Non-aqueous electrolyte secondary battery, and method for manufacturing non-aqueous electrolyte secondary battery

Abstract

This disclosed nonaqueous electrolyte secondary battery (10) includes a positive electrode (11), a negative electrode (12), a separator (13), and a nonaqueous electrolyte. The positive electrode (11) includes a positive electrode mixture layer. The positive electrode mixture layer contains a positive electrode active material and a conductive carbon material. The composition of the positive electrode active material is represented by the following compositional formula (1). Compositional formula (1):ＬｉａＮｉｂＣｏｃＭｎｄＭｅＯ２. (In the formula, ０＜ａ≦１．２、０．８≦ｂ＜１、０＜ｃ＋ｄ＜１－ｂ、０＜ｅ≦０．０５, and M is at least one element including Sr and / or W).　In a cross section of the positive electrode mixture layer, the ratio, Ｓｃ／Ｓｐ, of the area Sc of the conductive carbon material to the area Sp of the positive electrode active material is 0.059-0.091.

Description

Nonaqueous electrolyte secondary battery and method for manufacturing the same CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims the benefit of priority to Japanese Patent Application No. 2024-091514, filed on June 5, 2024, in the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a non-aqueous electrolyte secondary battery and a method for manufacturing a non-aqueous electrolyte secondary battery.

[0003] A non-aqueous electrolyte secondary battery, such as a lithium ion secondary battery, includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. Improvements in battery characteristics are required for non-aqueous electrolyte secondary batteries. Various proposals have been made to improve battery characteristics.

[0004] Claim 1 of Patent Document 1 (JP 2014-220074 A) describes, "A positive electrode for a non-aqueous electrolyte secondary battery, characterized by being produced using a positive electrode mixture slurry having a solids concentration of 68 mass % or more and 72 mass % or less in a mixed solvent containing at least a positive electrode active material in which the surface of a lithium transition metal compound is coated with a carbon material, a binder, and a solvent."

[0005] JP 2014-220074 A

[0006] Currently, there is a demand for improving the cycle characteristics of non-aqueous electrolyte secondary batteries. One of the objects of the present disclosure is to provide a non-aqueous electrolyte secondary battery with good cycle characteristics.

[0007] One aspect of the present disclosure is a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode includes a positive electrode mixture layer, and the positive electrode mixture layer includes a positive electrode active material and a conductive carbon material, and the composition of the positive electrode active material is represented by the following composition formula (1): Li ａ Ni ｂ Co ｃ Mn ｄ M ｅ O ２(1) (wherein 0<a≦1.2, 0.8≦b<1, 0<c+d<1−b, 0<e≦0.05, and M is at least one element including Sr and / or W), the ratio Sc / Sp of the area Sc of the conductive carbon material to the area Sp of the positive electrode active material in a cross section of the positive electrode mixture layer is 0.059 or more and 0.091 or less.

[0008] According to the present disclosure, a non-aqueous electrolyte secondary battery having excellent cycle characteristics 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.

[0009] 1 is a partially cutaway perspective view schematically illustrating a nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure.

[0010] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and other materials may be applied as long as the invention of the present disclosure can be implemented. 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 greater and numerical value B or less." In the following description, when lower and upper limits of numerical values ​​related to specific physical properties or conditions are exemplified, 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 greater than the upper limit. In the following description, when examples of components or methods are listed, only one of the listed examples may be used, or multiple of the listed examples may be used in combination, unless otherwise specified.

[0011] (Non-aqueous electrolyte secondary battery) The non-aqueous electrolyte secondary battery according to this embodiment may be referred to as a "nonaqueous electrolyte secondary battery (B)" or a "secondary battery (B)" below. The secondary battery (B) includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. The positive electrode includes a positive electrode mixture layer. The positive electrode mixture layer includes a positive electrode active material and a conductive carbon material. The composition of the positive electrode active material is represented by the following composition formula (1): Li ａ Niｂ Co ｃ Mn ｄ M ｅ O ２ (1) (Wherein, 0<a≦1.2, 0.8≦b<1, 0<c+d<1−b, 0<e≦0.05, and M is at least one element including Sr and / or W.) In a cross section of the positive electrode mixture layer, the ratio Sc / Sp of the area Sc of the conductive carbon material to the area Sp of the positive electrode active material is 0.059 or more and 0.091 or less. In formula (1), 0≦c and 0≦d may be satisfied, or 0<c and 0<d may be satisfied.

[0012] The secondary battery (B) uses the above-mentioned positive electrode active material, which allows for improved cycle characteristics and a higher capacity. On the other hand, when the above-mentioned positive electrode active material is used, the battery resistance may increase. In the secondary battery (B), the conductive carbon material is well dispersed. Therefore, an increase in the battery resistance can be suppressed. As a result, the configuration of the secondary battery (B) allows for suppression of an increase in the battery resistance and improvement in the cycle characteristics.

[0013] The element M in the composition formula (1) may contain Sr, may contain W, or may contain both Sr and W. The element M may contain an element other than Sr and W. For example, the element M may contain at least one element selected from the group consisting of Al, Zr, B, and rare earth elements. The positive electrode active material can be produced by a known method.

[0014] The ratio Sc / Sp in the cross section of the positive electrode mixture layer can be measured by the following method. First, an SEM image is obtained by photographing the cross section of the positive electrode mixture layer with a scanning electron microscope. Next, the SEM image is analyzed using image analysis software (Avizo). At this time, an area having a size of 80 μm × 120 μm or more is arbitrarily selected and analyzed. Specifically, first, based on the contrast difference within the image, the positive electrode active material portion, the void portion, the conductive carbon material, and the binder portion are identified. Note that, since it is not easy to distinguish the conductive carbon material portion from the binder portion in the SEM image, these are not distinguished. Then, the area Sp(1) of the positive electrode active material portion, the area Sv(1) of the void portion, and the area Scb(1) of the conductive carbon material and the binder portion are calculated. In this manner, analysis using image analysis software is performed.

[0015] Next, the area Sc(1) of the conductive carbon material portion is calculated by multiplying the area Scb(1) of the conductive carbon material and binder portion by the ratio Mc / (Mc+Mb). Mc is the mass of the conductive carbon material contained in the positive electrode mixture layer. Mb is the mass of the binder contained in the positive electrode mixture layer. The ratio Mc / (Mc+Mb) can be calculated from the mixing ratio of the components in the positive electrode slurry used to form the positive electrode mixture layer. Next, the ratio Sc(1) / Sp(1) of the area Sc(1) of the conductive carbon material to the area Sp(1) of the positive electrode active material is calculated.

[0016] The above analysis is performed at eight arbitrarily selected locations, and the ratio Sc(1) / Sp(1) at the eight locations is determined. The arithmetic average of these ratios is defined as the ratio Sc / Sp. The ratio Sc / Sp is the ratio of the area Sc of the conductive carbon material to the area Sp of the positive electrode active material in the cross section of the positive electrode mixture layer. In this manner, the ratio Sc / Sp is determined.

[0017] The conductive carbon material may contain carbon black or may be carbon black. The carbon black may be acetylene black. Carbon black (e.g., acetylene black) is preferred because of its high dispersibility and large specific surface area. The positive electrode mixture layer of the secondary battery (B) may or may not contain a conductive carbon material other than carbon black (e.g., carbon nanotubes, graphite, etc.).

[0018] (Method for manufacturing non-aqueous electrolyte secondary battery) The manufacturing method according to this embodiment may be referred to as "manufacturing method (M)" below. According to the manufacturing method (M), the above-described secondary battery (B) may be manufactured. The matters described for the secondary battery (B) may be applied to the manufacturing method (M), and therefore, redundant explanations may be omitted. The matters described for the manufacturing method (M) may also be applied to the secondary battery (B).

[0019] The manufacturing method (M) is a method for manufacturing a non-aqueous electrolyte secondary battery. The manufacturing method (M) includes a first step of preparing a conductive paste containing a conductive carbon material, a dispersant, and N-methyl-2-pyrrolidone; a second step of preparing a positive electrode slurry by mixing a positive electrode active material, the conductive paste, a binder, and a liquid medium; and a third step of forming a positive electrode mixture layer on a positive electrode current collector using the positive electrode slurry. The composition of the positive electrode active material is represented by the following composition formula (1): Li ａ Ni ｂ Co ｃ Mn ｄ M ｅ O ２ (1) (wherein 0<a≦1.2, 0.8≦b<1, 0<c+d<1−b, 0<e≦0.05, and M is at least one element including Sr and / or W.) In the conductive paste, the ratio Wb / Wa of the mass Wb of the conductive carbon material to the total mass Wa of the conductive carbon material, dispersant, and N-methyl-2-pyrrolidone is 0.135 or more and 0.185 or less.

[0020] In manufacturing method (M), a conductive paste containing a conductive carbon material, a dispersant, and N-methyl-2-pyrrolidone is prepared in advance. The conductive paste is prepared so that the ratio Wb / Wa is 0.135 or more and 0.185 or less. The conductive paste is then mixed with other materials to prepare a positive electrode slurry. By using the positive electrode slurry prepared in this manner, a positive electrode mixture layer in which the conductive carbon material is well dispersed can be formed. Therefore, even when a high-resistance positive electrode active material is used, an increase in the resistance of the positive electrode mixture layer can be suppressed. In other words, manufacturing method (M) makes it possible to manufacture a nonaqueous electrolyte secondary battery with relatively low resistance and good cycle characteristics.

[0021] (First Step) In the first step, a conductive paste containing a conductive carbon material, a dispersant, and N-methyl-2-pyrrolidone is prepared. The conductive carbon material is the conductive carbon material described above. Examples of dispersants include polyvinylpyrrolidone. N-methyl-2-pyrrolidone is a liquid medium (dispersion medium) that is removed during drying. The conductive paste can be prepared by mixing the conductive carbon material, the dispersant, and N-methyl-2-pyrrolidone.

[0022] (Second Step) In the second step, a positive electrode slurry is prepared by mixing a positive electrode active material, a conductive paste, a binder, and a liquid medium. The liquid medium (dispersion medium) used in the conductive paste may be used as the liquid medium. For example, N-methyl-2-pyrrolidone may be used. The binder may be a known binder used in the positive electrode mixture layer. Examples of binders include fluororesins and rubber-like materials (for example, styrene-butadiene copolymer (SBR)). Examples of fluororesins include polyvinylidene fluoride, polytetrafluoroethylene, etc.

[0023] (Third Step) In the third step, a positive electrode mixture layer is formed on the positive electrode current collector using the positive electrode slurry prepared in the second step. The method for forming the positive electrode mixture layer is not particularly limited, and known methods may be used. In one example of a method for forming the positive electrode mixture layer, the positive electrode slurry is applied to the positive electrode current collector. The method for applying the positive electrode slurry is not limited, and a doctor blade method, a die coating method, a gravure coating method, or the like may be used. The applied positive electrode slurry is then dried and, if necessary, rolled. At least a portion of the liquid medium is removed from the positive electrode mixture layer during the drying process. Similarly, at least a portion of the dispersant may also be removed from the positive electrode mixture layer during the drying process. In the third step, a positive electrode mixture layer is formed on one or both sides of the positive electrode current collector, depending on the configuration of the secondary battery (B). In this manner, a positive electrode is formed, including a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector.

[0024] The positive electrode formed in the third step is cut to a predetermined size as needed, and a positive electrode lead is connected to the positive electrode as needed.

[0025] An electrode group is formed using the formed positive electrode. The electrode group is formed by placing a positive electrode and a negative electrode facing each other with a separator sandwiched therebetween. For example, the electrode group may be formed by winding the positive electrode, the negative electrode, and the separator so that the separator is disposed between the positive electrode and the negative electrode. Next, the electrode group and the non-aqueous electrolyte are sealed in an outer casing. In this way, a non-aqueous electrolyte secondary battery is manufactured. The method for performing the steps other than the step of forming the positive electrode is not particularly limited, and known methods may be used.

[0026] The shape of the nonaqueous electrolyte secondary battery is not limited, and may be cylindrical or rectangular. The form of the electrode group of the nonaqueous electrolyte secondary battery is not limited, and may be wound or stacked. Examples of the components of the nonaqueous electrolyte secondary battery (B) are described below. However, the components of the secondary battery (B) are not limited to the following examples.

[0027] (Positive Electrode) The positive electrode includes a positive electrode mixture layer. The positive electrode may include a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode current collector is not particularly limited, and a positive electrode current collector used in a known non-aqueous electrolyte secondary battery may be used.

[0028] Examples of the material of the positive electrode current collector include metal materials containing Al, Ti, Fe, etc. Examples of the metal materials include Al, Al alloys, Ti, Ti alloys, and Fe alloys (such as stainless steel).

[0029] An example of the positive electrode mixture layer includes a positive electrode active material, a conductive carbon material, and a binder. The positive electrode mixture layer may further include other additives (such as a thickener). The positive electrode active material reversibly absorbs and releases lithium ions. The positive electrode active material may be a positive electrode active material whose composition is represented by composition formula (1).

[0030] (Negative Electrode) The negative electrode includes a negative electrode mixture layer. 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 includes a negative electrode active material and may further include other additives (such as a binder, a thickener, or a conductive material).

[0031] The negative electrode current collector is not particularly limited, and a known negative electrode current collector may be used. A conductive sheet (e.g., metal foil) may be used as the negative electrode current collector. For example, copper foil, copper alloy foil, a resin sheet (e.g., a polyethylene terephthalate sheet) on which copper is vapor-deposited, or a stainless steel foil on which copper is vapor-deposited may be used as the negative electrode current collector.

[0032] The negative electrode active material may be a material that reversibly absorbs and releases lithium ions. Examples of the negative electrode active material include silicon-containing materials and carbonaceous materials. The negative electrode mixture layer may contain only one type of negative electrode active material, or may contain two or more types of negative electrode active materials. Examples of the carbonaceous material include graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon).

[0033] Examples of silicon-containing materials include silicon oxide, silicon, and composite materials. The composite materials may contain multiple phases selected from a carbon phase, a lithium silicate phase, a silicon phase, and a silicon oxide phase. One example of a composite material contains a carbon phase and a particulate silicon phase dispersed in the carbon phase. Another example of a composite material contains a lithium silicate phase and a particulate silicon phase dispersed in the lithium silicate phase.

[0034] The components other than the negative electrode active material (e.g., conductive material) are not particularly limited, and known components may be used. The conductive material may be a material (conductive carbon material) exemplified as the conductive material of the positive electrode mixture layer. Examples of thickeners include carboxymethyl cellulose and carboxymethyl cellulose salts.

[0035] The negative electrode may be formed by a known method. In one example of a method for forming the negative electrode, first, a negative electrode slurry containing components of the negative electrode mixture layer and a liquid medium (dispersion medium) is prepared. Next, the negative electrode slurry is applied to a negative electrode current collector, followed by drying and rolling. In this manner, a negative electrode is formed, which includes a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector. The negative electrode mixture layer is formed on one or both sides of the negative electrode current collector. The formed negative electrode is cut to an appropriate size as needed. Furthermore, a negative electrode lead is connected to the negative electrode as needed.

[0036] (Separator) An insulating porous sheet is used for the separator. Examples of porous sheets include microporous membranes, woven fabrics, and nonwoven fabrics. The material of the separator is not particularly limited, and a polymeric material may be used. Examples of polymeric materials include polyolefin resins, polyamide resins, and cellulose. Examples of polyolefin resins include polyethylene, polypropylene, and ethylene-propylene copolymers. The separator may contain additives (such as inorganic fillers) as needed.

[0037] (Non-aqueous electrolyte) The non-aqueous electrolyte may be a non-aqueous electrolyte having lithium ion conductivity. The non-aqueous electrolyte contains a non-aqueous solvent and ions (lithium ions, anions, etc.) dissolved in the non-aqueous solvent. The non-aqueous electrolyte may be in a liquid or gel state.

[0038] The non-aqueous electrolyte can be prepared by dissolving a lithium salt in a non-aqueous solvent. When the lithium salt is dissolved in the non-aqueous solvent, lithium ions and anions are generated. Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO ４ , LiAlCl ４ , LiB １０ Cl １０ etc.), lithium salts of fluorine-containing acids (LiPF ６ , LiPF ２ O ２ , LiBF ４ , LiSbF ６ , LiAsF ６ , LiCF ３ SO ３ , LiCF ３ CO ２ etc.), lithium salt of fluorine-containing acid imide (LiN(FSO ２ ) ２ , LiN(CF ３ SO ２ ) ２ , LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ Lithium salts include lithium halides (LiCl, LiBr, LiI, etc.), and lithium halides (LiCl, LiBr, LiI, etc.). The lithium salts may be used alone or in combination of two or more. The concentration of the lithium salt in the non-aqueous electrolyte may be 0.5 mol / L or more and 3.5 mol / L or less.

[0039] The nonaqueous solvent is not particularly limited, and known nonaqueous solvents may be used. Examples of nonaqueous solvents include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC). Examples of chain carbonates include dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone. Examples of chain carboxylic acid esters include nonaqueous solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate, and ethyl propionate. One type of nonaqueous solvent may be used alone, or two or more types may be used in combination.

[0040] (Exterior Body) The exterior body (battery case) houses the electrode group and the non-aqueous electrolyte. The exterior body is not particularly limited, and any known exterior body may be used. The exterior body usually includes an exterior can and a sealing member that seals the opening of the exterior can. The exterior can functions as a negative electrode terminal, and the sealing member functions as a positive electrode terminal. The sealing member may include a sealing plate and a gasket.

[0041] An example of an embodiment according to the present disclosure will be specifically described below with reference to the drawings. The embodiment described below can be modified based on the above description. Furthermore, the matters described below may be applied to the above embodiment. Furthermore, in the embodiment described below, matters that are not essential to the invention according to the present disclosure may be omitted.

[0042] (Embodiment 1) Fig. 1 is a longitudinal cross-sectional view schematically illustrating an example of a nonaqueous electrolyte secondary battery according to Embodiment 1. The cylindrical nonaqueous electrolyte secondary battery 10 shown in Fig. 1 includes a cylindrical battery case, and an electrode group 14 and a nonaqueous electrolyte (not shown) housed in the battery case. The electrode group 14 is a wound electrode group and includes a positive electrode 11, a negative electrode 12, and a separator 13.

[0043] The battery case includes a case body 15, which is a cylindrical metal container with a bottom, and a sealing body 16 that seals the opening of the case body 15. A gasket 27 is disposed between the case body 15 and the sealing body 16. The gasket 27 ensures the airtightness of the battery case. Within the case body 15, insulating plates 17 and 18 are disposed at both ends of the electrode group 14 in the winding axis direction. The case body 15 has a step portion 21.

[0044] 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. The lower valve body 23 and the upper valve body 25 are connected at their respective centers. An insulating member 24 is disposed between the peripheral edge of the lower valve body 23 and the peripheral edge of the upper valve body 25. The filter 22 and the lower valve body 23 are connected at their respective peripheral edges. The upper valve body 25 and the cap 26 are connected at their respective peripheral edges. All of the components constituting the sealing body 16, except for the insulating member 24, are electrically connected.

[0045] A vent hole is formed in the lower valve body 23. Therefore, if the internal pressure of the battery case increases due to abnormal heat generation or the like, the upper valve body 25 bulges toward the cap 26 and separates from the lower valve body 23. This cuts off the electrical connection between the lower valve body 23 and the upper valve body 25. If the internal pressure increases further, the upper valve body 25 breaks, and gas is discharged from an opening formed in the cap 26.

[0046] The positive electrode 11 is electrically connected to a cap 26, which functions as a positive electrode terminal, via a positive electrode lead 19. The negative electrode 12 is electrically connected to a case body 15, which functions as a negative electrode terminal, via a negative electrode lead 20. The positive electrode 11 is the positive electrode described above.

[0047] (Supplementary Note) The above description discloses the following technology: (Technology 1) A non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode includes a positive electrode mixture layer, and the positive electrode mixture layer includes a positive electrode active material and a conductive carbon material, and the composition of the positive electrode active material is represented by the following composition formula (1): Li ａ Ni ｂ Co ｃ Mnｄ M ｅ O ２ (1) (wherein 0<a≦1.2, 0.8≦b<1, 0<c+d<1−b, 0<e≦0.05, and M is at least one element including Sr and / or W.) A non-aqueous electrolyte secondary battery, wherein in a cross section of the positive electrode mixture layer, a ratio Sc / Sp of an area Sc of the conductive carbon material to an area Sp of the positive electrode active material is 0.059 or more and 0.091 or less. (Technology 2) The non-aqueous electrolyte secondary battery according to Technology 1, wherein the conductive carbon material is carbon black. (Technology 3) The non-aqueous electrolyte secondary battery according to Technology 2, wherein the carbon black is acetylene black. (Technology 4) A method for manufacturing a non-aqueous electrolyte secondary battery, comprising: a first step of preparing a conductive paste containing a conductive carbon material, a dispersant, and N-methyl-2-pyrrolidone; a second step of preparing a positive electrode slurry by mixing a positive electrode active material, the conductive paste, a binder, and a liquid medium; and a third step of forming a positive electrode mixture layer on a positive electrode current collector using the positive electrode slurry, wherein the composition of the positive electrode active material is represented by the following composition formula (1): Li ａ Ni ｂ Co ｃ Mn ｄ M ｅ O ２ (1) (wherein 0<a≦1.2, 0.8≦b<1, 0<c+d<1−b, 0<e≦0.05, and M is at least one element including Sr and / or W), wherein in the conductive paste, a ratio Wb / Wa of a mass Wb of the conductive carbon material to a total mass Wa of the conductive carbon material, the dispersant, and N-methyl-2-pyrrolidone is 0.135 or more and 0.185 or less.

[0048] The nonaqueous electrolyte secondary battery (B) according to this embodiment will be described in more detail with reference to Examples. In these Examples, a plurality of nonaqueous electrolyte secondary batteries were fabricated and evaluated.

[0049] (Battery A1) Battery A1 was produced by the following method.

[0050] (1) Formation of Positive Electrode First, a conductive paste was prepared. The conductive paste was prepared by mixing a conductive carbon material (acetylene black), a dispersant, and N-methyl-2-pyrrolidone (NMP, liquid medium). The conductive paste was prepared so that the ratio Wb / Wa, where Wb is the mass of the conductive carbon material and Wb is the total mass Wa of the conductive carbon material, dispersant, and N-methyl-2-pyrrolidone, was the value shown in Table 1.

[0051] Next, a positive electrode active material, a conductive paste, polyvinylidene fluoride (PVDF, binder), and N-methyl-2-pyrrolidone (liquid medium) were mixed in a predetermined mass ratio to prepare a positive electrode slurry. The positive electrode active material, the conductive paste, and the PVDF were mixed in a mass ratio of 100:0.8:0.82. The positive electrode active material used had a composition represented by composition formula (1).

[0052] The positive electrode slurry was applied to both sides of an aluminum foil (positive electrode current collector), followed by drying and rolling to form a positive electrode including a positive electrode current collector and positive electrode mixture layers formed on both sides of the positive electrode current collector.

[0053] (2) Formation of Negative Electrode A negative electrode slurry was prepared by mixing the negative electrode active material, sodium carboxymethyl cellulose (CMC-Na, thickener), styrene butadiene rubber (SBR, binder), and water (liquid medium) in a predetermined mass ratio. The negative electrode active material was a mixture of graphite and a silicate compound in a mass ratio of 95:5.

[0054] Next, the negative electrode slurry was applied to both sides of a copper foil (negative electrode current collector), followed by drying and rolling to prepare a negative electrode including the negative electrode current collector and negative electrode mixture layers formed on both sides of the negative electrode current collector.

[0055] (3) Preparation of non-aqueous electrolyte: Ethylene carbonate and diethyl carbonate were mixed in a volume ratio of 3:7 to obtain a mixed solvent. LiPF was added to this mixed solvent to a concentration of 1.0 mol / L. ６ In this way, a non-aqueous electrolyte (electrolytic solution) was prepared.

[0056] (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. Next, the electrode assembly 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. In addition, the positive electrode tab was welded to a sealing member. Next, a nonaqueous electrolyte was injected into the external can. Next, the opening of the external can was sealed using a sealing member and a gasket. In this manner, a nonaqueous electrolyte secondary battery (battery A1) was obtained.

[0057] The following evaluations were performed on Battery A1 and its positive electrode. (1) Interface Resistance: The interface resistance between the positive electrode current collector and the positive electrode mixture layer of the rolled positive electrode of Battery A1 was measured using an electrode resistance measurement system RM26120 (manufactured by Hioki E.E. Corporation).

[0058] (2) Initial DC Resistance and DC Resistance After 100 Cycles The initial DC resistance of Battery A1 was measured. Next, the DC resistance of Battery A1 was measured after 100 charge-discharge cycles. The increase rate of the DC resistance after 100 cycles relative to the initial DC resistance was calculated.

[0059] (3) Measurement of Ratio Sc / Sp The ratio Sc / Sp of the area Sc of the conductive carbon material to the area Sp of the positive electrode active material in the cross section of the positive electrode mixture layer was measured by the method described above.

[0060] (4) Cycle Capacity Retention Rate The charge / discharge cycle was repeated 300 times, and the ratio of the discharge capacity at the 300th cycle to the discharge capacity at the first cycle (discharge capacity retention rate) was calculated.

[0061] (Battery A2) Battery A2 was produced in the same manner and under the same conditions as those for producing Battery A1, except that the ratio Wb / Wa was changed to the value shown in Table 1 when preparing the conductive paste, and the ratio Sc / Sp was changed to the value shown in Table 1.

[0062] (Batteries C1 and C2) Battery C1 was fabricated in the same manner and under the same conditions as Battery A1, except that the composition of the positive electrode active material was changed. Battery C2 was fabricated in the same manner and under the same conditions as Battery A2, except that the composition of the positive electrode active material was changed. A positive electrode active material containing no Sr or W was used as the positive electrode active material for Batteries C1 and C2.

[0063] The fabricated batteries A2, C1, and C2 were evaluated in the same manner as battery A1. The positive electrodes of batteries A2, C1, and C2 were evaluated in the same manner as battery A1. Some of the manufacturing conditions and the evaluation results are shown in Table 1. In Table 1, the interface resistance change rate of battery A2 indicates the rate of change in the interface resistance of battery A2 relative to the interface resistance of battery A1. The interface resistance change rate of battery C2 indicates the rate of change in the interface resistance of battery C2 relative to the interface resistance of battery C1. The initial DC resistance change rate of battery A2 indicates the rate of change in the initial DC resistance of battery A2 relative to the initial DC resistance of battery A1. The initial DC resistance change rate of battery C2 indicates the rate of change in the initial DC resistance of battery C2 relative to the initial DC resistance of battery C1.

[0064]

[0065] The interface resistance change rate and initial DC resistance change rate are preferably low. The DC resistance increase rate after 100 cycles is preferably low, and the capacity retention rate is preferably high. Batteries A1 and A2 are nonaqueous electrolyte secondary batteries (B) according to the present disclosure. Batteries C1 and C2 are comparative examples. As shown in Table 1, the DC resistance increase rate after 100 cycles of batteries A1 and A2 was low, and the capacity retention rates of batteries A1 and A2 were high. Furthermore, the interface resistance change rate and initial DC resistance change rate of battery A2 were lower than those of battery C2.

[0066] 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.

[0067] 10: Non-aqueous electrolyte secondary battery 11: Positive electrode 12: Negative electrode 13: Separator 14: Electrode group 15: Case body 16: Sealing body

Claims

1. A non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode includes a positive electrode mixture layer, the positive electrode mixture layer includes a positive electrode active material and a conductive carbon material, and the composition of the positive electrode active material is represented by the following composition formula (1): Li ａ Ni ｂ Co ｃ Mn ｄ M ｅ O ２ (1) (wherein 0<a≦1.2, 0.8≦b<1, 0<c+d<1−b, 0<e≦0.05, and M is at least one element including Sr and / or W), a ratio Sc / Sp of an area Sc of the conductive carbon material to an area Sp of the positive electrode active material in a cross section of the positive electrode mixture layer is 0.059 or more and 0.091 or less.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the conductive carbon material is carbon black.

3. The non-aqueous electrolyte secondary battery according to claim 2, wherein the carbon black is acetylene black.

4. A method for manufacturing a non-aqueous electrolyte secondary battery, comprising: a first step of preparing a conductive paste containing a conductive carbon material, a dispersant, and N-methyl-2-pyrrolidone; a second step of preparing a positive electrode slurry by mixing a positive electrode active material, the conductive paste, a binder, and a liquid medium; and a third step of forming a positive electrode mixture layer on a positive electrode current collector using the positive electrode slurry, wherein the composition of the positive electrode active material is represented by the following composition formula (1): Li ａ Ni ｂ Co ｃ Mn ｄ M ｅ O ２ (1) (wherein 0<a≦1.2, 0.8≦b<1, 0<c+d<1−b, 0<e≦0.05, and M is at least one element including Sr and / or W), wherein in the conductive paste, a ratio Wb / Wa of a mass Wb of the conductive carbon material to a total mass Wa of the conductive carbon material, the dispersant, and N-methyl-2-pyrrolidone is 0.135 or more and 0.185 or less.

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