JP3773514B2 - Negative electrode active material for lithium secondary battery, lithium secondary battery, and method for producing negative electrode active material for lithium secondary battery - Google Patents

Description

  The present invention relates to a negative electrode active material for a lithium secondary battery, a lithium secondary battery, and a method for producing a negative electrode active material for a lithium secondary battery, and in particular, for a lithium secondary battery capable of improving cycle characteristics. The present invention relates to a negative electrode active material.

Research on increasing the capacity of the negative electrode active material for lithium secondary batteries has been conducted before the battery system using the negative electrode active material as carbon has been put into practical use, and is still actively focused on metal materials such as Si, Sn, and Al. However, it has not yet been put to practical use. 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.
In addition, the state of the surface of a metal material such as Si, Sn, or Al is also important, and a decomposition reaction of the electrolytic solution may occur on the surface of the metal material during charge / discharge, and cycle characteristics may deteriorate.

Therefore, in order to solve these problems, an amorphous alloy as shown in Patent Document 1 below, or a Ni—Si alloy shown in Non-Patent Document 1 or Non-Patent Document 2 shown below is used. In addition, crystalline alloys made of a metal that can be alloyed with lithium and a metal that is not alloyed with lithium have 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

  However, even with the materials described in Patent Document 1 and Non-Patent Documents 1 and 2, pulverization due to expansion and contraction of the alloy volume during charging and discharging, peeling from the current collector, poor contact with the conductive material, etc. The conventional problem could not be solved completely.

  Conventionally, a method of reducing expansion and contraction during charging / discharging by making a metal material porous has been studied, but it has been insufficient to improve cycle deterioration.

  The present invention has been made in view of the above circumstances, and solves conventional problems such as pulverization due to expansion and contraction of the alloy volume during charging and discharging, peeling from the current collector, and poor contact with the conductive material. An object of the present invention is to provide a negative electrode active material for a lithium secondary battery, a lithium secondary battery, and a method for producing a negative electrode active material for a lithium secondary battery capable of suppressing decomposition of the electrolyte solution on the negative electrode surface To do.

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 is a powder containing at least one metal capable of being alloyed with lithium and hydrogen, and having pores formed therein. .

According to the above configuration, since the negative electrode active material contains hydrogen, reactions such as oxidation on the surface of the negative electrode material that occur during material production or before battery assembly are suppressed, and as a result, decomposition of the electrolyte solution is suppressed. The reaction can be suppressed to prevent the decomposition products from being deposited, and the cycle characteristics can be improved.
Furthermore, when the metal and lithium are alloyed, the volume expands. At this time, the pores inside the metal are crushed, so that the apparent volume expansion is offset. Thereby, pulverization of a negative electrode active material can be prevented and cycling characteristics can be improved. In addition, the pore can be impregnated with the electrolytic solution, and lithium ions can be efficiently diffused into the negative electrode active material, whereby the charge / discharge reaction can be smoothly advanced.

  In the negative electrode active material for a lithium secondary battery of the present invention, the pores preferably have an average pore diameter in the range of 1 nm to 5 μm. If the average pore diameter is less than 1 nm, the volume expansion during charging cannot be relaxed, which is not preferable. If the average pore diameter exceeds 5 μm, the amount of energy per volume decreases, which is not preferable.

  The negative electrode active material for a lithium secondary battery of the present invention is the negative electrode active material described above, wherein hydrogen is contained in the pore.

 According to the above configuration, the reaction in the pores, such as oxidation on the wall surfaces of the pores contained in the negative electrode active material, which occurs during the material production or before the assembly of the battery is suppressed, and as a result, the decomposition of the electrolyte solution The reaction can be suppressed to prevent the deposition of decomposition products. Thereby, cycle characteristics can be improved.

  Moreover, the negative electrode active material for a lithium secondary battery of the present invention is the negative electrode active material described above, wherein the metal that can be alloyed with lithium is Si, Al, or Sn. . According to this configuration, the charge / discharge capacity of the negative electrode active material can be increased.

  Moreover, the negative electrode active material for a lithium secondary battery of the present invention is the negative electrode active material described above, wherein the metal that can be alloyed with lithium is Si, and further, the Si phase and the SiM phase are contained in the metal. And having one or both of an X phase and a SiX phase. 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.

Since the element M is an element alloyed with Si and not alloyed with Li, the inclusion of the SiM phase in addition to the Si phase in the particle reduces the expansion / contraction amount of the particle itself as compared with the case of the Si layer alone. It is possible to reduce the number of particles, and it is possible to improve the cycle characteristics 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.

Moreover, in the negative electrode active material for lithium secondary batteries of the present invention, the peak intensity I (620) of 600 to 630 cm ⁻¹ derived from the bond between Si and H in the Raman spectrum, and 500 to 500 derived from crystalline Si. The intensity ratio I (620) / I (520) to the Raman shift intensity I (520) of 530 cm ⁻¹ is preferably 0.004 or more.

  The negative electrode active material for a lithium secondary battery of the present invention is obtained by dissolving hydrogen gas or a mixed gas of hydrogen gas and an inert gas in a molten metal composed of at least one metal that can be alloyed with lithium. Preferably, the molten metal is produced by solidifying the molten metal.

  Moreover, it is preferable that the negative electrode active material for lithium secondary batteries of the present invention has a pore aspect ratio of 1.2 or more by unidirectionally solidifying the molten metal in a mold.

  In addition, a lithium secondary battery of the present invention is characterized by including the negative electrode active material for a lithium secondary battery described above.

  The method for producing a negative electrode active material for a lithium secondary battery according to the present invention includes a lithium secondary battery comprising at least one metal capable of being alloyed with lithium and hydrogen, and having pores formed therein. A method for producing a negative electrode active material for use by dissolving hydrogen gas or a mixed gas of hydrogen gas and an inert gas in a molten metal composed of at least one kind of metal, and then solidifying the molten metal in one direction. In the solidified metal, pores containing hydrogen are formed.

The solubility of hydrogen in a metal is generally high when the metal is molten (liquid) and low when it is solidified (solid). Utilizing this difference in gas solubility, hydrogen is dissolved in a molten metal and then the metal is unidirectionally solidified, whereby hydrogen that is not solid-dissolved in a solid remains in the metal structure as bubbles.
A negative electrode active material in which a large number of pores filled with hydrogen are formed in a metal by dissolving hydrogen in a molten metal that can be alloyed with lithium such as Si, Al, Sn, etc. and then solidifying in one direction. Obtainable.

According to the negative electrode active material for a lithium secondary battery of the present invention, it is possible to solve problems such as pulverization due to expansion and contraction of the alloy volume during charge and discharge, separation from the current collector, poor contact with the conductive material, and negative electrode By suppressing the decomposition of the electrolyte solution on the surface, the charge / discharge capacity and cycle characteristics of the lithium secondary battery can be improved.
Further, according to the lithium secondary battery of the present invention, the charge / discharge capacity 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 a lithium secondary battery of the present embodiment is a powder made of a metal that can be alloyed with at least one kind of lithium, the metal powder contains hydrogen, and further has pores inside the metal. It is formed.
Examples of the metal that can be alloyed with lithium include Si, Al, and Sn. In addition to being capable of being alloyed with lithium, these metals have characteristics that the amount of hydrogen dissolved differs greatly between the molten state and the solidified state. Due to this characteristic, pores containing hydrogen can be easily formed inside the metal by a manufacturing method described later.

As the negative electrode active material contains hydrogen, reactions such as oxidation on the surface of the negative electrode active material or the wall of the pores contained in the active material that occur during material production or before battery assembly are suppressed. The decomposition reaction of the electrolyte during charging is suppressed, so that the decomposition products can be prevented from being deposited, and the cycle characteristics can be improved.
Further, when the metal and lithium are alloyed, the volume expands. At this time, the pore inside the metal is crushed, so that the apparent volume expansion is offset. Thereby, pulverization of a negative electrode active material can be prevented and cycling characteristics can be improved. In addition, the pore can be impregnated with the electrolytic solution, and lithium ions can be efficiently diffused into the negative electrode active material, whereby the charge / discharge reaction can be smoothly advanced.

  The negative electrode active material according to the present invention preferably has an average particle size of about 5 to 30 μm. In contrast, the average pore diameter of the pore is preferably in the range of 1 nm to 5 μm. If the average pore diameter of the pore is less than 1 nm, it is not preferable because the volume expansion during charging cannot be relaxed, and if the average pore diameter exceeds 5 μm, the amount of energy per volume decreases. The aspect ratio of the pore is preferably 1.2 or more.

  In addition, regarding the average particle diameter of the negative electrode active material, it is preferable to use a conductive additive because the metal powder generally containing Si has a higher resistance than the graphite powder used as the existing negative electrode material of the lithium ion battery. When the average particle size is 5 μm or less, the average particle size of the metal powder may be smaller than the particle size of the conductive additive, making it difficult to obtain the effect of the conductive additive and reducing battery characteristics such as capacity and cycle characteristics. Therefore, it is not preferable. 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.

  In the negative electrode active material according to the present invention, the metal that can be alloyed with lithium is Si. Furthermore, the negative electrode active material contains a Si phase and a SiM phase, and either the X phase or the SiX phase. Or it is preferable that both are included.

The Si phase is a phase involved in the charge / discharge reaction of the negative electrode active material, and forms an LiSi _x phase by alloying with lithium at the time of charge, and releases lithium to return to the Si single phase at the time of discharge.
In addition, the SiM phase does not react with lithium during charge / discharge, and maintains the shape of one metal particle to suppress expansion and contraction of the particle itself. The element M constituting the SiM phase is a metal element that is not alloyed with lithium, and is at least one element selected from Ni, Co, As, B, Cr, Cu, Fe, Mg, Mn, and Y It is. 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.

In addition, the X phase imparts conductivity to the metal powder to reduce the specific resistance of the negative electrode active material itself. The element X constituting the X phase 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 or together with the X phase, a SiX phase may be precipitated. Similar to the X phase, the SiX phase imparts conductivity to the multiphase alloy powder and reduces the specific resistance of the negative electrode active material itself.

  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, 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.

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 metal 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 negative electrode active material, and further the negative electrode active material powder is bound by removing the solvent. But you can. Alternatively, the negative electrode active material may be bound by mixing with the negative electrode active material and 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, those containing any one of LiPF ₆ , LiBF _4, LiN (CF ₃ SO ₂ ) _{2, and} LiN (C ₂ F ₅ SO ₂ ) ₂ are 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.
In addition, since the reaction such as oxidation is suppressed at the surface of the negative electrode active material powder or the pore wall contained in the active material before the material is manufactured or the battery is assembled, the decomposition reaction of the non-aqueous electrolyte is suppressed, The charge / discharge capacity can be improved and the cycle characteristics can be improved.
Furthermore, since a large number of pores are formed in the negative electrode active material powder, when used as a negative electrode active material for a lithium secondary battery, the pores can be impregnated with a non-aqueous electrolyte and the charge / discharge reaction can be carried out smoothly. it can.
In addition, when an X phase or SiX phase with high conductivity is included, lithium ions can be efficiently diffused inside the negative electrode active material, and high rate charge / discharge can be achieved.

  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 this embodiment includes a step of dissolving hydrogen gas in a molten metal, a step of solidifying the molten metal in one direction, and a step of pulverizing the solidified metal body. Has been.

  First, in the step of dissolving hydrogen gas in the molten metal, as shown in FIG. 1, at least one kind of metal that can be alloyed with lithium is heated in, for example, a high-frequency induction heating furnace 1 to form a molten metal 4. . The high frequency induction heating furnace 1 includes a crucible 2 and a high frequency induction coil 3 wound around the crucible 2. The metal to be dissolved may be one or more of Si, Al, and Sn, and element M and element X may be added in addition to Si. Further, the atmosphere for dissolving the metal is preferably a hydrogen gas atmosphere or a mixed gas atmosphere of hydrogen gas and inert gas. The high-frequency induction heating furnace 1 is desirably used in a state where it is housed in a chamber in which a hydrogen gas atmosphere or a mixed gas atmosphere of hydrogen gas and an inert gas is previously provided, as shown in FIG.

The hydrogen pressure (hydrogen partial pressure) in the atmosphere at the time of dissolution is preferably about 0.2 to 5 MPa. As the hydrogen partial pressure is increased, the hydrogen gas concentration in the molten metal 4 is increased. However, if the hydrogen concentration in the molten metal 4 is excessive, coarse pores may be formed during the solidification of the metal. On the other hand, if the hydrogen partial pressure is too low, the hydrogen gas concentration in the molten metal 4 is lowered, and sufficient pores are not formed during solidification, and the amount of hydrogen stored in the pores also decreases. Therefore, the hydrogen pressure (hydrogen partial pressure) is preferably in the above range.
The hydrogen pressure (partial pressure) is preferably maintained for at least about 3 minutes after the metal is completely dissolved. Thereby, hydrogen gas fully melt | dissolves in the molten metal 4, and the hydrogen gas concentration in the molten metal 4 can be raised.
Moreover, although the temperature of the molten metal 4 changes with kinds of metal, it is preferable to make it more than melting | fusing point of a metal, for example, in the case of Si, it is preferable to set it as 1410-1700 degreeC. The higher the temperature of the molten metal 4, the higher the solubility of hydrogen gas in the molten metal 4. However, when the temperature becomes too high, the hydrogen concentration in the molten metal 4 becomes excessive and coarse pores are formed during the solidification of the metal. This is not preferable because there is a risk of being lost.

  Next, in the step of solidifying the molten metal in one direction, a porous metal manufacturing apparatus 5 as shown in FIG. 2 is used. The porous metal manufacturing apparatus 5 includes a chamber 6, the above-described high frequency induction heating furnace 1 housed in the chamber 6, and a cooling device 7. The cooling device 7 includes a hollow cylindrical mold 8 whose upper and lower ends are open, and a cooling unit 9 mounted on the lower end side of the mold 8. The atmosphere control means 10 is connected to the chamber 6 so that the atmosphere in the chamber 6 can be adjusted by the atmosphere control means 10. The atmosphere control means 10 includes a pressure gauge 11, a three-way valve 12 attached to a pipe ahead of the pressure gauge 11, a vacuum pump 13 connected to a branch pipe ahead of the three-way valve 12, and a tip of the three-way valve 12. The gas cylinder 14 is connected to another branch pipe.

  Then, the molten metal 4 in the high frequency induction heating furnace 1 is poured into the mold 8 of the cooling device 7. Next, the cooling unit 9 is operated to cool the molten metal 4 in the mold 8 from below the mold 8. Then, solidification of the molten metal 4 starts from the lower side of the mold 8, and solidification of the molten metal 4 gradually proceeds toward the upper side of the mold 8. FIG. 2 shows a state in which the molten metal 4 on the lower side of the mold 8 is solidified to form a metal body 15 and the molten metal 4 is not solidified on the upper side of the mold 8. In this way, the molten metal is solidified in one direction.

The solubility of hydrogen in a metal is generally high when the metal is molten (liquid) and low when it is solidified (solid). Utilizing this difference in gas solubility, hydrogen is dissolved in a molten metal and then the metal is solidified in one direction, so that hydrogen not dissolved in the solid becomes bubbles (pores) and remains in the metal structure. . The pores are filled with hydrogen.
FIG. 3 shows a perspective view of the metal body 15 solidified in one direction. A number of pores 16 filled with hydrogen are formed in the metal body 15 that has been unidirectionally solidified. The pore 16 is a result of the solidification of the metal and the leaving of hydrogen when hydrogen in the form of bubbles is about to escape from the molten metal during unidirectional solidification. For this reason, the pore has an elongated shape along the cooling direction of unidirectional solidification. That is, the pore aspect ratio is 1.2 or more.

And the negative electrode active material which concerns on this invention is obtained by grind | pulverizing the obtained metal body 15 so that it may become about 5-30 micrometers in average particle diameter.
In addition, as an index representing the amount of hydrogen contained in the active material, in the measurement spectrum of Raman spectroscopy, a peak in the vicinity of 620 cm ⁻¹ derived from Si—H bond and 500 to 520 cm ⁻ derived from crystalline Si A peak ratio I (620) / I (520) of ¹ Raman shift was used. If I (620) / I (520) is 0.004 or more, the Si-H bond suppresses the oxidation of the alloy surface and the side reaction with the electrolyte does not easily occur. Improvement of battery characteristics can be expected.

Hereinafter, the present invention will be described in more detail with reference to examples.
Example 1
A mixed raw material was prepared by mixing 55 parts by weight of massive Si having a size of about 5 mm square, 35 parts by weight of Ni powder, and 10 parts by weight of Cu powder. And this mixed raw material was thrown into the crucible 2 of the high frequency induction heating furnace 1 in the chamber 6 shown in FIG. Next, the vacuum pump 13 is operated to depressurize the inside of the chamber 6 to 1 × 10 ⁻³ Pa, and after depressurization, a mixed gas of hydrogen and helium is introduced from the gas cylinder 14 so that the hydrogen partial pressure is 2.8 MPa. It adjusted so that it might become.

  Next, the high frequency induction heating furnace 1 was operated to melt the mixed raw material to obtain a molten metal 4. Then, the metal in the atmosphere was sufficiently dissolved in the molten metal by leaving it for 10 minutes while maintaining the temperature of the molten metal 4 at 1600 ° C.

Next, as shown in FIG. 2, the molten metal 4 was poured from the high-frequency induction heating furnace 1 into the mold 8 of the cooling device 7. The mold 8 is a carbon hollow cylindrical member having dimensions of an outer diameter of 100 mm, an inner diameter of 80 mm, and a height of 200 mm.
Next, the cooling unit 6 was operated to solidify the molten metal 4 in one direction. Thus, the metal body 15 having a large number of pores was produced. And this negative electrode active material of Example 1 with an average particle diameter of 10 micrometers was manufactured by grind | pulverizing this metal body 15. FIG.

(Example 2)
A negative electrode active material of Example 2 was produced in the same manner as in Example 1 except that the hydrogen partial pressure in the chamber was 1 MPa.

(Comparative Example 1)
A mixed raw material was prepared by mixing 55 parts by weight of massive Si having a size of about 5 mm square, 35 parts by weight of Ni powder, and 10 parts by weight of Cu powder. And this mixed raw material was thrown into the crucible 2 of the high frequency induction heating furnace 1 in the chamber 6 shown in FIG. Next, the vacuum pump 13 was operated to depressurize the inside of the chamber 6 to 1 × 10 ⁻³ Pa. After depressurization, only helium was introduced from the gas cylinder 14 and the helium pressure was adjusted to 2.8 MPa. .
Next, the high frequency induction heating furnace 1 was operated to melt the mixed raw material to form a molten metal, and the molten metal was left to cool in the crucible 2 to be solidified. After solidification, the negative electrode active material of Comparative Example 1 having an average particle size of 10 μm was produced by pulverization.

(Comparative Example 2)
A mixed raw material was prepared by mixing 55 parts by weight of massive Si having a size of about 5 mm square, 35 parts by weight of Ni powder, and 10 parts by weight of Cu powder. And this mixed raw material was thrown into the crucible 2 of the high frequency induction heating furnace 1 in the chamber 6 shown in FIG. Next, the vacuum pump 13 was operated to depressurize the inside of the chamber 6 to 1 × 10 ⁻³ Pa. After depressurization, only helium was introduced from the gas cylinder 14 and the helium pressure was adjusted to 2.8 MPa. .

  Next, the high frequency induction heating furnace 1 was operated to melt the mixed raw material to form a molten metal, and the molten metal was left to cool in the crucible 2 to be solidified. After solidification, the alloy powder was obtained by grinding. This alloy powder was placed in a 5N aqueous sodium hydroxide solution and impregnated for 1 hour with slow stirring at 60 ° C. to partially dissolve the Si phase on the surface of the alloy powder. Thereafter, the product was sufficiently washed with pure water so that no sodium remained, dried, and the particle size was adjusted to an average particle size of 10 μm. Thus, the negative electrode active material of Comparative Example 2 was produced.

(Comparative Example 3)
A commercially available Si powder having an average particle diameter of 1 μm was used as the negative electrode active material of Comparative Example 3.

A lithium secondary battery was manufactured using the negative electrode active materials of Examples 1 and 2 and Comparative Examples 1 to 3 described above. 70 parts by weight of each negative electrode active material, 20 parts by weight of graphite powder having an average particle size of 3 μm as a conductive material, and 10 parts by weight of polyvinylidene fluoride are mixed, and N-methylpyrrolidone is added, followed by stirring to create a slurry. did. 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 of EC: DEC = 3: 7 by volume ratio. 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 30 cycles in the battery voltage range of 0 V to 1.5 V. And the capacity | capacitance maintenance factor after 30 cycles was calculated | required. The results are shown in Table 1.
For each negative electrode active material, a ratio of peak intensity around 600 to 620 cm ⁻¹ and peak intensity between 500 and 520 cm ⁻¹ in Raman spectrum as an index indicating the bonding amount of Si and hydrogen I (620) / I (520 ) Was measured. The results are shown in Table 1. Moreover, the measurement result of a Raman spectrum is shown in FIG.4 and FIG.5.

As shown in Table 1, it was found that the lithium secondary batteries using the negative electrode active materials of Examples 1 and 2 had a good capacity retention rate. In general, it is known that the number of moles of hydrogen dissolved in the molten metal is proportional to the square root of the hydrogen pressure according to the Sieverts law. In particular, the negative electrode active material of Example 1 has a high hydrogen partial pressure. Is well dissolved in the molten alloy, and the pore diameter was reduced by the pressure, so it is considered that good characteristics were obtained.
Moreover, I (620) / I (520) of Examples 1 and 2 shows a higher value than Comparative Examples 1 to 3, and there are many Si—H bonds on the surface of the negative electrode active material. I understand.

On the other hand, since the negative electrode active material of Comparative Example 1 was allowed to cool and solidify in an atmosphere of 100% helium, it did not contain any pores and contained no hydrogen in the alloy. For this reason, the effect of impregnating the electrolytic solution with the pores or suppressing the decomposition reaction of the electrolytic solution with hydrogen cannot be obtained, and it is considered that the capacity retention rate is reduced.
Moreover, although the negative electrode active material of Comparative Example 2 was treated with a sodium hydroxide aqueous solution and the surface Si was dissolved into porous particles, hydrogen was not contained in the alloy as in Comparative Example 1. It is considered that the effect of suppressing the decomposition reaction of the electrolytic solution was not obtained and the capacity retention rate was lowered.
Furthermore, since the negative electrode active material of Comparative Example 3 contains only Si and does not contain a conductive metal such as Cu, it is considered that the capacity retention rate was lowered.

FIG. 1 is a process diagram for explaining a method for producing a negative electrode active material for a lithium secondary battery according to the present invention. FIG. 2 is a process diagram for explaining a method for producing a negative electrode active material for a lithium secondary battery according to the present invention. FIG. 3 is a process diagram for explaining a method for producing a negative electrode active material for a lithium secondary battery according to the present invention. FIG. 4 is a graph showing the Raman spectrum measurement results of Examples 1 and 2 and Comparative Examples 1 to 3. FIG. 5 is a partially enlarged view of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... High frequency induction heating furnace, 4 ... Molten metal (molten metal), 5 ... Porous metal manufacturing apparatus, 6 ... Chamber, 7 ... Cooling device, 8 ... Mold, 9 ... Cooling unit, 10 ... Atmosphere control means

Claims (10)

  A negative electrode active material for a lithium secondary battery, wherein the negative electrode active material is a powder comprising at least one metal capable of being alloyed with lithium and hydrogen and having pores formed therein.   2. The negative electrode active material for a lithium secondary battery according to claim 1, wherein an average pore diameter of the pore is in a range of 1 nm to 5 μm.   The negative electrode active material for a lithium secondary battery according to claim 1, wherein hydrogen is contained inside the pore.   2. The negative electrode active material for a lithium secondary battery according to claim 1, wherein the metal that can be alloyed with lithium is Si, Al, or Sn. 2. The metal capable of being alloyed with lithium is Si, and further has Si phase and SiM phase in the metal, and includes one or both of X phase and SiX phase. The negative electrode active material for lithium secondary batteries as described in 2.
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. Intensities of 600-630 cm ⁻¹ peak intensity I (620) derived from the bond of Si and H in the Raman spectrum and 500-530 cm ⁻¹ Raman shift intensity I (520) derived from crystalline Si. 2. The negative electrode active material for a lithium secondary battery according to claim 1, wherein the ratio I (620) / I (520) is 0.004 or more.   Manufactured by dissolving hydrogen gas or a mixed gas of hydrogen gas and inert gas in a molten metal composed of at least one metal that can be alloyed with lithium, and then solidifying the molten metal in one direction. The negative electrode active material for a lithium secondary battery according to claim 1, wherein:   The negative electrode active material for a lithium secondary battery according to claim 7, wherein the aspect ratio of the pore is 1.2 or more by solidifying the molten metal in one direction in a mold.   A lithium secondary battery comprising the negative electrode active material for a lithium secondary battery according to any one of claims 1 to 8. A method for producing a negative electrode active material for a lithium secondary battery comprising at least one kind of metal that can be alloyed with lithium and hydrogen and having pores formed therein,
Hydrogen is contained in the solidified metal by dissolving hydrogen gas or a mixed gas of hydrogen gas and an inert gas in at least one type of molten metal and then solidifying the molten metal in one direction. A method for producing a negative electrode active material for a lithium secondary battery, characterized by forming a pore.

Images