Nonaqueous electrolyte secondary battery

Abstract

A nonaqueous electrolyte secondary battery according to the present disclosure comprises: a positive electrode containing a positive electrode active material; a negative electrode containing a negative electrode active material; a separator disposed between the positive electrode and the negative electrode; and a nonaqueous electrolyte. The negative electrode active material includes a first silicon-containing material and a second silicon-containing material. The first silicon-containing material includes first silicon phases and a carbon phase in which the first silicon phases are dispersed. The second silicon-containing material includes second silicon phases and a silicon dioxide phase in which the second silicon phases are dispersed. The positive electrode active material includes a lithium-containing composite oxide. With respect to the lithium-containing composite oxide, the proportion of nickel in the elements other than oxygen and lithium is 85 at% or more.

Description

Nonaqueous electrolyte secondary battery

[0001] This disclosure relates to a non-aqueous electrolyte secondary battery.

[0002] Silicon-containing materials have been proposed as negative electrode active materials for non-aqueous electrolyte secondary batteries. Claim 1 of Patent Document 1 (International Publication No. 2023-053771) discloses a lithium-ion secondary battery comprising: a positive electrode, a negative electrode, a separator that isolates the positive electrode and the negative electrode from each other, and an electrolyte, wherein the negative electrode comprises a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector, the negative electrode mixture layer comprises a carbon-based material and a silicon-based material as negative electrode active materials, the discharge capacity of the negative electrode active materials is 400 mAh / g to 750 mAh / g, the thickness of the negative electrode current collector is 4 μm to 12 μm, the 1% proof stress of the negative electrode current collector is 300 MPa to 700 MPa, and the separator comprises a base layer, a filler layer formed on the surface of the base layer, and a resin layer formed on the surface of the base layer or the filler layer, the porosity of the resin layer is 30% to 80%.

[0003] International Publication No. 2023-053771

[0004] In non-aqueous electrolyte secondary batteries, there is a demand for higher capacity. To achieve this, the use of silicon-containing materials as the negative electrode active material has been proposed. However, simply using silicon-containing materials is not enough to obtain a battery with high performance. In this context, one of the objectives of this disclosure is to provide a non-aqueous electrolyte secondary battery with high capacity and relatively low internal resistance.

[0005] One aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery comprising: a positive electrode containing a positive electrode active material; a negative electrode containing a negative electrode active material; a separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte, wherein the negative electrode active material comprises a first silicon-containing material and a second silicon-containing material, the first silicon-containing material comprises a first silicon phase and a carbon phase in which the first silicon phase is dispersed, the second silicon-containing material comprises a second silicon phase and a silicon dioxide phase in which the second silicon phase is dispersed, the positive electrode active material comprises a lithium-containing composite oxide, and in the lithium-containing composite oxide, the proportion of nickel among elements other than lithium and oxygen is 85 atomic percent or more.

[0006] According to this disclosure, a non-aqueous electrolyte secondary battery with high capacity and relatively low internal resistance can be obtained. Novel features of the present invention are described in the appended claims, but the present invention, both in terms of its structure and content, and in conjunction with other objects and features of the present invention, will be better understood by the following detailed description in conjunction with the drawings.

[0007] This is a schematic cross-sectional view showing an example of a non-aqueous electrolyte secondary battery according to this embodiment.

[0008] Embodiments of the present invention will be described below with examples, but the present invention is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be given as examples, but other numerical values ​​and other materials may be applied as long as the invention relating to this disclosure can be carried out. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be read as "numerical value A or greater and numerical value B or less". In the following description, when lower and upper limits of numerical values ​​relating to specific physical properties or conditions are given as examples, either of the given lower limits and either of the given upper limits can be arbitrarily combined as long as the lower limit does not exceed the upper limit. In the following description, when examples of components or methods are listed, unless otherwise specified, only one of the listed examples may be used, or multiple of the listed examples may be used in combination.

[0009] (Nonaqueous electrolyte secondary battery) Hereinafter, the nonaqueous electrolyte secondary battery according to the present embodiment may be referred to as "nonaqueous electrolyte secondary battery (B)" or "secondary battery (B)". The secondary battery (B) includes a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, a separator disposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte. The negative electrode active material includes a first silicon-containing material and a second silicon-containing material. The first silicon-containing material includes a first silicon phase and a carbon phase in which the first silicon phase is dispersed. The second silicon-containing material includes a second silicon phase and a silicon dioxide phase in which the second silicon phase is dispersed. The positive electrode active material includes a lithium-containing composite oxide. In the lithium-containing composite oxide, the proportion of nickel in elements other than lithium and oxygen is 85 atomic percent or more.

[0010] Hereinafter, the first silicon-containing material may be denoted as "SiC". Hereinafter, the second silicon-containing material may be denoted as "SiO".

[0011] Since silicon-containing materials exhibit high lithium ion storage capacity, they are preferably used as negative electrode active materials for achieving higher capacity of nonaqueous electrolyte secondary batteries. However, since the first silicon-containing material (SiC) has high resistance, when only the first silicon-containing material is used, the internal resistance of the battery increases. In the secondary battery (B), the combined use of the first silicon-containing material and the second silicon-containing material increases the capacity of the negative electrode and suppresses an increase in the internal resistance of the battery. On the other hand, merely increasing the capacity of the negative electrode cannot achieve high capacity of the battery. Therefore, the above-described positive electrode active material is used in the secondary battery (B). With the above configuration, a nonaqueous electrolyte secondary battery having high capacity and relatively low internal resistance can be obtained.

[0012] (First silicon-containing material) The first silicon-containing material (SiC) includes a silicon phase and a carbon phase in which the silicon phase is dispersed. Using the first silicon-containing material enables particularly high capacity of the negative electrode. On the other hand, the first silicon-containing material has relatively high resistance.

[0013] (Second silicon-containing material) The second silicon-containing material (SiO) comprises a silicon phase and a silicon dioxide phase (SiO) in which the silicon phase is dispersed. ２ The second silicon-containing material includes the first silicon-containing material. The expansion and contraction of the second silicon-containing material during charging and discharging are smaller than those of the first silicon-containing material. Therefore, by using the second silicon-containing material, it is possible to suppress damage to the conductive network due to the miniaturization of the active material. In other words, by using the second silicon-containing material, it is possible to suppress the increase in the internal resistance of the battery.

[0014] As a result of their investigation, the inventors of the present invention have newly discovered that by adopting the configuration of secondary battery (B), a non-aqueous electrolyte secondary battery with high capacity and relatively low internal resistance can be obtained. This disclosure is based on this new finding.

[0015] The secondary battery (B) may satisfy the following conditions (1) and (2). By satisfying conditions (1) and (2), the positive electrode can be made to have high capacity and low resistance characteristics. (1) The discharge capacity density of the positive electrode active material is 221 mAh / g or more. (2) The positive electrode includes a positive electrode current collector and a positive electrode mixture layer formed on both sides of the positive electrode current collector. The average thickness of the portion of the positive electrode in which the positive electrode mixture layer is formed on both sides of the positive electrode current collector is 178 μm or less.

[0016] There is no upper limit to the discharge capacity density of the positive electrode active material, but it may be 250 mAh / g or less. The discharge capacity density of the positive electrode active material can be increased by increasing the nickel element content of the positive electrode active material. The discharge capacity density of the positive electrode active material can be measured by the following method.

[0017] The positive electrode for measuring discharge capacity density is prepared by the following method. First, a mixture is obtained by mixing the positive electrode active material, acetylene black (conductive material), and polyvinylidene fluoride (PVdF, binder) in a mass ratio of 100:0.82:0.82. An appropriate amount of N-methyl-2-pyrrolidone (NMP) is added to this mixture and kneaded to prepare a positive electrode mixture slurry. This positive electrode mixture slurry is applied to one side of the positive electrode current collector (aluminum foil) to form a positive electrode mixture layer. In this way, a laminate is formed containing the positive electrode current collector and the positive electrode mixture layer formed on one side of the positive electrode current collector. Next, the laminate is rolled and then shaped into a circle (area: 1.54 cm²) with a diameter of 14 mm. ２ Cut it into pieces. In this way, you obtain the positive electrode.

[0018] The prepared positive electrode is placed inside the bottom cover (a component for the commercially available coin-type battery R2032) with the aluminum foil facing downwards. Next, a separator (a porous polyethylene film) is placed on the positive electrode mixture surface. Then, the non-aqueous electrolyte is placed inside the bottom cover. The non-aqueous electrolyte is prepared by the following method: First, ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed in a volume ratio of 1:3 (EC:DMC), and vinylene carbonate is added to obtain a mixed solvent. Next, LiPF is added to the mixed solvent. ６ A non-aqueous electrolyte is obtained by dissolving [the substance]. The vinylene carbonate content in the non-aqueous electrolyte is 5% by mass. In addition, LiPF in the non-aqueous electrolyte ６ The concentration will be 1.5 mol / L.

[0019] Next, the negative electrode (lithium metal foil) is placed on top of the separator, the top cover is placed over it via a gasket, and it is crimped with a crimping machine. In this way, an R2032 type coin half cell is manufactured. The obtained half cell may be referred to as a "positive electrode half cell" below. The capacity density of the positive electrode active material is determined by dividing the discharge capacity of the positive electrode half cell by the mass of the positive electrode active material contained in the positive electrode mixture layer.

[0020] The discharge capacity of the positive electrode half-cell is obtained by performing an initial charge-discharge test under the following conditions. Note that "1C" refers to the current value that can discharge the battery's rated capacity in one hour. Test temperature: 25°C (Charging) Maximum charging voltage: 4.3V, charging current: 0.1C, constant current constant voltage charging (Discharging) Minimum discharge voltage: 2.5V, discharge current: 0.1C, constant current discharge

[0021] The thickness of the positive electrode is the sum of the thickness of the positive electrode current collector and the thickness of the positive electrode mixture layer. The average thickness of the portion of the positive electrode where the positive electrode mixture layer is formed on both sides of the positive electrode current collector may be 130 μm or more. The average thickness of the positive electrode under condition (2) can be measured by the following method. First, in the portion of the positive electrode where the positive electrode mixture layer is formed on both sides of the positive electrode current collector, the thickness of the positive electrode is measured at five points at 1 cm intervals in the width direction of the positive electrode and at ten points at 5 cm intervals in the length direction of the positive electrode. In other words, the thickness of 50 points on the positive electrode is measured. The thickness of the positive electrode can be measured using a contact-type thickness gauge. The arithmetic mean of the 50 measured thicknesses is taken as the average thickness of the positive electrode.

[0022] The secondary battery (B) may satisfy the following conditions (3) to (5). By satisfying conditions (3) to (5), the negative electrode can be made to have high capacity and low resistance characteristics. (3) The silicon content in the first silicon-containing material is 40% by mass or more. (4) The charge capacity density of the negative electrode active material is 500 mAh / g or more. (5) The negative electrode includes a negative electrode current collector and a negative electrode mixture layer formed on both sides of the negative electrode current collector. The average thickness of the portion of the negative electrode in which the negative electrode mixture layer is formed on both sides of the negative electrode current collector is 195 μm or less.

[0023] The secondary battery (B) preferably satisfies the above conditions (1) to (5). By satisfying conditions (1) to (5), it becomes possible to achieve cell characteristics that are particularly high in capacity and particularly low in resistance.

[0024] The silicon content in the first silicon-containing material may be 60% by mass or less. The upper limit of the charge capacity density of the negative electrode active material is not limited, but may be 600 mAh / g or less. The average thickness of the negative electrode may be 150 μm or more. The average thickness of the negative electrode can be measured in the same way as the average thickness of the positive electrode. The thickness of the negative electrode is the sum of the thickness of the negative electrode current collector and the thickness of the negative electrode mixture layer.

[0025] The charge capacity density of the negative electrode active material can be measured by the following method. First, the negative electrode is cut to a predetermined size (30 mm x 60 mm) and its mass W1 is measured. Also, the mass W2 of a negative electrode current collector (e.g., copper foil) of the same size as the cut negative electrode is measured. The mass of the negative electrode mixture layer is calculated from masses W1 and W2. A half-cell is made using the above negative electrode and lithium metal (counter electrode) and charged to 5 mV. The charging rate is continuously changed to 0.1 C, 0.05 C, and 0.01 C. The charge capacity density is obtained by dividing the sum of the charge capacities during charging by the mass of the negative electrode active material inside the negative electrode mixture layer.

[0026] The apparent density of the first silicon-containing material is 1.6 g / cm³. ３ ~2.2 g / cm ３ It may also be within this range. With this configuration, the expansion of silicon-containing materials during charging can be suppressed, and the cycle characteristics can be improved.

[0027] The apparent density of a silicon-containing material (for example, the first silicon-containing material) can be measured using a gas pycnometer by gas displacement. For example, the apparent density of a silicon-containing material can be measured using an Accupic II 1345TC-10CC (manufactured by Shimadzu Corporation) as the measuring device and helium as the gas.

[0028] The average size of the crystallites constituting the first silicon phase may be 15 nm or less, or 10 m or less. The average size may also be 1 m or more, or 3 nm or more. The average size of the crystallites constituting the first silicon phase can be varied by the firing temperature during the manufacturing of the first silicon-containing material.

[0029] The average size of the crystallites constituting the first silicon phase can be evaluated by XRD (X-ray diffraction). Specifically, using the full width at half maximum of the Si(111) plane peak that appears at approximately 28.5 degrees (2θ) based on the copper (Cu) Kα line (wavelength λ = 1.5406 Å), the average size of the crystallites can be calculated using Scherrer's formula.

[0030] The ratio of the discharge capacity of the negative electrode to the discharge capacity of the positive electrode (Cp) (Cn / Cp) may be in the range of 1.0 to 1.1. This configuration reduces the risk of overcharging and over-discharging, improving battery safety. This makes it possible to increase the reliability of the battery.

[0031] The shape of the non-aqueous electrolyte secondary battery (B) is not limited; it may be cylindrical or rectangular. The form of the electrode group of the secondary battery (B) is not limited, but a wound electrode group is preferably used. Examples of components of the secondary battery (B) are described below. However, the components of the secondary battery (B) are not limited to the examples below. Known components may be used for components other than those characteristic of the secondary battery (B) of this disclosure.

[0032] (Positive electrode) The positive electrode includes a positive electrode mixture layer. The positive electrode may also include a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode current collector is not particularly limited, and a positive electrode current collector used in known non-aqueous electrolyte secondary batteries may be used. The positive electrode may be manufactured by known methods.

[0033] Examples of materials for the positive electrode current collector include metallic materials such as Al, Ti, and Fe. Examples of metallic materials include Al, Al alloys, Ti, Ti alloys, and Fe alloys (such as stainless steel).

[0034] The positive electrode mixture layer contains a positive electrode active material, and may further contain additives (such as a conductive material, a binder, and a thickener). The positive electrode active material reversibly intercalates and deintercalates lithium ions. The positive electrode active material described above can be used as the positive electrode active material. Known additives may be used as the additive. A conductive carbonaceous material or the like may be used as the conductive material. Examples of the conductive carbonaceous material include carbon black, carbon nanotubes (CNT), graphite, and the like. Examples of carbon black include acetylene black, Ketjenblack, and the like. Examples of the binder include fluororesin (polyvinylidene fluoride) and the like. As the thickener, substances exemplified as the thickener for the negative electrode mixture layer may be used.

[0035] In the lithium-containing composite oxide constituting the positive electrode active material, the proportion Rni of nickel relative to elements other than lithium and oxygen is 85 atomic percent or more. The proportion may be 90 atomic percent or more, and is 100 atomic percent or less.

[0036] The lithium-containing composite oxide may be a lithium transition metal composite oxide. The lithium transition metal composite oxide may contain a element Me (for example, a metal element) other than Li, Ni, and oxygen. Examples of said element include Co, Mn, Al, Ti, Zr, Mg, Fe, Cu, Zn, Sn, Na, K, Ba, Sr, Ca, W, Mo, Nb, Si, and the like. The lithium transition metal composite oxide may or may not contain boron.

[0037] An exemplary composition of the lithium transition metal composite oxide is Li ａ Ni ｂ Me １－ｂ O ｄ represented by (wherein 0.8≦a≦1.2, b≧0.85, 1.5≦d≦2). In the formula, Me is an element other than Li, Ni, and oxygen, and may be any of the elements described above. The mole fraction of elements constituting the composite oxide can be measured by inductively coupled plasma (ICP) emission spectrometry.

[0038] (Negative electrode) The negative electrode includes a negative electrode mixture layer. The negative electrode may also include a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector. The negative electrode mixture layer includes a negative electrode active material and may further include additives (conductive material, binder, thickener, etc.). Known additives may be used. For the conductive material and binder, the substances exemplified as the conductive material and binder of the positive electrode mixture layer may be used. Examples of thickeners include cellulose derivatives. Examples of cellulose derivatives include carboxymethyl cellulose (CMC) and its modified forms, as well as methylcellulose. Examples of modified forms of CMC include salts of CMC. Examples of salts include alkali metal salts (e.g., sodium salts), ammonium salts, etc.

[0039] The negative electrode current collector is not particularly limited, and known negative electrode current collectors may be used. A conductive sheet (e.g., metal foil) can be used as the negative electrode current collector. For example, copper foil, copper alloy foil, a copper-deposited resin sheet (e.g., a polyethylene terephthalate sheet), or a copper-deposited stainless steel foil may be used as the negative electrode current collector.

[0040] The negative electrode active material can be a material that reversibly intercepts and releases lithium ions. Examples of negative electrode active materials include silicon-containing materials and carbonaceous materials. As described above, the negative electrode active material includes first and second silicon-containing materials. The negative electrode mixture layer may contain only one type of negative electrode active material or two or more types. Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon).

[0041] The carbonaceous material content in the negative electrode active material may be 80% by mass or more, or 90% by mass or more, or 95% by mass or less, or 93% by mass or less. The content of the first silicon-containing material in the negative electrode active material may be 3% by mass or more, or 5% by mass or more, or 15% by mass or less, or 10% by mass or less. The content of the second silicon-containing material in the negative electrode active material may be 3% by mass or more, or 5% by mass or more, or 15% by mass or less, or 10% by mass or less. In the negative electrode active material, the ratio of the mass of the first silicon-containing material to the total mass of the first silicon-containing material and the second silicon-containing material may be 6% or more, or 10% or more, or 30% or less, or 20% or less.

[0042] (Example of a method for producing the first silicon-containing material) The first silicon-containing material may be produced by crushing and mixing a carbon source and silicon, which are to be used as raw materials, in a ball mill or other agitator to obtain a mixture, and then firing this mixture under pressure in an inert atmosphere. Alternatively, the first silicon-containing material may be produced by heating the above mixture to a predetermined temperature, causing the silicon to neck in the mixture to obtain a sintered body, and then crushing this sintered body.

[0043] In the first silicon-containing material, the silicon phase content RSi may be 40% by mass or more and 80% by mass or less. RSi may be 70% by mass or less, or 60% by mass or less. By keeping RSi within the above numerical range, the first silicon-containing material can sufficiently absorb lithium ions while suppressing excessive expansion. This makes it possible to further achieve both high capacity and improved cycle characteristics. The silicon phase content RSi can be changed by the amount of Si deposited during the manufacturing of the first silicon-containing material.

[0044] In the first silicon-containing material, the carbon phase content RC may be 10% by mass or more and 60% by mass or less. RC may be 35% by mass or more, or 40% by mass or more. By keeping RC within the above numerical range, the first silicon-containing material can sufficiently absorb lithium ions while suppressing excessive expansion. This makes it possible to further achieve both high capacity and improved cycle characteristics.

[0045] The presence of both a silicon phase and a carbon phase in the first silicon-containing material can be confirmed by imaging and observing a cross-section of the first silicon-containing material using a scanning electron microscope (SEM). Cross-sectional observation of the first silicon-containing material can be performed, for example, by forming a cured product of a thermosetting resin filled with particles of the first silicon-containing material, obtaining a cross-section of this cured product using a cross-section polisher (CP), and then observing this cross-section with an SEM. Furthermore, both quantitative and qualitative analysis of both the silicon phase and the carbon phase can be performed on the cross-section of the particles of the first silicon-containing material by performing elemental mapping analysis using energy-dispersive X-ray (EDX). That is, both the silicon phase content RSi and the carbon phase content RC can be measured in the first silicon-containing material.

[0046] In the first silicon-containing material, the carbon phase may be formed by a porous carbon material. That is, at least a portion of the first silicon-containing material may have a carbon phase formed by a porous carbon material and a silicon phase dispersed within the carbon phase.

[0047] A porous carbon material can be any porous carbon material having multiple pores, but a carbon material having many micropores and mesopores with a pore diameter of 1 nm to 10 nm is preferred. Micropores and mesopores are suitable for arranging nano-sized silicon particles inside them. The more micropores and mesopores a porous carbon material has, the easier it is to arrange more silicon particles inside the pores. The volume ratio of micropores and mesopores to the total pore volume of the porous carbon material is preferably 50% or more, more preferably 70% or more, and most preferably 90% or more. Pores are classified into micropores (<2 nm), mesopores (2 nm to 50 nm), and macropores (>50 nm) according to their pore diameter. The volume ratio of micropores and mesopores to the total pore volume can be determined from the pore diameter distribution of the porous carbon material.

[0048] As the porous carbon material, for example, activated carbon can be used. Alternatively, the porous carbon material may be hard carbon or soft carbon, as described above. It is preferable that the porous carbon material has many mesopores. A commercially available porous carbon material having many mesopores is, for example, Knobel®, a porous carbon manufactured by Toyo Tanso Co., Ltd. It is preferable that the porous carbon material is at least one of hard carbon and soft carbon. In this case, the carbon phase is formed by at least one of hard carbon and soft carbon.

[0049] The first silicon-containing material having a carbon phase formed by a porous carbon material and a silicon phase dispersed within this carbon phase, as described above, can be produced by a manufacturing method comprising the steps of (i) preparing a precursor composite comprising a porous carbon material and siloxanes arranged in a plurality of pores of the porous carbon material, and (ii) contacting the precursor composite with magnesium vapor to reduce the siloxanes arranged in the pores to silicon.

[0050] In step (i), a siloxane can be produced from an organosilicon compound within multiple pores of a porous carbon material.

[0051] For example, a porous carbon material can be mixed with an organosilicon compound having a hydrolyzable functional group to hydrolyze the functional group and obtain a hydrolysis product. This hydrolysis product can then be subjected to dehydration condensation to obtain a precursor composite containing the porous carbon material and a siloxane. When the functional group is hydrolyzed, a hydroxyl group (-OH) is generated. Dehydration condensation of the hydroxyl groups between molecules of the hydrolysis product forms a siloxane bond (Si-O-Si), yielding a siloxane compound.

[0052] The conditions for carrying out the hydrolysis and dehydration condensation reactions of organosilicon compounds are not particularly limited. The hydrolysis and dehydration condensation reactions may proceed in the liquid phase or in the gas phase.

[0053] When hydrolysis and dehydration condensation reactions are carried out in the liquid phase, for example, a dispersion containing a porous carbon material, an organosilicon compound, an organic solvent, and water can be prepared, and the above-mentioned functional groups of the organosilicon compound can be hydrolyzed by stirring the dispersion at room temperature to 80°C. When preparing the dispersion, it is preferable to mix the porous carbon material, the organic solvent, and water, allow the mixture of the organic solvent and water to sufficiently permeate the pores of the porous carbon material, and then add the organosilicon compound. After that, the organic solvent and the silicon compound remaining in the organic solvent are removed by centrifugation, filtration, etc., and the solid content is recovered.

[0054] For example, toluene can be used as the organic solvent. The organic solvent is not particularly limited as long as it has high affinity for the porous carbon material and can dissolve a small amount of water. In the dispersion, the amount of organosilicon compound relative to the porous carbon material should be selected according to the amount of silicon phase to be contained in the pores. In the dispersion, the amount of organic solvent relative to the porous carbon material is not particularly limited, and should be an amount that sufficiently wets the surface of the porous carbon material with the organic solvent. In the dispersion, the amount of water relative to the porous carbon material is also not particularly limited, and should be an amount that sufficiently penetrates the pores of the porous carbon material with water molecules.

[0055] Next, the recovered solids are heated or dried to promote the dehydration condensation reaction of the hydrolysis products and generate siloxanes. For example, heating the dried solids under reduced pressure causes the hydrolysis products to undergo dehydration condensation, thereby accelerating the siloxane formation reaction.

[0056] When hydrolysis and dehydration condensation reactions are carried out in the gas phase, for example, a porous carbon material can be exposed to vapor and water vapor of an organosilicon compound having hydrolyzable functional groups, allowing molecules of the organosilicon compound and water molecules to penetrate into the pores of the porous carbon material, and then the porous carbon material can be heated. This heating promotes the hydrolysis reaction of the above-mentioned functional groups of the organosilicon compound within the pores of the porous carbon material to obtain a hydrolysis product, and then the dehydration condensation reaction can be promoted in this hydrolysis product.

[0057] For example, in the gas phase, first, a porous carbon material is placed in a reaction chamber under reduced pressure, and the porous carbon material is brought into contact with the vapor of an organosilicon compound within the reaction chamber, thereby allowing the organosilicon compound to penetrate into the pores of the porous carbon material. Next, the organosilicon compound is evacuated from the reaction chamber, and water vapor is introduced into the reaction chamber under reduced pressure, allowing water molecules to penetrate into the pores of the porous carbon material. By performing this operation one or more times, in addition to heating as appropriate, a precursor composite containing a porous carbon material and a siloxane can be obtained.

[0058] The type of organosilicon compound is not particularly limited, but from the viewpoint of being easily reduced to silicon, silicon compounds having hydrolyzable functional groups such as alkoxysilanes and chlorosilanes are preferred. Among these, alkoxysilanes are preferred because they are highly stable and easy to handle. Alkoxysilanes may or may not contain Si-C bonds (alkyl groups directly bonded to silicon atoms).

[0059] Examples of alkoxysilanes that do not contain Si-C bonds include tetramethyl orthosilicate (Si(OCH)). ３ ) ４ ), tetraethyl orthosilicate (Si(OC ２ H ５ )４ ), tetrapropyl orthosilicate (Si(OC ３ H ７ ) ４ ), tetrabutyl orthosilicate (Si(OC ４ H ９ ) ４ ) are some examples.

[0060] Examples of alkoxysilanes containing Si-C bonds include methyltriethoxysilane (CH ３ Si(OC) ２ H ５ ) ３ ), 3-aminopropylmethyldiethoxysilane (H ２ N(CH ２ ) ３ Si(CH ３ ) (OC ２ H5) ２ ) are some examples.

[0061] Since Si-C bonds are difficult to break at temperatures below 700°C, they may remain intact during the reduction reaction in step (ii). Therefore, it is more preferable that the alkoxysilane does not contain Si-C bonds. By using an alkoxysilane that does not contain Si-C bonds, a siloxane that does not contain Si-C bonds can be obtained with high efficiency. A siloxane that does not contain Si-C bonds is easily reduced to silicon, and silicon can be obtained with high efficiency.

[0062] In step (ii) of reducing siloxane to silicon, the precursor composite obtained in step (i) can be heated in a non-oxidizing atmosphere containing magnesium vapor. When siloxane is reduced with magnesium vapor, nano-sized silicon particles (nano-silicon particles) are generated. This forms a silicon phase within the pores of the porous carbon material. Magnesium oxide, silicon oxide, and silicon carbide are also generated. This allows the silicon phase formed by nano-silicon particles, silicon oxide, and silicon carbide to be arranged within the pores of the porous carbon material.

[0063] Silicon oxide and silicon carbide have the function of shielding the silicon phase from the non-aqueous electrolyte. Even if a gap forms between the silicon phase and the carbon phase within the pores, or if the silicon phase is damaged and an active surface of the silicon phase is formed, contact between the active surface of the silicon phase and the non-aqueous electrolyte is suppressed by silicon oxide and silicon carbide. As a result, the formation of SEI is suppressed, and the decrease in capacity of the non-aqueous electrolyte secondary battery can be further suppressed.

[0064] Silicon oxide may exist in close contact with the silicon phase, or it may exist as an integral part of the silicon phase. Silicon oxide may be particulate, or it may be a matrix. For example, the silicon phase may be present in a matrix formed by silicon oxide. From the viewpoint of relieving stress due to the expansion and contraction of the silicon phase, at least a portion of the silicon oxide may be located inside the silicon phase. Silicon carbide may exist in close contact with the porous carbon material or the silicon phase, or it may exist as an integral part of the porous material or the silicon phase. Silicon carbide may be particulate, or it may be a film interposed between the porous carbon material and the silicon phase. Silicon carbide may form a matrix together with silicon oxide. From the viewpoint of relieving stress due to the expansion and contraction of the silicon phase, at least a portion of the silicon carbide may be located inside the silicon phase.

[0065] A non-oxidizing atmosphere can be a vacuum, reduced pressure atmosphere, or inert gas atmosphere. Inert gases such as argon, noble gases, or nitrogen can be used. In an oxidizing atmosphere, the generated magnesium vapor is oxidized to produce magnesium oxide (MgO) or magnesium dioxide (MgO2), which may prevent sufficient reduction of the siloxane.

[0066] The melting point of magnesium is 650°C. Near its melting point, magnesium has an extremely high vapor pressure (372 Pa). When the precursor complex obtained in step (i) is heated in the presence of magnesium vapor, the chemical reaction represented by the following formula (1) proceeds, and the siloxane is reduced to silicon. At this time, magnesium oxide (MgO) is also produced. The higher the vapor pressure of magnesium, the more efficient the reduction reaction becomes.

[0067] 2Mg + -(O-Si-O)- → 2MgO + Si...(1)

[0068] To reduce siloxanes disposed within the pores of a porous carbon material with magnesium vapor, the precursor complex obtained in step (i) is mixed with magnesium, and the mixture is heated. The mixture of the precursor complex and magnesium can be heated in a container made of graphite, stainless steel, or the like. The form of magnesium mixed with the precursor complex is not particularly limited, as long as it can produce magnesium vapor. For example, magnesium in the form of powder, particles, ribbons, rods, pellets, etc., can be used.

[0069] The temperature at which the mixture of the precursor complex and magnesium is heated is not particularly limited, but the higher the temperature, the higher the vapor pressure of magnesium. However, at temperatures significantly above the melting point of magnesium, aggregation of molten magnesium occurs, reducing the surface area of ​​magnesium that can evaporate. The heating temperature is preferably, for example, near the melting point of magnesium.

[0070] The amount of magnesium relative to the precursor composite should be appropriately selected so that most of the magnesium can be vaporized and the inclusion of metallic magnesium into the first silicon-containing material is reduced.

[0071] In step (ii), it is preferable to remove the magnesium oxide disposed within the pores of the porous carbon material. Removal of magnesium oxide can be carried out, for example, by dissolving the magnesium oxide using an aqueous solution containing an acid or ammonium salt, thereby eluting the magnesium oxide from within the pores. Magnesium oxide, which is formed near the melting point of magnesium, dissolves readily in an aqueous solution containing an acid or ammonium salt. By removing magnesium oxide from within the pores, the space that mitigates the expansion of silicon particles within the pores can be increased. This significantly reduces the stress applied to the porous carbon material.

[0072] As the acid, inorganic acids such as sulfuric acid, boric acid, phosphoric acid, hydrochloric acid, and nitric acid may be used, or organic acids such as acetic acid, oxalic acid, succinic acid, and malonic acid may be used. Examples of ammonium salts include ammonium chloride.

[0073] (An example of a method for producing a second silicon-containing material) The second silicon-containing material may also be produced by heating silicon oxide, which is the raw material, in a non-oxidizing atmosphere (inert atmosphere) to allow a disproportionation reaction to proceed.

[0074] The negative electrode may be formed by known methods. In one example of a negative electrode formation method, first, a negative electrode slurry containing the components of the negative electrode mixture layer and a liquid medium (dispersion medium) is prepared. Next, the negative electrode slurry is applied to the negative electrode current collector, followed by drying and rolling. In this way, a negative electrode is formed, comprising the negative electrode current collector and the negative electrode mixture layer placed on the negative electrode current collector. The negative electrode mixture layer is formed on one or both sides of the negative electrode current collector. The formed negative electrode is cut to the appropriate size as needed. Furthermore, negative electrode leads are connected to the negative electrode as needed.

[0075] (Separator) A porous sheet with insulating properties is used as the separator. Examples of porous sheets include microporous membranes, woven fabrics, and nonwoven fabrics. The material of the separator is not particularly limited, and polymer materials may be used. Examples of polymer materials include polyolefin resins, polyamide resins, and cellulose. Examples of polyolefin resins include polyethylene, polypropylene, and ethylene-propylene copolymers. The separator may contain additives (such as inorganic fillers) as needed.

[0076] (Non-aqueous electrolyte) A non-aqueous electrolyte having lithium ion conductivity can be used as the non-aqueous electrolyte. The non-aqueous electrolyte contains a non-aqueous solvent and ions (lithium ions, anions, etc.) dissolved in the non-aqueous solvent. The non-aqueous electrolyte may be in liquid or gel form.

[0077] Non-aqueous electrolytes can be prepared by dissolving lithium salts in a non-aqueous solvent. When lithium salts dissolve in a non-aqueous solvent, lithium ions and anions are generated. Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO2). ４ LiAlCl ４ LiB １０ Cl １０ (e.g.), lithium salts of fluorine-containing acids (LiPF) ６ LiPF ２ O ２ LiBF ４ LiSbF ６ LiAsF ６ LiCF ３ SO ３ LiCF ３ CO ２ (etc.), lithium salts of fluorine-containing acidimides (LiN(FSO) ２ ) ２ ,LiN(CF ３ SO ２ ) ２ ,LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(C ２ F ５ SO ２) ２ This includes lithium halides (LiCl, LiBr, LiI, etc.), etc. A single lithium salt may be used alone, or two or more may be used in combination. The concentration of the lithium salt in the non-aqueous electrolyte may be 0.5 mol / L or more and 3.5 mol / L or less.

[0078] The non-aqueous solvent is not particularly limited, and any known non-aqueous solvent may be used. Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC). Examples of linear carbonate esters include dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone. Examples of linear carboxylic acid esters include non-aqueous solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate, and ethyl propionate. The non-aqueous solvent may be used alone or in combination of two or more types.

[0079] (Outer casing) The outer casing (battery case) houses the electrode group and the non-aqueous electrolyte. The outer casing is not particularly limited, and known outer casings may be used. The outer casing usually includes an outer can and a sealing body that seals the opening of the outer can. The outer can functions as the negative electrode terminal, and the sealing body functions as the positive electrode terminal. The sealing body may include a sealing plate and a gasket.

[0080] (Embodiment 1) Figure 1 is a schematic longitudinal cross-sectional view showing an example of a non-aqueous electrolyte secondary battery according to Embodiment 1. The cylindrical non-aqueous electrolyte secondary battery 10 shown in Figure 1 includes a cylindrical battery case and an electrode group 14 and a non-aqueous electrolyte (not shown) housed within the battery case. The electrode group 14 is a wound electrode group and includes a positive electrode 11, a negative electrode 12, and a separator 13.

[0081] The battery case includes a case body 15, which is a bottomed cylindrical metal container, and a sealing body 16 that seals the opening of the case body 15. A gasket 27 is placed between the case body 15 and the sealing body 16. The gasket 27 ensures that the battery case is airtight. Inside the case body 15, insulating plates 17 and 18 are placed at both ends of the electrode group 14 in the direction of the winding axis, respectively. The case body 15 has a stepped portion 21.

[0082] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26. All components of the sealing body 16, except for the insulating member 24, are electrically connected. The sealing body 16 acts as a safety valve when the internal pressure of the battery rises.

[0083] The positive electrode 11 is electrically connected to the cap 26, which functions as a positive electrode terminal, via the positive electrode lead 19. The negative electrode 12 is electrically connected to the case body 15, which functions as a negative electrode terminal, via the negative electrode lead 20. The positive electrode 11 includes the positive electrode active material described above. The negative electrode 12 includes the negative electrode active material described above.

[0084] (Note) The above description discloses the following technologies. (Technology 1) A non-aqueous electrolyte secondary battery comprising: a positive electrode containing a positive electrode active material; a negative electrode containing a negative electrode active material; a separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte, wherein the negative electrode active material comprises a first silicon-containing material and a second silicon-containing material; the first silicon-containing material comprises a first silicon phase and a carbon phase in which the first silicon phase is dispersed; the second silicon-containing material comprises a second silicon phase and a silicon dioxide phase in which the second silicon phase is dispersed; the positive electrode active material comprises a lithium-containing composite oxide; and in the lithium-containing composite oxide, the proportion of nickel among elements other than lithium and oxygen is 85 atomic percent or more. (Technology 2) The non-aqueous electrolyte secondary battery according to Technology 1, wherein the discharge capacity density of the positive electrode active material is 221 mAh / g or more, the positive electrode comprises a positive electrode current collector and a positive electrode mixture layer formed on both sides of the positive electrode current collector, and the average thickness of the portion of the positive electrode in which the positive electrode mixture layer is formed on both sides of the positive electrode current collector is 178 μm or less. (Technology 3) The non-aqueous electrolyte secondary battery according to Technology 1 or 2, wherein the silicon content in the first silicon-containing material is 40% by mass or more, the charge capacity density of the negative electrode active material is 500 mAh / g or more, the negative electrode comprises a negative electrode current collector and a negative electrode mixture layer formed on both sides of the negative electrode current collector, and the average thickness of the portion of the negative electrode in which the negative electrode mixture layer is formed on both sides of the negative electrode current collector is 195 μm or less. (Technology 4) The apparent density of the first silicon-containing material is 1.6 g / cm³ ３ ~2.2 g / cm ３ A non-aqueous electrolyte secondary battery according to any one of technologies 1 to 3, which falls within the range of (Technology 5). A non-aqueous electrolyte secondary battery according to any one of technologies 1 to 4, wherein the average size of the crystallites constituting the first silicon phase is 15 nm or less. (Technology 6). A non-aqueous electrolyte secondary battery according to any one of technologies 1 to 5, wherein the ratio Cn / Cp of the discharge capacity Cn of the negative electrode to the discharge capacity Cp of the positive electrode is in the range of 1.0 to 1.1.

[0085] The present disclosure will be described in detail below based on examples, but the present disclosure is not limited to the following examples.

[0086] (Battery A1) Battery A1 was manufactured using the following procedure. (1) Preparation of the positive electrode A positive electrode mixture was obtained by mixing the positive electrode active material, conductive agent and binder in a mass ratio of 98:1.4:0.6. N-methyl-2-pyrrolidone (dispersion medium) was added to this positive electrode mixture and stirred to obtain a positive electrode mixture slurry. The positive electrode active material was lithium nickel composite oxide (Li １．０５ Ni ０．８０ Co ０．１４ O ２ ) was used. Carbon black was used as the conductive agent. Polyvinylidene fluoride (PVDF) was used as the binder.

[0087] Next, a positive electrode mixture slurry was applied to both sides of an aluminum foil (positive electrode current collector, thickness: 15 μm) to form a coating. After drying the coating, the coating was compressed in the thickness direction using a roller. In this way, a laminate was formed containing the positive electrode current collector and the positive electrode mixture layers formed on both sides of the positive electrode current collector. The positive electrode was obtained by cutting the positive electrode laminate to a predetermined size.

[0088] (2) Preparation of the negative electrode A negative electrode mixture was obtained by mixing the negative electrode active material, a binder, and a thickener in a mass ratio of 98:1:1. A negative electrode mixture slurry was obtained by adding water (dispersion medium) to this negative electrode mixture and stirring. For the negative electrode active material, a mixture of graphite, a first silicon-containing material (SiC), and a second silicon-containing material (SiO) was used in a mass ratio of 92:4:4. The first silicon-containing material used had the physical properties shown in Table 1. Styrene-butadiene copolymer rubber (SBR) was used as the binder, and carboxymethylcellulose (CMC) was used as the thickener.

[0089] Next, a negative electrode mixture slurry was applied to both sides of a copper foil (negative electrode current collector, thickness: 15 μm) to form a coating. After drying the coating, it was compressed in the thickness direction using a roller. In this way, a laminate was obtained containing the negative electrode current collector and the negative electrode mixture layers formed on both sides of the negative electrode current collector. The negative electrode was obtained by cutting the laminate to a predetermined size. The negative electrode was manufactured to be slightly larger in dimensions than the positive electrode.

[0090] (3) Preparation of non-aqueous electrolyte Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a ratio of EC:DMC = 1:3 (by volume), and vinylene carbonate (VC) was added to prepare a non-aqueous solvent. LiPF was added to this non-aqueous solvent. ６ A non-aqueous electrolyte was prepared by dissolving [the substance]. The VC content in the non-aqueous electrolyte was set to 5% by mass. In addition, LiPF in the non-aqueous electrolyte was [the substance]. ６ The concentration was set to 1.5 mol / L.

[0091] (4) Fabrication of a non-aqueous electrolyte secondary battery One end of an aluminum positive electrode lead was attached to the positive electrode current collector by welding. One end of a nickel negative electrode lead was attached to the negative electrode current collector by welding. Then, in an inert gas atmosphere, the positive electrode and the negative electrode were stacked with a separator in between to obtain a laminate. A microporous polyethylene membrane was used as the separator. A wound-type electrode group was obtained by winding the laminate.

[0092] Next, as shown in Figure 1, a battery A1 (a non-aqueous electrolyte secondary battery) was fabricated by housing a wound electrode group (electrode group 14) and a non-aqueous electrolyte (non-aqueous electrolyte solution) in a cylindrical battery case. The cell capacity and internal resistance of the fabricated battery A1 were measured. The positive electrode and negative electrode were also evaluated using the method described above.

[0093] (Battery A2) Battery A2 was manufactured using the same method and conditions as Battery A1, except that the first silicon-containing material was changed to a material having the physical properties shown in Table 2.

[0094] (Battery C1 and Battery C2) Batteries C1 and C2 were manufactured using the same method and conditions as battery A1, except that the negative electrode active material was changed. The negative electrode active material of battery C1 was a mixture of graphite and a first silicon-containing material. The negative electrode active material of battery C2 was a mixture of graphite and a second silicon-containing material. The proportion of graphite in the negative electrode active material was the same as the proportion in the positive electrode active material of battery A1.

[0095] (Battery C3) Battery C3 was manufactured using the same method and conditions as battery A1, except that the positive electrode active material was changed. The positive electrode active material of battery C3 was lithium nickel composite oxide (Li １．０５ Ni ０．４３ Co ０．１４ O ２ ) was used.

[0096] The fabricated batteries A2 and C1-C3 were evaluated in the same manner as battery A1. The manufacturing conditions and evaluation results for each battery are shown in Table 1. In Table 1, "SiC / (SiC+SiO)" represents the ratio of silicon-containing material in the negative electrode active material. Specifically, it shows the ratio of the mass of the first silicon-containing material (SiC) to the total mass of the first silicon-containing material (SiC) and the second silicon-containing material (SiO). The average thickness of the positive electrode and the average thickness of the negative electrode are the average thickness of the portions on both sides of the current collector where the composite material layer is formed.

[0097] Batteries A1 and A2 are non-aqueous electrolyte secondary batteries (B) according to the present disclosure. Batteries C1 to C3 are comparative examples. As shown in Table 1, batteries A1 and A2 had high capacity and relatively low internal resistance.

[0098] This disclosure is applicable to non-aqueous electrolyte secondary batteries. Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be constrained. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be construed as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0099] 10: Non-aqueous electrolyte secondary battery 11: Positive electrode 12: Negative electrode 13: Separator

Claims

1. A non-aqueous electrolyte secondary battery comprising: a positive electrode containing a positive electrode active material; a negative electrode containing a negative electrode active material; a separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte, wherein the negative electrode active material comprises a first silicon-containing material and a second silicon-containing material; the first silicon-containing material comprises a first silicon phase and a carbon phase in which the first silicon phase is dispersed; the second silicon-containing material comprises a second silicon phase and a silicon dioxide phase in which the second silicon phase is dispersed; the positive electrode active material comprises a lithium-containing composite oxide; and in the lithium-containing composite oxide, the proportion of nickel among elements other than lithium and oxygen is 85 atomic percent or more.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the discharge capacity density of the positive electrode active material is 221 mAh / g or more, the positive electrode comprises a positive electrode current collector and a positive electrode mixture layer formed on both sides of the positive electrode current collector, and the average thickness of the portion of the positive electrode in which the positive electrode mixture layer is formed on both sides of the positive electrode current collector is 178 μm or less.

3. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the silicon content in the first silicon-containing material is 40% by mass or more, the charge capacity density of the negative electrode active material is 500 mAh / g or more, the negative electrode comprises a negative electrode current collector and a negative electrode mixture layer formed on both sides of the negative electrode current collector, and the average thickness of the portion of the negative electrode in which the negative electrode mixture layer is formed on both sides of the negative electrode current collector is 195 μm or less.

4. The apparent density of the first silicon-containing material is 1.6 g / cm³. ３ ~2.2 g / cm ３ A non-aqueous electrolyte secondary battery according to claim 1, which falls within the range of claim 1.

5. The non-aqueous electrolyte secondary battery according to claim 1, wherein the average size of the crystallites constituting the first silicon phase is 15 nm or less.

6. The non-aqueous electrolyte secondary battery according to claim 1, wherein the ratio Cn / Cp of the discharge capacity of the negative electrode to the discharge capacity Cp of the positive electrode is in the range of 1.0 to 1.1.

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