JP7059909B2 - All solid state battery - Google Patents

Description

The present disclosure relates to an all-solid-state battery.

The all-solid-state battery is a battery having a solid electrolyte layer between the positive electrode and the negative electrode, and has an advantage that the safety device can be easily simplified as compared with a liquid-based battery having an electrolytic solution containing a flammable organic solvent. ..

For example, Patent Document 1 discloses a lithium ion secondary battery having a negative electrode in which a Si-based active material, a conductive auxiliary agent, and a current collector are bonded by a metal bond. Further, Patent Document 2 describes composite particles having a sulfide solid electrolyte and a negative electrode active material, and the negative electrode active material having a carbon material containing Si or Sn, and the particle size of Si or Sn is 94 nm or less. Disclosed is a negative electrode for an all-solid-state battery in which the particle size of the negative electrode active material is 15 μm or less and the void ratio of the negative electrode is 5% to 30%.

Japanese Unexamined Patent Publication No. 2010-2188848 Japanese Unexamined Patent Publication No. 2017-054720

Since the volume of the Si-based active material changes greatly during charging / discharging, the resistance tends to increase every time the charging / discharging cycle is repeated. The present disclosure has been made in view of the above circumstances, and an object of the present disclosure is to provide an all-solid-state battery having good cycle characteristics.

In order to solve the above problems, in the present disclosure, the all-solid-state battery having a positive electrode layer, a negative electrode layer, and a solid electrolyte layer formed between the positive electrode layer and the negative electrode layer is the negative electrode layer. Contains a Si-based active material, and the Si _- based active material is a secondary particle having a plurality of primary particles, the volume of the secondary particle is VP, and the void volume of the secondary particle is _VV . When, the ratio of the _VV to the _{VP (VV /} VP) is 0.3 or more and 0.6 or less, and the Si _- based active material is placed between the plurality of primary particles. Provided is an all-solid-state battery containing a needle-shaped conductive material.

According to the present disclosure, an all-solid-state battery having good cycle characteristics due to the fact that _VV / VP is in a specific range and that the Si _- based active material contains a needle-shaped conductive material between a plurality of primary particles. Can be.

The all-solid-state battery in the present disclosure has the effect of having good cycle characteristics.

It is a schematic sectional drawing which shows an example of the all-solid-state battery in this disclosure. It is an image diagram of the cross section of the negative electrode active material in this disclosure. It is a graph which showed the result of an Example and a comparative example. It is a figure which showed schematically the relationship between the porosity and the resistance increase rate based on the result of an Example and a comparative example.

Hereinafter, the all-solid-state battery in the present disclosure will be described in detail.

FIG. 1 is a schematic cross-sectional view showing an example of an all-solid-state battery in the present disclosure. Further, FIG. 2 is an image diagram of a cross section of the Si-based active material in the present disclosure. The all-solid-state battery 10 shown in FIG. 1 has a positive electrode layer 1, a negative electrode layer 2, and a solid electrolyte layer 3 formed between the positive electrode layer 1 and the negative electrode layer 2. Further, the all-solid-state battery 10 has a positive electrode current collector 4 that collects electricity from the positive electrode layer 1 and a negative electrode current collector 5 that collects electricity from the negative electrode layer 2. The negative electrode layer 2 contains a Si-based active material which is a secondary particle having a plurality of primary particles. Further, as shown in FIG. 2, the Si-based active material in the present disclosure contains a needle-shaped conductive material between the primary particles and has voids inside the secondary particles. Further, when the volume of the secondary particles is _VP and the void volume of the secondary particles is V _V , V _V / _VP is in a specific range. In the present disclosure, _VV / _VP may be simply referred to as porosity.

According to the present disclosure, the _VV / VP is in a specific range, and the Si _- based active material contains a needle-shaped conductive material between the primary particles, whereby an all-solid-state battery having good cycle characteristics is obtained. be able to. Specifically, a Si-based active material, which is a secondary particle having voids at a specific ratio, is used, and a needle-shaped conductive material is contained between the primary particles to change the volume of the Si-based active material. For example, it is possible to suppress the generation of cracks between the Si-based active material and the solid electrolyte. As a result, an all-solid-state battery that can suppress an increase in resistance (suppress a decrease in ion conductivity) and has good cycle characteristics can be obtained. It is presumed that the reason why the volume change of the Si-based active material can be suppressed is that the needle-shaped conductive material exists between the plurality of primary particles, and the conductive material acts as a filler and contributes to the maintenance of the skeleton of the Si-based active material. Will be done.

1. 1. Negative electrode layer The negative electrode layer is a layer containing at least a negative electrode active material.

The negative electrode layer contains a Si-based active material as the negative electrode active material. The Si-based active material is preferably an active material that can be alloyed with Li. Examples of the Si-based active material include simple substances of Si, Si alloys, and Si oxides. The Si alloy preferably contains a Si element as a main component. The ratio of the Si element in the Si alloy may be, for example, 50 mol% or more, 70 mol% or more, or 90 mol% or more. Examples of the Si oxide include SiO.

The Si-based active material is a secondary particle having a plurality of primary particles. Further, the volume of the secondary particles is VP, the void volume of the secondary particles is _VV , and the _ratio of _VV to _VP is _VV / _VP . The value of V _V / _VP is 0.3 or more, for example, 0.35 or more, or 0.4 or more. On the other hand, the value of V _V / _VP is 0.6 or less, for example, 0.55 or less, or 0.5 or less. If the value of V _V / _VP is too small, the secondary particles cannot absorb the expansion and may increase the resistivity. Further, if the value of V _V / _VP is too large, the space not used in the secondary particles increases, and there is a risk that the primary particles are isolated and the contact between the Si-based active material and the solid electrolyte is easily broken.

_VV and VP can be determined by measuring a cross _- sectional image of a Si-based active material with a scanning electron microscope (SEM). In other words, VP can be approximated as the external area in the cross section of the Si _- based active material, and _VV can be approximated as the void area in the cross section of the Si-based active material. One secondary particle is specified from the cross _- sectional image, the outer shape area SP is obtained from the outer shape thereof, and the void area _SV is obtained from the voids. Obtain _{S V} / SP from these values. This operation is also performed for other secondary particles, and the average value of _SV / _SP is obtained. The number of samples is preferably large, for example, 20 or more, 50 or more, or 100 or more. The average value of _{S V} / _SP is adopted as V _V / VP.

Further, in the present disclosure, when the long side length of the primary particle is a, the short side length is b, and the ratio of b to a is b / a, the value of b / a is, for example, 0.5. It may be 0.6 or more, and may be 0.8 or more. On the other hand, the value of b / a may be 1 or less than 1. When such primary particles are used, an all-solid-state battery having better cycle characteristics can be obtained.

The long side length a and the short side length b can be obtained by measuring a cross-sectional image of the primary particle. Specifically, one primary particle is specified from the cross-sectional image, a straight line is drawn in the specified region, and the longest portion is defined as the long side a. On the other hand, a portion orthogonal to the long side a and having the smallest length is defined as the short side b. B / a is obtained from these values. This operation is also performed for other primary particles, and the average value of b / a is obtained. The number of samples is preferably large, for example, 20 or more, 50 or more, or 100 or more.

The average particle size (D ₅₀ ) of the primary particles is not particularly limited, but is, for example, 50 nm or more, and may be 100 nm or more. On the other hand, the average particle size (D ₅₀ ) of the primary particles is, for example, 1 μm or less, and may be 500 nm or less.

The Si-based active material in the present disclosure contains a needle-shaped conductive material between a plurality of primary particles. The needle shape in the present disclosure refers to an elongated shape in which the length of the long side is at least twice the length of the short side. Further, the needle shape may be a linear shape or a curved shape, and may be referred to as a rod shape or a fibrous shape. Further, a plurality of needle-shaped conductive materials may be entangled and exist. It is said that such a conductive material can play a role of a filler by being present between a plurality of primary particles, and can have a positive effect on the skeleton maintenance of the Si-based active material during expansion and contraction of the Si-based active material. Presumed.

The conductive material in the present disclosure is not particularly limited as long as it is needle-shaped, and examples thereof include VGCF (gas phase carbon fiber) and CNT (carbon nanotube).

In the needle-shaped conductive material in the present disclosure, the length of the long side is twice or more the length of the short side, and may be, for example, 10 times or more, or 50 times or more. On the other hand, the length of the long side may be 500 times or less or 100 times or less the length of the short side.

The long side of the needle-shaped conductive material in the present disclosure is, for example, 5 μm or more and 15 μm or less. Further, the short side of the needle-shaped conductive material in the present disclosure is, for example, 50 nm or more and 200 nm or less.

The Si-based active material in the present disclosure preferably does not contain a conductive material other than needle-like, such as granules, but may contain a small amount. Examples of the granular conductive material include acetylene black (AB) and Ketjen black (KB).

The ratio of the needle-shaped conductive material in the Si-based active material is not particularly limited, but is, for example, 0.5% by weight or more, and may be 1% by weight or more. The proportion of the conductive material is, for example, 10% by weight or less, and may be 5% by weight or less. If the proportion of the conductive material is too small, the effects in the present disclosure may not be fully enjoyed. On the other hand, if the proportion of the conductive material is too large, the productivity may deteriorate.

In the Si-based active material, a binder may be present between adjacent primary particles. Examples of the binder include polyimide. Further, a general binder used for the negative electrode layer may be used. The proportion of the binder contained in the Si-based active material is, for example, 0.5% by weight or more, and may be 1% by weight or more. On the other hand, the proportion of the binder contained in the Si-based active material is, for example, 5% by weight or less.

As a method for producing a Si-based active material, for example, a preparatory step for preparing a slurry containing primary particles of the Si-based active material, a conductive material (also referred to as a first conductive material), a binder, and a dispersion medium. , A method having a granulation step of performing a granulation treatment on the slurry can be mentioned. Examples of the dispersion medium used for the slurry include water. Further, as the binder, for example, a precursor of polyimide (polyamic acid) can be mentioned. As a method for forming a slurry, a mixture containing primary particles of a Si-based active material, a first conductive material, a binder, and a dispersion medium is kneaded using a kneading device such as a planetary mixer. There is a way to do it. The solid content concentration of the slurry is, for example, 5% by weight or more and 30% by weight or less.

On the other hand, examples of the granulation process include a process using a nozzle-type spray dryer. The amount of liquid to be sent is, for example, 20 mL / h or more and 200 mL / h or less. The spray gas pressure is, for example, 0.1 MPa or more and 0.4 MPa or less. The drying temperature is, for example, 140 ° C. or higher and 200 ° C. or lower. Further, for example, when the slurry contains a polyimide precursor as a binder, it is preferable to perform heat treatment after the granulation step to form the polyimide. The heat treatment temperature is, for example, 250 ° C. or higher and 350 ° C. or lower. The heat treatment time is, for example, 1 hour or more and 10 hours or less. The heat treatment atmosphere is preferably an inert atmosphere or a vacuum. This is because the oxidation of Si-based active substances can be prevented.

The voids of the Si-based active material can be adjusted by appropriately setting the production conditions. For example, when the solid content of the slurry is reduced, the voids tend to increase. On the other hand, for example, when the amount of the binder is increased, the voids tend to become smaller.

The negative electrode layer may contain only a Si-based active material as the negative electrode active material, or may contain another active material. In the latter case, the ratio of the Si-based active material in all the negative electrode active materials may be 50% by weight or more, 70% by weight or more, or 90% by weight or more.

The ratio of the negative electrode active material in the negative electrode layer is, for example, 20% by weight or more, 30% by weight or more, or 40% by weight or more. On the other hand, the ratio of the negative electrode active material in the negative electrode layer is 95% by weight or less, 90% by weight or less, or 80% by weight or less.

Further, the negative electrode layer may contain at least one of a solid electrolyte and a binder, if necessary. Examples of the solid electrolyte include inorganic solid electrolytes such as sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, and halide solid electrolytes. Examples of the sulfide solid electrolyte include Li element, X element (X is at least one of P, Si, Ge, Sn, B, Al, Ga, and In), and a solid electrolyte containing S element. Can be mentioned. Further, the sulfide solid electrolyte may further contain at least one of an O element and a halogen element. Examples of the oxide solid electrolyte include Li element, Y element (Y is at least one of Nb, B, Al, Si, P, Ti, Zr, Mo, W, and S), and O element. Examples thereof include solid electrolytes containing. Examples of the nitride solid electrolyte include Li 3N, and examples of the halide solid electrolyte include LiCl _, LiI, and LiBr. Examples of the binder include rubber-based binders such as butylene rubber (BR) and styrene-butadiene rubber (SBR), and fluoride-based binders such as polyvinylidene fluoride (PVDF).

Further, the negative electrode layer may contain a second conductive material in addition to the conductive material (first conductive material) contained in the Si-based active material. Examples of the second conductive material include a carbon material. Examples of the carbon material include particulate carbon materials such as acetylene black and ketjen black, and fibrous carbon materials such as carbon fibers, carbon nanotubes, and carbon nanofibers. The second conductive material may be different from the first conductive material and may be the same, but it is preferable that the second conductive material is the same.

When the negative electrode layer contains the first conductive material and the second conductive material, the ratio of the first conductive material in all the conductive materials in the negative electrode layer may be 50% by weight or more, 70% by weight. It may be 90% by weight or more, and may be 90% by weight or more.

The thickness of the negative electrode layer is, for example, 0.1 μm or more and 1000 μm or less. Examples of the method for forming the negative electrode layer include a method of applying and drying a slurry containing at least the SiO-based active material after the granulation treatment and the dispersion medium described above. The second conductive material may be added when preparing the above slurry.

2. 2. Positive electrode layer The positive electrode layer is a layer containing at least a positive electrode active material. Further, the positive electrode layer may contain at least one of a solid electrolyte, a conductive material and a binder, if necessary.

Examples of the positive electrode active material include an oxide active material. Examples of the oxide active material include rock salt layered active materials such as LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , LiVO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiMn ₂ O ₄ , Li ₄ Examples thereof include spinel-type active materials such as Ti ₅ O ₁₂ and Li (Ni _0.5 Mn _1.5 ) O ₄ , and olivine-type active materials such as LiFePO ₄ , LiMnPO ₄ , LiNiPO ₄ , and LiCoPO ₄ . Further, a coat layer containing a Li ion conductive oxide may be formed on the surface of the positive electrode active material. This is because the reaction between the positive electrode active material and the solid electrolyte can be suppressed.

Examples of the shape of the positive electrode active material include particles. The average secondary particles (D ₅₀ ) of the positive electrode active material are not particularly limited, but are, for example, 10 nm or more, and may be 100 nm or more. On the other hand, the average secondary particle diameter (D ₅₀ ) of the positive electrode active material is, for example, 50 μm or less, and may be 20 μm or less.

The ratio of the positive electrode active material in the positive electrode layer is, for example, 20% by weight or more, 30% by weight or more, or 40% by weight or more. On the other hand, the ratio of the positive electrode active material is, for example, 80% by weight or less, 70% by weight or less, or 60% by weight or less.

Since the solid electrolyte and the binder used for the positive electrode layer are the same as those described in "1. Negative electrode layer" above, the description thereof is omitted here.

Examples of the conductive material include a carbon material. Examples of the carbon material include particulate carbon materials such as acetylene black and ketjen black, and fibrous carbon materials such as carbon fibers, carbon nanotubes, and carbon nanofibers.

The thickness of the positive electrode layer is, for example, 0.1 μm or more and 1000 μm or less. Examples of the method for forming the positive electrode layer include a method in which a slurry containing at least a positive electrode active material and a dispersion medium is applied and dried.

3. 3. Solid electrolyte layer The solid electrolyte layer is a layer arranged between the positive electrode layer and the negative electrode layer. The solid electrolyte layer may contain at least a solid electrolyte and, if necessary, a binder. Since the solid electrolyte and the binder are the same as those described in "1. Negative electrode layer" above, the description thereof is omitted here. Above all, the solid electrolyte layer preferably contains a sulfide solid electrolyte as the solid electrolyte.

The thickness of the solid electrolyte layer is, for example, 0.1 μm or more and 1000 μm or less. Examples of the method for forming the solid electrolyte layer include a method of compression molding a solid electrolyte.

4. Other Members The all-solid-state battery in the present disclosure has at least the above-mentioned negative electrode layer, positive electrode layer and solid electrolyte layer. Further, usually, it has a positive electrode current collector that collects electricity from the positive electrode layer and a negative electrode current collector that collects electricity from the negative electrode layer. Examples of the material of the positive electrode current collector include SUS, aluminum, nickel, iron, titanium and carbon. On the other hand, examples of the material of the negative electrode current collector include SUS, copper, nickel and carbon. It is preferable to appropriately select the thickness and shape of the positive electrode current collector and the negative electrode current collector according to the application of the battery. Further, the all-solid-state battery in the present disclosure may have a battery case for accommodating the above-mentioned negative electrode layer, positive electrode layer and solid electrolyte layer.

5. All-solid-state battery The all-solid-state battery in the present disclosure is preferably an all-solid-state lithium battery. Further, the all-solid-state battery in the present disclosure may be a primary battery or a secondary battery, but a secondary battery is preferable. This is because it can be repeatedly charged and discharged and is useful as an in-vehicle battery, for example. Further, the all-solid-state battery in the present disclosure may be a single battery or a laminated battery. The laminated battery may be a monopolar type laminated battery (parallel connection type laminated battery) or a bipolar type laminated battery (series connection type laminated battery). Examples of the shape of the all-solid-state battery include a coin type, a laminated type, a cylindrical type, and a square type.

The present disclosure is not limited to the above embodiment. The above-described embodiment is an example, and any technique having substantially the same structure as the technical idea described in the claims of the present disclosure and exhibiting the same action and effect is the present invention. Included in the technical scope of the disclosure.

[Example 1]
(Preparation of positive electrode structure)
Positive electrode active material (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) and sulfide solid electrolyte (LiI-Li ₂ O-Li ₂ SP ₂ S ₅ ), positive electrode active material: sulfide solid electrolyte Weighed at a weight ratio of = 75:25. Then, the PVDF binder was weighed to 1.5 parts by weight and the conductive material (VGCF, manufactured by Showa Denko KK) was weighed to 3.0 parts by weight with respect to 100 parts by weight of the positive electrode active material. These materials were mixed and butyl butyrate was added to adjust the solid content to 63% by weight. Then, it was kneaded for 1 minute using an ultrasonic homogenizer to obtain a positive electrode slurry. The obtained positive electrode slurry was applied to the surface of a positive electrode current collector (Al foil, manufactured by Showa Denko KK) using an applicator (350 μm), and dried by heating. Then, it was roll-pressed at 25 ° C. and a linear pressure of 1 ton / cm to obtain a positive electrode structure having a positive electrode current collector and a positive electrode layer.

(Preparation of negative electrode active material)
Si particles (primary particles), a conductive material (VGCF, manufactured by Showa Denko KK) with a length of 6 μm and a diameter of 150 nm, a polyimide precursor (polyamic acid), and water are mixed and kneaded with a planetary mixer. And obtained a slurry. The obtained slurry was dried using a nozzle-type spray dryer and granulated. Then, the heat treatment was carried out in an inert atmosphere to obtain a negative electrode active material.

(Manufacturing of negative electrode structure)
The prepared negative electrode active material and sulfide solid electrolyte (LiI-Li ₂ O-Li ₂ SP ₂ S ₅ ) were weighed in a weight ratio of negative electrode active material: sulfide solid electrolyte = 58: 42. Then, the PVDF binder was weighed to 1.5 parts by weight and the conductive material (VGCF) was weighed to 5.0 parts by weight with respect to 100 parts by weight of the negative electrode active material. These materials were mixed and butyl butyrate was added to adjust the solid content to 63% by weight. Then, it was kneaded for 1 minute using an ultrasonic homogenizer to obtain a negative electrode slurry. The obtained negative electrode slurry was applied to the surface of the negative electrode current collector (Cu foil) using an applicator and dried by heating. Then, it was roll-pressed at 25 ° C. and a linear pressure of 1 ton / cm to obtain a negative electrode structure having a negative electrode current collector and a negative electrode layer. The thickness of the negative electrode layer was adjusted so that the negative electrode capacity was 2.5 with respect to the positive electrode capacity.

(Battery production)
A battery was obtained by arranging the positive electrode layer of the positive electrode structure and the negative electrode layer of the negative electrode structure so as to face each other via the solid electrolyte layer and pressing them at 5 tons.

[Examples 2 to 3]
As shown in Table 1, a battery was obtained in the same manner as in Example 1 except that the porosity of the negative electrode active material was changed.

[Examples 4 to 7]
As shown in Table 2, a battery was obtained in the same manner as in Example 2 (porosity 0.5) except that the content of the conductive material was changed.

[Comparative Examples 1 to 5]
As shown in Table 1, a battery was obtained in the same manner as in Example 1 except that the negative electrode active material did not contain a conductive material and the porosity of the negative electrode active material was changed.

[Comparative Examples 6 to 10]
As shown in Table 1, the same as in Example 1 except that the porosity was changed and the negative electrode active material did not contain the needle-shaped conductive material (VGCF) but contained only the granular conductive material (AB). I got a battery.

[Comparative Examples 11 to 12]
As shown in Table 1, a battery was obtained in the same manner as in Example 1 except that the porosity of the negative electrode active material was changed.

Cross-sectional images of the negative electrode active materials obtained in Examples 1 to 7 and Comparative Examples 1 to 12 were obtained by SEM, and _VV / _VP was obtained. The results are shown in Tables 1 and 2.

[evaluation]
The batteries obtained in Examples 1 to 7 and Comparative Examples 1 to 12 were charged and discharged for 500 cycles under the conditions of 60 ° C., a lower limit SOC of 10%, and an upper limit SOC of 90%. Note that SOC means state of charge. The internal resistance of the 1st cycle and 500th cycle batteries was measured by the DCIR method. The measurement conditions of the DCIR method are as follows.
OCV potential: 3.5V
Current density: 15mA / cm ²
Discharge time: 10 seconds The ratio of the resistance at the 500th cycle to the resistance at the 1st cycle was determined as the resistance increase rate ΔR. The results are shown in Table 1, Table 2, and FIG.

As shown in Table 1 and FIG. 3, ΔR of Examples 1 to 3 was smaller than that of Comparative Examples 1 to 12. As described above, it was confirmed that the V _V / VP is in a specific range and the Si _- based active material contains a needle-shaped conductive material, so that an all-solid-state battery having good cycle characteristics can be obtained.

From the results of Examples and Comparative Examples, the relationship between the porosity ( _VV / VP) and the resistance increase rate ( _ΔR ) is schematically shown in FIG. From FIGS. 3 and 4, if the ratio of the void volume was too small, ΔR became large. It is presumed that this is because the volume change of the Si-based active material due to charging and discharging could not be sufficiently mitigated. Further, even if the value of V _V / VP is too large, that is, even if the ratio of the void volume is too large, _ΔR becomes large. It is presumed that this is because the bonding between the Si-based active material and the solid electrolyte in the negative electrode layer is likely to come off. It was also suggested that the porosity is most preferably around 0.5.
Further, there was no difference in the resistance increase rate between the case where the Si-based active material did not contain the conductive material and the case where the Si-based active material contained only the granular conductive material. On the other hand, when the Si-based active material contains a needle-shaped conductive material, the resistance increase rate is remarkably suppressed. It is presumed that this is because the needle-shaped conductive material plays the role of a filler and has a positive effect on the skeleton maintenance of the weight-increasing body during expansion and contraction.

Further, as shown in Table 2, it was confirmed that ΔR becomes smaller as the amount of the conductive material increases, but from the viewpoint of production technology, 0.5% by weight or more and 10% by weight or less is appropriate. Conceivable.

1 ... Positive electrode layer 2 ... Negative electrode layer 3 ... Solid electrolyte layer 4 ... Positive electrode current collector 5 ... Negative electrode current collector 10 ... All-solid-state battery

Claims (1)

An all-solid-state battery having a positive electrode layer, a negative electrode layer, and a solid electrolyte layer formed between the positive electrode layer and the negative electrode layer.
The negative electrode layer contains a Si-based active material and contains
The Si-based active material is a secondary particle having a plurality of primary particles, and is a secondary particle.
The secondary particles are particles in which a binder exists between the adjacent primary particles and have voids inside.
When the volume of the secondary particles is VP and the void volume of the secondary particles is _VV , the ratio of the _VV to the _VP ( _VV / _VP ) is _0.3 or more, 0. .6 or less,
The Si-based active material contains a needle-shaped conductive material between the plurality of primary particles, and the Si-based active material contains a needle-shaped conductive material.
The _VP and the VV are all _- solid-state batteries obtained by measuring a cross-sectional image of the Si-based active material with a scanning electron microscope .

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