JP5644869B2 - Secondary battery electrode material - Google Patents

Description

  The present invention relates to a material suitably used for an electrode of a secondary battery.

  Secondary batteries such as lithium ion secondary batteries are small and have a large capacity, and are therefore used in a wide range of fields such as mobile phones and notebook computers. A lithium ion secondary battery has an active material capable of inserting and removing lithium (Li) in each of a positive electrode and a negative electrode. And it operate | moves because Li ion moves in the electrolyte solution provided between both electrodes.

  The performance of the secondary battery depends on the materials of the positive electrode, the negative electrode, and the electrolyte constituting the secondary battery. Among them, active research and development of active material that directly contributes to the delivery of electrons and acts as an active material is being actively conducted. For example, silicon (Si) or a material containing Si has been studied as a negative electrode active material. For example, silicon-based materials such as silicon and silicon oxide can occlude and release lithium, and are expected to be put to practical use.

  Incidentally, silicon-containing materials containing Si include silicon compounds such as silicon nitride and silicon oxynitride. However, Patent Document 1 describes silicon nitride and silicon oxynitride as materials that do not contribute to lithium adsorption / desorption. Patent Document 2 describes a negative electrode material containing silicon nitride. This silicon nitride is obtained by thermally decomposing a silazane compound having a Si—N bond, and a part of the silazane compound remains in the negative electrode material. In other words, this silicon nitride is different from inorganic silicon nitride, which has a clear crystal structure, strong covalent bonding, and stability, and it is assumed that lithium can be stored and released.

  In addition, it can be concluded from theoretical calculations such as first-principles calculations that silicon nitride has no function of inserting and extracting lithium.

  That is, the conventional common sense is that silicon nitride is basically unusable as an electrode material for a secondary battery.

JP 2005-56705 A JP 2008-103350 A

  However, it is known that when a silicon-based material such as silicon or silicon oxide is used as a negative electrode active material, the negative electrode active material expands and contracts due to a charge / discharge cycle. When the negative electrode active material expands or contracts, a load is applied to the binder that holds the negative electrode active material on the current collector, and the adhesion between the negative electrode active material and the current collector decreases. There is a problem that the conductive path in the electrode is broken and the capacity is remarkably reduced, or the negative electrode active material is distorted due to repeated expansion and contraction to be refined and detached from the electrode. Due to these various problems, a lithium ion secondary battery using a silicon-based material as an electrode material has not yet been put into practical use. Therefore, a novel silicon compound that can be used as an electrode material for a lithium ion secondary battery has been demanded.

  In view of the above problems, an object of the present invention is to provide a novel electrode material for a secondary battery containing silicon.

The present inventors paid attention to sialon which is a Si—Al—O—N compound in which aluminum (Al) and oxygen (O) are dissolved in silicon nitride (Si ₃ N ₄ ). It became clear that sialon occludes and releases lithium despite the basic composition of silicon nitride, which has no function to occlude and release lithium. Thus, the present inventors have completed various inventions described below.

  The electrode material for secondary batteries of the present invention is characterized by containing a silicon-based material containing at least sialon.

To silicon nitride can not occlude and release lithium ions, sialon although no reason is apparent that the lithium ion capable of occluding and releasing a solid solution of Al and O in the Si ₃ N _4, expressed in Si ₃ N ₄ This is thought to be due to the appearance of characteristics that do not.

  Further, sialon basically has characteristics similar to silicon nitride, and therefore has a hard and high mechanical characteristic. Since sialon is excellent in toughness, the use of the electrode material for a secondary battery according to the present invention may eliminate the above-described problems associated with conventional silicon-based materials such as silicon oxide. In addition, since sialon is excellent in corrosion resistance, stability in an electrolytic solution is also expected. Furthermore, since sialon contains lightweight Al, O, and N as constituent elements, the electrode material for secondary batteries of the present invention is lightweight.

  The electrode material for a secondary battery of the present invention is suitable as an electrode active material for a secondary battery, particularly as a negative electrode active material.

  The electrode material for a secondary battery of the present invention is lighter than conventional silicon-based materials (silicon and silicon oxide) and is excellent in mechanical characteristics.

The charging / discharging curve of the lithium ion secondary battery (half cell) which uses the electrode material for secondary batteries of this invention as a negative electrode active material is shown.

  Below, the best form for implementing the electrode material for secondary batteries of this invention is demonstrated. Unless otherwise specified, the numerical range “ab” described herein includes the lower limit “a” and the upper limit “b”. In addition, the numerical range can be configured by arbitrarily combining the numerical values described in the present specification within the numerical range.

  The electrode material for secondary batteries of the present invention is characterized by containing a silicon-based material containing at least sialon. First, sialon will be described.

As described above, sialon is a Si—Al—O—N compound in which Al and O are dissolved in silicon nitride (Si ₃ N ₄ ). The sialon includes β-sialon in which Al and O are substituted and dissolved in β-Si ₃ N _4, and Al and O are substituted and dissolved in α-Si ₃ N ₄ , and other atoms are present at interstitial positions. It can be broadly divided into two types: interstitial solid solution α-sialon. If these sialons are represented by a composition formula, β-sialon is Si _6-Z Al _Z O _Z N _8-Z (Z is 0 <Z ≦ 4.2), and α-sialon is M _X ( Si, Al) ₁₂ (O, N) ₁₆ (M is one or more selected from Mg, Ca, Y and rare earth elements, and X is 0.33 <X <0.67).

  The silicon-based material preferably contains β-sialon as an essential component among sialons. There is no particular limitation on the Z value of β-sialon, but if it is too small, the properties of silicon nitride become strong, and it is difficult to obtain capacity even when used as an electrode active material. The above is preferable.

  The silicon-based material containing sialon may be a silicon alloy containing sialon. As can be seen from the Z value and the X value, sialon is a solid solution having a wide composition range, and thus can exist as a structure of a silicon alloy. The silicon alloy containing sialon has a silicon (Si) content of 43 to 50 mass%, an aluminum (Al) content of 8.5 to 11.5 mass%, and an oxygen (O) content of 5.50 mass% when the total is 100 mass%. It is preferable to contain 5-9 mass%, 29-35.5 mass% of nitrogen (N), and inevitable impurities. Inevitable impurities are mainly impurities contained in raw materials (for example, metallic silicon) used when synthesizing silicon-based materials, and are iron (Fe), nickel (Ni), chromium (Cr), molybdenum (Mo). , Manganese (Mn), titanium (Ti), yttrium (Y), magnesium (Mg), calcium (Ca), zirconium (Zr), vanadium (V), boron (B), tungsten (W) and cobalt (Co) Is mentioned. Sialon may contain at least one of these as unavoidable impurities, but when the total is 100% by mass, the content of unavoidable impurities is 0.3% by mass or less, 0.2% by mass or less, and further 0.2%. It is good to set it as 15 mass% or less.

  For the composition analysis of the silicon alloy, a mass spectrometry method, a capacitance method, an infrared absorption method, a thermal conductivity method, an ICP emission analysis method, or the like may be used. Moreover, it can be confirmed by, for example, X-ray diffraction that the silicon alloy contains sialon and further β-sialon.

  The silicon-based material is preferably in a powder form. If it is in a powder form, when used as an electrode active material, it can be easily mixed with a conductive additive, and an electrode having a uniform active material layer can be easily obtained. Moreover, since the specific surface area will increase if it is a powder consisting of fine particles, occlusion and release of lithium ions and the like are performed efficiently. For example, the average particle size is preferably 1 μm or less, 850 nm or less, and further 750 nm or less. The lower limit of the particle size of the silicon-based material is not particularly limited, but when the average particle size is 100 nm or more, further 500 nm or more, it is easy to handle and easily obtain. As the average particle size, for example, an average particle size (d50) by laser diffraction particle size analysis can be adopted.

  At present, since various sialon sintered compact raw material powders are commercially available, it is relatively easy to obtain powdered sialon, but it can be produced by a known method. For example, as a method for synthesizing a silicon-based material containing sialon, a combustion synthesis method, a high-temperature melting synthesis method (Self-propagating High-temperature Synthesis; SHS method), and the like are known.

The combustion synthesis method is a method in which metallic silicon and aluminum are directly reacted in a chamber in a nitrogen atmosphere. Since the temperature of the reaction system is very high (1000 ° C. or higher), the reaction is carried out in the gas phase. As a raw material, metallic silicon, aluminum, alumina and / or silica as required are used, and a silicon alloy containing sialon in which Al and O are dissolved in Si ₃ N ₄ is cooled by a gas phase reaction. It is obtained by depositing on the surface.

  In the SHS method, metallic silicon, aluminum, alumina, and β-silicon nitride are mixed so as to be β-sialon having a target composition, and an SHS reaction is performed. The reaction is completed in a short time while controlling the generated heat of reaction (2000 ° C. or higher), and then seeds of crystals are added and cooled to obtain β-sialon as whiskers (needle crystals).

  A silicon-based material powder having a desired particle diameter can be obtained by pulverizing the silicon-based material obtained by the above method or the like by a normal method.

  Since the secondary battery electrode material of the present invention described above can occlude and release lithium or the like, it can be used by adding to the conventional secondary battery electrode. That is, the electrode material for a secondary battery of the present invention can be used as an active material for a non-aqueous electrolyte secondary battery and further a lithium ion secondary battery. The secondary battery electrode material of the present invention can be used alone as a positive electrode active material or a negative electrode active material, or the secondary battery electrode material of the present invention and a conventional positive electrode active material or negative electrode active material It is also possible to mix and use as a positive electrode active material or a negative electrode active material. Below, embodiment at the time of using the electrode material for secondary batteries of this invention as a negative electrode active material of a secondary battery is described.

  The negative electrode for a secondary battery mainly includes a negative electrode active material, a conductive additive, and a binder that binds the negative electrode active material and the conductive additive.

  The negative electrode active material is a secondary battery electrode material containing the above silicon-based material. In addition, after making said silicon-type material into the main negative electrode active material material, you may add and use other already well-known negative electrode active materials (for example, graphite, Sn, Si, etc.).

  As a conductive support material, a material generally used for an electrode of a secondary battery may be used. For example, it is preferable to use conductive carbon materials such as carbon black (carbonaceous fine particles) such as acetylene black and ketjen black, and carbon fibers. Besides these carbon materials, known conductive materials such as conductive organic compounds are also used. An auxiliary material may be used. One of these may be used alone or in combination of two or more. The blending ratio of the conductive additive is preferably a mass ratio of negative electrode active material: conductive additive = 1: 0.01 to 1: 0.5. This is because if the amount of the conductive aid is too small, an efficient conductive path cannot be formed, and if the amount of the conductive aid is too large, the moldability of the electrode is deteriorated and the energy density of the electrode is lowered.

  The binder is not particularly limited, and a known one may be used. For example, a resin that does not decompose even at a high potential, such as a fluorine-containing resin such as polytetrafluoroethylene or polyvinylidene fluoride, can be used. The blending ratio of the binder is preferably a mass ratio of negative electrode active material: binder = 1: 0.05 to 1: 0.5. This is because when the amount of the binder is too small, the moldability of the electrode is lowered, and when the amount of the binder is too large, the energy density of the electrode is lowered.

  In general, the negative electrode active material is used in a state where it is pressed against a current collector as an active material layer in the negative electrode. A metal mesh or metal foil can be used for the current collector. For example, a current collector made of copper or copper alloy may be used.

  There is no limitation in the manufacturing method of a negative electrode, What is necessary is just to follow the manufacturing method of the electrode for secondary batteries currently implemented generally. For example, the conductive additive and the binder are mixed with the negative electrode active material, and an appropriate amount of an organic solvent is added as necessary to obtain a paste-like electrode mixture. The electrode mixture is applied to the surface of the current collector, dried, and then pressed and pressed as necessary. According to this manufacturing method, the produced electrode becomes a sheet-like electrode. This sheet-like electrode may be cut into dimensions according to the specifications of the secondary battery to be manufactured.

  The secondary battery may be a non-aqueous electrolyte secondary battery including a positive electrode, the above-described negative electrode for a secondary battery, and a non-aqueous electrolyte obtained by dissolving an electrolyte material in an organic solvent. This non-aqueous electrolyte secondary battery includes a separator and a non-aqueous electrolyte sandwiched between a positive electrode and a negative electrode in addition to a positive electrode and a negative electrode, as in a general secondary battery.

  The separator separates the positive electrode and the negative electrode and holds the non-aqueous electrolyte, and a thin microporous film such as polyethylene or polypropylene can be used.

The nonaqueous electrolytic solution is obtained by dissolving an alkali metal salt as an electrolyte in an organic solvent. There is no limitation in particular in the kind of nonaqueous electrolyte solution used with a nonaqueous electrolyte secondary battery provided with said negative electrode for secondary batteries. As the non-aqueous electrolyte, one or more selected from aprotic organic solvents such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and the like. Can be used. As the electrolyte to be dissolved, an alkali metal salt that is soluble in an organic solvent such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiI, LiClO ₄ , NaPF ₆ , NaBF ₄ , NaAsF ₆ , LiBOB can be used.

  The negative electrode is as described above. The positive electrode includes a positive electrode active material in which lithium ions and the like can be inserted and removed, and a binder that binds the positive electrode active material. Further, a conductive aid may be included. The positive electrode active material, the conductive additive, and the binder are not particularly limited as long as they can be used in the nonaqueous electrolyte secondary battery.

A positive electrode active material containing Li can be used for the positive electrode. Specific examples include LiCoO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , and Li ₂ MnO ₂ . A positive electrode active material not containing Li can also be used. As the positive electrode active material not containing Li, for example, a nonmetal, a nonmetal compound, a metal compound, a polymer material, or the like can be used. Nonmetallic and nonmetallic compounds that do not contain Li include elemental sulfur (S) and a composite of sulfur and carbon. As carbon, acetylene black and mesoporous carbon are preferable. Examples of the metal compound not containing Li include oxides such as TiO ₂ , V ₂ O ₅ , and MnO ₂ , and sulfides such as MoS ₂ . Examples of the polymer material not containing Li include conductive polymers such as polyaniline and polythiophene. Particularly preferably, it is at least one of simple substance S, a composite of S and C, MnO ₂ and V ₂ O ₅ . By using these active materials for the positive electrode, a non-aqueous electrolyte secondary battery having a large battery capacity can be obtained.

  In addition, when using the active material which does not contain Li etc. for a positive electrode and a negative electrode, the lithium pre dope process which inserts Li etc. beforehand to either one or both of a negative electrode and a positive electrode is needed. The lithium pre-doping method may be a known method. For example, a method of inserting lithium by an electrolytic doping method in which a half battery is assembled using metallic lithium as the counter electrode and lithium is electrochemically doped into the other electrode, or an electrolytic solution in a state where the metallic lithium foil is in contact with the electrode There is a method of inserting lithium by a contact pre-doping method in which the material is left in the substrate and doped using diffusion of lithium to the electrode.

  The current collector may be any material that is generally used for the positive electrode of a nonaqueous electrolyte secondary battery, such as aluminum, nickel, and stainless steel.

  The shape of the nonaqueous electrolyte secondary battery is not particularly limited, and various shapes such as a cylindrical shape, a stacked shape, and a coin shape can be adopted. Regardless of the shape, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and the space between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal is used for current collection. After connecting using a lead or the like, the electrode body is sealed in a battery case together with a non-aqueous electrolyte to form a battery.

  The secondary battery described above can be suitably used not only in the field of communication devices such as mobile phones and personal computers and information-related devices, but also in the field of automobiles. For example, if this secondary battery is mounted on a vehicle, the secondary battery can be used as a power source for an electric vehicle.

  As mentioned above, although embodiment of the electrode material for secondary batteries of this invention was described, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art.

  Hereinafter, the electrode material for a secondary battery of the present invention will be specifically described with reference to examples.

<Example 1>
Example 1 is an example using the secondary battery electrode material of the present invention as an active material.

<Preparation of secondary battery electrode>
As an active material, “Meramix (registered trademark)” S1-NF manufactured by Isman Jay Co., Ltd., which is a commercially available β-sialon sintered material powder, was prepared. The dispersion of the composition of this powder by the 3σ method is, by mass, Si: 43.3 to 49.6%, Al: 8.7 to 11.3%, O: 5.8 to 8.7%, N: The composition (analytical value) of the powder used was Si 46.0% -Al 10.9% -O 8.84% -N 32.8% -Fe 0.04% -Mg0. .0031% -Ca0.023% -Zr0.052%. The average particle diameter (d50) of this powder was 700 nm or less. Although not shown, the X-ray diffraction pattern of this powder had a diffraction peak at a position substantially coincident with the standard diffraction line of Si ₅ AlON ₇ (that is, Z = 1). That is, it was found that the used powder was a silicon alloy containing a β-sialon phase.

  The above active material powder and ketjen black (KB) as a conductive additive were mixed to obtain a mixed powder. Further, N-methylpyrrolidone (NMP) and polyamideimide-silica hybrid resin as a binder (manufactured by Arakawa Chemical Industries, solvent composition: NMP / xylene = 4/1, cured residue 30.0%, cured residue) Silica: 2% (all proportions are mass ratios, viscosity 8700 mPa · S / 25 ° C.) were dissolved. This solution was mixed with a mixed powder of active material powder and KB to prepare a slurry. The mixing ratio of the active material powder, KB, and the binder (solid content) was 80: 5: 15 by mass ratio. The prepared slurry was applied to the surface of an electrolytic copper foil (current collector) having a thickness of 20 μm by using a doctor blade to form an active material layer on the copper foil. Then, it dried at 80 degreeC for 20 minute (s), and NMP was volatilized and removed from the active material layer. After drying, the current collector and the active material layer were firmly and closely joined with a roll press.

  This was heat-cured at 200 ° C. for 2 hours to obtain an electrode having an active material layer thickness of about 15 μm.

<Production of lithium ion secondary battery>
A lithium ion secondary battery (half cell) was produced using the electrode produced by the above procedure as an evaluation electrode. The counter electrode was a metal lithium foil (thickness 500 μm). The counter electrode was cut to 13 mm and the evaluation electrode was cut to 11 mm, and a separator (Hoechst Celanese glass filter and celgard 2400) was sandwiched between the two to form an electrode body. This electrode body was accommodated in a battery case (CR2032 coin cell manufactured by Hosen Co., Ltd.). In addition, a non-aqueous electrolyte in which LiPF ₆ was dissolved at a concentration of 1 M was injected into the battery case in a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at a ratio of 1: 1 (volume ratio). The battery case was sealed to obtain a lithium secondary battery.

<Evaluation>
In order to confirm that the produced lithium ion secondary battery (half cell) was chargeable / dischargeable, a charge / discharge test was performed.

  The charge / discharge test is conducted at a constant current of 0.05 mA up to a final discharge voltage of 2 V after charging at a constant current of 0.05 mA up to a final charge voltage of 0.01 V on a metal Li basis in a temperature environment of 25 ° C. Discharge was performed. “Charge” is the direction in which the active material of the evaluation electrode occludes Li, and “discharge” is the direction in which the active material of the evaluation electrode releases Li.

  The charge / discharge curve is shown in FIG. From FIG. 1, it was found that the lithium ion secondary battery manufactured in this example can be charged and discharged. It was found that a silicon alloy powder containing β-sialon can be used as an active material of a lithium ion secondary battery.

Claims (10)

An electrode material for a secondary battery comprising a silicon-based material containing at least sialon ,
The silicon-based material has 43 to 50% by mass of silicon (Si), 8.5 to 11.5% by mass of aluminum (Al), and oxygen (O) when the entire silicon-based material is 100% by mass. Is an electrode material for a secondary battery, which is a silicon alloy containing 5.5-9 mass%, nitrogen (N) 29-35.5 mass%, and inevitable impurities. 2. The electrode material for a secondary battery according to claim 1 , wherein the silicon-based material has an inevitable impurity content of 0.15 mass% or less. The inevitable impurities include iron (Fe), magnesium (Mg), an electrode material for a secondary battery according to claim 1 or 2 comprising at least one of calcium (Ca) and zirconium (Zr). The secondary battery electrode material according to any one of claims 1 to 3, wherein the sialon includes β-sialon. The β- sialon _{Si 6-Z Al Z O Z} N 8-Z (Z is 0 <Z ≦ 4.2) electrode material for a secondary battery according to claim 4 is. The β- sialon _{Si 6-Z Al Z O Z} N 8-Z (Z is 0.5 ≦ Z ≦ 4.2) electrode material for a secondary battery according to claim 4 is. The electrode material for a secondary battery according to any one of claims 1 to 6 , wherein the silicon-based material is in a powder form. Electrode active material for a secondary battery which comprises an electrode material for a secondary battery according to any one of claims 1-7. A nonaqueous electrolyte secondary battery comprising: a positive electrode; a negative electrode including the electrode active material for a secondary battery according to claim 8; and a nonaqueous electrolyte. A vehicle comprising the nonaqueous electrolyte secondary battery according to claim 9 .

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