JP3841779B2 - Negative electrode active material for lithium secondary battery, method for producing the same, and lithium secondary battery - Google Patents

Description

  The present invention relates to a negative electrode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery.

Research on increasing the capacity of the negative electrode active material of lithium secondary batteries has been conducted before the battery system using the current negative electrode active material as carbon has been put into practical use, and still focuses on metal materials such as Si, Sn, and Al. However, it has not been put into practical use yet. This is mainly because metals such as Si, Sn, and Al are alloyed with lithium during charge and discharge, resulting in volume expansion and contraction, which leads to pulverization of the metal, and the cycle characteristics are not solved. is there. Therefore, in order to solve this problem, an amorphous alloy as shown in Patent Document 1 below, or a Ni—Si based alloy shown in Non-Patent Document 1 or Non-Patent Document 2 shown below is used. A crystalline alloy made of a metal that can be alloyed with lithium and a metal that cannot be alloyed with lithium has been studied.
JP 2002-216746 A “Proceedings of the 42nd Battery Discussion Meeting”, Battery Technical Committee of the Electrochemical Society of Japan, November 21, 2001, p. 296-297 “Preliminary Collection of the 43rd Battery Discussion Meeting”, Battery Technical Committee of the Electrochemical Society of Japan, October 12, 2004, p. 326-327

  By the way, when Si powder is used as the negative electrode active material, the possibility of characteristic deterioration due to the decomposition reaction of the electrolytic solution on the surface of the Si powder is pointed out as a cause of deterioration of the cycle characteristics, in addition to the expansion and contraction of the powder itself. ing.

  The present invention has been made in view of the above circumstances, and it is possible to suppress the expansion and contraction of the powder itself, and to prevent the occurrence of the decomposition reaction of the electrolytic solution on the powder surface, the production method thereof, and the An object is to provide a lithium secondary battery using a negative electrode active material.

In order to achieve the above object, the present invention employs the following configuration.
The negative electrode active material for a lithium secondary battery of the present invention comprises a multiphase alloy powder that necessarily contains an Si phase and an SiM phase, and contains either one or both of an X phase and an SiX phase. the amount of Si phase in the surface rather less than the amount of Si phase in the grain interior, and the Si content a on the particle surface of the multi-phase alloy powder obtained from surface analysis of the electron microscope, the content of the particle cross sections Si b The ratio is 0.5 ≦ a / b ≦ 0.95, and the specific surface area is in the range of 0.2 m ² / g to 5 m ² / g . Where M is at least one element selected from Ni, Co, As, B, Cr, Cu, Fe, Mg, Mn, and Y, and element X is at least one element selected from Ag, Cu, and Au. In the above elements, Cu is not selected as the element M and the element X at the same time.

According to the above negative electrode active material, the amount of Si phase on the particle surface is less than the amount of Si phase inside the particle, or there is almost no Si phase, so the electrolyte solution by the Si phase that is the main active material phase The decomposition reaction is suppressed, and the cycle characteristics can be improved.
Further, since the element M is an element that is alloyed with Si and not alloyed with Li, the inclusion of the SiM phase in addition to the Si phase in the particle results in the expansion and contraction of the particle itself as compared with the case of the Si layer alone. The amount can be reduced, and the cycle characteristics can be improved by preventing the particles themselves from being pulverized.
In addition, since one or both of the X phase and the SiX phase, which have a lower resistance than the Si phase, are included, the specific resistance of the negative electrode active material can be reduced.
Note that Cu is an element having both properties of element M and element X because it is alloyed with Si and has a lower resistance than Si. Therefore, in the present invention, Cu is added to both the element M and the element X, but Cu is not selected for the element M and the element X at the same time.
Furthermore, with respect to the Si content a and the content b, it is difficult to quantitatively distinguish the alloy phase SiM phase from the single Si phase by surface analysis. Suppose that Si of this is also included.

  Moreover, the negative electrode active material for a lithium secondary battery according to the present invention is the negative electrode active material for a lithium secondary battery described above, wherein micropores are formed on the particle surface of the multiphase alloy powder. To do.

According to the negative electrode active material described above, since fine pores are formed on the particle surface, the specific surface area of the particle is increased, and lithium ions can be absorbed and released quickly, and high charge / discharge characteristics are achieved. Can be improved.
The average pore diameter of the fine pores is preferably in the range of 10 nm to 5 μm.

  Moreover, the negative electrode active material for a lithium secondary battery of the present invention is the negative electrode active material for a lithium secondary battery described above, and the multiphase alloy powder includes Ni, Co, As, B, Cr, Cu, and Fe. A molten alloy containing at least one element M of Mg, Mn, Y, at least one element X of Ag, Cu, Au, and Si is quenched to form a quenched alloy powder. The quenched alloy powder is formed by impregnating with an alkaline solution to remove a part or all of the Si phase on the particle surface. However, Cu is not selected as the element M and the element X at the same time.

  According to such a negative electrode active material, the amount of the Si phase on the particle surface can be made smaller than the amount of the Si phase inside the particle, the decomposition reaction of the electrolyte solution by the Si phase is suppressed, and the cycle characteristics can be improved. it can. In addition, since the Si phase having the highest resistance among the phases forming the alloy particles is reduced, the surface resistance of the particles can be reduced, and the high rate characteristics can be improved.

  Further, the negative electrode active material for a lithium secondary battery of the present invention is the negative electrode active material for a lithium secondary battery described above, and the molten alloy is rapidly cooled by any of a gas atomizing method, a water atomizing method, and a roll quenching method. It is characterized by that.

  According to such a negative electrode active material, it is possible to easily obtain a multiphase alloy powder that always contains a Si phase and a SiM phase, and contains either one or both of an X phase and a SiX phase. In particular, according to the gas atomization method or the water atomization method, since the spherical powder is obtained, the packing density of the negative electrode active material can be increased, and the energy density of the negative electrode active material can be increased.

Next, a lithium secondary battery of the present invention is characterized by including the negative electrode active material for a lithium secondary battery described above.
According to this lithium secondary battery, since the negative electrode active material is provided, the decomposition reaction of the electrolytic solution is suppressed, and the amount of expansion and contraction of the particles themselves can be reduced, thereby improving the cycle characteristics. be able to.

Next, the method for producing a negative electrode active material for a lithium secondary battery according to the present invention includes at least one element M of Ni, Co, As, B, Cr, Cu, Fe, Mg, Mn, and Y, A molten alloy containing at least one element X of Ag, Cu, and Au and Si is quenched to form a quenched alloy powder, and the quenched alloy powder is impregnated with an alkaline solution to form a Si phase on the particle surface. Impregnation treatment with alkaline solution until the specific surface area of the powder after removal of the Si phase becomes 1.2 to 50 times the specific surface area of the quenched alloy powder before the removal of the Si phase when removing part or all of it It is characterized by performing . However, Cu is not selected as the element M and the element X at the same time.

According to the above method for producing a negative electrode active material, by rapidly cooling the molten alloy containing the element M, the element X and the Si, the Si phase and the SiM phase are necessarily included, and either the X phase or the SiX phase or A quenched alloy powder containing both can be easily formed. The obtained quenched alloy powder is impregnated with an alkaline solution to remove a part or all of the Si phase on the particle surface, so that the amount of Si phase on the particle surface is less than the amount of Si phase inside the particle. can do. The negative electrode active material thus obtained suppresses the decomposition reaction of the electrolytic solution, can reduce the surface resistance of the particles, can reduce the amount of expansion and contraction of the particles themselves, and improves cycle characteristics. be able to.
In addition, since one or both of the X phase and the SiX phase, which have a lower resistance than the Si phase, are included, the specific resistance of the negative electrode active material can be reduced.
Note that Cu is an element having both properties of element M and element X because it is alloyed with Si and has a lower resistance than Si. Therefore, in the present invention, Cu is added to both the element M and the element X, but Cu is not selected for the element M and the element X at the same time.
Further, by performing the impregnation treatment until the specific surface area becomes 1.2 times or more of the original, Si on the particle surface is eluted, the reaction between Si and the electrolytic solution is suppressed, and the cycle characteristics can be improved. . In addition, when the specific surface area exceeds 50 times the initial value, the amount of Si as an active material is reduced, the capacity is greatly reduced, the particles are structurally brittle, and collapse due to expansion / contraction due to charge / discharge, resulting in cycle deterioration. This is not preferable.

  Further, the method for producing a negative electrode active material for a lithium secondary battery according to the present invention is the production method described above, wherein the molten alloy is rapidly cooled by any of a gas atomizing method, a water atomizing method, and a roll quenching method. And

  According to such a production method, it is possible to easily obtain a multiphase alloy powder that always contains the Si phase and the SiM phase, and that contains either one or both of the X phase and the SiX phase. In particular, according to the gas atomization method or the water atomization method, since the spherical powder is obtained, the packing density of the negative electrode active material can be increased, and the energy density of the negative electrode active material can be increased. In particular, in the case of gas atomization, by using helium gas as the cooling gas, each structure in the alloy particles can be made finer, so that the cycle characteristics can be further improved.

  As described above, according to the negative electrode active material for a lithium secondary battery of the present invention, the decomposition reaction of the electrolyte solution by the Si phase is suppressed, the surface resistance of the particles can be reduced, and the expansion of the particles themselves The amount of shrinkage can be reduced, the pulverization of the particles themselves can be prevented, and the cycle characteristics can be improved.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
The negative electrode active material for the lithium secondary battery of this embodiment is composed of a multiphase alloy powder that necessarily contains an Si phase and an SiM phase, and contains either one or both of an X phase and an SiX phase. The amount of the Si phase on the particle surface is configured to be smaller than the amount of the Si phase inside the particle. FIG. 1 shows an example of a schematic external view of one particle constituting the multiphase alloy powder, and FIG. 2 shows an example of a schematic cross-sectional view of the single particle shown in FIG.

  As shown in FIGS. 1 and 2, the structure of the multiphase alloy powder particles 1 constituting the negative electrode active material contains Si phase 2, SiM phase 3, and X phase or SiX phase 4.

The Si phase 2 is present more in the interior of the particle than in the particle surface. This Si phase 2 is alloyed with lithium during charging to form a LiSi _x phase, and during discharging, lithium is released to return to the Si single phase. In addition, since the Si phase on the particle surface does not exist or is small, the Si phase hardly touches the electrolytic solution, and the decomposition reaction of the electrolytic solution by the Si phase is suppressed.

Further, the SiM phase 3 does not react with lithium during charge and discharge, and maintains the shape of the one particle 1 to suppress the expansion and contraction of the particle 1 itself. The element M constituting the SiM phase 3 is a metal element that does not alloy with lithium, and is at least one selected from Ni, Co, As, B, Cr, Cu, Fe, Mg, Mn, and Y. It is an element. In particular, Ni is preferably used as the element M, and the composition of the SiM phase in this case is the Si ₂ Ni phase.

Further, the X phase 4 imparts conductivity to the multiphase alloy powder to reduce the specific resistance of the negative electrode active material itself. The element X constituting the X phase 4 is a metal element having a specific resistance of 3 Ω · m or less, and is at least one element selected from Ag, Cu, and Au. In particular, Cu is preferable because it does not alloy with lithium and has an effect of suppressing expansion. Further, since Ag hardly alloys with Si, it is preferable to select a metal that does not alloy with Ag as element M because Ag exists as a single phase and the conductivity of the particles can be improved.
Note that Cu is an element having both properties of element M and element X because it is alloyed with Si and has a lower resistance than Si. Therefore, in the present invention, Cu is added to both the element M and the element X, but Cu is not selected for the element M and the element X at the same time.

  Further, instead of the X phase 4 or together with the X phase 4, a SiX phase may be precipitated. Similar to the X phase 4, the SiX phase imparts conductivity to the multiphase alloy powder and reduces the specific resistance of the negative electrode active material itself.

  The crystal forms of the Si phase 2, the SiM phase 3, the X phase 4, and the SiX phase are determined by the rapid cooling rate, the alloy composition, and the presence or absence of heat treatment after the rapid cooling. In the negative electrode active material of the present embodiment, all of the phases may be a crystalline phase, an amorphous phase, or a mixture of a crystalline phase and an amorphous phase. Also good. In addition to the Si phase, SiM phase, X phase, and SiX phase, other alloy phases may be included.

  Next, referring to the alloy composition, since Si is an element that forms a Si single phase, a SiM phase, and further a SiX phase, judging from the state diagram of the alloy, even if the SiM phase and the SiX phase are formed, Si is still Si. By selecting the composition ratio so that a single phase is generated, the capacity of Si can be obtained. However, an excessive increase in the amount of Si is not preferable because a large amount of Si phase precipitates and the amount of expansion and contraction of the entire negative electrode active material during charge / discharge increases, and the negative electrode active material is pulverized to deteriorate cycle characteristics. Specifically, the composition ratio of Si in the negative electrode active material is preferably in the range of 30% by mass to 70% by mass.

  Since the element M is an element that forms a SiM phase together with Si, it is preferable to add the element M so that the entire amount thereof is alloyed with Si as judged from the phase diagram of the alloy. If the amount of M exceeds the amount that can be alloyed with Si, all of Si is alloyed, which causes a significant decrease in capacity, which is not preferable. On the other hand, when the amount of M is small, the SiM phase is decreased, the effect of suppressing the expansion of the Si phase is decreased, and the cycle inferior characteristics are deteriorated, which is not preferable. A plurality of M phases may exist such as different elements, such as M1, M2, M3, and so on. The composition ratio of M cannot be specifically limited because the solid solution limit with Si differs depending on the element, but the composition is considered so that the Si phase still exists even if Si and M are alloyed to the solid solution limit. The ratio is preferable. Further, since the element M is not alloyed with lithium, it does not have an irreversible capacity. Further, the element M is preferably insoluble in the alkaline solution.

  Further, when the composition ratio of X increases, although the specific resistance is reduced, the Si phase is relatively reduced and the charge / discharge capacity is reduced. On the other hand, when the composition ratio of X is small, the specific resistance of the negative electrode active material increases and the charge / discharge efficiency decreases. For this reason, it is preferable that the composition ratio of X in a negative electrode active material is the range of 1 mass% or more and 30 mass% or less. Further, the element X is preferably insoluble in the alkaline solution.

  The average particle size of the multiphase alloy powder is preferably in the range of 5 μm to 30 μm. In general, an alloy powder containing Si has a higher resistance than graphite powder used as an existing negative electrode material of a lithium ion battery. Therefore, it is preferable to use a conductive additive, but when the average particle size is 5 μm or less, the conductive additive is used. In some cases, the average particle size of the multiphase alloy powder becomes smaller than the particle size of the material, and it is difficult to obtain the effect of the conductive additive, and battery characteristics such as capacity and cycle characteristics are deteriorated. When the average particle size exceeds 30 μm, the packing density of the negative electrode active material in the lithium secondary battery is lowered, which is not preferable.

  As shown in FIGS. 1 and 2, a large number of fine holes 5 are formed on the particle surface of the multiphase alloy powder. The fine holes 5 are formed by quenching the molten alloy and then impregnating it with an alkaline solution, and are traces after elution of the Si phase exposed on the particle surface immediately after quenching. Thus, since Si is not exposed to the particle surface, the reaction with the electrolytic solution during charging is suppressed, and the formation of the micropores 5 increases the specific surface area of the multiphase alloy powder. The contact area becomes larger and the charge / discharge efficiency is improved.

The average pore diameter of the micropores 5 is preferably in the range of 10 nm to 5 μm. Further, the depth of the fine holes 5 is preferably in the range of 10 nm to 1 μm. Furthermore, the specific surface area of the multiphase alloy powder is preferably in the range of 0.2 m ² / g to 5 m ² / g.

  According to the above negative electrode active material, since the amount of Si phase on the particle surface is smaller than the amount of Si phase inside the particle, the decomposition reaction of the electrolyte solution by the Si phase is suppressed, and the cycle characteristics are improved. Can do. In addition, since the SiM phase and the X phase are included in addition to the Si phase in the particle, the amount of expansion and contraction of the particle itself can be reduced compared to the case of the Si phase alone, and the particle itself can be prevented from being pulverized. , Conductivity can be improved and cycle characteristics can be improved.

  Furthermore, since the micropores are formed on the particle surface, the specific surface area of the particle is increased, and lithium ions can be quickly occluded / released, so that high rate charge / discharge characteristics can be improved.

Next, a lithium secondary battery using the above negative electrode active material will be described. The lithium secondary battery includes at least a negative electrode including the negative electrode active material, a positive electrode, and an electrolyte.
Examples of the negative electrode of the lithium secondary battery include a material in which a multiphase alloy powder constituting the negative electrode active material is solidified and formed into a sheet by a binder. The negative electrode is not limited to a solidified sheet, but may be a pellet solidified into a columnar shape, a disk shape, a plate shape, or a column shape.

  The binder may be either organic or inorganic, but any binder can be used as long as it is dispersed or dissolved in a solvent together with the multiphase alloy powder and then the multiphase alloy powder is bound to each other by removing the solvent. But you can. Moreover, it may mix with multiphase alloy powder, and may bind multiphase alloy powders by performing solidification molding such as pressure molding. As such a binder, for example, vinyl resin, cellulose resin, phenol resin, thermoplastic resin, thermosetting resin and the like can be used, such as polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, styrene butadiene rubber, etc. Resins can be exemplified.

  In addition, in the negative electrode according to the present invention, carbon black, graphite powder, carbon fiber, metal powder, metal fiber, or the like may be added as a conductive additive in addition to the negative electrode active material and the binder.

Next, as the positive electrode, for example, LiMn ₂ O ₄ , LiCoO ₂ , LiNiO ₂ , LiFeO ₂ , V ₂ O ₅ , TiS, MoS, etc., and positive electrode capable of inserting and extracting lithium such as organic disulfide compounds and organic polysulfide compounds Examples include those containing an active material, and composite oxides such as Ni, Mn, and Co. In addition to the positive electrode active material, a binder such as polyvinylidene fluoride or a conductive aid such as carbon black may be added to the positive electrode.

  Specific examples of the positive electrode and the negative electrode include those obtained by applying the positive electrode or the negative electrode to a current collector made of a metal foil or a metal net and forming the sheet.

Further, examples of the electrolyte include an organic electrolytic solution in which a lithium salt is dissolved in an aprotic solvent.
As aprotic solvents, propylene carbonate, ethylene carbonate, butylene carbonate, benzonitrile, acetonitrile, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, dioxolane, 4-methyldioxolane, N, N-dimethylformamide, dimethylacetamide, dimethyl Sulfoxide, dioxane, 1,2-dimethoxyethane, sulfolane, dichloroethane, chlorobenzene, nitrobenzene, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, ethyl butyl carbonate, dipropyl carbonate, diisopropyl carbonate, dibutyl carbonate , Diethylene glycol, dimethyl An aprotic solvent such as ether, or a mixed solvent obtained by mixing two or more of these solvents can be exemplified, and in particular, any one of propylene carbonate (PC), ethylene carbonate (EC), and butylene carbonate (BC) In addition, it is preferable to always contain any one of dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC).

As the lithium _{salt, LiPF 6, LiBF 4, LiSbF} 6, LiAsF 6, LiClO 4, LiCF 3 SO 3, Li (CF 3 SO 2) 2 N, LiC 4 F 9 SO 3, LiSbF 6, LiAlO 4, _{LiAlCl 4, LiN (C x F} 2x + 1 SO 2) (C y F 2y _tens 1 _SO 2) (provided that x, y is a natural number), LiCl, by mixing one or more lithium salts of such LiI In particular, one containing any one of LiPF ₆ , LiBF _4, LiN (CF ₃ SO ₂ ) _{2, and} LiN (C ₂ F ₅ SO ₂ ) ₂ is preferable.

As another example of the electrolyte, a so-called polymer electrolyte such as a polymer obtained by mixing any of the above lithium salts with a polymer such as PEO or PVA, or a polymer having a high swellability impregnated with an organic electrolytic solution is used. It may be used.
Furthermore, the lithium secondary battery of the present invention is not limited to the positive electrode, the negative electrode, and the electrolyte, and may include other members as necessary. For example, the lithium secondary battery may include a separator that separates the positive electrode and the negative electrode. .

According to such a lithium secondary battery, since the negative electrode active material is provided and there is little expansion / contraction due to charging / discharging, there is no possibility that the negative electrode active material is pulverized or dropped from the current collector, and also conductive. Contact with the material is also maintained, the charge / discharge capacity can be improved, and the cycle characteristics can be improved.
Moreover, since the precipitation amount of the Si phase on the particle surface of the multiphase alloy powder is reduced, the decomposition reaction of the non-aqueous electrolyte is suppressed, the charge / discharge capacity can be improved and the cycle characteristics can be improved.
In addition, since a large number of micropores are formed on the particle surface of the multiphase alloy powder, when used as a negative electrode active material for a lithium secondary battery, the micropores are impregnated with the non-aqueous electrolyte and are electrically conductive. Since there is a high X phase, lithium ions can be diffused efficiently and high rate charge / discharge is possible.

  Next, the manufacturing method of the negative electrode active material for lithium secondary batteries of this embodiment is demonstrated. The method for producing a negative electrode active material for a lithium secondary battery according to the present embodiment includes a step of obtaining a quenched alloy powder containing Si, element M, and element X, and a step of impregnating the obtained quenched alloy powder with an alkaline solution. It is roughly composed of Hereinafter, each process is demonstrated in order.

  First, in the process of manufacturing a rapidly cooled alloy powder, a molten alloy containing Si, element M, and element X is rapidly cooled to obtain a rapidly cooled alloy powder. The molten alloy includes at least one element M of Ni, Co, As, B, Cr, Cu, Fe, Mg, Mn, and Y and at least one element X of Ag, Cu, and Au. And Si, and can be obtained by simultaneously dissolving these simple substances or alloys by, for example, a high frequency induction heating method.

  The Si content in the molten alloy is preferably in the range of 30% by mass to 70% by mass. If the Si content in the molten alloy is out of the above range, it is not preferable because there is too little Si and no Si phase is precipitated, or a negative electrode active material that is easily expanded and contracted due to too much Si.

  As a method for rapidly cooling the molten alloy, for example, a gas atomizing method, a water atomizing method, a roll quenching method, or the like can be used. In the gas atomization method and the water atomization method, a powdery quenching alloy is obtained, and in the roll quenching method, a ribbon-like quenching alloy is obtained. The ribbon-like quenched alloy is further pulverized into a powder. The average particle size of the quenched alloy powder thus obtained is the average particle size of the multiphase alloy powder to be finally obtained. Therefore, when obtaining a rapidly cooled alloy powder, it is necessary to adjust the average particle size in the range of 5 μm to 30 μm.

  The quenched alloy powder obtained from the molten alloy is a quenched alloy whose entire structure is an amorphous phase, or a quenched alloy whose part is an amorphous phase and the remainder is composed of crystalline phase grains, or the entire structure is crystalline. It becomes a quenched alloy that is a temperate phase. The quenched alloy powder always includes a SiX phase and a SiM phase, and includes one or both of an X phase and a SiX phase. Further, these Si phase, SiM phase, X phase, and SiX phase are uniformly mixed in the alloy structure.

  In addition, it is preferable that the rapid cooling rate at the time of rapid cooling is 100 K / second or more. If the quenching rate is less than 100 K / sec, the Si phase, SiM phase, X phase, and SiX phase may not precipitate uniformly in the alloy structure, and the crystal size of each phase becomes too large. It is not preferable because it is difficult to obtain a sufficient expansion suppressing effect and conductivity imparting effect.

  Next, in the step of impregnating the quenched alloy with an alkaline solution, the Si phase precipitated on the particle surface of the quenched alloy powder is eluted and removed. Specifically, the quenched alloy powder is impregnated with an alkaline solution, and then washed and dried. The impregnation condition is preferably a condition that is slowly stirred at room temperature for about 30 minutes to 5 hours. As the alkaline solution, for example, an aqueous solution of sodium hydroxide or potassium hydroxide is preferably used, and the concentration is preferably in the range of 1 to 5N.

  The impregnation conditions described here are only a guideline. In practice, it is possible to determine the impregnation conditions by confirming that only the Si phase precipitated on the particle surface is eluted and removed. Excessive impregnation treatment is not preferable because not only the surface but also the Si phase inside the particles are eluted and removed, and the charge / discharge capacity of the negative electrode active material is reduced. If the Si phase inside the particle is eluted, the strength of the particle itself is lowered, which is not preferable. Furthermore, if the impregnation conditions are insufficient, the Si phase remains on the particle surface, causing a decomposition reaction of the electrolytic solution, which is not preferable.

  Specifically, it is preferable to perform the impregnation treatment with the alkaline solution until the specific surface area of the powder after the Si phase removal becomes 1.2 times or more the specific surface area of the quenched alloy powder before the Si phase removal. By performing the impregnation treatment until the specific surface area becomes 1.2 times or more of the initial surface, part or all of Si on the surface can be removed, and the reaction with the electrolytic solution can be suppressed.

  Moreover, it is preferable to perform the impregnation treatment with an alkaline solution so that the specific surface area of the powder after the Si phase removal is at least 50 times the specific surface area of the quenched alloy powder before the Si phase removal. Thereby, dissolution of Si more than necessary can be prevented, and a decrease in battery capacity can be prevented.

By performing the above impregnation treatment, the Si phase precipitated on the surface of the quenched alloy powder particles is eluted and removed, and the SiM phase and the X phase or SiX phase remain on the particle surface. Micropores are formed in the portion where the Si phase has been removed. Furthermore, by removing the Si phase on the particle surface, the amount of Si phase on the particle surface becomes smaller than the amount of Si phase inside the particle.
The element M and the element X are insoluble in the alkaline solution, and the SiM phase and the SiX phase are also hardly soluble in the alkaline solution, so that the Si phase is eluted with priority.

  According to the above method for producing a negative electrode active material, the alloy melt containing the element M, the element X, and Si is quenched, so that the SiX phase and the SiM phase are always included, and either the X phase or the SiX phase is included. Alternatively, a quenched alloy powder having both can be easily formed. Then, by impregnating the obtained quenched alloy powder with an alkaline solution to remove the Si phase on the particle surface, the amount of Si phase on the particle surface can be made smaller than the amount of Si phase inside the particle. The negative electrode active material thus obtained can suppress the decomposition reaction of the electrolytic solution, reduce the amount of expansion and contraction of the particles themselves, and improve cycle characteristics.

  In addition, it is possible to easily obtain a multiphase alloy powder that always includes a SiX phase and a SiM phase, and includes one or both of the X phase and the SiX phase. In particular, according to the gas atomization method or the water atomization method, since the spherical powder is obtained, the packing density of the negative electrode active material can be increased, and the energy density of the negative electrode active material can be increased.

Prepare 55 parts by weight of massive Si with a size of about 5 mm square, 35 parts by weight of Ni powder, and 10 parts by weight of Ag powder, mix them, and dissolve them in a high-frequency heating method in an argon atmosphere. The alloy was melted. The molten alloy was quenched by a gas atomization method using helium gas at a pressure of 80 kg / cm ² to obtain a quenched alloy powder having an average particle size of 10 μm. The rapid cooling rate at this time was 1 × 10 ⁵ K / sec.
Next, the obtained quenched alloy powder was placed in a 5N aqueous sodium hydroxide solution and impregnated over 4 hours with slow stirring at room temperature. Then, after thoroughly washing with pure water so that no sodium remains, drying was performed, and the particle size was adjusted to an average particle size of 10 μm. Thus, the negative electrode active material of Experimental Example 1 was produced.

When X-ray diffraction was performed on the obtained powder, a structure in which a Si crystalline phase, a crystalline phase having a composition of NiSi2, and an Ag crystalline phase were mixed was confirmed.
Further, the morphology of the quenched alloy particles before the impregnation treatment (Comparative Example) and the quenched alloy particles after the impregnation treatment (Example) were observed with an electron microscope. The results are shown in FIGS. As shown in FIG. 3, it can be seen that the quenched alloy powder particles (comparative example) before the impregnation treatment are spherical and the surface is relatively smooth. The photographs shown in FIGS. 3 and 4 are secondary electron images. In FIG. 3, the Si phase is reflected in a relatively dark gray. As shown in FIG. 3, the Si phase has an elongated shape in appearance. Further, in FIG. 3, the NiSi ₂ phase is reflected in a relatively light gray. As shown in FIG. 3, the NiSi ₂ phase is located around the Si phase.

On the other hand, the quenched alloy powder particles (Example) after the impregnation treatment are found to have a large number of fine pores formed on the surface as shown in FIG. It can also be seen that the color of the particle surface is relatively uniform. It seems that the surface micropores were formed by elution of the Si phase exposed on the surface. Moreover, the reason why the color of the particle surface is relatively uniform is that, as a result of the elution of the Si phase, the surface is occupied by the NiSi ₂ phase, and the composition of the surface is uniform.

Thus, by rapidly cooling the molten alloy containing Ni, Ag and Si, a rapidly cooled alloy powder having a Si phase, a NiSi ₂ phase (SiM phase) and an Ag phase (X phase) can be easily formed. . Then, by impregnating the obtained quenched alloy powder with an alkaline solution and removing the Si phase on the particle surface, the Si phase on the particle surface can be reduced.

A negative electrode active material of Experimental Example 2 (Example) was manufactured in the same manner as Experimental Example 1 except that Cu powder was used instead of Ag powder.
Further, the negative electrode active material of Experimental Example 3 (Example) was manufactured in the same manner as Experimental Example 1 except that the treatment time for the impregnation treatment was 1 hour.
Furthermore, the negative electrode active material of Experimental Example 4 (Example) was manufactured in the same manner as in Experimental Example 1 except that the treatment temperature of the impregnation treatment was 40 ° C. and the treatment time was 1 hour.
Further, a negative electrode active material of Experimental Example 5 (Comparative Example) was manufactured in the same manner as Experimental Example 1 except that the treatment temperature of the impregnation treatment was 60 ° C. and the treatment time was 1 hour.
Further, a negative electrode active material of Experimental Example 6 (Comparative Example) was manufactured in the same manner as Experimental Example 1 except that the impregnation treatment was not performed.

A lithium secondary battery was manufactured using the negative electrode active material of Experimental Examples 2 to 6 and Experimental Example 1 manufactured in Example 1. After mixing 70 parts by weight of each of the negative electrode active materials of Experimental Examples 1 to 6, 20 parts by weight of graphite powder having an average particle diameter of 3 μm as a conductive material, and 10 parts by weight of polyvinylidene fluoride, N-methylpyrrolidone was added. A slurry was prepared by stirring. Next, this slurry was applied onto a copper foil having a thickness of 14 μm, dried, and rolled to prepare a negative electrode having a thickness of 40 μm. The prepared negative electrode was punched into a circle having a diameter of 13 mm, and a metallic polypropylene was stacked on the negative electrode with a porous polypropylene separator interposed therebetween. Further, LiPF ₆ was added to a mixed solvent having a volume ratio of EC: DEC = 3: 7. A coin-type lithium secondary battery was manufactured by injecting an electrolytic solution added at a concentration of 1.3 mol / L.

  The obtained lithium secondary battery was repeatedly charged and discharged with a current density of 0.2 C for 50 cycles in the battery voltage range of 0 V to 1.5 V. The initial discharge capacity at this time, the initial charge / discharge efficiency, and the capacity retention after 50 cycles were determined. The results are shown in Table 1. Table 1 also shows the specific surface area of the polar active materials of negative experimental examples 1 to 6, the Si content a on the particle surface of the multiphase alloy powder obtained by surface analysis with an electron microscope, and the Si content in the particle cross section. The ratio of b (a / b) is also shown. The amount of Si was measured based on the intensity of the characteristic X-ray of Si.

As shown in Table 1, the capacity retention rate of the lithium secondary batteries of Experimental Examples 1 to 4 (Examples) in which the negative electrode active material was impregnated was Experimental Example 6 in which the impregnation treatment was not performed (Comparative Example) It can be seen that it is higher than the capacity retention rate of the lithium secondary battery. This is considered to be because, in Experimental Examples 1 to 4 (Examples) , the Si phase on the surface of the negative electrode active material particles was removed by the impregnation treatment, so that the decomposition reaction of the electrolyte solution by the Si phase was suppressed. Moreover, it is considered that the fact that the Si phase having a high specific resistance is removed by the impregnation treatment and the surface resistance of the particle surface is lowered contributes to the improvement of the capacity retention rate. However, if the impregnation treatment is performed excessively as in Example 5 (comparative example) , Si inside the particles is also eluted, so that the strength of the particles is reduced and the expansion becomes impossible to withstand expansion, so that the cycle characteristics are deteriorated.

Further, comparing Experimental Examples 1 and 2 (both Examples) , it can be seen that the capacity retention rate of Experimental Example 2 is high. This is because Ag used as element X in Experimental Example 1 was alloyed with Li, so that the capacity retention rate of Experimental Example 1 was reduced, while Cu used as Element X in Experimental Example 2 was not alloyed with Li. It is thought that the capacity maintenance rate has improved.

Further, in Experimental Example 3 (Example), the impregnation time was shorter than in Experimental Example 1 (Example) , and therefore, the Si phase remained slightly on the particle surface. Therefore, it can be seen that the capacity retention ratio is lower than that of Experimental Example 1 ( Example), but the capacity retention ratio is superior to that of Experimental Example 6 (Comparative Example) .
Furthermore, in Experimental Example 4 (Example), the impregnation temperature was higher than in Experimental Example 1 (Example) , and thus the Si phase inside the particles was eluted. Therefore, the capacity retention rate is equivalent to that of Experimental Example 1 (Example) , but the initial capacity is slightly lower than that of Experimental Example 1.
Furthermore, in Experimental Example 5 (Comparative Example), the impregnation temperature was even higher than in Experimental Example 4 (Example) , and thus the Si phase inside the particles was eluted. Therefore, the capacity retention rate and the initial capacity were significantly lower than those of Experimental Example 1 (Example) .

  As described above, it can be seen that by appropriately performing the impregnation treatment, the Si phase on the particle surface can be eluted and removed, thereby improving the cycle life and charge / discharge capacity of the lithium secondary battery at the same time.

The schematic diagram which shows the negative electrode active material for lithium secondary batteries of embodiment of this invention. The cross-sectional schematic diagram which shows the negative electrode active material for lithium secondary batteries of embodiment of this invention. Electron micrograph (secondary electron image) of the negative electrode active material of Experimental Example 1 before the impregnation treatment Electron micrograph (secondary electron image) of the negative electrode active material of Experimental Example 1 after the impregnation treatment

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Particle | grains of multiphase alloy powder, 2 ... Si phase, 3 ... SiM phase, 4 ... X phase, 5 ... Micropore

Claims (8)

It consists of a multiphase alloy powder that necessarily contains an Si phase and an SiM phase, and contains either one or both of an X phase and an SiX phase, and the amount of Si phase on the particle surface of the multiphase alloy powder is the amount of Si phase inside the particle. less than the amount rather than,
The ratio of the Si content a on the particle surface of the multiphase alloy powder obtained by surface analysis with an electron microscope and the Si content b in the particle cross section is 0.5 ≦ a / b ≦ 0.95, A negative electrode active material for a lithium secondary battery, having a specific surface area of 0.2 m ² / g or more and 5 m ² / g or less .
Where M is at least one element selected from Ni, Co, As, B, Cr, Cu, Fe, Mg, Mn, and Y, and element X is at least one element selected from Ag, Cu, and Au. In the above elements, Cu is not selected as the element M and the element X at the same time.   2. The negative electrode active material for a lithium secondary battery according to claim 1, wherein micropores are formed on the particle surfaces of the multiphase alloy powder.   3. The negative electrode active material for a lithium secondary battery according to claim 2, wherein an average pore diameter of the micropores is in a range of 10 nm to 5 μm. The multiphase alloy powder includes at least one element M selected from Ni, Co, As, B, Cr, Cu, Fe, Mg, Mn, and Y, and at least one selected from Ag, Cu, and Au. The alloy melt containing the element X and Si was quenched to form a quenched alloy powder, and the quenched alloy powder was impregnated with an alkaline solution to remove part or all of the Si phase on the particle surface. The negative electrode active material for a lithium secondary battery according to any one of claims 1 to 3, wherein the negative electrode active material is a lithium secondary battery.
However, Cu is not selected as the element M and the element X at the same time.   The negative electrode active material for a lithium secondary battery according to claim 4, wherein the molten alloy is rapidly cooled by any one of a gas atomizing method, a water atomizing method, and a roll quenching method. A lithium secondary battery comprising the negative electrode active material for a lithium secondary battery according to any one of claims 1 to 5 . At least one element M of Ni, Co, As, B, Cr, Cu, Fe, Mg, Mn, Y, at least one element X of Ag, Cu, Au, and Si When the molten alloy containing is quenched to form a quenched alloy powder, and the quenched alloy powder is impregnated with an alkaline solution to remove part or all of the Si phase on the particle surface, the specific surface area of the powder after removal of the Si phase However, the impregnation process by an alkaline solution is performed until it becomes 1.2 times or more and 50 times or less the specific surface area of the quenched alloy powder before Si phase removal , The manufacturing method of the negative electrode active material of a lithium secondary battery characterized by the above-mentioned.
However, Cu is not selected as the element M and the element X at the same time. The method for producing a negative electrode active material for a lithium secondary battery according to claim 7 , wherein the molten alloy is rapidly cooled by any one of a gas atomizing method, a water atomizing method, and a roll rapid cooling method.

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