Negative electrode active material for secondary battery, and secondary battery

Abstract

A negative electrode active material (20) for a secondary battery disclosed herein comprises composite particles (23). The composite particles (23) contain a lithium silicate phase (21) and a silicon phase (22) dispersed in the lithium silicate 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 lithium silicate phase and a silicon phase dispersed within the lithium silicate phase, and fluorine being present on at least a portion of the 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 lithium silicate phase and a silicon phase dispersed within the lithium silicate 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 lithium silicate phase and a silicon phase (silicon particles) have been proposed as a negative electrode active material. 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 a negative electrode active material, 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 cycling. 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 forms 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 lithium silicate phase content and the silicon phase content may be 80% by mass or more, or 90% by mass or more. The lithium silicate phase content in the composite particle (P) may be in the range of 40 to 80% by mass (e.g., 50 to 70% by mass). By setting the lithium silicate phase content in the range of 40 to 80% by mass, high charge / discharge efficiency and cycle characteristics can be achieved. The silicon phase content in the composite particle (P) may be in the range of 40 to 80% by mass (e.g., 50 to 70% by mass). By setting the silicon phase content in the range of 40 to 80% by mass, it is possible to achieve both high discharge capacity and high cycle characteristics.

[0016] The ratio C(S) / C(LS) of the silicon phase content C(S) (mass%) in the composite particle (P) to the lithium silicate phase content C(LS) (mass%) in the composite particle (P) may be 0.2 or more, or 0.6 or more, or may be 1.5 or less, or 0.9 or less.

[0017] The composite particles (P) may contain phases other than the lithium silicate 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 carbon phase.

[0018] The fluorine (fluorine atom) content C(F) in the composite particle (P) may be 0.1% by mass or more, or 0.2% by mass or more. The content C(F) may be 5.0% by mass or less, or 3.0% by mass or less. The fluorine content C(F) may be in the range of 0.1 to 5.0% by mass, 0.2 to 5.0% by mass, 1.0 to 5.0% by mass, or 2.0 to 5.0% by mass. In any of these ranges, the upper limit may be 3.0% by mass, 2.0% by mass, 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 2.0% by mass. By setting the fluorine content C(F) to 0.2% by mass or more, the capacity retention rate during charge-discharge cycles can be particularly improved. By setting the fluorine content C(F) to 2.0% by mass or less, the decrease in initial discharge capacity can be suppressed. The "fluorine content C(F) in the composite particle (P)" may be read as the "fluorine content C(F) present in the vicinity of the surface of the silicon phase in the composite particle (P)." Here, "vicinity of the surface of the silicon phase" refers to a region at a distance of 0 to 10 nm from the surface of the silicon phase. The fluorine content C(F) can be measured using ion chromatography, which will be described later.

[0019] The lithium silicate phase is Li ２ Si ２ O ５ , Li ２ SiO ３ , and Li ４ SiO ４ The lithium silicate phase may contain at least one selected from the group consisting of Li ２ Si ２ O ５ , Li ２ SiO ３ , and Li ４ SiO ４ Among these, Li ２ Si ２ O ５ has an advantage that it is compatible with binders and its use improves cycle characteristics.

[0020] The average diameter D(Si) of the silicon phase may be 10 nm or more, or 20 nm or more, or 50 nm or less, or 40 nm or less. The average diameter D(Si) of the silicon phase may be in the range of 10 nm to 50 nm, 20 nm to 50 nm, or 30 nm to 50 nm. In any of these ranges, the upper limit may be 40 nm, 30 nm, or 20 nm, as long as the lower limit is not equal to or greater than the upper limit. The average diameter D(Si) of the silicon phase may be in the range of 10 nm to 50 nm. By setting the average diameter D(Si) to 10 nm or more, oxidation of the silicon phase can be suppressed, and charge / discharge efficiency can be improved. By setting the average diameter D(Si) to 50 nm or less, cycle characteristics are improved.

[0021] The silicon phase dispersed within the lithium silicate phase can be regarded as silicon particles (particulate silicon phase) dispersed within the lithium silicate phase. The average diameter D(Si) of the silicon phase in the cross section of the composite particle (P) is measured 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.

[0022] The average particle size of the composite particles (P) may be 1 μm or more, or 4 μm or less, or 25 μm or less, or 15 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.

[0023] The negative electrode active material (N) may further include a coating layer that coats at least a portion of the surface of the composite particle (P). The coating layer contains a carbon material. The presence of the coating layer containing a carbon material on the surface of the particle (P) improves the electronic conductivity of the composite particle (P). The coating layer also prevents electrolytes from penetrating the pores (voids) of the composite particle (P), thereby suppressing side reactions. The proportion of the carbon material in the total of the composite particle (P) and the coating layer is preferably 3% by mass or more and 5% by mass or less. In order to increase the conductivity of the coating layer, a conductive carbon material is preferably used as the carbon material constituting the coating layer. In other words, the coating layer is preferably conductive. A method for forming the coating layer will be described later. The composite particle (P) and the coating layer can also be collectively considered as the composite particle (P').

[0024] The proportion of the mass of the coating layer to the total mass of the composite particles (P) and the coating layer may be 2% by mass or more, or 4% by mass or more, and may be 10% by mass or less, or 5% by mass or less.

[0025] The lithium silicate phase is composed of a compound containing lithium (Li), silicon (Si), and oxygen (O). The atomic ratio of O to Si in the lithium silicate phase, O / Si, is, for example, greater than 2 and less than 4. When the O / Si ratio is greater than 2 and less than 4 (z in the formula described below is 0<z<2), this is advantageous in terms of the stability and lithium ion conductivity of the lithium silicate phase. Preferably, the O / Si ratio is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase, Li / Si, is, for example, greater than 0 and less than 4.

[0026] Lithium silicate is Li ２ｚ SiO ２＋ｚ It may have a composition expressed as (0<z<2). From the viewpoints of stability, ease of preparation, lithium ion conductivity, etc., it is preferable that 0<z≦1 is satisfied, and it is more preferable that z=1 / 2. For example, when z=1, lithium silicate is Li ２ SiO ３ When z = 1 / 2, Li ２ Si ２ O５ In a preferred example, Li in the lithium silicate phase ２ Si ２ The O5 content is 50 mass % or more (for example, 70 mass % or more or 90 mass % or more).

[0027] The lithium silicate phase may contain an element M other than lithium, silicon, and oxygen. When the lithium silicate phase contains the element M, it is possible to improve the chemical stability and lithium ion conductivity of the lithium silicate phase. Furthermore, when the lithium silicate phase contains the element M, it may be possible to suppress side reactions caused by contact between the lithium silicate phase and the electrolyte.

[0028] The element M may include at least one element selected from the group consisting of alkali metal elements (excluding lithium) and Group 2 elements (Group 2 elements of the long period periodic table), or may be the at least one element.

[0029] By including an alkali metal element other than lithium in the lithium silicate phase, the lithium silicate phase is less likely to crystallize and has higher fluidity at high temperatures. Therefore, in the heat treatment step, the lithium silicate phase is more likely to fill the gaps between the silicon particles, making it easier to produce dense composite particles (P). As the alkali metal element, sodium and / or potassium may be used because they are inexpensive.

[0030] Adding a Group 2 element to the lithium silicate phase can suppress the elution of alkali metals from the lithium silicate phase. Therefore, when the lithium silicate phase contains a Group 2 element, the slurry viscosity is easily stabilized when preparing a slurry containing a negative electrode active material. Furthermore, the need for treatment (e.g., acid treatment) to neutralize the alkaline component of the composite particles (P) is reduced. The Group 2 element may be Ca and / or Mg. Ca is preferred because it can increase the Vickers hardness of the lithium silicate phase and further improve the cycle characteristics.

[0031] The lithium silicate phase may contain at least one element selected from the group consisting of boron (B), aluminum (Al), zirconium (Zr), niobium (Nb), tantalum (Ta), vanadium (V), lanthanum (La), yttrium (Y), titanium (Ti), phosphorus (P), bismuth (Bi), zinc (Zn), tin (Sn), lead (Pb), antimony (Sb), cobalt (Co), erbium (Er), fluorine (F), and tungsten (W) as the element M. From the viewpoint of resistance to electrolytes and structural stability of the lithium silicate phase, it is preferable that the element M contains at least one element selected from the group consisting of Zr, Ti, P, Al, and B.

[0032] The lithium silicate phase may contain a rare earth element as element M. The inclusion of a rare earth element in the lithium silicate phase can improve the charge-discharge efficiency at the initial stage of the charge-discharge cycle. The rare earth element may be at least one selected from the group consisting of scandium (Sc), yttrium (Y), and lanthanoid elements. Note that lanthanum (La), yttrium (Y), and erbium (Er) are rare earth elements. The lithium silicate phase may contain at least one rare earth element selected from the group consisting of cerium (Ce), praseodymium (Pr), and neodymium (Nd). Lanthanum is preferred in terms of improving the lithium ion conductivity of the lithium silicate phase. The proportion of lanthanum in the rare earth elements contained in the lithium silicate phase is preferably 90 atomic % or more and 100 atomic % or less.

[0033] Boron (B) as element M has the advantage of having a low melting point and increasing the fluidity of the lithium silicate phase during sintering. Al, Zr, and La as element M can increase the hardness of the lithium silicate phase while maintaining ionic conductivity. Zr, Ti, P, Al, and B as element M have the effect of increasing resistance to electrolytes and the structural stability of the lithium silicate phase.

[0034] The lithium silicate phase may contain trace amounts of elements such as iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), and molybdenum (Mo).

[0035] The element M may form a compound. Examples of the compound include a silicate of the element M and an oxide of the element M. In the lithium silicate phase, the content of the element M relative to the total amount of elements other than oxygen may be 1 mol % or more and 40 mol % or less.

[0036] The silicon phase dispersed within the lithium silicate phase is a phase of simple silicon (Si) and is composed of a single or multiple crystallites. The silicon phase may be a particulate phase. From the viewpoint of reducing the volume change due to expansion and contraction of the silicon phase (silicon particles) during charging and discharging and facilitating improved cycle characteristics, the crystallite size of the silicon phase may be 50 nm or less, preferably 20 nm or less (e.g., 10 nm or less). The crystallite size of the silicon phase may be 5 nm or more. The crystallite size of the silicon phase is calculated using the Scherrer equation from the half-width of the diffraction peak assigned to the Si(111) plane in the X-ray diffraction (XRD) pattern.

[0037] 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 a negative electrode active material other than the negative electrode active material (N). The other negative electrode active material may be a material capable of reversibly absorbing and releasing lithium ions. The other negative electrode active material may be a known negative electrode active material. The proportion of the negative electrode active material (N) in the negative electrode active material of the secondary battery (B) may be 0.5 mass% or more, or 1.0 mass% or more. The proportion may be 30 mass% or less, 20 mass% or less, or 15 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 material include carbon materials (such as graphite), silicon oxide, and composite materials of a carbon phase and a silicon phase.

[0038] (Method for producing composite particles (P)) A manufacturing method (M), which is an example of a method for producing composite particles (P), will be described below. However, composite particles (P) may be produced by a manufacturing method other than the manufacturing method (M) described below. The matters described for composite particles (P) can be applied to the manufacturing method below. The matters described for the manufacturing method (M) below may also be applied to composite particles (P). The manufacturing method (M) includes the following first to fourth steps.

[0039] (First Step) The first step is a step of producing silicon microparticles having fluorine disposed on at least a portion of the surface. For example, a fluorine compound may be added to silicon particles and then pulverized to reduce the size of the silicon particles and to dispose fluorine on the surface of the micro silicon particles. The method for pulverizing the silicon particles is not particularly limited, and known methods may be used. For example, the silicon particles may be pulverized using a ball mill or the like.

[0040] The diameter of the silicon fine particles can be changed by changing the pulverization conditions, and the fluorine content in the composite particles (P) can be changed by changing the amount of the fluorine compound added during pulverization.

[0041] The fluorine compound to be added may be an organic fluorine compound or an inorganic fluorine compound. Examples of organic fluorine compounds include fluoroethylene carbonate (FEC), hexafluoro-2-propanol (HFIP), fluorobenzene, etc. Preferred examples of organic fluorine compounds include fluoroethylene carbonate (FEC). By using a liquid fluorine compound, it is possible to distribute fluorine evenly on the surface of the fine silicon particles. Therefore, it is preferable that the melting point of the fluorine compound is low (for example, 30°C or less or 25°C or less). Fluoroethylene carbonate is preferred because it has a melting point of less than 25°C.

[0042] Examples of inorganic fluorine compounds include lithium bis(trifluorosulfonyl)imide, lithium fluoride, magnesium fluoride, sodium fluoride, calcium fluoride, and the like.

[0043] (Second Step) The second step is a step of preparing a composite intermediate by combining lithium silicate particles with the fine silicon particles obtained in the first step. At this time, the fine silicon particles are pulverized to produce a silicon phase. The silicon phase is dispersed within the lithium silicate phase (matrix).

[0044] The method for producing the composite intermediate is not particularly limited, and known methods may be used. For example, the composite intermediate may be produced by grinding and mixing fine silicon particles and lithium silicate particles. Specifically, the composite intermediate may be produced by grinding a mixture of lithium silicate particles and fine silicon particles while applying shear force to the mixture. A grinding device such as a ball mill can be used to grind and mix the mixture. The grinding and mixing may be performed in a dry manner. Alternatively, a liquid may be added to the mixture and grind and mix in a wet manner. The liquid added to the mixture is not particularly limited, and alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, metal alkoxides, etc. may be used.

[0045] The diameter of the particles used as raw materials and the conditions for pulverization and mixing are selected depending on the physical properties of the desired composite particles (P).

[0046] The lithium silicate particles may be commercially available particles or may be prepared by a known method. An example of a method for preparing a lithium silicate phase is described below. First, a raw material containing Si and a raw material containing Li are mixed in a predetermined ratio to obtain a raw material mixture. At this time, a raw material containing element M is also mixed into the raw material mixture, if necessary. Next, the obtained raw material mixture is melted, and the melt is passed between two metal rolls to form flakes, thereby obtaining lithium silicate. The raw material mixture may also be fired at a temperature below the melting point without being melted, and lithium silicate may be synthesized by a solid-state reaction.

[0047] The Si-containing raw material may be silicon oxide (e.g., SiO2). The Li-containing raw material and the M-containing raw material may be carbonates, oxides, hydroxides, hydrides, nitrates, sulfates, etc. of lithium and M, respectively. Of these, carbonates, oxides, and hydroxides are preferably used.

[0048] In the second step, the composite intermediate may be produced by mixing the fine silicon particles obtained in the first step with lithium silicate nanoparticles without using a pulverizer. The lithium silicate nanoparticles can be produced by a known method such as a gas phase method (e.g., a plasma method) or a liquid phase method (e.g., a liquid phase reduction method).

[0049] (Step 3) The third step is a step of heat-treating the composite intermediate to produce a sintered body containing a lithium silicate phase and a silicon phase dispersed within the lithium silicate phase. In one example of Step 3, the composite intermediate is fired while applying pressure to the composite intermediate using a method such as hot pressing, thereby producing a sintered body. The composite intermediate may be fired in an inert gas atmosphere (such as an argon gas atmosphere or a nitrogen gas atmosphere). The firing temperature may be in the range of 450°C to 1000°C. A firing temperature within this range facilitates dispersion of minute silicon phases within the low-crystalline lithium silicate phase. During sintering, the lithium silicate phase softens and migrates to fill the gaps between the silicon particles. As a result, a dense block-shaped sintered body can be obtained, with the lithium silicate phase forming a sea portion and the silicon phase (silicon particles) forming islands. The firing temperature is preferably 550°C or higher and 900°C or lower, more preferably 650°C or higher and 850°C or lower. The firing time may be 1 hour or more and may be 10 hours or less.

[0050] (Step 4) Step 4 is a step of pulverizing the sintered body obtained in Step 3. By Step 4, composite particles (P) having a predetermined diameter can be obtained. By appropriately selecting the pulverization conditions, composite particles (P) having a predetermined average particle size can be obtained.

[0051] If necessary, after the fourth step, a step of forming a coating layer on the surface of the composite particles (P) is performed. The method for forming the coating layer is not particularly limited, and known methods may be used. The coating layer may be formed by mixing a raw material conductive carbon material with the composite particles and firing the mixture. The raw material conductive carbon material is carbonized by firing, forming a conductive coating layer containing the conductive carbon material. Examples of raw materials for the conductive carbon material include coal tar pitch, petroleum pitch, and phenolic resin. The mixture of the raw carbon material and the composite particles may be fired in an inert gas atmosphere (such as an argon gas atmosphere or a nitrogen gas atmosphere). The firing temperature may be in the range of 450°C to 1000°C. The firing time may be 1 hour or more and 10 hours or less.

[0052] The composition of the composite particles (P) can be determined by the analytical method described below. When analyzing the composite particles (P) 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 washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the electrolyte component, and then dried. Next, a cross-section polisher 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.

[0053] (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

[0054] (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).

[0055] 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

[0056] (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 remaining in the solution is filtered off. 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.

[0057] 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).

[0058] (ONH Analysis Method) The amount of oxygen in the composite particles (P) can be analyzed by ONH analysis method. Specifically, 1 g of the composite particles (P) is placed in a graphite crucible together with a flux, and melted and decomposed by resistance heating in an impulse furnace in a helium stream, and the oxygen and other elements in the composite particles (P) are quantified. Oxygen can be detected as carbon dioxide using an infrared detector. Hydrogen can be detected as water using an infrared detector. Nitrogen can be detected as nitrogen using a thermal conductivity detector. In this way, the oxygen and other elements in the composite particles (P) can be quantified. For example, an analyzer (ONH-2000) manufactured by ELTRA can be used as the analyzer.

[0059] The amounts of the lithium silicate phase and the silicon phase in the composite particles (P) can be distinguished and quantified by 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 can be a mixture containing a lithium silicate phase and a silicon phase in a predetermined ratio with known Si contents. An example of an Si-NMR apparatus and conditions is shown below.

[0060] (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

[0061] (Analysis of Carbon and Sulfur) The amount of carbon coating the surface of the composite particles (P) can be measured using a carbon / sulfur analyzer. Specifically, first, 0.1 g of a sample of the composite particles (P) is placed in a magnetic crucible and heated to a high temperature in an oxygen stream to oxidize it, thereby oxidizing the carbon (C) in the sample with carbon dioxide gas (CO２ These gases are sent to an infrared detector along with oxygen to measure CO ２ The infrared absorption of CO and CO is measured. The carbon concentration in the sample is then determined using a calibration curve prepared using standard substances with known carbon contents. The analyzer used may be, for example, a carbon / sulfur analyzer (EMIA-920V) manufactured by Horiba, Ltd.

[0062] (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.

[0063] 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.

[0064] 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."

[0065] 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.

[0066] (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.

[0067] 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.

[0068] 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 %.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] Examples of the dispersion medium for the negative electrode slurry include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), and mixtures thereof.

[0073] (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.

[0074] 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.

[0075] 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 NiO２ , 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.

[0076] 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.

[0077] 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.

[0078] (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).

[0079] 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.

[0080] As the solid electrolyte, for example, a solid electrolyte used in a known all-solid-state lithium ion secondary battery (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halide-based solid electrolyte, etc.) may be used.

[0081] 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.

[0082] 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.

[0083] As the non-aqueous solvent, cyclic ethers, chain ethers, nitriles such as acetonitrile, amides such as dimethylformamide, etc. may be used.

[0084] 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 ) ２ Lithium 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.

[0085] 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.

[0086] (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.

[0087] 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.

[0088] 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.

[0089] (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 and a coating layer 26 formed on the surface of the composite particle 23. The composite particle 23 includes a lithium silicate phase 21 and a silicon phase (silicon particles) 22 dispersed within the lithium silicate phase 21. The composite particle 23 has a sea-island structure in which fine silicon phases 22 are dispersed within the lithium silicate phase 21 (matrix). The surface of the composite particle 23 is covered with a conductive coating layer 26. Note that the negative electrode active material 20 does not necessarily include the coating layer 26.

[0090] As described above, fluorine is present on the surface of the silicon phase 22 (the interface between the silicon phase 22 and the lithium silicate phase 21). At least a portion of the surface of the silicon phase 22 may be covered with a layer containing fluorine. At least a portion of the surface of the silicon phase 22 may be coated with fluorine.

[0091] 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.

[0092] 2 is a schematic perspective view with a portion cut away of secondary battery 10. 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 nonaqueous electrolyte (not shown) housed in battery case 4.

[0093] 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.

[0094] 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)).

[0095] (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 lithium silicate phase and a silicon phase dispersed within the lithium silicate 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 fluorine content in the composite particles is in the range of 0.2 to 2.0 mass %. (Technology 3) The lithium silicate phase is Li ２ Si ２ O ５ , Li ２ SiO ３ , and Li ４ SiO ４The negative electrode active material according to Technology 1 or 2, comprising at least one selected from the group consisting of: (Technology 4) The negative electrode active material according to any one of Technology 1 to 3, wherein the average diameter of the silicon phase is in the range of 10 nm to 50 nm. (Technology 5) The negative electrode active material according to any one of Technology 1 to 4, further comprising a coating layer that coats at least a part of the surface of the composite particle, the coating layer containing a carbon material. (Technology 6) 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 Technology 1 to 5.

[0096] 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.

[0097] (Battery A1) Battery A1 was produced in the following manner.

[0098] (1) Preparation of Negative Electrode Active Material (Composite Particles) A ​​negative electrode active material (N) was prepared by the following method. First, silicon dioxide particles and Li ２ CO ３ The mixture was then calcined in air at 950°C for 10 hours to obtain lithium silicate, which was then pulverized to obtain lithium silicate particles (average particle size: 100 nm).

[0099] (1-1) First Step: Fluoroethylene carbonate (FEC) was added to silicon particles (average particle size: 10 μm). The silicon particles to which FEC had been added were then pulverized using a dry bead mill. The pulverization was continued until the average particle size of the silicon particles reached 30 nanometers. In this way, fine silicon particles having fluorine disposed on at least a portion of their surfaces were produced.

[0100] (1-2) Step 2 The lithium silicate particles and fine silicon particles produced by the above steps were mixed. In the mixture, the mass ratio of the lithium silicate particles to the fine silicon particles was 42:58. Next, the mixture was filled into a pot (SUS, volume: 500 mL) of a planetary ball mill (Fritsch, P-5), 24 SUS balls (diameter: 20 mm) were placed in the pot, the lid was closed, and the mixture was pulverized in an inert atmosphere at 200 to 300 rpm for 25 hours. In this way, a composite intermediate in which fine silicon particles were dispersed in lithium silicate was produced.

[0101] (1-3) Step 3 Next, the composite intermediate obtained in Step 2 was fired in an inert atmosphere to obtain a sintered body. The firing was carried out in a state where pressure was applied to the composite intermediate using a hot press.

[0102] (1-4) Fourth Step Next, the sintered body obtained in the third step was pulverized, and then the sintered body was sieved using a mesh to obtain composite particles PA1 having an average particle size (median diameter) of 6 μm. The main composition of the lithium silicate phase in the composite particles determined by the above-mentioned method was Li ２ Si ２ O ５ It was.

[0103] (1-5) Coating Layer Formation Step Next, the composite particles PA1 obtained in the fourth step and coal tar pitch were mixed in a mass ratio of 95:5 to obtain a mixture. The mixture was then fired at 800°C in an argon atmosphere. The coal tar pitch was converted to amorphous carbon by firing, resulting in the formation of a conductive coating layer covering at least a portion of the surface of the composite particles. In this way, a negative electrode active material NA1 containing the composite particles and the coating layer was obtained.

[0104] The ratio of the mass of the coating layer to the total mass of the composite particle and the coating layer was 3 mass %, which was determined by measuring the mass of the particle before and after forming the coating layer.

[0105] The obtained negative electrode active material NA1 was analyzed by the method described above to measure the fluorine content C(F) in the composite particles PA1.

[0106] (2) Preparation of Negative Electrode Negative electrode active material NA1 and graphite were mixed in a mass ratio of 5:95 and used as the negative electrode active material. This 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 a negative electrode mixture layer (density: 1.5 g / cm 3 ) formed on both sides of the copper foil were obtained. ３ ) and a negative electrode containing the same.

[0107] (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, N-methyl-2-pyrrolidone 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.

[0108] (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.

[0109] (5) Fabrication of Secondary Battery A wound electrode group was fabricated by winding a positive electrode and a negative electrode, each with a tab attached, with a separator interposed therebetween. The electrode group 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. In this way, a battery A1 (nonaqueous electrolyte secondary battery) was obtained.

[0110] 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).

[0111] (Batteries A2 to A4 and Battery C1) Negative electrode active materials NA2 to NA4 and NC1 were prepared using the same method and conditions as those for preparing negative electrode active material NA1, except that the amount of fluoroethylene carbon added to the silicon particles in the first step was changed. In preparing negative electrode active material NC1, fluoroethylene carbon was not added. The resulting negative electrode active materials were analyzed using the method described above to measure the fluorine content C(F) of the composite particles. Batteries A2 to A4 and Battery C1 were prepared using the same method and conditions as those for preparing Battery A1, except that negative electrode active materials NA2 to NA4 and NC1 were used instead of negative electrode active material NA1. For the prepared batteries, the initial discharge capacity DC(0) and the discharge capacity DC(500) after 500 cycles were measured using the same method as that for evaluating Battery A1. The discharge capacity retention rate was then calculated using the above formula.

[0112] 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.

[0113]

[0114] Batteries A1 to A4 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 A4.

[0115] As shown in Table 1, the discharge capacity retention rates of batteries A1 to A4, 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 2.0 mass% or less, and more preferably 1.0 mass% or less. In order to achieve a high discharge capacity retention rate, the fluorine content C(F) is preferably 0.2 mass% or more, more preferably 1.0 mass% or more, and even more preferably 2.0 mass% or more.

[0116] 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.

[0117] 1: Electrode group 10: Secondary battery 20: Negative electrode active material 21: Lithium silicate phase 22: Silicon phase 23: Composite particle 26: Coating layer

Claims

1. A negative electrode active material for a secondary battery, comprising composite particles, the composite particles containing a lithium silicate phase and a silicon phase dispersed within the lithium silicate 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 fluorine content in the composite particles is in the range of 0.2 to 2.0 mass %.

3. The lithium silicate phase is Li ２ S ２ O ５ , Li ２ SiO ３ , and Li ４ SiO ４ The negative electrode active material according to claim 1 , comprising at least one selected from the group consisting of:

4. The negative electrode active material according to claim 1, wherein the average diameter of the silicon phase is in the range of 10 nm to 50 nm.

5. The negative electrode active material according to claim 1, further comprising a coating layer that covers at least a portion of the surface of the composite particle, the coating layer containing a carbon material.

6. 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 5.

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