JP7230849B2 - Anode for lithium-ion batteries - Google Patents

Description

The present application relates to negative electrodes for lithium ion batteries.

Si has attracted attention as a negative electrode active material for secondary batteries. This is because Si has a high theoretical capacity and is effective in increasing the energy density of batteries. However, since Si undergoes a large volume change during charging and discharging, expansion and contraction cause cracks at the interface between the negative electrode active material and the solid electrolyte and between the solid electrolytes, resulting in a decrease in ionic conductivity. There is fear.

As a countermeasure for suppressing such expansion and contraction of Si, it is known to coat an active material containing Si with a solid electrolyte. Such a technique is described in Patent Document 1, for example. Patent Document 1 discloses composite active material particles obtained by coating active material particles with a sulfide-based solid electrolyte, a fibrous conductive material, and sulfide-based solid electrolyte particles having an average particle size smaller than that of the composite active material particles. and an electrode mixture is disclosed. Further, in the literature, the active material particles can also be used as a negative electrode active material, and lithium silicon oxides such as _{LixSiyOz and} metals such as Si are exemplified as specific examples _of the negative electrode active material. Further, FIG. 2 of the same document discloses a form in which an active material is covered with an oxide-based solid electrolyte and further covered with a sulfide-based solid electrolyte.

JP 2016-207418 A WO2008/81811 JP 2009-211910 A JP 2016-139461 A

According to the findings of the present inventors, by coating a negative electrode active material containing Si with a solid electrolyte, cracks occurring at the interface between the negative electrode active material and the solid electrolyte due to expansion and contraction of Si can be suppressed to some extent. However, cracks occurring at the interface between the solid electrolytes cannot be completely suppressed, and the effect is limited. If cracks occur at the interface between the negative electrode active material and the solid electrolyte or at the interface between the solid electrolyte and the solid electrolyte, there is a problem that the ionic conductivity is lowered and the capacity retention ratio of the battery is lowered.

Therefore, in view of the above circumstances, an object of the present application is to provide a negative electrode for a lithium ion battery that can suppress a decrease in capacity retention rate when using a negative electrode active material containing Si.

In order to solve the above problems, the present inventors have made extensive studies, and as a result, a negative electrode for a lithium ion battery having a negative electrode composite active material in which a negative electrode active material is coated with a sulfide solid electrolyte and a predetermined soft-viscous crystal Further, by adjusting the coverage of the sulfide solid electrolyte on the surface of the negative electrode composite active material and the volume ratio of the flexible crystal to the total of the solid electrolyte in the negative electrode within a predetermined range, the capacity retention rate of the battery is improved. It was found that the decrease can be suppressed.

The use of plastic crystals as an electrolyte is described in Patent Documents 2 to 4, for example. is different.

From the above, as one means for solving the above problems, the present application provides a negative electrode composite active material in which a negative electrode active material containing Si is coated with a sulfide solid electrolyte, pyrrolidinium, tetraalkylammonium and tetraalkylphosphonium and at least one cation selected from the group consisting of a sulfide solid electrolyte and a plastic crystal containing a carborane anion, and the coverage of the sulfide solid electrolyte on the surface of the negative electrode active material is 80% or more, and the sulfide Disclosed is a negative electrode for a lithium ion battery, wherein the volume ratio of the plastic crystal to the total of the solid electrolyte and the plastic crystal is 30 vol % to 75 vol %.

According to the lithium-ion battery negative electrode described above, even when a negative electrode active material containing Si is used, a decrease in the capacity retention rate can be suppressed.

It is a schematic diagram of the negative electrode 100 for lithium ion batteries. 1 is a schematic cross-sectional view of a lithium-ion all-solid-state battery 1000. FIG. FIG. 3 is a diagram showing the coverage of the second solid electrolyte on the surface of the negative electrode active material and the volume ratio of the first solid electrolyte to the electrolyte in the negative electrode.

[Negative electrode for lithium ion battery]
The negative electrode for a lithium ion battery of the present disclosure is a negative electrode composite active material obtained by coating a negative electrode active material containing Si with a sulfide solid electrolyte, and at least one selected from the group consisting of pyrrolidinium, tetraalkylammonium and tetraalkylphosphonium. Seed cations and plastic crystals containing carborane-based anions, the coverage of the sulfide solid electrolyte on the surface of the negative electrode active material is 80% or more, and the solid electrolyte with respect to the total of the sulfide solid electrolyte and the solid electrolyte is characterized by a volume ratio of 30 vol% to 75 vol%.

Since the negative electrode for a lithium ion battery of the present disclosure has the above configuration, even if a negative electrode active material containing Si is used, cracks occur at the interface between the negative electrode active material and the solid electrolyte and the interface between the solid electrolyte and the solid electrolyte. can be suppressed, it is possible to suppress a decrease in the capacity retention rate when the negative electrode is applied to a battery.

Hereinafter, the lithium ion battery negative electrode of the present disclosure will be described using a lithium ion negative electrode 100 that is one embodiment.

<Negative electrode 100 for lithium ion battery>
FIG. 1 shows a schematic diagram of a negative electrode 100 for a lithium ion battery (hereinafter sometimes referred to as "negative electrode 100"). As shown in FIG. 1 , the negative electrode 100 contains a negative electrode composite active material 10 obtained by coating a negative electrode active material 11 containing Si with a second solid electrolyte 12 , and a first solid electrolyte 20 .

(Negative electrode composite active material 10)
The negative electrode composite active material 10 is obtained by coating the negative electrode active material 11 with the second solid electrolyte 12 . By coating the negative electrode active material 11 containing Si with the solid electrolyte in this manner, the expansion and contraction of the negative electrode active material 11 due to charging and discharging can be suppressed, so that the occurrence of cracks near the surface of the negative electrode active material 11 can be suppressed. can do. Specifically, cracks occurring at the interface between the negative electrode active material 11 and the second solid electrolyte 12 and at the interface between the negative electrode composite active material 10 and the first solid electrolyte 20 can be suppressed.

The negative electrode active material 11 is not particularly limited as long as it is an active material containing Si. Examples thereof include Si metal and alloys containing Si. An active material containing Si element has an extremely high volume expansion coefficient. For example, when Si simple substance is used as the negative electrode active material, the volume expansion rate is about four times. Therefore, when a negative electrode active material containing Si element is used, the effect of the negative electrode 100 can be obtained at a higher level than when other active materials are used.

The second solid electrolyte 12 is a sulfide solid electrolyte. A sulfide solid electrolyte is a material containing sulfur (S) and having ion conductivity. A sulfide solid electrolyte containing Li, A (A is at least one of P, Si, Ge, Al and B) and S is preferred. Among them, the sulfide solid electrolyte preferably has an ortho-composition anion structure (PS43-structure, SiS44-structure, GeS44-structure, AlS33-structure, BS33-structure) as a main component of the anion. The proportion of the anion structure with the ortho composition is preferably 70 mol % or more, more preferably 90 mol % or more, relative to all anion structures in the sulfide solid electrolyte. The proportion of ortho-composition anionic structures can be determined by Raman spectroscopy, NMR, XPS, and the like. Furthermore, the sulfide solid electrolyte may further contain X (X is at least one of I, Br and Cl). In addition, part of S may be substituted with O in the sulfide solid electrolyte.

Specifically, the sulfide-based solid electrolytes include Li ₂ SP ₂ S ₅ , Li ₂ S—SiS ₂ , LiX—Li ₂ S—SiS ₂ , LiX—Li ₂ SP ₂ S ₅ , LiX -Li ₂ O-Li ₂ SP ₂ S ₅ , LiX-Li ₂ SP ₂ O ₅ , LiX-Li ₃ PO ₄ -P ₂ S ₅ , Li ₃ PS ₄ and the like. The above description of "Li ₂ SP ₂ S ₅ " means a material obtained by using a raw material composition containing Li ₂ S and P ₂ S ₅ , and the same applies to other descriptions. "X" in LiX above represents a halogen element. One or more kinds of LiX may be contained in the raw material composition containing LiX. When two or more types of LiX are included, the mixing ratio of the two or more types is not particularly limited.

The sulfide solid electrolyte may be sulfide glass, or sulfide glass-ceramics obtained by heat-treating the sulfide glass. Sulfide glass can be obtained, for example, by subjecting a raw material composition containing raw materials such as Li ₂ S and P ₂ S ₅ to an amorphization method. Amorphization methods include, for example, mechanical milling and melt quenching, with mechanical milling being preferred. This is because the process can be performed at room temperature, and the manufacturing process can be simplified. Mechanical milling is not particularly limited as long as it is a method of mixing raw material compositions while imparting mechanical energy, and examples thereof include ball mills, turbo mills, mechanofusion, and disk mills.

On the other hand, sulfide glass-ceramics can be obtained, for example, by heat-treating sulfide glass at a temperature equal to or higher than the crystallization temperature. That is, a sulfide glass-ceramic can be obtained by subjecting a raw material composition to an amorphization method and further to a heat treatment.

Here, the coverage of the second solid electrolyte (sulfide solid electrolyte) 12 on the surface of the negative electrode active material 11 is 80% or more. Preferably it is 90% or more. If the coverage of the second solid electrolyte 12 is less than 80%, the interaction between the uncovered portion and the first solid electrolyte results in high resistance and a reduced capacity retention rate. There is no particular upper limit for the coverage, and it may be 100%. The coverage can be measured by a known method such as cross-sectional observation of element distribution measurement.

The coating of the negative electrode active material 11 with the second solid electrolyte 12 can be performed by a known method using, for example, a particle compounding device. The conditions of the apparatus are appropriately set according to the purpose.

(First solid electrolyte 20)
The first solid electrolyte 20 is a plastic crystal. The plastic crystal has lithium ion conductivity and contains at least one cation selected from the group consisting of pyrrolidinium, tetraalkylammonium and tetraalkylphosphonium, and a carborane anion. Here, a plastic crystal is defined as a substance that is composed of a three-dimensional crystal lattice that is regularly aligned and that has orientational and rotational disorder at the level of molecular species or molecular ions. Since the plastic crystal has plasticity while taking the solid form of a crystal, it can change its form according to the fine unevenness of the interface between the electrode and the electrolyte. For example, the elastic modulus of general soft-viscous crystals is about 1/20 to 1/100 that of sulfide solid electrolytes. Therefore, the expansion and contraction of the negative electrode active material 11 containing Si can be absorbed, and the occurrence of cracks occurring at the interface between the solid electrolytes can be suppressed. Specifically, the occurrence of cracks at the interface between the first solid electrolyte and the second solid electrolyte and at the interface between the second solid electrolytes 12 can be suppressed.

The cation source of the first solid electrolyte 20 is preferably at least one selected from the group consisting of pyrrolidinium, tetraalkylammonium and tetraalkylphosphonium. By including a cation source having a long carbon chain in these alkyl groups, good plasticity is exhibited. The number of carbon atoms in the alkyl group is preferably 4 or more.

Examples of pyrrolidinium include N,N-dimethylpyrrolidinium, N-methyl-N-ethylpyrrolidinium, N-methyl-propylpyrrolidinium, N-methyl-butylpyrrolidinium, N-ethyl-methylpyrrolidinium dinium, N-butyl-methylpyrrolidinium, and the like.
Examples of tetraalkylammonium include tetramethylammonium, tetraethylammonium, tetrabutylammonium, and the like. Examples of tetraalkylphosphonium include tetramethylphosphonium, tetraethylphosphonium, tetrabutylphosphonium, and the like.

The first solid electrolyte 20 contains a carborane anion source. Here, the carborane anion is an anion in which, in carborane, which is a polyhedral cluster composed of boron atoms and carbon atoms, some of the boron atoms at the vertices of the polyhedron are replaced with carbon atoms. Carborane anions included in the carborane anion source in the present disclosure have the following general formula:
(C _x B _y M _z ) ⁻
(Wherein, x and y are each independently an integer of 1 or more, M is at least one of H, F, Cl and Br, and z is an integer of 0 or more)
is represented by Among them, the carborane anion is preferably at least one selected from CB ₉ H ₁₀ ^- and CB ₁₁ H ₁₂ ^- . This is because CB ₉ H ₁₀ ^- and CB ₁₁ H ₁₂ ^- have low reactivity with the sulfide solid electrolyte and can suppress an increase in interfacial resistance.

The first solid electrolyte (plastic crystal) has lithium ion conductivity. Although the method for imparting lithium ion conductivity to the plastic crystal is not particularly limited, for example, lithium ion conductivity can be expressed by using at least a lithium salt of carborane anion as a carborane anion source. The carborane anion lithium salt in the present disclosure is preferably at least one selected from CB ₉ H ₁₀ Li and CB ₁₁ H ₁₂ Li.

The content of the carborane anion lithium salt in the first solid electrolyte (plastic crystal) is 10 mol % or more, may be 20 mol % or more, or may be 30 mol % or more. Moreover, the upper limit is 90 mol % or less, may be 80 mol % or less, or may be 70 mol % or less. If the content of the lithium salt of the carborane anion in the first solid electrolyte is less than 10 mol %, the ionic conductivity decreases, resulting in a decrease in the capacity retention rate. On the other hand, when it is more than 90 mol %, the capacity retention ratio is lowered because the flexibility and viscosity are lowered.

A method for producing the first solid electrolyte is not particularly limited. For example, cations and anions are eutectic in ion-exchanged water, and the resulting precipitate is dissolved by heating. Subsequently, a lithium salt of an anion is dissolved in the solution in which the precipitate is dissolved, and the solution is cooled and collected by filtration to obtain a solid containing the first solid electrolyte. The desired first solid electrolyte can be obtained by pulverizing the solid.

In the negative electrode 100, the volume ratio of the first solid electrolyte (plastic crystal) 20 to the total of the first solid electrolyte (plastic crystal) 20 and the second solid electrolyte (sulfide solid electrolyte) 12 is 30 vol% to 75 vol%. %. When the volume ratio of the first solid electrolyte 20 is less than 30 vol %, it may not be possible to suppress the occurrence of cracks at the interface between the solid electrolytes in the negative electrode 10, and it may not be possible to suppress the decrease in the capacity retention rate. On the other hand, when the volume ratio of the first solid electrolyte 20 exceeds 75 vol %, the amount of the second solid electrolyte covering the negative electrode active material 11 decreases and the coating layer becomes thinner, so that sufficient effects are obtained. sometimes not. The volume fraction of the solid electrolyte can be calculated by analyzing images acquired using SEM (Scanning Electron Microscope) and EPMA (Electron Probe Microanalyzer). As representative elements, the first solid electrolyte (plastic crystal) contains N, B, and C, and the second solid electrolyte contains S and P.

(Optional component)
The negative electrode 100 may contain a lithium salt as a supporting salt. Lithium salts include Li(CF ₃ SO ₂ ) ₂ N (lithium bis(trifluoromethylsulfonyl)imide), Li(FSO ₂ ) ₂ N (lithium bis(fluorosulfonyl)imide), Li(C ₂ F ₅ SO ₂ ) _2N , _LiPF6 , _LiBF4 , _LiAsF6 , _LiTaF6 , _{LiClO4 , LiCF3SO3} and the like. Among them, at least one of lithium bis(trifluoromethylsulfonyl)imide and lithium bis(fluorosulfonyl)imide is preferred. The amount of lithium salt added is preferably 0.1 to 20 mol %, more preferably 2 to 20 mol %, relative to the negative electrode 100 . Within this range, high ionic conductivity can be exhibited.

The negative electrode 100 may contain a binder. Examples of the binder include rubber binders such as butadiene rubber, hydrogenated butadiene rubber, styrene butadiene rubber (SBR), hydrogenated styrene butadiene rubber, nitrile butadiene rubber, hydrogenated nitrile butadiene rubber, ethylene propylene rubber; polyvinylidene fluoride ( PVDF), polyvinylidene fluoride-polyhexafluoropropylene copolymer (PVDF-HFP), polytetrafluoroethylene, fluorine-based binders such as fluororubber; polyethylene, polypropylene, polyolefin-based thermoplastic resins such as polystyrene, polyimide, polyamide imide resins such as imide; amide resins such as polyamide; acrylic resins such as polymethyl acrylate and polyethyl acrylate; and methacrylic resins such as polymethyl methacrylate and polyethyl methacrylate.

The negative electrode 100 may contain a conductive material. Examples of conductive materials include carbon materials and metal materials. Examples of carbon materials include particulate carbon materials such as acetylene black (AB) and ketjen black (KB), carbon fibers such as VGCF, carbon nanotubes (CNT), and fibrous carbon materials such as carbon nanofibers (CNF). is mentioned. Examples of metal materials include Ni, Cu, Fe, and SUS. The metallic material is preferably particulate or fibrous.

Also, the negative electrode 100 may include a solid electrolyte other than the first solid electrolyte 20 and the second solid electrolyte 12 . For example, an oxide solid electrolyte. Examples of oxide solid electrolytes include lithium lanthanum zirconate, LiPON, Li _1+X Al _X Ge _2-X (PO ₄ ) ₃ , Li—SiO glass, Li—Al—SO glass, and the like. . Examples of sulfide solid electrolytes include Li ₂ SP ₂ S ₅ , Li ₂ S—SiS ₂ , LiI—Li ₂ S—SiS ₂ , LiI—Si ₂ SP ₂ S ₅ , Li ₂ SP ₂ S ₅ -LiI-LiBr, LiI-Li ₂ SP ₂ S ₅ , LiI-Li ₂ SP ₂ O ₅ , LiI-Li ₃ PO ₄ -P ₂ S ₅ , Li ₂ SP ₂ S ₅ - GeS ₂ and the like can be mentioned.

These optional components can be added as appropriate within a range in which the negative electrode 100 exhibits its effects.

(Negative electrode current collector)
The negative electrode 100 may include a negative electrode current collector. Examples of metals forming the negative electrode current collector include SUS, Cu, Al, Ni, Fe, Ti, Co, and Zn. Cu and Al are particularly preferred. The negative electrode current collector may have some kind of coating layer on its surface for adjusting the resistance. The thickness of the negative electrode current collector is not particularly limited.

(shape)
The shape of the negative electrode 100 is not particularly limited, but is preferably sheet-like. In this case, the thickness of the negative electrode 100 is, for example, preferably 0.1 μm or more and 1 mm or less, and more preferably 1 μm or more and 150 μm or less.

(Application)
The negative electrode 100 is suitable as a negative electrode for lithium ion batteries, and more suitable as a negative electrode for lithium ion all-solid-state batteries.

(Manufacturing method of negative electrode 100)
The negative electrode 100 can be manufactured by mixing the constituent materials and forming it into a desired shape. For example, if the materials that make up the negative electrode 100 are dry mixed, the negative electrode 100 can be manufactured by pressing the mixture. Further, when the materials constituting the negative electrode 100 are dispersed in an organic dispersion medium and mixed in a wet manner, the obtained slurry is applied to a base material such as a negative electrode current collector or a metal foil, and dried to obtain the negative electrode 100. can be manufactured. Since these methods are known, the details are omitted.

[Lithium-ion battery]
A lithium ion battery including the lithium battery negative electrode of the present disclosure will be described. Here, a case where the lithium battery negative electrode of the present disclosure is applied to a lithium ion all-solid-state battery will be described, but the lithium ion battery of the present disclosure is not limited thereto.
A lithium-ion all-solid-state battery 1000, which is one embodiment, will be described below.

<Lithium-ion all-solid-state battery 1000>
A schematic cross-sectional view of a lithium-ion all-solid-state battery 1000 (hereinafter sometimes referred to as "battery 1000") is shown in FIG. As shown in FIG. 2 , battery 1000 includes negative electrode 100 , positive electrode 200 , and solid electrolyte layer 300 disposed between negative electrode 100 and positive electrode 200 .
Since the negative electrode 100 has been described above, the description thereof is omitted.

(Positive electrode 200)
The positive electrode 200 contains at least a positive electrode active material, and may further contain at least one of a solid electrolyte, a conductive material and a binder as optional components.

Examples of positive electrode active materials include lithium composite oxides. Lithium composite oxides 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 and LiNi _0.5 Mn. Spinel-type active materials such as _1.5 O ₄ and olivine-type active materials such as LiFePO ₄ and LiMnPO ₄ can be used. Examples of the shape of the positive electrode active material include particulate. The positive electrode active material may be coated with a lithium ion conducting oxide such as _LiNbO3 .

As for the solid electrolyte, conductive material and binder, those described above can be used. Also, the content and the like can be appropriately set so as to obtain the desired performance.

The cathode 200 may comprise a cathode current collector. Examples of materials for the positive electrode current collector include Cr, Au, Pt, Al, Fe, Ti, and Zn.

The positive electrode 200 can be manufactured by a method similar to that of the negative electrode 100 .

(Solid electrolyte layer 300)
Solid electrolyte layer 300 contains at least a solid electrolyte, and may further contain a binder as an optional component. As the solid electrolyte and binder, those described above can be used. Also, the content and the like can be appropriately set so as to obtain the desired performance. The solid electrolyte layer 300 can be manufactured by a method similar to that of the negative electrode 100 .

(form)
The battery 1000 may have any battery case such as a SUS battery case. Examples of the shape of the battery 1000 include a coin shape, a laminate shape, a cylindrical shape, and a rectangular shape.

(Manufacturing method of battery 1000)
In the battery 1000, the negative electrode 100, the positive electrode 200, and the solid electrolyte layer 300 are pressure-molded by flat press, roll press, or the like to obtain a negative electrode 100-solid electrolyte layer 300-positive electrode 200 assembly. On the other hand, by attaching a current collector, an all-solid-state battery may be obtained. As another manufacturing method, the positive electrode 200 is obtained by coating and drying the positive electrode slurry on one surface of the positive electrode current collector, and the negative electrode 100 is obtained by coating and drying the negative electrode slurry on one surface of the negative electrode current collector. and a solid electrolyte layer 300 obtained by coating and drying the solid electrolyte slurry on one surface of the substrate, in the order of the positive electrode current collector, the positive electrode, the solid electrolyte layer, the negative electrode, and the negative electrode current collector. The battery 1000 may be formed by arranging them.

Hereinafter, the negative electrode for an all-solid-state battery and the like of the present disclosure will be further described using examples.

[Manufacturing of all-solid-state battery]
<Production of negative electrode>
The negative electrode active material Si and the second solid electrolyte sulfide solid electrolyte (LiI—Li ₂ O—Li ₂ SP ₂ S ₅ ) were weighed at the volume ratios shown in Table 1, and the particles were composited. Coating was performed using an apparatus (Nobilta, manufactured by Hosokawa Micron Corporation) to obtain a negative electrode composite active material.

As the first solid electrolyte (soft-viscous crystal), Pyr ₁₄ -CB ₁₁ H ₁₂ /LiCB ₁₁ H ₁₂ (1-butyl-1-methylpyrrolidinium CB ₁₁ H ₁₂ is melted with 30 mol % of LiCB ₁₁ H ₁₂ ) was used. In Comparative Example 1, Pyr ₁₂ -TFSI/LiTFSI (1-ethyl-1-methylpyrrolidinium TFSI melted with 30 mol % of LiTFSI) was used as the first solid electrolyte (soft-viscosity crystal).

Next, the obtained negative electrode composite active material and the first solid electrolyte (soft-viscous crystal) were weighed so as to have the volume ratio shown in Table 1, and a conductive material ( VGCF) was weighed to 5.0 parts, and the binder (PVdF) was weighed to 1.5 parts. These materials were dispersed in an organic dispersion medium (butyl butyrate) so that the solid content was 63 wt %, and kneaded for 1 minute using an ultrasonic homogenizer to prepare a negative electrode slurry. After that, the negative electrode slurry is applied to the surface of the negative electrode current collector (copper foil), and the negative electrode is formed through the process of heating and drying, and pressed at 25 ° C. with a linear pressure of 1 ton / cm to provide the negative electrode current collector. A negative electrode was produced. Here, the negative electrode was prepared by adjusting the coating conditions so that the positive/negative electrode capacity ratio was 2.5.

<Preparation of positive electrode>
A positive electrode active material (LiNi _0.33 Co _0.33 Mn _0.33 O ₂ ) and a sulfide solid electrolyte (LiI—Li ₂ O—Li ₂ SP ₂ S ₅ ) were mixed in a weight ratio of 75:25. In addition, 3.0 parts of the conductive material (VGCF) and 1.5 parts of the binder (PVdF) were weighed with respect to 100 parts of the positive electrode active material. These materials were dispersed in an organic dispersion medium (butyl butyrate) so that the solid content was 63 wt %, and kneaded for 1 minute using an ultrasonic homogenizer to prepare a positive electrode slurry. After that, the positive electrode slurry is applied to the surface of the positive electrode current collector (aluminum foil), and the positive electrode is formed through the process of heating and drying, and pressed at 25 ° C. with a linear pressure of 1 ton / cm to provide the positive electrode current collector. A positive electrode was produced. Here, the positive electrode was prepared by adjusting the coating conditions so that the positive/negative electrode capacity ratio was 2.5.

<Preparation of Solid Electrolyte Layer>
To 99.0 parts by weight of a sulfide solid electrolyte (LiI-LiO ₂ -Li ₂ SP ₂ S ₅ ), 1.0 parts by weight of a PVDF binder solution was added as a solid content to give a total composition of 100 parts by weight. got stuff The resulting composition was subjected to ultrasonic treatment for 60 seconds using an ultrasonic homogenizer to prepare a solid electrolyte layer slurry having a solid content of 63.0%. The base material (Al foil) was coated with this solid electrolyte layer slurry and dried by heating to obtain a base material and a solid electrolyte layer.

<Production of all-solid-state battery>
By pressing at 5 ton/cm after transferring so that the positive electrode and the negative electrode are opposed to each other with the solid electrolyte layer interposed therebetween. An all-solid-state battery was fabricated.

[evaluation]
The produced all-solid-state battery was charged and discharged at 3 to 4.2 V for 500 cycles, and the ratio of the discharge capacity at the 1st cycle and at the 500th cycle was calculated as the capacity retention ratio. The results are shown in Table 2 and FIG.

The capacity retention ratios of Examples 1 to 6 were 1.1 or more, which was a good result. Above all, the results were better when the coverage was 90% or more. On the other hand, the results of Comparative Examples 1 to 5 were inferior with a capacity retention ratio of less than 1.1. The results are further discussed below.

Comparative Example 2 is an example in which the plastic crystal, which is the first solid electrolyte, is not used. Comparing the results of Comparative Examples 2 and 3, the battery with the viscous crystal has a higher capacity retention ratio than the battery without the viscous crystal. It is considered that this is because cracks in the negative electrode can be suppressed by arranging the flexible crystals around the negative electrode composite active material.

Further, when comparing Comparative Example 1 and Example 1, Comparative Example 1, which uses TFSI-based plastic crystals, has a smaller capacity retention ratio than Example 4, which uses carborane-based plastic crystals. became. This is because a plastic crystal using imide-based ions such as TFSI has a large interfacial resistance with a sulfide solid electrolyte (second solid electrolyte), resulting in large deterioration of battery resistance.

In addition to the above, it is important that the coverage of the second solid electrolyte with respect to the negative electrode active material is 80% or more, and that the volume ratio of the first solid electrolyte is 30 vol % to 75 vol %. Do you get it. If the coverage of the second solid electrolyte with respect to the negative electrode active material is less than 80%, the interaction between the non-covered portion of the negative electrode active material and the soft-viscous crystals will result in high resistance, and the capacity retention ratio will decrease. be done. Further, when the volume ratio of the first solid electrolyte is less than 30 vol %, cracking of the solid electrolyte in the negative electrode cannot be completely suppressed, and the capacity retention ratio is not improved. When the volume ratio of the first solid electrolyte exceeds 75 vol%, the amount of the second solid electrolyte covering the negative electrode active material decreases, and the thickness of the coating layer becomes thin, resulting in cracks occurring between the negative electrode active material and the solid electrolyte. cannot be sufficiently suppressed.

10 negative electrode composite active material 11 negative electrode active material 12 second solid electrolyte (sulfide solid electrolyte)
20 First solid electrolyte (plastic crystal)
100 lithium ion battery negative electrode 200 positive electrode 300 solid electrolyte layer 1000 lithium ion all-solid battery

Claims (1)

a negative electrode composite active material obtained by coating a negative electrode active material containing Si with a sulfide solid electrolyte;
pyrrolidinium, at least one cation selected from the group consisting of tetraalkylammonium and tetraalkylphosphonium, and a plastic crystal containing a carborane anion,
The coverage of the sulfide solid electrolyte on the surface of the negative electrode active material is 80% or more,
The volume ratio of the plastic crystal to the total of the sulfide solid electrolyte and the plastic crystal is 30 vol% to 75 vol%,
Anode for lithium-ion batteries.

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