JP2004349237A - Cathode for lithium secondary battery, lithium secondary battery using cathode, deposition material used for cathode formation, and manufacturing method of cathode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To greatly improve the low level of an initial efficiency as its shortcoming without impairing the large level of an initial charge capacity which is a characteristic of a lithium secondary battery using SiO as a negative electrode. <P>SOLUTION: A film of a silicon oxide formed by vacuum vapor deposition or sputtering is formed on a surface of a current collector as a negative electrode active material layer. Preferably, the film is formed by an ion plating method. The silicon oxide is SiOx (0.5 ≤ x <1.0), and the thickness of the film is 0.1-50 μm. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a negative electrode used for a lithium secondary battery, a lithium secondary battery using the negative electrode, a film-forming material used for forming the negative electrode, and a method for manufacturing the negative electrode.

  Lithium secondary batteries, which charge and discharge by inserting and extracting lithium ions, have features such as high capacity, high voltage, and high energy density, making them suitable for OA equipment, especially mobile information devices such as mobile phones and personal computers. It is used very often as a power source. In this lithium secondary battery, lithium ions move from the positive electrode to the negative electrode during charging, and lithium ions stored in the negative electrode move to the positive electrode during discharging.

As a negative electrode active material constituting a negative electrode of a lithium secondary battery, carbon powder is frequently used. As will be described in detail later, this is because the comprehensive evaluation of various characteristics such as the capacity, initial efficiency, and cycle life of the carbon anode is high. The carbon powder is mixed with a binder solution to form a slurry. The slurry is applied to the surface of a current collector plate, dried, and then formed into a negative electrode sheet by a powder kneading application drying method in which pressure is applied. Incidentally, as a positive electrode active material constituting the positive electrode, an oxide of a transition metal containing lithium, mainly LiCoO ₂ or the like is used.

  One of the problems of the carbon anode which is frequently used at present is that its theoretical capacity is smaller than that of other anodes. Despite the small theoretical capacity, the reason why carbon anodes are frequently used is that the properties other than capacity, such as initial efficiency and cycle life, are high and the properties are well balanced.

  With respect to lithium secondary batteries, which are frequently used as power sources for portable information devices, further increase in capacity is required, and from this viewpoint, development of a negative electrode active material having a larger capacity than carbon powder is being promoted. One such negative electrode active material is SiO, and the theoretical capacity of SiO reaches several times that of carbon. Nevertheless, a SiO negative electrode has not been put to practical use. The biggest reason is that the initial efficiency of the SiO negative electrode is extremely low.

  The initial efficiency is a ratio of the initial discharge capacity to the initial charge capacity, and is one of important battery design factors. The fact that this is low means that the lithium ions injected into the negative electrode during the initial charge are not sufficiently released during the initial discharge, and if the initial efficiency is low, practical application is difficult even if the theoretical capacity is large. For this reason, various measures have been devised to increase the initial efficiency of the SiO negative electrode, and one of them is a method in which lithium is contained in SiO in advance described in Patent Document 1. Incidentally, the desired initial efficiency is 75% or more.

Japanese Patent No. 2997741

  Like the carbon anode, the SiO anode is prepared by mixing a fine powder of SiO with a binder solution to form a slurry, applying the slurry to the surface of a current collector plate, drying, and then applying pressure by a powder kneading application drying method. . In the case of a negative electrode containing lithium in SiO in advance, the negative electrode is manufactured by laminating powder on the surface of a current collector plate using the same powder kneading, coating and drying method.

  The lithium-containing SiO negative electrode thus produced is effective for increasing the initial efficiency of the lithium secondary battery. However, in the method of preliminarily containing lithium in SiO, the initial charge capacity is reduced by the content thereof, which substantially impairs the high theoretical capacity, which is an excellent characteristic of SiO. For this reason, measures to increase the initial efficiency without reducing the initial charge capacity of the SiO negative electrode have been desired.

  In addition, lithium secondary batteries are required to be further miniaturized. However, in the case of an SiO negative electrode manufactured by a powder kneading coating and drying method, since the SiO layer is a low-density porous body, the presence or absence of lithium is reduced. There is also a problem that miniaturization is difficult regardless of the above.

  An object of the present invention is to significantly improve the low initial efficiency, which is a disadvantage of the lithium secondary battery using SiO as a negative electrode, without impairing the initial charge capacity characteristic of the lithium secondary battery. The purpose is to reduce the size of the negative electrode.

  In order to achieve the above object, the present inventor has changed the idea and planned to form a dense layer of SiO on the surface of the current collector by vacuum evaporation. As a result, not only the capacity per unit volume is increased as compared with the conventional SiO layer formed by the powder kneading coating and drying method, but also the low initial efficiency, which is a problem in the SiO layer, is due to the initial charging capacity. It has been found that the improvement is drastically performed without a decrease in the amount. Also, among the vacuum deposition, the thin film formed by the ion plating method has a particularly high performance, the effect similar to the vacuum deposition film can be obtained even with the sputtering film, and as a film forming material used for the vacuum deposition. Has been found to be suitable for a precipitate of SiO or a sintered body produced from the precipitate, especially a special sintered body described later.

  The reason why the initial charge capacity decreases in the SiO powder kneaded coating and dried layer and does not decrease in the vacuum deposition layer and the sputtering layer is considered as follows.

The SiO powder is manufactured, for example, as follows. First, a mixture of Si powder and SiO ₂ powder is heated in a vacuum to generate a SiO gas, which is deposited in a low-temperature deposition section to obtain a SiO precipitate. The molar ratio of O to Si in the SiO precipitate obtained by this method is approximately 1. This SiO precipitate is pulverized to obtain SiO powder. However, when the powder is formed into a powder, the surface area is increased. Therefore, the powder is oxidized by oxygen in the air at the time of pulverization and when the powder is used. Molar ratio exceeds 1. In addition, when the SiO powder is laminated by the powder kneading coating and drying method, oxidation proceeds due to the large surface area of the SiO powder. Thus, the molar ratio of O to Si is increased in the dry layer of the powder kneaded and coated with SiO. If the molar ratio of O to Si in the SiO powder of the powder kneaded and dried layer is high, the lithium ions absorbed during the initial charge are less likely to be released during the discharge, and the initial efficiency is reduced.

  On the other hand, in the vacuum evaporation method and the sputtering method, since the film is formed in a vacuum, an increase in the oxygen molar ratio is suppressed, and as a result, a decrease in the initial efficiency is suppressed. In addition, a thin film formed by a vacuum evaporation method or a sputtering method is dense. On the other hand, the powder-kneaded and dried layer is merely a powder aggregate in which the powder is compacted, and has a low filling rate of SiO. Since the initial charge capacity is the amount of charge per unit volume of the negative electrode active material layer, a dense thin film has a higher initial charge capacity, and has a higher charge capacity even after the second cycle.

  The reason why the thin film formed by the ion plating method has a particularly high performance is that even when SiO having a molar ratio of O to Si of 1: 1 is used, the oxygen in the SiO tends to decrease. Is considered to be affected. That is, it is desirable that oxygen in SiO has as little oxygen as possible because of its strong binding with lithium ions. However, by using the ion plating method, the molar ratio of O to Si in the SiO film is about 0.5 at the maximum. Down to the point. Incidentally, the reason why the oxygen molar ratio is reduced by the ion plating method is unknown at present.

  Conversely, it is also possible to increase the oxygen molar ratio in SiO by increasing the amount of oxygen in the atmosphere during vacuum deposition or sputtering.

In vacuum deposition, a deposition source, that is, a film-forming material is heated and melted in a vacuum by resistance heating, induction heating, electron beam irradiation, or the like, and the vapor is adhered to the surface of a substrate. As the material for film formation here, for example, a mixed sintered body of Si powder and SiO ₂ powder is used. Further, the above-mentioned SiO precipitates and SiO sintered bodies manufactured from powders, granules, and lump of SiO obtained by pulverizing the precipitates are used. As a result of the investigation by the present inventors, as a film forming material when forming a dense layer of SiO by vacuum evaporation on the surface of the current collector, an SiO precipitate or SiO firing from a mixed sintered body of Si and SiO ₂ was used. It has been found that a sintered body is preferable in terms of the initial efficiency and the film forming rate, and among them, a powder sintered body manufactured by devising the powder particle size and the sintering atmosphere is particularly preferable.

That is, it is known that silicon monoxide has a higher evaporation rate than a mixed material of Si and SiO _{2 at the time} of vapor deposition for a film forming material used for vacuum deposition of silicon oxide. For this reason, the use of a silicon monoxide film-forming material makes it possible to increase the rate of forming a thin film. However, the evaporation characteristics of the film-forming material made of silicon monoxide produced by sintering depend on various conditions such as the particle size of the silicon monoxide powder used in the production and the production method. The evaporation rate of the film-forming material after sintering is lower than that of silicon oxide, and it is not expected that the productivity of the thin film is improved by using the film-forming material made of silicon monoxide.

  In view of such circumstances, the present inventor has studied a silicon monoxide sintered body that can maintain a high evaporation rate even when sintered, and a method of manufacturing the same. As a result, the following findings were obtained.

  First, the reason why the evaporation rate of silicon monoxide differs before and after sintering is that a slight variation occurs in the composition generated during sintering of silicon monoxide. On the other hand, silicon dioxide is a material that is more energy stable than silicon monoxide, and the evaporation rate of silicon dioxide is lower than that of silicon monoxide. Therefore, even when a material for forming a film of silicon monoxide is manufactured, it is inferred that the evaporation rate is reduced because silicon monoxide is locally oxidized and a part of the material is changed to silicon dioxide.

  Oxidation of silicon monoxide can occur during natural oxidation when left in the air or during sintering in an oxygen atmosphere. Therefore, natural oxidation is prevented by using silicon monoxide powder having a small surface area, and sintering of such silicon monoxide powder in a non-oxidizing atmosphere can suppress oxidation of silicon monoxide as much as possible. . The thus-produced sintered body of silicon monoxide has a high evaporation rate and an extremely small amount of evaporation residue when thermogravimetry is performed. In addition, when a negative electrode thin film in a lithium secondary battery is formed by vapor deposition, the initial efficiency in the lithium secondary battery is improved because the molar ratio of O to Si is reduced.

The present invention has been developed based on such knowledge, and has the following negative electrode for a lithium secondary battery, a lithium secondary battery, a material for film formation, and a method for manufacturing a lithium secondary battery. Alternatively, a negative electrode for a lithium secondary battery having a thin film of silicon oxide formed by sputtering on the surface of a current collector.
(2) A lithium secondary battery using the negative electrode.
(3) A film-forming material used for forming a silicon oxide thin film on the negative electrode, the film-forming material comprising a deposit of silicon monoxide or a sintered body produced from the deposit.
(3-1) In particular, when the thermogravimetric measurement of the sintered body sample is performed in a vacuum atmosphere having a heating temperature of 1300 ° C. and a pressure of 10 Pa or less, evaporation residue of the sample before measurement is obtained. A film-forming material having a mass of 4% or less.
(3-2) Alternatively, a film-forming material in which the powder has a mean particle size of 250 μm or more, which is a powder sintered body among the sintered bodies.
(4) A method for manufacturing a negative electrode for a lithium secondary battery, wherein a silicon oxide thin film is formed on the surface of a current collector by vacuum evaporation or sputtering.

  The thickness of the silicon oxide thin film is preferably from 0.1 to 50 μm. If it is less than 0.1 μm, the capacity per unit volume increases, but the capacity per unit area decreases. On the other hand, since this thin film is an insulating film, when the thickness exceeds 50 μm, there may be a problem that the current collection efficiency from the thin film to the current collector decreases. A particularly preferred film thickness is 0.1 to 20 μm.

  Among the vacuum depositions, the ion plating method is preferable. The reason is as described above.

  The molar ratio of O to Si in the silicon oxide forming the negative electrode active material layer is preferably 0.5 to 1.2, particularly preferably 0.5 or more and less than 1. That is, in the present invention, the molar ratio of O to Si in the silicon oxide forming the negative electrode active material layer can be made lower than that in the case of the powder kneading applied dry layer. Specifically, it can be reduced to less than 1, or can be increased intentionally. This molar ratio is preferably 0.5 to 1.2, which is sufficiently lower than that of the powder kneaded and dried layer, particularly preferably 0.5 or more and less than 1. In other words, as the silicon oxide, SiOx (0.5 ≦ x ≦ 1.2) is preferable, and SiOx (0.5 ≦ x <1) is particularly preferable. That is, from the viewpoint of suppressing the phenomenon in which lithium ions are combined with oxygen at the negative electrode, the molar ratio is preferably 1.2 or less, and particularly preferably less than 1. On the other hand, if it is less than 0.5, the volume expansion during lithium ion occlusion becomes remarkable, and the negative electrode active material layer may be broken.

  A thin metal plate is preferable as the current collector. Cu, Al, or the like can be used as the metal. The plate thickness is preferably 1 to 50 μm. If this is too thin, production becomes difficult, and reduction in mechanical strength also becomes a problem. On the other hand, when it is too thick, miniaturization of the negative electrode is hindered.

The positive electrode has a structure in which a positive electrode active material layer is formed on the surface of a current collector. As the positive electrode active material, a transition metal oxide containing lithium such as LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ is mainly used. As a method for producing the positive electrode, a powder kneading coating and drying method in which a fine powder of an oxide is mixed with a binder solution to form a slurry, the slurry is applied to the surface of a current collector plate, dried, and then pressurized, is generally used. However, it can also be formed by the same film formation as the negative electrode.

  As the electrolytic solution, for example, a non-aqueous electrolyte containing ethylene carbonate can be used.

  The film-forming material of the present invention is particularly effective for vacuum deposition, but is also effective for use in sputtering. Among the materials for film formation, the bulk density of the sintered body of silicon monoxide is not particularly limited, but is preferably 80% or more, and more preferably 95% or more from the viewpoint of effective suppression of splash and prevention of cracks and cracks during handling. More preferred.

  The negative electrode for a lithium secondary battery of the present invention has a structure in which a thin film of silicon oxide formed by vacuum evaporation or sputtering is provided on the surface of a current collector, and thus an initial characteristic characteristic of a lithium secondary battery using SiO for the negative electrode. Without impairing the charge capacity, the low initial efficiency, which is a drawback thereof, can be significantly improved, and it is highly effective in improving the performance and reducing the size of the lithium secondary battery.

  In addition, the use of the negative electrode of the lithium secondary battery of the present invention does not impair the initial charge capacity characteristic of the lithium secondary battery using SiO as the negative electrode, and the initial efficiency, which is a drawback, is low. The height can be greatly improved, and it has a significant effect on performance improvement and miniaturization.

  Further, the film-forming material of the present invention is used for forming a silicon oxide thin film in a negative electrode for a lithium secondary battery, and has an initial charge capacity characteristic of a lithium secondary battery using SiO as a negative electrode. However, the low initial efficiency, which is a drawback thereof, can be greatly improved without hindering the performance of the lithium secondary battery, thereby greatly improving the performance and reducing the size of the lithium secondary battery. Further, the evaporation rate is high, and the film formation rate can be improved.

  Further, the method for producing a negative electrode for a lithium secondary battery of the present invention is characterized by forming a thin film of silicon oxide on the surface of a current collector by vacuum evaporation or sputtering to form a lithium secondary battery using SiO for the negative electrode. A negative electrode with excellent characteristics that can significantly improve the low initial efficiency, which is a drawback of the negative electrode, without hindering the initial initial charge capacity, thereby improving the performance and miniaturizing lithium secondary batteries. It has a great effect.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. 1 is a longitudinal sectional view of a lithium secondary battery showing one embodiment of the present invention, FIG. 2 is a sectional view showing a configuration of a thermogravimeter used for thermogravimetry, and FIG. 4 is a graph showing a change in mass of a measurement sample.

  The lithium secondary battery of the present embodiment is a so-called button battery as shown in FIG. 1, and includes a circular flat case 10 forming a positive electrode surface. The case 10 is made of a metal, and accommodates therein a disk-shaped positive electrode 20 and a negative electrode 30 which are stacked in order from the bottom. The positive electrode 20 includes a current collector 21 formed of a circular thin metal plate, and a positive electrode active material layer 22 formed on the surface thereof. Similarly, the negative electrode 30 includes a current collector 31 made of a circular thin metal plate and a negative electrode active material layer 32 formed on the surface of the current collector 31. The two electrodes are stacked in a state where the respective active material layers face each other and the separator 40 is sandwiched between the facing surfaces, and are housed in the case 10.

  The case 10 also contains an electrolytic solution together with the positive electrode 20 and the negative electrode 30. The container is sealed in the case 10 by sealing the opening of the case 10 with the cover 60 via the seal member 50. The cover 60 also serves as a member forming the negative electrode surface, and is in contact with the current collector 31 of the negative electrode 30. The case 10 also serving as a member forming the positive electrode surface is in contact with the current collector 21 of the positive electrode 20.

  It should be noted that in the lithium secondary battery according to the present embodiment, the negative electrode active material layer 32 in the negative electrode 30 is formed by vacuum deposition or sputtering on the current collector 31 using SiO as a raw material, preferably an ion plating method which is a kind of vacuum deposition. This is a point made of a dense thin film of a silicon oxide formed by sputtering. The silicon oxide can be made SiOn (0.5 ≦ n <1.2) by controlling the oxygen concentration in the atmosphere, even though the starting material is SiO. The thickness of the thin film is suitably from 0.1 to 50 μm.

On the other hand, the positive electrode active material layer 22 in the positive electrode 20 is formed by mixing powder of a transition metal oxide containing lithium such as LiCoO ₂ with a binder solution to form a slurry, It is formed by a powder kneading coating and drying method in which the mixture is applied to the surface of the substrate, dried and then pressed.

  The features of the lithium secondary battery of the present embodiment are as follows.

  First, since the negative electrode active material layer 32 is made of silicon oxide, the theoretical capacity is much larger than that of the carbon powder layer. Second, the silicon oxide is a thin film formed by vacuum evaporation or sputtering, and has a low molar ratio of O to Si and is dense, so that the initial efficiency can be increased without reducing the initial charging capacity. . Third, since the capacity per unit volume of the thin film is large, miniaturization becomes easy.

The material for forming a film for forming the negative electrode active material layer 32 in the secondary battery negative electrode 30 is as follows.

  As described above, the material suitable for the film formation is a precipitate of silicon monoxide or a sintered body produced from the precipitate, particularly in a vacuum atmosphere at a heating temperature of 1300 ° C. and a pressure of 10 Pa or less. Is a sintered body whose evaporation residue when the thermogravimetric measurement of the sintered body sample is performed is 4% or less of the mass of the sample before the measurement.

  The thermogravimeter of FIG. 2 is used for the thermogravimetry of the sintered body. Specifically, the measurement sample 3 is loaded into the crucible 2 suspended on one side of the balance 1. On the other hand, a weight 4 having a mass balanced with the measurement sample 3 is arranged on the other side of the balance 1. The thermogravimeter includes a heating furnace 5, a gas inlet 6, a gas outlet 7, and the like, and controls the temperature and the atmosphere of the measurement sample 3 by these.

When the mass of the measurement sample 3 is reduced due to the evaporation of the measurement sample 3, an electric current is caused to flow through a feedback coil installed in a uniform magnetic field to generate an electromagnetic force, and the balance with the weight 4 is maintained. At this time, since the electromagnetic force and the current value are directly proportional, the change in mass of the measurement sample 3 can be measured from the current value.

The temperature of the measurement sample 3 is set to 1300 ° C., and thermogravimetry is performed in a vacuum atmosphere of 10 Pa or less. At this time, in the thermogravimetric measurement, a slight change in the temperature of the measurement sample 3 is inevitable, but a slight change in the temperature within the range of 1300 ± 50 ° C. is permissible. When thermogravimetry is performed under these conditions, the mass of the sintered body decreases as silicon monoxide evaporates. FIG. 3 shows the change in the mass of the sample at this time, assuming that the mass of the measurement sample before measurement is 100%, and the change in the amount of evaporation residue with the lapse of the measurement time. In the measurement shown in the figure, the temperature of the measurement sample was increased from room temperature to 1300 ° C.

  As shown in FIG. 3, when the measurement sample is charged into the crucible and the temperature is increased, the measurement sample starts to evaporate. After a certain period of time, the mass of the measurement sample does not substantially change, and the constant weight residue It can be grasped. A preferable film forming material is a silicon monoxide sintered body in which the mass of the evaporation residue of the sintered body at this time, that is, the mass of the constant weight residue is 4% or less of the mass before measurement.

  If this condition is satisfied, the evaporation rate of silicon monoxide is high, and the productivity of the silicon oxide thin film by vapor deposition by evaporation can be improved. In addition, the thin film has a low molar ratio of O to Si, so that the initial efficiency can be increased without reducing the initial charge capacity.

  Such a sintered body of silicon monoxide can be manufactured by sintering in a non-oxidizing atmosphere after press molding or while press molding SiO powder having a particle size of 250 μm or more.

  As the raw material of this powder sintered body, the use of SiO powder having an average particle diameter of 250 μm or more is because, in the case of SiO powder having a particle diameter of less than 250 μm, the surface area of the SiO powder is large and silicon dioxide is deposited on the particle surface by natural oxidation. This is because the silicon dioxide is reflected on the sintered body because it is formed, and the evaporation rate and the initial efficiency are reduced. The upper limit of the particle size is preferably 2000 μm or less. If it exceeds 2000 μm, the press formability and sinterability will be reduced.

  It is not necessary to make the particle diameter of the above-mentioned SiO powder as uniform as long as the average is 250 μm or more. For example, by mixing and sintering SiO particles having various particle diameters of 250 μm or more, the density of the sintered body can be increased. If the density of the sintered body is about 95% or less, the particle size of the SiO powder in the sintered body can be investigated by cross-sectional observation using an optical microscope. As a raw material of the sintered body, an SiO powder having an average particle size of 250 μm or more is used. You can check whether you have used.

Such SiO powder is sintered in a non-oxidizing atmosphere after or while press-forming into an arbitrary shape. When sintering is performed after press molding, the press molding method is not particularly limited as long as the desired shape can be formed by pressing. When the bonding property between SiO particles is poor, a small amount of water may be added to the SiO powder, and after press molding, water may be removed by a dehydration treatment. By applying a load of about 300 to 1500 kg per 1 cm ² , the SiO powder can be formed into an arbitrary shape.

On the other hand, when sintering is performed while press-molding, it is sufficient to apply a load of about 100 to 300 kg per 1 cm ² because the temperature of the SiO powder rises.

  Sintering is desirably performed in a non-oxidizing atmosphere. The non-oxidizing atmosphere is an atmosphere containing no oxygen, for example, a vacuum atmosphere or an inert atmosphere such as an argon gas. In particular, when sintering is performed in a vacuum atmosphere, the sintering can be performed in a vacuum atmosphere because the evaporation rate of the sintered body of silicon monoxide is not different from the evaporation rate of the SiO powder before sintering. preferable. When sintering is performed in an atmosphere containing oxygen, the SiO powder binds and the evaporation rate decreases.

  The sintering temperature is not particularly limited as long as the SiO particles are bonded to each other and the shape can be maintained. Sintering at 1200 to 1350 ° C. for 1 hour or more is sufficient.

  Next, examples of the present invention will be shown, and the effects of the present invention will be clarified by comparing with the conventional examples.

  When manufacturing the lithium secondary battery (size diameter 15 mm, thickness 3 mm) shown in FIG. 1, the configuration of the negative electrode was variously changed as follows.

As an example, silicon oxide was formed on a surface of a current collector made of a copper foil having a thickness of 10 μm as a negative electrode active material layer by an ion plating method, a normal vapor deposition method (resistance heating), a sputtering method, and a powder kneading coating and drying method. A thin film of the product was formed. In the ion plating method, a sintered silicon powder (tablet) is used as a material for film formation (evaporation source), and an EB gun is used as a heating source in a predetermined vacuum atmosphere [10 ⁻³ Pa (10 ⁻⁵ torr)]. An oxide thin film was formed.

As a material for film formation, in addition to the above-mentioned SiO powder sintered body, the above-mentioned SiO precipitate, that is, a mixture of Si powder and SiO ₂ powder is heated in a vacuum to generate a SiO gas, The crushed lump of the SiO precipitate obtained by precipitation in the precipitation section of No., a mixed sintered body of Si powder and SiO ₂ powder, and a silicon lump were used.

As the SiO powder sintered body, three kinds of powder having an average particle diameter of 250 μm, 1000 μm, and 10 μm were used. For each production method, performed sintering (1200 ° C. × 1.5 hours in vacuum) while 250μm things pressure press-in load of 100 kg / cm ^2, pressing pressurized with a load 100 kg / cm ² is that of 1000μm Sintering was performed (1200 ° C. × 1.5 hours in a vacuum), and sintering (1200 ° C. × 1.5 hours in a vacuum) was performed on a 10 μm-thick one while pressing with a load of 200 kg / cm ² .

  When the thermogravimetric measurement of the sintered body sample was performed in a vacuum atmosphere at a heating temperature of 1300 ° C. and a pressure of 10 Pa or less, the evaporation residue rates were 4%, 3%, and 8%, respectively. The thermometer shown in FIG. 2 was used for thermogravimetry. The heating temperature of 1300 ° C. is a temperature measured by a thermocouple 8 at a distance of about 1 mm from the measurement sample, and it is considered that the measurement sample is substantially heated to this temperature. The data obtained by the thermogravimetry were arranged, and the mass (the evaporation residue ratio) with respect to the mass before the measurement was calculated with the mass when the mass change of the measurement sample substantially disappeared as the mass of the evaporation residue (FIG. 3). reference).

  The two types of samples shown in FIG. 3 are two of the above-described three types of SiO powder sintered bodies, and specifically, an SiO powder sintered body having an average particle diameter of 250 μm ( Solid line: Example 3), and a sintered body of SiO powder having an average particle diameter of 10 μm (dotted line: Example 10). The former has an evaporation residue ratio of 4%, while the latter has an evaporation residue ratio of 8%.

Each of the prepared negative electrodes was combined with a positive electrode, and sealed in a case together with an electrolyte to complete a lithium secondary battery. The initial charge capacity, initial discharge capacity, and initial efficiency of the completed various batteries were measured. Note that a fine powder of LiCoO ₂ was used for the positive electrode, and a non-aqueous electrolyte containing ethylene carbonate was used for the electrolytic solution.

  Table 1 shows the initial charging capacity and the initial efficiency calculated from the initial charging capacity and the initial discharging capacity. The initial charge capacity is evaluated by the amount of current per unit volume, and is expressed as a ratio when the data in Example 3 is set to 1.

  In Examples 1 to 8, a SiO powder sintered body (vacuum sintered product) having an average powder particle size of 250 μm was used as a film forming material, and the film forming method was variously changed.

  In Examples 1 to 5, a film forming method used an ion plating method. The thickness of the thin film was set to five types: 0.05 μm, 0.1 μm, 1 μm, 20 μm, and 50 μm. In all the thin films, the molar ratio of O to Si in the silicon oxide was 0.5.

  In Example 6, when the thickness of the thin film was 1 μm, oxygen was added to the film formation atmosphere to intentionally increase the molar ratio of O to Si in the silicon oxide.

  In Example 7 and Example 8, a thin film of silicon oxide having a thickness of 1 μm was formed on the surface of the current collector by ordinary vacuum deposition (resistance heating) and sputtering.

  On the other hand, in Conventional Example 1, a fine powder of SiO was laminated on the surface of the current collector by a powder kneading coating and drying method to form a negative electrode active material layer having a thickness of 200 μm. In Conventional Example 2, a 200 μm-thick negative electrode active material layer in which lithium was previously contained in SiO was formed on the surface of the current collector by a powder kneading coating and drying method.

  When SiO was used as the negative electrode active material and a layer was formed by a powder kneading coating and drying method, the molar ratio of O to Si in the layer was increased to 1.4. Since the initial discharge capacity is smaller than the initial charge capacity, the initial efficiency is as low as 46% (conventional example 1). The incorporation of lithium in SiO beforehand increases the initial efficiency to 84%, but this is solely due to the decrease in the initial charge capacity, which results in the inhibition of the excellent theoretical capacity of SiO (Conventional Example 2). .

  Instead of the powder kneading, coating and drying method, SiO, which is a negative electrode active material, was formed by ion plating. The molar ratio of O to Si in the thin film dropped to 0.5. The initial efficiency was improved while the initial charging capacity was large (Examples 1 to 5). However, in Example 5 having a large film thickness, the initial charging capacity and the initial efficiency were slightly reduced. In Example 6 in which the molar ratio of O to Si in the thin film was increased to 0.99, although the initial efficiency was slightly lowered, it was still at a high level and the initial charging capacity was large.

  In Examples 7 and 8 in which a thin film was formed by ordinary vacuum deposition and sputtering, the molar ratio of O to Si in the thin film exceeded 1. Although the initial efficiency is slightly lower than that of ion plating, it is still at a high level, and the initial charging capacity is also at a high level. The film formation rate is lower in normal vacuum deposition than in the ion plating method, and further lower in sputtering.

  On the other hand, in Examples 9 to 13, the ion-plating method was used as the film-forming method, and the material for film-forming was variously changed. The film thickness was 1 μm.

  In Example 9, the material for film formation was an SiO powder sintered body (vacuum sintered product) having an average powder particle diameter of 1000 μm. In Example 10, an SiO powder sintered body (vacuum sintered product) having an average powder particle size of 10 μm was used. The evaporation residue rates are 3% and 8%, respectively. As compared with Example 3 having the same film thickness of 1 μm, the effect on the battery performance is saturated in Example 9, and the molar ratio of O to Si in the thin film is slightly reduced in Example 10. Therefore, the average particle size of the powder in the SiO powder sintered body is preferably 250 μm or more.

  In Example 11, a crushed lump of SiO precipitate (average particle size of about 5 cm) was used. Compared with Example 3 having the same film thickness of 1 μm, the same initial efficiency and initial charge capacity as the powder sintered body can be obtained even with the precipitate. On the other hand, in the case of the sintered body, since the number of splashes during the film formation is smaller than that of the precipitate, the film formation rate (evaporation rate) can be further increased. For this reason, a sintered body is more preferable in terms of productivity. Further, the sintered body has an advantage that the raw material can be easily supplied to the film forming apparatus continuously. The reason why the splash during the film formation of the SiO sintered body is small is considered that SiO, which is a material for film formation, is more strongly bonded than the SiO precipitate.

In Example 12, a mixed sintered body of Si powder and SiO ₂ powder was used. Although this film-forming material can provide an effect on battery performance, the film-forming rate is considerably slow. In the case of SiO precipitates and SiO sintered bodies, film formation is possible only by raising the heating temperature to the sublimation temperature of SiO. However, in the case of a mixed sintered body of Si powder and SiO ₂ powder, first, Si and SiO in the sintered body are mixed. It is necessary to react each other at the contact portion ₂ to generate SiO. Therefore, the mixed sintered body has a lower SiO generation rate than the SiO precipitate and the SiO sintered body. If a large amount of heat is applied, the generation rate increases, but this increases the splash at the time of film formation, so there is a restriction that the film formation rate must be reduced after all.

  In the thirteenth embodiment, a film is formed in an oxidizing atmosphere using a silicon lump cut from a silicon ingot manufactured by a casting method as a material. As for the battery performance, the same performance as that of the mixed sintered body of Example 12 can be obtained. The oxidizing atmosphere was formed by introducing oxygen gas. In this case, since the Si atoms need to be diffused from the material surface, splash is more likely to occur than in the mixed sintered body. For this reason, the film formation rate is even lower.

  In the above embodiment, the button battery is used as the battery type. However, in the present invention, since the negative electrode is thin, the capacity can be easily increased by lamination. For this reason, the present invention is particularly suitable for a stacked battery, and has a feature in that a small-sized and large-capacity battery can be provided at low cost by applying to the stacked battery. In the case of a stacked battery, a thin film formed by film formation can also be formed on the positive electrode active material layer, the current collector, the separator, and the like in the same manner as the negative electrode active material layer.

1 is a longitudinal sectional view of a lithium secondary battery showing one embodiment of the present invention. It is sectional drawing which shows the structure of the thermogravimeter used for thermogravimetry. 5 is a graph showing a change in mass of a measurement sample when thermogravimetry is performed.

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 1 Balance 2 Crucible 3 Measurement sample 4 Weight 5 Heating furnace 6 Gas inlet 7 Gas exhaust port 8 Thermocouple 10 Case 20 Positive electrode 21 Current collector 22 Positive electrode active material layer 30 Negative electrode 31 Current collector 32 Negative electrode active material layer 40 Separator 50 Sealing member 60 cover

Claims (12)

  A negative electrode for a lithium secondary battery having a silicon oxide thin film formed by vacuum evaporation or sputtering on the surface of a current collector.   The negative electrode for a lithium secondary battery according to claim 1, wherein the silicon oxide thin film has a thickness of 0.1 to 50 m.   The negative electrode for a lithium secondary battery according to claim 1, wherein the vacuum deposition is an ion plating method.   The negative electrode for a lithium secondary battery according to claim 1, wherein the silicon oxide is SiOx (0.5≤x≤1.2).   The negative electrode for a lithium secondary battery according to claim 4, wherein the silicon oxide is SiOx (0.5 ≦ x <1.0).   A lithium secondary battery having the negative electrode according to claim 1.   A film forming material used for forming a silicon oxide thin film in the negative electrode for a lithium secondary battery according to claim 1, comprising a deposit of silicon monoxide or a sintered body produced from the deposit. Materials for film formation.   Claims 1. A thermogravimetric measurement of a sintered body sample in a vacuum atmosphere having a heating temperature of 1300 ° C and a pressure of 10 Pa or less, the evaporation residue being 4% or less of the mass of the sample before measurement. Item 8. A film-forming material according to item 7.   The film forming material according to claim 7, which is formed of a powder sintered body, and has an average particle diameter of 250 µm or more.   A method for producing a negative electrode for a lithium secondary battery, wherein a silicon oxide thin film is formed on the surface of a current collector by vacuum deposition or sputtering.   The method for producing a negative electrode for a lithium secondary battery according to claim 10, wherein an ion plating method, which is a kind of the vacuum deposition, is used.   The method for producing a negative electrode for a lithium secondary battery according to claim 10, wherein the material for film formation according to any one of claims 7 to 9 is used as the material for film formation in the vacuum deposition.

Images