JP7152945B2 - Method for producing nanoparticle aggregate for negative electrode active material of lithium ion secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a nanoparticle aggregate for use as a negative electrode active material for lithium ion secondary batteries.

In recent years, as electronic devices have become smaller and lighter, there has been a demand for higher capacity secondary batteries. there is However, in a non-aqueous electrolyte lithium ion secondary battery, when the electrode layer is densified, the voids in the electrode layer decrease, and the electrolyte retention capacity of the electrode decreases, resulting in a decrease in the diffusion rate of lithium ions. is generated, and the charge/discharge characteristics of the secondary battery are also deteriorated.

Under these circumstances, cerium oxide has the ability to reversibly absorb and release lithium ions to an extent that is effective as a negative electrode active material for lithium ion secondary batteries, and is excellent in wettability with non-aqueous electrolytes. Since the charge-discharge characteristics of the secondary battery are unlikely to be deteriorated even when the cerium oxide is used as the negative electrode active material, it is possible to increase the capacity of the secondary battery.

On the other hand, cerium oxide undergoes a very large change in volume when it absorbs or releases lithium ions, which tends to cause deterioration in cycle characteristics. In order to improve this, for example, in Non-Patent Document 1, by using hollow cerium oxide particles, the volume change of cerium oxide during charging and discharging is mitigated by the internal voids, thereby suppressing deterioration of cycle characteristics. Techniques are disclosed. Also, in this document, an attempt is made to ensure the movement speed of lithium ions by miniaturizing the cerium oxide particles with an average particle size of 28±2 nm.

Manickam Sasidharan, et al., "CeO2 Hollow Nanospheres as Anode Materials for Lithium Ion Batteries" Chemistry Letters, 2012, Vol. 41, No. 4, p. 1-3

However, as shown in Figure 2 of the above non-patent document, finely divided cerium oxide easily aggregates, so when the negative electrode layer of a secondary battery is formed, coarsening due to the aggregation structure of such cerium oxide occurs. Such inter-particle voids are inherent in the negative electrode layer, making it impossible to sufficiently achieve a high capacity secondary battery.

Accordingly, an object of the present invention is to provide a lithium ion secondary battery that is composed of cerium oxide nanoparticles and that can effectively increase the volumetric energy density by increasing the density of the negative electrode layer of the lithium ion secondary battery and exhibiting excellent battery capacity. An object of the present invention is to provide a method for producing a nanoparticle aggregate for use as a negative electrode active material of a secondary battery.

Therefore, the present inventors have made various studies, and found that a production method in which cellulose nanofibers or lignocellulose nanofibers are used together with a cerium compound and subjected to a hydrothermal reaction, or a production method in which further calcination is performed, oxidation exhibits a unique shape. It was found that a nanoparticle aggregate composed of cerium nanoparticles can be obtained, and that the volume energy density can be effectively increased by constructing a negative electrode using this as a negative electrode active material for a lithium ion secondary battery. was completed.

That is, the present invention provides a nanoparticle assembly for a negative electrode active material of a lithium ion secondary battery, in which a large number of cerium oxide nanoparticles are arranged on the surface of cellulose nanofibers and/or lignocellulose nanofibers having an average outer diameter of 50 nm or less. comprising the following steps (I) to (II):
(I) a step of dispersing cellulose nanofibers and/or lignocellulose nanofibers having an average fiber diameter of 50 nm or less in a solvent to obtain a slurry (X); and (II) from the obtained slurry (X) and the cerium compound A negative electrode active material for a lithium ion secondary battery comprising a step of subjecting the prepared slurry (Y) to a hydrothermal reaction to obtain a nanoparticle aggregate composed of cerium oxide nanoparticles and cellulose nanofibers and/or lignocellulose nanofibers A method for producing a nanoparticle aggregate is provided.

The present invention also provides a method for producing a nanoparticle assembly for a negative electrode active material of a lithium ion secondary battery, comprising a large number of cerium oxide nanoparticles arranged on the surface of a carbon tubular body having an average outer diameter of 50 nm or less, The following steps (I)-(III):
(I) dispersing cellulose nanofibers and/or lignocellulose nanofibers having an average fiber diameter of 50 nm or less in a solvent to obtain a slurry (X);
(II) The obtained slurry (X) and the slurry (Y) prepared from the cerium compound are subjected to a hydrothermal reaction to obtain nanoparticle aggregates composed of cerium oxide nanoparticles and cellulose nanofibers and/or lignocellulose nanofibers. and (III) drying and then calcining the obtained nanoparticle aggregate to obtain a nanoparticle aggregate composed of carbon tubular bodies and cerium oxide nanoparticles. A method for producing a particle aggregate is provided.

According to the production method of the present invention, on the surface of cellulose nanofibers and/or lignocellulose nanofibers, or on the surface of carbon tubular bodies, nanoparticles exhibiting a unique shape in which a large number of cerium oxide nanoparticles are arranged. It is possible to obtain an aggregate, and by using this as a negative electrode active material for a lithium ion secondary battery, it is possible to construct a negative electrode with an increased volume energy density while effectively suppressing the aggregation of cerium oxide. As a result, a lithium ion secondary battery with excellent charge/discharge characteristics can be realized.

1 is a TEM image showing a nanoparticle aggregate (Z1-1) obtained in Example 1. FIG.

The present invention will be described in detail below.
In the method for producing a nanoparticle assembly for a negative electrode active material of a lithium ion secondary battery of the present invention, a large number of cerium oxide nanoparticles are arranged on the surface of cellulose nanofibers and/or lignocellulose nanofibers having an average outer diameter of 50 nm or less. A method for producing a nanoparticle aggregate (Z1) for a negative electrode active material of a lithium ion secondary battery comprising the following steps (I) to (II):
(I) a step of dispersing cellulose nanofibers and/or lignocellulose nanofibers having an average fiber diameter of 50 nm or less in a solvent to obtain a slurry (X); and (II) from the obtained slurry (X) and the cerium compound A step of subjecting the prepared slurry (Y) to a hydrothermal reaction to obtain a nanoparticle aggregate (Z1) composed of cerium oxide nanoparticles and cellulose nanofibers and/or lignocellulose nanofibers, which is used in the production of the present invention This is called method (Z1).
In addition, the method for producing a nanoparticle assembly for a negative electrode active material of a lithium ion secondary battery of the present invention is a lithium ion secondary battery in which a large number of cerium oxide nanoparticles are arranged on the surface of a carbon tubular body having an average outer diameter of 50 nm or less. A method for producing a nanoparticle aggregate (Z2) for a negative electrode active material of a next battery, comprising the following steps (I) to (III):
(I) dispersing cellulose nanofibers and/or lignocellulose nanofibers having an average fiber diameter of 50 nm or less in a solvent to obtain a slurry (X);
(II) The obtained slurry (X) and the slurry (Y) prepared from the cerium compound are subjected to a hydrothermal reaction to obtain nanoparticle aggregates ( and (III) drying and then calcining the resulting nanoparticle aggregate (Z1) to obtain a nanoparticle aggregate (Z2) composed of carbon tubular bodies and cerium oxide nanoparticles, is called the manufacturing method (Z2) of the present invention.

That is, the nanoparticle aggregate for a negative electrode active material of a lithium ion secondary battery obtained by the production method of the present invention has cerium oxide nanoparticles on the surface of cellulose nanofibers and/or lignocellulose nanofibers having an average outer diameter of 50 nm or less. is a nanoparticle assembly (Z1) in which a large number of are arranged, or a nanoparticle assembly (Z2) in which a large number of cerium oxide nanoparticles are arranged on the surface of a carbon tubular body having an average outer diameter of 50 nm or less. .
Each manufacturing method will be described below.

The nanoparticle aggregate (Z1) obtained by the production method (Z1) of the present invention includes cellulose nanofibers (a1) and/or lignocellulose nanofibers (a2) having an average fiber diameter of 50 nm or less (hereinafter referred to as "CNF (A)”) has a structure in which a large number of cerium oxide nanoparticles (C) are arranged on the surface of CNF (A). It has a structure in which the particles (C) are continuously arranged, and has a skewed dumpling-like or corn-like shape in appearance, in which a large number of cerium oxide nanoparticles (C) are linearly and continuously arranged. is presenting.

In the step (I) provided in the production method (Z1) of the present invention, the cellulose nanofibers (a1) and/or lignocellulose nanofibers (a2) having an average fiber diameter of 50 nm or less are dispersed in a solvent to form a slurry (X). is the process of obtaining

Cellulose nanofiber (a1) is a skeletal component that accounts for about 50% of all plant cell walls, and is a lightweight, high-strength fiber that can be obtained by defibrating plant fibers that make up such cell walls to nanosize. be. Lignocellulose nanofibers (a2) are cellulose nanofibers obtained without purification to remove lignin, and are obtained by coating cellulose nanofibers (a1) with lignin.
Both cellulose nanofibers (a1) and lignocellulose nanofibers (a2) have excellent dispersibility in water. In step (I), either one of these cellulose nanofibers (a1) and lignocellulose nanofibers (a2) may be used, or both may be used in combination.

The average fiber diameter of CNF (A) is 50 nm or less, preferably 20 nm or less, and more preferably 10 nm or less. Although the lower limit is not particularly limited, it is usually 1 nm or more. In addition, the average length of CNF (A) is preferably 100 nm to 100 μm, more preferably 1 μm to 100 μm, still more preferably 5 μm to 100 μm, from the viewpoint of efficiently processing the negative electrode.

As the solvent used in step (I), water is preferable from the viewpoint of good dispersibility of CNF (A) and from the viewpoint of handleability. Further, by using water as a solvent, the obtained slurry (X) can be used as it is without chemical modification or addition of a dispersant, even when subjected to a hydrothermal reaction in step (II) described later. Then, step (II) can be performed.

The total content of CNF (A) in the slurry (X) is preferably 0.01 parts by mass to 50 parts by mass in terms of carbon atoms of CNF (A) with respect to 100 parts by mass of the solvent in the slurry (X). parts, more preferably 0.05 to 40 parts by mass.
The obtained slurry (X) maintains a good dispersion state for several days, so that the slurry (X) can be prepared in advance. When the slurry (X) prepared appropriately is used in step (II) described later, it is preferable to use a high-concentration slurry (X) (masterbatch) with an increased content of CNF (A). The total content of CNF (A) in the high-concentration slurry (X) is preferably 5 parts by mass to 50 parts by mass in terms of carbon atoms of CNF (A) with respect to 100 parts by mass of the solvent in the slurry (X). parts by mass, more preferably 8 to 40 parts by mass.
When the slurry (X) is obtained by diluting the high-concentration slurry (X) with water, the slurry (X) may be sufficiently stirred with a disperser (homogenizer) before use. Examples of such a disperser include the aforementioned disaggregator, beater, low-pressure homogenizer, high-pressure homogenizer, grinder, cutter mill, ball mill, jet mill, short-screw extruder, twin-screw extruder, ultrasonic stirrer, juicer mixer for home use, and the like. can be used effectively.

In addition, when the resulting slurry (X) is used as it is in step (II), the total content of CNF (A) is the carbon atoms of CNF (A) with respect to 100 parts by mass of the solvent in slurry (X) The converted amount is preferably 0.01 to 50 parts by mass, more preferably 0.05 to 40 parts by mass.

Slurry (X) is preferably subjected to a hydrothermal reaction in advance before proceeding to step (II). At this time, it is preferable to stir the slurry (X) before subjecting it to the hydrothermal reaction, and it is preferable to use a disperser (homogenizer) for such stirring. Such dispersers include, for example, disaggregators, beaters, low-pressure homogenizers, high-pressure homogenizers, grinders, cutter mills, ball mills, jet mills, short-screw extruders, twin-screw extruders, ultrasonic stirrers, household juicer mixers, and the like. mentioned. Among them, an ultrasonic stirrer is preferable from the viewpoint of dispersion efficiency of CNF (A).

In the step (I), when the slurry (X) is previously subjected to a hydrothermal reaction before proceeding to the step (II), the temperature during the hydrothermal reaction may be 100 ° C. or higher, and 100 ° C. to 160 ° C. °C is preferred. The hydrothermal reaction is preferably carried out in a pressure vessel, and when the reaction is carried out at 100°C to 160°C, the pressure at this time is preferably 0.1 MPa to 0.7 MPa, and the reaction is carried out at 120°C to 160°C. The pressure when carrying out is preferably 0.2 MPa to 0.7 MPa. The hydrothermal reaction time is preferably 0.1 to 12 hours, more preferably 0.2 to 6 hours.

The CNFs (A) are well dispersed in the slurry (X) while they are present as single substances without unnecessary aggregation or sedimentation. The degree of dispersion of CNF (A) in the slurry (X) can be quantitatively evaluated, for example, by light transmittance using a UV/visible light spectrometer or viscosity using an E-type viscometer. Specifically, for example, a slurry (X) in which 2 parts by mass of CNF (A) in terms of carbon atoms are dispersed in 100 parts by mass of the solvent has a viscosity of 500 mPa at 25 ° C. measured by an E-type viscometer.・It maintains a good dispersion state of s to 3000 mPa·s.

The step (II) provided in the production method (Z1) of the present invention includes subjecting the obtained slurry (X) and the slurry (Y) prepared from the cerium compound to a hydrothermal reaction to obtain cerium oxide nanoparticles and cellulose nanofibers ( This is a step of obtaining nanoparticle aggregates (Z1) composed of a1) and/or lignocellulose nanofibers (a2).

The cerium compound used in step (II) is one or two selected from cerium oxide, cerium hydroxide, cerium carbonate, cerium acetate, cerium sulfate, cerium nitrate, cerium bromide, cerium boride and cerium chloride. The above are mentioned. Among them, one or more selected from cerium chloride, cerium nitrate and cerium sulfate is preferable from the viewpoint of efficiently proceeding the hydrothermal reaction.

The slurry (Y) is a slurry prepared from the slurry (X) and the cerium compound, and is prepared by mixing and stirring the slurry (X) and the cerium compound, or the slurry (X), the cerium compound and a solvent. good. As the solvent to be used, water is preferable, like the solvent to be used in step (I). The content of the cerium compound in the resulting slurry (Y) is preferably 0.1 parts by mass to 270 parts by mass, more preferably 0.5 parts by mass to 100 parts by mass of the solvent in the slurry (Y). 260 parts by mass, more preferably 1 to 250 parts by mass.

Further, the total content of CNF (A) in the slurry (Y) is preferably 0.01 parts by mass to 50 parts by mass in terms of carbon atoms of CNF (A) with respect to 100 parts by mass of the solvent in the slurry (Y). parts by mass, more preferably 0.05 to 40 parts by mass.
The mixing ratio of the slurry (X), the cerium compound and the solvent may be appropriately adjusted so that the contents of the cerium compound and CNF (A) in the slurry (Y) are as described above.

The slurry (Y) is preferably stirred in advance before being subjected to the hydrothermal reaction, and it is preferable to use a disperser (homogenizer) for such stirring. Examples of such a disperser include the aforementioned disaggregator, beater, low-pressure homogenizer, high-pressure homogenizer, grinder, cutter mill, ball mill, jet mill, short-screw extruder, twin-screw extruder, ultrasonic stirrer, juicer mixer for home use, and the like. can be used effectively.

The temperature during the hydrothermal reaction in step (II) may be 100°C or higher, preferably 130°C to 180°C. The hydrothermal reaction is preferably carried out in a pressure vessel, and when the reaction is carried out at 130°C to 180°C, the pressure at this time is preferably 0.2 MPa to 1 MPa, and when the reaction is carried out at 140°C to 160°C. is preferably 0.3 MPa to 0.6 MPa. The hydrothermal reaction time is preferably 0.5 hours to 24 hours, more preferably 0.5 hours to 15 hours.

The resulting hydrothermal reaction product is a nanoparticle assembly (Z1) composed of cellulose nanofibers (a1) and cerium oxide nanoparticles (C), or composed of lignocellulose nanofibers (a2) and cerium oxide nanoparticles (C). Nanoparticle aggregates (Z1), or nanoparticle aggregates (Z1) composed of cellulose nanofibers (a1), lignocellulose nanofibers (a2), and cerium oxide nanoparticles (C), and washed with water after filtration. It can be isolated by When washing the nanoparticle aggregates (Z1) with water, it is preferable to use 5 to 100 parts by mass of water with respect to 1 part by mass of the nanoparticle aggregates (Z1).

In addition, from the viewpoint of reducing aggregation of the obtained nanoparticle aggregates (Z1), the nanoparticle aggregates (Z1) are contained in a desired amount by repulping after washing the nanoparticle aggregates (Z1). A slurry (Z) may be prepared. The prepared slurry (Z) can be used as it is as a negative electrode slurry for producing a negative electrode for constructing a lithium ion secondary battery, or in the step (III) provided in the production method (Z2) of the present invention described later. , can be used as a slurry for obtaining the nanoparticle aggregate (Z2).
The content of the nanoparticle aggregates (Z1) in the slurry (Z) is preferably 0.5% by mass to 20% by mass in the slurry (Z) from the viewpoint of handling when preparing the negative electrode slurry of the secondary battery. %, more preferably 1% by mass to 15% by mass, and even more preferably 1.5% by mass to 12% by mass.

In the nanoparticle aggregate (Z1) obtained by the production method (Z1) of the present invention, the cerium oxide nanoparticles (C) arranged in large numbers on the surface of the CNF (A) have an average particle size of excellent rate characteristics. From the viewpoint of obtaining a lithium ion secondary battery, the thickness is preferably 1 nm to 100 nm, more preferably 1 nm to 70 nm, and still more preferably 1 nm to 50 nm.
The average particle size of the cerium oxide nanoparticles (C) is the value of the particle size (long axis length) of several tens of cerium oxide nanoparticles (C) measured by observation with an SEM or TEM electron microscope. means the average value of

In addition, the BET specific surface area of the cerium oxide nanoparticles (C) is preferably 5 m ² /g to 50 m ² /g, more preferably 15 m ² /g, from the viewpoint of obtaining a lithium ion secondary battery with excellent rate characteristics. 50 m ² /g, more preferably 20 m ² /g to 50 m ² /g. If the BET specific surface area exceeds 50 m ² /g, the volumetric energy density of the negative electrode may decrease.

In the nanoparticle assembly (Z1) for the negative electrode active material of the lithium ion secondary battery obtained by the production method (Z1) of the present invention, the content of CNF (A) is 100% by mass of the total amount of the nanoparticle assembly (Z1). Among them, it is preferably 0.3% by mass to 50% by mass, more preferably 1% by mass to 40% by mass.

Next, in the nanoparticle assembly (Z2) obtained by the production method (Z2) of the present invention, a large number of cerium oxide nanoparticles (C) are arranged on the surface of the carbon tubular body (B) having an average outer diameter of 50 nm or less. In other words, it has a structure in which a large number of cerium oxide nanoparticles (C) are continuously arranged so as to surround the carbon tubular body (B) as an axis, and the appearance is Like the nanoparticle aggregate (Z1), it has a skewered dumpling-like or corn-like shape in which a large number of cerium oxide nanoparticles (C) are linearly and continuously arranged.
The carbon tubular body (B) is a tubular structure composed of carbon derived from cellulose nanofibers (a1) and/or carbon derived from lignocellulose nanofibers (a2) having an average fiber diameter of 50 nm or less. be.

Steps (I) to (II) provided in the production method (Z2) of the present invention are the same as steps (I) to (II) provided in the production method (Z1) of the present invention. That is, the nanoparticle aggregates (Z1) obtained in the step (II) of the production method (Z1) are used as the nanoparticle aggregates (Z1) in the step (III) of the production method (Z2) of the present invention. Just do it. Here, as the nanoparticle aggregate (Z1), one isolated by washing with water after filtration may be used, or a slurry prepared by repulping after washing may be used as the nanoparticle aggregate. .

In the step (III) provided in the production method (Z2) of the present invention, the obtained nanoparticle aggregate (Z1) is dried and then calcined to obtain nanoparticles composed of carbon tubular bodies (B) and cerium oxide nanoparticles (C). This is the step of obtaining aggregates. As a drying means, constant temperature drying, freeze drying, spray drying, and vacuum drying are used, and freeze drying or spray drying is preferable from the viewpoint that the CNF (A) obtained after drying is less damaged.

Firing in step (III) is preferably carried out under an inert gas atmosphere or under reducing conditions. Thereby, the CNF (A) contained in the nanoparticle aggregate (Z1) is carbonized to form the carbon tubular body (B), and the carbon tubular body (B) and the cerium oxide nanoparticles (C) having good conductivity are formed. A nanoparticle aggregate (Z2) consisting of can be obtained.
In such firing, the firing temperature is preferably 400° C. to 800° C., more preferably 400° C. to 800° C., from the viewpoint of preventing enlargement of the cerium oxide nanoparticles (C) and unnecessary sintering of the cerium oxide nanoparticles (C). 500°C to 700°C. The firing time is preferably 0.5 hours to 24 hours, more preferably 6 hours to 18 hours. The apparatus used for firing is not particularly limited as long as it is capable of adjusting the firing atmosphere and temperature, and any of batch type, continuous type, and heating method (indirect or direct) can be used. . Examples of such apparatuses include firing furnaces such as externally heated kilns and roller hearths.

The average outer diameter of the carbon tubular body (B) that constitutes the obtained nanoparticle assembly (Z2) is formed by carbon derived from CNF (A), so similar to CNF (A), It is 50 nm or less, preferably 1 nm to 50 nm, more preferably 1 nm to 20 nm, even more preferably 1 nm to 10 nm. In addition, the average length of the carbon tubular body (B) also matches the average length of the CNF (A), and is preferably 100 nm to 100 μm, more preferably 1 μm to 100 μm, still more preferably 5 μm to 100 μm. .

The cerium oxide nanoparticles (C) arranged on the surface of the carbon tubular body (B) are the same as the cerium oxide nanoparticles (C) in the nanoparticle assembly (Z1), preferably 1 nm to 100 nm, and more It is preferably 1 nm to 70 nm, more preferably 1 nm to 50 nm, and the BET specific surface area is also preferably 5 m ² /g to 50 m ² /g, more preferably 15 m ² /g to 50 m ² /g. , more preferably 20 m ² /g to 50 m ² /g.

In the nanoparticle aggregates (Z2) for negative electrode active material of a lithium ion secondary battery obtained by the production method (Z2) of the present invention, the content of the carbon tubular body (B) is 100% of the total amount of the nanoparticle aggregates (Z2). In mass %, it is preferably 0.1 mass % to 25 mass %, more preferably 0.4 mass % to 20 mass %.

The method for producing the negative electrode of a lithium ion secondary battery by using the nanoparticle aggregate (Z1) or the nanoparticle aggregate (Z2) thus obtained as a negative electrode active material is not particularly limited, and a known method is used. can.
For example, such nanoparticle aggregates (Z) are mixed with additives such as binders and solvents to obtain negative electrode slurry. At this time, if necessary, a conductive aid may be added and mixed. Such a binder is not particularly limited, and any known agent can be used. Specific examples include styrene-butadiene rubber, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, ethylene propylene diene polymer and the like. Further, the conductivity aid is not particularly limited, and known agents can be used. Specific examples include acetylene black, ketjen black, natural graphite, artificial graphite, and fibrous carbon. Next, the negative electrode slurry is applied onto a negative electrode current collector such as a copper foil and dried to form a negative electrode.

The nanoparticle aggregate obtained by the production method of the present invention is useful in that it exhibits extremely excellent discharge capacity as a negative electrode of a lithium ion secondary battery when used as a negative electrode active material. A secondary battery to which such a negative electrode can be applied is not particularly limited as long as it essentially comprises a positive electrode, a negative electrode, an electrolytic solution, and a separator.

Here, regarding the positive electrode, the material configuration is not particularly limited as long as it can release lithium ions during charging and absorb lithium ions during discharging, and known material configurations can be used. For example, a positive electrode made of various polyanion-type positive electrode active materials obtained by hydrothermally reacting raw materials can be suitably used.

The electrolytic solution is obtained by dissolving a supporting salt in an organic solvent. The organic solvent is not particularly limited as long as it is an organic solvent that is usually used in the electrolyte of a lithium ion secondary battery, and examples thereof include carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones. , oxolane compounds and the like can be used.

The type of supporting salt is not particularly limited, but inorganic salts selected from LiPF ₆ , LiBF ₄ , LiClO ₄ and LiAsF ₆ , derivatives of the inorganic salts, LiSO ₃ CF ₃ , LiC(SO ₃ CF ₃ ) ₂ and LiN(SO ₃ CF ₃ ) ₂ , LiN(SO ₂ C ₂ F ₅ ) ₂ and LiN(SO ₂ CF ₃ )(SO ₂ C ₄ F ₉ ), and derivatives of said organic salts. is preferably at least one of

The separator serves to electrically insulate the positive electrode and the negative electrode and retain the electrolyte. For example, a porous synthetic resin film, particularly a porous film of polyolefin polymer (polyethylene, polypropylene) may be used.

The shape of the lithium-ion secondary battery having the above configuration is not particularly limited, and may be various shapes such as a coin shape, a cylindrical shape, a square shape, or an irregular shape enclosed in a laminated outer package. .

EXAMPLES The present invention will be specifically described below based on examples, but the present invention is not limited to these examples.

[Example 1]
23.8 g of Ce(NO ₃ ) ₂ .6H ₂ O, 19.29 g of cellulose nanofiber (TMa-10002 manufactured by Sugino Machine, water content of 98% by mass, average fiber diameter of 10 nm) and 55 mL of water were mixed for 5 minutes. A slurry a1 was produced. To the resulting slurry a1, 60 g of a 10% by mass NaOH aqueous solution was added and mixed for 5 minutes to obtain slurry b1.

The resulting slurry b1 was put into an autoclave and hydrothermally reacted at 130° C. and 0.3 MPa for 1 hour. The resulting hydrothermal reaction product was allowed to cool, filtered, and washed with water to obtain a nanoparticle aggregate (Z1-1) in which a large number of CeO ₂ nanoparticles were arranged on the surface of cellulose nanofibers. (CeO2 nanoparticles: cellulose nanofibers ₌ 96% by mass: 4% by mass)
A TEM photograph of the resulting nanoparticle aggregate (Z1-1) is shown in FIG. From FIG. 1, it was confirmed that the resulting nanoparticle aggregate (Z1-1) had a unique shape in which a large number of cerium oxide nanoparticles were arranged on the surface of cellulose nanofibers.

[Example 2]
_{Nanoparticle aggregates ( Z1-} 2) was obtained. (CeO2 nanoparticles: cellulose nanofibers ₌ 50% by mass: 50% by mass)

[Example 3]
_{A nanoparticle} aggregate _{( Z1-} 3) was obtained. (CeO2 nanoparticles: cellulose nanofibers ₌ 99.5% by mass: 0.5% by mass)

[Example 4]
After freeze-drying the nanoparticle aggregate (Z1-1) obtained in Example 1 using a freeze dryer (manufactured by EYELA, FDU-1200), cellulose was baked at 600 ° C. for 1 hour in a nitrogen atmosphere. A nanoparticle assembly (Z2-1) was obtained in which a large number of CeO ₂ nanoparticles were arranged on the surface of a carbon tubular body formed from nanofibers. ₍ CeO2 nanoparticles: carbon tubular body = 98.9% by mass: 1.1% by mass)

[Comparative Example 1]
CeO ₂ nanoparticles (E-1) were obtained in the same manner as in Example 1, except that the cellulose nanofibers were not added.

[Comparative Example 2]
3 g of the CeO ₂ nanoparticles (E-1) obtained in Comparative Example 1, 1.65 g of glucose, and 30 mL of water were mixed in a planetary ball mill to obtain a slurry, and then the resulting slurry was dried at 100° C. to obtain CeO Granules consisting of _two nanoparticles and glucose were obtained. The resulting granules were calcined at 600° C. for 1 hour in a nitrogen atmosphere to obtain CeO ₂ nanoparticles (E-2) coated with carbohydrated glucose. ₍ CeO2 nanoparticles: carbohydrated glucose = 90% by mass: 10% by mass)

Table 1 shows the physical properties and compositions of the resulting nanoparticle aggregates (Z1-1) to (Z1-4) and CeO ₂ nanoparticles (E-1) to (E-2).

<<Evaluation of charge/discharge characteristics>>
The obtained nanoparticle aggregates (Z1-1) to (Z1-4) and CeO ₂ nanoparticles (E-1) to (E-2) were used as negative electrode active materials for lithium ion secondary batteries to prepare negative electrodes. Then, the charge/discharge characteristics were evaluated.
Specifically, negative electrode active material for lithium ion secondary batteries (a), acetylene black (conductive aid, b), carboxymethyl cellulose (surfactant, c), styrene-butadiene rubber (binder, d) They were mixed at a:b:c:d (mass ratio) = 90:5:2.5:2.5, distilled water was added thereto and sufficiently kneaded to prepare a negative electrode slurry.
The obtained negative electrode slurry was applied to a current collector made of copper foil having a thickness of 10 μm using a coating machine, and vacuum-dried at 80° C. for 12 hours. After that, it was pressed with a roll press heated to 80° C., and punched out into a disk shape of φ14 mm to obtain a negative electrode.

Next, a coin-type lithium ion secondary battery was constructed using the above negative electrode. A lithium foil was used as the counter electrode. As the electrolytic solution, LIPF ₆ was dissolved at a concentration of 1 mol/L in a mixed solvent of ethylene carbonate:ethyl methyl carbonate (volume ratio)=3:7. Polypropylene was used for the separator.
These battery parts were incorporated and housed in an atmosphere with a dew point of -50°C or lower by a conventional method to produce a coin-type lithium ion secondary battery (CR-2032).

Cycle characteristics at a constant current density were evaluated using the obtained lithium ion secondary battery. Specifically, using a discharge capacity measuring device (manufactured by Hokuto Denko Co., Ltd., HJ-1001SD8), after discharging to 0.05 V at 200 mA / g at a temperature of 30 ° C., until the lower limit current of 20 mA / g is reached. Discharged at constant voltage. After that, charging up to 3 V at 200 mA/g was set as one cycle.
Also, the rate characteristics at different current densities were evaluated using the obtained lithium ion secondary battery. Specifically, after discharging to 0.05 V at 200 mA/g or 1000 mA/g in a temperature environment of 30° C., discharging was performed at a constant voltage until the lower limit current was 20 mA/g or 100 mA/g. After that, charging up to 3 V at 200 mA/g was set as one cycle.

In the evaluation of the cycle characteristics, the capacity retention rate (%) was obtained from the discharge capacity at the 2nd cycle and the discharge capacity at the 50th cycle.
Table 2 shows the results.
Cycle capacity retention rate (%) =
(discharge capacity at 50th cycle)/(discharge capacity at 2nd cycle)×100 (1)

Furthermore, in the evaluation of the rate characteristics, the rate capacity retention rate (%) was obtained from the discharge capacity when evaluated at 200 mA/g in the second cycle and the discharge capacity when evaluated at 1000 mA/g. Table 2 shows the results.
Rate capacity retention rate (%) =
(Discharge capacity when evaluated at 1000 mA/g)/(Discharge capacity when evaluated at 200 mA/g)×100 (2)

From the above results, as shown in FIG. 1, the lithium ion secondary battery produced by using the nanoparticle assembly of the example exhibiting a unique shape as the negative electrode active material of the lithium ion secondary battery is the CeO ₂ of the comparative example. It can be seen that both the cycle characteristics and the rate characteristics are superior to the lithium ion secondary battery produced using the nanoparticles as the negative electrode active material of the lithium ion secondary battery.

Claims (7)

A method for producing a nanoparticle assembly for a negative electrode active material of a lithium ion secondary battery, comprising a large number of cerium oxide nanoparticles arranged on the surface of cellulose nanofibers and/or lignocellulose nanofibers having an average outer diameter of 50 nm or less, , the following steps (I) to (II):
(I) a step of dispersing cellulose nanofibers and/or lignocellulose nanofibers having an average fiber diameter of 50 nm or less in a solvent to obtain a slurry (X); and (II) from the obtained slurry (X) and the cerium compound A negative electrode active material for a lithium ion secondary battery comprising a step of subjecting the prepared slurry (Y) to a hydrothermal reaction to obtain a nanoparticle aggregate composed of cerium oxide nanoparticles and cellulose nanofibers and/or lignocellulose nanofibers A method for producing a nanoparticle aggregate. A method for producing a nanoparticle assembly for a negative electrode active material for a lithium ion secondary battery, comprising a carbon tubular body having an average outer diameter of 50 nm or less and a large number of cerium oxide nanoparticles arranged on the surface thereof, the method comprising the following step (I): ~ (III):
(I) dispersing cellulose nanofibers and/or lignocellulose nanofibers having an average fiber diameter of 50 nm or less in a solvent to obtain a slurry (X);
(II) The obtained slurry (X) and the slurry (Y) prepared from the cerium compound are subjected to a hydrothermal reaction to obtain nanoparticle aggregates composed of cerium oxide nanoparticles and cellulose nanofibers and/or lignocellulose nanofibers. and (III) drying and then calcining the obtained nanoparticle aggregate to obtain a nanoparticle aggregate composed of carbon tubular bodies and cerium oxide nanoparticles. A method for producing a particle aggregate. The cerium compound used in step (II) is one or more selected from cerium oxide, cerium hydroxide, cerium carbonate, cerium acetate, cerium sulfate, cerium nitrate, cerium bromide, cerium boride and cerium chloride. 3. A method for producing a nanoparticle aggregate for a negative electrode active material of a lithium ion secondary battery according to claim 1 or 2. 4. The method according to any one of claims 1 to 3, wherein a large number of cerium oxide nanoparticles arranged on the surface of the cellulose nanofibers and/or lignocellulose nanofibers or on the surface of the carbon tubular body have an average particle size of 1 nm to 100 nm. A method for producing a nanoparticle aggregate for a negative electrode active material of a lithium ion secondary battery described above. The total content of cellulose nanofibers and lignocellulose nanofibers in the nanoparticle aggregate for negative electrode active material of a lithium ion secondary battery is 0.3% by mass to 50% by mass. A method for producing a nanoparticle aggregate for a negative electrode active material of a lithium ion secondary battery according to any one of claims 1 to 3. The method for producing a nanoparticle aggregate for a negative electrode active material of a lithium ion secondary battery according to any one of claims 2 to 4, wherein the sintering in step (III) is performed under an inert gas atmosphere or under reducing conditions. The negative electrode active material for the lithium ion secondary battery according to claim 6, wherein the content of the carbon tubular body in the nanoparticle aggregate for the negative electrode active material for the lithium ion secondary battery is 0.1% by mass to 25% by mass. A method for producing nanoparticle aggregates for use.

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