JP5073167B2 - Lithium secondary battery - Google Patents

Description

  The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery having excellent cycle characteristics.

A lithium secondary battery is generally a battery that uses LiCoO ₂ as a positive electrode active material, graphite as a negative electrode active material, and a non-aqueous solution as an electrolyte, such as a mobile phone, a digital still camera, a digital video camera, and a laptop computer. Widely used as a power source. In this lithium secondary battery, the electrolyte component is decomposed on the graphite surface during charge and discharge, and a LiF film is formed on the graphite surface. This LiF film formation is an irreversible reaction, and lithium ions are not generated from LiF. Therefore, if this LiF formation proceeds too much, the amount of lithium traveling between the positive electrode and the negative electrode decreases, thereby charging and discharging. The efficiency becomes worse. Further, when a thick film is formed on the surface, the impedance increases and the rate characteristic decreases.

On the other hand, recently, research on a negative electrode active material containing Si as a main component instead of graphite has been advanced. A negative electrode active material mainly composed of Si is promising as a future electrode material because it has a charge / discharge capacity nearly 10 times that of graphite. This Si has problems such as forming an alloy with lithium during charging and expanding its volume or decomposing the electrolytic solution. Recently, however, a negative electrode active material composed of a multiphase alloy powder containing Si and having Si removed from only the surface has been developed (Patent Document 1), and a negative electrode active material containing Si has been put to practical use. Is becoming a reality.
Japanese Patent Application No. 2003-299282

  However, since the negative electrode active material containing Si has larger volume expansion and contraction during charging than graphite, a stable coating is less likely to be formed when the same electrolyte is used compared to graphite. Phase surfaces are formed one after another and react with the electrolyte at the Si phase surface to cause a LiF film formation reaction. As a result, there is a problem in that lithium traveling between the positive electrode and the negative electrode is reduced, and charge / discharge cycle characteristics are deteriorated.

  This invention is made | formed in view of the said situation, and it aims at providing the lithium secondary battery which can suppress formation of LiF and can improve charging / discharging cycling characteristics.

In order to achieve the above object, the present invention employs the following configuration.
The lithium secondary battery of the present invention is a lithium secondary battery comprising at least a negative electrode containing a negative electrode active material, a positive electrode containing a positive electrode active material, and a non-aqueous electrolyte. A forming compound is provided at least, and an intensity ratio (SiF ⁺ / Li ⁺ ) between Li ⁺ ions and SiF ⁺ ions in a secondary ion mass spectrum with respect to the negative electrode after charging is 2.0 × 10 ⁻³ or more and 1.0 × 10 It is in the range of ⁻¹ or less.
The lithium secondary battery of the present invention includes at least a negative electrode containing a negative electrode active material containing Si, a non-aqueous electrolyte containing LiPF ₆ and a SiF-forming compound, and is a negative electrode after charging. It is desirable that the intensity ratio (SiF ⁺ / Li ⁺ ) of Li ⁺ ions and SiF ⁺ ions in the secondary ion mass spectrum with respect to is in the range of 2.0 × 10 ⁻³ or more and 1.0 × 10 ⁻¹ or less.
Moreover, in the lithium secondary battery of this invention, it is preferable that the component ratio of the LiF compound with respect to a total fluoride is 50% or less in the negative electrode after charge.
In the lithium secondary battery of the present invention, the SiF-forming compound is a silicone compound, silicon halide, silicon dioxide, silicon sulfide, silicon nitride, hexafluorosilicic acid, metasilicic acid, cycloxanic compound, silazane compound, or SiC _6. It is preferably at least one compound among compounds containing at least one kind of H ₅ group, SiCH ₃ group, and SiC ₂ H ₅ group.
Furthermore, the negative electrode active material in the lithium secondary battery of the present invention is at least one selected from Si, Ni, Co, As, B, Cr, Cu, Fe, Mg, Mn, Y, Ag, and Au. A material composed of the above elements is preferable.
Furthermore, the negative electrode active material in the 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, It is particularly preferable that the amount of Si phase on the particle surface of the phase alloy powder is smaller than the amount of Si phase inside the particle. 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.

In the above configuration, the negative electrode after charging is preferably one in which the open circuit voltage of the battery after charging is in a charged state of 3.5 V or more.
The secondary ion mass spectrum is particularly preferably a time-of-flight secondary ion mass spectrum (TOF-SIMS).
The SiF-forming compound refers to a compound that forms a SiF compound having a Si—F bond by combining with fluorine ions or fluoride ions, such as a silicone compound.
Further, the total fluoride refers to all fluorine-containing compounds such as LiF and SiF compounds formed in the negative electrode by charge / discharge reactions.
Furthermore, the LiF compound refers to all compounds containing lithium and fluorine, including lithium fluoride (LiF) formed in the negative electrode by a charge / discharge reaction.

According to the above lithium secondary battery, the SiF compound is formed by the SiF-forming compound even when a fresh Si phase surface is exposed during the charge / discharge process. In the charge / discharge process, the formed SiF is stable, and the intensity ratio of Li ⁺ ions to SiF ⁺ ions (SiF ⁺ / Li ⁺ ) is always within the above range. Will not accumulate. Thereby, there is no possibility that the lithium which goes back and forth between the positive electrode and the negative electrode of the lithium secondary battery is reduced, and deterioration of the charge / discharge cycle is prevented.
In addition, since the decomposition reaction of the electrolytic solution is suppressed by the formed SiF, generation of decomposition gas inside the battery is suppressed, and an increase in internal pressure can be prevented, and at the same time, charge / discharge efficiency of each cycle is improved. .

  Further, according to the above lithium secondary battery, since the component ratio of the LiF compound with respect to the total fluoride is 50% or less, there is no fear that lithium traveling between the positive electrode and the negative electrode decreases, and the charge / discharge cycle Deterioration is prevented.

  Further, according to the above lithium secondary battery, by using a silicone compound or the like as the SiF forming compound, it becomes possible to preferentially form the SiF compound before the LiF compound is formed, and between the positive electrode and the negative electrode. Therefore, there is no possibility that the lithium flowing back and forth decreases, and deterioration of the charge / discharge cycle is prevented.

Furthermore, according to the above lithium secondary battery, since the multiphase alloy powder in which the amount of Si phase on the particle surface is smaller than the amount of Si phase inside the particle is used as the negative electrode active material, the negative electrode active material of LiPF ₆ The decomposition reaction on the surface can be suppressed, the formation of the LiF compound is suppressed, and there is no possibility that lithium traveling between the positive electrode and the negative electrode is reduced, and deterioration of the charge / discharge cycle is prevented.

  Moreover, by adding the SiF-forming compound to the negative electrode, the SiF-forming compound can be reacted efficiently, and deterioration of the charge / discharge cycle is prevented.

  According to the negative electrode for the lithium secondary battery of the present invention, the SiF compound is formed before LiF is accumulated by the progress of the charge / discharge cycle, and the lithium traveling between the positive electrode and the negative electrode of the lithium secondary battery is reduced. Therefore, the cycle characteristics of the lithium secondary battery can be improved.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
The lithium secondary battery of the present invention comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte, and these are housed in battery cases of various shapes such as a cylindrical shape, a square shape, a coin shape, and a sheet shape. Has been. A separator is interposed between the positive electrode and the negative electrode. In addition, a SiF-forming compound is added to the lithium secondary battery of the present invention. This SiF-forming compound is preferably mixed in advance with the negative electrode, but it may be added to the non-aqueous electrolyte, applied to the inside of the battery case, and further applied to the separator. May be.

(Positive electrode)
As the positive electrode, a sheet-like electrode comprising a positive electrode mixture containing a positive electrode active material, a conductive additive and a binder, and a positive electrode current collector bonded to the positive electrode mixture can be used. . Moreover, the pellet type or sheet-like electrode formed by shape | molding said positive electrode compound material in a disk shape can also be used.

Examples of the positive electrode active material include compounds containing Li, oxides, and sulfides. Examples of the metal contained include at least one type of material such as Mn, Co, Ni, Fe, and Al. More specifically, LiMn ₂ O ₄ , LiCoO ₂ , LiNiO ₂ , LiFeO _2, LiNi _1/3 Co _1/3 Mn _1/3 O _2, LiNi _0.8 Co _0.2 O _{2 and the} like can be exemplified. Examples of the binder include polyvinylidene fluoride and polytetrafluoroethylene. Furthermore, examples of the conductive aid include carbonized materials such as carbon black, ketjen black, and graphite. Furthermore, examples of the positive electrode current collector include a metal foil or a metal net made of aluminum, stainless steel, or the like.

(Negative electrode)
The negative electrode includes a sheet-like electrode composed of a negative electrode mixture containing a negative electrode active material, a binder, and if necessary, a conductive additive, and a negative electrode current collector bonded to the negative electrode mixture. Can be used. Moreover, the pellet type or sheet-like electrode formed by shape | molding said negative electrode compound material in a disk shape can also be used.

  The binder for the negative electrode may be either organic or inorganic, but it is dispersed or dissolved in a solvent together with the multiphase alloy powder described below, and the multiphase alloy powder is bound by removing the solvent. Anything is acceptable. 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 to the negative electrode active material and the binder, carbon black, graphite powder, carbon fiber, metal powder, metal fiber, or the like may be added as a conductive additive. Furthermore, examples of the negative electrode current collector include a metal foil or a metal net made of copper.

  Next, the negative electrode active material is composed of a multiphase alloy powder that necessarily includes an Si phase and an SiM phase, and includes one or both of the X phase and the SiX phase, and the amount of the Si phase on the particle surface of the multiphase alloy powder is The amount is smaller than the amount of Si phase inside the particles. 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 11 constituting the negative electrode active material contains a Si phase 12, a SiM phase 13, and an X phase or a SiX phase 14.

The Si phase 12 is present in the interior of the particle more than the particle surface. The Si phase 12 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. Moreover, since the Si phase on the particle surface does not exist or decreases, the decomposition reaction of the electrolytic solution by the Si phase is suppressed.

In addition, the SiM phase 13 does not react with lithium during charge and discharge, and maintains the shape of the one particle 11 to suppress the expansion and contraction of the particle 11 itself. The element M constituting the SiM phase 13 is a metal element that is not alloyed 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.

The X phase 14 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 14 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 alloyed with Si, selecting a metal that does not alloy with Ag as element M is preferable 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 14 or together with the X phase 14, a SiX phase may be precipitated. The SiX phase, like the X phase 14, 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 12, the SiM phase 13, the X phase 14, 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 solubility 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 solubility 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 secondary battery. Therefore, it is preferable to use a conductive additive. However, when the average particle size is 5 μm or less, the conductive powder is used. In some cases, the average particle size of the multiphase alloy powder is smaller than the particle size of the material, and it becomes 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 15 are formed on the particle surface of the multiphase alloy powder. The fine holes 15 are formed by quenching the molten alloy and then impregnating it with an alkaline solution, and are traces after the Si phase exposed on the particle surface is eluted 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 15 is preferably in the range of 10 nm to 5 μm. Further, the depth of the fine holes 15 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.

This negative electrode active material can be manufactured, for example, by the following method.
The manufacturing method of a negative electrode active material is roughly comprised from the process of obtaining the quenching alloy powder containing Si, the element M, and the element X, and the process of impregnating the obtained quenching alloy powder in an alkaline solution. 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 contains the element M, the element X, and Si, and is obtained by simultaneously melting 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 manufacturing method, the alloy melt containing the element M, the element X, and Si is rapidly cooled, so that the SiX phase and the SiM phase are necessarily included, and either one or both of the X phase and the SiX phase are included. Quenched alloy powder is 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 the Si phase on the particle surface becomes smaller than the amount of the Si phase inside the particle. The negative electrode active material thus obtained can suppress the decomposition reaction of the electrolytic solution and can reduce the amount of expansion and contraction of the particles themselves, thereby improving the cycle characteristics.

  In addition, a multiphase alloy powder that always includes a SiX phase and a SiM phase and includes one or both of an X phase and a SiX phase can be easily obtained. In particular, according to the gas atomization method or the water atomization method, since 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.

(SiF-forming compound)
Next, the SiF-forming compound is a compound that combines with fluorine ions or fluoride ions to form a SiF compound having a Si-F bond. Specifically, silicone compounds, silicon halides, silicon dioxide, silicon sulfide , Silicon nitride, hexafluorosilicic acid, metasilicic acid, cycloxan compound, silazane compound, SiC ₆ H ₅ group or compound containing SiCH ₃ group or SiC ₂ H ₅ group, at least one kind of compound preferable. The SiF-forming compound may be mixed in the negative electrode, added to the non-aqueous electrolyte, applied to the battery case, or applied to the separator. In particular, liquid compounds such as silicone compounds are preferably added to the non-aqueous electrolyte. In the case of a solid such as silicon halide, it may be added to the negative electrode.

As the silicone compound, any one or two or more of the structures shown in the following [Chemical Formula 1] to [Chemical Formula 3] can be used. In the following [Chemical Formula 1] to [Chemical Formula 3], k is in the range of 0-50, m is a natural number in the range of 2-10, n is a natural number in the range of 1-50, and R is Either CH ₃ or C ₆ H ₅ and Z is either CH ₃ or C ₂ H ₅ .

If k exceeds 50, the thermal stability is improved, but the viscosity may be extremely high, and the ability to solvate with lithium ions is reduced, resulting in a decrease in ionic conductivity. Moreover, when m is less than 2, synthesis of the silicone compound becomes difficult, and when m exceeds 10, the viscosity increases and as a result, the ionic conductivity decreases, which is not preferable.
Further, when n is less than 1 (that is, n is 0), there is almost no polyether chain linked to the polysiloxane chain, and the compatibility with the solvent component contained in the electrolytic solution is lowered. Exceeding this is not preferable because the polyether chain becomes long and the viscosity becomes extremely high, and the ionic conductivity is lowered. Furthermore, when R is either CH ₃ or C ₆ H ₅ and Z is either CH ₃ or C ₂ H ₅ , the silicone compound can be easily synthesized.

In order to produce a silicone compound, for example, a polyether compound having a double bond such as (CH ₂ ═CH—) is subjected to a hydrosilylation reaction with respect to polysiloxane in which a part of R group is substituted with hydrogen. Can be obtained by combining.

Since these silicone compounds contain silicon (Si) in their molecules, they are easily compatible with Si fine particles constituting the negative electrode active material, and in some cases, the surface of the Si fine particles may be covered to form a protective layer. Thereby, decomposition | disassembly of the solute component in Si microparticle surface can be suppressed.
In addition, these silicone compounds react with F ions or PFx ions which are decomposition products of LiPF ₆ to form SiF compounds having Si—F bonds in which F is bonded to Si of the silicone compound. This SiF compound accumulates as a film on the surface of the negative electrode active material, and prevents contact between electrolysis and the Si phase in the negative electrode active material. Further, since the SiF compound is formed preferentially over the LiF compound, there is no possibility that Li ions traveling between the positive electrode and the negative electrode are reduced.

  When the silicone compound described in the above [Chemical Formula 1] to [Chemical Formula 3] is used as the SiF-forming compound, the blending ratio thereof is preferably in the range of 0.2% by mass or more and 20% by mass or less of the silicone compound in the electrolytic solution. If the compounding ratio of the silicon compound is lowered, the amount of SiF compound is decreased, Li ions are decreased, and the decomposition reaction of the lithium salt proceeds. Moreover, when the compounding ratio of the silicone compound is excessive, the amount of the negative electrode active material is relatively decreased, and the charge / discharge capacity is decreased, which is not preferable.

(Nonaqueous electrolyte)
Examples of the nonaqueous electrolytic solution include an organic electrolytic solution in which a lithium salt is dissolved in an aprotic solvent.
Examples of aprotic solvents include 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 LiPF ₆ are preferred.

In place of the non-aqueous electrolyte, a polymer such as PEO or PVA mixed with any of the lithium salts described above, or a polymer having a high swellability impregnated with an organic electrolyte, a so-called polymer electrolyte 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. .

Next, the charging reaction of the lithium secondary battery will be described.
The charging reaction proceeds as follows, for example. First, lithium ions are deintercalated from LiCoO ₂ which is a positive electrode active material and eluted into the non-aqueous electrolyte. The lithium ions are transported by the nonaqueous electrolytic solution and reach the surface of the negative electrode active material. Then, lithium ions are integrated with electrons in the negative electrode active material and precipitated as metal lithium, and this metal lithium is alloyed with the Si phase contained in the negative electrode active material to form a Li _x Si phase. When the charging reaction is continued, the Li _x Si phase gradually increases, whereby the alloy particles constituting the negative electrode active material expand and partly cracks in the alloy particles.
On the other hand, the lithium salt contained in the nonaqueous electrolytic solution decomposes to form F ions or PF _x ions when it comes into contact with the Si phase. The F ions or PF _x ions react with the SiF-forming compound to form a SiF compound, and are accumulated as a film on the surface of the Si phase of the negative electrode active material. Some of the F ions or PF _x ions may combine with lithium ions deintercalated from the positive electrode active material to form a LiF compound, but most of the F ions or PF _x ions are as described above. React with the SiF-forming compound to form a SiF compound. As a result, there is no possibility that lithium ions traveling between the positive electrode and the negative electrode will decrease.
In addition, the SiF compound accumulated on the surface of the Si phase also has an effect of preventing the decomposition reaction with the lithium salt by preventing the contact between the lithium salt and the Si phase. Thereby, the fall of the ionic conductivity of a non-aqueous electrolyte is prevented.

In such a lithium secondary battery, when the charged negative electrode is taken out and subjected to time-of-flight secondary ion mass spectrometry (TOF-SIMS) on its surface, various ion species are detected. Focusing on ⁺ ions and SiF ⁺ ions, the intensity ratio (SiF ⁺ / Li ⁺ ) of these ions in the secondary ion mass spectrum is in the range of 2.0 × 10 ⁻³ or more and 1.0 × 10 ⁻¹ or less. . Li ⁺ ions are derived from LiF formed by the reaction of F ions or PF _x ions with lithium ions. SiF ⁺ ions are derived from SiF compounds formed by reaction of F ions or PF _x ions with SiF-forming compounds. Accordingly, if the intensity ratio (SiF ⁺ / Li ⁺ ) is in the range of 2.0 × 10 ⁻³ or more and 1.0 × 10 ⁻¹ or less, the SiF compound is quantitatively larger than LiF. This suggests that the production of has priority over LiF production. When the intensity ratio (SiF ⁺ / Li ⁺ ) is less than 2.0 × 10 ⁻³ , the amount of LiF increases, suggesting that lithium ions traveling between the positive electrode and the negative electrode are decreasing. On the other hand, if the intensity ratio (SiF ⁺ / Li ⁺ ) exceeds 1.0 × 10 ⁻¹ , the amount of the SiF compound becomes excessive and the impedance of the negative electrode increases.
Therefore, in the negative electrode of this embodiment, the intensity ratio (SiF ⁺ / Li ⁺ ) between Li ⁺ ions and SiF ⁺ ions in the secondary ion mass spectrum with respect to the negative electrode after charging is 2.0 × 10 ⁻³ or more and 1.0 ×. It is desirable to adjust the compounding ratio, addition method, and the like of the negative electrode active material and the SiF-forming compound so that the range is 10 ⁻¹ or less. Although there are various types of secondary ion mass spectrometry (SIMS), it is preferable to use a time-of-flight secondary ion mass spectrum (TOF-SIMS) in terms of particularly excellent sensitivity.

  Moreover, when the negative electrode in a charged state is taken out and subjected to X-ray photoelectron spectroscopy (XPS) on its surface, elements derived from various compounds are detected. When the component ratio of the LiF compound is measured, the component ratio of the LiF compound is 50% or less. This suggests that the amount of LiF compound containing LiF is small, the amount of SiF compound is larger, and the generation of SiF compound is prioritized over the generation of LiF. When the component ratio of the LiF compound exceeds 50%, the amount of LiF increases, and lithium ions traveling between the positive electrode and the negative electrode decrease. Therefore, in the negative electrode of this embodiment, it is desirable to adjust the mixing ratio of the negative electrode active material and the SiF-forming compound, the addition method, etc. so that the component ratio of the LiF compound to the total fluoride is in the range of 50% or less.

As described above, according to the lithium secondary battery of the present embodiment, even when LiPF ₆ in the electrolytic solution is decomposed on the surface of the negative electrode active material, a SiF compound is formed by the SiF-forming compound, and Li ⁺ ions And the intensity ratio of SiF ⁺ ions (SiF ⁺ / Li ⁺ ) are always within the above range, so that LiF is not accumulated by the progress of the charge / discharge cycle. There is no possibility that the amount of lithium traveling between the two will decrease, and deterioration of the charge / discharge cycle can be prevented.
Moreover, since the component ratio of the LiF compound with respect to the total fluoride is 50% or less, there is no possibility that lithium flowing back and forth between the positive electrode and the negative electrode will decrease, and deterioration of the charge / discharge cycle can be prevented.
Furthermore, by using a silicone compound or the like as the SiF-forming compound, it becomes possible to preferentially form the SiF compound before the LiF compound is formed, and there is a risk that lithium traveling between the positive electrode and the negative electrode is reduced. Loss of charge and discharge cycle can be prevented.

Furthermore, since the multiphase alloy powder in which the amount of Si phase on the particle surface is smaller than the amount of Si phase inside the particle is used as the negative electrode active material, the decomposition reaction of lithium salt on the negative electrode active material surface is suppressed. Thus, the formation of the LiF compound is suppressed, and there is no possibility that the lithium flowing back and forth between the positive electrode and the negative electrode is reduced, and deterioration of the charge / discharge cycle can be prevented.
In addition, according to the above lithium secondary battery, the decomposition reaction of the lithium salt is also suppressed, so that the generation of decomposition gas inside the battery is suppressed, so that there is no gap between the positive electrode and the negative electrode, The heavy load characteristic can be improved by preventing the reaction area of each electrode from decreasing.

[Experimental Example 1]
A lithium secondary battery was manufactured and its cycle characteristics were evaluated.
Example 1
The battery was manufactured as follows. First, a positive electrode active material made of LiCoO _{2 having} an average particle size of 10 μm, a binder made of polyvinylidene fluoride, and a conductive additive made of carbon powder having an average particle size of 3 μm were mixed, and further N-methyl-2- Pyrrolidone was mixed to make a positive electrode slurry. The positive electrode slurry was applied onto a current collector made of an aluminum foil having a thickness of 20 μm by a doctor blade method, dried in a vacuum atmosphere at 120 ° C. for 24 hours to volatilize N-methyl-2-pyrrolidone, and then rolled. did. Thus, the positive electrode formed by laminating the composite material containing the positive electrode active material on the current collector was manufactured.

Moreover, the negative electrode active material which consists of multiphase alloy powder was manufactured in the following procedures. First, 60 parts by mass of massive Si having a size of about 5 mm square, 30 parts by mass of Ni powder, and 10 parts by mass of Ag powder are prepared, mixed, and then subjected to high-frequency heating in an argon atmosphere. It melted to obtain a molten alloy. 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, 30 g of the rapidly quenched alloy powder obtained was put into 500 ml of 5N sodium hydroxide aqueous solution and impregnated for 1 hour with slow stirring at room temperature. Then, after sufficiently washing with pure water so that no sodium remains, the particle size was adjusted to an average particle size of 12 μm. In this way, a negative electrode active material was produced. This negative electrode active material was observed for morphology by X-ray diffraction and an electron microscope.

70 parts by mass of the obtained negative electrode active material, 20 parts by mass of graphite powder having an average particle diameter of 3 μm, and 10 parts by mass of a binder made of polyvinylidene fluoride were mixed, and N-methyl-2-pyrrolidone was further added. It mixed and it was set as the negative electrode slurry. This negative electrode slurry was applied onto a current collector made of Cu foil having a thickness of 14 μm by a doctor blade method, dried in a vacuum atmosphere at 120 ° C. for 24 hours to volatilize N-methyl-2-pyrrolidone, and then rolled. did. In this way, a negative electrode was produced in which a composite material having a negative electrode active material and a density of 2.5 g / cm ³ was laminated on the current collector.

Further, LiPF ₆ was added to a mixed solvent obtained by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) at EC: DEC = 30: 70 so as to have a concentration of 1.3 mol / L. Furthermore, the silicone compound was added so that the concentration was 1% by mass. In this way, a non-aqueous electrolyte was prepared. The structural formula of the added silicone compound is that in the structural formula of [Chemical Formula 1] where k = 0, m = 4, n = 2, and R and Z are CH ₃ .

  The positive electrode is cut into a disk shape with a diameter of 14 mm, the negative electrode is cut into a disk shape with a diameter of 16 mm, a polypropylene porous separator is placed between the positive electrode and the negative electrode, and these are stored in a battery case. The battery case was sealed after injecting the above electrolytic solution to produce a coin-type lithium secondary battery of Example 1 having a diameter of 20 mm and a thickness of 1.6 mm.

(Example 2)
Example 2 is the same as Example 1 except that the mixing ratio of each raw material when producing the negative electrode active material is 50 parts by mass of Si, 40 parts by mass of Ni powder, and 10 parts by mass of Ag powder. A lithium secondary battery was manufactured.

(Example 3)
An example was prepared in the same manner as in Example 1 except that each raw material type and its blending ratio when producing the negative electrode active material were Si: 60 parts by mass, Ni powder: 30 parts by mass, and Cu powder: 10 parts by mass. 3 lithium secondary batteries were produced.

Example 4
A lithium secondary battery of Example 4 was manufactured in the same manner as in Example 1 except that 1% by mass of silicon nitride was added as an SiF forming compound instead of the silicone compound.

(Example 5)
A lithium secondary battery of Example 4 was produced in the same manner as in Example 1 except that 1% by mass of silicon dioxide was added to the electrolytic solution instead of the silicone compound as the SiF-forming compound.

(Comparative Example 1)
A lithium secondary battery of Comparative Example 1 was produced in the same manner as Example 1 except that no silicone compound was added.

(Comparative Example 2)
A lithium secondary battery of Comparative Example 2 was produced in the same manner as in Example 2 except that no silicone compound was added.

(Comparative Example 3)
A lithium secondary battery of Comparative Example 3 was produced in the same manner as Example 3 except that no silicone compound was added.

  The obtained lithium secondary battery was aged for 15 hours, charged at a constant current of 0.2C to 4.15V, and then charged at a constant voltage until the current value reached 0.01C. The initial charge / discharge was performed by performing a constant current discharge at 0.2C up to 2.75V.

Then, the lithium secondary battery after the initial charge / discharge is subjected to constant current / constant voltage charge in which constant current charge is performed until the current value becomes 0.01C after constant current charge to 4.15V at 1C (0.8 mA). Next, constant current discharge at 1 C (0.8 mA) to 2.75 V is defined as one cycle, and this charge / discharge cycle is performed up to 100 cycles, and the capacity retention rate after 100 cycles of the lithium secondary battery is investigated. did. The results are shown in Table 1.
In addition, the negative electrode after 100 cycles was taken out, subjected to time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS), and Li ⁺ ions and SiF ⁺ ions in the secondary ion mass spectrum. The intensity ratio (SiF ⁺ / Li ⁺ ) and the component ratio of the LiF compound to the total fluoride were analyzed. The results are shown in Table 1.

The time-of-flight secondary ion mass spectrometry was performed by decomposing the battery in an inert gas and taking out the negative electrode, further cutting the electrode to expose the cross section, and measuring this cross section within the range of 0 amu-1000 amu. . In addition, measurement was performed under the conditions of Ga ⁺ as primary ions, primary ion energy of 25 kV, pulse frequency of 8.3 kHz, and pulse width of 12 ns. The measurement vacuum was 4 × 10 ⁻⁷ Pa. For this time-of-flight secondary ion mass spectrometry, TFS-2000 manufactured by Physical Electronics was used.
The X-ray photoelectron spectroscopic analysis is performed by decomposing the battery in an inert gas, taking out the negative electrode, and setting the electrode in an X-ray photoelectron spectroscopic analyzer without touching the atmosphere. As a line, it measured on the conditions of output 10kV and 16.5mA. For X-ray photoelectron spectroscopy, ESCALAB220iXL manufactured by VG Scientific was used.

When X-ray diffraction was performed on the negative electrode active material, a structure in which a Si crystalline phase, a crystalline phase having a composition of NiSi _2, and an Ag crystalline phase were mixed was confirmed. Further, the Si content was reduced to 50% as a whole with respect to 60% of the raw material blending ratio.
Moreover, when the form observation was performed with the electron microscope about the negative electrode active material, it was confirmed that many micropores are formed in the surface. These micropores are thought to be formed by elution of the Si phase exposed on the surface. Furthermore, was subjected to elemental analysis by X-ray for the particle surface, the surface is occupied by NiSi ₂ phase, Si phase was hardly detected. This is because the Si phase on the surface was removed by the impregnation treatment with the alkaline solution. Therefore, the Si phase detected by X-ray diffraction is considered to exist inside the particles.

Moreover, as shown in Table 1, the battery of Example 1-3 to which the silicone compound was added had a strength ratio (SiF ⁺ / Li ⁺ ) of 2.0 × 10 ⁻³ or more and 1.0 × 10 ⁻¹ or less. It can be seen that the LiF component amount is in the range of 50% or less, and the capacity ratio and capacity retention ratio are excellent. On the other hand, in Comparative Example 1-3 in which no silicone compound was added, the strength ratio (SiF ⁺ / Li ⁺ ) was less than 2.0 × 10 ⁻³ and the LiF component amount exceeded 50%. It can be seen that the ratio and capacity retention rate are also low.
Thus, the capacity ratio is maintained by setting the strength ratio (SiF ⁺ / Li ⁺ ) in the range of 2.0 × 10 ^{−3 to} 1.0 × 10 ⁻¹ and the LiF component amount in the range of 50% or less. It has been found that the rate can be improved.

[Experiment 2]
(Examples 6 to 9 )
Lithium secondary batteries of Examples 6 to 9 were produced in the same manner as in Example 1 except that the addition amount of the silicone compound to the electrolytic solution was 3%, 7%, 15%, and 20%.
(Comparative Example 6)
A lithium secondary battery of Comparative Example 6 was produced in the same manner as in Example 1 except that the amount of silicone compound added to the electrolyte solution was 33%.
(Example 11)
An example was prepared in the same manner as in Example 1 except that each raw material type and its blending ratio when producing the negative electrode active material were Si: 48 parts by mass, Ni powder: 40 parts by mass, and Cu powder: 12 parts by mass. 11 lithium secondary batteries were produced.
(Example 12)
An example was prepared in the same manner as in Example 1 except that each raw material type and its blending ratio in producing the negative electrode active material were set to 45 parts by mass of Si, 35 parts by mass of Ni powder, and 20 parts by mass of Cu powder. Twelve lithium secondary batteries were produced.

(Comparative Example 4)
Each raw material type and its blending ratio when producing the negative electrode active material were Si: 70 parts by mass, Ni powder: 20 parts by mass, Cu powder: 10 parts by mass, except that no silicone compound was added. A lithium secondary battery of Comparative Example 4 was produced in the same manner as Example 1.
(Comparative Example 5)
Each raw material type and its blending ratio when producing the negative electrode active material were set to Si: 75 parts by mass, Ni powder: 20 parts by mass, Cu powder: 5 parts by mass, except that no silicone compound was added. A lithium secondary battery of Comparative Example 5 was produced in the same manner as Example 1.

  The obtained lithium secondary battery was aged for 15 hours, charged at a constant current of 0.2C to 4.15V, and then charged at a constant voltage until the current value reached 0.01C. The initial charge / discharge was performed by performing a constant current discharge at 0.2C up to 2.75V.

Then, the lithium secondary battery after the initial charge / discharge is subjected to constant current / constant voltage charge in which constant current charge is performed until the current value becomes 0.01C after constant current charge to 4.15V at 1C (0.8 mA). Next, constant current discharge at 1 C (0.8 mA) to 2.75 V is defined as one cycle, and this charge / discharge cycle is performed up to 100 cycles, and the discharge capacity and capacity maintenance after 100 cycles of the lithium secondary battery are performed. The rate was measured.
Moreover, the negative electrode after 100 cycles was taken out, time-of-flight secondary ion mass spectrometry (TOF-SIMS) was performed, and the intensity ratio (SiF ⁺ / Li ⁺ ) of Li ⁺ ions to SiF ⁺ ions in the secondary ion mass spectrum. Was analyzed. Furthermore, the extracted electrode was subjected to X-ray photoelectron spectroscopy (XPS) to analyze the component ratio of the LiF compound to the total fluoride.

Table 2 and FIG. 3 show the discharge capacity at the 100th cycle of the lithium secondary battery in which the intensity ratio (SiF ⁺ / Li ⁺ ) and the intensity ratio (SiF ⁺ / Li ⁺ ) are 1.8 × 10 ^−3. The relationship of the capacity ratio grinding of each lithium secondary battery in the case of the above is shown.
Table 3 and FIG. 4 show the relationship between the component amount of the LiF compound and the capacity retention rate.

As shown in Table 2 and FIG. 3, the discharge capacity of the lithium secondary battery is increased when the intensity ratio (SiF ⁺ / Li ⁺ ) is in the range of 2.0 × 10 ^{−3 to} 1.0 × 10 ^−1. I understand that.
Moreover, as shown in Table 3 and FIG. 4, it can be seen that the capacity retention ratio is high when the component ratio of the LiF compound to the total fluoride is 50% or less.

The schematic diagram which shows the negative electrode active material of the lithium secondary battery which is embodiment of this invention. The cross-sectional schematic diagram which shows the negative electrode active material of the lithium secondary battery which is embodiment of this invention. The intensity ratio (SiF ⁺ / Li ⁺ ) and the capacity ratio of each lithium secondary battery when the discharge capacity at the 100th cycle of the lithium secondary battery in which the intensity ratio is 1.8 × 10 ⁻³ is 100. A graph showing the relationship. The graph which shows the relationship between the amount of LiF components, and a capacity | capacitance maintenance factor.

Explanation of symbols

11 ... Particles of multiphase alloy powder, 12 ... Si phase, 13 ... SiM phase, 14 ... X phase, 15 ... Micropore

Claims (3)

A negative electrode including a negative electrode active material composed of a multiphase alloy powder containing Si, a positive electrode including a positive electrode active material, LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiSbF ₆ , LiN (C _x F _{2x + 1} SO ₂ ) (C _y F _{2y + 1} SO ₂ ) (where x and y are natural numbers) and at least lithium composed of a non-aqueous electrolyte solution In the secondary battery, at least a SiF-forming compound is provided in the battery, and the intensity ratio of Li ⁺ ions to SiF ⁺ ions (SiF ⁺ / Li in the secondary ion mass spectrum for the negative electrode in a charged state after 100 cycles of charge and discharge. ⁺ ) Is in the range of 2.0 × 10 ⁻³ or more and 1.0 × 10 ⁻¹ or less,
The SiF-forming compound is a silicone compound, silicon halide, silicon dioxide, silicon sulfide, silicon nitride, hexafluorosilicic acid, metasilicic acid, siloxane compound, silazane compound, or SiC ₆ H ₅ group, SiCH ₃ group, SiC ₂ H A lithium secondary battery comprising at least one compound among compounds containing at least one of _five groups.   2. The lithium secondary battery according to claim 1, wherein a component ratio of the LiF compound to the total fluoride is 50% or less in the negative electrode in a charged state after 100 cycles of charge and discharge.   The negative electrode active material is a material composed of Si and at least one element selected from Ni, Co, As, B, Cr, Cu, Fe, Mg, Mn, Y, Ag, and Au. The lithium secondary battery according to claim 1, wherein the lithium secondary battery is a lithium secondary battery.

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