JP2015185237A - Positive electrode for all-solid type lithium secondary batteries, and all-solid type lithium secondary battery arranged by use thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide means which enables the reduction in impedance at interface between a solid electrolyte and an active material, and enables the increase in all-solid type lithium secondary battery's charge/discharge efficiency and charge-discharge capacity.SOLUTION: A positive electrode for all-solid type lithium secondary batteries comprises: sulfide-based solid electrolyte particles (104) including LiS; lithium-containing transition metal oxide particles (106); and a SiOfilm (105) provided on the surface of the lithium-containing transition metal oxide particle (106) between the sulfide-based solid electrolyte particle (104) and the lithium-containing transition metal oxide particle (106). In the positive electrode for all-solid type lithium secondary batteries, the SiOfilm (105) with a thickness of 18 nm or less is formed by perhydropolysilazane.

Description

  The present invention relates to a positive electrode for an all-solid lithium secondary battery and an all-solid lithium secondary battery using the same.

  Lithium ion secondary batteries have features such as high energy density and high voltage operation. Therefore, lithium ion secondary batteries are used in information devices such as mobile phones as small and light secondary batteries. In recent years, the demand for lithium ion secondary batteries is increasing as a large power source such as a power source of a hybrid vehicle.

  A lithium ion secondary battery is composed of a positive electrode, a negative electrode, and an electrolyte disposed therebetween. The electrolyte is a non-aqueous electrolyte or a solid electrolyte. Since widely used electrolytes are flammable, a system for ensuring safety is required. On the other hand, since the solid electrolyte is nonflammable, the above system can be simplified. Hereinafter, a lithium ion secondary battery using a solid electrolyte is referred to as an “all-solid battery”.

Solid electrolytes are roughly classified into organic solid electrolytes and inorganic solid electrolytes. The former is also called a polymer solid electrolyte. Since the ionic conductivity of the organic solid electrolyte at room temperature is about 10 ⁻⁶ S / cm, it is difficult to operate an all solid state battery using the organic solid electrolyte at room temperature. On the other hand, the latter includes those made of oxide and those made of sulfide. Inorganic solid electrolytes made of oxides have problems that the grain boundary resistance is large and that the transition metal contained in the inorganic solid electrolyte is reduced by contact with lithium metal and the battery characteristics deteriorate. Therefore, in recent years, inorganic solid electrolytes made of sulfides, which are less subject to these problems, have been actively researched and developed.

  On the other hand, in recent years, as shown in Non-Patent Document 1, when an active material made of a lithium-containing transition metal oxide is brought into contact with a sulfide-based solid electrolyte, a high resistance layer that becomes a “space charge layer” at the interface between the two Was reported to form. Since this high resistance layer is formed, the output characteristics of the all-solid lithium secondary battery are considered to be inferior to that of a lithium ion battery using an organic electrolyte.

In order to improve the output characteristics of an all-solid lithium secondary battery, attempts have been made to coat the surfaces of lithium-containing transition metal oxide particles as an active material with a material such as an oxide-based solid electrolyte. For example, Non-Patent Document 1 discloses Li ₄ Ti ₅ O ₁₂ , LiNbO ₃ and LiTaO ₃ as coating materials. Non-Patent Document 2 discloses SiO ₂ and Li ₂ SiO ₃ as coating materials. It is said that when the surfaces of lithium-containing transition metal oxide particles are coated with these coating materials, the output characteristics of the battery are improved.

Material Integration, 122, 12, 9-14 2009 Electrochemical and Solid-State Letters, 11 1 A1-A3 2008

In the all-solid-state battery of Non-Patent Document 2, a positive electrode mixture obtained by mixing a sulfide-based solid electrolyte containing Li ₂ S and a transition metal oxide is used. An attempt to coat the surface of LiCoO ₂ particles with a Li ₂ SiO ₃ film or SiO ₂ film by a sol-gel method in order to reduce the impedance of the solid electrolyte / active material interface and increase the charge / discharge efficiency and charge / discharge capacity Has been made. However, Non-Patent Document 2 shows that LiCoO ₂ coated with a SiO ₂ film is not different from the uncoated one.

The present invention provides a positive electrode mixture containing a sulfide-based solid electrolyte containing Li ₂ S and a lithium-containing transition metal oxide, which reduces the impedance of the interface between the solid electrolyte and the active material, and can be used to charge an all-solid lithium secondary battery. The purpose is to increase discharge efficiency and charge / discharge capacity.

That is, this disclosure
A sulfide-based solid electrolyte particle containing Li ₂ S;
Lithium-containing transition metal oxide particles;
An SiO ₂ film provided between the sulfide-based solid electrolyte particles and the lithium-containing transition metal oxide particles, and in contact with the surfaces of the lithium-containing transition metal oxide particles;
With
Along the direction crossing the interface between the SiO ₂ film and the lithium-containing transition metal oxide particles, using a transmission electron microscope, near the interface between the SiO ₂ film and the lithium-containing transition metal oxide particles. The energy dispersive X-ray spectroscopic analysis is performed at a plurality of positions, and the Kα ray signal intensity and Kβ of the transition metal M (M is Mn, Ni, Co or Fe) obtained in the region of the lithium-containing transition metal oxide particles. The total value of the signal intensity of the line is defined as the signal intensity of the transition metal M, and the total value of the signal intensity of the Kα ray and the Kβ line obtained in the region of the SiO ₂ film is defined as the Si signal. When defined as strength,
All-solid lithium secondary in which the ratio (Is / Im) of the maximum signal intensity Is which is the maximum value of the Si signal intensity to the average signal intensity Im which is the average value of the signal intensity of the transition metal M is 0.2 or more A positive electrode for a battery is provided.

  According to the above technique, the impedance of the solid electrolyte / active material interface can be reduced, and the charge / discharge efficiency and charge / discharge capacity of the all-solid lithium secondary battery can be increased.

Configuration diagram of the all-solid lithium secondary battery of the present embodiment Sectional drawing of the electric power generation element of the all-solid-state lithium secondary battery shown in FIG. TEM image of cross section of LiCoO ₂ particles of Example 1 having SiO ₂ film formed using polysilazane Graph showing the results of TEM-EDS analysis of a cross section near the LiCoO ₂ particles of Example 1 TEM image of cross section of LiCoO ₂ particles of Comparative Example 2 having a SiO ₂ film formed using tetraethoxysilane Graph showing the results of TEM-EDS analysis of a cross section near the LiCoO ₂ particles of Comparative Example 2

  In Non-Patent Documents 1 and 2, a sol-gel method is employed as a method for coating transition metal oxide particles with a coating material. However, as a result of the study by the present inventors, it was found that the charge / discharge efficiency is low even when the transition metal oxide particles are coated with a coating material formed by a sol-gel method.

The first aspect of the present disclosure is:
A sulfide-based solid electrolyte particle containing Li ₂ S;
Lithium-containing transition metal oxide particles;
An SiO ₂ film provided between the sulfide-based solid electrolyte particles and the lithium-containing transition metal oxide particles, and in contact with the surfaces of the lithium-containing transition metal oxide particles;
With
Along the direction crossing the interface between the SiO ₂ film and the lithium-containing transition metal oxide particles, using a transmission electron microscope, near the interface between the SiO ₂ film and the lithium-containing transition metal oxide particles. The energy dispersive X-ray spectroscopic analysis is performed at a plurality of positions, and the Kα ray signal intensity and Kβ of the transition metal M (M is Mn, Ni, Co or Fe) obtained in the region of the lithium-containing transition metal oxide particles. The total value of the signal intensity of the line is defined as the signal intensity of the transition metal M, and the total value of the signal intensity of the Kα ray and the Kβ line obtained in the region of the SiO ₂ film is defined as the Si signal. When defined as strength,
All-solid lithium secondary in which the ratio (Is / Im) of the maximum signal intensity Is which is the maximum value of the Si signal intensity to the average signal intensity Im which is the average value of the signal intensity of the transition metal M is 0.2 or more A positive electrode for a battery is provided.

  According to the first aspect, the impedance of the solid electrolyte / active material interface can be reduced, and the charge / discharge efficiency and charge / discharge capacity of the all-solid lithium secondary battery can be increased.

According to a second aspect of the present disclosure, in addition to the first aspect, the SiO ₂ film is provided using a perhydropolysilazane, and provides a positive electrode for an all solid lithium secondary battery. The SiO ₂ film formed using perhydropolysilazane is dense.

A third aspect of the present disclosure provides a positive electrode for an all-solid lithium secondary battery, in addition to the first or second aspect, wherein the thickness of the SiO ₂ film is 18 nm or less. By appropriately controlling the thickness of the SiO ₂ film, high initial charge / discharge capacity and initial charge / discharge efficiency can be achieved.

  A fourth aspect of the present disclosure provides a positive electrode for an all solid lithium secondary battery, in addition to any one of the first to third aspects, the ratio (Is / Im) is 0.27 or more. According to the fourth aspect, the effect obtained in the first aspect can be further enhanced.

The fifth aspect of the present disclosure is:
Any one positive electrode of the first to fourth aspects;
A negative electrode,
A sulfide-based solid electrolyte disposed between the positive electrode and the negative electrode;
An all-solid lithium secondary battery is provided.

  If the positive electrode of the 1st-4th aspect is used, the all-solid-state lithium secondary battery with a high energy density and high reliability can be provided.

The sixth aspect of the present disclosure is:
Forming a SiO ₂ film on the surface of the lithium-containing transition metal oxide particles;
Mixing the lithium-containing transition metal oxide particles having the SiO ₂ film and the sulfide-based solid electrolyte particles;
Including
Along the direction crossing the interface between the SiO ₂ film and the lithium-containing transition metal oxide particles, using a transmission electron microscope, near the interface between the SiO ₂ film and the lithium-containing transition metal oxide particles. The energy dispersive X-ray spectroscopic analysis is performed at a plurality of positions, and the Kα ray signal intensity and Kβ of the transition metal M (M is Mn, Ni, Co or Fe) obtained in the region of the lithium-containing transition metal oxide particles. The total value of the signal intensity of the line is defined as the signal intensity of the transition metal M, and the total value of the signal intensity of the Kα ray and the Kβ line obtained in the region of the SiO ₂ film is defined as the Si signal. When defined as strength,
All-solid lithium secondary in which the ratio (Is / Im) of the maximum signal intensity Is which is the maximum value of the Si signal intensity to the average signal intensity Im which is the average value of the signal intensity of the transition metal M is 0.2 or more A method for producing a positive electrode for a battery is provided.

  According to the sixth aspect, the impedance of the solid electrolyte / active material interface can be reduced, and the charge / discharge efficiency and charge / discharge capacity of the all-solid lithium secondary battery can be increased.

A seventh aspect of the present disclosure, in addition to the sixth aspect, in the step of forming the SiO ₂ film, using a perhydropolysilazane as a raw material for the SiO ₂ film, a manufacturing method of a positive electrode for all-solid lithium secondary battery To do. By using perhydropolysilazane, a dense SiO ₂ film can be formed.

According to an eighth aspect of the present disclosure, in addition to the sixth or seventh aspect, the step of forming the SiO ₂ film includes a step of attaching perhydropolysilazane to the surface of the lithium-containing transition metal oxide particles, and a heat treatment. And a step of converting the perhydropolysilazane to SiO ₂ on the surface of the lithium-containing transition metal oxide particles by performing the method. By performing the heat treatment, perhydropolysilazane can be converted into SiO ₂ in a short time.

A ninth aspect of the present disclosure provides a method for producing a positive electrode for an all-solid lithium secondary battery, in addition to any one of the sixth to eighth aspects, wherein the thickness of the SiO ₂ film is 18 nm or less. By appropriately controlling the thickness of the SiO ₂ film, high initial charge / discharge capacity and initial charge / discharge efficiency can be achieved.

  A tenth aspect of the present disclosure provides the method for producing a positive electrode for an all-solid lithium secondary battery, in addition to any one of the sixth to ninth aspects, wherein the ratio (Is / Im) is 0.27 or more. To do. According to the tenth aspect, the effect obtained in the sixth aspect can be further enhanced.

An eleventh aspect of the present disclosure includes
A sulfide-based solid electrolyte particle containing Li ₂ S;
Lithium-containing transition metal oxide particles;
An SiO ₂ film provided between the sulfide-based solid electrolyte particles and the lithium-containing transition metal oxide particles, and in contact with the surfaces of the lithium-containing transition metal oxide particles;
With
The SiO ₂ film provides a positive electrode for an all solid lithium secondary battery, which is formed using perhydropolysilazane.

The SiO ₂ film formed using perhydropolysilazane can be denser than the SiO ₂ film formed using alkoxide such as tetraethoxysilane. Although the reason is not necessarily clear, the dense SiO ₂ film is suitable for improving the characteristics of the all-solid lithium secondary battery.

A twelfth aspect of the present disclosure provides the positive electrode for an all solid lithium secondary battery, in addition to the eleventh aspect, wherein the thickness of the SiO ₂ film is 18 nm or less. By appropriately controlling the thickness of the SiO ₂ film, high initial charge / discharge capacity and initial charge / discharge efficiency can be achieved.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to the following embodiments.

The present embodiment roughly includes the following five steps.
Step 1. Step of forming SiO ₂ film on the surface of lithium-containing transition metal oxide particles Step 2. Step of preparing sulfide-based solid electrolyte particles containing Li ₂ S Step 3. Step of preparing positive electrode mixture Step 4. Step of manufacturing all solid lithium secondary battery Step 5. Process for evaluating initial characteristics of all-solid-state lithium secondary batteries

  Hereafter, each process is demonstrated in detail in order.

[Step 1. Step of forming SiO ₂ film on the surface of lithium-containing transition metal oxide particles]
First, a perhydropolysilazane solution (coating agent) for forming a SiO ₂ film (SiO ₂ layer) on the surface of a predetermined amount of lithium-containing transition metal oxide particles whose specific surface area has been measured in advance by the BET method is prepared. . That is, perhydropolysilazane is used as a raw material for the SiO ₂ film. By using perhydropolysilazane, a dense SiO ₂ film can be formed. The dense SiO ₂ film is suitable for improving the characteristics of the all-solid lithium secondary battery.

The perhydropolysilazane solution contains perhydropolysilazane in an amount corresponding to the SiO ₂ film having a desired thickness. A solution is obtained by diluting perhydropolysilazane with a solvent. Examples of the diluting solvent include xylene, dibutyl ether, diethyl ether, solvesso, and terpene. Since perhydropolysilazane is converted to SiO ₂ when exposed to water and oxygen, it is important to use a sufficiently dehydrated solvent.

The reason for diluting perhydropolysilazane with a solvent is as follows. That is, the amount of perhydropolysilazane necessary for forming a SiO ₂ film having a thickness on the order of nanometers is extremely small. Since it is not easy to spread a small amount of perhydropolysilazane throughout the lithium-containing transition metal oxide powder, it is desirable to dilute the perhydropolysilazane with a solvent to an amount sufficient to wet the entire powder. By doing so, perhydropolysilazane can be uniformly attached to the surface of the lithium-containing transition metal oxide particle, and consequently, a SiO ₂ film having a uniform film thickness can be formed on the surface of the particle.

In this embodiment, a lithium-containing transition metal oxide is used as the positive electrode active material. Lithium-containing transition metal oxides include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiCoPO ₄ , LiMnPO ₄ , LiFePO ₄ , LiNiPO ₄ , by substituting transition metals of these compounds with one or two different elements. Examples thereof include compounds obtained. Examples of the compound obtained by substituting the transition metal of the above compound with one or two different elements include LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.8 Co _0.15 Al _0.05 O ₂ , LiNi _0.5 Mn _1.5 O _{2 and the} like.

  Next, while stirring the perhydropolysilazane solution, lithium-containing transition metal oxide particles are added to the solution little by little. After the total amount of lithium-containing transition metal oxide particles has been added to the solution, the liquid components are evaporated while stirring the solution on a hot plate until the particles can be visually confirmed to be dry. This allows perhydropolysilazane to adhere to the surface of the lithium-containing transition metal oxide particles, specifically, the particles can be coated with perhydropolysilazane. The temperature of the solution at the time of evaporating the liquid component (setting temperature of the hot plate) can be set as appropriate according to the solvent, but is preferably equal to or lower than the boiling point of the solvent. If the solution is heated above the boiling point of the solvent, the solution may suddenly boil and scatter. As a result, the raw material charge ratio may change.

Next, the particle lump is crushed with a mortar to loosen the particles. The operation so far is desirably performed in a low dew point environment, for example, in a dew point environment of −40 ° C. or lower. This is because in an atmosphere with a high dew point, perhydropolysilazane is converted to SiO ₂ in the solution in contact with water and oxygen in the air. When perhydropolysilazane is converted to SiO ₂ in the solution, the liquid phase of the solution may become cloudy. It is desirable to evaporate and remove the solvent without converting perhydropolysilazane to SiO ₂ as much as possible.

Thereafter, perhydropolysilazane is converted to SiO ₂ on the surface of the lithium-containing transition metal oxide particles. By heat-treating the lithium-containing transition metal oxide particles, perhydropolysilazane can be converted to SiO ₂ in a short time. In order to convert perhydropolysilazane to SiO ₂ , water and oxygen are required. Therefore, the atmosphere during the heat treatment is preferably a wet atmosphere in which oxygen and water are present. As the humid atmosphere, an air atmosphere is exemplified. In one example, the lithium-containing transition metal oxide particles are heat-treated in an air atmosphere at 150 to 450 ° C. for 1 to 2 hours. Although heat treatment is not essential, the heat treatment not only promotes the conversion of perhydropolysilazane to SiO ₂ but also increases the density of the SiO ₂ film. After the heat treatment, the lump of particles is crushed and loosened again. As a result, lithium-containing transition metal oxide particles coated with the SiO ₂ film are obtained.

[Step 2. Step of preparing sulfide-based solid electrolyte particles containing Li ₂ S]
Lithium sulfide (Li ₂ S) particles and niline pentasulfide (P ₂ S ₅ ) particles are mixed at a weight ratio of 80:20 to 70:30, and Li ₂ S is obtained by mechanical milling using a planetary ball mill. A sulfide-based solid electrolyte containing is synthesized. However, the mixing ratio of these particles is not particularly limited. The mechanical milling method can be performed under conditions of 200 to 600 rpm and 5 to 24 hours. _{Thus, Li 2 S-P 2 S} 5 glass solid electrolyte particles. After mechanical milling, the Li ₂ S—P ₂ S ₅ glass solid electrolyte particles are annealed under conditions of 200 to 300 ° C. and 1 to 10 hours in an inert atmosphere. _{Thus, Li 2 S-P 2 S} 5 glass ceramic solid electrolyte particles.

Other sulfide-based solid electrolytes containing Li ₂ S include Li ₂ S—SiS ₂ glass, Li ₂ S—B ₂ S ₃ glass, Li _3.25 Ge _0.25 P _0.75 S ₄ , Li ₁₀ GeP ₂ S _12. Etc. Further, in _{these, LiI, Li x MO y (} M: P, Si, Ge, B, Al, Ga or an In, x, y: integer) can be used which was added as an additive and the like.

  The method for synthesizing the sulfide-based solid electrolyte is not limited to the mechanical milling method. The sulfide-based solid electrolyte can also be synthesized by other methods such as a melt superquenching method and a sealed tube method. The melt super rapid cooling method is a method in which a raw material is melted and the melt is quenched by passing the melt through a twin roll or contacting the melt with liquid nitrogen, thereby obtaining a target material. The sealed tube method is a method in which a quartz tube containing raw materials is sealed under reduced pressure and heat-treated to obtain a target material.

[Step 3. Step of preparing positive electrode mixture]
Step 1. 1. Lithium-containing transition metal oxide particles coated with the SiO ₂ film prepared in step 2; A predetermined amount of each of the sulfide-based solid electrolyte particles containing Li ₂ S prepared in (1) is weighed and mixed thoroughly. Thereby, a positive electrode mixture is obtained. The mixing ratio is not particularly limited. In one example, the lithium-containing transition metal oxide: solid electrolyte = 5: 5 to 9: 1 by weight ratio.

A known mixing method can be used as the mixing method. For example, a method of mixing with a mortar, a method of mixing with a ball mill or a bead mill, a method of mixing with a jet mill, or the like is used. As a mixing method, any of a dry method and a wet method can be adopted. In the wet mixing method, it is necessary to use a liquid that does not react with any of the sulfide-based solid electrolyte containing SiO ₂ , lithium-containing transition metal oxide, and Li ₂ S. In addition, the liquid used in the wet mixing method needs to have water sufficiently removed. An example of such a liquid is dehydrated toluene. However, according to the above conditions, the liquid used in the wet mixing method is not limited to dehydrated toluene.

[Step 4. Process for producing an all-solid-state lithium secondary battery]
As shown in FIG. 1, the lower die 1 is inserted into the insulating tube 3. A sulfide-based solid electrolyte particle containing Li ₂ S is placed in the insulating tube 3. The upper die 2 is inserted into the insulating tube 3, and the sulfide-based solid electrolyte particles are pressurized to form the solid electrolyte layer 102. The upper die 2 is removed, and the positive electrode mixture is put into the insulating tube 3. The upper die 2 is inserted into the insulating tube 3 again, and the positive electrode mixture is pressurized to form the positive electrode mixture layer 101 on the solid electrolyte layer 102. The pressure applied to the positive electrode mixture when forming the positive electrode mixture layer 101 is desirably higher than the pressure applied to the solid electrolyte when forming the solid electrolyte layer 102. For example, it is desirable to form each layer by applying a pressure of 0.2 to 5 MPa when forming the solid electrolyte layer 102 and applying a pressure of 5 to 50 MPa when forming the positive electrode mixture layer 101.

  After forming the positive electrode mixture layer 101, the lower die 1 is removed, and a metal indium foil 103 (or metal lithium foil) punched into a disk shape is placed in the insulating tube 3. The lower die 1 is inserted again, and the metal indium foil 103 is pressurized. Thereby, the electric power generation element 10 is formed. The pressure applied at this time is not particularly limited, but if an excessive pressure is applied, the metal indium creeps up the interface between the insulating tube 3 and the solid electrolyte layer 102 and may lead to a short circuit.

As shown in FIG. 2, the power generation element 10 includes a positive electrode mixture layer 101 (positive electrode), a solid electrolyte layer 102, and a metal indium foil 103 (negative electrode). The solid electrolyte layer 102 is disposed between the positive electrode mixture layer 101 and the metal indium foil 103. Specifically, the solid electrolyte layer 102 is in contact with each of the positive electrode mixture layer 101 and the metal indium foil 103. The positive electrode mixture layer 101 is composed of sulfide-based solid electrolyte particles 104 and lithium-containing transition metal oxide particles 106. The surface of each particle 106 is covered with a SiO ₂ film 105. A part of the surface of the particle 106 may be covered with the SiO ₂ film 105, or the entire surface of the particle 106 may be covered with the SiO ₂ film 105. The particles 106 covered with the SiO ₂ film 105 have a relatively large particle size (average particle size). The particles 104 have a relatively small particle size (average particle size). One particle 106 is surrounded by a plurality of particles 104. That is, not only the particles 106 but also the particles 104 are in contact with the SiO ₂ film. The particles 104 have an average particle diameter of, for example, several tens of nm to 1 μm. The particle 106 has an average particle diameter of 1 μm to 20 μm, for example.

  Further, the shape of the particles 104 and 106 is not particularly limited. Typically, particles 104 and 106 are spherical in shape. The particles 104 and 106 may have other shapes such as scales and fibers.

  The average particle size of the particles 104 and 106 means a particle size (D50) corresponding to 50% volume accumulation in the particle size distribution measured by a laser diffraction particle size meter. The average particle diameter can also be obtained by actually measuring the particle diameter (major axis) of particles (for example, arbitrary 10 particles) in the TEM image and calculating the average. The value obtained by the latter method generally agrees with the value obtained by the former method.

  After forming the power generation element 10, the lower die 1 and the upper die 2 are fixed with an insulating tube 4, a bolt 5, and a nut 6. Thereby, an all-solid lithium secondary battery is obtained. It is desirable that the process for producing the all-solid lithium secondary battery is also performed under a low dew point environment (for example, under a dew point environment of −40 ° C. or lower).

In this embodiment, metallic indium or metallic lithium is used as the negative electrode active material. However, the negative electrode active material is not limited to these, and a known negative electrode active material such as a carbon material, Li ₄ Ti ₅ O ₁₂ , Si, SiO, Sn, or SnO can be used. Examples of the carbon material include graphite and hard carbon. When these negative electrode active materials are used, step 3. As described above, a negative electrode mixture can be obtained by mixing a negative electrode active material and a solid electrolyte. Regarding the pressure at the time of molding, when using these negative electrode active materials, it is necessary to apply a larger pressure than when using an indium foil or a lithium foil.

[Step 5. Process for evaluating initial characteristics of all-solid lithium secondary battery]
Step 4. The initial charge / discharge characteristics of the all-solid-state lithium secondary battery produced in step 1 can be evaluated by constant current charge / discharge. For example, charging / discharging is performed at a current value corresponding to 0.05 C of the theoretical capacity calculated from the weight of the positive electrode active material (lithium-containing transition metal oxide) in the positive electrode mixture layer. The charge and discharge end voltages may be 3.6V and 2.4V, respectively. Charge / discharge efficiency (%) can be calculated by dividing the amount of electricity discharged by the amount of electricity charged.

  The effects of the present invention will be described in detail using the following examples and comparative examples. However, the technical scope of the present invention is not limited to the following examples.

Example 1
25 μL of a 20% perhydropolysilazane-xylene solution (manufactured by AZ Materials, NP110-20) was diluted with 1 mL of ultra-dehydrated xylene (manufactured by Wako Pure Chemical Industries, Ltd.). To the resulting solution, 2 g of lithium cobaltate (LiCoO ₂ ) particles were added with stirring. The BET specific surface area of the LiCoO ₂ particles was 0.35 m ² / g. The liquid component was evaporated by heating the solution on a hot plate at 60 ° C. while stirring until it was confirmed visually that the LiCoO ₂ particles had been dried. The obtained particles were put into an agate mortar, and the lump of particles was crushed and loosened. The above operation was performed in a dry air atmosphere with a dew point of −60 ° C. Next, a treatment for converting perhydropolysilazane into SiO ₂ was performed. Specifically, LiCoO ₂ particles having a perhydropolysilazane coating were heat-treated at 150 ° C. for 1 hour in an air atmosphere. The resulting mass of particles was again crushed and loosened. Thereby, LiCoO ₂ particles coated with the SiO ₂ film were obtained.

LiCoO ₂ particles were cross-sectioned and the thickness of the SiO ₂ film was measured using a transmission electron microscope. As a result, the thickness of the SiO ₂ film was 4 nm. In the observation with a transmission electron microscope, the surface of the particles was coated with osmium and carbon for the purpose of increasing the contrast of the image.

Further, along with the thickness measurement, energy dispersive X-ray spectroscopic analysis (TEM-EDS analysis) was performed at a plurality of positions near the LiCoO ₂ / SiO ₂ interface along the thickness direction of the SiO ₂ film. The target elements were Co and Si. The results are shown in FIG. In addition, the apparatus and measurement conditions used for the measurement are as follows.

(Measuring method)
Thinning: FIB (Focused Ion Beam) method
EDS (Energy dispersive X-ray spectrometry) acceleration voltage: 200kV
(measuring device)
FIB: Hitachi, Ltd. FB2000A
TEM / STEM: HF-2200 manufactured by Hitachi, Ltd.
EDS: NORAN System Seven manufactured by Thermo Fisher Scientific
(Characteristic X-ray)
Si and Co characteristic X-ray types and their wavelengths (eV)
Si-Kα: 1.739 keV
Si-Kβ: 1.836 keV
Co-Kα: 8.040 keV
Co-Kβ: 7.648 keV

The horizontal axis in FIG. 4 represents the distance (measurement position) in the thickness direction of the SiO ₂ film. The vertical axis in FIG. 4 represents the net count, that is, the signal intensity (peak intensity) of the K line attributed to each element detected at each position in the thickness direction of the SiO ₂ film. The K-ray signal intensity is the total value of the Kα-ray signal intensity and the Kβ-ray signal intensity. In the following, the phrase “signal intensity” means “signal intensity of K-rays”.

From the result of the TEM-EDS analysis shown in FIG. 4, the Si / Co signal intensity ratio (spectral intensity ratio) was calculated. The Si / Co signal intensity ratio was 0.34. Specifically, the average signal intensity of Co obtained in the LiCoO ₂ particle area (area A shown in FIG. 4) is defined as Im, and the maximum Si signal intensity obtained in the SiO ₂ film area is defined as Is. The Si / Co signal intensity ratio is represented by the ratio of the maximum signal intensity Is of Si to the average signal intensity Im of Co (Is / Im). As the average signal intensity Im of Co, the average value of the signal intensity of Co in the region A where LiCoO ₂ is considered to be present was used. The signal intensity Is of Si was generally low. Therefore, the value of the portion with the highest signal intensity near the interface was used as the maximum signal intensity Is of Si. Here, the “near the interface” means the vicinity of the intersection of the Si spectrum and the Co spectrum.

FIG. 3 is a transmission electron microscope image (TEM image) of the cross section of the LiCoO ₂ particles of Example 1. In the TEM image shown in FIG. 3, the contrast between the LiCoO ₂ particle region and the SiO ₂ film region was relatively clear.

Next, using the positive electrode active material of Example 1 (actual LiCoO ₂ particles having a SiO ₂ film formed using polysilazane), an all-solid lithium secondary battery was prepared by the method described below, and its initial characteristics Evaluated.

(1) Preparation of solid electrolyte 0.8 g of lithium sulfide (Li ₂ S) and 0.2 g of diphosphorus pentasulfide (P ₂ S ₅ ) were used for zirconia for a planetary ball mill (Fritsch, P-7 type). It put into the pot (45 mL of internal volume), and milling was performed on the conditions of 510 rpm and 8 hours. The particles obtained by milling were annealed under an inert atmosphere at 270 ° C. for 2 hours. Thus, solid electrolyte particles were obtained. The lithium ion conductivity of the obtained solid electrolyte particles was 8 × 10 ⁻⁴ S / cm.

(2) Preparation of positive electrode mixture LiCoO ₂ particles (positive electrode active material) having a SiO ₂ film formed using perhydropolysilazane and solid electrolyte particles are sufficiently mixed in an agate mortar to obtain a positive electrode mixture. Obtained. The weight ratio of the positive electrode active material to the solid electrolyte was 6: 4.

(3) Production of all-solid lithium secondary battery 80 mg of solid electrolyte particles were placed in a cell container, and the pressure of 2 MPa was applied to preform the solid electrolyte particles to obtain a solid electrolyte layer. 10 mg of the positive electrode mixture particles were put in a cell container so as to cover the solid electrolyte layer, and the positive electrode mixture particles were formed by applying a pressure of 18 MPa to obtain a positive electrode mixture layer. Thereafter, a metal indium foil (diameter 10 mm, thickness 200 μm) is disposed on the side facing the positive electrode mixture layer with the solid electrolyte layer interposed therebetween, and a pressure of 2 MPa is applied to the positive electrode mixture layer, solid electrolyte layer and metal indium foil. These were integrated. In this way, an all solid lithium secondary battery of Example 1 was obtained.

(4) Evaluation of initial characteristics of all-solid lithium secondary battery The initial characteristics of the all-solid lithium secondary battery were examined by the following method. The all-solid lithium secondary battery is charged to 3.6 V with a constant current of 41 μA (equivalent to 0.05 C), and after a pause of 20 minutes, the all-solid lithium secondary battery is discharged to 2.4 V with a constant current of 41 μA. It was. The all-solid lithium secondary battery of Example 1 had a charge electricity amount of 110 mAh / g, a discharge electricity amount of 89 mAh / g, and a charge / discharge efficiency of 80.5%.

(Example 2)
LiCoO ₂ particles of Example 2 were prepared in the same manner as Example 1 except that the amount of perhydropolysilazane was changed to 50 μL in order to change the thickness of the SiO ₂ film. In the LiCoO ₂ particles of Example 2, the thickness of the SiO ₂ film was 9 nm. The Si / Co signal intensity ratio was 0.28. Then, the all-solid-state lithium secondary battery of Example 2 was produced by the same method as Example 1. The all-solid lithium secondary battery of Example 2 had a charge electricity amount of 106 mAh / g, a discharge electricity amount of 84 mAh / g, and a charge / discharge efficiency of 79.2%.

(Example 3)
LiCoO ₂ particles of Example 3 were prepared in the same manner as in Example 1 except that the amount of perhydropolysilazane was changed to 100 μL in order to change the thickness of the SiO ₂ film. In the LiCoO ₂ particles of Example 3, the thickness of the SiO ₂ film was 18 nm. The Si / Co signal intensity ratio was 0.27. Thereafter, an all solid lithium secondary battery of Example 3 was produced in the same manner as in Example 1. The all-solid lithium secondary battery of Example 3 had a charge electricity amount of 103 mAh / g, a discharge electricity amount of 80 mAh / g, and a charge / discharge efficiency of 78.1%.

(Comparative Example 1)
An all solid lithium secondary battery of Comparative Example 1 was produced in the same manner as in Example 1 using LiCoO ₂ particles having no SiO ₂ film as a positive electrode mixture. The all-solid lithium secondary battery of Comparative Example 1 had a charge electricity amount of 89 mAh / g, a discharge electricity amount of 61 mAh / g, and a charge / discharge efficiency of 68.0%.

(Comparative Example 2)
Instead of perhydropolysilazane, a SiO ₂ film was formed on the surface of LiCoO ₂ particles by a sol-gel method using tetraethoxysilane. A method for forming a SiO ₂ film using tetraethoxysilane will be described below.

16 μL of tetraethoxysilane (manufactured by Kojundo Chemical Co., Ltd.) was diluted with 1 mL of ultra-dehydrated ethanol (manufactured by Wako Pure Chemical Industries, Ltd.). 1 μL of hydrochloric acid (manufactured by Wako Pure Chemical Industries, Ltd.) was added to the resulting solution, and 2 g of LiCoO ₂ particles were further added with stirring. The BET specific surface area of the LiCoO ₂ particles was 0.35 m ² / g. The liquid component was evaporated by heating the solution on a hot plate at 30 ° C. while stirring until it was confirmed visually that the LiCoO ₂ particles had been dried. The obtained particles were put into an agate mortar, and the lump of particles was crushed and loosened. The above operation was performed in a dry air atmosphere with a dew point of −60 ° C. Further, the obtained particles were heat-treated at 350 ° C. for 1 hour in an air atmosphere. The resulting mass of particles was again crushed and loosened. Thereby, LiCoO ₂ particles coated with the SiO ₂ film were obtained.

In the same manner as in Example 1, the thickness of the SiO ₂ film was measured using a transmission electron microscope. The thickness of the SiO ₂ film was 3 nm. Along with the film thickness measurement, TEM-EDS analysis in the vicinity of the LiCoO ₂ / SiO ₂ interface was performed along the thickness direction of the SiO ₂ film. The results are shown in FIG. The Si / Co signal intensity ratio was 0.09.

FIG. 5 is a TEM image of a cross section of the LiCoO ₂ particles of Comparative Example 2. The contrast between the LiCoO ₂ particle region and the SiO ₂ film region in the TEM image shown in FIG. 5 was weaker than the contrast between those regions in the TEM image shown in FIG.

Next, using the LiCoO ₂ particles of Comparative Example 2 as the material for the positive electrode mixture, an all solid lithium secondary battery of Comparative Example 2 was produced in the same manner as in Example 1. The all-solid lithium secondary battery of Comparative Example 2 had a charge electricity amount of 116 mAh / g, a discharge electricity amount of 76 mAh / g, and a charge / discharge efficiency of 65.4%.

(Comparative Example 3)
LiCoO ₂ particles of Comparative Example 3 were prepared in the same manner as Comparative Example 2, except that the amount of tetraethoxysilane added was changed to 37 μL in order to change the thickness of the SiO ₂ film. In the LiCoO ₂ particles of Comparative Example 3, the thickness of the SiO ₂ film was 7 nm. The Si / Co signal intensity ratio was 0.09. Thereafter, an all solid lithium secondary battery of Comparative Example 3 was produced in the same manner as in Example 1. The all-solid lithium secondary battery of Comparative Example 3 had a charge electricity amount of 110 mAh / g, a discharge electricity amount of 76 mAh / g, and a charge / discharge efficiency of 68.9%.

(Comparative Example 4)
LiCoO ₂ particles of Comparative Example 4 were prepared by the same method as Comparative Example 2, except that the amount of tetraethoxysilane added was changed to 80 μL in order to change the thickness of the SiO ₂ film. In the LiCoO ₂ particles of Comparative Example 4, the thickness of the SiO ₂ film was 15 nm. The Si / Co signal intensity ratio was 0.07. Then, the all-solid-state lithium secondary battery of the comparative example 4 was produced by the same method as Example 1. The all-solid lithium secondary battery of Comparative Example 4 had a charge electricity amount of 108 mAh / g, a discharge electricity amount of 74 mAh / g, and a charge / discharge efficiency of 68.9%.

  The results are shown in Table 1.

The batteries of Examples 1 to 3 and Comparative Examples 2 to 4 showed larger initial charge electricity and initial discharge electricity than the battery of Comparative Example 1. In other words, the charge / discharge characteristics could be improved by forming the SiO ₂ film on the surface of the LiCoO ₂ particles. Furthermore, the batteries of Examples 1 to 3 showed a larger initial discharge electricity quantity than the batteries of Comparative Examples 2 to 4. That is, when the SiO ₂ film was formed using perhydropolysilazane, the initial charge / discharge efficiency superior to the case where the SiO ₂ film was formed using tetraethoxysilane could be achieved.

Moreover, the charge / discharge efficiency of the batteries of Examples 1 to 3 was higher than the charge / discharge efficiency of the batteries of Comparative Examples 2 to 4. As a difference between the SiO ₂ film in Examples 1 to 3 and the SiO ₂ film in Comparative Examples 2 to 4, there is a difference in contrast between the SiO ₂ film and LiCoO ₂ particles in the TEM image. As shown in FIG. 3, the contrast between the SiO ₂ film and the LiCoO ₂ particles in the TEM image of the LiCoO ₂ particles of Example 1 was relatively strong. As shown in FIG. 5, the contrast between the SiO ₂ film and the LiCoO ₂ particles in the TEM image of the LiCoO ₂ particles of Comparative Example 2 was relatively weak. The strength of contrast in the TEM image reflects the high electron density. That is, it can be said from the TEM images of FIGS. 3 and 5 that the SiO ₂ films of Examples 1 to 3 are denser than the SiO ₂ films of Comparative Examples 2 to 4.

The denseness of the SiO ₂ film can also be known from the Si / Co signal intensity ratio of EDS analysis. As shown in FIG. 6, the Si signal of the SiO ₂ film formed using tetraethoxysilane was very weak. From this, it can be said that the SiO ₂ film of Comparative Example 2 is a film having a low atomic density of Si, that is, a low density of SiO ₂ .

From the results of Examples 1 to 3, when the Si / Co signal intensity ratio is 0.20 or more, a battery having a sufficiently low impedance at the solid electrolyte / active material interface and excellent initial charge / discharge efficiency can be obtained. It can be said. That is, it is important for improving the characteristics of the all-solid-state lithium secondary battery to form a uniform and dense SiO ₂ film on the surface of the lithium-containing transition metal oxide particles.

The results shown in Table 1 indicate that high initial charge / discharge efficiency can be achieved by forming a SiO ₂ film having a high Si / Co signal intensity ratio on the surface of LiCoO ₂ particles. The Si / Co signal intensity ratio is desirably 0.20 or more, and more desirably 0.27 or more. A dense SiO ₂ film having a high Si / Co signal intensity ratio could be formed by using perhydropolysilazane. As described above, the technology disclosed in the present specification effectively contributes to improving the initial charge / discharge amount of the all-solid lithium secondary battery and increasing the charge / discharge efficiency. The upper limit of the Si / Co signal intensity ratio is not particularly limited. From the results of the examples, “1” can be exemplified as the upper limit of the Si / Co signal intensity ratio. However, the Si / Co signal intensity ratio increases as the density of the SiO ₂ film increases. The possibility of exceeding 1 cannot be denied.

When perhydropolysilazane is used, the SiO ₂ film can be formed in a low temperature range from room temperature to 300 ° C. Therefore, interdiffusion of elements is unlikely to occur at the interface between the active material particles and the coating (SiO ₂ film or a film containing a precursor thereof).

In the sol-gel method, a film is formed using a sol solution containing an alkoxide of silicon such as tetraethoxysilane (Si (OC ₂ H ₅ ) ₄ ). The alkoxide contained in the coating is hydrolyzed and the hydrolyzate is polymerized. An SiO ₂ film is obtained by heat-treating the obtained gel film. Since the series of reactions is a condensation reaction, the film shrinks and microcracks are likely to enter the film. Therefore, it is difficult to obtain a dense SiO ₂ film by the sol-gel method. On the other hand, since the conversion reaction of perhydropolysilazane to SiO ₂ is a substitution reaction of nitrogen and oxygen, film shrinkage hardly occurs. Therefore, a dense SiO ₂ film can be obtained relatively easily.

As the results of Examples 1 to 3 show, the initial charge / discharge capacity and the initial charge / discharge efficiency tend to decrease as the thickness of the SiO ₂ film formed using perhydropolysilazane increases. This tendency indicates that the resistance value of the SiO ₂ film increases as the thickness of the SiO ₂ film increases. That is, in order to obtain a high initial charge / discharge capacity and initial charge / discharge efficiency, it is desirable that the thickness of the SiO ₂ film is appropriately controlled. The upper limit of the thickness of the SiO ₂ film is, for example, 20 nm, and desirably 18 nm. The lower limit of the thickness of the SiO ₂ film is, for example, 3 nm, and desirably 7 nm.

Even if other lithium-containing transition metal oxides other than LiCoO ₂ are used, the transition metal M (M is Mn, Ni, Co or Fe) included in the other lithium-containing transition metal oxides. It is presumed that the ratio (Is / Im) of the maximum signal intensity Is of the Si K-line to the average signal intensity Im shows the same tendency as LiCoO ₂ . That is, regardless of the composition of the lithium-containing transition metal oxide, the Si / M signal intensity ratio is desirably 0.20 or more, and more desirably 0.27 or more.

Regardless of the composition of the lithium-containing transition metal oxide, by forming a SiO ₂ film on the surface of the lithium-containing transition metal oxide particles using perhydropolysilazane, good initial charge / discharge efficiency can be obtained. can get.

According to the technique disclosed in this specification, a homogeneous and dense SiO ₂ film can be easily formed on the surface of lithium-containing transition metal oxide particles. If lithium-containing transition metal oxide particles having a dense SiO ₂ film are used as the positive electrode active material, the impedance of the solid electrolyte / active material interface in the all-solid lithium secondary battery is reduced, and the charge / discharge efficiency and charge / discharge are reduced. Capacity can be increased. As a result, an all-solid lithium secondary battery having a high energy density and high reliability can be provided. The all-solid lithium secondary battery can be used for various applications such as portable devices, hybrid vehicles, plug-in hybrid vehicles, electric vehicles, and stationary power sources.

DESCRIPTION OF SYMBOLS 1 Lower die 2 Upper die 3 Insulating tube 4 Insulating tube 5 Bolt 6 Nut 10 Power generation element 101 Positive electrode mixture layer 102 Solid electrolyte layer 103 Negative electrode layer 104 Solid electrolyte particle 105 SiO ₂ film 106 Lithium-containing transition metal oxide particle

Claims (12)

A sulfide-based solid electrolyte particle containing Li ₂ S;
Lithium-containing transition metal oxide particles;
An SiO ₂ film provided between the sulfide-based solid electrolyte particles and the lithium-containing transition metal oxide particles, and in contact with the surfaces of the lithium-containing transition metal oxide particles;
With
Along the direction crossing the interface between the SiO ₂ film and the lithium-containing transition metal oxide particles, using a transmission electron microscope, near the interface between the SiO ₂ film and the lithium-containing transition metal oxide particles. The energy dispersive X-ray spectroscopic analysis is performed at a plurality of positions, and the Kα ray signal intensity and Kβ of the transition metal M (M is Mn, Ni, Co or Fe) obtained in the region of the lithium-containing transition metal oxide particles. The total value of the signal intensity of the line is defined as the signal intensity of the transition metal M, and the total value of the signal intensity of the Kα ray and the Kβ line obtained in the region of the SiO ₂ film is defined as the Si signal. When defined as strength,
All-solid lithium secondary in which the ratio (Is / Im) of the maximum signal intensity Is which is the maximum value of the Si signal intensity to the average signal intensity Im which is the average value of the signal intensity of the transition metal M is 0.2 or more Battery positive electrode. The positive electrode for an all-solid-state lithium secondary battery according to claim 1, wherein the SiO ₂ film is formed using perhydropolysilazane. The positive electrode for an all solid lithium secondary battery according to claim 1, wherein the thickness of the SiO ₂ film is 18 nm or less.   The positive electrode for an all-solid-state lithium secondary battery according to any one of claims 1 to 3, wherein the ratio (Is / Im) is 0.27 or more. The positive electrode according to any one of claims 1 to 4,
A negative electrode,
A sulfide-based solid electrolyte disposed between the positive electrode and the negative electrode;
An all-solid-state lithium secondary battery. Forming a SiO ₂ film on the surface of the lithium-containing transition metal oxide particles;
Mixing the lithium-containing transition metal oxide particles having the SiO ₂ film and the sulfide-based solid electrolyte particles;
Including
Along the direction crossing the interface between the SiO ₂ film and the lithium-containing transition metal oxide particles, using a transmission electron microscope, near the interface between the SiO ₂ film and the lithium-containing transition metal oxide particles. The energy dispersive X-ray spectroscopic analysis is performed at a plurality of positions, and the Kα ray signal intensity and Kβ of the transition metal M (M is Mn, Ni, Co or Fe) obtained in the region of the lithium-containing transition metal oxide particles. The total value of the signal intensity of the line is defined as the signal intensity of the transition metal M, and the total value of the signal intensity of the Kα ray and the Kβ line obtained in the region of the SiO ₂ film is defined as the Si signal. When defined as strength,
All-solid lithium secondary in which the ratio (Is / Im) of the maximum signal intensity Is which is the maximum value of the Si signal intensity to the average signal intensity Im which is the average value of the signal intensity of the transition metal M is 0.2 or more A method for producing a positive electrode for a battery. In the step of forming the SiO ₂ film, using a perhydropolysilazane as a raw material for the SiO ₂ film, a positive electrode manufacturing method for all-solid lithium secondary battery according to claim 6. The step of forming the SiO ₂ film includes
Attaching perhydropolysilazane to the surfaces of the lithium-containing transition metal oxide particles;
Converting the perhydropolysilazane to SiO ₂ on the surfaces of the lithium-containing transition metal oxide particles by performing a heat treatment;
The manufacturing method of the positive electrode for all-solid-state lithium secondary batteries of Claim 6 or 7 containing these. The thickness of the SiO ₂ film is not more than 18 nm, any one positive electrode manufacturing method for all-solid lithium secondary battery according to claim 6-8.   The manufacturing method of the positive electrode for all-solid-state lithium secondary batteries of any one of Claims 6-9 whose said ratio (Is / Im) is 0.27 or more. A sulfide-based solid electrolyte particle containing Li ₂ S;
Lithium-containing transition metal oxide particles;
An SiO ₂ film provided between the sulfide-based solid electrolyte particles and the lithium-containing transition metal oxide particles, and in contact with the surfaces of the lithium-containing transition metal oxide particles;
With
The SiO ₂ film is a positive electrode for an all solid lithium secondary battery, which is formed using perhydropolysilazane. The positive electrode for an all-solid-state lithium secondary battery according to claim 11, wherein the thickness of the SiO ₂ film is 18 nm or less.