Non-aqueous electrolyte secondary battery

Abstract

The disclosed non-aqueous electrolyte secondary battery (10) comprises a wound electrode group (14), a non-aqueous electrolyte, and a battery case that accommodates the electrode group (14) and the non-aqueous electrolyte. The electrode group (14) includes a positive electrode (11), a negative electrode (12), and a separator (13), which are wound. The tensile strength Sp (MPa) of the positive electrode (11), the tensile strength Sn (MPa) of the negative electrode (12), and the diameter D (mm) of a hollow section present along the central axis of the electrode group (14) satisfy the relationship of 4.0 ≤ D × (Sn / Sp) ≤ 20.

Description

Nonaqueous electrolyte secondary battery

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

[0002] Various proposals have been made for non-aqueous electrolyte secondary batteries including a wound electrode group. Claim 1 of Patent Document 1 (WO 2009 / 122717) describes "a non-aqueous electrolyte secondary battery comprising: a wound electrode group including a long positive electrode, a long negative electrode, and a separator disposed between the positive electrode and the negative electrode; a non-aqueous electrolyte; and a prismatic battery case accommodating these; wherein a cross section of the electrode group has a major axis and a minor axis; the positive electrode includes a positive electrode current collector and a positive electrode active material layer provided on the positive electrode current collector; the negative electrode includes a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector; and the tensile strength of the positive electrode is 15 N / cm or less when the elongation rate in the longitudinal direction of the positive electrode is 1%."

[0003] International Publication No. 2009 / 122717

[0004] Currently, there is a demand for higher capacity non-aqueous electrolyte secondary batteries. However, battery designs aimed at achieving higher capacity result in larger volume changes in the electrodes (positive electrode, negative electrode) during charging and discharging. Large volume changes in the electrodes make them more susceptible to deformation. Electrode deformation leads to a decrease in capacity retention. In this situation, one of the objects of the present disclosure is to provide a non-aqueous electrolyte secondary battery that can achieve higher capacity and a high capacity retention.

[0005] One aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery including: a wound electrode group; a non-aqueous electrolyte; and a battery case that houses the electrode group and the non-aqueous electrolyte, wherein the electrode group includes a wound positive electrode, a wound negative electrode, and a separator, and wherein a tensile strength Sp (MPa) of the positive electrode, a tensile strength Sn (MPa) of the negative electrode, and a diameter D (mm) of a hollow portion that exists along a central axis of the electrode group satisfy the relationship 4.0≦D×(Sn / Sp)≦20.

[0006] According to the present disclosure, a non-aqueous electrolyte secondary battery capable of achieving high capacity and high capacity retention 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.

[0007] FIG. 1 is a cross-sectional view schematically illustrating an example of a nonaqueous electrolyte secondary battery according to the present disclosure.

[0008] 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.

[0009] (Non-aqueous electrolyte secondary battery) The non-aqueous electrolyte secondary battery of this embodiment may be referred to as secondary battery (B) below. The secondary battery (B) includes a wound electrode group, a non-aqueous electrolyte, and a battery case that houses the electrode group and the non-aqueous electrolyte. The electrode group includes a wound positive electrode, a wound negative electrode, and a separator. That is, the electrode group is formed by winding the positive electrode, the wound negative electrode, and the separator. The tensile strength Sp (MPa) of the positive electrode, the tensile strength Sn (MPa) of the negative electrode, and the diameter D (mm) of a hollow portion existing along the central axis of the electrode group satisfy the relationship 4.0≦D×(Sn / Sp)×D≦20.

[0010] As a result of investigations, the present inventors have newly discovered that in nonaqueous electrolyte secondary batteries, high effects can be obtained by setting the value of D(Sn / Sp) in the range of 4.0 to 20. As will be explained in the examples, even when a silicon-containing material capable of increasing capacity is used as the negative electrode active material, a high capacity retention rate can be achieved by setting the value of D(Sn / Sp) in the range of 4.0 to 20. The present disclosure is based on this new finding.

[0011] The reason why the above effect is achieved by setting the D(Sn / Sp) value in the range of 4.0 to 20 is currently unclear, but the following can be considered. First, if the strength of the positive electrode plate is too low compared to the strength of the negative electrode plate, the positive electrode plate will deform and expand preferentially during charge and discharge, making it impossible to properly secure the opposing portions of the positive and negative electrodes. As a result, battery performance will deteriorate. On the other hand, if the strength of the negative electrode plate is similar to that of the positive electrode plate, the negative electrode plate will expand significantly during charging. However, if there is little space in the center of the electrode group at this time, the negative electrode cannot move toward the center, causing bending within the electrode group. As a result, charging and discharging will not be performed properly, and battery performance will deteriorate.

[0012] The value of D(Sn / Sp) may be 4.0 or greater, 5.3 or greater, or 8.4 or greater. The value of D(Sn / Sp) may be 20 or less, 15 or less, 13 or less, 12.3 or less, or 8.4 or less. The value of D(Sn / Sp) may be in the range of 4.0 to 20, 5.3 to 20, or 8.4 to 20. Within any of these ranges, the upper limit may be 15, 13, or 12.3.

[0013] The central axis of the electrode group is the central axis of the winding of the electrode group. The portion where the winding core existed when the electrode group was produced can be considered the central axis of the electrode group. The diameter D of the hollow portion existing along the central axis (winding axis) of the electrode group may be 3 mm or more, 5 mm or more, 6 mm or more, or 7 mm or more, and may be 8 mm or less, 7 mm or less, or 6 mm or less. The diameter D may be in the range of 3 to 8 mm, 5 to 8 mm, 6 to 8 mm, or 7 to 8 mm. In any of these ranges, the upper limit may be 7 mm or 6 mm, as long as the lower limit is not greater than the upper limit. Increasing the diameter D makes it easier to alleviate stress generated by expansion of the electrode plate. Reducing the diameter D enables even higher capacity.

[0014] The diameter D can be changed, for example, by changing the radius of the winding core used when winding the electrodes and separator to form the electrode group. By increasing the radius of the winding core, the diameter D can be increased.

[0015] The diameter D can be measured by the following method. First, computed tomography (CT) of the battery is performed to obtain an image of a plane perpendicular to the winding axis at the center of the battery in the height direction. Next, the circular equivalent diameter of the area of ​​the space present at the center of the battery in the obtained cross-sectional image is calculated. At this time, a portion of a predetermined component (positive electrode, negative electrode, separator) (for example, a portion sandwiched between two winding cores when the electrode group was produced) may protrude into the space present at the center. In this case, the area of ​​the portion protruding into the space present at the center is included in the area of ​​the space. The calculated circular equivalent diameter is used as the diameter D.

[0016] The (Sn / Sp) value may be 1.0 or more, 1.2 or more, or 1.5 or more, and may be 2.0 or less, or 1.8 or less. The (Sn / Sp) value may be in the range of 1.0 to 2.0, 1.2 to 2.0, or 1.5 to 2.0. In any of these ranges, the upper limit may be 1.8. By setting the (Sn / Sp) value in the range of 1.0 to 2.0, the difference between the deformation amount of the positive electrode and the deformation amount of the negative electrode due to charge and discharge can be reduced. Therefore, changes in the distance between the electrode plates due to charge and discharge can be suppressed, allowing for uniform and efficient charge and discharge. As a result, a high capacity retention rate can be maintained. Furthermore, by setting the (Sn / Sp) value to 2.0 or less, short circuits between the positive electrode and the negative electrode can be easily prevented.

[0017] The tensile strength Sp (MPa) of the positive electrode may be 50 or more, 100 or more, or 150 or more, and may be 350 or less, 250 or less, or 150 or less. The tensile strength Sp can be changed by changing the characteristics of the positive electrode current collector (thickness, material, shape, etc.) and the characteristics of the positive electrode mixture layer (thickness, composition, etc.). For example, the tensile strength Sp can be increased by increasing the proportion of the positive electrode current collector in the positive electrode. Furthermore, when comparing pure aluminum and aluminum alloys, aluminum alloys generally have higher tensile strength. Therefore, the tensile strength Sp can be increased by using an aluminum alloy for the positive electrode current collector. If the tensile strength Sp is too small, the expansion of the positive electrode during charge and discharge becomes relatively large, which may make short circuits between the positive electrode and the negative electrode more likely to occur.

[0018] The tensile strength Sn (MPa) of the negative electrode may be 150 or more, 250 or more, or 350 or more, and may be 650 or less, 550 or less, or 450 or less. The tensile strength Sn can be changed by changing the characteristics of the negative electrode current collector (thickness, material, shape, etc.) and the characteristics of the negative electrode mixture layer (thickness, composition, etc.). For example, the tensile strength Sp can be increased by increasing the proportion of the negative electrode current collector in the negative electrode. In addition, the tensile strength Sn can be increased by using a metal foil (e.g., copper foil) manufactured to have fine crystal grains as the negative electrode current collector.

[0019] The tensile strength of electrodes (positive electrode, negative electrode) can be determined by a tensile test in accordance with JIS (Japanese Industrial Standards) Z 2241. Specifically, first, a sample piece of a predetermined size is taken from the edge of the electrode. The test piece is a strip with a longitudinal length of 120 mm and a transverse width of 8 mm. The test piece is taken so that the longitudinal direction of the test piece and the longitudinal direction of the electrode are approximately aligned. The thickness of the taken test piece is measured. Next, the test piece is pulled in the longitudinal direction at a crosshead displacement rate of 5 mm / min. The maximum stress when the test piece is pulled until it breaks is taken as the tensile strength.

[0020] In one example of the secondary battery (B), the diameter D is in the range of 3 to 8 mm, and the value of (Sn / Sp) is in the range of 1.0 to 2.0. The diameter D and the value of (Sn / Sp) may each be in the ranges described above.

[0021] The negative electrode may include a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector. The negative electrode mixture layer may include a silicon-containing material as the negative electrode active material. In this case, the content C(Si) of the silicon-containing material in the negative electrode mixture layer may be 2% by mass or more. Compared to using a carbon material such as graphite as the negative electrode active material, the use of a silicon-containing material allows for a higher capacity. Therefore, by setting the content C(Si) to 2% by mass or more, a higher capacity can be achieved. On the other hand, because silicon-containing materials undergo large expansion and contraction during charge and discharge, the use of silicon-containing materials tends to reduce the capacity retention rate. However, in the secondary battery (B), the value of D(Sn / Sp) is set to a range of 4.0 to 20, and therefore a high capacity retention rate can be achieved even when using a silicon-containing material.

[0022] The content C(Si) may be 2% by mass or more, or 5% by mass or more, or 10% by mass or less, or 8% by mass or less. 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), the content C(Si) is calculated by taking the mass of the silicon-containing material as the mass of the entire composite particle.

[0023] The silicon-containing material is not particularly limited, and may be a known silicon-containing material used as a negative electrode active material in nonaqueous 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. In this case, the ion-conducting phase may include at least one selected from the group consisting of a silicate phase, a carbon phase, and a silicide phase, or may be at least one of the phases. The ion-conducting phase may include at least one selected from the group consisting of a silicate phase and a carbon phase, or may be at least one of the phases. Alternatively, the ion-conducting phase may be any of a silicate phase, a carbon phase, and a silicide phase.

[0024] The battery case may be cylindrical or rectangular. When the battery case is cylindrical, the electrode group has a substantially cylindrical shape. In this case, stress generated inside the electrode group due to expansion and contraction of the electrodes is easily alleviated. Therefore, when the battery case is cylindrical, a particularly high effect can be obtained.

[0025] There are no particular limitations on the size of the secondary battery (B). The diameter of the secondary battery (B) (diameter of the battery case) may be in the range of 10 to 80 mm (for example, in the range of 15 mm to 50 mm). The technology of the present disclosure can be preferably used when the diameter of the secondary battery (B) is large. In that case, the diameter D is generally also large. Therefore, the technology of the present disclosure can be preferably used when the diameter D is large. For example, the technology of the present disclosure can be preferably used when the diameter D is in the range of 6 to 8 mm.

[0026] (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) of this embodiment are not limited to the following examples. Known components may be used for components other than those characteristic of this embodiment. As described above, the secondary battery (B) includes a positive electrode, a negative electrode, a separator, a non-aqueous electrolyte, and a battery case.

[0027] (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.

[0028] 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.

[0029] 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).

[0030] Examples of the positive electrode active material include olivine-type lithium salt (LiFePO ４ Examples 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.

[0031] Examples of positive electrode active materials include Li ｘ CoO ２ , Li ｘ NiO ２ , Li ｘ MnO ２ , Li ｘCo 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.

[0032] Examples of conductive materials include carbon black (acetylene black, ketjen black, etc.) and carbon fiber. Examples of binders include fluororesin, polyacrylonitrile, polyimide resin, acrylic resin, polyolefin resin, rubber polymer, etc. Examples of fluororesin include polytetrafluoroethylene and polyvinylidene fluoride. Examples of thickeners include carboxymethyl cellulose salts.

[0033] (Negative Electrode (Negative Electrode Plate)) The negative electrode may include 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.

[0034] The negative electrode may be formed by a known method. In one example of the formation method, first, a negative electrode 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 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.

[0035] 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).

[0036] 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.

[0037] 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. A preferred example of a negative electrode contains a silicon-containing material and a carbonaceous material as the negative electrode active materials. The negative electrode active material is usually used in the form of particles.

[0038] 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.

[0039] (Composite Particles) The silicon-containing material (negative electrode active material) may include composite particles containing an 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, cracks and fractures in 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.

[0040] 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 ４ The composition may contain at least one selected from the group consisting of:

[0041] The atomic ratio O / Si of O to Si in the lithium silicate may be greater than 2 and less than 4. When the atomic ratio O / Si is greater than 2 and less than 4 (when z in the formula described below satisfies 0<z<2), this is advantageous in terms of the stability of the lithium silicate phase and lithium ion conductivity. The O / Si ratio may be greater than 2 and less than 3. The atomic ratio Li / Si of Li to Si in the lithium silicate may be greater than 0 and less than 4.

[0042] 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.

[0043] 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.

[0044] The silicate phase may contain, as element M, 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).

[0045] The silicate phase may contain a rare earth element as element M. The rare earth element may be any of scandium (Sc), yttrium (Y), and lanthanoid elements. Note that lanthanum (La), yttrium (Y), and erbium (Er) are rare earth elements. The silicate phase may contain at least one rare earth element selected from the group consisting of cerium (Ce), praseodymium (Pr), and neodymium (Nd).

[0046] The silicate phase may include elements such as iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), and molybdenum (Mo).

[0047] The element M may form a compound. The compound may be a silicate of the element M or an oxide of the element M. In the silicate phase, the proportion of the element M in the elements other than oxygen may be 1 mol % or more and 40 mol % or less.

[0048] 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.

[0049] 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).

[0050] 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.

[0051] In the description of the composite particles, the silicate phase may be replaced with a carbon phase or a silicide 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.

[0052] (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.

[0053] 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.

[0054] (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.

[0055] (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.

[0056] (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).

[0057] (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.

[0058] (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.

[0059] 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.

[0060] (Separator) The separator is disposed between the positive electrode and the negative electrode. The separator is not particularly limited, and a known separator used in non-aqueous electrolyte secondary batteries may be used. The separator may be a microporous film made of a polymer material. Examples of polymer materials include polyethylene, polypropylene, polyvinylidene fluoride, polyvinylidene chloride, polyacrylonitrile, polyacrylamide, polytetrafluoroethylene, polysulfone, polyethersulfone, polycarbonate, polyamide, polyimide, and cellulose-based compounds (carboxymethyl cellulose and hydroxypropyl cellulose). The separator may include multiple laminated layers.

[0061] (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.

[0062] 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.

[0063] Examples of electrolyte salts include LiPF ６ , LiBF ４ , LiClO４ , 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.

[0064] (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.

[0065] (Method for producing secondary battery (B)) The method for producing secondary battery (B) is not particularly limited, except that an electrode group is formed so as to satisfy 4.0≦D×(Sn / Sp)≦20. Except that the configurations of the positive electrode and the negative electrode are selected so as to satisfy the above formula, the secondary battery (B) may be produced using a known method.

[0066] In one example of the manufacturing method, first, a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte are prepared, and then the positive electrode, the negative electrode, and the separator are wound together so that the separator is disposed between the positive electrode and the negative electrode, thereby forming a wound electrode group.

[0067] An example of a method for forming an electrode assembly is described below. First, one end of at least one of the positive electrode, negative electrode, and separator is sandwiched between first and second winding cores. The first and second winding cores each have a semi-cylindrical shape. With the one end sandwiched between them, the first and second winding cores become approximately cylindrical. Then, while rotating the first and second winding cores, the positive electrode, negative electrode, and separator are wound around them to form a wound electrode assembly. Next, the ends of the electrode assembly are fixed with insulating tape or the like, and the winding core is then pulled out from the electrode assembly. After the winding core is pulled out, a hollow portion (a roughly cylindrical void overall) is formed. The hollow portion has a roughly cylindrical shape overall, but the one end sandwiched between the winding cores may cross the hollow portion. The diameter D described above can be changed by changing the radius of the first and second winding cores.

[0068] Next, the electrode group and the non-aqueous electrolyte are housed in a battery case, thus completing the production of the secondary battery (B).

[0069] 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.

[0070] (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.

[0071] 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 components except the insulating member 24 are electrically connected to one another. The sealing body 16 also functions as a safety valve in the event that the internal pressure inside the battery case becomes high. The positive electrode 11 is electrically connected to the cap 26, which functions as a positive electrode terminal, via a positive electrode lead 19. The negative electrode 12 is electrically connected to the case body 15, which functions as a negative electrode terminal, via a negative electrode lead 20.

[0072] 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 tensile strength Sp (MPa) of the positive electrode 11, the tensile strength Sn (MPa) of the negative electrode 12, and the diameter D (mm) of a hollow portion existing along the central axis of the electrode group 14 satisfy the relationship 4.0≦D×(Sn / Sp)≦20.

[0073] (Additional Notes) The above description discloses the following technologies. (Technology 1) A nonaqueous electrolyte secondary battery comprising: a wound electrode group; a nonaqueous electrolyte; and a battery case accommodating the electrode group and the nonaqueous electrolyte, wherein the electrode group comprises a wound positive electrode, a negative electrode, and a separator, and wherein the tensile strength Sp (MPa) of the positive electrode, the tensile strength Sn (MPa) of the negative electrode, and the diameter D (mm) of a hollow portion existing along the central axis of the electrode group satisfy the relationship 4.0≦D×(Sn / Sp)≦20. (Technology 2) The nonaqueous electrolyte secondary battery according to Technology 1, wherein the negative electrode comprises a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector, wherein the negative electrode mixture layer contains a silicon-containing material as a negative electrode active material, and wherein a content of the silicon-containing material in the negative electrode mixture layer is 2 mass% or more. (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 includes at least one phase selected from the group consisting of a silicate phase, a carbon phase, and a silicide phase. (Technology 4) The nonaqueous electrolyte secondary battery according to any one of Technology 1 to Technology 3, wherein the battery case is cylindrical.

[0074] 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.

[0075] (Preparation of Battery A1) (1) Preparation of Negative Electrode (Negative Electrode Plate) ２ CO ３ The mixture was mixed to obtain a mixture. The mixture was fired in air at 950°C for 10 hours to obtain a silicate. The obtained silicate was pulverized to an average particle size of 10 µm.

[0076] Next, the silicate was mixed with raw material silicon (average particle size: 10 μm) in a mass ratio of 42:58.

[0077] Next, the mixture and 24 stainless steel balls (diameter 20 mm) were placed in a pot (SUS, volume: 500 mL) of a planetary ball mill (Fritsch, P-5), and the pot was closed with a lid. Then, the mixture was milled and mixed for 25 hours in an inert atmosphere at 200 to 300 rpm.

[0078] Next, the powder mixture was removed from the pot in an inert atmosphere, and the resulting mixture was sintered in an inert atmosphere under pressure using a hot press machine, thereby obtaining a sintered body of the mixture.

[0079] Next, the obtained sintered body was pulverized to obtain particles. The particles were sieved through a 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 analyzed, and it was found that Li ２ Si ２ O ５ It was.

[0080] Next, the composite particles and coal tar pitch were mixed in a mass ratio of 95:5 to obtain a mixture. The mixture was fired at 800°C in an argon gas atmosphere to form a conductive coating layer covering at least a portion of the surface of the composite particles. By firing, at least a portion of the coal tar pitch was converted to amorphous carbon. In this way, a silicon-containing material (silicon-containing particles) was obtained.

[0081] The proportion of the coating layer relative to the entire silicon-containing particle including the coating layer was 3 mass %. The proportion of the coating layer was determined from the difference in mass between the composite particle before and after the coating layer was formed.

[0082] The silicon-containing particles and graphite were mixed in a mass ratio of 5:95 to obtain a negative electrode active material. A negative electrode slurry was prepared by mixing the negative electrode active material, a Na salt of carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), and water. The negative electrode active material, the Na salt of CMC, and the SBR were mixed in a mass ratio of 97.5:1:1.5. Next, the negative electrode slurry was applied to both sides of the negative electrode current collector to form a coating film, and the coating film was then dried to form a laminate including the negative electrode current collector and the negative electrode mixture layer formed on both sides of the negative electrode current collector. The coating film was 1 m thick. ２ The negative electrode mixture layer was formed so that the mass of the negative electrode mixture per layer was 190 g. Next, the laminate was rolled. In this way, the negative electrode current collector and the negative electrode mixture layer (density: 1.5 g / cm 3 ) formed on both sides of the negative electrode current collector were laminated. ３ A copper foil (thickness: 8 μm) was used as the negative electrode current collector.

[0083] (2) Preparation of Positive Electrode (Positive Electrode Plate) A positive electrode slurry was prepared by mixing lithium cobalt oxide (positive electrode active material), acetylene black, polyvinylidene fluoride (PVDF), and N-methyl-2-pyrrolidone (NMP, dispersion medium). The lithium cobalt oxide, acetylene black, and PVDF were mixed in a mass ratio of 95:2.5:2.5. Next, the positive electrode slurry was applied to both sides of the positive electrode current collector to form a coating film, and the coating film was then dried to form a laminate including the positive electrode current collector and positive electrode mixture layers formed on both sides of the positive electrode current collector. Next, the laminate was rolled. In this way, the positive electrode current collector and the positive electrode mixture layers (density: 3.6 g / cm ) formed on both sides of the positive electrode current collector were laminated. ３ For the positive electrode current collector, an aluminum foil (thickness: 15 μm) having a tensile strength such that the Sn / Sp value was 1.75 was used.

[0084] (3) 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] (4) Fabrication of Secondary Battery Tabs were attached to each of the positive and negative electrodes. Next, the positive and negative electrodes were wound with a separator interposed therebetween to fabricate a cylindrical electrode assembly having a central hollow. The diameter of the winding core used to form the electrode assembly was selected so that the diameter D of the hollow portion along the central axis of the electrode assembly was 3 mm. Next, the electrode assembly was inserted into a bottomed cylindrical case body (external can), and the negative electrode tab was welded to the inner bottom surface of the case body. The positive electrode tab was also welded to the internal terminal plate of the seal. A nonaqueous electrolyte was then poured into the external can, and the opening of the external can was sealed using a seal and a gasket. In this manner, a nonaqueous electrolyte secondary battery (Battery A1) was obtained.

[0086] 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

[0087] (ii) Dimensional Change Rate of Positive Electrode For Battery A1, CT imaging of the battery was performed before the charge / discharge test, and the width of the positive electrode (length in the height direction of the battery) was measured. Next, in a temperature environment of 25°C, constant current charging was performed at a current of 3 hour rate until the battery voltage reached 4.2 V, followed by constant voltage charging at a battery voltage of 4.2 V until the current reached a current of 50 hour rate. Then, constant current discharging was performed at a current of 3 hour rate until the battery voltage reached 3 V. This charge / discharge cycle was repeated 400 times. After the charge / discharge test, CT imaging of the battery was performed again, and the width of the positive electrode was measured. The dimensional change rate of the positive electrode was calculated using the following formula: Dimensional change rate (%) = {(Width of positive electrode after 400 cycles) / (Width of positive electrode before charge / discharge test) - 1} x 100

[0088] (Batteries A2-A3, C1-C2) Several nonaqueous electrolyte secondary batteries (Batteries A2-A3, C1-C2) were fabricated using the same method and conditions as Battery A1, except that the (Sn / Sp) ratio and / or diameter D were changed as shown in Table 1. The (Sn / Sp) ratio was changed by changing the material of the positive electrode current collector and / or the manufacturing method of the negative electrode current collector. For example, Battery C2, which had a high (Sn / Sp) ratio, used a positive electrode current collector with low tensile strength. The diameter D was changed by changing the diameter of the winding core used to form the electrode group.

[0089] The fabricated batteries A2 to A3, 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 values ​​in parentheses are values ​​before rounding. In Table 1, the capacity retention rate is a relative value when the capacity retention rate of battery A1 is set to 100. A higher capacity retention rate indicates better battery performance. A smaller dimensional change rate of the positive electrode indicates less likelihood of short circuiting.

[0090]

[0091] Batteries A1 to A3 are secondary batteries (B) according to the present disclosure, and batteries C1 and C2 are comparative examples. As shown in Table 1, batteries A1 to A3, which had a D(Sn / Sp) value of 4.0 or more and 20 or less, had higher capacity retention rates than battery C1. Battery C2 had a high capacity retention rate, but the dimensional change rate of the positive electrode was large due to the use of a positive electrode current collector with a low tensile strength Sp. A large dimensional change rate of the positive electrode increases the possibility of short circuits and reduces the average capacity retention rate. On the other hand, the dimensional change rate of the positive electrode of batteries A1 to A3 was small. Thus, batteries A1 to A3 were able to achieve high capacity retention rates even when using a silicon-containing material, which allows for high capacity, as the negative electrode active material.

[0092] 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.

[0093] 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: a wound electrode group; a non-aqueous electrolyte; and a battery case that accommodates the electrode group and the non-aqueous electrolyte, wherein the electrode group includes a wound positive electrode, a negative electrode, and a separator, and wherein a tensile strength Sp (MPa) of the positive electrode, a tensile strength Sn (MPa) of the negative electrode, and a diameter D (mm) of a hollow portion existing along a central axis of the electrode group satisfy the relationship: 4.0≦D×(Sn / Sp)≦20.

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

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 comprises at least one phase selected from the group consisting of a silicate phase, a carbon phase, and a silicide phase.

4. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the battery case is cylindrical.

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