JP7765214B2 - Tin oxide particles - Google Patents

Description

The present invention relates to tin oxide particles.

Lithium secondary batteries, such as lithium-ion batteries, are used as power sources for portable electronic devices such as laptops and mobile phones. In recent years, as these portable electronic devices have become more powerful and have longer lifespans, there is a demand for improved battery performance, such as higher capacity and the ability to maintain capacity for long periods of time.

Patent Document 1 discloses a battery separator made by applying an acrylic adhesive with dispersed tin oxide to both sides of a polyethylene film, with the aim of producing a thin, compact separator.

Patent Document 2 discloses a separator for ensuring battery safety, which includes a porous substrate with pores and a porous active layer formed by coating at least one surface of the substrate with a mixture of inorganic particles and a binder polymer. The document also discloses that this separator can be used in lithium secondary batteries.

Patent Document 3 discloses a lithium secondary battery equipped with a porous insulating layer containing a low-absorbency inorganic filler with relatively low lithium absorption and a high-absorbency inorganic filler with relatively high lithium absorption, with the aim of preventing internal short circuits in the battery.

Patent Document 4 discloses a battery that includes a positive electrode, a negative electrode, a separator, and an electrolyte containing particles with a predetermined flatness and refractive index, with the aim of ensuring capacity and safety.

Patent Document 5 discloses a battery that has a positive electrode, a negative electrode, a separator disposed between these electrodes, an electrolyte, and solid particles, with the solid particles disposed in predetermined regions of the electrodes, with the aim of improving overcharge resistance.

Japanese Patent Application Publication No. 11-213979 Special Publication No. 2009-518809 JP 2013-109866 A JP 2015-090777 A JP 2016-119213 A

Improving battery performance involves a variety of factors, including not only charge/discharge efficiency and capacity, but also cycle characteristics such as rate and discharge capacity retention. However, the separators containing particles or fillers described in Patent Documents 1 to 5, as well as the batteries equipped with such separators, do not address the issue of improving cycle characteristics.

Therefore, the objective of the present invention is to provide tin oxide particles that, when applied to separators for lithium-ion batteries, can improve the cycle characteristics of the batteries.

The present invention is characterized in that the volume cumulative particle size D90 at 90% by volume cumulative volume measured by a laser diffraction/scattering particle size distribution measurement method is 3 μm or less,
the ratio (S1/D50) of the BET specific surface area S1 to the volume cumulative particle size D50 at a cumulative volume of 50% by volume, as determined by a laser diffraction/scattering particle size distribution measurement method, is 10 or more and 100 or less;
The present invention provides tin oxide particles that are disposed in a separator for a lithium ion battery.

The present invention provides tin oxide particles that, when used in separators for lithium-ion batteries, can improve the cycle characteristics of the batteries.

FIG. 1 is a graph showing the charge/discharge curves of a battery when the tin oxide particles of Example 2 are used for the separator. FIG. 2 is a graph showing charge/discharge curves of a battery when the alumina particles of Comparative Example 2 are used for the separator.

The present invention will be described below based on preferred embodiments. The tin oxide particles of the present invention are composed of tin oxide and primarily comprise tin oxide represented by the chemical formula SnO _(2-X) (where 0≦X≦1). That is, the tin oxide particles include those composed of at least one of tin(II) oxide (SnO) and tin(IV) oxide ( _SnO2 ), or _SnO2 having oxygen deficiency. The tin oxide particles preferably contain 50 mol% or more of the tin oxide as _SnO2 . In addition, the tin oxide particles may contain elemental tin.
In either case, the tin oxide particles are suitable for use as a filler in a separator for a lithium ion battery.

Tin oxide particles are either composed of tin and oxygen and contain no other elements except for inevitable impurities, or they further contain elements other than tin and oxygen. The former form, i.e., tin and oxygen, is preferred, but the inclusion of inevitable impurity elements other than tin and oxygen is acceptable as long as the effects of the present invention are not impaired. In either form, the content of elements other than tin and oxygen in the tin oxide particles is preferably 2 mass% or less. The content of these elements can be measured, for example, by gas composition analysis or ICP atomic emission spectroscopy.

The tin element content in the tin oxide particles, as measured by ICP atomic emission spectroscopy, is preferably 76% by mass or more and 84% by mass or less, more preferably 76% by mass or more and 82% by mass or less, and even more preferably 76% by mass or more and 80% by mass or less. By having this range, when the tin oxide particles are used in a separator, the insulating properties required of the separator can be easily maintained.

Furthermore, the oxygen element content in the tin oxide particles, as measured by gas composition analysis, is preferably 13% by mass or more and 22% by mass or less, more preferably 15% by mass or more and 22% by mass or less, and even more preferably 17% by mass or more and 22% by mass or less. By having this range, when the tin oxide particles are used in a separator, it becomes easier to sufficiently maintain the insulating properties required of the separator.

The total tin oxide content in the tin oxide particles is preferably 98% by mass or more and 100% by mass or less, more preferably 99% by mass or more and 100% by mass or less, and even more preferably 99.5% by mass or more and 100% by mass or less. This range makes it easier to maintain sufficient insulating properties, which are important for separator function, when the tin oxide particles are used in a separator. The tin oxide content can be measured by a method such as a quantitative determination method using a displacement dissolution method using a copper(II) chloride potassium solution.

The volume cumulative particle size D90 of the tin oxide particles at 90% cumulative volume, as measured by a laser diffraction/scattering particle size distribution measurement method, is preferably 3.0 μm or less, more preferably 2.6 μm or less, and even more preferably 2.5 μm or less, with 1.0 μm or more being realistic.

The above-mentioned D90 is an index representing the size of relatively coarse particles present in tin oxide powder, which is an aggregate of tin oxide particles. Generally, lithium-ion batteries include a positive electrode, a negative electrode, and a separator disposed in a compressed state between these electrodes. When tin oxide particles are used as the separator, the separator is compressed together with the particles, causing mechanical damage to the separator. This can affect the insulating performance of the separator and cause micro-short circuits between the positive and negative electrodes. These micro-short circuits can contribute to a deterioration in the lithium ion supply balance between the positive and negative electrodes during charge and discharge, ultimately leading to a decrease in battery performance.
When the D90 is within the above range, even when the separator is compressed together with the particles, excessively coarse particles are not present, so that even when the particles are disposed in the separator, the occurrence of mechanical damage to the separator is reduced, the insulating performance of the separator is less affected, and the occurrence of micro-short circuits is also reduced. As a result, the separator has the heat resistance, electrical insulation, and ionic conductivity required, while improving the cycle characteristics of the battery, particularly the characteristics related to rate and capacity maintenance during charge/discharge cycles. Tin oxide particles satisfying the above D90 can be produced, for example, by the production method described below.

The volume cumulative particle diameter D90 can be measured using the following method. Specifically, 0.1 g of the tin oxide particles to be measured is mixed with 100 mL of a 20 mg/L aqueous solution of sodium hexametaphosphate, and the mixture is dispersed for 10 minutes using an ultrasonic homogenizer (Nippon Seiki Seisakusho US-300T). The particle size distribution is then measured using a laser diffraction/scattering particle size distribution analyzer (e.g., Horiba Ltd. LA-920).

The tin oxide particles preferably have an S1/D50 ratio ( ^m2 /g/μm), which is the ratio of the BET specific surface area S1 ( ^m2 /g) to the volume cumulative particle size D50 (μm) at 50% cumulative volume measured by a laser diffraction/scattering particle size distribution measurement method, within a predetermined range.
Specifically, the S1/D50 ratio in the tin oxide particles is preferably 10 or more and 100 or less, more preferably 20 or more and 90 or less, and even more preferably 45 or more and 90 or less.

The S1/D50 ratio is a parameter that represents the BET specific surface area per unit particle diameter. A higher S1/D50 ratio means a larger surface area per particle, for example, due to the particles being porous. By ensuring that the S1/D50 ratio is within the above-mentioned range, the electrolyte constituting the lithium-ion battery is more easily retained in the pores or between the particles, thereby improving the retention of the electrolyte in or near the separator (so-called liquid replenishment). As a result, even under harsh conditions such as high temperatures or after multiple charge/discharge cycles, loss of the electrolyte from the separator is prevented, allowing sufficient and reversible movement of lithium ions within the battery, thereby improving charge/discharge cycle durability. Tin oxide particles that satisfy this S1/D50 ratio can be produced, for example, by the production method described below.

The BET specific surface area S1 of the tin oxide particles is preferably 20 m ² /g or more and 70 m ² /g or less, more preferably 30 m ² /g or more and 60 m ² /g or less, and even more preferably 50 m ² /g or more and 60 m ² /g or less. By having the BET specific surface area S1 in this range, the separator can sufficiently retain the electrolyte even when the battery is exposed to significant temperature changes or when the internal battery pressure increases due to electrode expansion during charge and discharge. As a result, the battery performance required for a secondary battery, such as rate characteristics and cycle characteristics, can be maintained for a long period of time.

The BET specific surface area S1 can be measured by the nitrogen adsorption method based on the BET method, using, for example, a Monosorb MS-21 manufactured by Quantachrome. The amount of powder to be measured is 0.3 g, and the preliminary degassing conditions can be atmospheric air at 105°C for 60 minutes.

The volume cumulative particle diameter D50 is preferably 0.30 μm or more and 2.0 μm or less, more preferably 0.60 μm or more and 1.3 μm or less, and even more preferably 0.50 μm or more and 1.2 μm or less. By keeping the particle diameter within this range, the energy density of a battery including a separator in which tin oxide particles are disposed can be increased. In particular, when the tin oxide particles of the present invention are disposed on a separator by, for example, coating the surface of the separator, the thickness of the tin oxide particle layer can be made thin, which is advantageous in that the required insulation properties can be maintained while further increasing the energy density of the battery.

The volume cumulative particle size D10 of tin oxide particles at 10% cumulative volume, as measured by a laser diffraction/scattering particle size distribution measurement method, is preferably 0.30 μm or more and 0.80 μm or less, more preferably 0.30 μm or more and 0.70 μm or less, and even more preferably 0.50 μm or more and 0.70 μm or less. By keeping the particle size within this range, when the tin oxide particles are placed in a separator, appropriate voids are formed between the particles, improving electrolyte replenishment properties, and maintaining high rate characteristics of batteries including the separator.

The volume cumulative particle diameter D100 of the tin oxide particles at 100% cumulative volume as measured by a laser diffraction/scattering particle size distribution measurement method is preferably 3 μm or more and 6 μm or less, more preferably 3 μm or more and 5 μm or less, and even more preferably 3 μm or more and 4 μm or less. By setting the particle diameter in such a range, micro-short circuits caused by coarse particles can be suppressed when the tin oxide particles are disposed in a separator.
The D50, D10 and D100 of the tin oxide particles can each be measured by the same measurement method as the above-mentioned measurement of D90.

In a preferred embodiment, the tin oxide particles contain oxygen vacancies in the tin oxide _SnO2 . For example, the tin oxide particles preferably contain tin oxide represented by the chemical formula SnO _(2-X) (where 0<X<1).
In detail, the amount of oxygen vacancies contained in the tin oxide particles is preferably 1×10 ¹⁶ /g or more and 9×10 16 /g or less, more preferably 2×10 ¹⁶ /g or more and 6×10 ¹⁶ /g or less, and even more preferably 2×10 ¹⁶ /g or more and 4×10 ¹⁶ /g or less, per ^unit mass of the particles.
Such an amount of oxygen deficiency makes it possible to prevent unintended reactions with the electrolyte when the tin oxide particles are placed in the separator of a lithium ion battery while sufficiently maintaining the insulating performance of the separator, thereby maintaining stable charge/discharge voltage behavior and further improving cycle characteristics.

The amount of oxygen vacancy can be measured by electron spin resonance (ESR) analysis using the following method. A measuring device such as the JES-X330 manufactured by JEOL can be used. Details of the measurement procedure will be described in the examples below.

A suitable method for producing tin oxide particles is described below. In one embodiment of the production method, a water-soluble tin compound, which serves as the tin source, is dissolved in a solvent to produce a solution containing the tin compound (hereinafter also referred to as a tin solution), and intermediate particles are obtained by neutralizing the tin solution. The resulting intermediate particles are then calcined. This results in the desired tin oxide powder, which is composed of aggregates of tin oxide particles.

First, a tin solution is prepared (solution preparation step). The tin solution is preferably an aqueous tin solution using water as a solvent.
Examples of tin compounds include salts of stannic acid with alkali metals such as sodium and potassium, and inorganic tin salts such as tin(II) chloride and tin(IV) chloride. These tin compounds may be anhydrous or hydrated. The tin compounds can be used alone or in combination. Preferably, water-soluble inorganic tin salts such as alkali metal salts of stannic acid are used as the tin compound, which provides excellent handling during production, makes it easy to control the D90 within a predetermined range, and makes it easy to obtain particles that satisfy a predetermined S1/D50 ratio.

The tin compound is mixed so that the tin content in the tin solution is preferably 0.14 mol/L or more and 0.28 mol/L or less, and more preferably 0.17 mol/L or more and 0.24 mol/L or less. By adjusting the content in this manner, it becomes easier to control the D90 within the specified range, and particles that satisfy the specified S1/D50 ratio can be obtained with high productivity.

The tin solution may be heated or unheated. It is preferable that the solvent be heated before the tin solution is prepared. That is, the tin solution is preferably prepared by mixing a tin compound with a heated solvent. When water is used as the solvent, the solvent is heated to a temperature of preferably 30°C or higher and 70°C or lower, more preferably 40°C or higher and 65°C or lower. Heating to such a temperature allows the tin compound to be sufficiently dissolved, making it easier to produce particles that satisfy the specified D90 and S1/D50 ratio with high productivity.

Next, the tin solution obtained by the above method is neutralized to obtain a neutralized solution (neutralization step). The neutralizing agent used for neutralization can be an aqueous solution of various acids or bases. For example, when the tin solution is prepared using an alkali metal salt of stannic acid, the tin solution is alkaline. Therefore, an aqueous solution of a strong acid such as nitric acid or sulfuric acid is preferably used to neutralize the tin solution, and dilute sulfuric acid is more preferably used. The use of such a substance is advantageous in that the desired particles can be obtained efficiently in a fine particle state.
The concentration of the acid or base in the aqueous acid or base solution can be adjusted as appropriate, but is preferably 1 mol/L or more and 5 mol/L or less from the viewpoint of achieving both ease of handling and favorable physical properties of the resulting particles.

In the neutralization step, the neutralizing agent may be added all at once or may be added dropwise or in multiple batches. In this manufacturing method, it is preferable to add the neutralizing agent gradually, or to add the neutralizing agent while stirring the heated tin solution. The temperature of the tin solution when neutralizing it can be in the same range as the temperature of the solvent described above. In this case, the temperature of the tin solution may be the same as or different from the temperature of the solvent described above.

When the neutralizer is added gradually, it is preferable to add the entire amount of neutralizer continuously at a predetermined rate within the above-mentioned addition time range, provided that the concentration of the neutralizer is adjusted to the above-mentioned range. For example, as in Example 1 described below, in the case of a charge volume of 2.2 liters, the neutralizer addition rate can be preferably 12 mL/min or more and 73 mL/min or less, more preferably 18 mL/min or more and 37 mL/min or less. By using such an addition rate, particles that satisfy the specified D90 and S1/D50 ratio can be easily obtained with high productivity.

The time from the start to the end of neutralizing agent addition can be adjusted depending on the desired particle properties, but is preferably between 30 and 180 minutes, and more preferably between 60 and 120 minutes.

Furthermore, it is preferable to adjust the total amount of neutralizer added so that after adding the neutralizer, the pH of the neutralization solution at the solution temperature is preferably 2.0 or more and 4.0 or less, more preferably 2.5 or more and 3.5 or less. By adjusting the pH to this range, particle size variation is reduced, and particles that satisfy the specified D90 and S1/D50 ratio can be obtained with high productivity.

From the perspective of making the neutralization reaction occur more uniformly and making it easier to obtain homogeneous particles, it is preferable to add the neutralizing agent to the tin solution and then allow it to age for a predetermined period of time. Specifically, it is preferable to gradually add the entire amount of neutralizing agent to the tin solution, adjust the pH of the neutralized solution to the above-mentioned range, and then stir the neutralized solution in a heated state for a predetermined period of time. The stirring time is preferably 30 minutes or more and 180 minutes or less. Furthermore, it is preferable that the temperature of the neutralized solution be maintained at the conditions used when neutralizing the tin solution.

From the viewpoint of uniformly causing the neutralization reaction and obtaining particles with little variation in particle size, it is also preferable to continue stirring the neutralization solution at least from the start to the end of the neutralization step, more specifically, from the start of mixing the tin solution and the neutralizing agent to the end of aging, which is performed as needed. In this case, the stirring speed of the neutralization solution, expressed in terms of flow rate, is preferably 60 cm/s or more and 140 cm/s or less, more preferably 75 cm/s or more and 120 cm/s or less. The stirring speed of the neutralization solution may be constant or variable from the start to the end of the neutralization step. In addition, stirring may be performed at the above-mentioned stirring speed in the solution preparation step as well.
The stirring speed employed in the neutralization step is faster than that conventionally employed. By employing such a stirring speed, it is easy to control the particle size of the resulting particles, the particle size variation can be reduced, and particles satisfying a predetermined D90 and S1/D50 ratio can be easily obtained with high productivity.

Through the above steps, a plurality of intermediate particles dispersed in water are obtained, and these intermediate particles are typically tin hydroxide (Sn(OH) ₄ ).
The intermediate particles are preferably washed by pure water repulp washing, decantation, or the like before being subjected to the firing step described below. From the viewpoint of reducing impurities, washing is carried out until the conductivity of the aqueous phase after washing the intermediate particles is preferably 1000 μS/cm or less. Water is preferably used as the washing liquid.
In addition, after washing the intermediate particles, it is preferable to dry the solid content obtained by solid-liquid separation using a method such as dry heat drying or vacuum drying to obtain a dry powder that is an aggregate of intermediate particles. The dry powder of intermediate particles may be crushed using a known crushing device or sieved, and then subjected to subsequent steps.

Next, the dried powder of the intermediate particles is calcined (calcination step). The intermediate particles, which are tin hydroxide, are converted into tin oxide particles by calcining them under predetermined conditions.
The atmosphere in the firing step can be an oxygen-containing atmosphere such as air, an inert atmosphere, or a reducing atmosphere. Among these, from the viewpoint of efficiently obtaining tin oxide particles that satisfy a predetermined amount of oxygen deficiency while improving convenience, firing in an oxygen-containing atmosphere is preferred, and firing in an air atmosphere is more preferred.

The firing temperature in the firing step is preferably 400° C. or higher and 900° C. or lower, more preferably 500° C. or higher and 800° C. or lower. Furthermore, the firing time is preferably 20 minutes or higher and 150 minutes or lower, more preferably 40 minutes or higher and 120 minutes or lower, provided that the firing temperature is within the above-mentioned firing temperature range. By setting the firing time within such a range, the reaction from tin hydroxide to tin oxide is allowed to proceed sufficiently, and the BET specific surface area is easily controlled, making it easier to produce particles that satisfy a predetermined S1/D50 ratio with high productivity.
In addition, when the calcination is performed in an air atmosphere, tin oxide particles having a predetermined amount of oxygen vacancies can be efficiently obtained. Generally, calcination is performed in a reducing atmosphere to generate oxygen vacancies. However, the reason why a predetermined amount of oxygen vacancies can be generated by calcination in an air atmosphere as in the present production method is presumably because it is easier to control the amount of oxygen atoms released from the intermediate particles during temperature rise and the amount of oxygen atoms added to the intermediate particles during temperature fall than in a reducing atmosphere.

In order to remove coarse particles unintentionally generated during firing and efficiently obtain fine tin oxide powder, it is preferable to perform a process to remove the coarse particles by crushing or sieving the particles obtained after firing.

Through the above steps, the desired tin oxide particles can be obtained as a powder consisting of particle aggregates. The tin oxide particles thus obtained preferably contain tin oxide as a main component represented by the chemical formula SnO _(2-X) (where 0<X<1).
The tin oxide particles have a small particle size and are spherical even without a coating treatment to improve the dispersibility of the particles. This is because the degree of aggregation of the particles produced during production by this production method is thought to be low, allowing production while maintaining a small particle size. Note that, as long as the effects of the present invention are achieved, it is not excluded that the tin oxide particles may inevitably contain trace amounts of elements other than tin and oxygen, or that the surfaces of the tin oxide particles may inevitably be slightly oxidized.

Tin oxide particles can be used in the form of a powder consisting of an aggregate of particles without any special processing, and placed in the separator that constitutes a lithium-ion battery. Lithium-ion batteries are typically non-aqueous batteries that include a positive electrode made of a lithium-containing oxide, a negative electrode made of a carbon material, a separator placed between these electrodes, and an electrolyte solution containing an organic solvent and a lithium salt. One side of the separator is adjacent to the positive electrode, and the other side is adjacent to the negative electrode.

A separator for a lithium-ion battery is made of a porous resin film containing a polyolefin resin such as polyethylene or polypropylene, and is fluid permeable in the thickness direction. The separator may have a single-layer structure or a multi-layer structure.
The tin oxide particles are disposed in the separator in a state in which the particles are dispersed in a polyolefin resin during the separator manufacturing process, or are attached to at least one surface of a porous resin film and disposed on the surface of the film. This improves the heat resistance of the separator and the high-temperature stability of the battery. When the particles are disposed in the separator, the content of the tin oxide particles in the separator is usually about 30% by mass or more and 75% by mass or less.

When tin oxide particles are disposed on the surface of a porous resin film, the tin oxide particles are preferably disposed at least on the surface of the separator facing the positive electrode, and more preferably disposed adjacent to the surface as a particle layer containing tin oxide particles and a binder resin. This further improves the heat resistance of the separator and the high-temperature stability of the battery. Preferred examples of the binder resin constituting the particle layer include heat-resistant resins, such as polyimide resins and aromatic polyamide resins. When a particle layer is formed, the content of tin oxide particles in the particle layer is typically approximately 80% by mass or more and 95% by mass or less.

When tin oxide particles having the above configuration are placed in a separator for a lithium-ion battery, they reduce the occurrence of mechanical damage to the separator, while providing excellent electrolyte replenishment properties that make up the lithium-ion battery and maintaining sufficient lithium ion conductivity within the battery. As a result, they can be used as a filler to enhance the heat resistance and durability of the separator, and also improve charge/discharge cycle characteristics.

In addition to electrolytes such as lithium-containing compounds, sulfur-containing additives such as vinylene carbonate (VC) and 1,3-propane sultone (PS) may be added to the electrolyte solution constituting a lithium-ion battery in order to suppress decomposition of the electrolyte solution due to charging and discharging. PS, in particular, is often used when a compound with a high Ni ratio is used as the positive electrode in order to obtain a high-capacity lithium-ion battery. These additives contribute to suppressing decomposition of the electrolyte solution by forming a coating derived from PS or VC and PS at the electrode interface in the lithium-ion battery.
However, the present inventors have newly discovered that when alumina or magnesia, which have been conventionally used as inorganic fillers for separators in lithium-ion batteries equipped with electrolytes containing the above-mentioned additives, are used, the battery capacity may decrease and the charge/discharge cycle characteristics may be significantly affected. The reason for this adverse effect is presumed to be the generation of sulfides due to an unintended chemical reaction between the inorganic filler and PS.
In this regard, by using the tin oxide particles of the present invention as a constituent material of the separator, even when PS is added to the electrolyte, high battery capacity and high capacity retention can be achieved without interacting with compounds in the electrolyte, and charge/discharge cycle characteristics can also be sufficiently improved. These findings were newly discovered by the present inventors and are also evident from the results of the examples described below.

The present invention will be explained in more detail below using examples. However, the scope of the present invention is not limited to these examples. In the table, the "-" column indicates that the data was not measured.

Example 1
(1. Solution preparation process)
1.35 kg of sodium stannate trihydrate (purity 92% by mass) was dissolved as a tin source in 22.1 L of pure water heated to 50°C to obtain an aqueous tin solution with a sodium stannate concentration of 61 g/L (elemental tin concentration: 0.21 mol/L).

(2. Neutralization process)
Next, the tin aqueous solution was heated to 60° C., and in this state, 2.2 L of a 20% by mass diluted sulfuric acid aqueous solution was gradually and continuously added over 84 minutes (acid concentration: 2.3 mol/L, addition rate: 26 mL/min) to neutralize the solution until the pH at 60° C. reached 3.0, thereby obtaining a neutralized solution. This neutralized solution was then aged for 0.5 hours while maintaining the solution temperature at 60° C., and tin hydroxide particles were precipitated as intermediate particles.
Furthermore, the aqueous solution and the neutralization solution were continuously stirred at a constant speed of 110 cm/sec throughout the solution preparation step and the neutralization step.

(3. Firing process)
Subsequently, the aged neutralized solution was subjected to solid-liquid separation, and the obtained solid was washed with water until the water conductivity reached 580 μS/cm. The washed solid was subjected to solid-liquid separation and dried to obtain a dried product. This dried product was crushed in a mixer to obtain a dry powder, which was then fired in a box-type electric furnace. The firing conditions were 500°C for 2 hours in an air atmosphere. The fired particles were then sieved to remove coarse particles. Through this process, the desired tin oxide particles were obtained. The tin oxide particles contained 98% by mass or more of tin oxide represented by the chemical formula SnO _(2-X) (where 0<X<1).

Example 2
In the solution preparation step, 81 kg of sodium stannate trihydrate was dissolved as a tin source in 1,246 L of pure water heated to 30°C to obtain a tin aqueous solution with a sodium stannate concentration of 65 g/L (elemental tin concentration: 0.22 mol/L). The tin aqueous solution was then heated to 60°C, and in this state, 132 L of a 20% by mass diluted sulfuric acid aqueous solution was added successively over 82 minutes (acid concentration: 2.3 mol/L, addition rate: 1.61 L/min) to neutralize the solution until the pH at 60°C reached 3.1, thereby obtaining a neutralized solution. This neutralized solution was then aged for 1 hour while maintaining the solution temperature at 60°C, resulting in the precipitation of tin hydroxide particles as intermediate particles.
Furthermore, the target tin oxide particles were obtained under the same conditions as in Example 1, except that the stirring speed of the aqueous solution and the neutralization solution was changed to a constant speed of 100 cm/sec.

Example 3
The target tin oxide particles were obtained under the same conditions as in Example 2, except that the aqueous solution and neutralization solution were stirred at a constant speed of 80 cm/sec.

Example 4
The target tin oxide particles were obtained under the same conditions as in Example 2, except that in the firing step, the firing conditions were changed to 700° C. in an air atmosphere for 2 hours.

Example 5
The target tin oxide particles were obtained under the same conditions as in Example 2, except that in the firing step, the firing conditions were changed to a 3 vol% _H2 /97 vol% _N2 mixed atmosphere in a rotary kiln at 500°C for 45 minutes.

Comparative Example 1
As the tin oxide particles, model number SNO03PB manufactured by Kojundo Chemical Laboratory Co., Ltd. was used.

Comparative Example 2
Alumina (Al ₂ O ₃ ) particles manufactured by Wako Pure Chemical Industries, Ltd., model number: 013-11525, were used in place of the tin oxide particles.

Comparative Example 3
Instead of the tin oxide particles, magnesia (MgO) particles manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., model number: 131-10851, were used.

[Evaluation of particle properties]
The BET specific surface areas S1, D10, D50, D90, and D100 of the inorganic particles of the Examples and Comparative Examples were measured by the methods described above.

[Measurement of oxygen deficiency]
The amount of oxygen vacancy in the inorganic particles of the Examples and Comparative Examples was measured by the following method. A measurement sample was placed in a quartz tube with an outer diameter of 5 mm, and measurement was carried out under the following measurement conditions to obtain an ESR spectrum. For tin oxide particles, the absorption intensity of the obtained ESR spectrum was converted to a value per 1 g of sample. The number of unpaired electrons was calibrated using a 0.1 to 2 mM TEMPOL-toluene solution as a standard.
Sample mass: 120 mg
Quartz tube: JEOL Ltd. LST-5HS 300MM
Microwave output: 0.5mW
・Magnetic field sweep width: 338±25mT
Modulation magnetic field: 0.6 mT
Sensitivity: 100
・Time constant: 0.1 seconds ・Measurement time: 120 seconds ・Measurement temperature: 293K

[Evaluation of Battery Characteristics]
The battery characteristics were evaluated when the inorganic particles of the examples and comparative examples were disposed on the separator.
Specifically, N-methylpyrrolidone was used as a solvent. 90 parts by mass of inorganic particles and 10 parts by mass of a binder resin (polyvinylidene fluoride, hereinafter also referred to as PVdF) were mixed into the solvent to prepare an inorganic particle slurry. This slurry was dispersed using a paint shaker, and then coated onto one side of a polyolefin porous resin film using a bar coater to form a coating film, thereby producing a separator with a 3-4 μm thick inorganic particle layer formed on one side.

Next, ₈₉ parts by mass of lithium nickel manganese _cobalt composite oxide ( _{LiNi0.6Mn0.2Co0.2O2 )} as the positive electrode active material, 5% by mass of acetylene black as the conductive additive, and 6 parts by mass of PVdF as the binder were mixed, and N-methyl-2-pyrrolidone was further added to form a paste. This paste was applied to one side of aluminum foil to prepare a positive electrode sheet.

Separately, 96 parts by mass of graphite as a negative electrode active material and 2 parts by mass each of carboxymethyl cellulose and styrene-butadiene rubber as binders were mixed, and water was added to form a paste, which was then applied to one side of copper foil to prepare a negative electrode sheet.
The electrolyte used was a PS-free electrolyte, which was 1 mol/L LiPF ₆ (ethylene carbonate (EC):dimethyl carbonate (DMC) = 3:7 volume mixture) to which 0.5 mass % vinylene carbonate (VC) was added as an additive.

Six coin-type batteries were then fabricated, each containing a separator placed between the positive and negative electrode sheets and filled with either a PS-free electrolyte or a PS-added electrolyte. In each battery, the separator was positioned so that the inorganic particle layer faced the positive electrode sheet.

Each battery thus obtained was charged and discharged under the following conditions using a charge-discharge tester (manufactured by Toyo Systems Co., Ltd., "TOSCAT").
The charge-discharge cycle consisted of charging at a constant current of 0.1 C up to 4.3 V at 25° C., followed by constant voltage charging until the current decayed to 0.02 C, and then discharging at a current of 0.1 C until the voltage reached 3.0 V relative to the lithium potential. This cycle was repeated three times to determine the initial activity.
After initial activation, the charge-discharge cycle consisted of constant current charging at 55°C at a current value of 0.1 C to 4.3 V, followed by constant voltage charging until the current value decayed to 0.05 C. Next, one cycle of discharging at a current value of 0.1 C until the voltage reached 3.0 V relative to the lithium potential was performed. Subsequently, a cycle of constant current charging at a current value of 1 C to 4.3 V, constant voltage charging until the current value decayed to 0.05 C, and discharging at a current value of 1 C until the voltage reached 3.0 V relative to the lithium potential was repeated.

The rate characteristic (1C/0.1C [%]) was calculated by taking the discharge capacity at 0.1C in the first cycle at 55°C as 100% and then calculating the ratio of the discharge capacity at 1C in the 31st cycle. The higher the value, the better the rate characteristic.
Similarly, the ratio of the discharge capacity at 1 C in the 31st cycle to the discharge capacity at 1 C in the 2nd cycle, which was taken as 100%, was calculated and used as the capacity retention rate (1 C/1 C [%]). The higher the value, the better the capacity retention rate.

Separately, six batteries were fabricated using an electrolyte containing 5% by mass of propane sultone (PS) and otherwise configured similarly to the above. Rate characteristics and capacity retention were measured and calculated using the methods described above, and voltage behavior was also measured. The operating rates (percentages) of the fabricated batteries were evaluated based on the number of batteries fabricated according to the following criteria. The results are shown in Table 1.
1 and 2 show charge/discharge curves of the batteries using the inorganic particles of Example 2 and Comparative Example 2, respectively.

<Evaluation of operation rate>
A: The percentage of working batteries is 70% or more, and the battery has stable and excellent voltage behavior.
B: The percentage of working batteries is 30% or more and less than 70%, and the battery has voltage behavior that allows it to function as a battery.
C: The percentage of working batteries is less than 30% and the batteries do not function.

As shown in Table 1, when the tin oxide particles of each Example were used in a separator for a lithium ion battery, no short circuit was observed during the initial charge/discharge, and the rate characteristics and capacity retention rate were good, resulting in excellent charge/discharge cycle characteristics, even under high temperature conditions, compared to the case where the inorganic particles of the Comparative Examples were used.
In particular, the tin oxide particles of Examples 1 to 4 exhibit excellent operating rates and voltage behavior as batteries without any interactions even when the electrolyte contains propane sultone, as is evident from a comparison of the charge-discharge curves of Example 2 shown in Figure 1 and Comparative Example 2 shown in Figure 2.

Specifically, as shown in Figure 1, the battery using the tin oxide particles of Example 2 did not exhibit unstable voltage behavior during either the charge or discharge process. In contrast, as shown in Figure 2, the battery using the alumina particles of Comparative Example 2 reached the end-of-charge voltage in the early stages of charging, indicating an increase in the internal resistance of the battery. This suggests that by-products were generated by the chemical reaction between the propane sultone additive and alumina, and that these by-products accumulated throughout the separator, increasing the resistance between the electrodes and preventing the battery from performing as intended.

Claims (3)

The volume cumulative particle size D90 at 90% by volume of the cumulative volume measured by a laser diffraction/scattering particle size distribution measurement method is 3 μm or less,
the ratio (S1/D50) of the BET specific surface area S1 to the volume cumulative particle size D50 at a cumulative volume of 50% by volume, as determined by a laser diffraction/scattering particle size distribution measurement method, is 10 or more and 100 or less;
Tin oxide particles for use in a lithium ion battery separator ,
the tin oxide in the particles has oxygen vacancies;
Tin oxide particles, wherein the amount of oxygen deficiency per mass of the particles is 1×10 ¹⁶ /g or more and 9×10 ¹⁶ /g or less. 2. The tin oxide particles according to claim 1 , wherein the BET specific surface area S1 is 20 m ² /g or more and 70 m ² /g or less. 3. The tin oxide particles according to claim 1 , wherein the volume cumulative particle diameter D50 is 0.3 μm or more and 2.0 μm or less.