JP2013214421A - Carbon-solid electrolyte complex and manufacturing method of the same - Google Patents

Abstract

An all-solid-state lithium ion secondary battery using a solid electrolyte having lithium ion conductivity solves a problem related to a carbon material used as a negative electrode active material, and provides a negative electrode material having better performance.
A method of filling a conductive container with a mixture of a carbon material powder and a solid electrolyte powder, and sintering by applying a DC pulse current in a non-oxidizing atmosphere while the mixture is pressurized. According to the above, it is possible to obtain a carbon-solid electrolyte composite having improved lithium ion conductivity in which the carbon material powder and the solid electrolyte powder are joined to each other.
[Selection] Figure 2

Description

  The present invention relates to a composite of carbon and a solid electrolyte, a production method thereof, and an application thereof.

  Due to the high performance of portable electronic devices, hybrid vehicles, etc. in recent years, secondary batteries (particularly lithium ion secondary batteries) used therefor are required to have higher capacities. In the current lithium ion secondary battery, the increase in capacity of the positive electrode is delayed compared to the negative electrode, and the lithium nickelate-based material, which is said to have a relatively high capacity, is about 190 to 220 mAh / g.

  On the other hand, sulfur has a high theoretical capacity of about 1670 mAh / g and is one of the promising candidates for high capacity electrode materials. However, since sulfur alone does not contain lithium, lithium or an alloy containing lithium must be used for the negative electrode, and there is a disadvantage that the selection range of the negative electrode is narrow.

  On the other hand, since lithium sulfide contains lithium, alloys such as graphite and silicon can be used for the negative electrode, and the selection range of the negative electrode is greatly expanded, and a short circuit due to generation of dendrite by using metallic lithium. The dangers such as can be avoided. However, in a battery system using an organic electrolyte, lithium sulfide has a problem that it elutes into the electrolyte as lithium polysulfide at the time of charge / discharge (see Non-Patent Document 1 below). It is difficult to express capacity. Therefore, in order to improve the performance of a battery using lithium sulfide as the positive electrode, it is necessary to take measures such as devising an organic electrolytic solution for preventing elution and substituting with another electrolyte.

In the lithium ion conductive solid electrolyte, only Li ⁺ ions move in the solid. Therefore, in the all solid state battery using this for the electrolyte layer, when lithium sulfide is used for the positive electrode, the above-mentioned lithium polysulfide electrolyte is applied to the electrolyte. Elution can be prevented and the high capacity inherent in lithium sulfide can be realized. In fact, it has been reported that an all solid state battery using lithium sulfide as a positive electrode has no loss of active material due to elution and exhibits a relatively high discharge capacity (Patent Document 1 and Non-Patent Document 2 below).

In this all-solid-state battery, it is advantageous to use a sulfide-based electrolyte that has a relatively high electrical conductivity and does not easily react with lithium sulfide, which is a positive electrode active material, for example, a Li ₂ SP ₂ S _5- based electrolyte. is there. In addition, it is advantageous to use a carbon material such as graphite which shows a relatively low potential and does not easily react with the sulfide solid electrolyte for the negative electrode because the average voltage of the battery can be increased.

As the configuration of the all-solid-state battery, for example, as disclosed in the following Patent Document 2 and Non-Patent Document 2, each electrode layer is obtained by sandwiching a solid electrolyte layer and mixing and press-molding a positive electrode or negative electrode active material and a solid electrolyte. It becomes the structure which laminated | stacked each. Among them, the active material (Li ₂ S) and the sulfide solid electrolyte (Li ₂ SP ₂ S ₅ ) in the positive electrode layer are composed of similar elements, and their interface resistance is relatively low, but the negative electrode layer Active material (C) and sulfide solid electrolyte (Li ₂ SP ₂ S ₅ ) are composed of completely different elements, so a high resistance layer due to the space charge layer is formed at the interface between them ( Non-patent document 3 below). Therefore, in order to improve the performance of the battery, it is necessary to firmly join graphite and the solid electrolyte and reduce the electric resistance.

  As a method for improving the bonding between the active material and the solid electrolyte, a method of sintering both of them by heat treatment or the like is known, and for example, disclosed in Patent Documents 3 and 4 below. Further, as a method for reducing the interface resistance between the active material and the solid electrolyte, a method of covering the surface of the oxide positive electrode active material with a lithium ion conductive oxide is disclosed in Non-Patent Document 3 below. However, there is no report on a method for lowering electric resistance while improving the bonding between graphite and solid electrolyte for the negative electrode layer.

JP-A-6-275313 JP 2008-235227 A JP 2009-140911 JP 2011-192606

T. Takeuchi, H. Sakaebe, H. Kageyama, H. Senoh, T. Sakai, and K. Tatsumi, J. Power Sources, 195, 2928 (2010). T. Takeuchi, H. Kageyama, K. Nakanishi, M. Tabuchi, H. Sakaebe, T. Ohta, H. Senoh, T. Sakai, and K. Tatsumi, J. Electrochem. Soc., 157, A1196 (2010) . N. Ohta, K. Takada, L. Zhang, R. Ma, M. Osada, and T. Sasaki, Adv. Mater., 18, 2226 (2006).

  The present invention has been made in view of the current state of the prior art described above, and its main purpose is to be used as a negative electrode active material in an all solid lithium ion secondary battery using a solid electrolyte having lithium ion conductivity. An object of the present invention is to provide a negative electrode material having higher performance by solving the above-mentioned conventional problems relating to the carbon material.

  The present inventor has intensively studied to achieve the above-described object. As a result, carbon material powder such as graphite used as the negative electrode active material and solid electrolyte powder were used as raw materials, and this was filled in a conductive container, and a DC pulse current was applied under pressure in a non-oxidizing atmosphere. It has been found that according to the heating reaction method, a carbon-solid electrolyte composite in which the carbon material and the solid electrolyte are firmly bonded and the solid electrolyte is slightly reduced to increase the lattice volume can be obtained. And when this composite is used as a negative electrode layer of an all solid lithium ion secondary battery, the conductivity is improved, and it can exhibit excellent performance as a negative electrode layer for a high capacity all solid lithium secondary battery, It has been found that in an all-solid lithium ion secondary battery using a sulfide solid electrolyte as an electrolyte, it is possible to greatly improve cycle characteristics and output characteristics while utilizing the excellent performance of the composite. The present invention has been completed based on these findings.

That is, the present invention provides the following carbon-solid electrolyte composite, a method for producing the composite, and an all-solid lithium ion secondary battery including the composite.
Item 1. A mixture of a carbon material powder and a solid electrolyte powder is filled in a conductive container, and in a non-oxidizing atmosphere, the mixture is pressurized and sintered by applying a direct current pulse current. A method for producing a carbon-solid electrolyte composite.
Item 2. Item 2. The method according to Item 1, wherein the solid electrolyte is a sulfide-based solid electrolyte or an oxide-based solid electrolyte.
Item 3. Item 3. The method according to Item 1 or 2, wherein the mixture of the carbon material powder and the solid electrolyte powder contains 20 to 90% by weight of the carbon material powder based on the total amount of both.
Item 4. A composite of carbon material powder and solid electrolyte powder joined together,
(1) The amount of the carbon material is 20 to 90% by weight based on the total amount of the carbon material powder and the solid electrolyte powder,
(2) The tap density of the composite is 10% or more larger than the tap density of the mixture of the carbon material powder and the solid electrolyte powder used as the raw material.
A carbon-solid electrolyte composite characterized by the above.
Item 5. Item 5. The carbon-solid electrolyte composite according to item 4, wherein the solid electrolyte contained in the carbon-solid electrolyte composite has a lattice volume increased by 0.3% or more as compared with the unsintered solid electrolyte.
Item 6. Item 6. The carbon-solid electrolyte composite according to Item 4 or 5, wherein the solid electrolyte is a sulfide-based solid electrolyte or an oxide-based solid electrolyte, and the carbon material is graphite, mesoporous carbon, or a non-graphitizable carbon material.
Item 7. 7. A negative electrode material for an all solid lithium ion secondary battery comprising the carbon-solid electrolyte composite according to any one of Items 4 to 6.
Item 8. Item 8. An all solid lithium ion secondary battery having a negative electrode layer made of the negative electrode material according to Item 7.

  Hereinafter, first, the method for producing the carbon-solid electrolyte composite of the present invention will be described.

Method for Producing Carbon-Solid Electrolyte Composite The carbon-solid electrolyte composite of the present invention is obtained by filling a conductive container with a mixture of carbon material powder and solid electrolyte powder, and subjecting the mixture to a non-oxidizing atmosphere. It can be obtained by applying a direct current pulse current and sintering in a pressurized state. According to this method, the carbon material powder and the solid electrolyte powder are firmly joined to reduce the electrical resistance at the interface, and the solid electrolyte is slightly reduced. Hereinafter, this method will be specifically described.

(I) Carbon Material Powder In the present invention, the carbon material used as a raw material can be used without particular limitation as long as it can be used as a negative electrode active material in an all-solid lithium ion secondary battery. Specific examples of such a carbon material include graphite, mesoporous carbon, hard carbon (non-graphitizable carbon material), and the like.

  The particle size of the carbon material powder is not particularly limited, but is usually about 0.01 to 100 μm, preferably about 0.05 to 50 μm as an average particle size. In the present specification, the average particle size is a particle size at which the cumulative frequency distribution is 50% in the particle size distribution measurement by a dry laser diffraction / scattering method.

(Ii) solid electrolyte powder ,
The kind of the solid electrolyte powder is not particularly limited as long as it is a solid electrolyte having lithium ion conductivity that can be used in an all solid lithium ion secondary battery. Typical examples of such solid electrolytes include sulfide-based solid electrolytes and oxide-based solid electrolytes.

Among these, as the sulfide-based solid electrolyte, Li ₂ SP ₂ S ₅ solid electrolyte (for example, Li ₇ P ₃ S _{11 in} a 7: 3 ratio), Li ₂ S-SiS ₂ -Li ₃ PO ₄ -Based solid electrolyte (material with a molar ratio of Li ₂ S: SiS ₂ : Li ₃ PO ₄ = 63: 36: 1), thiolithicone-based solid electrolyte (Li ₄ SiS ₄ , Li ₄ GeS ₄ , Li ₃ PS ₄ Examples of such compounds include Li _3.25 Ge _0.25 P _0.75 S ₄ and Li ₁₀ GeP ₂ S ₁₂ . Oxide-based solid electrolytes include perovskite type ionic conductors (Li _0.35 La _0.55 TiO ₃ etc.), riccon type ionic conductors (Li ₁₄ Zn (GeO ₄ ) ₄ etc.), garnet type ionic conductors (Li ₇ La ₃ Zr ₂ O ₁₂ ) and the like.

  The solid electrolyte powder used in the production of the carbon-solid electrolyte composite includes a solid electrolyte used as an electrolyte for an all-solid lithium ion secondary battery in order to prevent a decrease in conductivity due to a reaction at the interface between the solid electrolyte layer and the negative electrode layer. It is preferable to use the same kind of solid electrolyte powder. For example, when a sulfide-based solid electrolyte is used as the solid electrolyte, it is preferable to use a sulfide-based solid electrolyte as the solid electrolyte powder used in the production of the carbon-solid electrolyte composite.

In the present invention, it is preferable to use a sulfide-based solid electrolyte in terms of high lithium ion conductivity. In particular, in a high-capacity all-solid-state lithium ion secondary battery using a sulfur-based material (Li ₂ S or the like) as a positive electrode active material, a solid electrolyte is used to suppress a decrease in lithium ion conductivity due to reaction with the positive electrode material. It is preferable to use a sulfide-based solid electrolyte as the layer. From this point, it is preferable to use a sulfide-based solid electrolyte powder as a raw material for the carbon-solid electrolyte composite.

  The particle diameter of the solid electrolyte powder is not particularly limited, but it is usually preferable to use a powder having an average particle diameter of about 0.1 to 50 μm.

(Iii) Manufacturing method of composite In the manufacturing method of the carbon-solid electrolyte composite of the present invention, first, a starting material composed of a carbon material powder and a solid electrolyte powder is sufficiently mixed and then filled into a conductive container. In a non-oxidizing atmosphere, with the mixture being pressurized, the raw material is obtained by an electric current sintering method in which a direct current pulse current is passed, which is called a discharge plasma sintering method, a pulse current sintering method, a plasma activated sintering method, etc. Sinter the mixture. Thereby, the target carbon-solid electrolyte composite can be obtained.

  Specifically, a container having electron conductivity is filled with a mixture of a carbon material powder as a raw material and a solid electrolyte powder, and a pulsed ON-OFF direct current is applied while applying pressure in a non-oxidizing atmosphere. , Current sintering can be performed.

The electric current sintering is performed in a non-oxidizing atmosphere, for example, in an inert gas atmosphere such as Ar or N ₂ or in a reducing atmosphere such as H ₂ . Further, a reduced pressure state in which the oxygen concentration is sufficiently low, for example, a reduced pressure state in which the oxygen partial pressure is about 20 Pa or less may be used.

  In the case of using a container that can ensure a sufficiently sealed state as a conductive container, the inside of the container may be a non-oxidizing atmosphere. In addition, the conductive container may not be completely sealed. When an incompletely sealed container is used, the container is accommodated in the reaction chamber, and the reaction chamber is filled with an inert gas atmosphere. A non-oxidizing atmosphere such as a reducing atmosphere may be used. Thereby, the reaction between the carbon material powder and the solid electrolyte powder can be performed in a non-oxidizing atmosphere. In this case, for example, the inside of the reaction chamber is preferably an inert gas atmosphere or a reducing gas atmosphere of about 0.1 MPa or more.

  The mixing ratio of the carbon material powder and the solid electrolyte powder is preferably about 20 to 90% by weight, more preferably about 40 to 80% by weight, based on the total amount of both. More preferably, it is about 50 to 70% by weight. By setting the mixing ratio in this range, a negative electrode material having both high energy density and good lithium ion conductivity can be obtained.

  The container having electron conductivity is not particularly limited as long as it has electron conductivity. Carbon, iron, iron oxide, copper, aluminum, tungsten carbide, carbon and / or iron oxide mixed with silicon nitride. What is formed from etc. can be used conveniently.

  By applying a direct current pulse current in such a state that the carbon conductive material and the solid electrolyte mixed powder are filled in such an electron conductive container, a discharge phenomenon that occurs in the particle gap of the filled mixed powder is utilized. Carbon material powder and solids act as the driving force for particle bonding, such as the activation of the purification of the particle surface by plasma, discharge shock pressure, etc., the electric field diffusion effect caused by the electric field, the thermal diffusion effect by Joule heat, and the plastic deformation pressure by pressurization The electrolyte powder is firmly bonded. At the same time, the solid electrolyte is reduced by conducting current sintering in a reducing atmosphere.

  The apparatus for conducting the current sintering is not particularly limited as long as the mixed powder of the carbon material powder and the solid electrolyte powder can be heated, cooled, pressurized, and the like and can apply a current necessary for discharge. For example, a commercially available electric current sintering apparatus (discharge plasma sintering apparatus) can be used. Such an electric current sintering apparatus and its principle are disclosed in, for example, Japanese Patent Laid-Open No. 10-251070.

  A specific example of the method for producing a carbon-solid electrolyte composite of the present invention will be described below with reference to FIG. 1 showing a schematic diagram of an electric current sintering apparatus.

  The electric sintering apparatus 1 includes a die (electron conductive container) 3 on which a sample 2 is loaded and a pair of upper and lower punches 4 and 5. The punches 4 and 5 are supported by punch electrodes 6 and 7, respectively, and a pulse current is supplied through the punch electrodes 6 and 7 while applying pressure to the sample 2 loaded in the die 3 as necessary. Can do. The material of the die 3 is not limited, and examples thereof include a carbon material such as graphite.

  In the apparatus shown in FIG. 1, the energization part including the above-described container 3 having electron conductivity, energization punches 4 and 5, and punch electrodes 6 and 7 is accommodated in a water-cooled vacuum chamber 8. It can be adjusted to a predetermined atmosphere by the control mechanism 15. Therefore, the atmosphere control mechanism 15 may be used to adjust the inside of the chamber to a non-oxidizing atmosphere.

  The control device 12 drives and controls the pressurization mechanism 13, the pulse power source 11, the atmosphere control mechanism 15, the water cooling mechanisms 16 and 10, and the temperature measurement device 17. The control device 12 is configured to drive the pressurizing mechanism 13 so that the punch electrodes 6 and 7 pressurize the raw material mixture at a predetermined pressure.

  The condition for the energization treatment may be a condition for forming a target composite having a strong bond. The temperature (heating temperature) of the die (electron conductive container) 3 at the time of specific energization treatment can be appropriately selected according to the types of carbon material powder and solid electrolyte powder used as raw materials and the particle diameter thereof. In general, the temperature may be about 50 to 800 ° C., preferably about 100 to 700 ° C. If the heating temperature is less than 50 ° C., the bonding between the carbon material powder and the solid electrolyte powder may be insufficient, and the reduction reaction of the solid electrolyte powder may not proceed sufficiently. On the other hand, when the heating temperature is higher than 800 ° C., the reduction of the solid electrolyte due to the reduction effect of the carbon powder or the electron conductive container proceeds excessively, which is not preferable. Accordingly, a heating temperature of about 100 to 700 ° C. is suitable.

  The pulse current applied for heating can be, for example, a pulsed ON-OFF direct current having a pulse width of about 2 to 3 milliseconds and a period of about 3 Hz to 300 kHz. Although the specific current value varies depending on the type and size of the electron conductive container, the specific current value may be determined so as to be in the temperature range described above. For example, when a graphite mold with an inner diameter of 15 mm is used, about 200 to 1000 A is preferable, and when a mold with an inner diameter of 100 mm is used, about 1000 to 8000 A is preferable. During processing, the current value may be controlled so that a predetermined temperature can be managed by increasing or decreasing the current value while monitoring the mold material temperature.

  The electric current sintering is preferably performed in a state where a raw material powder composed of a carbon material powder and a solid electrolyte powder is pressurized. As a specific method, for example, the raw material powder filled in the electron conductive container 3 may be pressurized through the punch electrodes 6 and 7. The pressure at the time of pressurizing the raw material powder is, for example, about 5 to 60 MPa, preferably about 10 to 50 MPa. A pressure of less than 5 MPa is not preferable because the bonding between the carbon material powder and the solid electrolyte powder becomes insufficient.

  The sintering time by electric current sintering varies depending on the amount of raw materials used, the sintering temperature, etc., and thus cannot be specified in general, but it is usually sufficient to heat until reaching the heating temperature range described above, and the temperature range described above. If it reaches | attains, you may cool immediately, or you may hold | maintain in this temperature range, for example to about 2 hours.

  After conducting the electric current sintering treatment at a predetermined temperature by the method described above, the electron conductive container is cooled, the formed composite is taken out from the container, and lightly pulverized with a mortar or the like as necessary. The carbon-solid electrolyte complex to be recovered can be recovered. When a large amount of current sintering treatment is performed, a large mold material is used, and the above process may be scaled up.

Carbon-solid electrolyte composite The carbon-solid electrolyte composite obtained by the above-described method is not a state in which the carbon material and the solid electrolyte are simply mixed, but a composite in which the two are firmly joined together. The density is greatly increased compared to Specifically, the tap density of the composite is 10% or more larger than the tap density of the mixture of the carbon material powder and the solid electrolyte powder used as the raw material. The upper limit of the tap density increase is not particularly limited, and it varies depending on the temperature, pressure, etc. during pressure electric current sintering, but usually increases to about 50% compared to the tap density of the raw material mixture. It becomes.

  The tap density in the present specification is as follows. After crushing a sample in a glove box with a dew point of -80 ° C in an argon gas atmosphere for 10 minutes or more in a mortar, collect about 1.2 g and put it into a 10 mL graduated cylinder. This is a value obtained by measuring the density after tapping 100 times.

  The ratio of the carbon material and the solid electrolyte in the composite is the same as the mixing ratio of the carbon material powder and the solid electrolyte powder in the raw material, and the amount of the carbon material is about 20 to 90% by weight based on the total amount of both. It is preferably about 40 to 80% by weight, more preferably about 50 to 70% by weight.

  The carbon-solid electrolyte composite obtained by the above-described method is obtained by reducing the solid electrolyte by conducting current sintering in a non-oxidizing atmosphere and introducing some electrons into the solid electrolyte. Binding energy between ions (sulfide ions, oxide ions, etc.) forming a skeleton structure of the solid electrolyte is lowered, and the mobility of lithium is increased. In addition, the concentration of lithium ions is slightly reduced, thereby creating vacancies at the lithium sites, increasing the mobility of lithium ions, in addition, increasing the ionic radius of the constituent elements of the solid electrolyte, and increasing the lattice volume. Increasingly, the size of the bottleneck in the lithium carrier path has increased and the lithium ion mobility has further increased. Usually, an increase of 0.3% or more in the lattice volume is recognized as compared with an unsintered solid electrolyte used as a raw material. The upper limit of the increase in the lattice volume varies depending on the type of the solid electrolyte, and thus cannot be defined unconditionally, but usually increases to about 10%.

  Thus, in the composite of the present invention, it is considered that the ion radius of the constituent element of the solid electrolyte is increased, the size of the bottleneck in the carrier path is increased, and the lithium ion conductivity is improved.

Use of carbon-solid electrolyte composite The carbon-solid electrolyte composite obtained by the method of the present invention is obtained by firmly joining a carbon material used as a negative electrode active material and a solid electrolyte, and is compared with a simple mixture. Then, the electrical resistance at the interface between the two is reduced. In addition, by strengthening the bonding between the carbon material and the solid electrolyte, separation from the solid electrolyte due to expansion and contraction of the carbon material accompanying charge / discharge is suppressed, and cycle characteristics are improved. Furthermore, the lattice volume of the solid electrolyte is increased by the electric current sintering, thereby improving the lithium ion conductivity.

  The carbon-solid electrolyte composite of the present invention has such excellent characteristics and can be effectively used as a negative electrode material for forming a negative electrode layer of an all-solid lithium ion secondary battery.

  The structure of the all solid lithium ion secondary battery using the carbon-solid electrolyte composite of the present invention is not particularly limited, and may be the same as a conventionally known one. The basic structure is a structure in which a positive electrode layer and a negative electrode layer are laminated with a lithium ion conductive solid electrolyte layer interposed therebetween, and the carbon-solid electrolyte composite of the present invention may be used as a negative electrode layer. In this case, as the lithium ion conductive solid electrolyte, a sulfide-based solid electrolyte, an oxide-based solid electrolyte, or the like can be used in the same manner as the solid electrolyte powder used as a raw material of the carbon-solid electrolyte composite. In particular, it is preferable to use the same type of solid electrolyte as that of the carbon-solid electrolyte complex in order to prevent a decrease in conductivity at the interface between the negative electrode layer made of the carbon-solid electrolyte complex and the solid electrolyte layer.

In the positive electrode layer, a known active material such as Li ₂ S, Li ₂ S ₂ , Li ₂ S ₄ , Li ₂ S ₈ or the like can be used as the positive electrode active material. Usually, these positive electrode active materials are used by mixing with a solid electrolyte.

  The carbon-solid electrolyte composite of the present invention is a composite in which a carbon material and a solid electrolyte are firmly joined and the solid electrolyte is slightly reduced, and this composite is used as a negative electrode layer of an all-solid lithium ion secondary battery. By using it, lithium ion conductivity improves and it can be set as a high capacity | capacitance all-solid-state lithium secondary battery. In particular, in an all solid lithium ion secondary battery using a sulfide solid electrolyte as an electrolyte, it is possible to greatly improve cycle characteristics and output characteristics by utilizing the excellent performance of the composite. The all-solid-state lithium ion secondary battery using the carbon-solid electrolyte composite of the present invention as a negative electrode layer can maintain a high discharge capacity even when the discharge current density is high, and has good rate characteristics. It is.

  Therefore, the carbon-solid electrolyte composite of the present invention is highly useful as a negative electrode material for an all solid lithium ion secondary battery.

  Moreover, according to the production method of the present invention, a composite having such excellent performance can be produced relatively easily.

Schematic of an example of an electric current sintering apparatus. The graph which shows the charging / discharging characteristic of the all-solid-state lithium ion secondary battery in Example 1 and Comparative Example 1. FIG. The graph which shows the rate characteristic of the all-solid-state lithium ion secondary battery in Example 1 and Comparative Example 1. FIG.

  The present invention will be specifically described below with reference to examples and comparative examples.

Example 1
0.3 g of commercially available graphite powder (average particle size 20 μm) and sulfide solid electrolyte (Li ₇ P ₃ S ₁₁ ) powder (average particle size 5 μm) 0.2 g (graphite: sulfide solid electrolyte = 3: 2 (weight ratio)) Was weighed in a glove box (dew point -80 ° C.) in an argon gas atmosphere, sealed in a zirconia pot, thoroughly mixed with a planetary ball mill, and filled in a graphite mold with an inner diameter of 15 mm in the glove box.

  Next, the graphite mold filled with the raw material was accommodated in an electric current sintering machine. The current-carrying part including the graphite mold and electrode part is housed in a vacuum chamber, and the chamber is filled with high-purity argon gas (oxygen concentration: about 0.2 ppm) to atmospheric pressure after vacuum (about 20 Pa) deaeration. did.

  Thereafter, a pulse current of about 100 A (pulse width 2.5 milliseconds, period 28.6 Hz) was applied while pressurizing the raw material filled in the graphite mold at about 30 MPa. The vicinity of the graphite mold was heated at a rate of about 100 ° C./min, and reached 100 ° C. 1 minute after the start of pulse current application. Then, after maintaining at 100 ° C. for 3 minutes, the current application and pressurization were stopped and the mixture was allowed to cool naturally.

  After cooling to room temperature, the graphite jig was transferred to a glove box with an argon gas atmosphere having a dew point of −80 ° C., and the composite of graphite and sulfide solid electrolyte was taken out of the mold. Samples were all crushed in the glove box.

In the X-ray diffraction pattern of the obtained composite, peaks derived from carbon and Li ₇ P ₃ S ₁₁ were observed, no other impurities were observed, and it was confirmed that they consisted of carbon and Li ₇ P ₃ S ₁₁ did it. The lattice constant of Li ₇ P ₃ S ₁₁ estimated by Rietveld analysis is a = 12.469 (4) Å, b = 6.0534 (15) Å, c = 12.516 (4) 4, and the lattice volume is V = 827.8 (4) a Å ^3, compared with the above-described electric current sintering pretreatment values (a = 12.460 (3) Å , b = 6.0386 (11) Å, c = 12.496 (3) Å, V = 824.5 (3) Å 3) It was found that Li ₇ P ₃ S ₁₁ was reduced by the energization treatment.

In addition, after pulverizing the obtained composite in a glove box with an argon gas atmosphere having a dew point of −80 ° C., about 1.2 g was collected, put into a 10 mL capacity graduated cylinder, tapped 100 times, and the density was adjusted. It was measured. As a result, the tap density was 0.79 g / cm ³ , which was increased by about 16% compared to the tap density (0.68 g / cm ³ ) of the raw material mixture before electric sintering measured in Comparative Example 1 described later. It was shown that the graphite and the sulfide solid electrolyte were joined and the density was increased.

Using the composite obtained by the above method as a negative electrode layer of an all-solid-state lithium ion secondary battery, using a lithium sulfide-carbon composite as a positive electrode, a titanium mesh as a current collector, and Li ₇ P ₃ S ₁₁ as an electrolyte layer An all-solid-state battery was constructed, and a charge / discharge test was performed by starting charging with constant current measurement at a cutoff of 0.5 to 3.5 V at a current density of 11.7 mA / g.

  The charge / discharge characteristics are as shown in FIG. 2. The initial discharge capacity is about 680 mAh / g, the discharge capacity after 10 cycles is about 670 mAh / g (capacity maintenance rate is about 98%). Compared to the value when the raw material mixture before electro-sintering measured in step 1 is used as the negative electrode layer (Figure 2, initial discharge capacity of about 710 mAh / g, discharge capacity after 10 cycles of about 500 mAh / g, capacity maintenance rate of about 70%) In particular, the cycle characteristics were remarkably improved.

  Furthermore, the rate characteristics are as shown in FIG. 3, and the discharge capacity is about 750 mAh / g at 0.01 C, about 720 mAh / g at 0.1 C, and about 600 mAh / g at 0.2 C, and the energization measured in Comparative Example 1 described later. Compared to the value when the raw material mixture before sintering is the negative electrode layer (approximately 750 mAh / g at 0.01 C, approximately 510 mAh / g at 0.1 C, approximately 320 mAh / g at 0.2 C), the discharge current density is high However, it was confirmed that the high discharge capacity was maintained and the rate characteristics were excellent.

  From the above results, it was confirmed that the cycle characteristics and rate characteristics can be greatly improved by combining graphite and a sulfide solid electrolyte by the electric current sintering method under the conditions employed in the present invention.

Comparative Example 1
As the negative electrode layer material, the same graphite and sulfide solid electrolyte (Li ₇ P ₃ S ₁₁ ) used in Example 1 were mixed in a ratio of graphite: sulfide solid electrolyte = 3: 2 (weight ratio). Using the mixture, an all-solid battery was produced in the same manner as in Example 1 without conducting current sintering, and a charge / discharge test was performed under the same conditions as in Example 1.

In the X-ray diffraction pattern of this mixture, as in Example 1, peaks derived from carbon and Li ₇ P ₃ S ₁₁ were observed, no other impurities were observed, and from carbon and Li ₇ P ₃ S ₁₁ It was confirmed that The lattice constant of Li ₇ P ₃ S ₁₁ estimated by Rietveld analysis is a = 12.437 (4) Å, b = 6.0428 (14) Å, c = 12.491 (4) Å, and the lattice volume is V = 823.3 (4) a Å ^3, before mixing value (a = 12.460 (3) Å , b = 6.0386 (11) Å, c = 12.496 (3) Å, V = 824.5 (3) Å 3) and shows good agreement It was found that the electrolyte was not reduced by mixing alone.

Further, the tap density of this mixture was measured in the same manner as in Example 1. As a result, it was 0.68 g / cm ³ , and it was confirmed that it was a low-density mixture as compared with the composite obtained in Example 1. It was.

  A charge / discharge test was conducted in the same manner as in Example 1 except that this mixture was used as the negative electrode layer. The charge / discharge characteristics are as shown in FIG. 2. The initial discharge capacity is about 710 mAh / g, the discharge capacity after 10 cycles is about 500 mAh / g, and the capacity retention rate is about 70%. The composite obtained in Example 1 is Compared with the negative electrode layer, the cycle characteristics were inferior. Furthermore, the rate characteristics are about 750 mAh / g at 0.01 C, about 510 mAh / g at 0.1 C, and about 320 mAh / g at 0.2 C, compared with the composite obtained in Example 1 as the negative electrode layer. Then, as the discharge current density increased, the discharge capacity decreased significantly, resulting in poor rate characteristics. From the above, it was found that the graphite and sulfide solid electrolyte could not be joined firmly only by mixing, and the electrolyte could not be reduced, and the performance of the all solid lithium ion battery could not be improved.

1 Electric current sintering equipment 2 Sample 3 Die (conductive container)
4, 5 Punch 6, 7 Punch electrode 8 Water-cooled vacuum chamber 9 Cooling water channel 10, 16 Water cooling mechanism 11 Power source for sintering 12 Controller 13 Pressurizing mechanism 14 Position measuring mechanism 15 Atmosphere controlling mechanism 17 Temperature measuring apparatus

Claims (8)

A mixture of a carbon material powder and a solid electrolyte powder is filled in a conductive container, and in a non-oxidizing atmosphere, the mixture is pressurized and sintered by applying a direct current pulse current. A method for producing a carbon-solid electrolyte composite. The method according to claim 1, wherein the solid electrolyte is a sulfide-based solid electrolyte or an oxide-based solid electrolyte. The method according to claim 1 or 2, wherein the mixture of the carbon material powder and the solid electrolyte powder contains 20 to 90% by weight of the carbon material powder based on the total amount of both. A composite of carbon material powder and solid electrolyte powder joined together,
(1) The amount of the carbon material is 20 to 90% by weight based on the total amount of the carbon material powder and the solid electrolyte powder,
(2) The tap density of the composite is 10% or more larger than the tap density of the mixture of the carbon material powder and the solid electrolyte powder used as the raw material.
A carbon-solid electrolyte composite characterized by the above. The carbon-solid electrolyte composite according to claim 4, wherein the solid electrolyte contained in the carbon-solid electrolyte composite has a lattice volume increased by 0.3% or more compared to the unsintered solid electrolyte. The carbon-solid electrolyte composite according to claim 4 or 5, wherein the solid electrolyte is a sulfide-based solid electrolyte or an oxide-based solid electrolyte, and the carbon material is graphite, mesoporous carbon, or a non-graphitizable carbon material. A negative electrode material for an all-solid-state lithium ion secondary battery comprising the carbon-solid electrolyte composite according to any one of claims 4 to 6. An all solid lithium ion secondary battery having a negative electrode layer made of the negative electrode material according to claim 7.

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