JP7388936B2 - Negative electrode active material for nonaqueous electrolyte secondary batteries, negative electrode material for nonaqueous electrolyte secondary batteries, and lithium ion secondary batteries - Google Patents

Description

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

In recent years, small electronic devices such as mobile terminals have become widespread, and there is a strong demand for further miniaturization, weight reduction, and longer life. In response to such market demands, development of secondary batteries that are particularly small, lightweight, and capable of obtaining high energy density is progressing. This secondary battery is being considered for application not only to small electronic devices, but also to large electronic devices such as automobiles, and power storage systems such as houses.

Among these, lithium ion secondary batteries are highly anticipated because they are easy to make small and have a high capacity, and can provide higher energy density than lead batteries and nickel cadmium batteries.

The above-mentioned 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 charge/discharge reactions.

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

However, when silicon is used as the main raw material for the negative electrode active material, the negative electrode active material expands and contracts during charging and discharging, making it easy to crack mainly near the surface layer of the negative electrode active material. In addition, an ionic substance is generated inside the active material, and the negative electrode active material becomes a material that is easily broken. When the surface layer of the negative electrode active material is cracked, a new surface is created thereby increasing the reaction area of the active material. At this time, a decomposition reaction of the electrolytic solution occurs on the new surface, and a film that is a decomposition product of the electrolytic solution is formed on the new surface, so that the electrolytic solution is consumed. For this reason, cycle characteristics tend to deteriorate.

To date, various studies have been conducted on negative electrode materials and electrode configurations for lithium-ion secondary batteries that are mainly made of silicon materials in order to improve initial battery efficiency and cycle characteristics.

Specifically, for the purpose of obtaining good cycle characteristics and high safety, silicon and amorphous silicon dioxide are deposited simultaneously using a gas phase method (see, for example, Patent Document 1). Furthermore, in order to obtain high battery capacity and safety, a carbon material (electronic conductive material) is provided on the surface layer of silicon oxide particles (see, for example, Patent Document 2). Furthermore, 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 with a high oxygen ratio near the current collector is formed ( For example, see Patent Document 3). In addition, in order to improve cycle characteristics, oxygen is contained in the silicon active material so that the average oxygen content is 40 at% or less, and the oxygen content is increased near the current collector. (For example, see Patent Document 4).

Moreover, in order to improve the initial charge/discharge efficiency, a nanocomposite containing a Si phase, SiO ₂ , and _MyO metal oxide is used (see, for example, Patent Document 5). Furthermore, in order to improve cycle characteristics, SiO _x (0.8≦x≦1.5, particle size range = 1 μm to 50 μm) and a carbon material are mixed and fired at a high temperature (for example, see Patent Document 6). In addition, in order to improve cycle characteristics, the molar ratio of oxygen to silicon in the negative electrode active material is set to 0.1 to 1.2, and the difference between the maximum value and the minimum value of the molar ratio near the interface between the active material and the current collector is The active material is controlled within a range where the value is 0.4 or less (see, for example, Patent Document 7). Furthermore, in order to improve battery load characteristics, a metal oxide containing lithium is used (see, for example, Patent Document 8). Furthermore, in order to improve 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).

Furthermore, in order to improve cycle characteristics, silicon oxide is used and a graphite film is formed on its surface layer to impart electrical conductivity (for example, see Patent Document 10). In Patent Document 10, regarding the shift value obtained from the RAMAN spectrum regarding the graphite coating, broad peaks appear at 1330 cm ^-1 and 1580 cm ^-1 , and their intensity ratio I ₁₃₃₀ /I ₁₅₈₀ is 1.5<I ₁₃₃₀ /I ₁₅₈₀ <3. Furthermore, in order to increase battery capacity and improve cycle characteristics, particles having a silicon microcrystalline phase dispersed in silicon dioxide are used (see, for example, Patent Document 11). Furthermore, in order to improve overcharge and overdischarge characteristics, silicon oxide is used in which the atomic ratio of silicon to oxygen is controlled to 1:y (0<y<2) (see, for example, Patent Document 12).

Japanese Patent Application Publication No. 2001-185127 JP2002-042806A Japanese Patent Application Publication No. 2006-164954 Japanese Patent Application Publication No. 2006-114454 JP2009-070825A JP2008-282819A Japanese Patent Application Publication No. 2008-251369 Japanese Patent Application Publication No. 2008-177346 Japanese Patent Application Publication No. 2007-234255 Japanese Patent Application Publication No. 2009-212074 Japanese Patent Application Publication No. 2009-205950 Japanese Patent Application Publication No. 6-325765

As mentioned above, in recent years, small electronic devices such as mobile terminals have become more sophisticated and multifunctional, and the lithium-ion secondary batteries that are their main power source are required to have increased battery capacity. ing. As one method to solve this problem, it is desired to develop a lithium ion secondary battery consisting of a negative electrode using silicon material as the main material. In addition, lithium-ion secondary batteries using silicon materials are expected to have cycle characteristics similar to those of lithium-ion secondary batteries using carbon-based active materials, but the decomposition of the electrolyte in the surface layer occurs at the beginning of the cycle. It was not enough to be promoted.

The present invention was made in view of the above circumstances, and provides a negative electrode active material and negative electrode material for non-aqueous electrolyte secondary batteries, which have excellent charge/discharge capacity and cycle characteristics and are particularly effective for lithium ion secondary batteries; An object of the present invention is to provide a lithium ion secondary battery having a negative electrode including this.

In order to solve the above object, the present invention provides a negative electrode active material for a non-aqueous electrolyte secondary battery that includes negative electrode active material particles, wherein the negative electrode active material particles are capable of occluding and releasing lithium ions. The negative electrode active material particles are powders containing silicon, and the negative electrode active material particles are determined to have a certain particle size D (μm) and the total amount of particles larger than that particle size based on the particle size distribution obtained by laser diffraction scattering particle size distribution measuring method. When the mass percentage of particles is R (%), the slope n of a straight line plotted on a Rosin-Rammler diagram with logD on the x-axis and log{log(100/R)} on the y-axis is 2. 5 or more, and the proportion of particles with a particle size of less than 1 μm in the negative electrode active material particles is 5% by mass or less based on the entire negative electrode active material particles. Provide active material.

The negative electrode active material of the present invention has excellent charge/discharge capacity by containing silicon, and the slope n of the straight line plotted on the Rosin-Rammler diagram of the particle size distribution of the negative electrode active material particles is 2.5 or more. By setting the proportion of particles with a particle size of less than 1 μm to 5% by mass or less based on the entire negative electrode active material particles, the particle sizes are appropriately uniform and Li is dissolved from the negative electrode active material particles. This makes it possible to suppress and improve cycle characteristics.

In this case, the mode diameter (mode) of the negative electrode active material particles is preferably 0.1 μm or more and 15.0 μm or less.

When this mode diameter is 0.1 μm or more, the reaction with the electrolyte is not promoted, so deterioration of battery characteristics can be prevented. When this mode diameter is 15.0 μm or less, expansion of the active material due to charging and discharging can be suppressed, and therefore, loss of electronic contacts can be prevented.

Further, it is preferable that the D _99.9 diameter of the negative electrode active material particles is 40 μm or less.

If this _D99.9 diameter is 40 μm or less, local variations in the diameter of the negative electrode active material in the electrode can be reduced, and cycle characteristics can be improved.

Moreover, it is preferable that the negative electrode active material particles include silicon compound particles containing a silicon compound containing oxygen.

By containing a silicon compound containing oxygen, the initial charge/discharge efficiency can be further improved.

Further, the ratio of silicon and oxygen constituting the silicon compound is preferably in the range of SiO _x :0.5≦x≦1.6.

In this way, when x is 0.5 or more, the oxygen ratio is higher than that of simple silicon, and therefore the cycle characteristics are good. If x is 1.6 or less, the resistance of silicon oxide will not become too high.

Further, it is preferable that at least a portion of the surface of the negative electrode active material particles is coated with a carbon material.

By coating the negative electrode active material particles with the carbon material in this manner, the negative electrode active material can have excellent conductivity.

Moreover, in order to achieve the above object, the present invention provides a negative electrode material for a non-aqueous electrolyte secondary battery, which is characterized by containing the above-described negative electrode active material for a non-aqueous electrolyte secondary battery.

If such a material is used, it becomes a negative electrode material for a non-aqueous electrolyte secondary battery having excellent charge/discharge capacity and cycle characteristics.

Moreover, in order to achieve the above object, the present invention provides a lithium ion secondary battery characterized by having a negative electrode containing the above negative electrode material for a non-aqueous electrolyte secondary battery.

Such a lithium ion secondary battery has excellent charge/discharge capacity and cycle characteristics.

The negative electrode active material of the present invention can provide high capacity and high cycle characteristics when used as a negative electrode active material of a secondary battery.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view showing an example of the structure of a negative electrode for a non-aqueous electrolyte secondary battery containing the negative electrode active material of the present invention. 1 is a particle size distribution chart of silicon compound particles before being coated with carbon by CVD in Example 1. 2 is a particle size distribution chart of silicon compound particles before being coated with carbon by CVD in Example 2. 3 is a particle size distribution chart of silicon compound particles before being coated with carbon by CVD in Example 3. 2 is a particle size distribution chart of silicon compound particles before carbon coating by CVD in Comparative Example 1. 2 is a particle size distribution chart of silicon compound particles before being coated with carbon by CVD in Comparative Example 2. FIG. 2 is a Rosin-Rammler diagram plotting the particle size distribution of silicon compound particles before carbon coating by CVD in Examples 1 to 3 and Comparative Examples 1 and 2.

Embodiments of the present invention will be described below, but the present invention is not limited thereto.

[Non-aqueous electrolyte secondary battery negative electrode active material]
As mentioned above, as one method for increasing the battery capacity of a lithium ion secondary battery, the use of a negative electrode using a silicon material as a main material as the negative electrode of a lithium ion secondary battery is being considered. Lithium ion secondary batteries using this silicon material are expected to have cycle characteristics similar to those of lithium ion secondary batteries using carbon-based active materials; However, it has not yet been possible to propose a negative electrode active material with cycle characteristics equivalent to that of .

Therefore, in order to obtain a negative electrode active material that has good cycle characteristics when used in a secondary battery, the present inventor conducted extensive research and determined the particle size distribution of negative electrode active material particles measured by a laser diffraction scattering particle size distribution measurement method. Based on this, when a certain particle size is D (μm) and the mass percentage of particles larger than that particle to all particles is R (%), the x axis is logD, and the y axis is log{log(100/R). } The slope n of the straight line plotted on a Rosin-Rammler diagram with graduations is 2.5 or more, and in the negative electrode active material particles, the proportion of particles with a particle size of less than 1 μm is It has been found that the cycle characteristics are improved when the amount is 5% by mass or less, and the present invention has been achieved.

That is, the present invention provides a negative electrode active material for a non-aqueous electrolyte secondary battery that includes negative electrode active material particles, wherein the negative electrode active material particles are powder containing silicon that is capable of occluding and releasing lithium ions. Based on the particle size distribution obtained by a laser diffraction scattering particle size distribution measurement method, the negative electrode active material particles have a certain particle size D (μm), and the mass percentage of particles larger than that particle size relative to the total particles is R. (%), the slope n of the straight line plotted on a Rosin-Ramler diagram with logD on the x-axis and log{log(100/R)} on the y-axis is 2.5 or more, and The present invention provides a negative electrode active material for a non-aqueous electrolyte secondary battery, characterized in that, in the negative electrode active material particles, the proportion of particles with a particle size of less than 1 μm is 5% by mass or less based on the entire negative electrode active material particles.

As described above, the definition of the particle size distribution of particles in the present invention is based on the laser diffraction scattering particle size distribution measuring method. As the laser diffraction scattering particle size distribution measuring device, for example, SALD-3100S manufactured by Shimadzu Corporation can be used.

The gradient n of the straight line plotted on the Rosin-Rammler diagram can be determined by, for example, measuring and plotting the particle size distribution with a particle size width of 1 μm, and then determining the regression line based on this distribution. The width of the particle size does not have to be constant, and it is sufficient to plot about 20 to 30 points.

Further, the slope n can be adjusted, for example, by changing the content of fine powder with a particle size of less than 1 μm.

As described below, a carbon film may be formed on the negative electrode active material particles in the negative electrode active material of the present invention. The particle size and particle size distribution obtained by the laser diffraction scattering particle size distribution measurement method, which is the standard standard for negative electrode active material particles in the negative electrode active material of the present invention, were measured without forming a carbon film. This also applies to the mode diameter and _D99.9 diameter of the negative electrode active material particles.

Moreover, it is preferable that the mode diameter (mode) of the negative electrode active material particles in the negative electrode active material of the present invention is 0.1 μm or more and 15.0 μm or less. When this mode diameter is 0.1 μm or more, the reaction with the electrolyte is not promoted, so deterioration of battery characteristics can be prevented. When this mode diameter is 15.0 μm or less, expansion of the active material due to charging and discharging can be suppressed, and therefore, loss of electronic contacts can be prevented. Moreover, this mode diameter is more preferably 3.0 μm or more and 12.0 μm or less.

Further, it is preferable that the D _99.9 diameter of the negative electrode active material particles in the negative electrode active material of the present invention is 40 μm or less. If this _D99.9 diameter is 40 μm or less, local variations in the diameter of the negative electrode active material in the electrode can be reduced, and cycle characteristics can be improved.

<Negative electrode for non-aqueous electrolyte secondary battery>
Next, a negative electrode for a non-aqueous electrolyte secondary battery (hereinafter also referred to as "negative electrode") containing the negative electrode active material of the present invention will be described. FIG. 1 is a cross-sectional view showing an example of the structure of a negative electrode for a non-aqueous electrolyte secondary battery containing the negative electrode active material of the present invention.

[Configuration of negative electrode]
As shown in FIG. 1, the negative electrode 10 has a negative electrode active material layer 12 on a negative electrode current collector 11. This negative electrode active material layer 12 may be provided on both sides of the negative electrode current collector 11 or only on one side. Furthermore, if the negative electrode is made from the negative electrode active material of the present invention, the negative electrode current collector 11 may not be provided.

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

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

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, 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 that is capable of intercalating and releasing lithium ions, and from the viewpoint of battery design, it also contains other materials such as a negative electrode binder and a conductive additive. May contain. The negative electrode active material includes negative electrode active material particles. Furthermore, it is preferable that the negative electrode active material particles include silicon compound particles containing a silicon compound containing oxygen.

Further, the negative electrode active material layer 12 may include a mixed negative electrode active material material containing the negative electrode active material of the present invention (silicon-based negative electrode active material) and a carbon-based active material. This reduces the electrical resistance of the negative electrode active material layer and makes it possible to alleviate expansion stress associated with charging. As the carbon-based active material, for example, pyrolytic carbons, cokes, glassy carbon fibers, fired organic polymer compounds, carbon blacks, etc. can be used.

Further, as described above, the negative electrode active material of the present invention includes negative electrode active material particles containing silicon that can insert and release lithium ions. The negative electrode active material particles include silicon compound particles, and the silicon compound particles are preferably a silicon oxide material containing a silicon compound containing oxygen. The ratio of silicon and oxygen constituting this silicon compound is preferably in the range of SiO _x :0.5≦x≦1.6. If x is 0.5 or more, the oxygen ratio will be higher than that of silicon alone, and the cycle characteristics will be good. If x is 1.6 or less, the resistance of the silicon oxide will not become too high, which is preferable. Among these, it is preferable for the composition of SiO _x that x is close to 1. This is because high cycle characteristics can be obtained. Note that the composition of the silicon compound in the present invention does not necessarily mean 100% purity, and may contain trace amounts of impurity elements.

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

As the negative electrode conductive additive, for example, one or more of carbon materials such as carbon black, acetylene black, graphite, Ketjen black, carbon nanotubes, and carbon nanofibers can be used.

The negative electrode active material layer is formed, for example, by a coating method. The coating method involves mixing a silicon-based negative electrode active material, the above-mentioned binder, etc., and if necessary, a conductive agent and a carbon-based active material, and then dispersing it in an organic solvent or water to form a negative electrode current collector. This is a method of applying it to etc.

[Manufacturing method of negative electrode]
First, negative electrode active material particles are produced. In the following, an example of producing silicon compound particles containing an oxygen-containing silicon compound, in particular, silicon oxide represented by SiO _x (0.5≦x≦1.6) is used as the oxygen-containing silicon compound. Let me explain the case.

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. At this time, a mixture of metal silicon powder and silicon dioxide powder can be used as the raw material. Considering the presence of surface oxygen of the metal silicon powder and trace oxygen in the reactor, 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, while the temperature inside the reactor is lowered to 100° C. or less, the silicon oxide deposit is taken out and pulverized using a ball mill, jet mill, etc., to form a powder. Silicon compound particles can be produced in the manner described above. Note that the Si crystallites in the silicon compound particles can be controlled by changing the vaporization temperature of the raw material that generates silicon oxide gas or by heat treatment after the silicon compound particles are produced.

Here, a layer of carbon material may be formed on the surface layer of the negative electrode active material particles (silicon compound particles). However, regarding the definition of the particle size in the negative electrode active material of the present invention, as described above, the particle size and particle size distribution of the negative electrode active material particles were measured without forming a layer of carbon material. A pyrolytic CVD method is preferable as a method for producing the carbon material layer. An example of a method for producing a carbon material layer using the pyrolysis CVD method will be described below.

First, negative electrode active material particles (silicon compound particles) are set in a furnace. Next, hydrocarbon gas is introduced into the furnace and the temperature inside the furnace is raised. The decomposition temperature is not particularly limited, but is preferably 1100°C or lower, more preferably 900°C or lower. By setting the decomposition temperature to 1100° C. or lower, unintended disproportionation of the active material particles can be suppressed. After raising the temperature in the furnace to a predetermined temperature, a carbon layer is generated on the surface of the silicon compound particles. Furthermore, the hydrocarbon gas that is the raw material for the carbon material is not particularly limited, but it is desirable that n≦3 in the C _n H _m composition. If n≦3, the manufacturing cost can be lowered and the physical properties of the decomposition product can be improved.

Further, it is better to have a two-layer structure consisting of a carbon layer formed to provide conductivity and a carbon layer formed above the carbon layer for the purpose of reducing reactivity with the electrolytic solution. Battery cycle characteristics are improved due to the effect of the reaction suppression layer, which is generated at a lower temperature than the layer that provides conductivity.

The total thickness of the carbon layer is required to be thinner and more uniform, but if the carbon layer has a thickness of 5 nm or more, both conductivity and reaction suppression layer can be achieved.

Further, it is desirable that the silicon compound contains as little crystalline Si as possible. This is because it has high reactivity with the electrolyte and deteriorates battery characteristics. The crystallite size of Si is preferably 7.5 nm or less, and is preferably substantially amorphous. This crystallite size can be obtained using the Scherrer equation from the half-value width of a peak derived from the Si (111) plane measured by X-ray diffraction using Cu-Kα radiation.

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, heating pressing or the like may be performed as necessary. A negative electrode can be manufactured in the manner described above.

[Positive electrode]
For example, the positive electrode has a positive electrode active material layer on both sides or one side of a positive electrode current collector, similar to the negative electrode 10 in FIG. 1 .

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

The positive electrode active material layer contains one or more types of positive electrode materials capable of intercalating and deintercalating lithium ions, and may also contain other materials such as a binder, a conductive aid, and a dispersant depending on the design. It's okay to stay. In this case, the details regarding the binder and the conductive aid are the same as, for example, the negative electrode binder and the negative conductive aid described above.

As the positive electrode material, a lithium-containing compound is desirable. Examples of the lithium-containing compound include a composite oxide containing lithium and a transition metal element, or a phosphoric acid compound containing lithium and a transition metal element. Among these positive electrode materials, compounds containing at least one of nickel, iron, manganese, and cobalt are preferred. 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 transition metal element. The values of x and y vary depending on the charging/discharging state of the battery, but are generally expressed 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 ), lithium nickel composite oxide (Li _x NiO ₂ ), and the like. Examples of phosphoric acid compounds containing lithium and a transition metal element include lithium iron phosphate compounds (LiFePO ₄ ) and lithium iron manganese phosphate compounds (LiFe _1-u Mn _u PO ₄ (0<u<1)). can be mentioned. This is because by using these positive electrode materials, not only high battery capacity can be obtained, but also excellent cycle characteristics can be obtained.

[Negative electrode]
The negative electrode has the same configuration as the negative electrode 10 for a lithium ion secondary battery shown in FIG. It is preferable that the negative electrode has a larger negative electrode charging capacity than the electric capacity (charging capacity as a battery) obtained from the positive electrode active material. This is because precipitation of lithium metal on the negative electrode can be suppressed.

The positive electrode active material layer is provided on a portion of both surfaces of the positive electrode current collector, and the negative electrode active material layer is also provided on a portion 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 has a region where the opposing positive electrode active material layer does not exist. This is to ensure stable battery design.

The non-opposed region, that is, the region where the negative electrode active material layer and the positive electrode active material layer do not face each other, is hardly affected by charging and discharging. Therefore, the state of the negative electrode active material layer is maintained as it is immediately after formation. This makes it possible to accurately investigate the composition of the negative electrode active material with good reproducibility, regardless of the presence or absence of charging and discharging.

[Separator]
The separator isolates the lithium metal and the negative electrode, and allows lithium ions to pass through while preventing current short circuits caused by contact between the two electrodes. The separator is formed of a porous membrane made of, for example, synthetic resin or ceramic, and may have a laminated structure in which two or more types of porous membranes are laminated. Examples of the synthetic resin include polytetrafluoroethylene, polypropylene, and polyethylene.

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

For example, a non-aqueous solvent can be used as the solvent. Examples of the nonaqueous 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 desirable to use at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethylmethyl 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, ethyl methyl carbonate, or diethyl carbonate. This is because the dissociation properties and ion mobility of the electrolyte salt are improved.

When using an alloy-based negative electrode, it is particularly desirable that the solvent contains at least one of a halogenated chain carbonate ester and a halogenated cyclic carbonate ester. As a result, a stable film is formed on the surface of the negative electrode active material during charging and discharging, particularly during charging. Here, the halogenated chain carbonate ester is a chain carbonate ester having a halogen as a constituent element (at least one hydrogen is replaced with a halogen). Further, the halogenated cyclic carbonate ester is a cyclic carbonate ester having a halogen as a constituent element (that is, at least one hydrogen is replaced with a halogen).

The type of halogen is not particularly limited, but fluorine is preferred. This is because it forms a film of better quality than other halogens. Further, the greater the number of halogens, the more desirable. This is because the resulting coating is more stable and the decomposition reaction of the electrolyte is reduced.

Examples of the halogenated chain carbonate ester include fluoromethylmethyl carbonate and difluoromethylmethyl carbonate. Examples of the halogenated cyclic carbonate ester include 4-fluoro-1,3-dioxolan-2-one and 4,5-difluoro-1,3-dioxolan-2-one.

It is preferable that an unsaturated carbon-bonded cyclic carbonate ester is included as a solvent additive. This is because a stable film is formed on the surface of the negative electrode during charging and discharging, and decomposition reactions of the electrolytic solution can be suppressed. Examples of the unsaturated carbon-bonded cyclic carbonate include vinylene carbonate and vinylethylene carbonate.

Moreover, it is preferable that sultone (cyclic sulfonic acid ester) is included as a solvent additive. This is because the chemical stability of the battery is improved. Examples of the sultone include propane sultone and propene sultone.

Furthermore, it is preferable that the solvent contains an acid anhydride. This is because the chemical stability of the electrolyte is improved. Examples of acid anhydrides include propane disulfonic anhydride.

The electrolyte salt can include, for example, one or more types of light metal salts such as lithium salts. 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 relative to the solvent. This is because high ionic conductivity can be obtained.

EXAMPLES Hereinafter, the present invention will be explained in more detail by showing Examples and Comparative Examples of the present invention, but the present invention is not limited to these Examples.

(Example 1)
First, negative electrode active material particles (silicon compound particles) were produced as follows. A raw material mixture of metallic silicon and silicon dioxide was introduced into a reactor, vaporized in a vacuum atmosphere of 10 Pa, and deposited on a precipitation plate. After cooling sufficiently, the deposit was taken out and coarsened with a jaw crusher. After crushing, it was pulverized using a jet mill KJ-200 manufactured by Kurimoto. The pulverizing conditions at this time were a pulverizing pressure of 0.6 MPa and a separator rotation speed of 7,700 rpm.

Figure 2 shows the results of particle size distribution measurement of the powder collected by the cyclone using SALD-3100S manufactured by Shimadzu Corporation. This powder (negative electrode active material particles) had a mode diameter of 7 μm in volume distribution, and the content of fine powder with a particle size of less than 1 μm was 0% by mass. Based on this particle size distribution, when a certain particle size is D (μm) and the mass percentage of particles larger than that particle to all particles is R (%), logD is plotted on the x-axis and log{log() is plotted on the y-axis. The slope n of the straight line plotted on the Rosin-Rammler diagram with a scale of 100/R) was 4.4. The results are shown in FIG.

Thereafter, thermal decomposition CVD was performed at 1000° C. to coat the surface of the negative electrode active material particles (silicon compound particles) with a carbon material. The amount of carbon material covered was 3.4% by mass.

(Example 2)
Negative electrode active material particles (silicon compound particles) coarsely crushed in the same manner as in Example 1 were crushed at a crushing pressure of 0.5 MPa and a separator rotation speed of 7500 rpm.

Particle size distribution measurement was performed in the same manner as in Example 1. Figure 3 shows the results. The powder collected by the cyclone had a mode diameter of 7 μm, and the content of fine powder with a particle size of less than 1 μm was 0.7% by mass. Further, the slope n of the straight line plotted on the Rosin-Rammler diagram of this particle size distribution was 4.1. The results are shown in FIG.

(Example 3)
The negative electrode active material particles (silicon compound particles) obtained in Example 1 were mixed with fine powder collected by a bag filter of a jet mill. In this way, the content of fine powder having a particle size of less than 1 μm in the mixed negative electrode active material particles (silicon compound particles) was about 3% by mass.

Regarding this powder, particle size distribution measurement was performed in the same manner as in Example 1. Figure 4 shows the results. This powder had a mode diameter of 7 μm, and a content of fine powder with a particle size of less than 1 μm was 3.2% by mass. Further, the slope n of the straight line plotted on the Rosin-Rammler diagram of this particle size distribution was 2.6. The results are shown in FIG.

(Comparative example 1)
In the same manner as in Example 3, the negative electrode active material particles (silicon compound particles) obtained in Example 1 were mixed with the fine powder collected by the bag filter of a jet mill. The mixture was mixed so that the content of fine powder with a particle size of less than 1 μm in the compound particles was about 6% by mass.

Regarding this powder, particle size distribution measurement was performed in the same manner as in Example 1. Figure 5 shows the results. This powder had a mode diameter of 7 μm and a content of fine powder with a particle size of less than 1 μm of 6.5% by mass. Further, the slope n of the straight line plotted on the Rosin-Rammler diagram of this particle size distribution was 2.0. The results are shown in FIG.

(Comparative example 2)
In the same manner as in Example 3, the fine powder collected by the bag filter of a jet mill was mixed with the negative electrode active material particles (silicon compound particles) obtained in Example 1. The mixture was mixed so that the fine powder content was approximately 12% by mass.

Regarding this powder, particle size distribution measurement was performed in the same manner as in Example 1. Figure 6 shows the results. This powder had a mode diameter of 7 μm and a content of fine powder with a particle size of less than 1 μm of 12.2% by mass. Further, the slope n of the straight line plotted on the Rosin-Rammler diagram of this particle size distribution was 0.9. The results are shown in FIG.

The powders of Examples 2 and 3 and Comparative Examples 1 and 2 were also subjected to pyrolytic CVD in the same manner as in Example 1, and the carbon coating amount was adjusted to 3 to 4% by mass.

Next, the produced negative electrode active material particles (silicon oxide compound particles), graphite particles, conductive aid 1 (carbon nanotubes, CNTs), conductive aid 2 (carbon fine particles with a median diameter of about 50 nm), sodium polyacrylate, Carboxymethyl cellulose (hereinafter referred to as CMC) was mixed at a dry mass ratio of 9.3:83.7:1:1:4:1, and then diluted with pure water to obtain a negative electrode mixture slurry.

Further, as the negative electrode current collector, an electrolytic copper foil with a thickness of 15 μm was used. This electrolytic copper foil contained carbon and sulfur each at a concentration of 70 mass ppm. 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. After drying, the amount of negative electrode active material layer deposited per unit area (also referred to as area density) on one side of the negative electrode was 7.0 mg/cm ² .

Next, after mixing solvents ethylene carbonate (EC) and dimethyl carbonate (DMC), an electrolyte salt (lithium hexafluorophosphate: LiPF ₆ ) was dissolved to prepare an electrolytic solution. In this case, the composition of the solvent was EC:DMC=30:70 in volume ratio, and the content of the electrolyte salt was 1 mol/kg with respect to the solvent. As additives, vinylene carbonate (VC) and fluoroethylene carbonate (FEC) were added in amounts of 1.0% by mass and 2.0% by mass, respectively.

Next, the coin battery was assembled as follows. First, as a counter electrode, a Li foil with a thickness of 1 mm was punched out to a diameter of 16 mm and attached to an aluminum cladding. Subsequently, polyethylene with a thickness of 20 μm was layered on the Li foil as a separator, and an electrolytic solution was poured into it. Next, the negative electrode punched out to a diameter of 15 mm, the spacer (thickness 1.0 mm) are stacked on the separator, and the electrolyte is injected, then the spring and the top cover of the coin battery are pumped up in that order, and crimped with an automatic coin cell crimping machine. A 2032 coin battery was produced.

The initial efficiency was measured under the following conditions. First, charging was performed in CCCV mode at a charging rate equivalent to 0.03C. The CV was 0V and the final current was 0.04mA. Similarly, CC discharge was performed at a discharge rate of 0.03C and a discharge voltage of 1.2V.

When examining the initial charge/discharge characteristics, initial efficiency (hereinafter sometimes referred to as initial efficiency) was calculated. The initial efficiency was calculated from the formula expressed as initial efficiency (%)=(initial discharge capacity/initial charge capacity)×100. Based on the initial data obtained, a counter-positive electrode was designed and the battery was evaluated.

The cycle characteristics of the secondary battery produced as described above were evaluated. The cycle characteristics were investigated as follows. First, in order to stabilize the battery, two cycles of charging and discharging were performed at 0.2C in an atmosphere of 25C, and the discharge capacity of the second cycle was measured. Subsequently, charging and discharging were performed until the total number of cycles reached 300 cycles, and the discharge capacity was measured each time. Finally, the capacity retention rate was calculated by dividing the discharge capacity at the 200th cycle and the 300th cycle by the discharge capacity at the 2nd cycle. In the normal cycle, that is, from the 3rd cycle to the 299th cycle, charging and discharging were performed at 0.7 C for charging and 0.5 C for discharging.

The charging voltage was 4.3V, the end-of-discharge voltage was 2.5V, and the end-of-charge rate was 0.07C.

Table 1 shows the charging and discharging test results of Examples 1 to 3 and Comparative Examples 1 and 2.

As shown in Table 1, in the particle size distribution of the negative electrode active material particles, in Examples 1 to 3 in which the n value was 2.5 or more and the content of fine powder with a particle size of less than 1 μm was 5% by mass or less, 300 Since the capacity retention rate after cycling is 80% or more, it can be said that it has high cycle characteristics.

Note that the present invention is not limited to the above embodiments. The above-mentioned embodiments are illustrative, and any embodiment having substantially the same configuration as the technical idea stated in the claims of the present invention and having similar effects may be included in the present invention. covered within the technical scope of.

10...Negative electrode, 11...Negative electrode current collector, 12...Negative electrode active material layer.

Claims (7)

A negative electrode active material for a non-aqueous electrolyte secondary battery comprising negative electrode active material particles,
The negative electrode active material particles are powders containing silicon that are capable of intercalating and deintercalating lithium ions,
The negative electrode active material particles are silicon compound particles containing a silicon compound containing oxygen (however, the silicon compound particles exclude those containing at least one of Li 2 SiO 3 _and Li ₄ SiO ₄ ) _. including;
The negative electrode active material particles have a certain particle size D (μm) and a mass percentage of particles larger than that particle size relative to all particles based on the particle size distribution obtained by laser diffraction scattering particle size distribution measuring method. ), the slope n of the straight line plotted on a Rosin-Ramler diagram with logD on the x-axis and log{log(100/R)} on the y-axis is 2.5 or more, and the negative electrode A negative electrode active material for a non-aqueous electrolyte secondary battery, characterized in that, in the active material particles, the proportion of particles with a particle size of less than 1 μm is 5% by mass or less based on the entire negative electrode active material particles. The negative electrode active material for a nonaqueous electrolyte secondary battery according to claim 1, wherein the mode diameter of the negative electrode active material particles is 0.1 μm or more and 15 μm or less. The negative electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the negative electrode active material particles have a D _99.9 diameter of 40 μm or less. The silicon compound according to any one of claims 1 to 3, wherein the ratio of silicon and oxygen constituting the silicon compound is in the range of SiO _x :0.5≦x≦1.6. Negative electrode active material for water electrolyte secondary batteries. The negative electrode active material for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 4 , wherein at least a part of the surface of the negative electrode active material particles is coated with a carbon material. A negative electrode material for a non-aqueous electrolyte secondary battery, comprising the negative electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5 . A lithium ion secondary battery comprising a negative electrode comprising the negative electrode material for a nonaqueous electrolyte secondary battery according to claim 6 .

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