JP2013161705A - Active material for secondary battery and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel active material for a secondary battery, excellent in initial charge/discharge efficiency, and a method for producing the active material for a secondary battery.SOLUTION: An active material for a secondary battery of the present invention contains a compound phase including a lithium metasilicate-based compound containing LiSiOas a basic composition and an Si-containing phase including Si fine particles, and the Si-containing phase is in a dispersion state. The active material for a secondary battery can be obtained through: a melting reaction step of allowing lithium hydroxide and a silicon-containing raw material at least including Si to react in an alkali metal nitrate-containing molten salt at 450°C or below; and a recovery step of recovering a reaction product from a mixture of the reaction product obtained in the melting reaction step and the molten salt.

Description

  The present invention relates to an active material for a secondary battery used for a lithium ion secondary battery and a method for producing the same.

  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. The performance of the lithium ion secondary battery depends on the materials of the positive electrode, the negative electrode, and the electrolyte constituting the secondary battery. In particular, research and development of active material materials contained in electrodes are being actively conducted. Currently, there is a carbon-based material such as graphite as a negative electrode active material that is generally used. Since the carbon negative electrode using graphite or the like as the negative electrode active material has an intercalation reaction, the cycle characteristics and output are good, but a significant increase in capacity cannot be expected in the future. On the other hand, with respect to lithium ion secondary batteries, there is an increasing demand for further capacity improvement as specifications and applications expand. Therefore, a negative electrode active material having a higher capacity than carbon, that is, a high energy density has been studied.

  Examples of the negative electrode active material capable of realizing a high energy density include silicon-based materials such as silicon and silicon oxide. Silicon has a large amount of occlusion and release of lithium ions per unit volume or unit mass, and exhibits a capacity 10 times or more that of carbon. However, although the silicon-based material has a large charge / discharge capacity, due to electrode destruction due to volume expansion during occlusion of lithium, silicon slid from the electrode due to silicon pulverization, and disconnection of conductive paths caused by them, etc. There is a problem that charge / discharge cycle characteristics are poor.

As measures for improving the charge / discharge cycle characteristics of silicon, it is known to use silicon oxide as a negative electrode active material. It is known that silicon oxide (SiO _x : x is about 0.5 ≦ x ≦ 1.5) decomposes into Si and SiO ₂ when heat-treated. This is called a disproportionation reaction, and if it is a homogeneous solid silicon monoxide SiO in which the ratio of Si to O is approximately 1: 1, it is separated into two phases of Si phase and SiO ₂ phase by solid internal reaction. To do. Of the two phases, the fine Si phase mainly stores and releases lithium. The SiO ₂ phase has a function of covering a plurality of minute Si phases and suppressing decomposition of the electrolytic solution. Therefore, the secondary battery using the negative electrode active material composed of SiO _x decomposed into the Si phase and the SiO ₂ phase has excellent cycle characteristics. On the other hand, since the conductivity of the oxide is low, lithium diffusion is insufficient. In addition, since the SiO ₂ phase electrochemically reacts with lithium to form silicate, irreversible capacity (capacity that cannot be taken out of the capacity charged during initial charging, that is, initial charge capacity and initial discharge capacity) Difference) and thus the initial charge / discharge efficiency is reduced.

  In order to improve the initial charge / discharge efficiency, for example, a method of compensating the irreversible capacity by electrochemically charging the irreversible capacity in advance or using a negative electrode attached with metallic lithium has been attempted. There is also a method of compensating for irreversible capacity by reacting a silicon-based material with lithium to synthesize a lithium-containing silicon-based material.

Patent Document 1 describes a lithium-containing silicon oxide powder in which lithium is fused with silicon oxide and partly crystallized. For example, in Example 1 of Patent Document 1, 0.08 mol of Li per mol of Si is used, and in Example 5, 0.16 mol of Li per mol of Si is a lithium-containing silicon oxide powder doped with silicon oxide. , Respectively. Although the X-ray diffraction measurement results are shown in FIG. 1 and FIG. 2, the presence of lithium silicate compounds such as Li ₂ SiO ₃ and Li ₄ SiO ₄ cannot be confirmed.

Patent Document 2 discloses a carbonaceous material, silicon oxide dispersed in the carbonaceous material, silicon dispersed in the silicon oxide, and lithium containing Li ₄ SiO ₄ as a main component contained in the silicon oxide. Composite particles having a silicate phase are disclosed.

  Both the lithium-containing silicon oxide powder described in Patent Document 1 and the composite particle described in Patent Document 2 are reacted from the surface of a raw material that remains solid by firing silicon oxide and a lithium salt as raw materials. It is synthesized by the solid phase method.

JP 2003-160328 A JP 2007-59213 A

In the solid phase method, the reaction proceeds in a state where powdery raw materials (for example, SiO powder and LiOH powder) are mixed. Silicates are sequentially formed from the surface of the SiO particles that are in contact with the LiOH powder to produce a lithium silicate compound. However, in the case of the solid phase method, unreacted SiO remains in a portion that does not come into contact with the LiOH powder, for example, in the inside of the SiO particles or a part of the surface of the SiO particles. The presence of unreacted SiO means that the doping amount of lithium is insufficient and the effect of reducing the irreversible capacity is small. In addition, when the surface layer of the SiO particles is sufficiently silicate but the inside is not silicate, there is unreacted SiO inside, and the Si phase responsible for charge / discharge is unevenly distributed inside the SiO ₂ phase. It is thought that. However, the lithium silicate compound in the surface layer formed by silicate formation may hinder the movement of electrolyte ions such as lithium ions, and may increase the irreversible capacity.

  An object of this invention is to provide the novel active material for secondary batteries excellent in initial stage charge / discharge efficiency, and its manufacturing method in view of said problem.

As a result of extensive research and trial and error, the present inventor has obtained a reaction product obtained by a molten salt method in which a SiO powder and a LiOH powder are reacted in a molten salt of LiNO _3. It has been found that it contains a ₂ SiO ₃ phase and Si fine particles and has a uniform structure in which Si fine particles involved in charge and discharge are dispersed. By developing this result, the present inventor has completed various inventions described below.

The active material for a secondary battery of the present invention includes a compound phase containing a lithium metasilicate compound having a basic composition of Li ₂ SiO ₃ and a Si-containing phase containing Si fine particles, and the Si fine particles are in a dispersed state. It is characterized by being.

  In addition, the “dispersed state”, in other words, is a state where the Si fine particles are present substantially uniformly in the active material for the secondary battery. For example, if the active material for a secondary battery is a powder, in addition to a state in which Si fine particles are uniformly mixed in the powder together with other particles, there is one Si fine particle in the matrix constituting one particle of the powder. The state which exists like this may be sufficient. When Si fine particles are unevenly distributed in the matrix constituting one particle, it can be regarded macroscopically that the Si fine particles are aggregated in the entire active material for a secondary battery. Is not in a distributed state.

  In addition, the term “basic composition” includes not only the composition represented by the composition formula but also those obtained by substituting some of the Li, Si, and O sites in the crystal structure with other elements. means. Further, it means that not only a stoichiometric composition but also a non-stoichiometric composition slightly deviating from the stoichiometric composition due to a deficiency in some elements.

  Since the active material for a secondary battery of the present invention is a silicon-based material containing lithium, the irreversible capacity inherent in the silicon-based material is supplemented. In the active material for a secondary battery of the present invention, Si fine particles are present in a dispersed manner without being unevenly distributed. Since the Si fine particles responsible for charge and discharge are in a dispersed state, the movement of electrolyte ions is facilitated, and as a result, an increase in irreversible capacity is suppressed.

Moreover, the method for producing an active material for a secondary battery of the present invention includes:
A melt reaction step in which a silicon-containing raw material containing at least Si and lithium hydroxide are reacted in a molten salt of 450 ° C. or lower containing an alkali metal nitrate;
A recovery step of recovering the reaction product from a mixture of the reaction product obtained in the melting reaction step and the molten salt;
It is characterized by including.

When only the silicon-containing raw material and lithium hydroxide are reacted, since lithium hydroxide is highly reactive, Si contained in the silicon-containing raw material is easily silicated to produce lithium orthosilicate (Li ₄ SiO ₄ ). Conceivable. If this reaction is carried out in a molten salt of alkali metal nitrate, it is presumed that the reactivity of lithium hydroxide is moderately suppressed. Therefore, all the Si contained in the silicon-containing raw material is not silicate, and Si can remain in the reaction product. Si positively contributes to charge and discharge. Then, a compound phase containing a lithium metasilicate compound having a basic composition of Li ₂ SiO ₃ is generated from a part of Si contained in the silicon-containing raw material. Further, in the molten salt, since the silicon-containing raw material and lithium hydroxide are finely dispersed, it is easy to form a uniform structure in which the whole raw material is sufficiently reacted. As a result, a lithium-containing silicon-based material containing a compound phase containing a lithium metasilicate compound having a basic composition of Li ₂ SiO ₃ and a Si-containing phase containing Si fine particles, in which the Si fine particles are in a dispersed state (ie, The secondary battery active material of the present invention is easily obtained.

  In the molten salt, the reaction between the silicon-containing raw material and the lithium hydroxide can proceed even at a low temperature. Therefore, it is advantageous in terms of energy compared to the solid phase method that requires the reaction at a high temperature.

  The active material for a secondary battery of the present invention is suitable as a negative electrode material for a secondary battery such as a lithium ion secondary battery.

  The active material for a secondary battery of the present invention is excellent in initial charge / discharge efficiency. Moreover, according to the manufacturing method of the active material for secondary batteries of this invention, the lithium containing silicon type material which shows the outstanding initial stage charge / discharge efficiency is obtained easily.

The result of the X-ray diffraction (XRD) measurement of the lithium containing silicon type material powder which is the active material for secondary batteries of this invention is shown. The result of having observed the cross section of the active material for secondary batteries of this invention using the scanning electron microscope (SEM) is shown. The XRD measurement result of the reaction product obtained when the synthesis | combination of the lithium containing silicon-type material powder by the conventional solid-phase method is performed at low temperature is shown. The XRD measurement result of the lithium containing silicon type material powder synthesize | combined by the conventional solid-phase method is shown. The result of having observed the lithium containing silicon system material powder synthesize | combined by the conventional solid-phase method using SEM is shown. It is a mimetic diagram showing the section of one particle of lithium content silicon system material powder. The XRD measurement result of the lithium containing silicon system material powder which is the active material for secondary batteries of this invention synthesize | combined by changing the mixture ratio of the lithium hydroxide with respect to a silicon containing raw material is shown. The XRD measurement result of the lithium containing silicon type material powder which is the active material for secondary batteries of this invention synthesize | combined on high temperature conditions is shown. The XRD measurement result of the reaction product obtained by making a silicon containing raw material and lithium hydroxide react under high temperature conditions in the molten salt of alkali metal nitrate is shown. The XRD measurement result of the reaction product obtained by making a silicon containing raw material react in the molten salt of alkali metal nitrate is shown.

  Below, the form for implementing the active material for secondary batteries of this invention and its manufacturing method is demonstrated. Unless otherwise specified, the numerical range “ab” described herein includes the lower limit “a” and the upper limit “b”. The numerical range can be configured by arbitrarily combining these upper limit value and lower limit value and the numerical values listed in the examples.

<Active materials for secondary batteries>
The active material for secondary battery of the present invention (sometimes abbreviated as “active material of the present invention”) contains a compound phase containing a lithium metasilicate compound and a Si-containing phase containing Si fine particles. The compound phase includes a lithium metasilicate compound having a basic composition of Li ₂ SiO ₃ . In the Si-containing phase, Si fine particles are in a dispersed state.

The compound phase includes a lithium metasilicate compound having a basic composition of Li ₂ SiO ₃ . The presence of a lithium metasilicate compound having a basic composition of Li ₂ SiO ₃ can be confirmed by XRD measurement. The compound phase is an inactive phase that does not contribute to the occlusion and release of electrolyte ions in the secondary battery, but has a function of relaxing the volume change of the active material accompanying charge / discharge. Therefore, the secondary battery using the active material of the present invention is excellent in cycle characteristics. In addition, lithium metasilicate compounds do not easily cause an electrochemical reaction that causes irreversible capacity with lithium or the like.

Further, compound phase is preferably for example not contain any other lithium silicate-based compounds such as the Li ₄ SiO ₄ orthosilicate lithium compound and basic composition essentially. Lithium metasilicate compounds can be said to be suitable as lithium silicate compounds contained in the compound phase because they are less prone to hydrolysis in the atmosphere and easier to handle than lithium orthosilicate compounds.

  In the lithium metasilicate compound, a part of Li may be substituted with another alkali metal element. The other alkali metal element is one or more of Na, K, Rb, and Cs. When Li contained in the lithium metasilicate compound is 100 atomic%, 10 atomic% or less, and 0.01 to 5 atomic% of Li may be substituted with another alkali metal element. In the lithium metasilicate compound, part of Si may be substituted with another metal element. Examples of other metal elements include P, Ge, Sn, Al, Mg, and Ca, and one or more of these are preferable. When Si contained in the lithium metasilicate compound is 100 atomic%, 10 atomic% or less, further 0.01 to 50 atomic% of Si may be substituted with another metal element.

  The Si-containing phase includes Si fine particles in a dispersed state. The Si fine particles are preferably present dispersed in the compound phase, and may be dispersed as a single crystal or may be dispersed as a polycrystal composed of two or more microcrystals. The presence of Si fine particles can also be confirmed by XRD measurement.

  The Si-containing phase has a crystal grain size of 20 nm or less calculated from Scherrer's equation from the half-value width of the diffraction peak of (111) plane (2θ exists at a position of 28 ° to 29 °) obtained by powder X-ray diffraction method. Further, it is preferably 1 to 10 nm, more preferably 1 to 7 nm. If the particle size of the crystals contained in the Si-containing phase is within this range, the stress caused by volume expansion and contraction when electrolyte ions such as lithium ions are occluded and released is relieved, and the particles are pulverized and accompanied by the associated fine particles. Electrode deterioration is suppressed.

  Moreover, it is preferable that the active material of this invention contains the composite particle comprised by a compound phase and the said Si containing phase. The composite particles are preferably formed by dispersing Si fine particles in the compound phase. Although the size of the composite particles is not particularly limited, the active material of the present invention preferably contains a composite particle powder having an average particle size of 0.5 to 50 μm, more preferably 1 to 5 μm. If the average particle diameter is in this range, it is preferable because the surface area involved in the occlusion and release of electrolyte ions can be sufficiently obtained. The average particle diameter is, for example, the maximum diameter of a plurality of particles measured by scanning microscope (SEM) observation and measured from an SEM image (the maximum value of the interval between parallel lines when the particles are sandwiched between two parallel lines). Average value.

The active material of the present invention preferably contains a compound phase and a Si-containing phase and substantially does not contain silicon oxide. For example, since SiO ₂ has low conductivity and binds to lithium ions to cause irreversible capacity, it is preferably not included in the active material of the present invention. Further, when the compound phase and the Si-containing phase constitute the composite particle, it is preferable that the compound phase is not unevenly distributed in the composite particle. “Unevenly distributed in the composite particles” corresponds to the case where the particles exist only in a part of the particles, for example, the case where the particles exist so as to cover the surface of the particles. According to the production method described in detail later, since the composite particles having a uniform structure in which the entire raw material is uniformly reacted can be produced, the compound phase can exist uniformly in the composite particles.

<Anode for secondary battery>
Although the electrode of a secondary battery is comprised using the active material of this invention, the active material of this invention is suitable as a negative electrode active material of a secondary battery. The negative electrode includes a current collector and a negative electrode active material layer that includes the active material of the present invention and is bound on the current collector. The negative electrode active material layer is prepared by adding a conductive additive and an appropriate amount of an organic solvent to a negative electrode active material and a binder resin, and mixing them into a slurry. A roll coating method, a dip coating method, a doctor blade method, It can be produced by applying onto a current collector by a method such as spray coating or curtain coating and curing the binder resin.

  The current collector can adopt a shape such as a metal mesh, foil, or plate, but is not particularly limited as long as it has a shape according to the purpose. For example, a copper foil or an aluminum foil can be suitably used as the current collector.

  Note that the negative electrode active material layer includes the active material of the present invention as an essential component, and may include other negative electrode active materials. Other negative electrode active materials are not particularly limited as long as they are simple substances or compounds that can occlude and release electrolyte ions. For example, lithium, elements that can be alloyed with lithium, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Ti, Ag, Zn, Cd, Al, Ga, In, A negative electrode material containing at least one of Ge, Sn, Pb, Sb, and Bi can be given. Specific examples include tin compounds such as Cu—Sn alloys and Co—Sn alloys, and carbon-based materials such as various graphites. One or more of these can be used together with the negative electrode active material.

  The conductive additive is added to increase the conductivity of the electrode. What is necessary is just to use the material generally used with the electrode of a secondary battery as a conductive support material. For example, it is preferable to use conductive carbon materials such as carbon black (carbonaceous fine particles) such as graphite, acetylene black and ketjen black, and carbon fiber. In addition to the conductive carbon material, conductive organic compounds, Ni flakes, etc. For example, a known conductive aid may be used. One of these may be used alone or in combination of two or more. The amount of the conductive aid used is not particularly limited, but can be, for example, about 20 to 100 parts by mass with respect to 100 parts by mass of the active material. When the amount of the conductive additive is less than 20 parts by mass, an efficient conductive path cannot be formed, and when it exceeds 100 parts by mass, the moldability of the electrode is deteriorated and the energy density is lowered. When silicon oxide particles having a coating layer made of a carbon material are used, the amount of conductive additive added can be reduced or not added.

  The binder resin is used as a binder for binding the active material and the conductive additive to the current collector. The binder resin is required to bind the active material and the like in as little amount as possible, and the amount is 0.5 to 50 mass when the negative electrode active material, the conductive additive and the binder resin are 100 mass% in total. % Is desirable. When the amount of the binder resin is less than 0.5% by mass, the moldability of the electrode is deteriorated. In addition, as binder resin, fluoropolymers such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), rubbers such as styrene butadiene rubber (SBR), imide polymers such as polyimide, polyamideimide, alkoxylsilyl groups Examples thereof include resins containing polyacrylic acid, polymethacrylic acid, polyitaconic acid, alginic acid, and gelatin. A copolymer of acrylic acid and an acid monomer such as methacrylic acid, itaconic acid, fumaric acid or maleic acid can also be used. Among them, a resin containing a carboxyl group such as polyacrylic acid is particularly desirable, and a resin having a higher carboxyl group content is more preferable.

<Method for producing active material for secondary battery>
Next, a method for producing a negative electrode active material capable of easily synthesizing the active material of the present invention (production method of the present invention) will be described. The production method of the present invention mainly includes a melting reaction step and a recovery step. Each process will be described below.

  The melting reaction step is a step in which a silicon-containing raw material containing at least Si and lithium hydroxide are reacted in a molten salt containing an alkali metal nitrate.

Lithium hydroxide becomes a Li source constituting the active material of the present invention. As lithium hydroxide, anhydrous lithium hydroxide (LiOH) and / or lithium hydroxide monohydrate (LiOH.H ₂ O) can be used.

The silicon-containing raw material becomes a Si source constituting the active material of the present invention. The silicon-containing raw material may be any of a simple substance, a compound, and an alloy as long as it contains Si. Specific examples include simple silicon, silicon monoxide (SiO _x : x is 0.9 ≦ x ≦ 1.9), silicon dioxide (SiO _y : y is 2), and the like. The silicon-containing raw material may include one or more of these, but silicon monoxide (SiO _x ) is particularly preferable. SiO _x can be used regardless of whether or not the above-mentioned disproportionation treatment is performed, but it is also possible to use a material that has been separated into two phases of SiO ₂ phase and Si phase by disproportionation treatment.

In silicon monoxide (SiO _x ), which is a homogeneous solid with an atomic ratio of Si and O of approximately 1: 1, SiO _x is disproportionated into two phases of SiO ₂ phase and Si phase due to the reaction inside the solid. . The silicon oxide powder obtained by disproportionation includes a SiO ₂ phase and a Si phase. Disproportionation proceeds by applying energy to SiO _x . Examples of the energy application means include application of thermal energy, application of mechanical energy by milling or the like.

When applying heat energy, it is preferable to heat SiO _x . In general, it is said that almost all silicon monoxide is disproportionated and separated into two phases at 800 ° C. or higher when oxygen is turned off. Specifically, a raw material powder containing amorphous silicon monoxide powder is subjected to heat treatment at 800 to 1200 ° C. for 1 to 5 hours in an inert atmosphere such as in a vacuum or an inert gas. A silicon oxide powder containing two phases of an amorphous SiO ₂ phase and a crystalline Si phase is obtained.

  In addition, when silicon monoxide is milled, the mechanical energy of milling contributes to chemical atomic diffusion at the solid phase interface of silicon monoxide, generating an oxide phase and a silicon phase. Milling may be performed in an inert gas atmosphere such as vacuum or argon gas. A V-type mixer, ball mill, attritor, jet mill, vibration mill, high energy ball mill, or the like can be used. Disproportionation of silicon monoxide may be further promoted by heating during or after milling of silicon monoxide.

  The silicon-containing raw material is preferably in powder form. Since the silicon-containing raw material is dissolved and finely dispersed in the molten salt, the particle size is not particularly limited, but a powder having an average particle size of 0.5 to 50 μm may be used. Alternatively, a silicon-containing raw material classified to 1 μm or less may be used.

  The lithium hydroxide and the silicon-containing raw material are preferably prepared so that Li contained in the lithium hydroxide and Si contained in the silicon-containing raw material have a molar ratio of about 1: 1. However, in the production method of the present invention, an active material having a desired composition can be obtained even if the proportion of lithium hydroxide is increased. Specifically, the lithium (Li / Si) contained in the lithium hydroxide with respect to Si contained in the silicon-containing raw material has a molar ratio of 0.3 to 10, and more preferably 0.4 to 5. A hydroxide and silicon-containing raw material may be prepared. If Li / Si is 0.3 or more, the effect of reducing the irreversible capacity by lithium doping is easily obtained. However, if the amount of lithium hydroxide used is excessive, the reactivity may be too high, and only a reaction product in which Si fine particles involved in charge / discharge do not remain sufficiently may be obtained. Therefore, it is desirable that Li / Si is 10 or less. However, even if Li / Si exceeds 10, it is possible to suppress the reaction by increasing the amount of alkali metal nitrate used.

  In the case where it is desired to add other elements of Li and Si to the active material to be synthesized, an additive element-containing material containing an additive element and a silicon-containing raw material and lithium hydroxide may be used. Examples of the additive element-containing material include alkali metal hydroxides, organic alkali salts, alkoxide salts, and metal nitrides. Alternatively, a silicon-containing raw material containing an additive element together with silicon may be used. One or more of these may be used depending on the composition of the target active material. Such a raw material is obtained as a precipitate by making an aqueous solution containing at least two metal elements containing Si alkaline.

  The alkali metal nitrate serves as a solvent for dispersing the silicon-containing raw material and lithium hydroxide in the synthesis by the molten salt method. The alkali metal nitrate is preferably a nitrate having a melting point lower than that of lithium hydroxide, and one or more of lithium nitrate, sodium nitrate, potassium nitrate, rubidium nitrate, and cesium nitrate may be used. Particularly desirable is lithium nitrate (melting point: 261 ° C.), which has the lowest melting point among alkali metal nitrates. When the lithium nitrate molten salt is mixed in the molten state, the melting point of the mixed molten salt is lowered to about 250 ° C. Therefore, the active material can be synthesized at a low temperature of 300 ° C. or lower.

  The molten salt serves to disperse the silicon-containing raw material and lithium hydroxide and to mix the lithium hydroxide in a molten state. The amount of the alkali metal nitrate is not particularly limited as long as the silicon-containing raw material and the lithium hydroxide can be dispersed in the molten salt. However, if the amount of alkali metal nitrate is too small, the reactivity may be too high and only a reaction product with few Si fine particles involved in charge / discharge may be obtained. Therefore, the ratio of alkali metal nitrate to lithium hydroxide (alkali metal nitrate / lithium hydroxide) is preferably 5 or more, 5 to 50, or 5 to 20 in terms of molar ratio. When the alkali metal nitrate / lithium hydroxide molar ratio exceeds 50, the amount of silicon-containing raw material and lithium hydroxide with respect to the amount of molten salt to be used is decreased, which is not desirable in terms of production efficiency.

  The reaction temperature in the melting reaction step corresponds to the temperature of the molten salt, and may be higher than the temperature at which the molten salt can exist in the molten state. Basically, the reaction may be performed at a temperature higher than the temperature at which it can exist in a molten state, and the higher the reaction temperature, the more efficiently the desired active material can be generated. However, if the reaction temperature is too high, the reactivity is too high and the Si fine particles involved in charge / discharge may be reduced. Moreover, when reaction temperature is too high, it will become easy to decompose | disassemble alkali metal nitrate. For this reason, the reaction temperature is preferably not lower than the temperature at which the molten salt can exist in a molten state and not higher than 450 ° C., 250 to 400 ° C., and more preferably 260 to 300 ° C. By reacting at the above reaction temperature for 60 minutes or longer, desirably 3 to 8 hours, an active material having a desired composition and structure can be obtained.

  In addition, the melt reaction step may be performed in the air, but may be performed in an inert atmosphere such as in argon gas.

  Prior to the melt reaction step, a drying step of drying at least a part of the silicon-containing raw material, lithium hydroxide, and alkali metal nitrate may be performed. The drying process is mainly intended to dehydrate lithium hydroxide monohydrate, but even when anhydrous lithium hydroxide is used, when using a highly hygroscopic compound as another raw material. Is valid. The water present in the molten salt containing lithium hydroxide in the melting reaction step has a very high pH. When the melt reaction step is performed in the presence of water having a high pH, the water may come into contact with the crucible, and depending on the type of the crucible, the amount of the crucible component may be eluted into the molten salt although the amount is small. In the drying process, moisture is removed from the raw material, which leads to suppression of elution of the crucible components. Further, by removing moisture from the raw material in the drying step, it is possible to prevent water from boiling in the melting reaction step and the molten salt from being scattered. If a vacuum dryer is used for a drying process, it is good to vacuum-dry at 80-150 degreeC for 2 to 24 hours.

  In the melt reaction step, it is presumed that the reaction represented by the following formulas (1) and (2) or formulas (3) and (3) 'proceeds mainly.

Formula (1) represents a disproportionation reaction of silicon monoxide in which x is 1. The disproportionation reaction is preliminarily applied to the silicon-containing raw material prior to the melting reaction step, or it is considered that a reaction equivalent to the disproportionation proceeds when silicon monoxide is finely dispersed in the molten salt. It is done. In either case, in the molten salt, as represented by the formula (2), it is presumed that SiO ₂ and LiOH preferentially react to produce Li ₂ SiO ₃ . Therefore, it is presumed that the Si-containing phase containing the Si fine particles generated by the disproportionation reaction remains without reacting with LiOH due to the reaction suppressing effect of the molten salt containing the alkali metal nitrate.

Formula (3) represents the reaction between elemental silicon and lithium hydroxide. Even if the silicon-containing raw material is simple silicon, it is presumed that the Si-containing phase remains due to the reaction suppression effect of the molten salt containing the alkali metal nitrate. However, unlike the reaction of SiO in which oxygen is excessively present as represented by the formula (2), Li ₄ SiO ₄ is generated in the reaction of Si and LiOH. And since a reaction product is what was produced | generated after the single-piece | unit silicon particle once melted in molten salt, it is guessed that Si microparticles | fine-particles are contained in a dispersed state as Si containing phase. In the molten salt method, washing of the reaction product is performed when recovering the reaction product. Therefore, Li ₄ SiO ₄ is hydrolyzed as represented by the formula (3) ′, and the recovered product contains Si and Li ₂ SiO ₃ .

By the way, Formula (4) represents the case where SiO (Si + SiO ₂ ) and LiOH are reacted at a high temperature by a solid phase method without using a molten salt containing an alkali metal nitrate. As already described, SiO that comes into contact with highly reactive LiOH generates Li ₄ SiO ₄ by the reaction. In the SiO that has not contacted LiOH, the Si phase and the SiO ₂ phase remain. However, when Equation (2) is compared with Equation (4), there is a difference in Si consumption before and after the reaction. In formula (4), it can be seen that Si that can remain as Si fine particles is reduced by half. Therefore, in the reaction product obtained by the solid phase method, Si fine particles involved in charge / discharge are reduced. Si fine particles are unevenly distributed in the unreacted portion.

  The recovery step is a step of recovering the reaction product from the mixture of the reaction product and molten salt obtained in the melt reaction step. The recovery step may include a cooling step and a separation step described below.

  The cooling step is a step of cooling the molten salt after the melting reaction step. In the cooling step, the high-temperature molten salt after completion of the reaction may be left in a heating furnace to be cooled, or may be taken out of the heating furnace and air-cooled at room temperature. Since the molten salt is solidified by cooling, a mixture of the synthesized lithium-containing silicon-based material (reaction product) and the molten salt is obtained as a solid after the cooling step.

  The separation step is a step of dissolving the molten salt solidified in the cooling step in a polar protic solvent and separating the reaction product (lithium-containing silicon material) from the molten salt. A polar protic solvent is used in this step because it can dissolve a solidified molten salt (that is, a molten salt raw material such as lithium nitrate). However, since it is a solvent having a proton donating property, lithium-containing silicon is used. Li deficiency tends to occur in the system material. However, a protic solvent has an effect of stabilizing ions as compared with an aprotic solvent, and is suitable for dissolving lithium nitrate, which is a molten salt, and thus is suitable for this step. Specific examples include pure water such as ion-exchanged water, alcohols such as ethanol, and mixed solvents containing these. The solidified molten salt is easily dissolved in the polar protic solvent, and the lithium-containing silicon-based material that is difficult to dissolve in the polar protic solvent remains undissolved in the solution. Therefore, the molten salt and the reaction product are easily separated. The method for collecting the reaction product is not particularly limited, but the solution can be collected by centrifugation or filtration. You may dry the reaction product after collection | recovery.

The separation step is desirably a step of separating the reaction product from the molten salt by dissolving the molten salt solidified after the melting reaction step in a solvent containing at least water. By using a solvent containing water, Li ₄ SiO ₄ contained in the reaction product is hydrolyzed (Li ₄ SiO ₄ + H ₂ O → Li ₂ MnO ₃ + 2LiOH) to become Li ₂ MnO ₃ . Therefore, unstable Li ₄ SiO ₄ produced by reacting single silicon with lithium hydroxide becomes stable Li ₂ MnO ₃ . LiOH, a by-product of hydrolysis, is removed along with water.

<Secondary battery>
The secondary battery of the present invention having the negative electrode containing the active material described above includes a known positive electrode, an electrolyte, and a separator that are not particularly limited, together with the negative electrode. The positive electrode only needs to be usable in a secondary battery such as a lithium ion secondary battery. The positive electrode includes a current collector and a positive electrode active material layer that includes the positive electrode active material and is bound on the current collector. The positive electrode active material layer includes a positive electrode active material and a binder resin, and may further include a conductive additive. The positive electrode active material, the conductive additive and the binder resin are not particularly limited as long as they can be used in the secondary battery.

As the positive electrode active material, LiCoO ₂ , LiNi _p Co _q Mn _r O ₂ (0 <p <1, 0 + p <q <1-p, 0+ (p + q) <r <1- (p + q)), Li ₂ MnO ₂ , Li ₂ MnO ₃ , LiNi _s Mn _t O ₂ (0 <s <1, 0 + s <t <1-s), LiFePO ₄ , Li ₂ FeSO ₄ , lithium-containing metal oxide having a basic composition of Li, or Examples thereof include a solid solution material containing one or more kinds. Further, as a positive electrode active material, an active material not containing an element such as lithium that becomes an electrolyte ion in charge and discharge, for example, sulfur-modified compounds in which S is introduced into an organic compound such as simple sulfur (S), polyacrylonitrile (PAN), or TiS. Metal sulfides such as ₂ , polyaniline, conjugated diacetate-based organic substances, and the like can also be used.

  The current collector can adopt a shape such as a metal mesh, foil, or plate, but is not particularly limited as long as it has a shape according to the purpose. The current collector is not particularly limited as long as it is generally used for the positive electrode of the secondary battery, such as aluminum, nickel, and stainless steel. As the binder resin and the conductive additive, the same materials as those described above for the negative electrode can be used.

  In addition, what is necessary is just to set the mixing | blending ratio of a positive electrode active material, a binder, and a conductive support material according to the conventional secondary battery.

  The positive electrode active material layer is usually prepared by adding a conductive additive and an appropriate amount of an organic solvent to the positive electrode active material and a binder resin, and mixing them into a slurry, which is then rolled, dip coated, doctor blade It can be produced by coating on a current collector by a method such as a spray coating method or a curtain coating method, and curing the binder resin. In addition, as long as it is a sheet-like sulfur-modified compound, the positive electrode active material layer may be directly pressure-bonded to the current collector.

As the electrolyte, an electrolytic solution in which a lithium metal salt that is an electrolyte is dissolved in an organic solvent may be used. The electrolytic solution is not particularly limited. As the organic solvent, an aprotic organic solvent such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) or the like is used. Can do. As the electrolytes _dissolved, it can be used _{LiPF 6, LiBF 4, LiAsF 6} , LiI, NaPF 6, NaBF 4, NaAsF 6, LiBOB, soluble alkali metal salt in an organic solvent and the like. The concentration of the electrolyte may be about 0.5 to 1.7 mol / L.

  The separator is disposed between the positive electrode and the negative electrode, allows ions to move between the positive electrode and the negative electrode, and prevents an internal short circuit between the positive electrode and the negative electrode. If the secondary battery is a sealed type, the separator is also required to have a function of holding the electrolyte. As the separator, it is preferable to use a thin, microporous or non-woven membrane made of polyethylene, polypropylene, PAN, aramid, polyimide, cellulose, glass or the like.

  The shape of the secondary battery is not particularly limited, and various shapes such as a cylindrical shape, a stacked shape, and a coin shape can be employed. 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 an 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 active material for secondary batteries of this invention and its manufacturing method 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 present invention will be described in detail by way of examples of the active material for secondary battery and the method for producing the same according to the present invention.

<Example 1>
Silicon monoxide (SiO powder having an average particle size of 50 μm: Sigma-Aldrich Japan Co., Ltd.), lithium hydroxide monohydrate (LiOH.H ₂ O: High Purity Chemical Research Laboratory Co., Ltd.) and lithium nitrate (LiNO ₃ : High Purity Chemical Research Laboratory Co., Ltd.). The SiO powder was not subjected to treatment such as heat treatment.

SiO powder (2 g), LiOH.H ₂ O (2 g) and LiNO ₃ (15 g) were weighed. At this time, the ratio (Li / Si) of Li contained in LiOH.H ₂ O to Si contained in the SiO powder was 1 in molar ratio. These powders were put into a crucible, transferred to an electric furnace at 270 ° C., and heated at 270 ° C. for 5 hours. The inside of the electric furnace was an argon atmosphere. At this time, the raw material in the crucible melted into a molten salt, and the reaction product was precipitated.

  Next, the crucible containing the molten salt was taken out of the electric furnace and cooled at room temperature. After the molten salt was sufficiently cooled and solidified, ethanol was added to dissolve the solidified molten salt. Since the reaction product is insoluble in ethanol, a suspension was obtained. The suspension was filtered and dried to obtain a powdery reaction product.

  Elemental analysis using inductively coupled plasma (ICP) was performed on the obtained powder, and as a result, it was found that 8.7% by mass of Li was contained with respect to 100% by mass of the reaction product.

Further, X-ray diffraction (XRD) measurement using CuKα rays was performed on the obtained powder. The results are shown in FIG. According to the XRD measurement, a diffraction peak derived from crystalline Li ₂ SiO ₃ (position indicated by ▽ in FIG. 1) and a halo derived from Si fine particles (position indicated by ◆ in FIG. 1) were observed. The Si fine particles had a crystal grain size of 4 nm calculated from Scherrer's equation from the half-value width of the diffraction peak on the (111) plane. Other diffraction patterns were not observed, suggesting that the Si fine particles were complexed with crystalline Li ₂ SiO ₃ . That is, it was found that the obtained powder was a lithium-containing silicon-based material powder containing composite particles of Si fine particles and Li ₂ SiO ₃ phase.

  Moreover, the particle | grain cross section of the obtained powder was exposed using the cross section polisher method, and the cross section was observed with the scanning electron microscope (SEM). The results are shown in FIG. The SEM image shown in FIG. 2 is a reflected electron image. If the particles contain Li, Si, and O, the portion containing a large amount of lithium is observed darker than the other portions. However, in the SEM image shown in FIG. 2, since the cross section of one particle is uniform with no shading, it can be said that the composition of one particle is almost uniform from the surface to the center. Further, uneven distribution of Si fine particles was not observed. From the SEM image, the average particle size of the obtained powder was calculated to be 2.5 μm.

In view of the above results, SiO powder having an average particle diameter of 50 μm was used as the silicon-containing raw material, so that SiO was dissolved in the molten salt to form particles having a size of about 2.5 μm and finely dispersed. , And disproportionated into SiO ₂ phase and Si fine particles dispersed in the SiO ₂ phase. And it can be said that the SiO ₂ phase contained in the disproportionated fine SiO particles and LiOH sufficiently react from the surface to the center to form composite particles as shown in the lower left of FIG. .

  In addition, from the SEM image, the presence of vacancies inside the particles was confirmed. The presence of pores can also be expected to reduce the volume expansion and contraction of Si that occurs due to the difference between Li occlusion and release.

<Comparative Example 1>
The above-mentioned SiO powder (2 g) and LiOH.H ₂ O (2 g) were pulverized and mixed using a ball mill. The obtained pulverized mixture was baked at 300 ° C. for 3 hours to obtain a powdered baked product.

XRD measurement using CuKα rays was performed on the obtained powder. The results are shown in FIG. According to the XRD measurement, a diffraction peak derived from crystalline LiOH (position indicated by x in FIG. 3) and a halo derived from Si fine particles (position indicated by ◆ in FIG. 3) were observed. The reaction did not proceed under the conditions of this comparative example. That is, it was found that the reaction between SiO and LiOH under a low temperature condition of about 300 ° C. does not proceed unless the molten salt method using LiNO ₃ molten salt is used.

<Comparative example 2>
The same pulverized mixture as in Comparative Example 1 was baked at 500 ° C. for 3 hours to obtain a powdered baked product.

XRD measurement using CuKα rays was performed on the obtained powder. The results are shown in FIG. According to the XRD measurement, a diffraction peak derived from crystalline Li ₄ SiO ₄ (position indicated by ▪ in FIG. 4) and a halo derived from Si fine particles (position indicated by ◆ in FIG. 1) were observed. Other diffraction patterns were not observed, suggesting that the Si fine particles were complexed with crystalline Li ₄ SiO ₄ . That is, it was found that the obtained powder was a lithium-containing silicon-based material powder containing composite particles of Si microcrystal and Li ₄ SiO ₄ phase.

Moreover, the particle | grain cross section of the obtained powder was exposed using the cross section polisher method, and the cross section was observed with the scanning electron microscope (SEM). The results are shown in FIG. The SEM image shown in FIG. 5 is a reflected electron image. The contrast of the cross section of one particle is dark at the surface layer and thin at the center. That is, it is the surface layer of the particles that contain Li ₄ SiO ₄ .

<About the structure of one particle of lithium-containing silicon-based material powder>
Differences between the lithium-containing silicon-based material powder of Example 1 and the lithium-containing silicon-based material powder of Comparative Example 2 estimated from the above results will be described with reference to FIG. FIG. 6 is a schematic view showing a cross section of one particle of the lithium-containing silicon-based material powder.

Example 1 is the synthesis of a lithium-containing silicon-based material powder by a molten salt method using LiNO ₃ molten salt. When the SiO particles are finely dispersed in the molten salt, a reaction equivalent to disproportionation proceeds, and it is considered that the SiO particles are separated into Si fine particles and SiO ₂ phase. In the molten salt, it is presumed that SiO _{2 of} SiO particles finely dispersed in the molten salt on the order of several hundred nm reacts preferentially with LiOH to produce Li ₂ SiO ₃ . At this time, since the SiO particles in the molten salt are fine, they react uniformly from the surface to the center. On the other hand, it is presumed that Si fine particles remain without reacting with LiOH due to the reaction suppressing effect of LiNO ₃ molten salt.

Comparative Example 2 is the synthesis of a lithium-containing silicon-based material powder by a solid phase method. It is considered that the SiO particles are disproportionated into an Si phase and an SiO ₂ phase by heating during firing. In the solid phase method, the reaction proceeds from the surface of the solid SiO particles. Therefore, the particle size of the SiO particles hardly changes during the reaction, and Li ₄ SiO ₄ is generated on the surface layer of the SiO particles in contact with LiOH. In the case of SiO particles on the order of several microns, it does not react up to the center of the particles that are not in contact with LiOH. Therefore, the Si phase is unevenly distributed inside the particles without being dispersed.

<Evaluation of initial charge / discharge efficiency>
A lithium secondary battery using the lithium-containing silicon-based material powder obtained in Example 1 and the lithium-containing silicon-based material powder obtained in Comparative Example 2 and the SiO powder used as a raw material as negative electrode active materials, respectively, It was produced by the procedure.

  42 parts by mass of any of the negative electrode active materials described above, 40 parts by mass of graphite, 3 parts by mass of acetylene black, 15 parts by mass of polyamideimide, and kneaded together with an appropriate amount of solvent (N-methylpyrrolidone) A slurry was obtained. Next, this slurry was applied onto a copper foil having a thickness of 20 μm using a bar coater. After drying at 80 ° C., roll pressing was performed and punched out to obtain an electrode having a diameter of 11 mm and a thickness of 40 μm.

A separator (Hoechst Celanese glass filter and celgard 2400) was sandwiched between the produced electrode (evaluation electrode) and a metal lithium foil (counter electrode: φ11 mm, thickness 0.5 μm) to form an electrode body battery. This electrode body battery was accommodated in a battery case (CR2032 coin cell manufactured by Hosen Co., Ltd.). Further, a non-aqueous electrolyte in which LiPF ₆ is dissolved at a concentration of 1.0 mol / L is injected into a mixed solvent in which ethylene carbonate and ethyl methyl carbonate are mixed at 1: 1 (volume ratio) into the battery case, and lithium A secondary battery (coin battery) was obtained.

  The prepared three types of coin batteries were held in a thermostatic bath at 25 ° C. for 1 hour, and then a charge / discharge test was performed. The test was performed at a constant current of 0.1 mA with the discharge end voltage being 0.01 V at the Li counter electrode and the charge end voltage being 1.0 V at the Li counter electrode. The initial charge / discharge efficiency was calculated as a ratio of the initial charge capacity to the initial discharge capacity (that is, (initial charge capacity) / (initial discharge capacity) × 100%). The results are shown in Table 1.

  The initial charge / discharge efficiency of the negative electrode active material of Example 1 was 68%. This was higher than when SiO powder, which is a raw material before reacting with lithium, was used as the negative electrode active material. That is, the negative electrode active material of Example 1 was able to reduce irreversible capacity by reacting lithium with a silicon-based material.

On the other hand, the initial charge / discharge efficiency of the negative electrode active material of Comparative Example 2 was 50%, and the irreversible capacity was large. This is because not only the conductivity of Li ₄ SiO _{4 on} the surface layer of the active material particles is low but also the Si fine particles contributing to charge and discharge are unevenly distributed in the center of the active material particles, so Diffusion became insufficient, irreversible capacity increased, and initial charge / discharge efficiency decreased.

<Example 2>
A powdery reaction product was obtained in the same manner as in Example 1 except that the amount of LiOH · H ₂ O was changed to 1 g. In this example, the ratio (Li / Si) of Li contained in LiOH.H ₂ O to Si contained in the SiO powder was 0.5.

<Example 3>
A powdery reaction product was obtained in the same manner as in Example 1 except that 4 g of LiOH.H ₂ O and 20 g of LiNO ₃ were used. In this example, the ratio (Li / Si) of Li contained in LiOH.H ₂ O to Si contained in the SiO powder was 2.

<Example 4>
A powdery reaction product was obtained in the same manner as in Example 1 except that 6 g of LiOH.H ₂ O and 25 g of LiNO ₃ were used. In this example, the ratio (Li / Si) of Li contained in LiOH.H ₂ O to Si contained in the SiO powder was 3 in molar ratio.

<Example 5>
A powdery reaction product was obtained in the same manner as in Example 1 except that 8 g of LiOH.H ₂ O and 30 g of LiNO ₃ were used. In this example, the ratio (Li / Si) of Li contained in LiOH.H ₂ O to Si contained in the SiO powder was 4 in molar ratio.

<X-ray diffraction measurement>
XRD measurement using CuKα rays was performed on each of the powders obtained in Examples 2 to 5. The results are shown in FIG. According to the XRD measurement, a diffraction peak derived from crystalline Li ₂ SiO ₃ (position indicated by ▽ in FIG. 7) and a halo derived from Si fine particles (position indicated by ◆ in FIG. 7) were observed in all powders. . The Si fine particles had a crystal grain size of 4 to 6 nm calculated from Scherrer's equation from the half width of the diffraction peak on the (111) plane. Other diffraction patterns were not observed, suggesting that the Si fine particles were complexed with crystalline Li ₂ SiO ₃ . That is, it was found that the obtained powder was a lithium-containing silicon-based material powder. Since almost the same diffraction pattern was obtained from the powder of any of the examples, it was found that the composition and structure of the obtained lithium-containing silicon-based material powder did not vary greatly depending on the value of Li / Si.

<Example 6>
A powdery reaction product was obtained in the same manner as in Example 1 except that the temperature of the electric furnace was set to 400 ° C. and the raw materials were reacted at 400 ° C. for 5 hours. In this example, the ratio (Li / Si) of Li contained in LiOH.H ₂ O to Si contained in the SiO powder was 1 in molar ratio.

XRD measurement using CuKα rays was performed on the powder obtained in Example 6. The results are shown in FIG. According to the XRD measurement, a diffraction peak derived from crystalline Li ₂ SiO ₃ (position indicated by ▽ in FIG. 8) and a halo derived from Si fine particles (position indicated by ◆ in FIG. 8) were observed. The Si fine particles had a crystal grain size of 5 nm calculated from Scherrer's equation from the half-value width of the diffraction peak on the (111) plane. Other diffraction patterns were not observed, suggesting that the Si fine particles were complexed with crystalline Li ₂ SiO ₃ .

Example 1 and Example 6 have different reaction temperatures. Comparing the XRD pattern of the powder obtained in Example 1 (FIG. 1) and the XRD pattern of the powder obtained in Example 6 (FIG. 8), in Example 6, the diffraction peak derived from Li ₂ SiO ₃ was observed. The ratio of the intensity of the halo derived from the Si fine particles to the intensity became smaller. The reaction at 400 ° C. as in Example 6 is more reactive than the reaction at 270 ° C. as in Example 1, and the Si fine particles are also liable to react with LiOH to generate Li ₂ SiO _3. Guessed.

  Moreover, although not shown in figure, the particle | grain cross section was exposed by the method similar to the above, and the cross section was observed by SEM. As a result, the cross section of one particle was uniform with no shading. That is, it can be said that the composition of one particle is uniform from the surface to the center. From the SEM image, the average particle size of the obtained powder was calculated to be 3 μm.

<Comparative Example 3>
A powdery reaction product was obtained in the same manner as in Example 5 except that the temperature of the electric furnace was set to 500 ° C and the raw materials were reacted at 500 ° C for 5 hours. In this example, the ratio (Li / Si) of Li contained in LiOH.H ₂ O to Si contained in the SiO powder was 4 in molar ratio.

XRD measurement using CuKα rays was performed on the powder obtained in Comparative Example 3. The results are shown in FIG. According to the XRD measurement, a diffraction peak derived from crystalline Li ₂ SiO ₃ was observed. Other diffraction patterns were not observed, and it was found that no Si fine particles were present.

Example 5 and Comparative Example 3 have different reaction temperatures. From the XRD pattern of the powder obtained in Comparative Example 3 (FIG. 9), the halo of Si fine particles disappeared because the reaction under high temperature conditions at 500 ° C. was highly reactive, and the Si fine particles reacted with LiOH. It is estimated that Li ₂ SiO ₃ was produced.

<Comparative example 4>
Silicon monoxide (SiO powder having an average particle diameter of 50 μm: Sigma-Aldrich Japan Co., Ltd.) and lithium nitrate (LiNO ₃ : High-Purity Chemical Laboratory Co., Ltd.) were prepared. The SiO powder was not subjected to treatment such as heat treatment.

SiO powder (2 g) and LiNO ₃ (20 g) were weighed. These powders were put into a crucible, transferred to an electric furnace at 270 ° C., and heated at 270 ° C. for 5 hours. The inside of the electric furnace was an argon atmosphere. At this time, the raw material in the crucible melted into a molten salt, and the reaction product was precipitated.

  Next, the crucible containing the molten salt was taken out of the electric furnace and cooled at room temperature. After the molten salt was sufficiently cooled and solidified, ethanol was added to dissolve the solidified molten salt. Since the reaction product is insoluble in ethanol, a suspension was obtained. The suspension was filtered and dried to obtain a powdery reaction product.

XRD measurement using CuKα rays was performed on the powder obtained in Comparative Example 4. The results are shown in FIG. According to the XRD measurement, Si fine particles contained in SiO used as a raw material and halo derived from amorphous SiO ₂ were observed. Other diffraction patterns were not observed.

Each Example and Comparative Example 4 differ depending on whether or not LiOH is used. In each example, LiOH having the highest basicity among lithium salts was involved in the reaction, and LiNO ₃ was considered to have simply played a role as a solvent for dispersing SiO and LiOH.

<Calculation of lithium doping amount>
As described above, the lithium-containing silicon-based material powder obtained in Example 1 contained 8.7% by mass of Li with respect to 100% by mass of the powder. This is an actual measurement value of the lithium doping amount, and a theoretical value is calculated as follows.

  The reaction of Example 1 in which the molar ratio of Li to Si contained in silicon oxide and lithium hydroxide used as raw materials was 1 (Li / Si = 1) is expressed as follows.

[Si + SiO ₂ ] + 2LiOH → Si + Li ₂ SiO ₃ (+ H ₂ O)
That is, the lithium-containing silicon-based material powder obtained in Example 1 is based on the overall composition: Li ₂ Si ₂ O ₃ (6.941 × 2 + 28.09 × 2 + 16.00 × 3 = 1118.062 g / mol). 6.941 × 2 = 13.882 g / mol of lithium is contained. Therefore, the lithium doping amount is calculated as 13.882 / 118.062 × 100 = 11.758%. Moreover, it turns out that simple substance Si before reaction remains after reaction from reaction formula.

  The reaction of Example 2 with Li / Si = 0.5 is expressed as follows.

2 [Si + SiO ₂ ] + 2LiOH → Si + Li ₂ SiO ₃ + [Si + SiO ₂ ] (+ H ₂ O)
That is, the lithium-containing silicon-based material powder obtained in Example 2 is based on the overall composition: Li ₂ Si ₄ O ₅ (6.941 × 2 + 28.09 × 4 + 16.00 × 5 = 206.242 g / mol). 6.941 × 2 = 13.882 g / mol of lithium is contained. Therefore, the lithium doping amount is calculated as 6.731%. Moreover, it turns out that simple substance Si before reaction remains after reaction from reaction formula.

  The reaction of Example 3 with Li / Si = 2 is expressed as follows.

2 [Si + SiO ₂ ] + 8LiOH → 2Si + 2Li ₂ SiO ₃ + 4LiOH (+ 2H ₂ O + H ₂ )
2Si + 2Li ₂ SiO ₃ + 4LiOH → Si + 2Li ₂ SiO ₃ + Li ₄ SiO ₄ (+ 2H ₂ )
[Si + 2Li ₂ SiO ₃ + Li ₄ SiO ₄ ] + H ₂ O → Si + 3Li ₂ SiO ₃ (+ 2LiOH)
That is, the lithium-containing silicon-based material powder obtained in Example 3 is based on the overall composition: Li ₆ Si ₄ O ₉ (6.941 × 6 + 28.09 × 4 + 16.00 × 9 = 298.006 g / mol). 6.941 × 6 = 41.646 g / mol of lithium is contained. Therefore, the lithium doping amount is calculated as 13.975%. Further, from the reaction formula, it can be seen that half of the single Si before the reaction remains after the reaction.

  The reaction of Example 4 with Li / Si = 3 is expressed as follows.

Si + SiO ₂ + 6LiOH → Si + Li ₂ SiO ₃ + 4LiOH (+ H ₂ O)
Si + Li ₂ SiO ₃ + 4LiOH → Li ₂ SiO ₃ + Li ₄ SiO ₄ + (2H ₂ )
Li ₂ SiO ₃ + Li ₄ SiO ₄ + H ₂ O → 2Li ₂ SiO ₃ (+ 2LiOH)
That is, the lithium-containing silicon-based material powder obtained in Example 4 is based on the total composition: Li ₄ Si ₂ O ₆ (6.941 × 4 + 28.09 × 2 + 16.00 × 6 = 179.944 g / mol). 6.941 × 4 = 27.764 g / mol of lithium is contained. Therefore, the lithium doping amount is calculated as 15.429%. Under this condition, it can be seen from the reaction formula that all of the Si before the reaction has changed to Li ₂ SiO ₃ .

  The doping amount (mass%) of lithium is about 30% less than the theoretical value, but the amount of lithium doping increases by increasing the amount of lithium hydroxide used. From the theoretical value, when the molar ratio of Li to Si contained in the raw material powder becomes 3 or more, all Si changes to a lithium metasilicate compound. However, the presence of Si fine particles was confirmed in the XRD patterns of the lithium-containing silicon-based material powders obtained in Examples 4 and 5. This is considered that the reaction between Si and LiOH was suppressed by the influence of other reaction conditions such as the amount of lithium nitrate used and the reaction temperature. Considering theoretical values, if the lithium doping amount is 1 to 15% by mass, further 5 to 13% by mass, and further 7 to 12% by mass, it can be said that the lithium-containing silicon-based material powder contains a sufficient amount of Si fine particles. .

Claims (16)

An active for a secondary battery comprising a compound phase containing a lithium metasilicate compound having a basic composition of Li ₂ SiO ₃ and a Si-containing phase containing Si fine particles, wherein the Si fine particles are in a dispersed state. material.   The active material for a secondary battery according to claim 1, comprising composite particles in which the Si fine particles are dispersed in the compound phase.   The secondary particle according to claim 1 or 2, wherein the Si-containing phase has a crystal grain size of 20 nm or less calculated from Scherrer's equation from the half-value width of the diffraction peak of the (111) plane obtained by powder X-ray diffraction. Battery active material.   4. The active material for a secondary battery according to claim 2, comprising a powder of the composite particles having an average particle size of 0.5 μm to 50 μm. 5. The lithium orthosilicate compound comprising the compound phase and the Si-containing phase and having a silicon oxide and / or Li ₄ SiO ₄ as a basic composition is substantially not contained. Secondary battery active material.   The active material for a secondary battery according to claim 2, comprising composite particles containing Li from the surface to the center.   The active material for a secondary battery according to claim 2, wherein the compound phase is not unevenly distributed in the composite particles.   The active material for a secondary battery according to any one of claims 1 to 7, comprising 1 to 15% by mass of Li when the total of the compound phase and the Si-containing phase is 100% by mass.   A secondary battery comprising: a positive electrode; a negative electrode including the active material for a secondary battery according to claim 1; and a nonaqueous electrolyte.   A vehicle comprising the secondary battery according to claim 9. A melt reaction step in which a silicon-containing raw material containing at least Si and lithium hydroxide are reacted in a molten salt of 450 ° C. or lower containing an alkali metal nitrate;
A recovery step of recovering the reaction product from a mixture of the reaction product obtained in the melting reaction step and the molten salt;
The manufacturing method of the active material for secondary batteries characterized by including.   The method for producing an active material for a secondary battery according to claim 11, wherein the molten salt includes a molten salt of lithium nitrate.   The method for producing an active material for a secondary battery according to claim 11 or 12, wherein a ratio of the alkali metal nitrate to the lithium hydroxide (alkali metal nitrate / lithium hydroxide) is 5 or more in molar ratio.   The ratio (Li / Si) of Li contained in the lithium hydroxide to Si contained in the silicon-containing raw material is 0.3 to 10 in terms of molar ratio. A method for producing an active material for a secondary battery.   15. The recovery step includes a separation and recovery step of dissolving the molten salt solidified after the melting reaction step in a solvent containing at least water and separating the reaction product from the molten salt. The manufacturing method of the active material for secondary batteries as described in 2 ..   The active material for secondary batteries obtained by the manufacturing method in any one of Claims 11-15.