JP5061437B2 - Lithium ion secondary battery - Google Patents

Description

The present invention relates to a lithium ion secondary battery including a composite oxide containing lithium (Li) and cobalt (Co) in a positive electrode and including natural graphite in a negative electrode.

  In recent years, many portable electronic devices such as a camera-integrated VTR (videotape recorder), a mobile phone, or a laptop computer have appeared, and their size and weight have been reduced. Accordingly, as a portable power source for these electronic devices, research and development for improving the energy density of batteries, particularly secondary batteries, are being actively promoted. Among them, a lithium ion secondary battery using lithium cobaltate for the positive electrode and a carbon material for the negative electrode has a higher energy density than a lead battery or nickel cadmium battery, which is a conventional aqueous electrolyte secondary battery. Because it is highly expected.

Carbon materials include natural graphite or artificial graphite such as mesocarbon microbeads (MCMB). Among these, natural graphite has a larger discharge capacity than artificial graphite, and has the advantage that the battery capacity can be increased. (For example, see Patent Documents 1 and 2).
JP-A-6-290781 JP-A-8-213020

  However, natural graphite has a portion where lithium can be occluded and released only at the edge, and has, for example, load characteristics compared to mesocarbon microbeads that can occlude and release lithium from the entire surface. Inferior, charge / discharge reaction tends to be non-uniform. For this reason, when lithium cobaltate with a large crystallite is used for the positive electrode, the charge / discharge performance is likely to deteriorate due to structural distortion of the positive electrode due to the non-uniform reaction during charge / discharge, and the discharge capacity after cycling is increased. There was a problem of being lowered.

The present invention has been made in view of such problems, and an object thereof is to provide a lithium ion secondary battery having a high capacity and excellent cycle characteristics.

A lithium ion secondary battery according to the present invention includes an electrolyte together with a positive electrode and a negative electrode. The positive electrode includes a lithium-cobalt composite oxide containing lithium and cobalt, and the negative electrode is spheroidized. include natural graphite, lithium - crystallite size of cobalt composite oxide, (003) vector direction is at 50nm or more 80nm or less, (110) vector direction Ri der than 100nm or less 60 nm, natural graphite, the surface of the It is formed so as to cover at least a part, and has a coating part made of either one of sodium glutamate or sodium carboxymethyl cellulose, and the ratio of the coating part to natural graphite (coating part / natural graphite) is In the range of 0.1% by mass to 2% by mass .

According to the lithium ion secondary battery of the present invention, since the negative electrode includes spherical natural graphite, the capacity can be increased. In addition, the positive electrode contains a lithium-cobalt composite oxide containing lithium and cobalt having a crystallite diameter in the (003) vector direction of 50 nm to 80 nm and a crystallite diameter of (110) vector direction of 60 nm to 100 nm. As a result, cycle characteristics can be improved.

Further, at least a portion of the surface of natural graphite, since to form a coating portion made of any one polymer compound of sodium glutamate or sodium carboxymethylcellulose at a predetermined ratio, to improve the ionic conductivity In addition, for example, the decomposition reaction can be suppressed even when the content of propylene carbonate is increased, so that the low-temperature characteristics can be improved.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows an exploded configuration of a secondary battery according to an embodiment of the present invention. In the secondary battery, a wound electrode body 20 to which a positive electrode lead 11 and a negative electrode lead 12 are attached is enclosed in a film-shaped exterior member 31.

  The positive electrode lead 11 and the negative electrode lead 12 are led out from the inside of the exterior member 31 to the outside, for example, in the same direction. The positive electrode lead 11 and the negative electrode lead 12 are each made of a metal material such as aluminum (Al), copper (Cu), nickel (Ni), or stainless steel, and each have a thin plate shape or a mesh shape.

  The exterior member 31 is made of, for example, a rectangular laminate film in which a nylon film, an aluminum foil, and a polyethylene film are bonded together in this order. The exterior member 31 is disposed, for example, so that the polyethylene film side and the wound electrode body 20 face each other, and the outer edge portions are in close contact with each other by fusion or an adhesive.

  The exterior member 31 may be made of a laminate film having another structure, a polymer film such as polypropylene, or a metal film instead of the above-described laminate film.

  FIG. 2 shows a cross-sectional structure taken along line II-II of the spirally wound electrode body 20 shown in FIG. The wound electrode body 20 is obtained by laminating a negative electrode 21 and a positive electrode 22 with a separator 23 interposed therebetween, and the outermost peripheral portion is protected by a protective tape 24.

  The negative electrode 21 includes, for example, a negative electrode current collector 21A and a negative electrode active material layer 21B provided on both surfaces or one surface of the negative electrode current collector 21A. In the negative electrode current collector 21A, there is an exposed portion where the negative electrode active material layer 21B is not provided at one end in the longitudinal direction, and the negative electrode lead 12 is attached to this exposed portion.

  The negative electrode current collector 21A preferably has good electrochemical stability, electrical conductivity, and mechanical strength, and is made of, for example, a metal material such as copper, nickel, or stainless steel.

  The negative electrode active material layer 21B includes, as the negative electrode active material, for example, a carbon material capable of inserting and extracting lithium, which is an electrode reactant, and optionally includes a binder such as polyvinylidene fluoride. May be. The carbon material capable of inserting and extracting lithium contains natural graphite. This is because the capacity is large and a high energy density can be obtained. Among them, flat natural graphite such as scales or scales, or natural graphite obtained by spheroidizing these flat natural graphites is preferable because the capacity and energy density can be further increased. One kind of natural graphite may be used alone, or a plurality of kinds may be mixed and used. Note that the spheroidized natural graphite can be produced, for example, as described in JP-A No. 11-263612, by causing the scaly natural graphite to circulate and collide with a jet stream.

  Moreover, these natural graphites may have a coating part made of a polymer compound formed so as to cover at least a part of the surface.

  The polymer compound constituting the coating portion has a structure represented by C═O or C—O, and may have both of these structures. This is because by having these structures, the ion conductivity can be improved and the decomposition reaction of the solvent can be suppressed. The ratio of the coating part to the natural graphite (coating part / natural graphite) is preferably in the range of 0.1% by mass to 2% by mass. This is because if the ratio is low, the effect is low, and if the ratio is high, the electrode reaction in the negative electrode 21 decreases, and the charge / discharge efficiency decreases. The polymer compound includes a salt.

  Specific examples of the polymer compound constituting the coating part include alginate propylene glycol ester, glycine, alanine, valine, leucine, isoleucine, serine, threonine, cysteine, cystine, methionine, lysine, arginine, phenylalanine, tyrosine, Histidine, tryptophan, proline, oxyproline, aminobutyric acid, aminobenzoic acid, aspartic acid, sodium aspartate, glutamic acid, sodium glutamate, or hydroxyalkyl starch such as acetate starch, phosphate starch, carboxymethyl starch or hydroxyethyl starch, or Viscous polysaccharides such as pullulan or dextrin, or carboxymethylcellulose, methylcellulose, hydroxyethylcellulose or hydroxy Water-soluble cellulose derivatives such as propyl cellulose, water-soluble cellulose derivatives such as carboxymethyl cellulose sodium salt or carboxymethyl cellulose ammonium salt, or polyuronides such as pectic acid or alginic acid, or salts of polyuronides such as sodium alginate or potassium alginate, or There are water-soluble synthetic resins such as water-soluble acrylic resins, water-soluble epoxy resins, water-soluble polyester resins, and water-soluble polyamide resins.

  Such a covering portion can be formed, for example, by introducing natural graphite particles into an aqueous solution in which these polymer compounds are dissolved, kneading with a planetary mixer, and then drying. Moreover, the coating amount of the polymer compound can be adjusted by adjusting the concentration of the aqueous solution.

  In addition to these carbon materials, other negative electrode active materials may be mixed and used as the negative electrode active material.

  The positive electrode 22 includes, for example, a positive electrode current collector 22A and a positive electrode active material layer 22B provided on both surfaces or one surface of the positive electrode current collector 22A. The positive electrode current collector 22A has an exposed portion where the positive electrode active material layer 22B is not provided at one end portion in the longitudinal direction, and the positive electrode lead 11 is attached to the exposed portion. The positive electrode current collector 22A is made of, for example, a metal material such as an aluminum foil, a nickel foil, or a stainless steel foil.

  The positive electrode active material layer 22B includes, as a positive electrode active material, for example, one or more of positive electrode materials capable of inserting and extracting lithium as an electrode reactant, and graphite or the like as necessary. And a binder such as polyvinylidene fluoride. The positive electrode material capable of inserting and extracting lithium contains a lithium-cobalt composite oxide containing lithium and cobalt, and among them, the lithium-cobalt composite oxide represented by Chemical Formula 1 is preferable. This is because the energy density can be increased.

(Chemical formula 1)
Li _x Co _1-y MI _y O ₂

  In chemical formula 1, MI is a group consisting of vanadium (V), chromium (Cr), iron (Fe), aluminum (Al), magnesium (Mg), zirconium (Zr), titanium (Ti) and tin (Sn). It represents at least one of them. The value of x varies depending on the state of charge / discharge, and 0.90 ≦ x ≦ 1.10. The value of y is 0 ≦ y ≦ 0.05, and preferably 0 ≦ y ≦ 0.02. This is because the energy density can be further increased.

  The crystallite diameter of this lithium-cobalt composite oxide is 50 nm or more and 80 nm or less in the (003) vector direction, and 60 nm or more and 100 nm or less in the (110) vector direction. This is because when the crystallite diameter in the (003) vector direction is large, expansion / contraction associated with insertion and extraction of lithium is increased, and capacity deterioration due to structural strain is increased. Further, if the crystallite diameter in the (003) vector direction is small, the stability of the crystal itself is lowered, and the structural deterioration accompanying charge / discharge is likely to proceed. On the other hand, if the crystallite size in the (110) vector direction is large, the lithium ion diffusion path in the solid phase becomes longer, the charge / discharge load characteristics are reduced, and side reactions are likely to occur, resulting in performance deterioration due to charge / discharge cycles. It is because it becomes easy to advance. Further, if the crystallite diameter in the (110) vector direction is small, the stability of the crystal itself is lowered, and the structural deterioration accompanying charge / discharge is likely to proceed.

This lithium-cobalt composite oxide can be produced, for example, by mixing lithium carbonate (Li ₂ CO ₃ ) and tricobalt tetroxide (Co ₃ O ₄ ) at a predetermined ratio and then firing. . At that time, by mixing at least one member selected from the group consisting of oxides of vanadium, chromium, iron, aluminum, magnesium, zirconium, titanium or tin as an additive, or by appropriately adjusting firing conditions, or Crystal growth can be suppressed by appropriately selecting the crystallinity of the raw material.

As positive electrode materials capable of inserting and extracting lithium, in addition to these positive electrode materials, one or more of other positive electrode materials may be mixed and used. Other positive electrode materials include, for example, metal sulfides or oxides not containing lithium, such as TiS ₂ , MoS ₂ , NbSe _2, or V ₂ O ₅ , or lithium oxides, lithium sulfides, or other lithium composite oxides Other lithium-containing compounds, or polymeric materials. As other lithium-containing compounds, for example, lithium and a lithium composite oxide containing at least one of nickel, manganese (Mn), iron (Fe), aluminum, vanadium, and titanium can obtain a high energy density. Specifically, LiNiO ₂ , Li _p Ni _q Co _1-q O ₂ (y and z differ depending on the charge / discharge state of the battery, and usually 0 <p <1, 0.7 <q <1 Or a LiMn ₂ O ₄ or the like. Further, a lithium phosphate compound such as LiMIIPO ₄ (MII is one or more transition metals) having an olivine type crystal structure is preferable because a high energy density can be obtained.

  The separator 23 is made of, for example, a porous film made of synthetic resin such as polytetrafluoroethylene, polypropylene, or polyethylene, or a porous film made of ceramic, and has a structure in which two or more kinds of porous films are laminated. May be.

  The separator 23 is impregnated with an electrolytic solution that is a liquid electrolyte. The electrolytic solution includes, for example, a solvent such as a nonaqueous solvent and a lithium salt that is an electrolyte salt dissolved in the solvent.

  As the solvent, various nonaqueous solvents conventionally used for nonaqueous electrolytes can be used. Specifically, ethylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3 -Dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methyl sulfolane, acetonitrile, propionitrile, acetate ester, butyrate ester, propionate ester and the like. In particular, from the viewpoint of oxidation stability, it is preferable to include a carbonate ester.

  Furthermore, as a solvent, propylene carbonate is also preferably mentioned. This is because the low temperature characteristics can be improved. Moreover, when using a propylene carbonate, it is preferable to use the natural graphite which has the coating | coated part mentioned above as a negative electrode active material. Propylene carbonate tends to undergo a decomposition reaction at the edge of natural graphite, and along with this, delamination tends to occur with natural graphite. In particular, when a large amount of propylene carbonate is used, the negative electrode 21 is likely to be deteriorated. However, if the above-described covering portion is formed on natural graphite, the decomposition reaction of propylene carbonate and natural properties can be achieved even if the content of propylene carbonate is increased. This is because delamination of graphite can be suppressed. The content of propylene carbonate in the solvent is preferably 30% by mass or more and 70% by mass or less. This is because if the content of propylene carbonate is small, the effect of improving the low-temperature characteristics is low, and if the content of propylene carbonate is too large, a decomposition reaction tends to occur on natural graphite.

Examples of the electrolyte salt include LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiClO ₄ , LiB (C ₆ H ₅ ) ₄ , LiCH ₃ SO ₃ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C _{2 F 5 SO 2) 2,} LiN (C 4 F 9 SO 2) (CF 3 SO 2), LiC (CF 3 SO 2) 3, LiC 4 F 9 SO 3, LiAlCl 4, LiSiF 6, LiCl or LiBr, etc. Any one of these or a mixture of two or more thereof may be used. Among these, LiPF ₆ is preferable because high conductivity can be obtained.

  The content (concentration) of the lithium salt in the electrolyte solution is preferably in the range of 0.1 mol / l or more and 2.0 mol / l or less, or in the range of 0.1 mol / kg or more and 2.0 mol / kg or less. . This is because good ionic conductivity can be obtained within these ranges.

  The electrolytic solution also preferably contains a cyclic carbonate of an unsaturated compound as an additive. This is because a stable film can be formed on the electrode surface, and the decomposition reaction of the solvent can be suppressed. Examples of the cyclic carbonate of the unsaturated compound include 1,3-dioxol-2-one, 4-vinyl-1,3-dioxolan-2-one, and derivatives thereof. In addition, although the cyclic carbonate of the unsaturated compound remaining without participating in the film formation also functions as a solvent, the content of propylene carbonate in the solvent described above is a solvent excluding the cyclic carbonate of the unsaturated compound. Calculate from

  For example, the secondary battery can be manufactured as follows.

  First, for example, a negative electrode active material and a binder are mixed to prepare a negative electrode mixture, and this negative electrode mixture is dispersed in a solvent such as N-methyl-2-pyrrolidone to obtain a paste-like negative electrode mixture slurry And Subsequently, the negative electrode mixture slurry is applied to the negative electrode current collector 21A, and the solvent is dried. Then, the negative electrode active material layer 21B is formed by compression molding using a roll press or the like, and the negative electrode 21 is manufactured.

  Further, for example, a positive electrode active material, a conductive agent, and a binder are mixed to prepare a positive electrode mixture, and the positive electrode mixture is dispersed in a solvent such as N-methyl-2-pyrrolidone to obtain a paste-like material. A positive electrode mixture slurry is obtained. Subsequently, the positive electrode mixture slurry is applied to the positive electrode current collector 22A and the solvent is dried. Then, the positive electrode active material layer 22B is formed by compression molding using a roll press or the like, and the positive electrode 22 is manufactured.

  Next, the negative electrode lead 12 is attached to the negative electrode current collector 21A, the positive electrode lead 11 is attached to the positive electrode current collector 22A, and the negative electrode 21 and the positive electrode 22 are laminated and wound via the separator 23, and the outermost peripheral part is wound. The protective electrode 24 is adhered to form the wound electrode body 20.

  Next, after sandwiching the wound electrode body 20 between the exterior members 31 made of a laminate film, the outer edge portions of the exterior member 31 are bonded to form a laminated bag shape leaving one side. At that time, the positive electrode lead 11 and the negative electrode lead 12 are led out of the exterior member 31.

  Subsequently, an electrolyte is injected into the exterior member 31 from the open side, and the separator 23 is impregnated. After that, the open side of the exterior member 31 is bonded. Thereby, the secondary battery shown in FIGS. 1 and 2 is completed.

  In the secondary battery, when charged, for example, lithium ions are released from the positive electrode 22 and inserted in the negative electrode 21 through the electrolytic solution. When discharging is performed, for example, lithium ions are extracted from the negative electrode 21 and inserted into the positive electrode 22 through the electrolytic solution. Here, since the positive electrode 22 includes the above-described lithium-cobalt composite oxide and the negative electrode 21 includes the above-described natural graphite, the capacity and cycle characteristics are improved.

  Thus, according to the secondary battery according to the present embodiment, since the negative electrode 21 contains natural graphite, the capacity can be increased. In addition, the positive electrode contains a lithium-cobalt composite oxide containing lithium and cobalt having a crystallite diameter in the (003) vector direction of 50 nm to 80 nm and a crystallite diameter of (110) vector direction of 60 nm to 100 nm. As a result, cycle characteristics can be improved.

  Further, ion conductivity can be improved by forming a covering portion made of a polymer compound containing a structure represented by C═O or C—O on at least a part of the surface of natural graphite. At the same time, for example, the decomposition reaction can be suppressed even when the content of propylene carbonate is increased, so that the low temperature characteristics can be improved.

  Furthermore, a higher effect can be obtained if the ratio of the covering portion to the natural graphite (covering portion / natural graphite) is in the range of 0.1% by mass to 2% by mass.

  Further, a specific embodiment of the present invention will be described in detail with reference to FIGS.

( Experimental Examples 1-1 to 1-4)
First, lithium carbonate (Li ₂ CO ₃ ) and tricobalt tetroxide (Co ₃ O ₄ ) are weighed so that the molar ratio of Li: Co is 1: 1, mixed in a mortar, and then in the air. Firing was obtained by firing. At that time, the crystallite diameter of the fired body was controlled by appropriately selecting the crystallinity of the raw material and the firing temperature. When the obtained fired body was subjected to X-ray diffraction measurement and ICP (Inductively Coupled Plasma) emission analysis, it was confirmed that it was represented by LiCoO ₂ . Further, when the crystallite diameters in the (003) vector direction and the (110) vector direction were examined by the Scherrer equation expressed by Equation 1, they were 70 nm and 80 nm, respectively. Thereafter, the fired body was pulverized to obtain an average particle size of 15 μm, which was used as a lithium-cobalt composite oxide as a positive electrode active material.

(Equation 1)
D (nm) = Kλ / (βsinθ)
D: Crystallite size K: Scherrer constant (when β is calculated from the integral width, it is 1.05)
λ: wavelength of the X-ray tube (CuKα1 = 0.154562 nm)
β: The width of the diffraction line spread depending on the crystallite size (radian)
θ: diffraction angle 2θ / 2 (degree)

  Subsequently, the lithium-cobalt composite oxide, ketjen black as a conductive agent, and polyvinylidene fluoride as a binder, lithium-cobalt composite oxide: ketjen black: polyvinylidene fluoride = 94: 3: 3. A positive electrode mixture is prepared by mixing in a mass ratio of the above, and dispersing the positive electrode mixture in N-methyl-2-pyrrolidone as a solvent to obtain a positive electrode mixture slurry. The positive electrode 22 was produced by uniformly applying and drying on both sides of the current collector 22A and compression molding with a roll press to form the positive electrode active material layer 22B.

In Experimental Examples 1-1 and 1-2, as the negative electrode active material, flaky natural graphite having an average particle diameter of 20 μm, and natural graphite obtained by granulating this flaky natural graphite (hereinafter referred to as spheroidized natural graphite). Each of these negative electrode active materials and polyvinylidene fluoride as a binder are mixed at a mass ratio of negative electrode active material: polyvinylidene fluoride = 90: 10 to prepare a negative electrode mixture. Prepared. This negative electrode mixture was dispersed in N-methyl-2-pyrrolidone as a solvent to form a negative electrode mixture slurry, which was then uniformly applied to the negative electrode current collector 21A made of copper foil, dried, and compressed by a roll press. The negative electrode 21 was fabricated by forming the negative electrode active material layer 21B.

Furthermore, in Experimental Examples 1-3 and 1-4, a negative electrode active material was produced as follows. First, flaky natural graphite or spheroidized natural graphite having an average particle diameter of 20 μm and an aqueous solution in which a polymer compound serving as a coating portion is dissolved are mixed by a planetary mixer and then dried at 120 ° C. for 12 hours or more. A coating was formed on the surface of natural graphite. The polymer compound was sodium glutamate, and the concentration of the aqueous solution was adjusted so that the ratio of the coating portion to each natural graphite was 1% by mass. The ratio of the covering portion was obtained from the mass loss by heating the obtained negative electrode active material at 400 ° C. for 1 hour. Subsequently, a negative electrode 21 was produced using these negative electrode active materials in the same manner as in Experimental Examples 1-1 and 1-2.

  After preparing the positive electrode 22 and the negative electrode 21, the positive electrode lead 11 made of aluminum is attached to the positive electrode 22, the negative electrode lead 12 made of nickel is attached to the negative electrode 21, and a microporous polyethylene stretched film having a thickness of 25 μm is used as the separator 23. The positive electrode 23, the separator 23, the negative electrode 21, and the separator 23 are laminated in this order and wound to obtain a wound electrode body 20.

  Next, after sandwiching the wound electrode body 20 between the exterior members 31 made of a laminate film, the outer edge portions of the exterior member 31 were bonded to form a bag shape with one side remaining. At that time, the positive electrode lead 11 and the negative electrode lead 12 were led out of the exterior member 31.

Subsequently, an electrolytic solution was injected into the exterior member 31 from the open side and impregnated in the separator 23, and then the open side of the exterior member 31 was bonded to obtain the secondary battery shown in FIG. 1 and FIG. . In the electrolytic solution, LiPF ₆ as an electrolyte salt is dissolved at a concentration of 1 mol / l in a solvent in which ethylene carbonate and diethyl carbonate are mixed in an equal mass, and 1,3-dioxol-2-one is electrolyzed as an additive. What mixed with content of 1 mass% with respect to the liquid was used.

As Comparative Example 1-1 with respect to Experimental Examples 1-1 to 1-4, except that a negative electrode was produced using mesocarbon microbeads as a negative electrode active material, the other was the same as Experimental Example 1-1 to 1-4. A secondary battery was manufactured.

The volume energy density of the fabricated secondary batteries of Experimental Examples 1-1 to 1-4 and Comparative Examples 1-1 and 1-2 was examined as follows. First, constant current and constant voltage charging with an upper limit voltage of 4.2 V, a current of 1 C, and a total charging time of 3 hours was performed in a thermostat set at 23 ° C. After that, constant current discharge up to 23 ° C., current 1 C, and final voltage 3.0 V was performed. The volume energy density was calculated from (average discharge voltage × discharge capacity) / (battery volume). The results are shown in Table 1.

As can be seen from Table 1, according to Experimental Examples 1-1 to 1-4 using flaky natural graphite or spheroidized natural graphite, the energy density is higher than that of Comparative Example 1-1 using mesocarbon microbeads. It was.

  That is, it was found that a high capacity can be obtained by using natural graphite as the negative electrode active material.

( Experimental Examples 2-1-1 to 2-1-3, 2-2-1 to 2-2-3)
A secondary battery was fabricated in the same manner as in Experimental Examples 1-1 and 1-2, except that the crystallite diameter was changed by appropriately changing the preparation conditions of the lithium-cobalt composite oxide. Specifically, the crystallite diameter in the (003) vector direction of the lithium-cobalt composite oxide was changed in the range of 50 nm to 80 nm. Incidentally, the negative electrode active material is a natural flake graphite in Examples 2-1-1～2-1-3 a spheroidized natural graphite In Example 2-2-1～2-2-3.

Comparative Examples 2-1-1, 2-1-2, 2-2-1,2-2-2 to Experimental Examples 2-1-1 to 2-1-3,2-2-1 to 2-2-3 2 except that the crystallite diameter in the (003) vector direction of the lithium-cobalt composite oxide was set to 40 nm or 90 nm, and the others were experimental examples 2-1-1 to 2-1-3, 2-2-1. A secondary battery was fabricated in the same manner as in 2-2-3. At that time, the crystallite size was changed by appropriately changing the preparation conditions of the lithium-cobalt composite oxide.

In addition, as Comparative Examples 2-3-1 to 2-3-5, mesocarbon microbeads were used as the negative electrode active material, and the lithium-cobalt composite oxide was appropriately changed by changing the preparation conditions of the lithium-cobalt composite oxide. A secondary battery was fabricated in the same manner as in Experimental Examples 2-1-1 to 2-1-3, 2-2-1 to 2-2-3, except that the crystallite diameter of the product was changed. . The crystallite diameter in the (003) vector direction of the lithium-cobalt composite oxide was 40 nm to 90 nm.

The cycle characteristics of the fabricated secondary batteries of the experimental example and the comparative example were examined as follows. First, in a thermostatic chamber set at 23 ° C., after performing constant current and constant voltage charging with an upper limit voltage of 4.2 V, a current of 1 C, and a total charging time of 3 hours, 23 ° C., a current of 1 C, and a final voltage of 3.0 V Up to a constant current discharge was performed. The cycle characteristics are obtained by repeating this charge and discharge, and the maintenance rate of the discharge capacity at the 300th cycle relative to the initial discharge capacity (the discharge capacity at the first cycle), that is, (discharge capacity at the 300th cycle / initial discharge capacity) × 100 (% ) The results are shown in Table 2 and FIG.

As can be seen from Table 2 and FIG. 3, Experimental Example 2-1-1 in which natural graphite was used as the negative electrode active material and the crystallite diameter in the (003) vector direction of the lithium-cobalt composite oxide was 50 nm or more and 80 nm or less. According to 2-1-3, 2-2-1 to 2-2-3, Comparative Example 2-1-1, 2-2-1 in which the crystallite diameter in the (003) vector direction is less than 50 nm, or (003) The discharge capacity retention ratio was dramatically improved over Comparative Examples 2-1-2 and 2-2-2 in which the crystallite diameter in the vector direction was more than 80 nm.

  On the other hand, in Comparative Examples 2-3-1 to 2-3-5 using mesocarbon microbeads as the negative electrode active material, the crystallite diameter in the (003) vector direction of the lithium-cobalt composite oxide was 50 nm or more and 80 nm or less. As a result, the discharge capacity retention rate was improved, but the degree was low.

  That is, even if natural graphite is used for the negative electrode active material, if the crystallite diameter in the (003) vector direction of the lithium-cobalt composite oxide containing lithium and cobalt is set to 50 nm or more and 80 nm or less, cycle characteristics are obtained. It was found that can improve.

( Experimental examples 3-1-1 to 1-3-1 and 3-2-1 to 2-3-2)
A secondary battery was fabricated in the same manner as in Experimental Examples 1-1 and 1-2, except that the crystallite diameter was changed by appropriately changing the preparation conditions of the lithium-cobalt composite oxide. Specifically, the crystallite diameter in the (110) vector direction of the lithium-cobalt composite oxide was changed in the range of 60 nm to 100 nm. Incidentally, the negative electrode active material is a natural flake graphite in Examples 3-1-1～3-1-3 a spheroidized natural graphite In Example 3-2-1～3-2-3.

Comparative Examples 3-1-1, 3-1-2, 3-2-1, 3-2- with respect to Experimental Examples 3-1-1 to 1-3-1 and 3-2-1 to 2-3-2-3 2 except that the crystallite diameter in the (110) vector direction of the positive electrode active material was set to 50 nm or 110 nm, and the others were Experimental Examples 3-1-1 to 1-3-1 and 3-2-1 to 3- A secondary battery was fabricated in the same manner as in 2-3. At that time, the crystallite size was changed by appropriately changing the preparation conditions of the lithium-cobalt composite oxide.

In addition, as Comparative Examples 3-3-1 to 3-3-5, mesocarbon microbeads were used as the negative electrode active material, and by appropriately changing the preparation conditions of the lithium-cobalt composite oxide, lithium-cobalt composite oxidation was performed. A secondary battery was fabricated in the same manner as in Experimental Examples 3-1-1 to 1-3-1 and 3-2-1 to 2-3-2-3, except that the crystallite diameter of the product was changed. . The crystallite diameter in the (110) vector direction of the lithium-cobalt composite oxide was 50 nm to 110 nm.

With respect to the fabricated secondary batteries of the experimental example and the comparative example, the cycle characteristics were examined in the same manner as in Experimental examples 2-1-1 to 2-1-3, 2-2-1 to 2-2-3. The results are shown in Table 3 and FIG.

As can be seen from Table 3 and FIG. 4, Experimental Example 3-1-1 in which natural graphite was used as the negative electrode active material and the crystallite diameter in the (110) vector direction of the lithium-cobalt composite oxide was 60 nm or more and 100 nm or less. According to 3-1-3, 3-2-1 to 2-3-2-3, Comparative Example 3-1-1, 3-2-1 having a crystallite diameter in the (110) vector direction of less than 60 nm, or (110) The discharge capacity retention rate was dramatically improved compared to Comparative Examples 3-1-2 and 3-2-2 in which the crystallite diameter in the vector direction was more than 100 nm.

  On the other hand, in Comparative Examples 3-3-1 to 3-3-5 using mesocarbon microbeads as the negative electrode active material, the crystallite diameter in the (110) vector direction of the lithium-cobalt composite oxide was 60 nm or more and 100 nm or less. As a result, the discharge capacity retention rate was improved, but the degree was low.

  That is, even when natural graphite is used for the negative electrode active material, the crystallite diameter in the (003) vector direction of the lithium-cobalt composite oxide containing lithium and cobalt is set to 50 nm to 80 nm and the crystal in the (110) vector direction It has been found that cycle characteristics can be improved by setting the diameter to 60 nm or more and 100 nm or less.

( Experimental examples 4-1-1 to 4-3-3, 5-1-1 to 5-2-3)
As Experimental Examples 4-1-1 to 4-1-3, 4-2-1 to 4-2-3, 4-3-1 to 4-3-3, a part of diethyl carbonate was replaced with propylene carbonate, A secondary battery was fabricated in the same manner as in Experimental Examples 1-2 and 1-4 except that a mixed solvent of ethylene carbonate: diethyl carbonate: propylene carbonate (mass ratio) = 50: 30: 20 was used. . At that time, the crystallite diameter in the (003) vector direction was changed in the range of 50 nm to 80 nm by appropriately changing the preparation conditions of the lithium-cobalt composite oxide as the positive electrode active material. Moreover, the negative electrode active material, an experimental example 4-1-1～4-1-3 the spherical natural graphite, in Example 4-2-1～4-2-3 consists of sodium glutamate as a polymer compound Spherical natural graphite having a coating portion on the surface was used, and in Experimental Examples 4-3-1 to 4-3-3, spherical natural graphite having a coating portion made of carboxymethyl cellulose sodium salt on the surface was used as the polymer compound. In Experimental Examples 4-2-1 to 4-2-3, 4-3-1 to 4-3-3, the ratio of the coating portion to the spherical natural graphite was 1% by mass.

Comparative Examples 4-1-1, 4-1 to Experimental Examples 4-1-1 to 4-1-3, 4-2-1 to 4-2-3, 4-3-1 to 4-3-3 The crystallite diameter in the (003) vector direction of the lithium-cobalt composite oxide was set to 40 nm or 90 nm as 2,4-2-1, 4-2-2, 4-3-1, 4-3-2. In the same manner as in Experimental Examples 4-1-1 to 4-1-3, 4-2-1 to 4-2-3, 4-3-1 to 4-3-3, Produced. At that time, the crystallite size was changed by appropriately changing the preparation conditions of the lithium-cobalt composite oxide.

As Experimental Examples 5-1-1 to 5-1-3, 5-2-1 to 5-2-3, except that propylene carbonate was used instead of diethyl carbonate, that is, ethylene carbonate and propylene carbonate were used. A secondary battery was fabricated in the same manner as in Experimental Example 1-4, except that a mixed solvent mixed at an equal mass was used. At that time, the crystallite diameter in the (003) vector direction was changed in the range of 50 nm to 80 nm by appropriately changing the preparation conditions of the lithium-cobalt composite oxide as the positive electrode active material. Moreover, the negative electrode active material in Examples 5-1-1～5-1-3, a spheroidized natural graphite having a coating portion made of sodium glutamate as a polymer compound on the surface, Experiment 5-2-1～ In 5-2-3, it was set as the spherical natural graphite which has the coating | coated part which consists of carboxymethylcellulose sodium salt as a high molecular compound on the surface. The ratio of the coating part to the spheroidized natural graphite was 1% by mass.

Comparative Example 5-1-1, 5-1-2, 5-2-1, 5-2 to Experimental Examples 5-1-1 to 5-1-3, 5-2-1 to 5-2-3 2 except that the crystallite diameter in the (003) vector direction of the lithium-cobalt composite oxide was set to 40 nm or 90 nm, except for Experimental Examples 5-1-1 to 5-1-3, 5-2-1. A secondary battery was produced in the same manner as ˜5-2-3. At that time, the crystallite size was changed by appropriately changing the preparation conditions of the lithium-cobalt composite oxide.

With respect to the fabricated secondary batteries of the experimental example and the comparative example, the cycle characteristics were examined in the same manner as in Experimental examples 2-1-1 to 2-1-3, 2-2-1 to 2-2-3. The results are shown in Tables 4 and 5 and FIGS.

As can be seen from Tables 4 and 5 and FIGS. 5 and 6, Experimental Example 4-1-1 in which propylene carbonate was used and the crystallite diameter in the (003) vector direction of the lithium-cobalt composite oxide was 50 nm or more and 80 nm or less. 4-1-3, 4-2-1 to 4-2-3, 4-3-1 to 4-3-3, 5-1-1 to 5-1-3, 5-2-1 to 5- According to 2-3, Comparative Examples 4-1-1, 4-2-1, 4-3-1, 5-1-1, 5-2 in which the crystallite diameter in the (003) vector direction was less than 50 nm. From Comparative Example 4-1-2, 4-2-2, 4-3-2, 5-1-2, 5-2-2 in which the crystallite diameter in the -1 or (003) vector direction is more than 80 nm However, the discharge capacity maintenance rate has improved dramatically.

  That is, even if other solvents are used, cycle characteristics can be improved if the crystallite diameter in the (003) vector direction of the lithium-cobalt composite oxide containing lithium and cobalt is 50 nm or more and 80 nm or less. I understood that I could do that.

( Experimental examples 6-1-1 to 6-3-3, 7-1-1 to 7-2-3)
As Experimental Examples 6-1-1 to 6-1-3, 6-2-1 to 6-2-3, 6-3-1 to 6-3-3, a part of diethyl carbonate was replaced with propylene carbonate, A secondary battery was fabricated in the same manner as in Experimental Examples 1-2 and 1-4 except that a mixed solvent of ethylene carbonate: diethyl carbonate: propylene carbonate (mass ratio) = 50: 30: 20 was used. . At that time, the crystallite diameter in the (110) vector direction was changed in the range of 60 nm to 100 nm by appropriately changing the preparation conditions of the lithium-cobalt composite oxide. Moreover, the negative electrode active material, an experimental example 6-1-1～6-1-3 the spherical natural graphite, in Example 6-2-1～6-2-3 consists of sodium glutamate as a polymer compound Spherical natural graphite having a coating portion on the surface was used, and in Experimental Examples 6-3-1 to 6-3-3, spherical natural graphite having a coating portion made of carboxymethyl cellulose sodium salt on the surface as a polymer compound was used. In Experimental Examples 6-2-1 to 6-2-3, 6-3-1 to 6-3-3, the ratio of the coating portion to the spherical natural graphite was 1% by mass.

Comparative Examples 6-1-1 and 6-1- for Experimental Examples 6-1-1 to 6-1-3, 6-2-1 to 6-2-3, 6-3-1 to 6-3-3 The crystallite diameter in the (110) vector direction of the lithium-cobalt composite oxide was set to 50 nm or 110 nm as 2,6-2-1, 6-2-2, 6-3-1, 6-3-2. In the same manner as in Experimental Examples 6-1-1 to 6-1-3, 6-2-1 to 6-2-3, 6-3-1 to 6-3-3, Produced. At that time, the crystallite size was changed by appropriately changing the preparation conditions of the lithium-cobalt composite oxide.

Further, as experimental examples 7-1-1 to 7-1-3, 7-2-1 to 7-2-3, except that propylene carbonate was used instead of diethyl carbonate, that is, ethylene carbonate and propylene carbonate A secondary battery was fabricated in the same manner as in Experimental Example 1-4, except that a mixed solvent in which an equal mass was mixed was used. At that time, the crystallite diameter in the (110) vector direction was changed in the range of 60 nm to 100 nm by appropriately changing the preparation conditions of the lithium-cobalt composite oxide. Moreover, the negative electrode active material in Examples 7-1-1～7-1-3, a spheroidized natural graphite having a coating portion made of sodium glutamate as a polymer compound on the surface, Experiment 7-2-1～ In 7-2-3, spherical natural graphite having a coating portion made of carboxymethyl cellulose sodium salt on the surface was used as a polymer compound. The ratio of the coating part to the spheroidized natural graphite was 1% by mass.

Comparative Examples 7-1-1, 7-1-2, 7-2-1 and 7-2-for Experimental Examples 7-1-1 to 7-1-3 and 7-2-1 to 7-2-3 2 except that the crystallite diameter in the (110) vector direction of the lithium-cobalt composite oxide was set to 50 nm or 110 nm, and the others were experimental examples 7-1-1 to 7-1-3, 7-2-1. A secondary battery was produced in the same manner as ˜7-2-3. At that time, the crystallite size was changed by appropriately changing the preparation conditions of the lithium-cobalt composite oxide.

With respect to the fabricated secondary batteries of the experimental example and the comparative example, the cycle characteristics were examined in the same manner as in Experimental examples 2-1-1 to 2-1-3, 2-2-1 to 2-2-3. The results are shown in Tables 6 and 7 and FIGS.

As can be seen from Tables 6 and 7 and FIGS. 7 and 8, Experimental Example 6-1-1 using propylene carbonate and having a crystallite diameter in the (110) vector direction of the lithium-cobalt composite oxide of 60 nm to 100 nm. 6-1-3, 6-2-1 to 6-2-3, 6-3-1 to 6-3-3, 7-1-1 to 7-1-3, 7-2-1 to 7- According to 2-3, Comparative Example 6-1-1, 6-2-1, 6-3-1, 7-1-1, 7-2 in which the crystallite diameter in the (110) vector direction was less than 60 nm. From Comparative Examples 6-1-2, 6-2-2, 6-3-2, 7-1-2, and 7-2-2 in which the crystallite diameter in the -1 or (110) vector direction exceeds 100 nm However, the discharge capacity maintenance rate has improved dramatically.

  That is, even if other solvents are used, the crystallite diameter in the (003) vector direction of the lithium-cobalt composite oxide containing lithium and cobalt is set to 50 nm to 80 nm, and the crystallite diameter in the (110) vector direction is set to It was found that cycle characteristics can be improved by setting the thickness to 60 nm or more and 100 nm or less.

( Experimental examples 8-1 to 8-5)
Using a mixed solvent of ethylene carbonate, diethyl carbonate and propylene carbonate, or a mixed solvent of ethylene carbonate and propylene carbonate as the solvent, and changing the content of propylene carbonate in the solvent in the range of 20% by mass to 80% by mass. Other than that, a secondary battery was fabricated in the same manner as in Experimental Example 1-4. In that case, the ratio by the mass ratio of each solvent was set to ethylene carbonate: diethyl carbonate: propylene carbonate = 50: 30: 20 or 50:20:30 in Experimental Examples 8-1 and 8-2, and Experimental Example 8- In 3-8-5, it was set as ethylene carbonate: propylene carbonate = 50: 50, 30:70 or 20:80. The crystallite diameter of the lithium-cobalt composite oxide is 70 nm in the (003) vector direction and 80 nm in the (110) vector direction.

As Comparative Examples 8-1-1 to 8-5-5 with respect to Experimental Examples 8-1 to 8-5, except that the crystallite diameter in the (003) vector direction of the lithium-cobalt composite oxide was set to 40 nm or 90 nm. Alternatively, a secondary battery was fabricated in the same manner as in Experimental Examples 8-1 to 8-5 except that the crystallite diameter in the (110) vector direction of the positive electrode active material was 50 nm or 110 nm.

With respect to the fabricated secondary batteries of the experimental example and the comparative example, the cycle characteristics were examined in the same manner as in Experimental examples 2-1-1 to 2-1-3, 2-2-1 to 2-2-3. The results are shown in Table 8, and the results of Experimental Examples 8-1 to 8-5 are shown in FIG.

Further, the low temperature characteristics of the secondary batteries of Experimental Examples 8-1 to 8-5 were examined as follows. First, in a thermostatic chamber set at 23 ° C., after performing constant current and constant voltage charging with an upper limit voltage of 4.2 V, a current of 1 C, and a total charging time of 3 hours, 23 ° C., a current of 1 C, and a final voltage of 3.0 V The discharge capacity at 23 ° C. was determined. In addition, after performing constant current and constant voltage charging with an upper limit voltage of 4.2 V, a current of 1 C, and a total charging time of 3 hours in a thermostat set at 23 ° C., −10 ° C., a current of 1 C, and a final voltage of 3. A constant current discharge to 0 V was performed, and the discharge capacity at -10 ° C was determined. The low temperature characteristics were determined from the ratio of the discharge capacity at −10 ° C. to the discharge capacity at 23 ° C., that is, (discharge capacity at −10 ° C./discharge capacity at 23 ° C.) × 100 (%). The results are shown in Table 8 and FIG.

As can be seen from Table 8 and FIG. 9, as the content of propylene carbonate increases, the discharge capacity retention rate tends to decrease and the low-temperature characteristics tend to increase. In particular, the content of propylene carbonate in the solvent is 30% by mass. In Experimental Examples 8-2 to 8-4 in which the content was set to% to 70% by mass, high values were obtained for both the discharge capacity retention ratio and the low temperature characteristics. That is, it was found that the content of propylene carbonate in the solvent is preferably 30% by mass or more and 70% by mass or less.

Further, the lithium - cobalt composite oxide (003) crystallite diameter of the vector direction 50nm or 80nm or less, the experimental examples 8-1 to 8-5 were less 100 nm (110) crystallite diameter of the vector direction 60nm or more According to Comparative Examples 8-1-1-1 to 8-1-5, 8-2-1 to 8-2-5 in which the (003) vector direction crystallite diameter is less than 50 nm or more than 80 nm, (110) The discharge capacity retention ratio is higher than that of Comparative Examples 8-3-1 to 8-3-5, 8-4-1 to 8-4-5 in which the crystallite diameter in the vector direction is less than 60 nm or more than 100 nm. Improved. That is, even when propylene carbonate is included in the solvent, the crystallite diameter in the (003) vector direction of the lithium-cobalt composite oxide containing lithium and cobalt is 50 nm or more and 80 nm or less, and the crystallite in the (110) vector direction. It has been found that cycle characteristics can be improved by setting the diameter to 60 nm or more and 100 nm or less.

( Experimental examples 9-1 to 9-5)
The concentration of the aqueous solution in which the polymer compound serving as the coating part was dissolved was adjusted, and the ratio of the coating part to natural graphite was changed in the range of 0.05% to 2.1% by mass as shown in Table 9. Except for this, a secondary battery was fabricated in the same manner as in Experimental Example 1-4. In that case, what mixed ethylene carbonate and propylene carbonate by equal mass was used for the solvent. The polymer compound is sodium glutamate.

For the secondary batteries of Experimental Examples 9-1 to 9-5 were prepared to examine the cycle characteristics in the same manner as in Experimental Example 2-1-1～2-1-3,2-2-1～2-2-3 It was. The results are shown in Table 9 and FIG.

  As can be seen from Table 9 and FIG. 10, the discharge capacity retention rate increased as the ratio of the coating portion to natural graphite increased, and a tendency to decrease after showing the maximum value was observed.

  That is, it was found that the ratio of the covering portion relative to natural graphite is preferably 0.1% by mass or more and 2% by mass or less.

  Although the present invention has been described with reference to the embodiments and examples, the present invention is not limited to the above embodiments and examples, and various modifications can be made. For example, in the above-described embodiment and examples, the case where the positive electrode 22 and the negative electrode 21 are wound has been described. However, a plurality of positive electrodes and negative electrodes may be stacked or folded. Furthermore, the present invention can also be applied to a battery of a cylindrical type, a square type, a coin type, a button type, etc. using a can as an exterior member.

  Moreover, although the case where electrolyte solution was used as it was was demonstrated in the said embodiment and Example, electrolyte solution may be hold | maintained to support bodies, such as a polymer compound, and you may make it make what is called a gel form. Examples of the polymer compound include a fluorine polymer compound such as polyvinylidene fluoride or a copolymer of vinylidene fluoride and hexafluoropropylene, an ether polymer compound such as polyethylene oxide or a crosslinked product containing polyethylene oxide, Examples include those containing acrylonitrile, polymethacrylate or polyacrylate as a repeating unit. In particular, a fluorine-based polymer compound is desirable from the viewpoint of redox stability. Any one of these polymer compounds may be used alone, or two or more thereof may be mixed and used.

It is a disassembled perspective view showing the structure of the secondary battery which concerns on one embodiment of this invention. It is sectional drawing showing the structure along the II-II line of the winding electrode body shown in FIG. It is a characteristic view showing the relationship between the crystallite diameter of the (003) vector direction and discharge capacity maintenance factor in lithium-cobalt composite oxide. It is a characteristic view showing the relationship between the crystallite diameter of a (110) vector direction and discharge capacity maintenance factor in lithium-cobalt complex oxide. It is another characteristic view showing the relationship between the crystallite diameter in the (003) vector direction and the discharge capacity retention rate in the lithium-cobalt composite oxide. It is another characteristic view showing the relationship between the crystallite diameter in the (003) vector direction and the discharge capacity retention rate in the lithium-cobalt composite oxide. It is another characteristic view showing the relationship between the crystallite diameter in the (110) vector direction and the discharge capacity retention ratio in the lithium-cobalt composite oxide. It is another characteristic view showing the relationship between the crystallite diameter in the (110) vector direction and the discharge capacity retention ratio in the lithium-cobalt composite oxide. It is a characteristic view showing the relationship between content of propylene carbonate, discharge capacity maintenance rate, and low temperature characteristics. It is a characteristic view showing the relationship between the quantity of a coating | coated part and a discharge capacity maintenance factor.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Positive electrode lead, 12 ... Negative electrode lead, 20 ... Winding electrode body, 21 ... Negative electrode, 21A ... Negative electrode collector, 21B ... Negative electrode active material layer, 22 ... Positive electrode, 22A ... Positive electrode collector, 22B ... Positive electrode active Material layer, 23 ... separator, 24 ... protective tape, 31 ... exterior member.

Claims (3)

Bei example a cathode, an anode,
The positive electrode includes a lithium-cobalt composite oxide containing lithium (Li) and cobalt (Co), and the negative electrode includes spherical natural graphite,
The lithium - crystallite size of cobalt composite oxide, (003) vector direction is at 50nm or more 80nm or less state, and are (110) vector direction is 60nm or 100nm or less,
The natural graphite is formed so as to cover at least a part of the surface, and has a coating portion made of any one polymer compound of sodium glutamate or sodium carboxymethyl cellulose,
The ratio of the said coating | coated part with respect to the said natural graphite (covering part / natural graphite) exists in the range of 0.1 mass% or more and 2 mass% or less, The lithium ion secondary battery. Before SL electrolyte comprises solvent containing propylene carbonate in the range of 30 wt% to 70 wt%, a lithium ion secondary battery according to claim 1, wherein. The lithium ion secondary battery according to claim 1, wherein the covering portion is made of sodium glutamate.

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