Non-aqueous electrolyte secondary battery and method for manufacturing the same

Abstract

Provided is a nonaqueous electrolyte secondary battery that uses a spinel-type manganese-containing composite oxide, in which capacity deterioration during repeated charge and discharge at high temperatures is suppressed. The nonaqueous electrolyte secondary battery disclosed herein includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. The positive electrode includes a positive electrode active material layer containing a positive electrode active material. The positive electrode active material has a spinel-type crystal structure and includes a lithium composite oxide containing Mn. The positive electrode active material layer contains orthophosphoric acid at 0.05 to 1.0 mass% relative to the positive electrode active material. The negative electrode includes a negative electrode active material layer containing a negative electrode active material. The negative electrode active material is graphite. The nonaqueous electrolyte contains a fluorine-containing lithium salt.

Description

Technical Field

[0001] This invention relates to a non-aqueous electrolyte secondary battery. It also relates to a method for manufacturing the non-aqueous electrolyte secondary battery. Background Technology

[0002] In recent years, non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries have been applied to portable power supplies for personal computers, mobile terminals, electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] In non-aqueous electrolyte secondary batteries, techniques for forming a protective film on the electrode to suppress the decomposition of the non-aqueous electrolyte are known. For example, Patent Document 1 discloses a method in a battery using a titanium-containing lithium transition metal compound with a spinel structure as the negative electrode active material, in which a protective film from the phosphorus compound is formed on the positive electrode by containing a phosphorus compound with a P-OH structure in the positive electrode or electrolyte. Patent Document 1 describes how this protective film can suppress the decomposition of the electrolyte near the positive electrode and suppress the increase in resistance.

[0004] Patent Document 1: Japanese Patent Application Publication No. 2013-152825 Summary of the Invention

[0005] However, the inventors conducted in-depth research and found that when lithium composite oxides (spinel-type manganese-containing composite oxides) with spinel-type crystal structure and containing Mn are used as the positive electrode active material of non-aqueous electrolyte secondary batteries in the above-mentioned prior art, there is a problem that the capacity of the non-aqueous electrolyte secondary battery deteriorates significantly when it is repeatedly charged and discharged at high temperatures.

[0006] Therefore, the purpose of this invention is to provide a non-aqueous electrolyte secondary battery, which is a non-aqueous electrolyte secondary battery obtained by using spinel-type manganese-containing composite oxide, and whose capacity degradation is suppressed during repeated charging and discharging at high temperature.

[0007] The non-aqueous electrolyte secondary battery disclosed herein includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode includes a positive electrode active material layer containing a positive electrode active material. The positive electrode active material comprises a lithium composite oxide having a spinel-type crystal structure and containing Mn. The positive electrode active material layer contains 0.05% to 1.0% by mass of orthophosphate relative to the positive electrode active material. The negative electrode includes a negative electrode active material layer containing a negative electrode active material. The negative electrode active material is graphite. The non-aqueous electrolyte contains a fluorinated lithium salt. By performing appropriate initial charging treatment on a non-aqueous electrolyte secondary battery with this configuration, it is possible to provide a non-aqueous electrolyte secondary battery in which capacity degradation is suppressed during repeated charge-discharge cycles at high temperatures.

[0008] In a preferred embodiment of the non-aqueous electrolyte secondary battery disclosed herein, the aforementioned positive electrode active material layer contains 0.1% to 0.5% by mass of orthophosphate relative to the aforementioned positive electrode active material. With this configuration, capacity degradation during repeated charge-discharge cycles at high temperatures can be further suppressed.

[0009] In a preferred embodiment of the non-aqueous electrolyte secondary battery disclosed herein, the non-aqueous electrolyte further contains an oxalate complex lithium salt. This configuration further suppresses capacity degradation during repeated charge-discharge cycles at high temperatures.

[0010] In a preferred embodiment of the non-aqueous electrolyte secondary battery disclosed herein, the aforementioned positive electrode active material layer further contains lithium triphosphate. This configuration further suppresses capacity degradation during repeated charge-discharge cycles at high temperatures.

[0011] On the other hand, the method for manufacturing a non-aqueous electrolyte secondary battery with a film on the surface of the positive electrode active material disclosed herein includes: a step of preparing the aforementioned non-aqueous electrolyte secondary battery, and a step of initially charging the prepared non-aqueous electrolyte secondary battery to a voltage of 4.7V or higher. Based on this configuration, it is possible to manufacture a non-aqueous electrolyte secondary battery in which capacity degradation is suppressed during repeated charge-discharge cycles at high temperatures.

[0012] From another perspective, the non-aqueous electrolyte secondary battery disclosed herein includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode includes a positive electrode active material layer containing a positive electrode active material. The positive electrode active material comprises a lithium composite oxide having a spinel-type crystal structure and containing Mn. The negative electrode includes a negative electrode active material layer containing a negative electrode active material. The negative electrode active material is graphite. The non-aqueous electrolyte contains a fluorinated lithium salt. The positive electrode active material has a coating on its surface. At least a portion of the coating has a multilayer structure comprising a first layer and a second layer located on the first layer. In the first layer, the content (atomic %) of P element relative to the total content (atomic %) of P and F elements, as determined by scanning transmission electron microscopy / energy dispersive X-ray diffraction analysis, is 67% or more. In the second layer described above, the content (atomic%) of F element relative to the total content (atomic%) of P and F elements, as determined by scanning transmission electron microscopy / energy dispersive X-ray diffraction analysis, is 38% or more. Based on this configuration, a non-aqueous electrolyte secondary battery in which capacity degradation is suppressed during repeated charge-discharge cycles at high temperatures can be provided. Attached Figure Description

[0013] Figure 1 This is a cross-sectional view schematically illustrating the internal structure of a lithium-ion secondary battery according to one embodiment of the present invention.

[0014] Figure 2 This is an exploded schematic diagram showing the configuration of the wound electrode body of a lithium-ion secondary battery according to one embodiment of the present invention.

[0015] Figure 3 This is a STEM-HAADF image of the lithium-ion secondary battery fabricated in Example 5.

[0016] Symbol Explanation

[0017] 20. Winded electrode body

[0018] 30 Battery casing

[0019] 36 Safety valve

[0020] 42 Positive extremes

[0021] 42a Positive Current Collector

[0022] 44 Negative extremes

[0023] 44a Negative Current Collector

[0024] 50 Positive Electrode Sheets (Positive Electrode)

[0025] 52 Positive current collector

[0026] 52a Non-forming portion of the positive electrode active material layer

[0027] 54 Positive electrode active material layer

[0028] 60 Negative electrode plate (negative electrode)

[0029] 62 Negative current collector

[0030] 62a Non-forming portion of the negative electrode active material layer

[0031] 64 Negative Electrode Active Material Layer

[0032] 70. Insulation plate (isolation component)

[0033] 80 Non-aqueous electrolyte

[0034] 100 Lithium-ion Secondary Battery Detailed Implementation

[0035] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. It should be noted that matters not mentioned in this specification and necessary for the implementation of the present invention can be grasped by those skilled in the art based on prior art in this field. The present invention can be implemented based on the disclosure in this specification and common technical knowledge in this field. Furthermore, in the following drawings, components and parts that perform the same function are labeled with the same symbols. Additionally, the dimensional relationships (length, width, thickness, etc.) in the drawings may not necessarily reflect actual dimensional relationships.

[0036] It should be noted that in this manual, "secondary battery" refers to an energy storage device capable of repeated charging and discharging, and is a term that includes energy storage components such as batteries and double-layer capacitors. Furthermore, in this manual, "lithium-ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and achieves charging and discharging through the transfer of charge accompanying lithium ions between the positive and negative electrodes.

[0037] Hereinafter, a first embodiment of the non-aqueous electrolyte secondary battery of the present invention (hereinafter also referred to as "non-aqueous electrolyte secondary battery (1)") will be described in detail using a flat square lithium-ion secondary battery having a flat-shaped wound electrode body and a flat-shaped battery casing as an example, but it is not intended to limit the present invention to the technical solutions described in the above embodiment.

[0038] Figure 1 The lithium-ion secondary battery 100 shown is a sealed battery constructed by housing a flat, wound electrode body 20 and a non-aqueous electrolyte 80 within a flat, square battery casing (i.e., an outer container) 30. The battery casing 30 is provided with a positive terminal 42 for external connection, a negative terminal 44, and a thin-walled safety valve 36 configured to release internal pressure when the internal pressure of the battery casing 30 rises above a predetermined level. Additionally, the battery casing 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolyte 80. The positive terminal 42 is electrically connected to a positive current collector 42a. The negative terminal 44 is electrically connected to a negative current collector 44a. As the material for the battery casing 30, lightweight and thermally conductive metal materials such as aluminum can be used, for example. It should be noted that... Figure 1 The amount of non-aqueous electrolyte 80 is not accurately indicated.

[0039] like Figure 1 and Figure 2As shown, the wound electrode body 20 has a shape in which a positive electrode sheet 50 and a negative electrode sheet 60 are overlapped and wound along their long sides, separated by two elongated insulating sheets 70. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed on one or both sides (here, both sides) of the elongated positive electrode current collector 52 along its long side. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed on one or both sides (here, both sides) of the elongated negative electrode current collector 62 along its long side. The non-formed portion 52a of the positive electrode active material layer (i.e., the portion where the positive electrode current collector 52 is exposed without forming the positive electrode active material layer 54) and the non-formed portion 62a of the negative electrode active material layer (i.e., the portion where the negative electrode current collector 62 is exposed without forming the negative electrode active material layer 64) are formed in such a way that they protrude outward from both ends of the winding axis of the wound electrode body 20 (i.e., the sheet width direction orthogonal to the aforementioned long side direction). The non-forming portion 52a of the positive electrode active material layer and the non-forming portion 62a of the negative electrode active material layer are respectively bonded with a positive electrode current collector 42a and a negative electrode current collector 44a.

[0040] As the positive current collector 52, a known positive current collector used in lithium-ion secondary batteries can be used, such as a sheet or foil made of a metal with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive current collector 52.

[0041] The size of the positive current collector 52 is not particularly limited, and can be appropriately determined according to the battery design. When aluminum foil is used as the positive current collector 52, its thickness is not particularly limited, for example, it is 5μm to 35μm, preferably 7μm to 20μm.

[0042] In this embodiment, the positive electrode active material uses a lithium composite oxide (spinel-type manganese-containing composite oxide) having a spinel-type crystal structure and containing Mn. Examples of such composite oxides include, for instance, lithium manganese oxide (LiMn2O4) with a spinel-type crystal structure, and composite oxides with a spinel-type crystal structure in which a portion of the manganese in lithium manganese oxide is replaced by lithium or other elements (e.g., LiNi). 0.5 Mn 1.5 O4, etc.

[0043] As a spinel-type manganese-containing composite oxide, specifically, for example, a composite oxide with the composition represented by the following formula (I) can be used.

[0044] Li x (M1 y M2 z Mn 2-x-y-z )O 4-δ ···(I)

[0045] In formula (I), M1 is at least one element selected from Ni, Co and Fe, preferably Ni. M2 is at least one element selected from Na, Mg, Al, P, K, Ca, Ba, Sr, Ti, V, Cr, Cu, Ga, Y, Zr, Nb, Mo, In, Ta, W, Re and Ce, preferably Ti, Al or Mg.

[0046] In equation (I), x satisfies 1.00≤x≤1.20, preferably 1.00≤x≤1.05, and more preferably 1.00. y satisfies 0≤y≤1.20, preferably 0≤y≤0.60, and more preferably 0. Z satisfies 0≤z≤0.5, preferably 0≤z≤0.10, and more preferably 0. δ satisfies 0≤δ≤0.20, preferably 0≤δ≤0.05, and more preferably 0.

[0047] In this embodiment, a spinel-type manganese-containing composite oxide with a specific composition can be used alone, or two or more spinel-type manganese-containing composite oxides with different compositions can be used in combination. When a non-aqueous electrolyte secondary battery using LiMn2O4 is repeatedly charged and discharged at high temperatures, its capacity deteriorates significantly. Therefore, in this embodiment, when the spinel-type manganese-containing composite oxide is LiMn2O4, the capacity deterioration suppression effect of the battery in this embodiment becomes more pronounced, which is advantageous. Furthermore, the use of LiMn2O4 can impart higher thermal stability to a non-aqueous electrolyte secondary battery using cathode 50, and also has the advantage of cost reduction.

[0048] In this embodiment, the spinel-type manganese-containing composite oxide may have cracked portions. These cracks can typically be generated through processes such as pressure treatment during the high-density enrichment of the positive electrode active material layer 54.

[0049] The average particle size (median particle size D50) of the positive electrode active material is not particularly limited, for example, it is 0.05 μm to 25 μm, preferably 0.5 μm to 23 μm, and more preferably 3 μm to 22 μm. It should be noted that, unless otherwise specified, the average particle size (median particle size D50) in this specification refers to the particle size that accounts for 50% of the cumulative degree from the smallest particle size side in the particle size distribution determined by laser diffraction scattering.

[0050] The positive electrode active material layer 54 may contain positive electrode active materials other than spinel-type manganese-containing composite oxides, within a range that does not significantly hinder the effects of the present invention. The content of the positive electrode active material is not particularly limited, but it is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more, in the positive electrode active material layer 54 (i.e., relative to the total mass of the positive electrode active material layer 54).

[0051] In this embodiment, the positive electrode active material layer 54 further contains orthophosphoric acid (H3PO4). Orthophosphoric acid is a component that contributes to the formation of the modified coating, which contains a phosphoric acid (P) component. To appropriately obtain an improved capacity degradation resistance effect based on this coating, the content of orthophosphoric acid relative to the positive electrode active material is 0.05% to 1.0% by mass. From the viewpoint of further improving capacity degradation resistance, the content of orthophosphoric acid relative to the positive electrode active material is preferably 0.08% by mass or more, more preferably 0.1% by mass or more. On the other hand, the content of orthophosphoric acid relative to the positive electrode active material is preferably 0.5% by mass or less.

[0052] The positive electrode active material layer 54 may contain components other than the positive electrode active material. Examples include lithium triphosphate, conductive materials, binders, and phosphonic acid.

[0053] Lithium triphosphate (Li3PO4) is also a component that helps form a film on the surface of the positive electrode active material. When the positive electrode active material layer 54 contains lithium triphosphate, the film formed on the surface of the positive electrode active material from orthophosphoric acid can be further modified. As a result, the capacity degradation resistance of the lithium-ion secondary battery 100 during repeated charge-discharge cycles at high temperatures can be further improved.

[0054] The particle size of lithium triphosphate is not particularly limited. The smaller the particle size of lithium triphosphate, the larger its specific surface area, and the easier it is to be consumed in the formation of a coating. That is, the smaller the particle size of lithium triphosphate particles, the more favorable it is for coating formation. Therefore, the average particle size (median particle size D50) of lithium triphosphate is preferably 10 μm or less, more preferably 5 μm or less, and even more preferably 3 μm or less. On the other hand, the average particle size of lithium triphosphate can be 0.05 μm or more, or 0.1 μm or more.

[0055] The content of trilithium phosphate in the positive electrode active material layer 54 is not particularly limited, but is, for example, 0.01% to 10% by mass relative to the positive electrode active material, preferably 0.1% to 5% by mass, more preferably 0.2% to 3% by mass, and even more preferably 0.2% to 1% by mass.

[0056] As a conductive material, carbon black such as acetylene black (AB) or other carbon materials (such as graphite) can be appropriately used. The content of the conductive material in the positive electrode active material layer 54 is not particularly limited, for example, it is 0.1% to 20% by mass, preferably 1% to 15% by mass, and more preferably 2% to 10% by mass.

[0057] As a binder, polyvinylidene fluoride (PVdF) can be used, for example. The content of the binder in the positive electrode active material layer 54 is not particularly limited, but is, for example, 0.5% to 15% by mass, preferably 1% to 10% by mass, and more preferably 1.5% to 8% by mass.

[0058] Phosphonic acid is also a component that helps form a film on the surface of the positive electrode active material. When the positive electrode active material layer 54 contains phosphonic acid, the film on the surface of the positive electrode active material formed by orthophosphoric acid can be further modified. As a result, the capacity degradation tolerance during repeated charge-discharge cycles of the lithium-ion secondary battery 100 can be further improved. The content of phosphonic acid is not particularly limited, for example, it is 0.05% to 1.0% by mass.

[0059] The density of the positive electrode active material layer 54 is not particularly limited. The density of the positive electrode active material layer 54 can be 2.0 g / cm³. 3 The above can also be 2.3 g / cm³. 3 The above results in a positive electrode active material layer 54 with a density of 2.6 g / cm³. 3 During the above process, numerous cracks can easily form in the lithium manganese oxide particles due to pressure treatment. Therefore, capacity degradation is likely to be significant. Thus, considering the particularly strong capacity degradation suppression effect based on the film, the density of the positive electrode active material layer 54 is preferably 2.6 g / cm³. 3 That's all. On the other hand, the density of the positive electrode active material layer 54 can be 3.3 g / cm³. 3 The following can also be 3.0 g / cm³ 3 It should be noted that, in this specification, the density of the positive electrode active material layer 54 refers to the apparent density of the positive electrode active material layer 54.

[0060] The thickness of the positive electrode active material layer 54 is not particularly limited, for example, it is 10μm to 300μm, preferably 20μm to 200μm.

[0061] As the negative electrode current collector 62 constituting the negative electrode sheet 60, a known negative electrode current collector used in lithium-ion secondary batteries can be used. Examples of such current collectors include sheets or foils made of metals with good conductivity (e.g., copper, nickel, titanium, stainless steel, etc.). Copper foil is preferred as the negative electrode current collector 62.

[0062] The size of the negative electrode current collector 62 is not particularly limited, and can be appropriately determined according to the battery design. When copper foil is used as the negative electrode current collector 62, its thickness is not particularly limited, for example, it is 5μm to 35μm, preferably 7μm to 20μm.

[0063] The negative electrode active material layer 64 contains a negative electrode active material. In this embodiment, graphite is used as the negative electrode active material. The graphite can be natural graphite, artificial graphite, or amorphous carbon-coated graphite in the form of graphite coated with amorphous carbon material.

[0064] The average particle size (median particle size D50) of the negative electrode active material is not particularly limited, for example, it is 0.1 μm to 50 μm, preferably 1 μm to 25 μm, and more preferably 5 μm to 20 μm.

[0065] In addition to graphite, the negative electrode active material layer 64 may also contain positive electrode active materials other than graphite, within a range that does not significantly hinder the effects of the present invention. The content of the negative electrode active material in the negative electrode active material layer 64 is not particularly limited, but is preferably 90% by mass or more, more preferably 95% by mass or more.

[0066] The negative electrode active material layer 64 may contain components other than the negative electrode active material, such as binders, thickeners, etc.

[0067] As a binder, for example, styrene-butadiene rubber (SBR) and its modifiers, acrylonitrile-butadiene rubber and its modifiers, acrylic rubber and its modifiers, fluororubber, etc., can be used. Among these, SBR is preferred. The content of the binder in the negative electrode active material layer 64 is not particularly limited, but is preferably 0.1% to 8% by mass, more preferably 0.2% to 3% by mass.

[0068] As a thickener, for example, cellulose-based polymers such as carboxymethyl cellulose (CMC), methyl cellulose (MC), cellulose acetate phthalate (CAP), and hydroxypropyl methyl cellulose (HPMC) can be used; polyvinyl alcohol (PVA), etc. are also suitable. Among these, CMC is preferred. The content of the thickener in the negative electrode active material layer 64 is not particularly limited, but is preferably 0.3% to 3% by mass, more preferably 0.4% to 2% by mass.

[0069] The thickness of the negative electrode active material layer 64 is not particularly limited, for example, it is 10μm to 300μm, preferably 20μm to 200μm.

[0070] Examples of insulating materials 70 include porous sheets (membranes) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. These porous sheets can be single-layered or multi-layered (e.g., a three-layered structure with PP layers laminated on both sides of a PE layer). A heat-resistant layer (HRL) can be provided on the surface of the insulating material 70.

[0071] The thickness of the spacer 70 is not particularly limited, for example, it is 5μm to 50μm, preferably 10μm to 30μm.

[0072] The non-aqueous electrolyte 80 contains a fluorinated lithium salt. Typically, the non-aqueous electrolyte 80 contains a non-aqueous solvent and a fluorinated lithium salt as the electrolyte salt (i.e., the supporting salt). As the non-aqueous solvent, various organic solvents used in common lithium-ion secondary battery electrolytes, such as carbonates, ethers, esters, nitriles, sulfones, and lactones, can be used without particular restriction. Among these, carbonates are preferred, and specific examples include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethylene monofluorocarbonate (MFEC), ethylene difluorocarbonate (DFEC), methyl monofluorodifluoromethyl carbonate (F-DMC), and dimethyl trifluorocarbonate (TFDMC). Such a non-aqueous solvent can be used alone or in appropriate combinations of two or more.

[0073] Examples of fluorinated lithium salts include LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI). Furthermore, fluorinated lithium salts are also components that contribute to the formation of a coating, which contains fluorine (F) from the fluorinated lithium salt. LiPF6 is preferred as the fluorinated lithium salt from the perspective of easily supplying a sufficient amount of F to the coating. The concentration of the fluorinated lithium salt is not particularly limited, but from the perspective of easily supplying a sufficient amount of F to the coating, it is preferably 0.8 mol / L or more, more preferably 1.0 mol / L or more. On the other hand, from the viewpoint of suppressing the increase in battery resistance caused by the increased viscosity of the non-aqueous electrolyte 80, the concentration of the fluorinated lithium salt is preferably 1.8 mol / L or less, more preferably 1.5 mol / L or less.

[0074] In the prior art, the decomposition of the non-aqueous electrolyte near the negative electrode is suppressed by using a spinel-structured, titanium-containing lithium transition metal oxide with low reactivity to the non-aqueous electrolyte as the negative electrode active material. In contrast, in this embodiment, graphite, which has higher reactivity to the non-aqueous electrolyte than the spinel-structured, titanium-containing lithium transition metal oxide, is used as the negative electrode active material. Therefore, to suppress the decomposition of the non-aqueous electrolyte near the negative electrode 60, it is preferable that the non-aqueous electrolyte 80 contains a lithium oxalate complex. The lithium oxalate complex functions as a negative electrode film-forming agent, suppressing the decomposition of the non-aqueous electrolyte near the negative electrode 60 by forming a film from the lithium oxalate complex on the negative electrode 60, and further improving the capacity degradation tolerance of the lithium-ion secondary battery 100 under repeated charge-discharge cycles at high temperatures.

[0075] As a lithium salt of oxalate complex, at least one oxalate ion (C2O4) can be used. 2－ A salt of lithium ions formed by the coordination bonding of a coordinating anion with a central element (also called a coordinating atom). Examples of central elements include boron (B) and phosphorus (P).

[0076] As a specific example of lithium salts containing oxalate complexes, one could cite those having at least one oxalate ion (C2O4) coordinated to boron (B) as the central atom. 2－ Compounds with a 4-coordinate structural moiety, such as lithium dioxolaneborate (Li[B(C2O4)2]; LiBOB) and lithium difluorooxolaneborate (Li[BF2(C2O4)]; LiDFOB); having at least one oxalate ion (C2O4) coordinated at phosphorus (P) as the central atom. 2－ Compounds with a 6-coordinate structure, such as lithium bis(oxalato) phosphate (Li[P(C2O4)3]) and lithium difluorobis(oxalato) phosphate (Li[PF2(C2O4)2]; LPFO), are preferred. Among these, LiBOB is preferred because it can form a more durable film on the surface of the negative electrode active material, significantly improving the resistance to capacity degradation of the lithium-ion secondary battery 100 during repeated charge-discharge cycles at high temperatures.

[0077] It should be noted that the above-mentioned non-aqueous electrolyte 80 may contain components other than those mentioned above, such as gas generating agents like biphenyl (BP) and cyclohexylbenzene (CHB), and various additives like thickeners, as long as it does not significantly impair the effects of the present invention.

[0078] By initially charging the lithium-ion secondary battery 100 constructed as described above to a voltage of 4.7V or higher, a modified film can be formed on the surface of the positive electrode active material. This suppresses capacity degradation of the lithium-ion secondary battery 100 during repeated charge-discharge cycles at high temperatures (e.g., around 60°C).

[0079] Specifically, Mn readily dissolves from spinel-type manganese-containing composite oxides, causing Li ions to deactivate and leading to capacity degradation. Therefore, the existing technology suffers from the following problem: lithium-ion secondary batteries using spinel-type manganese-containing composite oxides as the positive electrode active material experience significant capacity degradation during repeated charge-discharge cycles at high temperatures.

[0080] In contrast, the lithium-ion secondary battery 100 of this embodiment uses graphite as the negative electrode active material, enabling initial charging to a high voltage of 4.7V or higher. This voltage of 4.7V or higher is typically a high voltage that leads to the degradation of the positive electrode active material. However, in this embodiment, by presenting phosphoric acid in the positive electrode active material layer 54, the degradation of the positive electrode active material can be suppressed, and a phosphorus-enriched coating layer is formed on the surface of the positive electrode active material, thereby enabling the formation of a fluorine-enriched coating layer thereon. Based on this multilayer structure of phosphorus-enriched coating layer and fluorine-enriched coating layer, the dissolution of Mn from the positive electrode active material can be suppressed, and the deactivation of Li ions caused by the dissolved Mn can be suppressed. As a result, the capacity degradation of the secondary battery 100 due to repeated charge-discharge cycles at high temperatures can be suppressed.

[0081] Therefore, from other perspectives, the manufacturing method of the non-aqueous electrolyte secondary battery of this embodiment includes: a step of preparing the non-aqueous electrolyte secondary battery (i.e., the non-aqueous electrolyte secondary battery (1)) as described above (hereinafter also referred to as "step A"), and a step of applying an initial charge to the prepared non-aqueous electrolyte secondary battery to a voltage of 4.7V or higher (hereinafter also referred to as "step B"). Hereinafter, the manufacturing method will be described using the case where the non-aqueous electrolyte secondary battery (1) is the lithium-ion secondary battery 100 described above as an example.

[0082] First, process A will be explained. The lithium-ion secondary battery 100 can be prepared by manufacturing according to known methods.

[0083] Specifically, for example, a paste for forming the positive electrode active material layer is prepared by mixing a positive electrode active material containing a spinel-type manganese composite oxide, 0.05% to 1.0% by mass of orthophosphoric acid relative to the positive electrode active material, any component of the positive electrode active material layer 54 (e.g., lithium triphosphate, binder, etc.), and a solvent (e.g., N-methylpyrrolidone, etc.). This paste is then coated onto the positive electrode current collector 52 and dried to form the positive electrode active material layer 54. If necessary, the positive electrode active material layer 54 is subjected to pressure treatment to obtain the positive electrode sheet 50.

[0084] Here, cracks can be generated in the particles of spinel-type manganese-containing composite oxide through this pressure treatment. The pressure treatment conditions are preferably such that the density of the positive electrode active material layer 54 is 2.0 g / cm³. 3 The above, and more preferably 2.3 g / cm 3 The above, and more preferably 2.6 g / cm³ 3 The above method is used. The density of the positive electrode active material layer 54 after pressure treatment can be 3.3 g / cm³. 3 Below or 3.0g / cm 3 the following.

[0085] A paste for forming the negative electrode active material layer is prepared by mixing a graphite-containing negative electrode active material, any component of the negative electrode active material layer 64 (e.g., binder, thickener, etc.), and a solvent (e.g., water). This paste is then coated onto the negative electrode current collector 62 and dried to form the negative electrode active material layer 64. If necessary, the negative electrode active material layer 64 is subjected to pressure treatment to obtain the negative electrode sheet 60.

[0086] It should be noted that in this specification, "paste" refers to a mixture in which some or all of the solid components are dispersed in a solvent, including so-called "slurry," "ink," etc.

[0087] Prepare a separator 70, and overlap the positive electrode 50 and negative electrode 60 sandwiching the separator 70 to form an electrode body 20. The electrode body 20, together with the aforementioned non-aqueous electrolyte 80, is then housed in a battery casing 30 and sealed. This allows the fabrication of a lithium-ion secondary battery 100.

[0088] Specifically, for example, in the case where the electrode body 20 is a wound electrode body as shown in the example figure, such as Figure 2 As shown, a laminate is formed by overlapping positive electrode 50 and negative electrode 60 with two separators 70. After forming a winding body that is wound in the longitudinal direction, the winding body is flattened by pressure treatment or the like, thereby forming electrode body 20. When electrode body 20 is a laminated electrode body, electrode body 20 is formed by alternately stacking multiple positive electrode 50 and multiple negative electrode 60 with separators 70 sandwiched between them.

[0089] As the battery casing 30, for example, a battery casing is prepared having a casing body with an opening and a cover that seals the opening. The cover is provided with an injection port (not shown) for injecting a non-aqueous electrolyte 80.

[0090] A positive terminal 42 and a positive current collector 42a, and a negative terminal 44 and a negative current collector 44a are installed on the cover of the battery casing 30. The positive current collector 42a and the negative current collector 44a are welded to the non-formed portions 52a and 62a of the positive active material layer exposed at the ends of the electrode body 20, respectively. Then, the electrode body 20 is received inside the opening of the main body of the battery casing 30, and the main body of the battery casing 30 is welded to the cover.

[0091] Next, a non-aqueous electrolyte 80 is injected through the injection port, and then the injection port is sealed. This yields a lithium-ion secondary battery 100.

[0092] Next, process B will be explained. In process B, the lithium-ion secondary battery 100 is initially charged to a voltage of 4.7V or higher. The initial charging process can be performed using a known charger, etc.

[0093] By initially charging to such a high voltage, a modified film can be formed on the surface of the positive electrode active material. From the perspective of maximizing the capacity degradation suppression effect, the initial charging process is preferably performed to a voltage of 4.8V or higher.

[0094] As an example of the initial charging process, the device is first charged with a constant current, for example, 0.05C to 2C (preferably 0.05C to 1C), until a voltage of 4.7V or higher is achieved. There is no particular upper limit to the voltage during the initial charging. An upper limit is, for example, 5.1V, preferably 5.0V.

[0095] If charging is performed until the voltage reaches 4.7V or higher, a coating can be formed. However, to increase the amount of coating, constant voltage charging can be performed after constant current charging. The constant voltage charging time is not particularly limited, for example, it is 1 hour to 10 hours, preferably 3 hours to 7 hours.

[0096] By implementing the above procedures, a lithium-ion secondary battery 100 with a film formed on the surface of the positive electrode active material can be obtained.

[0097] At least a portion of the coating on the surface of the positive electrode active material has a multilayer structure comprising a lower layer enriched with phosphorus (P) and an upper layer enriched with fluorine (F). In the lower layer enriched with phosphorus, the content (atomic %) of P relative to the total content (atomic %) of P and F, as determined by scanning transmission electron microscopy / energy dispersive X-ray diffraction (STEM-EDX), can be 67% or more. Furthermore, in the upper layer enriched with fluorine, the content (atomic %) of F relative to the total content (atomic %) of P and F, as determined by STEM-EDX, can be 38% or more.

[0098] Therefore, from other perspectives, the second embodiment of the non-aqueous electrolyte secondary battery disclosed herein (hereinafter also referred to as "non-aqueous electrolyte secondary battery (2)") includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode includes a positive electrode active material layer containing a positive electrode active material. The positive electrode active material comprises a lithium composite oxide having a spinel-type crystal structure and containing Mn. The negative electrode includes a negative electrode active material layer containing a negative electrode active material. The negative electrode active material is graphite. The non-aqueous electrolyte contains a fluorinated lithium salt. The positive electrode active material has a coating on its surface. At least a portion of the coating has a multilayer structure having a first layer and a second layer located on the first layer. In the first layer, the content (atomic %) of P element relative to the total content (atomic %) of P element and F element, as determined by STEM-EDX, is 67% or more. In this second layer, the proportion of F element content (atomic%) relative to the total content (atomic%) of P and F elements, as determined by STEM-EDX, is 38% or more.

[0099] Taking the lithium-ion secondary battery 100 as an example, the non-aqueous electrolyte secondary battery (2) has the above-mentioned film formed on the surface of the positive electrode active material of the lithium-ion secondary battery 100.

[0100] In the non-aqueous electrolyte secondary battery (2), the content of orthophosphoric acid in the positive electrode active material layer 54 decreases due to the formation of the film and can be 0% by mass. In the non-aqueous electrolyte secondary battery (2), orthophosphoric acid is not an essential component. Therefore, the content of orthophosphoric acid in the positive electrode active material layer 54 relative to the positive electrode active material is more than 0% by mass and less than 1.0% by mass, and can be more than 0% by mass and less than 0.5% by mass.

[0101] In the first layer of the multilayer film, the content (atomic%) of P element relative to the total content (atomic%) of P and F elements, as determined by STEM-EDX, is 67% or more, preferably 70% or more, and more preferably 75% or more. Furthermore, the content (atomic%) of P element relative to the total content of P and F elements can be 90% or less, or 85% or less. Therefore, the content of F element relative to the total content of P and F elements is 33% or less, preferably 30% or less, and more preferably 25% or less. Additionally, the content of F element relative to the total content of P and F elements can be 10% or more, or 15% or more.

[0102] The content of F element in the first layer can be 0.5 atomic percent or more, 1.0 atomic percent or more, 1.5 atomic percent or more, or 2.0 atomic percent or more. On the other hand, the content of F element in the first layer can be 5.0 atomic percent or less, 4.0 atomic percent or less, or 3.5 atomic percent or less.

[0103] The content of P element in the first layer can be 7.0 atomic percent or more, 8.0 atomic percent or more, 9.0 atomic percent or more, or 9.5 atomic percent or more. On the other hand, the content of P element in the first layer can be less than 14.0 atomic percent, less than 12.0 atomic percent, or less than 11.0 atomic percent.

[0104] In the second layer of the multilayer film, the proportion of F element content (atomic %) relative to the total content (atomic %) of P and F elements, as determined by STEM-EDX, is 38% or more, preferably 40% or more, and more preferably 45% or more. Furthermore, the proportion of P element content (atomic %) relative to the total content of P and F elements can be 62% or less, or 60% or less. Therefore, the proportion of F element content relative to the total content of P and F elements is 62% or less, preferably 60% or less, and more preferably 55% or less. Furthermore, the proportion of P element content relative to the total content of P and F elements can be 38% or more, or 40% or more.

[0105] The content of F element in the second layer can be 3.0 atomic percent or more, 4.0 atomic percent or more, 5.0 atomic percent or more, or 6.0 atomic percent or more. On the other hand, the content of F element in the second layer can be less than 10.0 atomic percent, less than 9.0 atomic percent, or less than 8.0 atomic percent.

[0106] The content of P in the second layer can be 4.0 atomic percent or more, 5.0 atomic percent or more, 6.0 atomic percent or more, or 6.5 atomic percent or more. On the other hand, the content of P in the second layer can be less than 12.0 atomic percent, less than 10.0 atomic percent, or less than 9.0 atomic percent.

[0107] It should be noted that the content (atomic percent) of P and F elements in the first and second layers can be determined by obtaining a high-angle annular dark-field image (STEM-HAADF image) of the film using a scanning transmission electron microscope (STEM) equipped with an energy-dispersive X-ray spectroscopy analyzer, and then performing EDX analysis on the STEM-HAADF image. EDX analysis is performed, for example, by dividing the film into regions perpendicular to its thickness direction to analyze a distance of at least 20 nm. Therefore, the dimensions of the first and second layers in the direction perpendicular to the thickness direction can be greater than 20 nm.

[0108] In the case where the positive electrode active material has cracks, in the non-aqueous electrolyte secondary battery (2), a film is formed on the surface including the surface of the cracks in the positive electrode active material. In other words, the positive electrode active material has a film on its outer surface (or outer peripheral surface) and the surface of the cracks.

[0109] The coating formed on the surface of the positive electrode active material may have a multilayer structure in which a portion of it possesses a first layer with a high phosphorus content and a second layer with a high F content, as described above. Preferably, at least 25% (particularly 50% or more, further 75% or more) of the area covered along the surface direction (i.e., circumferential direction) of the positive electrode active material has this multilayer structure in the thickness direction. The coating may be dispersed on the surface of the positive electrode active material layer or may cover the entire surface of the positive electrode active material layer.

[0110] The thickness of the film formed on the surface of the positive electrode active material is, for example, less than 20 nm (especially 1 nm to 20 nm), but is not limited thereto.

[0111] It should be noted that the square lithium-ion secondary battery 100 with a flat, wound electrode body 20 is described as an example. However, the non-aqueous electrolyte secondary battery (1) and non-aqueous electrolyte secondary battery (2) disclosed herein can also be configured as lithium-ion secondary batteries with stacked electrode bodies (i.e., electrode bodies obtained by alternating stacking of multiple positive electrodes and multiple negative electrodes). In addition, the non-aqueous electrolyte secondary battery (1) and non-aqueous electrolyte secondary battery (2) disclosed herein can also be configured as cylindrical lithium-ion secondary batteries, laminated shell type lithium-ion secondary batteries, coin-shaped lithium-ion secondary batteries, etc. Furthermore, the non-aqueous electrolyte secondary battery (1) and non-aqueous electrolyte secondary battery (2) disclosed herein can also be configured as non-aqueous electrolyte secondary batteries other than lithium-ion secondary batteries according to known methods.

[0112] Non-aqueous electrolyte secondary batteries (1) and (2) can be used for various applications. Suitable applications include power supplies for driving electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Additionally, non-aqueous electrolyte secondary batteries (1) and (2) can be used as batteries for small-scale energy storage devices. Typically, non-aqueous electrolyte secondary batteries (1) and (2) can also be used in the form of battery packs consisting of multiple batteries connected in series and / or parallel.

[0113] The following describes embodiments of the present invention, but it is not intended to limit the present invention to the technical solutions shown in the above embodiments.

[0114] <Evaluation of Each Embodiment: Fabrication of a Lithium-ion Secondary Battery>

[0115] LiMn₂O₄, used as the positive electrode active material, and orthophosphoric acid in amounts shown in Table 1 relative to the positive electrode active material were mixed in N-methyl-2-pyrrolidone (NMP), and the LiMn₂O₄ was brought into contact with the orthophosphoric acid for surface treatment. Carbon black (CB), used as a conductive material, and polyvinylidene fluoride (PVDF), used as a binder, were added to this mixture in a mass ratio of LiMn₂O₄:CB:PVDF = 90:8:2 to disperse the solid components and prepare a slurry for forming the positive electrode active material layer. In Examples 4-8, trilithium phosphate (LPO) was further added in amounts shown in Table 1 relative to the positive electrode active material to prepare the slurry for forming the positive electrode active material layer. It should be noted that the orthophosphoric acid used was a reagent manufactured by Merck.

[0116] The positive electrode active material layer is formed by coating a slurry onto aluminum foil, drying it, and then performing a high-density treatment by rolling to produce the positive electrode sheet. The positive electrode sheet is then cut into dimensions of 120mm × 100mm.

[0117] In addition, spherical graphite (C) as the negative electrode active material, styrene-butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the thickener were mixed in water at a mass ratio of C:SBR:CMC = 98:1:1 to prepare a paste for forming the negative electrode active material layer. This paste was coated onto copper foil, dried, and then subjected to high-density treatment by rolling to produce a negative electrode sheet. The negative electrode sheet was then cut into dimensions of 122 mm × 102 mm.

[0118] A porous polyolefin sheet was prepared as a separator. An electrode body was fabricated using the aforementioned positive electrode, negative electrode, and separator. After the electrode terminals were installed on the electrode body, it was housed in a battery casing along with a non-aqueous electrolyte. The non-aqueous electrolyte was a solution obtained by dissolving LiPF6 at a concentration of 1.1 mol / L in a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3:4:3, and dissolving lithium bis(oxalato)borate (LiBOB) at a concentration of 0.5% by mass. In Example 9, LiBOB was not added. Evaluation lithium-ion secondary batteries for each example were thus fabricated.

[0119] The obtained evaluation lithium-ion secondary batteries were initially charged at 25°C with a constant current of 0.1C until the voltage shown in Table 1 was reached. Then, constant voltage charging was performed until the current reached 1 / 50C. This formed a film on the positive electrode. Subsequently, constant current discharge was performed at 0.1C until 3.0V was reached.

[0120] <Evaluation of Comparative Examples 1 and 2: Fabrication of Lithium-ion Secondary Batteries>

[0121] LiMn2O4, CB, and PVDF, serving as the positive electrode active materials, were mixed in NMP at a mass ratio of LiMn2O4:CB:PVDF = 90:8:2 to disperse the solid components and prepare a slurry for forming the positive electrode active material layer. Using this slurry, an evaluation lithium-ion secondary battery was fabricated using the same method as in Example 1, and an initial charging treatment was performed to form a coating on the positive electrode.

[0122] <Comparative Example 3 Evaluation: Fabrication of a Lithium-ion Secondary Battery>

[0123] The aforementioned LiMn2O4, CB, and PVDF, which are attached with orthophosphoric acid and serve as positive electrode active materials, are mixed in NMP at a mass ratio of LiMn2O4:CB:PVDF = 90:8:2 to disperse the solid components and prepare a slurry for forming a positive electrode active material layer. Using this slurry, an evaluation lithium-ion secondary battery is fabricated using the same method as in Example 1, and an initial charging treatment is performed to form a coating on the positive electrode.

[0124] <Comparative Example 4: Evaluation of the Fabrication of a Lithium-ion Secondary Battery>

[0125] A slurry for forming the positive electrode active material layer was prepared in the same manner as in Comparative Example 1. Using this slurry, without adding LiBOB to the non-aqueous electrolyte, an evaluation lithium-ion secondary battery was fabricated using the same method as in Example 1, and an initial charging treatment was performed to form a coating on the positive electrode.

[0126] <Cyclic Characteristic Evaluation>

[0127] The initial capacity was determined by measuring the discharge capacity after the initial charge. Each evaluation lithium-ion secondary battery that underwent the initial charge was placed in an environment of 60°C and subjected to 50 charge-discharge cycles, with one cycle defined as charging at a constant current of 0.5C to 4.2V and discharging at a constant current of 0.5C to 3.0V. The discharge capacity after 50 cycles was calculated using the same method as for the initial capacity. The capacity retention rate (%) was calculated as an indicator of cycle characteristics (capacity degradation tolerance) by multiplying (discharge capacity after 50 charge-discharge cycles / initial capacity) by 100. The results are shown in Table 1.

[0128] [Table 1]

[0129]

[0130] As shown in Table 1, it can be seen that in lithium-ion secondary batteries, when the positive electrode active material layer contains spinel-type manganese-containing composite oxide as the positive electrode active material and contains orthophosphate at a mass ratio of 0.05% to 1.0% relative to the positive electrode active material, and the lithium-ion secondary battery is initially charged to a voltage of 4.7V or higher, the capacity retention rate is significantly higher. Therefore, it can be seen that by performing appropriate initial charging treatment on the non-aqueous electrolyte secondary battery (1) described above, capacity degradation during repeated charge-discharge cycles at high temperatures can be suppressed. Furthermore, it can be seen that when the positive electrode active material layer further contains lithium triphosphate, capacity degradation can be further suppressed. Furthermore, it can be seen that when the non-aqueous electrolyte further contains LiBOB, capacity degradation can be further suppressed.

[0131] <STEM-EDX-based analysis of membrane>

[0132] The lithium-ion secondary battery used in Example 5, which underwent the above evaluation, was disassembled under an argon atmosphere, and the positive electrode was removed. The positive electrode was cleaned with ethyl methyl carbonate to remove the electrolyte and then dried. The positive electrode was then resin-embedded and cut using a focused ion beam (FIB) to prepare the material for measurement. It was observed using a scanning transmission electron microscope (Cs-STEM) with spherical aberration correction to obtain a STEM-HAADF image. The STEM-HAADF image is shown below. Figure 3 .like Figure 3 As shown, analytical regions 7–11 were defined, and the constituent elements and their contents (atomic %) in these regions were determined by energy-dispersive X-ray diffraction (EDX). The determination results for analytical regions 7–10 are shown in Table 2.

[0133] [Table 2]

[0134] Analysis area B C O F Al P S Mn 7 0.0 27.4 46.1 6.7 2.6 7.0 0.7 9.5 8 0.4 13.2 59.1 2.8 0.4 9.9 0.0 14.2 9 0.0 7.9 57.3 0.9 0.6 2.2 0.0 31.0 10 0.0 5.5 64.6 0.0 0.6 0.1 0.0 29.1

[0135] The numbers for each constituent element in the table are in atomic percentages.

[0136] according to Figure 3 As shown in Table 2, a layer enriched with phosphorus (P) was formed in analysis region 8, and a layer enriched with sulfur (F) was formed in analysis region 7. Therefore, it can be said that the special coating formed on the P-enriched layer and then enriched with F improves the capacity degradation tolerance of the lithium-ion secondary battery during repeated charge-discharge cycles. Furthermore, in analysis region 8, the P content (atomic %) relative to the total P and F content (atomic %) is 78%. In analysis region 7, the F content (atomic %) relative to the total P and F content (atomic %) is 48.9%.

[0137] Therefore, it can be seen that the non-aqueous electrolyte secondary battery (2) disclosed herein can suppress capacity degradation during repeated charging and discharging at high temperatures.

[0138] The specific examples of the present invention have been described in detail above, but these are merely illustrative and do not limit the scope of protection claimed. The technology described in the scope of protection includes technologies obtained by various modifications and alterations to the specific examples described above.

Claims

1. A non-aqueous electrolyte secondary battery, comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode has a positive electrode active material layer containing positive electrode active material. The positive electrode active material comprises a lithium composite oxide having a spinel-type crystal structure and containing Mn. The positive electrode active material layer contains 0.05% to 1.0% by mass of phosphoric acid relative to the positive electrode active material. The negative electrode has a negative electrode active material layer containing negative electrode active material. The negative electrode active material is graphite. The non-aqueous electrolyte contains fluorinated lithium salts. The non-aqueous electrolyte further contains lithium oxalate complex salt. The positive electrode active material layer further contains lithium triphosphate. The content of lithium triphosphate in the positive electrode active material layer is 0.2% to 0.8% by mass relative to the positive electrode active material.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein, The positive electrode active material layer contains 0.1% to 0.5% by mass of phosphoric acid relative to the positive electrode active material.

3. A method for manufacturing a non-aqueous electrolyte secondary battery, wherein the non-aqueous electrolyte secondary battery has a coating on the surface of the positive electrode active material, the manufacturing method comprising the following steps: The process of preparing the non-aqueous electrolyte secondary battery according to claim 1 or 2, and The prepared non-aqueous electrolyte secondary battery is subjected to an initial charging process to a voltage of 4.7V or higher.

4. A non-aqueous electrolyte secondary battery, comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode has a positive electrode active material layer containing positive electrode active material. The positive electrode active material comprises a lithium composite oxide having a spinel-type crystal structure and containing Mn. The negative electrode has a negative electrode active material layer containing negative electrode active material. The negative electrode active material is graphite. The non-aqueous electrolyte contains fluorinated lithium salts. The positive electrode active material has a film on its surface. At least a portion of the coating has a multilayer structure comprising a first layer and a second layer located on the first layer. In the first layer, the proportion of P element content relative to the total content of P and F elements, as determined by scanning transmission electron microscopy / energy dispersive X-ray diffraction analysis, is 67% or more. The unit of content is atomic percentage. In the second layer, the proportion of F element content relative to the total content of P and F elements, as determined by scanning transmission electron microscopy / energy dispersive X-ray diffraction analysis, is 38% or more, where the content is measured in atoms.

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