JP2016103411A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery improved in discharge capacity and load characteristics.SOLUTION: The lithium ion secondary battery includes: positive electrode particles each having a positive electrode active material particle, a first coating layer coating the surface of the positive electrode active material particle and formed of a carbon material, and a second coating layer coating the first coating layer and formed of a lithium-containing compound; and a sulfide-based solid electrolyte containing at least LiS and PSand being in contact with the positive electrode particles.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a lithium ion secondary battery.

  In recent years, all-solid-state lithium ion secondary batteries using a solid electrolyte having lithium ion conductivity have attracted attention. As a solid electrolyte of such an all solid lithium ion secondary battery, for example, a sulfide-based solid electrolyte having high lithium ion conductivity has been proposed.

  However, the sulfide-based solid electrolyte may react with the positive electrode active material during charging and generate a resistance component at the interface with the positive electrode active material. In such a case, since the resistance at the interface between the positive electrode active material and the solid electrolyte increases and the conductivity of lithium ions decreases, the output of the all solid lithium ion secondary battery decreases.

For example, Patent Document 1 discloses a technique for coating the surface of a positive electrode active material with a coating material made of LiTi ₂ (PO ₄ ) ₃ in order to suppress a reaction that occurs at the interface between the positive electrode active material and a solid electrolyte. ing. In Patent Documents 2 and 3, a buffer layer that suppresses the reaction by relaxing the unevenness of lithium ions at the interface between the positive electrode layer and the solid electrolyte layer, a positive electrode active material between both layers, and A technique for providing an intermediate layer that suppresses mutual diffusion of solid electrolytes is disclosed.

Japanese Patent No. 498866 JP 2010-40439 A JP 2011-44368 A

  However, the techniques disclosed in Patent Documents 1 to 3 have a problem that they are insufficient to suppress the reaction at the interface between the positive electrode active material and the solid electrolyte.

  Accordingly, the present invention has been made in view of the above problems, and an object of the present invention is to further suppress the reaction at the interface between the positive electrode active material and the solid electrolyte, thereby reducing discharge capacity and load characteristics. It is an object of the present invention to provide a new and improved lithium ion secondary battery capable of improving the resistance.

In order to solve the above-described problems, according to an aspect of the present invention, positive electrode active material particles, a first coating layer that covers a surface of the positive electrode active material particles and is formed of a carbon material, and the first A positive electrode particle that covers the coating layer and has a second coating layer formed of a lithium-containing compound, and a sulfide-based solid electrolyte that includes at least Li ₂ S and P ₂ S ₅ and is in contact with the positive electrode particle; A lithium ion secondary battery is provided.

  According to this viewpoint, the discharge capacity and load characteristics of the lithium ion secondary battery can be improved.

The carbon material may be carbon or amorphous carbon containing a hydrocarbon represented by C _{4n + 6} H _{4n + 12} (where n is an integer of 0 or more).

  According to this viewpoint, the discharge capacity and load characteristics of the lithium ion secondary battery can be further improved.

  The carbon material may be diamond-like carbon.

  According to this viewpoint, the discharge capacity and load characteristics of the lithium ion secondary battery can be further improved.

  The diamond-like carbon may contain 1 atom% or more and 50 atom% or less of hydrogen atoms.

  According to this viewpoint, the discharge capacity and load characteristics of the lithium ion secondary battery can be further improved.

The diamond-like carbon includes carbon atoms with sp ² hybridized bond, a carbon atom having an sp ³ hybrid bonds, of the carbon atoms contained in the diamond-like carbon, carbon atoms having a sp ³ hybridized bonds The ratio may be 10% or more and 100% or less.

  According to this viewpoint, the discharge capacity and load characteristics of the lithium ion secondary battery can be further improved.

  The thickness of the first covering layer may be 1 nm or more and 50 nm or less.

  According to this viewpoint, the discharge capacity and load characteristics of the lithium ion secondary battery can be further improved.

  The lithium-containing compound may be a lithium-containing oxide or a lithium-containing phosphorus oxide.

  According to this viewpoint, the discharge capacity and load characteristics of the lithium ion secondary battery can be further improved.

The lithium-containing oxide may be aLi ₂ O—ZrO ₂ (where 0.1 ≦ a ≦ 2.0).

  According to this viewpoint, the discharge capacity and load characteristics of the lithium ion secondary battery can be further improved.

  The total thickness of the first coating layer and the second coating layer may be 1 nm or more and 500 nm or less.

  According to this viewpoint, the discharge capacity and load characteristics of the lithium ion secondary battery can be further improved.

  The positive electrode active material particles may include a lithium salt of a transition oxide having a layered rock salt type structure.

  According to this viewpoint, the battery characteristics of the lithium ion secondary battery can be improved.

The lithium salt of the transition oxide having the layered rock salt structure is LiNi _x Co _y M _z O ₂ (where M is Al or Mn, 0 <x <1, 0 <y <1, x + y + z = 1). ).

  According to this viewpoint, the battery characteristics of the lithium ion secondary battery can be further improved.

  As described above, according to the present invention, it is possible to further improve the discharge capacity and load characteristics of the lithium ion secondary battery.

It is sectional drawing which shows typically the laminated constitution of the lithium ion secondary battery which concerns on one Embodiment of this invention. It is sectional drawing which showed typically the structure of the positive electrode particle which concerns on the same embodiment. It is a graph which shows an example of the TEM-EELS spectrum measured about the coating layer of the positive electrode particle, and an example of a fitting result.

  Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In addition, in this specification and drawing, about the component which has the substantially same function structure, duplication description is abbreviate | omitted by attaching | subjecting the same code | symbol.

<1. Overview of lithium-ion secondary battery>
A lithium ion secondary battery according to an embodiment of the present invention is an all-solid lithium ion secondary battery using a solid electrolyte as an electrolyte.

  Here, the all-solid-state lithium ion secondary battery using a solid electrolyte has a positive electrode active material and an electrolyte that are solid, so that the inside of the positive electrode active material is compared with a lithium ion secondary battery using an organic solvent as the electrolyte. The electrolyte is difficult to penetrate. Therefore, in an all-solid-state lithium ion secondary battery, the area of the interface between the positive electrode active material and the electrolyte tends to be small, and it is necessary to ensure a sufficient movement path for lithium ions and electrons between the positive electrode active material and the solid electrolyte. there were.

  Therefore, for example, a technique for increasing the area of the interface between the positive electrode active material and the solid electrolyte by forming the positive electrode layer as a mixed layer of the positive electrode active material and the solid electrolyte has been proposed.

  However, when a sulfide-based solid electrolyte is used, a reaction occurs at the interface between the positive electrode active material and the solid electrolyte during charging and a resistance component is generated, so that the interface between the positive electrode active material and the solid electrolyte is generated. Resistance sometimes increased. In addition, such a reaction at the interface between the positive electrode active material and the solid electrolyte particularly when the load on the all solid lithium ion secondary battery is large (for example, charging the all solid lithium ion secondary battery to a higher voltage). Or when the all-solid-state lithium ion secondary battery is discharged with a large current).

  Therefore, in an all-solid-state lithium ion secondary battery using a sulfide-based solid electrolyte, it has been required to suppress the generation of a resistance component at the interface between the positive electrode active material and the solid electrolyte.

  In the lithium ion secondary battery 1 according to this embodiment, the surface of the positive electrode active material particles 101 is covered with a first coating layer 102 formed of a carbon material, and the first coating layer 102 is formed of a lithium-containing compound. The second coating layer 103 is used. According to this configuration, the direct contact between the positive electrode active material particles 101 and the solid electrolyte 300 is suppressed by the first coating layer 102 and the second coating layer 103, and therefore the interface between the positive electrode active material particles 101 and the solid electrolyte 300. The generation of the resistance component in can be suppressed.

  The first coating layer 102 formed of the carbon material suppresses the structural change of the positive electrode active material particles 101, and the second coating layer 103 formed of the lithium-containing compound includes the positive electrode active material particles 101 and the solid electrolyte. While suppressing the reaction with 300, it has lithium ion conductivity. Therefore, the first coating layer 102 and the second coating layer 103 can secure a lithium ion movement path between the positive electrode active material particles 101 and the solid electrolyte 300, so that the battery characteristics of the lithium ion secondary battery 1 can be improved. Can be improved.

  In particular, in the lithium ion secondary battery 1 according to the present embodiment, the first coating layer 102 that covers the surfaces of the positive electrode active material particles 101 is coated with a carbon material (more specifically, non-thermally stable). In this case, the structural change of the positive electrode active material particles 101 due to charging / discharging can be effectively suppressed. In addition, the lithium ion secondary battery 1 according to the present embodiment forms the second coating layer 103 that covers the outermost surface of the positive electrode particle 100 with a lithium-containing compound, whereby the interface between the positive electrode particle 100 and the solid electrolyte 300 is formed. Generation | occurrence | production of a high resistance layer can be suppressed and lithium ion conductivity can be improved. With these configurations, the lithium ion secondary battery 1 according to the present embodiment can improve the discharge capacity and the load characteristics.

  As demonstrated in the examples described later, the first coating layer 102 that covers the surfaces of the positive electrode active material particles 101 is formed of a lithium-containing compound, and the second coating layer 103 that covers the first coating layer 102 is formed. Is made of a carbon material, the above effect is not exhibited and the battery characteristics deteriorate, which is not preferable.

<2. Configuration of lithium ion secondary battery>
Next, with reference to FIG. 1 and FIG. 2, the structure of the lithium ion secondary battery which concerns on this embodiment is demonstrated. FIG. 1 is a cross-sectional view schematically showing a layer configuration of a lithium ion secondary battery 1 according to this embodiment. FIG. 2 is a cross-sectional view schematically showing the configuration of the positive electrode particles 100 of the lithium ion secondary battery 1 according to this embodiment.

  As shown in FIG. 1, the lithium ion secondary battery 1 has a structure in which a positive electrode layer 10, a negative electrode layer 20, and a solid electrolyte layer 30 positioned between the positive electrode layer 10 and the negative electrode layer 20 are stacked.

[Positive electrode layer]
The positive electrode layer 10 includes positive electrode particles 100 and a solid electrolyte 300. Further, the positive electrode layer 10 may further include a conductive auxiliary agent in order to supplement electronic conductivity. The solid electrolyte 300 will be described later in the solid electrolyte layer 30.

  Here, as shown in FIG. 2, the positive electrode particle 100 includes a positive electrode active material particle 101, a first coating layer 102 that covers the surface of the positive electrode active material particle 101, and a second coating that further covers the first coating layer 102. And a coating layer 103.

(Positive electrode active material particles)
The positive electrode active material particles 101 are formed of a positive electrode active material that has a higher charge / discharge potential than a negative electrode active material included in the negative electrode particles 200 described later and can reversibly occlude and release lithium ions.

  For example, the positive electrode active material particles 101 include lithium cobalt oxide (hereinafter referred to as LCO), lithium nickelate, lithium nickel cobaltate, nickel cobalt lithium aluminumate (hereinafter referred to as NCA), nickel cobalt lithium manganate (hereinafter referred to as “LCO”). NCM), lithium salts such as lithium manganate and lithium iron phosphate, nickel sulfide, copper sulfide, sulfur, iron oxide, vanadium oxide, and the like. These positive electrode active materials may be used alone or in combination of two or more.

  Moreover, it is preferable that the positive electrode active material particle 101 is formed including a lithium salt of a transition oxide having a layered rock salt type structure among the lithium salts described above. Here, “layered” represents a thin sheet-like shape. The “rock salt structure” refers to a sodium chloride structure, which is a kind of crystal structure. Specifically, the face-centered cubic lattice formed by each cation and anion is a unit lattice. It represents a structure that is displaced by a half of the edge.

The lithium salt of a transition oxides having such a layered rock-salt structure, for _{example, LiNi x Co y Al z O} 2 (NCA), or _{LiNi x Co y Mn z O 2} (NCM) ( however, 0 <x Examples thereof include lithium salts of ternary transition oxides such as <1, 0 <y <1, 0 <z <1, and x + y + z = 1).

  When the positive electrode active material particle 101 includes a lithium salt of a ternary transition oxide having the above layered rock salt structure, the energy density and thermal stability of the lithium ion secondary battery 1 can be improved. .

  Here, the positive electrode particle 100 according to the present embodiment has a structure change of the positive electrode active material particle 101 and an interface between the positive electrode active material particle 101 and the solid electrolyte 300 by the first coating layer 102 and the second coating layer 103. Since the reaction is suppressed, the battery characteristics of the lithium ion secondary battery 1 can be further improved.

  When the positive electrode active material particle 101 is formed of a lithium salt of a ternary transition oxide such as NCA or NCM and contains nickel (Ni) as the positive electrode active material, the capacity of the lithium ion secondary battery 1 It is possible to increase the density and reduce metal elution from the positive electrode active material in the charged state. Thereby, the lithium ion secondary battery 1 which concerns on this embodiment can improve the long-term reliability in a charge condition, and a cycle (cycle) characteristic.

  Here, examples of the shape of the positive electrode active material particle 101 include particle shapes such as a true sphere and an elliptic sphere. The average particle diameter of the positive electrode active material particles 101 is preferably, for example, 0.1 μm or more and 50 μm or less. The “average particle diameter” represents the number average diameter in the particle size distribution of particles obtained by a scattering method or the like, and can be measured by a particle size distribution meter or the like.

  In addition, the content of the positive electrode active material particles 101 in the positive electrode layer 10 is, for example, preferably from 10% by mass to 99% by mass, and more preferably from 20% by mass to 90% by mass.

(First coating layer)
The first coating layer 102 covers the surface of the positive electrode active material particles 101 and is formed of a carbon material. Since the carbon material has high thermal and chemical stability, the structural change of the positive electrode active material particles 101 can be suppressed.

  In particular, in the present embodiment, the first coating layer 102 that covers the surface of the positive electrode active material particles 101 is formed of a carbon material having high thermal and chemical stability instead of the second coating layer 103 described later. The structural change of the positive electrode active material particles 101 can be effectively suppressed.

  Any carbon material can be used as long as it is a chemically stable carbon material. For example, amorphous carbon such as diamond like carbon (DLC) is used. can do.

Specifically, the first coating layer 102 is preferably a film formed of amorphous carbon. Here, the amorphous carbon is composed of carbon mainly composed of carbon and having a sp ³ hybrid orbital bond corresponding to a diamond structure and a sp ² hybrid orbital bond corresponding to a graphite structure. It is a material having an amorphous structure in which carbon is randomly mixed. In the following description, a bond by sp ³ hybrid orbital is simply expressed as sp ³ hybrid bond, and a bond by sp ² hybrid orbit is simply expressed by sp ² hybrid bond.

For example, the first coating layer 102 may be formed of a hydrocarbon represented by C _{4n + 6} H _{4n + 12} (where n is an integer of 0 or more) and amorphous carbon such as DLC. Here, as the hydrocarbon represented by C _{4n + 6} H _{4n + 12} (where n is an integer of 0 or more), for example, adamantane, diamantane, triamantane, tetramantane, A pentamantane etc. are mentioned. The amorphous carbon forming the first coating layer 102 is other than carbon such as hydrogen (H) or silicon (Si) if carbon is the main component (for example, if carbon is 50 atomic% or more). May be included.

  The first coating layer 102 is more preferably a film formed by DLC. The DLC film is an amorphous hard film made of only carbon and hydrogen or carbon, in which a diamond structure and a graphite structure are mixed. According to such a first coating layer 102, the structural change of the positive electrode active material particles 101 can be further suppressed.

  In this embodiment, it is preferable that DLC which forms the 1st coating layer 102 contains the hydrogen atom. Further, the content of hydrogen atoms contained in DLC is preferably 1 atom% or more and 50 atoms or less, and more preferably 10 atom% or more and 30 atom% or less. In addition, when content of the hydrogen atom contained in the 1st coating layer 102 is too high, since stability of the 1st coating layer 102 may fall, it is unpreferable.

  The content of hydrogen atoms contained in the DLC forming the first coating layer 102 described above is measured by, for example, IPC (Inductively Coupled Plasma) analysis (for example, ICP emission spectroscopy or ICP mass spectrometry). can do.

  The thickness of the first coating layer 102 is preferably 1 nm or more and 50 nm or less. When the thickness of the 1st coating layer 102 is contained in the above-mentioned range, the structural change of the positive electrode active material particle 101 can be suppressed, without reducing the electroconductivity of lithium ion. On the other hand, when the thickness of the 1st coating layer 102 is less than 1 nm, since the effect of suppressing the structural change is not sufficient, it is not preferable. Moreover, when the thickness of the 1st coating layer 102 exceeds 50 nm, since the lithium ion conductivity between the positive electrode active material particle 101 and the solid electrolyte 300 falls, it is unpreferable.

  The thickness of the first covering layer 102 described above can be measured using, for example, a cross-sectional image by a transmission electron microscope (TEM).

In the present embodiment, the DLC forming the first coating layer 102 includes carbon atoms having sp ² hybrid bonds and carbon atoms having sp ³ hybrid bonds, as described above. Here, the proportion of carbon atoms having sp ³ hybrid bonds among carbon atoms contained in DLC forming the first coating layer 102 is preferably 10% or more and 100% or less, and 30% or more and 100% or less. It is more preferable that it is 40% or more and 100% or less.

When the proportion of carbon atoms having sp ³ hybrid bonds in the DLC is in the above range, the structural change of the positive electrode active material particles 101 can be effectively suppressed. Thus, according to this configuration, the discharge capacity and load characteristics of the lithium ion secondary battery 1 can be further improved.

The ratio of carbon atoms having sp ² hybrid bonds and carbon atoms having sp ³ hybrid bonds in the DLC described above is obtained by, for example, electron energy loss spectroscopy (Electron Energy-Loss Spectroscopy: EELS) using TEM. be able to.

Hereinafter, a method for obtaining the ratio of carbon atoms having sp ² hybrid bonds and carbon atoms having sp ³ hybrid bonds in DLC will be specifically described with reference to FIG. FIG. 3 is a graph illustrating an example of a TEM-EELS measurement result of the positive electrode particle 100 with respect to the first coating layer 102 and an example of a fitting result thereof.

  In FIG. 3, the electron energy is in the range of 0.280 keV to 0.295 keV in the EELS spectrum near the K loss edge of carbon. In FIG. 3, the horizontal axis indicates the loss energy (keV), and the vertical axis indicates the spectrum intensity.

  In FIG. 3, the solid line indicates the EELS spectrum measured for the first coating layer 102. As shown in FIG. 3, the DLC EELS spectrum has a first peak near 0.284 keV to 0.286 keV and a second peak near 0.292 keV to 0.295 keV. Here, the first peak is a peak corresponding to a π bond of a carbon atom, and the second peak is a peak corresponding to a σ bond of a carbon atom.

  First, in FIG. 3, the first peak and the second peak are separated from the measured EELS spectrum (solid line). Specifically, by calculation, a spectrum of only the first peak (one-dot chain line) and a spectrum of only the second peak (two-dot chain line) are calculated, and a spectrum obtained by adding both spectra to the EELS spectrum (solid line). Fit it. Subsequently, a peak area ratio (first peak area / second peak area) between the first peak (one-dot chain line) and the second peak (two-dot chain line) in the spectrum (dashed line) for which fitting has been completed is calculated. .

Further, the peak area ratio is calculated for diamond and graphite (not shown). Here, the carbon atoms contained in diamond are bonded only by sp ³ hybrid bonds, and the carbon atoms contained in graphite can be regarded as bonded only by sp ² hybrid bonds. Therefore, by calculating the relative value of the peak area ratio in DLC with the peak area ratio in diamond being 0 and the peak area ratio in graphite being 100, carbon atoms having sp ² hybrid bonds, carbon atoms having sp ³ hybrid bonds, and The ratio can be calculated.

A method for determining the ratio of carbon atoms having a sp ³ hybridized bonds and carbon atoms with sp ² hybridized bonds in DLC to form the first coating layer 102 is not limited to the method using the TEM-EELS described above. For example, the ratio of carbon atoms having sp ² hybrid bonds to carbon atoms having sp ³ hybrid bonds in DLC may be determined by a method using X-ray photoelectron spectroscopy, Raman spectroscopy, or the like.

(Second coating layer)
The second coating layer 103 covers the first coating layer 102 and is formed of a lithium-containing compound. By further covering the positive electrode active material particles 101 with the second coating layer 103, the reaction between the positive electrode active material particles 101 and the solid electrolyte 300 can be further suppressed.

  In particular, in the present embodiment, positive electrode active material particles are formed by forming the second coating layer 103 that covers not the first coating layer 102 but the outermost surface of the positive electrode particles 100 with a lithium-containing compound having high lithium ion conductivity. The reaction between 101 and the solid electrolyte 300 can be suppressed, and lithium ion conductivity can be maintained.

Specifically, such a lithium-containing compound is preferably a lithium-containing oxide or a lithium-containing phosphorus oxide. Examples of the lithium-containing oxide include lithium zirconium oxide (Li—Zr—O), lithium niobium oxide (Li—Nb—O), lithium titanium oxide (Li—Ti—O), lithium aluminum oxide ( Li-Al-O). Examples of the lithium-containing phosphorus oxide include lithium titanium phosphorus oxide (Li—Ti—PO ₄ ) and lithium zirconium phosphorus oxide (Li—Zr—PO ₄ ). According to such a second coating layer 103, it is possible to suppress the formation of a high resistance layer at the interface between the positive electrode particle 100 and the solid electrolyte 300, and therefore, between the positive electrode active material particle 101 and the solid electrolyte 300. Lithium ion conductivity can be further improved.

More specifically, the second coating layer 103 may be formed of aLi ₂ O—ZrO ₂ (where 0.1 ≦ a ≦ 2.0). Since aLi ₂ O—ZrO ₂ (hereinafter also referred to as LZO) is chemically stable, the positive electrode active material particles 101 are formed by forming the second coating layer 103 with such aLi ₂ O—ZrO ₂ . And the solid electrolyte 300 can be further suppressed.

Here, aLi ₂ O—ZrO ₂ is a composite oxide of Li ₂ O and ZrO _2, and the range of a is preferably 0.1 ≦ a ≦ 2.0. By setting a to the above-described range, the battery characteristics of the lithium ion secondary battery 1 can be further improved.

The second coating layer 103 preferably covers the positive electrode active material particles 101 so that the ratio of aLi ₂ O—ZrO ₂ to the positive electrode active material particles 101 is 0.1 mol% or more and 2.0 mol% or less. . When the coating amount of the second coating layer 103 is in the above range, the discharge capacity and load characteristics can be further improved. On the other hand, when the coating amount of the second coating layer 103 is less than 0.1 mol%, the effect of suppressing the reaction between the positive electrode active material particles 101 and the solid electrolyte 300 is not sufficient, which is not preferable. Moreover, when the coating amount of the second coating layer 103 exceeds 2.0 mol%, the lithium ion conductivity between the positive electrode active material particles 101 and the solid electrolyte 300 is lowered, which is not preferable.

  Moreover, it is preferable that the total thickness of the 1st coating layer 102 and the 2nd coating layer 103 is 1 nm or more and 500 nm or less. When the total thickness of the first coating layer 102 and the second coating layer 103 is included in the above-described range, the reaction between the positive electrode active material particles 101 and the solid electrolyte 300 is further suppressed without reducing the lithium ion conductivity. be able to. On the other hand, when the total thickness of the first coating layer 102 and the second coating layer 103 is less than 1 nm, the effect of suppressing the reaction between the positive electrode active material particles 101 and the solid electrolyte 300 is not sufficient, which is not preferable. Moreover, when the total thickness of the 1st coating layer 102 and the 2nd coating layer 103 exceeds 500 nm, since the lithium ion conductivity between the positive electrode active material particle 101 and the solid electrolyte 300 falls, it is unpreferable.

  In the above, the first coating layer 102 and the second coating layer 103 only need to cover at least part of the positive electrode active material particles 101. That is, the entire surface of the positive electrode active material particles 101 may be coated with the first coating layer 102 and the second coating layer 103, and a part of the surface of the positive electrode active material particles 101 may be coated with the first coating layer 102 and the first coating layer 102. Two coating layers 103 may be covered.

  In addition to the positive electrode particles 100 and the solid electrolyte 300 described above, the positive electrode layer 10 is appropriately mixed with additives such as a conductive agent, a binder, a filler, a dispersant, and an ionic conductive agent. May be.

  Examples of the conductive agent that can be blended in the positive electrode layer 10 include graphite, carbon black, acetylene black, ketjen black, carbon fiber, and metal powder. Examples of the binder that can be blended in the positive electrode layer 10 include polytetrafluoroethylene, polyvinylidene fluoride, and polyethylene. Furthermore, as a filler, a dispersing agent, an ionic conductive agent, and the like that can be blended in the positive electrode layer 10, known materials that are generally used for electrodes of lithium ion secondary batteries can be used.

[Negative electrode layer]
As shown in FIG. 1, the negative electrode layer 20 includes negative electrode particles 200 and a solid electrolyte 300. The solid electrolyte 300 will be described later in the solid electrolyte layer 30.

  The negative electrode particle 200 is a negative electrode active material having a lower charge / discharge potential than the positive electrode active material contained in the positive electrode active material particle 101 and capable of being alloyed with lithium or reversibly occluding and releasing lithium. Composed.

  For example, as the negative electrode active material, a metal active material, a carbon active material, or the like can be given. Examples of the metal active material include metals such as lithium (Li), indium (In), aluminum (Al), tin (Sn), and silicon (Si), and alloys thereof. Examples of the carbon active material include artificial graphite, graphite carbon fiber, resin-fired carbon, pyrolytic vapor-grown carbon, coke, mesocarbon microbeads (MCMB), and furfuryl alcohol (furfuryl alcohol) resin. Examples thereof include carbon, polyacene, pitch-based carbon fiber, vapor-grown carbon fiber, natural graphite, and non-graphitizable carbon. These negative electrode active materials may be used independently and may be used in combination of 2 or more type.

  Moreover, in addition to the negative electrode particles 200 and the solid electrolyte 300 described above, for example, additives such as a conductive agent, a binder, a filler, a dispersant, and an ionic conductive agent may be appropriately mixed in the negative electrode layer 20. .

  In addition, as an additive mix | blended with the negative electrode layer 20, the thing similar to the additive mix | blended with the positive electrode layer 10 mentioned above can be used.

[Solid electrolyte layer]
The solid electrolyte layer 30 is formed between the positive electrode layer 10 and the negative electrode layer 20 and includes a solid electrolyte 300.

Solid electrolyte 300 is formed of a sulfide solid electrolyte material. Examples of the sulfide solid electrolyte material include Li ₂ S—P ₂ S ₅ , Li ₂ S—P ₂ S ₅ —LiX (X is a halogen element), Li ₂ S—P ₂ S ₅ —Li ₂ O, Li _{2 S-P 2 S 5 -Li} 2 O-LiI, Li 2 S-SiS 2, Li2 S -SiS 2 -LiI, Li 2 S-SiS 2 -LiBr, Li 2 S-SiS 2 -LiCl, Li 2 S _{-SiS 2 -B 2 S 3 -LiI,} Li 2 S-SiS 2 -P 2 S 5 -LiI, Li 2 S-B 2 S 3, Li 2 S-P 2 S 5 -Z m S n (m, n is a positive number, Z is one of Ge, Zn or _{Ga), Li 2 S-GeS} 2, Li 2 S-SiS 2 -Li 3 PO 4, Li 2 S-SiS 2 -Li p MO q (p , Q is a positive number, M is P, Si, Ge, B, Al, Ga. Mention may be made of either), etc. In.

Further, in the solid electrolyte 300, of the above sulfide solid electrolyte material, sulfur (S) as at least an element, it is preferable to use a material containing phosphorus (P) and lithium (Li), at least Li 2 _S-P it is more preferable to use those containing ₂ S _5.

Here, the case of using those containing _Li 2 _S-P 2 _{S 5} as a sulfide solid electrolyte material to form a solid electrolyte 300, the mixing molar ratio of Li ₂ S and _P 2 _{S 5,} for example, _Li 2 S : P ₂ S ₅ = 50: 50 to 90:10.

  In addition, examples of the shape of the solid electrolyte 300 include particle shapes such as a true sphere and an oval sphere. Moreover, the particle diameter of the solid electrolyte 300 is not particularly limited, but the average particle diameter of the solid electrolyte 300 is preferably 0.01 μm or more and 30 μm or less, and more preferably 0.1 μm or more and 20 μm or less. The average particle diameter represents the number average diameter in the particle size distribution of particles obtained by a scattering method or the like as described above.

  The configuration of the lithium ion secondary battery 1 according to this embodiment has been described in detail above.

<2. Manufacturing method of lithium ion secondary battery>
Then, the manufacturing method of the lithium ion secondary battery 1 which concerns on this embodiment is demonstrated. The lithium ion secondary battery 1 according to this embodiment can be manufactured by stacking the above layers after manufacturing the positive electrode layer 10, the negative electrode layer 20, and the solid electrolyte layer 30, respectively.

[Preparation of positive electrode layer]
First, the positive electrode particles 100 are produced by sequentially forming the first coating layer 102 and the second coating layer 103 on the surface of the positive electrode active material particles 101.

The positive electrode active material particles 101 can be produced by a known method. For example, when NCA is used as the positive electrode active material particles 101, first, Ni (OH) ₂ powder, Co (OH) ₂ powder, Al ₂ O ₃ .H ₂ O powder so that the composition ratio is equal to NCA to be generated. And LiOH.H ₂ O powder are mixed and pulverized by a ball mill or the like. Next, the mixed and pulverized raw material powder is mixed with a predetermined dispersant, a binder, and the like, adjusted for viscosity, and then formed on a sheet. Furthermore, the positive electrode active material particles 101 can be produced by firing the sheet-like molded body at a predetermined temperature and pulverizing the fired molded body with a sieve or the like. Here, the particle diameter of the positive electrode active material particles 101 can be adjusted by changing the fineness of the sieve used for pulverizing the compact.

  Then, the 1st coating layer 102 is formed with respect to the surface of the positive electrode active material particle 101 produced above. The first coating layer 102 is formed by, for example, a plasma chemical vapor deposition method, an ion plating method, a sputtering method, a chemical vapor deposition (CVD) method such as a thermal CVD method or a plasma CVD method. Further, it can be formed by a physical vapor deposition method such as a pulse laser deposition (PLD) method or an electron beam evaporation method.

  For example, when forming the 1st coating layer 102 by plasma CVD method, first, the positive electrode active material particle 101 is arrange | positioned in a vacuum vessel, and hydrocarbon gas and carrier gas (carrier gas) are introduce | transduced in a vacuum vessel. Next, plasma is generated in the vacuum vessel by discharge, and a hydrocarbon gas ionized by the plasma is attached to the surface of the positive electrode active material particle 101, so that the surface of the positive electrode active material particle 101 is made of a carbon material. The first covering layer 102 can be formed.

Here, as the hydrocarbon gas, for example, methane (CH ₄ ), ethane (C ₂ H ₆ ), ethylene (C ₂ H ₄ ), acetylene (C ₂ H ₂ ), benzene (benzene), toluene (toluene) , Xylene, naphthalene, cyclohexane and the like can be used. By changing the type and flow rate of the hydrocarbon gas, the content of hydrogen atoms in the carbon material constituting the first coating layer 102, the ratio of carbon atoms having sp ² hybrid bonds and carbon atoms having sp ³ hybrid bonds Etc. can be adjusted. In addition, as for hydrocarbon gas, any 1 type of the said gas may be used independently, and 2 or more types may be mixed and used for it.

As the carrier gas, for example, hydrogen (H ₂ ) gas or inert gas (eg, argon (Ar) gas) can be used. Further, it is possible to adjust the hydrogen atom content and the like in the first coating layer 102 by changing the type and flow rate ratio of the carrier gas. For example, when hydrogen gas is used as the carrier gas, it is possible to adjust the hydrogen atom content in the first coating layer 102 by changing the flow rate ratio of the hydrogen gas to the hydrocarbon gas.

  In addition, the pressure in the vacuum vessel when forming the first coating layer 102 may be, for example, about 0.01 Pa to 1 Pa, and the temperature may be, for example, about 100 ° C. to 500 ° C. In addition, in order to clean and activate the surface of the positive electrode active material particles 101, an ion bombardment process may be performed on the positive electrode active material particles 101 before forming the first coating layer 102. .

  As described above, the first coating layer 102 can also be formed by a PVD method. When the first coating layer 102 is formed by the PVD method, a solid material such as graphite, glassy carbon, or diamond-like carbon can be used as the carbon source. When a solid material that does not contain hydrogen atoms is used as the carbon source, for example, hydrogen gas can be contained in the first coating layer 102 by supplying hydrogen gas or the like into the atmosphere.

  Subsequently, a second coating layer 103 is further formed on the positive electrode active material particles 101 on which the first coating layer 102 is formed as described above. The second coating layer 103 can be formed by the following method, for example.

  Specifically, first, lithium alkoxide and zirconium alkoxide are stirred and mixed in a solvent composed of an organic solvent such as alcohol and water to prepare a coating solution for the second coating layer 103. . Subsequently, the positive electrode active material particles 101 on which the first coating layer 102 is formed are added to the adjusted coating solution and mixed by stirring. Thereafter, the solvent is distilled off by heating or reducing pressure while irradiating the mixed solution with ultrasonic waves. By baking the positive electrode active material particles 101 after evaporation of the solvent at a predetermined baking temperature for a predetermined time, a second coating layer 103 made of a lithium-containing compound can be formed on the first coating layer 102.

  The calcination temperature of the positive electrode active material particles 101 after evaporation of the solvent may be, for example, 750 ° C. or less, and the calcination time may be about 0.5 hours to 3 hours.

  By the above method, the positive electrode particles 100 sequentially coated with the first coating layer 102 and the second coating layer 103 can be produced.

  Subsequently, the produced positive electrode particles 100, the solid electrolyte 300 produced by the method described later, and various additives are mixed, and added to a solvent such as water or an organic solvent to form a slurry or paste. Form. Furthermore, the obtained slurry or paste is applied to a current collector, dried, and then rolled, whereby the positive electrode layer 10 can be obtained.

[Preparation of negative electrode layer]
The negative electrode layer 20 can be produced by the same method as the positive electrode layer. Specifically, the negative electrode particles 200, the solid electrolyte 300 produced by the method described later, and various additives are mixed and added to a solvent such as water or an organic solvent to form a slurry or paste. Furthermore, the obtained slurry or paste is applied to a current collector, dried, and then rolled, whereby the negative electrode layer 20 can be obtained. The negative electrode particles 200 can be produced by a known method using a negative electrode active material.

  Here, as the current collector used in the positive electrode layer 10 and the negative electrode layer 20, for example, indium (In), copper (Cu), magnesium (Mg), stainless steel, titanium (Ti), iron (Fe), A plate-like body or a foil-like body made of cobalt (Co), nickel (Ni), zinc (Zn), aluminum (Al), germanium (Ge), lithium (Li), or an alloy thereof can be used. Note that the positive electrode layer 10 or the negative electrode layer 20 may be formed by compacting a mixture of the positive electrode particles 100 or the negative electrode particles 200 and various additives into a pellet shape without using a current collector. .

[Production of solid electrolyte layer]
The solid electrolyte layer 30 can be produced by a solid electrolyte 300 formed of a sulfide solid electrolyte material.

  First, a sulfide-based solid electrolyte material is manufactured by a melt quenching method or a mechanical milling method.

For example, when the melt quenching method is used, a predetermined amount of Li ₂ S and P ₂ S ₅ are mixed, and the pellets are reacted at a predetermined reaction temperature in a vacuum, and then rapidly cooled to form a sulfide system. A solid electrolyte material can be produced. The reaction temperature of the mixture of Li ₂ S and P ₂ S ₅ is preferably 400 ° C. to 1000 ° C., more preferably 800 ° C. to 900 ° C. Moreover, reaction time becomes like this. Preferably it is 0.1 to 12 hours, More preferably, it is 1 to 12 hours. Furthermore, the quenching temperature of the reaction product is usually 10 ° C. or less, preferably 0 ° C. or less, and the quenching rate is usually about 1 ° C./sec to 10000 ° C./sec, preferably 1 ° C./sec to 1000 ° C. It is about ° C / sec.

In the case of using the mechanical milling method, a predetermined amount of Li ₂ S and P ₂ S ₅ are mixed, and stirred and reacted using a ball mill or the like, whereby a sulfide-based solid electrolyte material can be produced. In addition, although the stirring speed and stirring time in the mechanical milling method are not particularly limited, the faster the stirring speed, the faster the generation rate of the sulfide-based solid electrolyte material, and the longer the stirring time, the more the sulfide-based solid electrolyte material. The conversion rate of the raw material can be increased.

  Thereafter, the sulfide-based solid electrolyte material obtained by the melt quenching method or the mechanical milling method is heat-treated at a predetermined temperature, and then pulverized to produce the particulate solid electrolyte 300.

  Subsequently, the solid electrolyte 300 obtained by the above-described method is subjected to, for example, a blast method, an aerosol deposition method, a cold spray method, a sputtering method, a CVD method, a thermal spraying method, or the like. The solid electrolyte layer 30 can be manufactured by forming a film using a known film forming method. The solid electrolyte layer 30 may be produced by pressurizing the solid electrolyte 300 alone. Moreover, the solid electrolyte layer 30 may produce the solid electrolyte layer 30 by mixing the solid electrolyte 300 and a solvent, a binder, or a support body, and pressurizing. Here, the binder or the support is added for the purpose of reinforcing the strength of the solid electrolyte layer 30 or preventing a short circuit of the solid electrolyte 300.

[Manufacture of lithium ion secondary batteries]
Furthermore, the positive electrode layer 10, the negative electrode layer 20, and the solid electrolyte layer 30 manufactured by the above method are stacked so that the solid electrolyte layer 30 is sandwiched between the positive electrode layer 10 and the negative electrode layer 20, and then pressed. The lithium ion secondary battery 1 according to the embodiment can be manufactured.

<3. Example>
Hereinafter, the lithium ion secondary battery according to the present embodiment will be specifically described with reference to Examples and Comparative Examples. In addition, the Example shown below is an example to the last, and the lithium ion secondary battery which concerns on this embodiment is not limited to the following example.

Example 1
LiNi _0.8 Co _0.15 Al _0.05 O ₂ (NCA: manufactured by Nippon Chemical Co., Ltd.) is used as the positive electrode active material particle 101, and the surface of the positive electrode active material particle 101 is made of DLC by plasma CVD. Layer 102 was formed. Specifically, after performing ion bombardment treatment for 3 minutes using a PIG plasma CVD apparatus, gas pressure 0.06 Pa, gas flow C ₂ H ₂ / Ar (flow rate 150 sccm / 10 sccm), film formation temperature At 200 ° C., a first coating layer 102 made of DLC was formed on the surface of the positive electrode active material.

Note that the carbon having sp ² hybridized bond of the first coating layer 102 formed on the surface of the positive electrode active material particles 101, the ratio of carbon with sp ³ hybridization binding was measured by the method described above, sp ² hybrid The ratio of carbon having a bond was approximately 50 to 55%. Further, when the content of hydrogen atoms in the first coating layer 102 formed on the surface of the positive electrode active material particles 101 was measured by IPC analysis (for example, ICP emission spectroscopy or ICP mass spectrometry), approximately 20 atoms were obtained. % To 30 atomic%. Furthermore, when the thickness of the 1st coating layer 102 was measured, it was about 5 nm.

Next, lithium methoxide and zirconium propoxide were mixed in an ethanol solution for 10 minutes. Here, the positive electrode active material particles 101 on which the first coating layer 102 is formed are added to the mixed solution so that the ratio of Li ₂ O—ZrO ₂ (LZO) is 0.5 mol% with respect to NCA. And mixed for 15 minutes with stirring. Furthermore, the solvent was distilled off with a rotary evaporator while irradiating the mixed solution with ultrasonic waves. The positive electrode active material particles 101 after the evaporation of the solvent were baked at 350 ° C. for 1 hour in an air atmosphere to form a second coating layer 103 made of LZO on the first coating layer. The positive electrode particle 100 was produced by the above method.

Next, Li ₂ S and P ₂ S ₅ were mixed at a molar ratio of 80:20, and a solid electrolyte 300 was produced by mechanical milling treatment.

  Subsequently, 15 mg of graphite, the above solid electrolyte 300, and carbon nanofiber (conductive material) mixed at a mass ratio of 60: 35: 5 are stacked in a cell container, and the surface is prepared with a molding machine. It was set to 20. In addition, 70 mg of the solid electrolyte 300 was laminated on the negative electrode layer 20, and the surface was adjusted with a molding machine to obtain the solid electrolyte layer 30. Next, 15 mg of a mixture of the positive electrode particle 100 produced above, the solid electrolyte 300, and the carbon nanofiber (conductive material) at a mass ratio of 60: 35: 5 is laminated on the solid electrolyte layer 30, The surface was prepared with a molding machine to obtain a positive electrode layer 10.

Furthermore, the negative electrode layer 20, the solid electrolyte layer 30, and the positive electrode layer 10 laminated in the cell container were pressed at a pressure of 3 t / cm ² to produce pellets, and a test cell (cell) according to Example 1 was produced. .

(Comparative Example 1)
A test cell according to Comparative Example 1 was produced in the same manner as in Example 1 except that the positive electrode active material particles 101 in which the first coating layer 102 and the second coating layer 103 were not formed were used as the positive electrode particles 100. .

(Comparative Example 2)
A test cell according to Comparative Example 2 in the same manner as in Example 1 except that the positive electrode active material particles 101 in which only the second coating layer 103 was formed without forming the first coating layer 102 were used as the positive electrode particles 100. Was made.

(Comparative Example 3)
The test cell according to Comparative Example 3 was prepared in the same manner as in Example 1 except that the positive electrode active material particles 101 in which the stacking order of the first coating layer 102 and the second coating layer 103 was reversed were used as the positive electrode particles 100. Produced. That is, the positive electrode particle 100 in which the second coating layer 103 made of Li ₂ O—ZrO ₂ is formed on the surface of the positive electrode active material particle 101 and the first coating layer 102 made of DLC is formed on the second coating layer 103 is used. Thus, a test cell according to Comparative Example 3 was produced.

(Comparative Example 4)
Test cell according to Comparative Example 4 in the same manner as in Example 1 except that the positive electrode active material particles 101 in which only the first coating layer 102 was formed without forming the second coating layer 103 were used as the positive electrode particles 100. Was made.

(Evaluation)
First, the test cells according to Example 1 and Comparative Examples 1 to 3 were charged at 25 ° C. with a constant current of 0.05 C to an upper limit voltage of 4.0 V, and then with a constant current of 0.05 C and a lower limit voltage of 2. The initial discharge capacity was evaluated by discharging to 5V. In addition, the test cells according to Example 1 and Comparative Examples 1 and 2 were charged and discharged at constant currents of 0.05 C, 0.5 C, and 1.0 C, respectively, discharge capacity was measured, and load characteristics were evaluated. In addition, the test cells according to Example 1 and Comparative Examples 1, 2, and 4 perform impedance measurements in a state where they are charged to the upper limit voltages of 4.0V, 4.1V, and 4.2V, and the internal resistance is measured. evaluated.

  In the column of “Coating material” in Tables 1 to 3 below, “DLC” represents that DLC was formed on the surface of the positive electrode active material particle 101, and “LZO” represents LZO on the surface of the positive electrode active material particle 101. Represents the formation of “DLC / LZO” indicates that DLC is formed on the surface of the positive electrode active material particle 101 and LZO is formed on the DLC. Note that “−” indicates that the coating layer is not formed on the surface of the positive electrode active material particle 101.

  The evaluation results of the initial discharge capacity are shown in Table 1 below.

  Referring to the results in Table 1, it was found that Example 1 had an increased initial discharge capacity compared to Comparative Examples 1 to 3. Specifically, Example 1 in which the surface of the positive electrode active material particle 101 is coated with the first coating layer 102 made of DLC and the second coating layer 103 made of LZO is a comparative example that is not covered with these two layers. It was found that the initial discharge capacity increased with respect to 1 and 2. Further, it was found that the initial discharge capacity of Example 1 was increased compared to Comparative Example 3 in which the stacking order of the first coating layer 102 and the second coating layer 103 was reversed.

  Next, the evaluation results of load characteristics are shown in Table 2 below. The “load characteristic (0.5 C / 0.05 C)” is a value obtained by dividing the discharge capacity of 0.5 C by the discharge capacity of 0.05 C, and “load characteristic (1.0 C / 0.05 C)”. Is a value obtained by dividing a discharge capacity of 1.0 C by a discharge capacity of 0.05 C.

  Referring to the results in Table 2, it was found that the load characteristics of Example 1 were improved with respect to Comparative Examples 1 and 2. Specifically, in Example 1, a decrease in discharge capacity when discharging with a large current is suppressed as compared with Comparative Examples 1 and 2, and the load on the lithium ion secondary battery is large (the discharge current is large). ), It was found that the battery had good battery characteristics.

  Subsequently, the evaluation results of the internal resistance are shown in Table 3 below.

  Referring to the results in Table 3, it was found that Example 1 had a lower internal resistance than Comparative Examples 1, 2, and 4. That is, in Example 1, the reaction at the interface between the positive electrode active material particles 101 and the solid electrolyte 300 is suppressed as compared with Comparative Examples 1, 2, and 4, and the generation of resistance components is suppressed. all right. Specifically, Example 1 in which the surface of the positive electrode active material particle 101 is coated with the first coating layer 102 made of DLC and the second coating layer 103 made of LZO is a comparative example that is not covered with these two layers. It was found that the internal resistance was lower for 1, 2 and 4.

  In particular, even when Example 1 is charged to a higher upper limit voltage, the internal resistance of Comparative Example is kept low compared to Comparative Examples 1, 2, and 4, and the load on the lithium ion secondary battery is large (charging) It was found that the battery had good battery characteristics even when the voltage at the time was high.

  As can be seen from the evaluation results described above, the lithium ion secondary battery 1 according to the present embodiment covers the positive electrode active material particles 101 in order by the first coating layer 102 and the second coating layer 103, whereby the positive electrode active material particles Generation of a resistance component at the interface between 101 and the solid electrolyte 300 can be suppressed. In addition, the lithium ion secondary battery 1 can ensure good lithium ion conductivity between the positive electrode active material particles 101 and the solid electrolyte 300 by the first coating layer 102 and the second coating layer 103. Therefore, the lithium ion secondary battery 1 according to the present embodiment can improve the discharge capacity and the load characteristics.

  The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present invention.

DESCRIPTION OF SYMBOLS 1 Lithium ion secondary battery 10 Positive electrode layer 20 Negative electrode layer 30 Solid electrolyte layer 100 Positive electrode particle 101 Positive electrode active material particle 102 1st coating layer 103 2nd coating layer 200 Negative electrode particle 300 Solid electrolyte

Claims (11)

Positive electrode active material particles, a first coating layer that covers the surface of the positive electrode active material particles and is formed of a carbon material, and a second coating that is formed of a lithium-containing compound that covers the first coating layer. Positive electrode particles having a layer;
A sulfide-based solid electrolyte containing at least Li ₂ S and P ₂ S ₅ and in contact with the positive electrode particles;
Lithium ion secondary battery. 2. The lithium ion secondary battery according to claim 1, wherein the carbon material is amorphous carbon containing hydrocarbons represented by carbon or C _{4n + 6} H _{4n + 12} (where n is an integer of 0 or more).   The lithium ion secondary battery according to claim 1, wherein the carbon material is diamond-like carbon.   The lithium ion secondary battery according to claim 3, wherein the diamond-like carbon contains hydrogen atoms in an amount of 1 atomic% to 50 atomic%. The diamond-like carbon includes a carbon atom having an sp ² hybrid bond and a carbon atom having an sp ³ hybrid bond,
4. The lithium ion secondary battery according to claim ³ , wherein a ratio of carbon atoms having the sp ³ hybrid bond among carbon atoms contained in the diamond-like carbon is 10% or more and 100% or less.   The thickness of the said 1st coating layer is a lithium ion secondary battery as described in any one of Claims 1-5 which are 1 nm or more and 50 nm or less.   The lithium ion secondary battery according to any one of claims 1 to 6, wherein the lithium-containing compound is a lithium-containing oxide or a lithium-containing phosphorus oxide. The lithium ion secondary battery according to claim 7, wherein the lithium-containing oxide is aLi ₂ O—ZrO ₂ (where 0.1 ≦ a ≦ 2.0).   The lithium ion secondary battery according to any one of claims 1 to 8, wherein a total thickness of the first coating layer and the second coating layer is 1 nm or more and 500 nm or less.   The lithium ion secondary battery according to any one of claims 1 to 9, wherein the positive electrode active material particles include a lithium salt of a transition oxide having a layered rock salt structure. The lithium salt of the transition oxide having the layered rock salt structure is LiNi _x Co _y M _z O ₂ (where M is Al or Mn, 0 <x <1, 0 <y <1, x + y + z = 1). The lithium ion secondary battery according to claim 10, wherein

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