JP5150966B2 - Non-aqueous electrolyte secondary battery positive electrode and non-aqueous electrolyte secondary battery using the same - Google Patents

Description

  The present invention relates to a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the same, and more particularly, a lithium ion battery having high output and long life characteristics, particularly improved cycle characteristics at high temperatures. The present invention relates to a positive electrode for a non-aqueous electrolyte secondary battery to be used for the above and a non-aqueous electrolyte secondary battery using the same.

  Lithium-ion secondary batteries are a composite oxidation of lithium and transition metal oxides with a negative electrode using a carbonaceous material that can be doped or dedoped with lithium, or a metal material that forms an alloy with lithium and lithium. A positive electrode using an active material as an active material is used, and a belt-shaped negative electrode current collector and a positive electrode current collector, which are applied and laminated via a separator, are covered with an exterior material or laminated. A battery is manufactured by accommodating a wound body in which a product is wound in a spiral shape in a battery can. As the positive electrode active material used for the positive electrode, a composite oxide of lithium and a transition metal such as lithium cobaltate, lithium nickelate, and lithium manganate is used.

  Nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries are widely used as power sources for mobile phones, notebook computers, camcorders, and the like. These non-aqueous electrolyte secondary batteries have a larger volume or weight capacity density and can take out a higher voltage than secondary batteries using aqueous electrolyte such as conventional lead storage batteries and alkaline storage batteries. Therefore, it is widely adopted as a power source for small devices, and greatly contributes to the development of today's mobile devices.

  On the other hand, in recent years, the shift to a clean energy society and the establishment of environmental technology have attracted attention due to the growing awareness of environmental issues, and it is suitable for power storage applications, uninterruptible power supply (UPS) applications, mobile power supply applications, etc. Early realization of high performance secondary batteries is required. Lithium ion secondary batteries are being actively studied for such large-scale batteries due to the above-mentioned characteristics of high energy density. However, in order to spread a wide range of applicable products, the life cycle cost of existing products is high. Superiority is essential, and price reduction per unit energy is an essential factor.

  In other words, if a lithium ion secondary battery having a high operating voltage capable of flowing a large charge / discharge current can be used continuously for a long period of time, a high-performance UPS, hybrid vehicle (HEV), or electric vehicle can be used. (EV) realization, and in turn, can contribute to the construction of an advanced information society and a clean energy society. Against this background, high capacity, high output and long life of lithium ion secondary batteries are being actively studied.

  Patent Documents 1 and 2 propose increasing the specific surface area of the positive electrode active material with the aim of increasing the electrode reaction area as a study for higher output. Specifically, increasing the specific surface area means reducing the average particle diameter. However, with a powder having a small average particle diameter, the desired output can be obtained, but due to the increase in the reaction area, it is easily affected by the acid generated by the moisture contained on the electrode surface, causing the output characteristics of the active material particles to deteriorate. . In addition, in order to suppress inactivation due to the liberation of the active material particles, a large amount of binder is required for binding to the current collector, which causes a decrease in energy density. In addition, since a large amount of solvent is required at the time of slurry preparation for application to the current collector, there is a problem that leads to deterioration in productivity.

  Patent Document 3 proposes to use a powder having two peaks in the particle size distribution of the positive electrode active material as a technique for controlling the particle diameter in the positive electrode active material. However, in this case, particles with a small particle size present on the electrode surface are easily affected by the acid generated by the contained water, and the life performance can be expected to some extent due to the inactivation due to the liberation of the active material particles. There was a risk that the characteristics would be significantly degraded.

  In addition, in order to reduce internal resistance for higher output and to ensure electronic conductivity between the positive electrode active materials, carbon such as carbon black is used as a conductive material on the surface of the lithium-containing composite oxide that is the positive electrode active material. Japanese Patent Application Laid-Open No. H10-228867 describes a proposal to deposit a material and form a good conductive path. By attaching a carbon material as a conductive material to the particle surface and interposing between the particles, the direct current resistance of the electrode can be reduced, but it contains a lot of conductive carbon that does not contribute to charge and discharge, so the electrode itself There was a problem that led to a decrease in energy density.

  When the average particle size of the positive electrode active material is increased, the specific surface area can be kept low, and the electrode reaction area is reduced, so that deterioration and elution of the active material on the electrode surface can be suppressed, thereby extending the life. Connected. However, when the battery is charged / discharged at a large current value, reaction unevenness of the active material may occur greatly in the thickness direction of the positive electrode active material layer. That is, the lithium ion insertion / desorption reaction associated with the charge / discharge reaction concentrates on the active material located on the side closer to the current collector, and the active material located on the side far from the current collector cannot contribute effectively. As a result, the output is reduced.

JP 7-335220 A JP 2002-373654 A JP 2000-82466 A JP 2001-250553 A

  As described above, the compatibility between the high output and long life of lithium secondary batteries under active use conditions that require high output, such as automotive applications, has been actively studied, but both are satisfactory. It was difficult to achieve both. Accordingly, an object of the present invention is to provide a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the same, which can achieve both excellent high output characteristics and long-term life characteristics.

  As a result of scrutinizing the conventional technology and repeating various studies to achieve the above object, the inventors of the present invention have found that the positive electrode active material powder has a small average particle size and a small average particle size. A positive electrode active material layer mainly composed of a powder having an average particle size intermediate between a small particle size powder and a large particle size powder is formed on a positive electrode active material layer centered on a large large particle size powder. It has been found that the use of a positive electrode having a layer structure has a great influence on the compatibility between high output characteristics and long-term life characteristics, and the present invention has been achieved.

According to the present invention, the first positive electrode active material layer formed on the surface of the positive electrode current collector, the second positive electrode active material layer formed on the first positive electrode active material layer, only the first positive electrode active material layer electrode active material powder C average particle diameter as the positive electrode active material powder a mean particle size of less than 4μm than 0.5μm is 12μm or more 50μm or less in the included as a positive electrode active material In the second positive electrode active material layer, only the positive electrode active material powder B having an average particle diameter of 4 μm or more and less than 12 μm is included as the positive electrode active material, and the positive electrode active material powder A and the positive electrode active material powder C The BET specific surface area P of the mixed particles is 0.5 ≦ P / Q ≦ 2 with respect to the BET specific surface area Q of the positive electrode active material powder B, and the positive electrode active material powder A, the positive electrode active material powder B, and the non active material powder C is characterized in that the same positive electrode material Positive electrode electrolyte secondary battery Ru obtained.

Furthermore, according to the present invention, in a non-aqueous electrolyte secondary battery comprising at least a negative electrode capable of inserting / extracting lithium and a positive electrode disposed opposite to the negative electrode via a non-aqueous electrolyte, the positive electrode A non-aqueous electrolyte secondary battery characterized by being a positive electrode for a non-aqueous electrolyte secondary battery according to the present invention is obtained.

The positive electrode of the present invention includes a powder having a small average particle diameter, that is, a powder A having an average particle diameter of 0.5 μm or more and less than 4 μm, contained in the first positive electrode active material layer disposed on the current collector side. When charging / discharging the battery at the value, the powder A having a small average particle diameter, that is, the powder A having a large specific surface area promptly causes the insertion / desorption reaction of lithium ions, so that high output can be achieved. The powder having a large average particle size contained in the first positive electrode active material layer, that is, the powder C having an average particle size of 12 μm or more and 50 μm or less has an amount of binder for binding the first positive electrode active material layer to the current collector. The amount of the solvent can be reduced, and the solvent for preparing the slurry to be applied to the current collector can be reduced. In addition, since the powder C is contained only in the first positive electrode active material layer disposed on the current collector side, lithium ions associated with the thickness direction of the positive electrode active material layer at the time of charge / discharge of the battery at a large current There is no unevenness in the insertion / desorption reaction, and all particles can efficiently contribute to charge / discharge.

In addition, the second positive electrode active material layer of the present invention is arranged on the first positive electrode active material layer, so that the above-described effects can be effectively exhibited. A powder having an average particle diameter in the middle, that is, a powder B having an average particle diameter of 4 μm or more and less than 12 μm is included only in the second positive electrode active material layer. Inactivation can be suppressed, and the influence of the acid generated by the contained water can be minimized. Further, it is possible to alleviate reaction unevenness of lithium ion insertion / extraction reaction due to charging / discharging of the battery with a large current.

  Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram showing the configuration of the positive electrode for a non-aqueous electrolyte secondary battery of the present invention. A positive electrode current collector 11, a first positive electrode active material layer 12 containing a positive electrode active material powder A14 and a positive electrode active material powder C16, and a positive electrode active material powder B15 formed on the first positive electrode active material layer The second positive electrode active material layer 13 is contained. In the present invention, the drawings are exaggerated for convenience of explanation, and the technical scope of the present invention is not limited to the forms shown in the drawings.

  FIG. 2 is a schematic diagram showing the configuration of the nonaqueous electrolyte secondary battery of the present invention. A positive electrode current collector 21, a positive electrode 22 described in FIG. 1 containing a positive electrode active material capable of occluding and releasing lithium ions, a negative electrode 23 containing a negative electrode active material occluding and releasing lithium ions, and a negative electrode current collector It is comprised from the body 24, the electrolyte solution 25, and the separator 26 containing this.

(Positive electrode)
In the positive electrode for a non-aqueous electrolyte secondary battery according to an embodiment of the present invention, a 4V class positive electrode material or other positive electrode material exhibiting a charge / discharge potential can be used as the positive electrode active material.

As the 4V class positive electrode material, for example, a lithium-containing metal oxide such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ can be used. Among these, a spinel type lithium manganese composite oxide represented by LiMn ₂ O ₄ is preferably used. When LiMn ₂ O ₄ is used, the amount of Li can be excessive, or trivalent Mn can be replaced with other elements. For example, the composition formula ^{_{Li x M 1 y Mn 2-}} x _over y O ₄ (M ¹ is Al, B, Cr, Co, Ni, Ti, Fe, Mg, Ba, Zn, Ge, 1 or more selected from Nb , 0 ≦ x ≦ 0.3, 0 ≦ y ≦ 0.3)). Thereby, structural stability can be improved.

As a positive electrode material other than the 4V class positive electrode material, for example, a lithium-containing composite oxide is preferably used. Examples of the lithium-containing composite oxide include spinel type lithium manganese composite oxide represented by LiMn _1-x M ² _x O ₄ (0 ≦ x <1, M ² = Ni, Co, Cr, Cu, Fe), LiM Examples include olivine-type lithium-containing composite oxides represented by ³ PO ₄ (M ³ = Co, Ni, Fe), and reverse spinel-type lithium-containing composite oxides such as LiNiVO ₄ .

  About the mixing ratio of the positive electrode active material powder A and the positive electrode active material powder C contained in the first positive electrode active material layer, the mixed positive electrode active material obtained after mixing the positive electrode active material powder A and the positive electrode active material powder C is used. It is preferable that the BET specific surface area P is mixed with the BET specific surface area Q of the positive electrode active material powder B within a range of 0.5 ≦ P / Q ≦ 2. When the specific surface area P deviates from the above range, it is difficult to optimally balance the output characteristics and the life characteristics, but deviates from the above ranges only when priority is given to either the output characteristics or the life characteristics. There may be cases.

  Moreover, the component ratio contained in the positive electrode active material layer is not particularly limited. In addition, the positive electrode active material may include a plurality of types of positive electrode active materials.

  In addition, there is no restriction | limiting in particular as an electroconductivity imparting agent, What is used normally, such as carbon black, acetylene black, natural graphite, artificial graphite, carbon fiber, can be used. Also, as the binder, commonly used ones such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) can be used.

Preferably, the addition amount of the conductivity imparting agent is about 1 to 10% by mass with respect to the mass of the mixture, and the addition amount of the binder is also about 1 to 10% by mass with respect to the mass of the mixture. This is because the capacity per unit weight increases as the proportion of the positive electrode active material for the non-aqueous electrolyte secondary battery increases. If the ratio between the conductivity-imparting agent and the binder is too small, the conductivity may not be maintained, or a problem of electrode peeling may occur. Moreover, it is preferable that the density of the mixture which comprises the formed secondary battery positive electrode except the electrical power collector shall be 2.55-3.05 g / cm < ³ >. When the density of the mixture is the above value, the discharge capacity at the time of use at a high discharge rate is preferably improved.

(Negative electrode)
The negative electrode active material is made of a material that can occlude and release lithium, such as lithium metal or carbon material. As the carbon material, graphite that occludes lithium, amorphous carbon, diamond-like carbon, fullerene, carbon nanotube, carbon nanohorn, or a composite thereof can be used. When lithium metal is used as the negative electrode active material, a melt cooling method, a liquid quenching method, an atomizing method, a vacuum deposition method, a sputtering method, a plasma CVD method, a photo CVD method, a thermal CVD method, a sol-gel method, etc. Thus, the layer 23 to be the negative electrode can be obtained. In the case of carbon materials, carbon and a binder such as polyvinylidene fluoride (PVDF) are mixed, dispersed and kneaded in a solvent such as NMP, and this is applied onto a substrate such as copper foil. The negative electrode 23 can be obtained by a method such as vapor deposition, CVD, or sputtering.

(Current collector)
Aluminum, stainless steel, nickel, titanium, or an alloy thereof can be used as the positive electrode current collectors 11 and 21, and copper, stainless steel, nickel, titanium, or an alloy thereof can be used as the negative electrode current collector 24. Can do.

(Separator)
As the separator 26, a woven fabric, a nonwoven fabric, a porous film, or the like can be used. In particular, a polypropylene or polyethylene-based porous film is preferably used in terms of a thin film and a large area, film strength and film resistance.

(Electrolyte)
Examples of the electrolytic solution in the present invention include propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), cyclic carbonates such as vinylene carbonate (VC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl Chain carbonates such as methyl carbonate (EMC) and dipropyl carbonate (DPC), aliphatic carboxylic acid esters such as methyl formate, methyl acetate and ethyl propionate, γ-lactones such as γ-butyrolactone, 1, 2 -Chain ethers such as diethoxyethane (DEE) and ethoxymethoxyethane (EME), cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, acetamide , Dimethylformamide, dioxolane, acetonitrile, propylnitrile, nitromethane, ethyl monoglyme, phosphoric acid triester, trimethoxymethane, dioxolane derivative, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl- Mixing one or more aprotic organic solvents such as 2-oxazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ethyl ether, 1,3-propane sultone, anisole, N-methylpyrrolidone, fluorinated carboxylic acid ester Can be used. Of these, propylene carbonate, ethylene carbonate, γ-butyl lactone, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate and the like are preferably used alone or in combination.

In these organic solvents, a lithium salt is dissolved as a supporting salt. Examples of the lithium salt include LiPF ₆ , LiAsF ₆ , LiAlCl ₄ , LiClO ₄ , LiBF ₄ , LiSBF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ CO ₃ , LiC (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiB ₁₀ Cl ₁₀ , lower aliphatic carboxylic acid lithium carboxylate, lithium chloroborane, lithium tetraphenylborate, LiBr, LiI, LiSCN, LiCl, imides, etc. can give. Further, a polymer electrolyte may be used instead of the electrolytic solution. The electrolyte concentration is, for example, 0.5 mol / L to 1.5 mol / L. If the concentration is too high, density and viscosity increase. If the concentration is too low, the electrical conductivity may decrease.

  In the lithium secondary battery according to the present invention, the negative electrode 23 and the positive electrode 22 are laminated via the separator 26 in a dry air or an inert gas atmosphere, or the laminated one is wound, and then inserted into the outer package, After impregnating the liquid 25, the battery outer package is sealed.

  There are no restrictions on the battery shape, and it can take the form of a positive electrode and negative electrode facing each other with a separator in between, a wound type, a laminated type, etc., and the cell can also be a coin type, laminate pack, square cell, cylinder Type cells can be used.

  Hereinafter, the present invention will be described using the following examples and comparative examples. The present invention is not limited to the following examples.

(Preparation of positive electrode)
As positive electrode active material powder B, spinel type lithium manganate (LiMn ₂ O ₄ ) having an average particle size of 9.8 μm and a BET specific surface area of 0.8 m ² / g is prepared, and dry mixed with carbon black as a conductivity imparting agent. The slurry was uniformly dispersed in N-methyl-2-pyrrolidone (NMP) in which vinylidene fluoride resin (PVDF) as a binder was dissolved to prepare slurry B. The solid content ratio in the slurry B was positive electrode active material: conductivity imparting agent: PVDF = 89: 4: 7 (mass%).

Subsequently, spinel type lithium manganate having an average particle size of 1.4 μm and a specific surface area of 3.1 m ² / g was prepared as the positive electrode active material A, and an average particle size of 21.2 μm and a specific surface area of 0 as the positive electrode active material C. A spinel type lithium manganate of 3 m ² / g was prepared. The positive electrode active material powder A and the positive electrode active material powder C were mixed so as to be positive electrode active material powder A: positive electrode active material powder C = 20: 80. The specific surface area P of the obtained mixed positive electrode active material powder was 0.86 m ² / g. Next, slurry A was prepared by the same method as that for preparing slurry B.

  The first positive electrode active material layer having the positive electrode active material powder A and the positive electrode active material powder C by evaporating NMP after applying the slurry A onto the aluminum metal foil (thickness 20 μm) serving as the positive electrode current collector (Film thickness 55 μm) was prepared on an aluminum metal foil, and this was used as a positive electrode sheet A. Next, after applying the slurry B prepared above on the positive electrode sheet A, NMP is evaporated, whereby the second positive electrode active material layer (film) having the positive electrode active material powder B on the first positive electrode active material layer. A positive electrode sheet having a thickness of 110 μm was prepared. Moreover, the positive electrode sheet which changed the specific surface area P of mixed positive electrode active material powder by changing the mixing ratio of the positive electrode active material powder A and the positive electrode active material powder C using the same method, and produced Example 1 It was set to 5. Table 1 shows the specific surface area P of the mixed positive electrode active material powder of the positive electrode sheet produced in the example. In addition, what was specifically illustrated above is Example 3. Table 1 also shows the specific surface area P of the mixed positive electrode active material contained in the first positive electrode active material layer and the ratio (P / Q) of the specific surface area P to the specific surface area Q of the positive electrode active material B. Table 1 also summarizes the evaluation results of the batteries described below.

  As a comparative example, a slurry prepared using only the positive electrode active material powders A, B, and C, or a positive electrode sheet having a thickness of 110 μm was prepared from the slurry A. In either case, the same method as the above slurry preparation was used. Comparative Examples 1-3 produced positive electrode sheets using only positive electrode active material powders A, B, and C, respectively, and Comparative Examples 4-8 were a mixture of positive electrode active material powders A, C used in Examples 1-5. A positive electrode sheet is prepared using a positive electrode active material powder.

The negative electrode active material is made of a carbon material, mixed so as to have a ratio of carbon: PVDF = 90: 10 (mass%), dispersed in NMP, and coated on a copper foil (thickness 10 μm) serving as a negative electrode current collector. Thus, a negative electrode sheet having a film thickness of 65 μm was produced. As the electrolyte solution, 1 mol / L LiPF ₆ as an electrolyte was used. Thereafter, the negative electrode and the positive electrode sheet were laminated via a separator made of polyethylene to produce a cylindrical nonaqueous electrolyte secondary battery.

  The high-temperature cycle characteristics of the produced cylindrical nonaqueous electrolyte secondary battery were evaluated. At a temperature of 60 ° C., the charge rate was 1.0 C, the discharge rate was 1.0 C, the charge end voltage was 4.3 V, and the discharge end voltage was 2.5 V. The capacity retention rate (%) is a value obtained by dividing the discharge capacity (mAh) after 300 cycles by the discharge capacity (mAh) at the 10th cycle. The results are shown in Table 1.

  Moreover, the output characteristics of the produced cylindrical nonaqueous electrolyte secondary battery were evaluated. At room temperature, the battery was charged with a constant current to 4.2 V at 0.2 C, and then charged with a constant voltage for 2 hours. Thereafter, constant current discharge was performed at 0.1 C, and the capacity at this time (hereinafter referred to as “low current discharge capacity”) was measured for each cylindrical non-aqueous electrolyte secondary battery. Subsequently, constant current charging up to 4.2 V at 0.2 C and constant voltage charging for 2 hours at room temperature is performed, and then constant current discharging is performed at 10 C, and the capacity at this time (hereinafter referred to as “high current discharging”). Capacity ") was measured for each lithium ion secondary battery. In addition, since the design capacity | capacitance of each lithium ion secondary battery is 1.4Ah, 1C at the time of performing said charging / discharging was set to 1.4A. Based on the above measurement results, the percentage of the high current discharge capacity to the low current discharge capacity (hereinafter referred to as “discharge capacity ratio”) was calculated. The results are shown in Table 1. It can be said that the larger the discharge capacity ratio, the higher the output characteristics.

  From the comparison between each example and the comparative example, the first positive electrode active material layer mainly composed of the small particle size powder having a small average particle size and the large particle size having a large average particle size among the positive electrode active material powders of the present invention. On top of that, by using a positive electrode having a two-layer structure of a second positive electrode active material layer mainly composed of a powder having an average particle size intermediate between a small particle size powder and a large particle size powder, It can be seen that both high output characteristics and long life characteristics are achieved. Therefore, it can be seen that the present invention can effectively contribute to the improvement of the output characteristics and long-term life characteristics of the battery.

The schematic diagram which shows the structure of the positive electrode for nonaqueous electrolyte secondary batteries of this invention. The schematic diagram which shows the structure of the nonaqueous electrolyte secondary battery of this invention.

Explanation of symbols

11, 21 Positive electrode current collector 12 First positive electrode active material layer 13 Second positive electrode active material layer 14 Positive electrode active material powder A
15 Positive electrode active material powder B
16 Positive electrode active material powder C
22 Positive electrode 23 Negative electrode 24 Negative electrode current collector 25 Electrolytic solution 26 Separator

Claims (2)

A first positive electrode active material layer formed on the surface of the positive electrode current collector; a second positive electrode active material layer formed on the first positive electrode active material layer; The material layer includes only the positive electrode active material powder A having an average particle diameter of 0.5 μm or more and less than 4 μm and the positive electrode active material powder C having an average particle diameter of 12 μm or more and 50 μm or less as the positive electrode active material. Only the positive electrode active material powder B having an average particle diameter of 4 μm or more and less than 12 μm is included as the positive electrode active material in the positive electrode active material layer, and the BET of the particles after mixing the positive electrode active material powder A and the positive electrode active material powder C The specific surface area P is 0.5 ≦ P / Q ≦ 2 with respect to the BET specific surface area Q of the positive electrode active material powder B, and the positive electrode active material powder A, the positive electrode active material powder B, and the positive electrode active material powder C are non-aqueous electrolyte secondary battery, characterized in that the same positive electrode material The positive electrode. 2. The non-aqueous electrolyte secondary battery comprising at least a negative electrode capable of inserting and removing lithium and a positive electrode disposed opposite to the negative electrode via a non-aqueous electrolyte, wherein the positive electrode is according to claim 1 . A non-aqueous electrolyte secondary battery, which is a positive electrode for a non-aqueous electrolyte secondary battery.

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