JP6746526B2 - Negative electrode active material, mixed negative electrode active material, and method for producing negative electrode active material - Google Patents

Description

The present invention relates to a negative electrode active material, a mixed negative electrode active material, and a method for producing a negative electrode active material.

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 high energy density, is under way. This secondary battery is not limited to small electronic devices, but is also being considered for application to large electronic devices typified by automobiles and power storage systems typified by houses.

Among them, a lithium ion secondary battery is easily expected to be small in size and high in capacity, and can obtain a higher energy density than a lead battery or a nickel-cadmium battery, and thus is highly expected.

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

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 a silicon material as a negative electrode active material is being studied not only for silicon itself, but also for alloys and compounds represented by oxides. Further, the active material shape has been studied from a standard coating type for carbon-based active materials to an integrated type 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 cracked. When the surface layer of the negative electrode active material is broken, a new surface is generated thereby, and the reaction area of the active material is increased. At this time, a decomposition reaction of the electrolytic solution occurs on the new surface, and a coating film, which is a decomposition product of the electrolytic solution, is formed on the new surface, so that the electrolytic solution is consumed. Therefore, the cycle characteristics are likely to deteriorate.

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

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 Patent Document 1, for example). Further, in order to obtain high battery capacity and safety, a carbon material (electron conductive material) is provided on the surface layer of the silicon oxide particles (for example, refer to Patent Document 2). Further, in order to improve 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 in a place near the current collector. (For example, refer to 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 (see, for example, Patent Document 6). In order to improve the 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). Further, a metal oxide containing lithium is used to improve the 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 the cycle characteristics, silicon oxide is used, and conductivity is imparted by forming a graphite film 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 spectra 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. In addition, particles having a silicon microcrystalline phase dispersed in silicon dioxide are used in order to improve high battery capacity and cycle characteristics (see, for example, 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 Japanese Patent Laid-Open No. 2002-042806 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 improved 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 an initial efficiency and cycle characteristics that are similar to those of a lithium ion secondary battery using a carbon-based active material. However, it has not been possible to propose a negative electrode active material having the same initial efficiency and cycle stability as a lithium ion secondary battery using a carbon-based active material.

The present invention has been made in view of the above problems, and when used as a negative electrode active material of a secondary battery, a negative electrode active material capable of improving initial charge/discharge characteristics and cycle characteristics, and the negative electrode active material. An object is to provide a mixed negative electrode active material containing a substance, a negative electrode having a negative electrode active material layer formed of this negative electrode active material, and a lithium ion secondary battery using the negative electrode active material of the present invention. .. Moreover, it aims at providing the method of manufacturing the negative electrode active material of this invention which can improve initial charging/discharging characteristics and cycle characteristics.

To achieve the above object, the present invention provides a negative electrode active material including negative electrode active material particles, wherein the negative electrode active material particles include a silicon compound (SiO _x : 0.5≦x≦1.6). Silicon compound particles are contained, the silicon compound particles contain at least one kind of Li ₂ SiO ₃ and Li ₄ SiO ₄ , and the negative electrode active material particles contain at least one of Cl ion and SO ₄ ion. One of them is contained on the surface, the content of the Cl ions is 5 mass ppm or more and 200 mass ppm or less with respect to the mass of the negative electrode active material particles, and the content of SO ₄ ions is the negative electrode active material particles. The present invention provides a negative electrode active material, which satisfies at least one of 5 mass ppm or more and 200 mass ppm or less with respect to the mass.

Thus, comprises either or both of the anode active material particles Cl ions and SO ₄ ions to the surface, and the content of either or both of the Cl ions and SO ₄ ions, the anode active material particles When the content is 5 mass ppm or more and 200 mass ppm or less with respect to the mass, the negative electrode active material has good electron conductivity. Therefore, the negative electrode active material of the present invention can improve the initial efficiency and cycle characteristics of the secondary battery. When the content is less than 5 mass ppm, the effect of improving the electronic conductivity cannot be sufficiently obtained. Further, when the content exceeds 200 mass ppm, the viscosity of the slurry increases and it becomes difficult to produce a good electrode.

At this time, the negative electrode active material particles contain both Cl ions and SO ₄ ions on the surface, and the content of the Cl ions is 5 mass ppm or more and 200 mass ppm or less with respect to the mass of the negative electrode active material particles. And that the content of SO ₄ ions is 5 mass ppm or more and 200 mass ppm or less with respect to the mass of the negative electrode active material particles.

The negative electrode active material of the present invention contains both Cl ions and SO ₄ ions on the surface, and both contents satisfy 5 mass ppm or more and 200 mass ppm or less with respect to the mass of the negative electrode active material particles. Especially, the initial efficiency and cycle characteristics of the secondary battery can be improved.

In addition, the negative electrode active material particles include LiOH and Li ₂ CO ₃ on the surface, and the content of LiOH is 0.01% by mass or more and 5.00% by mass or less with respect to the mass of the negative electrode active material particles. It is preferable that the content of Li ₂ CO ₃ satisfies 0.01% by mass or more and 5.00% by mass or less with respect to the mass of the negative electrode active material particles.

If the content of Li ₂ CO ₃ and LiOH on the surface of the negative electrode active material particles is 0.01% by mass or more with respect to the mass of the negative electrode active material, the amount of Li used as a medium when Li diffuses Since it is sufficiently present, the electron conductivity is further improved. Further, when the content of Li ₂ CO ₃ and the content of LiOH are each 5.00 mass% or less, the amount of these Li compounds is an appropriate amount, so that the effect of improving the electron conductivity can be reliably obtained. Such a negative electrode active material of the present invention can further improve cycle characteristics.

Further, it is preferable that the negative electrode active material particles have at least one of LiCl and Li ₂ SO _{4 on} the surface.

The negative electrode active material in which the negative electrode active material particles have the above-described particles on the surface thereof can further improve the cycle characteristics.

Further, the silicon compound particles have a full width at half maximum (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.

If the negative electrode active material having silicon compound particles having the above-mentioned silicon crystallinity is used as the negative electrode active material of a lithium ion secondary battery, better cycle characteristics and initial charge/discharge characteristics can be obtained.

In addition, the negative electrode active material of the present invention has a maximum peak intensity value A of Si and Li silicate regions, which is obtained from ²⁹ Si-MAS-NMR spectrum and is given as a chemical shift value of −60 to −95 ppm in the silicon compound particles. And it is preferable that the peak intensity value B in the SiO ₂ region given as a chemical shift value of −96 to −150 ppm satisfies the relationship of A>B.

In the silicon compound particles, if the amounts of Si and Li ₂ SiO ₃ are larger than that of the SiO ₂ component, the negative electrode active material can sufficiently obtain the effect of improving the battery characteristics by inserting Li.

Further, a test cell composed of a negative electrode containing a mixture of the negative electrode active material and a carbon-based active material and a counter electrode lithium was prepared, and in the test cell, charging was performed by passing an electric current so as to insert lithium into the negative electrode active material. , Charging and discharging consisting of discharging a current so as to desorb lithium from the negative electrode active material is carried out 30 times, and the discharge capacity Q in each charge and discharge is differentiated with the potential V of the negative electrode with reference to the counter lithium. When a graph showing the relationship between the differentiated value dQ/dV and the potential V is drawn, the potential V of the negative electrode during the X-th discharge (1≦X≦30) and thereafter is 0.40 V to 0. It preferably has a peak in the range of 55V.

The above-mentioned peak in the V-dQ/dV curve is similar to the peak of the silicon material, and the discharge curve on the higher potential side rises sharply, which makes it easy to develop the capacity when designing the battery. In addition, if the above-mentioned peak is manifested within 30 times of charge/discharge, the negative electrode active material will have a stable bulk.

The negative electrode active material particles preferably have a median diameter of 1.0 μm or more and 15 μm or less.

When the median diameter is 1.0 μm or more, it is possible to suppress an increase in battery irreversible capacity due to an increase in surface area per mass. On the other hand, when the median diameter is 15 μm or less, the particles are less likely to be broken, and the new surface is less likely to appear.

Further, it is preferable that the negative electrode active material particles include a carbon material in the surface layer portion.

As described above, the negative electrode active material particles include the carbon material in the surface layer portion thereof, whereby the conductivity can be improved.

The average thickness of the carbon material is preferably 10 nm or more and 5000 nm or less.

When the average thickness of the carbon material is 10 nm or more, improved conductivity can be obtained. If the average thickness of the carbon material to be coated is 5000 nm or less, a sufficient amount of silicon compound particles can be secured by using a negative electrode active material containing such negative electrode active material particles in a lithium ion secondary battery. It is possible to suppress a decrease in battery capacity.

Provided is a mixed negative electrode active material, comprising the above negative electrode active material and a carbon-based active material.

Thus, the conductivity of the negative electrode active material layer can be improved by including the carbon active material together with the negative electrode active material (silicon negative electrode active material) of the present invention as the material for forming the negative electrode active material layer. At the same time, it becomes possible to relieve the expansion stress associated with charging. In addition, the battery capacity can be increased by mixing the silicon-based negative electrode active material with the carbon-based active material.

Further, in order to achieve the above object, the present invention includes the above mixed negative electrode active material, and the ratio of the mass of the negative electrode active material to the total mass of the negative electrode active material and the carbon-based active material is 6 Provided is a negative electrode for a non-aqueous electrolyte secondary battery, the content of which is at least mass%.

If the ratio of the mass of the negative electrode active material (silicon-based negative electrode active material) to the total mass of the negative electrode active material (silicon-based negative electrode active material) and the carbon-based active material is 6% by mass or more, the battery capacity is further improved. It is possible to improve.

In order to achieve the above object, the present invention has a negative electrode active material layer formed of the above mixed negative electrode active material, and a negative electrode current collector, wherein the negative electrode active material layer is the negative electrode current collector. Provided is a negative electrode for a non-aqueous electrolyte secondary battery, which is formed on a body, in which the negative electrode current collector 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 carbon and sulfur in the amounts described above, so that the deformation of the negative electrode during charging can be suppressed.

Further, to achieve the above object, the present invention provides a lithium ion secondary battery using a negative electrode containing the above negative electrode active material.

A lithium-ion secondary battery using a negative electrode containing such a negative electrode active material has high capacity and good cycle characteristics and initial charge/discharge characteristics.

In order to achieve the above object, the present invention is a method for producing a negative electrode active material containing negative electrode active material particles containing silicon compound particles, wherein the silicon compound (SiO _x : 0.5≦x≦1 Negative electrode active material particles by a step of producing silicon compound particles containing (6.6) and a step of inserting lithium into the silicon compound particles to contain at least one or more of Li ₂ SiO ₃ and Li ₄ SiO _4. From the negative electrode active material particles, the content of the Cl ions is 5 mass ppm or more and 200 mass ppm or less with respect to the mass of the negative electrode active material particles, and the content of the SO ₄ ions is A step of selecting at least one of 5 mass ppm or more and 200 mass ppm or less with respect to the mass of the negative electrode active material particles, and using the selected negative electrode active material particles, a negative electrode Provided is a method for producing a negative electrode active material, which comprises producing an active material.

By selecting the silicon-based active material particles in this way to produce the negative electrode active material, the high capacity and good cycle characteristics and initial charge/discharge characteristics when used as the negative electrode active material of a lithium ion secondary battery are obtained. It is possible to manufacture a negative electrode active material having

At this time, the method for producing the negative electrode active material may further include a step of containing at least one of Cl ions and SO ₄ ions on the surface of the silicon compound particles.

Thus, at least one of Cl ion and SO ₄ ion may be added to the surface of the silicon compound particles.

Further, in order to achieve the above object, the present invention is to manufacture a negative electrode using the negative electrode active material manufactured by the above-described method for manufacturing a negative electrode active material, and to manufacture a lithium ion secondary battery using the manufactured negative electrode. A method for manufacturing a lithium ion secondary battery is provided.

By using the negative electrode active material manufactured as described above, it is possible to manufacture a lithium-ion secondary battery having a high capacity and good cycle characteristics and initial charge/discharge characteristics.

When the negative electrode active material of the present invention is used as a negative electrode active material of a secondary battery, it has high capacity and good cycle characteristics and initial charge/discharge characteristics. Further, the same effect can be obtained in the mixed negative electrode active material containing the negative electrode active material, the negative electrode, and the lithium ion secondary battery. 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 having good cycle characteristics and initial charge/discharge characteristics when used as a negative electrode active material of a secondary battery.

It is sectional drawing which shows the structure of the negative electrode for non-aqueous electrolyte secondary batteries of this invention. It is an example of a ²⁹ Si-MAS-NMR spectrum measured from silicon compound particles when modified by a redox method. It is an example of a ²⁹ Si-MAS-NMR spectrum measured from silicon compound particles when modified by a thermal doping method. It is a figure showing the structural example (laminate film type) of the lithium secondary battery of this invention. 3 is a graph showing the relationship between the ratio of silicon-based active material particles to the total amount of negative electrode active material and the rate of increase in battery capacity of the secondary battery.

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, it has been considered to use a negative electrode using a silicon material as a main material as a negative electrode of a lithium-ion secondary battery. A lithium-ion secondary battery using this silicon material is desired to have initial charge/discharge characteristics and cycle characteristics that are similar to those of a lithium-ion secondary battery using a carbon-based active material. It has not been possible to propose a negative electrode active material having the same initial charge/discharge characteristics and cycle characteristics as those of the lithium ion secondary battery.

Therefore, the present inventors have conducted extensive studies to obtain a negative electrode active material which, when used in a secondary battery, has a high battery capacity and good cycle characteristics and initial efficiency, and has arrived at the present invention.

The negative electrode active material of the present invention contains negative electrode active material particles. The negative electrode active material particles contain silicon compound particles containing a silicon compound (SiO _x : 0.5≦x≦1.6). The silicon compound particles contain at least one lithium silicate of Li ₂ SiO ₃ and Li ₄ SiO ₄ . The negative electrode active material particles contain at least one of Cl ions and SO ₄ ions on the surface, and the content of Cl ions is 5 mass ppm or more and 200 mass ppm or less with respect to the mass of the negative electrode active material particles. And that the content of SO ₄ ions is 5 mass ppm or more and 200 mass ppm or less with respect to the mass of the negative electrode active material particles.

Since such a negative electrode active material contains negative electrode active material particles containing silicon compound particles (also referred to as silicon-based active material particles), the battery capacity can be improved. In addition, since the SiO ₂ component part, which becomes unstable when lithium is inserted and released during charge/discharge of the battery in the silicon compound, is modified in advance to lithium silicate, the irreversible capacity generated during charging is reduced. can do. Further, if such a negative electrode active material, one or both of the Cl ions and SO ₄ ions include on the surface of the anode active material particles, and containing either or both of the Cl ions and SO ₄ ions When the amount is 5 mass ppm or more and 200 mass ppm or less with respect to the mass of the negative electrode active material particles, the electron conductivity becomes good. Therefore, the negative electrode active material of the present invention can improve the initial efficiency and cycle characteristics of the secondary battery.

In addition, in particular, the negative electrode active material particles include both Cl ions and SO ₄ ions on the surface, and the content of Cl ions is 5 mass ppm or more and 200 mass ppm or less with respect to the mass of the negative electrode active material particles, It is preferable that the content of SO ₄ ions is both 5 mass ppm or more and 200 mass ppm or less with respect to the mass of the negative electrode active material particles. The negative electrode active material containing such negative electrode active material particles can particularly improve the initial efficiency and cycle characteristics of the secondary battery.

<Negative electrode for non-aqueous electrolyte secondary battery>
First, the negative electrode for non-aqueous electrolyte secondary battery will be described. FIG. 1 shows a cross-sectional structure of a negative electrode for a non-aqueous electrolyte secondary battery (hereinafter, also referred to as “negative electrode”) according to an embodiment of the present invention.

[Negative electrode configuration]
As shown in FIG. 1, 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. The negative electrode active material layer 12 may be provided on both surfaces of the negative electrode current collector 11 or only on 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 a 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). The 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 is improved. In particular, in the case where the active material layer which expands at the time of charging is included, if the current collector contains any of the above elements, deformation of the electrode including the current collector can be suppressed. The content of each of the above-mentioned contained elements is not particularly limited, but is preferably 100 ppm by mass or less. This is because a higher deformation suppressing effect can be obtained. The cycle characteristic can be further improved by such a deformation suppressing effect.

Further, the surface of the negative electrode current collector 11 may or may not be roughened. The roughened negative electrode current collector is, for example, a metal foil 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, and from the viewpoint of battery design, further, other materials such as a negative electrode binder (binder) and a conductive auxiliary agent. May be included. The negative electrode active material includes negative electrode active material particles, and the negative electrode active material particles include silicon compound particles containing a silicon compound (SiO _x : 0.5≦x≦1.6).

In addition, the negative electrode active material layer 12 may include a mixed negative electrode active material material including the negative electrode active material (silicon-based negative electrode active material) of the present invention and a carbon-based active material. As a result, the electric resistance of the negative electrode active material layer is reduced, and the expansion stress associated with charging can be relieved. Examples of the carbon-based active material that can be used include pyrolytic carbons, cokes, glassy carbon fibers, organic polymer compound fired bodies, and carbon blacks.

In the negative electrode of the present invention, the mass ratio of the negative electrode active material (silicon-based negative electrode active material) to the total mass of the negative electrode active material (silicon-based negative electrode active material) and carbon-based active material of the present invention is 6% by mass. The above is preferable. When the ratio of the mass of the negative electrode active material of the present invention to the total mass of the negative electrode active material of the present invention and the carbon-based active material is 6% by mass or more, it is possible to reliably improve the battery capacity.

Further, as described above, the negative electrode active material of the present invention contains silicon compound particles, and the silicon compound particles are silicon oxide raw materials containing a silicon compound (SiO _x : 0.5≦x≦1.6). The composition is preferably such that x is close to 1. This is because high cycle characteristics can be obtained. The composition of the silicon compound in the present invention does not necessarily mean a purity of 100%, and may contain a slight amount of impurity element.

In the negative electrode active material of the present invention, the silicon compound particles contain at least one kind of Li ₂ SiO ₃ and Li ₄ SiO ₄ . The battery characteristics are improved by the presence of at least one kind of Li ₄ SiO ₄ and Li ₂ SiO ₃ inside the bulk of the silicon compound particles, but the battery characteristics are further improved when the above two kinds of lithium silicate are coexisted. These lithium silicates can be quantified by NMR (Nuclear Magnetic Resonance) or XPS (X-ray photoelectron spectroscopy: X-ray photoelectron spectroscopy). The XPS and NMR measurements can be performed, for example, under the following conditions.

XPS
・Device: X-ray photoelectron spectrometer,
・X-ray source: monochromatic Al Kα ray,
・X-ray spot diameter: 100 μm
-Ar ion gun sputtering conditions: 0.5 kV/2 mm x 2 mm.
²⁹ Si MAS NMR (Magic Angle Rotation Nuclear Magnetic Resonance)
-Apparatus: 700 NMR spectrometer manufactured by Bruker,
・Probe: 4 mm HR-MAS rotor 50 μL,
・Sample rotation speed: 10 kHz,
-Measuring environment temperature: 25°C.

In addition, the silicon compound particles have a full width at half maximum (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 compound in the silicon compound particles, the better. Particularly, if the amount of Si crystals present is small, the battery characteristics can be improved and a stable Li compound can be produced.

Further, the negative electrode active material of the present invention has a maximum peak intensity value A of Si and Li silicate regions, which is obtained from ²⁹ Si-MAS-NMR spectrum and is given as a chemical shift value of −60 to −95 ppm in silicon compound particles. It is preferable that the peak intensity value B in the SiO ₂ region given as a chemical shift value of −96 to −150 ppm satisfies the relation of A>B. In the silicon compound particles, if the amount of the silicon component or Li ₂ SiO ₃ is relatively large when the SiO ₂ component is used as a reference, the effect of improving the battery characteristics by inserting Li can be sufficiently obtained. The measurement conditions of ²⁹ Si-MAS-NMR may be the same as above.

Further, in the negative electrode active material of the present invention, the negative electrode active material particles preferably include a carbon material in the surface layer portion. By including a carbon material in the surface layer of the negative electrode active material particles, it is possible to obtain improved conductivity, and thus when using a negative electrode active material containing such negative electrode active material particles as a negative electrode active material of a secondary battery. The battery characteristics can be improved.

The average thickness of the carbon material in the surface layer of the negative electrode active material particles is preferably 10 nm or more and 5000 nm or less. When the average thickness of the carbon material is 10 nm or more, the conductivity is improved, and when 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 used as a lithium ion electrolyte. When used as a negative electrode active material for a 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 particles are 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 confirmed 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 the measurement position widely and randomly without concentrating on a specific place as much as possible. Finally, the average value of the above-mentioned 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.

In addition, the negative electrode active material particles include LiOH and Li ₂ CO ₃ on the surface, and the content of LiOH is 0.01% by mass or more and 5.00% by mass or less with respect to the mass of the negative electrode active material particles, and It is preferable that the content of Li ₂ CO ₃ satisfies 0.01 mass% or more and 5.00 mass% or less with respect to the mass of the negative electrode active material particles. When the negative electrode active material particles contain Li ₂ CO ₃ and LiOH on the surface with such a content, the electron conductivity is further improved. As a result, the cycle characteristics of the secondary battery can be further improved.

Further, the negative electrode active material particles preferably include at least one of LiCl and Li ₂ SO _{4 on} the surface. By replacing part of LiOH on the surface of the negative electrode active material particles with LiCl and Li ₂ SO ₄ , Li conductivity on the surface of the negative electrode active material particles is improved, and cycle characteristics can be further improved. Become.

In addition, the median diameter (D ₅₀ : particle diameter when the cumulative volume becomes 50%) of the negative electrode active material particles is preferably 1.0 μm or more and 15 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 particles are less likely to break. When the median diameter is 1.0 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 15 μm or less, the particles are less likely to be broken, and the new surface is less likely to appear.

Further, the negative electrode active material of the present invention (silicon-based active material), a test cell composed of a negative electrode and a counter electrode lithium containing a mixture of the silicon-based active material and the carbon-based active material, to prepare a test cell, Charging/discharging consisting of charging with a current for inserting lithium into the silicon-based active material and discharging with a current for desorbing lithium from the silicon-based active material was carried out 30 times, and the discharge capacity Q at each charging/discharging was measured. When a graph showing the relationship between the differential value dQ/dV obtained by differentiating with the potential V of the negative electrode with reference to the counter electrode lithium and the potential V is drawn, the negative electrode at the time of discharging after the Xth time (1≦X≦30) It is preferable that the potential V of the electrode has a peak in the range of 0.40V to 0.55V. The above-mentioned peak in the V-dQ/dV curve is similar to the peak of the silicon material, and the discharge curve on the higher potential side rises sharply, which makes it easy to develop the capacity when designing the battery. Moreover, it can be judged that a stable bulk is formed if it is a negative electrode active material in which the above-mentioned peak is developed by charging and discharging within 30 times.

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

[Negative electrode manufacturing method]
The negative electrode can be manufactured, for example, by the following procedure. First, a method for manufacturing the negative electrode active material used for the negative electrode will be described. First, silicon compound particles containing a silicon compound (SiO _x : 0.5≦x≦1.6) are prepared. Next, lithium is inserted into the silicon compound particles to contain at least one of Li ₂ SiO ₃ and Li ₄ SiO ₄ . Next, the surface of the silicon compound particles may be made to contain at least one of Cl ion and SO ₄ ion. The step of containing Cl ions or SO ₄ ions on the surface of the silicon compound particles may be performed simultaneously with the step of inserting lithium into the silicon compound particles. Thereby, negative electrode active material particles are produced. Alternatively, the silicon compound particles may be coated with a carbon material and then lithium may be inserted into the silicon compound particles.

Next, from the negative electrode active material particles, the content of Cl ions is 5 mass ppm or more and 200 mass ppm or less based on the mass of the negative electrode active material particles, and the content of SO ₄ ions of the negative electrode active material particles is A material satisfying at least one of 5 mass ppm to 200 mass ppm is selected. Then, the negative electrode active material is manufactured using the selected negative electrode active material particles.

More specifically, the negative electrode active material can be manufactured as follows. First, a raw material for generating silicon oxide gas is heated in the presence of an inert gas under reduced pressure in a temperature range of 900° C. to 1600° C. to generate silicon oxide gas. 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.

The generated silicon oxide gas is solidified and deposited on the adsorption plate. Next, a silicon oxide deposit 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. Silicon compound particles can be produced as described above. The Si crystallites in the silicon compound particles can be controlled by changing the vaporization temperature or by heat treatment after generation.

Here, a carbon material layer may be formed on the surface layer of the silicon compound particles. A pyrolysis CVD method is desirable as a method for forming the carbon material layer. A method of forming a carbon material layer by the thermal decomposition CVD method will be described.

First, silicon compound particles are set in a 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 active material particles can be suppressed. After raising the temperature in the furnace to a predetermined temperature, a carbon layer is formed on the surface of the silicon compound 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 production cost can be reduced and the physical properties of the decomposition product can be improved.

Next, Li is inserted into the negative electrode active material particles containing the silicon active material particles produced as described above to contain at least one or more of Li ₂ SiO ₃ and Li ₄ SiO ₄ . Li is preferably inserted by a thermal doping method.

The modification by the thermal doping method can be modified, for example, by mixing silicon compound particles with LiH and heating the mixture in an inert atmosphere. At this time, if it contains Cl or SO ₄ as an impurity in the silicon compound particles (SiOx) or LiH, it may contain Cl or SO ₄ on the surface of the negative electrode active material particle. As the inert atmosphere, for example, an Ar atmosphere, an N ₂ atmosphere, a mixed atmosphere of Ar and N ₂ or the like can be used. More specifically, first, LiH powder and silicon oxide powder are sufficiently mixed in an Ar or N ₂ atmosphere, sealed, and the sealed container is agitated to homogenize it. Then, heating and baking are performed in the range of 700° C. to 800° C. for modification. Note that when the modification is performed in an N ₂ atmosphere, the surface of the obtained negative electrode active material particles contains Li ₃ N. Then, if the negative electrode active material particles are washed with pure water or the like, Li ₂ CO ₃ and LiOH can be generated on the surface. The amount of pure water can be, for example, four times equivalent to that of the powder. The cleaning time can be adjusted according to the powder. Then, by drying, silicon compound particles having an appropriate amount of ions can be obtained.

Further, the modification may be performed by a redox method. In the modification by the redox method, for example, lithium can be inserted by first immersing the silicon compound particles in a solution in which lithium is dissolved in an ether solvent. This solution may further contain a polycyclic aromatic compound or a linear polyphenylene compound. Also in this case, if it contains Cl or SO ₄ as an impurity in the silicon compound particles (SiOx) or the like, may be contained Cl or SO ₄ on the surface of the negative electrode active material particle. If N ₂ is bubbled into this solution, Li ₃ N can be produced at the same time when lithium is inserted. After that, the active lithium may be desorbed from the silicon compound particles by immersing the silicon compound particles in a solution containing an alcohol solvent, a carboxylic acid solvent, water, or a mixed solvent thereof. In this way, by removing the active lithium after inserting lithium, a negative electrode active material having higher water resistance can be obtained. Then, it may be washed by a method of washing with alcohol, alkaline water in which lithium carbonate is dissolved, weak acid, pure water, or the like.

When the modification is performed by the thermal doping method, the ²⁹ Si-MAS-NMR spectrum obtained from the silicon compound particles is different from that when the redox method is used. FIG. 2 shows an example of a ²⁹ Si-MAS-NMR spectrum measured from silicon compound particles when modified by the redox method. In FIG. 2, the peak given in the vicinity of −75 ppm is the peak derived from Li ₂ SiO _3, and the peak given in −80 to −100 ppm is the peak derived from Si. In some cases, the peak of Li silicate other than Li ₂ SiO ₃ and Li ₄ SiO ₄ may be present from −80 to −100 ppm.

Further, FIG. 3 shows an example of a ²⁹ Si-MAS-NMR spectrum measured from silicon compound particles when the modification is performed by the thermal doping method. In FIG. 3, the peak given in the vicinity of −75 ppm is the peak derived from Li ₂ SiO _3, and the peak given in −80 to −100 ppm is the peak derived from Si. In some cases, the peak of Li silicate other than Li ₂ SiO ₃ and Li ₄ SiO ₄ may be present from −80 to −100 ppm. Note that the peak of Li ₄ SiO ₄ can be confirmed from the XPS spectrum.

Further, a step of containing at least one of Cl ions and SO ₄ ions on the surface of the silicon compound particles may be performed. In this step, for example, a substance serving as a Cl ion source or an SO ₄ ion source may be added to the surface of the silicon compound particles. When a carbon coating is formed on the surface of the silicon compound particles, Cl ion or SO ₄ ion may be contained on the surface of the carbon coating or the surface of the silicon compound. Further, a step of containing ions may be performed at the same time as the above lithium insertion step. For example, when the thermal doping method is selected, a substance serving as a Cl ion source or an SO ₄ ion source may be added to the mixture of silicon compound particles and LiH. In addition, for example, when the redox method is selected, a substance serving as a Cl ion source or SO ₄ ion source may be added to the solution used for lithium insertion and desorption. In addition, the amounts of Cl ions and SO ₄ ions contained on the surface of the silicon compound particles can be adjusted by adjusting the addition amounts of the Cl ion source and the SO ₄ ion source. Negative electrode active material particles can be produced as described above.

Next, from the negative electrode active material particles, the content of Cl ions is 5 mass ppm or more and 200 mass ppm or less based on the mass of the negative electrode active material particles, and the content of SO ₄ ions of the negative electrode active material particles is A material satisfying at least one of 5 mass ppm to 200 mass ppm is selected. At this time, the content of Cl ions and SO ₄ ions can be quantified as follows. For example, 5.00 g of negative electrode active material particles and 50.00 g of pure water are weighed into a beaker and dispersed for 5 minutes using a magnetic stirrer. Next, the dispersion liquid is filtered, and the resulting filtrate is analyzed by ion chromatography, whereby Cl ion and SO ₄ ion can be quantified. The amount of Li ions can also be quantified by analyzing the filtrate with ICP. Therefore, regarding LiCl and Li ₂ SO ₄ , if Cl ions, SO ₄ ions, and Li ions are detected by the above method, it can be estimated that LiCl and Li ₂ SO ₄ are present on the surface of the negative electrode active material.

The selection of the negative electrode active material particles does not necessarily have to be performed each time the negative electrode active material is manufactured, and the content of Cl ions is 5 mass ppm or more and 200 mass ppm or less with respect to the mass of the negative electrode active material particles. And selecting the production conditions by which the negative electrode active material particles satisfying at least one of the content of SO ₄ ions being 5 mass ppm or more and 200 mass ppm or less with respect to the mass of the negative electrode active material particles are obtained. For example, thereafter, the negative electrode active material can be manufactured under the same conditions as the selected conditions.

The negative electrode active material produced 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 to form a slurry. Next, the above slurry is applied to the surface of the negative electrode current collector and dried to form a negative electrode active material layer. 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 negative electrode containing the negative electrode active material of the present invention. Here, as a specific example, a laminate film type lithium ion secondary battery is taken as an example.

[Structure of Laminated Film Type Lithium Ion Secondary Battery]
The laminated film type lithium-ion secondary battery 20 shown in FIG. 4 mainly includes a wound electrode body 21 housed inside a sheet-shaped exterior member 25. 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 22 is attached to the positive electrode and the negative electrode lead 23 is attached to the negative electrode. The outermost peripheral portion of the electrode body is protected by a protective tape.

The positive and negative leads are led out in one direction from the inside to the outside of the exterior member 25, for example. The positive electrode lead 22 is formed of a conductive material such as aluminum, and the negative electrode lead 23 is formed of a conductive material such as nickel and copper.

The exterior member 25 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 is formed of two films so that the fusion layer faces the electrode body 21. The outer peripheral edge portions of the fusion layer are fused or adhered to each other with an adhesive or the like. The fused portion is a film of polyethylene or polypropylene, for example, and the metal portion is an aluminum foil or the like. The protective layer is, for example, nylon.

An adhesive film 24 is inserted between the exterior member 25 and the positive and negative electrode leads to prevent outside air from entering. This material is, for example, polyethylene, polypropylene, or 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, for example, similar to 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 inserting and extracting lithium ions, and also contains other materials such as a binder, a conductive auxiliary agent, and a dispersant depending on the design. You can go out. In this case, the details of the binder and the conductive auxiliary agent are the same as those of the negative electrode binder and the negative electrode conductive auxiliary agent described above, for example.

A lithium-containing compound is desirable as the positive electrode material. The lithium-containing compound may be, for example, a composite oxide including lithium and a transition metal element, or a phosphoric acid compound including lithium and a transition metal element. Among the positive electrode materials described above, compounds containing at least one of nickel, iron, manganese, and cobalt are preferable. These chemical formulas are represented by, for example, Li _x M1O ₂ or Li _y M2PO ₄ . In the formula, M1 and M2 represent at least one kind of transition metal element. The values of x and y show different values depending on the charging/discharging state of the battery, but are generally shown as 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 ) and lithium nickel composite oxide (Li _x NiO ₂ ). Examples of the phosphoric acid compound having lithium and a transition metal element include a lithium iron phosphoric acid compound (LiFePO ₄ ) or a lithium iron manganese phosphoric acid compound (LiFe _1-u Mn _u PO ₄ (0<u<1)). Is mentioned. By using these positive electrode materials, a high battery capacity and excellent cycle characteristics can be obtained.

[Negative electrode]
The negative electrode has the same structure as the negative electrode 10 for the lithium ion secondary battery in FIG. 1 described above, and has, for example, the negative electrode active material layers 12 on both surfaces of the current collector 11. 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 is because the deposition of lithium metal on the negative electrode can be suppressed.

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 non-opposing region, that is, in the region in which the negative electrode active material layer and the positive electrode active material layer do not face each other, the influence of charging/discharging is hardly exerted. Therefore, the state of the negative electrode active material layer is maintained as it is immediately after formation. Thereby, 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 and 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, ethylmethyl carbonate, methylpropyl carbonate, 1,2-dimethoxyethane, and tetrahydrofuran. Among these, it is preferable 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, more superior properties can be obtained by combining a high-viscosity solvent such as ethylene carbonate or propylene carbonate with a low-viscosity solvent such as dimethyl carbonate, ethylmethyl carbonate or diethyl carbonate. This is because the dissociation property and ion mobility of the electrolyte salt are improved.

When using the alloy-based negative electrode, it is particularly preferable that the solvent contains at least one of halogenated chain carbonic acid ester and halogenated cyclic carbonic acid ester. 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 with better quality than other halogens. Also, 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, difluoromethyl methyl carbonate and the like. 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 carbonate include vinylene carbonate and vinyl ethylene carbonate.

Further, it is 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.

Further, 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 any one or more of light metal salts such as 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 secondary battery]
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.

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. Subsequently, 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 manufactured by forming a negative electrode active material layer on the negative electrode current collector by using the same procedure as that for manufacturing the negative electrode 10 for the lithium ion secondary battery 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 application lengths of both surfaces may be deviated in both electrodes (see FIG. 1).

Subsequently, the electrolytic solution is adjusted. Subsequently, the positive electrode lead 22 is attached to the positive electrode current collector and the negative electrode lead 23 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 21, and a protective tape is adhered to the outermost peripheral portion thereof. Next, the wound body is formed into a flat shape. Subsequently, the wound electrode body is sandwiched between the folded film-shaped outer casing members 25, and then the insulating portions of the outer casing members are adhered to each other by a heat fusion method, and the 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 adjusted electrolytic solution is charged from the open part and vacuum impregnation is performed. After the impregnation, the open portion is adhered by the vacuum heat fusion method. As described above, the laminated film type lithium ion secondary battery 20 can be manufactured.

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

(Example 1-1)
The laminated film type lithium ion secondary battery 20 shown in FIG. 4 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, which is a lithium nickel cobalt composite oxide, at 95% by mass, a positive electrode conduction aid 2.5% by mass, and a positive electrode binder (polyvinylidene fluoride). : PVDF) 2.5 mass% was mixed to obtain 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. First, a negative electrode active material was produced as follows. A raw material in which metallic silicon and silicon dioxide were mixed was introduced into a reaction furnace, vaporized in an atmosphere having a vacuum degree of 10 Pa, deposited on an adsorption plate, sufficiently cooled, and then the deposit was taken out and pulverized by a ball mill. .. The value of x of SiO _x of the silicon compound particles thus obtained was 0.5. Then, the particle size of the silicon compound particles was adjusted by classification. Then, thermal decomposition CVD was performed to coat the surface of the silicon compound particles with the carbon material.

Subsequently, lithium was inserted into the silicon compound particles by a thermal doping method to modify the particles. First, the silicon compound particles after coating with the carbon material are thoroughly mixed with LiH powder in an N ₂ atmosphere, LiCl and Li ₂ SO ₄ are added, sealing is performed, and the sealed container is stirred to homogenize it. did. Then, it heated and baked in the range of 700 to 800 degreeC. Through the above treatment, lithium was inserted into the silicon compound particles. In this way, negative electrode active material particles were produced. Further, during this thermal doping, lithium nitride was generated on the surface of the negative electrode active material particles. Next, the negative electrode active material particles were washed with pure water in an amount of 4 times the equivalent amount of the negative electrode active material particles. By this washing, LiOH and Li ₂ CO ₃ were produced on the surface of the negative electrode active material particles. Then, the negative electrode active material particles were dried.

A part of the negative electrode active material particles produced here was taken out, and the contents of Cl ions and SO ₄ ions contained in the surfaces of the negative electrode active material particles were measured. As a result, the content of Cl ions was 165 mass ppm with respect to the mass of the negative electrode active material particles, and the content of SO ₄ ions was 170 mass ppm with respect to the mass of the negative electrode active material particles.

Further, the content of LiOH was 0.65 mass% with respect to the mass of the negative electrode active material particles, and the content of Li ₂ CO ₃ was 0.45 mass% with respect to the mass of the negative electrode active material particles. It was

Moreover, LiCl and Li ₂ SO ₄ were contained on the surface of the negative electrode active material particles.

Next, the negative electrode active material particles and the carbon-based active material were mixed in a mass ratio of 1:9 to prepare a mixed negative electrode active material. Here, as the carbon-based active material, a mixture of natural graphite and artificial graphite coated with a pitch layer in a mass ratio of 5:5 was used. The median diameter of the carbon-based active material was 20 μm.

Next, the prepared mixed negative electrode active material, conduction aid 1 (carbon nanotube, CNT), conduction aid 2 (fine carbon particles having a median diameter of about 50 nm), styrene butadiene rubber (styrene butadiene copolymer, hereinafter referred to as SBR) , Carboxymethyl cellulose (hereinafter referred to as CMC) were mixed at a dry mass ratio of 92.5:1:1:2.5:3, and then diluted with pure water to obtain a negative electrode mixture slurry. The above SBR and CMC are negative electrode binders (negative electrode binders).

Moreover, as the negative electrode current collector, an electrolytic copper foil having a thickness of 15 μm was used. This electrolytic copper foil contained carbon and sulfur at a concentration of 70 mass ppm each. Finally, the negative electrode mixture slurry was applied to the negative electrode current collector and dried at 100° C. for 1 hour in a vacuum atmosphere. The amount of the negative electrode active material layer deposited (also referred to as areal density) per unit area on one surface of the negative electrode after drying was 5 mg/cm ² .

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: LiPF6) 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 1.2 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 were 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. Then, 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. An aluminum laminate film in which a nylon film, an aluminum foil, and a polypropylene film were laminated was used as the exterior member. Subsequently, the adjusted 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, the battery was charged/discharged at 0.2 C 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 499 cycles, and the discharge capacity was measured each time. Finally, the discharge capacity at the 500th cycle obtained by charging/discharging at 0.2C was divided by the discharge capacity at the 2nd cycle to calculate the capacity retention rate (hereinafter, also simply referred to as the retention rate). During the normal cycle, that is, from the third cycle to the 499th cycle, charging/discharging was performed at a charge of 0.7 C and a discharge of 0.5 C.

When examining the initial charge/discharge characteristics, the initial efficiency (hereinafter sometimes referred to as the initial efficiency) was calculated. The initial efficiency was calculated from the formula represented by initial efficiency (%)=(initial discharge capacity/initial charge capacity)×100. The ambient temperature was the same as when examining the cycle characteristics.

(Example 1-2 to Example 1-3, Comparative Examples 1-1 and 1-2)
A secondary battery was manufactured in the same manner as in Example 1-1, except that the oxygen content in the bulk of the silicon compound was adjusted. In this case, the amount of oxygen was adjusted by changing the ratio of metallic silicon and silicon dioxide in the raw material of the silicon compound and the heating temperature. The values of x of the silicon compounds represented by SiO _x in Examples 1-1 to 1-3 and Comparative Examples 1-1 and 1-2 are shown in Table 1.

At this time, the negative electrode active material particles (silicon-based negative electrode active material particles) of Examples 1-1 to 1-3 and Comparative Examples 1-1 and 1-2 had the following properties. The median diameter of the negative electrode active material particles was 4 μm. Li ₂ SiO ₃ and Li ₄ SiO ₄ were contained inside the silicon compound particles. Further, the silicon compound has a full width at half maximum (2θ) of the diffraction peak due to the Si(111) crystal plane obtained by X-ray diffraction of 1.755°, and the crystallite size due to the Si(111) crystal plane. Was 4.86 nm.

Further, in all of the above Examples and Comparative Examples, peaks in the Si and Li silicate regions, which were given at -60 to -95 ppm as the chemical shift values obtained from the ²⁹ Si-MAS-NMR spectrum, were developed. Further, in all of the above Examples and Comparative Examples, the maximum peak intensity value A in the Si and Li silicate regions given at -60 to -95 ppm as the chemical shift value obtained from the ²⁹ Si-MAS-NMR spectrum, and -96 to The relationship with the peak intensity value B in the SiO ₂ region given at −150 ppm was A>B.

The average thickness of the carbonaceous material contained in the negative electrode active material particles was 100 nm.

Further, a 2032 size coin battery type test cell was produced from the negative electrode and the counter electrode lithium produced as described above, and the discharge behavior thereof was evaluated. More specifically, first, constant-current constant-voltage charging was performed with the counter electrode Li to 0 V, and the charging was stopped when the current density reached 0.05 mA/cm ² . After that, constant current discharge was performed up to 1.2V. The current density at this time was 0.2 mA/cm ² . This charging/discharging was repeated 30 times, and from the data obtained in each charging/discharging, a graph was drawn with the rate of change of capacity (dQ/dV) on the vertical axis and the voltage (V) on the horizontal axis, and V was 0.4 to 0. It was confirmed whether a peak was obtained in the range of 0.55 (V). As a result, the above peak was not obtained in Example 1-1 and Comparative Example 1-1 in which x of SiOx was 0.5 or less. In the other Examples and Comparative Examples, the above-mentioned peak was obtained within 30 times of charging/discharging, and the above-mentioned peak was obtained in all charging/discharging from the first charging/discharging to the 30th charging/discharging.

Table 1 shows the evaluation results of Examples 1-1 to 1-3 and Comparative examples 1-1 and 1-2.

As shown in Table 1, in the silicon compound 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 there is not enough oxygen (x=0.3), the initial efficiency is improved, but the capacity retention ratio is significantly deteriorated. On the other hand, as shown in Comparative Example 1-2, when the amount of oxygen was large (x=1.8), the conductivity was lowered and the capacity of the silicon oxide was not substantially expressed, so the evaluation was stopped.

(Example 2-1 and Example 2-2)
Secondary batteries were produced under the same conditions as in Example 1-2, except that the type of lithium silicate contained in the silicon compound particles was changed as shown in Table 2, and the cycle characteristics and initial efficiency were evaluated.

(Comparative Example 2-1)
A secondary battery was produced under the same conditions as in Example 1-2, except that lithium was not inserted into the silicon compound particles, and cycle characteristics and initial efficiency were evaluated.

Table 2 shows the results of Example 2-1, Example 2-2, and Comparative example 2-1.

Since the silicon compound contained stable lithium silicate such as Li ₂ SiO ₃ and Li ₄ SiO ₄ , the capacity retention rate and the initial efficiency were improved. In particular, when both lithium silicates of Li ₂ SiO ₃ and Li ₄ SiO ₄ were included, the capacity retention rate and the initial efficiency were further improved. On the other hand, in Comparative Example 2-1 in which the silicon compound was not modified and lithium was not included, the capacity retention rate and the initial efficiency decreased.

(Examples 3-1 to 3-6)
A secondary battery was produced under the same conditions as in Example 1-2, except that the content of Cl ions and SO ₄ ions on the surface of the negative electrode active material particles was changed as shown in Table 3, and the cycle characteristics and the initial efficiency were determined. evaluated. The content of Cl ion and SO ₄ ion was changed by adjusting the amount of the added ion source. Moreover, in Examples 3-1 to 3-6, at least one of the Cl ion content and the SO ₄ ion content was adjusted to satisfy 5 mass ppm to 200 mass ppm. Moreover, in Examples 3-1 to 3-4, the surface of the negative electrode active material particles contained LiCl and Li ₂ SO ₄ . Moreover, in Example 3-5, Li ₂ SO ₄ was contained on the surface of the negative electrode active material particles. Moreover, in Example 3-6, LiCl was contained on the surface of the negative electrode active material particles.

(Examples 3-7 to 3-12)
A secondary battery was produced under the same conditions as in Example 1-2, except that the method for modifying the silicon compound particles was changed to the redox method, and the contents of Cl ions and SO ₄ ions were changed as shown in Table 3. Then, the cycle characteristics and the initial efficiency were evaluated.

Modification of the silicon compound particles by the redox method in these examples was performed as follows. First, the silicon compound particles coated with a carbon material, which were produced in the same manner as in Example 1-2, were immersed in a solution prepared by dissolving lithium pieces and naphthalene in tetrahydrofuran (hereinafter also referred to as THF). This solution was prepared by dissolving naphthalene in a THF solvent at a concentration of 1.0 mol/L, and then adding 10 mass% of lithium pieces to the mixed solution of THF and naphthalene. The temperature of the solution when the silicon compound particles were immersed was 20° C., and the immersion time was 20 hours. Then, the negative electrode active material particles were collected by filtration. Through the above treatment, lithium was inserted into the negative electrode active material particles.

Next, the negative electrode active material particles after lithium insertion were immersed in a solution in which naphthalene was dissolved in THF. This solution was prepared by dissolving naphthalene in a THF solvent at a concentration of 2 mol/L. The temperature of the solution when the negative electrode active material particles were immersed was 20° C., and the immersion time was 20 hours. Then, the negative electrode active material particles were collected by filtration.

In this lithium insertion and desorption treatment, LiCl and Li ₂ SO ₄ were added to the above Li insertion solution to contain Cl ions and SO ₄ ions on the surface of the negative electrode active material particles. The content of Cl ions and SO ₄ ions was changed by adjusting the amount of Cl ion source and SO ₄ ion source. Moreover, also in Examples 3-7 to 3-12, LiCl and Li ₂ SO ₄ were contained on the surface of the negative electrode active material particles.

Next, the negative electrode active material particles were washed, and the washed negative electrode active material particles were dried under reduced pressure. Negative electrode active material particles were produced as described above.

(Comparative Example 3-1)
A secondary battery was produced under the same conditions as in Example 1-2, except that neither Cl ion nor SO ₄ ion was contained on the surface of the negative electrode active material particles, and the cycle characteristics and the initial efficiency were evaluated.

(Comparative Example 3-2)
Secondary battery under the same conditions as in Example 1-2, except that the content of both Cl ions and SO ₄ ions on the surface of the negative electrode active material particles was larger than 200 mass ppm with respect to the mass of the negative electrode active material particles. Was prepared and the cycle characteristics and the initial efficiency were evaluated.

(Comparative Example 3-3)
The method for modifying the silicon compound particles was changed to a redox method, and the content of Cl ions and SO ₄ ions on the surface of the negative electrode active material particles was 200 mass ppm with respect to the mass of the negative electrode active material particles. A secondary battery was produced under the same conditions as in Example 1-2 except that the size was increased, and the cycle characteristics and the initial efficiency were evaluated.

As can be seen from Table 3, when the Cl ion content and the SO ₄ ion content were both 5 mass ppm to 200 mass ppm, the capacity retention ratio and the initial efficiency were highest (Examples 1-2 and Example 1). Examples 3-1 to 3-2, 3-7 to 3-8). Also, when either one of the Cl ion content and the SO ₄ ion content satisfies 5 mass ppm to 200 mass ppm, good capacity retention rate and initial efficiency were obtained (Examples 3-3 to 3-6). , 3-9 to 3-12). On the other hand, when the Cl ion and SO ₄ ion contents are both less than 5 mass ppm or both the Cl ion and SO ₄ ion content exceeds 200 mass ppm, The capacity retention rate and the initial efficiency were low (Comparative Examples 3-1 to 3-3).

(Examples 4-1 to 4-6)
A secondary battery was prepared under the same conditions as in Example 1-2, except that the contents of LiOH and Li ₂ CO _{3 on} the surface of the negative electrode active material particles were changed as shown in Table 4, and the cycle characteristics and the initial efficiency were evaluated. did.

As shown in Table 4, Example 4 satisfying the condition that the content of Li ₂ CO _{3 and} the content of LiOH are both 0.01% by mass or more and 5.00% by mass or less with respect to the mass of the negative electrode active material particles. In −2 to 4-5, the capacity retention ratio was improved as compared with Examples 4-1 and 4-6 in which this condition was not satisfied.

(Examples 5-1 to 5-6)
A secondary battery was produced under the same conditions as in Example 1-2, except that the crystallinity of the silicon compound particles was changed as shown in Table 5, and the cycle characteristics and the initial efficiency were evaluated. The crystallinity in the silicon compound particles can be controlled by changing the vaporization temperature of the raw material and heat treatment after the silicon compound particles are produced.

In particular, a high capacity retention rate was obtained with a low crystalline material having a half width of 1.2° or more and a crystallite size of 7.5 nm or less due to the Si(111) plane.

(Example 6-1)
The silicon compound was used under the same conditions as in Example 1-2 except that the relation between the maximum peak intensity value A in the Si and Li silicate regions and the peak intensity value B derived from the SiO ₂ region was A<B. A secondary battery was produced, and cycle characteristics and initial efficiency were evaluated. In this case, by reducing the amount of insertion of lithium during reforming _to reduce the amount of Li 2 SiO _3, it has a small intensity A of a peak derived from the Li 2 SiO _3.

As can be seen from Table 6, the battery characteristics were improved when the peak intensity relationship was A>B.

(Example 7-1)
In the V-dQ/dV curve obtained by charging/discharging 30 times in the test cell, a negative electrode active material was used in which V did not show a peak in the range of 0.40V to 0.55V at any charging/discharging. Other than that, a secondary battery was produced under the same conditions as in Example 1-2, and cycle characteristics and initial efficiency were evaluated.

In order for the shape of the discharge curve to rise more sharply, the silicon compound (SiOx) must exhibit the same discharge behavior as that of silicon (Si). Since the silicon compound has a relatively gentle discharge curve in which no peak appears in the above range after 30 times of charge and discharge, the initial efficiency of the secondary battery is slightly lowered when it is used as a secondary battery. If the peak was exhibited by charge/discharge within 30 times, a stable bulk was formed, and the capacity retention rate and the initial efficiency were improved.

(Examples 8-1 to 8-6)
A secondary battery was produced under the same conditions as in Example 1-2, except that the median diameter of the silicon compound was changed as shown in Table 8, and the cycle characteristics and the initial efficiency were evaluated.

When the median diameter of the silicon compound was 1.0 μm or more, the retention rate was improved. It is considered that this is because the surface area per mass of the silicon compound was not too large and the area where the side reaction occurred could be reduced. On the other hand, when the median diameter is 15 μm or less, particles are less likely to break during charging and SEI (solid electrolyte interface) due to a newly-formed surface is less likely to be generated during charging/discharging, so that loss of reversible Li can be suppressed. Further, when the median diameter of the silicon-based active material particles is 15 μm or less, the expansion amount of the silicon compound particles during charging does not increase, so that physical and electrical destruction of the negative electrode active material layer due to expansion can be prevented.

(Examples 9-1 to 9-5)
Secondary batteries were produced under the same conditions as in Example 1-2, except that the average thickness of the carbon material coated on the surface of the silicon-based active material particles was changed, and the cycle characteristics and the initial efficiency were evaluated. The average thickness of the carbon material can be adjusted by changing the CVD conditions.

As can be seen from Table 9, the conductivity is particularly improved when the thickness of the carbon layer is 10 nm or more, so that the capacity retention rate and the initial efficiency can be improved. On the other hand, when the thickness of the carbon layer is 5000 nm or less, a sufficient amount of silicon compound particles can be ensured in battery design, so that the battery capacity does not decrease.

(Example 10-1)
A secondary battery was produced under the same conditions as in Example 1-2, except that the mass ratio of the silicon-based active material particles in the negative electrode active material was changed, and the rate of increase in battery capacity was evaluated.

FIG. 5 is a graph showing the relationship between the ratio of silicon-based active material particles to the total amount of negative electrode active material and the increase rate of the battery capacity of the secondary battery. The graph shown by A in FIG. 5 shows the rate of increase in battery capacity when the ratio of silicon compound particles was increased in the negative electrode active material of the negative electrode of the present invention. On the other hand, the graph indicated by B in FIG. 5 shows the rate of increase in battery capacity when the proportion of silicon compound particles not doped with Li was increased. As can be seen from FIG. 5, when the proportion of the silicon compound is 6% by mass or more, the rate of increase in battery capacity is larger than that in the conventional case, and the volume energy density is remarkably increased.

The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the invention having substantially the same configuration as the technical idea described in the scope of the claims of the present invention and exhibiting the same action and effect is the present invention Within the technical scope of.

10... Negative electrode, 11... Negative electrode collector, 12... Negative electrode active material layer,
20... Lithium secondary battery (laminate film type), 21... Winding electrode body,
22... Positive electrode lead, 23... Negative electrode lead, 24... Adhesive film,
25... Exterior member.

Claims (12)

A negative electrode active material containing negative electrode active material particles,
The negative electrode active material particles contain silicon compound particles containing a silicon compound (SiO _x : 0.5≦x≦1.6),
The silicon compound particles contain at least one kind of Li ₂ SiO ₃ and Li ₄ SiO ₄ ,
The negative electrode active material particles include both Cl ions and SO ₄ ions on the surface,
The content of the Cl ions is 5 mass ppm or more and 200 mass ppm or less with respect to the mass of the negative electrode active material particles, and the content of SO ₄ ions is 5 with respect to the mass of the negative electrode active material particles. A negative electrode active material, which satisfies both of mass ppm or more and 200 mass ppm or less. The negative electrode active material particles include LiOH and Li ₂ CO ₃ on the surface, and the content of LiOH is 0.01% by mass or more and 5.00% by mass or less with respect to the mass of the negative electrode active material particles, and The negative electrode active material according to claim 1, wherein the content of Li ₂ CO ₃ satisfies 0.01 mass% or more and 5.00 mass% or less with respect to the mass of the negative electrode active material particles. material. The negative electrode active material according to claim 1, wherein the negative electrode active material particles have at least one of LiCl and Li ₂ SO _{4 on} the surface. The silicon compound particles have a full width at half maximum (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 7 It is 0.5 nm or less, The negative electrode active material according to any one of claims 1 to 3. In the silicon compound particles, the maximum peak intensity value A of the Si and Li silicate region, which is obtained from the ²⁹ Si-MAS-NMR spectrum as a chemical shift value of −60 to −95 ppm, and the chemical shift value of −96 to − 5. The negative electrode active material according to claim 1, wherein the peak intensity value B in the SiO ₂ region given at 150 ppm satisfies the relationship of A>B. A test cell composed of a negative electrode and a counter electrode lithium containing a mixture of the negative electrode active material and a carbon-based active material was prepared, and in the test cell, charging was performed by passing an electric current so as to insert lithium into the negative electrode active material, A charge and discharge consisting of discharge in which an electric current is passed so as to desorb lithium from the negative electrode active material was carried out 30 times, and the discharge capacity Q in each charge and discharge was differentiated by the potential V of the negative electrode with reference to the counter lithium. When a graph showing the relationship between the value dQ/dV and the potential V is drawn, the potential V of the negative electrode is 0.40 V to 0.55 V during the Xth discharge (1≦X≦30) and thereafter. The negative electrode active material according to any one of claims 1 to 5, which has a peak in the range. The negative electrode active material according to any one of claims 1 to 6, wherein the negative electrode active material particles have a median diameter of 1.0 µm or more and 15 µm or less. The negative electrode active material according to any one of claims 1 to 7, wherein the negative electrode active material particles include a carbon material in a surface layer portion. The negative active material of claim 8, wherein the carbon material has an average thickness of 10 nm or more and 5000 nm or less. A mixed negative electrode active material, comprising the negative electrode active material according to any one of claims 1 to 9 and a carbon-based active material. A method for producing a negative electrode active material containing negative electrode active material particles containing silicon compound particles,
A step of producing silicon compound particles containing a silicon compound (SiO _x : 0.5≦x≦1.6),
Inserting lithium into the silicon compound particles to contain at least one or more of Li ₂ SiO ₃ and Li ₄ SiO ₄ .
To produce negative electrode active material particles,
From the negative electrode active material particles, the content of Cl ions on the surface of the negative electrode active material particles is 5 ppm or more and 200 ppm or less with respect to the mass of the negative electrode active material particles, and SO on the surface of the negative electrode active material particles Including a step of selecting ones satisfying both that the content of ₄ ions is 5 ppm or more and 200 ppm or less with respect to the mass of the negative electrode active material particles,
A method for producing a negative electrode active material, comprising producing a negative electrode active material using the selected negative electrode active material particles. The method for producing a negative electrode active material according to claim 11, further comprising the step of containing at least one of Cl ions and SO ₄ ions on the surface of the silicon compound particles.

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