Negative electrode for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery

Abstract

A negative electrode for a non-aqueous electrolyte secondary battery comprises a negative electrode mixture layer containing a negative electrode active material and carbon nanotubes. The negative electrode active material comprises a first negative electrode active material and a second negative electrode active material. The first and second negative electrode active materials comprise a lithium silicate phase containing lithium, silicon, and oxygen, and silicon particles dispersed within the lithium silicate phase. The molar ratio of oxygen to silicon (O / Si) in each lithium silicate phase varies. The carbon nanotubes have a diameter of 1 nm to 5 nm. The mass ratio of the first negative electrode active material to the total mass of the first and second negative electrode active materials is 60% or less.

Description

Technical Field

[0001] The present disclosure relates to a negative electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery. Background Art

[0002] As a negative electrode active material for a high-capacity secondary battery, a silicon-based material that can absorb and release more lithium ions than carbon materials such as graphite has been studied. However, since the silicon-based material has a large volume change during the absorption and release of lithium ions, the conductive path inside the negative electrode mixture layer is cut off by repeated charge and discharge, and there is a problem of deterioration of the cycle characteristics of the secondary battery. Patent Document 1 discloses a secondary battery in which the cycle characteristics are improved by containing carbon nanotubes of a specified size in the negative electrode mixture layer at a ratio of 0.1 mass% to 2 mass%.

[0003] Prior Art Documents

[0004] Patent Documents

[0005] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2016-110876 Summary of the Invention

[0006] In the secondary battery disclosed in Patent Document 1, the diameter of the carbon nanotubes in the negative electrode mixture layer is large and the content of the carbon nanotubes is large. Therefore, even if the cycle characteristics are improved, the initial discharge capacity sometimes decreases. Patent Document 1 has not studied the balance between the initial discharge capacity and the cycle characteristics, and there is still room for improvement.

[0007] The negative electrode for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes: a negative electrode current collector; and a negative electrode mixture layer formed on the surface of the negative electrode current collector and containing a negative electrode active material and carbon nanotubes. The negative electrode active material includes a first negative electrode active material and a second negative electrode active material. The first negative electrode active material includes: a first lithium silicate phase containing lithium, silicon, and oxygen, and first silicon particles dispersed in the first lithium silicate phase. A1 representing the molar ratio of oxygen to silicon (O / Si) in the first lithium silicate phase satisfies the relationship of 2 < A1 ≤ 3. The second negative electrode active material includes: a second lithium silicate phase containing lithium, silicon, and oxygen, and second silicon particles dispersed in the second lithium silicate phase. A2 representing the molar ratio of oxygen to silicon (O / Si) in the second lithium silicate phase satisfies the relationship of 3 < A2 ≤ 4. The diameter of the carbon nanotubes is 1 nm to 5 nm, and the ratio of the mass of the first negative electrode active material to the total mass of the first negative electrode active material and the second negative electrode active material is 60% or less.

[0008] The non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes: the above-mentioned negative electrode for a non-aqueous electrolyte secondary battery; a positive electrode containing a positive electrode active material; and a non-aqueous electrolyte.

[0009] According to one embodiment of the present disclosure, the initial discharge capacity of a secondary battery can be increased, and the cycle characteristics can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 1 is a longitudinal sectional view of a nonaqueous electrolyte secondary battery as an example of an embodiment. DETAILED DESCRIPTION

[0011] As mentioned above, silicon-based materials are negative electrode active materials that can achieve high capacity in secondary batteries, but the volume change caused by charging and discharging is large, so the reduction in discharge capacity caused by charge and discharge cycles sometimes becomes a problem. For example, in the method disclosed in Patent Document 1, in which a large amount of carbon nanotubes as a conductive additive are included in the negative electrode mixture layer to ensure a conductive path even after charge and discharge cycles, the content of the negative electrode active material in the negative electrode mixture layer is reduced. Therefore, even if the cycle characteristics can be improved, the initial discharge capacity may sometimes be reduced. In addition, as described in Patent Document 1, if the negative electrode mixture slurry contains a large amount of large-diameter carbon nanotubes, the dispersibility may also deteriorate. On the other hand, the negative electrode active material containing silicon particles in the lithium silicate phase containing lithium, silicon, and oxygen in the silicon-based material can store and release a large amount of lithium ions, but it will break due to charge and discharge cycles, becoming prone to side reactions such as reactions with the electrolyte, which can easily lead to a reduction in cycle characteristics. Therefore, the present inventors conducted in-depth research to solve the above-mentioned problems and found that by preparing two types of particles containing a lithium silicate phase with silicon particles dispersed inside, combining and using a negative electrode active material obtained by mixing them in an appropriate ratio with carbon nanotubes with smaller diameters, the initial discharge capacity of the secondary battery can be specifically increased and the cycle characteristics can be improved.

[0012] Below, an example of an embodiment of the non-aqueous electrolyte secondary battery disclosed herein is described in detail. Below, a cylindrical battery is illustrated in which a wound electrode body is housed in a cylindrical outer shell. However, the electrode body is not limited to a wound type and may also be a stacked type in which multiple positive electrodes and multiple negative electrodes are alternately stacked one by one through a separator. In addition, the outer shell is not limited to a cylindrical shape and may be, for example, a square shape, a coin shape, or a battery shell composed of a laminated sheet including a metal layer and a resin layer.

[0013] Figure 1 It is a longitudinal cross-sectional view of a cylindrical secondary battery 10 as an example of an embodiment. Figure 1 The illustrated secondary battery 10 includes an electrode body 14 and a nonaqueous electrolyte housed in an outer case 15. The electrode body 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. Note that, for ease of explanation, the sealing body 16 side is referred to as "upper" and the bottom side of the outer case 15 is referred to as "lower."

[0014] The open end of the upper part of the outer shell 15 is blocked by the sealing body 16, so that the interior of the secondary battery 10 is sealed. Insulating plates 17 and 18 are provided above and below the electrode body 14, respectively. The positive lead 19 extends upward through the through hole of the insulating plate 17 and is welded to the bottom plate of the sealing body 16, that is, the lower surface of the partially opened metal plate 22. In the secondary battery 10, the top plate of the sealing body 16, that is, the cover 26, which is electrically connected to the partially opened metal plate 22, becomes the positive terminal. On the other hand, the negative lead 20 extends to the bottom side of the outer shell 15 through the outside of the insulating plate 18 and is welded to the bottom inner surface of the outer shell 15. In the secondary battery 10, the outer shell 15 becomes the negative terminal.

[0015] The outer shell 15 is, for example, a cylindrical metal can with a bottom. A gasket 27 is provided between the outer shell 15 and the sealing member 16 to ensure the airtightness of the interior of the secondary battery 10. The outer shell 15 has a groove 21 formed by applying pressure to the side surface from the outside to support the sealing member 16. The groove 21 is preferably formed in an annular shape along the circumference of the outer shell 15, and the upper surface of the groove supports the sealing member 16 via the gasket 27.

[0016] The sealing body 16 includes: a partially open metal plate 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a lid 26, which are stacked in sequence from the electrode body 14 side. The components constituting the sealing body 16 are, for example, in the shape of a circular plate or a ring, and the components except the insulating member 24 are electrically connected to each other. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective central portions, and an insulating member 24 is sandwiched between their respective peripheral portions. When the internal pressure of the battery rises due to abnormal heat, for example, the lower valve body 23 breaks, whereby the upper valve body 25 expands toward the lid 26 side and detaches from the lower valve body 23, thereby cutting off the electrical connection between the two. When the internal pressure rises further, the upper valve body 25 breaks, and the gas is discharged from the opening 26a of the lid 26.

[0017] Hereinafter, the positive electrode 11 , the negative electrode 12 , the separator 13 , and the non-aqueous electrolyte constituting the secondary battery 10 , particularly the negative electrode mixture layer 41 constituting the negative electrode 12 , will be described in detail.

[0018] [positive electrode]

[0019] The positive electrode 11 includes a positive electrode current collector 30 and a positive electrode mixture layer 31 formed on the surface of the positive electrode current collector 30. The positive electrode current collector 30 can be made of a foil of a metal that is stable within the potential range of the positive electrode 11, such as aluminum, or a thin film having the metal disposed on the surface. The positive electrode mixture layer 31 can include a positive electrode active material, a binder, a conductive agent, and the like. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a binder, a conductive agent, and the like to the surface of the positive electrode current collector 30, drying the coating, and then compressing the coating to form the positive electrode mixture layer 31 on both sides of the positive electrode current collector 30.

[0020] The positive electrode active material contained in the positive electrode mixture layer 31 can include a lithium transition metal oxide as a main component. The positive electrode active material can be composed essentially only of the lithium transition metal oxide, or it can be formed by attaching inorganic compound particles, etc., to the surface of lithium transition metal oxide particles. The lithium transition metal oxide can be used alone or in combination of two or more.

[0021] Lithium transition metal oxides can be formulated as Li a Ni x Co y M 1-x-y O 2-b (wherein, 0.97≤a≤1.2, 0.8≤x≤1.0, 0≤y≤0.1, 0≤b<0.05, and M contains at least one element selected from Ca, Mn, Al, B, W, Sr, Mg, Mo, Nb, Ti, Si, and Zr).

[0022] Examples of the conductive agent contained in the positive electrode mixture layer 31 include carbon materials such as carbon black (CB), acetylene black (AB), Ketjen black, carbon nanotubes, carbon nanofibers, and graphite. These may be used alone or in combination of two or more.

[0023] Examples of the binder contained in the positive electrode mixture layer 31 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. These may be used alone or in combination of two or more.

[0024] [negative electrode]

[0025] The negative electrode 12 includes a negative electrode current collector 40 and a negative electrode mixture layer 41 formed on the surface of the negative electrode current collector 40. The negative electrode current collector 40 can be made of a metal foil such as copper that is stable within the potential range of the negative electrode 12, or a thin film having the metal disposed on the surface. The negative electrode mixture layer 41 contains a negative electrode active material and carbon nanotubes. The negative electrode 12 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, carbon nanotubes, etc., to the surface of the negative electrode current collector 40, drying the coating, and then compressing the coating to form the negative electrode mixture layer 41 on both surfaces of the negative electrode current collector 40.

[0026] The diameter of the carbon nanotubes is 1 nm to 5 nm, preferably 1 nm to 3 nm. By using carbon nanotubes with such a relatively small diameter, a conductive path between the negative electrode active material can be ensured with a lower content than when using carbon nanotubes with larger diameters. This allows for a higher content of the negative electrode active material in the negative electrode mixture layer 41. Furthermore, the use of these carbon nanotubes improves the dispersibility of the negative electrode mixture slurry. The carbon nanotubes contained in the negative electrode mixture layer 41 can be, for example, single-walled carbon nanotubes.

[0027] The length of the carbon nanotubes may be 5 μm or longer. This increases the aspect ratio of the carbon nanotubes and improves their electrical conductivity, thereby ensuring conductive paths between the negative electrode active materials with a smaller content.

[0028] In the negative electrode mixture layer 41, the mass ratio of the carbon nanotubes to the mass of the negative electrode active material can be 0.005% to 0.05%. A carbon nanotube mass ratio of 0.005% or greater ensures a conductive path between the negative electrode active material, thereby achieving excellent cycle characteristics. A carbon nanotube mass ratio of 0.05% or less allows the negative electrode mixture layer 41 to contain an appropriate amount of the negative electrode active material, which is advantageous from the perspective of discharge capacity.

[0029] The negative electrode mixture layer 41 may further include a conductive additive other than carbon nanotubes. Examples of conductive additives other than carbon nanotubes include carbon black (CB), acetylene black (AB), Ketjen black, and carbon nanofibers. These may be used alone or in combination of two or more. To ensure that the negative electrode mixture layer 41 contains an appropriate amount of negative electrode active material, the total mass of the conductive additive including carbon nanotubes is preferably 0.05% or less relative to the mass of the negative electrode active material.

[0030] The negative electrode mixture layer 41 may further contain a binder. As the binder, similar to the case of the positive electrode 11, a fluororesin, PAN, polyimide resin, acrylic resin, polyolefin resin, etc. may be used. They may be used alone or in combination of two or more. In the case of preparing the negative electrode mixture slurry using an aqueous solvent, it is preferable to use carboxymethyl cellulose (CMC) or its salt, styrene-butadiene rubber (SBR), polyacrylic acid (PAA) or its salt, polyvinyl alcohol, etc. in combination.

[0031] The negative electrode active material contained in the negative electrode mixture layer 41 includes a first negative electrode active material and a second negative electrode active material. Both the first negative electrode active material and the second negative electrode active material are particles having a median particle diameter (D50) on a volume basis, for example, of 1 μm to 25 μm, preferably 4 μm to 15 μm. By pulverizing the negative electrode active material with a ball mill, the value of D50 can be reduced. D50 means: in the particle size distribution on a volume basis, the particle diameter at which the cumulative frequency becomes 50% starting from the smaller particle diameter, and is also called the median diameter. The particle size distribution of the negative electrode active material can be measured using a laser diffraction type particle size distribution measuring device (for example, LA-750 manufactured by Horiba, Ltd.) with water as the dispersion medium.

[0032] The first negative electrode active material includes: a first lithium silicate phase containing lithium, silicon, and oxygen, and first silicon particles dispersed in the first lithium silicate phase. A1 representing the molar ratio of oxygen to silicon (O / Si) in the first lithium silicate phase satisfies the relationship of 2 < A1 ≤ 3. It should be noted that in the first negative electrode active material, phases other than the first lithium silicate phase may be included within the scope not impairing the object of the present disclosure, but in the present embodiment, the first negative electrode active material contains only the first lithium silicate phase as the phase.

[0033] The first negative electrode active material may have a conductive layer covering at least a part of the surface. The conductive layer contains a conductive material. Thereby, the conductivity of the first negative electrode active material can be improved. As the conductive material, for example, a carbon material can be used. It should be noted that the conductive layer can be thin enough not to affect the average particle diameter of the first negative electrode active material.

[0034] The first lithium silicate phase can be represented by the general formula Li2Si x O 2x+1 (x ≥ 1). From the viewpoints of the stability and lithium ion conductivity of the first lithium silicate phase, x preferably satisfies the relationship of 1 ≤ x ≤ 2. It should be noted that B1 representing the molar ratio of lithium to silicon (Li / Si) in the first lithium silicate phase can satisfy the relationship of 0 < B1 ≤ 2, or can be 1 ≤ B1 ≤ 2.

[0035] The first lithium silicate phase may further contain at least one element selected from the group consisting of Na, K, Mg, Ca, Ba, Zr, Nb, Ta, V, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, F, W, Al, La, and B. The content of these elements in the first lithium silicate phase is preferably such that the discharge capacity is not affected. For example, the molar ratio relative to the total amount of Li, Si, and O in the first lithium silicate phase may be 0.01 or less. The content of these elements, such as Na, in the first lithium silicate phase can be measured by dissolving the entire negative electrode active material in hot nitric and hydrofluoric acid (a mixture of heated hydrofluoric acid and nitric acid), filtering and removing the remaining carbon, and analyzing the resulting filtrate using inductively coupled plasma atomic emission spectrometry (ICP-AES).

[0036] The first silicon particles dispersed in the first lithium silicate phase suppress volume changes in the first negative electrode active material during charge and discharge. The crystallite size of the first silicon particles can be, for example, 10 nm or larger. This reduces the surface area of ​​the first silicon particles, thereby suppressing degradation of the first silicon particles. The crystallite size of the first silicon particles can be calculated from the half-value width of the diffraction peak attributable to the Si(111) plane in the X-ray diffraction (XRD) spectrum of the first silicon particles according to the Scherrer equation.

[0037] The first silicon particles are aggregates of multiple microcrystals. From the viewpoint of suppressing the breakage of the first silicon particles, before the initial charge, the average particle size of the first silicon particles is preferably 500 nm or less, more preferably 200 nm or less, and particularly preferably 50 nm or less. After the initial charge, the average particle size of the first silicon particles is preferably 400 nm or less, more preferably 100 nm or less. By miniaturizing the first silicon particles, the volume change during charge and discharge can be further suppressed. The average particle size of the first silicon particles can be obtained as follows: the cross-sectional SEM image obtained by observing the cross section of the first negative electrode active material with a scanning electron microscope (SEM) is analyzed, thereby obtaining the average particle size. Specifically, the grain boundaries of 100 randomly selected primary particles are observed from the cross-sectional SEM image, and after determining the outer shape of the primary particles, the long diameter (longest diameter) of each of the 100 primary particles is obtained, and their average value is taken as the average particle size of the first silicon particles.

[0038] The content of the first silicon particles in the first negative electrode active material is preferably 30% by mass or more, more preferably 50% by mass or more, and particularly preferably 55% by mass or more, in order to improve the diffusibility of lithium ions and achieve a high capacity. On the other hand, in order to suppress the reaction between the electrolyte and the first silicon particles by covering the surface of the first silicon particles with the first lithium silicate phase, the content of the first silicon particles in the first negative electrode active material is preferably 95% by mass or less, more preferably 80% by mass or less, and particularly preferably 75% by mass or less, for example.

[0039] The content of the first silicon particles can be measured using Si-NMR (INOVA-400 manufactured by Varian) under the following measurement conditions.

[0040] Probe: Varian 7mm CPMAS-2

[0041] MAS: 4.2kHz

[0042] MAS speed: 4kHz

[0043] Pulse: DD (45° pulse + signal collection time 1H decoupling)

[0044] Repeat time: 1200 seconds

[0045] Observation amplitude: 100kHz

[0046] Observation center: around -100ppm

[0047] Signal collection time: 0.05 seconds

[0048] Cumulative number of times: 560

[0049] Sample size: 207.6 mg

[0050] The first lithium silicate phase Li2Si x O 2x+1 The composition of (x≥1) can be analyzed, for example, by the following method: It should be noted that the following describes the case where the first lithium silicate phase does not contain the aforementioned elements such as Na, but if it contains elements such as Na, the content thereof is preliminarily subtracted for calculation.

[0051] (1) The mass of a sample of the first negative electrode active material is measured.

[0052] (2) The contents of carbon, oxygen, and lithium in the sample were calculated as follows.

[0053] (3) Subtract the carbon content and silicon content from the mass of the sample to calculate the proportion of lithium and oxygen in the remainder. From the molar ratio of lithium to oxygen, find the ratio of 2 to (2x + 1) and calculate x.

[0054] The carbon content of the sample can be measured using a carbon / sulfur analyzer (EMIA-520 type manufactured by Horiba, Ltd.). The sample is weighed on a magnetic plate, a combustion aid is added, and it is inserted into a combustion furnace heated to 1350 °C (carrier gas: oxygen). The amount of carbon dioxide gas generated during combustion is detected based on infrared absorption. For example, a standard curve is prepared using carbon steel (carbon content 0.49%) manufactured by Bureau of Analysed Sampels Ltd., and the carbon content of the sample is calculated. It should be noted that the carbon content of the sample mainly comes from the conductive layer.

[0055] The oxygen content of the sample can be measured using an oxygen / nitrogen / hydrogen analyzer (EGMA-830 type manufactured by Horiba, Ltd.). The sample is placed in a Ni capsule, and Sn pellets and Ni pellets as a soldering aid are added together into a carbon crucible heated at a power of 5.75 kW, and the released carbon monoxide gas is detected. A standard curve is prepared using the standard sample Y2O3, and the oxygen content of the sample is calculated.

[0056] The lithium content of the sample is measured as follows: The sample is completely dissolved in hot hydrofluoric acid (a mixed acid of hydrofluoric acid and nitric acid after heating), the carbon in the dissolved residue is filtered and removed, and the obtained filtrate is analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES), whereby it can be measured. A standard curve is prepared using a commercially available standard solution, and the lithium content of the sample is calculated.

[0057] The amount obtained by subtracting the carbon content, oxygen content, and lithium content from the mass of the sample of the first negative electrode active material is the silicon content of the sample. This silicon content includes both silicon present in the form of silicon particles and silicon present in the form of lithium silicate. By subtracting the content of silicon particles determined by the above Si-NMR measurement from this silicon content, the content of silicon present in the form of lithium silicate can be obtained.

[0058] The second negative electrode active material includes: a second lithium silicate phase containing lithium, silicon, and oxygen, and second silicon particles dispersed in the second lithium silicate phase. A2 representing the molar ratio (O / Si) of oxygen to silicon in the second lithium silicate phase satisfies the relationship 3 < A2 ≤ 4. It should be noted that in the second negative electrode active material, phases other than the second lithium silicate phase may be included within the scope that does not impair the object of the present disclosure, but in this embodiment, the second negative electrode active material only includes the second lithium silicate phase as a phase. The second negative electrode active material may have a conductive layer covering at least a part of the surface and containing a conductive material such as a carbon material. It should be noted that the conductive layer can be thin enough not to affect the average particle size of the second negative electrode active material.

[0059] The second lithium silicate phase can be represented, for example, by the general formula Li2Si y O 2y+1It is expressed as (y < 1). Preferably, y satisfies the relationship of 0.5 ≤ y < 1. It should be noted that B2, which represents the molar ratio of lithium to silicon (Li / Si) in the second lithium silicate phase, can satisfy the relationship of 2 < B2 ≤ 4. In addition, besides lithium, silicon, and oxygen, the second lithium silicate phase may contain trace elements such as Na exemplified in the first lithium silicate phase.

[0060] The second silicon particles dispersed in the second lithium silicate phase suppress the volume change of the second negative electrode active material during charge and discharge. The crystallite size of the second silicon particles can be, for example, 10 nm or more. The second silicon particles are an aggregate of multiple crystallites. From the viewpoint of suppressing the breakage of the second silicon particles, before the first charge, the average particle size of the second silicon particles is preferably 500 nm or less, more preferably 200 nm or less, and particularly preferably 50 nm or less. After the first charge, the average particle size of the second silicon particles is preferably 400 nm or less, more preferably 100 nm or less.

[0061] From the viewpoint of achieving high capacity, the lower limit value of the content of the second silicon particles in the second negative electrode active material is preferably, for example, 30% by mass, more preferably 50% by mass, and particularly preferably 55% by mass. In addition, from the viewpoint of suppressing side reactions, the upper limit value of the content of the first silicon particles in the first negative electrode active material is preferably, for example, 95% by mass, more preferably 80% by mass, and particularly preferably 75% by mass. The content of the second silicon particles can be measured in the same manner as the content of the first silicon particles. In addition, the composition of the second lithium silicate phase can be calculated in the same manner as the composition of the first lithium silicate phase.

[0062] The second negative electrode active material may contain a precursor of the second negative electrode active material before the first charge. The composition of the precursor of the second negative electrode active material can be expressed, for example, as SiO z (0.5 ≤ Z ≤ 1.5). SiO z may contain: a SiO2 phase and silicon particles dispersed in the SiO2 phase. Through the first charge, the SiO2 phase stores lithium ions and forms the second lithium silicate phase. The second lithium silicate phase can be, for example, Li4SiO4. Thus, a second negative electrode active material containing the second lithium silicate phase and the second silicon particles dispersed in the second lithium silicate phase is formed.

[0063] The ratio of the mass of the first negative electrode active material to the total mass of the first negative electrode active material and the second negative electrode active material is 60% or less, and more preferably 20% or less. By mixing the two negative electrode active materials, the initial discharge capacity of the secondary battery can be increased, and the cycle characteristics can be improved. The first negative electrode active material and the second negative electrode active material have different timings for absorbing and releasing lithium ions during charge and discharge. Therefore, by mixing the two negative electrode active materials in an appropriate ratio, the above-mentioned effect can be obtained. In addition, the hardness of the first negative electrode active material and the second negative electrode active material is also different. Therefore, by mixing them in the above-mentioned ratio, the conductive path between the negative electrode active materials can be well maintained.

[0064] In addition to the first negative electrode active material and the second negative electrode active material, the negative electrode active material may include a carbon material such as graphite (natural graphite, artificial graphite) that can absorb and release lithium ions, a metal such as tin alloyed with lithium, a metal compound containing tin, a lithium-titanium composite oxide and other non-carbon materials as a third negative electrode active material. As the third negative electrode active material, graphite is preferred because it has a small volume change during charge and discharge. The ratio of the third negative electrode active material in the entire negative electrode active material can be, for example, 80% to 95% by mass. If it is within this range, the volume change of the silicon-based negative electrode active material during charge and discharge can be alleviated, thereby improving the cycle characteristics.

[0065] [Separator]

[0066] A porous sheet with ion permeability and insulation can be used in the separator 13. Specific examples of the porous sheet include microporous membranes, woven fabrics, non-woven fabrics, and the like. Suitable materials for the separator 13 include olefin resins such as polyethylene and polypropylene, and cellulose. The separator 13 can be a single-layer structure or a laminated structure. A heat-resistant layer containing a heat-resistant material can be formed on the surface of the separator 13. Examples of the heat-resistant material include polyamide resins such as aliphatic polyamides and aromatic polyamides (aramid), polyimide resins such as polyamide-imide and polyimide, and the like.

[0067] [Non-aqueous electrolyte]

[0068] As the non-aqueous solvent (organic solvent) of the non-aqueous electrolyte, carbonates, lactones, ethers, ketones, esters, etc. can be used, and these solvents can be mixed and used in combination of two or more. As the electrolyte salt of the non-aqueous electrolyte, LiPF6, LiBF4, LiCF3SO3, etc. and mixtures thereof can be used. The amount of the electrolyte salt dissolved in the non-aqueous solvent can be set to, for example, 0.5 to 2.0 mol / L.

[0069] <Example>

[0070] Hereinafter, the present disclosure will be further described based on examples, but the present disclosure is not limited to these examples.

[0071] <Example 1>

[0072] [Synthesis of the First Negative Electrode Active Material]

[0073] Silica and lithium carbonate were mixed in a Si / Li molar ratio of 1.05, and the mixture was calcined in air at 950°C for 10 hours to obtain lithium silicate represented by Li2Si2O5. The obtained lithium silicate was crushed in such a way that the average particle size became 10 μm. After that, Li2Si2O5 was mixed with raw silicon (3N, average particle size 10 μm) in a mass ratio of 45:55, and the mixture was filled into a planetary ball mill (manufactured by Fritsch, P-5) (SUS, volume: 500 mL). 24 SUS balls (diameter 20 mm) were placed in the jar, the lid was sealed, and the pulverization treatment was carried out at 200 rpm in an inert atmosphere for 50 hours.

[0074] Next, the powdered mixture after the pulverization process was taken out and calcined at 800°C for 4 hours in an inert atmosphere under pressure from a hot press to obtain a sintered body of the mixture. Afterwards, the sintered body was pulverized, passed through a 40 μm sieve, and mixed with coal tar (MCP250 manufactured by JFE Chemical Corporation). The mixture was calcined at 800°C in an inert atmosphere and the surface was covered with conductive carbon to form a conductive layer. The coverage of the conductive layer was 5% by mass relative to the mass of the first negative electrode active material. Afterwards, sieving was performed to obtain a first negative electrode active material having an average particle size of 5 μm with a conductive layer on the surface.

[0075] The crystallite size of the silicon particles in the first negative electrode active material is 15 nm. The first lithium silicate phase has an O / Si ratio of 2.5 and a Si / Li ratio of 1.0. The first lithium silicate phase contains 45% Li₂Si₂O₅ and 55% silicon particles. It should be noted that the first negative electrode active material does not contain elements such as sodium.

[0076] [Preparation of Precursor of Second Negative Electrode Active Material]

[0077] SiO particles (average particle size 5 μm) with silicon particles dispersed within a SiO2 phase were mixed with coal tar (MCP250, manufactured by JFE Chemical Corporation). The mixture was calcined at 800°C in an inert atmosphere, and the surface of the SiO particles was coated with conductive carbon to form a conductive layer. The coating amount of the conductive layer was 5% by mass relative to the total mass of the SiO particles and the conductive layer. In this manner, SiO particles (precursor of the second negative electrode active material) with an average particle size of 5 μm and a conductive layer on the surface were obtained.

[0078] After the initial charge, the SiO2 phase of the SiO particles primarily transforms into the second lithium silicate phase, Li4SiO4, and the precursor becomes the second negative electrode active material comprising Li4SiO4 and silicon particles dispersed within the Li4SiO4. After the initial charge, the second negative electrode active material contains 42% Li4SiO4 by mass, and the content of silicon particles occluded with lithium ions is 58% by mass. It should be noted that the content of silicon particles other than the occluded lithium ions is 29% by mass. It should be noted that the first negative electrode active material does not contain elements such as sodium.

[0079] [Production of negative electrode]

[0080] 1 part by mass of the first negative electrode active material, 4 parts by mass of the precursor of the second negative electrode active material, and 95 parts by mass of graphite as the third negative electrode active material were mixed to produce a mixed negative electrode active material. Next, this mixed negative electrode active material was mixed with carbon nanotubes (CNTs) with a diameter of 1.2 to 2 nm, an average diameter of 1.6 nm, and a length of 5 μm, styrene-butadiene rubber (SBR), and sodium carboxymethylcellulose (CMC-Na) in a mass ratio of 100:0.01:1:1. This mixture was then kneaded with an N-methyl-2-pyrrolidone (NMP) solution to prepare a negative electrode mixture slurry. This negative electrode mixture slurry was applied to a copper negative electrode current collector. After drying, the coating was rolled with a roller and cut into the desired electrode size, resulting in a negative electrode having a negative electrode mixture layer formed on both sides of the negative electrode current collector.

[0081] [Production of positive electrode]

[0082] Using lithium nickel composite oxide (LiNi 0.8 Co 0.18 Al 0.02) as the positive electrode active material. This positive electrode active material is mixed with acetylene black and polyvinylidene fluoride (PVdF) at a solid content mass ratio of 95:2.5:2.5. After adding an appropriate amount of N-methyl-2-pyrrolidone (NMP), the mixture is kneaded to prepare a positive electrode mixture slurry. This positive electrode mixture slurry is applied to both sides of an aluminum positive electrode collector. After the coating is dried, it is rolled using a roller and cut into the specified electrode size, resulting in a positive electrode with a positive electrode mixture layer formed on both sides of the positive electrode collector.

[0083] [Preparation of non-aqueous electrolyte]

[0084] LiPF 6 as an electrolyte salt was dissolved at 1.0 mol / L in a non-aqueous solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) mixed at a volume ratio of 1:3 to form a non-aqueous electrolyte solution as a liquid non-aqueous electrolyte.

[0085] [Battery Production]

[0086] An aluminum lead was attached to the positive electrode, and a nickel lead was attached to the negative electrode. The positive and negative electrodes were spirally wound with a polyolefin separator interposed therebetween, and then radially press-formed to produce a flat, wound electrode assembly. This electrode assembly was housed in an outer case made of aluminum laminate, and the nonaqueous electrolyte was injected. The opening of the outer case was then sealed to produce an evaluation cell.

[0087] <Example 2>

[0088] In the preparation of the negative electrode, a battery was prepared in the same manner as in Example 1 except that 3 parts by mass of the first negative electrode active material, 3 parts by mass of the precursor of the second negative electrode active material, and 94 parts by mass of graphite as the third negative electrode active material were mixed to obtain a mixed negative electrode active material.

[0089] <Example 3>

[0090] In the preparation of the negative electrode, a battery was prepared in the same manner as in Example 1 except that 6 parts by mass of the first negative electrode active material, 4 parts by mass of a precursor of the second negative electrode active material, and 90 parts by mass of graphite as the third negative electrode active material were mixed to obtain a mixed negative electrode active material.

[0091] <Comparative Example 1>

[0092] In the preparation of the negative electrode, a battery was prepared in the same manner as in Example 1 except that 4.5 parts by mass of the first negative electrode active material, 2.5 parts by mass of the precursor of the second negative electrode active material, and 93 parts by mass of graphite as the third negative electrode active material were mixed to obtain a mixed negative electrode active material.

[0093] <Comparative Example 2>

[0094] In the preparation of the negative electrode, a battery was prepared in the same manner as in Example 1 except that 4 parts by mass of the precursor of the second negative electrode active material and 96 parts by mass of graphite as the third negative electrode active material were mixed to obtain a mixed negative electrode active material.

[0095] For each of the above-mentioned batteries, the initial discharge capacity and capacity retention rate were evaluated using the following method. The initial discharge capacity represents a relative value when the value of Comparative Example 1 is set to 100. The capacity retention rate represents a relative value when the value of Comparative Example 2 is set to 1. The batteries of the embodiments and comparative examples were evaluated by multiplying the initial discharge capacity by the capacity retention rate. Table 1 shows the evaluation results, the respective contents of the first negative electrode active material and the second negative electrode active material, and the ratio of the mass of the first negative electrode active material to the total mass of the first negative electrode active material and the second negative electrode active material.

[0096] [Measurement of initial discharge capacity]

[0097] For the batteries of the Examples and Comparative Examples, the batteries were charged at a constant current of 0.05C until the battery voltage reached 0V under a temperature environment of 25°C, and then discharged at a constant current of 0.05C until the battery voltage reached 1V, and the initial discharge capacity was measured. It should be noted that the discharge here refers to the discharge in a battery that combines the negative electrodes of the Examples and Comparative Examples with a commonly used positive electrode such as LiNiO2. In the batteries of the Examples and Comparative Examples, the negative electrode is used as the working electrode and metallic lithium (Li) is used as the counter electrode. Therefore, they should be charged, but according to the charge and discharge behavior of the negative electrode in the battery that combines the commonly used positive and negative electrodes, the charge and discharge directions are opposite. That is, charging means that the current flows in a manner that reduces the potential of the negative electrode that serves as the working electrode, and discharging means that the current flows in a manner that increases the potential of the negative electrode that serves as the working electrode.

[0098] [Evaluation of Capacity Retention Rate after Cycle Test]

[0099] The battery in the initial state was subjected to the following cycle test. The discharge capacity at the 1st cycle and the discharge capacity at the 300th cycle of the cycle test were determined, and the capacity retention rate was calculated according to the following formula.

[0100] Capacity retention rate (%) = (discharge capacity at the 300th cycle ÷ discharge capacity at the 1st cycle) × 100

[0101] <Cycle test>

[0102] First, the initial battery was charged at a constant current of 0.05 C at a temperature of 25°C until the battery voltage reached the set voltage of 0 V. Then, it was discharged at a constant current of 0.05 C until the battery voltage reached 1 V. This charge and discharge cycle was repeated 300 times.

[0103] [Table 1]

[0104]

[0105] As shown in Table 1, the batteries of the Examples exhibited a greater product of initial discharge capacity and capacity retention than the batteries of the Comparative Examples, demonstrating superior overall performance. While the first negative electrode active material has a high capacity, increasing its content tends to decrease the capacity retention. However, Example 1 achieved a capacity retention comparable to that of Comparative Example 2. When the mass ratio of the first negative electrode active material to the total mass of the first and second negative electrode active materials was 20% or less, the capacity retention remained substantially constant, suggesting excellent cycle characteristics.

[0106] Description of Reference Numerals

[0107] 10 Secondary Batteries

[0108] 11. Positive electrode

[0109] 12 negative electrode

[0110] 13 Dividers

[0111] 14 Electrode body

[0112] 15 outer shell

[0113] 16 Sealing body

[0114] 17, 18 insulation board

[0115] 19 Positive lead

[0116] 20 Negative lead

[0117] 21 Groove

[0118] 22 Metal plates with partial openings

[0119] 23 Lower valve body

[0120] 24 Insulation components

[0121] 25 Upper valve body

[0122] 26 lid

[0123] 26a Opening

[0124] 27 gasket

[0125] 30 positive electrode collector

[0126] 31. Positive electrode mixture layer

[0127] 40 negative electrode collector

[0128] 41 negative electrode mixture layer

Claims

1. A negative electrode for a non-aqueous electrolyte secondary battery, comprising: a negative electrode current collector; and a single-layer negative electrode mixture layer formed on both surface sides of the negative electrode current collector and containing a negative electrode active material and carbon nanotubes. The negative electrode active material includes a first negative electrode active material and a second negative electrode active material. The first negative electrode active material includes: a first lithium silicate phase containing lithium, silicon, and oxygen, and first silicon particles dispersed in the first lithium silicate phase, and A1 representing the molar ratio of oxygen to silicon (O / Si) in the first lithium silicate phase satisfies the relationship of 2 < A1 ≤ 3. The second negative electrode active material includes: a second lithium silicate phase containing lithium, silicon, and oxygen, and second silicon particles dispersed in the second lithium silicate phase, and A2 representing the molar ratio of oxygen to silicon (O / Si) in the second lithium silicate phase satisfies the relationship of 3 < A2 ≤ 4. The diameter of the carbon nanotubes is 1 nm to 5 nm. The mass ratio of the first negative electrode active material is 60% or less with respect to the total mass of the first negative electrode active material and the second negative electrode active material. In the negative electrode mixture layer, the mass ratio of the carbon nanotubes with respect to the mass of the negative electrode active material is 0.005% to 0.05%.

2. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein The mass ratio of the first negative electrode active material is 20% or less with respect to the total mass of the first negative electrode active material and the second negative electrode active material.

3. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein The first lithium silicate phase further includes: at least one element selected from the group consisting of Na, K, Mg, Ca, Ba, Zr, Nb, Ta, V, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, F, W, Al, La, and B.

4. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein The length of the carbon nanotubes is 5 μm or more.

5. A non-aqueous electrolyte secondary battery, comprising: The negative electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 4; A positive electrode containing a positive electrode active material; and A non-aqueous electrolyte.

6. The nonaqueous electrolyte secondary battery according to claim 5, wherein The positive electrode active material comprises the general formula Li a Ni x Co y M 1-x-y O 2-b The lithium transition metal oxide shown in the formula, wherein 0.97≤a≤1.2, 0.8≤x≤1.0, 0≤y≤0.1, 0≤b<0.05, and M contains at least one element selected from Ca, Mn, Al, B, W, Sr, Mg, Mo, Nb, Ti, Si, and Zr.

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