JP2015088383A - Positive electrode for lithium ion secondary batteries, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery having a sulfide-based solid electrolyte arranged to suppress an interface reaction of a positive electrode active material and the solid electrolyte.SOLUTION: A positive electrode layer 10 of a lithium ion secondary battery includes: positive electrode particles 100 each including a positive electrode active material 101 including a lithium salt, and a coating film 102 composed of an amorphous carbon film including no lithium ion and covering the surface of the positive electrode active material 101; and a solid electrolyte 301 including a solid sulfide material and provided in contact with the coating film 102 of each positive electrode particle 100.

Description

  The present invention relates to a positive electrode of a lithium ion secondary battery and a lithium ion secondary battery.

Recently, all solid lithium ion secondary batteries using a solid electrolyte having lithium ion conductivity as an electrolyte have attracted attention.
As a solid electrolyte of a lithium ion secondary battery, a sulfide-based solid electrolyte is most expected from the viewpoint of lithium ion conductivity. However, when a sulfide-based solid electrolyte is used, for example, when a lithium ion secondary battery is charged, a reaction occurs at the interface between the positive electrode active material and the solid electrolyte, and a resistance component is generated at this interface. There is. When a resistance component is generated at the interface between the positive electrode active material and the solid electrolyte, the resistance (interface resistance) when lithium ions move through the interface between the positive electrode active material and the solid electrolyte increases. Due to the increase in the interfacial resistance, the lithium ion conductivity is lowered, which causes a problem that the output of the lithium ion secondary battery is lowered.

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 an interfacial reaction between the positive electrode active material and a solid electrolyte. .
Patent Document 2 and Patent Document 3 disclose that lithium in the vicinity of the interface between the positive electrode layer and the sulfide-based solid electrolyte layer between the positive electrode layer and the sulfide-based solid electrolyte layer in order to suppress the interfacial reaction between the positive electrode active material and the solid electrolyte. A technique is disclosed in which a buffer layer for buffering ion bias and an intermediate layer for suppressing interdiffusion between both layers are provided.

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

  However, even when the technique disclosed in the above-mentioned prior art document is adopted, it is insufficient to suppress the reaction at the interface between the positive electrode active material and the solid electrolyte, further reducing the resistance component, Improvement of the characteristics of lithium ion secondary batteries is also desired.

  An object of the present invention is to suppress an interface reaction between a positive electrode active material and a solid electrolyte in a lithium ion secondary battery having a sulfide-based solid electrolyte.

The present invention that achieves the above object includes a positive electrode active material that includes a lithium salt, and a positive electrode particle that includes an amorphous carbon film that does not include lithium ions and a coating film that covers the surface of the positive electrode active material. A positive electrode of a lithium ion secondary battery comprising a solid sulfide material and having a sulfide solid electrolyte provided in contact with the coating film of the positive electrode particles.
Here, the coating film may include an alicyclic hydrocarbon represented by C _{4n + 6} H _{4n + 12} (n is a positive integer).
Further, the coating film can be characterized by being composed of diamond-like carbon.
Furthermore, the coating film may include hydrogen atoms, and the content of the hydrogen atoms may be in the range of 1 atomic% to 50 atomic%.
Furthermore, the coating film includes carbon atoms bonded by sp ² hybrid bonds and carbon atoms bonded by sp ³ hybrid bonds, and among the carbon atoms contained in the coating film, the sp ³ hybrid bonds It may be characterized in that the proportion of bonded carbon atoms is in the range of 10% to 100%.
The coating film may be formed by a plasma CVD method or a PVD method.

From another viewpoint, the present invention provides a positive electrode having positive electrode particles in which a coating film composed of an amorphous carbon film not containing lithium ions is formed on the surface of a positive electrode active material containing a lithium salt. A lithium ion battery comprising: a layer; a negative electrode layer having a negative electrode active material; and a sulfide solid electrolyte layer having a sulfide solid electrolyte containing a solid sulfide material and provided between the positive electrode layer and the negative electrode layer. Next battery.
Here, the positive electrode active material may include a lithium salt of a transition metal oxide having a layered rock salt structure.
Further, the lithium salt of the transition metal oxide having a layered rock-salt _{structure, LiNi x Co y Al z O} 2 or _{LiNi x Co y Mn z O 2} (0 <x <1,0 <y <1,0 < z <1, x + y + z = 1).
Further, the sulfide solid electrolyte may include at least sulfur, phosphorus, and lithium.

  ADVANTAGE OF THE INVENTION According to this invention, the interface reaction of a positive electrode active material and a solid electrolyte can be suppressed in the lithium ion secondary battery which has a sulfide type solid electrolyte.

(A) (b) is the figure which showed an example of the lithium ion secondary battery with which this Embodiment is applied. It is the figure which showed an example of the TEM-EELS spectrum measured about the coating film of the positive electrode particle, and an example of the fitting result. It is the figure which showed the initial stage charge / discharge curve in Example 1 and a comparative example. (A) (b) is the figure which showed the rate characteristic in Example 1 and a comparative example. It is the figure which showed the measurement result of the impedance in 4.0V in Example 1, Example 2, and a comparative example. (A) (b) is the figure which showed the measurement result of the impedance in 4.0V, 4.1V, and 4.2V about Example 1 and Example 2. FIG. It is the figure which showed the measurement result of the impedance in 4.0V, 4.1V, and 4.2V about a comparative example.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the accompanying drawings.
[Configuration of Lithium Ion Secondary Battery 1]
FIGS. 1A and 1B are diagrams illustrating an example of a lithium ion secondary battery 1 to which the exemplary embodiment is applied. FIG. 1A is a schematic cross-sectional view showing a layer configuration of a lithium ion secondary battery 1 to which the present embodiment is applied, and FIG. 1B is a lithium ion to which the present embodiment is applied. 2 is a schematic cross-sectional view showing a configuration of positive electrode particles 100 of secondary battery 1. FIG.
As shown to Fig.1 (a), the lithium ion secondary battery 1 to which this Embodiment is applied is an all-solid-state battery using a solid electrolyte as electrolyte. As shown in FIG. 1A, the lithium ion secondary battery 1 of the present embodiment includes a positive electrode layer 10, a negative electrode layer 20, and a solid electrolyte layer 30 located between the positive electrode layer 10 and the negative electrode layer 20. And have a laminated structure.

[Positive electrode layer 10]
As shown in FIG. 1A, the positive electrode layer 10 according to the present embodiment includes positive electrode particles 100 and a solid electrolyte 301. The solid electrolyte 301 is a particle constituting the solid electrolyte layer 30, and the detailed configuration thereof will be described in the stage of the solid electrolyte layer 30 described later.

(Positive electrode particle 100)
As shown in FIG. 1B, the positive electrode particle 100 of this embodiment includes a positive electrode active material 101 and a coating film 102 that covers the surface of the positive electrode active material 101. Hereinafter, the positive electrode active material 101 and the coating film 102 will be described in detail.

(Positive electrode active material 101)
The positive electrode active material 101 of this embodiment has a higher charge / discharge potential than a negative electrode active material 201 described later, and is formed of a material capable of reversibly occluding or releasing lithium ions.
Examples of the positive electrode active material 101 include lithium cobaltate (hereinafter sometimes referred to as “LCO”), lithium nickelate, lithium nickel cobaltate, and lithium nickel cobaltaluminate (hereinafter referred to as “NCA”). ), Nickel cobalt lithium manganate (hereinafter sometimes referred to as “NCM”), lithium salts such as lithium manganate and lithium iron phosphate, nickel sulfide, copper sulfide, sulfur, iron oxide, and vanadium oxide. Etc. can be used. Each of these may be used alone as the positive electrode active material 101, or two or more of them may be used in combination.

In addition, the positive electrode active material 101 is preferably a lithium salt of a transition metal oxide having a layered rock salt structure, among the lithium salts described above. Here, “layered” means a thin sheet-like shape. The “rock salt type structure” is a sodium chloride type structure which is a kind of crystal structure, and the face-centered cubic lattice formed by each of the cation and the anion is 1 / of the edge of the unit cell. Refers to a structure shifted by two.
The lithium salt of a transition metal oxide 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) (0 <x <1 , 0 <y <1, 0 <z <1, and x + y + z = 1). A lithium salt of a ternary transition metal oxide represented by:

Thus, the lithium ion secondary battery 1 excellent in energy density and thermal stability is obtained by using the lithium salt of the ternary transition metal oxide having a layered rock salt type structure as the positive electrode active material 101. be able to.
In addition, lithium salt particles of ternary transition metal oxides such as NCA and NCM have a smaller particle size than particles such as LCO, and thus have a large specific surface area (about 10 times). Thereby, the contact area of the positive electrode particle 100 and the solid electrolyte 301 can be enlarged. As a result, as described later, by suppressing the reaction between the positive electrode active material 101 and the solid electrolyte 301 by the coating film 102, the positive electrode particles 100 (the positive electrode active material 101) are compared with the case where this configuration is not adopted. And the solid electrolyte 301, the lithium ion conductivity is improved, and the power of the lithium ion secondary battery 1 can be increased.

  Further, when the lithium salt of the ternary transition metal oxide having a layered rock salt structure is used as the positive electrode active material 101, the positive electrode active material 101 contains Ni as a constituent element. In this case, the capacity density of the lithium ion secondary battery 1 increases, and metal elution in a charged state tends to decrease. Thereby, compared with the case where this structure is not employ | adopted, the long-term reliability of the lithium ion secondary battery 1 in a charging state can be improved, and the cycling characteristics of the lithium ion secondary battery 1 can be improved.

Examples of the shape of the positive electrode active material 101 include particle shapes such as a true sphere and an elliptic sphere. Moreover, when the positive electrode active material 101 is a particle shape, it is preferable that the average particle diameter is the range of 0.1 micrometer-50 micrometers, for example. The “average particle diameter” refers to 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.
Further, the content of the positive electrode active material 101 in the positive electrode layer 10 is preferably in the range of 10 wt% to 99 wt%, for example, and more preferably in the range of 20 wt% to 90 wt%.

(Coating film 102)
The coating film 102 is provided on the surface of the positive electrode active material 101. As a result, direct contact between the positive electrode active material 101 and the solid electrolyte 301 is suppressed, and a reaction at the interface between the positive electrode active material 101 and the solid electrolyte 301 can be suppressed.

The coating film 102 of the present embodiment is composed of an amorphous carbon film that does not contain lithium ions. Here, in this embodiment, the amorphous carbon film is a film composed of carbon or containing carbon as a main component, carbon having a bond by sp ³ hybrid orbital corresponding to a diamond structure, and a graphite structure. Is a film having an amorphous structure in which carbon having bonds by sp ² hybrid orbital corresponding to is irregularly mixed. In the following description, a bond based on sp ³ hybrid orbital is sometimes simply referred to as sp ³ hybrid bond, and a bond based on sp ² hybrid orbital is simply referred to as sp ² hybrid bond.
Note that the coating film 102 (amorphous carbon film) of this embodiment may contain atoms other than carbon, such as hydrogen and silicon, as long as carbon is the main component.

Examples of the amorphous carbon film that can be used as the coating film 102 include alicyclic hydrocarbons represented by the general formula C _{4n + 6} H _{4n + 12} and diamond-like carbon (hereinafter referred to as “DLC”). Or the like.)).
Examples of the alicyclic hydrocarbon represented by the general formula C _{4n + 6} H _{4n + 12} include, but are not limited to, adamantane, diamantane, triamantane, tetramantane, pentamantane, and the like. .

In the coating film 102 of the present embodiment, the amorphous carbon film preferably contains hydrogen atoms. When DLC is used as the amorphous carbon film constituting the coating film 102, hydrogen is used. It is preferable to use the contained DLC. The content of hydrogen atoms contained in the amorphous carbon film is preferably in the range of 1 atomic% to 50 atomic%, and more preferably in the range of 10 atomic% to 30 atomic%.
When the content of hydrogen atoms in the coating film 102 is excessively high, the stability of the coating film 102 may be reduced.

The thickness of the coating film 102 is preferably selected from the range of 1 nm to 100 nm, and more preferably selected from the range of 5 to 20 nm. By setting the thickness of the coating film 102 within this range, the solid electrolyte 301 and the positive electrode active material are suppressed while suppressing a decrease in lithium ion conductivity as compared with the case where the thickness of the coating film 102 is outside this range. Interfacial reaction with the substance 101 can be suppressed.
When the thickness of the coating film 102 is excessively large, the lithium ion conductivity between the solid electrolyte 301 and the positive electrode active material 101 through the coating film 102 may be reduced.
On the other hand, when the thickness of the coating film 102 is excessively thin, the solid electrolyte 301 and the positive electrode active material 101 may react.
Note that the thickness of the coating film 102 can be measured using a transmission electron microscope (TEM) image of the positive electrode particle 100 or the like.

As described above, the amorphous carbon film used as the coating film 102 of the present embodiment includes carbon atoms bonded by sp ² hybrid bonds and carbon atoms bonded by sp ³ hybrid bonds. In the present embodiment, the coating film 102 includes a carbon-carbon bond composed of sp ² hybrid bonds, a carbon-carbon bond composed of sp ³ hybrid bonds, a carbon-hydrogen bond composed of sp ² hybrid bonds, and sp ^3. It preferably contains a carbon-hydrogen bond consisting of a bond.

In the case of using the DLC as a coating film 102 of the present embodiment, of the carbon contained in the DLC, the proportion of carbon having sp ³ hybrid bonds, is preferably in the range of 10% to 100%, 30 More preferably, it is in the range of% to 70%, and further preferably in the range of 40% to 60%. When the proportion of carbon having sp ³ hybrid bonds contained in the amorphous carbon film is within this range, the reaction between the positive electrode active material 101 and the solid electrolyte 301 is effectively suppressed, and the lithium ions in the coating film 102 are suppressed. Therefore, the lithium ion secondary battery 1 can have good discharge capacity, load characteristics, and cycle characteristics.

Here, the ratio of carbon having sp ² hybrid bonds and carbon having sp ³ hybrid bonds contained in DLC used as the coating film 102 is, for example, an electron using a transmission electron microscope (TEM). It can obtain | require by an energy loss spectroscopy (Electron Energy-Loss Spectroscopy; EELS).
Hereinafter, an example of a method for obtaining the ratio of carbon having sp ² hybrid bonds and carbon having sp ³ hybrid bonds contained in the coating film 102 will be specifically described.

  FIG. 2 is a diagram showing an example of a TEM-EELS spectrum measured for the coating film 102 of the positive electrode particle 100 and an example of the fitting result. In the example shown in FIG. 2, DLC is used as the coating film 102. FIG. 2 shows a range of electron energy of 0.280 keV to 0.295 keV in the EELS spectrum near the K loss end of carbon. In FIG. 2, the horizontal axis represents loss energy (keV), and the vertical axis represents spectrum intensity.

In FIG. 2, the solid line indicates the EELS spectrum measured for the coating film 102. As shown by a solid line in FIG. 2, the EELS spectrum of DLC has peaks at around 0.284 keV to 0.286 keV and around 0.292 keV to 0.295 keV, respectively. In the following description, a peak around 0.284 keV to 0.286 keV is called a first peak, and a peak around 0.292 keV to 0.295 keV is called a second peak.
Here, in the DLC EELS spectrum, the first peak corresponds to the π bond of carbon atoms, and the second peak corresponds to the σ bond of carbon atoms.

In the present embodiment, first, the first peak and the second peak are separated from the EELS spectrum indicated by the solid line in FIG. In FIG. 2, the curve indicated by the alternate long and short dash line corresponds to the separated first peak, and the curve indicated by the alternate long and two short dashes line corresponds to the separated second peak. Moreover, the curve shown with a broken line in FIG. 2 respond | corresponds to the EELS spectrum fitted with the isolate | separated 1st peak and the isolate | separated 2nd peak.
Subsequently, the intensity ratio (peak area ratio; area of peak 1 / area of peak 2) between the separated first peak and the separated second peak is determined.

Although not shown, for each of diamond and graphite, similarly, the EELS spectrum is measured, the first peak and the second peak are separated, and the separated first peak is separated from the separated second peak. Obtain the peak area ratio.
Here, the carbon atoms contained in the diamond are bonded by sp ³ hybrid bonds, and the carbon atoms contained in the graphite are bonded by sp ² hybrid bonds.
Therefore, for example, when the peak area ratio between the first peak and the second peak in diamond is 0 and the peak area ratio between the first peak and the second peak in graphite is 100, the first peak and the second peak in DLC By calculating the relative value of the peak area ratio with respect to the peak, the ratio of carbon atoms having sp ² hybrid bonds to carbon atoms having sp ³ hybrid bonds (in this case, the ratio of carbon atoms having sp ² hybrid bonds) (%)).

  Table 1 shows the analysis results in the EELS spectrum of the DLC shown in FIG.

As shown in Table 1, in the example shown in FIG. 2, the relative value of the peak area is 51, and the DLC includes carbon atoms having sp ² hybrid bonds and carbon atoms having sp ³ hybrid bonds. , Sp ² : sp ³ = 51: 49.

Note that the method for obtaining the ratio of carbon atoms having sp ² hybrid bonds and carbon atoms having sp ³ hybrid bonds in the coating film 102 is not limited to the method using TEM-EELS described above. For example, it may be obtained by a method using an x-ray photoelectron spectroscopy spectrum, a Raman spectrum, or the like.

(Other additives)
In the positive electrode layer 10, in addition to the positive electrode particles 100 (the positive electrode active material 101 and the coating film 102) and the solid electrolyte 301, for example, additives such as a conductive agent, a binder, a filler, a dispersant, and an ionic conductive agent are included. It may be appropriately selected and blended.

  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. Moreover, as a binder which can be mix | blended with the positive electrode layer 10, polytetrafluoroethylene, a polyvinylidene fluoride, polyethylene etc. are mentioned, for example. 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 substances that are generally used for electrodes of lithium ion secondary batteries can be used.

[Negative electrode layer 20]
Subsequently, the negative electrode layer 20 will be described.
The negative electrode layer 20 of this Embodiment is comprised including the negative electrode active material 201 and the solid electrolyte 301 mentioned later, as shown to Fig.1 (a).
As described above, the solid electrolyte 301 is a particle constituting the solid electrolyte layer 30. The configuration of the solid electrolyte 301 will be described later.

(Negative electrode active material 201)
The negative electrode active material 201 of this embodiment has a lower charge / discharge potential than the positive electrode active material 101, and is formed of a material that can be alloyed with lithium or reversibly occluded and released. .
Examples of the negative electrode active material 201 include a metal active material and a carbon active material. Examples of the metal active material include metals such as lithium, indium, aluminum, tin, and silicon, 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), furfuryl alcohol resin-fired carbon, polyacene, and pitch-based carbon. Examples thereof include fibers, vapor-grown carbon fibers, natural graphite, and non-graphitizable carbon. These may be used alone as the negative electrode active material 201, or two or more of them may be used in combination.

(Other additives)
In the negative electrode layer 20, in addition to the negative electrode active material 201 and the solid electrolyte 301, for example, additives such as a conductive agent, a binder, a filler, a dispersant, and an ionic conductive agent may be appropriately selected and blended.
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 30]
As shown in FIG. 1A, the solid electrolyte layer 30 is a layer formed between the positive electrode layer 10 and the negative electrode layer 20, and is composed of a solid electrolyte 301 made of a sulfide solid electrolyte material.

(Solid electrolyte 301)
The solid electrolyte 301 of the present embodiment is made 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 —LiI, Li ₂ S—P ₂ S ₅ —Li ₂ O, and Li ₂ S—P ₂ S. _{5 -Li 2 O-LiI, Li} 2 S-SiS 2, Li 2 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 ( however, m, n are positive the number of, Z is, Ge, Zn, one of _{Ga.), Li 2 S-} GeS 2, Li 2 S-SiS 2 -Li 3 PO 4, Li 2 S-SiS 2 -Li p MO q ( where p and q are positive numbers, and M is any one of P, Si, Ge, B, Al, Ga, and In.).

Among these, as the sulfide solid electrolyte material constituting the solid electrolyte 301, it is preferable to use a material containing at least sulfur, phosphorus and lithium as constituent elements, and particularly Li ₂ S—P ₂ S ₅ . It is preferable to use a material containing the same.
When a material containing Li ₂ S—P ₂ S ₅ is used as the sulfide solid electrolyte material constituting the solid electrolyte 301, the mixing ratio of Li ₂ S and P ₂ S ₅ (Li ₂ S: P ₂ S ₅ ) is For example, the molar ratio is selected from the range of 50:50 to 90:10.

  Examples of the shape of the solid electrolyte 301 include particle shapes such as a true sphere and an oval sphere. Moreover, the particle diameter of the solid electrolyte 301 is not particularly limited, but the average particle diameter is preferably 0.01 to 1 μm, and more preferably 0.1 to 0.5 μm.

Incidentally, in general, an all-solid lithium ion secondary battery using, for example, a solid electrolyte layer made of a solid electrolyte having a particle shape as the electrolyte layer may cause the following problems.
That is, in an all-solid-state secondary battery using a solid electrolyte, since the positive electrode active material and the electrolyte are solid, the electrolyte is less likely to penetrate into the positive electrode active material than when an organic electrolyte is used as the electrolyte. For this reason, since the area of the interface between the positive electrode active material and the electrolyte tends to be small, it is difficult to secure a sufficient movement path of lithium ions and electrons between the positive electrode active material and the electrolyte.
In order to solve such a problem, there is a technique for increasing the area of the interface between the positive electrode active material and the solid electrolyte by making the positive electrode layer a mixed layer in which the positive electrode active material and the solid electrolyte are mixed.

However, when a sulfide-based solid electrolyte is used as the solid electrolyte, when a lithium ion secondary battery is charged, a reaction occurs at the interface between the positive electrode active material and the solid electrolyte, and a resistance component is generated at this interface. As a result, a high resistance layer is formed on the surface of the positive electrode active material. As a result, the interface resistance between the positive electrode active material and the solid electrolyte tends to increase.
Note that the high resistance layer is a layer made of a resistance component formed on the surface of the positive electrode active material when the positive electrode active material and the solid electrolyte are brought into contact with each other, and includes the inside of the positive electrode active material and the solid electrolyte. As compared with, it means a layer in which resistance when lithium ions move increases.
Thus, when the area of the interface between the positive electrode active material and the solid electrolyte is increased, a movement path of lithium ions and electrons can be secured between the positive electrode active material and the solid electrolyte, but a high resistance layer is formed. It becomes easy to be done. Then, the movement of lithium ions from the positive electrode active material to the solid electrolyte is hindered by the high resistance layer, and the lithium ion conductivity is lowered, so that the output of the lithium ion secondary battery is lowered.

  On the other hand, in the present embodiment, a coating film 102 made of an amorphous carbon film not containing lithium ions is provided on the surface of the positive electrode active material 101. Thereby, the positive electrode active material 101 and the solid electrolyte 301 can be prevented from being in direct contact by the coating film 102. As a result, the reaction at the interface between the positive electrode active material 101 and the solid electrolyte 301 can be suppressed, and the generation of a resistance component at the interface between the positive electrode active material 101 and the solid electrolyte 301 can be suppressed. For example, even when the charging / discharging of the lithium ion secondary battery 1 is repeated, a movement path of lithium ions between the positive electrode active material 101 and the solid electrolyte 301 is secured, and the lithium ion secondary battery 1 A decrease in output can be suppressed, and the discharge capacity / load characteristics, cycle characteristics, and the like of the lithium ion secondary battery 1 can be improved.

  In particular, in this embodiment, an amorphous carbon film that does not contain lithium ions such as DLC is used as the coating film 102, so that the coating film 102 can be coated as compared with the case where another substance is used. The thermal stability and chemical stability of the film 102 can be increased, and deterioration of the coating film 102 can be suppressed. Thereby, the lithium ion conduction between the positive electrode active material 101 and the solid electrolyte 301 is kept good while suppressing the reaction at the interface between the positive electrode active material 101 and the solid electrolyte 301 for a long period of time. Can do. As a result, the load characteristics, cycle characteristics, etc. of the lithium ion secondary battery 1 can be improved.

[Method for Manufacturing Lithium Ion Secondary Battery 1]
Then, the manufacturing method of the lithium ion secondary battery 1 of this Embodiment is demonstrated. Lithium ion secondary battery 1 of the present embodiment can be manufactured by fabricating positive electrode layer 10, negative electrode layer 20, and solid electrolyte layer 30, and then laminating these layers.
Hereinafter, each process which manufactures the lithium ion secondary battery 1 is demonstrated in order.

[Preparation of Positive Electrode Layer 10]
In the present embodiment, when the positive electrode layer 10 is manufactured, the positive electrode particles 100 are first manufactured. The positive electrode particle 100 can be formed by forming a coating film 102 on the surface of the positive electrode active material 101.

(Formation of positive electrode active material 101)
The positive electrode active material 101 can be manufactured by a known method.
For example, when NCA is used as the positive electrode active material 101, the positive electrode active material 101 can be manufactured as follows.
First, the composition ratio of the mixture, generated to be equal to the composition ratio of NCA to, Ni (OH) ₂ powder, Co (OH) _{2 powder, Al 2 O 3 · H 2} O powder and LiOH · H ₂ O powder Are mixed and pulverized with a ball mill or the like. Subsequently, the mixed and pulverized raw material powder is mixed with a predetermined dispersant, a binder and the like to prepare a viscosity and the like, and then formed into a sheet shape. Thereafter, the sheet-like molded body is fired at a predetermined temperature, and the fired molded body is pulverized using a sieve (mesh) or the like, whereby the particulate positive electrode active material 101 can be obtained. Here, for example, the particle diameter of the positive electrode active material 101 can be adjusted by changing the sieve (mesh) used for pulverizing the compact.

(Formation of coating film 102)
The coating film 102 of the present embodiment is formed by, for example, a known chemical vapor deposition (CVD) method such as plasma chemical vapor deposition, ion plating, or sputtering, or physical vapor deposition (PVD). It can be formed by the Vapor Deposition method. The CVD method includes various methods such as a thermal CVD method and a plasma CVD method, and the PVD method includes a resistance heating vapor deposition method, an electron beam vapor deposition method, and a pulse laser deposition (PLD) method. Of these, the plasma CVD method and the PLD method are preferably used.

  In the case where the coating film 102 is formed by the plasma CVD method, the positive electrode active material 101 that is the target for forming the coating film 102 is placed in a vacuum vessel, and a reaction gas and a carrier gas are introduced into the vacuum vessel. Next, plasma is generated in the vacuum vessel by discharge of the reactive gas, and a gas ionized by the plasma is attached to the surface of the positive electrode active material 101, thereby forming the coating film 102 made of an amorphous carbon film. Can do.

A hydrocarbon gas can be used as the reaction gas. Examples of the hydrocarbon gas used in the present embodiment include acetylene, methane, benzene, toluene, xylene, naphthalene, and cyclohexane. Further, a gas containing an alicyclic hydrocarbon represented by C _{4n + 6} H _{4n + 12} such as adamantane or diamantane may be used as a reaction gas.
The reaction gas can be selected so that the amorphous carbon film formed as the coating film 102 has a desired composition. That is, by changing the type and flow rate of the hydrocarbon gas used as the reaction gas, the content of hydrogen atoms in the amorphous carbon film constituting the coating film 102, the carbon atoms having sp ² hybrid bonds, and the sp ³ It becomes possible to adjust the proportion of carbon atoms having a hybrid bond.
In addition, any 1 type may be used individually for the hydrocarbon gas used as reaction gas, and 2 or more types may be mixed and used for it.

Here, acetylene has a carbon atom having an sp hybrid bond as a carbon atom. For example, when acetylene is used as a reactive gas, the discharge causes decomposition or reaction of carbon atoms having sp hybrid bonds contained in acetylene, and thus having carbon atoms having sp ² hybrid bonds or sp ³ hybrid bonds. Carbon atoms will be generated. As a result, it is possible to obtain a coating film 102 composed of an amorphous carbon film having carbon atoms having sp ² hybrid bonds and carbon atoms having sp ³ hybrid bonds.

  As the carrier gas, hydrogen gas or argon gas can be used. The type and flow ratio of the carrier gas can be selected so that the amorphous carbon film formed as the coating film 102 has a desired composition. For example, when hydrogen gas is used as the carrier gas, the content of hydrogen atoms in the coating film 102 can be adjusted by changing the flow rate ratio of the hydrogen gas to the source gas.

The pressure in the atmosphere for forming the coating film 102 can be, for example, in the range of 0.01 Pa to 1 Pa.
Moreover, the film forming temperature of the coating film 102 can be made into the range of 100 to 500 degreeC, for example. In addition, when the film forming temperature is excessively high, defects may occur in the coating film 102. Moreover, when the film forming temperature is excessively low, the discharge in the reaction vessel may become unstable.

  Note that in the case where the coating film 102 is formed by a plasma CVD method, before the coating film 102 is formed, the positive electrode active material 101 is subjected to ion bombardment to clean and activate the surface of the positive electrode active material 101. It is preferable to make it.

Further, as described above, the coating film 102 of the present embodiment made of an amorphous carbon film may be formed by various PVD methods.
When the coating film 102 is formed by the PVD method, examples of the carbon source include solid materials such as graphite, glassy carbon, and diamond-like carbon. When a solid material that does not contain hydrogen atoms is used as the carbon source in the PVD method, when the coating film 102 contains hydrogen atoms, for example, a gas containing hydrogen atoms such as hydrogen gas in the atmosphere is used. What is necessary is just to supply.

As described above, the positive electrode particles 100 in which the coating film 102 made of an amorphous carbon film not containing lithium ions is formed on the surface of the positive electrode active material 101 can be obtained.
Then, the produced positive electrode particle 100, a solid electrolyte 301 produced by a method described later, for example, and various additives are mixed and added to a solvent such as water or an organic solvent to form a slurry or paste, and the obtained slurry Alternatively, the positive electrode layer 10 can be obtained by applying the paste to a current collector and drying it, followed by pressure contact with a rolling roll or the like.

[Preparation of Negative Electrode Layer 20]
The negative electrode layer 20 can be produced by a known method.
For example, the negative electrode active material 201, for example, a solid electrolyte 301 produced by a 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, and the obtained slurry or The negative electrode layer 20 can be obtained by applying the paste to the current collector and drying it, followed by pressure contact with a rolling roll or the like.
Examples of the current collector used in this case include a plate-like body and a foil-like body made of indium, copper, magnesium, stainless steel, titanium, iron, cobalt, nickel, zinc, aluminum, germanium, lithium, or an alloy thereof. Etc.
Note that the negative electrode layer 20 may be formed by compacting a mixture of the negative electrode active material 201 and various additives into a pellet shape without using a current collector. Further, when a metal or an alloy thereof is used as the negative electrode active material 201, a metal sheet or a metal foil may be used as it is as the negative electrode layer 20.

[Production of Solid Electrolyte Layer 30]
The solid electrolyte layer 30 can be produced, for example, by the following method.
When producing the solid electrolyte layer 30, first, the solid electrolyte 301 constituting the solid electrolyte layer 30 is produced. For production of the solid electrolyte 301 of the present embodiment composed of a sulfide solid electrolyte material, for example, a melt quenching method or a mechanical milling method (MM method) can be used. In the following, a manufacturing method will be described by taking as an example a case where a sulfide solid electrolyte material having Li ₂ S and P ₂ S ₅ is used as the solid electrolyte 301.

First, in the case of the melt quenching method, a mixture of a predetermined amount of Li ₂ S and P ₂ S ₅ in a pellet form is reacted at a predetermined reaction temperature in a vacuum, and then rapidly cooled to form a sulfide. System solid electrolyte can be obtained.
The reaction temperature at this time is preferably selected from the range of 400 ° C to 1000 ° C, and more preferably selected from the range of 800 ° C to 900 ° C. The reaction time is preferably selected from the range of 0.1 hour to 12 hours, and more preferably selected from the range of 1 hour to 12 hours. Furthermore, the quenching temperature of the reaction product is usually 10 ° C. or less, preferably 0 ° C. or less, and the cooling rate is usually about 1 K / sec to 10000 K / sec, from the range of 1 K / sec to 1000 K / sec. Preferably it is selected.

On the other hand, in the case of the MM method, a predetermined amount of Li ₂ S and P ₂ S ₅ are mixed and reacted with strong stirring using an attritor or a ball mill for a predetermined time to obtain a sulfide-based solid electrolyte. be able to.
The MM method using the above raw materials has an advantage that the reaction can be performed at room temperature. According to the MM method, since a sulfide-based solid electrolyte can be produced at room temperature, the raw material is hardly decomposed, and a sulfide solid electrolyte having a charged composition can be obtained.
The rotation speed and rotation time by the MM method are not particularly limited, but the higher the rotation speed, the higher the production rate of the solid electrolyte, and the longer the rotation time, the higher the conversion rate of the raw material to the solid electrolyte.

  Thereafter, the sulfide-based solid electrolyte obtained by the melt quenching method or the MM method is heat-treated at a predetermined temperature and then pulverized to obtain a particulate solid electrolyte 301.

The particulate solid electrolyte 301 thus obtained is subjected to a known film forming method such as a blast method, an aerosol deposition method, a cold spray method, a sputtering method, a vapor deposition method (CVD), or a thermal spraying method. The solid electrolyte layer 30 can be obtained by forming into a film.
In addition, the solid electrolyte 301, a solvent, a binder (binder, polymer compound, etc.), a support (strengthening the strength of the solid electrolyte layer 30, or a material or a compound for preventing a short circuit of the solid electrolyte 301 itself) ) May be mixed and pressed to form the solid electrolyte layer 30.

(Preparation of lithium ion secondary battery 1)
And the positive electrode layer 10, the solid electrolyte layer 30, and the negative electrode layer 20 which were produced by the method mentioned above are laminated | stacked in this order, and the lithium ion secondary battery 1 which concerns on this Embodiment can be obtained by pressing.

Then, this invention is demonstrated based on an Example. However, the present invention is not limited to the following examples.
This inventor formed the test cell of the lithium ion secondary battery 1 as follows, and evaluated the characteristic.

Example 1
First, 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 101, and the surface of the positive electrode active material 101 made of NCA is coated with DLC by plasma CVD. A film 102 was formed, and positive electrode particles 100 were obtained.
Specifically, a plasma CVD apparatus is used, and the conditions are a heating temperature of 200 ° C., a gas pressure in the apparatus of 0.06 Pa, an ion bombardment treatment for 3 minutes, an Ar gas of 10 sccm as a carrier gas, The coating film 102 was formed by supplying 150 sccm of acetylene gas as a reaction gas and performing plasma treatment.

Note that the ratio of the carbon having sp ² hybrid bonds and the carbon having sp ³ hybrid bonds in the coating film 102 formed on the surface of the positive electrode active material 101 was measured by the above-described method, and as a result, sp ² : sp ³ = 52: 48. Moreover, when the thickness of the coating film 102 was measured, it was 5 nm-10 nm.

Subsequently, Li ₂ S and P ₂ S ₅ were mixed at a molar ratio of 80:20 and subjected to mechanical milling (MM treatment) to obtain a solid electrolyte 301 (SE; solid electrolyte).
And the ratio of 60/35/5 mass% of the graphite used as the negative electrode active material 201, the said solid electrolyte 301 (SE), and the vapor growth carbon fiber (VGCF; vapor grown carbon fiber) which is a electrically conductive agent. 80 mg of what was mixed in was laminated on a cell container, and the surface was adjusted with a molding machine to obtain a negative electrode layer 20.

  Next, 100 mg of the above-mentioned solid electrolyte 301 (SE) was laminated on the negative electrode layer 20 of the cell container, and the surface was prepared with a molding machine to obtain the solid electrolyte layer 30.

Subsequently, the positive electrode particles 100 in which the coating film 102 made of DLC is formed on the surface of the positive electrode active material 101 obtained as described above, the above-described solid electrolyte 301 (SE), VGCF as a conductive agent, Was mixed at a ratio of 60/35/5% by mass on the solid electrolyte layer 30 of the cell container to obtain a positive electrode layer 10.
And the negative electrode layer 20, the solid electrolyte layer 30, and the positive electrode layer 10 in a cell container were pressed at a pressure of 3 t / cm ^{2 in} a laminated state to produce a pellet, and a test cell was obtained.

(Example 2)
First, 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 101, and the coating film made of DLC is applied to the surface of the positive electrode active material 101 made of NCA by PVD 102 was formed, and positive electrode particles 100 were obtained.
Specifically, graphite was used as a carbon ionization deposition source, and the pressure of the atmosphere was set to 10 to 4 Pa or less to form the coating film 102.

Note that the ratio of carbon having sp ² hybrid bonds to carbon having sp ³ hybrid bonds in the coating film 102 formed on the surface of the positive electrode active material 101 was measured by the above-described method, and as a result, sp ² : sp ³ = It was 40:60. Moreover, when the thickness of the coating film 102 was measured, it was 5 nm-10 nm.

Thereafter, as in Example 1, Li ₂ S and P ₂ S ₅ were mixed at a molar ratio of 80:20 and subjected to mechanical milling (MM treatment) to obtain a solid electrolyte 301 (SE; solid electrolyte). It was.
And the ratio of 60/35/5 mass% of the graphite used as the negative electrode active material 201, the said solid electrolyte 301 (SE), and the vapor growth carbon fiber (VGCF; vapor grown carbon fiber) which is a electrically conductive agent. 80 mg of what was mixed in was laminated on a cell container, and the surface was adjusted with a molding machine to obtain a negative electrode layer 20.

  Next, 100 mg of the above-mentioned solid electrolyte 301 (SE) was laminated on the negative electrode layer 20 of the cell container, and the surface was prepared with a molding machine to obtain the solid electrolyte layer 30.

Subsequently, the positive electrode particles 100 in which the coating film 102 made of DLC is formed on the surface of the positive electrode active material 101 obtained as described above, the above-described solid electrolyte 301 (SE), VGCF as a conductive agent, Was mixed at a ratio of 60/35/5% by mass on the solid electrolyte layer 30 of the cell container to obtain a positive electrode layer 10.
And the negative electrode layer 20, the solid electrolyte layer 30, and the positive electrode layer 10 in a cell container were pressed at a pressure of 3 t / cm ^{2 in} a laminated state to produce a pellet, and a test cell was obtained.

(Comparative example)
A negative electrode layer 20, a solid electrolyte layer 30, and a positive electrode layer 10 are laminated in a cell container in the same manner as in Example 1 except that the coating film 102 is not formed on the surface of the positive electrode active material 101, and a pressure of 3 t / cm ² . Was pressed to produce pellets, and a test cell was obtained.

(Evaluation)
Each test cell obtained in Example 1, Example 2 and Comparative Example was charged at 25 ° C. with a constant current of 0.05 C to an upper limit voltage of 4.0 V, and the initial discharge capacity was measured. 0.05 C was discharged. Thereafter, charging and discharging were performed in the same manner even at a constant current of 0.5 C and 1 C. Moreover, the alternating current impedance in 4.0V, 4.1V, and 4.2V was measured, respectively. Based on the above measurement results, the battery characteristics in Example 1, Example 2, and Comparative Example were evaluated.

  Table 2 shows the measurement results for the test cells of Example 1, Example 2, and Comparative Example.

  Moreover, FIG. 3 is a figure which showed the initial stage charge / discharge curve in Example 1 and a comparative example. Further, FIGS. 4A and 4B are graphs showing rate characteristics in the example 1 and the comparative example. 4 (a) and 4 (b) show charge / discharge curves when charge / discharge is performed at a constant current of 0.05C, 0.5C, and 1C as rate characteristics, and FIG. The rate characteristic in Example 1 and the rate characteristic in the comparative example are shown in FIG. 4B. In FIGS. 3 and 4 (a) and 4 (b), the horizontal axis indicates the charge / discharge capacity (Capacity), and the vertical axis indicates the voltage (Voltage).

  FIG. 5 is a diagram showing impedance measurement results at 4.0 V in Example 1, Example 2, and Comparative Example. 6 (a) and 6 (b) are diagrams showing impedance measurement results at 4.0V, 4.1V and 4.2V for Example 1 and Example 2, and are shown in FIG. 6 (a). The measurement result of Example 1 and the measurement result of Example 2 are shown in FIG. FIG. 7 is a diagram showing the measurement results of impedance at 4.0 V, 4.1 V, and 4.2 V for the comparative example. In FIGS. 5, 6A, 6B, and 7, the horizontal axis represents the real component of the complex impedance, and the vertical axis represents the imaginary component of the complex impedance.

  As shown in Table 2 and FIG. 3, in Example 1 and Example 2 in which the coating film 102 made of DLC was formed on the surface of the positive electrode active material 101, the coating film 102 was formed on the surface of the positive electrode active material 101. It was confirmed that the initial discharge capacity was larger than that of the comparative example.

  Further, as shown in Table 2 and FIG. 4, in Example 1 and Example 2 in which the coating film 102 made of DLC was formed on the surface of the positive electrode active material 101, the coating film 102 was formed on the surface of the positive electrode active material 101. It was confirmed that the rate characteristics were good as compared with the comparative example which was not formed. Specifically, in Example 1 and Example 2, it was confirmed that a discharge capacity of 60% or more of a current discharge at 0.05 C can be maintained even in a current discharge at 1 C. On the other hand, in the comparative example, it was 33%, and it was confirmed that the discharge capacity was lowered. That is, it was confirmed that by forming the coating film 102 made of DLC on the surface of the positive electrode active material 101, the capacity decrease when the current density is increased is reduced as compared with the case where this configuration is not adopted. .

  Further, as shown in Table 2 and FIG. 5, in the test cells of Example 1 and Example 2 in which the coating film 102 made of DLC was formed on the surface of the positive electrode active material 101, the surface of the positive electrode active material 101 was coated on the surface. It was confirmed that the impedance was low as compared with the test cell of Comparative Example 1 in which the covering film 102 was not formed. Furthermore, as shown in FIGS. 6A and 6B and FIG. 7, it was confirmed that Example 1 and Example 2 can suppress an increase in impedance when the potential is high as compared with the comparative example. .

  Moreover, although omitted for details, it was confirmed that in the test cells of Example 1 and Example 2, it was possible to suppress deterioration after repeated charging and discharging as compared with the test cell of the comparative example.

Further, comparing Example 1 and Example 2, the coating film 102 is formed by the plasma CVD method, and the ratio of carbon having sp ² hybrid bonds to carbon having sp ³ hybrid bonds is sp ² : sp. ^In Example 1 where ³ = 52: 48, the coating film 102 is formed by the PVD method, and the ratio of carbon having sp ² hybrid bonds to carbon having sp ³ hybrid bonds is sp ² : sp ³ = Compared with Example 2 which is 60:40, it was confirmed that the discharge capacity was high, the impedance was low, and the rate characteristics were good.

  As is clear from the above examples, the surface of the positive electrode active material 101 is covered with a coating film 102 made of an amorphous carbon film that does not contain lithium ions, so that the positive electrode active material 101 and the solid electrolyte 301 are separated from each other. It was confirmed that the interface reaction can be suppressed, and as a result, the discharge capacity, load characteristics, and cycle characteristics of the lithium ion secondary battery 1 can be improved.

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, 102 ... Coating film, 201 ... Negative electrode active material, 301 ... Solid Electrolytes

Claims (10)

Positive electrode particles having a positive electrode active material containing a lithium salt and a coating film that is composed of an amorphous carbon film not containing lithium ions and covers the surface of the positive electrode active material;
A positive electrode of a lithium ion secondary battery comprising a solid sulfide material, and a sulfide solid electrolyte provided in contact with the coating film of the positive electrode particles. 2. The positive electrode of a lithium ion secondary battery according to claim 1, wherein the coating film contains an alicyclic hydrocarbon represented by C _{4n + 6} H _{4n + 12} (n is a positive integer).   The positive electrode of the lithium ion secondary battery according to claim 1, wherein the coating film is made of diamond-like carbon.   4. The lithium ion secondary battery according to claim 1, wherein the coating film contains hydrogen atoms, and the content of the hydrogen atoms is in the range of 1 atom% to 50 atom%. Positive electrode. The coating film includes carbon atoms bonded by sp ² hybrid bonds and carbon atoms bonded by sp ³ hybrid bonds, and among the carbon atoms contained in the coating film, carbons bonded by the sp ³ hybrid bonds. 5. The positive electrode of a lithium ion secondary battery according to claim 1, wherein an atomic ratio is in a range of 10% to 100%.   The positive electrode of the lithium ion secondary battery according to claim 1, wherein the coating film is formed by a plasma CVD method or a PVD method. A positive electrode layer having positive electrode particles in which a coating film composed of an amorphous carbon film not containing lithium ions is formed on the surface of a positive electrode active material containing a lithium salt;
A negative electrode layer having a negative electrode active material;
A lithium ion secondary battery having a sulfide solid electrolyte layer provided between the positive electrode layer and the negative electrode layer and having a sulfide solid electrolyte containing a solid sulfide material.   The lithium ion secondary battery according to claim 7, wherein the positive electrode active material includes a lithium salt of a transition metal oxide having a layered rock salt structure. The lithium salt of the transition metal oxide having a layered rock salt structure is LiNi _x Co _y Al _z O ₂ or LiNi _x Co _y Mn _z O ₂ (0 <x <1, 0 <y <1, 0 <z < The lithium ion secondary battery according to claim 8, wherein 1 and x + y + z = 1).   The lithium ion secondary battery according to any one of claims 7 to 9, wherein the sulfide solid electrolyte contains at least sulfur, phosphorus, and lithium.

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