Negative electrode active material for secondary battery, and secondary battery

Abstract

A negative electrode active material disclosed herein is a negative electrode active material for a secondary battery. The negative electrode active material (20) comprises composite particles (23). The composite particles (23) include a carbon phase (21) and a silicon phase (22) dispersed in the carbon phase (21). Fluorine is present on at least a portion of the surface of the silicon phase (22).

Description

Negative electrode active material for secondary battery and secondary battery

[0001] The present disclosure relates to a negative electrode active material for a secondary battery and a secondary battery.

[0002] The negative electrode of a secondary battery, such as a lithium-ion secondary battery, uses a negative electrode active material capable of absorbing and releasing lithium ions. Graphite is commonly used as the negative electrode active material. In recent years, silicon-containing composite materials have been investigated as negative electrode active materials. Silicon-containing composite materials are characterized by high capacity density. Various proposals have been made regarding such composite materials.

[0003] Claim 1 of Patent Document 1 (WO 2017-077986) describes a lithium ion secondary battery comprising: a negative electrode, a positive electrode, a separator provided between the negative electrode and the positive electrode, and an electrolyte solution, wherein the electrolyte solution contains fluoroethylene carbonate; the negative electrode having a negative electrode active material having particles containing silicon and particles containing carbon, and a coating containing fluorine formed on a surface of the negative electrode active material, wherein the surface area of ​​the silicon-containing particles and the content of fluorine contained in the coating satisfy the following formula (1): ２ ≦ Fluorine content in the coating (g) / surface area of ​​silicon-containing particle (m ２ )≦0.015g / m ２ ...(1)" is stated.

[0004] International Publication No. 2017-077986

[0005] Silicon-containing negative electrode active materials undergo large expansion and contraction during charge and discharge, which tends to induce side reactions. Therefore, secondary batteries using silicon-containing negative electrode active materials have a problem of low capacity retention during charge and discharge cycles. One object of the present disclosure is to provide a silicon-containing negative electrode active material that can be used to produce secondary batteries with a high capacity retention.

[0006] One aspect of the present disclosure relates to a negative electrode active material for a secondary battery, the negative electrode active material including composite particles, the composite particles containing a carbon phase and a silicon phase dispersed within the carbon phase, and fluorine being present on at least a portion of a surface of the silicon phase.

[0007] Another aspect of the present disclosure relates to a secondary battery including a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode contains the negative electrode active material according to the present disclosure.

[0008] According to the present disclosure, there is provided a negative electrode active material containing silicon, which can be used to manufacture a secondary battery having a high capacity retention rate. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0009] 1 is a cross-sectional view schematically illustrating an example of a negative electrode active material according to an embodiment of the present disclosure, and FIG. 2 is a schematic perspective view, with a portion cut away, of a secondary battery according to an embodiment of the present disclosure.

[0010] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits are exemplified for numerical values ​​of specific physical properties or conditions, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more materials may be used in combination.

[0011] (Negative electrode active material for secondary battery) The negative electrode active material according to this embodiment may be referred to as "negative electrode active material (N)" hereinafter. The negative electrode active material (N) is used in a secondary battery. The negative electrode active material (N) includes composite particles. The composite particles may be referred to as "composite particles (P)" hereinafter. The composite particles (P) contain a carbon phase and a silicon phase dispersed within the carbon phase. Fluorine is present on at least a portion of the surface of the silicon phase.

[0012] In one aspect, the present disclosure provides composite particles (P) for use in a negative electrode active material of a secondary battery.

[0013] Composite particles containing a carbon phase and a silicon phase (silicon particles) have been proposed as negative electrode active materials. The composite particles may undergo a side reaction with the electrolyte, causing battery degradation. This side reaction forms a layer called a solid electrolyte interface (SEI) on the surface of the composite particles. The SEI layer formed on the surface of the composite particles suppresses further side reactions, and maintaining the SEI layer suppresses battery degradation. However, the silicon phase expands and contracts significantly during charge and discharge, making the SEI vulnerable to destruction. Therefore, when conventional composite particles are used as negative electrode active materials, the discharge capacity significantly decreases with charge and discharge cycles.

[0014] As a result of investigations, the present inventors have newly discovered that protecting the surface of the silicon phase with fluorine can significantly suppress the decrease in discharge capacity associated with charge-discharge cycles. The present disclosure is based on this new finding. It is currently unclear what state the fluorine (fluorine atoms) present on the surface of the silicon phase is in. At least a portion of the fluorine may be bonded to the silicon phase. It is also possible that at least a portion of the fluorine exists in the form of a fluorine compound. It is also possible that at least a portion of the fluorine is contained in a coating layer that covers at least a portion of the surface of the silicon phase.

[0015] In the composite particle (P), the sum of the carbon phase content and the silicon phase content may be 80% by mass or more, or 90% by mass or more. The carbon phase content in the composite particle (P) may be in the range of 30 to 80% by mass (e.g., 40 to 70% by mass). By setting the carbon phase content in the range of 30 to 80% by mass, the carbon phase can easily penetrate into voids that are subsequently generated due to charge and discharge, making it easier to maintain electrical connection between the composite particle (P) and its surroundings. The silicon phase content in the composite particle (P) may be in the range of 30 to 80% by mass (e.g., 40 to 70% by mass). By setting the silicon phase content in the range of 30 to 80% by mass, it is possible to achieve both high discharge capacity and high cycle characteristics.

[0016] The ratio C(S) / C(C) of the silicon phase content C(S) (mass%) in the composite particle (P) to the carbon phase content C(C) (mass%) in the composite particle (P) may be 0.1 or more, 0.3 or more, or 0.6 or more, or may be 2.7 or less, 1.8 or less, or 0.9 or less.

[0017] The composite particles (P) may contain phases other than the carbon phase and the silicon phase. However, the content of the other phases is, for example, 10% by mass or less, and may be 5% by mass or less. Examples of the other phases include a silicon oxide phase and a lithium silicate phase.

[0018] The silicon phase is preferably amorphous. Because amorphous materials have a random structure, amorphous silicon phases can absorb larger volume changes. As a result, the cycle life and performance stability of the battery can be improved. The amorphous silicon phase can be formed by a vapor deposition method (e.g., evaporation).

[0019] The fluorine (fluorine atom) content C(F) in the composite particles (P) may be 0.1% by mass or more, 0.2% by mass or more, 1.0% by mass or more, or 5.0% by mass or more. The content C(F) may be 15% by mass or less, 11% by mass or less, 10% by mass or less, 5.0% by mass or less, or 1.0% by mass or less. The fluorine content C(F) may be in the range of 0.1 to 15% by mass, 0.2 to 15% by mass, 1.0 to 15% by mass, or 5.0 to 15% by mass. In any of these ranges, the upper limit may be 11%, 10%, 5.0%, or 1.0% by mass, as long as the lower limit is not equal to or greater than the upper limit. The fluorine content C(F) may be in the range of 0.2 to 10% by mass (e.g., 1.0 to 5.0% by mass). By setting the fluorine content C(F) to 0.2 mass% or more, the capacity retention rate during charge / discharge cycles can be particularly improved. By setting the fluorine content C(F) to 10 mass% or less, the decrease in initial discharge capacity can be suppressed. Note that the "fluorine content C(F) in the composite particle (P)" may be read as the "fluorine content C(F) present on the surface of the silicon phase in the composite particle (P)." The fluorine content C(F) can be measured by ion chromatography, which will be described later.

[0020] The density of the composite particles (P) is 1.5 g / cm ３ ~2.0 g / cm ３ By increasing the density of the composite particles (P), the initial discharge capacity can be increased. On the other hand, if the density of the composite particles (P) is too high, the expansion and contraction of the composite particles (P) during charge and discharge increases, resulting in a decrease in charge and discharge cycle characteristics. The density of the composite particles (P) may be in the range of 1.5 g / cm. ３ ~2.0 g / cm ３ By setting the range, it is possible to achieve both a high initial discharge capacity and good charge-discharge cycle characteristics.

[0021] The density of the composite particles (P) is evaluated by a gas displacement method using a gas displacement pycnometer. That is, the density is measured by the gas displacement method. Apparatus: Accupyc II 1345TC-10CC (Shimadzu Corporation) Gas type: Helium

[0022] A first layer containing fluorine and a second layer containing carbon may be laminated in this order on at least a portion of the surface of the silicon phase. That is, the first layer and the second layer may be laminated in the order silicon phase / first layer / second layer. The presence of the second layer can suppress elution of the first layer. As a result, the effect of the first layer (e.g., the effect of improving charge / discharge cycle characteristics) is more likely to be sustained.

[0023] The carbon content of the second layer of the composite particle (P) may be in the range of 1.0 to 6.0 mass %, which can sufficiently suppress elution of the first layer and also suppress a decrease in discharge capacity.

[0024] The average diameter D(Si) of the silicon phase may be 1.0 nm or more, or 2.0 nm or more, or may be 10 nm or less, or 5.0 nm or less. By setting the average diameter D(Si) to 10 nm or less, the effects of expansion and contraction of the silicon phase due to charge and discharge can be suppressed, and the capacity retention rate during charge and discharge cycling can be increased. The average diameter D(Si) of the silicon phase may be in the range of 1.0 nm to 10 nm, 2.0 nm to 10 nm, 3.0 nm to 10 nm, or 5.0 nm to 10 nm. Within these ranges, the upper limit may be 5.0 nm or 3.0 nm, as long as the lower limit is not equal to or greater than the upper limit.

[0025] The silicon phase dispersed within the carbon phase can be regarded as silicon particles (particulate silicon phase) dispersed within the carbon phase. The average diameter D(Si) of the silicon phase in the cross section of the composite particle (P) can be determined by observing the cross section of the composite particle (P) using an electron microscope (SEM or TEM). Specifically, it is determined by arithmetically averaging the maximum diameters of 100 silicon phases arbitrarily selected in the cross section.

[0026] The average particle size of the composite particles (P) may be 1.0 μm or more, or 5.0 μm or more, or may be 20 μm or less, 15 μm or less, or 10 μm or less. Having the average particle size of the composite particles (P) within these ranges makes it easier to obtain good battery characteristics. The average particle size of the composite particles (P) is the median diameter (D50) at which the cumulative volume becomes 50% in the volume-based particle size distribution. The average particle size (median diameter) is determined using a laser diffraction / scattering particle size distribution analyzer.

[0027] The average value Fs of the single particle fracture strength of the composite particles may be 550 MPa or less, 500 MPa or less, or 400 MPa or less. The average value Fs may be 300 MPa or more, 350 MPa or more, or 400 MPa or more. When the average value Fs is 400 MPa or less, the particles are flexible and easily deformed. Therefore, even if the volume of the silicon phase changes due to charge and discharge, cracks are unlikely to occur inside the particles. As a result, it is possible to increase the capacity retention rate during charge and discharge cycles.

[0028] The method for controlling the single particle breaking strength of the composite particles (P) is not particularly limited. For example, the strength of the composite particles (P) may be controlled by changing the porosity of the porous carbon before silicon deposition. The strength of the composite particles (P) can be reduced by increasing the porosity of the porous carbon. The strength of the composite particles (P) can be increased by decreasing the porosity of the porous carbon.

[0029] The single particle breaking strength of the composite particles (P) can be measured using a commercially available measuring device (for example, a microcompression tester (MCT-W201) manufactured by Shimadzu Corporation) according to the following procedure.

[0030] (1) The composite particles (P) are dispersed on the lower pressure plate (SKS flat plate) of the measuring device. (2) Using an optical microscope, one composite particle (P) with a maximum diameter in the range of 5 to 7 μm is selected. (3) A diamond flat indenter with a diameter of 50 μm is used as the upper pressure tool, and only one particle selected as the sample is placed between this upper pressure tool and the lower pressure tool. (4) The upper pressure tool is slowly lowered, and from the point where it comes into contact with the sample (the lowering speed changes), the load is increased at a constant rate (specifically, 2.7 mN / sec).

[0031] (5) The relationship between the load and the deformation of the sample is measured, and the point at which the deformation of the sample changes suddenly (the inflection point of the load-deformation profile) is taken as the breaking point. The breaking strength (single particle breaking strength) is calculated from the load and particle size (maximum diameter) at that time based on the following formula:

[0032] St = 2.8P / πd ２ St: Breaking strength [MPa or N / mm ２ ] P: Load [N] d: Particle size [mm]

[0033] The above measurement is carried out five times. That is, the single particle fracture strength is measured for five selected particles. The average value Fs of the single particle fracture strength is calculated by arithmetically averaging the five measured values.

[0034] When measuring the single particle fracture strength of the composite particles (P) in a secondary battery, the composite particles (P) are separated from the battery and measured. Specifically, first, the secondary battery is disassembled to remove the negative electrode, and the negative electrode mixture is peeled off from the negative electrode. Next, the negative electrode mixture is washed with anhydrous ethyl methyl carbonate or dimethyl carbonate, and then crushed in a mortar. Next, the crushed negative electrode mixture is dried in a dry atmosphere for 1 hour and then immersed in heated hydrochloric acid (concentration: 6 mol / L) for 10 minutes. Next, the negative electrode mixture is washed with ion-exchanged water, and the solid content is filtered and dried at 200 ° C. for 1 hour. Next, the composite particles (P) are separated from the solid content by centrifugation. The composite particles (P) obtained in this manner are analyzed.

[0035] The composite particles (P) contain a carbon phase and a silicon phase dispersed within the carbon phase. The composite particles (P) are flexible and highly conductive. Therefore, a negative electrode containing the composite particles (P) is likely to maintain a good conductive network. Specifically, even if voids are formed around the composite particles (P) or cracks occur in the composite particles (P), parts of the composite particles (P) are unlikely to become isolated, and contact between the composite particles (P) and their surroundings is likely to be maintained. Therefore, capacity loss due to charge / discharge cycles is likely to be suppressed. In addition, expansion of the negative electrode is unlikely to occur.

[0036] The composite particles (P) may have a sea-island structure. In this case, the silicon phase (islands) are dispersed in a matrix (sea) of carbon phase. The sea-island structure limits contact between the silicon phase and the electrolyte, thereby suppressing side reactions. Furthermore, stress caused by expansion and contraction of the silicon phase is alleviated by the carbon phase matrix.

[0037] The carbon phase may be composed of amorphous carbon. Examples of amorphous carbon that constitutes the carbon phase include hard carbon, soft carbon, and other amorphous carbon. Amorphous carbon is a carbon material in which the average interplanar spacing d002 of the (002) plane measured by X-ray diffraction exceeds 0.34 nm.

[0038] The composite particles (P) can reversibly absorb and release lithium ions. The negative electrode active material may be composed solely of the negative electrode active material (N). The negative electrode active material may include other negative electrode active materials besides the negative electrode active material (N). The other negative electrode active materials may be materials capable of reversibly absorbing and releasing lithium ions. The other negative electrode active materials may be known negative electrode active materials. The proportion of the negative electrode active material (N) in the negative electrode active material of the secondary battery (B) may be 0.5% by mass or more, or 1.0% by mass or more. This proportion may be 30% by mass or less, 20% by mass or less, or 15% by mass or less. When the negative electrode active material (N) is the negative electrode active material of a nonaqueous electrolyte secondary battery, examples of the other negative electrode active materials include carbon materials (such as graphite), silicon oxide, and composite materials of a lithium silicate phase and a silicon phase.

[0039] (Method for producing composite particles (P)) There is no particular limitation on the method for producing the composite particles (P). The composite particles (P) may be produced by the following production method including a first step and a second step.

[0040] In the first step, a silicon phase dispersed within a carbon phase is formed. In one example of the first step, a silicon-containing source gas is flowed into a heated reactor while the porous carbon is stirred in the reactor. This causes amorphous silicon to be deposited inside the pores of the porous carbon. The deposition forms a silicon phase dispersed within the carbon phase (porous carbon). In the first step, the reactor may be heated to a temperature in the range of 500 to 700°C (e.g., 600°C). An example of the source gas is monosilane (SiH ４ The porous carbon starting material may have an average pore size in the range of 2 to 10 nm and an average particle size (median diameter D50) in the range of 5 to 7 μm. Examples of porous carbon that can be used include activated carbon, carbon aerogel, carbon foam, graphite, and nanoporous carbon.

[0041] In the second step, fluorine is disposed on the surface of the silicon phase formed in the first step. In one example of the second step, a fluorine-containing source gas is flowed into a heated reactor while stirring the porous carbon. This allows a fluorine-containing layer and a carbon layer to be formed in this order on the amorphous silicon. The first and second steps may be performed consecutively in the same reactor. In the second step, the reactor may be heated to a temperature in the range of 500 to 700°C (e.g., 600°C). An example of the source gas is a mixed gas of a fluorocarbon (e.g., tetrafluoroethylene) and a carrier gas (e.g., nitrogen gas).

[0042] The fluorocarbon may be a hydrofluorocarbon containing hydrogen and fluorine, or a perfluorocarbon containing fluorine but no hydrogen. Examples of fluorocarbons include tetrafluoroethylene, trifluoromethylacetylene, 1,1,1,2-tetrafluoroethane, (trifluoromethyl)acetylene, 1,1,1,2-tetrafluoroethane, 1,1-difluoroethane, pentafluoroethane, and 1,2-difluoroethylene.

[0043] The composition of the composite particles (P) can be determined by the following analytical method. When analyzing the composite particles (P) contained in the negative electrode of a secondary battery, the analysis is performed on the negative electrode removed by disassembling a fully discharged battery. The removed negative electrode is first washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the electrolyte component, and then dried. Next, a cross-section polisher (CP) is used to expose the cross-section of the negative electrode mixture layer, and the cross-section of the negative electrode mixture layer is analyzed.

[0044] (ICP) A sample of the composite particles (P) is dissolved in a heated acid solution (a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid), and the carbon residue in the solution is removed by filtration. The filtrate is then analyzed by inductively coupled plasma emission spectroscopy (ICP) to measure the spectral intensity of each element. Next, a calibration curve is created using commercially available standard solutions of the elements, and the content of each element contained in the composite particles is calculated.

[0045] The contents of Na, K, Al, Li, Si, and B contained in the composite particles (P) can be quantitatively analyzed in accordance with JIS (Japanese Industrial Standards) R3105 (1995) (method for analyzing borosilicate glass).

[0046] When the composite particles (P) contain a lithium silicate phase, the amounts of the lithium silicate phase and the silicon phase in the composite particles (P) can be separately quantified using Si-NMR. The Si content obtained by the above method is the sum of the amount of Si constituting the silicon phase and the amount of Si in the lithium silicate phase. The amount of Si contained in the composite particles (P) is distributed into the amount of Si in the lithium silicate phase and the amount of Si in the silicon phase using the results of quantitative analysis by Si-NMR. Note that the standard substance required for quantification may be a mixture containing a lithium silicate phase and a silicon phase in a predetermined ratio with known Si contents. An example of a desirable Si-NMR apparatus and conditions is shown below.

[0047] (Si-NMR measurement conditions) Measurement device: Varian solid-state nuclear magnetic resonance spectrometer (INOVA-400) Probe: Varian 7 mm CPMAS-2 MAS: 4.2 kHz MAS speed: 4 kHz Pulse: DD (45° pulse + signal acquisition time 1H decoupled) Repetition time: 1200 sec to 3000 sec Observation width: 100 kHz Observation center: near -100 ppm Signal acquisition time: 0.05 sec Number of accumulations: 560

[0048] (STEM-EDX analysis) The locations where fluorine is present in the composite particles (P) can be clarified by STEM-EDX analysis. Specifically, first, 10 composite particles (P) with a maximum diameter of 5 μm or more are randomly selected. Next, the cross section of the selected particles is exposed using a focused ion beam (FIB). Next, elemental analysis of the particle cross section is performed using STEM-EDX. The presence or absence of a fluorine (F) peak is confirmed from the integrated spectrum obtained by the analysis. At that time, the presence or absence of fluorine is confirmed by pinpointing the vicinity of the Si (silicon phase) and the silicate phase. For example, the following equipment can be used to prepare and analyze the sample. (1) Sample preparation using a focused ion beam (FIB) - Equipment: NX5000 (Hitachi High-Tech Science Corporation) - Acceleration voltage: 30 kV (2) Cross-sectional observation using a transmission electron microscope (TEM) - Equipment: JEM-F200 (JEOL Ltd.) - Acceleration voltage: 200 kV (3) Elemental analysis using energy dispersive X-ray spectroscopy (EDX) - Equipment: JED-2300T (JEOL Ltd.: attached to JEM-F200) - Acceleration voltage: 200 kV (4) State analysis using electron energy loss spectroscopy (EELS) - Equipment: Quantum ER (Gatan Co., Ltd.: attached to JEM-F200) - Acceleration voltage: 200 kV

[0049] (Ion Chromatography) The fluorine (F) inside the composite particles (P) can be quantitatively evaluated by the following method. First, the mass of the composite particles (P) is measured and placed on a sample boat. (1) Next, the composite particles (P) are heated in an automatic combustion device to absorb free fluorine into an absorption liquid, and the free fluorine is quantified as fluoride ions by ion chromatography. (2) Next, WO ３ The same procedure as in (1) is repeated to quantify the amount of hardly liberated fluorine. The total value of the two measurements is the amount of fluorine in the composite particles (P).

[0050] Ion chromatography can be performed, for example, using the following apparatus and conditions: Ion chromatography: Integration manufactured by ThermoFisherScientific Automatic combustion apparatus: AQF-100 manufactured by Analytech, Mitsubishi Chemical (now Nitto Seiko) Measurement conditions (combustion, absorption) Heating temperature: inlet 900°C, outlet 1000°C Gas: Ar / O ２ : 200 / 400 ml / min Absorbent: Na ２ CO ３ (3 mM) + H ２ O ２ (900mg / l) (Standard solution: Br － ) 5 to 15 ml Measurement conditions (ion chromatography) Column: AG + AS12A Eluent: Na ２ CO ３ (2.7mM)+NaHCO ３ (0.3mM) (1.5ml / min) Suppressor: ADRS600 (25mA) Injection volume: 100μL

[0051] (Secondary Battery) The secondary battery according to this embodiment may be referred to as "secondary battery (B)" hereinafter. The secondary battery (B) includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode contains the negative electrode active material (N) according to the present disclosure. That is, the negative electrode contains the composite particles (P) as the negative electrode active material. The negative electrode active material (N) and the composite particles (P) have been described above, and therefore a redundant description will be omitted.

[0052] By using the composite particles (P), decomposition of the electrolyte caused by the negative electrode active material can be suppressed, and therefore the secondary battery (B) exhibits a small decrease in discharge capacity due to charge-discharge cycles.

[0053] The secondary battery (B) is typically a non-aqueous electrolyte secondary battery (e.g., a lithium ion secondary battery). In this case, a non-aqueous electrolyte is used as the electrolyte. When the secondary battery (B) is a non-aqueous electrolyte secondary battery, the term "electrolyte" can be read as "nonaqueous electrolyte."

[0054] Known components may be used as the components other than the negative electrode active material (N). Examples of components when the secondary battery (B) is a non-aqueous electrolyte secondary battery are described below. However, the secondary battery (B) is not limited to the following examples.

[0055] (Negative Electrode) The negative electrode includes a negative electrode mixture layer containing a negative electrode active material. The negative electrode may include a negative electrode mixture layer and a negative electrode current collector supporting the negative electrode mixture layer. In one example of a method for forming the negative electrode mixture layer, first, the components of the negative electrode mixture layer are dispersed in a dispersion medium to prepare a negative electrode slurry. Next, the negative electrode slurry is applied to the surface of the negative electrode current collector and dried to form the negative electrode mixture layer. The dried coating may be rolled as necessary. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector.

[0056] The negative electrode mixture layer contains the negative electrode active material (N) as an essential component. The negative electrode mixture layer may contain additives (binders, conductive agents, thickeners, etc.) as optional components. The silicon phase in the composite particles (P) can absorb many lithium ions, contributing to a high capacity of the negative electrode. The content of the composite particles (P) in the negative electrode mixture layer may be 0.5% by mass or more and 50% by mass or less.

[0057] As described above, the negative electrode active material may contain another active material (a negative electrode active material other than the negative electrode active material (N)) that reversibly absorbs and releases lithium ions. As such, a carbon-based active material is preferable. Since the composite particles (P) expand and contract in volume with charge and discharge, a large proportion of the composite particles (P) in the negative electrode active material tends to cause poor contact between the negative electrode active material and the negative electrode current collector with charge and discharge. On the other hand, the combined use of the composite particles (P) and a carbon-based active material makes it possible to achieve both high discharge capacity and excellent cycle characteristics. The proportion of the composite particles (P) in the total of the composite particles (P) and the carbon-based active material is preferably in the range of 0.5 to 15 mass %, more preferably in the range of 1 to 5 mass %.

[0058] Examples of carbon-based active materials include graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon). Graphite is preferred because it has excellent charge / discharge stability and low irreversible capacity. Graphite is a material having a graphite-type crystalline structure. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. One type of carbon-based active material may be used alone, or two or more types may be used in combination.

[0059] As the negative electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet) is used. Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy. The thickness of the negative electrode current collector is not particularly limited. From the viewpoint of balancing the strength and weight of the negative electrode, the thickness of the negative electrode current collector is preferably in the range of 1 to 50 μm, and more preferably in the range of 5 to 20 μm.

[0060] Examples of binders include fluororesin, polyolefin resin, polyamide resin, polyimide resin, vinyl resin, styrene-butadiene copolymer rubber (SBR), polyacrylic acid and its derivatives, etc. Examples of conductive agents include carbon black, conductive fibers, carbon fluoride, organic conductive materials, etc. Examples of thickeners include carboxymethyl cellulose (CMC), CMC salts, polyvinyl alcohol, etc.

[0061] Examples of the dispersion medium for the negative electrode slurry include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), and mixtures thereof.

[0062] (Positive Electrode) The positive electrode includes a positive electrode mixture layer. The positive electrode may include a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector. In one example of a method for forming the positive electrode mixture layer, first, the components of the positive electrode mixture layer are dispersed in a dispersion medium to prepare a positive electrode slurry. Next, the positive electrode slurry is applied to the surface of the positive electrode current collector and dried to form the positive electrode mixture layer. The dried coating may be rolled as necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector.

[0063] The positive electrode mixture layer contains a positive electrode active material as an essential component, and may contain additives (such as a binder, a conductive agent, or a thickener) as optional components.

[0064] As the positive electrode active material, a lithium transition metal composite oxide can be used. Examples of the lithium transition metal composite oxide include Li ａ CoO ２ , Li ａ NiO ２ , Li ａ MnO ２ , Li ａ Co ｂ Ni １－ｂ O ２ , Li ａ Co ｂ M １－ｂ O ｃ , Li ａ Ni １－ｂ M ｂ O ｃ , Li ａ Mn ２ O ４ , Li ａ Mn ２－ｂ M ｂ O ４ , LiMePO ４ , Li ２ MeP.O. ４ Examples of the lithium-ion battery include Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. Mg includes at least one transition element. For example, Mg may include at least one element selected from the group consisting of Mn, Fe, Co, and Ni. Here, 0≦a≦1.2, 0≦b≦0.9, and 2.0≦c≦2.3. The value a, which indicates the molar ratio of lithium, increases or decreases with charge and discharge.

[0065] The binder, conductive agent, and thickener may be the same as those exemplified for the negative electrode. Graphite may be used as the conductive agent for the positive electrode mixture layer.

[0066] The shape and thickness of the positive electrode current collector can be selected from the shape and range corresponding to those of the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0067] (Electrolyte) The electrolyte may be a liquid electrolyte (electrolytic solution), a gel electrolyte, or a solid electrolyte. The liquid electrolyte may be an electrolytic solution (nonaqueous electrolyte) containing a nonaqueous solvent and a salt dissolved in the nonaqueous solvent. The concentration of the salt in the electrolytic solution is, for example, 0.5 mol / L or more and 2 mol / L or less. The electrolyte may contain an additive (for example, a known additive).

[0068] The gel electrolyte may include a salt and a matrix polymer. Alternatively, the gel electrolyte may include a salt, a non-aqueous solvent, and a matrix polymer. The matrix polymer may be a polymer material that absorbs the non-aqueous solvent and gels. The polymer material may be a fluororesin, an acrylic resin, a polyether resin, polyethylene oxide, or the like.

[0069] As the solid electrolyte, for example, a solid electrolyte used in a known all-solid-state lithium ion secondary battery (e.g., an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halide-based solid electrolyte, etc.) may be used.

[0070] A liquid non-aqueous electrolyte can be prepared by dissolving a salt in a non-aqueous solvent. The salt is an electrolyte salt that ionically dissociates in the electrolyte, and may include, for example, a lithium salt. The electrolyte may contain various additives. The electrolyte is usually used in its liquid state, but its fluidity may be limited by a gelling agent or the like.

[0071] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), and vinylene carbonate (VC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0072] As the non-aqueous solvent, cyclic ethers, chain ethers, nitriles such as acetonitrile, amides such as dimethylformamide, etc. may be used.

[0073] Examples of the lithium salt include lithium salts of chlorine-containing acids (LiClO ４ , LiAlCl ４ , LiB １０ Cl １０ etc.), lithium salts of fluorine-containing acids (LiPF ６ , LiBF ４ , LiSbF ６ , LiAsF ６ , LiCF ３ SO ３ , LiCF ３ CO ２ etc.), lithium salt of fluorine-containing acid imide (LiN(CF ３ SO ２ ) ２ , LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(C ２ F ５ SO ２ ) ２Lithium salts such as lithium halides (LiCl, LiBr, LiI, etc.) can be used. One type of lithium salt can be used alone, or two or more types can be used in combination.

[0074] The concentration of the lithium salt in the electrolyte may be 1 mol / L or more and 2 mol / L or less, or 1 mol / L or more and 1.5 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte having excellent ionic conductivity and appropriate viscosity can be obtained.

[0075] (Separator) A separator is usually disposed between the positive electrode and the negative electrode. The separator may be made of a microporous thin film, a woven fabric, a nonwoven fabric, or the like. The separator may be made of polyolefin (such as polypropylene or polyethylene) or other polymers.

[0076] The secondary battery (B) may include an electrode group, an electrolyte, and an exterior housing that houses them. The electrode group includes a positive electrode and a negative electrode, and may further include a separator. The electrode group may be a wound electrode group or a stacked electrode group. A wound electrode group may be formed by winding a positive electrode and a negative electrode with a separator interposed therebetween. A stacked electrode group may be formed by stacking a positive electrode and a negative electrode in one direction with a separator interposed therebetween. The shape of the secondary battery (B) is not limited, and may be cylindrical, prismatic, coin-shaped, button-shaped, laminate-shaped, or the like.

[0077] An example of the composite particle (P) and an example of the secondary battery (B) according to this embodiment will be specifically described below with reference to the drawings. The components described above can be applied to the components of the examples described below. Furthermore, the components of the examples described below can be modified based on the above description. Furthermore, the matters described below may be applied to the above embodiment. Furthermore, in the examples described below, components that are not essential for the composite particle (P) and secondary battery (B) according to the present disclosure may be omitted.

[0078] (Embodiment 1) In Embodiment 1, an example of a negative electrode active material (N) and a composite particle (P) contained therein will be described. FIG. 1 schematically shows a cross section of a negative electrode active material 20 of Embodiment 1. The negative electrode active material 20 includes a composite particle 23. The composite particle 23 includes a carbon phase 21 and a silicon phase (silicon particles) 22 dispersed within the carbon phase 21. The silicon phase of one example of the composite particle 23 is disposed within the pores of the carbon phase 21. From one perspective, it can be considered that the fine silicon phase 22 is dispersed within the matrix of the carbon phase 21.

[0079] As described above, fluorine (not shown) is present on the surface of the silicon phase 22. At least a portion of the surface of the silicon phase 22 may be covered with a layer containing fluorine.

[0080] Second Embodiment In a second embodiment, a prismatic non-aqueous electrolyte secondary battery (secondary battery 10) will be described as an example of a secondary battery (B) according to the present disclosure.

[0081] 1 is a schematic perspective view with a portion cut away of a secondary battery 10. The secondary battery 10 includes a battery case 4 in the shape of a rectangular cylinder with a bottom, and an electrode group 1 and a non-aqueous electrolyte (not shown) housed in the battery case 4.

[0082] The electrode group 1 includes a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator disposed therebetween. The negative electrode current collector of the negative electrode is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector of the positive electrode is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. The periphery of the sealing plate 5 is fitted into the open edge of the battery case 4, and the fitting portion is laser welded. That is, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The sealing plate 5 has an injection hole for a non-aqueous electrolyte. The injection hole is closed with a seal 8 after the non-aqueous electrolyte is injected.

[0083] The negative electrode includes a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector. The negative electrode mixture layer contains the above-described negative electrode active material (N) (composite particles (P)).

[0084] (Additional Notes) The above description discloses the following technologies. (Technology 1) A negative electrode active material for a secondary battery, comprising composite particles, the composite particles containing a carbon phase and a silicon phase dispersed within the carbon phase, and fluorine being present on at least a portion of the surface of the silicon phase. (Technology 2) The negative electrode active material according to Technology 1, wherein the silicon phase is amorphous. (Technology 3) The negative electrode active material according to Technology 1 or 2, wherein the fluorine content in the composite particles is in the range of 0.2 to 10 mass %. (Technology 4) The density of the composite particles is 1.5 g / cm ３ ~2.0 g / cm ３ (Technology 5) The negative electrode active material according to any one of Technologies 1 to 4, wherein a first layer containing fluorine and a second layer containing carbon are laminated in this order on at least a part of the surface of the silicon phase. (Technology 6) The negative electrode active material according to any one of Technologies 1 to 5, wherein the silicon phase has an average diameter of 10 nm or less. (Technology 7) The negative electrode active material according to any one of Technologies 1 to 6, wherein the composite particles have an average single particle fracture strength of 400 MPa or less. (Technology 8) A secondary battery including a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode contains the negative electrode active material according to any one of Technologies 1 to 7.

[0085] EXAMPLES Examples of the present disclosure will be specifically described below, but the present disclosure is not limited to the following examples. In these examples, a plurality of non-aqueous electrolyte secondary batteries were fabricated and evaluated.

[0086] (Battery A1) Battery A1 was produced in the following manner.

[0087] (1) Preparation of Negative Electrode Active Material (Composite Particles) Negative electrode active material (composite particles (P)) was prepared by the following method including the first and second steps. In the first step, a silicon-containing raw material gas was flowed into a reactor heated to 600°C while stirring porous carbon in the reactor. This resulted in the deposition of amorphous silicon inside the pores of the porous carbon. The deposition formed a silicon phase dispersed in the carbon phase (porous carbon). The raw material gas contained monosilane (SiH４ A mixed gas of HCl and a carrier gas (e.g., nitrogen gas) was used. The starting porous carbon (specifically, activated carbon) had an average pore size in the range of 2 to 10 nm and an average particle size (median diameter D50) in the range of 5 to 7 μm.

[0088] In the second step, fluorine was placed on the surface of the silicon phase formed in the first step. Specifically, after the above step, a fluorine-containing raw material gas was flowed into a reactor heated to 600°C while stirring the porous carbon. This resulted in the formation of a fluorine-containing layer and a carbon layer in this order on the amorphous silicon. A mixed gas of fluorocarbon (e.g., tetrafluoroethylene) and a carrier gas (e.g., nitrogen gas) was used as the raw material gas. In this way, composite particles PA1 (negative electrode active material NA1) were obtained.

[0089] The resulting composite particles PA1 (negative electrode active material NA1) were analyzed by the method described above to measure the fluorine content C(F) of the composite particles PA1. The resulting composite particles PA1 were also analyzed by the method described above to measure the density of the composite particles PA1.

[0090] (2) Preparation of Negative Electrode Composite particles PA1 (negative electrode active material NA1) and graphite were mixed in a mass ratio of 5:95 and used as the negative electrode active material. The negative electrode active material, a Na salt of carboxymethyl cellulose, and styrene butadiene rubber were mixed in a mass ratio of 97.5:1:1.5 to obtain a negative electrode mixture. Next, water was added to the negative electrode mixture and stirred to prepare a negative electrode slurry. Next, a 1 m thick film was applied to the surface of copper foil. ２ The negative electrode slurry was applied so that the mass of the negative electrode mixture per electrode was 190 g, and the coating was dried and then rolled. In this way, a copper foil (negative electrode current collector) and negative electrode mixture layers (density: 1.5 g / cm 3 ) formed on both sides of the copper foil were obtained. ３ ) and a negative electrode containing the same.

[0091] (3) Preparation of Positive Electrode A positive electrode mixture was obtained by mixing lithium cobalt oxide, acetylene black, and polyvinylidene fluoride in a mass ratio of 95:2.5:2.5. Next, NMP was added to the positive electrode mixture and stirred to prepare a positive electrode slurry. Next, the positive electrode slurry was applied to the surface of an aluminum foil, and the coating was dried and then rolled. In this way, an aluminum foil (positive electrode current collector) and positive electrode mixture layers (density: 3.6 g / cm ) formed on both sides of the aluminum foil were prepared. ３ ) and a positive electrode containing the same.

[0092] (4) Preparation of non-aqueous electrolyte: LiPF6 was dissolved in a mixed solvent containing ethylene carbonate and diethyl carbonate in a volume ratio of 3:7. ６ A non-aqueous electrolyte (electrolytic solution) was prepared by dissolving the above in a concentration of 1.0 mol / L.

[0093] (5) Fabrication of Secondary Battery A wound electrode assembly was fabricated by winding a positive electrode and a negative electrode, each with a tab attached, with a separator interposed therebetween. The electrode assembly was inserted into an exterior case made of aluminum laminate film and vacuum dried at 105°C for 2 hours. A nonaqueous electrolyte was then injected into the exterior case, and the opening of the exterior case was sealed. Battery A1 was thus obtained.

[0094] The initial discharge capacity DC(0) of Battery A1 was measured. Battery A1 was also subjected to 500 charge / discharge cycles, and the discharge capacity DC(500) of Battery A1 after 500 cycles was measured. The discharge capacity retention rate was calculated using the following formula: Discharge capacity retention rate (%) = 100 × DC(500) / DC(0).

[0095] (Batteries A2 to A5 and Battery C1) Negative electrode active materials NA2 to NA5 were produced using the same method and conditions as those for producing negative electrode active material NA1 (composite particle PA1), except that the amount of tetrafluoroethylene flowing into the reactor in the second step was changed. The second step was not performed in the production of negative electrode active material NC1. The resulting negative electrode active materials were analyzed using the method described above to measure the fluorine content C(F) in the composite particles. Batteries A2 to A5 and Battery C1 were produced using the same method and conditions as those for producing Battery A1, except that negative electrode active materials NA2 to NA5 and NC1 were used instead of negative electrode active material NA1. For the produced batteries, the initial discharge capacity DC(0) and the discharge capacity DC(500) after 500 cycles were measured using the same method as the evaluation method for Battery A1. The discharge capacity retention rate was then calculated using the above formula.

[0096] The evaluation results are shown in Table 1. It is preferable that the discharge capacity retention rate and the initial discharge capacity DC(0) are high.

[0097]

[0098] Batteries A1 to A5 are secondary batteries (B) according to the present disclosure. Battery C1 is a comparative example. Analysis revealed that fluorine was present on the surface of the silicon phase in the composite particles of batteries A1 to A5.

[0099] As shown in Table 1, the discharge capacity retention rates of batteries A1 to A5, in which fluorine was present on the surface of the silicon phase, were high. On the other hand, the initial discharge capacity decreased as the fluorine content C(F) increased. In order to suppress the decrease in initial discharge capacity, the fluorine content C(F) is preferably 10% by mass or less, and more preferably 5.0% by mass or less (e.g., 1.0% by mass or less). In order to achieve a high discharge capacity retention rate, the fluorine content C(F) is preferably 0.2% by mass or more, more preferably 1.0% by mass or more, and even more preferably 5.0% by mass or more.

[0100] The present disclosure can be used for negative electrode active materials for secondary batteries and secondary batteries. Although the present invention has been described with reference to presently preferred embodiments, such disclosure should not be interpreted as limiting. Various modifications and alterations will undoubtedly become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. Therefore, the appended claims should be construed to cover all modifications and alterations without departing from the true spirit and scope of the present invention.

[0101] 1: Electrode group 10: Secondary battery 20: Negative electrode active material 21: Carbon phase 22: Silicon phase 23: Composite particle

Claims

1. A negative electrode active material for a secondary battery, comprising composite particles, the composite particles containing a carbon phase and a silicon phase dispersed within the carbon phase, and fluorine being present on at least a portion of a surface of the silicon phase.

2. The negative electrode active material according to claim 1, wherein the silicon phase is amorphous.

3. The negative electrode active material according to claim 1, wherein the fluorine content in the composite particles is in the range of 0.2 to 10 mass %.

4. The density of the composite particles is 1.5 g / cm ３ ~2.0g / cm ３ The negative electrode active material according to claim 1 , wherein the content of the negative electrode active material is in the range of 5. The negative electrode active material according to claim 1, wherein a first layer containing fluorine and a second layer containing carbon are laminated in this order on at least a portion of the surface of the silicon phase.

6. The negative electrode active material according to claim 1, wherein the average diameter of the silicon phase is 10 nm or less.

7. The negative electrode active material according to claim 1, wherein the composite particles have an average single particle breaking strength of 400 MPa or less.

8. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode contains the negative electrode active material according to any one of claims 1 to 7.