JP5994688B2 - Negative electrode active material for lithium ion secondary battery and lithium ion secondary battery using the negative electrode active material - Google Patents

Description

  The present invention relates to a negative electrode active material having a high capacity and excellent cycle characteristics, a lithium ion secondary battery using the negative electrode active material, and a method for producing the negative electrode active material.

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 has been studied and developed in which tin is used as a metal alloying with lithium, cobalt is used as a metal not alloying with lithium, and these alloy thin films are used as a negative electrode active material layer. For example, in the previous application, the present inventors have disclosed a composite containing tin and cobalt at a predetermined ratio as a negative electrode active material capable of producing a long-life lithium ion secondary battery having a higher capacity and superior cycle characteristics than the conventional application. A negative electrode active material composed of particles, which has a plurality of pores communicating with the surface of the composite particles, and has a structure in which cobalt is unevenly distributed on the outer surface of the composite particles and the inner surface of the pores. For example, see Patent Document 1). In this negative electrode active material, the volume expansion of composite particles due to pores, or the structure in which cobalt having relatively high hardness and conductivity is unevenly distributed on the outer surface of tin or the inner surface of pores, the capacity and cycle characteristics, etc. are higher than before. It is said that an excellent lithium ion secondary battery can be manufactured.

JP 2012-164463 A (claim 1, paragraph [0021])

  As a result of intensive studies, the inventors have further improved the capacity and cycle characteristics of the lithium ion secondary battery by further improving the negative electrode active material and the like disclosed in the above-mentioned conventional Patent Document 1. We have succeeded in developing negative electrode materials that can be used.

  An object of the present invention is to provide a negative electrode active material capable of producing a long-life lithium ion secondary battery having high capacity and excellent cycle characteristics and output characteristics.

  Another object of the present invention is to provide a long-life lithium ion secondary battery having a high capacity and excellent cycle characteristics.

  A first aspect of the present invention is composed of composite particles containing tin (Sn) and cobalt (Co), the composite particles are arranged centering on tin (Sn), and cobalt (Co) is present on the outer surface of the tin (Sn). Lithium ion two-dimensional 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 (Co) is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore. A negative electrode active material for a secondary battery, wherein the composite particles further include bismuth as a constituent element, and the content of bismuth is 0.05 to 1.2 atomic% relative to 100 atomic% of tin (Sn) content It is the negative electrode active material for lithium ion secondary batteries characterized by these.

  The second aspect of the present invention is the invention based on the first aspect, and further, the ratio of cobalt (Co) to the total amount of tin (Sn) and cobalt (Co) is 5 to 40 atomic%. Features.

  According to a third aspect of the present invention, in a lithium ion secondary battery including a negative electrode having a negative electrode active material, a positive electrode having a positive electrode active material, and a non-aqueous electrolyte, the negative electrode active material includes tin (Sn) and cobalt. It is composed of composite particles containing (Co), and the composite particles have a structure in which tin (Sn) is arranged in the center and cobalt (Co) is unevenly distributed on the outer surface of the tin (Sn), or the composite particles are combined at the cut surface. The structure has a plurality of pores communicating with the surface of the particle, and cobalt (Co) is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore. The composite particle further contains bismuth as a constituent element, and the content of bismuth is tin. It is characterized by being contained in an amount of 0.05 to 1.2 atomic% with respect to a content of (Sn) of 100 atomic%.

  4th viewpoint of this invention is invention based on 3rd viewpoint, Comprising: The ratio of cobalt (Co) with respect to the total amount of tin (Sn) and cobalt (Co) is 5-40 atomic%. Features.

  The negative electrode active material according to the first aspect of the present invention is composed of composite particles containing tin (Sn) and cobalt (Co), and the composite particles are arranged centering on tin, and cobalt is unevenly distributed on the outer surface of tin. For example, an unevenly distributed tin layer of cobalt having a relatively high hardness and conductivity is formed on the outer surface of tin, which is a base material and has a relatively low hardness and conductivity, so that stress due to volume expansion / contraction during charge / discharge Can be relaxed and conductivity can be secured. Further, when the composite particle has a plurality of pores communicating with the surface of the composite particle at the cut surface and cobalt (Co) is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore, this negative electrode active material was used. When the lithium ion secondary battery is repeatedly charged and discharged, the pores in the composite particles 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. Thus, a tin layer with a relatively high hardness and conductivity is formed on the outer surface or the inner surface of the tin having a plurality of pores and a relatively low hardness and conductivity. The stress due to volume expansion / contraction at the time can be relieved and conductivity can be secured. As a result, the original performance of tin that tin reacts efficiently with lithium can be brought out. Therefore, in the negative electrode active material of the present invention, for example, compared with a lithium ion secondary battery in which cobalt that is not alloyed with lithium is alloyed with tin with a substantially uniform composition to produce a negative electrode, the cycle characteristics and the output characteristics are excellent. A lithium ion secondary battery having a long life and a high capacity can be produced. Furthermore, in the negative electrode active material of the present invention, bismuth is further contained in the composite particles in a predetermined ratio in addition to tin (Sn) and cobalt (Co) as constituent elements. That is, in the reduction reaction for precipitating the composite particles, bismuth ions that form the nuclei of the particles are added so that the bismuth content in the composite particles after the precipitation becomes a predetermined ratio, and the bismuth ions form a plurality of nuclei. Is controlled to a very small particle size. Thereby, compared with the negative electrode active material which consists of a conventional composite particle, the cycling characteristics of a lithium ion secondary battery can be improved more especially. In addition, since the composite particles are controlled to have a very small particle size, the negative electrode using the composite particles as a negative electrode active material can increase the amount of the conductive auxiliary agent present on the surface of the negative electrode active material. A good conductive path is ensured. Moreover, the effect which prevents the crack of the negative electrode active material accompanying charging / discharging is acquired by controlling to a small particle size.

  In the negative electrode active material according to the second aspect of the present invention, the ratio of cobalt (Co) to the total amount of tin (Sn) and cobalt (Co) is 5 to 40 atomic%. Thus, the presence of cobalt having a relatively high hardness and electrical conductivity in a desired ratio further enhances the effect of relieving stress due to volume expansion / contraction during charge / discharge and the effect of ensuring conductivity.

  A lithium ion secondary battery according to a third aspect of the present invention includes the above-described tin (Sn) and cobalt (Co) as a negative electrode active material, and is arranged around tin (Sn) and the tin (Sn) outer surface. In this case, composite particles having a structure in which cobalt (Co) is unevenly distributed or having a plurality of pores communicating with the surface at the cut surface and having a structure in which cobalt (Co) is unevenly distributed on the outer surface and the inner surface of the pore are used. Therefore, for the reasons described above, compared with a lithium ion secondary battery in which cobalt that is not alloyed with lithium is alloyed with tin with a substantially uniform composition to produce a negative electrode, it has excellent cycle characteristics and output characteristics, has a long life, and has a capacity. Is expensive. Further, since the composite particles used as the negative electrode active material further contain bismuth as a constituent element at a predetermined ratio and are controlled to have a very small desired particle size, lithium using conventional composite particles as the negative electrode active material Compared to an ion secondary battery, cracking of the negative electrode active material accompanying charge / discharge is suppressed, and in particular, cycle characteristics are excellent. Moreover, since a favorable conductive path is obtained in the negative electrode, the battery capacity can be increased.

  The lithium ion secondary battery according to the fourth aspect of the present invention includes the composite particles as a negative electrode active material, and the ratio of cobalt (Co) to the total amount of tin (Sn) and cobalt (Co) in the composite particles. Is 5 to 40 atomic%. Thus, since cobalt with comparatively high hardness and electrical conductivity exists in a desired ratio, the effect of relieving stress due to volume expansion / contraction during charge / discharge and the effect of ensuring conductivity are further enhanced in the negative electrode.

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 Example 4, and a cobalt concentration, respectively.

  Next, an embodiment for carrying out the present invention will be described with reference to the drawings.

The negative electrode active material for a lithium ion secondary battery of the present invention is composed of composite particles containing tin (Sn) and cobalt (Co). The ratio of cobalt (Co) to the total amount of tin (Sn) and cobalt (Co) is preferably 5 to 40 atomic%. Here, the reason why the ratio of cobalt in the composite particles is preferably within the above range is that, when the ratio of cobalt is less than 5 atomic%, the hardness formed on the outer surface of tin having a relatively low hardness is relatively high. The tin layer in which cobalt is unevenly distributed becomes thin, and it is difficult to obtain the effect of relieving the stress due to volume expansion / contraction at the time of charging / discharging of the secondary battery using this negative electrode active material, and the cycle characteristics of the secondary battery are deteriorated. This is because there is a tendency. On the other hand, even if the proportion of cobalt exceeds 40 atomic%, the secondary battery using this negative electrode active material has good cycle characteristics, but the amount of cobalt increases and the amount of tin that reacts with lithium relatively decreases. This is because the initial discharge capacity tends to be small. Of these, the ratio of cobalt (Co) to the total amount of tin (Sn) and cobalt (Co) is particularly preferably 10 to 30 atomic%.
Further, the composite particle has a structure in which tin (Sn) is arranged at the center and cobalt (Co) is unevenly distributed on the outer surface of the tin (Sn), or communicates with the surface of the composite particle at the cut surface of the composite particle. The structure has a plurality of pores and cobalt (Co) is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore. 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.

  When the composite particles are arranged around tin (Sn) and have a structure in which cobalt (Co) is unevenly distributed on the outer surface of tin (Sn), the outer surface of tin, which is a base material and has relatively low hardness and conductivity, Since a cobalt-distributed tin layer having relatively high hardness and conductivity is formed, stress due to volume expansion / contraction during charge / discharge can be relieved and conductivity can be ensured.

On the other hand, when the composite particle has a plurality of pores communicating with the surface of the composite particle at the cut surface and cobalt (Co) is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore, the composite particle is combined at the cut surface. By having a plurality of pores communicating with the surface of the particle, when the lithium ion secondary battery using this negative electrode active material is repeatedly charged and discharged, the pore in the composite particle expands the volume of the composite particle during charging. It can be absorbed and relaxed, and cobalt is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore, so that the hardness of the outer surface of the tin or the inner surface of the tin is relatively low in hardness and conductivity. In addition, since a tin layer in which cobalt having a relatively high conductivity is unevenly distributed is formed, stress due to volume expansion / contraction during charge / discharge can be relieved and conductivity can be ensured.
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 of the present invention is excellent in cycle characteristics and output characteristics, has a long life, and has a high capacity.

  The negative electrode active material of the present invention further contains bismuth as a constituent element of the composite particles in addition to tin (Sn) and cobalt (Co) at a predetermined ratio. This bismuth is obtained by reducing the bismuth ions added when the composite particles are deposited by a reduction reaction or the like in the composite particle manufacturing process described later and remaining as a constituent element in the manufactured composite particles. Thus, bismuth ions are first reduced in this reduction reaction by adding bismuth ions when reducing tin ions and cobalt ions so that bismuth remains in a predetermined ratio in the composite particles after production. With this as the core of the particle, tin ions and cobalt ions can be reduced and deposited. Since the oxidation-reduction potential of bismuth ions is more noble than that of tin ions and cobalt ions, bismuth ions are reduced prior to tin ions and cobalt ions. That is, if the amount of bismuth ions added at this time is increased, the number of nuclei is increased, and the number of tin ions and cobalt ions that can be reduced and precipitated with respect to one nucleus is decreased, so that a finer particle size can be controlled. . On the other hand, if the amount of bismuth is reduced, the number of tin ions and cobalt ions increases and the particle size is controlled to be larger. It is not yet clear where the bismuth remains in the composite particles at this stage, but the bismuth ions added when reducing tin ions and cobalt ions are reduced. Are not added and mixed separately after the production of the slag, but are contained as constituent elements at least inside the particles.

The negative electrode active material of the present invention has a content of tin (Sn) of 100 atomic% as a constituent element in the composite particles after production by adding bismuth ions at a predetermined ratio when reducing the above-described tin ions and cobalt ions. Bismuth is contained at a ratio of 0.05 to 1.2 atomic% with respect to the above. That is, it is included so that the atomic ratio of tin and bismuth is 100: 0.05 to 1.2 (tin: bismuth). Thereby, the composite particle which comprises a negative electrode active material is controlled by the very fine particle diameter which has an average particle diameter in the range of preferably 0.05-10 micrometers, More preferably 0.1-5 micrometers. Therefore, in particular, the cycle characteristics of the lithium ion secondary battery can be further improved as compared with the negative electrode active material made of conventional composite particles. The reason why the cycle characteristics can be particularly improved by controlling the composite particles constituting the negative electrode active material to a fine particle size is that the negative electrode active material cracks due to charge / discharge due to being controlled to a small particle size. It is because it is suppressed. In addition, since the specific surface area increases as the particle size of the composite particles decreases, the negative electrode using this as the negative electrode active material can increase the amount of the conductive auxiliary agent present on the surface of the negative electrode active material. A conductive path is ensured. Here, the amount of bismuth contained as a constituent element in the composite particles is limited to the above range because the proportion of bismuth contained in the composite particles is less than the lower limit, that is, added when reducing tin ions and cobalt ions. This is because if the amount of bismuth ions is too small, the average particle size of the composite particles exceeds 10 μm for the reasons described above, and the above-described effects cannot be obtained. On the other hand, if the proportion of bismuth contained in the composite particles exceeds the upper limit, that is, if the amount of bismuth ions added when reducing tin ions and cobalt ions is too large, the average particle size of the composite particles will be less than 0.1 μm. This is because it becomes difficult to make a slurry and cannot be applied to the manufacturing process of an existing lithium ion secondary battery. Among these, the amount of bismuth contained as a constituent element in the composite particles is preferably in the range of 0.1 to 1.1 atomic%. The average particle size of the composite particles means a volume-based average particle size D ₅₀ measured using a particle size distribution measuring device (LA-950 manufactured by Horiba, Ltd.). The contents of tin, cobalt, bismuth, chromium, zinc, etc. in the composite particles constituting the negative electrode active material of the present invention can be determined by quantitative analysis using ICP (inductively coupled plasma). In addition, this composite particle is a powder whose particle size is controlled within a desired range, and the negative electrode active material can be slurried and applied to the negative electrode current collector plate. The process can be applied.

  The negative electrode active material has a chromium (Cr) content of unavoidable components of 1.0% or less by mass ratio with respect to the total mass of the negative electrode active material, and a zinc (Zn) content of 50 ppm or less by mass ratio. It is preferable that This is because if the chromium content is 1% or the zinc content exceeds 50 ppm, the strength of the tin layer in which cobalt is unevenly distributed is reduced, so that the protective effect is lowered and the cycle characteristics tend to be lowered. Moreover, the diffusion of lithium ions into the composite particles becomes insufficient, and the initial discharge capacity tends to decrease. Among these, it is more preferable that the content of chromium (Cr) is less than 0.001% by mass, and the content of zinc (Zn) is less than 2 ppm by mass.

  The negative electrode active material preferably 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. Furthermore, it is preferable to add a conductive auxiliary agent made of carbon nanofiber (CNF) to the negative electrode active material. By adding this conductive auxiliary agent, the conductive auxiliary agent covers the particles, and a conductive path can be formed in a network form on the entire negative electrode, so that the cycle characteristics can be further improved.

  Next, the manufacturing method of the said negative electrode active material for lithium ion secondary batteries is demonstrated. First, an aqueous solution containing tin ions and cobalt ions and a solution containing bismuth ions are prepared, mixed, and further mixed with a reducing agent aqueous solution containing divalent chromium ions. Tin ions, cobalt ions and bismuth ions are reduced. The aqueous solution containing tin ions and cobalt ions is prepared by dissolving a tin compound such as tin (II) chloride and a cobalt compound such as cobalt (II) chloride in a solvent such as ion-exchanged water. The solution containing bismuth ions is prepared by dissolving a bismuth compound such as bismuth (III) chloride in a solvent such as hydrochloric acid.

  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 ratio of each component when adjusting the aqueous solution containing tin ions and cobalt ions is such that the cobalt ratio with respect to the total of tin and cobalt in the composite particles obtained by synthesis is 5 to 40 atomic%, preferably 10 to 30 atomic%. Adjust so that it is within the range. And as above-mentioned, the content of bismuth is 0.05-1.2 atom with respect to 100 atomic% of tin (Sn) content as for the aqueous solution containing a tin ion and cobalt ion, and the solution containing a bismuth ion. %, Preferably 0.1 to 1.1 atomic%, to prepare a solution containing tin ions, cobalt ions and bismuth ions.

  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 a solution containing tin ions, cobalt ions and bismuth 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 pH 0-2. This is because when the pH exceeds the upper limit value, a problem that trivalent chromium ions precipitate as hydroxides easily occurs.

  In order to obtain a composite particle having a two-layer structure in which tin (Sn) is arranged at the center and cobalt (Co) is unevenly distributed on the outer surface of tin (Sn), a solution containing tin ion, cobalt ion and bismuth ion and divalent chromium ion are used. It is suitable that the temperature of the mixed solution is set to 15 to 50 ° C., preferably 25 to 40 ° C., and the pH is controlled to 0 to 4, preferably 0 to 2. . Here, the reason why the temperature of the mixed solution is limited to the above range is that if the temperature is less than the lower limit, the introduction of the cooling device is costly, whereas if the upper limit is exceeded, dissolution of the precipitated composite particles is promoted. In addition, the pH of the mixed solution was limited to the above range. If the pH is less than 0, 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. This is because if the pH exceeds 4, there is a problem that the cycle characteristics are deteriorated.

  The treatment time of the mixed solution is 6 to 48 hours, more preferably 12 to 24 hours, and the stirring speed is set to 0.2 to 1.5 m / second, preferably 0.4 to 1.0 m / second. Is done. The processing time of the mixed solution refers to the stirring and holding time of the mixed solution. When the stirring and holding time is less than 6 hours, elution of tin is insufficient, and there may be a problem that the effect of absorbing and relaxing the volume expansion of the composite particles during charging is reduced. On the other hand, if it exceeds 48 hours, the production efficiency may be reduced. In addition, the stirring speed of the mixed solution is limited to the above range because the mixing of the solution is insufficient when the amount is less than the lower limit, and the composite particles having a predetermined shape cannot be obtained with good reproducibility. This is because there is a problem that the realization at the time of scale-up involves difficulty. The stirring speed refers to the average flow rate of the mixed liquid when the mixed liquid flows due to the rotation of the stirring blade.

  When the above mixture is stirred and mixed, and tin ions, cobalt ions, and bismuth ions are reduced in the mixture, bismuth having an oxidation-reduction potential nobler than that of tin ions and cobalt ions is first reduced, and then this is converted into nuclei. As a result, 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. Thereby, composite particles having a base material made of tin and a tin layer in which cobalt is unevenly distributed on the outer surface of the base material are deposited.

  On the other hand, when adjusting at least one of the temperature, pH, treatment time or stirring speed of the mixed solution during the reduction reaction, a plurality of pores communicating with the surface of the particles are formed in the composite particles, Cobalt is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore, 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. 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.

  In order to obtain composite particles having this structure, the temperature of the mixed solution obtained by mixing the solution containing tin ions, cobalt ions and bismuth ions with the reducing agent aqueous solution containing divalent chromium ions is 15 to 50 ° C., preferably 25 to 40 ° C. The temperature of the mixed solution is set to 0 to 4, preferably 0 to 2. 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. 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. . In addition, the pH of the mixed solution was limited to the above range. If the pH is less than 0, 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. If the pH exceeds 4, the generation of hydrogen from the cobalt side due to the contact between tin and cobalt is difficult to occur, and the tin inside the composite particle is difficult to dissolve, so that it is difficult to form a plurality of pores in the composite particle. This is because the effect of absorbing and relaxing the volume expansion of the composite particles during charging is reduced. 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, that is, the particle size decreases, and the specific surface area of the composite particles tends to increase as the pH 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.

  As described above, the manufacturing method of the present invention 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 not required, and the manufacturing cost is suppressed. it can.

  In addition, the content of chromium (Cr) and zinc (Zn) in the negative electrode active material is used when preparing a reducing agent aqueous solution that increases or decreases the mixing ratio of a solution containing tin ions, cobalt ions, and bismuth ions and a reducing agent aqueous solution. The content of chromium and zinc can be controlled by techniques such as increasing or decreasing the amount of metallic zinc used in the process, or adding zinc chloride to an aqueous solution containing tin ions and cobalt ions or an aqueous reducing agent solution.

Next, a method for producing a lithium ion secondary battery using the negative electrode active material of the present invention will be described. Specifically, first, the negative electrode active material, the conductive auxiliary agent, and the binder are mixed in a predetermined ratio, and then a predetermined ratio (for example, the negative electrode active material, the conductive auxiliary agent, and the binder is added to the mixture). The slurry of the composition for negative electrodes is prepared by mixing a solvent in 35-60 mass%) with respect to 100 mass% of total amounts. Next, the slurry for the negative electrode composition is applied onto the negative electrode current collector by a technique such as a doctor blade method and then dried to prepare a negative electrode.
The conductive additive, binder, solvent, and negative electrode current collector used for the production of the negative electrode are not particularly limited, and those generally used conventionally can be used. For example, examples of the conductive assistant include carbon black such as acetylene black and ketjen black, VGCF, or metal powder that is difficult to alloy with lithium such as copper and titanium. 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). Examples of the solvent include N-methylpyrrolidone and water. 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 active material is mixed with a binder and a conductive additive at a predetermined ratio to prepare a slurry of the positive electrode composition. Next, the slurry for the positive electrode composition is applied onto the positive electrode current collector by a technique such as a doctor blade method, and then dried to produce 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 obtained by forming the negative electrode active material layer on the negative electrode current collector, the separator, and the positive electrode obtained by forming the positive electrode active material layer on the positive electrode current collector are combined into the positive electrode and the negative electrode. Lamination is performed with the active material surfaces facing each other to form a laminate. 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 as described above, when the composite particles have a structure in which tin (Sn) is arranged at the center and cobalt (Co) is unevenly distributed on the outer surface of the tin (Sn), the volume during charge / discharge It is possible to relieve stress due to expansion and contraction and secure conductivity. On the other hand, when the composite particle has a plurality of pores communicating with the surface of the composite particle at the cut surface and cobalt (Co) is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore, the composite particle is combined at the cut surface. By having a plurality of pores communicating with the surface of the particles, when the lithium ion secondary battery is repeatedly charged and discharged, the pores in the composite particles can absorb and relax the volume expansion of the composite particles during charging, and When cobalt is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore, stress due to volume expansion / contraction during charging / discharging can be relieved and conductivity can be secured.

  As a result, the original performance of tin that tin reacts efficiently with lithium can be brought out, so the lithium ion secondary battery of the present invention has excellent cycle characteristics and output characteristics, has a long life, and has a high capacity. Become. And since the composite particle which comprises a negative electrode active material is controlled by the very fine particle diameter by containing bismuth in a predetermined | prescribed ratio, it is excellent in cycling characteristics etc. especially. In addition, when a conductive auxiliary agent made of carbon nanofiber (CNF) 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. Therefore, the initial discharge capacity and cycle characteristics per active material can be further improved.

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

<Example 1-1>
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%, and stirred and dissolved. I let you. Polyacrylic acid was used as the dispersant. Separately, a hydrochloric acid solution containing bismuth ions in which bismuth (III) chloride is dissolved in hydrochloric acid is prepared, and the bismuth content is 0.5 atomic% with respect to 100 atomic% of tin ( After adding and mixing in the aqueous solution containing tin ions and cobalt ions so that the atomic ratio of tin: bismuth = 100: 0.5), 35% by mass hydrochloric acid was further added to adjust the pH to 0.8. It was adjusted.

  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, cobalt ions, and bismuth ions and a reducing agent aqueous solution were mixed at a predetermined ratio, and stirred and held for 24 hours to cause a reduction reaction of tin ions, cobalt ions, and bismuth ions. The temperature of this liquid mixture was 30 degreeC, pH was 0.9, and the stirring speed was 1.4 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 so that the ratio of cobalt to the total amount of tin and cobalt is 20 atomic%, tin (Sn) is arranged at the center, and cobalt (Co) is disposed on the outer surface of tin (Sn). Were obtained, and a negative electrode active material containing 0.5 atomic% of bismuth was obtained.

<Examples 1-2 to 1-4 and Comparative Examples 1-1 and 1-2>
Hydrochloric acid containing bismuth ions so that the bismuth content of the composite particles obtained by synthesis when adding a hydrochloric acid solution containing bismuth ions to an aqueous solution containing tin ions and cobalt ions is the ratio shown in Table 1 below. A negative electrode active material was obtained in the same manner as in Example 1-1 except that the addition amount of the solution was adjusted.

<Examples 2-1 to 2-7>
Add tin (II) chloride and cobalt (II) chloride so that the ratio of cobalt to the total of tin and cobalt in the composite particles obtained by synthesis is the ratio shown in Table 2 below, and contain tin ions and cobalt ions. A negative electrode active material was obtained in the same manner as in Example 1-3 except that an aqueous solution was prepared. For comparison, Example 1-3 is shown in Table 2 as Example 2-3.

<Examples 3-1 to 3-8>
Examples except that the mixing ratio of the aqueous solution containing tin ions, cobalt ions and bismuth ions and the reducing agent aqueous solution was adjusted so that the concentrations of chromium and zinc in the negative electrode active material were the ratios shown in Table 3 below. A negative electrode active material was obtained in the same manner as in 1-3.

<Example 4>
A negative electrode active material was obtained in the same manner as in Example 1-3, except that the conditions of the treatment time were changed among the temperature, pH, treatment time or stirring speed of the mixed solution. Thereby, the ratio of cobalt to the total amount of tin and cobalt is 20 atomic%, and this composite particle has a plurality of pores communicating with the surface of the composite particle at the cut surface (FIG. 1). A composite particle having a structure unevenly distributed on the outer surface and the inner surface of the pore was obtained, and further, a negative electrode active material containing 0.5 atomic% of bismuth with respect to a tin content of 100 atomic% was obtained.

<Comparative Example 2-1>
The proportion of cobalt with respect to the total of tin and cobalt is 20 atomic%, and the particles have a substantially uniform composition with no deviation between the central portion and the outer peripheral portion of the particles, and the bismuth has a tin content of 100 atomic percent. The negative electrode active material contained in 0.5 atomic% was set as Comparative Example 2-1.

<Comparative Example 2-2>
A negative electrode active material was obtained in the same manner as in Example 4 except that the hydrochloric acid solution containing bismuth ions was not added.

<Comparison test and evaluation>
The negative electrode active materials of Examples 1-1 to 1-4 and Comparative Examples 1-1 to 2-2 were subjected to ICP quantitative analysis, and included in each content of tin, cobalt, and bismuth in the composite particles, and the negative electrode active material powder. The concentration of chromium and zinc was determined. These results are shown in the following Tables 1 to 4. In the table, “<0.001” and “<2” indicate that the measured values were below the detection limit of ICP. In the table, “composite particle structure”, “two layers” indicates a structure in which tin (Sn) is arranged at the center and cobalt (Co) is unevenly distributed on the outer surface of the tin (Sn). Further, in the table, in the “pore” of “composite particle structure”, “present” indicates that the composite particle constituting the negative electrode active material has a plurality of pores, and “Co position” in “composite particle 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 composite particle and an electron micrograph of a cross section of the composite particle.

  Moreover, the average particle diameter of the composite particle which comprises the negative electrode active material of Examples 1-1 to 4 and Comparative Examples 1-1 to 2-2 was measured. The average particle size is a volume-based average particle size measured using a particle size distribution measuring device (LA-950 manufactured by Horiba, Ltd.).

  Moreover, using the negative electrode active materials of Examples 1-1 to 1-4 and Comparative Examples 1-1 to 2-2, 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, carbon nanofiber (CNF), polyvinylidene fluoride (PVdF) and n-methylpyrrolidinone (NMP) have a mass ratio of 80: 5: 5: 10: 100. Thus, the slurry was prepared by weighing and kneading using a kneader.

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 form a negative electrode. Produced.

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 defined as one cycle, and a charge / discharge test is performed up to 50 cycles. The discharge capacity per active material weight for the first time and the ratio of the discharge capacity at the 50th cycle to the initial discharge capacity Performance was evaluated as a characteristic. The obtained evaluation results are shown in the following Tables 1 to 4.

  As is clear from Table 1, when Examples 1-1 to 1-4 and Comparative Examples 1-1 and 1-2 are compared, in Comparative Example 1-1, the bismuth content is less than 0.05 atomic%. Although the initial discharge capacity showed a high value, the life characteristics deteriorated as compared with Examples 1-1 to 1-4. On the other hand, in Comparative Example 1-2 in which the bismuth content exceeds 1.2 atomic%, the initial discharge capacity and the life characteristics are lower than those in Examples 1-1 to 1-4. On the other hand, in Examples 1-1 to 1-4, in the charge / discharge cycle test, all showed a high initial discharge capacity, and the life characteristics were very excellent. From this, it was confirmed that the negative electrode active materials of Examples 1-1 to 1-4 can improve the output characteristics and cycle characteristics of the lithium ion secondary battery.

  As is clear from Table 2, when Examples 2-1 to 2-5 and Examples 2-6 and 2-7 are compared, in Example 2-6 in which the cobalt content is less than 5 atomic%, Although the discharge capacity showed a relatively high value, the life characteristics were deteriorated as compared with Examples 2-1 to 2-4. On the other hand, in Example 2-7 in which the cobalt content exceeds 40 atomic%, although the life characteristics were highly evaluated, the initial discharge capacity was lower than those in Examples 2-1 to 2-4. On the other hand, in Examples 2-1 to 2-4, in the charge / discharge cycle test, all showed a high initial discharge capacity, and the life characteristics were very excellent. From this, it was confirmed that the negative electrode active materials of Examples 2-1 to 2-4 can improve the output characteristics and cycle characteristics of the lithium ion secondary battery.

  As is apparent from Table 3, when Examples 3-1 to 3-3 and Example 3-7 are compared, Example 3-7, in which the chromium concentration exceeds 1.0 mass%, shows Example 3- It can be seen that the initial discharge capacity and the life characteristics are slightly lowered as compared with 1-3. Moreover, when Examples 3-4 to 3-6 and Example 3-8 are compared, in Example 3-8 in which the concentration of zinc exceeds 50 ppm, compared to Examples 3-4 to 3-6, the first time It can be seen that the discharge capacity and life characteristics are slightly reduced. This shows that the chromium concentration contained in the negative electrode active material is preferably 1.0% by mass or less and the zinc concentration is preferably 50 ppm or less.

  As is apparent from Table 4, in Comparative Example 2-1, in which the particles having a substantially uniform composition with no bias in the composition of cobalt and tin at the center and the outer periphery of the particles were used as the negative electrode active material, the initial discharge capacity was low. Value, and the life characteristics were significantly reduced. Moreover, in Comparative Example 2-2 to which bismuth was not added, the average particle size of the composite particles increased and the life characteristics were deteriorated. On the other hand, in Example 4, in the charge / discharge cycle test, the initial discharge capacity showed a high value, and the life characteristics were very excellent. From this, it was confirmed that the negative electrode active material of Example 4 can improve the output characteristics and cycle characteristics of the lithium ion secondary battery.

Claims (4)

It is composed of composite particles containing tin (Sn) and cobalt (Co), the composite particles are arranged around the tin (Sn), and cobalt (Co) is unevenly distributed on the outer surface of the tin (Sn), Alternatively, the lithium ion secondary battery has a structure in which the composite particle has a plurality of pores communicating with the surface of the composite particle at a cut surface and the cobalt (Co) is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore. Negative electrode active material for
The composite particles further include bismuth as a constituent element, and the content of the bismuth is 0.05 to 1.2 atomic% with respect to 100 atomic% of tin (Sn). Negative electrode active material for secondary battery.   2. The negative electrode active material according to claim 1, wherein a ratio of the cobalt (Co) to a total amount of the tin (Sn) and cobalt (Co) is 5 to 40 atomic%. In a lithium ion secondary battery comprising a negative electrode having a negative electrode active material, a positive electrode having a positive electrode active material, and a non-aqueous electrolyte,
The negative electrode active material is composed of composite particles containing tin (Sn) and cobalt (Co), the composite particles are arranged around the tin (Sn), and cobalt (Co) is unevenly distributed on the outer surface of the tin (Sn). Or 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 the cobalt (Co) is unevenly distributed on the outer surface of the composite particle and the inner surface of the pore. Yes,
The composite particles further include bismuth as a constituent element, and the content of the bismuth is 0.05 to 1.2 atomic% with respect to 100 atomic% of tin (Sn). Next battery.   The lithium ion secondary battery according to claim 3, wherein a ratio of the cobalt (Co) to a total amount of the tin (Sn) and cobalt (Co) is 5 to 40 atomic%.

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