JP5855988B2 - Positive electrode active material powder for lithium secondary battery, and lithium secondary battery - Google Patents

Description

  The present invention relates to a positive electrode active material powder for a lithium secondary battery and a lithium secondary battery using the same.

  As a positive electrode active material constituting a positive electrode of a lithium secondary battery (sometimes referred to as a lithium ion secondary battery), a spinel-type lithium manganate containing lithium and manganese as constituent elements is known. The present applicant has already made a proposal to obtain a high rate characteristic by setting the ratio of single particles to a predetermined ratio or more in this kind of positive electrode active material powder (Japanese Patent Laid-Open No. 2010-212260). reference.). Here, the detailed definition of “single particle” will be described later. However, in general, a single single crystal primary particle exists substantially alone without forming a polycrystalline particle or an agglomerated particle. (However, it is possible that other fine particles may adhere to the single crystal primary particles.)

  An object of the present invention is to further improve the charge / discharge capacity, durability and rate characteristics in a lithium secondary battery, and to further improve the charge / discharge capacity, durability and rate characteristics in a lithium secondary battery. An object of the present invention is to provide a positive electrode active material powder for a lithium secondary battery (hereinafter simply referred to as “positive electrode active material powder”).

  The lithium secondary battery of the present invention includes a positive electrode including a large number of positive electrode active material particles and a negative electrode including a negative electrode active material. The positive electrode active material powder of the present invention is an aggregate of a large number of the positive electrode active material particles and is used when the positive electrode is manufactured. The positive electrode active material particles are spinel lithium manganate particles containing lithium and manganese as constituent elements.

The feature of the present invention is that
The volume ratio of particles having a particle size larger than 2d is 5% or less of the total,
The volume ratio of particles having a particle size smaller than 0.5d is 5 to 35% of the whole,
40% by area or more of the particles having a particle diameter larger than 0.5d is a single particle.

Here, “d” is a mode diameter (maximum frequency particle diameter) in a volume-based particle size distribution by a laser diffraction method. “Area%” is the percentage of a specific particle in a large number of particles expressed as a percentage based on the area occupied by the plane. That is, “area%” is expressed by the following equation, where A is the area occupied by all particles whose particle size can be measured, and a is the area occupied by particles having specific (focused) characteristics. Calculated. Note that these areas can be acquired, for example, by processing an image taken with an electron microscope or the like with commercially available image processing software.
100 · a / A

  “Single particle” means a particle that substantially exists alone by satisfying both of the following (1) and (2): (1) Spinel structure having a size of 1/2 or more of the particle diameter Contains primary particles of lithium manganate single crystal. (2) No aggregation is formed with other spinel lithium manganate particles having the same size as the single crystal primary particles, or between the other lithium manganate particles of the single crystal primary particles. The adhering portion of the single crystal becomes a length of 1/5 or less of the circumference of the single crystal primary particle. However, the “single particles” include those in which deposits such as fine particles having a smaller diameter than the single crystal primary particles are adhered.

Specifically, for example, bismuth compounds such as Bi ₂ O ₃ and Bi ₂ Mn ₄ O ₁₀ can adhere to the single crystal primary particles as the deposit. In this case, the positive electrode active material particles contain the bismuth compound (for example, Bi ₂ O ₃ and / or Bi ₂ Mn ₄ O ₁₀ ) in a range where the ratio of bismuth to manganese is 0.005 to 0.5 mol%. Contains.

The positive electrode active material particles preferably have high crystallinity. Specifically, the positive electrode active material particles are preferably formed so that the value of the lattice strain (η) of lithium manganate in the powder X-ray diffraction pattern is 0.7 × 10 ⁻³ or less. is there.

It is preferable that the positive electrode active material particles have few aggregated particles. Specifically, the ratio D ₅₀ / D _BET between the median diameter D ₅₀ [μm] in the positive electrode active material powder and D _BET [μm] calculated from the BET specific surface area using the following formula (1) is: The positive electrode active material particles are preferably formed so as to be 1-6.
D _BET = 6 / ( _MS ) (1)
(In said formula (1), M shows the true density [g / cm < ³ >] of the said positive electrode active material powder, and S shows a BET specific surface area [m < ² > / g].)

  The positive electrode active material powder is preferably formed to have a large particle diameter (specifically, the mode diameter d is 8 to 25 μm).

  The reason for the effect of improving the charge / discharge capacity, durability, and rate characteristics according to the present invention is under intensive study and is not completely clarified at present, but is estimated as follows.

  (A) Increase in active material packing density: the positive electrode active material particles having a relatively small particle size (specifically, those having a particle size smaller than 0.3 d: hereinafter referred to as “small particles”); The positive electrode active material particles having a particle size larger than this (that is, the particle size is in the vicinity of d: hereinafter referred to as “large particles”) coexist at a constant rate, so that a gap between the adjacent large particles is present. The small particles enter into. Thereby, the packing density of the positive electrode active material in the positive electrode is increased, and thus the charge / discharge capacity per unit volume is improved.

  (B) Suppression of crack generation in particles during electrode preparation: when the positive electrode is prepared using only the large particles made into a single particle, compared with the case where polycrystalline particles having the same particle diameter are used. Thus, cracks in the particles are likely to occur. Such cracks are mainly generated when particles collide during a process such as mixing. In particular, when an adhering substance such as a bismuth compound adheres to the large particles, a stress is concentrated on the adhering portion of the adhering substance, so that cracks are more likely to occur.

  In this regard, in the present invention, the occurrence of cracks in the positive electrode active material particles during the positive electrode preparation step is favorably suppressed by the buffering action of the small particles that have entered the gaps between the adjacent large particles. Is done. Therefore, a decrease in charge / discharge capacity due to the occurrence of cracks inside the positive electrode active material particles is satisfactorily suppressed. In particular, the effect of improving the rate characteristics by using the positive electrode active material particles (the large particles) that have been made into single particles is more favorably expressed.

  (C) Suppression of crack generation in particles during charge / discharge: As is well known, the positive electrode active material particles expand and contract in volume during charge / discharge. At this time, cracks may occur inside the positive electrode active material particles. In this regard, the positive electrode active material particles that have been made into single particles do not undergo stress relaxation at the grain boundaries unlike the polycrystalline particles having the same particle size, and therefore cracks associated with volume expansion and contraction during charging and discharging occur. It is usually easy.

  On the other hand, in the present invention, the small particles enter the gaps between the adjacent large particles, so that the consolidation characteristics in the positive electrode (positive electrode active material layer) are improved, and the large particles The isotropic compressive stress received by the is increased. Thereby, generation | occurrence | production of the crack accompanying the volume expansion / contraction at the time of charging / discharging is suppressed, and durability improves. Further, even if a crack is generated, the generated crack is closed by compressive stress, so that the conductivity due to the movement of lithium ions in the positive electrode active material particles is maintained, and thus the charge / discharge capacity Is well suppressed.

  (D) Suppression of void generation along the particle surface during charging / discharging: The positive electrode active material particles in the positive electrode are expanded and contracted during the charge / discharge of the positive electrode active material particles as described above. There can be voids along the surface. Such voids are generated, for example, by peeling between the positive electrode active material particles and the conductive binder that supports them in a dispersed state. When such voids are generated, the electron conductivity on the surface of the positive electrode material is impaired, it becomes difficult for lithium to enter and leave the positive electrode material, and the charge / discharge capacity decreases. In this regard, the positive electrode active material particles that have been made into single particles have less surface irregularities than polycrystalline particles having the same particle size, and thus the above-described voids (peeling) are likely to occur.

  On the other hand, in the present invention, the small particles enter the gaps between the adjacent large particles, thereby improving the consolidation in the positive electrode (in the positive electrode active material layer) as described above. The isotropic compressive stress received by the is increased. Thereby, generation | occurrence | production of the above voids (peeling) is suppressed favorably, and thus durability is improved. Moreover, since the conductivity by movement of lithium ions in the positive electrode is improved, rate characteristics are improved.

FIG. 1A is a cross-sectional view showing a schematic configuration of a lithium secondary battery according to an embodiment of the present invention. FIG. 1B is an enlarged cross-sectional view of the positive electrode plate shown in FIG. 1A. FIG. 2 is a graph showing the particle size distribution of the positive electrode active material powder as an aggregate of the positive electrode active material particles (Example 3) shown in FIG. 1B. FIG. 3A is a schematic diagram of the positive electrode active material particles shown in FIG. 1B. FIG. 3B is a schematic view of the positive electrode active material particles shown in FIG. 1B. FIG. 3C is a schematic view of the positive electrode active material particles shown in FIG. 1B. FIG. 3D is a schematic view of the positive electrode active material particles shown in FIG. 1B. FIG. 4 is a scanning electron micrograph showing the appearance of the positive electrode active material particles shown in FIG. 1B.

  Hereinafter, preferred embodiments of the present invention will be described using examples and comparative examples. In addition, the description about the following embodiment is specific to the extent possible, merely an example of the embodiment of the present invention in order to satisfy the description requirement (description requirement / practicability requirement) of the specification required by law. It is only what is described in.

  Therefore, as will be described later, it is quite natural that the present invention is not limited to the specific configurations of the embodiments and examples described below. Examples of the various modifications that can be made to this embodiment or example are inserted into the description of the embodiment, as they interfere with the understanding of the consistent description of the embodiment, and as much as possible. It is listed together at the end.

1. Schematic Configuration of Lithium Secondary Battery FIG. 1A is a cross-sectional view showing a schematic configuration of a lithium secondary battery 1 that is an embodiment of the present invention. Referring to FIG. 1A, the lithium secondary battery 1 of the present embodiment is a so-called liquid type, and includes a battery case 2, a separator 3, an electrolyte 4, a negative electrode 5, and a positive electrode 6.

  The separator 3 is provided so as to bisect the inside of the battery case 2. In the battery case 2, a liquid electrolyte 4 is accommodated, and a negative electrode 5 and a positive electrode 6 are provided so as to face each other with the separator 3 interposed therebetween.

  As the electrolyte 4, 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. However, a polymer electrolyte, a gel electrolyte, an organic solid electrolyte, and an inorganic solid electrolyte can also be used as the electrolyte 4 without problems.

  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 , LiCnF _{2n + 1} SO ₃ (n ≧ 2), LiN (RfOSO ₂ ) 2 [ Here, Rf and Rf ′ are fluoroalkyl groups], and the like 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 organic lithium 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 5 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 or an oxide 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 6 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 6 is easy to control.

2. Configuration of Positive Electrode FIG. 1B is an enlarged cross-sectional view of the positive electrode 6 shown in FIG. 1A. Referring to FIG. 1B, the positive electrode 6 includes a positive electrode current collector 61 and a positive electrode active material layer 62. The positive electrode active material layer 62 is composed of a binder 621 and positive electrode active material particles 622 and 623. The basic configuration of the lithium secondary battery 1 and the positive electrode 6 shown in FIGS. 1A and 1B (the battery case 2, the separator 3, the electrolyte 4, the negative electrode 5, the positive electrode current collector 61, and the binder 621). Is included in the present specification, and a detailed description thereof is omitted.

  The positive electrode active material particles 622 are relatively large particles, of which a predetermined ratio is a single particle. On the other hand, the positive electrode active material particles 623 are relatively small particles, and are single particles, polycrystalline particles, or aggregate particles. These are supported in a dispersed state in the conductive binder 621.

In the present embodiment, the positive electrode active material powder that is an aggregate of the positive electrode active material particles 622 and 623 has the following characteristics.
The mode diameter d is 8-25 μm,
The volume ratio of particles having a particle size larger than 2d is 5% or less of the total,
The volume ratio of particles having a particle size smaller than 0.5d is 5 to 35% of the whole,
Of the particles having a particle size larger than 0.5d, 40 area% or more is a single particle.
Preferably, the volume ratio of particles having a particle size smaller than 0.5d is 10 to 30% of the whole.

  Here, the mode diameter d is the maximum frequency particle diameter in the volume-based particle size distribution by the laser diffraction method. FIG. 2 is a graph showing the particle size distribution of the positive electrode active material powder (specifically, Example 3 described later) as an aggregate of the positive electrode active material particles 622 and 623 shown in FIG. 1B. In FIG. 2, the positions of 0.5d, d, and 2d are indicated by arrows.

  3A to 3D are schematic views of the positive electrode active material particles 622 shown in FIG. 1B. Hereinafter, the significance of the “single particle” in the present invention will be described in detail with reference to FIGS. 3A to 3D.

  As shown in FIG. 3A, the positive electrode active material particles 622 that exist as “single particles” typically do not form aggregates with other lithium manganate particles and exist alone. Lithium single crystal primary particles 622a are included. Specifically, the positive electrode active material particles 622 existing as “single particles” may be composed of only the lithium manganate single crystal primary particles 622a that do not form aggregates and exist alone. As shown in 3A, deposits 622b1, 622b2, 622b3... Made of a material different from lithium manganate may be adhered to lithium manganate single crystal primary particles 622a.

  However, as shown in FIG. 3B, lithium manganate crystal fine particles 622c1, 622c2 (which may be single crystal or polycrystal) may be included in lithium manganate single crystal primary particles 622a. In the case where it is attached and the attached amount is slight, or as shown in FIG. 3C, the lithium manganate single crystal primary particles 622a are bonded together through a few attached portions, There is almost no adverse effect on the rate characteristics. On the other hand, as shown in FIG. 3D, when the adhering part between the lithium manganate single crystal primary particles 622a is large, diffusion of lithium ions in the adhering part (grain boundary part) is inhibited, Rate characteristics are degraded.

  Therefore, as shown in FIGS. 3A to 3D, when the positive electrode active material particles 622 are composed of a plurality of particles, whether or not this is a “single particle” is determined as follows: The length of the adhering portion AD with other particles with respect to the circumference of one lithium manganate single crystal primary particle 622a constituting the positive electrode active material particle 622 (in a plan view or a sectional view of a predetermined section) In the case where the total number of the positive electrode active material particles is 1/5 or less, the positive electrode active material particles 622 are “single particles” including the lithium manganate single crystal primary particles 622a as “mother particles”. Based on this standard, the positive electrode active material particles 622 shown in FIGS. 3A to 3C are “single particles”, and the lithium manganate crystal fine particles 622c1 and 622c2 in FIG. 3B are the above “mother particles”. In other words, the positive electrode active material particles 622 shown in FIG. 3D are not “single particles”.

  FIG. 4 is a scanning electron micrograph showing the appearance of the positive electrode active material particles 622 (large particles A2 described later) shown in FIG. 1B. As shown in FIG. 4, the lithium manganate single crystal primary particles 622a as the “mother particles” in the positive electrode active material particles 622 existing as “single particles” Other lithium manganate crystal fine particles 622c1, deposits 622b1 made of a bismuth compound, and the like are adhered.

3. Outline of Manufacturing Method Hereinafter, an outline of a manufacturing method of the positive electrode active material powder (positive electrode active material particles 622 and 623) of the present embodiment will be described.

First, a mixed powder containing a lithium compound, a manganese compound, and a bismuth compound is prepared. Note that as the lithium compound and the manganese compound, a compound containing both lithium and manganese (for example, LiMn ₂ O ₄ ) may be used. As the manganese compound and bismuth compound, a compound containing both manganese and bismuth (for example, Bi ₂ Mn ₄ O ₁₀ ) may be used. Furthermore, a seed crystal composed of lithium manganate having a spinel structure can be added to the above-described mixed powder in order to promote grain growth. This seed crystal functions as a nucleus during grain growth. At this time, the bismuth compound may adhere to the seed crystal.

Examples of the lithium compound include Li ₂ CO ₃ , LiNO ₃ , LiOH, Li ₂ O ₂ , Li ₂ O, CH ₃ COOLi, Li (OCH ₃ ), Li (OC ₂ H ₅ ), and Li (OC ₃ H _7. ), Li (OC ₄ H ₉ ), Li (C ₁₁ H ₁₉ O ₂ ), Li ₂ C ₂ O ₄ , LiCl, and the like. The manganese _{compound, MnO 2, MnO, Mn 2} O 3, Mn 3 O 4, MnCO 3, MnOOH, Mn (OCH 3) 2, Mn (OC 2 H 5) 2, Mn (OC 3 H 7) 2, MnC ₂ O ₄ , Mn (CH ₃ COO) ₂ , MnCl ₂ , Mn (NO ₃ ) ₂ , etc. can be used. In addition, when substituting manganese with substitution elements other than lithium, an aluminum compound, a magnesium compound, a nickel compound, a cobalt compound, a titanium compound, a zirconium compound, a cerium compound, etc. are contained in the mixed powder raw material.

Examples of the aluminum compound include α-Al ₂ O ₃ , γ-Al ₂ O ₃ , AlOOH, Al (OH) ₃ , Al (OCH ₃ ) ₃ , Al (OC ₂ H ₅ ) ₃ , Al (OC ₃ H ₇ ) ₃ , Al (OC ₄ H ₉ ) ₃ , AlOCl, Al (NO ₃ ) ₃ , etc. may be used. Examples of the magnesium compound include MgO, Mg (OH) ₂ , MgCO ₃ , Mg (OCH ₃ ) ₂ , Mg (OC ₂ H ₅ ) ₂ , Mg (OC ₃ H ₇ ) ₂ , Mg (OC ₄ H _{9) 2, Mg (C 11} H 19 O 2) 2, MgCl 2, Mg (C 2 H 3 O 2) 2, Mg (NO 3) 2, MgC 2 O 4, may equally be used. As the nickel compound, for example, NiO, Ni (OH) ₂ , NiNO ₃ , Ni (C ₂ H ₃ O ₂ ) ₂ , NiC ₂ O ₄ , NiCO ₃ , NiCl ₂ , etc. can be used. As the cobalt compound, for example, Co ₃ O ₄ , CoO, Co (OH) ₃ , CoCO ₃ , CoC ₂ O ₄ , CoCl ₂ , Co (NO ₃ ) ₂ , Co (OC ₃ H ₇ ) ₂ , etc. are used. Can be. Examples of the titanium compound include TiO, TiO ₂ , Ti ₂ O ₃ , Ti (OCH ₃ ) ₄ , Ti (OC ₂ H ₅ ) ₄ , Ti (OC ₃ H ₇ ) ₄ , and Ti (OC ₄ H ₉ ) _4. TiCl ₄ , etc. can be used. Examples of the zirconium compound include ZrO ₂ , Zr (OH) ₄ , ZrO (NO ₃ ) ₂ , Zr (OCH ₃ ) ₄ , Zr (OC ₂ H ₅ ) ₄ , Zr (OC ₃ H ₇ ) ₄ , Zr ( OC ₄ H ₉ ) ₄ , ZrOCl ₂ , etc. may be used. The cerium compound, for _{example, CeO 2, Ce (OH)} 4, Ce (NO 3) 3, etc. may be used.

  A grain growth promoting aid may be added to the mixed powder as necessary. Grain growth promoting aids (flux aids or low melting point aids) include low melting point oxides, chlorides, borides, carbonates, nitrates, hydroxides, oxalates, acetates, alkoxides, permanganates. , Etc. can be used. The grain growth promoting aid may be added separately from the seed crystal, may be added in a state of adhering to the seed crystal, or both may be used together, but both are used together. It is preferable to add in. Grain growth promoting aid adheres to the seed crystal, so that the seed crystal effectively grows from the point where it adheres. On the other hand, by adding it separately from the seed crystal, there are some parts where there is no seed crystal. The grain growth of the seed crystal proceeds and the seed crystal is prevented from growing abnormally. Thereby, the grain size of crystal grains becomes more uniform.

  You may grind | pulverize the above-mentioned mixed powder as needed. For example, the particle size of the mixed powder is preferably 10 μm or less. For this reason, when the particle diameter of mixed powder is larger than 10 micrometers, it is preferable to grind | pulverize the above-mentioned mixed powder so that a particle size may become 10 micrometers or less by a dry-type or wet grinding method. The pulverization method is not particularly limited, but a pot mill, a bead mill, a hammer mill, a jet mill or the like can be used.

Next, it shape | molds into a molded object of a suitable shape using the above-mentioned mixed powder. The molding method is not particularly limited, and for example, a conventionally known molding method can be used. Specifically, when obtaining a tape-like, sheet-like, or flaky shaped body,
・ Doctor blade method ・ Screen printing method ・ Slurry of raw material particle powder is applied onto a heated drum and the dried material is scraped off with a scraper. Drum dryer method ・ On the surface of a heated disk of the raw material particle powder slurry It is possible to use a molding method such as an extrusion molding method in which the coated and dried material is scraped off with a scraper, and the clay containing the raw material particle powder is extruded into a die provided with a slit. In addition, you may make it raise the density by pressurizing the molded object obtained by the above-mentioned shaping | molding method with a roller etc. further.

  Among these molding methods, the doctor blade method is preferred because a uniform sheet-like molded body can be obtained. This doctor blade method is performed, for example, by applying a slurry to a flexible plate (for example, an organic polymer plate such as a polyethylene terephthalate (PET) film) and drying and solidifying the applied slurry to form a molded body. Is called. And the molded object before baking is obtained by peeling this molded object and a board. The slurry is preferably prepared so as to have a viscosity of 500 to 4000 mPa · s, and is preferably defoamed under reduced pressure.

  Moreover, a hollow granulated body can be produced by appropriately setting the conditions of the spray dryer. Examples of a method for producing a granular molded body (bulk molded body) having a diameter of 10 to 30 μm include, for example, a spray drying method, a method of pressing raw material particle powder with a roller, an extruded molded rod-shaped or sheet-shaped molded body. The method etc. which cut | disconnect can be used. Moreover, as a method for producing a honeycomb-shaped or rod-shaped molded body, for example, an extrusion molding method or the like can be used. Moreover, as a method of producing a roll-shaped molded body, for example, a drum dryer method or the like can be used.

  Subsequently, the above-described molded body is fired (heat treatment) at 830 to 1050 ° C. Thereby, a molded object turns into a sintered body of spinel type lithium manganate (positive electrode active material). If the firing temperature is less than 830 ° C., grain growth may be insufficient. On the other hand, when the firing temperature exceeds 1050 ° C. (for example, reaches about 1100 ° C.), spinel-type lithium manganate may decompose into lithium manganate and manganese oxide having a layered rock salt structure by releasing oxygen. is there.

  The firing step can be performed, for example, by putting the above-described molded body into an alumina crucible or sheath and further placing the crucible or sheath into a furnace. When the above-described molded body is put into a crucible or a sheath, the above-described molded body can be processed (bent or cut) in advance so as to have an appropriate length or shape. Or a sheet-like molded object can be baked in the state put on the setter for every sheet so that the overlap between sheets may become small.

  Note that the firing atmosphere may be an oxygen atmosphere (a state where the oxygen partial pressure is high) (in this case, the oxygen partial pressure is preferably 50% or more of the atmospheric pressure of the firing atmosphere, for example). As a result, oxygen release from the spinel type lithium manganate is less likely to occur, so that the above-described decomposition and generation of oxygen vacancies are effectively suppressed. Further, in the firing, the presence of the above-mentioned grain growth promoting aid and seed crystal promotes grain growth even when the firing temperature is relatively low (for example, about 900 ° C.), thereby improving the crystallinity. It is assumed that is played.

  By appropriately adjusting the rate of temperature rise during firing (for example, 50 to 500 ° C./hour), the particle size of the primary particles after firing can be made uniform. Further, by maintaining the temperature in a low temperature range and then firing at the firing temperature, the primary particles can be uniformly grown. In this case, as a low temperature range, when a calcination temperature is 900 degreeC, it can be set as 400-800 degreeC, for example. The primary particles can also be grown uniformly by maintaining the temperature higher than the firing temperature and firing the crystal at the firing temperature after forming crystal nuclei. In this case, the temperature higher than the firing temperature can be, for example, 1000 ° C. when the firing temperature is 900 ° C.

  Firing can be performed in two stages. For example, lithium manganate can be formed by forming a mixed powder of manganese oxide and alumina into a sheet and firing it, and then adding a lithium compound and further firing. Moreover, after forming a lithium manganate crystal having a high lithium content, lithium manganate can be formed by adding manganese oxide or alumina and further firing.

  Subsequently, the obtained sintered body is crushed and / or classified. Crushing is performed to such an extent that cleavage occurs at the adhering portion (grain boundary portion) between adjacent primary particles while the primary particles are not destroyed by wet or dry processing. The crushing method is not particularly limited, and a method of crushing by pressing against a mesh or screen having a predetermined opening diameter, a method using a pot mill, a bead mill, a hammer mill, a jet mill, or the like can be used. The classification method is not particularly limited. For example, a method of sieving with a mesh having a predetermined opening diameter, a method using a water tank, a method of using an airflow classifier, a sieve classifier, an elbow jet classifier, etc. Etc. can be used.

  The positive electrode active material powder having a predetermined particle size distribution and a predetermined single particle ratio can be obtained by appropriately setting the above-described crushing and / or classification conditions. Alternatively, the positive electrode active material powder having a predetermined particle size distribution and a predetermined single particle ratio can be obtained by mixing a plurality of types of powders obtained by the above-mentioned crushing and / or classification. It is. That is, for example, a positive electrode active material having a predetermined particle size distribution and a predetermined single particle ratio by mixing a plurality of types of powders having different crushing and / or classification conditions for the obtained sintered body It is possible to obtain a powder.

  The obtained positive electrode active material powder is heat-treated again at a temperature lower than the above firing temperature, thereby repairing oxygen deficiency and recovering disorder of crystallinity during crushing (however, Such re-heat treatment is not essential.) However, when the temperature is lowered from the firing temperature in the firing step prior to the crushing treatment, holding at a desired temperature for a certain time has an effect as a reheat treatment because the oxygen deficiency is thereby repaired. When reheat treatment is performed after the crushing treatment (or after the classification treatment), the reheated powder may be crushed and classified again. In this case, the above-described method or the like can be used for the second crushing / classifying process.

4). Example (Example)
Hereinafter, the specific example of the manufacturing method of the positive electrode plate 2 of this embodiment and its evaluation result are demonstrated.

4-1. Method for producing positive electrode active material powder (1) Slurry preparation Li ₂ CO ₃ powder (manufactured by Honjo Chemical Co., Ltd., fine grade, average particle size so as to have a chemical formula of Li _1.08 Mn _1.83 Al _0.09 O ₄ 3 μm diameter), MnO ₂ powder (manufactured by Tosoh Corporation, electrolytic manganese dioxide, FM grade, average particle diameter 5 μm, purity 95%), and Al (OH) ₃ powder (manufactured by Showa Denko KK, product model number “H-43M” The average particle size 0.8 μm) was weighed, and Bi ₂ O ₃ powder (average particle size 0.3 μm, manufactured by Taiyo Mining Co., Ltd.) was added to the manganese contained in the MnO ₂ raw material. 1 was weighed to the amount described in 1. 100 parts of this weighed product and 100 parts of an organic solvent (mixed liquid in which equal amounts of toluene and isopropyl alcohol are mixed) as a dispersion medium are placed in a cylindrical wide-mouth bottle made of synthetic resin, and a ball mill (zirconia ball having a diameter of 5 mm). 16 hours wet mixing and grinding.

  100 parts by weight of the powder obtained by the above-described wet mixing and pulverization, 10 parts by weight of a binder (polyvinyl butyral: product model “BM-2” manufactured by Sekisui Chemical Co., Ltd.), and a plasticizer (DOP: Di (2-ethylhexyl) ) Phthalate, manufactured by Kurokin Kasei Co., Ltd.) 4 parts by weight and 2 parts by weight of a dispersant (product name “Reodol SP-O30” manufactured by Kao Corporation) are mixed, and the mixture is further stirred under reduced pressure. As a result, the slurry was prepared by adjusting the viscosity to 4000 mPa · s (viscosity was measured with an LVT viscometer manufactured by Brookfield. The same applies hereinafter).

(2) Molding The slurry prepared as described above was molded on a PET film by a doctor blade method so that the thickness after drying was a value described in Table 1. A large number of square shaped bodies were produced by cutting the sheet-like shaped bodies peeled from the PET film into 300 mm squares with a cutter.

(3) Heat treatment (firing)
The obtained square shaped product was put in an alumina sheath (dimensions: 90 mm × 90 mm × height 60 mm) in a state of being crumpled and opened with a lid open (that is, in an air atmosphere). Degreasing was carried out at 2 ° C. for 2 hours, followed by baking at 900 ° C. for 12 hours.

(4) Crushing / Classification The fired sheet-like molded body was placed on a polyester mesh having an average opening diameter selected under the following conditions, and lightly pressed on a sieve with a spatula to be crushed. Thereafter, the pulverized powder is dispersed in ethanol, subjected to ultrasonic treatment (38 kHz, 5 minutes) with an ultrasonic cleaner, and then passed through a polyester mesh having an average opening diameter of 5 μm to leave the powder remaining on the mesh. By collecting, fine particles having a particle size of 5 μm or less generated during firing or grinding were removed. Thereby, large particles A1 to A3 and B1 to B3 described in Table 1 were obtained. On the other hand, small particles C shown in Table 1 were obtained by dry crushing the fired sheet-shaped molded body for 3 hours with a pot mill using nylon cobblestone having a diameter of 5 mm.
Level of sheet-shaped molded product having a thickness of 10 μm or less: average opening diameter of 10 μm
Level of sheet-shaped molded product with a thickness of 11 to 20 μm: average opening diameter of 20 μm
Level of sheet-like molded body with a thickness exceeding 20 μm: average opening diameter of 40 μm

Table 1 shows the median diameter D ₅₀ [μm] and the BET specific surface area [m ² / g] of the large particles A1 to A3 and B1 to B3 and the small particles C obtained as described above. In addition, the measurement of the particle size distribution relationship including the median diameter D ₅₀ [μm] is performed using a laser diffraction particle size distribution measuring device (product model number MT3300EXII manufactured by Nikkiso Co., Ltd.), and a particle size of 132 in the range of 0.02 μm to 2000 μm. This was done by logarithmic division into categories. The specific surface area was measured with a flow-type specific surface area measuring device (product name “Flowsorb III2305” manufactured by Shimadzu Corporation) as an adsorption gas.

(5) Mixing After weighing the large particles A1 to A3 and B1 to B3 and the small particles C obtained as described above in the weight ratio shown in Table 2, they are put into a glass container and shaken. The positive electrode active material powders of Examples 1 to 6 and Comparative Examples 1 to 5 were obtained by mixing and dry mixing. Note that “Bi content ratio” and “Bi compound type” in Table 2 indicate the measurement results of the following “bismuth content ratio” and “bismuth compound type”.

4-2. Evaluation Method (1) Bismuth Content Ratio A solution sample prepared by adding hydrochloric acid to a positive electrode active material powder sample (Examples 1 to 6 and Comparative Examples 1 to 5) and decomposing it under pressure was measured using an ICP emission spectroscopic analyzer (stock) Quantitative analysis of lithium, manganese, and bismuth by the product name “ULTIMA2” manufactured by HORIBA, Ltd. Based on this quantitative analysis, bismuth contained in bismuth compounds with respect to manganese contained in lithium manganate The content ratio was calculated.

(2) Bismuth compound type Identified using an X-ray diffractometer (product name “Geiger Flex RAD-IB” manufactured by Rigaku Corporation). If the amount is too small to be confirmed by X-ray diffraction (XRD), bismuth is detected by EPMA (Electron Probe Micro-Analysis) using an electron beam microanalyzer (product model number “JXA-8800” manufactured by JEOL Ltd.). When other components are detected in a region, bismuth is assumed to exist as a component and a compound.

(3) Particle size distribution The mode diameter d, the volume ratio of particles having a particle diameter larger than 2d (D> 2d), the volume ratio of particles having a particle diameter smaller than 0.5d (D <0.5d), and the median diameter D ₅₀ Was measured using a laser diffraction particle size distribution analyzer as described above. Note that the mode diameter d and the median diameter D ₅₀ are the center values of the corresponding particle size classification in the volume-based particle size distribution histogram. Moreover, the ratio [area%] of single particles among particles having a particle size larger than 0.5 d was measured as follows.

  A positive electrode active material powder sample (Examples 1 to 6 and Comparative Examples 1 to 5) and a conductive resin (trade name “Technobit 5000” manufactured by Kultzer) are mixed and cured, and then mechanically polished. Furthermore, a cross-sectional sample for observation with a scanning electron microscope was prepared by ion polishing using a cross-sectional sample preparation device (product model number “SM-09010” manufactured by JEOL Ltd.). The reflection electron image of the cross-sectional sample thus prepared was observed using a scanning electron microscope (product name “ULTRA55”, manufactured by ZEISS).

  In the reflected electron image, when the crystal orientation is different, the contrast is different due to the channeling effect. For this reason, when the grain boundary part is included in the observed particles, the grain boundary part becomes clear or unclear when the observation direction (inclination of the sample) of the sample is slightly changed. Using this property, the presence of the grain boundary portion can be confirmed. In this way, it is possible to identify whether the observed particle is a single particle, or a polycrystalline particle or an agglomerated particle in which primary particles having different crystal orientations are connected.

  In addition, in the particle form as shown in FIGS. 3A to 3D, whether or not it is a single particle is identified using image processing software (trade name “Image-Pro” manufactured by Media Cybernetics). The length of the adhering portion (the sum of the lengths of all the adhering portions when there are a plurality of adhering portions) is 1/5 with respect to the circumference of the particle estimated from the reflected electron image. If it was 5 or less, the particle was considered a single particle.

  And about the particle | grains in a cross-section reflected electron image, when the average value of two diameters mutually orthogonal is made into a particle size [micrometer], all the area (B) which the particle | grains used as particle size> 0.5d occupy, The area (b) occupied by a single particle among the particles having a particle size> 0.5d is measured using image processing software (trade name “photoshop (registered trademark)” manufactured by Adobe), and (b By calculating the value of / B) × 100, the ratio (area%) of single particles in the particles having a particle size> 0.5d was obtained.

(4) Lattice strain The powder X-ray diffraction patterns of the positive electrode active material powder samples (Examples 1 to 6 and Comparative Examples 1 to 5) were measured using an X-ray diffractometer (product name “D8 ADVANCE” manufactured by Bruker AXS). The lattice strain (η) was calculated by measuring under the following conditions and analyzing by the WPPD (Whole-Powder-Pattern Decomposition) method.
X-ray output: 40 kV x 40 mA
Goniometer radius: 250mm
Divergent slit: 0.6 °
Scattering slit: 0.6 °
Light receiving slit: 0.1 mm
Solar slit: 2.5 ° (incident side, light receiving side)
Measurement method: 2θ / θ method using a sample horizontal type concentrated optical system (2θ = 15 to 140 ° is measured, step width is 0.01 °)
Scanning time: set so that the intensity of the main peak ((111) plane) is about 10,000 counts.

A specific analysis procedure will be described below.
1. Starts up total profile fitting software (product name “DIFFRAC ^plus TOPAS”) and reads measurement data.
2. Emission Profile setting (Cu tube, Bragg Brentano concentrated optical system is selected).
3. Background setting (Legendre's polynomial is used as the profile function, and the number of terms is set to 8 to 20).
4). Instrument setting (use Fundamental Parameter, input slit condition, filament length, sample length).
5). Corrections setting (use sample displacement. If the sample packing density in the sample holder is low, also use Absorption. In this case, Absorption is fixed by the linear absorption coefficient of the sample).
6). Crystal structure setting (set to space group F-d3m. Use lattice constant, crystallite diameter, and lattice strain. Set spread of profile due to crystallite diameter and lattice strain to Lorentz function).
7). Calculation (background, sample displacement, diffraction intensity, lattice constant, crystallite diameter, lattice strain refined).
8). If the standard deviation of the crystallite diameter is 6% or less of the refined value, the analysis is completed. If greater than 6%, go to step 9.
9. Set the spread of the profile due to lattice distortion to a Gaussian function (the crystallite diameter remains the Lorentz function).
10. Calculation (background, sample displacement, diffraction intensity, lattice constant, crystallite diameter, lattice strain refined).
11. If the standard deviation of the crystallite diameter is 6% or less of the refined value, the analysis is completed. If it is greater than 6%, analysis is not possible.
12 By multiplying the obtained lattice strain value by π / 180, η is obtained.

(5) Battery characteristics Positive electrode active material powder samples (Examples 1 to 6 and Comparative Examples 1 to 5), acetylene black as a conductive agent, PVDF as a binder, and NMP as a solvent are 90: 5: 5. A positive electrode paste was prepared by mixing using a hybrid mixer at a weight ratio of 100. The positive electrode paste was placed on an aluminum foil as a positive electrode current collector, and then coated with an applicator adjusted to a gap of 150 μm, and dried at 80 ° C. for 10 hours. The laminated body of the positive electrode paste and aluminum foil after drying was punched out into a substantially circular shape in a plan view with a diameter of 14 mm, and press-formed at ² t / cm ² to prepare a positive electrode.

The positive electrode thus prepared, the negative electrode formed by laminating a negative electrode layer made of a lithium metal plate and a negative electrode current collector made of a stainless steel plate, and a separator made of a polyethylene film having lithium ion permeability were used as an aluminum foil in the positive electrode. A lithium secondary battery (coin cell) was fabricated by laminating the positive electrode, the separator, the negative electrode layer, and the negative electrode current collector plate in this order with the side facing outward (the side opposite to the separator), and filling the laminate with the electrolytic solution. 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 lithium secondary battery (coin cell) thus manufactured, the initial capacity and cycle characteristics were evaluated in the following manner.

(5-1) Initial capacity [mAh / g]
The cycle charge / discharge with one cycle of the following charge / discharge operation at a test temperature of 20 ° C. was repeated for a total of 3 cycles: (1) Constant current until the battery voltage reached 4.3 V at a current value of 0.1 C rate. (2) Charge at a constant voltage until the current value drops to 1/20 under the current condition of maintaining the battery voltage at 4.3 V. (3) After 10 minutes of rest, the battery voltage at the current value of 1C rate Is discharged at a constant current until the voltage reaches 3.0 V, and then rests for 10 minutes. The measured value of the discharge capacity at the third cycle was taken as the initial capacity.

(5-2) Cycle characteristics [%]
The test temperature was set to 60 ° C., and cycle charge / discharge with the following charge / discharge operation as one cycle was repeated for a total of 100 cycles: after charging to 4.3 V at a constant current of 5C rate, 3. The battery was discharged to 0V. A value obtained by dividing the discharge capacity at the 100th time by the discharge capacity at the first time as a percentage was defined as the cycle characteristics.

4-3. Evaluation Results Tables 3 and 4 show the results of evaluating the positive electrode active material powder samples of Examples 1 to 6 and Comparative Examples 1 to 5 by the above-described procedure. Hereinafter, the evaluation results will be discussed with reference to Tables 1 to 4. The cycle characteristics are “good” when 80% or more and “bad” when less than 75%.

Examples 1-6 are
The mode diameter d is 8-25 μm,
The volume ratio of particles having a particle size larger than 2d is 5% or less of the total,
The volume ratio of particles having a particle size smaller than 0.5d is 5 to 35% of the whole,
The particle size of the large particles and the mixing ratio of the large particles and the small particles are changed so that 40% by area or more of the particles having a particle size larger than 0.5d satisfy the property of being a single particle. It is. According to the positive electrode active material powder samples of Examples 1 to 6, good initial capacity and cycle characteristics were achieved.

  On the other hand, in Comparative Example 1 in which the small particles C were not blended, the volume ratio of the particles having a particle size smaller than 0.5d was insufficient, and the predetermined amount due to the small particles entering the gap between adjacent large particles. The effect was not manifested and the cycle characteristics were poor. On the other hand, in Comparative Example 2 in which the compounding amount of the small particles C is excessive, the cycle characteristics were slightly lowered because the volume ratio of the particles having a particle size smaller than 0.5d was too large.

Moreover, in Comparative Example 3, the mode diameter d is larger than the predetermined range in Comparative Example 3 in which the particle diameter of the large particles is too large (B1: median diameter D ₅₀ = 26 μm), and Comparative Example 4 (B2 : Median diameter D ₅₀ = 6 μm), the mode diameter d was smaller than the above predetermined range, and both the cycle characteristics were slightly lowered. Furthermore, in Comparative Example 5 in which the ratio of single particles among particles having a particle size larger than 0.5d was low, the cycle characteristics were poor.

5). Modifications 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, as described above. The present invention should not be limited by the above-described 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.

  The configuration of the lithium secondary battery 1 to which the present invention is applied is not limited to the configuration described above. Specifically, for example, a gel polymer electrolyte can be used as the electrolyte. Also, there is no problem with the composition of the positive electrode active material other than the above specific examples (examples).

  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 1 Lithium secondary battery 2 Battery case 3 Separator 4 Electrolyte 5 Negative electrode 6 Positive electrode 61 Positive electrode collector 62 Positive electrode active material layer 621 Binder 622 Positive electrode active material particle 622a Lithium manganate single crystal primary particle 622b1 Adherent 622b2 Adherent 622b3 Deposit 622c1 Lithium manganate crystal fine particles 622c2 Lithium manganate crystal fine particles 623 Positive electrode active material particles

JP 2010-212260 A

Claims (2)

A positive electrode active material powder of a lithium secondary battery, which is an aggregate of a large number of positive electrode active material particles,
The positive electrode active material particles are lithium manganate particles having a spinel structure containing lithium and manganese as constituent elements,
The mode diameter which is the maximum frequency particle size in the volume-based particle size distribution by laser diffraction method is d,
The percentage of specific ones in a number of positive electrode active material particles, expressed as a percentage based on the area occupied in a given plane, is area% ,
For each of the plurality of positive electrode active material particles in the predetermined plane, when the average value of two diameters orthogonal to each other is the particle size ,
The volume ratio of particles having a particle size larger than 2d is 5% or less of the total,
The volume ratio of particles having a particle size smaller than 0.5d is 5 to 35% of the whole,
Particle size less than 40 area% of the particles larger than 0.5d is Ri Oh single particles,
The single particle is
Other spinel-structured lithium manganate particles comprising spinel-structure lithium manganate single-crystal primary particles having a size of 1/2 or more of the particle size of the single particles, and having the same size as the single particles Particles that do not form aggregates with
Or
Other spinel-structured lithium manganate particles comprising spinel-structure lithium manganate single-crystal primary particles having a size of 1/2 or more of the particle size of the single particles, and having the same size as the single particles The adhering portion between and the single crystal primary particle is a particle having a length of 1/5 or less of the circumference of the single crystal primary particle,
The mode diameter d is 8 to 25 μm.
The positive electrode active material particle powder of a lithium secondary battery characterized by the above-mentioned. A positive electrode comprising a number of positive electrode active material particles;
A negative electrode containing a negative electrode active material;
A lithium secondary battery comprising:
The positive electrode active material particles are lithium manganate particles having a spinel structure containing lithium and manganese as constituent elements,
The mode diameter which is the maximum frequency particle diameter in the volume-based particle size distribution by laser diffraction of a large number of the positive electrode active material particles is defined as d,
The percentage of specific ones in a number of positive electrode active material particles, expressed as a percentage based on the area occupied in a given plane, is area% ,
For each of the plurality of positive electrode active material particles in the predetermined plane, when the average value of two diameters orthogonal to each other is the particle size ,
The volume ratio of particles having a particle size larger than 2d is 5% or less of the total,
The volume ratio of particles having a particle size smaller than 0.5d is 5 to 35% of the whole,
Particle size less than 40 area% of the particles larger than 0.5d is Ri Oh single particles,
The single particle is
Other spinel-structured lithium manganate particles comprising spinel-structure lithium manganate single-crystal primary particles having a size of 1/2 or more of the particle size of the single particles, and having the same size as the single particles Particles that do not form aggregates with
Or
Other spinel-structured lithium manganate particles comprising spinel-structure lithium manganate single-crystal primary particles having a size of 1/2 or more of the particle size of the single particles, and having the same size as the single particles The adhering portion between and the single crystal primary particle is a particle having a length of 1/5 or less of the circumference of the single crystal primary particle,
The mode diameter d is 8 to 25 μm.
A lithium secondary battery characterized by the above.