Lithium secondary battery and electrode for lithium secondary battery

Abstract

Disclosed is a lithium secondary battery comprising: an electrode group including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and negative electrode; and a non-aqueous electrolyte. At the negative electrode, lithium metal is deposited during charging, and the lithium metal dissolves into 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 contains a resin and an inorganic filler. The inorganic filler contains boehmite particles. In an X-ray diffraction pattern of the protective layer, the ratio I1 / I2 of the peak intensity I1 of a diffraction peak attributed to the (020) plane of the boehmite particles to the peak intensity I2 of a diffraction peak attributed to the (130) plane of the boehmite particles is 5.0 or more.

Description

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

[0001] This disclosure claims priority with respect to Japanese Patent Application No. 2025-025321, 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 this disclosure relates to 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 resin and an inorganic filler, the inorganic filler comprises boehmite particles, and the ratio I1 / I2 of the peak intensity of the diffraction peak attributed to the (020) plane and the peak intensity of the diffraction peak attributed to the (130) plane of the boehmite particles in the X-ray diffraction pattern of the protective layer is 5.0 or more.

[0008] Another aspect of this disclosure relates to 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 comprises a resin and an inorganic filler, the inorganic filler comprises boehmite particles, and the ratio I1 / I2 of the peak intensity of the diffraction peak attributed to the (020) plane and the peak intensity of the diffraction peak attributed to the (130) plane of the boehmite particles in the X-ray diffraction pattern of the protective layer is 5.0 or greater.

[0009] According to this disclosure, lithium secondary batteries with good charge-discharge cycle characteristics can be obtained. Novel features of the present invention are described in the appended claims, but the present invention, in conjunction with other objects and features of the present invention, will be better understood by the following detailed description in conjunction with the drawings, both in terms of structure and content.

[0010] This is a schematic cross-sectional view showing an example of a lithium secondary battery relating to this disclosure. This is a schematic cross-section of an electrode group. This is a schematic 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 specific physical properties and conditions 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 be hereinafter 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 first 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 through the separator. 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, and the protective layer includes a resin and an inorganic filler, and the inorganic filler includes boehmite particles.

[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 of the lithium secondary battery (hereinafter sometimes simply referred to as "cycle characteristics").

[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 can be formed naturally or intentionally during the manufacturing process of the negative electrode. Lithium compounds can also be produced by reactions within the battery. Therefore, 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 comprises a resin and an inorganic filler, the inorganic filler comprising boehmite particles. Boehmite is a monohydrated aluminum oxide (α-Al ２ O ３・H ２ O), and can also be represented by the composition of Al(OH)O. Boehmite generates water vapor (H ２ O) at high temperatures and absorbs heat, so it can also contribute to improving the safety of the battery. It is preferable that 80% or more, or 90% or more, of the surface of the negative electrode is covered with the protective layer, and it is more preferable that 99% to 100% of the surface of the negative electrode is covered with the protective layer.

[0017] In the X-ray diffraction pattern of the protective layer, the ratio of the peak intensity I1 of the diffraction peak attributed to the (020) plane of the boehmite particles to the peak intensity I2 of the diffraction peak attributed to the (130) plane: I1 / I2 is 5.0 or more.

[0018] The X-ray diffraction pattern of the protective layer is the X-ray diffraction pattern when X-rays are incident on the surface of the protective layer by the θ / 2θ method. When the X-ray diffraction pattern of the protective layer satisfies the above conditions, it can be said that the flat surfaces of the boehmite particles are oriented in a direction parallel to the surface of the negative electrode. The larger the intensity ratio I1 / I2, the greater the degree of orientation of the flat surfaces of the boehmite particles in the direction parallel to the negative electrode surface. The intensity ratio I1 / I2 may be 10 or more, or 20 or more. The following shows the preferable conditions when measuring the X-ray diffraction pattern. For the measuring device, for example, D8 ADVANCE manufactured by Bruker is used. Target Kβ removal: Cu·Ni filter (Disc. Lev. 0.20V) Tube voltage·Tube current: 40 kV·40 mA Step width: 0.02° Scanning speed: 5 sec / step Slit width (DS / SS / RS): 0.5° / None / 0.1 mm Measuring range: 15° to 90° Measuring temperature: Room temperature Analysis: HighScore Plus Ver. 4.8 manufactured by PANalytical

[0019] The boehmite particles contained in the protective layer may have a plate-like shape with flat surfaces. Generally, when lithium metal is deposited in a dendrite shape on the negative electrode during charging, the protective layer may be pressured, compressed, and deformed by the dendritic lithium metal. In that case, the voids in the protective layer are blocked and the porosity decreases.

[0020] On the other hand, when the flat surfaces that boehmite particles may have are oriented parallel to the surface of the negative electrode, changes in porosity are suppressed. That is, the voids formed within the protective layer are less likely to decrease. As a result, the Li ion conductivity of the protective layer is maintained to a high degree, the fluidity of the non-aqueous electrolyte is ensured, and the increase in reaction resistance is suppressed. This is thought to be because the orientation of boehmite particles with flat surfaces oriented parallel to the surface of the negative electrode is less likely to fluctuate, and the voids between the particles are more easily maintained.

[0021] When the flat surfaces of the boehmite particles are oriented parallel to the surface of the negative electrode, gaps are less likely to form at the interface between the protective layer and the negative electrode surface. This means that the adhesion between the protective layer and the negative electrode surface is high. The higher the adhesion between the protective layer and the negative electrode surface, the more the deposition of dendrite-like lithium metal on the negative electrode during charging is suppressed. When the flat surfaces of the boehmite particles are oriented parallel to the surface of the negative electrode, it is thought that the contact area between the boehmite particles and the negative electrode surface is increased.

[0022] By covering at least a portion of the negative electrode surface with a protective layer that satisfies the above conditions, the porosity of the protective layer is maintained at a high level, and the adhesion between the protective layer and the negative electrode surface is improved. As a result, the deposition of dendrite-like lithium metal is significantly suppressed, and the cycle characteristics of the lithium secondary battery (B) are greatly improved.

[0023] From the viewpoint of suppressing the deposition of dendrite-like lithium metal, as described above, a high degree of 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 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.

[0024] 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.

[0025] (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.

[0026] The negative electrode and protective layer to be measured are 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%.

[0027] (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.

[0028] (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".

[0029] The protective layer must be lithium-ion conductive. If the protective layer contains inorganic fillers, many voids are formed between the filler particles, so the protective layer may have a porous structure. Within a porous protective layer, liquid non-aqueous electrolytes can move easily, so the protective layer has high lithium-ion conductivity.

[0030] 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%.

[0031] The porosity B is measured from a secondary battery with a depth of discharge (DOD) of 90% or more after 10 or more charge-discharge cycles in a 25°C environment. Charging and discharging are performed under the following conditions: A 20-minute pause is taken between charging and discharging. (Charging) 10mA / cm until the voltage reaches 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.

[0032] 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.

[0033] 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, for example, by adding a 100 μm layer to the cross-sectional SEM image. ２ The image will be taken so that the cross-section of the protective layer described above can be observed.

[0034] Next, the obtained cross-sectional SEM image is subjected to binarization processing so as to be divided into a void region and a non-void region. The area of the region to be binarized is, for example, 100 μm ２ or more. The ratio of the area of the void region to the area of the binarized region is measured as the void fraction B. The same measurement is performed 5 times using different cross-sections respectively, and the obtained void fractions B are averaged to obtain the void fraction B.

[0035] In the negative electrode, when lithium metal is deposited during charging and dissolved in the non-aqueous electrolyte during discharging, the change in the thickness of the negative electrode accompanying charge and discharge (expansion and contraction in the thickness direction of the negative electrode) is large. Therefore, the protective layer is subjected to a strong pressure within the electrode group. To sufficiently withstand such a pressure, a considerable proportion of the volume of the inorganic filler in the volume of the protective layer (hereinafter referred to as "ratio R(F)") is required.

[0036] The ratio R(F) is, for example, 10% or more, may be 20% or more, is 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 is preferably 70% or less. The ratio R(F) is preferably, for example, 30% or more and 70% or less.

[0037] When the ratio R(F) is within the above range, even when the protective layer is subjected to a strong pressure, the deformation of the protective layer in which the boehmite particles are oriented is significantly suppressed. On the other hand, if the ratio R(F) is within the above range, in the protective layer, the amount of the resin forming the protective layer is sufficiently ensured, and since the remaining space is large, a large void fraction is ensured.

[0038] The ratio of the volume of the inorganic filler in the volume of the protective layer can be determined at an arbitrary cross-section S of the protective layer in a direction parallel to the thickness direction of the protective layer.

[0039] First, the cross-section of the protective layer is exposed and an arbitrary cross-section is photographed with a scanning electron microscope (SEM). The photographing by SEM is performed, for example, at a magnification of 20,000 times. The cross-sectional SEM image is photographed so that, for example, cross-sections of the protective layer of 100 μm ２ or more are observed in the cross-sectional SEM image.

[0040] 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. ２ The 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).

[0041] The resin forming the protective layer functions as a binder that binds the inorganic filler particles together, and also 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.

[0042] 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.

[0043] The side of the protective layer opposite to the side facing the negative electrode faces the separator. The protective layer may be bonded to the separator. The resin forming the protective layer may 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.

[0044] 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.

[0045] 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.).

[0046] The solvents used in the coating solution can include acetone, dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, and tripropylene glycol.

[0047] 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 resin and an inorganic filler, the inorganic filler comprises boehmite particles, and in the X-ray diffraction pattern of the protective layer, the ratio I1 / I2 of the peak intensity of the diffraction peak attributed to the (020) plane of the boehmite particles to the peak intensity of the diffraction peak attributed to the (130) plane, respectively, is 5.0 or greater."

[0048] The resin forming the protective layer is required to be stable within the lithium secondary battery (B). The resin forming the protective layer is also required to have adhesive properties. The type of resin is not particularly limited as long as it has such physical properties. Examples include fluororesins, polyimide resins, acrylic resins, polyolefin resins, and rubbery polymers. Examples of fluororesins include polytetrafluoroethylene, polyvinylidene fluoride, and poly-N-vinylacetamide. Examples of rubbery polymers include styrene-butadiene copolymers. As high-strength resins, polyimide resins and polyamide resins (especially aromatic polyamide resins) can be used.

[0049] The mass content of the resin in the protective layer may be, for example, 1% or more, 3% or more, preferably 10% or more, 15% or more, or 20% or more. The content of the vinylidene fluoride polymer in the protective layer is preferably 50% or less, may be 40% or less, or 30% or less.

[0050] In particular, it is preferable to use a vinylidene fluoride polymer synthesized by polymerizing monomers containing vinylidene fluoride as the resin that forms the protective layer. Polyvinylidene fluoride is a typical example. The vinylidene fluoride polymer has high adhesion to the negative electrode and high stability within the lithium secondary battery (B).

[0051] Vinylidene fluoride polymers contain constituent units derived from vinylidene fluoride. The proportion of VDF units (constituent units derived from vinylidene fluoride) to the total constituent units of the vinylidene fluoride polymer is 50 mol% or more and 100 mol% or less. This proportion may be 75 mol% or more, or 90 mol% or more, and may be 99.5 mol% or less, or 95 mol% or less. Examples of monomers copolymerized with vinylidene fluoride include tetrafluoroethylene and hexafluoropropylene.

[0052] The vinylidene fluoride polymer may be polyvinylidene fluoride. Alternatively, the vinylidene fluoride polymer may be a copolymer synthesized by polymerizing monomers containing vinylidene fluoride and hexafluoropropylene. Since hexafluoropropylene has a bulkier molecular structure than vinylidene fluoride, copolymers synthesized by polymerizing monomers containing vinylidene fluoride and hexafluoropropylene have a lower density than vinylidene fluoride polymers. By using monomers containing hexafluoropropylene, polymer crystallization is suppressed, and a flexible adhesive layer is obtained. In addition, the retention of non-aqueous electrolytes in the adhesive layer is improved, and lithium-ion conductivity is enhanced. As a result, the resistance of the battery is reduced, and the cycle characteristics can be further improved.

[0053] The vinylidene fluoride polymer may be a copolymer of vinylidene fluoride and hexafluoropropylene (HFP). The proportion of HFP units (constituent units derived from hexafluoropropylene) to the total constituent units of the vinylidene fluoride polymer may be in the range of 0 to 50 mol% (for example, in the range of 0.5 to 25 mol% or 2 to 10 mol%).

[0054] High-strength resins such as polyimide and polyamide are also preferred as resins for forming the protective layer. Because these resins have a rigid molecular structure, they have a strong effect in inhibiting the growth of dendrite-like lithium metal. Furthermore, resins with such molecular structures have excellent adhesion to lithium metal.

[0055] High-strength resins are also heat-resistant resins with high heat resistance. A heat-resistant resin refers to a resin with a high heat distortion temperature, for example. The heat distortion temperature of a heat-resistant resin is preferably 260°C or higher. Here, the heat distortion temperature is the deflection temperature determined by a load of 1.82 MPa in accordance with ASTM-D648. The protective layer containing the heat-resistant resin can maintain its covering of the negative electrode surface without shrinking even if the separator shrinks due to heat. Therefore, it also serves to ensure the safety of the battery when the battery becomes hot enough for the separator to shrink due to heat.

[0056] Examples of heat-resistant resins include polyamides, polyamide-imides, polyimides, cellulose, cellulose acetate, polysulfones, polyethersulfones, polyetherimides, polyetheretherketones, amorphous polyarylates, polyphenylene sulfide, polypyromellitrimide, and polycarbonates. Among these, aromatic polyamides, aromatic polyamide-imides, and aromatic polyimides are preferred, and aramid (all-aromatic polyamide) is more preferred. All-aromatic polyamides may be meta-aromatic polyamides or para-aromatic polyamides. The heat distortion temperature of these heat-resistant resins can be 260°C or higher. Among these, aramid has a rigid molecular structure consisting of a benzene ring and an amide bond, which further inhibits the growth of dendritic lithium metal and provides excellent resistance to reducing atmospheres caused by lithium metal. Aramid has high polarity and excellent adhesion to lithium metal.

[0057] The inorganic filler may contain boehmite particles, but may also contain particles other than boehmite. However, the mass content of boehmite particles in the inorganic filler may be, for example, 50% or more, 60% or more, 80% or more, 90% or more, 99% or more, or 100%.

[0058] As previously described, boehmite particles are plate-shaped particles with a flat surface. The average aspect ratio of boehmite particles is, for example, 2 or more, and may also be 3.5 or more. The aspect ratio of a particle is the ratio of the length of the major axis to the minor axis (the length in the direction perpendicular to the major axis at the center of the major axis) of the boehmite particle observed in the secondary electron image of a cross-sectional SEM at a magnification of 10,000x. The aspect ratio is obtained by averaging the values ​​calculated for each of 15 or more arbitrarily selected boehmite particles.

[0059] The mass content of the inorganic filler in the protective layer may be, for example, 50% or more, preferably 60% or more, may be 70% or less, or may be 80% or more. The mass content of the inorganic filler in the protective layer may be 99% or less, preferably 97% or less, may be 90% or less, or may be 85% or less. This makes it easy to set the ratio of the area of ​​the inorganic filler to the cross-section S (ratio R(F)) to, for example, 30% to 70%, making it easier to obtain a protective layer with excellent durability.

[0060] Other particles that can be used as inorganic fillers (hereinafter also referred to as "second inorganic particles") may include metal oxides, metal hydroxides, metal nitrides, metal carbides, metal sulfides, etc. Examples of materials for the second inorganic particles include α-alumina, magnesium oxide, titanium oxide, zirconium oxide, silicon oxide, zinc oxide, silicon nitride, aluminum nitride, titanium nitride, silicon carbide, aluminum hydroxide, lithium phosphate, lithium fluoride, carbon black (such as acetylene black), amorphous silica, and crystalline silica. Solid electrolyte particles such as LLZO (lithium lanthanum zirconium oxide) and LATP (lithium aluminum titanium phosphate) may also be used.

[0061] 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.

[0062] The thickness of the protective layer may be, for example, in the range of 0.2 μm to 10 μm (e.g., 1 μm to 8 μm, 2 μm to 4 μm, or 4 μm to 10 μm). The thickness of the protective layer may be 0.2 μm or more, 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 may be 10 μm or less, 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.

[0063] (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.

[0064] 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.

[0065] 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.

[0066] 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).

[0067] 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 (non-solvent-induced phase separation method), but the method for producing a porous heat-resistant layer is not particularly limited. In the NIPS method, for example, a coating solution is prepared by dissolving a resin in the solvent (good solvent) of the coating solution described above, the coating solution is applied to the surface of a separator to form a coating film, and then, before the coating film dries, the coating film is immersed in a poor solvent to make the coating film porous and form a porous heat-resistant layer.

[0068] Poor solvents that can be used include water, methanol, ethanol, isopropyl alcohol, or mixtures thereof. Furthermore, by mixing the above poor solvents with acetone, dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, tripropylene glycol, etc., the porosity of the protective layer (coating film) can be altered.

[0069] 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.

[0070] 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.

[0071] For the second separator, materials such as the porous sheet described as the first separator can be used.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] (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.

[0076] (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.

[0077] (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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] (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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] Conductive materials can include carbon materials. Examples of carbon materials include carbon black (acetylene black, Ketjenblack, etc.), carbon nanotubes, and graphite.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] (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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] Fluorination rate (%) = 100 × (number of fluorine atoms in the hydrofluoroether) / (total number of fluorine and hydrogen atoms in the hydrofluoroether)

[0096] 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.

[0097] 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.

[0098] 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.

[0099] 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.

[0100] (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.

[0101] 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.

[0102] (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.

[0103] 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.

[0104] 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.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] (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 in 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 resin and an inorganic filler, the inorganic filler comprises boehmite particles, and in the X-ray diffraction pattern of the protective layer, the ratio I1 / I2 of the peak intensity of the diffraction peak attributed to the (020) plane of the boehmite particles to the peak intensity of the diffraction peak attributed to the (130) plane, respectively, is 5.0 or more. (Technology 2) The lithium secondary battery according to Technology 1, wherein the porosity at the interface between the negative electrode and the protective layer is 10% or less. (Technology 3) A lithium secondary battery according to Technology 1 or 2, wherein the porosity within the protective layer is 5% or more and 40% or less. (Technology 4) A lithium secondary battery according to any one of Technology 1 to 3, wherein in any cross-section S of the protective layer in a direction parallel to the thickness direction of the protective layer, the ratio of the area of ​​the inorganic filler to the arbitrary cross-section S is 30% or more and 70% or less. (Technology 5) A lithium secondary battery according to any one of Technology 1 to 4, 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 6) 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 comprises a resin and an inorganic filler, the inorganic filler comprises boehmite particles, and in the X-ray diffraction pattern of the protective layer, the ratio I1 / I2 of the peak intensity of the diffraction peak attributed to the (020) plane of the boehmite particles and the peak intensity of the diffraction peak attributed to the (130) plane, respectively, is 5.0 or more.

[0110] 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.

[0111] (Batteries A1 to A16) (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.

[0112] 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.

[0113] (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.

[0114] (3) Preparation of non-aqueous electrolytes LiPF ６ 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.

[0115] (4) Preparation of protective layer and separator A 10 μm thick polyethylene microporous membrane was prepared as a separator (first separator). A protective layer (2 μm thick) containing polyvinylidene fluoride and boehmite particles was formed on one side of the microporous membrane. Next, the lithium-containing metal layer of the negative electrode and the microporous membrane were laminated with the protective layer in between, and the negative electrode, protective layer and first separator were integrated by hot rolling press. Thus, the protective layer is adhered to the surface of the separator and also adhered to the surface of the lithium-containing metal layer of the negative electrode.

[0116] The boehmite particles were selected so that the intensity ratio I1 / I2 in the X-ray diffraction pattern of the protective layer would be the value shown in Table 1.

[0117] 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, protective layer, and separator.

[0118] The porosity B within the protective layer was controlled to the values ​​shown in Table 1 by changing the mixing ratio of polyvinylidene fluoride and boehmite particles, and the conditions of the heated rolling press.

[0119] The ratio (ratio R(F)) of the area of ​​inorganic filler (boehmite particles) in any cross-section S of the protective layer in a direction parallel to the thickness direction of the protective layer was controlled to the values ​​shown in Table 1 by changing the mixing ratio of polyvinylidene fluoride and boehmite particles and the conditions of the heated rolling press.

[0120] (5) Battery Fabrication Next, a wound electrode assembly was fabricated by stacking and winding an integrated negative electrode, protective layer, separator, and positive electrode in an inert gas atmosphere. Then, the electrode assembly and the non-aqueous electrolyte were housed in an outer casing, and the outer casing was sealed to fabricate batteries A1 to A17 (lithium secondary batteries) of the example. A bag-shaped outer casing made of a laminate sheet containing an aluminum layer was used for the outer casing.

[0121] [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.

[0122] (Batteries B1, B2) Except for changing the boehmite particles to barium sulfate or alumina, and setting the strength ratio I1 / I2, porosity A, B, and ratio R(F) to the values ​​shown in Table 1, batteries B1 and B2 were fabricated and evaluated in the same manner as battery A1.

[0123] (Batteries B3, B4) Except for setting the strength ratio I1 / I2, porosity A, B, and ratio R(F) to the values ​​shown in Table 1, batteries B3 and B4 were fabricated and evaluated in the same manner as battery A1.

[0124] Table 1 shows some of the manufacturing conditions 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.

[0125]

[0126] Batteries A1 to A16 are lithium secondary batteries (B) according to this disclosure. Batteries B1 to B4 are comparative examples. As shown in Table 1, batteries A1 to A16 had better cycle characteristics than batteries B1 to B4. Generally, there was a tendency for cycle characteristics to improve as the intensity ratio I1 / I2 increased. When the porosity A was 10% or less (and even 5% or less), the cycle characteristics were better. Also, when the porosity B was 5% or more and 40% or less, the cycle characteristics were better. When the ratio R(F) was 30% or more and 70% or less, the cycle characteristics were even better.

[0127] This disclosure can be used in lithium secondary batteries.

[0128] 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.

[0129] 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 resin and an inorganic filler, the inorganic filler comprises boehmite particles, and the ratio I1 / I2 of the peak intensity of the diffraction peak attributed to the (020) plane and the peak intensity of the diffraction peak attributed to the (130) plane of the boehmite particles in the X-ray diffraction pattern of the protective layer is 5.0 or more.

2. 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.

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

4. The lithium secondary battery according to claim 1, wherein in any cross-section S of the protective layer in a direction parallel to the thickness direction of the protective layer, the ratio of the area of ​​the inorganic filler to the area of ​​the arbitrary cross-section S is 30% or more and 70% or less.

5. 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.

6. 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 comprises a resin and an inorganic filler, the inorganic filler comprises boehmite particles, and the ratio I1 / I2 of the peak intensity of the diffraction peak attributed to the (020) plane and the peak intensity of the diffraction peak attributed to the (130) plane of the boehmite particles in the X-ray diffraction pattern of the protective layer is 5.0 or more.

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