Non-aqueous electrolyte secondary battery

Abstract

A non-aqueous electrolyte secondary battery (10) disclosed herein includes: an electrode group (14) including a positive electrode (11), a negative electrode (12), and a separator (13); and a non-aqueous electrolyte. The negative electrode (12) includes a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector. The activation energy at 100-135°C of the separator 13 measured via dynamic viscoelasticity measurement is 400-700 kJ / mol.

Description

Nonaqueous electrolyte secondary battery

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

[0002] Silicon-containing materials are capable of absorbing more lithium ions than carbon materials (such as graphite), and therefore their use as negative electrode active materials for non-aqueous electrolyte secondary batteries has been proposed.

[0003] Claim 1 of Patent Document 1 (International Publication No. 2016 / 035290) states, "Li ２ｚ SiO （２＋ｚ） (z<2) and silicon particles dispersed in the lithium silicate phase.

[0004] International Publication No. 2016 / 035290

[0005] However, silicon-containing materials undergo large expansion and contraction during charge and discharge. Therefore, when a silicon-containing material is used as a negative electrode active material, the pressure inside the electrode plate increases with charge and discharge. As a result, as charge and discharge cycles progress, a portion of the separator is crushed, resulting in greater variation in the depth of charge within the electrode assembly. As a result, the cycle capacity retention rate is likely to decrease. Therefore, there is a demand for a battery that uses a negative electrode active material that can achieve high capacity and has a high cycle capacity retention rate. Furthermore, there is a demand for non-aqueous electrolyte secondary batteries that can suppress temperature rise during abnormal conditions. In this context, one object of the present disclosure is to provide a non-aqueous electrolyte secondary battery that has a high capacity and / or cycle capacity retention rate and a small temperature rise during a specified abnormal condition.

[0006] One aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery including: an electrode group including a positive electrode, a negative electrode, and a separator; and a non-aqueous electrolyte, wherein the negative electrode includes a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector; and the separator has an activation energy of 400 kJ / mol or more and 700 kJ / mol or less at 100 to 135°C as measured by dynamic viscoelasticity measurement.

[0007] According to the present disclosure, a non-aqueous electrolyte secondary battery having a high capacity and / or cycle capacity retention rate and a small temperature rise under a predetermined abnormal condition can be obtained. 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.

[0008] 1 is a diagram schematically illustrating an example of a manufacturing apparatus for a separator used in a nonaqueous electrolyte secondary battery according to the present disclosure. 2 is a diagram schematically illustrating an example of a part of the manufacturing apparatus shown in FIG. 1. 3 is a cross-sectional view schematically illustrating an example of a nonaqueous electrolyte secondary battery according to the present disclosure.

[0009] Below, embodiments according to the present disclosure will be described using examples, but the embodiments according to the present disclosure are not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and other materials may be applied as long as the invention according to the present disclosure can be implemented. In this specification, the expression "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.

[0010] (Nonaqueous Electrolyte Secondary Battery) The nonaqueous electrolyte secondary battery according to this embodiment may be referred to hereinafter as a "nonaqueous electrolyte secondary battery (B)" or a "secondary battery (B)." The secondary battery (B) includes a nonaqueous electrolyte and an electrode group including a positive electrode, a negative electrode, and a separator. The negative electrode includes a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector. The activation energy of the separator measured by dynamic viscoelasticity measurement at 100 to 135°C is 400 kJ / mol or more and 700 kJ / mol or less. The activation energy of the separator measured by dynamic viscoelasticity measurement at 100 to 135°C may be referred to hereinafter as "activation energy (E)."

[0011] As a result of investigation, the present inventors have newly discovered that by using the separator, it is possible to obtain a nonaqueous electrolyte secondary battery that has high capacity and / or cycle capacity retention rate and that exhibits small temperature rise under predetermined abnormal conditions. The present disclosure is based on this new finding. Examples of predetermined abnormalities include abnormalities that result in a state equivalent to a forced short-circuit test using an external resistor.

[0012] Dynamic viscoelasticity can be measured using a viscoelasticity measuring device. Specifically, it can be measured using a viscoelasticity measuring device (DMS6100) manufactured by Seiko Instruments Inc. The measurement is performed by clamping a 10 mm wide separator between two chucks. At this time, the separator is clamped between the two chucks so that the MD direction of the separator is parallel to the direction connecting the two chucks. The strain amplitude is 10 μm (0.04%). The temperature is increased from 25° C. to 160° C. at a rate of 1° C. / min. During the temperature increase process, strain is applied so that a cycle of strain frequencies of 10 Hz, 5 Hz, 1 Hz, and 0.5 Hz is repeated. Dynamic viscoelasticity is measured under the above conditions.

[0013] (Method of Calculating Activation Energy (E)) The activation energy (E) can be determined by the following procedure. (1) The frequency dependence of the dynamic viscoelasticity of the separator is measured at different temperatures. Specifically, the frequency dependence of the dynamic viscoelasticity is measured at 10°C intervals from 30°C to 130°C and at 135°C. This measurement determines the storage modulus of the separator. (2) Next, the measurement results at each temperature are plotted as curves on a graph where the X axis is the logarithm of frequency and the Y axis is the logarithm of storage modulus. Next, each curve is shifted in the frequency direction so that they overlap, creating a master curve. At this time, the frequency shift amount is recorded for the curve at each temperature. (3) Next, an Arrhenius plot is drawn by plotting the relationship between the temperature and the frequency shift amount recorded in (2) above on a graph where the X axis is the reciprocal of temperature and the Y axis is the logarithm of the shift amount. At this time, a line is drawn for a region below temperature T and a region above temperature T, with a temperature T of approximately 100°C as the boundary. The slope of the line in the region of 100° C. or higher and 135° C. or lower is calculated. From the slope of the line, the activation energy (E) of the separator in the range of 100 to 135° C. is calculated based on the Arrhenius equation.

[0014] The temperature range of 100°C to 135°C is near or below the melting point of the separator material (e.g., polyethylene), and the activation energy in this range is thought to be related to the thermal motion of molecular chains within the crystalline phase. Crystallinity correlates with the strength of the polymer. Therefore, the activation energy (E) determined by the above procedure is thought to indicate the magnitude of dynamic viscoelasticity (i.e., the magnitude of creep resistance). For the separator of secondary battery (B), it is important that the activation energy (E) is 400 kJ / mol or more and 700 kJ / mol or less.

[0015] (Separator) A porous film can be used for the separator. The porous film is not a film formed of fibers, but a film having many pores. A polymer can be used as the material for the separator. The separator may be a separator whose main component is polyolefin. Unless otherwise specified in this specification, "main component" means a content of 50% by mass or more. The separator may be a separator whose main component is polyethylene. The polyethylene content in the separator may be 50% by mass or more, 90% by mass or more, or 95% by mass or more. The separator may be made of polyethylene. A porous film made of polyethylene may be used for the separator. The following mainly describes an example in which the separator is made of polyethylene.

[0016] High-density polyethylene, which has high crystallinity, is preferably used as the polyethylene. The density of the polyethylene (high-density polyethylene) may be in the range of 0.93 to 0.97 (for example, in the range of 0.95 to 0.97). By using polyethylene with high crystallinity, it is possible to increase the activation energy (E) of the separator. The crystallinity of the polyethylene can be changed by the film-forming method and film-forming conditions. The same applies to polyolefins other than polyethylene.

[0017] The thickness of the separator may be 3 μm or more, or 5 μm or more, or 15 μm or less, or 13 μm or less. By making the thickness of the separator 3 μm or more, reliability can be improved. By making the thickness of the separator 15 μm or less, high capacity can be achieved. The thickness of the separator may be 3 μm or more and 15 μm or less.

[0018] The activation energy (E) is 400 kJ / mol or more, and may be 500 kJ / mol or more. The activation energy (E) is 700 kJ / mol or less, 600 kJ / mol or less, or 500 kJ / mol or less. By setting the activation energy (E) to 400 kJ / mol or more, which is higher than that of conventional separators, the cycle capacity retention rate can be significantly improved. This is thought to be because the creep resistance of the separator is improved by setting the activation energy (E) to 400 kJ / mol or more. When the creep resistance of the separator is high, deformation of the separator during repeated charge and discharge is reduced, and variation in the depth of charge is reduced.

[0019] By setting the activation energy (E) to 700 kJ / mol or less, it is possible to suppress temperature rise during abnormal conditions such as short circuits. This is thought to be because the fluidity of the separator at high temperatures can be increased, thereby improving the shutdown function of the separator.

[0020] The negative electrode includes a negative electrode mixture layer. The negative electrode mixture layer includes a negative electrode active material. The negative electrode active material preferably includes a silicon-containing material. The silicon-containing material includes elemental silicon or a silicon compound. Examples of silicon compounds include silicate, silicon oxide, silicon carbide, silicon nitride, and silicide. The silicon-containing material may be composite particles composed of at least one of elemental silicon and a silicon compound and a silicon-free substance.

[0021] The content C(Si) of the silicon-containing material in the negative electrode mixture layer may be 3% by mass or more, 6% by mass or more, or 15% by mass or more. The content C(Si) may be 25% by mass or less, 20% by mass or less, or 15% by mass or less. By setting the content C(Si) to 6% by mass or more, a high capacity can be achieved. By setting the content C(Si) to 20% by mass or less, a high cycle capacity retention rate can be achieved. The content C(Si) may be 6% by mass or more and 20% by mass or less.

[0022] The content C(Si) can be determined from the proportion of silicon-containing material when preparing the negative electrode mixture (or negative electrode mixture slurry). The content C(Si) in the negative electrode mixture layer of the produced battery can be determined by disassembling the battery, removing the negative electrode mixture layer, and analyzing the negative electrode mixture layer using the method described below. When the silicon-containing material is a composite particle containing a silicon-containing phase (silicate phase, silicon phase, silicide phase, etc.) and a silicon-free phase (carbon phase, etc.), the content C(Si) is calculated using the mass of the entire composite particle as the mass of the silicon-containing material.

[0023] The silicon-containing material may include an ion-conducting phase and a silicon phase dispersed in the ion-conducting phase. Specifically, the silicon-containing material may be a composite particle including an ion-conducting phase and a silicon phase dispersed in the ion-conducting phase. The ion-conducting phase may be at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. The use of such a silicon-containing material can increase capacity.

[0024] The expansion rate of the negative electrode during charging may be 15% or more, 20% or more, or 30% or more, and may be 45% or less, 40% or less, or 30% or less. The expansion rate tends to increase as the content of the silicon-containing material in the negative electrode mixture layer increases. The expansion rate may be 20% or more and 40% or less.

[0025] The negative electrode mixture layer may contain carbon nanotubes. By including carbon nanotubes in the negative electrode mixture layer, a good conductive path can be formed even when the negative electrode active material (e.g., a silicon-containing material) expands and contracts during charging and discharging. As a result, the cycle capacity retention rate can be improved. The carbon nanotube content in the negative electrode mixture layer may be 0.01% by mass or more and 0.05% by mass or less. Within this range, the effect of the carbon nanotubes is particularly enhanced.

[0026] In the electrode group, a positive electrode and a negative electrode are stacked with a separator interposed therebetween. The electrode group may be a wound type. That is, in the electrode group, the positive electrode, the negative electrode, and the separator may be wound. Alternatively, the electrode group may be an electrode group in which a plurality of flat positive electrodes and a plurality of flat negative electrodes are alternately stacked in one direction. In this case, a separator is also disposed between the positive electrode and the negative electrode.

[0027] The shape of the nonaqueous electrolyte secondary battery (B) may be cylindrical or prismatic. The present disclosure is preferably used for a cylindrical secondary battery (B) having a wound electrode group. The following describes the case where the electrode group is a wound electrode group.

[0028] The negative electrode may have a non-facing portion at its inner peripheral end that does not face the positive electrode across the separator. The number of turns of the non-facing portion may be 0.4 or more and 1.2 or less (e.g., 0.5 or more and 0.8 or less). This configuration can suppress pressure fluctuations inside the electrode plate during charge and discharge, thereby preventing the separator from being crushed. As a result, smooth charge and discharge behavior is possible even in the later stages of cycling, and a high cycle capacity retention rate can be achieved.

[0029] The number of turns in the portion where the positive electrode and the negative electrode face each other may be 15 to 60 (for example, 18 to 55). This configuration makes it possible to achieve both good load characteristics and high capacity of the battery.

[0030] (Method for manufacturing a separator) The separator can be formed by melting the separator material and forming a film. After film formation, the components are removed to form pores. The film formation method is not limited, and may be performed using a cast molding method, a biaxial stretching method, or the like. A component for forming pores (e.g., liquid paraffin) is mixed into the separator material. In this way, a separator that is a porous film is obtained. The components can be removed by a known method. For example, liquid paraffin may be removed using a solvent such as methylene chloride.

[0031] An example of a separator manufacturing apparatus is shown schematically in Figure 1. The manufacturing apparatus 100 in Figure 1 includes a kneading / extrusion section 101, a longitudinal stretching section 102, a transverse stretching section 103, a solvent extraction section 104, a transverse stretching / heat setting section 105, and a winding section 106.

[0032] An example of the configuration of the kneading / extrusion section 101 is shown schematically in FIG. 2. The kneading / extrusion section 101 includes a T-die 101a and a cast roll 101b. The molten material 201 is discharged from the T-die 101a and cooled on the cast roll 101b to form a film 202. The crystallinity of the separator can be controlled by the difference between the temperature of the discharged material and the temperature of the cast roll 101b. If the temperature difference is large, the crystallinity will be low. By increasing the temperature difference, the crystallinity can be increased. By increasing the crystallinity, the activation energy (E) can be increased.

[0033] The formed membrane 202 is stretched in the longitudinal stretching section 102 and the transverse stretching section 103. By changing the stretching ratio in these sections, the crystallinity and activation energy (E) of the separator can be controlled. Specifically, by increasing the stretching ratio, it is possible to increase the crystallinity and activation energy (E).

[0034] In the solvent extraction section 104, components that form pores are removed by a solvent. After the solvent extraction section 104, the separator 203 is produced through a transverse stretching / heat setting section 105. The produced separator 203 is wound up on a roll in a winding section 106. The separator 203 is cut to a predetermined size to obtain the separator 13.

[0035] (Method for manufacturing non-aqueous electrolyte secondary battery (B)) The method for manufacturing the secondary battery (B) is not particularly limited, except for using the separator described above. In one example of the manufacturing method, a positive electrode and a negative electrode are formed, and a non-aqueous electrolyte is prepared. Next, an electrode group is formed using the positive electrode, the negative electrode, and the separator. Next, the electrode group and the non-aqueous electrolyte are housed in a battery case. In this manner, the secondary battery (B) can be manufactured.

[0036] (Examples of Components) Examples of other components of the secondary battery (B) are specifically described below. Note that the components described below are merely examples, and the components of the secondary battery (B) are not limited to the following examples. Known components may be used for components other than those characteristic of this embodiment. The secondary battery (B) typically includes a positive electrode, a negative electrode, a separator, a non-aqueous electrolyte, and a battery case.

[0037] (Positive Electrode (Positive Electrode Plate)) The positive electrode may include a positive electrode current collector and positive electrode mixture layers disposed on both sides of the positive electrode current collector. The positive electrode mixture layer includes a positive electrode active material and may also include other components (such as a conductive material, a binder, and a thickener). These components are not particularly limited, and known components used in positive electrodes of non-aqueous electrolyte secondary batteries may be used.

[0038] The positive electrode may be formed by a known method. In one example of the formation method, positive electrode components and a dispersion medium are first mixed to prepare a positive electrode slurry. The dispersion medium may be water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), or a mixture thereof. Next, the positive electrode slurry is applied to a positive electrode current collector and then dried to form a laminate including the positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector. Next, the laminate is rolled as necessary. In this manner, the positive electrode is formed.

[0039] A conductive sheet can be used for the positive electrode current collector. The positive electrode current collector may be a non-porous conductive sheet (e.g., a non-porous metal foil). Alternatively, the positive electrode current collector may be a porous conductive sheet, such as a metal mesh sheet or a punched metal. Examples of materials for the positive electrode current collector include metal materials containing Al, Ti, Fe, etc. The metal material may be Al, an Al alloy, Ti, a Ti alloy, an Fe alloy (e.g., stainless steel), etc. The thickness of the positive electrode current collector may be in the range of 5 to 50 μm (e.g., in the range of 10 to 30 μm).

[0040] Examples of the positive electrode active material include olivine-type lithium salt (LiFePO ４Examples of lithium-containing composite metal oxides include metal oxides containing lithium and a transition metal. A portion of the transition metal in the metal oxide may be substituted with a different element. Examples of the different element include Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, etc. Preferred examples of the different element include Mn, Al, Co, Ni, Mg, etc. The different element may be one type or two or more types. The positive electrode active material is usually used in the form of particles.

[0041] Examples of positive electrode active materials include Li ｘ CoO ２ , Li ｘ NiO ２ , Li ｘ MnO ２ , Li ｘ Co ｙ Ni １－ｙ O ２ , Li ｘ Co ｙ M １－ｙ O ｚ , Li ｘ Ni １－ｙ M ｙ O ｚ , Li ｘ Mn ２ O ４ , Li ｘ Mn ２－ｙ M ｙ O ４ , LiMPO ４ , and Li ２ MPO ４ and F. In each formula, M represents at least one element selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, V, and B. x = 0 to 1.2, y = 0 to 0.9, and z = 2.0 to 2.3.

[0042] Examples of conductive materials include carbon black (acetylene black, ketjen black, etc.), carbon nanotubes, carbon fibers, etc. Examples of binders include fluororesins, polyacrylonitrile, polyimide resins, acrylic resins, polyolefin resins, rubber-like polymers, etc. Examples of fluororesins include polytetrafluoroethylene, polyvinylidene fluoride, etc. Examples of thickeners include carboxymethyl cellulose salts, etc.

[0043] (Negative Electrode (Negative Electrode Plate)) The negative electrode includes a negative electrode current collector and a negative electrode mixture layer disposed on both sides of the negative electrode current collector. The negative electrode mixture layer includes a negative electrode active material and may also include other components (such as a conductive material, a binder, and a thickener). These components are not particularly limited, and known components used in negative electrodes of non-aqueous electrolyte secondary batteries may be used. The conductive material, binder, and thickener may be the same materials as those exemplified for the conductive material, binder, and thickener of the positive electrode, respectively.

[0044] The negative electrode may be formed by a known method. In one example of the formation method, first, a negative electrode mixture slurry is prepared by mixing the components of the negative electrode with a dispersion medium. The dispersion medium may be water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), or a mixture thereof. Next, the negative electrode mixture slurry is applied to a negative electrode current collector and then dried to form a laminate including the negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector. Next, the laminate is rolled as necessary. In this manner, the negative electrode is formed.

[0045] A conductive sheet can be used for the negative electrode current collector. The negative electrode current collector may be a non-porous conductive sheet (e.g., a non-porous metal foil). Alternatively, the negative electrode current collector may be a porous conductive sheet, such as a metal mesh sheet or a punched metal. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys. The thickness of the negative electrode current collector may be in the range of 1 to 50 μm (e.g., in the range of 5 to 20 μm).

[0046] The negative electrode mixture layer contains a negative electrode active material, for example, a negative electrode active material and additives (such as a binder, a conductive material, and a thickener). The negative electrode can be formed by a known method. For example, first, a negative electrode mixture slurry containing the negative electrode active material and additives is prepared. Next, the negative electrode mixture slurry is applied to a negative electrode current collector and then dried to form a coating film. Next, a laminate consisting of the negative electrode current collector and the coating film is rolled to obtain a negative electrode. The formed negative electrode is cut to a predetermined size as needed. The thickness of the negative electrode mixture layer may be 3 μm or more, or 5 μm or more, or may be 200 μm or less, or 150 μm or less.

[0047] The negative electrode active material contained in the negative electrode mixture layer can be a material capable of reversibly absorbing and releasing lithium ions. Examples of such negative electrode active materials include carbonaceous materials and silicon-containing materials. The negative electrode active material may contain or be a silicon-containing material. Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon). Examples of silicon-containing materials include elemental silicon, silicon alloys, silicon compounds (such as silicon oxides), and composite materials in which a silicon phase is dispersed within a lithium ion conductive phase (matrix). The negative electrode may contain only one type of negative electrode active material, or may contain two or more types of negative electrode active materials. The negative electrode active material is usually used in the form of particles.

[0048] The additives contained in the negative electrode mixture layer are not particularly limited. The binder, conductive material, and thickener may be the same as those exemplified for the binder, conductive material, and thickener of the positive electrode mixture layer. For example, the negative electrode mixture layer may contain carbon nanotubes.

[0049] (Silicon-Containing Material) The silicon-containing material is not particularly limited, and may be a known silicon-containing material used as a negative electrode active material in non-aqueous electrolyte secondary batteries. The silicon-containing material may be silicon oxide. Alternatively, the silicon-containing material may be a silicon-containing composite material (composite particles). For example, the silicon-containing material may include an ion-conducting phase and a silicon phase dispersed in the ion-conducting phase. The ion-conducting phase may include at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase, or may be at least one of these phases. Alternatively, the ion-conducting phase may be any of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase.

[0050] The silicon-containing material (negative electrode active material) may include composite particles containing the ion-conducting phase (matrix) and a silicon phase dispersed in the ion-conducting phase. In the composite particles, the stress associated with the expansion and contraction of the silicon phase during charge and discharge is alleviated by the ion-conducting phase. Therefore, cracking and fracture of the particles are suppressed. As a result, it is possible to achieve both high capacity due to the inclusion of silicon and improved cycle characteristics.

[0051] The silicate phase is composed of a compound containing a metal element, silicon (Si), and oxygen (O). Examples of the metal element include alkali metal elements (such as lithium) and Group 2 elements (Group 2 elements in the long periodic table). The silicate phase preferably contains lithium silicate, and may be a lithium silicate phase. The lithium silicate phase is preferred in that lithium ions can easily enter and exit the silicate phase. The lithium silicate phase has a smaller irreversible capacity than the silicon oxide phase. The ion-conducting phase may contain the lithium silicate phase as a main component (content: 50 mass% or more), or may further contain a silicon oxide phase. The lithium silicate phase is preferably Li ２ Si ２ O ５ , Li ２ SiO ３ , and Li ４ SiO ４The composition may contain at least one selected from the group consisting of:

[0052] The silicate phase may further contain another element M in addition to Li, Si, and O. When the silicate phase contains another element M, it is possible to improve the chemical stability and lithium ion conductivity of the silicate phase. Furthermore, when the silicate phase contains another element M, it is possible to suppress side reactions between the silicate phase and the non-aqueous electrolyte.

[0053] The silicate phase may contain, as element M, at least one element selected from the group consisting of alkali metal elements (excluding lithium) and Group 2 elements. Na and / or K are preferably used as inexpensive alkali metal elements. The Group 2 element may be Ca and / or Mg.

[0054] From the viewpoint of increasing capacity and improving cycle characteristics, the content of the silicon phase in the composite particles may be 45% by mass or more and 70% by mass or less, 50% by mass or more and 70% by mass or less, or 58% by mass or more and 70% by mass or less.

[0055] The silicon oxide phase is composed of a compound of Si and O. The main component (for example, 95 to 100 mass%) of the silicon oxide phase may be silicon dioxide. ｘ It may be a phase represented by the formula (0.5≦x<1.6).

[0056] The average particle size of the composite particles may be 1 μm or more and 25 μm or less, 4 μm or more and 15 μm or less, or 6 μm or more and 8 μm or less. Within these ranges, good battery performance is likely to be obtained. The average particle size of the composite particles is the median diameter (D50) at which the cumulative volume reaches 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.

[0057] In the description of the composite particles, the silicate phase may be replaced with a carbon phase, a silicide phase, or a silicon oxide phase. The silicide constituting the silicide phase may be an intermetallic compound of silicon and a metal element Me. The metal element Me may be at least one element selected from the group consisting of Al, Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Mo.

[0058] (Coating layer) At least a portion of the surface of the composite particle (silicon-containing material) may be covered with a coating layer. The proportion of the coating layer relative to the entire composite particle including the coating layer is preferably 3% by mass or more and 5% by mass or less. The coating layer may contain a conductive carbon material. From the viewpoint of ensuring conductivity, the thickness of the coating layer is preferably 1 nm or more. The thickness of the coating layer may be 200 nm or less, 100 nm or less, or 10 nm or less.

[0059] In one example of a method for forming a conductive coating layer, a mixture is first obtained by mixing a carbonaceous raw material with composite particles (silicon-containing material). The mixture is then fired to carbonize the carbonaceous raw material, thereby forming a conductive coating layer on the surface of the composite particles. Examples of the carbonaceous raw material include coal pitch, petroleum pitch, and phenolic resin. The firing temperature may be 450°C or higher and 1000°C or lower. The firing time may be 1 hour or higher and 10 hours or lower.

[0060] (Method for producing composite particles) As an example of composite particles (silicon-containing material), an example of a method for producing composite particles containing a lithium silicate phase will be described below. The composite particles are produced, for example, by the following production method including the following first to fourth steps. However, the composite particles may also be produced by a method other than the following production method.

[0061] (First Step) The first step is a step of preparing or synthesizing lithium silicate (a compound that forms an ion-conducting phase). Commercially available lithium silicate may be used as the lithium silicate. Alternatively, the lithium silicate may be synthesized by a known method.

[0062] (Second Step) The second step is a step of forming a composite intermediate containing a lithium silicate phase and a silicon phase dispersed in the lithium silicate phase by combining lithium silicate and raw silicon. In one example of the second step, first, a mixture is obtained by mixing lithium silicate (a compound that forms an ion-conducting phase) with raw silicon. Next, the mixture is pulverized (and stirred) while applying shear force to obtain a finely divided composite intermediate. At this time, the raw silicon is pulverized to form a silicon phase. The silicon phase is dispersed in the lithium silicate (matrix) phase. The pulverization can be performed using a pulverizing device (e.g., a ball mill).

[0063] (Third Step) The third step is a step of forming a sintered body containing a lithium silicate phase and a silicon phase dispersed in the lithium silicate phase by heat-treating the composite intermediate. In one example of the third step, the composite intermediate (e.g., a micronized composite intermediate) obtained in the second step is fired while applying pressure using a hot press or the like to form a sintered body. The firing of the composite intermediate may be carried out in an inert atmosphere (e.g., an argon gas atmosphere or a nitrogen gas atmosphere). The firing temperature is preferably 450°C or higher and 1000°C or lower. The firing time may be 1 hour or higher and 10 hours or shorter.

[0064] (Step 4) In step 4, the sintered body is pulverized to obtain composite particles containing a silicate phase and a silicon phase dispersed in the silicate phase. By selecting the pulverization conditions, composite particles having a predetermined average particle size can be obtained.

[0065] When the ion-conducting phase is a phase other than the lithium silicate phase (such as a carbon phase or a silicide phase), a substance other than the lithium silicate (such as a carbon material or a silicide) may be prepared in the first step. The substance other than the lithium silicate may be a commercially available substance or may be synthesized by a known method. In the steps from step 2 onward, the substance may be used instead of the lithium silicate.

[0066] The content of the silicon-containing material in the negative electrode mixture layer can be determined by the following method. First, a cross section of the negative electrode is exposed by a predetermined method (ion milling, cross-section polisher, etc.). Next, the exposed cross section is observed using a transmission electron microscope (SEM). Next, the ratio of Si to other elements (e.g., carbon) in the silicon-containing material present in the cross section is calculated using EDS (energy dispersive X-ray analysis). Furthermore, after dissolving the negative electrode mixture layer with hydrofluoric acid, the amount of Si is quantified using ICP atomic emission spectroscopy. Through these analyses, the content of the silicon-containing material in the negative electrode mixture layer can be determined.

[0067] (Separator) The separator may be any of the separators described above.

[0068] (Non-aqueous electrolyte) The non-aqueous electrolyte may be a non-aqueous electrolyte containing a non-aqueous solvent and an electrolyte salt. The electrolyte may be a gel electrolyte using a gel polymer or the like.

[0069] Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, etc. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, etc. Examples of chain carbonates include dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, etc.

[0070] Examples of electrolyte salts include LiPF ６ , LiBF ４ , LiClO ４ , LiAsF ６ , LiCF ３ SO ３ , LiN(SO ２ CF ３ ) ２ , LiN(SO ２ C ２ F ５ ) ２ , and LiC(SO ２ CF ３ ) ３ The non-aqueous solvent and the electrolyte salt may each be used alone or in combination of two or more.

[0071] (Battery Case) The battery case of the secondary battery (B) is not particularly limited, and a known battery case may be used. The battery case of the secondary battery (B) may be cylindrical or rectangular. The battery case may include a cylindrical case body with a bottom, a sealing body that seals the opening of the case body, and a gasket disposed between the case body and the sealing body. The case body may be made of metal, and for example, a case body made of a metal primarily composed of iron may be used.

[0072] An example of the secondary battery (B) will be specifically described below with reference to the drawings. The components described above can be applied to the components of the example described below. The components of the example described below can be modified based on the above description. The matters described below may also be applied to the above embodiment. In the example described below, components that are not essential for the secondary battery (B) may be omitted.

[0073] (Embodiment 1) In Embodiment 1, an example of a cylindrical secondary battery (B) will be described. FIG. 1 schematically shows a cross section of an example of a nonaqueous electrolyte secondary battery 10 of Embodiment 1. The secondary battery 10 includes a cylindrical battery case, a wound electrode group 14, and a nonaqueous electrolyte (not shown) housed in the battery case. The battery case includes a case body 15 and a sealing body 16 that seals the opening of the case body 15. The case body 15 is a cylindrical case with a bottom made of metal. The case body 15 has a step portion 21. A gasket 27 is disposed between the case body 15 and the sealing body 16. Within the case body 15, insulating plates 17 and 18 are disposed at both ends of the electrode group 14, respectively.

[0074] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26. All of the members except for the insulating member 24 are electrically connected to one another. The sealing body 16 also functions as a safety valve when the internal pressure inside the battery case becomes high.

[0075] The electrode group 14 is formed by winding the positive electrode 11, the negative electrode 12, and the separator 13 so that the separator 13 is disposed between the positive electrode 11 and the negative electrode 12. The positive electrode 11, the negative electrode 12, and the separator 13 are each strip-shaped. The positive electrode 11 includes a positive electrode current collector and a positive electrode mixture layer formed on both sides of the positive electrode current collector. The negative electrode 12 includes a negative electrode current collector and a negative electrode mixture layer formed on both sides of the negative electrode current collector. The separator 13 is the separator described above.

[0076] (Additional Notes) The above description discloses the following technologies. (Technology 1) A nonaqueous electrolyte secondary battery comprising: an electrode group including a positive electrode, a negative electrode, and a separator; and a nonaqueous electrolyte, wherein the negative electrode includes a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector, and wherein the separator has an activation energy of 400 kJ / mol or more and 700 kJ / mol or less at 100 to 135°C measured by dynamic viscoelasticity measurement. (Technology 2) The nonaqueous electrolyte secondary battery according to Technology 1, wherein the negative electrode mixture layer contains a silicon-containing material, and wherein the content of the silicon-containing material in the negative electrode mixture layer is 6 mass% or more and 20 mass% or less. (Technology 3) The nonaqueous electrolyte secondary battery according to Technology 2, wherein the silicon-containing material includes an ion-conducting phase and a silicon phase dispersed in the ion-conducting phase, and the ion-conducting phase is at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. (Technology 4) The nonaqueous electrolyte secondary battery according to any one of Technology 1 to 3, wherein the expansion coefficient of the negative electrode during charge is 20% or more and 40% or less. (Technology 5) The nonaqueous electrolyte secondary battery according to any one of Technology 1 to 4, wherein the negative electrode mixture layer includes carbon nanotubes, and wherein the content of the carbon nanotubes in the negative electrode mixture layer is 0.01% by mass or more and 0.05% by mass or less. (Technology 6) The nonaqueous electrolyte secondary battery according to any one of Technology 1 to 5, wherein the thickness of the separator is 3 μm or more and 15 μm or less. (Technology 7) The nonaqueous electrolyte secondary battery according to any one of Techniques 1 to 6, wherein in the electrode group, the positive electrode, the negative electrode, and the separator are wound together. (Technology 8) The nonaqueous electrolyte secondary battery according to Technique 7, wherein the negative electrode has, at an inner peripheral end side, a non-facing portion that does not face the positive electrode across the separator, and the number of turns of the non-facing portion is 0.4 to 1.2. (Technology 9) The nonaqueous electrolyte secondary battery according to Technique 7 or 8, wherein the number of turns of the portion where the positive electrode and the negative electrode face each other is 15 to 60.

[0077] The nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail below with reference to examples. In these examples, a plurality of nonaqueous electrolyte secondary batteries were fabricated and evaluated as follows.

[0078] (Preparation of Battery A1) (1) Preparation of Composite Particles Silicon dioxide and Li ２ CO ３ The mixture was mixed and fired in air at 950°C for 10 hours to obtain silicate. The obtained silicate was pulverized to an average particle size of 10 μm. Next, the silicate was mixed with raw silicon (average particle size: 10 μm). In the mixture, the mass ratio of silicate to raw silicon was 42:58.

[0079] The mixture was charged 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 pot was closed with a lid, and the mixture was milled at 200 to 300 rpm for 25 hours in an inert atmosphere.

[0080] Next, the powder mixture was taken out in an inert atmosphere and fired under pressure using a hot press machine in an inert atmosphere to obtain a sintered body of the mixture. Next, the obtained sintered body was crushed and passed through a 40 μm mesh to obtain composite particles with an average particle size (median diameter) of 6 μm. The composition of the main component of the silicate phase in the composite particles was Li ２ Si ２ O ５ It was.

[0081] Next, the composite particles and coal tar pitch were mixed in a mass ratio of 95:5, and then calcined at 800°C in an argon atmosphere to form a conductive coating layer covering at least a portion of the surface of the composite particles, thereby obtaining silicon-containing particles. By calcining, the coal tar pitch was converted into amorphous carbon.

[0082] (2) Preparation of Negative Electrode Silicon-containing particles (silicon-containing material) and graphite were mixed in a predetermined mass ratio and used as the negative electrode active material. A negative electrode mixture containing a negative electrode active material, a Na salt of CMC, and SBR in a mass ratio of 97.5:1:1.5 was added with water and stirred to prepare a negative electrode mixture slurry. The content of the silicon-containing material in the negative electrode mixture layer is shown in Table 1. Next, the negative electrode mixture slurry was applied to both sides of a negative electrode current collector (copper foil) to form a coating film, and the coating film was then dried to form a laminate including the negative electrode current collector and a negative electrode mixture layer formed on both sides of the negative electrode current collector. Next, the laminate was rolled. In this way, a negative electrode including a negative electrode current collector and a negative electrode mixture layer formed on both sides of the negative electrode current collector was prepared.

[0083] (3) Preparation of Positive Electrode A positive electrode mixture slurry was prepared by adding NMP to a positive electrode mixture containing lithium cobalt oxide, acetylene black, and PVDF in a mass ratio of 95:2.5:2.5 and stirring the mixture. The positive electrode mixture slurry was then applied to the surface of an aluminum foil, the coating was dried, and the mixture was rolled to form a positive electrode mixture slurry having a density of 3.6 g / cm on both sides of the aluminum foil. ３ A positive electrode having the positive electrode mixture layer formed thereon was fabricated.

[0084] (4) Preparation of non-aqueous electrolyte: LiPF in a non-aqueous solvent ６ A non-aqueous electrolyte (electrolytic solution) was prepared by dissolving the above in a solution of 1.0 mol / L. The non-aqueous solvent used was a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 3:7.

[0085] (5) Fabrication of Secondary Battery Tabs were attached to each of the positive and negative electrodes. Next, a wound electrode group was fabricated by winding the positive and negative electrodes with a separator interposed therebetween. A porous film (thickness: 12 μm) made of high-density polyethylene having the activation energy (E) shown in Table 1 was used as the separator. The number of windings of the non-facing portion of the negative electrode, which did not face the positive electrode across the separator at the inner peripheral end side of the negative electrode, was 0.6 turns.

[0086] Next, the electrode group was inserted into a cylindrical case body (external can) with a bottom, and the negative electrode tab was welded to the inner bottom surface of the case body. The positive electrode tab was welded to the internal terminal plate of the sealing body. A nonaqueous electrolyte was then poured into the external can, and the opening of the external can was sealed using a sealing body and a gasket. In this manner, a nonaqueous electrolyte secondary battery (Battery A1) was obtained.

[0087] The fabricated battery A1 was evaluated as follows. (i) Measurement of capacity retention rate Battery A1 was subjected to constant current charging at a 3-hour rate in an environment of 25°C until the battery voltage reached 4.2 V, followed by constant voltage charging with a cut-off current of 50-hour rate at a battery voltage of 4.2 V. Thereafter, constant current discharging was performed at a 3-hour rate until the battery voltage reached 3 V. This charge / discharge cycle was repeated 400 times. The capacity retention rate was calculated using the following formula: Capacity retention rate (%) = {(discharge capacity at 400th cycle) / (discharge capacity at 1st cycle)} × 100

[0088] (ii) Expansion Coefficient of Negative Electrode The expansion coefficient of the negative electrode was measured by the following method. First, the thickness Tna of the negative electrode formed in the above step (2) was measured. Next, the fabricated battery A1 was charged at a constant current of 0.3 It until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.02 It. The current It is a current value expressed as It (A) = rated capacity (Ah) / 1 (h). After charging, the battery A1 was disassembled, and the thickness Tnb of the negative electrode was measured. The thickness of the negative electrode was measured using a digital length measuring instrument (MH-15M) manufactured by Nikon Solutions Corporation. The measurement was performed using a flat-tipped probe (diameter: 5 mm). The expansion coefficient of the negative electrode during charging was calculated using the following formula: Expansion coefficient of negative electrode during charging (%) = 100 × (Tnb - Tna) / Tna

[0089] (iii) Forced Short-Circuit Test The forced short-circuit test was conducted as follows. First, Battery A1 was charged at a constant current of 0.3 It until the battery voltage reached 4.2 V. Next, the charged Battery A1 was placed in a thermostatic chamber at 60°C, and both electrodes were short-circuited via an external resistance of 50 mΩ, thereby forcing Battery A1 to be externally short-circuited. The surface temperature of the side surface of the battery's outer can was measured, and a test was conducted to see if the maximum temperature reached 140°C or higher. The above forced short-circuit test was conducted on five Battery A1s.

[0090] (Batteries A2 to A7, Batteries C1 to C2) Batteries A2 to A7 and Batteries C1 to C2 were fabricated using the same method and conditions as those for fabricating Battery A1, except that the activation energy (E) of the separator and / or the silicon-containing material content C(Si) were changed. The separator activation energy (E) and content C(Si) are shown in Table 1. The separator material and thickness were the same as those used in fabricating Battery A1. The activation energy (E) was changed by changing the cooling rate and / or the draw ratio of the separator material when it was extruded from the die head (T die) and formed into a film on a casting roll.

[0091] The fabricated batteries A2 to A7, C1, and C2 were evaluated in the same manner as battery A1. Table 1 shows some of the fabrication conditions and the evaluation results for each battery. In Table 1, the initial discharge capacity is the discharge capacity at the first cycle of the charge-discharge cycle used to measure the capacity retention rate. The silicon-containing material content C(Si) is the content of the silicon-containing material in the negative electrode mixture layer. The number shown in Table 1 for the forced short circuit test indicates the number of batteries out of the five tested that reached a surface temperature of 140°C or higher. It is preferable that the initial discharge capacity and cycle capacity retention rate are high. The cycle capacity retention rate is required to be 40% or higher, and preferably 75% or higher.

[0092]

[0093] Batteries A1 to A7 are secondary batteries (B) according to the present disclosure, and batteries C1 and C2 are comparative examples. As shown in Table 1, by setting the activation energy to 400 kJ / mol or more, the cycle capacity retention rate could be increased. Furthermore, by setting the activation energy to 700 kJ / mol or less, the temperature rise of the battery during forced short circuiting could be suppressed. By setting the content of silicon-containing material C(Si) to 6 mass% or more, a high capacity could be achieved. By setting the content of C(Si) to 20 mass% or less, a high cycle capacity retention rate could be achieved.

[0094] The present disclosure can be used in non-aqueous electrolyte 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.

[0095] 10: Non-aqueous electrolyte secondary battery 11: Positive electrode 12: Negative electrode 13: Separator 14: Electrode group

Claims

1. A non-aqueous electrolyte secondary battery comprising: an electrode group including a positive electrode, a negative electrode, and a separator; and a non-aqueous electrolyte, wherein the negative electrode includes a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector, and the activation energy of the separator at 100 to 135°C measured by dynamic viscoelasticity measurement is 400 kJ / mol or more and 700 kJ / mol or less.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode mixture layer contains a silicon-containing material, and the content of the silicon-containing material in the negative electrode mixture layer is 6 mass % or more and 20 mass % or less.

3. The nonaqueous electrolyte secondary battery according to claim 2, wherein the silicon-containing material comprises an ion-conducting phase and a silicon phase dispersed in the ion-conducting phase, and the ion-conducting phase is at least one phase selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase.

4. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the expansion rate of the negative electrode during charging is 20% or more and 40% or less.

5. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the negative electrode mixture layer contains carbon nanotubes, and the content of the carbon nanotubes in the negative electrode mixture layer is 0.01 mass % or more and 0.05 mass % or less.

6. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the thickness of the separator is 3 μm or more and 15 μm or less.

7. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein in the electrode group, the positive electrode, the negative electrode, and the separator are wound together.

8. The nonaqueous electrolyte secondary battery according to claim 7, wherein the negative electrode has a non-facing portion at its inner peripheral end that does not face the positive electrode across the separator, and the number of turns of the non-facing portion is 0.4 to 1.

2.

9. The nonaqueous electrolyte secondary battery according to claim 7, wherein the number of turns in the portion where the positive electrode and the negative electrode face each other is 15 or more and 60 or less.

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