JP6680531B2 - Negative electrode active material manufacturing method and lithium ion secondary battery manufacturing method - Google Patents

Description

  The present invention relates to a negative electrode active material, a negative electrode, a lithium ion secondary battery, a method for manufacturing a negative electrode active material, and a method for manufacturing a lithium ion secondary battery.

  In recent years, small electronic devices typified by mobile terminals have become widespread, and there is a strong demand for further size reduction, weight reduction, and longer life. In response to such market demand, development of a secondary battery, which is particularly small and lightweight and capable of obtaining a high energy density, is in progress. The application of this secondary battery to not only small electronic devices but also large electronic devices typified by automobiles and power storage systems typified by houses is being considered.

  Among them, a lithium ion secondary battery is easily expected to be small in size and have a high capacity, and has a higher energy density than that of a lead battery or a nickel-cadmium battery.

  The lithium-ion secondary battery includes a positive electrode, a negative electrode, and a separator, and an electrolytic solution, and the negative electrode contains a negative electrode active material involved in charge / discharge reactions.

  While carbon-based active materials are widely used as the negative electrode active material, recent market demands call for further improvements in battery capacity. In order to improve the battery capacity, the use of silicon as the negative electrode active material has been studied. This is because the theoretical capacity of silicon (4199 mAh / g) is 10 times or more larger than the theoretical capacity of graphite (372 mAh / g), and therefore a significant improvement in battery capacity can be expected. The development of silicon materials as a negative electrode active material is being studied not only for silicon alone but also for compounds such as alloys and oxides. In addition, the active material shape has been studied from a standard coating type for a carbon-based active material to an integral type that is directly deposited on a current collector.

  However, when silicon is used as the main raw material as the negative electrode active material, the negative electrode active material expands and contracts during charge and discharge, so that the negative electrode active material is prone to crack mainly in the vicinity of the surface layer. Further, an ionic substance is generated inside the active material, and the negative electrode active material becomes a material that is easily broken. When the surface layer of the negative electrode active material is cracked, a new surface is generated thereby, and the reaction area of the active material is increased. At this time, the decomposition reaction of the electrolytic solution occurs on the new surface, and the electrolytic solution is consumed because a film which is a decomposition product of the electrolytic solution is formed on the new surface. Therefore, the cycle characteristics are likely to deteriorate.

  In order to improve the battery initial efficiency and cycle characteristics, various studies have been made so far on negative electrode materials for lithium-ion secondary batteries mainly composed of a silicon material and electrode configurations.

  Specifically, for the purpose of obtaining good cycle characteristics and high safety, silicon and amorphous silicon dioxide are simultaneously deposited using a vapor phase method (see, for example, Patent Document 1). Further, in order to obtain high battery capacity and safety, a carbon material (electroconductive material) is provided on the surface layer of the silicon oxide particles (for example, refer to Patent Document 2). Furthermore, in order to improve the cycle characteristics and obtain high input / output characteristics, an active material containing silicon and oxygen is produced, and an active material layer having a high oxygen ratio in the vicinity of the current collector is formed ( For example, see Patent Document 3). Further, in order to improve the cycle characteristics, oxygen is contained in the silicon active material, the average oxygen content is 40 at% or less, and the oxygen content is increased near the current collector. (For example, see Patent Document 4).

Further, Si phase, (for example, see Patent Document 5) by using a nanocomposite containing SiO 2, _M y _O metal oxide in order to improve the initial charge and discharge efficiency. Further, in order to improve cycle characteristics, SiO _x (0.8 ≦ x ≦ 1.5, particle size range = 1 μm to 50 μm) and a carbon material are mixed and fired at a high temperature (for example, see Patent Document 6). Further, in order to improve cycle characteristics, the molar ratio of oxygen to silicon in the negative electrode active material is set to 0.1 to 1.2, and the difference between the maximum value and the minimum value of the molar ratio in the vicinity of the interface between the active material and the current collector. The active material is controlled within a range of 0.4 or less (for example, refer to Patent Document 7). In addition, a metal oxide containing lithium is used to improve battery load characteristics (see, for example, Patent Document 8). Further, in order to improve the cycle characteristics, a hydrophobic layer such as a silane compound is formed on the surface layer of the silicon material (see, for example, Patent Document 9). Further, in order to improve cycle characteristics, silicon oxide is used, and conductivity is imparted by forming a graphite coating on the surface layer thereof (see, for example, Patent Document 10). In Patent Document 10, with respect to the shift value obtained from the RAMAN spectrum for graphite coating, with broad peaks appearing at 1330 cm ^-1 and 1580 cm ^-1, their intensity ratio _I 1330 _{/ I 1580} is _{1.5 <I 1330} / _{I 1580} <3. Further, in order to improve high battery capacity and cycle characteristics, particles having a silicon microcrystalline phase dispersed in silicon dioxide are used (for example, see Patent Document 11). Further, in order to improve the overcharge and overdischarge characteristics, a silicon oxide in which the atomic ratio of silicon and oxygen is controlled to 1: y (0 <y <2) is used (for example, see Patent Document 12).

JP 2001-185127 A JP 2002-042806 A JP, 2006-164954, A JP, 2006-114454, A JP, 2009-070825, A JP, 2008-282819, A JP, 2008-251369, A JP, 2008-177346, A JP, 2007-234255, A JP, 2009-212074, A JP, 2009-205950, A Japanese Patent No. 2997741

  As described above, in recent years, small mobile devices typified by electronic devices have been enhanced in performance and multifunction, and the lithium-ion secondary battery, which is the main power source thereof, is required to have an increased battery capacity. There is. As one method for solving this problem, development of a lithium ion secondary battery including a negative electrode using a silicon material as a main material is desired. In addition, a lithium ion secondary battery using a silicon material is desired to have cycle characteristics close to those of a lithium ion secondary battery using a carbon-based active material. However, no negative electrode active material having the same cycle stability as that of a lithium ion secondary battery using a carbon-based active material has been proposed.

  The present invention has been made in view of the above problems, when used as a negative electrode active material of a secondary battery, the negative electrode active material capable of increasing the battery capacity and improving the cycle characteristics, An object is to provide a negative electrode having a negative electrode active material layer containing a negative electrode active material, and a lithium ion secondary battery using this negative electrode. Moreover, it aims at providing the manufacturing method of the negative electrode active material which can increase a battery capacity and can improve a cycle characteristic. Moreover, it aims at providing the manufacturing method of the lithium ion secondary battery which uses such a negative electrode active material.

In order to achieve the above object, the present invention provides a negative electrode active material containing negative electrode active material particles, wherein the negative electrode active material particles are silicon oxides represented by SiO _x (0.5 ≦ x ≦ 1.6). Containing a compound,
The negative electrode active material containing the negative electrode active material particles was used as a negative electrode of a secondary battery having metallic lithium as a counter electrode, and 0 V constant current constant voltage charging and 1.2 V constant current discharging cycles of the secondary battery were repeated X times ( (X ≧ 0) After repeating, the secondary battery was further charged with 0 V constant current and constant voltage (however, charging was stopped 60 hours after reaching 0 V), and the negative electrode active material after charging was ⁷ Provided is a negative electrode active material having a chemical shift value having a peak in a range of 25 to 55 ppm and a range of 0 to 3 ppm, which is obtained from a Li-MAS-NMR spectrum.

Thus, the negative electrode active material contains negative electrode active material particles containing a silicon compound (SiO _x : 0.5 ≦ x ≦ 1.6, hereinafter also referred to as silicon oxide), and When the negative electrode active material has the above-mentioned two types of peaks after the end of charging, it has a high battery capacity and good cycle characteristics when used as a negative electrode active material of a lithium ion secondary battery. To be

  Further, the peak in the range of 25 to 55 ppm is preferably such that the X is expressed within 49 times.

  With such a negative electrode active material, when this negative electrode active material is used as the negative electrode active material of a lithium ion secondary battery, the time (cycle number) until the secondary battery is stabilized is further reduced. You can

  Further, it is preferable that the peak in the range of 0 to 3 ppm is expressed by the X within 9 times.

  With such a negative electrode active material, when this negative electrode active material is used as a negative electrode active material of a lithium ion secondary battery, the time until a stable Li compound is generated inside the negative electrode active material particles (the number of cycles). ) Can be less.

  Further, the peak in the range of 25 to 55 ppm and the peak in the range of 0 to 3 ppm are preferably expressed when the X is 0 times.

  With such a negative electrode active material, when this negative electrode active material is used as a negative electrode active material of a lithium-ion secondary battery, the time until the secondary battery is stabilized and the stability inside the negative electrode active material particles are stable. It is possible to further reduce the time until the Li compound is formed.

  Further, it is preferable that the peak in the range of 25 to 55 ppm decreases during the cycle of the 0V constant current constant voltage charging and the 1.2V constant current discharging repeated 49 times or less.

  With such a negative electrode active material, the bulk state of silicon oxide is further stabilized by repeating insertion and desorption of Li.

  Further, the negative electrode active material has a half value width (2θ) of a diffraction peak due to a Si (111) crystal plane obtained by X-ray diffraction of 1.2 ° or more, and a crystallite size corresponding to the crystal plane. Is preferably 7.5 nm or less.

  Since the negative electrode active material contains a silicon oxide having the crystallinity of the above Si crystallite, when using such a negative electrode active material as a negative electrode active material of a lithium ion secondary battery, better cycle characteristics are obtained. And initial charge and discharge characteristics are obtained.

  The median diameter of the negative electrode active material particles is preferably 0.5 μm or more and 20 μm or less.

  When the median diameter of the negative electrode active material particles is within the above range, when the negative electrode active material containing such negative electrode active material particles is used as the negative electrode active material of a lithium ion secondary battery, better cycle characteristics are obtained. And initial charge and discharge characteristics are obtained.

  The negative electrode active material particles preferably include a carbon material in the surface layer portion.

  As described above, since the negative electrode active material particles include the carbon material in the surface layer thereof, the conductivity can be improved. Therefore, the negative electrode active material containing such negative electrode active material particles is used as the negative electrode active material of the lithium ion secondary battery. When used as a substance, the battery characteristics can be improved.

  The average thickness of the carbon material is preferably 1 nm or more and 5000 nm or less.

  If the average thickness of the carbon material to be coated is 1 nm or more, the conductivity is improved, and if the average thickness of the carbon material to be coated is 5000 nm or less, the negative electrode active material containing such negative electrode active material particles is treated with lithium. When used as a negative electrode active material for an ion secondary battery, it is possible to suppress a decrease in battery capacity.

  Furthermore, the present invention provides a negative electrode comprising the negative electrode active material of the present invention.

  With such a negative electrode, when this negative electrode is used as a negative electrode of a lithium ion secondary battery, it has high battery capacity and good cycle characteristics.

Further, the negative electrode, a negative electrode active material layer containing the negative electrode active material,
Having a negative electrode current collector,
The negative electrode active material layer is formed on the negative electrode current collector,
The negative electrode current collector preferably contains carbon and sulfur, and the content of each of them is 100 mass ppm or less.

  As described above, the negative electrode current collector that constitutes the negative electrode contains the above-described contents of carbon and sulfur, so that the deformation of the negative electrode during charging can be suppressed.

  Furthermore, the present invention provides a lithium-ion secondary battery characterized by using the above-mentioned negative electrode of the present invention as a negative electrode.

  A lithium-ion secondary battery using such a negative electrode has high capacity and good cycle characteristics.

Further, in the present invention, a method for producing a negative electrode active material containing negative electrode active material particles,
A step of preparing negative electrode active material particles containing a silicon compound represented by the general formula SiO _x (0.5 ≦ x ≦ 1.6),
A step of producing a secondary battery having a negative electrode containing a negative electrode active material containing the negative electrode active material particles, and a counter electrode made of metallic lithium;
After the cycle of 0V constant current constant voltage charging and 1.2V constant current discharging of the secondary battery is repeated X times (X ≧ 0), the secondary battery is further charged to 0V constant current constant voltage (provided that 0V is reached. 60 hours after the end of the process of charging)
Measuring the negative electrode active material by ⁷ Li-MAS-NMR in the state of termination of charging;
And a step of selecting a negative electrode active material having a peak in a range of 25 to 55 ppm and a range of 0 to 3 ppm as a chemical shift value, obtained from the ⁷ Li-MAS-NMR spectrum. A method for manufacturing the same is provided.

  By selecting the negative electrode active material in this manner and producing the negative electrode active material, a negative electrode active material having a high capacity and good cycle characteristics when used as the negative electrode active material of a lithium ion secondary battery is obtained. It can be manufactured.

  Furthermore, in the present invention, a negative electrode is manufactured using the negative electrode active material manufactured by the above-described method for manufacturing a negative electrode active material of the present invention, and a lithium ion secondary battery is manufactured using the manufactured negative electrode. Provided is a method for manufacturing an ion secondary battery.

  In this manufacturing method, a lithium ion secondary battery having a high capacity and good cycle characteristics can be manufactured by using the negative electrode active material selected as described above.

  As described above, when the negative electrode active material of the present invention is used as a negative electrode active material of a lithium ion secondary battery, it has high capacity and good cycle characteristics. Further, according to the method for producing a negative electrode active material of the present invention, it is possible to produce a negative electrode active material for a lithium ion secondary battery having good cycle characteristics.

It is a ⁷ Li-MAS-NMR spectra measured in Examples 1-3 of the present invention. It is sectional drawing which shows an example of a structure of the negative electrode of this invention. It is an exploded view showing an example of composition of a lithium ion secondary battery (laminate film type) of the present invention. Is a ⁷ Li-MAS-NMR spectra were measured using a general silicon single negative electrode containing silicon simple substance as an anode active material.

  Hereinafter, embodiments of the present invention will be described, but the present invention is not limited thereto.

  As described above, as one method of increasing the battery capacity of a lithium ion secondary battery, the use of a negative electrode using a silicon material as a main material as a negative electrode of a lithium ion secondary battery has been studied. A lithium ion secondary battery using this silicon material is expected to have cycle characteristics close to those of a lithium ion secondary battery using a carbon-based active material, but a lithium ion secondary battery using a carbon-based active material is desired. It has not been possible to propose a negative electrode active material exhibiting a cycle characteristic equivalent to that of.

Therefore, the inventors of the present invention have made extensive studies on a negative electrode active material that can obtain good cycle characteristics when used as a negative electrode of a lithium ion secondary battery. As a result, the negative electrode active material includes negative electrode active material particles, and the negative electrode active material particles contain a silicon compound represented by SiO _x (0.5 ≦ x ≦ 1.6). A negative electrode active material containing particles was used as a negative electrode of a secondary battery having metallic lithium as a counter electrode, and 0 V constant current constant voltage charging and 1.2 V constant current discharging cycles of this secondary battery were performed X times (X ≧ 0). After repeating, the secondary battery was further charged with 0V constant current and constant voltage (however, charging was stopped 60 hours after reaching 0V), and the negative electrode active material after charging was ⁷ Li-MAS-NMR. High battery capacity and good cycle characteristics are obtained when using a negative electrode active material having a chemical shift value having a peak in a range of 25 to 55 ppm and a range of 0 to 3 ppm obtained from a spectrum. Heading the door, the present invention has been accomplished.

<Negative electrode>
First, the negative electrode (negative electrode for non-aqueous electrolyte secondary battery) will be described. FIG. 2 is a sectional view showing an example of the configuration of the negative electrode of the present invention (hereinafter, also referred to as negative electrode).

[Negative electrode configuration]
As shown in FIG. 2, the negative electrode 10 has a structure in which the negative electrode active material layer 12 is provided on the negative electrode current collector 11. Further, the negative electrode active material layer 12 may be provided on both surfaces of the negative electrode current collector 11 or on only one surface. Further, the negative electrode current collector 11 may be omitted if the negative electrode active material of the present invention is used.

[Negative electrode current collector]
The negative electrode current collector 11 is made of an excellent conductive material and has high mechanical strength. Examples of the conductive material that can be used for the negative electrode current collector 11 include copper (Cu) and nickel (Ni). Further, this conductive material is preferably a material that does not form an intermetallic compound with lithium (Li).

  The negative electrode current collector 11 preferably contains carbon (C) and sulfur (S) in addition to the main elements. This is because the physical strength of the negative electrode current collector 11 is improved. In particular, when the negative electrode has an active material layer that expands at the time of charging, if the current collector contains the above element, it is effective in suppressing deformation of the electrode including the current collector. The content of each of the above-mentioned contained elements is not particularly limited, but is preferably 100 mass ppm or less. This is because a higher deformation suppressing effect can be obtained. The cycle characteristics can be further improved by such a deformation suppressing effect.

  The surface of the negative electrode current collector 11 may be roughened or may not be roughened. The roughened negative electrode current collector is, for example, a metal foil or the like that has been subjected to electrolytic treatment, embossing treatment, or chemical etching treatment. The non-roughened negative electrode current collector is, for example, a rolled metal foil.

[Negative electrode active material layer]
The negative electrode active material layer 12 contains the negative electrode active material of the present invention capable of occluding and releasing lithium ions. From the viewpoint of battery design, other materials such as a negative electrode binder (binder) and a conductive auxiliary agent are further included. May be included.

  The negative electrode active material of the present invention contains negative electrode active material particles. The negative electrode active material particles have a core portion capable of inserting and extracting lithium ions. When the negative electrode active material particles contain a carbon material in the surface layer portion, the negative electrode active material particle further has a carbon coating portion capable of obtaining conductivity.

The negative electrode active material particles contain a silicon compound (SiO _x : 0.5 ≦ x ≦ 1.6), and the composition of the silicon compound preferably has x close to 1. This is because stable battery characteristics can be obtained. The composition of the silicon compound in the present invention does not necessarily mean 100% purity, and may contain a trace amount of impurity element.

The negative electrode active material of the present invention is used as a negative electrode of a secondary battery having metallic lithium as a counter electrode, and the cycle of 0 V constant current constant voltage charging and 1.2 V constant current discharge of the secondary battery (hereinafter, After repeating 0V-1.2V cycle X times (X ≧ 0), the secondary battery is further charged with 0V constant current and constant voltage (however, charging is terminated 60 hours after reaching 0V) (hereinafter, 60V). The negative electrode active material after the end of charging has a peak in a range of 25 to 55 ppm and a range of 0 to 3 ppm as a chemical shift value, which is obtained from a ⁷ Li-MAS-NMR spectrum in a state of performing (time charging). Is characterized by. Hereinafter, the 0V-1.2V cycle to the charging for 60 hours will be collectively referred to as a charging condition A.

First, the details of the charging condition A will be described. First, 0V constant current / constant voltage charging (0VCCCV) means that after a secondary battery having lithium as a counter electrode is manufactured, the battery is charged to 0V in a constant current (current density: 0.5 mA / cm ² ) mode and then charged from 0V. This means that charging is terminated after the voltage mode is reached and the current density reaches 0.1 mA / cm ² . Next, 1.2 V constant current discharge means discharging in a constant current (current density: 0.5 mA / cm ² ) mode, and ending the discharge after the potential reaches 1.2 V. Next, 60-hour charging means charging to 0 V in a constant current (current density: 0.5 mA / cm ² ) mode, changing from 0 V to a constant voltage mode, and ending charging after 60 hours have passed from 0 V. Means

  As described above, the negative electrode active material of the present invention is an active material that can obtain the above-mentioned two types of peaks when charged under the above-mentioned charging condition A. As described above, if the negative electrode active material contains negative electrode active material particles containing a silicon compound, and has the above-mentioned two types of peaks after the end of charging, this negative electrode active material When used as a negative electrode active material of a lithium-ion secondary battery, it has a high battery capacity and good cycle characteristics.

  Here, it is assumed that the peak in the range of 25 to 55 ppm indicates the existence of Li-Si bond. In the negative electrode active material from which this peak is obtained, it is easy to stabilize the bulk state of silicon oxide by repeating Li insertion and desorption. Therefore, when the negative electrode active material that gives this peak is used as the negative electrode active material of a lithium ion secondary battery, stable battery characteristics, particularly stable cycle characteristics can be obtained.

  On the other hand, the peak in the range of 0 to 3 ppm is presumed to represent the presence of the Li silicate layer (Li—O bond). With respect to the negative electrode active material from which this peak is obtained, stable Li compounds are easily generated inside the negative electrode active material particles by repeating Li insertion and desorption. Therefore, in the negative electrode active material from which this peak is obtained, Li easily diffuses in the bulk of the silicon oxide. Therefore, the negative electrode active material from which this peak is obtained becomes a stable battery material and can improve the cycle characteristics.

  The number of X's in the 0V-1.2V cycle is not particularly limited. For example, the upper limit of X can be 99. That is, the range of X can be set to 0 ≦ X ≦ 99. The range of X is more preferably 0 ≦ X ≦ 49, further preferably 0 ≦ X ≦ 9, and particularly preferably X = 0.

When the upper limit of X is 99, the 0V-1.2V cycle was performed 0 times or more and 99 times or less, and then the negative electrode active material after charging was terminated was ⁷ Li-MAS-NMR spectrum in a state of being charged for 60 hours. As long as it has a chemical shift value having a peak in the range of 25 to 55 ppm and in the range of 0 to 3 ppm. In this case, for example, after performing the 0V-1.2V cycle 19 times, these peaks may be expressed at the 20th cycle for 60 hours, or after performing the 0V-1.2V cycle 49 times, These peaks may appear during 60-hour charging at the 50th cycle.

  In the present invention, a peak in the range of 0 to 3 ppm may be expressed first, and then a peak in the range of 25 to 55 ppm may be expressed in addition to this peak. For example, a peak in the range of 0 to 3 ppm appears at the time of 60-hour charging at the first cycle (that is, X = 0), and a range of 0 to 3 ppm at the time of 60-hour charging at the 10th cycle (that is, X = 9). A peak in the range of 25 to 55 ppm may be expressed together with the peak of.

  In the present invention, it is particularly preferable that two types of peaks are expressed within the number of times shown below.

  First, the peak in the range of 25 to 55 ppm is preferably expressed in X within 49 times.

  With such a negative electrode active material, when this negative electrode active material is used as the negative electrode active material of a lithium ion secondary battery, the time (cycle number) until the secondary battery is stabilized is further reduced. You can As a result, the cycle deterioration rate at the beginning of the charge / discharge cycle becomes smaller, and the cycle characteristics are further improved. A secondary battery including such a negative electrode active material has stable cycle characteristics.

  Moreover, it is preferable that the peak in the range of 0 to 3 ppm is expressed within 9 times of X.

  With such a negative electrode active material, when this negative electrode active material is used as a negative electrode active material of a lithium ion secondary battery, the time until a stable Li compound is generated inside the negative electrode active material particles (the number of cycles). ) Can be less. This makes it easier to diffuse Li in the bulk of the silicon oxide.

  Further, it is preferable that the peak in the range of 25 to 55 ppm and the peak in the range of 0 to 3 ppm are expressed when X is 0 times.

  With such a negative electrode active material, when this negative electrode active material is used as a negative electrode active material of a lithium-ion secondary battery, the time until the secondary battery is stabilized and the stability inside the negative electrode active material particles are stable. It is possible to further reduce the time until the Li compound is formed.

  Further, it is preferable that the peak in the range of 25 to 55 ppm is reduced while the cycle of 0V constant current constant voltage charging and 1.2V constant current discharging is repeated 49 times or less. That is, it is preferable that the negative electrode active material of the present invention develops and decreases a peak in the range of 25 to 55 ppm while repeating the 0 V to 1.2 V cycle within 49 times. In particular, it is preferable that the peak in the range of 25 to 55 ppm appears and disappears. As a specific example of this aspect, a peak in the range of 25 to 55 ppm appears at the time of 60-hour charging at the first cycle (that is, X = 0), and at the time of 60-hour charging at the 10th cycle (that is, X = 9). In the aspect, the peak in the range of 25 to 55 ppm decreases.

  With such a negative electrode active material, the bulk state of silicon oxide is further stabilized by repeating insertion and desorption of Li. That is, by repeating insertion and desorption of Li, the state of the active material becomes a state suitable for charging and discharging.

  In particular, the negative electrode active material of the present invention not only develops and decreases (especially disappears) peaks in the range of 25 to 55 ppm during repeated 0V-1.2V cycles, but this peak also exhibits 0V-1.2V cycles. It is preferable to gradually shift to a direction close to 0 ppm while repeating. Such a negative electrode active material can create a more stable bulk state by repeating Li insertion and desorption.

  The reason why the negative electrode active material in which the peak in the range of 25 to 55 ppm and the like develops as described above improves the cycle characteristics has not been completely clarified, but at least the charging was performed under the above charging condition A. In this case, it is clear that the active material which gives the above-mentioned two types of peaks improves the cycle characteristics.

  Further, the negative electrode active material has a half value width (2θ) of the diffraction peak due to the Si (111) crystal plane obtained by X-ray diffraction of 1.2 ° or more, and the crystallite size corresponding to the crystal plane is It is preferably 7.5 nm or less.

  The lower the silicon crystallinity of the silicon oxide is, the better. Since the Si crystallites in the silicon oxide have the above-mentioned crystallinity, the negative electrode active material containing such silicon oxide can be used as the negative electrode active material of a lithium ion secondary battery. When used as a substance, better cycle characteristics and initial charge / discharge characteristics are obtained.

The median diameter (D ₅₀ : particle diameter when the cumulative volume reaches 50%) of the negative electrode active material particles is not particularly limited, but is preferably 0.5 μm or more and 20 μm or less. This is because when the median diameter is in the above range, lithium ions are likely to be absorbed and released during charging and discharging, and the particles are less likely to be broken. When the median diameter is 0.5 μm or more, the surface area per mass can be reduced and the increase in battery irreversible capacity can be suppressed. On the other hand, when the median diameter is 20 μm or less, the particles are less likely to be cracked and the new surface is less likely to appear.

  Further, the negative electrode active material particles preferably include a carbon material in the surface layer portion.

  As described above, since the negative electrode active material particles include the carbon material in the surface layer thereof, the conductivity can be improved. Therefore, the negative electrode active material containing such negative electrode active material particles is used as the negative electrode active material of the lithium ion secondary battery. When used as a substance, the battery characteristics can be improved.

  The average thickness of this carbon material is preferably 1 nm or more and 5000 nm or less.

  If the average thickness of the carbon material to be coated is 1 nm or more, the conductivity is improved, and if the average thickness of the carbon material to be coated is 5000 nm or less, the negative electrode active material containing such negative electrode active material particles is treated with lithium. When used as a negative electrode active material for an ion secondary battery, it is possible to suppress a decrease in battery capacity.

  The average thickness of this carbon material can be calculated, for example, by the following procedure. First, the negative electrode active material is observed with a TEM (transmission electron microscope) at an arbitrary magnification. This magnification is preferably such that the thickness of the carbon material can be visually checked so that the thickness can be measured. Then, the thickness of the carbon material is measured at arbitrary 15 points. In this case, it is preferable to set measurement positions widely and randomly without concentrating on a specific place as much as possible. Finally, the average value of the above 15 carbon material thicknesses is calculated.

  The coverage of the carbon material is not particularly limited, but it is desirable that the coverage be as high as possible. When the coverage is 30% or more, the electrical conductivity is further improved, which is preferable. The carbon material coating method is not particularly limited, but a sugar carbonization method and a hydrocarbon gas thermal decomposition method are preferable. This is because the coverage can be improved.

  Further, as the negative electrode binder contained in the negative electrode active material layer 12, for example, one or more kinds of polymer materials, synthetic rubbers and the like can be used. The polymer material is, for example, polyvinylidene fluoride, polyimide, polyamideimide, aramid, polyacrylic acid, lithium polyacrylate, carboxymethyl cellulose, or the like. The synthetic rubber is, for example, styrene-butadiene rubber, fluorine-based rubber, ethylene propylene diene, or the like.

  As the negative electrode conduction aid, for example, any one or more of carbon materials such as carbon black, acetylene black, graphite, ketjen black, carbon nanotubes and carbon nanofibers can be used.

  The negative electrode active material layer 12 may include a carbon-based active material in addition to the negative electrode active material (silicon-based active material) of the present invention. This makes it possible to reduce the electric resistance of the negative electrode active material layer 12 and to relieve the expansion stress associated with charging. As the carbon-based active material, for example, pyrolytic carbons, cokes, glassy carbon fibers, organic polymer compound fired bodies, carbon blacks and the like can be used.

  The negative electrode active material layer 12 is formed by, for example, a coating method. The coating method is a method in which a silicon-based active material is mixed with the above-mentioned binder, etc., and if necessary, a conductive auxiliary agent and a carbon-based active material are mixed and then dispersed in an organic solvent or water and then coated.

[Method of manufacturing negative electrode]
The negative electrode 10 can be manufactured by the following procedure, for example. First, a method for manufacturing the negative electrode active material used for the negative electrode will be described. First, negative electrode active material particles containing a silicon compound represented by the general formula SiO _x (0.5 ≦ x ≦ 1.6) are prepared. Next, a secondary battery having a negative electrode containing the negative electrode active material containing the negative electrode active material particles and a counter electrode made of metallic lithium is prepared. Next, after repeating the cycle of 0V constant current constant voltage charging and 1.2V constant current discharging of the secondary battery thus manufactured X times (X ≧ 0), the secondary battery further 0V constant current Constant voltage charging (however, charging is terminated 60 hours after reaching 0 V) is performed. Next, in the state of termination of charging, the negative electrode active material is measured by ⁷ Li-MAS-NMR. Next, a negative electrode active material having peaks in the chemical shift values of 25 to 55 ppm and 0 to 3 ppm obtained from the ⁷ Li-MAS-NMR spectrum is selected.

The negative electrode active material particles containing silicon oxide (SiO _x : 0.5 ≦ x ≦ 1.6) can be produced, for example, by the following method. First, a raw material that generates a silicon oxide gas is heated under reduced pressure in the temperature range of 900 ° C. to 1600 ° C. in the presence of an inert gas to generate a silicon oxide gas. At this time, a mixture of metallic silicon powder and silicon dioxide powder can be used as the raw material. Considering the surface oxygen of the metal silicon powder and the presence of a trace amount of oxygen in the reaction furnace, it is desirable that the mixing molar ratio is in the range of 0.8 <metal silicon powder / silicon dioxide powder <1.3.

  Next, the generated silicon oxide gas is solidified and deposited on the adsorption plate (deposition plate). Next, a deposit of silicon oxide is taken out in a state where the temperature inside the reaction furnace is lowered to 100 ° C. or lower, and pulverized and pulverized by using a ball mill, a jet mill or the like. As described above, the negative electrode active material particles can be produced.

  The Si crystallites in the negative electrode active material particles change the vaporization temperature of the raw material that generates silicon oxide gas, the deposition plate temperature, the injection amount of gas (inert gas, reducing gas) with respect to the deposition flow of silicon oxide gas, or It can be controlled by the type, the heat treatment after the negative electrode active material particles are generated, or the temperature or time at which the carbon material described below is deposited.

  The cycle number at which the peak in the range of 25 to 55 ppm and the peak in the range of 0 to 3 ppm are expressed is the temperature of the deposition plate at the time of depositing silicon oxide, the heating temperature or the time at which the carbon material is deposited by the CVD described later, or It can be controlled by the pulverization conditions of the silicon oxide deposit. For example, when the temperature of the deposition plate at the time of depositing silicon oxide is increased, these peaks (particularly peaks in the range of 25 to 55 ppm) are likely to be obtained quickly. However, if this temperature is set too high, the expression of these peaks may be delayed.

  Next, a carbon material is formed on the surface layer portion of the prepared negative electrode active material particles. However, this step is not essential. A pyrolysis CVD method is desirable as a method of forming the carbon material layer. An example of a method of forming a carbon material layer by the thermal decomposition CVD method will be described below.

First, the negative electrode active material particles are set in the furnace. Next, a hydrocarbon gas is introduced into the furnace to raise the temperature inside the furnace. The decomposition temperature is not particularly limited, but it is preferably 1200 ° C or lower, and more preferably 950 ° C or lower. By setting the decomposition temperature to 1200 ° C. or lower, unintentional disproportionation of the negative electrode active material particles can be suppressed. After raising the temperature in the furnace to a predetermined temperature, a carbon material is generated on the surface layer portion of the negative electrode active material particles. The hydrocarbon gas used as the raw material of the carbon material is not particularly limited, but it is desirable that n ≦ 3 in the C _n H _m composition. When n ≦ 3, the manufacturing cost can be reduced and the physical properties of the decomposition product can be improved.

  As described above, by coating the negative electrode active material particles with the carbon material, the compound state inside the bulk can be made more uniform, the stability as an active material is improved, and a higher effect can be obtained.

  Next, a secondary battery having a negative electrode containing the negative electrode active material containing the above negative electrode active material particles and a counter electrode made of metallic lithium is prepared. Here, as a specific example of the secondary battery for this test, a 2032 type coin battery is taken as an example.

  First, the negative electrode used for the 2032 type coin battery is prepared. The negative electrode may contain the negative electrode active material of the present invention. For example, the negative electrode shown in FIG. The method for producing this negative electrode can be the same as the method for producing the negative electrode of the present invention. In addition, a counter electrode made of metallic lithium is prepared. A specific example is a metallic lithium foil having a thickness of 0.5 mm. Next, an electrolytic solution and a separator are prepared. Specific examples thereof include the same as those used for the secondary battery of the present invention described later.

  Subsequently, the bottom lid of the 2032 type coin battery, the lithium foil, and the separator are stacked, the electrolytic solution is injected, and then the negative electrode and the spacer (for example, 1.0 mm in thickness) are stacked and the electrolytic solution is injected. Then, the spring and the upper lid of the coin battery are sequentially pulled up and crimped by an automatic coin cell caulking machine, whereby a 2032 type coin battery can be manufactured.

  Next, after repeating the cycle of 0V constant current constant voltage charging and 1.2V constant current discharging of the secondary battery thus manufactured X times (X ≧ 0), the secondary battery further 0V constant current Constant voltage charging (however, charging is terminated 60 hours after reaching 0 V) is performed. The details of the charging condition A are as described above. The upper limit of X can be appropriately set according to the quality of the negative electrode active material to be manufactured (for example, 99), but it is preferable that it is as small as possible as described above.

Next, in the state of termination of charging, the negative electrode active material is measured by ⁷ Li-MAS-NMR. The negative electrode active material is measured by solid state ⁷ Li-MAS-NMR. At this time, the apparatus to be used is not particularly limited, but a 700 NMR spectroscope manufactured by Bruker Co. may be mentioned. At this time, a rotor having a diameter of 2.5 mm is used as the probe, the sample rotation speed is 16 kHz, and the measurement environment temperature can be 25 ° C.

In the following, as an example, a peak in the range of 0 to 3 ppm and a peak in the range of 25 to 55 ppm appear at the time of 60-hour charging in the first cycle (that is, X = 0), as an example, ⁷ Li-MAS-NMR measurement procedure Will be explained.

  About 20 coin batteries are usually required for each cycle to measure the NMR of the negative electrode active material. This is to ensure sufficient negative electrode active material to be packed in the NMR rotor. Therefore, in the case of the above example, 20 coin batteries may be prepared. First, 20 negative electrodes containing the negative electrode active material manufactured under the same manufacturing conditions are prepared. Next, 20 2032 type coin batteries having this negative electrode are manufactured, and these coin batteries are charged under charging condition A (X = 0). Next, 20 coin batteries in the state where charging is terminated are disassembled in the glove box, the negative electrode active material is peeled from the negative electrode, and packed in one NMR rotor. Next, NMR measurement of the negative electrode active material packed in the NMR rotor in this way is performed. By this measurement, it was confirmed that a peak in the range of 0 to 3 ppm and a peak in the range of 25 to 55 ppm were developed, and the test was terminated. In addition, when two types of peaks appear in the second cycle, a total of 40 coin batteries are manufactured. With such a measuring method, it is possible to reliably determine in what cycle the two types of peaks are developed.

Next, a negative electrode active material having peaks in the chemical shift values of 25 to 55 ppm and 0 to 3 ppm obtained from the ⁷ Li-MAS-NMR spectrum is selected. For example, when a negative electrode active material is manufactured under a certain manufacturing condition and NMR measurement of this negative electrode active material is performed and it is confirmed that two kinds of peaks are obtained within 9 times of X, the negative electrode active material manufactured under the same manufacturing condition is The negative electrode active material can be selected on the assumption that all the substances have two types of peaks in which X is expressed within 9 times. In addition, by changing the conditions for producing the negative electrode active material or the conditions for carbon coating and performing measurement by ⁷ Li-MAS-NMR each time, how long it takes to obtain two types of peaks under each production condition ( It can also be determined whether the number of cycles) takes.

  In the negative electrode active material manufactured by such a manufacturing method, the silicon dioxide component present in the bulk of the silicon oxide during reaction with Li changes to a stable Li compound, and the silicon-lithium bond state is changed to the secondary battery. Will be guided to a state suitable for.

  The negative electrode active material manufactured (sorted) as described above is mixed with other materials such as a negative electrode binder and a conductive additive to form a negative electrode mixture, and then an organic solvent or water is added. Use slurry. Next, the negative electrode mixture slurry is applied to the surface of the negative electrode current collector 11 and dried to form the negative electrode active material layer 12. At this time, a heating press or the like may be performed if necessary. The negative electrode can be manufactured as described above.

<Lithium-ion secondary battery>
Next, the lithium ion secondary battery of the present invention will be described. The lithium ion secondary battery of the present invention uses the above negative electrode of the present invention as the negative electrode. Here, as a specific example, a laminate film type lithium ion secondary battery will be described as an example.

[Structure of laminated film type secondary battery]
The laminated film type lithium ion secondary battery 30 shown in FIG. 3 mainly includes a wound electrode body 31 housed inside a sheet-shaped exterior member 35. This wound body has a separator between the positive electrode and the negative electrode and is wound. There is also a case where a separator is provided between the positive electrode and the negative electrode to house the laminate. In both electrode bodies, the positive electrode lead 32 is attached to the positive electrode and the negative electrode lead 33 is attached to the negative electrode. The outermost peripheral portion of the electrode body is protected by a protective tape.

  The positive and negative electrode leads are led out in one direction from the inside to the outside of the exterior member 35, for example. The positive electrode lead 32 is formed of a conductive material such as aluminum, and the negative electrode lead 33 is formed of a conductive material such as nickel and copper.

  The exterior member 35 is, for example, a laminate film in which a fusion layer, a metal layer, and a surface protection layer are laminated in this order. This laminate film has two sheets so that the fusion layer faces the spirally wound electrode body 31. The outer peripheral edge portions of the fusion layer of the film are fused or adhered to each other with an adhesive or the like. The fused portion is, for example, a film of polyethylene or polypropylene, and the metal portion is aluminum foil or the like. The protective layer is, for example, nylon or the like.

  An adhesive film 34 is inserted between the exterior member 35 and the positive and negative electrode leads to prevent outside air from entering. This material is, for example, polyethylene, polypropylene, or a polyolefin resin.

[Positive electrode]
The positive electrode has a positive electrode active material layer on both surfaces or one surface of the positive electrode current collector, as in the case of the negative electrode 10 in FIG.

  The positive electrode current collector is made of, for example, a conductive material such as aluminum.

  The positive electrode active material layer contains any one kind or two or more kinds of positive electrode materials capable of occluding and releasing lithium ions, and also contains other materials such as a binder, a conductive aid, and a dispersant depending on the design. You can leave. In this case, the details of the binder and the conductive additive can be the same as those of the negative electrode binder and the negative electrode conductive additive already described, for example.

A lithium-containing compound is desirable as the positive electrode material. Examples of the lithium-containing compound include a composite oxide including lithium and a transition metal element, or a phosphoric acid compound including lithium and a transition metal element. Among these positive electrode materials, compounds containing at least one of nickel, iron, manganese, and cobalt are preferable. Chemical formulas of these positive electrode materials, for _example, Li x M1O _2, or _is represented by Li y M2PO _4. In the above chemical formula, M1 and M2 represent at least one or more kinds of transition metal elements, and the values of x and y are different depending on the charge / discharge state of the battery, but generally 0.05 ≦ x ≦ 1. 10 and 0.05 ≦ y ≦ 1.10.

Examples of the composite oxide containing lithium and a transition metal element include lithium cobalt composite oxide (Li _x CoO ₂ ), lithium nickel composite oxide (Li _x NiO ₂ ), and the like. Examples of the phosphoric acid compound having: include a lithium iron phosphate compound (LiFePO ₄ ), a lithium iron manganese phosphate compound (LiFe _1-u Mn _u PO ₄ (0 <u <1)), and the like. This is because when the above positive electrode material is used, a high battery capacity can be obtained and also excellent cycle characteristics can be obtained.

[Negative electrode]
The negative electrode has the same configuration as the negative electrode 10 in FIG. 2 described above, and has, for example, negative electrode active material layers on both surfaces of the current collector. It is preferable that the negative electrode has a larger negative electrode charge capacity than the electric capacity (charge capacity as a battery) obtained from the positive electrode active material agent. This makes it possible to suppress the precipitation of lithium metal on the negative electrode.

  The positive electrode active material layer is provided on a part of both surfaces of the positive electrode current collector, and the negative electrode active material layer is also provided on a part of both surfaces of the negative electrode current collector. In this case, for example, the negative electrode active material layer provided on the negative electrode current collector is provided with a region in which the facing positive electrode active material layer does not exist. This is for stable battery design.

  In the region where the negative electrode active material layer and the positive electrode active material layer do not face each other, there is almost no influence of charge / discharge. Therefore, the state of the negative electrode active material layer is maintained as it is immediately after the formation, and thus the composition and the like of the negative electrode active material can be accurately investigated with good reproducibility without depending on the presence or absence of charge / discharge.

[Separator]
The separator separates the positive electrode and the negative electrode, prevents current short circuit due to contact between both electrodes, and allows lithium ions to pass through. This separator is formed of, for example, a porous film made of synthetic resin or ceramic, and may have a laminated structure in which two or more kinds of porous films are laminated. Examples of synthetic resins include polytetrafluoroethylene, polypropylene, and polyethylene.

[Electrolyte]
At least a part of the active material layer or the separator is impregnated with a liquid electrolyte (electrolytic solution). This electrolytic solution has an electrolyte salt dissolved in a solvent and may contain other materials such as additives.

  As the solvent, for example, a non-aqueous solvent can be used. Examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, 1,2-dimethoxyethane, and tetrahydrofuran. Among these, it is desirable to use at least one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. This is because better characteristics can be obtained. Further, in this case, by using a high-viscosity solvent such as ethylene carbonate or propylene carbonate and a low-viscosity solvent such as dimethyl carbonate, ethylmethyl carbonate or diethyl carbonate in combination, the dissociation property and ion mobility of the electrolyte salt are improved. be able to.

  When the alloy-based negative electrode is used, it is particularly preferable that the solvent contains at least one of halogenated chain ester carbonate or halogenated cyclic ester carbonate. As a result, a stable coating film is formed on the surface of the negative electrode active material during charge / discharge, especially during charge. Here, the halogenated chain ester carbonate is a chain ester carbonate having halogen as a constituent element (at least one hydrogen is replaced by halogen). The halogenated cyclic carbonic acid ester is a cyclic carbonic acid ester having halogen as a constituent element (that is, at least one hydrogen is replaced by halogen).

  The type of halogen is not particularly limited, but fluorine is preferable. This is because it forms a film of higher quality than other halogens. The larger the number of halogens, the more desirable. This is because the resulting coating is more stable and the decomposition reaction of the electrolytic solution is reduced.

  Examples of the halogenated chain ester carbonate include fluoromethyl methyl carbonate and difluoromethyl methyl carbonate. Examples of the halogenated cyclic carbonic acid ester include 4-fluoro-1,3-dioxolan-2-one and 4,5-difluoro-1,3-dioxolan-2-one.

  The solvent additive preferably contains an unsaturated carbon bond cyclic ester carbonate. This is because a stable coating film is formed on the surface of the negative electrode during charge / discharge, and the decomposition reaction of the electrolytic solution can be suppressed. Examples of the unsaturated carbon bond cyclic ester carbonate include vinylene carbonate and vinyl ethylene carbonate.

  It is also preferable that the solvent additive contains sultone (cyclic sulfonic acid ester). This is because the chemical stability of the battery is improved. Examples of the sultone include propane sultone and propene sultone.

  Furthermore, the solvent preferably contains an acid anhydride. This is because the chemical stability of the electrolytic solution is improved. Examples of the acid anhydride include propanedisulfonic acid anhydride.

The electrolyte salt may include, for example, one or more light metal salts such as a lithium salt. Examples of the lithium salt include lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), and the like.

  The content of the electrolyte salt is preferably 0.5 mol / kg or more and 2.5 mol / kg or less with respect to the solvent. This is because high ionic conductivity can be obtained.

[Method for manufacturing laminated film type lithium ion secondary battery]
First, a positive electrode is manufactured using the positive electrode material described above. First, a positive electrode active material and, if necessary, a binder, a conductive auxiliary agent, and the like are mixed to form a positive electrode mixture, which is then dispersed in an organic solvent to form a positive electrode mixture slurry. Next, the mixture slurry is applied to the positive electrode current collector with a coating device such as a knife roll or a die coater having a die head, and dried with hot air to obtain a positive electrode active material layer. Finally, the positive electrode active material layer is compression-molded with a roll press or the like. At this time, heating may be performed, or heating or compression may be repeated a plurality of times.

  Next, a negative electrode is prepared by forming a negative electrode active material layer on the negative electrode current collector by using the same procedure as that for preparing the negative electrode 10 described above.

  When manufacturing the positive electrode and the negative electrode, the respective active material layers are formed on both surfaces of the positive electrode and the negative electrode current collector. At this time, the active material coating lengths on both surfaces of both electrodes may be different (see FIG. 2).

  Then, an electrolytic solution is prepared. Subsequently, the positive electrode lead 32 is attached to the positive electrode current collector and the negative electrode lead 33 is attached to the negative electrode current collector by ultrasonic welding or the like. Subsequently, the positive electrode and the negative electrode are laminated or wound with a separator interposed therebetween to produce a wound electrode body 31, and a protective tape is adhered to the outermost peripheral portion thereof. Next, the wound body is formed into a flat shape. Subsequently, after sandwiching the spirally wound electrode body between the folded film-shaped exterior members, the insulating parts of the exterior member 35 are adhered to each other by a heat fusion method, and the spirally wound electrode body is released only in one direction. Is enclosed. An adhesive film is inserted between the positive electrode lead and the negative electrode lead and the exterior member. A predetermined amount of the above-prepared electrolytic solution is charged from the open part and vacuum impregnation is performed. After the impregnation, the open part is adhered by the vacuum heat fusion method. As described above, the laminated film type lithium ion secondary battery 30 can be manufactured.

  Hereinafter, the present invention will be described more specifically by showing Examples and Comparative Examples, but the present invention is not limited to the following Examples.

(Example 1-1)
The laminated film type lithium ion secondary battery 30 shown in FIG. 3 was produced by the following procedure.

First, the positive electrode was produced. The positive electrode active material was LiNi _0.7 Co _0.25 Al _0.05 O (lithium nickel cobalt aluminum composite oxide: NCA), which is a lithium nickel cobalt composite oxide, at 95 mass%, and a positive electrode conduction aid was 2.5 mass. % And 2.5 mass% of a positive electrode binder (polyvinylidene fluoride: PVDF) were mixed to prepare a positive electrode mixture. Then, the positive electrode mixture was dispersed in an organic solvent (N-methyl-2-pyrrolidone: NMP) to obtain a paste-like slurry. Subsequently, the slurry was applied to both surfaces of the positive electrode current collector with a coating device having a die head, and dried with a hot air dryer. At this time, the positive electrode current collector had a thickness of 15 μm. Finally, compression molding was performed with a roll press.

  Next, a negative electrode was produced. As the negative electrode active material, a raw material in which metallic silicon and silicon dioxide were mixed was introduced into a reaction furnace, vaporized in an atmosphere with a vacuum degree of 10 Pa, deposited on an adsorption plate, sufficiently cooled, and then the deposit was taken out. It was crushed with a ball mill. After adjusting the particle size, pyrolysis CVD was performed to form a carbon material on the surface layer of the negative electrode active material particles. Subsequently, the negative electrode active material particles and the precursor of the negative electrode binder (polyamic acid), the conductive auxiliary agent 1 (scaly graphite) and the conductive auxiliary agent 2 (acetylene black) were mixed in a dry mass of 80: 8: 10: 2. After mixing in a ratio, it was diluted with NMP to obtain a paste-like negative electrode mixture slurry. In this case, NMP was used as the solvent for the polyamic acid. Subsequently, the negative electrode mixture slurry was applied to both surfaces of the negative electrode current collector with a coating device and then dried. An electrolytic copper foil (thickness = 15 μm) was used as the negative electrode current collector. Finally, it was baked at 400 ° C. for 1 hour in a vacuum atmosphere. Thereby, the negative electrode binder (polyimide) was formed. Further, in this way, the negative electrode active material layers were formed on both surfaces of the negative electrode current collector. At this time, the negative electrode current collector contained carbon and sulfur, and the content of each of them was 100 mass ppm or less.

Next, after mixing a solvent (4-fluoro-1,3-dioxolan-2-one (FEC), ethylene carbonate (EC) and dimethyl carbonate (DMC)), an electrolyte salt (lithium hexafluorophosphate: LiPF 6) is mixed. ₆ ) was dissolved to prepare an electrolytic solution. In this case, the composition of the solvent was FEC: EC: DMC = 10: 20: 70 by volume ratio, and the content of the electrolyte salt was 1.0 mol / kg with respect to the solvent.

  Next, a secondary battery was assembled as follows. First, an aluminum lead was ultrasonically welded to one end of the positive electrode current collector, and a nickel lead was welded to one end of the negative electrode current collector. Then, the positive electrode, the separator, the negative electrode, and the separator were laminated in this order and wound in the longitudinal direction to obtain a wound electrode body. The end of winding was fixed with a PET protective tape. As the separator, a laminated film (thickness 12 μm) sandwiched between a film containing porous polypropylene as a main component and a film containing porous polyethylene as a main component was used. Subsequently, after sandwiching the electrode body between the exterior members, the outer peripheral edge portions except one side were heat-sealed to house the electrode body inside. As the exterior member, an aluminum laminated film in which a nylon film, an aluminum foil and a polypropylene film were laminated was used. Subsequently, the prepared electrolytic solution was injected from the opening and impregnated in a vacuum atmosphere, followed by heat fusion and sealing.

  The cycle characteristics and the initial charge / discharge characteristics of the secondary battery manufactured as described above were evaluated.

The cycle characteristics were examined as follows. First, in order to stabilize the battery, charge and discharge were performed for 2 cycles in an atmosphere of 25 ° C., and the discharge capacity at the 2nd cycle was measured. Subsequently, charging / discharging was performed until the total number of cycles reached 100 cycles, and the discharge capacity was measured each time. Finally, the discharge capacity at the 100th cycle was divided by the discharge capacity at the second cycle to calculate the capacity retention rate. As cycling conditions, a constant current density until reaching 4.2V, 2.5 mA / cm was charged with ^2, the current density at 4.2V constant voltage at the stage of reaching the voltage 4.2V 0.25 mA / cm ² Charged until reaching. Further, during discharging, the battery was discharged at a constant current density of 2.5 mA / cm ² until the voltage reached 2.5V.

  Regarding the initial charge / discharge characteristics, the initial efficiency (initial efficiency) (%) = (initial discharge capacity / initial charge capacity) × 100 was calculated. The atmosphere and temperature were the same as those when the cycle characteristics were examined, and the charging and discharging conditions were 0.2 times those when the cycle characteristics were examined.

Next, a 2032 type coin battery was assembled as a secondary battery for ⁷ Li-MAS-NMR measurement test. For each cycle, 20 coin batteries having a negative electrode containing a negative electrode active material manufactured under the same manufacturing conditions were prepared.

As the negative electrode, one prepared by the same procedure as the negative electrode of the laminate film type lithium ion secondary battery 30 in Example 1-1 was used. The deposition amount (also referred to as areal density) of the negative electrode active material layer per unit area on one surface of this negative electrode was 2.5 mg / cm ² .

  As the electrolytic solution, one prepared by the same procedure as that of the electrolytic solution of the laminated film type lithium ion secondary battery 30 in Example 1-1 was used.

  A 0.5 mm thick metallic lithium foil was used as the counter electrode. As the separator, polyethylene having a thickness of 20 μm was used.

  Subsequently, the bottom lid of the 2032 type coin battery, the lithium foil, and the separator are stacked, 150 mL of the electrolytic solution is injected, and subsequently, the negative electrode and the spacer (thickness 1.0 mm) are stacked, and 150 mL of the electrolytic solution is injected. Then, the spring and the upper lid of the coin battery were sequentially lifted up and crimped by an automatic coin cell caulking machine to prepare a 2032 type coin battery.

The measurement conditions of ⁷ Li-MAS-NMR were the same as the charging conditions A described above. That is, 0V-1.2V cycle was performed a predetermined number of times, and then charging was performed for 60 hours. Thereby, the measurement result of NMR in each cycle was obtained. The NMR measurement of the negative electrode active material was performed by disassembling a coin battery containing the negative electrode active material in a glove box, peeling the negative electrode active material from the negative electrode, and packing it in an NMR rotor. Further, the upper limit of X in the 0V-1.2V cycle in this example is set to 99. For example, a negative electrode active material in which a peak in the range of 25 to 55 ppm was not expressed by the time of 60-hour charging at the 100th cycle was regarded as a negative electrode active material in which this peak was not obtained, and "25 to 55 ppm in the table" was used. “None” is described.

(Example 1-2 to Example 1-10, Comparative Examples 1-1 to 1-4)
Oxygen in the bulk of silicon oxide, half-value width (half-value width) of the diffraction peak due to the Si (111) crystal plane obtained by X-ray diffraction (this also affects the crystallite size calculated from this half-value width). The presence / absence of a peak in the range of 25 to 55 ppm, the presence / absence of a peak in the range of 0 to 3 ppm, the presence / absence of "expression and decrease within 50 times" (that is, 0V-1.2V cycle is repeated within 49 times) Same as Example 1-1, except that the number of cycles (indicated by Cy) at the time of peak expression in the range of 25 to 55 ppm was changed. , A secondary battery was manufactured. Table 1 shows the results of Example 1-1 to Example 1-10 and Comparative Examples 1-1 to 1-4.

  The negative electrode active materials of Examples 1-1 to 1-10 and Comparative examples 1-1 to 1-4 had the following properties. The median diameter of the negative electrode active material particles was 4 μm. The negative electrode active material particles contained a carbon material having an average thickness of 100 nm in the surface layer portion. In particular, in all the negative electrode active materials of Examples 1-1 to 1-10, peaks in the range of 0 to 3 ppm were developed during the first cycle of 60-hour charging, and peaks in the range of 0 to 3 ppm that were once expressed were There was no decrease during repeated 0V-1.2V cycles. As will be described later, the peak appearing in the range of 25 to 55 ppm at the beginning of the charge / discharge cycle may shift toward 0 ppm by repeating the charge / discharge cycle. At this time, since the peak shift value of the shifted peak is larger than the peak shift value of the peak in the range of 0 to 3 ppm, the peak in the range of 0 to 3 ppm may be buried in the shifted peak.

  As shown in Table 1, in the silicon oxide represented by SiOx, when the value of x was out of the range of 0.5 ≦ x ≦ 1.6, the battery characteristics were deteriorated. For example, as shown in Comparative Example 1-1, when oxygen is insufficient (x = 0.3), a peak in the range of 25 to 55 ppm does not appear, and a stable Li silicate peak (range of 0 to 3 ppm) is obtained. Could not be obtained. Therefore, the capacity maintenance ratio of the secondary battery is significantly deteriorated. Although the initial efficiency of the secondary battery was a high value, the cycle characteristics were significantly reduced, and it was concluded comprehensively that the battery characteristics were poor. On the other hand, as shown in Comparative Example 1-3, when the amount of oxygen increased (x = 1.8), the electronic resistance and the ionic diffusion resistance increased, the battery evaluation was difficult, and the initial battery efficiency significantly decreased. Therefore, evaluation of cycle characteristics was stopped. Further, in Comparative Example 1-2, x of SiOx was 0.5, but two types of peaks were not expressed, so the results of cycle characteristics and initial efficiency were poor.

On the other hand, the secondary batteries using the negative electrode active material of the present invention (Examples 1-1 to 1-10) had good cycle characteristics. FIG. 1 is a ⁷ Li-MAS-NMR spectrum measured in Example 1-3 of the present invention. The peak in the range of 25 to 55 ppm in FIG. 1 is presumed to represent the presence of Li-Si bond.

On the other hand, FIG. 4 is a ⁷ Li-MAS-NMR spectrum measured using a general negative electrode containing silicon as a negative electrode active material. This spectrum was also obtained under the same conditions as in Example 1-3. That is, this spectrum shows that a 2032 type coin battery was produced in the same manner as in Example 1-3, and was charged only for 60 hours without performing 0V-1.2V cycle, and the negative electrode after charging for 60 hours was used as an NMR rotor. It was obtained by packing and performing NMR measurement.

As shown in FIG. 4, the peak value obtained from ⁷ Li-MAS-NMR when a general negative electrode of silicon simple substance is used appears around 10 ppm. On the other hand, as shown in FIG. 1, when the negative electrode containing silicon oxide was measured, the peak value was largely shifted to the plus side depending on the bulk condition of silicon oxide. It is speculated that this is due to the large distance between silicon atoms. After obtaining this peak value, this peak gradually shifts to a direction close to 0 ppm during repeated Li insertion and desorption (repetition of 0V-1.2V cycle), so that a stable bulk condition can be created. it can.

The peak in the range of 0 to 3 ppm shown in FIG. 1 indicates the reaction between the oxygen side of the silicon oxide and Li and is presumed to indicate the presence of the Li silicate layer. In the negative electrode active material from which this peak is obtained, a stable Li compound is easily generated inside the negative electrode active material particles by charge and discharge. Therefore, in the negative electrode active material from which this peak is obtained, Li easily diffuses in the bulk of the silicon oxide. Therefore, the negative electrode active material from which this peak is obtained becomes a stable battery material and can improve the cycle characteristics. The sharp peak around 0 ppm in FIG. 1 indicates the presence of LiPF ₆ , and is not essential. On the other hand, as shown in Comparative Example 1-4, the negative electrode active material in which a peak in the range of 0 to 3 ppm is not obtained cannot obtain a sufficient silicate layer even if Li insertion and desorption are repeated. It is considered that such a negative electrode active material has swelled SiO ₂ in the bulk and is difficult to store Li. Therefore, when this negative electrode active material is used as a negative electrode active material of a lithium-ion secondary battery, cycle characteristics are deteriorated.

  Further, as in Examples 1-1 to 1-6 and Examples 1-8 to 1-10, it is preferable that the peak in the range of 25 to 55 ppm be X within 49 times. With such a negative electrode active material, when this negative electrode active material is used as the negative electrode active material of a lithium ion secondary battery, the time (cycle number) until the secondary battery is stabilized is further reduced. You can As a result, the cycle deterioration rate at the beginning of the charge / discharge cycle becomes smaller, and the cycle characteristics are further improved. A secondary battery including such a negative electrode active material has stable cycle characteristics.

  In addition, as described above, charging and discharging are repeated for a state in which a peak that is considered to show a Li-Si bond is largely shifted to the positive side, and the peak becomes closer to 0, so that the inside of the bulk of the silicon oxide is stabilized. Therefore, like Example 1-1 to Example 1-6, Example 1-8, and Example 1-9, Li-Si that was once expressed during the 0V-1.2V cycle repeated 49 times or less. It is desirable that peaks showing binding (peaks in the range of 25 to 55 ppm) be reduced. In particular, it is preferable that this peak disappear (disappear). Thus, by "developing and decreasing within 50 times", the bulk of silicon oxide becomes stable earlier in the charge / discharge cycle.

  For example, comparing Example 1-5 and Example 1-10 in which the expression cycle of this peak is the same (40th cycle), after this peak was expressed within 50 times, more than 50 cycles (specifically, Example 1-5 in which “expression and decrease within 50 times” suppress the decrease in battery retention rate at the beginning of the cycle, and have cycle characteristics, as compared with Example 1-10 in which the cycle characteristic decreases 70 times). To improve. Therefore, when comprehensively judged, it is desirable that the above-mentioned peak is expressed and reduced at the earliest possible cycle.

(Example 2-1 to Example 2-6)
Secondary batteries were produced under the same conditions as in Example 1-3, except that the median diameter of the negative electrode active material particles was changed, and the cycle characteristics and the initial efficiency were evaluated. Table 2 shows the results. The results of Examples 1-3 above are also shown in Tables 2 to 4 below.

  As shown in Table 2, when the particle size of the negative electrode active material particles was 0.5 μm or more, an increase in the surface area could be suppressed, so that both the battery maintenance ratio and the initial efficiency tended to be better. Further, it has been found that when the particle size is 20 μm or less, the negative electrode active material is less likely to expand during charging and the negative electrode active material is less likely to be broken, so that the battery characteristics can be improved.

(Example 3-1)
A secondary battery was produced under the same conditions as in Example 1-3, except that the negative electrode current collector did not contain carbon and sulfur, and the cycle characteristics and the initial efficiency were evaluated. The results are shown in Table 3.

  As shown in Table 3, when the negative electrode current collector contains carbon and sulfur in an amount of 100 ppm by mass or less, it is possible to suppress the deformation of the negative electrode during charging. As a result, it was found that the battery maintenance rate was improved.

(Examples 4-1 to 4-7)
Secondary batteries were produced under the same conditions as in Example 1-3, except that the thickness of the carbon material was changed, and the cycle characteristics and the initial efficiency were evaluated. The results are shown in Table 4.

  As a result of evaluating the battery characteristics by changing the thickness of the carbon material, both the initial efficiency and the maintenance rate of the battery were lowered when the carbon material was not deposited. It is presumed that the carbon material has an effect of suppressing the decomposition of the electrolytic solution. The battery characteristics are stabilized by increasing the thickness of the carbon material, but it becomes difficult to improve the battery capacity as the carbon material becomes thicker. Even with a thickness of about 5 μm (5000 nm), it is difficult to improve the battery capacity. Further, in the experiment in which the thickness of the carbon material was about 7 μm, the capacity did not develop further. Based on these results, it is considered that the thickness of the carbon material is preferably 5 μm or less.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, has substantially the same configuration as the technical idea described in the scope of the claims of the present invention, and has any similar effect to the present invention. It is included in the technical scope of the invention.

10 ... Negative electrode, 11 ... Negative electrode collector, 12 ... Negative electrode active material layer,
30 ... Laminated film type lithium ion secondary battery, 31 ... Winding electrode body,
32 ... Positive electrode lead, 33 ... Negative electrode lead, 34 ... Adhesive film,
35 ... Exterior member.

Claims (2)

A method for producing a negative electrode active material containing negative electrode active material particles,
A step of preparing negative electrode active material particles containing a silicon compound represented by the general formula SiO _x (0.5 ≦ x ≦ 1.6),
A step of producing a secondary battery having a negative electrode containing a negative electrode active material containing the negative electrode active material particles, and a counter electrode made of metallic lithium;
After the cycle of 0V constant current constant voltage charging and 1.2V constant current discharging of the secondary battery is repeated X times (X ≧ 0), the secondary battery is further charged to 0V constant current constant voltage (provided that 0V is reached. 60 hours after the end of the process of charging)
Measuring the negative electrode active material by ⁷ Li-MAS-NMR in the state of termination of charging;
And a step of selecting a negative electrode active material having a peak in a range of 25 to 55 ppm and a range of 0 to 3 ppm as a chemical shift value, obtained from the ⁷ Li-MAS-NMR spectrum. Manufacturing method. A lithium ion secondary battery characterized by producing a negative electrode using the negative electrode active material produced by the method for producing a negative electrode active material according to claim 1 , and producing a lithium ion secondary battery using the produced negative electrode. Battery manufacturing method.

Images