JP2012089418A - Negative active material for lithium ion secondary battery, lithium ion secondary battery using the same, and manufacturing method of negative active material for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative active material consisting of composite powder and used in forming a lithium ion secondary battery, which allows the production of a long-life lithium ion secondary battery achieving much smaller volumetric expansion and shrinkage during charge and discharge, a high capacity, and good cycle characteristics, and to provide a lithium ion secondary battery using the negative active material.SOLUTION: The negative active material of the invention comprises composite particles each having a double layer structure including a core of tin at the center thereof and a nickel coating covering up the core. The percentage of the nickel to a total amount of the tin and nickel is 5-40 atom%. The method for manufacturing the composite particles comprises mixing an aqueous solution containing tin and nickel ions and a reducer solution containing divalent chromium ions, and keeping stirring the mixture solution, thereby causing reductive reactions of tin and nickel ions in the mixture solution, whereby the composite particles each having a double layer structure including a core of tin at the center thereof and a nickel coating covering up the core can be obtained.

Description

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

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, when tin is used as the negative electrode active material of a lithium ion secondary battery, the volume greatly changes when alloying or dealloying with lithium during charging and discharging. For this reason, the negative electrode active material is peeled off from the current collector, the contact with the conductive auxiliary agent is lost, the conductivity is greatly impaired, or the inside of the electrode is destroyed and the battery life is extremely shortened. Occurs. In order to eliminate such problems, conventionally, a non-aqueous electrolyte secondary battery formed using a tin-nickel alloy powder as a negative electrode active material has been disclosed (for example, see Patent Document 1). In the non-aqueous electrolyte secondary battery of Patent Document 1, a material obtained by pulverizing an arc-melted tin-nickel alloy is used as a negative electrode active material, and a material including a Sn ₄ Ni ₃ phase and a Sn phase contained in the negative electrode, and a carbon material. Of these, by adjusting the mass ratio of the Sn ₄ Ni ₃ phase and the Sn phase to a predetermined range, the expansion and contraction of the material containing the Sn ₄ Ni ₃ phase and the Sn phase is suppressed, and the battery swells when used as a battery pack Is suppressed. In a negative electrode using such a tin-nickel alloy powder as a negative electrode active material, the ratio of tin and nickel is considered to have a large effect on battery characteristics such as capacity and battery life (for example, Non-Patent Document 1). reference.). According to Non-Patent Document 1, in a tin-nickel alloy thin film used for a negative electrode of a lithium ion secondary battery, a tin / nickel ratio of 62:38 is particularly excellent in capacity and charge / discharge cycle characteristics. The results show that a large number of Ni ₃ Sn ₄ phases were found in the negative electrode formed at the above ratio. This suggests that the Ni ₃ Sn ₄ phase contained in the negative electrode greatly affects the capacity and charge / discharge cycle characteristics of the lithium ion secondary battery.

Japanese Patent Laying-Open No. 2004-296149 (Claim 1, paragraph [0036], paragraph [0037])

H. Mukaibo, et. Al. , J .; Power Sources 146 (2005) 457

  However, in recent years, there has been a demand for further increase in capacity and long life of charge / discharge cycles in lithium ion secondary batteries, and the tin-nickel alloy powder shown in Patent Document 1 or Non-Patent Document 1 above. Even those using a battery cannot be said to have sufficient capacity and battery life.

  An object of the present invention is a composite powder used as a negative electrode active material when forming a lithium ion secondary battery, and the volume expansion / contraction during charge / discharge is extremely small because nickel is coated around tin. Another 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 a method for producing the same.

  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 a composite particle having a two-layer structure in which tin (Sn) is arranged at the center and the periphery thereof is coated with nickel (Ni), and the ratio of nickel to the total amount of tin and nickel Is a negative electrode active material for a lithium ion secondary battery.

  A second aspect of the present invention is an invention based on the first aspect, and further includes at least one of chromium (Cr) and zinc (Zn) as a constituent element.

  A third aspect of the present invention is an invention based on the second aspect, wherein the chromium content is 0.005 to 1% by mass, and the zinc content is 5 to 50 ppm by mass. It is characterized by being.

  A fourth aspect of the present invention is an invention based on the first to third aspects, further comprising at least one selected from the group consisting of polyacrylic acid, water-soluble cellulose and polyvinylpyrrolidone. And

  According to a fifth 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 is mainly tin (Sn). It is characterized by being composed of composite particles having a two-layer structure in which the periphery is coated with nickel (Ni), and the ratio of nickel to the total amount of tin and nickel is 5 to 40 atomic%.

  A sixth aspect of the present invention is the invention based on the fifth aspect, further comprising at least one of chromium (Cr) and zinc (Zn) as a constituent element of the negative electrode active material, To do.

  7th viewpoint of this invention is invention based on 6th viewpoint, Comprising: Content of chromium is 0.005-1% by mass ratio, and content of zinc is 5-50 ppm by mass ratio. It is characterized by being.

  The eighth aspect of the present invention is an invention based on the fifth to seventh aspects, and further comprises at least one selected from the group consisting of polyacrylic acid, water-soluble cellulose and polyvinylpyrrolidone as the negative electrode active material. It is characterized by including.

  According to a ninth aspect of the present invention, an aqueous solution containing tin ions and nickel ions and a reducing agent aqueous solution containing divalent chromium ions are mixed and held by stirring, thereby causing tin ions and nickel ions to undergo a reduction reaction in the mixed solution, It is a method for producing a negative electrode active material for a lithium ion secondary battery, wherein composite particles having a two-layer structure in which tin is arranged at the center and the periphery thereof is coated with nickel are obtained.

  A tenth aspect of the present invention is the invention based on the ninth aspect, and further includes at least one dispersant selected from the group consisting of polyacrylic acid, water-soluble cellulose and polyvinylpyrrolidone in the liquid. It is characterized by that.

  In the negative electrode active material according to the first aspect of the present invention, the composite particles having a two-layer structure in which the periphery of tin coordinated at the center is coated with nickel is used to relieve stress due to volume expansion and contraction during charge and discharge. Since the effect of ensuring conductivity is obtained, and as a result, the original performance of tin can be extracted, the capacity and cycle characteristics are superior to those using powders obtained by pulverizing conventional graphite or tin-nickel alloy. A lithium ion secondary battery can be manufactured.

It is a cross-sectional schematic when the negative electrode active material powder cross section obtained in Example 3 is observed with an electron microscope.

  Next, the form for implementing this invention is demonstrated.

  The negative electrode active material for a lithium ion secondary battery of the present invention is composed of composite particles having a two-layer structure in which tin (Sn) is arranged at the center and the periphery thereof is coated with nickel (Ni). In other words, as is conventionally known, there is no uniform alloyed form in which the composition of the tin-nickel is not biased between the center and the outer periphery of the particle. And the ratio of nickel with respect to the total amount of tin and nickel is 5-40 atomic%, It is characterized by the above-mentioned.

  In the negative electrode active material configured in this way, the stress due to volume expansion and contraction during charge and discharge is alleviated by nickel covering the periphery of tin coordinated at the center, and conductivity is ensured. As a result, the original performance of tin can be extracted, so that it is possible to manufacture a lithium ion secondary battery having higher capacity and superior cycle characteristics than those using powders obtained by pulverizing conventional graphite or tin-nickel alloy. . Moreover, the ratio of nickel in the composite particles is regulated within the above range because when the nickel ratio is less than 5 atomic%, the effect of nickel coating is reduced, and the cycle characteristics of a battery using this negative electrode active material are deteriorated. Even if the nickel ratio exceeds 40 atomic%, the cycle characteristics of the battery using this negative electrode active material are good, but the amount of nickel to be coated increases, and the center is relatively central. This is because the amount of tin to be coordinated is reduced, so that the initial discharge capacity is reduced.

  The negative electrode active material preferably 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, and the zinc content is 5 to 50 ppm by mass. If the chromium content is 0.005% or more or the zinc content is not 5 ppm or more, the effect of increasing the initial discharge capacity is not exhibited. If the chromium content is 1% or the zinc content exceeds 50 ppm, tin As a result, the strength of the nickel coating is reduced, the protective effect is reduced, and the cycle characteristics are deteriorated.

  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-mentioned type of dispersant, the dispersant covers the particles, thereby enhancing the expansion and contraction suppressing effect by the nickel coating and improving the cycle characteristics.

  The composite particles constituting the negative electrode active material of the present invention are preferably controlled in terms of particle size in the range of 0.1 to 20 μm in average particle size for ease of handling. The powder whose particle size is controlled in this way can be slurried and applied to the negative electrode current collector, and a conventional lithium secondary battery manufacturing process can be applied. If the average particle size is less than 0.1 μm, slurrying becomes difficult, and there is a problem that it cannot be applied to the existing lithium ion secondary battery manufacturing process. If it exceeds 20 μm, the expansion suppression effect by nickel coating becomes insufficient, and the cycle characteristics deteriorate. There is a bug to do.

  In addition, the average particle diameter used the particle size distribution measuring apparatus (LA-950 by Horiba, Ltd.), and made the volume reference average particle diameter the average particle diameter.

  The contents of tin, nickel, chromium and zinc in the composite particles constituting the negative electrode active material of the present invention can be determined by quantitative analysis using ICP.

  Next, the manufacturing method of the said negative electrode active material for lithium ion secondary batteries is demonstrated.

  The method for producing a negative electrode active material for a lithium ion secondary battery according to the present invention comprises mixing an aqueous solution containing tin ions and nickel ions and a reducing agent aqueous solution containing divalent chromium ions, and stirring and holding them in the mixed solution. And a composite ion having a two-layer structure in which tin is arranged at the center and the periphery thereof is covered with nickel. 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, a composite particle having a two-layer structure in which nickel particles are reduced and grown to a certain particle size as a nucleus, nickel is grown around the nucleus, and the tin is covered with nickel. It becomes.

  The aqueous solution containing tin ions and nickel 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 nickel 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.

  It is preferable that the pH of the mixed solution obtained by mixing the aqueous solution containing tin ions and nickel ions and the reducing agent aqueous solution containing divalent chromium ions is controlled to 1 to 4. This is because when the pH of the mixed solution is high, nickel or tin once reduced by the reduction reaction is oxidized and returned to an ionic state again.

  As described above, the production method of the present invention is a wet method, and both the preparation of the aqueous solution and the reduction reaction can be performed at a temperature of about room temperature, so that no special apparatus that requires a large initial cost is required.

  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, a mixing ratio of an aqueous solution containing tin ions and nickel ions and an aqueous reducing agent solution is set. Control chromium and zinc contents by increasing or decreasing, increasing or decreasing the amount of metallic zinc used in preparing an aqueous reducing agent solution, or adding zinc chloride to an aqueous solution containing tin ions and nickel ions or an aqueous reducing agent solution. be able to.

  A lithium ion secondary battery is produced using the negative electrode active material of the present invention thus obtained.

  A negative electrode slurry is prepared by mixing the above-obtained negative electrode active material, conductive additive, binder and solvent in a predetermined ratio. Next, the prepared negative electrode slurry is applied onto a negative electrode current collector by a technique such as a doctor blade method and dried to prepare a negative electrode.

  In addition, the conductive support agent, the binder, the solvent, and the 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, ketjen black, VGCF, or metal powder that is difficult to be alloyed with lithium such as copper or titanium. 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.

  On the other hand, a positive electrode active material is mixed with a binder and a conductive additive at a predetermined ratio to prepare a positive electrode slurry. 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 LiFe ₂ O ₄ . Examples of the conductive assistant include carbon black such as acetylene black, ketjen black, VGCF, and graphite. 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 ].

  The lithium ion secondary battery manufactured in this way uses a negative electrode active material composed of composite particles having a two-layer structure in which tin is arranged at the center and the periphery thereof is coated with nickel. The nickel covering the periphery of the tin relieves stress due to volume expansion and contraction during charge and discharge, and ensures conductivity. As a result, the original performance of tin can be brought out, so that it has higher capacity and better cycle characteristics than a lithium ion secondary battery using a powder obtained by pulverizing conventional graphite or tin-nickel alloy as a negative electrode active material.

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

<Example 1>
First, a dispersant, tin chloride (II) and nickel chloride (II) are added to ion-exchanged water in such a ratio that the nickel ratio with respect to the total of tin and nickel in the composite particles obtained by synthesis is 5 atomic%, The mixture was dissolved with stirring, and hydrochloric acid was further added to adjust the pH to 1.0. 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), 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 nickel ions and a reducing agent aqueous solution were mixed at a predetermined ratio, and stirred and held for 30 minutes to cause the tin ions and nickel ions to undergo a reduction reaction.

  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 was vacuum-dried to obtain a negative electrode active material composed of composite particles having a two-layer structure in which tin is arranged in the center and nickel is coated around the tin.

<Example 2>
An aqueous solution containing tin ions and nickel ions was prepared by adding tin (II) chloride and nickel (II) chloride in such a ratio that the nickel ratio of the composite particles obtained by synthesis was 10 atomic% with respect to the total of tin and nickel. A negative electrode active material was obtained in the same manner as Example 1 except for the above.

<Example 3>
An aqueous solution containing tin ions and nickel ions was prepared by adding tin (II) chloride and nickel (II) chloride in such a ratio that the nickel ratio of the composite particles obtained by synthesis was 20 atomic% with respect to the total of tin and nickel. A negative electrode active material was obtained in the same manner as Example 1 except for the above.

<Example 4>
An aqueous solution containing tin ions and nickel ions was prepared by adding tin (II) chloride and nickel (II) chloride in such a ratio that the nickel ratio of the composite particles obtained by synthesis was 30 atomic% with respect to the total of tin and nickel. A negative electrode active material was obtained in the same manner as Example 1 except for the above.

<Example 5>
An aqueous solution containing tin ions and nickel ions was prepared by adding tin (II) chloride and nickel (II) chloride in such a ratio that the nickel ratio of the composite particles obtained by synthesis was 40 atomic% with respect to the total of tin and nickel. A negative electrode active material was obtained in the same manner as Example 1 except for the above.

<Comparative Example 1>
An aqueous solution containing tin ions and nickel ions was prepared by adding tin (II) chloride and nickel (II) chloride in such a ratio that the nickel ratio of the composite particles obtained by synthesis was 3 atomic% with respect to the total of tin and nickel. A negative electrode active material was obtained in the same manner as Example 1 except for the above.

<Comparative example 2>
An aqueous solution containing tin ions and nickel ions was prepared by adding tin (II) chloride and nickel (II) chloride at a ratio such that the nickel ratio in the composite particles obtained by synthesis was 45 atomic% with respect to the total of tin and nickel. A negative electrode active material was obtained in the same manner as Example 1 except for the above.

<Comparative Example 3>
The negative electrode active material was a tin-nickel powder having a nickel ratio of 20 atomic% with respect to the total of tin and nickel and having no compositional deviation in the center portion and the outer peripheral portion and having a substantially uniform composition.

<Example 6>
The same procedure as in Example 3 was performed except that the mixing ratio of the aqueous solution containing tin ions and nickel 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 7>
The same procedure as in Example 3 was performed except that the mixing ratio of the aqueous solution containing tin ions and nickel 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 8>
A negative electrode was prepared in the same manner as in Example 3 except that the mixing ratio of the aqueous solution containing tin ions and nickel ions and the reducing agent aqueous solution was adjusted so that the composite particles obtained by synthesis further contained 1% chromium by mass. An active material was obtained.

<Example 9>
A negative electrode active material was obtained in the same manner as in Example 3 except that the amount of metal zinc added 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. It was.

<Example 10>
A negative electrode active material was obtained in the same manner as in Example 3 except that the amount of metal zinc added in preparing the reducing agent aqueous solution was adjusted so that the composite particles obtained by synthesis further contained 25 ppm of zinc by mass. It was.

<Example 11>
A negative electrode active material was obtained in the same manner as in Example 3 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 50 ppm of zinc by mass. It was.

<Example 12>
The composite zinc particles obtained by synthesis contain 0.1% chromium and 25 ppm zinc in a mass ratio, respectively. A negative electrode active material was obtained in the same manner as in Example 3 except that the mixing ratio of the aqueous solution containing the aqueous solution and the reducing agent aqueous solution was adjusted.

<Example 13>
The same as in Example 3 except that the mixing ratio of the aqueous solution containing tin ions and nickel ions and the reducing agent aqueous solution was adjusted so that the composite particles obtained by synthesis further contained 1.5% chromium by mass. Thus, a negative electrode active material was obtained.

<Example 14>
A negative electrode active material was obtained in the same manner as in Example 3 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 75 ppm of zinc by mass. It was.

<Example 15>
A negative electrode active material was obtained in the same manner as in Example 3 except that water-soluble cellulose was used instead of polyacrylic acid as a dispersant.

<Example 16>
A negative electrode active material was obtained in the same manner as in Example 3 except that polyvinylpyrrolidone was used in place of polyacrylic acid as a dispersant.

<Example 17>
A negative electrode active material was obtained in the same manner as in Example 3 except that no dispersant was added.

<Comparison test and evaluation 1>
In order to observe the distribution of tin and nickel in the composite particles as the negative electrode active material obtained in Example 3, the cross section of the particles was observed with an electron microscope. As shown in the schematic diagram of FIG. 11 was arranged in the center, and it was confirmed that it has a two-layer structure in which the periphery is covered with nickel 12.

  For other examples and comparative examples, the cross section of the particles was similarly observed with an electron microscope, and as a result of observing the distribution of tin and nickel in the negative electrode active material, all examples except for Comparative Example 3 were mainly tin. It was confirmed that it had a two-layer structure in which the periphery thereof was covered with nickel, and only Comparative Example 3 was substantially uniform with no uneven distribution of tin and nickel.

<Comparison test and evaluation 2>
ICP quantitative analysis was performed about the negative electrode active material of Examples 1-17 and Comparative Examples 1-3, and each content of tin in a composite particle, nickel, chromium, and zinc was calculated | required. The obtained results are shown in the following Tables 1 to 3, respectively. In Tables 1 to 3, “<2” indicates that the measured value was below the detection limit of ICP.

<Comparative test and evaluation 3>
Using the negative electrode active materials of Examples 1 to 17 and Comparative Examples 1 to 3, 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, polyvinylidene fluoride (PVdF) and n-methylpyrrolidinone (NMP) were weighed so as to have a mass ratio of 8: 1: 1: 10, and a kneader was used. And kneading 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 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 .

Discharging was conducted 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 the following Tables 1 to 3, respectively.

表１から明らかなように、２層構造とした実施例１〜５及び比較例１，２では、いずれも高い寿命特性を示したのに対し、粒子中のニッケルとスズとを略均一とした比較例３では、寿命特性が２５％と非常に低い結果であった。この結果から、スズを中心に配位し、その周囲にニッケルを被覆した２層構造をとる複合粒子は非常に高いサイクル特性が得られることが確認された。

As is clear from Table 1, in Examples 1 to 5 and Comparative Examples 1 and 2 having a two-layer structure, all showed high life characteristics, whereas nickel and tin in the particles were made substantially uniform. In Comparative Example 3, the lifetime characteristics were as low as 25%. From this result, it was confirmed that the composite particles having a two-layer structure in which tin is coordinated at the center and nickel is coated around the tin can obtain very high cycle characteristics.

  In addition, when Examples 1 to 5 and Comparative Examples 1 and 2 having a two-layer structure and varying the nickel ratio are compared, the initial ratio is high when the nickel ratio of Examples 1 to 5 is in the range of 5 atomic% to 40 atomic%. While the discharge capacity was obtained and the life characteristics were high, when the nickel ratio was low as in 3 atomic% of Comparative Example 1, the life characteristics deteriorated, such as 45 atomic% of Comparative Example 2. However, when the nickel owl ratio increased, the initial discharge capacity tended to decrease. From these results, it was confirmed that an appropriate range exists for the nickel ratio in the particles.

表２から明らかなように、クロムを０．００５〜１％又は亜鉛を５〜５０ｐｐｍ更に含有させた実施例６〜１１では、クロムの含有量が０．００１％未満で、亜鉛の含有量が２ｐｐｍ未満と低い実施例３の結果に比べ、高い初回放電容量が得られた。同様に、クロムを０．１％及び亜鉛を２５ｐｐｍそれぞれ更に含有させた実施例１２でも、実施例３の結果に比べ、高い初回放電容量が得られた。一方、クロムの含有量が１．５％又は亜鉛の含有量が７５ｐｐｍと高い実施例１３，１４では、実施例３の結果と同程度の高い初回放電容量が得られているが、寿命特性が低い結果となった。これらの結果から、クロムや亜鉛を所定量含有することで初回放電容量値を高めることができる一方、クロムや亜鉛を含有し過ぎると、その特性が低下してしまうことから、クロムと亜鉛の含有量には適切な範囲が存在することが確認された。

As apparent from Table 2, in Examples 6 to 11 in which chromium was further contained in 0.005 to 1% or zinc in an amount of 5 to 50 ppm, the chromium content was less than 0.001%, and the zinc content was Compared with the result of Example 3 which is less than 2 ppm, a high initial discharge capacity was obtained. Similarly, in Example 12 in which 0.1% chromium and 25 ppm of zinc were further contained, a higher initial discharge capacity was obtained as compared with the result of Example 3. On the other hand, in Examples 13 and 14, where the chromium content is as high as 1.5% or the zinc content is as high as 75 ppm, the initial discharge capacity as high as the result of Example 3 is obtained, but the life characteristics are The result was 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.

表３から明らかなように、分散剤種類を変化させた実施例１５，１６は、初回放電容量、寿命特性ともに、実施例３の結果と同程度の優れた数値を示した。この結果から、分散剤として使用した水溶性セルロース、ポリビニルピロリドンは、実施例３で使用したポリアクリル酸と同程度の効果が得られることが確認された。

As is apparent from Table 3, Examples 15 and 16 in which the type of the dispersant was changed showed excellent numerical values comparable to the results of Example 3 in both initial discharge capacity and life characteristics. From this result, it was confirmed that the water-soluble cellulose and polyvinyl pyrrolidone used as the dispersant can obtain the same effect as the polyacrylic acid used in Example 3.

  In Example 17, in which no dispersant was used, the initial discharge capacity was similar to that in Example 3, but the life characteristics were inferior. From this result, it was confirmed that high life characteristics can be maintained by including a dispersant.

10 Composite particles (negative electrode active material)
11 Tin 12 Nickel

Claims (10)

  It is composed of composite particles having a two-layer structure in which tin (Sn) is arranged at the center and the periphery thereof is coated with nickel (Ni), and the ratio of nickel to the total amount of tin and nickel is 5 to 40 atomic% A negative electrode active material for a lithium ion secondary battery.   The negative electrode active material according to claim 1, further comprising at least one of chromium (Cr) and zinc (Zn) as a constituent element.   The negative electrode active material according to claim 2, wherein the chromium content is 0.005 to 1% by mass, and the zinc content is 5 to 50 ppm by mass.   The negative electrode active material according to any one of claims 1 to 3, further comprising at least one selected from the group consisting of polyacrylic acid, water-soluble cellulose, and polyvinylpyrrolidone. 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 having a two-layer structure in which tin (Sn) is arranged at the center and the periphery thereof is coated with nickel (Ni), and the ratio of nickel to the total amount of tin and nickel is 5 to 5 A lithium ion secondary battery characterized by being 40 atomic%.   The lithium ion secondary battery according to claim 5, further comprising at least one of chromium (Cr) and zinc (Zn) as a constituent element of the negative electrode active material.   The lithium ion secondary battery according to claim 6, wherein the chromium content is 0.005 to 1% by mass, and the zinc content is 5 to 50 ppm by mass.   The lithium ion secondary battery according to any one of claims 5 to 7, further comprising at least one selected from the group consisting of polyacrylic acid, water-soluble cellulose, and polyvinylpyrrolidone in the negative electrode active material.   An aqueous solution containing tin ions and nickel ions and a reducing agent aqueous solution containing divalent chromium ions are mixed and held by stirring to cause the tin ions and nickel ions to undergo a reduction reaction in the mixed solution. A method for producing a negative electrode active material for a lithium ion secondary battery, comprising obtaining composite particles having a two-layer structure, the periphery of which is coated with nickel.   The production method according to claim 9, further comprising at least one dispersant selected from the group consisting of polyacrylic acid, water-soluble cellulose, and polyvinylpyrrolidone in the liquid.

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