JP2012164631A - Composite for negative electrode of lithium ion secondary battery, and negative electrode of lithium ion secondary battery comprising the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a long-life lithium ion secondary battery having high capacity and excellent cycle characteristics and output characteristics, in which composite particles contain a plurality of pores to relax the cubical expansion during charging, and cobalt is predominantly distributed to the outer surfaces of the composite particles and the inner surfaces surrounding the pores to relax the stress due to the cubical expansion and contraction during charging and discharging and to ensure conductivity.SOLUTION: A composite for a negative electrode contains a negative electrode active material, a conducting assistant, and a binder. The negative electrode active material comprises composite particles in which the percentage of cobalt (Co) is in the range of 5 to 40 atom% with respect to the total amount of tin (Sn) and cobalt (Co). The composite particles have a plurality of pores communicating with the surfaces of the composite particles in a section, cobalt (Co) is predominantly distributed to the outer surfaces of the composite particles and the inner surfaces surrounding the pores, and the porosity of the inside of the composite particles is in the range of 20 to 80%. The conducting assistant is composed of a carbon material such as carbon nanofibers, and adheres to the outer surfaces of the composite particles or somewhere else, in a reticular form.

Description

  The present invention relates to a negative electrode composition having high capacity and excellent cycle characteristics, and a negative electrode of a lithium ion secondary battery using the negative electrode composition.

In recent years, along with the development of portable electronic devices such as mobile phones and laptop computers, and the practical application of electric vehicles, secondary batteries with small size and light weight and high capacity have been required. Currently, lithium ion batteries using a lithium-containing composite oxide such as LiCoO ₂ as a positive electrode material and a carbon-based material as a negative electrode active material are commercialized as high-capacity secondary batteries that meet this requirement. When this carbon material is used for the negative electrode, its theoretical capacity is 372 mAh / g, which is only about 1/10 the capacity of metallic lithium, and its theoretical density is as low as 2.2 g / cc. In addition, the density further decreases. For this reason, it is desired to use a material having a higher capacity per volume as the negative electrode from the viewpoint of increasing the capacity of the battery.

  On the other hand, metals or metalloids such as Al, Ge, Si, Sn, Zn, and Pb are known to be alloyed with lithium, and secondary batteries using these metals or metalloids as negative electrode active materials are known. It is being considered. These materials have a high capacity and a high energy density, and can absorb and desorb more lithium ions than a negative electrode using a carbon-based material. Therefore, by using these materials, a high capacity and a high energy density can be obtained. It is believed that a battery can be made. For example, pure tin is known to exhibit a high theoretical capacity of 993 mAh / g.

  However, since the cycle characteristics are inferior to those of carbon-based materials, it has not yet been put into practical use. The reason for this is that if tin is used as a negative electrode active material for a lithium ion secondary battery as it is, it will be pulverized due to a large volume change associated with charge and discharge, and may be peeled off from the current collector plate or lost contact with the conductive auxiliary agent. Therefore, there arises a problem that sufficient cycle characteristics cannot be obtained.

  As a technique for solving such a problem, a negative electrode material having a small volume change has been researched and developed by adding other substances to inorganic particles such as silicon and tin. Specifically, a technique in which tin is used as a metal alloying with lithium and cobalt is used as a metal not alloying with lithium and these alloy thin films are used as a negative electrode active material layer has been researched and developed (for example, patents). References 1 and 2). In Patent Document 1, an Sn—Co alloy layer is provided on a current collector by wet plating, and the Sn—Co alloy layer is made amorphous so as to improve cycle characteristics. In Patent Document 2, cycle characteristics are improved by generating an Sn—Co alloy thin film on a current collector plate using an electrochemical method such as electrolytic plating or electroless plating.

JP 2009-245794 A (paragraph [0042], paragraph [0043]) JP 2002-373647 A (Claim 1, claim 15, paragraph [0006], paragraph [0011])

  However, in the negative electrodes shown in the above-mentioned conventional patent documents 1 and 2, although deterioration of cycle characteristics has been suppressed by alloying cobalt, which is not alloyed with lithium, with tin, they have a substantially uniform composition and a capacity. In addition, the cycle characteristics were not sufficient.

  It is an object of the present invention to be able to relieve stress due to volume expansion / contraction during charge / discharge and ensure conductivity, because cobalt is unevenly distributed on the outer surface of the composite particle constituting the negative electrode active material and the inner surface of the pore, and the composite particle The composition for negative electrode which can relieve the volume expansion at the time of charging due to the presence of a plurality of pores, can produce a long-life lithium ion secondary battery with high capacity and excellent cycle characteristics and output characteristics, and the same The object is to provide a negative electrode. Another object of the present invention is to provide a composition for a negative electrode capable of producing a lithium ion secondary battery that can further improve cycle characteristics and output characteristics by adding carbon nanofibers, acetylene black, or the like as a conductive additive, and the use thereof. It is to provide a negative electrode.

  A first aspect of the present invention is a composition for a negative electrode of a lithium ion secondary battery comprising a negative electrode active material, a conductive additive and a binder, wherein the negative electrode active material comprises tin (Sn) and cobalt (Co ) Is a composite particle having a ratio of cobalt (Co) to the total amount of 5 to 40 atomic%, and the composite particle has a plurality of pores communicating with the surface of the composite particle at the cut surface. One type selected from the group consisting of carbon nanofibers, acetylene black and ketjen black, wherein the composite particles are unevenly distributed on the outer surface of the composite particles and the inner surface of the pores, and the space ratio inside the composite particles is 20 to 80%. Or it is 2 or more types of carbon materials, and it was comprised so that it might adhere to the outer surface of a composite particle, or the outer surface of a composite particle, and the inner surface of a pore at mesh shape.

  A second aspect of the present invention is an invention based on the first aspect, wherein the content ratio of the negative electrode active material is 70 to 95% by mass, and the content ratio of the conductive assistant is 2 to 20% by mass. When the content of the binder is 3 to 15% by mass, the content of the conductive auxiliary agent is X% by mass, and the content of the binder is Y% by mass, X / Y is 0.4. It is set in the range of -3, When a conductive support agent contains carbon nanofiber, the content rate of carbon nanofiber is 2-15 mass%, It is characterized by the above-mentioned.

  A third aspect of the present invention is a negative electrode of a lithium ion secondary battery in which the negative electrode composition described in the first or second aspect is coated on a negative electrode current collector.

  In the negative electrode composition according to the first aspect of the present invention, the composite particles constituting the negative electrode active material in which the ratio of cobalt to the total amount of tin and cobalt is 5 to 40 atomic% communicate with the surface of the composite particles at the cut surface. Therefore, when the lithium ion secondary battery using this negative electrode active material is repeatedly charged and discharged, the pores in the composite particles can absorb and relax the volume expansion of the composite particles during charging. In addition, since cobalt is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore, that is, a base material having a plurality of pores and a relatively low hardness and conductivity of tin or the inner surface of the pore, Since a high cobalt unevenly distributed tin layer is formed, stress due to volume expansion / contraction during charge / discharge can be relieved and conductivity can be ensured. Furthermore, since the composite particle has a plurality of pores communicating with the surface of the composite particle at the cut surface and the space ratio inside the composite particle is relatively large as 20 to 80%, the reaction area of tin with lithium is relatively wide. . As a result, the original performance of tin that tin reacts efficiently with lithium can be brought out. Therefore, the negative electrode of the present invention is compared with a conventional lithium ion secondary battery in which sufficient capacity and cycle characteristics are not obtained because cobalt which is not alloyed with lithium is alloyed with tin with a substantially uniform composition to produce a negative electrode. The lithium ion secondary battery using the composition for use is excellent in cycle characteristics and output characteristics, has a long life, and has a high capacity. In addition, by adding carbon nanofibers as a conductive aid, the cycle characteristics of lithium ion secondary batteries can be further improved, and by adding acetylene black as a conductive aid, the output characteristics of lithium ion secondary batteries are further improved. it can.

It is a figure which shows the change of the tin concentration with respect to the change of the depth from the surface of the composite particle of this invention embodiment, and a cobalt concentration, respectively. It is the figure which showed the relative difference of the acid concentration of the liquid mixture which mixed the aqueous solution containing the tin ion and cobalt ion of Examples 1-4 and the reducing agent aqueous solution in a predetermined ratio. FIG. 3 is an electron micrograph of a surface obtained by cutting composite particles of Example 1. It is a figure which shows the range of the mixture ratio of the composition for negative electrodes (solid content) of this invention embodiment.

  Next, the form for implementing this invention is demonstrated. The composition for a negative electrode of a lithium ion secondary battery of the present invention includes a negative electrode active material, a conductive additive and a binder. The negative electrode active material is composed of composite particles in which the ratio of cobalt (Co) to the total amount of tin (Sn) and cobalt (Co) is 5 to 40 atomic%, preferably 10 to 30 atomic%. Here, the proportion of cobalt in the composite particles is limited to the above range because when the proportion of cobalt is less than 5 atomic%, the hardness formed on the outer surface of the tin or the inner surface of the pore having a relatively low hardness is relatively high. Because the tin layer with uneven distribution of cobalt becomes thin, stress due to volume expansion / contraction during charging / discharging of the secondary battery using this negative electrode active material cannot be relieved, resulting in a problem that the cycle characteristics of the secondary battery deteriorate. Even if the proportion of cobalt exceeds 40 atomic%, the cycle characteristics of the secondary battery using this negative electrode active material are good, but the amount of cobalt increases and the amount of tin that reacts relatively with lithium increases. This is because the first discharge capacity decreases due to the decrease.

  The composite particle has a plurality of pores communicating with the surface of the composite particle at the cut surface. Cobalt (Co) is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore, and the space ratio inside the composite particle is 20 to 80%, preferably 30 to 70%. As a result, the negative electrode active material of the present invention does not take an almost uniform alloyed form in which there is no bias in the composition of tin-cobalt between the central part and the outer peripheral part of the particles as conventionally known. Here, the space ratio inside the composite particles is limited to the above range, and when the space ratio is less than 20%, the cycle characteristics of the lithium ion secondary battery using the negative electrode active material of the present invention are deteriorated. This is because when the space ratio exceeds 80%, the cycle characteristics are good, but the amount of tin decreases and the initial discharge capacity decreases. Here, the cycle characteristics of the secondary battery can be improved when the space ratio inside the composite particles is within the above range and the number of pores communicating with the surface of the composite particles is larger. As a method for measuring the space ratio inside the composite particles, it is preferable to use a method of measuring the mass of the void area photograph relative to the mass of the entire sectional photograph in the cross-sectional photograph of the particles by C.P.Auger.

  In the negative electrode active material configured in this manner, the composite particle has a plurality of pores communicating with the surface of the composite particle at the cut surface, and therefore when a lithium ion secondary battery using this negative electrode active material is repeatedly charged and discharged Further, the pores in the composite particles can absorb and relax the volume expansion of the composite particles during charging. In addition, since cobalt is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore, that is, a base material having a plurality of pores and a relatively low hardness and conductivity of tin or the inner surface of the pore, Since a high cobalt unevenly distributed tin layer is formed, stress due to volume expansion / contraction during charge / discharge can be relieved and conductivity can be ensured. Furthermore, since the composite particle has a plurality of pores communicating with the surface of the composite particle at the cut surface and the space ratio inside the composite particle is relatively large, the reaction area of tin with lithium is relatively wide. As a result, the original performance of tin that tin reacts efficiently with lithium can be brought out. Therefore, the lithium ion secondary battery using the negative electrode active material has excellent cycle characteristics and output characteristics, has a long life, and has a high capacity.

  In addition, the composite particles constituting the negative electrode active material preferably have an average particle diameter of 0.1 to 20 μm, preferably 0.5 to 10 μm in view of ease of handling. The powder composed of a plurality of composite particles whose particle diameters are controlled in this way can be slurried and applied to the negative electrode current collector, and a conventional lithium ion secondary battery manufacturing process can be applied. Here, the average particle size of the composite particles is limited to the above range because when the average particle size is less than 0.1 μm, it becomes difficult to form a slurry, and there is a problem that it cannot be applied to an existing lithium ion secondary battery manufacturing process. This is because the expansion suppression effect by the tin layer in which cobalt is unevenly distributed becomes insufficient and the cycle characteristics deteriorate. The average particle size of the composite particles was measured using a particle size distribution measuring device (LA-950 manufactured by Horiba, Ltd.), and the volume-based average particle size was defined as the average particle size.

  Furthermore, it is preferable that the negative electrode active material further includes at least one of chromium (Cr) and zinc (Zn) as a constituent element. By including chromium or zinc, the initial discharge capacity can be increased. Although the details of this reason are unknown, it is presumed that lithium can be rapidly diffused to the particle center by containing chromium or zinc. The chromium content is 0.005 to 1% by mass, preferably 0.1 to 0.9%, and the zinc content is 5 to 50 ppm, preferably 10 to 30 ppm by mass. Here, the chromium and zinc contents are limited to the above range because the effect of increasing the initial discharge capacity is not exhibited unless the chromium content is 0.005% or more or the zinc content is 5 ppm or more. If the content of 1% or the content of zinc exceeds 50 ppm, the strength of the tin layer in which cobalt is unevenly distributed is lowered, the protective effect is lowered, and the cycle characteristics are deteriorated. In addition, each content of tin, cobalt, chromium, and zinc in the composite particles constituting the negative electrode active material of the present invention can be obtained by quantitative analysis using ICP (inductively coupled plasma).

  On the other hand, the conductive additive is one or more carbon materials selected from the group consisting of carbon nanofibers, acetylene black, and ketjen black. The conductive additive is configured to adhere to the outer surface of the composite particle, or the outer surface of the composite particle and the inner surface of the pore in a mesh shape. Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene-diene copolymer (EPDM), and styrene-butadiene rubber (SBR). In addition, the content ratio of the negative electrode active material is preferably 70 to 95 mass%, more preferably 80 to 93 mass%, and the content ratio of the conductive auxiliary agent is preferably 2 to 20 mass%, more preferably 3 to 10 mass%. The content ratio of the binder is preferably 3 to 15% by mass, and more preferably 4 to 10% by mass (portion surrounded by a broken line in FIG. 2). Further, when the content ratio of the conductive auxiliary agent is X mass% and the content ratio of the binder is Y mass%, X / Y is preferably in the range of 0.4 to 3, more preferably 0.5 to 2. (The portion between the straight lines A and B indicated by the one-dot chain line in FIG. 2). Thereby, the hatched portion of FIG. 2 is in a range indicating the respective content ratios of the negative electrode active material, the conductive additive and the binder. Furthermore, when a conductive support agent contains carbon nanofiber, the content rate of carbon nanofiber becomes like this. Preferably it is 2-15 mass%, More preferably, it is 3-11 mass%.

  Here, the content ratio of the negative electrode active material is limited to the range of 70 to 95% by mass. When the content is less than 70% by mass, the ratio of the negative electrode active material that undergoes charge / discharge reaction with lithium decreases, and the energy density of the battery is small. If the amount exceeds 95% by mass, the binder and the conductive aid for expressing battery performance are relatively insufficient, and the discharge capacity, cycle characteristics, and output characteristics are degraded. is there. The content of the conductive auxiliary agent is limited to the range of 2 to 20% by mass. If the content is less than 2% by mass, the construction of the electron conduction path from the negative electrode active material to the current collector is insufficient, and the discharge capacity and output characteristics. This is because when the amount exceeds 20% by mass, the proportion of the negative electrode active material is relatively reduced, and the energy density of the battery is reduced. The content ratio of the binder is limited to the range of 3 to 15% by mass. When the content is less than 3% by mass, the negative electrode active material and the conductive additive cannot be sufficiently retained on the sheet, and the cycle characteristics are deteriorated. If the amount exceeds 15% by mass, the proportion of the negative electrode active material is relatively reduced, the battery energy density is reduced, and the contact between the negative electrode active material and the conductive additive is hindered, resulting in a decrease in discharge capacity and output characteristics. Because there is. Moreover, the X / Y was limited to the range of 0.4 to 3. If the ratio is less than 0.4, the effect of the binder hindering the contact between the conductive auxiliary agent and the negative electrode active material becomes remarkable, and the discharge capacity and output characteristics are lowered. This is because when the ratio exceeds 3, there is a problem in that the holding power of the binder conductive auxiliary agent and the negative electrode active material is significantly reduced and the cycle characteristics are deteriorated. Furthermore, the content ratio of the carbon nanofibers is limited to the range of 2 to 15% by mass. If the amount is less than 2% by mass, the carbon nanofibers cannot sufficiently cover the surface of the fine negative electrode active material, and charge / discharge reactions are prevented. There is a problem in that the negative electrode active material that cannot contribute and the discharge capacity decreases, and the surface of the negative electrode active material can be sufficiently covered with 15% by mass of carbon nanofibers. This is because the effect is not obtained, and the ratio of the negative electrode active material relatively decreases, so that the energy density of the battery decreases.

  Moreover, the said composition for negative electrodes contains solvents, such as n-methylpyrrolidinone and water. As a result, the negative electrode composition is slurried and can be applied (applied) to the negative electrode current collector. This solvent evaporates in the drying step after the slurry-like negative electrode composition is applied to the negative electrode current collector. Furthermore, it is preferable that the negative electrode composition further includes at least one dispersant selected from the group consisting of polyacrylic acid, water-soluble cellulose, and polyvinylpyrrolidone. By including the above type of dispersant, the dispersant covers the particles, thereby enhancing the expansion and contraction suppressing effect by the tin layer in which cobalt is unevenly distributed, and improving the cycle characteristics.

Next, the manufacturing method of the said composition for negative electrodes is demonstrated. First, a negative electrode active material is manufactured. The negative electrode active material is prepared by mixing an aqueous solution containing tin ions and cobalt ions with an aqueous reducing agent solution containing divalent chromium ions, and adjusting at least one of the temperature, acid concentration, treatment time or stirring speed of the mixed solution. Manufactured by. Specifically, by mixing the aqueous solution containing tin ions and cobalt ions and the reducing agent aqueous solution containing divalent chromium ions, the composite particles are synthesized by reducing tin ions and cobalt ions in the mixed solution, By adjusting at least one condition of the temperature of the mixed solution, the acid concentration, the treatment time, or the stirring speed, tin (Sn) inside the composite particles is eluted to control the space ratio inside the composite particles. Accordingly, the composite particles are composed of composite particles in which the ratio of cobalt to the total amount of tin and cobalt is 5 to 40 atomic%, the composite particles have a plurality of pores communicating with the surface of the composite particles at the cut surface, and the cobalt is composite particles. Negative electrode active material that is unevenly distributed on the outer surface and the inner surface of the pores and that has a porosity of 1.0 to 6.0 m ² / g inside the composite particles.

  When the reduction reaction is performed in the mixed solution, first, tin ions are reduced to produce uniform tin particles, and the tin particles grow to a certain particle size. Subsequently, cobalt ions are reduced and tin particles grown to a certain particle size are used as a base material, and the cobalt enters the periphery of the base material, resulting in composite particles in which cobalt is unevenly distributed on the outer surface of the tin particles. When adjusting at least one condition of the temperature, acid concentration, treatment time or stirring speed of the mixed liquid, a plurality of pores communicating with the surface of the composite particle are formed in the composite particle, and cobalt is formed on the outer surface of the composite particle and A tin layer in which cobalt is unevenly distributed is formed on the outer surface of the composite particle and the inner surface of the pore, unevenly distributed on the inner surface of the pore. The cobalt concentration in this unevenly distributed tin layer gradually decreases from the outer surface of the composite particle and the inner surface of the pore toward the inner side of the tin base material, and the tin concentration decreases from the outer surface of the composite particle and the inner surface of the pore to the inner side of the tin base material. It is formed so as to gradually become higher as it goes (FIG. 1). In addition, it is guessed that a several pore is formed in the said composite particle for the following reason. Since the hydrogen overvoltage of tin is high, the dissolution reaction of tin hardly occurs. On the other hand, since the hydrogen overvoltage of cobalt is lower than that of tin, hydrogen is generated from the cobalt side when tin and cobalt come into contact with each other. As a result, tin inside the composite particle is very easily dissolved, and a plurality of pores are formed in the composite particle.

  The aqueous solution containing tin ions and cobalt ions preferably contains a dispersant that suppresses aggregation of the resulting composite particles. Examples of the dispersant include at least one selected from polyacrylic acid, water-soluble cellulose, and polyvinyl pyrrolidone.

  The divalent chromium ion contained in the reducing agent aqueous solution has a function as a reducing agent. Since the divalent chromium ions are unstable, the reducing agent aqueous solution is preferably prepared each time when it is mixed with an aqueous solution containing tin ions and cobalt ions. Specifically, for example, the second chromium chloride solution is brought into contact with metallic zinc in a non-oxidizing atmosphere, preferably a nitrogen gas atmosphere, or the chromium is electrochemically reduced to obtain a first chromium chloride solution. Use a good one. The second chromium chloride solution is preferably adjusted to an acid concentration of 0.0079 to 1.26 [a.u.] (arbitrary units: arbitrary units; hereinafter referred to as [a.u.]). This is because when the acid concentration is below the lower limit, a problem that trivalent chromium ions are precipitated as a hydroxide tends to occur.

  The temperature of the mixed solution obtained by mixing the aqueous solution containing tin ions and cobalt ions and the reducing agent aqueous solution containing divalent chromium ions is set to 15 to 50 ° C, preferably 25 to 40 ° C, and the acid concentration of the mixed solution is 0.0035. It is set to ˜0.50 [au], preferably 0.06 to 0.3 [au]. The processing time of the above mixed solution is set to 1 to 40 hours, preferably 3 to 30 hours, and the stirring speed of the mixed solution is 0.2 to 1.5 m / second, preferably 0.4 to 1.0 m / second. Set to The processing time of the mixed solution refers to the stirring and holding time of the mixed solution. Moreover, the stirring speed of the said liquid mixture says the average flow velocity of a liquid mixture when a liquid mixture flows by rotation of a stirring blade. Here, the temperature of the mixed solution is limited to the above range. If the temperature is lower than 15 ° C., tin hardly dissolves and a plurality of pores are hardly formed in the composite particle, so that the volume expansion of the composite particle during charging is absorbed. When the temperature exceeds 50 ° C., elution of tin is promoted and the cycle characteristics are good, but the amount of tin is reduced and the initial discharge capacity is reduced. . Moreover, the acid concentration of the mixed solution is limited to the above range because when the acid concentration exceeds 0.50 [au], elution of tin is promoted and the cycle characteristics are good, but the amount of tin is small. Thus, the initial discharge capacity is reduced, and when the acid concentration is less than 0.0035 [au], hydrogen generation from the cobalt side due to contact between tin and cobalt is difficult to occur, and tin inside the composite particles is difficult to dissolve. Therefore, it becomes difficult for a plurality of pores to be formed in the composite particle, thereby reducing the effect of absorbing and relaxing the volume expansion of the composite particle during charging. In addition, the treatment time of the mixed solution is limited to the above range because the number of pores formed in the composite particle is reduced if it is less than 1 hour, and therefore the effect of absorbing and relaxing the volume expansion of the composite particle during charging. This is because, if it exceeds 40 hours, it takes too much time to obtain the target negative electrode active material, which increases the manufacturing cost and decreases the production efficiency. Furthermore, the stirring speed of the mixed solution is limited to the above range because if it is less than 0.2 m / second, the composition of the mixed solution cannot be kept uniform, and a desired negative electrode active material cannot be obtained. If it exceeds 0.5 m / sec, excessive energy will be input to obtain the desired negative electrode active material, resulting in wasted energy costs. Note that the specific surface area of the composite particles increases as the temperature of the mixed solution increases, and the specific surface area of the composite particles tends to increase as the acid concentration of the mixed solution decreases. In addition, the specific surface area of the composite particles increases as the treatment time of the mixed solution increases, and the specific surface area of the composite particles tends to increase as the stirring speed of the mixed solution increases.

  Thus, the negative electrode active material manufacturing method is a wet method, and both the aqueous solution preparation and the reduction reaction can be performed at a temperature of about room temperature. Therefore, special equipment that requires a large initial cost is unnecessary, and the manufacturing cost is reduced. Can be suppressed.

  In addition, when further including at least one of chromium (Cr) and zinc (Zn) as a constituent element of the negative electrode active material to be manufactured, the mixing ratio of the aqueous solution containing tin ions and cobalt ions and the reducing agent aqueous solution is Control chromium and zinc contents by increasing or decreasing, increasing or decreasing the amount of metallic zinc used in preparing the reducing agent aqueous solution, adding zinc chloride to an aqueous solution containing tin ions and cobalt ions, or an aqueous reducing agent solution. be able to.

  Next, after mixing the negative electrode active material, the conductive additive, and the binder in a predetermined ratio, the mixture is mixed with a predetermined ratio (for example, the total amount of the negative electrode active material, the conductive auxiliary, and the binder is 100% by mass). The slurry of the composition for negative electrodes is prepared by mixing a solvent in 35-60 mass%). Next, the slurry of the negative electrode composition is applied (applied) to the negative electrode current collector and then dried to produce a negative electrode.

  In addition, the negative electrode collector used for preparation of a negative electrode is not specifically limited, The thing generally used conventionally can be used. For example, examples of the negative electrode current collector include copper foil, stainless steel foil, and nickel foil.

A lithium ion secondary battery is produced using the negative electrode thus obtained. A positive electrode slurry is prepared by mixing the positive electrode active material with a binder and a conductive additive at a predetermined ratio. Next, the prepared positive electrode slurry is applied onto a positive electrode current collector by a technique such as a doctor blade method and dried to prepare a positive electrode. In addition, the positive electrode active material, the binder, the conductive auxiliary agent, and the positive electrode current collector used for the production of the positive electrode are not particularly limited, and those conventionally used can be used. For example, examples of the positive electrode active material include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiMnO ₂ , and LiFePO ₄ . Examples of the conductive assistant include carbon black such as acetylene black and ketjen black, VGCF, graphite and the like. Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene-diene copolymer (EPDM), and styrene-butadiene rubber (SBR). Examples of the positive electrode current collector include aluminum foil, stainless steel foil, and nickel foil.

  Next, the negative electrode, the separator, and the positive electrode are stacked with the active material surfaces of the positive electrode and the negative electrode facing each other to form a stacked body. The separator is formed from a synthetic resin nonwoven fabric, a polyethylene porous film, a polypropylene porous film, or the like. Then, one end of the mesh material is connected to each of the positive electrode-side back surface and the negative electrode-side back surface of the laminate, and the laminate is loaded so that the other end of the mesh material protrudes into the bag-shaped aluminum laminate material. Next, a lithium ion secondary battery is obtained by adding a non-aqueous electrolyte from the opening of the laminate and heat-sealing the opening of the laminate while evacuating. An aluminum mesh material is used as the mesh material connected to the back surface on the positive electrode side, and a nickel mesh material is used as the mesh material connected to the back surface on the negative electrode side.

For the non-aqueous electrolyte, a solvent in which an electrolyte is dissolved in a non-aqueous solvent is used. Non-aqueous solvents include cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), chain carbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC), dimethoxyethane, Chain ethers such as ethoxyethane and ethoxymethoxyethane, cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran, fatty acid esters such as crown ether and γ-butyrolactone, nitrogen compounds such as acetonitrile, sulfides such as sulfolane and dimethyl sulfoxide, etc. Is exemplified. The non-aqueous electrolyte may be used alone or as a mixed solvent in which two or more kinds are mixed. Examples of the electrolyte include lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium borofluoride (LiBF ₄ ), lithium arsenic hexafluoride (LiAsF ₆ ), lithium trifluorometasulfonate ( Examples thereof include lithium salts such as LiCF ₃ SO ₃ ) and bistrifluoromethylsulfonylimide lithium [LiN (CF ₃ SO ₂ ) ₂ ].

  In the lithium ion secondary battery manufactured in this way, since the composite particle has a plurality of pores communicating with the surface of the composite particle at the cut surface, when the lithium ion secondary battery is repeatedly charged and discharged, The pores can absorb and relax the volume expansion of the composite particles during charging, and cobalt is unevenly distributed on the outer surface of the composite particles and the inner surface of the pores. In addition, since the composite particle has a plurality of pores communicating with the surface of the composite particle at the cut surface and the space ratio inside the composite particle is relatively large, the reaction area of tin with lithium is relatively wide. As a result, the original performance of tin that tin reacts efficiently with lithium can be brought out, so that the lithium ion secondary battery has excellent cycle characteristics and output characteristics, has a long life, and has a high capacity. In addition, when a conductive additive made of carbon nanofibers is added to the negative electrode active material, the conductive auxiliary agent covers the particles, and a conductive path can be formed in a net shape throughout the negative electrode. The initial discharge capacity and cycle characteristics can be further improved. When a conductive additive made of acetylene black is added to the negative electrode active material, since acetylene black is larger than carbon nanofibers, a large current can be taken out, thereby further improving output characteristics. Therefore, when a conductive additive in which carbon nanofibers and acetylene black are mixed in a mass ratio within the range of (10:90) to (80:20) is added to the negative electrode active material, cycle characteristics and output characteristics can be further improved.

  Next, examples of the present invention will be described in detail together with comparative examples.

<Example 1>
First, a dispersant, tin (II) chloride and cobalt (II) chloride are added to ion-exchanged water at a ratio such that the cobalt ratio with respect to the total of tin and cobalt in the composite particles obtained by synthesis is 20 atomic%. The mixture was dissolved by stirring, and hydrochloric acid was further added to adjust the acid concentration to 0.050 [au]. Polyacrylic acid was used as the dispersant.

  On the other hand, chromium (III) chloride is added to ion-exchanged water and dissolved by stirring. By adding metal zinc (Zn) to this, chromium ions are reduced from trivalent to divalent, and divalent in all chromium ions. The chromium ratio was adjusted to 70% or more. This was designated as a reducing agent aqueous solution.

  Next, an aqueous solution containing tin ions and cobalt ions and a reducing agent aqueous solution were mixed at a predetermined ratio, and the stirring and holding time was 24 hours to cause the tin ions and cobalt ions to undergo a reduction reaction. The temperature of this mixed solution was 25 ° C., the acid concentration was 0.050 [a.u.] (FIG. 2), and the stirring speed was 0.5 m / sec.

  Then, the liquid which was stirred and mixed was left still, the synthesized particle | grains were settled, and the supernatant liquid was removed. Subsequently, ion-exchanged water was added to the precipitate, and the operations of stirring and washing, standing sedimentation and supernatant removal were repeated several times, and finally stirring and washing with ethanol, standing sedimentation and supernatant removal were performed. The obtained precipitate is vacuum-dried to form composite particles in which the ratio of cobalt to the total amount of tin and cobalt is 20 atomic%, and the composite particles communicate with the surface of the composite particles at the cut surface. (FIG. 3), cobalt was unevenly distributed on the outer surface of the composite particle and the inner surface of the pore, and a negative electrode active material having a space ratio of 20% inside the composite particle was obtained.

<Example 2>
A negative electrode active material in the same manner as in Example 1 except that the acid concentration of the mixed solution in which the aqueous solution containing tin ions and cobalt ions and the reducing agent aqueous solution were mixed at a predetermined ratio was 0.100 [au] (FIG. 2). Got. The space ratio inside the composite particles of this negative electrode active material was 30%.

<Example 3>
A negative electrode active material in the same manner as in Example 1 except that the acid concentration of the mixed solution in which the aqueous solution containing tin ions and cobalt ions and the reducing agent aqueous solution were mixed at a predetermined ratio was 0.195 [au] (FIG. 2). Got. The space ratio inside the composite particles of this negative electrode active material was 40%.

<Example 4>
A negative electrode active material in the same manner as in Example 1 except that the acid concentration of the mixed solution in which the aqueous solution containing tin ions and cobalt ions and the reducing agent aqueous solution were mixed at a predetermined ratio was 0.269 [au] (FIG. 2). Got. The space ratio inside the composite particles of this negative electrode active material was 50%.

<Example 5>
A negative electrode active material in the same manner as in Example 1 except that the acid concentration of the mixed solution in which the aqueous solution containing tin ions and cobalt ions and the reducing agent aqueous solution were mixed at a predetermined ratio was 0.372 [au] (FIG. 2). Got. The space ratio inside the composite particles of this negative electrode active material was 70%.

<Example 6>
A negative electrode active material in the same manner as in Example 1 except that the acid concentration of the mixture obtained by mixing an aqueous solution containing tin ions and cobalt ions and an aqueous reducing agent solution at a predetermined ratio was 0.457 [au] (FIG. 2). Got. The space ratio inside the composite particles of this negative electrode active material was 80%.

<Comparative Example 1>
A negative electrode active material in the same manner as in Example 1 except that the acid concentration of the mixed solution in which the aqueous solution containing tin ions and cobalt ions and the reducing agent aqueous solution were mixed at a predetermined ratio was 0.032 [au] (FIG. 2). Got. The space ratio inside the composite particles of this negative electrode active material was 10%.

<Comparative example 2>
A negative electrode active material in the same manner as in Example 1 except that the acid concentration of the mixed solution in which the aqueous solution containing tin ions and cobalt ions and the reducing agent aqueous solution were mixed at a predetermined ratio was 0.562 [au] (FIG. 2). Got. The space ratio inside the composite particles of this negative electrode active material was 90%.

<Comparative test 1 and evaluation>
ICP quantitative analysis was performed about the negative electrode active material of Examples 1-6 and Comparative Examples 1 and 2, and each content of tin, cobalt, chromium, and zinc in a composite particle was calculated | required. The obtained results are shown in Table 1 below. In Table 1, “<0.001” and “<2” indicate that the measured values were below the ICP detection limit. Further, in “pore” of “structure of negative electrode active material” in Table 1, “present” indicates that the composite particles of the negative electrode active material have a plurality of pores, and “Co position” of “structure of negative electrode active material”. In the graph, “uneven distribution” indicates that cobalt is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore. The presence of the plurality of pores and the uneven distribution of cobalt were confirmed by an electron micrograph of the negative electrode active material and an electron micrograph of a cross section of the negative electrode active material.

  Moreover, using the negative electrode active materials of Examples 1 to 6 and Comparative Examples 1 and 2, the negative electrode active material powder was mixed with a conductive additive, a binder, and a solvent to prepare slurries. That is, the synthesized negative electrode active material powder, acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: Denka Black), carbon nanofiber (CNF), polyvinylidene fluoride (PVdF: binder), and n-methylpyrrolidinone ( NMP) was weighed in a mass ratio of 80: 5: 5: 10: 100 and kneaded using a kneader to prepare a slurry.

Next, the obtained slurry was applied on a copper foil using an applicator so that the active material density was 5 mg / cm ² , dried, rolled, and cut into a width of 3 cm and a length of 3 cm to produce a negative electrode. did.

A half battery was assembled using the produced negative electrode, and a charge / discharge cycle test was conducted. Lithium metal was used for the counter electrode and the reference electrode, and equal volume solvents of ethylene carbonate (EC) and diethyl carbonate (DEC) in which lithium hexafluorophosphate (LiPF ₆ ) was dissolved at a concentration of 1 M were used for the electrolyte. Charging was performed under a constant current condition of 0.5 mA / cm ² until the voltage reached 5 mV, and then under a constant voltage condition of 5 mV until the current reached 0.01 mA / cm ² .

The discharge was carried out under a constant current condition of 0.5 mA / cm ² until the voltage reached 1V. The state in which charging and discharging are performed once is regarded as one cycle, a charge / discharge test up to 100 cycles is performed, and the discharge capacity per active material weight for the first time and the ratio of the discharge capacity at the 100th cycle to the initial discharge capacity are lifetimes. Performance was evaluated as a characteristic. The obtained evaluation results are shown in Table 1 below.

As can be seen from Table 1, in Comparative Example 1 in which the space ratio inside the composite particles is 10%, the initial discharge capacity is as low as 555 mAh / g, and in Comparative Example 2 where the space ratio inside the composite particles is 90%. In Examples 1-6, in which the initial discharge capacity was as low as 490 mAh / g, but the porosity inside the composite particles was in the range of 20 to 80%, the initial discharge capacity was 573 to 803 mAh / g. It became bigger. From these results, it was confirmed that there was an optimum range for increasing the initial discharge capacity in the space ratio inside the composite particles. In Comparative Example 1, the initial discharge capacity was low because the reaction area of tin with lithium was too small. In Comparative Example 2, the initial discharge capacity was low because the amount of tin that reacted with lithium was too small. On the other hand, in Examples 1-6, since the reaction area with lithium was wide and the amount of tin reacting with lithium was relatively large, it is presumed that the initial discharge capacity was increased.

  Further, in Comparative Example 1 in which the space ratio inside the composite particles was 10%, the lifetime characteristics were as low as 89.0%, whereas the space ratio inside the composite particles was in the range of 20 to 90%. In 1 to 6 and Comparative Example 2, the life characteristics were as high as 94.0 to 98.5%. In Comparative Example 1, the composite particles did not have pores communicating with the particle surface, and the volume expansion of the composite particles during battery charging could not be absorbed. In 1 to 6 and Comparative Example 2, the composite particles have a plurality of pores communicating with the particle surface, and the volume expansion of the composite particles at the time of battery charging was absorbed by the plurality of pores so that the life characteristics were high. It is presumed that

<Example 7>
An aqueous solution containing tin ions and cobalt ions was prepared by adding tin (II) chloride and cobalt (II) chloride in such a ratio that the cobalt ratio of the composite particles obtained by synthesis was 5 atomic% with respect to the total of tin and cobalt. A negative electrode active material was obtained in the same manner as in Example 4 except that.

<Example 8>
An aqueous solution containing tin ions and cobalt ions was prepared by adding tin (II) chloride and cobalt (II) chloride in such a ratio that the cobalt ratio in the composite particles obtained by synthesis was 10 atomic% with respect to the total of tin and cobalt. A negative electrode active material was obtained in the same manner as in Example 4 except that.

<Example 9>
In the same manner as in Example 4, tin ions and cobalt were added by adding tin (II) chloride and cobalt (II) chloride in such a ratio that the ratio of cobalt to the total of tin and cobalt in the composite particles obtained by synthesis was 20 atomic%. An aqueous solution containing ions was prepared to obtain a negative electrode active material.

<Example 10>
An aqueous solution containing tin ions and cobalt ions was prepared by adding tin (II) chloride and cobalt (II) chloride at a ratio such that the cobalt ratio in the composite particles obtained by synthesis was 30 atomic% with respect to the total of tin and cobalt. A negative electrode active material was obtained in the same manner as in Example 4 except that.

<Example 11>
An aqueous solution containing tin ions and cobalt ions was prepared by adding tin (II) chloride and cobalt (II) chloride in such a ratio that the cobalt ratio in the composite particles obtained by synthesis was 40 atomic% with respect to the total of tin and cobalt. A negative electrode active material was obtained in the same manner as in Example 4 except that.

<Comparative Example 3>
An aqueous solution containing tin ions and cobalt ions was prepared by adding tin (II) chloride and cobalt (II) chloride in such a ratio that the cobalt ratio in the composite particles obtained by synthesis was 3 atomic% with respect to the total of tin and cobalt. A negative electrode active material was obtained in the same manner as in Example 4 except that.

<Comparative example 4>
An aqueous solution containing tin ions and cobalt ions was prepared by adding tin (II) chloride and cobalt (II) chloride in such a ratio that the cobalt ratio in the composite particles obtained by synthesis was 45 atomic% with respect to the total of tin and cobalt. A negative electrode active material was obtained in the same manner as in Example 4 except that.

<Comparative Example 5>
The negative electrode active material was a tin-cobalt powder having a cobalt ratio of 20 atomic% with respect to the total of tin and cobalt, having no compositional deviation in the center portion and the outer peripheral portion, and having a substantially uniform composition.

<Comparative test 2 and evaluation>
About the negative electrode active material of Examples 7-11 and Comparative Examples 3-5, similarly to the said comparative test 1, ICP quantitative analysis was performed and each content of tin in the composite particle, cobalt, chromium, and zinc was calculated | required. The obtained results are shown in Table 2 below. In Table 2, “<0.001” and “<2” indicate that the measured values were below the ICP detection limit. In addition, in the “pore” of “Negative electrode active material structure” in Table 2, “Yes” indicates that the composite particles of the negative electrode active material have a plurality of pores, and “Co position” of “Negative electrode active material structure”. In the graph, “uneven distribution” indicates that cobalt is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore. The presence of the plurality of pores and the uneven distribution of cobalt were confirmed by an electron micrograph of the negative electrode active material and an electron micrograph of a cross section of the negative electrode active material.

  Using the negative electrode active materials of Examples 7 to 11 and Comparative Examples 3 to 5, negative electrodes were produced in the same manner as in Comparative Test 1 above. In addition, using this negative electrode, as in Comparative Test 1 above, a half-cell was assembled, a charge / discharge cycle test was performed, and the discharge capacity per active material weight for the first time and the ratio of the discharge capacity at the 100th cycle to the initial discharge capacity Was evaluated as a life characteristic. The obtained evaluation results are shown in Table 2 below.

As apparent from Table 2, the composite particles have a plurality of pores communicating with the surface of the composite particles at the cut surface, and cobalt is unevenly distributed on the outer surface of the composite particles and the inner surface of the pores. In Comparative Examples 3 and 4, both showed high life characteristics, whereas in Comparative Example 5 in which cobalt and tin in the particles were substantially uniform, the life characteristics were very low at 40.0%. It was. From this result, composite particles having a structure in which the composite particle has a plurality of pores communicating with the surface of the composite particle at the cut surface and cobalt is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore have very high cycle characteristics. It was confirmed that

  In addition, the composite particles had a plurality of pores communicating with the surface of the composite particles at the cut surface, and the cobalt was unevenly distributed on the outer surface of the composite particles and the inner surface of the pores, and Examples 7 to 11 in which the cobalt ratio was varied Comparing Comparative Examples 3 and 4, when the cobalt ratio of Examples 7 to 11 was in the range of 5 to 40 atomic%, a high initial discharge capacity was obtained, and the life characteristics were also high. When the cobalt ratio was low as in 3 atom% of 3, the life characteristics were lowered, and when the cobalt ratio was high as in 45 atom% of Comparative Example 4, the initial discharge capacity tended to be low. From these results, it was confirmed that an appropriate range exists for the cobalt ratio in the particles.

<Example 12>
The same as in Example 4 except that the mixing ratio of the aqueous solution containing tin ions and cobalt ions and the reducing agent aqueous solution was adjusted so that the composite particles obtained by synthesis further contained 0.005% chromium by mass. Thus, a negative electrode active material was obtained.

<Example 13>
The same as in Example 4 except that the mixing ratio of the aqueous solution containing tin ions and cobalt ions and the reducing agent aqueous solution was adjusted so that the composite particles obtained by synthesis further contained 0.1% chromium by mass. Thus, a negative electrode active material was obtained.

<Example 14>
Except that the mixing ratio of the aqueous solution containing tin ions and cobalt ions and the reducing agent aqueous solution was adjusted so that the composite particles obtained by synthesis further contained 1% chromium by mass ratio, the same as in Example 4. A negative electrode active material was obtained.

<Example 15>
The negative electrode active material was prepared in the same manner as in Example 4 except that the amount of zinc metal in preparing the reducing agent aqueous solution was adjusted so that the composite particles obtained by synthesis further contained 5 ppm of zinc by mass ratio. Obtained.

<Example 16>
The negative electrode active material was prepared in the same manner as in Example 4 except that the amount of metal zinc input when preparing the reducing agent aqueous solution was adjusted so that the composite particles obtained by synthesis further contained 25 ppm of zinc by mass. Obtained.

<Example 17>
The negative electrode active material was prepared in the same manner as in Example 4 except that the amount of metal zinc input in preparing the reducing agent aqueous solution was adjusted so that the composite particles obtained by synthesis further contained 50 ppm of zinc by mass. Obtained.

<Example 18>
The same as in Example 4 except that the mixing ratio of the aqueous solution containing tin ions and cobalt ions and the reducing agent aqueous solution was adjusted so that 1.5% by mass of chromium was further contained in the composite particles obtained by synthesis. Thus, a negative electrode active material was obtained.

<Example 19>
The negative electrode active material was prepared in the same manner as in Example 4 except that the amount of metal zinc added when adjusting the reducing agent aqueous solution was adjusted so that the composite particles obtained by synthesis further contained 75 ppm of zinc by mass. Obtained.

<Comparative test 3 and evaluation>
About the negative electrode active material of Examples 12-19, similarly to the said comparative test 1, ICP quantitative analysis was performed and each content of tin, cobalt, chromium, and zinc in a composite particle was calculated | required. The results obtained are shown in Table 3 below. In Table 3, “<0.001” and “<2” indicate that the measured values were below the detection limit of ICP. In “Pore” of “Structure of negative electrode active material” in Table 3, “Yes” indicates that the composite particle of the negative electrode active material has a plurality of pores, and “Co position” of “Structure of negative electrode active material”. In the graph, “uneven distribution” indicates that cobalt is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore. The presence of the plurality of pores and the uneven distribution of cobalt were confirmed by an electron micrograph of the negative electrode active material and an electron micrograph of a cross section of the negative electrode active material.

  Using the negative electrode active materials of Examples 12 to 19, negative electrodes were produced in the same manner as in Comparative Test 1 above. In addition, using this negative electrode, as in Comparative Test 1 above, a half-cell was assembled, a charge / discharge cycle test was performed, and the discharge capacity per active material weight for the first time and the ratio of the discharge capacity at the 100th cycle to the initial discharge capacity Was evaluated as a life characteristic. The obtained evaluation results are shown in Table 3 below.

As is apparent from Table 3, in Examples 12 to 17 in which chromium was further contained in 0.005 to 1% or zinc in 5 to 50 ppm, the chromium content was less than 0.001% and the zinc content was 2 ppm. A high initial discharge capacity was obtained as compared with the result of Example 4 which is less than the lower value. On the other hand, in Example 18 with a high chromium content of 1.5% and Example 19 with a high zinc content of 75 ppm, a high initial discharge capacity similar to the result of Example 4 was obtained. The life characteristics were low. From these results, it is possible to increase the initial discharge capacity value by containing a predetermined amount of chromium and zinc. On the other hand, if too much chromium and zinc are contained, the characteristics will deteriorate, so the inclusion of chromium and zinc. It was confirmed that there was an appropriate range for the amount.

<Example 20>
Using the negative electrode active material of Example 3, the negative electrode active material was mixed with a binder, a conductive additive and a solvent to prepare a slurry. That is, a negative electrode active material, polyvinylidene fluoride (PVdF: binder), a conductive assistant and n-methylpyrrolidinone (NMP) were weighed so as to have a mass ratio of 80: 10: 10: 100, and kneader The slurry was produced by kneading using Here, the conductive assistant was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 5: 5. Next, the obtained slurry was applied on a copper foil using an applicator so that the active material density was 5 mg / cm ² , dried, rolled, and cut into a width of 3 cm and a length of 3 cm to produce a negative electrode. did. This negative electrode was designated as Example 20.

<Example 21>
A negative electrode active material, polyvinylidene fluoride (PVdF: binder), a conductive additive and n-methylpyrrolidinone (NMP) are weighed so as to have a mass ratio of 95: 3: 2: 100, and a kneader is used. And kneading to prepare a slurry. Here, only the carbon nanofiber (CNF) was used for the conductive support agent. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 21.

<Example 22>
A negative electrode active material, polyvinylidene fluoride (PVdF: binder), a conductive additive and n-methylpyrrolidinone (NMP) were weighed to a mass ratio of 88: 3: 9: 100, and a kneader was used. And kneading to prepare a slurry. Here, the conductive assistant was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 4: 5. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 22.

<Example 23>
The negative electrode active material, polyvinylidene fluoride (PVdF: binder), conductive additive and n-methylpyrrolidinone (NMP) are weighed so that the mass ratio is 80: 5: 15: 100, and a kneader is used. And kneading to prepare a slurry. Here, the conductive auxiliary was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 7: 8. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 23.

<Example 24>
The negative electrode active material, polyvinylidene fluoride (PVdF: binder), conductive additive and n-methylpyrrolidinone (NMP) were weighed so that the mass ratio was 75: 5: 20: 100, and a kneader was used. And kneading to prepare a slurry. Here, the conductive assistant was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 2:18. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 24.

<Example 25>
The negative electrode active material, polyvinylidene fluoride (PVdF: binder), conductive additive and n-methylpyrrolidinone (NMP) were weighed so that the mass ratio was 75: 5: 20: 100, and a kneader was used. And kneading to prepare a slurry. Here, the conductive assistant was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 5:15. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 25.

<Example 26>
The negative electrode active material, polyvinylidene fluoride (PVdF: binder), conductive additive and n-methylpyrrolidinone (NMP) were weighed so that the mass ratio was 75: 5: 20: 100, and a kneader was used. And kneading to prepare a slurry. Here, the conductive assistant was weighed so that carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 10:10. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 26.

<Example 27>
The negative electrode active material, polyvinylidene fluoride (PVdF: binder), conductive additive and n-methylpyrrolidinone (NMP) were weighed so that the mass ratio was 75: 5: 20: 100, and a kneader was used. And kneading to prepare a slurry. Here, the conductive assistant was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 15: 5. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 27.

<Example 28>
A negative electrode active material, polyvinylidene fluoride (PVdF: binder), a conductive assistant and n-methylpyrrolidinone (NMP) are weighed so as to have a mass ratio of 70: 10: 20: 100, and a kneader is used. And kneading to prepare a slurry. Here, the conductive assistant was weighed so that carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 10:10. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 28.

<Example 29>
A negative electrode active material, polyvinylidene fluoride (PVdF: binder), conductive additive and n-methylpyrrolidinone (NMP) are weighed so as to have a mass ratio of 70: 15: 15: 100, and a kneader is used. And kneading to prepare a slurry. Here, the conductive auxiliary was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 7: 8. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 29.

<Example 30>
The negative electrode active material, polyvinylidene fluoride (PVdF: binder), conductive additive and n-methylpyrrolidinone (NMP) were weighed to a mass ratio of 79: 15: 6: 100, and a kneader was used. And kneading to prepare a slurry. Here, the conductive assistant was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 3: 3. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 30.

<Example 31>
A negative electrode active material, polyvinylidene fluoride (PVdF: binder), a conductive assistant and n-methylpyrrolidinone (NMP) are weighed so as to have a mass ratio of 86: 10: 4: 100, and a kneader is used. And kneading to prepare a slurry. Here, the conductive assistant was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 2: 2. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 31.

<Example 32>
The negative electrode active material, polyvinylidene fluoride (PVdF: binder), conductive additive and n-methylpyrrolidinone (NMP) are weighed so that the mass ratio is 88: 7: 5: 100, and a kneader is used. And kneading to prepare a slurry. Here, the conductive assistant was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: Denka Black) were in a mass ratio of 2: 3. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 32.

<Example 33>
The negative electrode active material, polyvinylidene fluoride (PVdF: binder), conductive additive and n-methylpyrrolidinone (NMP) are weighed so that the mass ratio is 73: 12: 15: 100, and a kneader is used. And kneading to prepare a slurry. Here, the conductive auxiliary was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 7: 8. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 33.

<Example 34>
A negative electrode active material, polyvinylidene fluoride (PVdF: binder), a conductive additive and n-methylpyrrolidinone (NMP) are weighed to a mass ratio of 80: 10: 10: 100, and a kneader is used. And kneading to prepare a slurry. Here, the conductive assistant was weighed so that the carbon nanofiber (CNF) and Ketjen Black (KB, manufactured by Ketjen Black International Co., Ltd., trade name: Ketjen Black EC300J) had a mass ratio of 2: 8. . A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 34.

<Example 35>
A negative electrode active material, polyvinylidene fluoride (PVdF: binder), a conductive additive and n-methylpyrrolidinone (NMP) are weighed so as to have a mass ratio of 95: 3: 2: 100, and a kneader is used. And kneading to prepare a slurry. Here, the conductive assistant was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were 1: 1 by mass ratio. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 35.

<Example 36>
The negative electrode active material, polyvinylidene fluoride (PVdF: binder), conductive additive and n-methylpyrrolidinone (NMP) were weighed so that the mass ratio was 75: 5: 20: 100, and a kneader was used. And kneading to prepare a slurry. Here, the conductive assistant was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 1:19. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 36.

<Example 37>
The negative electrode active material, polyvinylidene fluoride (PVdF: binder), conductive additive and n-methylpyrrolidinone (NMP) were weighed so that the mass ratio was 75: 5: 20: 100, and a kneader was used. And kneading to prepare a slurry. Here, the conductive assistant was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 18: 2. Only carbon nanofiber (CNF) was used. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 37.

<Example 38>
The negative electrode active material, polyvinylidene fluoride (PVdF: binder), conductive additive and n-methylpyrrolidinone (NMP) are weighed so that the mass ratio is 78: 4: 18: 100, and a kneader is used. And kneading to prepare a slurry. Here, the conductive assistant was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 9: 9. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was referred to as Example 38.

<Example 39>
A negative electrode active material, polyvinylidene fluoride (PVdF: binder), a conductive additive and n-methylpyrrolidinone (NMP) are weighed so as to have a mass ratio of 70: 7: 23: 100, and a kneader is used. And kneading to prepare a slurry. Here, the conductive assistant was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 11:12. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 39.

<Example 40>
A negative electrode active material, polyvinylidene fluoride (PVdF: binder), a conductive additive and n-methylpyrrolidinone (NMP) are weighed to a mass ratio of 65: 15: 20: 100, and a kneader is used. And kneading to prepare a slurry. Here, the conductive assistant was weighed so that carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 10:10. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 40.

<Example 41>
A negative electrode active material, polyvinylidene fluoride (PVdF: binder), conductive additive and n-methylpyrrolidinone (NMP) are weighed so as to have a mass ratio of 70: 20: 10: 100, and a kneader is used. And kneading to prepare a slurry. Here, the conductive assistant was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: Denka Black) were in a mass ratio of 5: 5. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was referred to as Example 41.

<Example 42>
A negative electrode active material, polyvinylidene fluoride (PVdF: binder), a conductive additive and n-methylpyrrolidinone (NMP) are weighed to a mass ratio of 88: 10: 2: 100, and a kneader is used. And kneading to prepare a slurry. Here, only the carbon nanofiber (CNF) was used for the conductive support agent. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was referred to as Example 42.

<Example 43>
A negative electrode active material, polyvinylidene fluoride (PVdF: binder), a conductive additive and n-methylpyrrolidinone (NMP) were weighed so as to have a mass ratio of 95: 4: 1: 100, and a kneader was used. And kneading to prepare a slurry. Here, the conductive auxiliary was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 0.5: 0.5. . A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was made Example 43.

<Example 44>
A negative electrode active material, polyvinylidene fluoride (PVdF: binder), a conductive additive and n-methylpyrrolidinone (NMP) were weighed so as to have a mass ratio of 97: 2: 1: 100, and a kneader was used. And kneading to prepare a slurry. Here, the conductive auxiliary was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 0.5: 0.5. . A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 44.

<Example 45>
A negative electrode active material, polyvinylidene fluoride (PVdF: binder), a conductive additive and n-methylpyrrolidinone (NMP) were weighed so as to have a mass ratio of 95: 1: 4: 100, and a kneader was used. And kneading to prepare a slurry. Here, the conductive assistant was weighed so that the carbon nanofiber (CNF) and acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: DENKA BLACK) were in a mass ratio of 2: 2. A negative electrode was produced in the same manner as in Example 20 except for the above. This negative electrode was designated as Example 45.

<Comparative test 4 and evaluation>
A half battery was assembled using the negative electrodes of Examples 20 to 45, and a charge / discharge cycle test was conducted. Lithium metal was used for the counter electrode and the reference electrode, and equal volume solvents of ethylene carbonate (EC) and diethyl carbonate (DEC) in which lithium hexafluorophosphate (LiPF ₆ ) was dissolved at a concentration of 1 M were used for the electrolyte. Charging was performed under a constant current condition of 0.5 mA / cm ² until the voltage reached 5 mV, and then under a constant voltage condition of 5 mV until the current reached 0.01 mA / cm ² .

The discharge was carried out under a constant current condition of 0.5 mA / cm ² until the voltage reached 1V. The state in which charging and discharging are performed once is regarded as one cycle, a charge / discharge test up to 100 cycles is performed, and the discharge capacity per active material weight for the first time and the ratio of the discharge capacity at the 100th cycle to the initial discharge capacity are lifetimes. Performance was evaluated as a characteristic. Further, the discharge capacity per unit weight of the negative electrode mixture sheet (negative electrode active material + conductive auxiliary agent + binder) was measured. Furthermore, the ratio of the discharge capacity obtained at a current density of 5 mA / cm ² that was 10 times the discharge capacity obtained at a current density of 0.5 mA / cm ² was measured as output characteristics. The obtained evaluation results are shown in the following Tables 4 and 5.

As is apparent from Tables 4 and 5, in Examples 35 and 36 in which the carbon nanofiber (CNF) of the conductive additive is below the preferred range (2 to 15% by mass), the initial discharge capacity per active material is 666 mAh / g. And 671 mAh / g and relatively low life characteristics of 88.4% and 88.6%, while carbon nanofiber (CNF) as a conductive aid is in a preferable range (2 to 15% by mass). In Examples 20 to 34, the initial discharge capacity per active material was relatively high at 727 to 770 mAh / g, and the life characteristics were also relatively high at 89.5 to 97.0%. In Example 37, in which the carbon nanofiber (CNF) of the conductive additive exceeded the preferred range (2 to 15% by mass), the life characteristics and output characteristics were relatively low, 89.4% and 49%, respectively. In Examples 20 to 34 in which the carbon nanofiber (CNF) of the conductive additive is in the preferable range (2 to 15% by mass), the life characteristics and the output characteristics are 89.5 to 97.0% and 56 to 77%, respectively. It turned out to be relatively high. Furthermore, in Example 38 where the ratio X / Y of the content ratio X of the conductive auxiliary agent and the content ratio Y of the binder deviates from the preferred range (0.4 to 3), the life characteristics were relatively low at 87.0%. On the other hand, in Examples 20 to 34 in which the ratio X / Y of the content ratio X of the conductive auxiliary agent and the content ratio Y of the binder is within the preferable range (0.4 to 3), the life characteristic is 89.5. It was found to be relatively high at ˜97.0%.

  On the other hand, in Example 39 in which the content of the conductive auxiliary exceeds the preferable range (2 to 20% by mass), the initial discharge capacity per sheet is relatively low at 527 mAh / g, and the life characteristics are also relatively low at 88.4%. On the other hand, in Examples 20 to 34 in which the content of the conductive auxiliary agent is within a preferable range (2 to 20% by mass), the initial discharge capacity per sheet is relatively high at 536 to 691 mAh / g, and the life characteristics are increased. Was also found to be relatively high at 89.5 to 97.0%. Further, in Example 43 in which the content ratio of the conductive auxiliary agent falls below the preferred range (2 to 20% by mass), the initial discharge capacity per active material was relatively low at 614 mAh / g, whereas the output characteristics were also low at 32%. In Examples 20 to 34 in which the content of the conductive auxiliary agent is within a preferable range (2 to 20% by mass), the initial discharge capacity per active material is relatively high at 727 to 770 mAh / g, and the output characteristics are 56 to It was found to be relatively high at 77%.

  In Example 40 in which the content ratio of the negative electrode active material exceeds the preferable range (70 to 95% by mass), the initial discharge capacity per sheet was relatively low at 490 mAh / g, whereas the content ratio of the negative electrode active material was preferable. In Examples 20 to 34 within (70 to 95% by mass), the initial discharge capacity per sheet was found to be relatively high at 536 to 691 mAh / g. Further, in Example 44 in which the content ratio of the negative electrode active material is lower than the preferred range (70 to 95% by mass), the life characteristics were as low as 81.9% and the output characteristics were as low as 37%. In Examples 20 to 34 in which the ratio is within a preferable range (70 to 95% by mass), the life characteristics are relatively high as 89.5 to 97.0%, and the output characteristics are also relatively high as 56 to 77%. I understood.

  Furthermore, in Example 41 in which the binder content exceeds the preferred range (3 to 15% by mass), the initial discharge capacity per sheet was relatively low at 517 mAh / g, whereas the binder content was preferred. In Examples 20 to 34 within the range (3 to 15% by mass), the initial discharge capacity per sheet was found to be relatively high at 536 to 691 mAh / g. Further, in Example 45 in which the content ratio of the binder is less than the preferred range (3 to 15% by mass), the life characteristics are as low as 80.0% and the output characteristics are as low as 42%, whereas the content of the binder is included. In Examples 20 to 34 in which the ratio is within a preferable range (3 to 15% by mass), the life characteristics are relatively high as 89.5 to 97.0%, and the output characteristics are also relatively high as 56 to 77%. I understood.

  On the other hand, in Example 42 where the ratio X / Y of the content ratio X of the conductive auxiliary agent and the content ratio Y of the binder deviates from the preferred range (0.4 to 3), the initial discharge capacity and output characteristics per active material are The ratio X / Y of the content ratio X of the conductive auxiliary agent and the content ratio Y of the binder is within a preferable range (0.4 to 3), while they were relatively low at 676 mAh / g and 37%, respectively. In Examples 20 to 34, it was found that the initial discharge capacity and output characteristics per active material were relatively high at 727 to 770 mAh / g and 56 to 77%, respectively. Further, when acetylene black (AB, manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: Denka Black) is increased from carbon nanofiber (CNF) from Examples 24-29, the output characteristics are improved, and carbon nanofiber (CNF) is converted to acetylene black. It was found that when the amount was the same as (AB, manufactured by Denki Black, trade name: Denka Black) or more than acetylene black (AB, trade name: Denka Black), the life characteristics were improved.

Claims (3)

A composition for a negative electrode of a lithium ion secondary battery comprising a negative electrode active material, a conductive additive and a binder,
The negative electrode active material is composed of composite particles in which the ratio of cobalt (Co) to the total amount of tin (Sn) and cobalt (Co) is 5 to 40 atomic%, and the composite particles are cut at the surface of the composite particles. The cobalt (Co) is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore, and the space ratio inside the composite particle is 20 to 80%,
The conductive additive is one or more carbon materials selected from the group consisting of carbon nanofibers, acetylene black, and ketjen black, and the outer surface of the composite particle or the outer surface of the composite particle and the inner surface of the pore A composition for a negative electrode of a lithium ion secondary battery, characterized in that it is configured to adhere to a network.   The content ratio of the negative electrode active material is 70 to 95 mass%, the content ratio of the conductive auxiliary agent is 2 to 20 mass%, the content ratio of the binder is 3 to 15 mass%, When the content ratio of the conductive auxiliary agent is X and the content ratio of the binder is Y, X / Y is set within a range of 0.4 to 3, and the conductive auxiliary agent includes carbon nanofibers. 2. The composition for a negative electrode of a lithium ion secondary battery according to claim 1, wherein the content of the carbon nanofiber is 2 to 15% by mass.   A negative electrode of a lithium ion secondary battery, wherein the negative electrode composition according to claim 1 or 2 is coated on a negative electrode current collector.