JP7816566B2 - Negative electrode for secondary battery and secondary battery - Google Patents

Description

This technology relates to negative electrodes for secondary batteries and secondary batteries.

With the widespread use of a wide variety of electronic devices, such as mobile phones, development is underway on secondary batteries as a power source that is small, lightweight, and offers high energy density. These secondary batteries contain a positive electrode, a negative electrode (secondary battery negative electrode), and an electrolyte, and various studies are being conducted on the configuration of these secondary batteries.

Specifically, the negative electrode contains first and second active material particles (see, for example, Patent Document 1). The first active material particles contain silicon or a silicon alloy, and the second active material particles contain silicon oxide (SiO _x (x=0.5 to 1.5)).

JP 2015-232921 A

Various studies have been conducted on the configuration of secondary batteries, but the battery characteristics of these batteries are still insufficient and there is room for improvement.

There is a demand for negative electrodes for secondary batteries and secondary batteries that can achieve excellent battery characteristics.

In one embodiment of the present technology, a secondary battery negative electrode includes a plurality of first negative electrode active material particles and a plurality of second negative electrode active material particles. The first negative electrode active material particles include an oxide of silicon, and the second negative electrode active material particles include at least one of elemental silicon and a silicon alloy. Cross-sectional observation using an electron microscope revealed that the median diameter of the first negative electrode active material particles was 5 μm or more and 8 μm or less, and the median diameter of the second negative electrode active material particles was 0.12 μm or more and 1.2 μm or less, the ratio of the median diameter of the first negative electrode active material particles to the median diameter of the second negative electrode active material particles was 5 or more and 50 or less, and the ratio of the total area of the first negative electrode active material particles to the total area of the second negative electrode active material particles was 0.6 or more and 0.9 or less.

A secondary battery according to one embodiment of the present technology comprises a positive electrode, a negative electrode, and an electrolyte, and the negative electrode has a configuration similar to that of the negative electrode for a secondary battery according to one embodiment of the present technology described above.

The following four types of configuration conditions are identified based on the results of observing the cross section of the negative electrode using an electron microscope, as described above. The details of the procedure for identifying these four types of configuration conditions will be described later.

(1) Range of median diameters of the plurality of first negative electrode active material particles; (2) Range of median diameters of the plurality of second negative electrode active material particles; (3) Range of ratio of median diameters of the plurality of first negative electrode active material particles to median diameters of the plurality of second negative electrode active material particles; (4) Range of ratio of total area of the plurality of first negative electrode active material particles to total area of the plurality of second negative electrode active material particles.

According to one embodiment of the present technology, the secondary battery negative electrode or secondary battery includes a plurality of first negative electrode active material particles and a plurality of second negative electrode active material particles, the first negative electrode active material particles include an oxide of silicon, and the second negative electrode active material particles include at least one of elemental silicon and a silicon alloy, and since the above-mentioned four configuration conditions are satisfied, excellent battery characteristics can be obtained.

Note that the effects of this technology are not necessarily limited to the effects described here, but may be any of a series of effects related to this technology described below.

1 is a cross-sectional view illustrating a configuration of a negative electrode for a secondary battery according to an embodiment of the present technology. 2 is a cross-sectional view illustrating the configuration of each of a plurality of first negative electrode active material particles and a plurality of second negative electrode active material particles illustrated in FIG. 1. FIG. 1 is a perspective view illustrating a configuration of a secondary battery according to an embodiment of the present technology. FIG. 4 is a cross-sectional view illustrating the configuration of the battery element shown in FIG. 3. FIG. 1 is a block diagram illustrating a configuration of an application example of a secondary battery. FIG. 2 is a cross-sectional view illustrating the configuration of a test secondary battery.

Hereinafter, an embodiment of the present technology will be described in detail with reference to the drawings. The description will be made in the following order.

1. Negative electrode for secondary battery 1-1. Structure 1-2. Operation 1-3. Manufacturing method 1-4. Action and effect 2. Secondary battery 2-1. Structure 2-2. Operation 2-3. Manufacturing method 2-4. Action and effect 3. Modification 4. Uses of secondary battery

<1. Negative electrode for secondary batteries>
First, a negative electrode for a secondary battery (hereinafter simply referred to as "negative electrode") according to one embodiment of the present technology will be described.

The negative electrode described here is used in a secondary battery, which is an electrochemical device. However, the negative electrode may also be used in electrochemical devices other than secondary batteries. Examples of other electrochemical devices include primary batteries and capacitors.

This negative electrode absorbs and releases electrode reactants during the operation of the electrochemical device (electrode reaction). The type of electrode reactant is not particularly limited, but examples include light metals such as alkali metals and alkaline earth metals. Specific examples of alkali metals include lithium, sodium, and potassium, while specific examples of alkaline earth metals include beryllium, magnesium, and calcium.

In the following, we will use an example where the electrode reactant is lithium. As a result, lithium is absorbed and released in ionic form at the negative electrode during the electrode reaction.

<1-1. Structure>
FIG. 1 shows the cross-sectional structure of a negative electrode 100, which is an example of a negative electrode, and FIG. 2 shows the cross-sectional structures of a plurality of first negative electrode active material particles 121 and a plurality of second negative electrode active material particles 122 shown in FIG. 1.

As shown in Figure 1, this negative electrode 100 comprises a negative electrode current collector 110 and a negative electrode active material layer 120.

[Negative electrode current collector]
1, the negative electrode current collector 110 is a conductive support member that supports the negative electrode active material layer 120, and has a pair of surfaces (upper and lower surfaces) on which the negative electrode active material layer 120 is provided. The negative electrode current collector 110 contains one or more types of conductive materials such as metal materials, and a specific example of the conductive material is copper.

It is preferable that the surface of the negative electrode current collector 110 is roughened using an electrolytic method. This is because the so-called anchor effect is utilized to improve the adhesion of the negative electrode active material layer 120 to the negative electrode current collector 110.

However, the negative electrode current collector 110 may be omitted. That is, the negative electrode 100 may include only the negative electrode active material layer 120.

[Negative electrode active material layer]
1 , the negative electrode active material layer 120 is a layer that absorbs and releases lithium, and is provided on one surface (upper surface or lower surface) of the negative electrode current collector 110. However, the negative electrode active material layer 120 may be provided on both surfaces (upper surface and lower surface) of the negative electrode current collector 110.

This negative electrode active material layer 120 contains two types of particulate negative electrode active material. Specifically, as shown in FIG. 2, the negative electrode active material layer 120 contains a plurality of first negative electrode active material particles 121 and a plurality of second negative electrode active material particles 122. The plurality of first negative electrode active material particles 121 and the plurality of second negative electrode active material particles 122 are mixed together within the negative electrode active material layer 120. Most of the plurality of first negative electrode active material particles 121 and the plurality of second negative electrode active material particles 122 are dispersed, but some of the plurality of first negative electrode active material particles 121 and the plurality of second negative electrode active material particles 122 may be agglomerated together.

However, in Figure 2, to simplify the illustration, the cross-sectional shape of the first negative electrode active material particles 121 is shown as circular. However, the cross-sectional shape of the first negative electrode active material particles 121 is not particularly limited, and may be any shape other than circular. Of course, the cross-sectional shape of each of the multiple first negative electrode active material particles 121 is not limited to one type, and may be two or more types.

What has been explained here regarding the cross-sectional shape of the first negative electrode active material particle 121 also applies to the cross-sectional shape of the second negative electrode active material particle 122. That is, in Figure 2, the cross-sectional shape of the second negative electrode active material particle 122 is circular, but the cross-sectional shape of the second negative electrode active material particle 122 is not particularly limited and may be a shape other than circular.

(First negative electrode active material particles)
The first negative electrode active material particles 121 have a relatively large median diameter, more specifically, a median diameter larger than the median diameter of the second negative electrode active material particles 122 .

The median diameter of the multiple first negative electrode active material particles 121 described here is so-called D50 (μm). Hereinafter, the median diameter of the multiple first negative electrode active material particles 121 will be referred to as the "median diameter MD1."

The first negative electrode active material particles 121 contain one or more types of silicon oxides. This is because in a secondary battery using the negative electrode 100, the discharge capacity is less likely to decrease even after repeated charging and discharging.

The type of silicon oxide is not particularly limited. In particular, it is preferable that the silicon oxide contains one or more of the compounds represented by formula (1). This is because the irreversible capacity is sufficiently reduced.

SiO _x (1)
(x satisfies 0.5<x<1.2.)

The detailed configuration of the multiple first negative electrode active material particles 121 (four types of configuration conditions) will be described later.

(Second negative electrode active material particles)
The second negative electrode active material particles 122 have a relatively small median diameter, more specifically, a median diameter smaller than the median diameter of the first negative electrode active material particles 121 .

The median diameter of the plurality of second negative electrode active material particles 122 described here is the so-called D50 (μm), similar to the median diameter of the plurality of first negative electrode active material particles 121. Hereinafter, the median diameter of the plurality of second negative electrode active material particles 122 will be referred to as the "median diameter MD2."

The second negative electrode active material particles 122 contain one or more types of silicon elemental element and silicon alloys. This is because a high energy density can be obtained. The second negative electrode active material particles 122 may contain only silicon elemental element, only a silicon alloy, or both silicon elemental element and a silicon alloy. When the second negative electrode active material particles 122 contain a silicon alloy, the type of silicon alloy may be only one type, or two or more types.

Elemental silicon refers only to the general elemental silicon. Therefore, elemental silicon may contain trace amounts of impurities. In other words, the purity of elemental silicon is not necessarily limited to 100%.

The type of silicon alloy is not particularly limited. Specifically, silicon alloys contain, in addition to silicon, one or more of the following metal elements: tin, nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium.

Specific examples of silicon alloys include _SiB4 , _SiB6 , Mg2Si, _Ni2Si , TiSi2, MoSi2, _CoSi2 , _NiSi2 , _CaSi2 , _CrSi2 , _Cu5Si , _{FeSi2 , MnSi2} , _NbSi2 , _{TaSi2, VSi2 , WSi2} , _{ZnSi2 ,} and _SiC . However, the composition of the silicon alloys exemplified here (the mixing ratio of silicon to metal elements) can be changed as desired.

The detailed configuration of the multiple second negative electrode active material particles 122 (four types of configuration conditions) will be described later.

(Other materials)
The negative electrode active material layer 120 may further contain one or more of the other materials.

The other material is the negative electrode binder, which contains one or more of the following materials: synthetic rubber, polymeric compound, etc. Specific examples of synthetic rubber include styrene-butadiene rubber, fluororubber, and ethylene-propylene-diene. Specific examples of polymeric compounds include polyvinylidene fluoride, polyimide, polyacrylic acid, and carboxymethyl cellulose.

The other material is a negative electrode conductive agent, which contains one or more conductive materials such as carbon materials, metal materials, and conductive polymer compounds. Specific examples of carbon materials include graphite, carbon black, acetylene black, and ketjen black.

Furthermore, the other material is a plurality of particulate third negative electrode active materials (a plurality of third negative electrode active material particles), and the third negative electrode active material particles contain one or more types of carbon materials. Since the crystal structure of the carbon material changes very little when lithium is absorbed and released, a high energy density can be obtained stably. In addition, since the carbon material also functions as a negative electrode conductor, the conductivity of the negative electrode active material layer 120 is improved.

Specific examples of carbon materials include graphitizable carbon, non-graphitizable carbon, and graphite. This graphite may be natural graphite, artificial graphite, or both. The interplanar spacing of the (002) plane of non-graphitizable carbon is not particularly limited, but is typically 0.37 nm or greater. The interplanar spacing of the (002) plane of graphite is not particularly limited, but is typically 0.34 nm or less.

Specific examples of carbon materials include pyrolytic carbons, cokes, glassy carbon fibers, organic polymer compound calcined bodies, activated carbon, and carbon blacks. Examples of cokes include pitch coke, needle coke, and petroleum coke. Organic polymer compound calcined bodies are produced by calcining (carbonizing) polymer compounds such as phenolic resins and furan resins at an appropriate temperature. Alternatively, the carbon material may be low-crystalline carbon heat-treated at temperatures below approximately 1000°C, or amorphous carbon. The shape of the carbon material is not particularly limited, but may be one or more of the following: fibrous, spherical, granular, and flaky.

When the negative electrode active material layer 120 contains a plurality of third negative electrode active material particles along with a plurality of first negative electrode active material particles 121 and a plurality of second negative electrode active material particles 122, damage and detachment of the negative electrode active material layer 120 are suppressed while the battery capacity is guaranteed, particularly during the electrode reaction (charging and discharging) of a secondary battery using the negative electrode 100.

Specifically, silicon-containing materials (silicon oxides, silicon elemental elements, and silicon alloys) have the advantage of high theoretical capacity, but have the drawback of being prone to severe expansion and contraction during charging and discharging. On the other hand, carbon materials have the drawback of low theoretical capacity, but have the advantage of being less prone to expansion and contraction during charging and discharging. Therefore, by using a silicon-containing material in combination with a carbon material, a high theoretical capacity can be obtained while suppressing the expansion and contraction of the anode active material layer 120 during charging and discharging. As a result, as described above, damage and shedding of the anode active material layer 120 are suppressed while maintaining the battery capacity.

[Detailed configuration]
Here, the detailed configuration of each of the plurality of first negative electrode active material particles 121 and the plurality of second negative electrode active material particles 122 will be described.

(4 types of configuration conditions)
Predetermined conditions are satisfied with respect to the respective configurations of the plurality of first negative electrode active material particles 121 and the plurality of second negative electrode active material particles 122. Specifically, the results of observation of a cross section of the negative electrode 100 using an electron microscope show that the following four types of configuration conditions are satisfied.

Below, we will explain the procedure for obtaining the observation results of the cross section of the negative electrode 100 using an electron microscope, and then explain the details of the four types of conditions.

When observing the cross section of the negative electrode 100 using an electron microscope, the negative electrode 100 is cut in the thickness direction (the vertical direction in Figure 1) to expose the cross section of the negative electrode 100. This exposes the cross section of the negative electrode active material layer 120, which contains a plurality of first negative electrode active material particles 121 and a plurality of second negative electrode active material particles 122.

Then, the cross section of the negative electrode active material layer 120 is observed using an electron microscope to obtain an observation result (electron microscope photograph) of the cross section. As a result, as shown in Figure 2, a state in which a plurality of first negative electrode active material particles 121 and a plurality of second negative electrode active material particles 122 coexist is observed. The type of electron microscope is not particularly limited, but specifically, it may be one or more of a scanning electron microscope, a transmission electron microscope, or the like. As a result, an observation result of the cross section of the negative electrode 100 using the electron microscope is obtained.

The cutting method and conditions for the negative electrode 100 are not particularly limited, and the conditions for observing the cross section of the negative electrode 100 are not particularly limited. Specifically, when cutting the negative electrode 100, an ion milling device is used as the cutting device, and the cutting conditions are an acceleration voltage of 6 kV, a discharge voltage of 1.5 kV, and a cutting time of 3 hours. When observing the cross section of the negative electrode 100, the observation conditions are a magnification of 2000x to 5000x and an acceleration voltage of 3 kV.

The first condition is that the median diameter MD1 is within a predetermined range. Specifically, the median diameter MD1 is 5 μm to 8 μm. The value of this median diameter MD1 is rounded to two decimal places.

The second condition is that the median diameter MD2 is within a predetermined range. Specifically, the median diameter MD2 is 0.12 μm to 1.2 μm. The value of this median diameter MD2 is rounded to two decimal places.

The third condition is that the ratio of median diameter MD1 to median diameter MD2 (particle size ratio RD) is within a specified range. Specifically, the particle size ratio RD (= MD1/MD2) is 5 to 50. The value of this particle size ratio RD is rounded to two decimal places.

The fourth condition is that the ratio (area ratio RS) of the total area S1 of the plurality of first negative electrode active material particles 121 to the total area S2 of the plurality of second negative electrode active material particles 122 is within a predetermined range. Specifically, the area ratio RS (= S1/S2) is 0.6 to 0.9. The value of this area ratio RS is rounded to one decimal place.

The area ratio RS is an index that represents the mixing ratio of the plurality of first negative electrode active material particles 121 and the plurality of second negative electrode active material particles 122 based on the abundance of the plurality of first negative electrode active material particles 121 and the abundance of the plurality of second negative electrode active material particles 122.

The four configuration conditions described above are satisfied because, unlike when the four configuration conditions are not satisfied, the relationship between the median diameter MD1 and the median diameter MD2 is optimized, and the mixture ratio between the plurality of first negative electrode active material particles 121 and the plurality of second negative electrode active material particles 122 is also optimized. As a result, a secondary battery using the negative electrode 100 can obtain sufficient discharge capacity from the first charge/discharge, and the discharge capacity is less likely to decrease even with repeated charge/discharge thereafter.

In particular, the third condition, particle size ratio RD (= 5 to 50), tends to be as explained below.

When the particle size ratio RD is less than 5, the difference between the median diameter MD1 and the median diameter MD2 is too small, making it difficult for the multiple first negative electrode active material particles 121 to be arranged so as to cover the surfaces of the second negative electrode active material particles 122. This makes it difficult to obtain sufficient discharge capacity from the first charge/discharge, and the discharge capacity is likely to decrease with subsequent repeated charge/discharge cycles.

On the other hand, when the particle size ratio RD is greater than 50, the difference between the median diameter MD1 and the median diameter MD2 is too large, making it difficult for the multiple first negative electrode active material particles 121 to be arranged so as to cover the surfaces of the second negative electrode active material particles 122, as in the case where the particle size ratio RD is less than 5. As a result, the amount of coverage of the surfaces of the second negative electrode active material particles 122 by the multiple first negative electrode active material particles 121 is insufficient, making it difficult to obtain sufficient discharge capacity from the first charge/discharge, and the discharge capacity is likely to decrease with subsequent repeated charge/discharge.

In particular, it is preferable that the particle size ratio RD is between 10 and 30. This is because a more optimized particle size ratio RD makes it easier to obtain sufficient discharge capacity from the first charge/discharge, and the discharge capacity is less likely to decrease even with subsequent repeated charge/discharge.

(Element distribution)
The element distribution on the surface of the first negative electrode active material particles 121 is not particularly limited. In particular, in the results of analyzing the surface of the first negative electrode active material particles 121 using X-ray photoelectron spectroscopy (XPS), it is preferable that the ratio of the amount of carbon present E2 to the amount of all elements present E1 (carbon ratio RE = E2/E1) is less than 1%. This is because a coating layer containing carbon as a constituent element is hardly provided on the surface of the first negative electrode active material particles 121, and therefore the coating layer hardly covers the surface of the first negative electrode active material particles 121. The value of this carbon ratio RE is rounded to one decimal place.

Here, "total elements" refers to all types of elements detected in the surface analysis of the first negative electrode active material particles 121 using XPS, including carbon. Therefore, the carbon fraction RE is an indicator of how much carbon is present on the surface of the first negative electrode active material particles 121.

(Calculation and Identification Procedures)
To calculate the median diameter MD1, first, a plurality of first negative electrode active material particles 121 are identified in an electron microscope photograph. Next, 300 first negative electrode active material particles 121 are arbitrarily selected from the plurality of first negative electrode active material particles 121, and the particle diameter (μm) of each of the 300 first negative electrode active material particles 121 is measured. In this case, the particle diameter may be measured manually or mechanically using any image processing software. Note that, if the particle diameter varies depending on the measurement direction, the maximum particle diameter is selected. Finally, the median diameter MD1 is calculated based on the number (= 300 particles) and the measurement results of the plurality of particle diameters.

The procedure for calculating the median diameter MD2 is the same as the procedure for calculating the median diameter MD1 described above, except that focus is on a plurality of second negative electrode active material particles 122 instead of a plurality of first negative electrode active material particles 121.

To calculate the particle size ratio RD, first calculate the median diameters MD1 and MD2 using the procedure described above, and then calculate the particle size ratio RD using the formula RD (= MD1/MD2).

When examining the area ratio RS, first, 300 first negative electrode active material particles 121 are arbitrarily selected from the plurality of first negative electrode active material particles 121 in the electron microscope photograph. Next, the area (μm ² ) of each of the 300 first negative electrode active material particles 121 is measured, and then the areas of the 300 particles are added together to calculate the total area S1 (μm ² ). In other words, the total area S1 is the sum of the areas of the 300 first negative electrode active material particles 121.

Next, the total area S2 (μm 2 ) is calculated using the same procedure as for calculating the total area S1 described above, except that the focus is on the plurality of second negative electrode active material particles 122 instead of the plurality of first negative electrode active material particles 121. In other words, the total area ^S2 is the sum of the areas of the 300 second negative electrode active material particles 122.

Finally, the area ratio RS (= S1/S2) is calculated using the formula:

The procedure for calculating the carbon ratio RE is as follows. The following describes the case where the negative electrode active material layer 120 contains a plurality of first negative electrode active material particles 121 and a plurality of second negative electrode active material particles 122, as well as a negative electrode binder and a negative electrode conductor.

First, the negative electrode 100 is disassembled to recover multiple first negative electrode active material particles 121.

In this case, the negative electrode current collector 110 is peeled off from the negative electrode active material layer 120, and then the negative electrode active material layer 120 is placed in a solvent. This solvent is either an aqueous solvent or an organic solvent that can dissolve the negative electrode binder. A specific example of a non-aqueous solvent is water, and a specific example of an organic solvent is N-methyl-2-pyrrolidone. In this case, the negative electrode binder dissolves, but the multiple first negative electrode active material particles 121, the multiple second negative electrode active material particles 122, and the negative electrode conductive agent do not. As a result, the negative electrode binder is dissolved and removed, and a mixture of the multiple first negative electrode active material particles 121 and the multiple second negative electrode active material particles 122 is recovered.

The mixture is then poured into a sieve with multiple holes to separate the first negative electrode active material particles 121 from the mixture. In this case, the size of the holes is set so that the first negative electrode active material particles 121 can pass through while the second negative electrode active material particles 122 and the negative electrode conductive agent cannot. This allows the first negative electrode active material particles 121 to pass through the holes, while the second negative electrode active material particles 122 and the negative electrode conductive agent cannot. Therefore, the first negative electrode active material particles 121 are separated from the second negative electrode active material particles 122 and the negative electrode conductive agent.

Next, the abundances of a series of elements are measured by performing a surface analysis of the first negative electrode active material particles 121 using XPS. This allows the abundances E1 of all elements to be calculated, as well as the abundance E2 of carbon.

Finally, the carbon ratio RE is calculated using the formula: carbon ratio RE = (E2/E1) x 100.

<1-2. Operation＞
This negative electrode 100 operates as follows during the electrode reaction.

In the negative electrode active material layer 120, lithium is absorbed in an ionic state into each of the plurality of first negative electrode active material particles 121 and the plurality of second negative electrode active material particles 122. In addition, in the negative electrode active material layer 120, lithium is released in an ionic state from each of the plurality of first negative electrode active material particles 121 and the plurality of second negative electrode active material particles 122.

<1-3. Manufacturing method＞
The negative electrode 100 is manufactured by the following example procedure.

First, a plurality of first negative electrode active material particles 121, a plurality of second negative electrode active material particles 122, a negative electrode binder, and a negative electrode conductive agent are mixed together to form a negative electrode mixture.

In this case, the median diameters MD1 and MD2 are adjusted so that the first and second conditions are satisfied. Specifically, by using a plurality of first negative electrode active material particles 121 whose median diameter MD1 satisfies the first condition and a plurality of second negative electrode active material particles 122 whose median diameter MD2 satisfies the second condition, the third condition is satisfied with respect to the particle size ratio RD.

In addition, by adjusting the mixing ratio of the plurality of first negative electrode active material particles 121 and the plurality of second negative electrode active material particles 122, the fourth condition regarding the area ratio RS is satisfied.

Next, the negative electrode mixture is poured into a solvent to prepare a paste-like negative electrode mixture slurry. This solvent may be an aqueous solvent or an organic solvent.

Finally, the negative electrode mixture slurry is applied to one side of the negative electrode current collector 110 to form the negative electrode active material layer 120. The negative electrode active material layer 120 may then be compression molded using a roll press or the like. In this case, the negative electrode active material layer 120 may be heated, or compression molding may be repeated multiple times.

This forms the negative electrode active material layer 120 on the negative electrode current collector 110, thereby completing the negative electrode 100.

<1-4. Actions and Effects>
According to this negative electrode 100, the negative electrode 100 includes a plurality of first negative electrode active material particles 121 and a plurality of second negative electrode active material particles 122, the first negative electrode active material particles 121 including an oxide of silicon, and the second negative electrode active material particles 122 including either or both of elemental silicon and an alloy of silicon.

Furthermore, the plurality of first negative electrode active material particles 121 and the plurality of second negative electrode active material particles 122 satisfy the four configuration conditions described above. Specifically, the median diameter MD1 is 5 μm to 8 μm (first condition). The median diameter MD2 is 0.12 μm to 1.2 μm (second condition). The particle size ratio RD is 5 to 50 (third condition). The area ratio RS is 0.6 to 0.9 (fourth condition).

In this case, as described above, the relationship between the median diameters MD1 and MD2 is optimized, and the mixture ratio between the plurality of first negative electrode active material particles 121 and the plurality of second negative electrode active material particles 122 is also optimized. As a result, in a secondary battery using the negative electrode 100, sufficient discharge capacity can be obtained from the first charge/discharge, and the discharge capacity is less likely to decrease even with subsequent repeated charge/discharge. Therefore, a secondary battery with excellent battery characteristics can be realized using the negative electrode 100.

In particular, if the particle size ratio RD is between 10 and 30, sufficient discharge capacity is more likely to be obtained from the first charge/discharge, and the discharge capacity is less likely to decrease even with subsequent repeated charge/discharge, resulting in greater effectiveness.

Furthermore, if the silicon oxide contains the compound shown in formula (1), the irreversible capacity is sufficiently reduced, thereby achieving a greater effect.

Furthermore, if the carbon ratio RE is less than 1%, the surface of the first negative electrode active material particles 121 is hardly covered with carbon, thereby achieving a greater effect.

Furthermore, if the negative electrode 100 further contains a plurality of third negative electrode active material particles, and these third negative electrode active material particles contain a carbon material, the battery capacity is guaranteed during charging and discharging of a secondary battery using the negative electrode 100, while damage and detachment of the negative electrode active material layer 120 are suppressed, thereby achieving even greater effects.

<2. Secondary battery>
Next, a secondary battery according to an embodiment of the present technology to which the negative electrode 100 is applied will be described.

The secondary battery described here is a secondary battery that obtains battery capacity by utilizing the absorption and release of electrode reactants, and is equipped with a positive electrode, a negative electrode, and an electrolyte.

The charge capacity of the negative electrode is preferably greater than the discharge capacity of the positive electrode. In other words, the electrochemical capacity per unit area of the negative electrode is preferably greater than the electrochemical capacity per unit area of the positive electrode. This is to prevent electrode reactants from depositing on the surface of the negative electrode during charging.

In the following, we will use the example where the electrode reactant is lithium, as mentioned above. Secondary batteries that obtain battery capacity by utilizing the absorption and release of lithium are known as lithium-ion secondary batteries. In these lithium-ion secondary batteries, lithium is absorbed and released in the ionic state.

<2-1. Structure>
Fig. 3 shows a perspective view of the secondary battery, and Fig. 4 shows a cross-sectional view of the battery element 20 shown in Fig. 3. However, Fig. 4 shows a state in which the exterior film 10 and the battery element 20 are separated from each other, and the cross section of the battery element 20 along the XZ plane is shown by a broken line.

As shown in Figures 3 and 4, this secondary battery comprises an exterior film 10, a battery element 20, a positive electrode lead 31, a negative electrode lead 32, and sealing films 41 and 42.

The secondary battery described here is a so-called laminate film type secondary battery, as it uses a flexible or pliable exterior film 10 as the exterior member for housing the battery element 20, as described above.

[Exterior film]
3, the exterior film 10 has a sealed bag-like structure in which the battery element 20 is housed, thereby housing a positive electrode 21, a negative electrode 22, a separator 23, and an electrolyte solution, which will be described later.

Here, the exterior film 10 is a single film-like member that is folded in the folding direction F. This exterior film 10 has a recessed portion 10U (a so-called deep-drawn portion) for accommodating the battery element 20.

Specifically, the exterior film 10 is a three-layer laminate film in which a fusion layer, a metal layer, and a surface protection layer are laminated in this order from the inside out. When the exterior film 10 is folded, the outer edges of the opposing fusion layers are fused together. The fusion layer contains a polymer compound such as polypropylene. The metal layer contains a metal material such as aluminum. The surface protection layer contains a polymer compound such as nylon.

However, the configuration (number of layers) of the exterior film 10 is not particularly limited, and may be one or two layers, or four or more layers.

[Battery element]
The battery element 20 is housed in an exterior film 10. The battery element 20 is a so-called power generating element, and as shown in Figures 3 and 4, includes a positive electrode 21, a negative electrode 22, a separator 23, and an electrolyte (not shown).

Here, the battery element 20 is a so-called wound electrode body. That is, the positive electrode 21 and the negative electrode 22 are wound around a winding axis P while facing each other via a separator 23. As is clear from Figure 3, this winding axis P is an imaginary axis extending in the Y-axis direction.

The three-dimensional shape of the battery element 20 is not particularly limited. Here, the battery element 20 has a flat three-dimensional shape, and therefore the shape of the cross section (cross section along the XZ plane) of the battery element 20 intersecting the winding axis P is a flat shape defined by the major axis J1 and the minor axis J2.

The major axis J1 is an imaginary axis extending in the X-axis direction and has a length greater than that of the minor axis J2. The minor axis J2 is an imaginary axis extending in the Z-axis direction intersecting the X-axis direction and has a length less than that of the major axis J1. Here, the three-dimensional shape of the battery element 20 is a flattened cylinder, and therefore the cross-sectional shape of the battery element 20 is a flattened, approximately elliptical shape.

(positive electrode)
As shown in FIG. 4, the positive electrode 21 includes a positive electrode current collector 21A and a positive electrode active material layer 21B.

The positive electrode current collector 21A has a pair of surfaces on which the positive electrode active material layer 21B is provided. This positive electrode current collector 21A contains a conductive material such as a metal material, and a specific example of such a conductive material is aluminum.

Positive electrode active material layer 21B contains one or more types of positive electrode active materials capable of absorbing and releasing lithium. However, positive electrode active material layer 21B may also contain one or more types of other materials, such as a positive electrode binder and a positive electrode conductor. The method for forming positive electrode active material layer 21B is not particularly limited, but examples include coating methods.

Here, the positive electrode active material layer 21B is provided on both sides of the positive electrode current collector 21A, so the positive electrode 21 includes two positive electrode active material layers 21B. However, since the positive electrode active material layer 21B is provided on only one side of the positive electrode current collector 21A, on the side where the positive electrode 21 faces the negative electrode 22, the positive electrode 21 may include only one positive electrode active material layer 21B.

The type of positive electrode active material is not particularly limited, but specific examples include lithium-containing compounds. This lithium-containing compound is a compound that contains lithium as well as one or more transition metal elements as constituent elements, and may also contain one or more other elements as constituent elements. The type of other elements is not particularly limited, but specific examples include elements belonging to groups 2 to 15 of the long period periodic table. The type of lithium-containing compound is not particularly limited, but specific examples include oxides, phosphate compounds, silicate compounds, and borate compounds.

Specific examples of _oxides include _LiNiO2 , _LiCoO2 , _{LiCo0.98Al0.01Mg0.01O2 , LiNi0.5Co0.2Mn0.3O2 , and LiMn2O4 .} Specific _{examples of phosphate} compounds include _LiFePO4 , _{LiMnPO4 ,} and _{LiFe0.5Mn0.5PO4} .

Details regarding the positive electrode binder and the positive electrode conductor are the same as those regarding the negative electrode binder and the negative electrode conductor described above.

(Negative electrode)
The negative electrode 22 has a configuration similar to that of the negative electrode 100. That is, the negative electrode 22 includes a negative electrode current collector 22A and a negative electrode active material layer 22B, as shown in Fig. 4. The configurations of the negative electrode current collector 22A and the negative electrode active material layer 22B are similar to the configurations of the negative electrode current collector 110 and the negative electrode active material layer 120, respectively.

Here, the negative electrode active material layer 22B is provided on both sides of the negative electrode current collector 22A. However, the negative electrode active material layer 22B may be provided on only one side of the negative electrode current collector 22A.

(separator)
4, the separator 23 is an insulating porous film interposed between the positive electrode 21 and the negative electrode 22, and allows lithium to pass through in an ionic state while preventing the occurrence of a short circuit due to contact between the positive electrode 21 and the negative electrode 22. The separator 23 contains a polymer compound such as polyethylene.

(electrolyte)
The electrolytic solution is a liquid electrolyte, and is impregnated into each of the positive electrode 21, the negative electrode 22, and the separator 23. The electrolytic solution contains a solvent and an electrolyte salt.

The solvent contains one or more non-aqueous solvents (organic solvents), and the electrolyte containing the non-aqueous solvent is a so-called non-aqueous electrolyte.

This non-aqueous solvent is an ester or ether, and more specifically, one or more of the following compounds: carbonate ester compounds, carboxylic acid ester compounds, and lactone compounds. This improves the dissociation of the electrolyte salt and the mobility of ions.

Carbonate compounds include cyclic carbonates and chain carbonates. Specific examples of cyclic carbonates include ethylene carbonate and propylene carbonate, while specific examples of chain carbonates include dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

Carboxylic acid ester compounds include chain carboxylic acid esters. Specific examples of chain carboxylic acid esters include ethyl acetate, ethyl propionate, propyl propionate, and ethyl trimethylacetate.

Lactone compounds include lactones. Specific examples of lactones include gamma-butyrolactone and gamma-valerolactone.

The ethers may also be 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, and 1,4-dioxane.

The nonaqueous solvent is one or more of the following: unsaturated cyclic carbonates, fluorinated cyclic carbonates, sulfonates, phosphates, acid anhydrides, nitrile compounds, and isocyanate compounds. This is because the dissociation of the electrolyte salt and the mobility of ions are similarly improved.

Specific examples of unsaturated cyclic carbonates include vinylene carbonate, vinylethylene carbonate, and methyleneethylene carbonate. Specific examples of fluorinated cyclic carbonates include monofluoroethylene carbonate and difluoroethylene carbonate. Specific examples of sulfonic acid esters include propane sultone and propene sultone. Specific examples of phosphate esters include trimethyl phosphate and triethyl phosphate. Specific examples of acid anhydrides include succinic anhydride, 1,2-ethanedisulfonic anhydride, and 2-sulfobenzoic anhydride. Specific examples of nitrile compounds include succinonitrile. Specific examples of isocyanate compounds include hexamethylene diisocyanate.

Of course, the composition of the non-aqueous solvent is not particularly limited as long as it contains one or more of the non-aqueous solvents listed above, and can be set as desired.

The electrolyte salt contains one or more light metal salts, such as lithium salts.

Specific examples of lithium salts include lithium hexafluorophosphate ( _LiPF ), lithium tetrafluoroborate (LiBF), lithium trifluoromethanesulfonate ( _LiCFSO ), lithium _bis (fluorosulfonyl)imide (LiN( _FSO ), lithium _bis (trifluoromethanesulfonyl)imide (LiN( _CFSO ), lithium tris(trifluoromethanesulfonyl)methide (LiC( _CFSO ), lithium _{bis ( oxalato ) borate} (LiB( _{CO )} ), lithium monofluorophosphate ( _LiPFO ), _and lithium _{difluorophosphate} ( _{LiPFO )} . These lithium salts are preferred because they can provide high battery capacities.

The content of the electrolyte salt is not particularly limited, but specifically, it is 0.3 mol/kg to 3.0 mol/kg relative to the solvent. This is because high ionic conductivity is obtained.

[Positive electrode lead]
3 and 4, the positive electrode lead 31 is a positive electrode wiring connected to the positive electrode current collector 21A of the positive electrode 21, and is led out of the exterior film 10. The positive electrode lead 31 contains a conductive material such as a metal material, and a specific example of the conductive material is aluminum. The positive electrode lead 31 has a shape such as a thin plate or a mesh.

[Negative electrode lead]
As shown in Figures 3 and 4 , the negative electrode lead 32 is a negative electrode wiring connected to the negative electrode current collector 22A of the negative electrode 22, and is led out of the exterior film 10. Here, the lead-out direction of the negative electrode lead 32 is the same as the lead-out direction of the positive electrode lead 31. This negative electrode lead 32 contains a conductive material such as a metal material, and a specific example of the conductive material is copper. Note that the details regarding the shape of the negative electrode lead 32 are the same as the details regarding the shape of the positive electrode lead 31.

[Sealing film]
The sealing film 41 is inserted between the exterior film 10 and the positive electrode lead 31, and the sealing film 42 is inserted between the exterior film 10 and the negative electrode lead 32. However, one or both of the sealing films 41 and 42 may be omitted.

The sealing film 41 is a sealing member that prevents outside air and other substances from entering the interior of the exterior film 10. This sealing film 41 contains a polymer compound such as polyolefin that has adhesion to the positive electrode lead 31, and a specific example of such a polymer compound is polypropylene.

The configuration of the sealing film 42 is the same as that of the sealing film 41, except that it is a sealing member that has adhesion to the negative electrode lead 32. That is, the sealing film 42 contains a polymer compound such as polyolefin that has adhesion to the negative electrode lead 32.

<2-2. Operation>
This secondary battery operates as follows during charging and discharging.

During charging, lithium is released from the positive electrode 21 of the battery element 20 and is absorbed into the negative electrode 22 via the electrolyte. During discharging, lithium is released from the negative electrode 22 of the battery element 20 and is absorbed into the positive electrode 21 via the electrolyte. During both discharging and charging, lithium is absorbed and released in an ionic state.

<2-3. Manufacturing method＞
When manufacturing a secondary battery, the positive electrode 21 and the negative electrode 22 are each produced and an electrolytic solution is prepared according to the procedure described below as an example. Then, the positive electrode 21, the negative electrode 22, and the electrolytic solution are used to assemble a secondary battery, and a stabilization process is performed on the assembled secondary battery.

[Preparation of Positive Electrode]
First, a cathode active material, a cathode binder, and a cathode conductive agent are mixed together to form a cathode mixture. Next, the cathode mixture is poured into a solvent to prepare a paste-like cathode mixture slurry. This solvent may be an aqueous solvent or an organic solvent. Finally, the cathode mixture slurry is applied to both surfaces of the cathode current collector 21A to form the cathode active material layer 21B. The cathode active material layer 21B may then be compression-molded using a roll press or the like. In this case, the cathode active material layer 21B may be heated, or the compression molding may be repeated multiple times. This results in the formation of the cathode active material layer 21B on both surfaces of the cathode current collector 21A, thereby producing the cathode 21.

[Fabrication of negative electrode]
The negative electrode 22 is produced by the same procedure as that for producing the negative electrode 100. In this case, the negative electrode active material layers 22B are formed on both surfaces of the negative electrode current collector 22A.

[Preparation of Electrolyte Solution]
An electrolyte salt is added to a solvent, whereby the electrolyte salt is dispersed or dissolved in the solvent, thereby preparing an electrolytic solution.

[Assembly of secondary battery]
First, the positive electrode lead 31 is connected to the positive electrode current collector 21A of the positive electrode 21 using a joining method such as welding, and the negative electrode lead 32 is connected to the negative electrode current collector 22A of the negative electrode 22 using a joining method such as welding. Next, the positive electrode lead 31 is connected to the positive electrode current collector 21A of the positive electrode 21 using a joining method such as welding, and the negative electrode lead 32 is connected to the negative electrode current collector 22A of the negative electrode 22 using a joining method such as welding.

Next, the positive electrode 21 and the negative electrode 22 are stacked together with the separator 23 interposed therebetween, and then the positive electrode 21, the negative electrode 22, and the separator 23 are wound together to form a wound body (not shown). The wound body is then pressed using a press or the like to form a flat shape. The wound body after this formation has a configuration similar to that of the battery element 20, except that the positive electrode 21, the negative electrode 22, and the separator 23 are not impregnated with electrolyte.

Next, after the roll is placed inside the recess 10U, the exterior film 10 (adhesive layer/metal layer/surface protection layer) is folded so that the exterior films 10 face each other. Next, the outer edges of two of the opposing adhesive layers are joined together using an adhesive method such as heat fusion, thereby placing the roll inside the bag-shaped exterior film 10.

Finally, electrolyte is poured into the bag-shaped exterior film 10, and the outer edges of the remaining opposing fusion layers are joined together using an adhesive method such as heat fusion. In this case, a sealing film 41 is inserted between the exterior film 10 and the positive electrode lead 31, and a sealing film 42 is inserted between the exterior film 10 and the negative electrode lead 32.

As a result, the wound body is impregnated with electrolyte, forming the battery element 20, which is a wound electrode body. The battery element 20 is then sealed inside the bag-shaped exterior film 10, and a secondary battery is assembled.

[Stabilization of secondary battery]
The assembled secondary battery is charged and discharged. Various conditions, such as the ambient temperature, the number of charge/discharge cycles (number of cycles), and the charge/discharge conditions, can be set as desired. This electrochemically stabilizes the states of the positive electrode 21 and the negative electrode 22, thereby completing the secondary battery.

<2-4. Actions and Effects>
In this secondary battery, the negative electrode 22 has a configuration similar to that of the negative electrode 100. Therefore, for the reasons described above, a sufficient discharge capacity can be obtained from the first charge/discharge, and the discharge capacity is less likely to decrease even after repeated charge/discharge, thereby achieving excellent battery characteristics.

In particular, if the secondary battery is a lithium-ion secondary battery, sufficient battery capacity can be obtained stably by utilizing the absorption and release of lithium, thereby achieving even greater effects.

3. Modified Examples
The configuration of the secondary battery described above can be modified as appropriate as described below, although the series of modifications described below may be combined with each other.

[Modification 1]
A porous film separator 23 was used. However, although not specifically shown here, a laminated separator including a polymer compound layer may also be used.

Specifically, the laminated separator includes a porous membrane with a pair of surfaces and a polymer compound layer provided on one or both surfaces of the porous membrane. This is because the separator's adhesion to each of the positive electrode 21 and negative electrode 22 is improved, thereby preventing misalignment (winding) of the battery element 20. This prevents swelling of the secondary battery even if a side reaction, such as the decomposition reaction of the electrolyte, occurs. The polymer compound layer includes a polymer compound such as polyvinylidene fluoride. This is because the polymer compound layer provides excellent physical strength and excellent electrochemical stability.

One or both of the porous film and the polymer compound layer may contain a plurality of insulating particles. This is because the plurality of insulating particles promotes heat dissipation when the secondary battery generates heat, thereby improving the safety (heat resistance) of the secondary battery. The plurality of insulating particles contain one or more types of insulating materials, such as inorganic materials and resin materials. Specific examples of inorganic materials include aluminum oxide, aluminum nitride, boehmite, silicon oxide, titanium oxide, magnesium oxide, and zirconium oxide. Specific examples of resin materials include acrylic resin and styrene resin.

When fabricating a laminated separator, a precursor solution containing a polymer compound and a solvent is prepared, and then the precursor solution is applied to one or both sides of a porous membrane. In this case, multiple insulating particles may be added to the precursor solution, if necessary.

Even when this laminated separator is used, the same effect can be achieved because lithium ions can move between the positive electrode 21 and the negative electrode 22. In this case, as mentioned above, the safety of the secondary battery is particularly improved, resulting in even greater effects.

[Modification 2]
An electrolyte solution, which is a liquid electrolyte, was used, but although not specifically shown here, an electrolyte layer, which is a gel electrolyte, may also be used.

In a battery element 20 using an electrolyte layer, positive electrodes 21 and negative electrodes 22 are alternately stacked with separators 23 and electrolyte layers interposed between them. The electrolyte layer is interposed between the positive electrode 21 and separator 23, and also between the negative electrode 22 and separator 23.

Specifically, the electrolyte layer contains a polymer compound along with an electrolyte solution, and the electrolyte solution is held in place by the polymer compound. This is because leakage of the electrolyte solution is prevented. The composition of the electrolyte solution is as described above. The polymer compound includes polyvinylidene fluoride, etc. To form the electrolyte layer, a precursor solution containing the electrolyte solution, the polymer compound, and a solvent is prepared, and then the precursor solution is applied to one or both surfaces of each of the positive electrode 21 and the negative electrode 22.

Even when this electrolyte layer is used, the same effect can be achieved because lithium ions can move between the positive electrode 21 and the negative electrode 22 via the electrolyte layer. In this case, even greater effects can be achieved, particularly because leakage of the electrolyte is prevented, as described above.

<4. Uses of secondary batteries>
The use (application example) of the secondary battery is not particularly limited. The secondary battery used as a power source may be a main power source or an auxiliary power source in electronic devices, electric vehicles, etc. The main power source is a power source that is used preferentially regardless of the presence or absence of other power sources. The auxiliary power source may be a power source used in place of the main power source or a power source that can be switched from the main power source.

Specific examples of uses for secondary batteries are as follows: Electronic devices such as video cameras, digital still cameras, mobile phones, laptop computers, headphone stereos, portable radios, and portable information terminals. Storage devices such as backup power supplies and memory cards. Power tools such as power drills and power saws. Battery packs installed in electronic devices. Medical electronic devices such as pacemakers and hearing aids. Electric vehicles such as electric cars (including hybrid cars). Power storage systems such as home or industrial battery systems that store power in preparation for emergencies. In these applications, one secondary battery may be used, or multiple secondary batteries may be used.

The battery pack may use a single cell or a battery pack. An electric vehicle is a vehicle that runs on a secondary battery as a driving power source, and may also be a hybrid vehicle that also has a driving source other than the secondary battery. In a home power storage system, household electrical appliances can be powered using the electricity stored in the secondary battery, which is the power storage source.

Here, we will explain in detail an example of an application of a secondary battery. The configuration of the application example described below is merely an example and can be modified as appropriate.

Figure 5 shows the block diagram of a battery pack. The battery pack described here is a battery pack (a so-called soft pack) that uses a single secondary battery, and is installed in electronic devices such as smartphones.

As shown in Figure 5, this battery pack comprises a power supply 71 and a circuit board 72. The circuit board 72 is connected to the power supply 71 and includes a positive terminal 73, a negative terminal 74, and a temperature detection terminal 75.

The power source 71 includes one secondary battery. The positive electrode lead of this secondary battery is connected to the positive electrode terminal 73, and the negative electrode lead is connected to the negative electrode terminal 74. The power source 71 can be connected to the outside via the positive electrode terminal 73 and the negative electrode terminal 74, and is therefore capable of charging and discharging. The circuit board 72 includes a control unit 76, a switch 77, a thermosensitive resistor (PTC element) 78, and a temperature detection unit 79. However, the PTC element 78 may be omitted.

The control unit 76 includes a central processing unit (CPU) and memory, and controls the operation of the entire battery pack. This control unit 76 detects and controls the usage status of the power source 71 as necessary.

When the voltage of the power source 71 (secondary battery) reaches the overcharge detection voltage or overdischarge detection voltage, the control unit 76 turns off the switch 77 to prevent charging current from flowing through the current path of the power source 71. The overcharge detection voltage is not particularly limited, but is specifically 4.20V ± 0.05V, and the overdischarge detection voltage is not particularly limited, but is specifically 2.40V ± 0.1V.

Switch 77 includes a charge control switch, a discharge control switch, a charge diode, and a discharge diode, and switches the connection between power supply 71 and an external device on or off in response to instructions from control unit 76. Switch 77 includes a metal oxide semiconductor field effect transistor (MOSFET), and the charge/discharge current is detected based on the ON resistance of switch 77.

The temperature detection unit 79 includes a temperature detection element such as a thermistor. This temperature detection unit 79 measures the temperature of the power supply 71 using the temperature detection terminal 75 and outputs the temperature measurement result to the control unit 76. The temperature measurement result measured by the temperature detection unit 79 is used when the control unit 76 controls charging and discharging in the event of abnormal heat generation, and when the control unit 76 performs correction processing when calculating the remaining capacity.

An example implementation of this technology is described below.

<Examples 1 to 21 and Comparative Examples 1 to 13>
As will be described below, after the secondary batteries were manufactured, the battery characteristics of the secondary batteries were evaluated.

[Manufacturing of secondary batteries]
Here, a test secondary battery was fabricated to perform a simple evaluation of the battery characteristics. Fig. 6 shows the cross-sectional structure of the test secondary battery (a coin-type lithium ion secondary battery).

As shown in Figure 6, this secondary battery comprises a test electrode 61, a counter electrode 62, a separator 63, an outer cup 64, an outer can 65, a gasket 66, and an electrolyte (not shown). Here, the test electrode 61 corresponds to the negative electrode, and the counter electrode 62 corresponds to the positive electrode.

The test electrode 61 is housed in an outer cup 64, and the counter electrode 62 is housed in an outer can 65. The test electrode 61 and counter electrode 62 are stacked together with a separator 63 interposed therebetween, and the test electrode 61, counter electrode 62, and separator 63 are each impregnated with electrolyte. The outer cup 64 and outer can 65 are crimped together with a gasket 66, so that the test electrode 61, counter electrode 62, and separator 63 are sealed within the outer cup 64 and outer can 65.

(Preparation of test electrode (negative electrode))
To fabricate the test electrode 61, first, 80 parts by mass of the negative electrode active material, 10 parts by mass of a negative electrode binder (polyacrylic acid), and 10 parts by mass of a negative electrode conductive agent (carbon black) were mixed together to form a negative electrode mixture. The negative electrode active materials used were a plurality of first negative electrode active material particles 121 (silicon oxide (SiO) which is an oxide of silicon) and a plurality of second negative electrode active material particles 122 (silicon element (Si) or silicon-iron-nickel alloy (SiFeNi) which is a silicon alloy). In this case, silicon element and a silicon alloy were used in combination as necessary.

Next, the negative electrode mixture was added to a solvent (pure water, an aqueous solvent), and the aqueous solvent was mixed using a planetary mixer. This dispersed the negative electrode mixture in the aqueous solvent, resulting in a paste-like negative electrode mixture slurry. Next, the negative electrode mixture slurry was applied to one side of the negative electrode current collector 22A (copper foil with a thickness of 12 μm) using a coating device, and the negative electrode mixture slurry was then dried to form the negative electrode active material layer 22B. In this case, the negative electrode mixture slurry was heated and dried (drying temperature = 120°C), and then vacuum-dried. Finally, the negative electrode active material layer 22B was compression-molded using a roll press. This resulted in the formation of the negative electrode active material layer 22B on one side of the negative electrode current collector 22A, resulting in the production of the test electrode 61.

(Preparation of counter electrode)
As described above, in order to perform a simple evaluation of the battery characteristics using a test secondary battery (the coin-type lithium ion secondary battery shown in FIG. 6), a lithium metal plate was used as the counter electrode 62 .

(Preparation of Electrolyte)
An electrolyte salt (lithium hexafluorophosphate (LiPF ₆ )) was added to a solvent (ethylene carbonate, a cyclic carbonate ester, and ethyl methyl carbonate, a chain carbonate ester), and the solvent was then stirred. In this case, the mixing ratio (weight ratio) of the solvents was 50:50, and the content of the electrolyte salt relative to the solvent was 1 mol/L (=1 mol/dm ³ ). In this way, an electrolytic solution was prepared.

(Secondary battery assembly)
First, the test electrode 61 was placed in an exterior cup 64, and the counter electrode 62 was placed in an exterior can 65. Next, the test electrode 61 placed in the exterior cup 64 and the counter electrode 62 placed in the exterior can 65 were stacked together with a separator 63 (a microporous polyethylene film with a thickness of 25 μm) impregnated with an electrolyte solution interposed therebetween. In this case, the negative electrode active material layer 22B and the counter electrode 62 faced each other with the separator 63 interposed therebetween. Finally, with the test electrode 61 and the counter electrode 62 stacked together with the separator 63 interposed therebetween, the exterior cup 64 and the exterior can 65 were crimped together with a gasket 66. As a result, the test electrode 61 and the counter electrode 62 were enclosed by the exterior cup 64 and the exterior can 65, and a secondary battery was assembled.

(Stabilization of secondary batteries)
The secondary battery was subjected to one cycle of charge and discharge in a room temperature environment (temperature = 23°C). During charging, the battery was charged at a constant current of 0.1 C until the voltage reached 0.02 V, and then charged at a constant voltage of 0.02 V until the current reached 0.025 C. During discharging, the battery was discharged at a constant current of 0.1 C until the voltage reached 1.5 V. 0.1 C is the current value at which the battery capacity (theoretical capacity) is fully discharged in 10 hours, and 0.025 C is the current value at which the battery capacity is fully discharged in 40 hours.

As a result, the states of the test electrode 61 and the counter electrode 62 were electrochemically stabilized, and the secondary battery was completed.

(Median diameters MD1, MD2, size relationship, particle size ratio RD, area ratio RS, and carbon ratio RE)
After the secondary battery was completed, the secondary battery was disassembled to recover the test electrode 61, and the median diameters MD1 and MD2 (μm), the size relationship, the particle size ratio RD, and the area ratio RS were examined. The results are shown in Tables 1 to 3.

Here, "size relationship" refers to the size relationship between the median diameters MD1 and MD2. "Large" in the "size relationship" column means that the median diameter MD1 is larger than the median diameter MD2. On the other hand, "small" in the "size relationship" column means that the median diameter MD1 is smaller than the median diameter MD2.

When preparing the test electrode 61, the surface of the first negative electrode active material particles 121 was not coated with carbon. As a result, after recovering the test electrode 61 from the secondary battery, the surface of the first negative electrode active material particles 121 was analyzed using XPS, and the carbon fraction RE was found to be less than 1%.

The calculation and determination procedures for the median diameters MD1, MD2 (μm), size relationship, particle size ratio RD, area ratio RS and carbon ratio RE are as described above.

[Evaluation of Battery Characteristics]
The initial charge/discharge characteristics and cycle characteristics were evaluated as the battery characteristics, and the results shown in Tables 1 to 3 were obtained.

(Initial charge/discharge characteristics)
First, the secondary battery was charged in a room temperature environment (temperature = 23°C) to measure the charge capacity (charge capacity at the first cycle). Next, the secondary battery was discharged in the same environment to measure the discharge capacity (discharge capacity at the first cycle). Finally, the initial efficiency, which is an index for evaluating the initial charge/discharge characteristics, was calculated based on the formula: initial efficiency (%) = (discharge capacity at the first cycle/discharge capacity at the first cycle) × 100. The charge/discharge conditions were the same as those during stabilization of the secondary battery.

The initial efficiency values shown in Table 1 are normalized with the initial efficiency value in Comparative Example 2 set to 1. The initial efficiency values shown in Table 2 are normalized with the initial efficiency value in Comparative Example 7 set to 1. The initial efficiency values shown in Table 3 are normalized with the initial efficiency value in Comparative Example 11 set to 1.

(Cycle characteristics)
First, the discharge capacity (discharge capacity at the first cycle) was measured by charging and discharging the secondary battery in a room temperature environment (temperature = 23°C). The charge and discharge conditions were the same as those during stabilization of the secondary battery.

Next, the secondary battery was repeatedly charged and discharged in the same environment until the total number of cycles reached 100, and the discharge capacity (discharge capacity at the 100th cycle) was measured. The charge and discharge conditions were the same as those used during stabilization of the secondary battery, except that the charge and discharge currents were both changed to 0.7 C.

Finally, the capacity retention rate, which is an indicator for evaluating cycle characteristics, was calculated based on the formula: capacity retention rate (%) = (discharge capacity at 100th cycle / discharge capacity at 1st cycle) x 100.

The capacity retention values shown in Table 1 are normalized values with the capacity retention value in Comparative Example 2 set to 1. The capacity retention values shown in Table 2 are normalized values with the capacity retention value in Comparative Example 7 set to 1. The capacity retention values shown in Table 3 are normalized values with the capacity retention value in Comparative Example 11 set to 1.

[Consideration]
As shown in Tables 1 to 3, the initial efficiency and capacity retention rate each varied greatly depending on the configuration of the test electrode 61.

Specifically, when the following four configuration conditions were met (Examples 1 to 21), a higher initial efficiency and a higher capacity retention rate were obtained compared to when the four configuration conditions were not met (Comparative Examples 1 to 13).

Median diameter MD1 = 5 μm to 8 μm
Median diameter MD2 = 0.12 μm to 1.2 μm
Particle size ratio RD=5~50
Area ratio RS=0.6~0.9

In particular, when four configuration conditions were met (Examples 1 to 21), the trends described below were observed. First, high initial efficiency and high capacity retention were obtained regardless of the material of the second negative electrode active material particles 122. In this case, similar trends were obtained when either silicon alone or a silicon alloy was used, and also when both silicon alone and a silicon alloy were used. Second, when the particle size ratio RD was 10 to 30, a high capacity retention was obtained while maintaining a high initial efficiency.

[summary]
The results shown in Tables 1 to 3 indicate that when the test electrode 61 includes a plurality of first anode active material particles 121 and a plurality of second anode active material particles 122, the first anode active material particles 121 include an oxide of silicon, and the second anode active material particles 122 include either or both of elemental silicon and a silicon alloy, and the four structural conditions described above are satisfied, high initial efficiency and high capacity retention are also obtained. Therefore, both the initial charge/discharge characteristics and the cycle characteristics are improved, and excellent battery characteristics are obtained in the secondary battery.

The present technology has been described above using one embodiment and example, but the configuration of the present technology is not limited to the configuration described in the one embodiment and example, and can be modified in various ways.

Specifically, the battery structure of the secondary battery has been described as being laminate film type and coin type. However, the battery structure of the secondary battery is not particularly limited, and may be cylindrical, rectangular, button type, etc.

The battery element has been described as having a wound structure. However, the structure of the battery element is not particularly limited, and may be a stacked structure or a zigzag structure. In a stacked structure, positive and negative electrodes are alternately stacked with separators in between, while in a zigzag structure, the positive and negative electrodes are folded in a zigzag pattern while facing each other with the separator in between.

Furthermore, although the electrode reactant has been described as being lithium, the electrode reactant is not particularly limited. Specifically, as mentioned above, the electrode reactant may be other alkali metals such as sodium and potassium, or alkaline earth metals such as beryllium, magnesium, and calcium. Alternatively, the electrode reactant may be other light metals such as aluminum.

The effects described in this specification are merely examples, and the effects of this technology are not limited to those described in this specification. Therefore, other effects may be obtained with this technology.

The present technology can also be configured as follows.

<1>
a positive electrode, a negative electrode, and an electrolyte;
the negative electrode includes a plurality of first negative electrode active material particles and a plurality of second negative electrode active material particles;
the first negative electrode active material particles contain an oxide of silicon,
the second negative electrode active material particles contain at least one of silicon elemental element and a silicon alloy,
As a result of observing a cross section of the negative electrode using an electron microscope,
the plurality of first negative electrode active material particles have a median diameter of 5 μm or more and 8 μm or less;
the plurality of second negative electrode active material particles have a median diameter of 0.12 μm or more and 1.2 μm or less;
a ratio of a median diameter of the plurality of first negative electrode active material particles to a median diameter of the plurality of second negative electrode active material particles is 5 or more and 50 or less;
a ratio of a total area of the plurality of first negative electrode active material particles to a total area of the plurality of second negative electrode active material particles is 0.6 or more and 0.9 or less;
Secondary battery.
<2>
a ratio of a median diameter of the first negative electrode active material particles to a median diameter of the second negative electrode active material particles is 10 or more and 30 or less;
The secondary battery according to <1>.
<3>
The oxide of silicon includes a compound represented by formula (1):
The secondary battery according to <1> or <2>.
SiO _x (1)
(x satisfies 0.5<x<1.2.)
＜4＞
an analysis result of the surface of the first negative electrode active material particle using X-ray photoelectron spectroscopy shows that the ratio of the amount of carbon present to the amount of all elements present, including carbon, is less than 1%;
<1><3> The secondary battery according to any one of <1> to <3>.
<5>
the negative electrode further includes a plurality of third negative electrode active material particles,
the third negative electrode active material particles contain a carbon material;
<4> The secondary battery according to any one of <1> to <4>.
<6>
It is a lithium-ion secondary battery.
<5> The secondary battery according to any one of <1> to <5>.
＜７＞
a plurality of first negative electrode active material particles and a plurality of second negative electrode active material particles;
the first negative electrode active material particles contain an oxide of silicon,
the second negative electrode active material particles contain at least one of silicon elemental element and a silicon alloy,
Cross-sectional observation using an electron microscope revealed that
the plurality of first negative electrode active material particles have a median diameter of 5 μm or more and 8 μm or less;
the plurality of second negative electrode active material particles have a median diameter of 0.12 μm or more and 1.2 μm or less;
a ratio of a median diameter of the plurality of first negative electrode active material particles to a median diameter of the plurality of second negative electrode active material particles is 5 or more and 50 or less;
a ratio of a total area of the plurality of first negative electrode active material particles to a total area of the plurality of second negative electrode active material particles is 0.6 or more and 0.9 or less;
Negative electrode for secondary batteries.

Claims (7)

a positive electrode, a negative electrode, and an electrolyte;
the negative electrode includes a plurality of first negative electrode active material particles and a plurality of second negative electrode active material particles;
the first negative electrode active material particles contain an oxide of silicon,
the second negative electrode active material particles contain at least one of silicon elemental element and a silicon alloy,
As a result of observing a cross section of the negative electrode using an electron microscope,
the plurality of first negative electrode active material particles have a median diameter of 5 μm or more and 8 μm or less;
the plurality of second negative electrode active material particles have a median diameter of 0.12 μm or more and 1.2 μm or less;
a ratio of a median diameter of the plurality of first negative electrode active material particles to a median diameter of the plurality of second negative electrode active material particles is 5 or more and 50 or less;
a ratio of a total area of the plurality of first negative electrode active material particles to a total area of the plurality of second negative electrode active material particles is 0.6 or more and 0.9 or less;
Secondary battery. a ratio of a median diameter of the first negative electrode active material particles to a median diameter of the second negative electrode active material particles is 10 or more and 30 or less;
The secondary battery according to claim 1 . The oxide of silicon includes a compound represented by formula (1):
The secondary battery according to claim 1 or 2.
SiO _x (1)
(x satisfies 0.5<x<1.2.) an analysis result of the surface of the first negative electrode active material particle using X-ray photoelectron spectroscopy shows that the ratio of the amount of carbon present to the amount of all elements present, including carbon, is less than 1%;
The secondary battery according to claim 1 or 2 . the negative electrode further includes a plurality of third negative electrode active material particles,
the third negative electrode active material particles contain a carbon material;
The secondary battery according to claim 1 or 2 . It is a lithium-ion secondary battery.
The secondary battery according to claim 1 or 2 . a plurality of first negative electrode active material particles and a plurality of second negative electrode active material particles;
the first negative electrode active material particles contain an oxide of silicon,
the second negative electrode active material particles contain at least one of silicon elemental element and a silicon alloy,
Cross-sectional observation using an electron microscope revealed that
the plurality of first negative electrode active material particles have a median diameter of 5 μm or more and 8 μm or less;
the plurality of second negative electrode active material particles have a median diameter of 0.12 μm or more and 1.2 μm or less;
a ratio of a median diameter of the plurality of first negative electrode active material particles to a median diameter of the plurality of second negative electrode active material particles is 5 or more and 50 or less;
a ratio of a total area of the plurality of first negative electrode active material particles to a total area of the plurality of second negative electrode active material particles is 0.6 or more and 0.9 or less;
Negative electrode for secondary batteries.