Lithium secondary battery and electrode for lithium secondary battery

Abstract

This lithium secondary battery comprises: an electrode group including a positive electrode, a negative electrode, and a separator arranged between the positive electrode and the negative electrode; and a nonaqueous electrolyte. In the negative electrode, lithium metal is precipitated during charging, lithium metal is dissolved in the nonaqueous electrolyte during discharging, and at least a part of a surface of the negative electrode is covered with a protective layer. The protective layer includes a vinylidene fluoride-based polymer. The vinylidene fluoride-based polymer includes a crystal portion. The crystal portion includes at least one of a beta crystal phase and a gamma crystal phase, and the ratio of the total of the number of moles (M beta) of monomer units constituting the beta crystal phase and the number of moles (M gamma) of monomer units constituting the gamma crystal phase to the number of moles (Mt) of all monomer units constituting the vinylidene fluoride-based polymer is 6 mol% or more.

Description

Lithium-ion batteries and electrodes for lithium-ion batteries Cross-reference of related applications

[0001] This disclosure claims priority rights to Japanese Patent Application No. 2025-025244, filed with the Japan Patent Office on 19 February 2025, and the entirety of the said patent application is incorporated herein by reference.

[0002] This disclosure relates to lithium secondary batteries and electrodes for lithium secondary batteries.

[0003] Lithium-ion batteries (lithium metal secondary batteries) are used in a variety of applications as high-capacity secondary batteries. In lithium-ion batteries, lithium metal is deposited on the negative electrode during charging. The deposited lithium metal dissolves in the non-aqueous electrolyte during discharge. Various proposals have been made regarding lithium-ion batteries.

[0004] Patent Document 1 describes a lithium secondary battery comprising a positive electrode, a negative electrode, and a polymer layer disposed on the surface of the negative electrode, wherein the negative electrode includes a negative electrode current collector and a negative electrode active material layer, the negative electrode current collector has a larger planar size than the positive electrode, and the negative electrode current collector has a first region that overlaps with the positive electrode and a second region that does not overlap with the positive electrode, the negative electrode active material layer is selectively provided in the first region of the negative electrode current collector, and the polymer layer includes a copolymer of polyvinylidene fluoride (PVDF) and hexafluoropropylene (HFP) and an ionic liquid, and covers the negative electrode current collector via the negative electrode active material layer in the first region and covers the negative electrode current collector without the negative electrode active material layer in the second region.

[0005] Japanese Patent Publication No. 2020-95931

[0006] In lithium-ion secondary batteries, a problem arises when lithium metal precipitates in a dendrite-like manner during charging. This problem leads to a decrease in charge-discharge cycle characteristics. Currently, there is a need to improve the charge-discharge cycle characteristics of lithium-ion secondary batteries. One of the objectives of this disclosure is to provide a lithium-ion secondary battery with good charge-discharge cycle characteristics.

[0007] One aspect of the present disclosure provides a lithium secondary battery including a positive electrode, a negative electrode, an electrode group including a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. In the negative electrode, lithium metal is deposited during charging and dissolved in the non-aqueous electrolyte during discharging. At least a part of the surface of the negative electrode is covered with a protective layer. The protective layer includes a vinylidene fluoride-based polymer. The vinylidene fluoride-based polymer includes a crystalline portion. The crystalline portion includes at least one of a β crystal phase and a γ crystal phase. The ratio of the total number of moles of monomer units (Mβ + Mγ) constituting the β crystal phase and the γ crystal phase to the total number of moles of all monomer units (Mt) constituting the vinylidene fluoride-based polymer is 6 mol% or more.

[0008] Another aspect of the present disclosure provides a negative electrode for a lithium secondary battery including a negative electrode current collector, a lithium metal-containing layer supported on the negative electrode current collector, and a protective layer formed on at least a part of the surface of the lithium metal-containing layer. The protective layer includes a vinylidene fluoride-based polymer. The vinylidene fluoride-based polymer includes a crystalline portion. The crystalline portion includes at least one of a β crystal phase and a γ crystal phase. The ratio of the total number of moles of monomer units (Mβ + Mγ) constituting the β crystal phase and the γ crystal phase to the total number of moles of all monomer units (Mt) constituting the vinylidene fluoride-based polymer is 6 mol% or more.

[0009] According to the present disclosure, a lithium secondary battery with good charge-discharge cycle characteristics can be obtained. The novel features of the present invention are described in the appended claims. The present invention will be better understood from the following detailed description in conjunction with the drawings, together with other objects and features of the present invention with respect to both the structure and the content.

[0010] It is a cross-sectional view schematically showing an example of a lithium secondary battery according to the present disclosure. It is a diagram schematically showing a cross-section of an electrode group. It is a diagram schematically showing a cross-section of another electrode group.

[0011] Hereinafter, embodiments according to the present disclosure will be described with examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values and materials may be exemplified, but other numerical values and materials may be applied as long as the effects of the present disclosure can be obtained. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be read as "numerical value A or more and numerical value B or less". In the following description, when the lower limit and the upper limit of a numerical value regarding a specific physical property or condition are exemplified, any combination of any of the exemplified lower limits and any of the exemplified upper limits can be made as long as the lower limit is not more than the upper limit. In the following description, when examples of components and methods are listed, unless otherwise specified, only one of the listed examples may be used, or a plurality of the listed examples may be used in combination.

[0012] (Lithium secondary battery) The lithium secondary battery according to this embodiment may hereinafter be referred to as "lithium secondary battery (B)". The lithium secondary battery (B) includes an electrode group including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The electrode group may be an electrode group in which the positive electrode and the negative electrode are wound with the separator interposed therebetween. In the negative electrode, lithium metal is deposited during charging and dissolved in the non-aqueous electrolyte during discharging. That is, the lithium secondary battery (B) is a "lithium metal secondary battery". At least a part of the surface of the negative electrode is covered with a protective layer.

[0013] In lithium secondary batteries, for example, 70% or more of the rated capacity is due to the deposition and dissolution of lithium metal. The movement of electrons in the negative electrode during charging and discharging is mainly due to the deposition and dissolution of lithium metal in the negative electrode. Specifically, 70-100% (e.g., 80-100% or 90-100%) of the movement of electrons (or current, in other words) in the negative electrode during charging and discharging is due to the deposition and dissolution of lithium metal. In other words, the negative electrode of the lithium secondary battery according to this disclosure differs from a negative electrode where the movement of electrons in the negative electrode during charging and discharging is mainly due to the intercalation and release of lithium ions by the negative electrode active material (such as graphite). For example, the negative electrode of the lithium secondary battery according to this disclosure does not need to contain a negative electrode active material (such as graphite) that intercalates and releases lithium ions.

[0014] Generally, in lithium secondary batteries, lithium metal tends to deposit in a dendrite-like manner on the negative electrode during charging. When lithium metal deposits in a dendrite-like manner, it can lead to internal short circuits and an increase in lithium metal that does not contribute to charging and discharging, thus degrading the charge-discharge cycle characteristics (hereinafter sometimes simply referred to as "cycle characteristics") of the lithium secondary battery. When dendrite-like lithium metal deposits on the negative electrode, the protective layer is subjected to strong pressure.

[0015] (Negative Electrode) The negative electrode has a negative electrode current collector, and lithium metal is deposited on the negative electrode during charging. Therefore, the surface of the negative electrode covered by the protective layer may be the surface of the negative electrode current collector, but usually it is the surface of the lithium metal-containing layer present on the surface of the negative electrode current collector. The lithium metal-containing layer is composed of lithium metal or lithium alloy. The surface of the lithium metal-containing layer may contain lithium compounds (carbonates, oxides, hydroxides, etc.). Lithium compounds may be formed naturally or intentionally during the manufacturing process of the negative electrode. Lithium compounds can also be produced by reactions within the battery. The protective layer is usually in contact with the lithium metal or lithium alloy contained in the lithium metal-containing layer, or in contact with lithium compounds (carbonates, oxides, hydroxides, etc.).

[0016] (Protective layer) The protective layer covering at least a portion of the surface of the negative electrode contains a vinylidene fluoride polymer having the following characteristics (A) to (C).

[0017] (A) The vinylidene fluoride polymer contains a crystalline portion. (B) The crystalline portion contains at least one of a β-crystalline phase and a γ-crystalline phase. (C) The ratio of the total number of moles of monomer units constituting the β-crystalline phase (Mβ) and the number of moles of monomer units constituting the γ-crystalline phase (Mγ) to the total number of moles of monomer units constituting the vinylidene fluoride polymer (Mt) is 6 mol% or more.

[0018] By including a vinylidene fluoride polymer having characteristics (A) to (C) in the protective layer (hereinafter also referred to as "vinylidene fluoride polymer (P)"), the deposition of lithium metal in the planar direction on the negative electrode surface is promoted, reducing the reaction resistance and suppressing the deposition of dendrite-like lithium metal.

[0019] The protective layer must be lithium ion conductive. The protective layer may have lithium ion conductivity by having, for example, a porous structure. If the protective layer contains inorganic fillers, many voids are formed between the filler particles, so the protective layer can have a porous structure. A porous protective layer can also be formed, for example, by the NIPS method (non-solvent-induced phase separation method). Within a porous protective layer, liquid non-aqueous electrolytes can move easily, so the protective layer has high lithium ion conductivity.

[0020] When a vinylidene fluoride polymer possessing characteristic (A) includes a crystalline portion, the protective layer exhibits high strength and durability. By increasing the durability of the protective layer, deformation of the protective layer is suppressed even under pressure, and the pores within the protective layer are more easily maintained. As a result, the lithium ion conductivity of the protective layer is highly maintained, ensuring the fluidity of the non-aqueous electrolyte and suppressing an increase in reaction resistance. The crystalline portion is composed of monomer units derived from vinylidene fluoride in the vinylidene fluoride polymer.

[0021] Among vinylidene fluoride polymers possessing characteristic (A), those possessing characteristic (B) have a β-crystalline phase or a γ-crystalline phase in which fluorine atoms that readily react with lithium are characteristically arranged. Hereinafter, "at least one of the β-crystalline phase and the γ-crystalline phase" will also be referred to as the "β-γ crystalline phase." Vinylidene fluoride polymers having a β-γ crystalline phase are highly polar and have a high affinity for lithium metal. It is thought that some of the fluorine atoms in the β-γ crystalline phase can react with lithium. It is also speculated that stable components are generated at the interface between the protective layer and the negative electrode due to the reaction between fluorine atoms and lithium, and that such components promote the deposition of lithium metal.

[0022] Among vinylidene fluoride polymers possessing characteristic (B), vinylidene fluoride polymers possessing characteristic (C) (i.e., vinylidene fluoride polymer (P)) contain a large amount of β-γ crystalline phase. By including vinylidene fluoride polymer (P) in the protective layer, the adhesion between the protective layer and the negative electrode surface is significantly enhanced. As a result, the deposition of lithium metal in the planar direction on the negative electrode surface is promoted, the deposition of dendrite-like lithium metal is significantly suppressed, and the reaction resistance is further reduced. Therefore, the cycle characteristics of the lithium secondary battery (B) are greatly improved.

[0023] The ratio of the total number of moles of monomer units constituting the β-crystalline phase (Mβ) and the total number of moles of monomer units constituting the γ-crystalline phase (Mγ) to the total number of moles of monomer units constituting the vinylidene fluoride polymer (Mt) (hereinafter also referred to as "ratio (Mβγ / Mt)") is preferably 7 mol% or more, and more preferably 8 mol% or more.

[0024] The crystalline portion of vinylidene fluoride polymers includes α-crystalline phase, β-crystalline phase, and γ-crystalline phase. The α-crystalline phase is nonpolar, while the β-crystalline phase is the most polar. The γ-crystalline phase has moderate polarity. The β-crystalline phase is easily formed by stretching the α-crystalline phase or copolymerizing vinylidene fluoride with other monomers. The γ-crystalline phase tends to form when the α-crystalline phase is heated at high temperatures. Furthermore, when producing a porous protective layer by the NIPS method, the formation of the β-γ crystalline phase can be promoted by adjusting the NIPS conditions.

[0025] The NIPS method is a technique in which a coating solution containing, for example, a vinylidene fluoride polymer is prepared, a film is formed from the coating solution, and then, before the film dries, the film is immersed in a poor solvent to make the film porous. Porous solvents that can be used include water, methanol, ethanol, isopropyl alcohol, or mixtures thereof. By mixing acetone, dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, or tripropylene glycol with the poor solvent, the porosity of the protective layer film can be altered.

[0026] From the viewpoint of increasing the polarity of the vinylidene fluoride polymer, it is preferable that the crystalline portion contains as many β-crystalline phases as possible. Specifically, the ratio of the number of moles of monomer units constituting the β-crystalline phase (Mβ) to the total number of moles of monomer units constituting the vinylidene fluoride polymer (Mt) (hereinafter also referred to as "ratio (Mβ / Mt)") is preferably 4.5 mol% or more, more preferably 5 mol% or more, and even more preferably 7 mol% or more.

[0027] From the viewpoint of maximizing the durability of the protective layer and ensuring a sufficient amount of the β-γ crystalline phase, it is preferable that the vinylidene fluoride polymer contains a considerable amount of crystalline material. On the other hand, if the amount of crystalline material is excessive, the flexibility of the protective layer may decrease, and the adhesion between the protective layer and the negative electrode may decrease. Considering the balance between the flexibility and rigidity of the protective layer, the ratio of the number of moles of monomer units constituting the crystalline portion (Mc) to the total number of moles of monomer units constituting the vinylidene fluoride polymer (Mt) (hereinafter also referred to as "ratio (Mc / Mt)") is preferably controlled to, for example, 30 mol% or more and 60 mol% or less, and more preferably to 40 mol% or more and 50 mol% or less.

[0028] A vinylidene fluoride polymer is a polymer synthesized by polymerizing monomers containing vinylidene fluoride. Polyvinylidene fluoride is a typical example of a vinylidene fluoride polymer. As mentioned above, vinylidene fluoride polymers have excellent adhesion to the negative electrode and high stability within lithium secondary batteries (B).

[0029] A vinylidene fluoride polymer is a polymer containing monomer units derived from vinylidene fluoride. The proportion of VDF units (monomer units derived from vinylidene fluoride) to the total monomer units of a vinylidene fluoride polymer is, for example, 50 mol% or more, may be 75 mol% or more, may be 90 mol% or more, or may be 100 mol%. In other words, the vinylidene fluoride polymer may be polyvinylidene fluoride. This proportion may be 99.5 mol% or less, or 95 mol% or less. Examples of monomers copolymerized with vinylidene fluoride include tetrafluoroethylene, hexafluoropropylene, trifluoroethylene, and chlorotrifluoroethylene.

[0030] The vinylidene fluoride polymer may be a copolymer of vinylidene fluoride with at least one selected from the group consisting of hexafluoropropylene, trifluoroethylene, and chlorotrifluoroethylene. In such copolymers, it is easy to suppress the content of the crystalline portion from becoming too high, and it is easy to control the ratios (Mβγ / Mt), (Mβ / Mt), and (Mc / Mt) within a preferred range.

[0031] In particular, the vinylidene fluoride polymer is preferably a copolymer synthesized by polymerizing monomers containing vinylidene fluoride and hexafluoropropylene (HFP) (hereinafter also referred to as "VDF-HFP copolymer"). Since hexafluoropropylene has a bulkier molecular structure than vinylidene fluoride, the VDF-HFP copolymer is lower in density and more flexible than polyvinylidene fluoride. Therefore, by using a VDF-HFP copolymer as the vinylidene fluoride polymer, a protective layer with an excellent balance of flexibility and rigidity can be formed. A flexible protective layer also has excellent retention of non-aqueous electrolytes, thus improving lithium ion conductivity.

[0032] The proportion of HFP units (monomer units derived from hexafluoropropylene) to the total monomer units of the VDF-HFP copolymer is, for example, 0 to 50 mol%, and may also be 0.5 mol% to 25 mol%, or 2 mol% to 10 mol%.

[0033] (Method for measuring the ratio (Mβγ / Mt), ratio (Mβ / Mt), and ratio (Mc / Mt)) The ratio (Mβγ / Mt), ratio (Mβ / Mt), and ratio (Mc / Mt) can be determined by solid-state nuclear magnetic resonance spectroscopy (NMR). Specifically, each ratio can be analyzed from the F-NMR spectrum of vinylidene fluoride polymers. Here, a sample of vinylidene fluoride polymer is packed into a 2.5 mmΦ sample tube, and then rotated at 32 kHz while １９ This section explains how to perform F-NMR measurements.

[0034] The measurement conditions are as follows: Analytical equipment (solid-state NMR): Brecker Biospin, AVANCE ＩＩＩ -400WB (manufactured by Bruker Corporation)

[0035] Observation frequency: 376 MHz ( １９ F) Measurement temperature: Room temperature; Sample tube diameter: 2.5 mmΦ; MAS rotation speed: 32 kHz; Chemical shift reference: External standard PTFE (-122.0 ppm)

[0036] solid １９ In F-NMR, in the case of the α-crystalline phase, the monomer unit of vinylidene fluoride (-(CH)) ２ -CF ２ ) ｎ Two fluorine atoms are detected at different positions, approximately -80 ppm and -98 ppm, respectively. In principle, the peak areas around -80 ppm and -98 ppm are the same, since each peak corresponds to one fluorine atom. The peak around -80 ppm is easier to analyze (calculation of area value) than the peak around -98 ppm. The area value for one fluorine atom is calculated from the peak around -80 ppm, and the area value of the α-crystal phase is obtained by doubling that area value.

[0037] Four peaks of the γ crystal phase are detected with similar intensity: γ1 (around -80 ppm), γ2 (around -84 ppm), γ3 (around -94 ppm), and γ4 (around -101 ppm). The total area value is calculated by multiplying the average of the four area values ​​by four.

[0038] One peak of the β-crystalline phase is detected around -98 ppm. Therefore, the area value of the α-crystalline phase and the area value of γ3 are obtained by subtracting them from the area value of the peak around -98 ppm.

[0039] Since one peak of the amorphous part is detected around -89 ppm, the area value of the peak is calculated. Also, since two peaks of the hetero-bonded part are detected at approximately the same intensity around -112 ppm and -114 ppm, the sum of the area values of the two peaks is calculated. The "hetero-bonded part" refers to, for example, -CH ２ -CF ２ -CF ２ -CH ２ - such as the head-to-head bond or tail-to-tail bond of vinylidene fluoride.

[0040] Each area value can be calculated with high precision by waveform separation of the solid １９ F-NMR spectrum. For waveform separation, the Sola program (Fit Solid NMR Methods) installed on Topspin manufactured by Bruker Corporation can be used, but the software that can be used is not limited. From each area value, the area value (1F) corresponding to one F atom of each area value is calculated. The ratio of each area value (1F) (α-crystalline phase: β-crystalline phase: γ-crystalline phase: amorphous part: hetero-bonded part) corresponds to the mol% of the monomer unit. The ratios (Mβγ / Mt), (Mβ / Mt), and (Mc / Mt) are obtained.

[0041] The weight average molecular weight of the vinylidene fluoride-based polymer may be, for example, 1 million or more, 1.1 million or more, 1.2 million or more, or 1.3 million or more. The weight average molecular weight of the vinylidene fluoride-based resin may be 2 million or less, 1.8 million or less. The weight average molecular weight (Mw) is the polystyrene equivalent value measured by gel permeation chromatography (GPC).

[0042] As part of the resin forming the protective layer, resins other than vinylidene fluoride polymers may be used. Such resins are required to be stable within the lithium secondary battery (B). The type of resin is not particularly limited, but examples include fluororesins other than vinylidene fluoride polymers, high-strength resins, rubbery polymers, acrylic resins, polyolefin resins, etc. Examples of fluororesins include polytetrafluoroethylene. Examples of rubbery polymers include styrene-butadiene copolymers. Examples of high-strength resins include polyimide resins and polyamide resins (especially aromatic polyamide resins). However, the mass content of resins other than vinylidene fluoride polymers in the protective layer is preferably limited to, for example, 10% or less, and more preferably to 3% or less.

[0043] From the viewpoint of suppressing the deposition of dendrite-like lithium metal, higher adhesion between the negative electrode and the protective layer is desirable. The adhesion between the negative electrode and the protective layer can be evaluated by the porosity at the interface between the negative electrode and the protective layer. The smaller the gap at the interface between the protective layer and the negative electrode surface, the higher the adhesion between the protective layer and the negative electrode surface. The higher the adhesion between the protective layer and the negative electrode surface, the more the deposition of dendrite-like lithium metal is suppressed.

[0044] The porosity at the interface between the negative electrode and the protective layer (hereinafter referred to as "porosity A") is preferably 10% or less, and more preferably 5% or less. By reducing the gap at the interface between the protective layer and the negative electrode in this way, the diffusion of lithium deposition on the negative electrode towards the surface of the negative electrode during charging can be promoted. Therefore, the deposition of dendrite-like lithium metal is suppressed to a greater extent.

[0045] The porosity A at the interface between the negative electrode and the protective layer is defined in the SEM image of the cross-section of the negative electrode and the protective layer. Specifically, it is measured by the following procedure.

[0046] (1) Formation of the cross-section First, prepare the negative electrode to be measured. The cross-section of the negative electrode and protective layer is formed by cutting the lithium metal-containing layer and the protective layer together along the thickness direction of the negative electrode. The lithium metal-containing layer may be cut together with the negative electrode current collector on which the lithium metal-containing layer is arranged. At this time, a thermosetting resin may be applied to the negative electrode and protective layer and the thermosetting resin may be cured. The obtained cross-section may be processed by, for example, the CP (cross-section polisher) method, the FIB (focused ion beam) method, etc.

[0047] The negative electrode to be measured is taken from a secondary battery with a depth of discharge (DOD) of 90% or more. Depth of discharge (DOD) is the ratio of the amount of discharged electricity to the amount of electricity held in a fully charged battery. Note that the amount of electricity charged when charging a battery from a completely discharged state (DOD = 100%) to a fully charged state (DOD = 0%) corresponds to the rated capacity. The voltage of a fully charged battery corresponds to the charge termination voltage. The voltage of a completely discharged battery corresponds to the discharge termination voltage. Note that DOD = 0% corresponds to SOC (State of Discharge) = 100%.

[0048] (2) Acquisition of cross-sectional SEM images Next, the cross-section of the formed negative electrode and protective layer is imaged using an SEM (scanning electron microscope). The SEM imaging is performed at a magnification of, for example, 5000x. The cross-sectional SEM image is taken so that the interface between the negative electrode and the protective layer is observed in the cross-sectional SEM image. The cross-sectional SEM image is taken five times at different locations on the negative electrode and protective layer.

[0049] (3) Image Analysis By image processing along the interface between the negative electrode and the protective layer observed in the five cross-sectional SEM images, the portion where the negative electrode and the protective layer are not in contact is identified. Then, the ratio of the total length of the portion where the negative electrode and the protective layer are not in contact (total length of the five cross-sectional SEM images) to the total length of the interface between the negative electrode and the protective layer observed in the five cross-sectional SEM images (total length of the five cross-sectional SEM images) is calculated. The obtained ratio is defined as "porosity A at the interface between the negative electrode surface and the protective layer".

[0050] The lithium ion conductivity of the protective layer is preferably as high as possible. Therefore, it is preferable that the protective layer has a porous structure. Since liquid non-aqueous electrolytes can easily move within a porous protective layer, the protective layer has high lithium ion conductivity.

[0051] From the viewpoint of ensuring high lithium-ion conductivity and reducing reaction resistance to improve cycle characteristics, the porosity within the protective layer (hereinafter referred to as "porosity B") is preferably 5% or more, more preferably 10% or more, and may also be 15% or more, or 20% or more. The porosity B within the protective layer is preferably 40% or less, and may also be 30% or less. By setting an upper limit for the porosity B within the protective layer in this way, sufficiently high strength of the protective layer is ensured. Therefore, even when the protective layer is subjected to strong pressure, sufficient voids within the protective layer are easily maintained. The porosity B is preferably 5% to 40%, may also be 10% to 40%, or 15% to 30%.

[0052] The void ratio B within the protective layer can be determined at any cross-section S of the protective layer in a direction parallel to the thickness direction of the protective layer.

[0053] First, the cross-section of the protective layer is exposed and an arbitrary cross-section is imaged using a scanning electron microscope (SEM). The SEM image is taken, for example, at a magnification of 20,000x. The cross-sectional SEM image is then modified by adding, for example, a 100 μm layer to the cross-sectional SEM image. ２ The image is taken so that the cross-section of the protective layer described above can be observed.

[0054] Next, the obtained cross-sectional SEM image is binarized so that it is divided into regions of voids and regions without voids. The area of ​​the region to be binarized is, for example, 100 μm. ２ The above procedure is followed. The ratio of the area of ​​the void region to the area of ​​the binarized region is measured as the porosity B. The same measurement is performed five times using different cross-sections, and the obtained porosity B is averaged to determine the porosity B.

[0055] In the negative electrode, when lithium metal is deposited during charging and dissolves in the non-aqueous electrolyte during discharge, the change in the thickness of the negative electrode (expansion and contraction in the thickness direction of the negative electrode) associated with charging and discharging is large. Therefore, the protective layer is subjected to strong pressure within the electrode group. From the viewpoint of adequately resisting such pressure, it is preferable that the protective layer contains inorganic fillers (inorganic particles).

[0056] From the viewpoint of improving the durability of the protective layer, the ratio of the volume of inorganic filler to the volume of the protective layer (hereinafter referred to as "ratio R(F)") is, for example, 10% or more, may be 20% or more, preferably 30% or more, and may be 35% or more. The ratio R(F) is, for example, 85% or less, may be 80% or less, may be 75% or less, and preferably 70% or less. The ratio R(F) is, for example, preferably 30% or more and 70% or less.

[0057] When the ratio R(F) is within the above range, deformation of the protective layer is suppressed even when the protective layer is subjected to strong pressure. On the other hand, when the ratio R(F) is within the above range, a sufficient amount of vinylidene fluoride polymer forming the protective layer is secured within the protective layer, and a large porosity is secured due to the large remaining space.

[0058] The ratio of the volume of inorganic filler to the volume of the protective layer can be determined at any cross-section S of the protective layer in a direction parallel to the thickness direction of the protective layer.

[0059] First, the cross-section of the protective layer is exposed and an arbitrary cross-section is imaged using a scanning electron microscope (SEM). The SEM image is taken, for example, at a magnification of 1000x. The cross-sectional SEM image is then modified, for example, by adding a 100 μm layer to the cross-sectional SEM image. ２ The image is taken so that the cross-section of the protective layer described above can be observed.

[0060] Next, the obtained cross-sectional SEM image is binarized so that it is separated into the inorganic filler region and the other region. The area of ​​the region to be binarized is, for example, 100 μm. ２The above procedure is followed. The ratio of the area of ​​the inorganic filler region to the area of ​​the binarized region is measured as the ratio of the area of ​​the inorganic filler to the cross-section S. The same measurement is performed five times using different cross-sections, and the obtained ratios are averaged to obtain the ratio R(F).

[0061] Inorganic fillers include, for example, oxides, oxide hydrates, hydroxides, nitrides, carbides, and sulfides. Examples of oxides and oxide hydrates include aluminum oxide (alumina) and boehmite (Al ２ O ３ ・H ２ Examples of nitrides include 0 or AlOOH, magnesium oxide, titanium oxide, zirconium oxide, silicon oxide, yttrium oxide, zinc oxide, etc. Examples of nitrides include silicon nitride, aluminum nitride, boron nitride, titanium nitride, etc. Examples of carbides include silicon carbide, boron carbide, etc. Examples of sulfides include barium sulfate, etc. Examples of hydroxides include aluminum hydroxide, etc. Porous aluminosilicates such as zeolites, layered silicates such as talc, barium titanate (BaTiO) ３ ), strontium titanate (SrTiO ３ You may also use ) etc.

[0062] In the volume-based particle size distribution of inorganic fillers measured using a laser diffraction / scattering particle size distribution analyzer, the median diameter (D) at which the cumulative volume reaches 50% is ５０ The median diameter of the inorganic filler is, for example, 0.1 μm to 4 μm, and may also be 1.5 μm to 2.5 μm. When the median diameter of the inorganic filler is within this range, it is easy to form a thin protective layer with high porosity.

[0063] The vinylidene fluoride polymer that forms the protective layer functions as a binder that binds the inorganic filler particles together and also functions as a binder that fixes the protective layer to the negative electrode surface. In other words, the protective layer may adhere to at least a portion of the negative electrode surface.

[0064] Furthermore, if the porosity A at the interface between the negative electrode and the protective layer is, for example, 3% or less, then the protective layer can be said to be substantially entirely bonded to the surface of the negative electrode.

[0065] The side of the protective layer opposite the side facing the negative electrode faces the separator. The protective layer may be bonded to the separator. The vinylidene fluoride polymer forming the protective layer can also function as a binder that fixes the protective layer to the separator surface. The protective layer may be formed on the separator surface in advance. In one preferred embodiment, the protective layer is bonded to the surface of the separator and to at least a portion of the surface of the negative electrode. In this case, the strength of the protective layer is increased by the separator integrated with the protective layer, and the durability of the protective layer is significantly improved.

[0066] In the battery manufacturing process, if a protective layer is pre-formed on the surface of the separator, a process may be performed to bond the surface of the protective layer opposite to the surface facing the separator to the surface of the negative electrode. In this case, an integrated product of the negative electrode, protective layer, and separator is obtained. However, the protective layer may also be pre-formed on the surface of the negative electrode.

[0067] The protective layer may be formed, for example, by applying a coating solution containing the components of the protective layer to the surface of the separator and then drying it. The method of applying the coating solution is not limited, and known methods may be used. For example, the coating may be applied using a spray method, a method using rollers or dies (gravure coating method, die coating method, etc.), or a printing method (screen printing method, inkjet method, etc.).

[0068] The solvents used in the coating solution can include acetone, dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, and tripropylene glycol.

[0069] The thickness of the protective layer is, for example, 0.2 μm or more, but may also be 0.3 μm or more, 0.5 μm or more, 1 μm or more, 2 μm or more, 3 μm or more, or 4 μm or more. The thickness of the protective layer is, for example, 10 μm or less, but may also be 8 μm or less, 7 μm or less, or 5 μm or less. When the thickness of the protective layer is 0.2 μm or more, the effect of suppressing the dendritic deposition of lithium metal is enhanced. When the thickness of the protective layer is 10 μm or less, the internal resistance of the battery can be kept low. The thickness of the protective layer is, for example, 0.2 μm to 10 μm, but may also be 1 μm to 8 μm, 2 μm to 4 μm, or 4 μm to 10 μm.

[0070] Another aspect of this disclosure relates to the negative electrode for lithium secondary batteries described above, namely, "a negative electrode comprising a negative electrode current collector, a lithium metal-containing layer supported on the negative electrode current collector, and a protective layer formed on at least a portion of the surface of the lithium metal-containing layer, wherein the protective layer comprises a vinylidene fluoride polymer, the vinylidene fluoride polymer comprises a crystalline portion, the crystalline portion comprises at least one of a β-crystalline phase and a γ-crystalline phase, and the ratio of the sum of the number of moles of monomer units constituting the β-crystalline phase (Mβ) and the number of moles of monomer units constituting the γ-crystalline phase (Mγ) to the total number of moles of monomer units constituting the vinylidene fluoride polymer (Mt) is 6 mol% or more."

[0071] (Separator) The separator is a porous sheet having ion permeability and insulating properties. The separator has at least a first base layer. The first base layer may be a microporous membrane, a woven fabric, a nonwoven fabric, etc. A microporous membrane is preferred among these. A microporous membrane is a film having fine pores. The microporous membrane may also be a resin sheet manufactured by uniaxial stretching or biaxial stretching. Fine pores can be formed by stretching and other treatments. A resin sheet may be formed from a film raw material that has been pre-impregnated with a pore-forming agent, and then the pore-forming agent may be removed to form fine pores.

[0072] The separator may consist only of the first base layer. The separator may be a laminate of the first base layer and a layer other than the first base layer. The separator may be, for example, a laminate of the first base layer and a porous heat-resistant layer that has higher heat resistance than the first base layer. The separator may be bonded to the surface of the negative electrode by a protective layer, as described above.

[0073] The first base layer may contain polyolefin. The first base layer may also contain materials other than polyolefin. Examples of polyolefins include polyethylene, polypropylene, and copolymers of ethylene and propylene. Examples of materials other than polyolefins include inorganic additives such as inorganic fillers and organic additives such as paraffin. The polyolefin content in the first base layer is, for example, 80% by mass or more, and preferably 90% by mass or more. As the separator, a separator used in known lithium secondary batteries may be used.

[0074] The porous heat-resistant layer is formed from a heat-resistant resin. The heat-resistant resin may be a resin whose heat distortion temperature is 260°C or higher (a heat deflection temperature of 260°C or higher, as determined by a load of 1.82 MPa in accordance with ASTM-D648).

[0075] Examples of heat-resistant resins include polyamide, polyamide-imide, polyimide, cellulose, cellulose acetate, polysulfone, polyethersulfone, polyetherimide, polyetheretherketone, amorphous polyarylate, polyphenylene sulfide, polypyromelillimide, and polycarbonate. Among these, aromatic polyamide, aromatic polyamide-imide, and aromatic polyimide are preferred, and aramid (all aromatic polyamide) is more preferred.

[0076] The porous heat-resistant layer is preferably porous from the viewpoint of ensuring good lithium-ion conductivity. A porous heat-resistant layer can be formed, for example, by the NIPS method, but the method for manufacturing a porous heat-resistant layer is not particularly limited.

[0077] The lithium secondary battery (B) may further include another separator (hereinafter also referred to as the "second separator") between the positive electrode and the separator. In this case, the previously described separator (hereinafter referred to as the "first separator") and the second separator are stacked and placed between the positive electrode and the negative electrode. The first separator and the second separator usually have the same shape. However, as long as they are placed between the positive electrode and the negative electrode, the shape of the second separator may differ from the shape of the first separator. The first separator and the second separator may be bonded together, but it is preferable that they are not bonded together. By not bonding the first separator and the second separator together, it becomes easier to relieve the stress generated inside the electrode group due to the expansion of the negative electrode.

[0078] The thickness of the second separator may be greater than that of the first separator. In other words, the first separator may be made relatively thinner to reinforce the protective layer, while the second separator may function as a reservoir to relieve the stress generated inside the electrode group during charging and discharging, and to hold a sufficient amount of non-aqueous electrolyte. In lithium secondary batteries, lithium metal is deposited on the negative electrode during charging, which tends to increase the amount of expansion of the negative electrode. If the amount of expansion of the negative electrode (especially the expansion in the thickness direction of the negative electrode) is large, it becomes easier for the circulation of the non-aqueous electrolyte to decrease and for the negative electrode current collector to break due to increased stress, thus degrading the cycle characteristics of the lithium secondary battery. In contrast, by using a thicker second separator, it becomes easier to avoid the decrease in the circulation of the non-aqueous electrolyte and the breakage of the negative electrode current collector.

[0079] For the second separator, materials such as the porous sheet described as the first separator can be used.

[0080] Even when the lithium secondary battery (B) does not have a second separator, the combined thickness of the first separator and the second separator may be, for example, 30 μm to 60 μm or 40 μm to 55 μm. When the lithium secondary battery (B) does not have a second separator, the thickness of the first separator alone may be 30 μm to 60 μm or 40 μm to 55 μm.

[0081] When the lithium secondary battery (B) comprises a first separator and a second separator, the ratio of the thickness T2 of the second separator to the thickness T1 of the first separator (T2 / T1) may be, for example, 1.1 or more, 1.3 or more, or 1.5 or more, or 3.0 or less, or 2.0 or less. By setting the ratio Ts2 / Ts1 to 1.5 or more, the expansion of the negative electrode is more easily absorbed by the second separator. In addition, the mechanical strength of the protective layer is increased, and a lithium secondary battery (B) with excellent circulation of the non-aqueous electrolyte can be obtained.

[0082] The thickness of each separator is determined by taking the arithmetic mean of the thicknesses at 15 points. The thickness of the separator can be measured according to the method compliant with JIS (Japanese Industrial Standards) K6250. Specifically, the thickness can be measured using a test piece thickness gauge (SDA-12 type) manufactured by Polymer Instruments Co., Ltd., which complies with JIS K6250. The measurement is performed using a measuring probe with a diameter of 5 mm and a pressure of 22 kPa. The thickness is measured at 15 intersection points of five lines that divide the separator into six equal parts in the longitudinal direction and three lines that divide it into four equal parts in the width direction. The arithmetic mean of the 15 measured thicknesses is then taken as thickness T1 or T2.

[0083] (Examples of Components) Examples of other components of the lithium secondary battery (B) are described below in detail. Note that the components described below are illustrative, and the components of the lithium secondary battery (B) of this embodiment are not limited to the following examples. Known components may be used for components other than those characteristic of this embodiment.

[0084] (Lithium-ion secondary battery (B)) Examples of other components of lithium-ion secondary battery (B) are described below in detail. Note that the components described below are illustrative, and the components of lithium-ion secondary battery (B) in this embodiment are not limited to the following examples. Known components may be used for components other than those characteristic of this embodiment.

[0085] (Negative electrode) The negative electrode includes a negative electrode current collector. In lithium secondary batteries, lithium metal is deposited on the negative electrode during charging. The deposited lithium metal dissolves as lithium ions in the non-aqueous electrolyte during discharge.

[0086] The negative electrode current collector is usually composed of a conductive sheet. The conductive sheet may be composed of a conductive material other than lithium metal and lithium alloys. The conductive material may be a metallic material such as a metal or alloy. The metallic material may be a material that does not react with lithium (a material that does not form any alloys or intermetallic compounds with lithium). Examples of such metallic materials are copper, nickel, iron, and alloys containing these metallic elements. As for alloys, copper alloys and stainless steel may also be used. From the viewpoint of easily ensuring high capacity and high charge / discharge efficiency by having high conductivity, the metallic material may include at least one of copper and copper alloys. The conductive sheet may contain one of these conductive materials or two or more.

[0087] Conductive sheets can be foils, films, etc. Conductive sheets may be porous. From the viewpoint of easily ensuring high conductivity, conductive sheets may be metal foils, or metal foils containing copper. Such metal foils may be copper foils or copper alloy foils.

[0088] Furthermore, since it is easier to ensure a high volumetric energy density, the negative electrode may consist only of a negative electrode current collector in the fully discharged state of the lithium metal secondary battery. Also, from the viewpoint of easily ensuring high charge and discharge efficiency, in the fully discharged state, the negative electrode may consist of a negative electrode current collector and a negative electrode active material layer arranged on the surface of the negative electrode current collector. When assembling the battery, only the negative electrode current collector may be used as the negative electrode, or a negative electrode consisting of a negative electrode active material layer and a negative electrode current collector may be used.

[0089] Examples of negative electrode active materials included in the negative electrode active material layer include metallic lithium, lithium alloys, and materials that reversibly intercalate and release lithium ions. The negative electrode active material may be one used in lithium-ion batteries. Examples of lithium alloys include lithium-aluminum alloys. Examples of materials that reversibly intercalate and release lithium ions include carbon materials and alloying materials. Examples of carbon materials include graphite materials, soft carbon, hard carbon, and amorphous carbon. Examples of alloying materials include silicon-containing materials and tin-containing materials. Examples of alloying materials include elemental silicon, silicon alloys, silicon compounds, elemental tin, tin alloys, and tin compounds. Examples of silicon compounds and tin compounds include oxides and nitrides, respectively. The negative electrode active material layer may contain one type of negative electrode active material, or a combination of two or more types.

[0090] The negative electrode active material layer may be formed by depositing the negative electrode active material onto the surface of the negative electrode current collector using a gas-phase method such as electrodeposition or vapor deposition. Alternatively, it may be formed by coating the surface of the negative electrode current collector with a negative electrode mixture containing the negative electrode active material and a binder.

[0091] (Positive Electrode) The positive electrode includes a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode mixture layer includes a positive electrode active material, for example, a positive electrode active material and additives (conductive material, binder, etc.). The positive electrode mixture layer is formed on both sides of the positive electrode current collector. The positive electrode can be formed by known methods. For example, first, a positive electrode mixture slurry containing the positive electrode active material and additives is prepared. Next, a coating film is formed by applying the positive electrode mixture slurry to the positive electrode current collector and then drying it. Next, the positive electrode is obtained by rolling the laminate consisting of the positive electrode current collector and the coating film. The formed positive electrode can be cut to a predetermined size as needed.

[0092] The thickness of the positive electrode mixture layer may be 50 μm or more, or 100 μm or more, or 300 μm or less, or 250 μm or less.

[0093] The positive electrode active material can be a substance capable of reversibly intercalating and releasing lithium ions. Examples of positive electrode active materials include lithium-containing transition metal oxides, transition metal fluorides, polyanions, fluorinated polyanions, and transition metal sulfides. Lithium-containing transition metal oxides are preferred because they have low manufacturing costs and a high average discharge voltage.

[0094] Examples of transition metal elements included in lithium-containing transition metal oxides include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, and W. Lithium-containing transition metal oxides may contain only one transition metal element or two or more. The transition metal element may be at least one selected from the group consisting of Co, Ni, and Mn. Lithium-containing transition metal oxides may also contain one or more main group elements. Examples of main group elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, Bi, and B.

[0095] Conductive materials can include carbon materials. Examples of carbon materials include carbon black (acetylene black, Ketjenblack, etc.), carbon nanotubes, and graphite.

[0096] Examples of binders include fluororesins, polyimide resins, acrylic resins, polyolefin resins, and rubbery polymers. Examples of fluororesins include polytetrafluoroethylene and vinylidene fluoride polymers such as polyvinylidene fluoride.

[0097] A conductive sheet can be used as the positive electrode current collector. Examples of conductive sheets include metal foil. The surface of the positive electrode current collector may also be coated with a carbon material.

[0098] Examples of materials for the positive electrode current collector (conductive sheet) include metallic materials containing Al, Ti, Fe, etc. The metallic material may be Al, Al alloy, Ti, Ti alloy, Fe alloy (e.g., stainless steel), etc. The thickness of the positive electrode current collector is not particularly limited and may be in the range of 5 to 300 μm.

[0099] (Non-aqueous electrolyte) A non-aqueous electrolyte having lithium ion conductivity can be used as the non-aqueous electrolyte. The non-aqueous electrolyte may be in liquid or gel form. A liquid non-aqueous electrolyte (electrolyte) can be prepared by dissolving a lithium salt in a non-aqueous solvent.

[0100] The non-aqueous solvent may contain an ether compound. The content of the ether compound in the non-aqueous solvent is, for example, 50% by mass or more, but may also be 80% by mass or more, or 90% by mass or more. The non-aqueous solvent may consist only of the ether compound. The ether compound has a high affinity for paraffin, which may be contained in the first separator. Therefore, by using a non-aqueous solvent containing an ether compound, the cycle characteristics can be further significantly improved. In addition, the ether compound has the effect of suppressing the dendritic deposition of lithium metal at the negative electrode during charging.

[0101] The ether compound may be a linear ether, a fluorinated linear ether, a cyclic ether, or a fluorinated cyclic ether. That is, the ether compound may be a fluoroether containing a fluoro group (-F), or a hydrofluoroether. A hydrofluoroether contains a carbon atom to which hydrogen and fluorine atoms are bonded. By using a hydrofluoroether, the reduction resistance of the non-aqueous electrolyte is improved, and decomposition of the non-aqueous electrolyte on the negative electrode surface becomes less likely. The hydrofluoroether content in the non-aqueous solvent may be 80% by mass or more, or 90% by mass or more. The non-aqueous solvent may consist solely of hydrofluoroether.

[0102] The fluorination rate of the hydrofluoroether is preferably 60% or more, and more preferably 65% ​​or more. The fluorination rate may also be 95% or less, 90% or less, or 80% or less. The fluorination rate of a single hydrofluoroether is defined by the following formula.

[0103] Fluorination rate (%) = 100 × (number of fluorine atoms in the hydrofluoroether) / (total number of fluorine and hydrogen atoms in the hydrofluoroether)

[0104] The hydrofluoroether used as the non-aqueous solvent may be at least one selected from the group consisting of 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0105] Other known solvents besides those listed above may be used as non-aqueous solvents. Such non-aqueous solvents may include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC). Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of linear carboxylic acid esters include ethyl acetate, methyl propionate, and methyl fluoropropionate. Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of linear ethers include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether, methylphenyl ether, benzyl ethyl ether, diphenyl ether, dibenzyl ether, 1,2-diethoxyethane, and diethylene glycol dimethyl ether. Non-aqueous solvents may be used individually or in combination of two or more.

[0106] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO2). ４ LiAlCl ４ LiB １０ Cl １０ (e.g.), lithium salts of fluorine-containing acids (LiPF) ６ LiPF ２ O ２ LiBF ４ LiSbF ６ LiAsF６ LiCF ３ SO ３ LiCF ３ CO ２ (etc.), lithium salts of fluorine-containing acidimides (LiN(FSO) ２ ) ２ ,LiN(CF ３ SO ２ ) ２ ,LiN(CF ３ SO ２ ) (FSO ２ ), LiN (CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ (etc.), lithium halides (LiCl, LiBr, LiI, etc.), oxalate complex-containing lithium salts (LiB(C) ２ O ４ ) ２ LiBF ２ (C ２ O ４ ), LiPF ４ (C ２ O ４ ), LiPF ２ (C ２ O ４ ) ２ These include (etc.). Lithium salts may be used individually or in combination of two or more types.

[0107] The concentration of lithium salt in the non-aqueous electrolyte may be 0.5 mol / L or higher, 1.0 mol / L or higher, or 1.5 mol / L or higher, and may be 3.5 mol / L or lower, 2.0 mol / L or lower, or 1.5 mol / L or lower.

[0108] (Outer casing) The outer casing houses the non-aqueous electrolyte and the electrode group. The outer casing is not particularly limited, and known outer casings can be used. The outer casing may include a bottomed cylindrical battery case and a sealing body and gasket that seal the opening of the battery case.

[0109] In the following, an example of the lithium secondary battery (B) of this embodiment will be specifically described with reference to the drawings. The components of the lithium secondary battery example described below can be the components described above. Furthermore, the components of the example described below can be modified based on the above description. In addition, the matters described below may be applied to the above embodiment. Furthermore, in the lithium secondary battery described below, components that are not essential to the lithium secondary battery (B) according to this disclosure may be omitted.

[0110] (Embodiment 1) Figure 1 is a schematic longitudinal cross-sectional view showing an example of a lithium secondary battery according to Embodiment 1. The cylindrical lithium secondary battery 10 shown in Figure 1 includes a cylindrical battery case and an electrode group 14 and a non-aqueous electrolyte (not shown) housed within the battery case. The electrode group 14 includes a positive electrode 11, a negative electrode 12, and a separator 13. The electrode group 14 is a wound-type electrode group formed by winding the positive electrode 11, the negative electrode 12, and the separator 13. The separator 13 is positioned between the positive electrode 11 and the negative electrode 12.

[0111] The battery case includes a case body 15, which is a bottomed cylindrical metal container, and a sealing body 16 that seals the opening of the case body 15. A gasket 27 is placed between the case body 15 and the sealing body 16. The gasket 27 ensures that the battery case is airtight. Inside the case body 15, insulating plates 17 and 18 are placed at both ends of the electrode group 14 in the direction of the winding axis, respectively. The case body 15 has a stepped portion 21.

[0112] 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. The insulating member 24 is positioned 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 components of the sealing body 16, except for the insulating member 24, are electrically connected.

[0113] The lower valve body 23 has a ventilation hole. Therefore, if the internal pressure of the battery case rises due to abnormal heat generation or the like, the upper valve body 25 bulges towards the cap 26 and separates from the lower valve body 23. This disconnects the electrical connection between the lower valve body 23 and the upper valve body 25. If the internal pressure rises further, the upper valve body 25 ruptures, and gas is released through the opening formed in the cap 26.

[0114] The positive electrode 11 is electrically connected to the cap 26, which functions as a positive terminal, via the positive lead 19. The negative electrode 12 is electrically connected to the case body 15, which functions as a negative terminal, via the negative lead 20.

[0115] Figure 2 schematically shows the cross-sectional structure of an example of an electrode group. The separator 13 consists only of a first separator 131. The negative electrode 12 comprises a negative electrode current collector 121 and a lithium-containing metal layer 122. A protective layer 132 is provided on the first main surface 131s of the first separator 131 on the negative electrode 12 side, and the protective layer 132 is bonded to the first main surface 131s. The protective layer 132 is bonded to at least a portion of the surface of the lithium-containing metal layer 122. That is, the protective layer 132 is bonded to the surface of the separator 13 and to at least a portion of the surface of the negative electrode 12. In other words, the separator 13 is bonded to the lithium-containing metal layer 122 by the protective layer 132.

[0116] Figure 3 schematically shows the cross-sectional structure of another example of an electrode group. The separator 13 in Figure 3 is composed of a first separator 131 and a second separator 132. The first separator 131 is thinner than the second separator 132, and the second separator 132 is formed to be considerably thicker than the first separator 131. Except for this point, the electrode group in Figure 3 has the same configuration as the electrode group in Figure 2.

[0117] (Note) The above description discloses the following technologies. (Technology 1) A lithium secondary battery comprising an electrode group including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein lithium metal is deposited on the negative electrode during charging, and the lithium metal dissolves in the non-aqueous electrolyte during discharge, at least a portion of the surface of the negative electrode is covered with a protective layer, the protective layer comprises a vinylidene fluoride polymer, the vinylidene fluoride polymer comprises a crystalline portion, the crystalline portion comprises at least one of a β-crystalline phase and a γ-crystalline phase, and the ratio of the sum of the number of moles of monomer units constituting the β-crystalline phase (Mβ) and the number of moles of monomer units constituting the γ-crystalline phase (Mγ) to the total number of moles of monomer units constituting the vinylidene fluoride polymer (Mt) is 6 mol% or more. (Technology 2) The lithium secondary battery according to Technology 1, wherein the ratio of the number of moles of monomer units constituting the β-crystal phase (Mβ) to the total number of moles of monomer units constituting the vinylidene fluoride polymer (Mt) is 4.5 mol% or more. (Technology 3) The lithium secondary battery according to Technology 1 or 2, wherein the ratio of the number of moles of monomer units constituting the crystalline portion (Mc) to the total number of moles of monomer units constituting the vinylidene fluoride polymer (Mt) is 40 mol% or more and 50 mol% or less. (Technology 4) The lithium secondary battery according to any one of Technology 1 to 3, wherein the vinylidene fluoride polymer is a copolymer of vinylidene fluoride and at least one selected from the group consisting of hexafluoropropylene, trifluoroethylene, and chlorotrifluoroethylene. (Technology 5) The lithium secondary battery according to any one of Technology 1 to 4, wherein the porosity at the interface between the negative electrode and the protective layer is 10% or less. (Technical 6) A lithium secondary battery according to any one of Technical 1 to 5, wherein the porosity within the protective layer is 5% or more and 40% or less. (Technical 7) A lithium secondary battery according to any one of Technical 1 to 6, wherein the protective layer is bonded to the surface of the separator and to at least a portion of the surface of the negative electrode.(Technical 8) A negative electrode for a lithium secondary battery, comprising: a negative electrode current collector; a lithium metal-containing layer supported on the negative electrode current collector; and a protective layer formed on at least a part of the surface of the lithium metal-containing layer, wherein the protective layer contains a vinylidene fluoride polymer, the vinylidene fluoride polymer contains a crystalline portion, the crystalline portion contains at least one of a β crystalline phase and a γ crystalline phase, and the ratio of the sum of the number of moles of monomer units constituting the β crystalline phase (Mβ) and the number of moles of monomer units constituting the γ crystalline phase (Mγ) to the total number of moles of monomer units constituting the vinylidene fluoride polymer (Mt) is 6 mol% or more.

[0118] The lithium secondary battery relating to this disclosure will be described in detail below with reference to examples. However, this disclosure is not limited to the following examples.

[0119] (Batteries A1-A21, B1-B3) (1) Preparation of the positive electrode A positive electrode slurry was prepared by mixing lithium-containing transition metal oxide (positive electrode active material), acetylene black (AB, conductive material), and polyvinylidene fluoride (PVDF, binder) in a mass ratio of positive electrode active material:AB:PVDF = 95:2.5:2.5, and then adding an appropriate amount of N-methyl-2-pyrrolidone (NMP) and stirring. A lithium-containing transition metal oxide containing Li, Ni, Co, and Al was used as the positive electrode active material.

[0120] Next, the positive electrode mixture slurry was applied to both sides of the positive electrode current collector (aluminum foil), dried, and then rolled using a roller to create the coating of the positive electrode mixture. Finally, the resulting laminate of the positive electrode current collector and the positive electrode mixture was cut to a predetermined electrode size. In this way, a positive electrode was fabricated, comprising a positive electrode current collector and positive electrode mixture layers formed on both sides of the positive electrode current collector. Next, an aluminum tab was attached to the fabricated positive electrode.

[0121] (2) Fabrication of the negative electrode A negative electrode containing a copper foil and a lithium-containing metal layer was fabricated by pressing a lithium alloy foil (thickness: 25 μm) onto each of the two sides of a copper foil (thickness: 10 μm). Next, a nickel tab was attached to the negative electrode.

[0122] (3) Preparation of non-aqueous electrolytes LiPF６ The concentration becomes 1 mol / L and LiBF ２ (C ２ O ４ A non-aqueous electrolyte was prepared by dissolving the following substances in a non-aqueous solvent to a concentration of 0.1 mol / L. Dimethyl carbonate (carbonate ester) was used as the non-aqueous solvent.

[0123] (4) Formation of protective layer A protective layer (2 μm thick) containing a predetermined vinylidene fluoride polymer and inorganic filler (boehmite particles) as shown in Table 1 was formed on one side of a polyethylene substrate with a thickness of 10 μm by the NIPS method. The composition of the crystalline portion of the vinylidene fluoride polymer was controlled by changing the conditions of the NIPS method and the type and mol% of monomer copolymerized with vinylidene fluoride.

[0124] The vinylidene fluoride polymers used are listed below: PVDF: polyvinylidene fluoride VDF-HFP: copolymer of vinylidene fluoride and hexafluoropropylene VDF-TrFE: copolymer of vinylidene fluoride and trifluoroethylene VDF-ClFE: copolymer of vinylidene fluoride and chlorotrifluoroethylene

[0125] Next, the lithium-containing metal layer of the negative electrode and the protective layer were laminated together, and then heated and roll-pressed to bond the negative electrode and the protective layer. After that, the substrate was peeled off. This integrated the negative electrode and the protective layer.

[0126] (5) Preparation of the separator A microporous polyethylene membrane with a thickness of 10 μm was prepared as the separator.

[0127] The porosity A at the interface between the negative electrode and the protective layer was controlled to the values ​​shown in Table 1 by changing the conditions of the heated rolling press during the process of integrating the negative electrode and the protective layer.

[0128] The porosity B within the protective layer was controlled to the values ​​shown in Table 1 by changing the mixing ratio of the vinylidene fluoride polymer and the inorganic filler, as well as the conditions of the heated rolling press.

[0129] (6) Battery Fabrication Next, a wound electrode group was fabricated by winding the integrated negative electrode and protective layer and the positive electrode with a separator in an inert gas atmosphere. Then, the electrode group and the non-aqueous electrolyte were housed in an outer casing and the outer casing was sealed to fabricate the batteries A1 to A21 of the examples and the batteries B1 to B3 of the comparative examples (lithium secondary batteries). A bag-shaped outer casing made of a laminate sheet containing an aluminum layer was used for the outer casing.

[0130] [Evaluation] (Charge-Discharge Cycle Test) Charge-discharge cycle tests were performed on each battery under conditions of 25°C. Charging and discharging were performed under the following conditions: A 20-minute pause was taken between charging and discharging cycles. The charge-discharge cycle was repeated 100 times, and the discharge capacity at 100 cycles was measured. The ratio of the discharge capacity at 100 cycles to the initial discharge capacity was then calculated as the capacity retention rate. (Charging) 10mA / cm until the voltage reached 4.1V ２ Constant current charging is performed, and then the current is 1 mA / cm². ２ Constant voltage charging was performed at 4.1V until the voltage reached 3V. (Discharge) 10mA / cm until the voltage reached 3V. ２ Constant current discharge was performed.

[0131] Table 1 shows some of the components and evaluation results for each battery. In Table 1, the capacity retention rate of each battery after 100 cycles is shown as a relative value with the capacity retention rate of battery A1 after 100 cycles set to 100.

[0132]

[0133] Batteries A1 to A21 are lithium secondary batteries (B) of the embodiments according to the present disclosure. Batteries B1 to B3 are comparative examples. As shown in Table 1, batteries A1 to A21 had better cycle characteristics compared to batteries B1 to B3. When the porosity A was 10% or less (furthermore, 5% or less), the cycle characteristics were even better. Also, when the porosity B was 5% or more and 40% or less, the cycle characteristics were even better.

[0134] This disclosure can be used in lithium secondary batteries.

[0135] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0136] 10: Lithium secondary battery 11: Positive electrode 12: Negative electrode 121: Negative electrode current collector 122: Lithium-containing metal layer 13: Separator 131: First separator 131s: First main surface 132: Protective layer 132: Second separator 14: Electrode group 15: Case body 16: Sealing body 17, 18: Insulating plate 19: Positive electrode lead 20: Negative electrode lead 21: Step 22: Filter 23: Lower valve body 24: Insulating member 25: Upper valve body 26: Cap 27: Gasket

Claims

1. A lithium secondary battery comprising an electrode group including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein lithium metal is deposited on the negative electrode during charging, and the lithium metal dissolves in the non-aqueous electrolyte during discharge, and at least a portion of the surface of the negative electrode is covered with a protective layer, the protective layer comprises a vinylidene fluoride polymer, the vinylidene fluoride polymer comprises a crystalline portion, the crystalline portion comprises at least one of a β-crystalline phase and a γ-crystalline phase, and the ratio of the sum of the number of moles of monomer units constituting the β-crystalline phase (Mβ) and the number of moles of monomer units constituting the γ-crystalline phase (Mγ) to the total number of moles of monomer units constituting the vinylidene fluoride polymer (Mt) is 6 mol% or more.

2. The lithium secondary battery according to claim 1, wherein the ratio of the number of moles of monomer units constituting the β-crystalline phase (Mβ) to the total number of moles of monomer units constituting the vinylidene fluoride polymer (Mt) is 4.5 mol% or more.

3. The lithium secondary battery according to claim 1, wherein the ratio of the number of moles (Mc) of monomer units constituting the crystalline portion to the total number of moles (Mt) of monomer units constituting the vinylidene fluoride polymer is 40 mol% or more and 50 mol% or less.

4. The lithium secondary battery according to claim 1, wherein the vinylidene fluoride polymer is a copolymer of vinylidene fluoride with at least one selected from the group consisting of hexafluoropropylene, trifluoroethylene, and chlorotrifluoroethylene.

5. The lithium secondary battery according to claim 1, wherein the porosity at the interface between the negative electrode and the protective layer is 10% or less.

6. The lithium secondary battery according to claim 1, wherein the porosity within the protective layer is 5% or more and 40% or less.

7. The lithium secondary battery according to claim 1, wherein the protective layer is bonded to the surface of the separator and to at least a portion of the surface of the negative electrode.

8. A negative electrode for a lithium secondary battery, comprising: a negative electrode current collector; a lithium metal-containing layer supported on the negative electrode current collector; and a protective layer formed on at least a portion of the surface of the lithium metal-containing layer, wherein the protective layer contains a vinylidene fluoride polymer, the vinylidene fluoride polymer contains a crystalline portion, the crystalline portion contains at least one of a β-crystalline phase and a γ-crystalline phase, and the ratio of the sum of the number of moles of monomer units constituting the β-crystalline phase (Mβ) and the number of moles of monomer units constituting the γ-crystalline phase (Mγ) to the total number of moles of monomer units constituting the vinylidene fluoride polymer (Mt) is 6 mol% or more.

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