JP6876256B2 - Sulfide solid electrolyte secondary battery - Google Patents

Description

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

In recent years, the development of solid electrolyte secondary batteries has been accelerating as the reliability of secondary batteries has been improved and the cost has been reduced. 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-A-2017-079126

However, according to the study of the present inventor, in the solid electrolyte secondary battery having the above configuration, the crystal structure of the metal oxide contained in the positive electrode becomes unstable due to charging and discharging, and the metal oxide is formed. Oxide elements may be desorbed. Since the oxygen element once desorbed from the metal oxide is difficult to be incorporated into the metal oxide again, the oxygen concentration on the positive electrode side gradually increases by repeating charging and discharging. By creating an oxidizing atmosphere on the positive electrode side in this way, the sulfide solid electrolyte is oxidatively deteriorated. As a result, there is a problem that the battery capacity is reduced and the cycle characteristics are lowered.

The present invention has been made in view of the above points, and an object of the present invention is to provide a sulfide solid electrolyte secondary battery having 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 aggregating primary particles of a metal oxide. _{Ce 2} O ₃ is arranged inside the secondary particles.

In the positive electrode active material having the above configuration, when the oxygen element is desorbed from the metal oxide during charging and discharging, the desorbed oxygen is occluded (trap _{) in Ce 2} O _{3 arranged in the vicinity of the metal oxide.} )can do. In other words, the oxygen desorbed from the positive electrode active material can be taken into the inside again. As a result, it is possible to suppress an oxidative atmosphere on the positive electrode side and suppress oxidative deterioration of the sulfide solid electrolyte. As a result, the cycle characteristics can be improved.

It is the SEM observation image of the cross section of the positive electrode active material before the reduction treatment which concerns on Example 1, (A) is the observation image of 2000 times magnification, (B) is the observation image of 20000 times magnification. It is an XRD chart of the positive electrode active material before the reduction treatment which concerns on Example 1. FIG. It is an EPMA observation image of the cross section of the positive electrode active material before the reduction treatment which concerns on 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. It should be noted that matters other than those specifically mentioned in the present specification (for example, the configuration of the positive electrode) and necessary for carrying out the present invention (for example, components of a battery that do not characterize the present invention and general matters). The manufacturing process) can be grasped as a design matter of a person skilled in the art based on the prior art in the field. The present invention can be carried out based on the contents disclosed in the present specification and common general technical knowledge in the art. In addition, in this specification, the expression "A to B (where A and B are arbitrary values)" shall include the values of A and B (lower limit value and upper limit value) unless otherwise specified.

The sulfide solid electrolyte secondary battery disclosed herein includes 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 the positive electrode active material. The positive electrode active material is a material capable of reversibly occluding and releasing charge carriers. The positive electrode may be in 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 suitable.

The positive electrode active material contains a metal oxide. A metal oxide is a compound containing one or more kinds of metal elements and an oxygen element. The metal oxide may be a compound containing a lithium element, one or more kinds of transition metal elements, and an oxygen element. As a preferred example of the metal oxide, lithium transition such as lithium nickel-containing composite oxide, lithium cobalt-containing composite oxide, lithium nickel cobalt-containing composite oxide, lithium manganese-containing composite oxide, and lithium nickel cobalt manganese-containing composite oxide. Examples include metal composite oxides.

The metal oxide includes the form of secondary particles formed by aggregating a plurality of primary particles. Secondary particles are composed of a plurality of primary particles aggregated by a physical or chemical bonding force. The properties of the secondary particles are not particularly limited, but for example, the average particle size based on the number 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 arranged inside the secondary particles of the metal composite oxide. _{In other words, Ce 2} O ₃ is arranged 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 techniques disclosed herein at a higher level, when the total amount of the metal oxide is 100 mol%, Ce ₂ O ₃ is approximately 0.1 mol% or more, for example, 1 mol% or more. It is preferable that they are arranged at a ratio of 1.5 mol% or more, more preferably 2 mol% or more. The upper limit is not particularly limited, but for example, when the total amount of the metal oxide is 100 mol%, Ce ₂ O ₃ may be approximately 10 mol% or less, for example, 5 mol% or less.

The method for producing such a positive electrode active material is not particularly limited. For example, a positive electrode active material having secondary particles of a lithium transition metal composite oxide and Ce ₂ O ₃ arranged 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 with 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, a carbonate of Ce, a hydroxide, or the like can be used. The metal source compound may be mixed so that the molar ratio of the metal element in the raw material mixture becomes the stoichiometric ratio of the desired metal oxide.
Next, by contacting the raw material mixed solution with an alkaline substance, a hydroxide (precursor) of a metal element and Ce is precipitated 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 or the like can be used.
Next, the precursor and the Li source compound are mixed so that the metal elements have a desired stoichiometric ratio, and the mixture is calcined in the air. As the Li source compound, for example, lithium carbonate, lithium hydroxide and the like can be used. The maximum temperature at the time of firing may be, for example, 600 to 1000 ° C. As a result, a fired body having secondary particles of the lithium transition metal composite oxide and CeO ₂ (cerium (IV) oxide) arranged inside the secondary particles (positive electrode active material before reduction treatment). Is obtained.
Next, the fired body is heated in a reducing atmosphere. As a result, CeO ₂ (Ce ⁴⁺ ) is reduced 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 composite oxide and Ce ₂ O ₃ are mixed at the primary particle level can be produced.

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, if necessary, in addition to the positive electrode active material. Examples of the solid electrolyte material include sulfide solid electrolyte materials described later. Examples of the binder include vinyl halide 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 fibers and carbon black.

The negative electrode contains at least the negative electrode active material. The negative electrode active material is a material capable of reversibly storing and releasing charge carriers. The negative electrode may be in 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, for example, copper, is suitable.

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, if necessary, in addition to the negative electrode active material. Examples of the solid electrolyte material include sulfide solid electrolyte materials described later. Examples of the binder include vinyl halide 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 ionic 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 S ₅ -Li ₂ like O-LiI of _Li 2 _S-P 2 _{S 5} based material is exemplified. The sulfide solid electrolyte may contain other components such as a binder and various additives, if necessary, in addition to the sulfide solid electrolyte material. Examples of the binder include vinyl halide 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 for various purposes. Preferred application targets include, for example, a power source (driving power source) for a plug-in hybrid vehicle, a hybrid vehicle, an electric vehicle, or the like.

Hereinafter, some examples of the present invention will be described, but the present invention is not intended to be limited to those shown in such specific examples.

<< Example 1 >>
[Synthesis of positive electrode active material]
First, nickel nitrate, cobalt nitrate, and manganese nitrate were weighed by 0.15 mol each and dissolved in 600 ml of distilled water. A raw material mixture was prepared by adding 0.006 mol of cerium nitrate and mixing the mixture. Further, 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 mixture was added dropwise to the sodium carbonate aqueous solution to precipitate and precipitate a hydroxide (precursor) containing Ni, Co, Mn and Ce. The precursor was filtered off, washed 5 times with distilled water, and then dried at 120 ° C. to obtain a dried product.
Next, the obtained dried product was pulverized in a mortar, 0.45 mol of lithium carbonate was added, and the mixture was dried and mixed. This was fired at 600 ° C. for 10 hours in an air atmosphere, and then further fired at 900 ° C. for 10 hours. As a result, a fired body having a lithium nickel cobalt manganese composite oxide (LiNi _1/3 Co _1/3 Mn _1/3 O _{2 , NCM111) and CeO 2} corresponding to 3 mol% of the entire NCM111 (before reduction treatment). (Positive electrode active material) was obtained.

In order to investigate the structure of the obtained fired body (positive electrode active material before reduction treatment), scanning electron microscope (SEM) observation of the cross section, X-ray diffraction (XRD) measurement, and electron probe microanalyzer (EPMA) of the cross section were performed. ) Observation and. The results are shown in FIGS. 1-3.
1 (A) and 1 (B) are SEM observation images of a cross section of a fired body. In this observation image, the portion of _{CeO 2} having a relatively large atomic weight as compared with NCM111 appears white. From FIG. 1 (A), it was confirmed that a plurality of primary particles were aggregated to form homogeneous secondary particles. Also, from the FIG. 1 (B), the inside of the secondary particles of NCM111, CeO ₂ is interposed between the primary particles of NCM111, it was confirmed that the NCM111 and CeO ₂ is in contact with 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 NCM111, a single peak of _{CeO 2 could be confirmed.} In FIG. 2, _{a single peak of CeO 2} is indicated by an asterisk.
FIG. 3 is an EPMA observation image of a cross section of the fired body. From the low-magnification observation image on the upper side of FIG. 3, it was confirmed that a plurality of primary particles were aggregated to form homogeneous secondary particles. Further, from the high-magnification observation image on the lower side of FIG. 3, _{it was confirmed that CeO 2} was finely arranged at the primary particle level.

[Reduction treatment]
Next, in order to reduce _{CeO 2} ^{to Ce 4+} → Ce ³⁺ , the obtained fired body was placed in a gas atmosphere tube furnace, and under an atmosphere in which _{N 2 gas containing 5% CO was circulated.} At 300 ° C. for 10 minutes. Then, by confirming the production of _{CO 2} gas with a gas analyzer, the state change _{from CeO 2} to Ce ₂ O _{3 was confirmed.} As a result, _{a positive electrode active material in which NCM111 and Ce 2} O ₃ were mixed at the primary particle level was obtained.

[Preparation of positive electrode]
First, a cathode active material obtained above, and _{Li 2 S-P 2 S 5} -Li 2 O-LiI as sulfide solid electrolyte material, the mass ratio of the positive electrode active material: sulfide solid electrolyte material = 75 The mixture was mixed so as to have a ratio of 25 to prepare a mixed powder. Next, PVdF-HFP as a binder and vapor-grown carbon fiber as a conductive material were weighed so as to be 3.0 parts by mass with respect to 100 parts by mass of the positive electrode active material, respectively. Then, the mixed powder, the binder, and the conductive material are mixed with butyl n-butylate as a solvent and kneaded with an ultrasonic homogenizer for 1 minute to form a paste-like positive electrode composition (solid content of the positive electrode active material 63). %) Was prepared. This positive electrode composition was applied to the surface of an aluminum foil (positive electrode current collector) using an applicator (350 μm gap), air-dried for 5 minutes, and then heat-dried at 100 ° C. for 5 minutes. As a result, a positive electrode in which the positive electrode active material layer was fixed on the positive electrode current collector was produced.

[Preparation of negative electrode]
First, a hard carbon as a negative electrode active material, and _{Li 2 S-P 2 S 5} -Li 2 O-LiI as sulfide solid electrolyte material, the mass ratio of the negative electrode active material: sulfide solid electrolyte material = 58 The mixture was mixed so as to be: 42, and the mixed powder was prepared. 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-butylate as a solvent and kneaded with an ultrasonic homogenizer for 1 minute to prepare a paste-like negative electrode composition (solid content 63%). This negative electrode composition was applied to the surface of a copper foil (negative electrode current collector) using an applicator (350 μm gap), air-dried for 5 minutes, and then heat-dried at 100 ° C. for 5 minutes. As a result, 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 As described above, the mixture was further mixed with butyl n-butyrate as a solvent, and kneaded with an ultrasonic homogenizer to prepare a slurry for forming a solid electrolyte (solid content ratio: 35%). This solid electrolyte forming slurry was applied to the surface of the 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 then the aluminum foil was peeled off from the sulfide solid electrolyte. Then, the positive electrode and the negative electrode produced above were overlapped with the sulfide solid electrolyte sandwiched between them, and pressed at a pressure of 4.3 tons to construct a sulfide solid electrolyte secondary battery (Example 1).

<< Example 2 >>
Except for the fact that the amount of cerium nitrate added was changed to 0.003 mol during the synthesis of the positive electrode active material to obtain a fired body (positive electrode active material before reduction treatment) having _{CeO 2 corresponding to 1.5 mol% of the total NCM111.} Constructed a sulfide solid electrolyte secondary battery (Example 2) in the same manner as in Example 1 above.

<< 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, the sulfide solid electrolyte secondary battery (Comparative Example 1) was used in the same manner as in Examples 1 and 2 above, except that the fired body before the reduction treatment was used as it was as the positive electrode active material (without the reduction treatment). 2) was constructed.

<< 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 obtained in the same manner as in Example 1 above, except that a calcined product containing no _{CeO 2} was obtained and used as the positive electrode active material when synthesizing the positive electrode active material without adding cerium nitrate. A secondary battery (Comparative Example 3) was constructed.

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

As shown in FIG. 4, _{Examples 1 and 2 containing Ce 2} O ₃ in the positive electrode active material are _{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. Compared with Example 3, the battery capacity after the cycle test was relatively high, and the cycle characteristics were excellent. The reason for this is that when the positive electrode active material is synthesized, Ce ₂ O ₃ is placed inside the secondary particles of NCM 111, and the NCM 111 and Ce ₂ O ₃ are brought into contact with each other at the primary particle level, so that the NCM 111 can be charged and discharged. It is considered that the gradually released oxygen element _{could be easily incorporated into Ce 2} O ₃ , and as a result, the oxidative deterioration of the sulfide solid electrolyte could be delayed.
Such results show the significance of the techniques disclosed herein.

Although the present invention has been described in detail above, the above-described embodiments and examples are merely examples, and the inventions disclosed herein include various modifications and modifications of the above-mentioned specific examples.

Claims (1)

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 and contains a positive electrode active material.
The positive electrode active material contains secondary particles formed by aggregating primary particles of a metal oxide.
A sulfide solid electrolyte secondary battery in which _{Ce 2} O ₃ is arranged inside the secondary particles.

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