JP2019102321A - Sulfide solid electrolyte secondary battery - Google Patents

Abstract

A sulfide solid electrolyte secondary battery having improved cycle characteristics is provided. According to the present invention, there is provided a sulfide solid electrolyte secondary battery including a positive electrode, a negative electrode, and a sulfide solid electrolyte. The positive electrode contains a positive electrode active material. The positive electrode active material includes secondary particles formed by aggregating primary particles of metal oxide. Ce2O3 is arranged inside the secondary particles. [Selection diagram] Figure 1

Description

  The present invention relates to a sulfide solid electrolyte secondary battery.

  In recent years, development of a solid electrolyte secondary battery has been accelerated while improvement in reliability and cost reduction of the secondary battery are required. For example, Patent Document 1 discloses a solid electrolyte secondary battery having a positive electrode containing a metal oxide as a positive electrode active material, a negative electrode containing a negative electrode active material, and a sulfide solid electrolyte.

JP, 2017-079126, A

  However, according to the study of the present inventor, in the solid electrolyte secondary battery having the above-described configuration, the crystal structure of the metal oxide contained in the positive electrode becomes unstable due to charge and discharge, and the metal oxide is formed Oxygen elements may be eliminated. Since the oxygen element once desorbed from the metal oxide is difficult to be taken into the metal oxide again, the oxygen concentration on the positive electrode side is gradually increased by repeating the charge and discharge. Thus, the sulfide solid electrolyte is oxidized and degraded because the positive electrode side is in an oxidizing atmosphere. As a result, there is a problem that the battery capacity decreases and the cycle characteristics deteriorate.

  The present invention has been made in view of the foregoing, and an object thereof is to provide a sulfide solid electrolyte secondary battery with improved cycle characteristics.

The present invention provides a sulfide solid electrolyte secondary battery comprising a positive electrode, a negative electrode, and a sulfide solid electrolyte. The positive electrode contains a positive electrode active material. The positive electrode active material includes secondary particles formed by aggregation of primary particles of metal oxides. Ce ₂ O ₃ is disposed inside the secondary particles.

In the positive electrode active material having the above structure, when oxygen element is desorbed from the metal oxide during charge and discharge, the desorbed oxygen is stored in Ce ₂ O ₃ disposed in the vicinity of the metal oxide (trap )can do. In other words, the oxygen desorbed from the positive electrode active material can be taken into the inside again. By this, it can suppress that the positive electrode side becomes an oxidizing atmosphere, and can suppress the oxidation degradation of a sulfide solid electrolyte. As a result, cycle characteristics can be improved.

It is a SEM observation image of the cross section of the positive electrode active material before the reduction process which concerns on Example 1, (A) is 2000 times magnification, (B) is a 20000 times magnification observation image. 5 is an XRD chart of a positive electrode active material before reduction treatment according to Example 1. FIG. 6 is an EPMA observation image of a cross section of a positive electrode active material before reduction treatment according to Example 1. FIG. It is the graph which compared the charge / discharge capacity after a cycle test.

  Hereinafter, preferred embodiments of the present invention will be described. In addition, matters other than the matters particularly referred to in the specification (for example, the configuration of the positive electrode) and matters necessary for the practice of the present invention (for example, components of the battery and the like which do not characterize the present invention) The manufacturing process can be understood as a design matter of a person skilled in the art based on prior art in the art. The present invention can be implemented based on the contents disclosed in the present specification and common technical knowledge in the field. In the present specification, the expression "A to B (where A and B are arbitrary values)" includes the values of A and B (lower limit and upper limit) unless otherwise specified.

  The sulfide solid electrolyte secondary battery disclosed herein comprises a positive electrode, a negative electrode, and a sulfide solid electrolyte. Hereinafter, each component will be described in order.

  The positive electrode contains at least a positive electrode active material. The positive electrode active material is a material capable of reversibly absorbing and desorbing charge carriers. The positive electrode may have a form including a positive electrode current collector and a positive electrode active material layer fixed to the positive electrode current collector and containing a positive electrode active material. As the positive electrode current collector, a conductive member made of a metal having good conductivity, for example, aluminum is preferable.

  The positive electrode active material contains a metal oxide. The metal oxide is a compound containing one or more metal elements and an oxygen element. The metal oxide may be a compound containing a lithium element, one or more transition metal elements, and an oxygen element. Lithium transition, such as lithium nickel containing complex oxide, lithium cobalt containing complex oxide, lithium nickel cobalt containing complex oxide, lithium manganese containing complex oxide, lithium nickel cobalt manganese containing complex oxide, as a suitable example of a metal oxide Metal complex oxides can be mentioned.

  The metal oxide includes the form of secondary particles in which a plurality of primary particles are aggregated. The secondary particles are constituted by aggregation of a plurality of primary particles by physical or chemical bonding force. The properties of the secondary particles are not particularly limited, but for example, the average particle diameter on a number basis based on SEM observation is approximately 1 to 50 μm, for example, 3 to 20 μm.

In the present embodiment, Ce ₂ O ₃ is disposed inside the secondary particles of the metal composite oxide. In other words, Ce ₂ O ₃ is disposed between the primary particles of the metal oxide. Ce ₂ O ₃ functions as a so-called oxygen storage material. Ce ₂ O ₃ is in direct contact with the metal oxide at the primary particle level. As a result, the structure of the metal oxide becomes unstable with charge and discharge, and when the oxygen element is desorbed from the metal oxide, the oxygen element can be easily incorporated into Ce ₂ O ₃ . From the viewpoint of exerting the effects of the technology disclosed herein at a higher level, Ce ₂ O ₃ is generally 0.1 mol% or more, for example, 1 mol% or more, based on 100 mol% of the entire metal oxide. Preferably, the ratio is 1.5 mol% or more, more preferably 2 mol% or more. Although the upper limit is not particularly limited, Ce ₂ O ₃ may be about 10 mol% or less, for example, 5 mol% or less, for example, when the entire metal oxide is 100 mol%.

In addition, the manufacturing method of such a positive electrode active material is not specifically limited. For example, a positive electrode active material having secondary particles of a lithium transition metal composite oxide and Ce ₂ O ₃ disposed inside the secondary particles can be produced by the following method.
That is, first, a metal source compound containing a desired metal element and a Ce source compound are mixed in an aqueous solvent to obtain a raw material mixed solution. As the metal source compound, for example, nitrates, acetates, sulfates and the like containing a desired metal element can be used. As the Ce source compound, for example, carbonates, hydroxides and the like of Ce can be used. The metal source compound may be mixed so that the molar ratio of the metal elements in the raw material mixture liquid is the desired stoichiometric ratio of the metal oxide.
Next, the raw material mixed solution is brought into contact with an alkaline substance to precipitate a hydroxide (precursor) of a metal element and Ce from the raw material mixed solution. As the alkaline substance, for example, an aqueous solution of sodium carbonate, an aqueous solution of sodium hydroxide, ammonia gas and the like can be used.
Next, the above precursor and a Li source compound are mixed so that the metal element has a desired stoichiometric ratio, and the mixture is fired in the air. As a Li source compound, lithium carbonate, lithium hydroxide etc. can be used, for example. The maximum temperature at the time of firing may be, for example, 600 to 1000 ° C. As a result, a fired body (a positive electrode active material before reduction treatment) having secondary particles of a lithium transition metal complex oxide and CeO ₂ (cerium oxide (IV)) disposed inside the secondary particles Is obtained.
Next, the fired body is heated in a reducing atmosphere. As a result, CeO ₂ (Ce ⁴⁺ ) is reduced to change to the state of Ce ₂ O ₃ (cerium (III) oxide, Ce ³⁺ ).
As described above, a positive electrode active material in which the lithium transition metal complex oxide and Ce ₂ O ₃ are mixed at the primary particle level can be manufactured.

  The positive electrode active material layer may contain other components, for example, a solid electrolyte material, a binder, a conductive material, various additives, and the like, as necessary, in addition to the positive electrode active material. As a solid electrolyte material, the sulfide solid electrolyte material mentioned later is illustrated, for example. Examples of the binder include halogenated vinyl resins such as polyvinylidene fluoride (PVdF) and a copolymer of polyvinylidene fluoride and hexafluoropropylene (PVdF-HFP). Examples of the conductive material include carbon materials such as vapor grown carbon fiber and carbon black.

  The negative electrode contains at least a negative electrode active material. The negative electrode active material is a material capable of reversibly absorbing and desorbing charge carriers. The negative electrode may have a form including a negative electrode current collector and a negative electrode active material layer fixed to the negative electrode current collector and containing a negative electrode active material. As the negative electrode current collector, a conductive member made of a metal having good conductivity such as copper is preferable.

  Examples of the negative electrode active material include carbon materials such as hard carbon and graphite. The negative electrode active material layer may contain other components, for example, a solid electrolyte material, a binder, a thickener, various additives, and the like, as necessary, in addition to the negative electrode active material. As a solid electrolyte material, the sulfide solid electrolyte material mentioned later is illustrated, for example. Examples of the binder include halogenated vinyl resins such as polyvinylidene fluoride (PVdF) and a copolymer of polyvinylidene fluoride and hexafluoropropylene (PVdF-HFP).

The sulfide solid electrolyte contains a sulfide solid electrolyte material. The sulfide solid electrolyte material has ion conductivity. Typically, it has Li ion conductivity. As a preferred example of the sulfide solid electrolyte _{material, Li 2 S-P 2 S} 5, Li 2 S-P 2 S 5 -LiI, Li 2 S-P 2 S 5 -Li 2 O, Li 2 S-P 2 Examples include Li ₂ S—P ₂ S ₅ based materials such as S ₅ —Li ₂ O—LiI. The sulfide solid electrolyte may contain, in addition to the sulfide solid electrolyte material, other components, for example, a binder, various additives, etc., as necessary. Examples of the binder include halogenated vinyl resins such as polyvinylidene fluoride (PVdF) and a copolymer of polyvinylidene fluoride and hexafluoropropylene (PVdF-HFP).

  The sulfide solid electrolyte secondary battery disclosed herein can be used in various applications. Preferred applications include, for example, power sources (driving power sources) such as plug-in hybrid vehicles, hybrid vehicles, and electric vehicles.

  The following examples illustrate some of the embodiments of the present invention, but are not intended to limit the present invention to those shown in the specific examples.

Example 1
[Synthesis of positive electrode active material]
First, each of 0.15 mol of nickel nitrate, cobalt nitrate and manganese nitrate was weighed and dissolved in 600 ml of distilled water. A raw material mixed solution was prepared by adding 0.006 mol of cerium nitrate thereto and mixing. In addition, 1.2 mol of sodium carbonate was dissolved in 800 ml of distilled water to prepare an aqueous sodium carbonate solution.
Next, the raw material mixed solution was dropped into the aqueous sodium carbonate solution to precipitate and precipitate a hydroxide (precursor) containing Ni, Co, Mn and Ce. The precursor was separated by filtration, washed five times with distilled water, and dried at 120 ° C. to obtain a dried product.
Next, the resulting dried product was ground in a mortar, and then 0.45 mol of lithium carbonate was added and dry mixed. The resultant was fired at 600 ° C. for 10 hours in an air atmosphere, and further fired at 900 ° C. for 10 hours. Thus, a sintered body having lithium nickel cobalt manganese composite oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , NCM 111) and CeO ₂ corresponding to 3 mol% of the entire NCM 111 (before reduction treatment The positive electrode active material of

In order to investigate the structure of the obtained sintered body (positive electrode active material before reduction treatment), scanning electron microscope (SEM) observation of the cross section, X-ray diffraction (XRD) measurement, and electron beam microanalyzer (EPMA of the cross section) ) And did the observation. The results are shown in FIGS.
1 (A) and 1 (B) are SEM observation images of the cross section of the fired body. In this observation image, a portion of CeO ₂ having a relatively large atomic weight as compared to NCM 111 is shown in white. From FIG. 1 (A), it could be confirmed that a plurality of primary particles are gathered to form homogeneous secondary particles. Further, from FIG. 1 (B), it was confirmed that CeO ₂ was interposed between primary particles of NCM 111 in the secondary particles of NCM 111, and NCM 111 and CeO ₂ were in contact at the primary particle level. .
FIG. 2 is an XRD chart of the fired body. From this XRD chart, in addition to the layered structure of NCM 111, a single peak of CeO ₂ could be confirmed. In FIG. 2, the CeO ₂ single peak is indicated by an asterisk.
FIG. 3 is an EPMA observation image of the cross section of the fired body. From the low-magnification observation image in the upper side of FIG. 3, it can be confirmed that a plurality of primary particles are gathered to form a homogeneous secondary particle. In addition, it can be confirmed from the high-magnification observed image on the lower side of FIG. 3 that CeO ₂ is finely arranged at the primary particle level.

[Reduction treatment]
Next, in order to reduce CeO ₂ to Ce ^{4 +} → Ce ^{3 +} , the above-obtained fired body is placed in a gas atmosphere tubular furnace, and an atmosphere in which N ₂ gas containing 5% of CO is circulated And heated at 300 ° C. for 10 minutes. Then, the state change from CeO ₂ to Ce ₂ O ₃ was confirmed by confirming the generation of CO ₂ gas by a gas analyzer. Thus, a positive electrode active material in which NCM 111 and Ce ₂ O ₃ were mixed at the primary particle level was obtained.

[Production of positive electrode]
First, the mass ratio of the obtained positive electrode active material and Li ₂ S-P ₂ S ₅ -Li ₂ O-LiI as a sulfide solid electrolyte material is as follows: positive electrode active material: sulfide solid electrolyte material = 75 It mixed so that it might become: 25, and prepared the mixed powder. Next, PVdF-HFP as a binder and vapor grown carbon fiber as a conductive material were respectively weighed to be 3.0 parts by mass with respect to 100 parts by mass of the positive electrode active material. Then, the mixed powder, the binder, and the conductive material are mixed with n-butyl butylate as a solvent, and the mixture is kneaded for 1 minute with an ultrasonic homogenizer to obtain a paste-like positive electrode composition (solid fraction 63 of positive electrode active material). %) Was prepared. This positive electrode composition was coated on the surface of an aluminum foil (positive electrode current collector) using an applicator (350 μm gap), naturally dried for 5 minutes, and then heat dried at 100 ° C. for 5 minutes. Thus, a positive electrode in which the positive electrode active material layer was fixed on the positive electrode current collector was produced.

[Fabrication of negative electrode]
First, the mass ratio of hard carbon as a negative electrode active material and Li ₂ S-P ₂ S ₅ -Li ₂ O-LiI as a sulfide solid electrolyte material: negative electrode active material: sulfide solid electrolyte material = 58 It mixed so that it might become: 42, and prepared the mixed powder. Next, PVdF-HFP as a binder was weighed so as to be 3.0 parts by mass with respect to 100 parts by mass of the negative electrode active material. Then, the mixed powder and the binder were mixed with butyl n-butyrate as a solvent, and the mixture was kneaded for 1 minute with an ultrasonic homogenizer to prepare a paste-like negative electrode composition (solid content 63%). This negative electrode composition was coated on the surface of a copper foil (negative electrode current collector) using an applicator (350 μm gap), naturally dried for 5 minutes, and then heat dried at 100 ° C. for 5 minutes. Thus, a negative electrode in which the negative electrode active material layer was fixed on the negative electrode current collector was produced.

[Construction of sulfide solid electrolyte secondary battery]
First, in an inert gas atmosphere, and _{Li 2 S-P 2 S 5} -Li 2 O-LiI as sulfide solid electrolyte material, and PVdF-HFP as a binder, the mass ratio is 100: a 1 The slurry was mixed as described above, further mixed with butyl n-butyrate as a solvent, and kneaded with an ultrasonic homogenizer to prepare a slurry (solid content 35%) for forming a solid electrolyte. The solid electrolyte forming slurry was applied to the surface of an aluminum foil using an applicator and dried to form a sulfide solid electrolyte. Next, the aluminum foil and the sulfide solid electrolyte were punched to a size of 1 cm ² , and the aluminum foil was peeled off from the sulfide solid electrolyte. And the sulfide positive electrolyte secondary battery (Example 1) was constructed | assembled by superposing the produced said positive electrode and negative electrode on both sides of a sulfide solid electrolyte, and pressing them with the pressure of 4.3 ton.

Example 2
At the time of synthesis of the positive electrode active material, the addition amount of cerium nitrate was changed to 0.003 mol, and except that a fired body (positive electrode active material before reduction treatment) having CeO ₂ corresponding to 1.5 mol% of the entire NCM 111 was obtained. In the same manner as in Example 1 above, a sulfide solid electrolyte secondary battery (Example 2) was constructed.

«Comparative Examples 1 and 2»
Comparative Examples 1 and 2 are test examples in which CeO ₂ is contained in the positive electrode active material. That is, a sulfide solid electrolyte secondary battery (Comparative Example 1) was prepared in the same manner as in Examples 1 and 2 except that the fired body before reduction treatment (without reduction treatment) was used as a positive electrode active material as it was. , 2) built.

«Comparative Example 3»
Comparative Example 3 is a test example in which the positive electrode active material does not contain cerium oxide (CeO ₂ and Ce ₂ O ₃ ). A sulfide solid electrolyte was prepared in the same manner as in Example 1 except that at the time of synthesis of the positive electrode active material, no cerium nitrate was added to obtain a fired body not containing CeO ₂ and used as a positive electrode active material. A secondary battery (Comparative Example 3) was constructed.

«Evaluation of battery performance»
A cycle test was performed in which the above-constructed sulfide solid electrolyte secondary battery was subjected to 100 cycles of charge and discharge at a charge and discharge rate of 0.5 C in a voltage range of 3.0 to 4.2 V under an environment of 25 ° C. . Then, the charge / discharge capacity (battery capacity) after 100 cycles was relatively evaluated. The results are shown in FIG. FIG. 4 shows relative values when the charge / discharge capacity after 100 cycles of Example 1 is 100.

As shown in FIG. 4, in Examples 1 and _{2 in} which the positive electrode active material contains Ce ₂ O ₃ , Comparative Examples 1 and _{2 in} which the positive electrode active material contains CeO _2, and a comparison in which the positive electrode active material does not contain cerium oxide As compared to Example 3, the battery capacity after the cycle test was relatively high, and the cycle characteristics were excellent. The reason for this is that by arranging Ce ₂ O ₃ inside the secondary particles of NCM 111 at the time of synthesis of the positive electrode active material and bringing NCM 111 and Ce ₂ O ₃ into contact at the primary particle level, The gradually released oxygen element can be easily incorporated into Ce ₂ O ₃ , and as a result, it is considered that the oxidation deterioration of the sulfide solid electrolyte could be delayed.
Such results show the significance of the technology disclosed herein.

  The present invention has been described in detail above, but the above embodiments and examples are merely examples, and the invention disclosed herein includes various modifications and alterations of the specific example described above.

Claims (1)

A sulfide solid electrolyte secondary battery comprising a positive electrode, a negative electrode, and a sulfide solid electrolyte,
The positive electrode includes a positive electrode active material,
The positive electrode active material includes secondary particles formed by aggregation of primary particles of metal oxide,
A sulfide solid electrolyte secondary battery, wherein Ce ₂ O ₃ is disposed inside the secondary particles.