JP6738121B2 - Lithium ion secondary battery - Google Patents

Description

The present invention relates to a lithium ion secondary battery.

Lithium-ion secondary batteries have large charge/discharge capacity, high operating potential, and excellent charge/discharge cycle characteristics, so they can be used to power portable information terminals, portable electronic devices, household small power storage devices, and motors. There is an increasing demand for applications such as motorcycles, electric vehicles, hybrid electric vehicles, and the like. In a lithium ion secondary battery, a non-aqueous electrolytic solution in which a lithium salt is dissolved in an organic solvent is used as an electrolyte, but such a non-aqueous electrolytic solution is easy to ignite and leaks of the electrolytic solution. From the problem of, there is concern about safety. Therefore, in recent years, for the purpose of improving the safety of the lithium-ion secondary battery, an all-solid-state lithium-ion secondary battery (hereinafter, also referred to as “all-solid-state secondary battery”) using a solid electrolyte made of an inorganic material that is an incombustible material. Research is being actively conducted.

Although sulfides, oxides, and the like can be used as the solid electrolyte of the all-solid secondary battery, the sulfide-based solid electrolyte is the most promising material from the viewpoint of lithium ion conductivity. However, when a sulfide-based solid electrolyte is used, a reaction occurs at the interface between the positive electrode active material particles and the solid electrolyte particles during charging, and a resistance component is generated at this interface, which results in positive electrode active material particles. The resistance (hereinafter, also referred to as “interface resistance”) when lithium ions move at the interface between the solid electrolyte particles and the solid electrolyte particles is likely to increase. Due to this increase in interfacial resistance, the lithium ion conductivity is reduced, and there is a problem that the output of the lithium ion secondary battery is reduced.

For such a problem, it has been studied to coat the surface of positive electrode active material particles such as LiCoO ₂ (hereinafter, also referred to as “LCO”) with another substance to reduce the interface resistance.

For example, Non-Patent Document 1 discloses a technique of coating LCO with SiO ₂ or Li ₂ SiO ₃ , and Non-Patent Document 2 discloses a technique of coating LCO with Li ₂ Ti ₂ O ₅ . Further, Patent Documents 1 and 2 disclose a technique of coating particles of a positive electrode active material such as LCO with ZrO ₂ . Further, in Patent Document 3, the surface of the positive electrode active material particles is coated with an oxide such as aluminum oxide (aluminium), zirconium oxide (zirconium), titanium oxide (titanium), boron oxide (boron), or silicon oxide (silicon). Techniques for doing so are disclosed.

Further, in Patent Documents 4 and 5, instead of coating the particle surface of the positive electrode active material, the deviation of lithium ions in the vicinity of the interface between the positive electrode layer and the sulfide-based solid electrolyte layer is prevented. A technique of providing a buffer layer for buffering and an intermediate layer for suppressing mutual diffusion between both layers is disclosed.

Japanese Patent Publication No. 2009-541938 Special table 2011-515139 gazette JP, 2008-103204, A JP, 2010-40439, A JP, 2011-44368, A

J. Power sources, 189, pp. 527-530, 2009 J. Power sources, 195, pp. 599-603, 2010

However, as in the technologies disclosed in Non-Patent Documents 1 and 2 and Patent Documents 1 to 5, the surface of the positive electrode active material particles is coated with an oxide such as SiO _2, or the positive electrode layer and the solid electrolyte layer are formed. It is not enough to suppress the reaction at the interface between the positive electrode active material particles and the solid electrolyte particles simply by providing a buffer layer or an intermediate layer between them, and further reduction of the resistance component is desired. There is.

Therefore, the present invention has been made in view of the above circumstances, and provides a lithium ion secondary battery capable of further suppressing the reaction at the interface between the positive electrode active material particles and the sulfide-based solid electrolyte particles. The purpose is to do.

In order to solve the above problems, according to an aspect of the present invention, a coating particle including a coating layer that covers the positive electrode active material particles and the positive electrode active material particles, and a sulfide-based solid electrolyte particle that contacts the coating particles, The coating layer includes, of the elements other than lithium and oxygen, a highly reactive element having a higher reactivity with the sulfide-based solid electrolyte particles than the transition metal element in the positive electrode active material particles, and the layer thickness of the coating layer. And a diameter of the positive electrode active material particles (a value obtained by dividing the layer thickness of the coating layer by the diameter of the positive electrode active material particles) is 0.0010 to 0.25. Provided. It is preferably 0.0016 to 0.1. More preferably, it is 0.0016 to 0.01.

From this viewpoint, since the highly reactive element in the coating layer reacts preferentially with the sulfur element in the sulfide-based solid electrolyte particles, the reaction between the transition metal element and the sulfur element in the positive electrode active material particles (sub-element) Reaction) can be suppressed. That is, it becomes possible to further suppress the reaction at the interface between the positive electrode active material particles and the sulfide-based solid electrolyte particles.

Here, the highly reactive element may have a lower standard enthalpy of sulfide formation than the transition metal element in the positive electrode active material particles.

From this viewpoint, it becomes possible to further suppress the reaction at the interface between the positive electrode active material particles and the sulfide-based solid electrolyte particles.

The standard enthalpy of sulfide formation of highly reactive elements may be less than -80 kJ/mol.

From this viewpoint, it becomes possible to further suppress the reaction at the interface between the positive electrode active material particles and the sulfide-based solid electrolyte particles.

In addition, while performing the first DSC test in which the coated particles and the sulfide-based solid electrolyte particles are mixed and heated at a mass ratio of 1:1, the positive electrode active material particles and the sulfide-based solid particles not covered with the coating layer are performed. When the second DSC test in which the electrolyte particles are mixed at a mass ratio of 1:1 and heated is performed, the starting temperature of the exothermic reaction in the first DSC test is the starting temperature of the exothermic reaction in the second DSC test. May be higher than.

From this viewpoint, it becomes possible to further suppress the reaction at the interface between the positive electrode active material particles and the sulfide-based solid electrolyte particles.

The starting temperature of the exothermic reaction in the first DSC test may be higher than 250°C.

From this viewpoint, it becomes possible to further suppress the reaction at the interface between the positive electrode active material particles and the sulfide-based solid electrolyte particles.

The temperature at which the amount of heat generated in the first DSC test is maximum may be higher than 330°C.

From this viewpoint, it becomes possible to further suppress the reaction at the interface between the positive electrode active material particles and the sulfide-based solid electrolyte particles.

Further, some of the highly reactive elements may be solid-dissolved in the positive electrode active material particles.

From this viewpoint, it becomes possible to further suppress the reaction at the interface between the positive electrode active material particles and the sulfide-based solid electrolyte particles.

Further, the highly reactive element may be at least one selected from the group consisting of aluminum, cobalt, manganese, and magnesium.

From this viewpoint, it becomes possible to further suppress the reaction at the interface between the positive electrode active material particles and the sulfide-based solid electrolyte particles.

In addition, the sulfide-based solid electrolyte particles may contain phosphorus.

From this viewpoint, it becomes possible to further suppress the reaction at the interface between the positive electrode active material particles and the sulfide-based solid electrolyte particles.

As described above, according to the present invention, it is possible to further suppress the reaction at the interface between the positive electrode active material particles and the sulfide-based solid electrolyte particles.

It is an explanatory view showing typically composition of a lithium ion secondary battery concerning a preferred embodiment of the present invention. 3 is a graph obtained by performing differential scanning calorimetry (DSC) on a mixture of positive electrode active material particles and sulfide-based solid electrolyte particles. It is a graph which shows the evaluation result of the impedance (impedance) of an Example and a comparative example. It is explanatory drawing which shows a mode that the interface resistance in the conventional all-solid-state lithium ion secondary battery increases.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and the drawings, constituent elements having substantially the same functional configuration are designated by the same reference numerals, and a duplicate description will be omitted.

<1. Problems when using a solid electrolyte>
First, with reference to FIG. 4, before describing the lithium-ion secondary battery according to the preferred embodiment of the present invention, problems in the case of using a solid electrolyte will be described. FIG. 4 is an explanatory diagram showing a schematic configuration of a conventional lithium ion secondary battery 100.

The lithium-ion secondary battery 100 has a structure in which a positive electrode layer 110, a negative electrode layer 120, and a solid electrolyte layer 130 are laminated. The positive electrode layer 110 is composed of mixed particles in which positive electrode active material particles 111 and sulfide-based solid electrolyte particles 131 (hereinafter, also referred to as “solid electrolyte particles 131”) are mixed. Similarly, the negative electrode layer 120 is composed of mixed particles obtained by mixing the negative electrode active material particles 121 and the solid electrolyte particles 131. The solid electrolyte layer 130 is provided between the positive electrode layer 110 and the negative electrode layer 120. The solid electrolyte layer 130 is composed of solid electrolyte particles 131.

In the lithium-ion secondary battery 100 using the sulfide-based solid electrolyte, since the positive electrode active material and the electrolyte are solid, the electrolyte is less likely to penetrate into the positive electrode active material than in the case of using the organic electrolytic solution as the electrolyte, Since the area of the interface between the positive electrode active material and the electrolyte tends to decrease, it is difficult to secure a sufficient migration path for lithium ions and electrons. Therefore, as shown in FIG. 4, the positive electrode layer 110 is constituted by mixed particles obtained by mixing the positive electrode active material particles 111 and the sulfide-based solid electrolyte particles 131, and the negative electrode active material particles 121 and the sulfide-based solid electrolyte particles 131 are formed. The negative electrode layer 120 is constituted by mixed particles obtained by mixing the above. This increases the area of the interface between the active material and the solid electrolyte.

However, as described above, a reaction occurs at the interface between the positive electrode active material particles 111 and the solid electrolyte particles 131 during charging, and the high resistance layer 150 is formed. Specifically, the high resistance layer 150 is generated by the reaction between the transition metal element existing on the surface of the positive electrode active material particles 111 and the sulfur element existing on the surface of the solid electrolyte particles 131. Here, the “high resistance layer 150 ”is a layer composed of a resistance component formed at the interface between the positive electrode active material particles 111 and the solid electrolyte particles 131, and the inside of the positive electrode active material particles 111 and the sulfide-based solid. It means a layer having a higher resistance when lithium ions move than the electrolyte particles 131. Therefore, the interface resistance between the positive electrode active material particles 111 and the solid electrolyte particles 131 is likely to increase. When the area of the interface between the positive electrode active material particles 111 and the solid electrolyte particles 131 is increased, the movement paths of lithium ions and electrons can be secured, but the high resistance layer 150 is easily formed. Therefore, the high resistance layer 150 inhibits the movement of lithium ions from the positive electrode active material particles 111 to the solid electrolyte particles 131. As a result, the lithium ion conductivity is reduced, so that the output of the lithium ion secondary battery 100 is reduced.

<2. Examination by the Inventor>
It was thought that the high resistance layer 150 was generated due to a difference in chemical potential between lithium ions in the positive electrode active material particles 111 and lithium ions in the solid electrolyte particles 131. However, until now, a technique capable of sufficiently suppressing the formation of the high resistance layer 150 has not been established.

Therefore, the present inventor considered that there may be factors other than the difference in the chemical potential of lithium ions that affect the formation of the high resistance layer 150, and investigated thermodynamic data of various metal sulfides. As a result, the present inventor has found that the reactivity between the metal element contained in the positive electrode active material particles 111 and the sulfur element contained in the solid electrolyte particles 131 has a great influence on the formation of the high resistance layer 150.

Then, the present inventor has found that the reactivity with the sulfur element contained in the solid electrolyte particles 131 (hereinafter, also simply referred to as “reactivity with the solid electrolyte particles 131”) is higher than that of the transition metal element in the positive electrode active material particles 111. It has been found that by coating the positive electrode active material particles 11 with a high metal element (hereinafter, such a metal element is also referred to as “highly reactive element”), the generation of the high resistance layer 150 is significantly suppressed.

With respect to this phenomenon, the present inventor preferentially reacts the highly reactive element with the sulfur element in the solid electrolyte particles 131 rather than the transition metal element in the positive electrode active material particles 111, so that the transition metal element and the sulfur element I think that the reaction of is suppressed.

Further, the present inventor has examined an index for classifying a metal element having high reactivity with the solid electrolyte particles 131 (that is, a highly reactive element) and a metal element having low reactivity, and the sulfide standard formation enthalpy of the metal element is an index. I found that. That is, the present inventor has found that the lower the sulfide standard enthalpy of formation of a metal element (the larger the negative enthalpy), the higher the reactivity of the metal element with the sulfide-based solid electrolyte particles 13.

The present inventor has arrived at the lithium-ion secondary battery according to the present embodiment based on the above findings. As shown in FIG. 1, in the lithium-ion secondary battery 1 according to the present embodiment, by covering the positive electrode active material particles 11 with the coating layer 12 containing a highly reactive element, generation of a high resistance layer can be suppressed. it can. Hereinafter, the lithium-ion secondary battery 1 according to this embodiment will be described in detail.

<3. Lithium-ion secondary battery configuration>
Subsequently, the configuration of the lithium-ion secondary battery according to the preferred embodiment of the present invention will be described in detail with reference to FIG. FIG. 1 is an explanatory diagram schematically showing the configuration of the lithium-ion secondary battery 1 according to this embodiment.

As shown in FIG. 1, the lithium-ion secondary battery 1 according to the present embodiment is an all-solid-state lithium-ion secondary battery, and includes a positive electrode layer 10, a negative electrode layer 20, a positive electrode layer 10 and a negative electrode layer 20. It has a structure in which a solid electrolyte layer 30 provided therebetween is laminated.

(2.1. Positive electrode layer 10)
The positive electrode layer 10 contains mixed particles obtained by mixing the coated particles 10a and the sulfide-based solid electrolyte particles 31 (hereinafter, also referred to as “solid electrolyte particles 31”). The coated particle 10 a includes a positive electrode active material particle 11 and a coating layer 12 that covers the surface of the positive electrode active material particle 11. Therefore, the coating layer 12 contacts the solid electrolyte particles 31. As described above, in the lithium ion secondary battery 100 using the solid electrolyte particles 131, the interface resistance increases due to the reaction at the interface between the positive electrode active material particles 111 and the solid electrolyte particles 131, and the battery output decreases. There's a problem. However, according to the all-solid-state lithium ion secondary battery 1 according to the present embodiment, since the surface of the positive electrode active material particles 11 is covered with the coating layer 12 containing the highly reactive element, the coating layer 12 concerned. Can prevent the reaction (side reaction) between the sulfur element in the solid electrolyte particles 31 and the transition metal element in the positive electrode active material particles 11. Therefore, the resistance component (high resistance layer) is less likely to be generated at the interface between the positive electrode active material particles 11 and the solid electrolyte particles 31.

At least a part of the surface of the positive electrode active material particles 11 may be covered with the coating layer 12. That is, the entire surface of the positive electrode active material particles 11 may be covered with the coating layer 12, or the surface of the positive electrode active material particles 11 may be partially covered with the coating layer 12.

In addition, the fact that the coating layer 12 containing a highly reactive element is formed on the particle surface of the positive electrode active material particles 11 may cause a difference in contrast due to a structural difference between the positive electrode active material particles 11 and the coating layer 12, for example. The difference can be confirmed by a method such as a microscope image analysis (field emission scanning electron microscope (FE-SEM) or transmission electron microscope (TEM) image) analysis. Hereinafter, the positive electrode active material particles 11 and the coating layer 12 included in the positive electrode layer 10 will be described in detail.

(Cathode active material particles 11)
The positive electrode active material forming the positive electrode active material particles 11 is not particularly limited as long as it is a substance capable of reversibly occluding and releasing lithium ions, and examples thereof include lithium cobalt oxide (LCO) and lithium nickel oxide. Lithium nickel cobalt oxide, lithium nickel cobalt aluminum oxide (hereinafter sometimes referred to as “NCA”), lithium nickel cobalt manganate (hereinafter sometimes referred to as “NCM”), lithium manganate, iron phosphate Examples thereof include lithium, nickel sulfide, copper sulfide, sulfur, iron oxide and vanadium oxide. These positive electrode active materials may be used alone or in combination of two or more kinds.

Among the examples of the positive electrode active material mentioned above, the positive electrode active material particles 11 are particularly preferably a lithium salt of a transition metal oxide having a layered rock salt structure. The term "layered" as used herein means a thin sheet-like shape, and the "rock salt type structure" means a sodium chloride type structure which is one of the crystal structures, and is composed of cations and anions. The face-centered cubic lattices formed by each point are shifted from each other by ½ of the edge of the unit lattice. Examples of the lithium salt of the transition metal oxide having such a layered rock salt type structure include, for example, Li _{2−x−y−z} Ni _x Co _y Al _z O ₂ (NCA) or Li _{2−x−y−z} Ni. Lithium salt of ternary transition metal oxide represented by _x Co _y Mn _z O ₂ (NCM) (0<x<1, 0<y<1, 0<z<1, and x+y+z is 1 or less) Are listed.

As described above, by using the lithium salt of the ternary transition metal oxide as the positive electrode active material particles 11, an all-solid-state lithium ion battery having excellent energy density and thermal stability can be obtained. Further, particles of a lithium salt of a ternary transition metal oxide such as NCA or NCM (present as aggregates of primary particles) are smaller than the particle size and larger in specific surface area than particles such as LCO. (About 10 times). Therefore, the contact area between the positive electrode active material particles 11 and the solid electrolyte particles 31 is increased and the lithium ion conductivity is improved, so that the output of the battery is increased. Further, by including Ni as a constituent element of the positive electrode active material particles 11, the capacity density of the lithium ion secondary battery 1 is increased, and since the metal elution in the charged state is small, the lithium ion secondary battery in the charged state is small. The long term reliability of 1 can be improved.

(Coating layer 12)
The coating layer 12 is a layer containing a highly reactive element as described above. Preferably, the coating layer 12 is composed only of a highly reactive element. Among the elements other than lithium and oxygen, the highly reactive element is more reactive with the sulfur element in the solid electrolyte particles 31 than the transition metal element in the positive electrode active material particles 11 (hereinafter, simply referred to as “solid electrolyte particles 31 It is an element having a high reactivity). More specifically, the highly reactive element is an element having a lower standard enthalpy of sulfide formation than the transition metal element in the positive electrode active material particles 11 among the elements other than lithium and oxygen. When the positive electrode active material particles 11 include a plurality of types of transition metal elements, the highly reactive elements are all transition metal elements included in the positive electrode active material particles 11 (when the positive active material particles 11 include highly reactive elements). , Except for highly reactive elements), has a lower standard enthalpy of sulfide formation. Further, when a plurality of types of sulfides can be produced from highly reactive elements, it is preferable that the standard enthalpies of formation of all sulfides satisfy the above conditions. Further, the coating layer 12 may include a plurality of types of highly reactive elements.

Specifically, the value of the sulfide standard enthalpy of formation of the highly reactive element is preferably -80.0 kJ/mol or less, and preferably -250 kJ/mol. When the value of the sulfide standard enthalpy of formation of the highly reactive element is within these ranges, the formation of the high resistance layer is more reliably suppressed. As described above, the highly reactive element needs to have a lower standard enthalpy of sulfide formation than all transition metal elements contained in the positive electrode active material particles 11. Therefore, the sulfide standard enthalpy of formation of the highly reactive element is preferably a value within the above numerical range while satisfying this condition.

By covering the positive electrode active material particles 11 with such a coating layer 12, the reaction between the positive electrode active material particles 11 and the sulfide solid electrolyte particles 31 is suppressed. The suppression of the reaction can be confirmed by, for example, the DSC test described below. In other words, whether or not a certain metal element is a highly reactive element can be determined based on the results of the following DSC test.

Specifically, the first DSC test in which the coated particles 10a and the solid electrolyte particles 31 are mixed and heated at a mass ratio of 1:1 is performed. Similarly, the second DSC test is performed in which the positive electrode active material particles 11 not covered with the coating layer 12 and the sulfide-based solid electrolyte particles 31 are mixed and heated at a mass ratio of 1:1. As a result, the starting temperature of the exothermic reaction in the first DSC becomes higher than the starting temperature of the exothermic reaction in the second DSC test.

That is, the exothermic reaction is a reaction between the transition metal element in the positive electrode active material particles 11 and the sulfur element in the solid electrolyte particles 31, that is, a side reaction. Therefore, it can be said that the higher the starting temperature of this exothermic reaction, the less likely the side reaction occurs. The onset temperature of the exothermic reaction in the first DSC test is preferably higher than 250°C. When the starting temperature of the exothermic reaction has a value within this range, the formation of the high resistance layer is more reliably suppressed.

Further, the temperature at which the heat generation amount (heat generation amount of exothermic reaction) in the first DSC test is maximum, that is, the peak temperature of so-called exothermic reaction is preferably higher than 330°C, more preferably higher than 350°C. When the peak temperature of the exothermic reaction has a value within these ranges, the formation of the high resistance layer is more reliably suppressed. Table 1 shows examples of highly reactive elements, sulfides of highly reactive elements, and standard enthalpies of formation of sulfides. For reference, the sulfide standard formation enthalpy of nickel element is -53 kJ/mol. Therefore, all of the elements listed in Table 1 have higher reactivity with the solid electrolyte particles 31 than the nickel element.

The coating layer 12 cannot exert its effect if it is too thick or too thin with respect to the diameter of the positive electrode active material particles 11. The ratio between the layer thickness of the coating layer 12 and the diameter of the positive electrode active material particles 11 (the equivalent spherical diameter of the primary particles) (hereinafter, also simply referred to as “diameter layer thickness ratio”) is 0.0010 to 0.25. .. It is preferably 0.0016 to 0.1. More preferably, it is 0.0016 to 0.01. The diameter layer thickness ratio is obtained, for example, by dividing the arithmetic average value of the layer thickness of the coating layer 12 by D50 (median diameter) of the positive electrode active material particles 11. The arithmetic mean value of the layer thickness is calculated by the following method. That is, some of the coated particles 10a are sampled. Then, the layer thickness of the coating layer 12 is calculated for each sampled coated particle 10a. Specifically, some measurement points are set on the coating layer 12, and the layer thickness at this measurement point is measured. Then, the layer thickness of the coating layer 12 is measured by arithmetically averaging the layer thickness at each measurement point. Then, the arithmetical mean value of the layer thickness of the coating layer 12 is calculated (measured) by arithmetically averaging the layer thickness measured for each coated particle 10a. In Examples described later, the arithmetic mean value of the layer thickness was measured by this method. The layer thickness at each measurement point is the cross-sectional observation of the coated particles 10a by a field emission scanning electron microscope (for example, S-4800 manufactured by Hitachi High Technology Co., Ltd.) and energy dispersive X-ray analysis (for example, manufactured by Horiba Ltd.). It can be measured based on the result of elemental analysis by EMAX ENERGY E-350). The D50 of the positive electrode active material particles 11 can be measured by a laser diffraction/scattering particle size distribution measuring device (for example, Microtrack MT-3000II manufactured by Nikkiso Co., Ltd.).

Further, some of the highly reactive elements may be solid-solved in the positive electrode active material particles 11. That is, the highly reactive element may be a constituent element of the positive electrode active material particles 11. However, the concentration of the highly reactive element in the coating layer 12 is higher than the concentration in the positive electrode active material particles 11. It should be noted that the highly reactive element is dissolved in the positive electrode active material particles 11 and the concentration of the highly reactive element can be measured by X-ray photoelectron spectroscopy (XPS) or secondary ion mass spectrometry (SIMS). Is. When the high-reactivity element is solid-solved in the positive electrode active material particles 11, the coated particles 10a include a coating layer 12 in order from the surface of the coated particles 10a, a layer in which the high-reactivity element is solid-solved in the positive electrode active material, and a positive electrode It is composed of layers (particles) made of an active material. Therefore, an element having high reactivity with sulfide can be arranged at a high concentration on the surface side.

As described above, the highly reactive element may be dissolved in the positive electrode active material particles 11 as a solid solution, but it is necessary that the highly reactive element is necessarily unevenly distributed on the surface of the positive electrode active material particles 11. In the all-solid-state lithium-ion secondary battery 1, since the electrolyte is solid, that is, the electrolyte particles 31, it does not enter the positive electrode active material particles 11. Therefore, the side reaction between the electrolyte particles 31 and the positive electrode active material particles 11 occurs at the interface between the solid electrolyte particles 31 and the positive electrode active material particles 11, that is, the surface of the positive electrode active material particles 11. Therefore, it is necessary to care for the surface of the positive electrode active material particles 11. Therefore, in this embodiment, the highly reactive element is unevenly distributed on the surface of the positive electrode active material particles 11 (specifically, the surface of the positive electrode active material particles 11 is covered with the highly reactive element).

(Other additives)
In the positive electrode layer 10, in addition to the coated particles 10a, for example, additives such as a conductive agent, a binder, an electrolyte, a filler, a dispersant, and an ion conductive agent may be appropriately selected and blended.

Examples of the conductive agent include graphite, carbon black, acetylene black, Ketjen black, carbon fiber, and metal powder. Examples of the binder include polytetrafluoroethylene, polyvinylidene fluoride, and polyethylene. Are listed. Examples of the electrolyte include a sulfide-based solid electrolyte described below. As the filler, the dispersant, the ionic conductive agent, etc., known substances that are usually used for electrodes of lithium ion secondary batteries can be used.

(2.2. Negative electrode layer 20)
(Negative electrode active material particles 21)
The negative electrode active material particles 21 included in the negative electrode layer 20 according to the present embodiment are not particularly limited as long as they can be alloyed with lithium or a material capable of reversibly occluding and releasing lithium, for example, lithium , Metals such as indium, tin, aluminum, and silicon and alloys thereof, transition metal oxides such as Li _4/3 Ti _5/3 O ₄ , SnO, artificial graphite, graphite carbon fiber, resin-fired carbon, and thermal decomposition Vapor grown carbon, coke, mesocarbon microbeads (MCMB), furfuryl alcohol resin fired carbon, polyacene, pitch carbon fiber, vapor grown carbon fiber, carbon materials such as natural graphite and non-graphitizable carbon, etc. To be These negative electrode active material particles 21 may be used alone or in combination of two or more kinds.

(Other additives)
The negative electrode layer 20 contains, in addition to the particles of the negative electrode active material particles 21, additives such as a conductive agent, a binder, an electrolyte, a filler, a dispersant, and an ion conductive agent, which are appropriately selected and mixed. Good. Specific examples of these include the same substances as those of the positive electrode layer 10 described above.

(2.3. Solid electrolyte layer 30)
The solid electrolyte layer 30 according to this embodiment includes solid electrolyte particles 31. The solid electrolyte particles 31 are not particularly limited as long as they are sulfide-based solid electrolyte particles. The solid electrolyte particles 31 are preferably sulfide-based solid electrolyte particles containing at least Li, P and S. It is known that this sulfide-based solid electrolyte has higher lithium ion conductivity than other inorganic compounds, and in addition to Li ₂ S and P ₂ S ₅ , SiS ₂ , GeS ₂ , B ₂ S _{3, and the} like. It may contain a sulfide. Further, Li ₃ PO ₄ , halogen, halogen compound, or the like may be added to the solid electrolyte particles 31 as appropriate.

(3. Method for manufacturing lithium-ion secondary battery)
The configuration of the lithium-ion secondary battery 1 according to the preferred embodiment of the present invention has been described above in detail. Next, a method for manufacturing the lithium-ion secondary battery 1 having the above-described configuration will be described. The lithium ion secondary battery 1 can be manufactured by forming the positive electrode layer 10, the negative electrode layer 20, and the solid electrolyte layer 30, and then stacking these layers. Hereinafter, each step will be described in detail.

(3.1. Preparation of coated particles 10a)
First, a method for producing the coated particles 10a will be described. In this example, the coated particles 10a are produced by the so-called coprecipitation method. Of course, the method for producing the coated particles 10a is not limited to this example, and any method may be used as long as it can coat the positive active material particles with the highly reactive element.

First, an aqueous urea solution is added to a nitrate aqueous solution of a highly reactive element, and a transition metal hydroxide that is a raw material of a positive electrode active material is dispersed in the aqueous solution.

Next, urea is decomposed by maintaining the transition metal dispersion liquid at 100° C. in a nitrogen atmosphere. As a result, the pH in the transition metal dispersion liquid rises, so that the hydroxide of the highly reactive element is deposited on the surface of the transition metal hydroxide particles.

The sample obtained is dried and then the sample is mixed with lithium hydroxide powder. The mixture is then fired in air. The firing temperature is not particularly limited, but may be about 1000° C., for example. The coated particles 10a are manufactured by the above steps. Here, the layer thickness of the coating layer 12 is adjusted by fixing at least the concentration of the nitrate aqueous solution of the highly reactive element and adjusting at least one of the mass and the reaction time of the transition metal hydroxide added to the nitrate aqueous solution. Alternatively, it is adjusted by adjusting the firing time. At least one of the mass and the reaction time of the transition metal hydroxide and the firing time may be adjusted. In addition, a part of the highly reactive element in the coating layer 10a may be solid-dissolved in the positive electrode active material particles 11 due to firing. The higher the calcination temperature and the longer the calcination time, the more highly reactive elements will form a solid solution in the positive electrode active material particles 11. However, in this manufacturing method, since the coating layer 12 is composed only of the highly reactive element, the concentration of the highly reactive element in the coating layer 12 is higher than the concentration of the highly reactive element in the positive electrode active material particles 11. ..

(3.2. Preparation of solid electrolyte particles 31)
The method for producing the solid electrolyte particles 31 is not particularly limited, and any conventional method can be applied. For example, the solid electrolyte particles 31 can be produced by a melt quenching method or a mechanical milling method (MM method). Hereinafter, as an example of a method for producing the solid electrolyte particles 31, a method for producing the solid electrolyte particles 31 containing Li ₂ S and P ₂ S ₅ will be described.

In the case of the melt-quenching method, a mixture of a predetermined amount of Li ₂ S and P ₂ S ₅ in the form of pellets is reacted at a predetermined reaction temperature in vacuum and then rapidly cooled to obtain a sulfide-based compound. A solid electrolyte can be obtained. The reaction temperature at this time is preferably 400°C to 1000°C, more preferably 800°C to 900°C. The reaction time is preferably 0.1 hours to 12 hours, more preferably 1 to 12 hours. Further, the quenching temperature of the above reaction product is usually 10° C. or lower, preferably 0° C. or lower, and the cooling rate thereof is usually about 1 to 10000 K/sec, preferably 1 to 1000 K/sec.

In the case of using the MM method, a sulfide-based solid electrolyte can be obtained by mixing a predetermined amount of Li ₂ S and P ₂ S ₅ and reacting them by a mechanical milling method for a predetermined time. The mechanical milling method using the above raw materials has an advantage that the reaction can be performed at room temperature. According to the MM method, since the solid electrolyte can be produced at room temperature, the raw material is not thermally decomposed and the solid electrolyte having the charged composition can be obtained. The rotation speed and rotation time of the MM method are not particularly limited, but the faster the rotation speed, the faster the production rate of the solid electrolyte, and the longer the rotation time, the higher the conversion rate of the raw material into the solid electrolyte.

Then, the obtained solid electrolyte is heat-treated at a predetermined temperature and then pulverized to obtain solid electrolyte particles 31. The mixing ratio of Li ₂ S and sulfide containing P ₂ S ₅ is usually 50:50 to 80:20, and preferably 60:40 to 75:25 in terms of molar ratio.

(3.3. Preparation of positive electrode layer 10)
A mixture of the coated particles 10a, the solid electrolyte particles 31, and various additives is added to the solvent to prepare a slurry or paste positive electrode mixture. Here, the solvent is not particularly limited as long as it can be used for producing the positive electrode mixture, but a nonpolar solvent is particularly preferable. This is because the nonpolar solvent does not easily react with the solid electrolyte particles 31. Then, the obtained positive electrode mixture is applied to a current collector using a doctor blade or the like and dried. Next, the current collector and the positive electrode material mixture layer are consolidated with a rolling roll or the like to obtain the positive electrode layer 10.

Examples of the current collector that can be used at this time include plate-shaped bodies and foil-shaped bodies made of stainless steel, titanium, aluminum, or alloys thereof. Note that the positive electrode mixture may be compacted into pellets to form the positive electrode layer 10 without using the current collector.

(3.4. Fabrication of Negative Electrode Layer 20)
The method for producing the negative electrode layer 20 is as follows. For example, a mixture of the negative electrode active material particles 21, the solid electrolyte particles 31, and various additives is added to the solvent to prepare a slurry or paste negative electrode mixture. Here, the solvent is not particularly limited as long as it can be used for preparing the negative electrode mixture, but a nonpolar solvent is particularly preferable. This is because the nonpolar solvent does not easily react with the solid electrolyte particles 31. Then, the obtained negative electrode mixture is applied to a current collector using a doctor blade or the like and dried. Next, the current collector and the negative electrode mixture layer are consolidated with a rolling roll or the like to obtain the negative electrode layer 20.

Examples of the current collector that can be used at this time include a plate-shaped body or a foil-shaped body made of copper, stainless steel, nickel, or an alloy thereof. The negative electrode layer 20 may be formed by compacting a mixture of the negative electrode active material particles 21 and various additives into a pellet without using a current collector. When a metal or an alloy thereof is used as the negative electrode active material particles 21, a metal sheet (foil) may be used as it is.

(3.5. Preparation of solid electrolyte layer 30)
The method for producing the solid electrolyte layer 30 is as follows. The solid electrolyte particles 31 are formed into a solid by forming the solid electrolyte particles 31 by a known film forming method such as a blast method, an aerosol deposition method, a cold spray method, a sputtering method, a vapor phase growth method (CVD), and a thermal spray method. The electrolyte layer 30 can be produced. Alternatively, a method of applying a solution in which the solid electrolyte particles 31 and a solvent or binder (binder, polymer compound, etc.) are mixed and then removing the solvent to form a film may be used. In addition, the solid electrolyte particles 31 themselves or the solid electrolyte particles 31 and the binder (binder, polymer compound, etc.) or the support (solid electrolyte layer 30 to strengthen the strength, or to prevent the short circuit of the solid electrolyte particles 31 itself. It is also possible to form a film by pressing an electrolyte mixed with materials and compounds.

(3.6. Lamination of each layer)
By stacking the positive electrode layer 10, the solid electrolyte layer 30, and the negative electrode layer 20 obtained as described above in this order and pressing, the lithium ion secondary battery 1 according to the present embodiment can be manufactured. ..

Next, examples of this embodiment will be described. Of course, the present invention is not limited to the following examples.

(1. Preparation example 1 of coated particles)
In Production Example 1 of coated particles, coated particles 10a were produced by the following steps. 100 ml of 0.16 mol/L urea solution was added to 100 ml of 0.15 mol/L aluminum nitrate aqueous solution. Then, 60 g of a transition metal hydroxide ((Mn, Co, Ni) _1/3 (OH) ₂ ) as a raw material of the active material was dispersed in this aqueous solution.

Then, this dispersion is kept at 100° C. under a nitrogen atmosphere. Thereby, urea was decomposed. As a result, the pH in the dispersion liquid increased, and aluminum hydroxide was deposited on the surface of the transition metal hydroxide particles.

The obtained sample was dried, mixed with lithium hydroxide powder, and baked in the atmosphere at 1000° C. for 10 hours. As a result, coated particles 10a according to Preparation Example 1 (hereinafter, also referred to as "coated particles 10a-1") were obtained. The positive electrode active material particles 11 of the coated particles 10a-1 are composed of LiNi _1/3 Co _1/3 Mn _1/3 0 ₂ (NCM333), and the coating layer 12 of the coated particles 10a-1 is composed of aluminum.

When D50 (median diameter) of the positive electrode active material particles 11 was measured using Microtrack MT-3000II manufactured by Nikkiso Co., Ltd., it was 5.0 μm. Further, the arithmetic mean value of the layer thickness of the coated particles 10a was measured by the above-mentioned method and was found to be 8.0 nm. Here, the layer thickness at each measurement point is the cross-sectional observation of the coated particles 10a by a field emission scanning electron microscope (S-4800 manufactured by Hitachi High Technology Co., Ltd.) and energy dispersive X-ray analysis (EMAX manufactured by Horiba Ltd.). It measured based on the result of the elemental analysis by ENERGY E-350). Therefore, the diameter layer thickness ratio was 0.0016. Moreover, it was confirmed by XPS that a part of the coating layer 12 was solid-dissolved in the positive electrode active material particles 11.

(2. Production Example 2 of Coated Particles)
By performing the same treatment as in Production Example 1 of coated particles except that the amount of the transition metal hydroxide used in Production Example 1 of coated particles was 10 g, the coated particles 10a according to Production Example 2 (hereinafter, "coated particles") will be described. 10a-2”). Further, the diameter layer thickness ratio of the coated particles 10a-2 was measured in the same manner as in Preparation Example 1, and it was 0.01.

(3. Preparation Example 3 of Coated Particles)
The coated particles 10a according to Preparation Example 3 (hereinafter, referred to as "Coated Particle Preparation Example 1") were subjected to the same treatment as in Coated Particle Preparation Example 1 except that the amount of the transition metal hydroxide used was 2.5 g. Coated particles 10a-3" were also prepared). Further, the diameter layer thickness ratio was measured in the same manner as in Preparation Example 1, and was 0.10.

(4. Production Example 4 of Coated Particles)
According to Production Example 4, the same treatment as in Production Example 1 of coated particles was performed except that the amount of the transition metal hydroxide used was 2.5 g and the firing time was 24 hours in Production Example 1 of coated particles. A coated particle 10a (hereinafter, also referred to as "coated particle 10a-4") was produced. Further, the diameter layer thickness ratio was measured in the same manner as in Production Example 1, and it was 0.25.

(5. Preparation Example 5 of Coated Particles)
The coated particles 10a according to Preparation Example 5 (hereinafter, referred to as “coated particles”) are the same as in Preparation Example 1 of coated particles except that the amount of the transition metal hydroxide used in Preparation Example 1 of coated particles is 80 g. 10a-5"). Further, the diameter layer thickness ratio was measured in the same manner as in Preparation Example 1, and it was 0.0010.

(6. Preparation Example 6 of Coated Particles)
Preparation of coated particles except that a 0.15 mol/L aluminum nitrate aqueous solution was used as a mixed solution in which both aluminum nitrate and magnesium nitrate were dissolved at 0.075 mol/L and the amount of transition metal hydroxide used was 10 g. By performing the same treatment as in Example 1, coated particles 10a according to Production Example 6 (hereinafter, also referred to as "coated particles 10a-6") were produced. The coating layer 12 of Production Example 6 of coated particles is composed only of aluminum and magnesium. The molar ratio of aluminum and magnesium forming the coating layer 12 is 1:1. The diameter layer thickness ratio was measured in the same manner as in Production Example 1, and it was 0.010.

(7. Preparation Example 7 of Coated Particles)
In Preparation Example 1 of coated particles, the 0.15 mol/L aluminum nitrate aqueous solution was changed to a 0.15 mol/L cobalt nitrate aqueous solution, the transition metal hydroxide was changed to nickel hydroxide, and the amount of the transition metal hydroxide used The coated particles 10a according to Production Example 7 (hereinafter, also referred to as "coated particles 10a-7") were produced by performing the same treatment as in Production Example 1 of coated particles except that the amount was 10 g. The positive electrode active material particles 11 are made of lithium nickel oxide, and the coating layer 12 is made of cobalt. The diameter layer thickness ratio was measured in the same manner as in Preparation Example 1, and it was 0.009.

(8. Preparation Example 8 of Coated Particles)
Coated particles according to Production Example 8 were the same as in Production Example 1 of coated particles except that the amount of the transition metal hydroxide used was 100 g and the firing time was 2 hours in Production Example 1 of coated particles. 10a (hereinafter, also referred to as "coated particles 10a-8") was produced. When the diameter layer thickness ratio was measured in the same manner as in Preparation Example 1, it was less than 0.0010.

(9. Production Example 9 of Coated Particles)
According to Production Example 9, the same treatment as in Production Example 1 of coated particles was performed except that the amount of the transition metal hydroxide used was 2.5 g and the firing time was 50 hours in Production Example 1 of coated particles. A coated particle 10a (hereinafter, also referred to as "coated particle 10a-9") was produced. Moreover, when the diameter layer thickness ratio was measured in the same manner as in Preparation Example 1, it was a value larger than 0.25.

(8. DSC evaluation)
(8.1. DSC test of Preparation Examples 1 and 6)
Next, in order to evaluate the reactivity between the coated particles 10a-1 and 10a-6 and the solid electrolyte particles 31, a DSC test described below was performed. That is, as the solid electrolyte particles 31, those obtained by subjecting Li ₂ S—P ₂ S ₅ (80-20 mol%) to mechanical milling (MM treatment) were prepared. Then, the coated particles 10a-1 and the solid electrolyte particles 31 were mixed in the glove box at a mass ratio of 1:1. Then, the temperature at which the exothermic reaction of the mixture started was evaluated using a differential scanning calorimeter (THERMO plus EVO II/DSC8230 manufactured by Rigaku Corporation). The same evaluation was performed on the coated particles 10a-6. In addition, NCM333 particles (positive electrode active material particles 11) not covered with the coating layer 12 were prepared, and the same evaluation was performed. The results are shown in Figure 2. The horizontal axis of FIG. 2 represents temperature and the vertical axis represents heat flow. In FIG. 2, “Li(Ni,Mn,Co)O ₂ +Al” indicates the coated particle 10a-1, and “Li(Ni,Mn,Co)O ₂ +Al/Mg” indicates the coated particle 10a-6. “Li(Ni,Mn,Co)O ₂ ”denotes NCM333 (the positive electrode active material particles 11 not covered with the coating layer 12).

As is clear from FIG. 2, the starting temperature of the exothermic reaction of the coated particles 10a-1 and 10a-6 was higher than the starting temperature of the exothermic reaction of the NCM333 particles. Specifically, the starting temperature of the exothermic reaction of the coated particles 10a-1 and 10a-6 was about 290°C, whereas the starting temperature of the exothermic reaction of the NCM333 particles was about 210°C. Further, the peak temperature of the exothermic reaction of the coated particles 10a-1 and 10a-6 was about 350 to 380°C, while the peak temperature of the exothermic reaction of the NCM333 particles was 310°C.

The exothermic reaction is a reaction between the transition metal in the positive electrode active material particles 11 and the sulfur element in the solid electrolyte particles 31, that is, a side reaction. Therefore, the coated particles 10a-1 and 10a-6 are less likely to cause a side reaction than the positive electrode active material particles 11 (that is, NCM333 particles) not covered with the coating layer 12. As a result, it was confirmed that by covering the positive electrode active material particles 11 with the coating layer 12 made of a highly reactive element, a side reaction is less likely to occur (that is, generation of a high resistance layer is suppressed).

(8.2. DSC test of Production Examples 2 to 5)
8.1. The same DSC test was performed by changing the coated particles 10a-1 of Example 1 to the coated particles 10a-2 to 10a-5. As a result, 8.1. Similar results were obtained.

(8.3. DSC evaluation of Preparation Example 7)
Coated particles 10a-7 and 8.1. The solid electrolyte particles 31 prepared in 1. were mixed in a mass ratio of 1:1. And 8.1. The same evaluation as was done. Further, lithium nickel oxide particles (positive electrode active material particles 11) not covered with the coating layer 12 are prepared, and 8.1. The same evaluation as was done. As a result, 8.1. Similar results were obtained.

(9. Example 1)
The all-solid-state lithium ion secondary battery 1 was produced by the following steps. Li foil (thickness 0.03 mm) used as the negative electrode layer 20 was punched out with a diameter of 13 mm and set in a cell container. On top of that, 8.1. 80 mg of the solid electrolyte particles 31 prepared in 1. was laminated, and the surface was lightly conditioned with a molding machine. Thereby, the electrolyte layer 30 was formed. Then, the coated particles 10a-1 and 8.1. A mixture of the solid electrolyte particles 31 prepared in 1 above and vapor-grown carbon fiber (VGCF) as a conductive agent in a ratio of 60/35/5 mass% was laminated on SE as a positive electrode mixture. Then, the stack was pressed at a pressure of 3 t/cm ² to prepare pellets. That is, the test cell according to Example 1 was obtained.

The obtained test cell was charged at a constant current of 0.02 C to an upper limit voltage of 4.0 V at a temperature of 25° C., and the discharge end voltage of 2.5 V was discharged at 0.1 C for 30 charge/discharge cycles. It was Then, the impedance of the lithium ion secondary battery 1 was measured, and the interface resistance was calculated from the result. The impedance was measured by the AC impedance method.

(10. Examples 2 to 7)
The same treatment as in Example 1 was carried out except that the coated particles 10a-1 in Example 1 were changed to the coated particles 10a-2 to 10a-7.

(11. Comparative Examples 1 to 4)
The same treatment as in Example 1 was performed except that the coated particles 10a-1 of Example 1 were changed to coated particles 10a-8, 10a-9, NCM333 particles, and lithium nickelate particles.

(12. Evaluation of interface resistance)
Table 2 collectively shows the diameter layer thickness ratio and the interface resistance of Examples 1 to 7 and Comparative Examples 1 to 4.

According to Table 2, the interface resistances of Examples 1 to 6 were lower than the interface resistances of Comparative Examples 1 to 3. Therefore, in Examples 1 to 6, it was confirmed that the generation of the high resistance layer was suppressed more than in Comparative Examples 1 to 3. FIG. 3 shows the impedance of Example 6 (expressed as “Li(Ni,Mn,Co)O ₂ +Al/Mg”) and the impedance of Comparative Example 3 (expressed as “Li(Ni,Mn,Co)O ₂ ”). And contrasted with. The horizontal axis of FIG. 3 represents the real part of the impedance, and the vertical axis represents the imaginary part. That is, FIG. 3 is a complex impedance plot diagram (Nyquist diagram). As is clear from FIG. 3, the interface resistance of Example 6 is lower than the interface resistance of Comparative Example 3. Furthermore, comparing Examples 1 and 2 with Examples 3 to 5, the interface resistance of Examples 1 and 2 was lower than the interface resistance of Examples 3 to 5. Further, comparing Example 3 with Examples 4 and 5, the interface resistance of Example 3 was smaller than that of Examples 4 and 5. Therefore, it was found that the preferable range of the diameter layer thickness ratio is 0.0016 to 0.1, and the more preferable range is 0.0016 to 0.01.

Further, when the impedance of Example 7 and the impedance of Comparative Example 4 were compared, the impedance of Example 7 was lower than the impedance of Comparative Example 4. Therefore, in Example 7, it was confirmed that generation of the high resistance layer was suppressed as compared with Comparative Example 4.

The preferred embodiments of the present invention have been described above in detail with reference to the accompanying drawings, but the present invention is not limited to these examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present invention.

DESCRIPTION OF SYMBOLS 1 Lithium ion secondary battery 10 Positive electrode layer 10a Coated particles 11 Positive electrode active material particles 12 Coating layer 20 Negative electrode layer 21 Negative electrode active material particles 30 Electrolyte layer 31 Solid electrolyte particles

Claims (6)

Coated particles having a coating layer that covers the positive electrode active material particles and the positive electrode active material particles, and sulfide-based solid electrolyte particles that contact the coated particles,
Among the elements other than lithium and oxygen, the coating layer contains a highly reactive element having a higher reactivity with the sulfide-based solid electrolyte particles than the transition metal element in the positive electrode active material particles,
The ratio of the diameter of the layer thickness of the coating layer the positive active material particles Ri from 0.0010 to 0.25 der,
While conducting a first DSC test in which the coating particles and the sulfide-based solid electrolyte particles are mixed and heated at a mass ratio of 1:1, the positive electrode active material particles not covered with the coating layer and the sulfide are carried out. When the second DSC test in which the system solid electrolyte particles are mixed and heated at a mass ratio of 1:1 is performed, the starting temperature of the exothermic reaction in the first DSC test is the exothermic reaction in the second DSC test. Characterized by being higher than the starting temperature of the reaction,
The highly reactive element is at least one selected from the group consisting of aluminum, cobalt, manganese, and magnesium,
A part of the highly reactive element is solid-dissolved in the positive electrode active material particles, wherein the lithium ion secondary battery is characterized. The lithium ion secondary battery according to claim 1, wherein the highly reactive element has a lower standard enthalpy of sulfide formation than the transition metal element in the positive electrode active material particles. The lithium ion secondary battery according to claim 2, wherein the standard enthalpy of sulfide formation of the highly reactive element is less than -80 kJ/mol. The onset temperature of the exothermic reaction in the first DSC test, being higher than 250 ° C., a lithium ion secondary battery according to claim 1, wherein. The temperature at which the heat generation amount is maximum in the first DSC test, being higher than 330 ° C., a lithium ion secondary battery according to claim 1, wherein. The sulfide-based solid electrolyte particles is characterized by containing a phosphorus, lithium ion secondary battery according to any one of claims 1-5.

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