), the rate characteristics are lowered. Further, in Experimental Examples 7 and 8 where the plate-like particle blending ratio is too low, and in Experimental Example 15 where the plate-like particle blending ratio is too high, the active material filling rate decreased.

  On the other hand, favorable rate characteristics were obtained in Experimental Examples 2 to 6 in which the peak intensity ratio [003] / [104] was 1.6 or less and the plate-like particle mixture ratio was 50%. In Experimental Examples 9 to 14 in which the plate-like particle blending rate is 5 to 80%, a good active material filling rate was obtained.

<Effect by embodiment>
As described above, in the positive electrode active material plate-like particle 15b2, the lithium ion entrance / exit surface (for example, the (104) surface) is oriented so as to be parallel to the plate surface, so that most of the surface is exposed. . On the other hand, the (003) plane where lithium ions cannot enter and exit is only slightly exposed at the end face (see FIG. 2A). That is, in the positive electrode active material plate-like particles 15b2, the exposure of the lithium ion entrance / exit surface to the electrolyte 13 (including those permeating the binder 15b1) is increased, and lithium ions cannot enter and exit (003). ) The exposure ratio of the surface is extremely low.

  Further, the positive electrode active material plate-like particles 15b2 and the positive electrode active material equiaxed particles 15b3 are dispersed in the positive electrode active material layer 15b at an appropriate mixing ratio so as to fill a gap between the positive electrode active material plate-like particles 15b2. Positive electrode active material equiaxed particles 15b3 are arranged. Thereby, the filling rate of the positive electrode active material particles in the positive electrode active material layer 15b is increased. That is, the ratio of the binder 15b1 in the positive electrode active material layer 15b can be made as low as possible. Further, the lithium ion entrance / exit surface exposed on the surface of the positive electrode active material equiaxed particles 15b3 can also contribute to the charge / discharge operation.

  Therefore, according to the configuration of the present embodiment, the rate characteristics are improved by increasing the exposure of the lithium ion entrance and exit surfaces as much as possible, and the particle strength of the positive electrode active material plate-like particles 15b2 is increased. The improvement in durability accompanying the improvement and the increase in capacity by increasing the filling rate are simultaneously achieved.

  In particular, a lithium ion secondary battery for mobile devices mounted on a mobile phone or a notebook PC is required to have a capacity that can be used for a long time. From such a viewpoint, it is preferable to increase the filling rate by using large particles having a particle diameter of 10 μm or more as the positive electrode active material plate-like particles 15b2.

  In this regard, in the prior art, when the particle size is increased, on the other hand, a surface (003) where lithium ions cannot enter and exit is widely exposed on the surface (see FIG. 2C), and the output characteristics may be deteriorated.

  On the other hand, in the positive electrode active material plate-like particle 15b2 according to this embodiment, the lithium ion entry / exit surface is widely exposed on the surface. For this reason, according to this embodiment, the positive electrode active material plate-like particle 15b2 can be enlarged without adversely affecting the output characteristics. Further, as described above, the positive electrode active material equiaxed particles 15b3 are arranged so as to fill the gaps between the positive electrode active material plate-like particles 15b2. Therefore, according to the present embodiment, it is possible to provide a positive electrode that has a higher filling rate than conventional ones and that is compatible with higher capacity.

  In addition, the thickness of the positive electrode active material plate-like particle 15b2 is 2 to 100 μm, more preferably 5 to 50 μm, and still more preferably 5 to 20 μm. If it is thicker than 100 μm, it is not preferable from the viewpoint of the rate characteristics decreasing and the sheet formability. The thickness of the positive electrode active material plate-like particle 15b2 is desirably 2 μm or more. If it is thinner than 2 μm, it is not preferable in that the effect of increasing the filling rate is reduced.

  The aspect ratio of the positive electrode active material plate-like particles 15b2 is desirably 2.1 to 20. If the aspect ratio is too small, the effect of expanding the entrance / exit surface of lithium ions by orientation will be small. When the aspect ratio is larger than 20, when the positive electrode active material plate-like particle 15b2 is filled so that the plate surface of the positive electrode active material plate-like particle 15b2 is parallel to the in-plane direction of the positive electrode active material layer 15b, the positive electrode active material Since the diffusion path of lithium ions in the thickness direction of the layer 15b becomes extremely long, the rate characteristics deteriorate, which is not preferable.

<List of examples of modification>
It should be noted that the above-described embodiments and specific examples are merely examples of realization of the present invention that the applicant has considered to be the best at the time of filing of the present application, and the present invention is not limited to the above. It should not be limited at all by the embodiments and specific examples. Therefore, it goes without saying that various modifications can be made to the above-described embodiments and specific examples without departing from the essential part of the present invention.

  Hereinafter, some modifications will be exemplified. In the following description of the modification, components having the same configurations and functions as the components in the above-described embodiment are given the same name and the same reference numerals in this modification. And about description of the said component, description in the above-mentioned embodiment shall be used suitably in the range which is not inconsistent.

  However, it goes without saying that the modified examples are not limited to the following. The limited interpretation of the present invention based on the description of the above-described embodiment and the following modifications unfairly harms the interests of the applicant (especially rushing the application under the principle of prior application), but improperly imitates the imitator. It is beneficial and not allowed.

  It goes without saying that the configuration of the above-described embodiment and the configuration described in each of the following modifications can be combined in an appropriate manner within a technically consistent range.

  As the electrolyte, an inorganic solid, an organic polymer, or a gel polymer (a gel-like material in which an organic polymer is impregnated with an electrolytic solution) can be used.

  The manufacturing method of the positive electrode active material plate-like particle 15b2 is not limited to the above specific example. For example, the positive electrode active material plate-like particles 15b2 can be manufactured by a manufacturing method (for example, International Application No. PCT / JP2009 / 071838) according to the prior application of the present applicant, which is different from a conventionally known manufacturing method.

  In the above specific example, the positive electrode active material equiaxed particles 15b3 are obtained by pulverizing the positive electrode active material plate-like particles 15b2 having a high degree of orientation (peak intensity ratio [003] / [104] is 1.6 or less). However, the present invention is not limited to this. That is, for example, as the positive electrode active material equiaxed particles 15b3, those produced by a conventionally known production method (with a crystal structure similar to that shown in FIG. 2C and having an equiaxial shape) are also used. obtain.

  Other modifications not specifically mentioned are naturally included in the technical scope of the present invention without departing from the essential part of the present invention.

  In addition, in each element constituting the means for solving the problems of the present invention, elements expressed functionally and functionally include the specific structures disclosed in the above-described embodiments and modifications, It includes any structure that can realize this action / function. Furthermore, the contents of the prior application and each publication (including the specification and the drawings) cited in the present specification may be incorporated as appropriate as part of the present specification.

DESCRIPTION OF SYMBOLS 10 ... Lithium secondary battery 11 ... Battery case 12 ... Separator 13 ... Electrolyte 14 ... Negative electrode 15 ... Positive electrode 15a ... Positive electrode collector 15b ... Positive electrode active material layer 15b1 ... Binder 15b2 ... Positive electrode active material plate-like particle 15b3 ... Positive electrode active material Equiaxial particle A ... Plate surface (upper surface) B ... Plate surface (lower surface)
C ... End face

Japanese Patent Laid-Open No. 9-22893 JP 2003-132877 A JP 2003-346809 A

Claims (10)

It has a layered rock salt structure, and the peak intensity ratio [003] / [104], which is the ratio of the diffraction intensity by the (003) plane to the diffraction intensity by the (104) plane in X-ray diffraction, is 1.6 or less, and the thickness A plate-like particle having an aspect ratio of 2.1 to 20, which is a value obtained by dividing the particle diameter defined by the minimum dimension in the plate surface direction orthogonal to the plate thickness direction defining
An equiaxed particle having a layered rock salt structure and having an aspect ratio of 2 or less;
A positive electrode for a lithium secondary battery, comprising: The positive electrode of the lithium secondary battery according to claim 1,
The positive electrode of a lithium secondary battery, wherein a particle diameter of the equiaxed particles is equal to or less than the thickness of the plate-like particles. The positive electrode of the lithium secondary battery according to claim 1 or 2,
The positive electrode of a lithium secondary battery, wherein the thickness of the plate-like particles is 5 to 100 μm. The positive electrode of the lithium secondary battery according to any one of claims 1 to 3,
The positive electrode of a lithium secondary battery, wherein a plate-like particle content ratio, which is a volume percent of the plate-like particles with respect to a total of the plate-like particles and the equiaxed particles, is 5 to 80%. The positive electrode of the lithium secondary battery according to any one of claims 1 to 4,
A positive electrode of a lithium secondary battery, comprising a positive electrode active material film formed by dispersing the plate-like particles and the equiaxed particles in a binder. It has a layered rock salt structure, and the peak intensity ratio [003] / [104], which is the ratio of the diffraction intensity by the (003) plane to the diffraction intensity by the (104) plane in X-ray diffraction, is 1.6 or less, and the thickness A plate-like particle having an aspect ratio of 2.1 to 20 which is a value obtained by dividing a particle diameter defined by a minimum dimension in a plate surface direction perpendicular to the plate thickness direction defining the thickness by the thickness, and a layered rock salt structure A positive electrode comprising, as positive electrode active material particles, equiaxed particles having an aspect ratio of 2 or less,
A negative electrode containing a carbonaceous material or a lithium storage material as a negative electrode active material;
An electrolyte provided to be interposed between the positive electrode and the negative electrode;
A lithium secondary battery comprising: The lithium secondary battery according to claim 6,
The lithium secondary battery, wherein the equiaxed particles have a particle diameter equal to or less than the thickness of the plate-like particles. The lithium secondary battery according to claim 6 or 7,
The lithium secondary battery, wherein the thickness of the plate-like particles is 5 to 100 µm. A lithium secondary battery according to any one of claims 6 to 8,
Lithium secondary battery characterized in that a plate-like particle mixture ratio, which is the volume% of the plate-like particles with respect to the sum of the plate-like particles and the equiaxed particles in the positive electrode, is 5 to 80%. . The positive electrode of the lithium secondary battery according to any one of claims 6 to 9,
The positive electrode of a lithium secondary battery, wherein the positive electrode includes a positive electrode active material film formed by dispersing the plate-like particles and the equiaxed particles in a binder.

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