JP2009140910A - All-solid battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an all-solid battery for charge and discharge operation even with all solid by expanding an interface area where an electrode active material and an all-solid electrolyte are jointed and by lowering the interface reaction resistance. <P>SOLUTION: The all-solid battery combining an electrode active material and a solid electrolyte includes a solid electrolyte calcined body 14 of plate-shape consisting of ceramics containing a solid electrolyte 12, a first electrode layer 18 (for example, a positive electrode) which is formed by calcining and integrating on one face of the solid electrolyte calcined body 14 and by mixing and heating calcination of an electrode active material 16 and the solid electrolyte 12, and a second electrode layer 20 (for example, a negative electrode) which is formed by calcining and integrating on the other face of the solid electrolyte calcined body 14 and by mixing and heating calcination of the electrode active material 16 and the solid electrolyte 12. The solid electrolyte material added in the first electrode layer 18 and the second electrode layer 20 is an amorphous polyanion compound. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an all-solid battery using a combination of an electrode active material and a solid electrolyte.

  In recent years, with the development of portable devices such as personal computers and mobile phones, the demand for batteries as power sources has been greatly expanded. In a battery used for such an application, a liquid electrolyte (electrolytic solution) such as an organic solvent using a flammable organic solvent as a diluent solvent has been conventionally used as a medium for moving ions. A battery using such an electrolytic solution may cause problems such as leakage of the electrolytic solution, ignition, and explosion.

  In order to solve these problems, in order to ensure intrinsic safety, development of an all-solid-state battery in which a solid electrolyte is used instead of a liquid electrolyte and all other elements are made of solid is being promoted. ing. Such an all-solid battery is made of sintered ceramics whose electrolyte is solid, so there is no concern of ignition or leakage, and problems such as deterioration of battery performance due to corrosion are unlikely to occur. is there. In particular, all-solid lithium secondary batteries have been actively studied in various fields as secondary batteries that can easily have a high energy density (see, for example, Patent Documents 1 and 2 and Non-Patent Document 1). ).

JP 2000-311710 A JP 2005-63958 A Yusuke Fukushima and 4 others, "Preparation of electrode-electrolyte interface of all-solid-state battery using soft fusion of Li2S-P2S5 glass electrolyte" Vol. 9th, Page. Issued 51-52 2007.6.11

  Patent Document 2 described above discloses a thin film solid lithium ion secondary battery. The secondary battery described in Patent Document 2 is a thin film battery characterized in that a positive and negative active material is formed by sputtering on a surface of a flexible solid electrolyte and can be bent. Since this battery is realized by forming the electrode with a thin film, there is a problem that it is difficult to obtain a capacity due to the amount of active material used for the electrode portion.

  Next, Non-Patent Document 1 reports on the production of an electrode-electrolyte interface using a composite of an electrolyte and an electrode active material utilizing softening and fusion of a glass electrolyte. In this report, it has been reported that the resistance between the electrolyte particles is reduced due to the fusion effect of the glass electrolyte. There is a report that no heterogeneous phase has been confirmed for the reaction between the electrolyte and the active material.

  However, there is no description of a configuration having positive and negative electrodes as an all-solid-state battery, and it is unclear whether the reaction resistance has been reduced at the electrolyte-electrode active material interface. There is also no particular description of the relationship, and it is unclear as to whether or not charge / discharge is possible in an all solid state battery. In addition, it uses sulfide electrolytes and is expected to be unstable to the atmosphere (air). There is a possibility that toxic gas may be generated when exposed to the atmosphere due to damage or the like, and there has been a problem that safety has not been established.

  The problem to be solved by the present invention is the resistance (hereinafter referred to as interfacial reaction resistance) when electrons and Li ions move along the charge / discharge at the interface between the electrode active material that occupies one end of the internal resistance of the battery and the solid electrolyte. The present invention relates to a technique for reducing the resistance in an all-solid battery system using a solid electrolyte.

  For example, in a lithium ion battery using a conventional electrolytic solution, the electrode active material is a solid, but the electrolyte is a liquid dissolved in an organic solvent, so that it can easily penetrate between the particles of the electrode active material. An electrolyte network is formed in the electrode layer as described above, and a low interfacial reaction resistance is realized.

  Regarding the interfacial reaction resistance which is a problem in the present invention, the reaction resistance per unit area to be joined at the particle level is determined to some extent by the combination of the active material and the electrolyte material to be used. The larger the area that is joined between the particles, the more resistance is connected in parallel in terms of equivalent circuit, and the interfacial reaction resistance when viewed from the whole battery is lowered, and the internal resistance as a battery is lowered. become. Therefore, as a method of reducing the interfacial reaction resistance between the electrolyte and the active material, (1) selecting a combination of materials that can move Li ions more smoothly, and (2) electrolyte in the same electrode volume— This is done by two points of expanding the interface area where the active materials are joined.

  The present invention relates to (1) using a combination of an electrode active material and a solid electrolyte having a common polyanion, or a combination of an electrode active material comprising a phosphoric acid compound and a solid electrolyte. When a network is formed in the electrode layer with the mixed electrolyte, the interfacial area where the electrode active material and the solid electrolyte are joined is drastically increased, so that the interfacial reaction resistance can be lowered.

  On the other hand, Patent Document 1 discloses a “solid electrolyte battery characterized by interposing an inorganic oxide made of a solid electrolyte material between particles of an electrode active material so as to form a three-dimensional network”. . Therefore, the present inventors selected a combination of an electrode active material made of a phosphate compound having a common polyanion and a solid electrolyte as a combination of materials that can move Li ions more smoothly. An attempt was made to produce an all-solid battery having an electrode structure interposed between electrode active material particles. However, the result is that the electrode layer is baked in a mixed state with the electrode active material, so that the active material reacts with the electrolyte and the peak intensity of the electrode active material is reduced or a different phase is generated on XRD (X-ray diffraction) observation. A phenomenon that happened. As a result of measuring the charge / discharge capacity of the active material in an ideal system using the active material in this state with an electrolyte, the chargeable / dischargeable capacity is greatly reduced, and the active material can be charged / discharged up to the theoretical capacity. Can no longer charge or discharge. That is, there is a problem that the capacity of the electrode active material itself is reduced.

  Therefore, this time, in order to suppress the reaction between the electrode active material and the solid electrolyte, an attempt was made to lower the temperature during baking, but this time the solid electrolyte particles were not sintered and the solid electrolyte particles did not progress. The intergranular resistance between the electrode active material and the solid electrolyte is increased, and the interfacial area where the electrode active material and the solid electrolyte are joined is not enlarged. As a result, the grain boundary resistance in the solid electrolyte and the interfacial reaction resistance between the electrode active material and the solid electrolyte are increased. As a result, there was a problem that the charge / discharge capacity was not obtained (not charged / discharged) as an all-solid-state battery.

  The present invention has been made in consideration of such problems, and in the electrode layer of the all-solid-state battery, while reducing the intergranular resistance between the solid electrolyte particles, the present invention has been achieved between the solid electrolyte and the electrode active material. Suppressing the capacity drop due to the reaction, and as a result, it is possible to form an electrolyte network in the electrode layer, and at the same time, the interface area where the electrode active material and the solid electrolyte are joined to each other is drastically expanded. An object of the present invention is to provide an all-solid-state battery that can be charged and discharged even in an all-solid state.

  In the process of studying an all-solid battery using an electrode structure in which a solid electrolyte and an electrode active material are mixed, the present inventor has developed an electrode active material by a reaction between the solid electrolyte material and the electrode active material used. Based on this, it has been found that due to the lowering of crystallinity and the formation of heterogeneous phases, the electrode active material can only obtain charge / discharge capacity up to a capacity lower than the theoretical capacity that can be charged / discharged. The temperature between Ty and Tz is a combination of materials in which the relationship of Ty> Tz is established between the temperature (Ty) at which the electrode active material causes a decrease in capacity and the firing shrinkage start temperature (Tz) of the solid electrolyte. In addition to realizing an electrolyte network that enables low resistance in the electrode layer in the region, it is possible to expand the bonding area while suppressing the reaction between the electrolyte and the electrode active material. Resulted in reduction of the interface reaction resistance in case the interface, it was found to achieve a low internal resistance all solid state battery.

  And in this invention, the combination of the electrode active material and solid electrolyte which consist of a phosphoric acid compound which has a common polyanion is selected as a combination of the material which can perform the movement of Li ion more smoothly, and consists of a phosphoric acid compound The solid electrolyte material was vitrified. As a specific example, in a case where a NASICON type LAGP solid electrolyte, which is said to have high ionic conductivity among phosphoric acid compounds, is vitrified, Tg (glass transition point) is about 480 ° C., Tx (crystallization temperature) ) Has a low transition temperature of about 590 ° C. (see FIG. 10). When the firing shrinkage start temperature of this glass material was confirmed, it was confirmed that it was between 550 ° C. and 600 ° C. Next, as a result of investigating the reactivity between the vitrified solid electrolyte and the electrode active material, it was found that there was no decrease in crystallinity up to a temperature range sufficiently higher than the firing shrinkage start temperature, and no heterogeneous phase was generated. The present inventors have newly found a combination of phosphate compounds based on a common polyanion and having a relationship of Ty> Tz.

  As a result, within the range where there is no hindrance to the bonding between the solid electrolyte particles, a condition range that avoids a decrease in the charge / discharge capacity of the electrode active material due to the reaction between the electrode active material and the solid electrolyte was created, and the problem was solved .

  And by using this as a mixed electrode, in the electrode layer of the all-solid-state battery, while reducing the intergranular resistance between the solid electrolyte particles, the capacity reduction of the electrode active material is suppressed, and the electrode layer By forming the electrolyte network, the interface area where the electrode active material and the solid electrolyte are joined can be greatly expanded, and the interface reaction resistance can be lowered. As a result, all solids can be charged and discharged. A solid battery could be obtained.

  That is, the all solid state battery according to the first aspect of the present invention is an all solid state battery comprising a positive and negative electrode part containing an electrode active material, an electrolyte part made of a solid electrolyte, and a positive and negative electrode current collector. In an all solid state battery using a combination of an electrode active material having a common polyanion and a solid electrolyte, either the positive or negative electrode part or the positive and negative electrode part is a mixture of the electrode active material and the solid electrolyte. The electrode portion is formed by mixing a solid electrolyte material made of an amorphous polyanion compound with an electrode active material and heating and firing.

  An all solid state battery according to a second aspect of the present invention is an all solid state battery comprising a positive and negative electrode part containing an electrode active material, an electrolyte part made of a solid electrolyte, and a positive and negative current collector part, In an all-solid-state battery comprising a combination of an electrode active material made of a phosphoric acid compound and a solid electrolyte, either one of the positive and negative electrode parts or both positive and negative electrode parts are configured by mixing the electrode active material and the solid electrolyte. The electrode portion is characterized in that a solid electrolyte material made of an amorphous phosphate compound is mixed with an electrode active material and heated and fired.

And, when Ty is the temperature at which the electrode active material undergoes capacity reduction due to the reaction between the solid electrolyte material and the electrode active material, and Tz is the temperature at which the solid electrolyte material is fired and contracted,
Ty> Tz
Have the relationship.

  The state of firing shrinkage that determines the temperature at which shrinkage occurs here refers to the temperature at which the shrinkage temperature is reduced to a relative density of 70% or more with respect to the theoretical density of the material. And as temperature range which satisfy | fills Ty> Tz, it is further more desirable that it is temperature Tz from which this baking shrinkage becomes a relative density of 80% or more.

  Next, the capacity reduction that determines the temperature at which the capacity of the electrode active material decreases is the temperature at which the charge / discharge capacity can be obtained only to a capacity that is less than 50% of the theoretical capacity of the theoretical capacity of the electrode active material. Refers to that. The temperature range satisfying Ty> Tz is more preferably a temperature Ty that can secure a charge / discharge capacity of 80% or more of the theoretical capacity.

In the second aspect of the present invention, the solid electrolyte material made of a phosphoric acid compound may be a NASICON type material after being heated and fired. In this case, the solid electrolyte material made of a phosphoric acid compound is LAGP: Li _{1 + x} Al _x Ge _2-x (PO ₄ ) ₃ or LATP: Li _{1 + x} Al _x Ti _2-x (PO ₄ ) ₃ . Can be used. Here, x is 0 ≦ x ≦ 1.

In the second aspect of the present invention, the electrode active material may be made of a phosphate compound and may be a NASICON type material. In this case, the electrode active material comprising a phosphate _{compound, LVP: Li m V 2 (} PO 4) 3 may be used. Here, m is 1 ≦ m ≦ 5.

In the second aspect of the present invention, the positive electrode active material of the electrode active material may be a phosphate compound and may be an olivine type material. In this case, the positive electrode active material comprising a phosphate _{compound, LNP: Li n NiPO 4,} LCP: Li n CoPO 4, LMP: Li n MnPO 4, LFP: be used either Li _n FePO ₄ it can. Here, n is 0 ≦ n ≦ 1.

  In the second aspect of the present invention, the solid electrolyte material and the electrode active material may be NASICON type materials after being heated and fired together.

In the second aspect of the present invention, the solid electrolyte material and the electrode active material are both NASICON type materials, and the solid electrolyte material is LAGP: Li _{1 + x} Al _x Ge _2-x (PO ₄ ). _{3. The} electrode active material may have a symmetry structure in which both positive and negative electrodes are LVP: Li _m V ₂ (PO ₄ ) ₃ . Here, x is 0 ≦ x ≦ 1, more preferably 0.3 ≦ x ≦ 0.7, and m is 1 ≦ m ≦ 5.

  In the first and second aspects of the present invention described above, the electrode portion may be configured to be heated and fired in a pressurized state. In this case, by firing under pressure, it is possible to construct a fine microstructure inside the electrode part, and the interface area between the electrode active material and the solid electrolyte is increased, and the effect of reducing the interface charge transfer resistance is obtained.

  In the first and second aspects of the present invention described above, either one of the positive and negative electrode portions or the positive and negative electrode portions is formed of a printing paste, and is heated and fired in an inert atmosphere. May be. In this case, the electron conductivity in the electrode portion can be ensured by carbonization of the binder component. Electron conductivity can be imparted to the electrode portion without positively adding a carbon member used as an electron conduction aid.

  As described above, according to the all solid state battery of the present invention, in the electrode layer of the all solid state battery, while suppressing the grain boundary resistance between the solid electrolyte particles, it suppresses the capacity reduction of the electrode active material. Can do.

  In addition, since an electrolyte network can be formed in the electrode layer, the interface area where the electrode active material and the solid electrolyte are joined can be dramatically increased, thereby reducing the interfacial reaction resistance and reducing the total solid However, charge / discharge operation is possible.

  Embodiments of an all-solid battery according to the present invention will be described below with reference to FIGS.

  As shown in FIG. 1, an all-solid battery 10 according to the present embodiment is an all-solid battery that is a combination of an electrode active material and a solid electrolyte, and is a plate-like solid electrolyte made of ceramics containing a solid electrolyte 12. The first electrode layer 18 (for example, positive electrode) formed by firing and integrating with the fired body 14 and one surface of the solid electrolyte fired body 14, mixing the electrode active material 16 and the solid electrolyte 12, and heating and firing. A second electrode layer 20 (for example, a negative electrode) formed by mixing and firing the electrode active material 16 and the solid electrolyte 12 on the other surface of the solid electrolyte fired body 14; A first collector electrode 24 electrically connected to the electrode layer 18 and a second collector electrode 26 electrically connected to the second electrode layer 20 are included.

The solid electrolyte fired body 14 is disposed so as to separate the positive electrode and the negative electrode in the all-solid-state battery 10 and is a substantial solid electrolyte portion. There is no restriction | limiting in particular about the kind of solid electrolyte 12 contained in the ceramics which comprise the solid electrolyte sintered body 14, A conventionally well-known solid electrolyte can be used. For example, a material containing lithium as a mobile ion can be preferably used, and Li ₃ PO ₄ , Li ₃ PO _{4 in} which nitrogen is mixed with Li ₃ PO ₄ , Li ₂ S—SiS ₂ , Li ₂ S—P ₂ S _5. Lithium ion conductive glassy solid electrolytes such as Li ₂ S—B ₂ S ₃ , and lithium ion conductive solids doped with lithium halides such as LiI and lithium oxyacid salts such as Li ₃ PO _{4 on} these glasses An electrolyte etc. can be mentioned. In particular, a titanium oxide type solid electrolyte containing lithium, titanium and oxygen, for example, Li _x La _y TiO ₃ (where x is 0 ≦ x ≦ 1, y is 0 ≦ y ≦ 1) and NASICON type phosphoric acid. Compounds such as Li _{1 + x} Al _x Ge _2-x (PO ₄ ) ₃ and Li _{1 + x} Al _x Ti _2-x (PO ₄ ) ₃ (where x is 0 ≦ x ≦ 1) are in an oxygen atmosphere. It is preferable because it exhibits stable performance even in baking at.

  The thickness of the solid electrolyte fired body 14 is not particularly limited, but is preferably 5 μm to 1 mm, and more preferably 5 μm to 100 μm.

  The first electrode layer 18 and the second electrode layer 20 are porous in which many powder particles constituting the solid electrolyte 12 are connected by sintering, and a large number of pores communicating three-dimensionally from the surface to the inside are formed. Further, the electrode active material 16 is filled in a large number of pores of the porous body. A porous body formed by sintering a large number of powder particles constituting the solid electrolyte 12 is also referred to as an “electrolyte network”.

  The thicknesses of the first electrode layer 18 and the second electrode layer 20 are not particularly limited, but are preferably 5 μm to 1 mm, and more preferably 5 μm to 500 μm.

  As a method for forming the first electrode layer 18 and the second electrode layer 20 on the solid electrolyte fired body 14, a first paste for constituting the first electrode layer 18 and a first paste for constituting the second electrode layer 20 are used. The two pastes are printed on the solid electrolyte fired body 14 by a screen printing method or the like to form electrode patterns to be the first electrode layer 18 and the second electrode layer 20.

  The first paste and the second paste can be prepared by adding an appropriate amount of a binder dissolved in an organic solvent to an electrode active material material powder and a solid electrolyte material powder, which will be described later, and kneading them.

  Then, the first electrode layer is formed by firing the electrode pattern formed by printing the first paste and the second paste on the solid electrolyte fired body 14 at a temperature lower than the temperature at which the solid electrolyte fired body 14 is produced. 18 and the second electrode layer 20. At this time, the 1st electrode layer 18 and the 2nd electrode layer 20 become a form where the electrode active material 16 was filled with many pores of the porous body.

  In the above example, the first electrode layer 18 and the second electrode layer 20 formed on the solid electrolyte fired body 14 are both made of ceramics in which the electrode active material 16 and the solid electrolyte 12 are mixed. However, as in the all-solid battery 10a according to another embodiment shown in FIG. 2, for example, the second electrode layer 20 may be constituted by a metal film 22 such as metal Li or Li alloy.

  In the present embodiment, the first electrode layer 18 and the second electrode layer 20 are such that the solid electrolyte material added to the first electrode layer 18 and the second electrode layer 20 is an amorphous polyanion compound, and It is configured by heating and baking.

  In the present embodiment, the first electrode layer 18 and the second electrode layer 20 are such that the solid electrolyte material added to the first electrode layer 18 and the second electrode layer 20 is an amorphous phosphate compound, And it is configured by heating and firing.

The solid electrolyte material made of a phosphoric acid compound can be a NASICON type material after being heated and fired. In particular, LAGP: Li _{1 + x} Al _x Ge _2-x (PO ₄ ) ₃ or LATP: Li ₁ It is preferable to use _{+ x} Al _x Ti _2-x (PO ₄ ) ₃ . Here, x is 0 ≦ x ≦ 1.

The electrode active material comprising a phosphate compound, may be a Nasicon type material, in particular, LVP: it is preferable to use a _{Li m V 2 (PO 4)} 3. Here, m is 1 ≦ m ≦ 5.

Positive electrode active material comprising a phosphate compound, may be an olivine type material, in _{particular, LNP: Li n NiPO 4,} LCP: Li n CoPO 4, LMP: Li n MnPO 4, LFP: the Li _n FePO ₄ It is preferable to use either one. Here, n is 0 ≦ n ≦ 1.

In the present embodiment, as the solid electrolyte material made of a phosphoric acid compound and the electrode active material, a NASICON type material can be used after being heated and fired together. In this case, the solid electrolyte material is LAGP: Li _{1 + x} Al _x Ge _2-x (PO ₄ ) ₃ , and the electrode active material is LVP: Li _m V ₂ (PO ₄ ) ₃ for both positive and negative electrodes. It is preferable to use it. Here, x is 0 ≦ x ≦ 1, more preferably 0.3 ≦ x ≦ 0.7, and m is 1 ≦ m ≦ 5.

  As described above, in the present embodiment, in the first electrode layer 18 and the second electrode layer 20 of the all solid state battery 10, the electrode active material 16 and the electrode active material 16 are reduced while reducing the intergranular resistance between the solid electrolyte particles. It is possible to suppress the generation of a heterogeneous phase due to the reaction between.

  Moreover, since the electrolyte network into the 1st electrode layer 18 and the 2nd electrode layer 20 can be formed, the interface area which the electrode active material 16 and the solid electrolyte 12 join can be expanded greatly, Thereby, The interface reaction resistance can be lowered, and the all-solid battery 10 can be charged and discharged.

  Furthermore, the first electrode layer 18 and the second electrode layer 20 are preferably configured by being heated and fired in a pressurized state. As a result, by firing under pressure, it is possible to construct a dense microstructure inside the electrode part, and the interface area between the electrode active material and the solid electrolyte is increased, and the effect of reducing the interface charge transfer resistance is obtained.

  The method of heating and baking in a pressurized state includes a method of performing heat treatment while simultaneously applying high temperature and isotropic pressure to the mixture (HIP: Hot Isostatic Pressing), and storing in a baking jig and pressing in a uniaxial direction. There is a method (hot press method) of heat-treating the entire firing jig while applying pressure. In the HIP method, for example, an isotropic pressure can be applied to the mixture by using a gas such as argon as a pressure medium.

  Further, the first electrode layer 18 and / or the second electrode layer 20 is formed by printing paste and is heated and fired in an inert atmosphere such as Ar, whereby the first electrode layer is formed by carbonization of the binder component. 18 and / or electron conductivity in the second electrode layer 20 can be ensured. In this case, electron conductivity can be imparted to the first electrode layer 18 and / or the second electrode layer 20 without positively adding a carbon member used as an electron conduction aid.

  Next, examples of the all solid state battery 10 according to the present embodiment will be described in detail.

In this example, a NASICON phosphoric acid compound was used as both the solid electrolyte material and the electrode active material. Specifically, it is as follows.
Solid electrolyte material: LAGP: Li _1.5 Al _0.5 Ge _1.5 (PO ₄ ) ₃
Electrode active material: LVP: Li ₃ V ₂ (PO ₄ ) ₃

[Preparation of crystal powder]
First, a solid phase synthesis method in which powders of Li ₂ CO ₃ , GeO ₂ , Al ₂ O _3, and NH ₄ H ₂ (PO ₄ ) ₃ are mixed in a stoichiometric composition and fired at 900 ° C. in the atmosphere is performed. An electrolyte material “Li _1.5 Al _0.5 Ge _1.5 (PO ₄ ) ₃ ” (LAGP) crystal powder (hereinafter referred to as LAGP crystal powder) was obtained.

Further, by a solid phase synthesis method in which powders of Li ₂ CO ₃ , V ₂ O ₃ and NH ₄ H ₂ (PO ₄ ) ₃ are mixed in a stoichiometric composition and fired at 930 ° C. in an Ar (argon) stream. A crystal powder of the positive electrode (negative electrode) active material “Li ₃ V ₂ (PO ₄ ) ₃ ” (LVP) (hereinafter referred to as LVP crystal powder) was obtained.

[Production of solid electrolyte fired body]
The LAGP crystal powder obtained as described above was molded into compacted pellets having a diameter of 16 mm and a thickness of about 1 mm by die press molding. The pressing pressure was 500 kg / cm ² . The pellet was fired at 840 ° C. in an air atmosphere to obtain a LAGP solid electrolyte fired body pellet.

[Preparation of Glass Powder (Vitrification of LAGP Solid Electrolyte)]
The LAGP crystal powder obtained by the solid phase method is put in a Pt crucible and put in an atmospheric furnace heated to 1200 ° C. After holding for 1 hour, it was taken out and rapidly cooled with ice water to obtain a vitrified LAGP. This was pulverized with a mortar, a ball mill or the like to obtain a finely divided LAGP glass powder.

[Comparison of sinterability of solid electrolytes]
Solid electrolyte pellets using LAGP crystal powder (hereinafter referred to as crystalline LAGP solid electrolyte) and solid electrolyte pellets using LAGP glass powder (hereinafter referred to as vitrified LAGP solid electrolyte) were prepared, and Ar (argon) ) Comparison of sintering states at different firing temperatures in the atmosphere. 3 to 9 show the results of cross-sectional SEM observation of the respective fracture surfaces, and FIGS. 11 and 12 show graphs summarizing the firing shrinkage under Ar atmosphere firing.

  3 to 5 show cross-sectional SEM photographs when the firing temperature of the crystalline LAGP solid electrolyte is 600 ° C, 700 ° C, and 800 ° C. 3 to 5, each upper row shows an SEM photograph at a magnification of × 1000, and each lower row shows an SEM photograph at a magnification of × 5000. 6 to 9 show cross-sectional SEM photographs when the firing temperature of the vitrified LAGP solid electrolyte is 550 ° C, 600 ° C, 650 ° C, and 700 ° C. 6-9, each upper stage shows the SEM photograph by the magnification of x1000, and each lower stage shows the SEM photograph by the magnification of x5000.

FIG. 10 shows the DTA (differential thermal analysis) characteristics of the vitrified LAGP solid electrolyte in an inert atmosphere (N ₂ atmosphere). From FIG. 10, it can be seen that the vitrified LAGP solid electrolyte is a material having a low transition temperature of Tg (glass transition point) of about 480 ° C. and Tx (crystallization temperature) of about 590 ° C.

  Further, FIG. 11 shows changes in the firing shrinkage (%) with respect to the firing temperature of the crystalline LAGP solid electrolyte and the internal impedance of the solid electrolyte. FIG. 12 shows changes in the firing shrinkage (%) with respect to the firing temperature of the vitrified LAGP solid electrolyte and the internal impedance of the solid electrolyte. The firing shrinkage of the LAGP solid electrolyte is shown by a plot “●”, and the internal impedance of the solid electrolyte is shown by a bar graph divided into intragranular resistance and grain boundary resistance.

  From FIG. 3 to FIG. 5 and FIG. 11, in the crystalline LAGP solid electrolyte, the shape of the powder particles remains as it is up to 700 ° C., and the sintering does not proceed, and the grain boundary resistance remains very high even in the internal impedance. Recognize. On the other hand, at the level using LAGP glass powder (vitrified LAGP solid electrolyte), as shown in FIG. 6 to FIG. 9 and FIG. It can be seen that the grain boundary resistance is greatly reduced even in the internal impedance.

[Relationship between reactivity between electrolyte and electrode active material and charge / discharge capacity of electrode active material]
Next, acetylene black is added to the mixture of the LAGP crystal powder and the LVP crystal powder as an electron conduction auxiliary agent for later evaluation with an electrolytic solution to produce a mixed powder pellet, which is fired at different firing temperatures in an Ar atmosphere. Thus, a fired body was obtained. First, XRD (X-ray diffraction) measurement was performed on the obtained fired body. The measurement results are shown in FIG. In FIG. 13, the part indicated by ■ indicates the peak of Li ₃ Fe ₂ (PO ₄ ) ₃ that identifies the crystal structure of LVP, and the part indicated by ▲ indicates LiGe ₂ (PO ₄ ) ₃ that identifies the crystal structure of LAGP. The peak is shown. In addition, since LAGP and LVP have no registration of ICDD data, identification was performed with these substances having the same crystal structure. From this measurement result, it was confirmed that at 700 ° C. and 800 ° C. where the sintering was confirmed with the LAGP crystal powder, heterogeneous peaks derived from a plurality of phosphate condensed salts other than the peaks caused by LAGP and LVP began to occur. .

Next, the relationship between the charge / discharge capacity of the electrode active material, the peak intensity of the positive electrode active material, and the heterophase peak intensity was confirmed. Specifically, the sintered powder pellets of the mixed powder fired at each temperature are pulverized and used as a positive electrode, and an electrolyte (1 mol / liter concentration LiClO ₄ / EC + DEC (volume ratio 1: 1) solution) and The charge / discharge capacity (capacity) as a positive electrode active material was measured with the same configuration as a liquid lithium ion battery using metal Li as the negative electrode. Incidentally, LVP is a material whose chargeable / dischargeable theoretical capacity is about 130 mAh / g. The results of this measurement were confirmed by XRD measurement (see FIG. 13). The peak intensity (peak height) of the main peak (a) of the positive electrode active material and the LiVP ₂ identified by ○ in FIG. The relationship with the intensity (peak height) of the main peak (b) of O ₇ is shown in FIG. In FIG. 14, the change in the charge / discharge capacity of the positive electrode active material is indicated by a plot of “■”. Further, the main peak intensity of the positive electrode active material (a) and the heterophasic peak intensity (b) are shown in a bar graph. From this measurement result, the decrease in the peak intensity of the positive electrode active material itself, the appearance of the heterogeneous phase related to the material constituting the positive electrode active material: vanadium, and the decrease in the charge / discharge capacity of the electrode active material coincide, and the solid at high temperature firing It was confirmed that the capacity decrease due to the reaction between the electrolyte and the electrode active material caused the charge / discharge capacity of the electrode active material to decrease. Probably, the electrode active material is considered to have changed to LiVP ₂ O ₇ previously identified. And at 700 ° C and 800 ° C where the progress of the sintering was confirmed with the LAGP crystal powder, it was confirmed that only a charge / discharge capacity significantly lower than the theoretical capacity at which the charge / discharge of the electrode active material can be achieved is obtained. did it.

  This time, acetylene black is added to the mixture of the LAGP glass powder and the LVP crystal powder as an electron conduction auxiliary agent for later evaluation with an electrolytic solution to produce a mixed powder pellet, which is fired at different firing temperatures in an Ar atmosphere. Thus, a fired body was obtained. The XRD (X-ray diffraction) measurement of the obtained fired body and the charge / discharge capacity evaluation of the electrode active material measured in the electrolyte system using this fired body were performed. An XRD measurement result is shown in FIG. 15, and charge / discharge capability evaluation is shown in FIG. The way of viewing the plot is the same as in FIGS. 13 and 14.

  As can be seen from FIG. 15, in the vicinity of 600 ° C., good bonding between the particles can be confirmed with the LAGP glass powder, and the peak intensity of the electrode active material is maintained as shown in FIG. Thus, the charge / discharge capacity close to the theoretical capacity at which the electrode active material was originally chargeable / dischargeable was confirmed. As a result, the combination of these materials composed of a phosphate compound having a common polyanion has the relationship that the temperature at which the electrode active material undergoes capacity reduction due to the reaction between the solid electrolyte and the electrode active material> the firing shrinkage start temperature of the solid electrolyte. It was found to be a material system.

[Production of all-solid-state batteries]
Next, an electrode in which an electrode active material and a solid electrolyte are mixed by a combination of these materials, and the solid electrolyte material added to the electrode is an amorphous polyanion compound and a LAGP solid electrolyte that is a phosphate compound, An all-solid battery having an electrode formed of a mixed electrode of a solid electrolyte and an electrode active material and having been heated and fired was produced. As a comparative example, an all-solid battery was fabricated using a crystalline LAGP solid electrolyte. Specific configurations of Examples and Comparative Examples are shown below.

(Example 1)
An appropriate amount of a binder dissolved in an organic solvent was added to LAGP glass powder and LVP crystal powder and kneaded in a mortar to obtain an electrode paste for screen printing. A positive electrode and a negative electrode were formed by printing and drying an electrode pattern having a diameter of 12 mm on both surfaces of a solid electrolyte fired body having a diameter of 13 mm and a thickness of 1 mm as a substrate, using the electrode paste prepared as described above.

  Next, the electrode was baked on both surfaces of the solid electrolyte substrate by baking at 600 ° C. for 2 hours in a baking furnace in an Ar atmosphere.

Next, a gold (Au) sputtered film having a thickness of about 500 angstroms was formed on both surfaces of the obtained fired body for the purpose of current collection.

  The film thickness of the positive electrode after firing was about 20 μm, and the amount of active material was about 2 mg. The charge / discharge capacity per unit weight was calculated from the amount of the positive electrode active material and plotted.

(Example 2)
An appropriate amount of a binder dissolved in an organic solvent was added to LAGP glass powder and LVP crystal powder and kneaded in a mortar to obtain an electrode paste for screen printing. As described above, the electrode pattern was printed and dried using the prepared electrode paste on both surfaces of the solid electrolyte fired body to be the base, thereby forming positive and negative electrodes.

  Next, firing was performed at 600 ° C. for 40 hours in a firing furnace in an Ar atmosphere, the electrodes were baked on both surfaces of the solid electrolyte substrate, and Au sputtered films were formed on both surfaces of the obtained fired body.

  Similarly, the thickness of the positive electrode after firing was about 20 μm, and the amount of active material was about 2 mg.

(Comparative Example 1)
An appropriate amount of a binder dissolved in an organic solvent was added to the LAGP crystal powder and the LVP crystal powder, and kneaded in a mortar to obtain an electrode paste for screen printing. As described above, the electrode pattern was printed and dried using the prepared electrode paste on both surfaces of the solid electrolyte fired body to be the base, thereby forming positive and negative electrodes.

  Next, firing was performed at 600 ° C. for 2 hours in a firing furnace in an Ar atmosphere, and the electrodes were baked on both surfaces of the solid electrolyte substrate, and Au sputtered films were formed on both surfaces of the obtained fired body.

  Similarly, the film thickness of the positive electrode after firing was about 20 μm, and the amount of active material was about 2 mg.

(Comparative Example 2)
An appropriate amount of a binder dissolved in an organic solvent was added to the LAGP crystal powder and the LVP crystal powder, and kneaded in a mortar to obtain an electrode paste for screen printing. As described above, the electrode pattern was printed and dried using the prepared electrode paste on both surfaces of the solid electrolyte fired body to be the base, thereby forming positive and negative electrodes.

  Next, firing was performed at 700 ° C. for 2 hours in a firing furnace in an Ar atmosphere, and the electrodes were baked on both surfaces of the solid electrolyte base, and Au sputtered films were formed on both surfaces of the obtained fired body.

  Similarly, the film thickness of the positive electrode after firing was about 20 μm, and the amount of active material was about 2 mg.

[Measurement of AC impedance]
The measurement of AC impedance was performed using a combination of Solartron 1287 type potentio / galvanostat (trade name) and 1255B type frequency response analyzer (trade name). The measurement frequency was 1 MHz to 0.1 Hz, and measurement was performed at a measurement signal voltage of 10 mV.

[Evaluation of charge / discharge characteristics]
The obtained all solid state battery was charged and discharged by a CCCV (Constant Current Constant Voltage) method, and the charge and discharge evaluation of the all solid state battery was performed. Specifically, for Examples 1 and 2, was charged at a constant current 9 .mu.A / cm ² to 2.4V cutoff charged at 2.4V constant voltage until the current value of 0.9μA / cm ² and, discharge characteristics, and was discharged at a constant current of 9 .mu.A / cm ² to 0.1V cutoff was discharged at 0.1V constant voltage until the current value of 0.9μA / cm ^2. Comparative Example 1 and Comparative Example 2, was charged at a constant current 0.9μA / cm ² to 2.4V cutoff was charged at 2.4V constant voltage until the current value of 0.45μA / cm ^2, discharge properties, and was discharged at a constant current of 0.9μA / cm ² to 0.1V cutoff was discharged at 0.1V constant voltage until the current value of 0.45μA / cm ^2.

(Evaluation)
The all-solid-state ceramic battery cell comprising the obtained mixed electrode was subjected to electrical evaluation in a state where it was vacuum-heated and dried and incorporated in a 2032 type coin battery type package in a glove box. The charge / discharge characteristics of Example 1, Example 2, Comparative Example 1 and Comparative Example 2 are shown in FIGS. 17, 19, 21 and 23. Moreover, the alternating current impedance of Example 1, Example 2, Comparative example 1, and Comparative example 2 is shown in FIG.18, FIG.20, FIG.22 and FIG. In the AC impedance waveform, the horizontal axis indicates the real part Z ′ of the impedance, the vertical axis indicates the imaginary part Z ″ of the impedance, and the measurement frequencies of 1 kHz and 1 Hz are indicated by ●.

(Discussion)
Comparing the charge / discharge capacities, both Comparative Examples 1 and 2 were in a state where internal resistance was high and charge / discharge could hardly be performed. In Comparative Example 1, since the arc formed on the higher frequency side than 1 kHz corresponding to the grain boundary resistance is larger than the AC impedance waveform, the solid electrolyte is not sufficiently bonded between particles. Therefore, it is considered that charging / discharging is hardly performed due to the insufficient reaction interface area between the solid electrolyte and the electrode active material. On the other hand, in Comparative Example 2, since the arc formed on the low frequency side of 1 kHz or less corresponding to the reaction interface resistance is larger than the AC impedance waveform, the generation of a different phase at the solid electrolyte-electrode active material junction interface, It is thought that charging / discharging is hardly performed due to a decrease in the capacity of the active material.

  On the other hand, in Example 1, the internal resistance was low and charging / discharging was also about 20 mAh / g, and in Example 2, the result was clearly about 40 mAh / g. This is because the AC impedance waveform realizes low impedance in both grain boundary resistance and interfacial reaction resistance, and in the region where no abnormalities such as generation of heterogeneous phase due to reaction or capacity reduction of active material occur between the solid electrolyte and electrode active material In addition, the particles between the solid electrolytes in the electrode layer are bonded, and at the same time, the bonding interface between the solid electrolyte and the electrode active material is well formed. As a result, it is considered that charging / discharging became possible.

(Example 3)
Next, an all solid state battery according to Example 3 was produced, and its charge / discharge characteristics and AC impedance characteristics were measured.

  In the all solid state battery according to Example 3, first, as in Example 1 described above, an appropriate amount of a binder dissolved in an organic solvent is added to LAGP glass powder and LVP crystal powder, and the mixture is kneaded in a mortar for screen printing. Electrode paste. A positive electrode and a negative electrode were formed by printing and drying an electrode pattern having a diameter of 12 mm on both surfaces of a solid electrolyte fired body having a diameter of 13 mm and a thickness of 1 mm as a substrate, using the electrode paste prepared as described above.

Next, firing was performed in a hot press furnace in an Ar atmosphere with a firing profile of 600 hours at 600 ° C. with a load of 500 kg / cm ² applied in the thickness direction, and the electrode portions were baked on both surfaces of the solid electrolyte substrate. A gold (Au) sputtered film having a thickness of about 500 angstroms was formed on both surfaces of the obtained fired body for the purpose of current collection.

  The film thickness of the positive electrode after firing was about 20 μm as in Example 1, and the amount of active material was about 2 mg.

[Measurement of AC impedance]
In the same manner as in Example 1, the AC impedance was measured by combining a 1287 type potentio / galvanostat (trade name) manufactured by Solartron and a 1255B type frequency response analyzer (trade name). The measurement frequency was 1 MHz to 0.1 Hz, and measurement was performed at a measurement signal voltage of 10 mV.

[Evaluation of charge / discharge characteristics]
The obtained all solid state battery was charged and discharged by the CCCV method, and the charge and discharge evaluation of the all solid state battery was performed. Specifically, for example 3, was charged at a constant current 90 .mu.A / cm ² to 2.4V cutoff was charged at 2.4V constant voltage until the current value of 0.9μA / cm ^2, discharge characteristics , and was discharged at a constant current of 90 .mu.A / cm ² to 0.1V cutoff was discharged at 0.1V constant voltage until the current value of 0.9μA / cm ^2.

(Evaluation)
The all-solid-state ceramic battery cell comprising the obtained mixed electrode was subjected to electrical evaluation in a state where it was vacuum-heated and dried and incorporated in a 2032 type coin battery type package in a glove box. The charge / discharge characteristics of Example 3 are shown in FIG. 25, and the AC impedance is shown in FIG. In the AC impedance waveform, the horizontal axis indicates the real part Z ′ of the impedance, the vertical axis indicates the imaginary part Z ″ of the impedance, and the measurement frequencies of 1 kHz and 1 Hz are indicated by ●.

(Discussion)
In Example 3, as can be seen from FIG. 26, the internal resistance is reduced. The reduction effect of the internal resistance obtained is mostly due to the decrease in the reaction resistance (interfacial charge transfer resistance) portion of the internal resistance, so that the densification has progressed and the junction interface area between the electrode active material and the solid electrolyte is reduced. This is thought to be due to expansion.

  The all-solid-state battery according to the present invention is not limited to the above-described embodiment, and it is needless to say that various configurations can be adopted without departing from the gist of the present invention.

It is sectional drawing which shows typically the structure of the all-solid-state battery which concerns on this Embodiment. It is sectional drawing which shows typically the structure of the modification of the all-solid-state battery which concerns on this Embodiment. It is a figure which shows the cross-sectional SEM photograph of the sintered state by the sintering temperature (600 degreeC) from which crystal | crystallization LAGP solid electrolyte differs by Ar atmosphere. It is a figure which shows the cross-sectional SEM photograph of the sintering state by the sintering temperature (700 degreeC) from which a crystalline LAGP solid electrolyte is different in Ar atmosphere. It is a figure which shows the cross-sectional SEM photograph of the sintered state by the sintering temperature (800 degreeC) from which crystal | crystallization LAGP solid electrolyte differs by Ar atmosphere. It is a figure which shows the cross-sectional SEM photograph of the sintering state by the calcination temperature (550 degreeC) from which vitrification LAGP solid electrolyte is different in Ar atmosphere. It is a figure which shows the cross-sectional SEM photograph of the sintering state by the calcination temperature (600 degreeC) from which vitrification LAGP solid electrolyte differs by Ar atmosphere. It is a figure which shows the cross-sectional SEM photograph of the sintering state by the calcination temperature (650 degreeC) from which vitrification LAGP solid electrolyte differs by Ar atmosphere. It is a figure which shows the cross-sectional SEM photograph of the sintering state by vitrification temperature (700 degreeC) from which vitrification LAGP solid electrolyte is different in Ar atmosphere. It is a graph which shows the characteristic of DTA (differential thermal analysis) of vitrification LAGP solid electrolyte. It is the characteristic view which looked at the relationship of the baking shrinkage with respect to the calcination temperature of a crystalline LAGP solid electrolyte, and the internal impedance. It is the characteristic view which looked at the relationship of the baking shrinkage with respect to the calcination temperature of a vitrified LAGP solid electrolyte, and the internal impedance. It is a figure which shows the XRD (X-ray diffraction) characteristic of the sintered body by the mixed powder pellet of a LAGP crystal powder and a LVP crystal powder. The intensity (peak height) of the main peak of the positive electrode active material by the sintered body of the mixed powder pellet of LAGP crystal powder and LVP crystal powder, and the intensity (peak) of the main peak of LiVP ₂ O ₇ identified in the heterophasic peak FIG. 6 is a characteristic diagram showing a relationship between the height and the discharge capacity of the positive electrode active material. It is a figure which shows the XRD (X-ray diffraction) characteristic of the sintered body by the mixed powder pellet of a LAGP glass powder and a LVP crystal powder. The peak intensity (peak height) of the positive electrode active material by the fired body of the mixed powder pellet of LAGP glass powder and LVP crystal powder, and the intensity (peak) of the main peak of LiVP ₂ O ₇ identified in the heterophasic peak FIG. 6 is a characteristic diagram showing a relationship between the height and the discharge capacity of the positive electrode active material. It is a graph which shows the charging / discharging characteristic of Example 1 using a LAGP glass powder. 3 is a graph showing AC impedance characteristics of Example 1. It is a graph which shows the charging / discharging characteristic of Example 2 using a LAGP glass powder. 6 is a graph showing AC impedance characteristics of Example 2. It is a graph which shows the charging / discharging characteristic of the comparative example 1 using a LAGP crystal powder. 5 is a graph showing AC impedance characteristics of Comparative Example 1. It is a graph which shows the charging / discharging characteristic of the comparative example 2 using a LAGP crystal powder. 10 is a graph showing AC impedance characteristics of Comparative Example 2. 6 is a graph showing charge / discharge characteristics of Example 3. 6 is a graph showing AC impedance characteristics of Example 3.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10, 10a ... All-solid-state battery 12 ... Solid electrolyte 14 ... Solid electrolyte sintered body 16 ... Electrode active material 18 ... 1st electrode layer (positive electrode) 20 ... 2nd electrode layer (negative electrode)
24 ... 1st collector electrode 26 ... 2nd collector electrode

Claims (17)

An all-solid battery comprising a positive and negative electrode part containing an electrode active material, an electrolyte part made of a solid electrolyte, and a positive and negative current collector part,
In an all-solid-state battery using a combination of an electrode active material with a common polyanion and a solid electrolyte,
One of the positive and negative electrode parts, or both positive and negative electrode parts are configured by mixing the electrode active material and the solid electrolyte,
The all-solid-state battery, wherein the electrode part is formed by mixing a solid electrolyte material made of an amorphous polyanion compound with an electrode active material and heating and firing. An all-solid battery comprising a positive and negative electrode part containing an electrode active material, an electrolyte part made of a solid electrolyte, and a positive and negative current collector part,
In an all-solid-state battery using a combination of an electrode active material composed of a phosphate compound and a solid electrolyte,
One of the positive and negative electrode parts, or both positive and negative electrode parts are configured by mixing the electrode active material and the solid electrolyte,
The all-solid battery, wherein the electrode part is formed by mixing a solid electrolyte material made of an amorphous phosphate compound with an electrode active material and heating and firing. The all-solid-state battery according to claim 1 or 2,
When the temperature at which the electrode active material undergoes a capacity decrease due to the reaction between the solid electrolyte material and the electrode active material is Ty, and the temperature at which the solid electrolyte material is fired and contracted is Tz,
Ty> Tz
All-solid-state battery characterized by having the following relationship. The all-solid-state battery according to claim 3,
The all-solid-state battery, wherein the solid electrolyte material has a relationship in which the temperature at which the solid electrolyte material is fired and contracted is Tz, the temperature at which the solid electrolyte material contracts to a relative density of 70% or more with respect to the theoretical density of the solid electrolyte material. The all solid state battery according to claim 2,
An all-solid battery, wherein the solid electrolyte material made of a phosphoric acid compound is a NASICON type material after being heated and fired. The all-solid-state battery according to claim 5,
The solid electrolyte material made of a phosphoric acid compound is LAGP: Li _{1 + x} Al _x Ge _2-x (PO ₄ ) ₃
All-solid-state battery characterized by being.
[However, x is 0 ≦ x ≦ 1. ] The all-solid-state battery according to claim 5,
The solid electrolyte material made of a phosphoric acid compound is LATP: Li _{1 + x} Al _x Ti _2-x (PO ₄ ) ₃
All-solid-state battery characterized by being.
[However, x is 0 ≦ x ≦ 1. ] In the all-solid-state battery of any one of Claims 2-7,
The all-solid-state battery, wherein the electrode active material is made of a phosphoric acid compound and is a NASICON type material. The all-solid-state battery according to claim 8,
The electrode active material is LVP comprising the phosphate _{compound: Li m V 2 (PO 4} ) 3
[However, m is 1 ≦ m ≦ 5. ]
All-solid-state battery characterized by being. In the all-solid-state battery of any one of Claims 2-7,
An all-solid-state battery characterized in that, among the electrode active materials, a positive electrode active material is made of a phosphate compound and is an olivine type material. The all-solid-state battery according to claim 10,
Wherein comprising the phosphate compound positive electrode active material is LNP: Li _n NiPO ₄
LCP: Li _n CoPO ₄
LMP: Li _n MnPO ₄
LFP: Li _n FePO ₄
All-solid-state battery characterized by the above-mentioned.
[However, n is 0 ≦ n ≦ 1. ] In the all-solid-state battery of any one of Claims 2-5,
The solid electrolyte material and the electrode active material are both NASICON type materials. In the all-solid-state battery of any one of Claims 2-5,
The solid electrolyte material and the electrode active material are both NASICON type materials after being heated and fired,
The solid electrolyte material is _{LAGP: Li 1 + x Al x} Ge 2-x (PO 4) 3, wherein the electrode active material is LVP both positive and negative _{electrodes: Li m V 2 (PO 4} ) having a symmetry structure is ₃ All-solid battery characterized by.
[However, x is 0 ≦ x ≦ 1, and m is 1 ≦ m ≦ 5. ] The all-solid-state battery according to claim 1,
An all-solid-state battery, wherein the solid electrolyte material used for the electrolyte portion is an amorphous polyanion compound and is fired and fired. The all solid state battery according to claim 2,
The all-solid-state battery, wherein the solid electrolyte material used for the electrolyte portion is an amorphous phosphoric acid compound and is heated and fired. In the all-solid-state battery of any one of Claims 1-15,
The all-solid-state battery, wherein the electrode part is configured by being heated and fired in a pressurized state. In the all-solid-state battery of any one of Claims 1-16,
An all-solid-state battery characterized in that any one of the positive and negative electrode portions or both positive and negative electrode portions is formed of a printing paste and is heated and fired in an inert atmosphere.

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