JP7453037B2 - all solid state battery - Google Patents

Description

The present invention relates to an all-solid-state battery system with high capacity and excellent output characteristics.

In recent years, with the development of portable electronic devices such as mobile phones and notebook personal computers, and the practical use of electric vehicles, there has been a need for small, lightweight, high-capacity, and high-energy-density secondary batteries. It has become to.

Currently, lithium secondary batteries that can meet this demand, especially lithium ion secondary batteries, use lithium-containing composite oxides such as lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide (LiNiO ₂ ) as positive electrode active materials. , graphite or the like is used as the negative electrode active material, and an organic electrolyte containing an organic solvent and a lithium salt is used as the nonaqueous electrolyte.

With the further development of equipment to which lithium ion secondary batteries are applied, there is a need for lithium ion secondary batteries to have a longer lifespan, higher capacity, and higher energy density. Lithium-ion secondary batteries with higher capacity and higher energy density are also required to be highly reliable.

However, the organic electrolyte used in lithium-ion secondary batteries contains an organic solvent, which is a flammable substance, so when an abnormal situation such as a short circuit occurs in the battery, the organic electrolyte can generate abnormal heat. there is a possibility. Furthermore, with the recent trend towards higher energy densities in lithium ion secondary batteries and an increase in the amount of organic solvents in organic electrolytes, there is a demand for even greater reliability in lithium ion secondary batteries.

Under the above circumstances, all-solid-state lithium secondary batteries (all-solid-state batteries) that do not use organic solvents are attracting attention. All-solid-state batteries use molded solid electrolytes that do not use organic solvents instead of conventional organic solvent-based electrolytes, and are highly safe because there is no risk of abnormal heat generation of the solid electrolytes.

Patent Document 1 describes argyrodite having a specific composition as a solid electrolyte for lithium ion batteries that can suppress the generation of hydrogen sulfide when exposed to the atmosphere and maintain high conductivity even when left in dry air. A sulfide-based solid electrolyte with an (Argurodite) type crystal structure has been proposed.

International Publication No. 2016/104702

Currently, the field of application of all-solid-state batteries is rapidly expanding, and for example, applications that require discharge at large current values are considered, so it is necessary to improve the output characteristics to meet this demand. is required.
Further, in an all-solid-state battery system using the all-solid-state battery, it is also required to use the capacity of the all-solid-state battery to the maximum extent possible.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an all-solid-state battery system with high capacity and excellent output characteristics.

The all-solid-state battery system of the present invention charges the all-solid-state battery to a voltage of 2.8V as an upper limit, and the all-solid-state battery has a positive electrode, a negative electrode, and a space between the positive electrode and the negative electrode. and a solid electrolyte layer interposed therein, the positive electrode has a molded positive electrode mixture containing a positive electrode active material, a solid electrolyte, and a conductive additive, and the positive electrode active material contains lithium cobalt oxide. ,
The solid electrolyte has the following general formula (1)
Li _7-x PS _6-x Cl _y Br _z (1)
[In the general formula (1), x=y+z, 1.0<x≦1.8, 0.1≦z/y≦10.0] is a sulfide-based solid electrolyte,
The content of the positive electrode active material in the positive electrode mixture is 50 to 80% by mass.

According to the present invention, it is possible to provide an all-solid-state battery system with high capacity and excellent output characteristics.

FIG. 1 is a cross-sectional view schematically showing an example of an all-solid-state battery used in the all-solid-state battery system of the present invention.

The all-solid-state battery used in the all-solid-state battery system of the present invention includes a positive electrode having a molded positive electrode mixture containing a positive electrode active material, a solid electrolyte, and a conductive additive, a negative electrode, and a space between the positive electrode and the negative electrode. and a solid electrolyte layer interposed therein.

In the all-solid-state battery used in the present invention, lithium cobalt oxide is used as the positive electrode active material. Since lithium cobalt oxide has a high true density and a high working potential, it is possible to increase the energy density per volume by using it as a positive electrode active material. Moreover, a specific solid electrolyte (represented by general formula (1)) with high output characteristics is used as the solid electrolyte of the positive electrode. Furthermore, by setting the content of the positive electrode active material in the positive electrode mixture to a specific amount, it is possible to obtain a well-balanced all-solid-state battery with high capacity and high output.

(positive electrode)
The positive electrode of an all-solid-state battery has a molded body of a positive electrode mixture containing a positive electrode active material, a solid electrolyte, a conductive additive, etc. For example, a positive electrode consisting only of the molded body, or a positive electrode consisting of the molded body and a current collector. Examples include a positive electrode with a structure in which the two are integrated.

In the present invention, lithium cobalt oxide is used as the positive electrode active material.

As the lithium cobalt oxide, one represented by the general compositional formula (2) can be employed.
Li _x Co _y M _z O ₂ (2)
[In the general formula (2), M is Al, Mg, Ni, Mn, Na, Fe, Cu, Zr, Ti, Bi, Ca, F, P, Sr, W, Ba, Nb, Si, Zn, At least one element selected from the group consisting of Mo, V, Sn, Sb, Ta, Ge, Cr, K, S and Er, 0.9<x<1.1, 0<y≦1, 0≦z<0.1]

In this way, by including elements that can be substituted in Co sites and Li sites, it is possible to suppress the expansion and contraction of lithium cobalt oxide during charging and discharging, and even when charging and discharging are repeated, lithium cobalt oxide Good contact between the solid electrolyte and the solid electrolyte can be maintained, and the internal resistance can be kept low, so the output characteristics can be improved.

In particular, in general formula (2), Al is an element substituted into Co sites, and Mg and Ni are elements substituted with Li sites, and both have the effect of reducing the amount of expansion of lithium cobalt oxide during charging. have.

As the positive electrode active material, lithium cobalt oxide alone may be used, or lithium cobalt oxide and other positive electrode active materials may be used in combination. Other positive electrode active materials that can be used in combination with lithium cobalt oxide include active materials that can intercalate and release lithium ions, similar to those used as positive electrode active materials in conventionally known lithium ion secondary batteries. Examples include substances. However, when lithium cobalt oxide and other positive electrode active materials are used together as the positive electrode active material, the proportion of lithium cobalt oxide in the total positive electrode active material is preferably 60% by mass or more. Note that since all of the positive electrode active material may be lithium cobalt oxide, the upper limit of the preferable content of lithium cobalt oxide in the total positive electrode active material is 100% by mass.

The average particle diameter of the positive electrode active material is preferably 1 μm or more, more preferably 2 μm or more, and preferably 10 μm or less, more preferably 8 μm or less. Note that the positive electrode active material may be primary particles or secondary particles obtained by agglomerating primary particles. When a positive electrode active material having an average particle diameter in the above range is used, a large number of interfaces with the solid electrolyte can be obtained, so that the load characteristics of the battery are further improved.

The average particle diameter of the positive electrode active material referred to in this specification is defined as the integral volume when calculating the integral volume from particles with a small particle size distribution using a particle size distribution measuring device (such as Microtrac particle size distribution measuring device "HRA9320" manufactured by Nikkiso Co., Ltd.). It means the value of the 50% diameter (d ₅₀ ) in the volume-based integrated fraction.

The content of the positive electrode active material in the positive electrode mixture is 50% by mass or more and 80% by mass or less. It seems that the higher the content of the positive electrode active material in the positive electrode mixture, the higher the capacity, but on the other hand, the positive electrode active material expands and contracts with charging and discharging, and each time the positive electrode active material and the solid electrolyte It may not be possible to maintain a contact point with the terminal, resulting in a decrease in output characteristics. This is a problem specific to all-solid-state batteries.
Therefore, by using lithium cobalt oxide, which has a high energy density per volume, as the positive electrode active material and keeping the content of the positive electrode active material in the positive electrode mixture below a certain amount, an all-solid-state battery with high capacity and high output can be obtained. The inventors discovered that.
The content of the positive electrode active material in the positive electrode mixture is preferably 60% by mass or more, and preferably 70% by mass or less.
By using a sulfide-based solid electrolyte represented by the general formula (1) described later as the solid electrolyte of the positive electrode, the output characteristics can be synergistically improved.

The positive electrode active material has a reaction suppression layer on its surface for suppressing reaction with the solid electrolyte.

The reaction suppression layer may be made of a material that has ionic conductivity and can suppress the reaction between the positive electrode active material and the solid electrolyte. Examples of materials that can constitute the reaction suppression layer include, for example, oxides containing Li and at least one element selected from the group consisting of Nb, P, B, Si, Ge, Ti, and Zr; Examples include Nb-containing oxides such as LiNbO ₃ , Li ₃ PO ₄ , Li ₃ BO ₃ , Li ₄ SiO ₄ , Li ₄ GeO ₄ , LiTiO ₃ , LiZrO ₃ and the like. The reaction suppression layer may contain only one type of these oxides, or may contain two or more types of these oxides, and may further contain multiple types of these oxides in a composite compound. may be formed. Among these oxides, it is preferable to use Nb-containing oxides, and it is more preferable to use LiNbO ₃ .

The reaction suppression layer is preferably present on the surface in an amount of 0.1 to 1.0 parts by mass based on 100 parts by mass of the positive electrode active material. Within this range, the reaction between the lithium cobalt oxide and the solid electrolyte can be suppressed well, and deterioration of load characteristics can be prevented.

Examples of methods for forming a reaction suppression layer on the surface of the positive electrode active material include a sol-gel method, a mechanofusion method, a CVD method, and a PVD method.

A sulfide-based solid electrolyte represented by general formula (1) is used as the solid electrolyte of the positive electrode.

Li _7-x PS _6-x Cl _y Br _z (1)
[In the general formula (1), x=y+z, 1.0<x≦1.8, 0.1≦z/y≦10.0]
The solid electrolyte represented by the general formula (1) has particularly excellent ionic conductivity among solid electrolytes that can be used in all-solid-state batteries, and can contribute to improving the output characteristics of the battery. Moreover, by using this not only for the positive electrode but also for the negative electrode and solid electrolyte layer, the output characteristics of the battery are further improved.

Although only the sulfide-based solid electrolyte represented by general formula (1) may be used for the positive electrode, other solid electrolytes may also be used together with the sulfide-based solid electrolyte. Solid electrolytes that can be used in combination with the sulfide solid electrolyte include hydride solid electrolytes, oxide solid electrolytes, and the like.

Examples of the hydride solid electrolyte include LiBH ₄ and a solid solution of LIBH ₄ and the following alkali metal compound (for example, one in which the molar ratio of LiBH ₄ and the alkali metal compound is 1:1 to 20:1). Can be mentioned. Examples of the alkali metal compounds in the solid solution include lithium halides (LiI, LiBr, LiF, LiCl, etc.), rubidium halides (RbI, RbBr, RbiF, RbCl, etc.), and cesium halides (CsI, CsBr, CsF, CsCl, etc.). , lithium amide, rubidium amide, and cesium amide.

Examples of the oxide solid electrolyte include Li ₇ La ₃ Zr ₂ O ₁₂ , LiTi(PO ₄ ) ₃ , LiGe(PO ₄ ) ₃ , and LiLaTiO ₃ .

However, the proportion of solid electrolytes other than the sulfide-based solid electrolyte in the total amount of solid electrolytes used in the positive electrode mixture is preferably 30% by mass or less.

The content of the solid electrolyte in the positive electrode mixture is 15% by mass or more in the molded body of the positive electrode mixture of the positive electrode for an all-solid-state battery, from the viewpoint of further improving the output characteristics of the all-solid-state battery and further increasing the capacity. It is preferably at least 20% by mass, even more preferably at least 25% by mass, and is preferably at most 45% by mass, more preferably at most 40% by mass, More preferably, it is 35% by mass or less.

A carbon material such as carbon black can be used as a conductive additive for the positive electrode. In particular, by using fibrous carbon and granular carbon together as conductive additives, it is possible to form a good conductive network within the molded positive electrode mixture. While achieving high capacity, it is also possible to ensure excellent load characteristics.
When fibrous carbon and granular carbon are used together as a conductive agent for a positive electrode for an all-solid-state battery, the fibrous carbon used has a ratio of fiber length to fiber diameter (fiber diameter) of 20 or more. Preferably. The fiber length of the fibrous carbon is more preferably 3 to 600 μm, and the fiber diameter is even more preferably 1 to 300 nm.

The fiber length and fiber diameter of the fibrous carbon referred to in this specification are determined by selecting 50 fibers whose outlines can be confirmed in an image of carbon observed at 30,000 times using a scanning electron microscope (SEM). This value is obtained by measuring the particle size of fibers using a two-point method and calculating the average value (number average) of all fibers.

Specific examples of fibrous carbon include vapor grown carbon fibers, carbon nanofibers, carbon nanotubes, and the like. As the fibrous carbon, only one type of the above-mentioned examples may be used, or two or more types may be used in combination.

When fibrous carbon and granular carbon are used together as a conductive agent for a positive electrode for an all-solid-state battery, the granular carbon in the state of primary particles has a ratio of the length of the longest diameter to the length of the shortest diameter, It is preferable that it has a value of 1 to 1.3. Further, the average particle diameter of the granular carbon is preferably 10 nm to 1000 nm.

The particle diameter of the primary particles of granular carbon as used herein is a value determined as follows. Using a scanning electron microscope (SEM), select 50 particles whose outline can be confirmed in an image of carbon observed at 30,000 times, and measure the longest diameter and shortest diameter of the selected particles using a two-point method. The longest diameter of the granular carbon is the average value (number average) of all the measured longest diameters, and the shortest diameter is the average value (number average) of all the measured shortest diameters. Further, the average particle diameter of the granular carbon is the longest diameter (average value of all longest diameters) determined as described above.

Specific examples of granular carbon include highly crystalline carbon materials such as graphite (natural graphite, artificial graphite) and graphene (single-layer graphene, multilayer graphene); low-crystalline carbon materials such as carbon black; etc. .

When fibrous carbon and granular carbon are used together as a conductive agent for a positive electrode for an all-solid-state battery, it is preferable to use granular carbon containing a hydrophilic portion in a proportion of 10% by mass or more. . By using granular carbon containing a hydrophilic portion in a proportion of 10% by mass or more, it becomes easier to lower the porosity and increase the density of the molded body of the positive electrode mixture. Further, as described later, it is more preferable that the granular carbon forms a composite with the fibrous carbon, but by using the granular carbon containing a hydrophilic portion in a proportion of 10% by mass or more, the fibrous carbon It also becomes easier to form a complex with

The "hydrophilic portion" in granular carbon as used herein is as follows. Add 0.1 g of granular carbon to 20 mL of ammonia aqueous solution with pH = 11, perform ultrasonic irradiation for 1 minute, and leave the resulting liquid for 5 hours to precipitate the solid phase portion. At this time, the portion that does not precipitate but is dispersed in the liquid phase portion (supernatant liquid) corresponds to the “hydrophilic portion”.

Furthermore, the proportion of the "hydrophilic portion" in the total amount of granular carbon as used herein is a value determined by the following method. A supernatant liquid is removed from the liquid after precipitation of the solid phase portion, the remaining portion is dried, and the weight of the solid after drying is measured. The value obtained by subtracting the obtained weight from the weight of the granular carbon initially added (0.1 g) becomes the weight of the "hydrophilic portion" dispersed in the supernatant liquid. Then, the value obtained by dividing the weight of the "hydrophilic portion" by the weight of the granular carbon initially added (0.1 g) and expressed as a percentage corresponds to the proportion of the "hydrophilic portion" in the total amount of granular carbon.

In addition, in the case of granular carbon in which the proportion of the hydrophilic portion is 10% by mass or more, the average particle diameter of the primary particles is preferably 10 nm or more, and preferably 20 nm or more, from the viewpoint of further improving the moldability of the positive electrode mixture. On the other hand, since it is easy to increase the proportion of the hydrophilic part, the average particle diameter of the primary particles in the case of granular carbon with the proportion of the hydrophilic part of 10% by mass or more is preferably 400 nm or less, More preferably, the thickness is 300 nm or less.

In electrodes of batteries such as lithium ion secondary batteries, conductive carbon such as graphite, carbon black, and carbon nanotubes, which are generally used as conductive aids, have a hydrophilic portion of 5% by mass or less. By subjecting such conductive carbon particles to oxidation treatment, hydroxy groups, carboxy groups, ether bonds, etc. are introduced, and the conjugated double bonds of carbon are oxidized to become single bonds, and some carbon bonds are formed. Since hydrophilic portions are generated by cutting the bonds, it is possible to obtain granular carbon in which the proportion of hydrophilic portions satisfies the above value.

A more specific method for producing granular carbon particles in which the proportion of the hydrophilic portion satisfies the above value includes, for example, carbon raw materials having voids (porous carbon powder, Ketjen black, furnace black having voids, carbon nano Fibers, carbon nanotubes, etc.) are treated with acids (nitric acid, nitric acid/sulfuric acid mixtures, hypochlorous acid aqueous solution, etc.), and then transition metal compounds (transition metal halides, transition metal inorganic salts, transition metal This mixture is subjected to a mechanochemical reaction, and the reaction product is heated in a non-oxidizing atmosphere (nitrogen atmosphere, argon atmosphere, etc.), and the transition metal compound is extracted from the heated product. For example, the reaction product of a transition metal compound may be removed by dissolving it in an acid, followed by washing and drying.

In addition, the carbon raw material having the above-mentioned voids is mixed with the above-mentioned transition metal compound, and this is heated in an oxidizing atmosphere (in an oxygen-containing atmosphere such as air), and the transition metal compound and the transition metal are extracted from the heated product. Particulate carbon in which the proportion of the hydrophilic portion satisfies the above value can also be obtained by a method in which the reaction product of the compound is removed by dissolving it in an acid, etc., and then washed and dried.

Note that details of the method and conditions for producing granular carbon in which the proportion of the hydrophilic portion satisfies the above value are disclosed in International Publication No. 2015/133586, and may be produced according to the description thereof.

Note that fibrous carbon tends to aggregate, and even when mixed with a positive electrode active material and the like during the preparation of a positive electrode mixture, it often remains aggregated without being disintegrated. When such an electrode mixture is used, since the aggregated fibrous carbon is bulky, it becomes difficult to form a compact of the positive electrode mixture with few voids and high density. Therefore, it is preferable to use the fibrous carbon as a composite with granular carbon. When fibrous carbon forms a composite with granular carbon, aggregation of the fibrous carbon is suppressed by the granular carbon adhering to the surface of the fibrous carbon. This facilitates the formation of a molded body of positive electrode mixture that has low porosity and high density, for example.

A composite of fibrous carbon and granular carbon can be obtained by dry mixing fibrous carbon and granular carbon, as described below.

Whether or not a "composite of fibrous carbon and granular carbon" is used in the positive electrode mixture molded body can be determined by the following method, for example, if the granular carbon is of the low crystallinity mentioned above. . The molded body of the positive electrode mixture is taken out from the battery, and the cross section of the positive electrode is subjected to mapping measurement using micro-Raman spectroscopy (measurement range: 80×80 μm, 2 μm steps). Both fibrous carbon and granular carbon have peaks observed at 1340 cm ^-1 and 1590 cm ^-1 , but the peak intensity of fibrous carbon at 1340 cm ^-1 is about 1/10 to 1/5 of that of granular carbon. It is. Therefore, when fibrous carbon and granular carbon exist separately, it is possible to determine which type of carbon is present by observing the peak intensity at 1340 cm ⁻¹ . When fibrous carbon and granular carbon form a composite, the peak intensity at 1340 cm ^-1 is about 1/3 to 3/4 of that of granular carbon, so the carbon being observed is a fiber. It can be determined that it is a composite of granular carbon and granular carbon.

In the molded body of the positive electrode mixture in the positive electrode for an all-solid-state battery, the ratio of fibrous carbon to granular carbon is set to 100 parts by mass of fibrous carbon to 100 parts by mass of fibrous carbon. The amount of carbon is preferably 10 parts by mass or more, more preferably 30 parts by mass or more. In addition, from the perspective of limiting the specific surface area of the entire conductive additive and better suppressing oxidation of the sulfide solid electrolyte in the molded positive electrode mixture, the ratio of fibrous carbon to granular carbon is as follows: The amount of granular carbon is preferably 100 parts by mass or less, more preferably 70 parts by mass or less, based on 100 parts by mass of fibrous carbon.

The ratio of fibrous carbon to granular carbon in the molded body of the positive electrode mixture can be determined by the following method. Take out the electrode stack from the battery and separate only the positive electrode mixture molded body using a precision knife. After the separated molded positive electrode mixture is placed in ion-exchanged water, the same amount of toluene as the ion-exchanged water is added and subjected to ultrasonication. The liquid subjected to this treatment is separated into an aqueous phase containing granular carbon and a toluene phase containing fibrous carbon. This liquid is separated into an aqueous phase and a toluene phase. The resulting aqueous phase was centrifuged at a centrifugal acceleration of 50,000 G, and the supernatant was replaced with ion-exchanged water three times, and the remaining sample was dried to recover the solid content. Measure its mass Y (mg). Next, the solid content whose mass Y was measured was subjected to thermogravimetric (TG) analysis in an air atmosphere to determine the amount of change in mass from 120°C to 700°C. Let it be Z (mg).

The resulting toluene layer was centrifuged at a centrifugal acceleration of 50,000 G, and the supernatant was replaced with toluene three times. The remaining sample was dried to recover the solid content, and its mass ( mg). Next, thermogravimetric (TG) analysis is performed on the solid content whose mass W was measured in an air atmosphere to determine the amount of change in mass from 120°C to 800°C. Let the mass be X (mg).

Furthermore, the mass P (mg) of the positive electrode active material in the positive electrode mixture can be calculated by subtracting Z from Y.

In the molded product of the positive electrode mixture of the positive electrode for an all-solid-state battery, the total amount of the conductive additive is preferably 1 to 10% by mass.

The positive electrode mixture does not need to contain a resin binder, or may contain it. Examples of the resin binder include fluororesins such as polyvinylidene fluoride (PVDF). However, since the resin binder acts as a resistance component in the positive electrode mixture, it is desirable that its amount be as small as possible. Therefore, it is preferable that the positive electrode mixture does not contain a resin binder, or if it contains a resin binder, the content thereof is 0.5% by mass or less. The content of the resinous binder in the positive electrode mixture is more preferably 0.3% by mass or less, and even more preferably 0% by mass (that is, no resinous binder is contained).

When a current collector is used for the positive electrode, metal foil such as aluminum or stainless steel, punched metal, net, expanded metal, foamed metal, carbon sheet, etc. can be used as the current collector.

The molded positive electrode mixture is prepared by, for example, mixing a positive electrode active material, a solid electrolyte, a conductive additive, and a binder added depending on the non-vaginal use, and then molding the positive electrode mixture by pressure molding or the like. It can be formed by compression.

In the case of a positive electrode having a current collector, it can be manufactured by bonding a molded body of the positive electrode mixture formed by the method described above to the current collector by pressing or the like.

The thickness of the positive electrode mixture molded body (in the case of a positive electrode with a current collector, the thickness of the positive electrode mixture molded body per one side of the current collector; the same applies hereinafter) is determined from the viewpoint of increasing the capacity of the battery. , 200 μm or more. Further, the thickness of the molded body of the positive electrode mixture is usually 2000 μm or less.

(Negative electrode)
The negative electrode of an all-solid-state battery has a molded body of a negative electrode mixture containing lithium titanium oxide as a negative electrode active material, a solid electrolyte, a conductive additive, etc. For example, a negative electrode consisting only of the molded body, Examples include a negative electrode having a structure in which a molded body and a current collector are integrated.

Examples of the lithium titanium oxide include those represented by the following general compositional formula (3).

Li[Li _1/3-a M ¹ _s Ti _5/3-t M ² _t ]O ₄ (3)

In the general composition formula (3), M ¹ is at least one element selected from the group consisting of Na, Mg, K, Ca, Sr and Ba, and M ² is Al, V, Cr, Fe, At least one element selected from the group consisting of Co, Ni, Zn, Ym, Zr, Nb, Mo, Ta, and W, and 0≦s<1/3, 0≦t<5/3.

That is, in the lithium titanium oxide represented by the general compositional formula (3), some of the Li sites may be substituted with element ^M1 . However, in the general compositional formula (3), s representing the ratio of element M ¹ is preferably less than 1/3. In the lithium titanium oxide represented by the general compositional formula (3), Li does not need to be substituted with the element M ¹ , so s representing the ratio of the element M ¹ may be 0.

In addition, in the lithium titanium oxide represented by the general compositional formula (3), the element ^M2 is a component for increasing the electronic conductivity of the lithium titanium oxide, and t, which represents the ratio of the element ^M2 , is 0. When ≦t<5/3, the effect of improving electronic conductivity can be satisfactorily ensured.

For the negative electrode active material, a negative electrode active material other than lithium titanium oxide used in lithium ion secondary batteries etc. can also be used together with lithium titanium oxide. However, the proportion of negative electrode active materials other than lithium titanium oxide in the total amount of negative electrode active materials is preferably 30% by mass or less.

The content of the negative electrode active material in the negative electrode mixture is 40% by mass or more, preferably 45% by mass or more, from the viewpoint of improving the output characteristics of the all-solid-state battery and further increasing the capacity. The content is 60% by mass or less, preferably 55% by mass.

It is preferable to use a sulfide-based solid electrolyte as the solid electrolyte of the negative electrode. Sulfide-based solid electrolytes have particularly excellent ionic conductivity among solid electrolytes that can be used in all-solid-state batteries, and as mentioned above, by using them not only for the negative electrode but also for the positive electrode and solid electrolyte layer, Improves battery output characteristics.

As the sulfide-based solid electrolyte, it is more preferable to use the same solid electrolyte as exemplified above as the solid electrolyte that can be used for the positive electrode.

Although only a sulfide-based solid electrolyte may be used for the negative electrode, other solid electrolytes may also be used together with the sulfide-based solid electrolyte. Examples of the solid electrolyte that can be used in combination with the sulfide solid electrolyte include the same hydride solid electrolytes and oxide solid electrolytes as those exemplified above as those that can be used in the positive electrode.

However, the proportion of solid electrolytes other than the sulfide-based solid electrolyte in the total amount of solid electrolytes used in the negative electrode mixture is preferably 30% by mass or less.

The content of the solid electrolyte in the negative electrode mixture is 50 parts by mass or more when the content of the negative electrode active material is 100 parts by mass, from the viewpoint of further enhancing the output characteristics of the all-solid-state battery and further increasing the capacity. It is preferably 60 parts by mass or more, more preferably 70 parts by mass or more, and preferably 130 parts by mass or less, more preferably 120 parts by mass or less. , more preferably 110 parts by mass or less.

A carbon material such as carbon black can be used as a conductive additive for the negative electrode. The content of the conductive additive in the negative electrode mixture is 10 parts by mass when the content of the negative electrode active material is 100 parts by mass, from the viewpoint of further enhancing the output characteristics of the all-solid-state battery and further increasing the capacity. It is preferably at least 12 parts by mass, more preferably at least 15 parts by mass, further preferably at most 30 parts by mass, and more preferably at most 25 parts by mass. The amount is preferably 22 parts by mass or less, and more preferably 22 parts by mass or less.

The negative electrode mixture does not need to contain a resin binder, or may contain it. Examples of the resin binder include fluororesins such as polyvinylidene fluoride (PVDF). However, since the resin binder also acts as a resistance component in the negative electrode mixture, it is desirable that its amount be as small as possible. Therefore, similarly to the positive electrode mixture, it is preferable that the negative electrode mixture does not contain a resin binder, or if it does contain a resin binder, the content thereof is 0.5% by mass or less. The content of the resinous binder in the negative electrode mixture is more preferably 0.3% by mass or less, and even more preferably 0% by mass (that is, no resinous binder is contained).

When a current collector is used for the negative electrode, copper or nickel foil, punched metal, net, expanded metal, foamed metal, carbon sheet, etc. can be used as the current collector.

The negative electrode mixture molded body is prepared by, for example, mixing a negative electrode active material, a solid electrolyte, a conductive aid, and a binder added depending on the non-vaginal use, and then molding the negative electrode mixture by pressure molding or the like. It can be formed by compression.

In the case of a negative electrode having a current collector, it can be manufactured by bonding a molded negative electrode mixture formed by the method described above to the current collector by pressing or the like.

The thickness of the molded body of the negative electrode mixture (in the case of a negative electrode with a current collector, the thickness of the molded body of the positive electrode mixture per one side of the current collector; the same applies hereinafter) is determined from the viewpoint of increasing the capacity of the battery. , 200 μm or more. Note that the output characteristics of a battery are generally easily improved by making the positive electrode and negative electrode thinner, but according to the present invention, the output characteristics can be improved even when the negative electrode mixture molded body is as thick as 200 μm or more. It is possible. Therefore, in the present invention, the effect becomes more pronounced when the thickness of the molded body of the negative electrode mixture is, for example, 200 μm or more. In the present invention, the effect becomes particularly remarkable when the thickness of the molded body of the positive electrode mixture is 200 μm or more and the thickness of the molded body of the negative electrode mixture is 200 μm or more. Further, the thickness of the molded body of the negative electrode mixture is usually 3000 μm or less.

(solid electrolyte layer)
It is also preferable to use a sulfide-based solid electrolyte as the solid electrolyte in the solid electrolyte layer of an all-solid-state battery. Sulfide-based solid electrolytes have particularly excellent ionic conductivity among solid electrolytes that can be used in all-solid-state batteries, and as mentioned above, by using them not only for the solid electrolyte layer but also for the positive and negative electrodes, Improves battery output characteristics.

As the sulfide-based solid electrolyte, it is more preferable to use the same solid electrolyte as exemplified above as the solid electrolyte that can be used for the positive electrode.

Although only a sulfide-based solid electrolyte may be used in the solid electrolyte layer, other solid electrolytes may also be used together with the sulfide-based solid electrolyte. Examples of the solid electrolyte that can be used in combination with the sulfide solid electrolyte include the same hydride solid electrolytes and oxide solid electrolytes as those exemplified above as those that can be used in the positive electrode.

However, the proportion of solid electrolytes other than the sulfide-based solid electrolyte in the total amount of solid electrolytes used in the solid electrolyte layer is preferably 30% by mass or less.

The solid electrolyte layer is prepared by dispersing the solid electrolyte in a solvent, applying the composition for forming the solid electrolyte layer onto the base material, the positive electrode, and the negative electrode, drying it, and performing pressure forming such as press treatment as necessary. It can be formed by doing.

The solvent used in the solid electrolyte layer forming composition is preferably selected from a solvent that does not easily deteriorate the solid electrolyte. In particular, sulfide-based solid electrolytes and hydride-based solid electrolytes cause chemical reactions with minute amounts of water, so hydrocarbon solvents such as hexane, heptane, octane, nonane, decane, decalin, toluene, and xylene are typical. Preference is given to using non-polar aprotic solvents. In particular, it is more preferable to use a super dehydrated solvent with a water content of 0.001% by mass (10 ppm) or less. In addition, fluorinated solvents such as "Vertrell (registered trademark)" manufactured by Mitsui-DuPont Fluorochemicals, "Zeorolla (registered trademark)" manufactured by Nippon Zeon, and "Novec (registered trademark)" manufactured by Sumitomo 3M, Non-aqueous organic solvents such as , dichloromethane and diethyl ether can also be used.

The thickness of the solid electrolyte layer is preferably 15 to 300 μm.

(electrode body)
The positive electrode and the negative electrode can be used in a battery in the form of a laminated electrode body laminated with a solid electrolyte layer in between, or a wound electrode body in which this laminated electrode body is further wound.

Note that when forming the electrode body, it is preferable to press-mold the positive electrode, the negative electrode, and the solid electrolyte layer in a laminated state from the viewpoint of increasing the mechanical strength of the electrode body.

(Battery form)
A cross-sectional view schematically showing an example of the all-solid-state battery of the present invention is shown in FIG. The battery 1 shown in FIG. 1 includes a positive electrode 10, a negative electrode 20, and a positive electrode 10 and a negative electrode 20 in an external case formed of an external can 40, a sealed can 50, and a resin gasket 60 interposed between these. A solid electrolyte layer 30 interposed therebetween is enclosed.

The sealing can 50 is fitted into the opening of the outer can 40 via a gasket 60, and the open end of the outer can 40 is tightened inward, causing the gasket 60 to come into contact with the sealing can 50. The opening of the outer can 40 is sealed, so that the inside of the element has a hermetically sealed structure.

Stainless steel can be used for the outer can and sealed can. In addition, polypropylene, nylon, etc. can be used as the material for the gasket, and if heat resistance is required due to battery usage, materials such as tetrafluoroethylene-perfluoroalkoxyethylene copolymer (PFA) can be used. Heat-resistant materials with melting points exceeding 240°C such as fluororesin, polyphenylene ether (PEE), polysulfone (PSF), polyarylate (PAR), polyether sulfone (PES), polyphenylene sulfide (PPS), and polyether ether ketone (PEEK) Resins can also be used. Furthermore, when the battery is used in applications requiring heat resistance, a glass hermetic seal can also be used to seal the battery.

The form of an all-solid-state battery is one that has an exterior body consisting of an exterior can, a sealed can, and a gasket, as shown in Figure 1, that is, a type that is generally referred to as a coin-shaped battery or a button-shaped battery. Examples include, but are not limited to, those having an exterior made of a resin film or a metal-resin laminate film, or a metallic exterior can with a cylindrical shape (cylindrical or rectangular shape) and its opening sealed. It may have an exterior body having a sealing structure for sealing.

The all-solid-state battery of the present invention can be applied to the same applications as conventionally known secondary batteries, but since it has a solid electrolyte instead of an organic electrolyte, it has excellent heat resistance and high temperature It can be preferably used in applications where it is exposed to

<All-solid-state battery system>
The all-solid-state battery system of the present invention includes the all-solid-state battery of the present invention and a charging device, and the upper limit of the voltage applied by the charging device to the all-solid-state battery is 2.8 V or less (preferably 2.5V or higher). This allows the all-solid-state battery used in such a system to exhibit high capacity and good output characteristics. The charging device related to the all-solid-state battery system of the present invention may be of any type as long as it can charge the all-solid-state battery under the condition that the final voltage is 2.8 V or less (preferably 2.5 V or more). For example, a charging device that can perform constant voltage charging after constant current charging or a charging device that can perform pulse charging can be used.

The all-solid-state battery system of the present invention can be applied to the same applications as conventionally known secondary batteries and secondary battery systems, but since it has a solid electrolyte instead of an organic electrolyte, It has excellent heat resistance and can be preferably used in applications that are exposed to high temperatures.

Hereinafter, the present invention will be described in detail based on Examples. However, the following examples do not limit the present invention.

Example 1
<Preparation of positive electrode>
Carbon black whose primary particles have an average particle diameter of 200 nm and pores of 2 nm or less: 9 parts by mass, Co(CH ₃ COO) _{2.4H 2} O: 99.6 parts by mass, LiOH.H ₂ O: After stirring for 1 hour, the mixture was filtered to obtain a mixture containing carbon black.

Next, 30 parts by mass of LiOH.H ₂ O was added to the mixture and heated in air at 250° C. for 30 minutes using an evaporator to obtain a composite in which a lithium cobalt compound was supported on carbon black. This complex was placed in a mixed aqueous solution of 98% concentrated sulfuric acid, 70% concentrated nitric acid, and 30% hydrochloric acid in a volume ratio of 1:1:1, and ultrasonic waves were irradiated to dissolve the complex. The lithium cobalt compound was dissolved and the remaining solid was filtered, washed with water and dried.

By repeating the steps of dissolving the lithium cobalt compound with the mixed aqueous solution, filtration, washing with water, and drying, the lithium cobalt compound was completely removed, and granular carbon containing a hydrophilic portion in a proportion of 10% by mass or more was obtained. The obtained granular carbon had a ratio of the longest diameter to the shortest diameter in the state of primary particles of 1.1, and the average particle diameter of the primary particles of the granular carbon was 200 nm.

Carbon nanotubes [VGCF (trade name) manufactured by Showa Denko Co., Ltd., fibrous carbon, ratio of fiber length to fiber diameter of 30 or more] and the granular carbon were mixed using a planetary ball mill at a mass ratio of 2: Dry mixing was carried out for 60 minutes at a ratio of 1:1 to obtain a composite of fibrous carbon and granular carbon.

LiCo _0.98 Al _0.01 Mg _0.01 O ₂ (positive electrode active material) having an average particle size of 5 μm and a layer made of LiNbO ₃ on the surface, and a sulfide solid electrolyte having an argyrodite structure with an average particle size of 3 μm. (Li _5.4 PS _4.4 Cl _0.8 Br _0.8 ) and the composite of fibrous carbon and granular carbon (conductivity aid) in a mass ratio of 65:31:4. A positive electrode mixture was prepared by mixing.
Next, 110 mg of the positive electrode mixture was put into a powder molding mold with a diameter of 10 mm, and pressure molded using a press machine to produce a positive electrode consisting of a cylindrical positive electrode mixture molded body. Note that the amount of the layer consisting of LiNbO ₃ on the surface of LiCo _0.98 Al _0.01 Mg _0.01 O ₂ is 0 per 100 parts by mass of LiCo _0.98 Al _0.01 Mg _0.01 O ₂ It was .5 parts by mass.

<Formation of solid electrolyte layer>
Next, 15 mg of the sulfide solid electrolyte was placed on top of the positive electrode mixture molded body in the powder molding mold, and pressure molding was performed using a press machine. A solid electrolyte layer was formed on the surface.

<Preparation of negative electrode>
The mass of lithium titanate (Li ₄ Ti ₅ O ₁₂ , negative electrode active material) with an average particle diameter of 7 μm, the sulfide solid electrolyte, and carbon nanotubes as a conductive agent [VGCF (trade name) manufactured by Showa Denko K.K. They were mixed in a ratio of 50:41:9 and thoroughly kneaded to prepare a negative electrode mixture. Next, 140 mg of the negative electrode mixture was placed on top of the solid electrolyte layer in the powder molding mold, and pressure molded using a press, so that a molded negative electrode mixture was placed on top of the solid electrolyte layer. By forming a negative electrode consisting of the following, a laminate in which a positive electrode, a solid electrolyte layer, and a negative electrode were stacked was produced.

A copper foam base material made by Sumitomo Electric Co., Ltd. [Copper Celmet (trade name), thickness: 1 mm, porosity: 97%] was punched out to a size of 6 mmφ and placed on the inner bottom surface of a stainless steel sealed can. Then, on top of that, the laminate of the positive electrode/solid electrolyte layer/negative electrode was stacked with the negative electrode facing the base material side, and then an aluminum foam base manufactured by Sumitomo Electric Industries, Ltd., punched out to the same size as above was added. After placing the material [Celmet (trade name) made of aluminum, thickness: 1 mm, porosity: 97%] on the positive electrode of the laminate, a stainless steel exterior can is covered and sealed. , we fabricated a flat all-solid-state battery.
This flat type all-solid-state battery was combined with a charging/discharging device (a device for charging and discharging the battery) to construct an all-solid-state battery system with a charging upper limit voltage of 2.6V.

Example 2
A flat all-solid-state battery was produced in the same manner as in Example 1, except that the ratio of the positive electrode active material, sulfide-based solid electrolyte, and conductive additive in the positive electrode mixture was changed to 50:44:6 in terms of mass ratio. An all-solid-state battery system was constructed in the same manner as in Example 1 except that this flat-type all-solid-state battery was used.

Example 3
A flat all-solid-state battery was produced in the same manner as in Example 1, except that the ratio of the positive electrode active material, sulfide-based solid electrolyte, and conductive additive in the positive electrode mixture was changed to 80:18:2 in terms of mass ratio. An all-solid-state battery system was constructed in the same manner as in Example 1 except that this flat-type all-solid-state battery was used.

Example 4
A flat all-solid-state battery was created in the same manner as in Example 1. This flat type all-solid-state battery was combined with a charging/discharging device (device for charging and discharging the battery) to construct an all-solid-state battery system with a charging upper limit voltage of 2.8V.

Example 5
Except that the positive electrode active material in the positive electrode mixture was LiCo _0.98 Al _0.01 Mg _0.01 O ₂ (which does not have a layer consisting of LiNbO ₃ on the surface) with an average particle size of 5 μm. A flat all-solid-state battery was produced in the same manner as in Example 1. An all-solid-state battery system was constructed in the same manner as in Example 1 except that this flat-type all-solid-state battery was used.

Comparative example 1
A flat all-solid-state battery was produced in the same manner as in Example 1, except that the ratio of the positive electrode active material, sulfide-based solid electrolyte, and conductive additive in the positive electrode mixture was changed to 45:49:6 in terms of mass ratio. . An all-solid-state battery system was constructed in the same manner as in Example 1 except that this flat-type all-solid-state battery was used.

Comparative example 2
A flat all-solid-state battery was produced in the same manner as in Example 1, except that the ratio of the positive electrode active material, sulfide-based solid electrolyte, and conductive additive in the positive electrode mixture was changed to 85:13:2 in terms of mass ratio. . An all-solid-state battery system was constructed in the same manner as in Example 1 except that this flat-type all-solid-state battery was used.

Comparative example 3
A flat all-solid-state battery was produced in the same manner as in Example 1 except that Li _7.0 PS _5.4 Cl _1.2 was used as the solid electrolyte in the positive electrode mixture. An all-solid-state battery system was constructed in the same manner as in Example 1 except that this flat-type all-solid-state battery was used.

Comparative example 4
A flat all-solid-state battery was created in the same manner as in Example 1. This flat type all-solid-state battery was combined with a charging/discharging device (a device for charging and discharging the battery) to construct an all-solid-state battery system with a charging upper limit voltage of 3.0V.

For the all-solid-state battery systems of Examples and Comparative Examples, initial capacity measurements and output characteristics were evaluated using the following methods.

<Initial capacity measurement>
Each all-solid-state battery system of the example and comparative example was charged at a constant current with a current value of 0.02C until the voltage reached the charging upper limit voltage of each all-solid-state battery system, and then the current value was 0.002C. The battery was charged at a constant voltage until the voltage reached 1V, and then discharged at a current value of 0.02C until the voltage reached 1V, and the discharge capacity (initial capacity) at that time was measured.

<Output characteristics evaluation>
For each all-solid-state battery system of Examples and Comparative Examples, constant current charging and constant voltage charging were performed under the same conditions as when measuring the initial capacity, and then discharged at a current value of 0.2C until the voltage reached 1V. The discharge capacity (0.2C discharge capacity) at this time was measured.

Then, for each battery, the value obtained by dividing the 0.2C discharge capacity by the initial capacity was expressed as a percentage to determine the capacity retention rate, and the output characteristics were evaluated.

<Evaluation of output characteristics after storage>
For each all-solid-state battery system in Examples and Comparative Examples, the initial capacity was measured by performing constant-current charging and constant-voltage charging under the same conditions as when measuring the initial capacity, and then the all-solid-state battery in each all-solid-state battery system was was stored at 85°C for 5 hours. Thereafter, the temperature of each battery was returned to room temperature, and then discharged at a current value of 0.2C until the voltage reached 1V, and the discharge capacity at this time (0.2C discharge capacity after storage) was measured.
Then, for each battery, the output characteristics after storage were evaluated by dividing the 0.2C discharge capacity after storage by the initial capacity and expressing it as a percentage.

As shown in Table 1, by setting the amount of the positive electrode active material in the positive electrode mixture within a specific range and using a sulfide-based solid electrolyte with a specific composition as the solid electrolyte of the positive electrode, an all-solid-state battery with high output characteristics and capacity can be obtained. Furthermore, it can be seen that by specifying the charging upper limit voltage in the all-solid-state battery system of the present invention, the all-solid-state battery can exhibit high capacity and good output characteristics.

1 All solid battery 10 Positive electrode 20 Negative electrode 30 Solid electrolyte layer 40 Exterior can 50 Sealing can 60 Gasket

Claims (3)

In an all-solid-state battery system that includes an all-solid-state battery and a charging device,
Charging the all-solid-state battery to a voltage of 2.8V as an upper limit,
The all-solid-state battery includes a positive electrode, a negative electrode, and a solid electrolyte layer interposed between the positive electrode and the negative electrode,
The positive electrode has a molded positive electrode mixture containing a positive electrode active material, a solid electrolyte, and a conductive additive,
The positive electrode active material contains lithium cobalt oxide,
The solid electrolyte has the following general formula (1)
Li _7-x PS _6-x Cl _y Br _z (1)
[In the general formula (1), x=y+z, 1.0<x≦1.8, 0.1≦z/y≦10.0]
It is a sulfide-based solid electrolyte represented by
The content of the positive electrode active material in the positive electrode mixture is 50 to 80% by mass,
The negative electrode has a molded body of a negative electrode mixture containing a negative electrode active material, a solid electrolyte, and a conductive additive,
An all-solid-state battery system , wherein the negative electrode active material contains lithium titanium oxide . The all-solid-state battery system according to claim 1, wherein the all-solid-state battery is charged to a voltage of 2.6V as an upper limit. 3. The all-solid-state battery system according to claim 1, wherein the lithium cobalt oxide has a reaction suppression layer on at least a portion of the surface thereof to suppress a reaction between the lithium cobalt oxide and the solid electrolyte.

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