JP2015179608A - Negative electrode active material for lithium ion secondary batteries, manufacturing method thereof, and lithium ion secondary battery arranged by use thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable the relaxation of a stress caused by the repetition of metal tin particles' volume expansion and shrinkage at charging and discharging, and to enhance a discharge capacity and cycle characteristics by allowing carbon nanofibers (CNF) binding broken metal tin particles to one another to ensure conducting paths even if the metal tin particles are broken owing to the repetition of metal tin particles' volume expansion and shrinkage.SOLUTION: A negative electrode active material 10 for lithium ion secondary batteries of the present invention comprises: metal tin particles 11; a carbon nanofiber 12, a part of which is located in the metal tin particles 11 and the rest of which is outside the metal tin particle 11; a carbon nanofiber 12 of which the whole is located in the metal tin particle 11; and a carbon nanofiber 12 of which the whole is located outside the metal tin particle 11. The content of the carbon nanofibers 12 is 1-20 mass% to 100 mass% of the metal tin particles. Further, the metal tin particles 11 have an average particle diameter of 5-150 nm.

Description

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

  For this reason, secondary batteries using metals or metalloids such as Al, Ge, Si, Sn, Zn, and Pb, which are known to be alloyed with lithium, as negative electrode active materials have been studied. 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.

  On the other hand, a plurality of particulate first carbon materials having an average particle diameter of 5 μm to 40 μm and a planar graphite network having an average diameter of 10 nm to 500 nm are stacked, and the graphite network is substantially perpendicular to the longitudinal axis of the fiber. Disclosed are negative electrode materials (negative electrode active materials) each including a second carbon material containing carbon nanofibers (CNF) as a main component and further including particulate aggregates made of carbon fine powder having a graphite structure in addition to CNF. (For example, refer to Patent Document 1). In this negative electrode material, CNF contained in the second carbon material has a length of 1000 nm or more and an aspect ratio of 10 or more. Further, the first carbon material is composed of 98 wt% to 70 wt%, the second carbon material is composed of 2 wt% to 30 wt%, and the second carbon material is a void formed by the first carbon material. The second carbon material has a CNF content of 80% to 99.5% by weight and a particulate aggregate content of 0.5% to 20% by weight.

  In the negative electrode material (negative electrode active material) configured as described above, the first electrode material having a large average particle diameter and the negative electrode material each including the nano-sized second carbon material were used. The void formed by the carbon material is filled with the second carbon material, and the packing density of the carbon material in the electrode is effectively improved. In addition, since CNF having a length of 1000 nm or more and an aspect ratio of 10 or more, which is the main component of the second carbon material, exposes many edge surfaces of the graphite network, the second carbon material containing CNF as a main component, By using negative electrode materials each including a first carbon material, which is a carbon material, lithium ion insertion and desorption reactions associated with charge / discharge proceed more smoothly than when only a carbon material is used as a negative electrode material. The high rate charge / discharge characteristics are improved. In addition, since the second carbon material is a material having a smaller average diameter than the conventionally used carbon material, when the battery electrode is manufactured, it is possible to charge the battery at a high density and to improve the energy density of the battery. It leads to. Furthermore, the negative electrode material of the present invention includes a particulate aggregate in which the second carbon material is further aggregated into CNF in addition to the CNF, so that the contact between the CNFs as the main components becomes good, and the high rate Charge / discharge characteristics are further improved.

JP 2009-59713 A (Claim 1, paragraph [0013])

  However, a lithium ion secondary battery using a negative electrode active material containing the above-described conventional metal tin (Sn), or a lithium ion secondary battery using the negative electrode material (negative electrode active material) disclosed in Patent Document 1 above. Then, since the particles are pulverized by a large volume change of Sn particles and carbon particles in the negative electrode active material due to repeated charge and discharge, the particles are peeled off from the current collector plate, or contact between the particles and the conductive auxiliary agent occurs. There is a problem that it is lost and a sufficient cycle characteristic cannot be obtained. In addition, the lithium ion secondary battery using the negative electrode material (negative electrode active material) disclosed in the above-described conventional Patent Document 1 still has a problem of low discharge capacity.

  The first object of the present invention is to relieve stress due to repeated volume expansion and contraction of metal tin particles during charging and discharging, and even when cracks occur due to repeated volume expansion and contraction of metal tin particles, Conductive path can be ensured by carbon nanofiber (CNF) connecting metal tin particles, thereby improving discharge capacity and cycle characteristics, and negative electrode active material for lithium ion secondary battery, method for producing the same, and use of the negative electrode active material It is to provide a lithium ion secondary battery. The second object of the present invention is to synthesize a negative electrode active material by a wet method, thereby making it possible to eliminate the need for special devices that require a large initial cost. It is in providing the manufacturing method of.

  A first aspect of the present invention is a metal tin particle, a carbon nanofiber partly located inside the metal tin particle and the rest located outside the metal tin particle, and all inside the metal tin particle. The carbon nanofibers are located and the carbon nanofibers are entirely located outside the metal tin particles, and the content of the carbon nanofibers is 1 to 20% by mass with respect to 100% by mass of the metal tin particles. It is a negative electrode active material for lithium ion secondary batteries in which the average particle diameter of tin particles is 5 to 150 nm.

  The second aspect of the present invention includes a step of preparing a carbon nanofiber-dispersed tin aqueous solution by mixing an aqueous solution in which carbon nanofibers are dispersed in ammonia water with a tin ion-containing aqueous solution; A negative electrode active material for a lithium ion secondary battery, wherein a negative electrode active material is prepared by reducing tin ions in the presence of carbon nanofibers, and a step of mixing with a reducing agent aqueous solution containing a reducing agent having a potential lower than the oxidation-reduction potential. It is a manufacturing method of a substance.

  A third aspect of the present invention includes a negative electrode having a negative electrode active material, a positive electrode having a positive electrode active material, and a non-aqueous electrolyte, and the negative electrode active material has metal tin particles and one inside of the metal tin particles. A carbon nanofiber in which the portion is located and the remainder is located outside the metal tin particle, a carbon nanofiber that is entirely located inside the metal tin particle, and a carbon nanofiber that is located entirely outside the metal tin particle The lithium ion secondary battery has a carbon nanofiber content of 1 to 20% by mass with respect to 100% by mass of metal tin particles, and an average particle size of the metal tin particles of 5 to 150 nm.

  In the negative electrode active material for a lithium ion secondary battery according to the first aspect of the present invention, the metal tin particles are nano-sized so that the metal tin particles are not charged during charging / discharging of the lithium ion secondary battery using the negative electrode active material. Even if the volume expansion and contraction are repeated, the stress generated at this time can be relaxed. Further, a plurality of carbon nanofibers (CNF), CNF partially located inside the metal tin particles and the remainder located outside the metal tin particles, CNF entirely located inside the metal tin particles, Since it consists of CNF that is entirely located outside the metal tin particles, even if cracks occur due to repeated volume expansion and contraction of the metal tin particles, a conductive path can be secured by the CNF that connects the broken metal tin particles. . As a result, the original performance of tin (Sn) can be extracted, and the discharge capacity and cycle characteristics of the lithium ion secondary battery can be improved as compared with the negative electrode active material using a conventional carbon material having a graphite structure.

  In the method for producing a negative electrode active material for a lithium ion secondary battery according to the second aspect of the present invention, since the negative electrode active material is synthesized by a wet method, special devices such as a sputtering device that requires a large initial cost are used. It can be made unnecessary.

  Since the lithium ion secondary battery according to the third aspect of the present invention is a lithium ion secondary battery using the negative electrode active material according to the first aspect, the metal tin particles are nanosized in the same manner as described above. Even when the metal tin particles repeat volume expansion and contraction during charging and discharging of the lithium ion secondary battery, the stress generated at this time can be relieved. Further, a plurality of carbon nanofibers (CNF), CNF partially located inside the metal tin particles and the remainder located outside the metal tin particles, CNF entirely located inside the metal tin particles, Since it consists of CNF that is entirely located outside the metal tin particles, even if cracks occur due to repeated volume expansion and contraction of the metal tin particles, a conductive path can be secured by the CNF that connects the broken metal tin particles. . As a result, the original performance of tin (Sn) can be extracted, and the discharge capacity and cycle characteristics of the lithium ion secondary battery can be improved as compared with the negative electrode active material using a conventional carbon material having a graphite structure.

It is a schematic diagram of the negative electrode active material for lithium ion secondary batteries of this invention embodiment.

  Next, an embodiment for carrying out the present invention will be described with reference to the drawings. As shown in FIG. 1, the negative electrode active material 10 of the present invention includes metal tin particles 11, and carbon nanofibers in which a part is located inside the metal tin particles 11 and the remainder is located outside the metal tin particles 11. (Hereinafter referred to as CNF) 12, CNF 12 that is entirely located inside the metal tin particle 11, and CNF 12 that is located entirely outside the metal tin particle 11. Here, the structure in which the CNF 12 is partially located inside the metal tin particle 11 and the rest is located outside the metal tin particle 11 is that the center of the CNF 12 is located inside the metal tin particle 11 and both ends of the CNF 12 are A structure (penetrating structure) located outside the metal tin particles 11 and a structure (piercing structure) where one end of the CNF 12 is located inside the metal tin particles 11 and the other end of the CNF 12 is located outside the metal tin particles 11. Refers to the structure containing. Further, the structure in which the CNF 12 is entirely located inside the metal tin particles 11 refers to a structure in which the CNF 12 is included in the metal tin particles 11 (encapsulation structure). Further, the structure in which the CNF 12 is entirely located outside the metal tin particle 11 is the entire CNF 12 located outside the metal tin particle 11 in a state of being separated from or contacting the metal tin particle 11. Structure (external position structure).

  The content ratio of the CNF 12 is 1 to 20% by mass, preferably 1 to 10% by mass when the metal tin particles 11 are 100% by mass. Moreover, the average particle diameter of the metal tin particle 11 is 5-150 nm, Preferably it is 6-20 nm. Here, the content ratio of CNF12 is limited to the range of 1 to 20% by mass, and if it is less than 1% by mass, when cracks due to repeated volume expansion and contraction of the metal tin particles 11 occur, this cracked metal When the amount of CNF 12 that plays a role of connecting the tin particles 11 is extremely small and the cycle characteristics are deteriorated, and it exceeds 20% by mass, the cycle characteristics are good, but the first discharge capacity is reduced. Because it ends up. Moreover, the average particle diameter of the metal tin particles 11 is limited to the range of 5 to 150 nm. If the thickness is less than 5 nm, slurry coating becomes difficult when the negative electrode is produced. This is because the metal tin particles break due to repeated volume expansion and contraction of the tin particles, and the cycle characteristics are deteriorated. In addition, the average particle diameter of the metal tin particles 11 is a particle diameter measured using a particle size distribution measuring apparatus (LA-950 manufactured by Horiba, Ltd.), and is a volume-based average particle diameter.

  On the other hand, it is preferable that the average diameter and average length of CNF12 are 10-20 nm and 130-190 nm, respectively. Here, the average diameter of CNF12 is limited to the range of 10 to 20 nm because the resistance during electron conduction is increased below 10 nm, and when it exceeds 20 nm, effective contact with the negative electrode active material important for electron conduction is achieved. This is because the number of points is reduced. Further, the average length of CNF 12 is limited to the range of 130 to 190 nm because if it is less than 130 nm, it becomes impossible to secure the connection of the conductive path. Because it will increase. The average diameter and average length of the CNF 12 are values measured using a transmission electron microscope apparatus (JEM-2010F manufactured by JEOL Ltd.), and 100 samples arbitrarily selected from an arbitrary field of view. It is a value obtained by measuring the diameter and length.

A method for manufacturing the negative electrode active material 10 configured as described above will be described. A tin ion-containing aqueous solution is prepared in advance. Specifically, a dispersant and tin (II) chloride (SnCl ₂ ) are added to ion-exchanged water and dissolved by stirring, and hydrochloric acid with a concentration of 35% by mass is added to bring the pH within a range of 0.7 to 1.3. By adjusting, a tin ion-containing aqueous solution is prepared. Here, examples of the dispersant include polyvinyl pyrrolidone, polyacrylic acid, and water-soluble cellulose. Moreover, the reducing agent aqueous solution containing the reducing agent which has a potential lower than the oxidation-reduction potential of tin ion is prepared. Specifically, an aqueous reducing agent solution is prepared by adding the reducing agent to ion-exchanged water and dissolving with stirring. Here, examples of the reducing agent include sodium borohydride (NaBH ₄ ), potassium borohydride, hydrazine and the like.

  First, a CNF-dispersed tin aqueous solution is prepared by mixing an aqueous solution in which CNF is dispersed in ammonia water with the tin ion-containing aqueous solution. Next, the CNF-dispersed tin aqueous solution is mixed with the reducing agent aqueous solution and stirred and mixed for 10 to 60 minutes. And this liquid mixture is solidified by hold | maintaining at the temperature of 40-60 degreeC for 10 to 24 hours with a hot air dryer. Thereby, tin ions are reduced in the presence of CNF. Next, after milling the obtained dried solid product, a step of adding ion-exchanged water to the crushed product and stirring and washing, a step of solid-liquid separation of the stirred and washed product by centrifugation, The contained salt is removed by repeating the step of removing the separated upper solution several times. Further, the precipitate from which the salt has been removed is vacuum-dried. As a result, a particulate negative electrode active material 10 having metal tin particles 11 and CNFs 12 exhibiting a penetrating structure, a piercing structure, an inclusion structure, and an external position structure with respect to the metal tin particles 11 is obtained. As described above, since the negative electrode active material is synthesized by a wet method, special devices such as a sputtering device that requires a large initial cost can be eliminated. The average particle diameter of the particulate negative electrode active material 10 is substantially the same as the average particle diameter of the metal tin particles 11.

A negative electrode manufacturing method using the particulate negative electrode active material 10 manufactured in this way will be described. First, a conductive auxiliary agent, a binder and a solvent are mixed with the particulate negative electrode active material 10 by a kneading apparatus to prepare a slurry. Here, examples of the conductive assistant include carbon black such as acetylene black and ketjen black, VGCF (Vaper-Grown Carbon Fiver), or metal powder that is difficult to alloy with lithium such as copper and titanium, and the like. Examples thereof include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), and the solvent includes n-methylpyrrolidinone. (NMP), water and the like. Examples of the kneading apparatus include Awatori Nertaro (trade name of a mixer manufactured by Sinky), a shaker mill, a homogenizer, and a planetary mixer. Next, the slurry is applied to a copper foil with an applicator or the like so that the active material density is 5 mg / cm ² , dried, further rolled, and then cut into a predetermined size to obtain a negative electrode.

Then, the manufacturing method of a lithium ion secondary battery is demonstrated. This lithium ion secondary battery includes a negative electrode having a negative electrode active material 10, a positive electrode having a positive electrode active material, and a non-aqueous electrolyte. First, the negative electrode active material 10 and the negative electrode are produced by the method described above. Next, a positive electrode active material is mixed with a binder and a conductive additive at a predetermined ratio to prepare a positive electrode slurry, and this positive electrode slurry is applied onto a positive electrode current collector by a technique such as a doctor blade method and dried. Is made. Here, examples of the positive electrode active material include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , and examples of the conductive assistant include carbon black such as acetylene black and ketjen black, VGCF, graphite, and the like. Examples thereof include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and an ethylene-propylene-diene copolymer (EPDM). Examples of the positive electrode current collector include aluminum foil, stainless steel foil, and nickel foil.

Next, the negative electrode, the separator, and the positive electrode are stacked with the active material surfaces of the positive electrode and the negative electrode facing each other to form a stacked body. The separator is formed of 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. Furthermore, a lithium ion secondary battery can be obtained by adding a non-aqueous electrolyte from the opening of the laminate and thermally fusing 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. As the non-aqueous electrolyte, an electrolyte dissolved in a non-aqueous solvent is used. Examples of the non-aqueous solvent include cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), and chain carbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC). . Examples of the electrolyte include lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium borofluoride (LiBF ₄ ), lithium arsenic hexafluoride (LiAsF ₆ ), and trifluorometasulfone. Examples thereof include lithium salts such as lithium acid lithium (LiCF ₃ SO ₃ ) and bistrifluoromethylsulfonylimide lithium [LiN (CF ₃ SO ₂ ) ₂ ].

  Since the lithium ion secondary battery manufactured in this way is a lithium ion secondary battery using the negative electrode active material 10, the metal tin particles 11 are nanosized so that the lithium ion secondary battery can be charged and discharged. Even if the metal tin particle 11 repeats volume expansion and contraction, the stress generated at this time can be relaxed. In addition, since a plurality of CNFs 12 are composed of a CNF 12 having a penetrating structure, a CNF 12 having a piercing structure, a CNF 12 having an inclusion structure, and a CNF 12 having an external position structure, cracking occurs due to repeated volume expansion and contraction of the metal tin particles 11. Even in this case, a conductive path can be secured by the CNF 12 that connects the broken metal tin particles 11 to each other. As a result, the original performance of tin (Sn) can be extracted, and the discharge capacity and cycle characteristics of the lithium ion secondary battery can be improved as compared with the negative electrode active material using a conventional carbon material having a graphite structure.

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

<Example 1>
By previously adding polyvinylpyrrolidone (dispersing agent) and tin (II) chloride (SnCl ₂ ) to ion-exchanged water and dissolving by stirring, and adding hydrochloric acid with a concentration of 35% by mass to adjust the pH to 1.0, A tin ion-containing aqueous solution was prepared. In addition, a reducing agent aqueous solution was prepared by adding sodium borohydride (NaBH ₄ ) as a reducing agent having a potential lower than the oxidation-reduction potential of tin ions to ion-exchanged water and dissolving with stirring.

  First, the CNF dispersion | distribution tin aqueous solution was prepared by mixing the aqueous solution which disperse | distributed CNF in ammonia water with the said tin ion containing aqueous solution. Next, the CNF-dispersed tin aqueous solution was mixed with the reducing agent aqueous solution and mixed with stirring for 30 minutes. And this liquid mixture was dried by hold | maintaining at the temperature of 50 degreeC with a hot air dryer for 12 hours. As a result, tin ions were reduced in the presence of CNF. Next, after milling the obtained dried solid product, a step of adding ion-exchanged water to the crushed product and stirring and washing, a step of solid-liquid separation of the stirred and washed product by centrifugation, The salt contained was removed by repeating the step of removing the separated upper solution several times. Further, the precipitate from which the salt was removed was vacuum-dried. As a result, a particulate negative electrode active material having an average particle diameter of 10 nm having metal tin particles and CNF having a penetrating structure, a piercing structure, an inclusion structure, and an external position structure with respect to the metal tin particles 11 was obtained. This negative electrode active material was designated as Example 1. In addition, the content rate of CNF was 2 mass% with respect to 100 mass% of metal tin particles. The content ratio of CNF was determined by gas quantitative analysis.

<Example 2>
A negative electrode active material was produced in the same manner as in Example 1 except that the content ratio of CNF was 1% by mass with respect to 100% by mass of the metal tin particles.

<Example 3>
A negative electrode active material was produced in the same manner as in Example 1 except that the content ratio of CNF was 5% by mass with respect to 100% by mass of the metal tin particles.

<Example 4>
A negative electrode active material was produced in the same manner as in Example 1 except that the CNF content was 10% by mass with respect to 100% by mass of the metal tin particles.

<Example 5>
A negative electrode active material was produced in the same manner as in Example 1 except that the CNF content ratio was 15% by mass with respect to 100% by mass of the metal tin particles.

<Example 6>
A negative electrode active material was produced in the same manner as in Example 1 except that the CNF content ratio was 20% by mass with respect to 100% by mass of the metal tin particles.

<Comparative Example 1>
A negative electrode active material was produced in the same manner as in Example 1 except that the CNF content was 0.5% by mass with respect to 100% by mass of the metal tin particles.

<Comparative Example 2>
A negative electrode active material was produced in the same manner as in Example 1 except that the CNF content was 30% by mass with respect to 100% by mass of the metal tin particles.

<Comparative Example 3>
After synthesizing the metal tin particles, a negative electrode active material was prepared in the same manner as in Example 1 except that CNF was mixed with the metal tin particles.

<Comparative test 1 and evaluation>
Negative electrodes were prepared using the negative electrode active materials of Examples 1 to 6 and Comparative Examples 1 to 3, and half-cells were assembled using these negative electrodes to perform a charge / discharge test. First discharge capacity, 50th cycle (51 cycles) Eye) life characteristics and 50th (51st cycle) Coulomb efficiency were measured. Specifically, the negative electrode was produced as follows. First, 0.5 g of acetylene black (conducting aid), 0.5 g of polyvinylidene fluoride (binder), and 5 g of n-methylpyrrolidinone (solvent) were added to 4 g of the negative electrode active material. (Product name of manufactured mixer) to prepare a slurry. Next, this slurry was applied to a copper foil with an applicator so that the active material density was 5 mg / cm ² and dried. Furthermore, after rolling the copper foil which dried this coating film, it cut | disconnected in the square shape whose length and width are 3 cm each, and produced the negative electrode. In addition, lithium metal (Li) is used as the counter electrode and the reference electrode of the half-cell, and ethylene carbonate (EC: ethylene carbonate) and carbonic acid in which lithium hexafluorophosphate (LiPF ₆ ) is dissolved at a concentration of 1 M as the electrolytic solution. An equal volume solvent of diethyl (DEC: diethyl carbonate) was used.

On the other hand, the half-cell is charged by applying a constant current of 0.5 mA / cm ² until the voltage reaches 5 mV, and then applying a constant voltage of 5 mV until the current reaches 0.01 mA / cm ^2. Carried out. Further, the half-cell was discharged by flowing a constant current of 0.5 mA / cm ² until the voltage reached 1V. A state in which the above charging and discharging are performed once is defined as one cycle, a charge / discharge test up to 50 cycles is performed, the first cycle is defined as an activation step, and the second cycle is defined as the first cycle test. The discharge capacity per gram of the negative electrode active material for the first time, the life characteristic that is the ratio of the 50th (after the 51st cycle test) discharge capacity to the first discharge capacity, and the ratio of the 50th discharge capacity to the 50th charge capacity Each coulomb efficiency was measured. These results are shown in Table 1 together with the CNF content ratio and the structure of the negative electrode active material.

  In the structure of the negative electrode active material in Table 1, “A” indicates that CNF is a metal in a structure in which a part of CNF is located inside the metal tin particles and the rest of the CNF is located outside the metal tin particles. A structure penetrating tin particles (penetrating structure) is shown. In the structure of the negative electrode active material in Table 1, “B” indicates that CNF is a metal in a structure in which a part of CNF is located inside the metal tin particles and the rest of the CNF is located outside the metal tin particles. The structure (piercing structure) pierced into the tin particle is shown. In the structure of the negative electrode active material in Table 1, “C” indicates a structure in which all of CNF is located inside the metal tin particles, that is, a structure in which CNF is included in the metal tin particles (encapsulation structure). Further, in the structure of the negative electrode active material in Table 1, “D” indicates a structure in which all of CNF is located outside the metal tin particles (external position structure).

  As is apparent from Table 1, in Comparative Example 1 having a low CNF content ratio of 0.5% by mass, the first discharge capacity was as large as 563 mAh / g, but the life characteristics were slightly low at 87% and the Coulomb efficiency was 97%. It was as low as 9%. Further, in Comparative Example 2 where the CNF content ratio was as large as 30.0% by mass, the life characteristics were as high as 91% and the Coulomb efficiency was as high as 99.2%, but the first discharge capacity was as small as 396 mAh / g. In contrast, in Examples 1 to 6 in which the CNF content ratio is within an appropriate range of 1.0 to 20.0 mass%, the first discharge capacity is increased to 453 to 561 mAh / g, and the life characteristics are 88 to 88%. The coulombic efficiency was as high as 98.2 to 99.2%.

  On the other hand, in Comparative Example 3 in which the content ratio of CNF is 2.0 mass% and the negative electrode active material has a structure in which all of CNF is located outside the metal tin particles (D: external position structure), the first discharge capacity Was 553 mAh / g, but the life characteristics were as low as 85% and the coulomb efficiency was as low as 94.6%. On the other hand, the content ratio of CNF is 2.0% by mass, and the negative electrode active material has a structure in which CNF penetrates metal tin particles (A: penetration structure) and a structure in which CNF penetrates metal tin particles (B : Piercing structure), a structure in which CNF is included in metal tin particles (C: inclusion structure), and a structure in which all of CNF is located outside the metal tin particles (D: external position structure). In No. 1, the first discharge capacity increased to 555 mAh / g, the life characteristics increased to 89%, and the coulomb efficiency increased to 98.8%.

<Example 7>
The negative electrode active material was prepared in the same manner as in Example 1 except that the negative electrode active material (metal tin particles) was prepared by adjusting the mixing method of the tin ion-containing aqueous solution and the reducing agent aqueous solution so that the average particle size was 5 nm. The material was made.

<Example 8>
The negative electrode active material was prepared in the same manner as in Example 1 except that the negative electrode active material (metal tin particles) was prepared by adjusting the mixing method of the tin ion-containing aqueous solution and the reducing agent aqueous solution so that the average particle size was 6 nm. The material was made.

<Example 9>
The negative electrode active material was prepared in the same manner as in Example 1 except that the negative electrode active material (metal tin particles) was prepared by adjusting the mixing method of the tin ion-containing aqueous solution and the reducing agent aqueous solution so that the average particle size was 20 nm. The material was made.

<Example 10>
The negative electrode active material was prepared in the same manner as in Example 1 except that the negative electrode active material (metal tin particles) was prepared by adjusting the mixing method of the tin ion-containing aqueous solution and the reducing agent aqueous solution so that the average particle size was 150 nm. The material was made.

<Comparative Example 4>
The negative electrode active material was prepared in the same manner as in Example 1 except that the negative electrode active material (metal tin particles) was prepared by adjusting the mixing method of the tin ion-containing aqueous solution and the reducing agent aqueous solution so that the average particle size was 3 nm. The material was made.

<Comparative Example 5>
The negative electrode active material was prepared in the same manner as in Example 1 except that the negative electrode active material (metal tin particles) was prepared by adjusting the mixing method of the tin ion-containing aqueous solution and the reducing agent aqueous solution so that the average particle size was 160 nm. The material was made.

<Comparative test 2 and evaluation>
Using the negative electrode active materials of Examples 7 to 10, Comparative Example 4 and Comparative Example 5, the first discharge capacity, the life characteristics and the Coulomb efficiency were measured in the same manner as in Comparative Test 1 above. These results are shown in Table 2 together with the average particle diameter of the metal tin particles.

  As apparent from Table 2, in Comparative Example 4 in which the particle size of the metal tin particles is as small as 3 nm, the life characteristics were as high as 88%, but the first discharge capacity was considerably small as 434 mAh / g, and the metal tin particles In Comparative Example 5 in which the particle diameter of the metal tin was as large as 160 nm, the discharge capacity at the first time was relatively small at 550 mAh / g and the life characteristics were relatively low at 79%, whereas the particle diameter of the metal tin particles was 3 In Examples 7 to 10 in an appropriate range of ˜150 nm, the first discharge capacity was increased to 553 to 557 mAh / g, and the life characteristics were increased to 80 to 91%. Here, when the particle size of the metal tin particles is increased, the life characteristics are reduced because the cracking of the negative electrode active material proceeds as the battery cycle progresses, and the electronic conductive path is cut off. Similarly to the life characteristics, the Coulomb efficiency increases as the particle size of the metal tin particles increases, the cracking of the negative electrode active material proceeds as the battery cycle progresses, and SEI (Solid Electrolyte Interphase) ) And the like are formed and thus become smaller. Furthermore, the reason why the first discharge capacity of Comparative Example 4 was as small as 434 mAh / g was that it was extremely difficult to apply the electrode slurry at the time of battery preparation, and thus the battery could not be manufactured properly.

  The negative electrode active material of the present invention can be used as a negative electrode material for lithium ion secondary batteries.

10 Negative electrode active material for lithium ion secondary battery 11 Metal tin particle 12 Carbon nanofiber (CNF)

Claims (3)

A metal tin particle, a carbon nanofiber partially located inside the metal tin particle and a remainder located outside the metal tin particle, a carbon nanofiber entirely located inside the metal tin particle, Carbon nanofibers that are all located outside the metal tin particles,
The negative electrode active material for lithium ion secondary batteries whose content rate of the said carbon nanofiber is 1-20 mass% with respect to 100 mass% of said metal tin particles, and whose average particle diameter of the said metal tin particle is 5-150 nm. Preparing a carbon nanofiber-dispersed tin aqueous solution by mixing an aqueous solution in which carbon nanofibers are dispersed in ammonia water with a tin ion-containing aqueous solution;
Mixing the carbon nanofiber-dispersed tin aqueous solution with a reducing agent aqueous solution containing a reducing agent having a potential lower than the oxidation-reduction potential of the tin ions,
The manufacturing method of the negative electrode active material for lithium ion secondary batteries which produces a negative electrode active material by reducing the said tin ion in coexistence of the said carbon nanofiber. 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 metal tin particles, carbon nanofibers that are partly located inside the metal tin particles and the rest located outside the metal tin particles, and all located inside the metal tin particles. Carbon nanofibers, and carbon nanofibers that are entirely located outside the metal tin particles,
The lithium ion secondary battery whose content rate of the said carbon nanofiber is 1-20 mass% with respect to 100 mass% of the said metal tin particles, and whose average particle diameter of the said metal tin particle is 5-150 nm.

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