JP5369708B2 - Anode material for secondary battery and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode material for a secondary battery which is an Si-SiO<SB>2</SB>-magnesium silicate-carbon based composite material, and superior in cycle characteristics and charge-discharge efficiency. <P>SOLUTION: In the negative electrode material for the secondary battery containing crystalline Si, amorphous SiO<SB>2</SB>, a crystalline Mg<SB>2</SB>SiO<SB>4</SB>, and a carbonaceous material, it is preferable that atomic ratio of Mg atom and Si atom in the negative electrode material is from 1:2 to 1:40. The negative material is manufactured through a mixing process 1 of silicon oxide SiO<SB>X</SB>(0&lt;X&lt;2), and magnesium compound, a process 2 of making Mg-Si based compound containing a magnesium silicate phase by heating treatment of a mixture 1, a process 3 of making Mg-Si based composite powder by pulverization treatment of the Mg-Si based compound, a process 4 of mixing the Mg-Si based composite and carbon precursor, a process 5 of obtaining curing treated body by heating treatment of carbon precursor mixture obtained in the process 4, a process 6 of obtaining curing treated powder by pulverization treatment of the curing treated body, and a process 7 of obtaining active material by carbonization treatment of the curing treated body powder. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a negative electrode material for a lithium ion secondary battery using a non-aqueous electrolyte and a method for producing the same.

  With higher functionality of portable devices, high-capacity secondary batteries represented by lithium ion secondary batteries are required to have higher energy density than ever before. Conventionally, a graphite-based material has been generally used as a negative electrode material of a lithium ion secondary battery (hereinafter simply referred to as a secondary battery), but the capacity of the graphite negative electrode approaches its theoretical capacity (372 mAh / g). Therefore, development of a new high capacity negative electrode material is demanded. Note that in this specification, the negative electrode material and the active material are used interchangeably.

  In recent years, attempts have been made to use metals such as Si and Sn that form alloys with Li as negative electrode materials. Although these metal negative electrodes have a very large theoretical capacity of several to 10 times that of graphite, the volume change during storage (alloying) and release (dealloying) of Li ions is large. There is a problem that a crack is generated inside the active material and the capacity is drastically reduced in several tens of cycles.

In order to overcome this problem, a nano-sized Si—SiO ₂ composite material obtained when a lower oxide of Si such as silicon monoxide (SiO) is heated in a non-oxidizing atmosphere (disproportionation reaction) is a negative electrode material. It is attracting attention as. However, since this composite material has poor electrical conductivity, composite (Si—SiO ₂ —C) with carbon has been attempted for the purpose of imparting electrical conductivity and improving cycle characteristics (Patent Documents 1 and 2).

In addition, in the Si—SiO ₂ composite material, when Li changes to Li ₂ O or Li silicate during the charging process, Li desorption becomes very difficult during discharge, and the balance between the charge amount of Li and the discharge amount (charge) A problem that the discharge efficiency is reduced has also been pointed out. Therefore, for the purpose of improving the charge / discharge efficiency, Li is introduced in advance into the Si—SiO ₂ composite material, and the composite structure oxide composed of three phases of Si—SiO ₂ —Li ₄ SiO ₄ and carbon are combined into a negative electrode A material (Si—SiO ₂ —Li ₄ SiO ₄ —C) has been proposed (Patent Document 3). However, even the proposed composite materials are not always sufficient in terms of improving cycle characteristics and charge / discharge efficiency.

JP 2004-47404 A JP 2004-119176 A JP 2007-59213 A

An object of the present invention is to provide a negative electrode material for a secondary battery which is a Si—SiO ₂ -magnesium silicate-carbon composite material and has excellent cycle characteristics and charge / discharge efficiency.

The present invention provides a negative electrode material for a secondary battery comprising crystalline Si, amorphous SiO ₂ , crystalline Mg ₂ SiO _4, and a carbonaceous material.

In another embodiment of the present invention, silicon oxide SiO _X (0 <X <2) and a magnesium compound are mixed so that the atomic ratio of Mg atoms to Si atoms is 1: 2 to 1:40. Step 1 for obtaining a Mg—Si based composite containing a magnesium silicate phase by heat-treating the mixture 1 at a holding temperature of 800 ° C. to 1500 ° C. in an inert atmosphere, and the Mg—Si based composite Obtained by pulverizing the product to obtain Mg-Si based composite powder, Step 4 to mix the Mg-Si based composite powder and the carbon precursor to obtain a carbon precursor mixture, and Step 4 Step 7 including heat treatment of the carbon precursor mixture at a holding temperature of 100 to 500 ° C. to obtain a cured product, and step 6 to pulverize the cured product to obtain a cured product powder. As said hardened body powder as an inert atmosphere Lower, to provide a method of manufacturing a negative electrode material for a secondary battery, characterized in that by carbonizing the cured powder is heat treated at a holding temperature 700 ° C. to 1300 ° C. and the active material.

The negative electrode material for a secondary battery of the present invention (hereinafter referred to as the present negative electrode material) includes crystalline Si, amorphous SiO ₂ , crystalline Mg ₂ SiO _4, and a carbonaceous material, and these phases. In the state in which the constituent elements of each other are diffused and the phase interface is bonded, that is, each phase is in a bonded state at the atomic level, so the volume change at the time of insertion and extraction of Li ions is small, Cracks are less likely to occur in the active material. Therefore, even if the number of cycles is large, the capacity is hardly reduced, and the cycle characteristics are excellent because there is no sudden capacity decrease at a small number of cycles as in the conventional one.

  This negative electrode material is also characterized in that each phase is in a bonded state at the atomic level, so that the desorption of Li ions during discharge is smooth, the balance between the charge amount and the discharge amount of Li ions is good, and the charge / discharge efficiency is high. The charge / discharge efficiency is the ratio of the amount of electricity that can be discharged to the amount of electricity required for charging, and indicates the proportion of Li ions that can be taken out during discharge out of Li ions taken into the negative electrode active material during charging. Naturally, the higher the charge / discharge efficiency, the better.

In the method for producing a negative electrode material for a secondary battery according to the present invention, crystalline Mg ₂ SiO ₄ is produced by reacting silicon oxide with a magnesium compound without using forsterite powder, and pulverizing this to produce powder. Therefore, it is excellent in homogeneity without being unevenly distributed as a large lump.

  In addition, in the conventional method of carbonizing the carbon precursor and then pulverizing it into an active material, microcracks remain in the active material particles due to the pulverization after carbonization, resulting in fine powder during use as an electrode material. However, the present invention is hardened by a temperature treatment up to 500 ° C. before the final carbonization treatment, and then pulverized and then carbonized, so that it enters the active material. There is an advantage that the remaining of microcracks can be prevented.

  In particular, when the carbon precursor mixture obtained in step 4 is in a powder state, the cured body needs to be pulverized by slowing the heating rate of the curing process and holding it at a predetermined temperature for a long time. Since it can be pulverized as easily as possible, the prevention of deterioration due to microcracks becomes remarkable, and the durability is remarkably improved.

That is, crystalline Si, amorphous SiO ₂ , crystalline Mg ₂ SiO _4, and carbonaceous material are mixed with a very high homogeneity, so it is simply compared with a mixture of each constituent material. Thus, an electrode material with stable characteristics and high performance can be manufactured.

The transmission electron micrograph of the Mg-Si type composite obtained by this invention. It can be seen that three phases of an amorphous SiO ₂ phase matrix, crystalline Si of 10 nm or less, and large crystalline Mg ₂ SiO ₄ of several tens of nm or more are bonded at an atomic level. The transmission electron micrograph of the active material which combined the Mg-Si type complex and amorphous carbon obtained by this invention. It can be seen that the outer surface of the Mg—Si based composite having a three-phase structure is covered with amorphous carbon. 3 is an X-ray diffraction pattern of a composite structure oxide composed of a three-phase structure of crystalline Si, magnesium silicate, and amorphous SiO ₂ according to the present invention. Crystalline Si and magnesium silicate and the X-ray diffraction pattern of the active material complexed with an amorphous carbon composite structure oxide of three-phase structure of an amorphous SiO ₂ according to the present invention. Composite structure oxide X-ray diffraction pattern of before and after the heat treatment of 1000 ° C. -2 h _SiO X to that used for the synthesis (X = 1.0) of the present invention. It can be seen that heating at 1000 ° C. for 2 hours causes a disproportionation reaction between crystalline Si and amorphous SiO ₂ to cause decomposition. The X-ray diffraction pattern of the carbon material obtained by carbonizing the resol type phenol resin used for the one part Example of this invention at 800 degreeC. It can be seen that the obtained carbon material is amorphous carbon.

The present negative electrode material includes crystalline Si, amorphous SiO ₂ , crystalline Mg ₂ SiO _4, and a carbonaceous material. Here, crystalline Si means that which shows a crystal peak of Si by X-ray diffraction, and amorphous SiO ₂ is peculiar to amorphous called halo at a diffraction angle of 2Θ = 22 ° in X-ray diffraction. What shows a diffraction pattern is said, and a carbonaceous material means what shows the presence of C by chemical analysis.

In this negative electrode material, crystalline Si, amorphous SiO ₂ , crystalline Mg ₂ SiO _4, and carbonaceous material are in a state in which the constituent elements of these phases diffuse to each other and the phase interface is bonded That is, it is preferable that each phase is in a bonded state at the atomic level.

The negative electrode material may contain crystalline Si, amorphous SiO ₂ , crystalline Mg ₂ SiO _4, and elements and compounds other than the carbonaceous material, but for high performance, crystalline Si And amorphous SiO ₂ , crystalline Mg ₂ SiO _4, and a carbonaceous material. Here, “substantially constituted” means that it does not contain any inevitable impurities other than crystalline Si, amorphous SiO ₂ , crystalline Mg ₂ SiO _4, and carbonaceous material. Meaning.

  Hereinafter, the configuration of the negative electrode material will be described in detail. In the present negative electrode material, the crystalline Si is preferably dispersed in the negative electrode material in nm size. More preferably, the crystallite size of the crystalline Si is preferably 100 nm or less, and more preferably 2 to 30 nm. The crystallite size of crystalline Si is particularly preferably 3 to 10 nm. In the present specification, the crystallite size of crystalline Si is calculated from the Scherrer equation for the diffraction peak from the (220) plane.

In the present negative electrode material, crystalline Mg ₂ SiO ₄ is magnesium silicate having a mineral phase of forsterite and showing a Pbnm-type crystal structure. The crystalline Mg ₂ SiO ₄ is preferably uniformly dispersed in the negative electrode material. The crystallite size of the crystalline Mg ₂ SiO ₄ is preferably 100 nm or less, more preferably 5 to 50 nm, and particularly preferably the crystallite size of the crystalline MgSiO ₄ is 10 to 30 nm. In this specification, the crystallite size of crystalline MgSiO ₄ is calculated from the Scherrer equation for the diffraction peak from the (031) plane.

In this negative electrode material, Mg atoms: Si atoms = 1: 2 to 1:40 are preferable. It is more preferable that Mg atoms: Si atoms in the negative electrode material = 1: 4 to 1:20. It is particularly preferable that Mg atom: Si atom = 1: 6 to 1:10 in the negative electrode material. When the atomic ratio of Mg and Si is smaller than 1:40, the amount of Mg ₂ SiO ₄ type magnesium silicate phase formed is small, and the effect of improving the cycle characteristics during the charge / discharge test is small. On the other hand, when the atomic ratio of Mg and Si is larger than 1: 2 (the amount of Mg added is large), although the charge / discharge cycle characteristics are excellent, the initial charge / discharge capacity is decreased, which is not preferable. The reason why the initial charge / discharge capacity is reduced when the atomic ratio of Mg and Si is larger than 1: 2 is that the SiO atom contained in SiO _X reacts with the originally added Si atom and the Mg atom. This is probably because an excessive amount of Mg ₂ SiO ₄ that does not easily react with Li atoms is formed.

  In the present negative electrode material, the carbonaceous material can be either crystalline or amorphous. The content of the carbonaceous material is preferably 5 to 75% by mass (hereinafter, mass% is simply abbreviated as%), and the content of the carbonaceous material is more preferably 20 to 50%. If the amount of the carbon material is too small, sufficient conductivity during charging / discharging cannot be secured, the initial capacity becomes small, and the capacity deterioration after the cycle elapses becomes large. If the carbon material is too much, the charge / discharge cycle characteristics are improved, but the charge capacity per mass of the carbon material itself is about one third to one fifth of the capacity of the composite structure oxide material. The charge / discharge capacity of the whole negative electrode material becomes small, which is not preferable.

In the present negative electrode material, it is preferable that the total amount of crystalline Si, amorphous SiO ₂ and crystalline Mg ₂ SiO ₄ is 25 to 95% because a high charge / discharge capacity and cycle characteristics can be balanced. In the present negative electrode material, the total amount is more preferably 80 to 50%.

In the present negative electrode material, the specific surface area is preferably 1 to 400 (m ² / g). The specific surface area of the present negative electrode material is more preferably ² to 100 (cm ² / g). The specific surface area of the present negative electrode material is particularly preferably 5 to 250 (m ² / g). If the specific surface area of the negative electrode material is excessively large, the electrolyte tends to decompose on the negative electrode surface during the first charge, which is not preferable. Further, if the specific surface area is excessively small, the internal resistance of the battery tends to increase, which is not preferable. In the present specification, the value of the specific surface area is a value measured by the BET method.

  In the present negative electrode material, an average particle diameter (hereinafter, the particle diameter is abbreviated as a particle size) of 10 to 100 μm is preferable because a high electrode layer density is obtained when the negative electrode material is applied to a current collector plate. The average particle size of the negative electrode material is more preferably 10 to 50 μm. In this specification, the value of average particle diameter shall mean the value measured with the laser diffraction type particle size distribution measuring device.

  This negative electrode material is characterized in that it contains magnesium silicate. However, since the magnesium silicate phase is difficult to react with Li ions, the amount of expansion / contraction of the electrodes when Li ions are occluded is reduced. Thus, it is presumed that the cycle characteristics are improved.

  Next, a method for producing the present negative electrode material (hereinafter referred to as the present production method) will be described. This manufacturing method is a method of performing the following steps 1 to 7 in order. This will be described in the order of each process.

First, in step 1, silicon oxide SiO _X (0 <X <2) and a magnesium compound are mixed so that the atomic ratio of Mg atoms to Si atoms is 1: 2 to 1:40 to obtain mixture 1. It is a process.

As described above, the silicon oxide of step 1 has an amorphous structure or a nano-sized structure when a wide-angle X-ray diffraction analysis is performed, among which the composition formula can be expressed as SiO _X (0 <X <2). Those having a unique halo diffraction pattern and not having a sharp diffraction pattern are preferred. This is because high capacity and cycle stability can be obtained by using silicon oxide having such a composition as a raw material. A silicon oxide material exhibiting such characteristics is more preferably represented by SiO _X (0.8 <X <1.2), and particularly preferably SiO _X (0.9 <X <1.1). .

  Further, the ratio of Si atom to Mg atom in the mixture 1 is more preferably Mg atom: Si atom = 1: 4 to 1:20, and Mg atom: Si atom = 1: 5 to 1:15. Further preferred. It is preferable that the average particle diameter of silicon oxide is 0.5 to 10 μm because the distribution of the magnesium silicate phase obtained in step 2 becomes uniform. The average particle size of silicon oxide is more preferably 0.5 to 5 μm.

  As the magnesium compound in step 1, a compound containing divalent magnesium is preferable. Specific examples of such compounds include inorganic magnesium salts such as magnesium oxide, magnesium hydroxide, magnesium carbonate, magnesium nitrate, and magnesium chloride. Specific examples of the magnesium compound include organic magnesium compounds such as magnesium acetate, magnesium oxalate, magnesium ethoxide, and acetylacetone magnesium.

  Among these magnesium compounds, magnesium nitrate, magnesium acetate, etc., which are soluble in water or organic solvents, are mixed with silicon oxide in a solution state and further desolvated to obtain a uniform mixed state of silicon oxide and magnesium compound. Therefore, it is preferable.

When the magnesium compound is mixed with silicon oxide in the form of powder, it is preferable that the particle size be approximately the same as the particle size of the silicon oxide powder because the homogeneity of the resulting mixture is increased. When the average particle diameter of the magnesium compound powder is 0.5 to 10 μm, it is preferable because the homogeneity in the mixture is increased, and it is more preferable that the average particle diameter of the magnesium compound powder is 0.5 to 5 μm.
As a mixing means, a generally used mixing method such as a ball mill, a V-type mixer, a Henschel mixer, or the like is appropriately employed.

Next, Step 2 is performed by heating the mixture 1 obtained in Step 1 at 800 to 1500 ° C. under an inert atmosphere to obtain an Mg—Si based composite containing a magnesium silicate phase. In this heat treatment, a disproportionation reaction of SiO _X → Si + SiO ₂ and a formation reaction of magnesium silicate of 2MgO + SiO ₂ → Mg ₂ SiO ₄ occur.

  There exists a possibility that a silicon oxide and a magnesium compound may not react that heat processing temperature is less than 800 degreeC. On the other hand, when the heat treatment temperature exceeds 1500 ° C., the crystallite diameter of crystalline Si generated by the disproportionation reaction of silicon oxide becomes coarse, and the cycle characteristics may be deteriorated when an electrode material is used.

Since the heat treatment temperature in step 2 is 700 to 1500 ° C., some of the Si atoms and Mg atoms in the silicon oxide react with each other to effectively form Mg ₂ SiO ₄ that is a magnesium silicate. By the disproportionation reaction from silicon oxide, crystalline Si and amorphous SiO ₂ can be effectively formed. The heat treatment temperature in step 2 is preferably 750 to 1100 ° C. from the viewpoint of forming the magnesium silicate. The heat treatment temperature in step 2 is more preferably 800 to 1000 ° C.

In Step 2, when SiO _X and the magnesium compound are mixed and then heat-treated at a high temperature of 800 ° C. or higher, the SiO _X and the magnesium compound react to form magnesium silicate (Mg ₂ SiO ₄ having a Pbnm type crystal structure). As a result, an Mg—Si based composite composed of metallic Si and amorphous SiO ₂ is obtained. In the structure of the Mg-Si composite, the constituent elements of the raw materials are mutually diffused during the heat treatment, so that crystalline Si is homogeneously dispersed in a matrix mainly composed of amorphous SiO ₂ and crystalline Mg ₂ SiO _4. Organization. Step 2 is one of the steps characteristic of the present invention.

  Furthermore, in step 2, the holding time at the heat treatment temperature is preferably 1 to 20 hours. This is because when the holding time is as short as less than 1 hour, the above reaction may not proceed sufficiently. On the other hand, when the holding time exceeds 20 hours, the crystalline Si crystallite diameter becomes coarse. This is because when the electrode material is used, the cycle characteristics may deteriorate.

  Next to step 2, step 3 is a step of pulverizing the Mg-Si composite to form an Mg-Si composite powder. This is because the Mg—Si based composite obtained by the heat treatment in step 2 becomes a kind of sintered state and the powder becomes coarse, so that the particle size is adjusted to obtain an Mg—Si based composite powder. As the Mg—Si based composite powder, an average particle size of 3 to 300 μm is preferable because the required time is short, labor is not required, and handling property in the subsequent process is also good. As such particle size adjusting means, general particle size adjusting means for ceramic powder such as a ball mill, a vibration mill, and a jet mill can be appropriately employed. The average particle size of the Mg—Si based composite powder is more preferably 5 to 200 μm, and the average particle size of the Mg—Si based composite powder is particularly preferably 10 to 100 μm. In addition, as the pulverization, it is easy to adopt a dry method. On the other hand, when a wet method is adopted, there is an advantage that the homogeneity in the mixed powder becomes high although an extra labor of drying is required.

  The next step 4 is a step of mixing the Mg—Si based composite powder and a carbon precursor to obtain a carbon precursor mixture. Here, the carbon precursor to be mixed with the Mg—Si based composite powder is not particularly limited as long as it becomes carbon by heat treatment. Examples of the carbon precursor include thermosetting resins such as phenol resin, furan resin, epoxy resin, and xylene resin.

  Alternatively, a thermoplastic resin may be used. Preferred examples of such thermoplastic resins include polyvinyl chloride, polyacrylonitrile, polyvinylidene chloride, polyvinyl acetate, polyvinyl alcohol, and polyvinyl pyrrolidone. Among these, polyvinylpyrrolidone is particularly preferable because it has a high carbonization yield and is excellent in handleability. Preferable examples other than thermosetting resins and thermoplastic resins include pitches such as petroleum pitch and coal pitch.

  The carbon precursor to be mixed with the Mg—Si based composite powder is preferably in a liquid form because it is easy to improve the homogeneity of the carbon precursor mixture. As the mixing means in this case, wet mixing is suitable, and then drying is preferably performed to remove the liquid component.

  Specifically, when polyvinyl pyrrolidone or a resol type phenol resin is adopted as a carbon precursor, an organic solvent such as an alcohol such as ethanol, isopropyl alcohol, or industrial alcohol is used as the polyvinyl pyrrolidone or resol type phenol resin. An example of a liquid is preferable. In addition, if the carbon precursor mixture is processed into a thin sheet, processed into a small particle of about several millimeters, or processed into a powder of about several μm to several tens of μm, processing in the subsequent process is quick and homogeneity is improved. preferable.

  These carbon precursors may be used alone or in combination. Further, a part of the carbon precursor may be replaced with a carbon material such as carbon black, graphite, or carbon fiber. When the carbon precursor is replaced with a carbon material, it is preferable that the carbon precursor is 50% or less in terms of solid content in order to improve cycle characteristics. As a means for mixing, it is preferable to employ the mixing means mentioned in Step 1.

  Step 5 is a step of heat-treating the carbon precursor mixture at 100 to 500 ° C. to cure the carbon precursor in the mixture to obtain a cured body. A part of the carbon precursor may be carbonized (hereinafter also referred to as partial carbonization) during curing.

  The atmosphere of the heat treatment is not particularly limited in the case of a thermosetting resin, but in the case of a thermoplastic resin or a pitch-based material, the atmosphere includes an oxygen-containing atmosphere (hereinafter referred to as an oxygen atmosphere). Since oxygen is taken in and hardening is accelerated | stimulated, it is preferable.

  If the heat treatment temperature in step 5 exceeds 500 ° C., it is not preferable because carbonization proceeds excessively in a non-oxygen atmosphere, while it is not preferable because partial combustion proceeds in an oxygen atmosphere. In addition, when the heat treatment temperature is less than 100 ° C., curing does not occur, and in the final carbonization process, the carbon precursor is once melted and carbonized as a large lump, and in the process of pulverizing it Micro cracks may enter the active material, and as a result, the durability of the electrode material may be reduced. As heat-processing temperature of the process 5, it is preferable in it being 150-400 degreeC, and it is further more preferable in it being 200-300 degreeC.

  In the heat treatment of step 5, the holding time at the heat treatment temperature is preferably 1 to 30 hours. If the holding time is less than 1 hour, the above curing may be insufficient. On the other hand, if the holding time exceeds 30 hours, the cured part may be lost as a gas in an oxygen atmosphere, or the amount of disappearance may be increased and the yield of the final active material may be greatly reduced. Come out. More preferably, the holding time is 1 to 10 hours.

  In the heat treatment of step 5, when the rate of temperature rise is 5 ° C./hour to 100 ° C./hour, curing is ensured and the cured body can be easily crushed by hand. The pulverizing step of Step 6 is easy. In addition, since the carbon precursor does not melt during the final carbonization treatment, generation of microcracks in the active material can be prevented. Furthermore, it is preferable because the balance with productivity is good.

  More preferably, the temperature increase rate in step 5 is 5 ° C./hour to 50 ° C./hour. More preferably, the rate of temperature rise is 5 ° C./hour to 25 ° C./hour. In addition, when employ | adopting a thermosetting resin as a carbon precursor, when there exists a hardening | curing agent with respect to the said resin, you may add a hardening | curing agent before the process 5. FIG.

  By curing the carbon precursor in step 5, the carbon precursor is prevented from becoming a melt and a large block in the temperature rising process of the carbonization treatment step which is the final step. Once the carbon precursor is melted and a large block is formed, empirically it becomes a hard agglomerate, which is not easy to crush, and even if it breaks, microcracks remain in the particles. As a result, it may be finely powdered during use as an electrode material, causing deterioration, and may also adversely affect the homogeneity of the electrode material. That is, Step 5 is a carbon precursor curing and a non-melting treatment of the carbon precursor during carbonization, which is the final step.

  In step 6, the cured body obtained in step 5 is pulverized to obtain a cured body powder. As a pulverizing means, a ball mill, a V-type mixer, a Henschel mixer, a jaw crusher, or the like is appropriately employed. In addition, when a hardening process body is soft, it is good also as pulverizing with a hand or a mortar etc. to make the said powder.

  The average particle size of the cured body powder is preferably 10 to 100 μm because it is excellent in handling at the time of manufacture and requires a short carbonization time. It is also one of the features of the present invention that the carbon precursor in the carbon precursor mixture is made into a powder whose surface is hardened before carbonization treatment in Step 5 and Step 6. By performing Step 5 and Step 6, the homogeneity of the distribution of the carbonaceous material in the negative electrode material is improved, and an electrode material with stable performance can be obtained. In addition, it is preferable to cool the cured body with liquid nitrogen or the like before the pulverization process in Step 6 because it can be easily pulverized and the generation of microcracks in the particles can be reduced.

  After step 6 is completed, step 7 is performed in which the cured powder is heat-treated at 700 ° C. to 1300 ° C. in an inert atmosphere to carbonize (carbonize) the carbon precursor to obtain an active material. If the maximum temperature holding time is too long, the crystallite diameter of crystalline Si may be too large, and therefore it is preferably 0.5 to 5 hours. In the case of proceeding directly from step 4 to step 7, it is preferable to adjust the carbon precursor mixture to an average particle size of about 10 to 300 μm because the homogeneity of the electrode material is improved.

  In step 7, the rate of temperature rise is preferably 10 ° C./hour to 300 ° C./hour because a homogeneous cured product is easily obtained. More preferably, the rate of temperature rise is 50 ° C./hour to 200 ° C./hour. After step 7, when the active material is a lump that is too large to be used directly as a negative electrode material, the active material may be crushed so as not to have a mechanical impact as much as the pulverization.

  In this production method, as Step 4, the Mg-Si based composite powder and the carbon precursor are mixed in water or an organic solvent to form a slurry-like carbon precursor mixture, and further, the slurry-like carbon precursor mixture is obtained. You may dry and granulate by the spray-drying method. In the present production method 2, when the step 4 is a spray drying method, the particle size is adjusted by granulation at the same time as drying, so that the step 6 can be performed simultaneously with the step 4. In this case, step 4 and step 6 are performed simultaneously, and then step 5 and then step 7 are performed.

  Examples of the present invention (Examples 1 to 8: Examples, Examples 9 to 11: Comparative Examples) will be described in detail below, but the present invention is not limited to these Examples.

[Example 1]
(1) Production of Mg-Si based composite powder 20 g of commercially available silicon oxide powder (SiO _x , X = 1.0, average particle size 3 μm) and 14.5 g of magnesium nitrate hexahydrate are mixed with an atomic ratio of Mg and Si. Weighed to be 1: 8. Next, 80 g of ion-exchanged water was added to these to prepare a slurry, which was sufficiently mixed using a magnetic stirrer, and then ion-exchanged water was dried under reduced pressure using a rotary evaporator to obtain a mixture 1 of silicon oxide powder and magnesium nitrate. Next, this mixture 1 was put into an alumina boat, heated to 1000 ° C. at a heating rate of 200 ° C./hour in an annular furnace at a heating rate of 200 ° C./hour, held for 2 hours, then cooled to room temperature, and Mg—Si system A composite was obtained.

Next, the Mg—Si based composite was pulverized in a dry manner for 4 hours using a ball mill (a zirconia pot and ball) to obtain a Mg—Si based composite powder having an average particle size of 3 μm. In addition, unless otherwise indicated, the average particle diameter in an Example was measured using the laser diffraction type particle size distribution measuring apparatus (The Nikkiso Co., Ltd. make, brand name: Microtrac MK-I). Elemental analysis of the obtained Mg-Si based composite powder with a fluorescent X-ray apparatus (manufactured by Rigaku Corporation, trade name: RIX3000 or less, the same unless otherwise noted) revealed an element ratio of Mg to Si of 1: 8. Further, the Mg—Si based composite powder was X-rayed in a range of 2Θ = 10 ° to 60 ° with a wide angle X-ray diffractometer (trade name: TTR-III, manufactured by Rigaku Corporation) using CuKα rays. The diffraction pattern was measured. As shown in FIG. 3, the X-ray diffraction pattern shows a diffraction peak from a crystalline metal Si phase, a diffraction peak from a crystalline magnesium silicate (Mg ₂ SiO ₄ ) phase, and a diffraction angle of 2Θ = 22 °. A halo peak from crystalline SiO ₂ was shown.

  The crystallite diameter of the diffraction peak from the (220) plane of crystalline silicon was calculated by the Scherrer equation and found to be 5 nm. When this reaction product was observed with a transmission electron microscope (manufactured by JEOL Ltd., trade name: JEM1230 or less, the same unless otherwise noted), the amorphous phase and the crystalline phase were bonded at the atomic level as shown in FIG. It was found to be a composite material.

(2) Production of carbon precursor mixture and cured body Mg-Si composite powder obtained above: 10 g and resol type phenolic resin (manufactured by Showa Polymer Co., Ltd., a thermosetting resin as a carbon precursor material) Name: BRL-120Z); 23 g and 10 g of ethanol were mixed and stirred while irradiating ultrasonic waves to obtain a uniform slurry. Next, this slurry was heated at 70 ° C. under reduced pressure to remove ethanol to obtain a carbon precursor mixture. This carbon precursor mixture is heated to 90 ° C. at a heating rate of 50 ° C./hour in the atmosphere and held at 90 ° C. for 15 hours, and then heated to 180 ° C. at a heating rate of 7.5 ° C./hour and held for 2 hours. Then, the phenol resin component in the carbon precursor mixture was cured to prepare a cured product.

Next, the obtained cured treated body was pulverized using a rotary blade type pulverizer and a ball mill while being cooled with liquid nitrogen to obtain a powder (powder product 2) having an average particle size of 30 μm. Using a small rotary kiln, the obtained powder 2 was heated to 800 ° C. at a temperature rising rate of 200 ° C./hour in an argon stream and held for 2 hours to carbonize the phenol resin component, and the content of the carbon material was A 50% active material (negative electrode material) was obtained. The active material that had been carbonized was slightly agglomerated during heating to form a lump, but it could be easily crushed using a mortar, resulting in a powder with an average particle size of about 30 μm. The specific surface area of this active material powder measured by the BET method using a nitrogen adsorption measuring apparatus (manufactured by Cantachrome, trade name: Autosorb 3 or less, the same unless otherwise specified) was 200 m ² / g.

  The amount of phenol resin added should be measured from the ratio of the mass of the obtained carbon material to the mass of the initial phenol resin when a mixture of phenol resin and ethanol is treated in advance under the same conditions as above. Determined.

When wide-angle X-ray diffraction measurement using CuKα rays of the active material thus obtained was performed, diffraction peaks from crystalline Si and crystalline Mg ₂ SiO ₄ were observed as shown in FIG. A halo peak from SiO ₂ and amorphous carbon was observed superimposed on around 2Θ = 22 °. At this time, the crystallite size of crystalline Si was 6 nm, and the crystallite size of crystalline Mg ₂ SiO ₄ was 15 nm.

For reference, FIG. 5 shows a silicon oxide used in the experiment and a wide-angle X-ray diffraction pattern after heating the mixture at 1000 ° C. for 2 hours to cause a disproportionation reaction, and FIG. 6 shows a phenol resin alone. A wide-angle X-ray pattern of a carbon material obtained by carbonization under the same conditions as in the above process was shown. In FIG. 4, the crystalline Si produced by the disproportionation reaction of SiO _{X and} the amorphous SiO ₂ peak showing a halo peak at 2Θ = 23 ° are observed. FIG. 6 shows that the X-ray diffraction pattern of the carbon material obtained by carbonizing the phenol resin is a carbon material showing a halo pattern peculiar to the amorphous structure.

[Example 2]
Example 1 except that silicon oxide powder (SiO _X , X = 1.0, average particle size 3 μm) and magnesium nitrate hexahydrate were prepared so that the atomic ratio of Mg and Si was 1: 4 in Example 1. Similarly, an active material having a carbon material content of 50% was produced.

[Example 3]
In Example 1, magnesium acetate tetrahydrate was used as the magnesium source, and the silicon oxide powder (SiO _X , X = 1.0, average particle size 3 μm) was prepared so that the atomic ratio of Mg and Si was 1: 2. An active material was prepared in the same manner as in Example 1 except that.

[Example 4]
In Example 1, except that silicon oxide powder (SiO _X , X = 1.0, average particle size 3 μm) and magnesium nitrate hexahydrate were prepared so that the atomic ratio of Mg and Si was 1:15. In the same manner as described above, an active material having a carbon material content of 50% was obtained.

[Example 5]
In Example 1, an active material was produced in the same manner as in Example 1 except that the content of the carbon material in the active material was 70%. It was 250 m < ² > / g when the specific surface area of this active material powder was measured by BET method.

[Example 6]
Mg-Si based composite powder with an atomic ratio of Mg and Si of 1: 8 produced in the same manner as in Example 1; 50 g and the resol type phenolic resin used in Example 1 (trade name: BRL-120Z, manufactured by Showa Polymer Co., Ltd.) 115 g and 1485 g of ethanol were mixed and mixed for 1 hour using a rotary ball mill (alumina pot / alumina ball) to obtain a slurry with a solid content of 10%.

  Furthermore, this slurry was dried using an explosion-proof spray dryer (drying chamber inlet temperature 150 ° C., outlet temperature 70 ° C.) to obtain granules having an average particle diameter of 20 μm. The obtained granule is heated to 90 ° C. at a temperature rising rate of 50 ° C./hour in the atmosphere and kept at 90 ° C. for 15 hours to remove residual ethanol, and then to 180 ° C. at a temperature rising rate of 7.5 ° C./hour. The mixture was heated and held for 2 hours to cure the phenol resin component of the mixture. Since the granules in which the phenol resin component was cured had some aggregates in the course of the curing treatment, they were crushed using a rotary blade crusher to such an extent that the granule shape was not destroyed.

The granular cured product thus obtained was heated to 800 ° C. at a heating rate of 200 ° C./hour in an argon stream using a small rotary kiln, and held for 2 hours to carbonize the phenol resin component, thereby producing a carbon material. An active material having a content of 30% was obtained. The active material that had been carbonized was slightly agglomerated during heating and became a lump, but it could be easily loosened by lightly crushing using a mortar, resulting in a powder with an average particle size of about 20 μm. When the specific surface area of this active material powder was measured by the BET method, it was 100 m ² / g.

[Example 7]
Mg-Si based composite powder prepared in the same manner as in Example 1 and having an atomic ratio of Mg to Si of 1: 8; 10 g of polyvinyl pyrrolidone (trade name: K-90, manufactured by Junsei Chemical Co., Ltd.) as a carbon precursor material and ethanol 70 g was added and mixed with a ball mill for 3 hours to obtain a slurry. Next, ethanol was removed from the slurry under reduced pressure using a rotary evaporator to obtain a carbon precursor mixture. Next, this carbon precursor mixture was held in the atmosphere at 90 ° C. for 15 hours, and then heated to 300 ° C. at a temperature increase rate of 100 ° C./hour and held for 2 hours to be cured to produce a cured product.

The obtained cured product was pulverized using a jaw crusher and a ball mill to obtain a powder having an average particle size of 15 μm. The powder was heated to 200 ° C. at a heating rate of 100 ° C./hour in the atmosphere, held for 2 hours, and cured (infusibilized) again. The obtained cured product powder was heated to 800 ° C. at a temperature increase rate of 200 ° C./hour and held for 2 hours in a stream of argon using a small rotary kiln to carbonize the polyvinylpyrrolidone component. The active material that had been carbonized was slightly agglomerated during heating and became agglomerated, so it was lightly crushed using a mortar to obtain a powder having an average particle size of about 18 μm. It was 20 m < ² > / g when the specific surface area was measured for this active material powder by BET method.

[Example 8]
Mg-Si composite powder having an atomic ratio of Mg and Si of 1: 8 produced in the same manner as in Example 1; resol type phenol resin used in Example 1 for 10 g (manufactured by Showa Polymer Co., Ltd., trade name: BRL-120Z) 23 g and 10 g of ethanol were mixed and stirred while irradiating with ultrasonic waves to obtain a uniform slurry.

  Next, the slurry was heated at 70 ° C. under reduced pressure to remove ethanol, and further heated in the atmosphere to 90 ° C. at a temperature rising rate of 50 ° C./hour and held at 90 ° C. for 15 hours, and then the temperature rising rate was changed to 7. The mixture was heated to 180 ° C. at 5 ° C./hour and held for 2 hours to cure the phenol resin component of the mixture to obtain a block-shaped cured product.

  Next, the cured product was pulverized with a jaw crusher and sized into particles having a particle size of about 3 mm. The obtained particles were heated to 800 ° C. in a stream of argon using a small rotary kiln (temperature rising rate: 200 ° C./hour) for 2 hours to carbonize the phenol resin component and have a carbon material content of 50. % Active material was obtained.

  Next, this active material was pulverized using a planetary ball mill (using a zirconia pot and zirconia balls) to obtain an active material powder having an average particle size of 30 μm. In Examples 1 to 11, the active material was pulverized only in this example. In Examples 1 to 7, almost no microcracks were observed inside the active material, but in this example, relatively many microcracks were observed inside the active material. The reason why the discharge capacity retention rate after 50 cycles in this example is lower than that in Examples 1 to 7 seems to be related to the presence or absence of microcracks.

[Example 9 (comparative example)]
Example 1 was the same as Example 1 except that magnesium nitrate was not used. Specifically, 10 g of commercially available silicon oxide powder (SiO _X , X = 1.0, average particle size 3 μm), 23 g of the resol type phenol resin used in Example 1 and 10 g of ethanol are mixed and stirred while irradiating with ultrasonic waves. A uniform slurry was obtained. The following was performed in the same manner as in Example 1. In this example, since no magnesium compound is added to SiO _X , the formation of magnesium silicate phase is not recognized in the phase structure determined from the X-ray diffraction results, and the performance deterioration due to the charge / discharge cycle is as in Examples 1 to 8. It was big compared.

[Example 10 (comparative example)]
Example 1 was the same as Example 1 except that silicon oxide powder (SiO _X , X = 1.0, average particle size 3 μm) and magnesium nitrate were prepared so that the atomic ratio of Mg and Si was 1: 1/3. Thus, an active material was obtained. In this example, since a large amount of Mg is added to Si, the phase structure determined from the X-ray diffraction result of the Mg—Si based composite powder is a magnesium silicate (Mg ₂ SiO ₄ ) phase and a MgO phase. Si phase was not observed. As a result, the initial charge capacity was much smaller than in Example 1.

[Example 11 (comparative example)]
In Example 1, the heat treatment conditions for obtaining the Mg—Si based composite were changed from 1000 ° C. × 2 hours to 700 ° C. × 2 hours, and the carbonization treatment conditions were changed from 800 ° C. × 2 hours to 700 ° C. × 2 hours. An active material was obtained in the same manner as in Example 1 except that.

In this example, since the heating temperature at the time of synthesizing the composite structure oxide was as low as 700 ° C., the magnesium silicate (Mg ₂ SiO ₄ ) phase was not formed, and the disproportionation reaction of SiO _X did not proceed. From the X-ray diffraction result of the composite structure oxide, only an amorphous phase having a peak in the vicinity of magnesia (MgO) and 2Θ = 23 ° was observed. Furthermore, since the heating temperature at the time of compounding with the carbon material was 700 ° C., the disproportionation reaction of SiO _X did not proceed even in this step, and the final phase structure of the active material identified from X-ray diffraction was also It was a two-phase structure of a magnesia (MgO) phase and an amorphous phase. The cycle characteristics of the active material obtained as a result were inferior to those of Example 1 having a three-phase structure.

  The preparation conditions of the active materials of Examples 1 to 11 are summarized in Tables 1 and 2. Table 1 shows the preparation conditions of the Mg—Si based composite. Since the production heating conditions for the Mg—Si based composite in Table 1 are constant for 2 hours, only the holding temperature is shown.

Table 2 shows the carbon precursor carbonization conditions and the active material particle size adjustment conditions. The carbon precursors in Table 2 are abbreviated as PF for phenol resin and PVP for polyvinylpyrrolidone resin, respectively. Since the carbonization treatment condition is changed only in the treatment temperature and the holding time is constant for 2 hours, only the treatment temperature is described. The crystallite diameters of Si and Mg ₂ SiO ₄ in Table 2 are values after carbonization.

[Preparation of negative electrode for Li ion secondary battery]
The obtained active material, polyvinylidene fluoride resin as a binder, and acetylene black as a conductive agent were weighed at a mass ratio of 8: 1: 1, and well mixed with N-methylpyrrolidone as a solvent to form a slurry. . Next, this slurry was applied to a copper foil having a thickness of 30 μm using a bar coater. After drying the solvent at 120 ° C. in the air, the coating layer was consolidated by a roll press, and then cut into strips having a width of 10 mm and a length of 40 mm.

  The coating layer was peeled off leaving a 10 × 10 mm portion of the strip-shaped copper foil, and this was used as an electrode. The thickness of the coating layer after roll pressing of the obtained electrode was 50 μm. The obtained electrode was vacuum-dried at 150 ° C., then carried into a glove box filled with purified argon gas, and opposed to a lithium foil counter electrode pressed against a nickel mesh with a porous polyethylene film separator on both sides. Was fixed with a polyethylene plate.

  This counter electrode was put into a polyethylene beaker, and a nonaqueous electrolytic solution in which lithium perchlorate was dissolved in a mixed solvent of ethylene carbonate and diethylene carbonate (1: 1 volume ratio) at a concentration of 1 mol / L was sufficiently impregnated. It was. The electrode after impregnation with the electrolytic solution was taken out from the beaker, put in an aluminum laminate film bag, the lead wire part was taken out and sealed to form a half battery.

[Evaluation of Li-ion secondary battery characteristics]
The obtained half-cell was placed in a constant temperature bath at 25 ° C. and connected to a constant current charge / discharge tester (manufactured by Hokuto Denko) to conduct a charge / discharge test. For the current density, the current value per mass of the electrode active material (the mass amount excluding the conductive material and the binder) was 75 mA / g, and the first charge (Li ion occlusion reaction) was performed.

  The end-of-charge potential was 0.01 V with respect to the Li counter electrode, and discharge (Li ion desorption reaction) started immediately after reaching the end voltage. The end-of-discharge voltage was 1.5 V with respect to the Li counter electrode. This charge / discharge cycle was repeated up to 100 cycles, and the charge capacity, discharge capacity, and charge / discharge efficiency (discharge capacity / percentage of charge capacity) were measured.

  The measurement results are shown in Table 3 as initial charge capacity, initial discharge capacity, initial charge / discharge efficiency, discharge capacity after 50 cycles, and discharge capacity retention ratio after 50 cycles (= discharge capacity after 50 cycles / initial discharge capacity).

  The present invention relates to a non-aqueous electrolyte secondary battery that can be used as a power source for portable equipment such as a mobile phone or a notebook computer, or a storage element for a hybrid vehicle, particularly a high-capacity and long-life anode for a lithium ion secondary battery. Provide material (active material).

Claims (10)

A negative electrode material for a secondary battery comprising crystalline Si, amorphous SiO ₂ , crystalline Mg ₂ SiO _4, and a carbonaceous material.   The negative electrode material for a secondary battery according to claim 1, wherein an atomic ratio of Mg atoms to Si atoms in the negative electrode material is 1: 2 to 1:40. _{2. The} total amount of crystalline Si, amorphous SiO ₂ , and crystalline Mg ₂ SiO ₄ in the negative electrode material is 25 to 95 mass% and the carbonaceous content is 5 to 75 mass%. Or the negative electrode material for secondary batteries of 2.   The negative electrode material for a secondary battery according to claim 1, 2 or 3, wherein the crystalline Si has a crystallite size of 2 to 30 nm. 5. The negative electrode material for a secondary battery according to claim 1, wherein the crystalline Mg ₂ SiO _{4 has} a crystallite size of 5 to 50 nm. Step 1 of mixing silicon oxide SiO _X (0 <X <2) and a magnesium compound so that the atomic ratio of Mg atoms to Si atoms is 1: 2 to 1:40 to obtain a mixture 1, and the mixture 1 is heated at a holding temperature of 700 ° C. to 1500 ° C. in an inert atmosphere to obtain a Mg—Si based composite containing a magnesium silicate phase, and the Mg—Si based composite is pulverized to form Mg— Step 3 for preparing Si-based composite powder, Step 4 for mixing the Mg-Si-based composite powder and the carbon precursor to form a carbon precursor mixture, and holding the carbon precursor mixture obtained in Step 4 at a holding temperature. Including a step 5 of heat-treating at 100 ° C. to 500 ° C. to obtain a cured product and a step 6 of crushing the cured product to obtain a cured powder. Holding temperature 700 ° C under atmosphere A method for producing a negative electrode material for a secondary battery, characterized by heat-treating at -1300 ° C. and carbonizing the cured powder to obtain an active material.   The heat treatment conditions in step 5 are as follows: a temperature increase rate from room temperature to a holding temperature in an oxygen atmosphere is 5 ° C / hour to 100 ° C / hour, and a holding time at the holding temperature is 1 to 30 hours. Item 7. A method for producing a negative electrode material for a secondary battery according to Item 6.   The magnesium compound is at least one selected from the group consisting of magnesium oxide, magnesium hydroxide, magnesium carbonate, magnesium nitrate, magnesium chloride, magnesium acetate, magnesium oxalate, magnesium ethoxide, and acetylacetone magnesium. 8. A method for producing a negative electrode material for a secondary battery according to 7.   The method for producing a negative electrode material for a secondary battery according to claim 6, wherein a part of the carbon precursor is substituted with a carbonaceous material. The negative electrode for a secondary battery according to claim 6, wherein the negative electrode material for a secondary battery includes crystalline Si, amorphous SiO ₂ , crystalline Mg ₂ SiO _4, and a carbonaceous material. Material manufacturing method.

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