Lithium secondary batteries

Abstract

This invention provides a lithium secondary battery with high energy density and excellent cycle characteristics. The invention relates to lithium secondary batteries, etc., wherein the lithium secondary battery comprises a positive electrode, a negative electrode without a negative electrode active material, a separator disposed between the positive and negative electrodes, and a fibrous or porous buffer layer with ion conductivity formed on the surface of the separator opposite to the negative electrode. The positive electrode comprises a positive electrode active material and a lithium-containing compound that undergoes oxidation within the charge / discharge potential range of the positive electrode active material but does not substantially undergo reduction. In the particle size distribution measured by laser diffraction scattering of the lithium-containing compound, the particle size D corresponding to 50% of the cumulative frequency is... 50 (S) is a particle size D corresponding to the 95% cumulative frequency of lithium-containing compounds, where 1.0 μm is greater than or equal to 20 μm. 95 (S) is 1.0μm or more and 30μm or less.

Description

Technical Field

[0001] This invention relates to lithium secondary batteries. Background Technology

[0002] In recent years, technologies that convert natural energy such as sunlight or wind power into electrical energy have attracted much attention. Along with this, various types of rechargeable batteries have been developed as energy storage devices that are highly safe and capable of storing a large amount of electrical energy.

[0003] Among these, secondary batteries that are known to perform charging and discharging by moving metal ions between the positive and negative electrodes exhibit high voltage and high energy density; lithium-ion secondary batteries are a typical example. As a typical lithium-ion secondary battery, an active material capable of retaining lithium is introduced into both the positive and negative electrodes, and charging and discharging are performed by giving and receiving lithium ions between the positive and negative electrode active materials. Furthermore, as a secondary battery that does not use an active material at the negative electrode, lithium metal secondary batteries have been developed that retain lithium by depositing lithium metal on the surface of the negative electrode.

[0004] For example, Patent Document 1 discloses a high-energy-density, high-output lithium metal anode secondary battery that, when discharged at a rate of at least 1C at room temperature, has a volumetric energy density exceeding 1000 Wh / L and / or a gravimetric energy density exceeding 350 Wh / kg. Patent Document 1 discloses the use of an extremely thin lithium metal anode to achieve such a lithium metal anode secondary battery.

[0005] Furthermore, Patent Document 2 discloses a lithium secondary battery comprising a positive electrode, a negative electrode, a separator sandwiched between them, and an electrolyte. In this battery, metal particles are formed on a negative electrode current collector at the negative electrode, and these particles move from the positive electrode during charging, forming lithium metal on the negative electrode current collector within the negative electrode. Patent Document 2 discloses a lithium secondary battery that solves problems caused by the reactivity of lithium metal and issues arising during assembly, providing a lithium secondary battery with improved performance and lifespan.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Publication No. 2019-517722

[0009] Patent Document 2: Japanese Patent Publication No. 2019-537226 Summary of the Invention

[0010] However, the inventors have studied in detail existing batteries, including those described in the aforementioned patent documents, and found that at least one of the energy density, cycle characteristics, and rate characteristics is insufficient.

[0011] For example, typical secondary batteries that charge and discharge by giving and receiving lithium ions between the positive and negative electrode active materials have insufficient energy density. Furthermore, conventional lithium metal secondary batteries, as described in the aforementioned patent documents, which retain lithium by depositing lithium metal on the negative electrode surface, are prone to forming dendritic lithium metal on the negative electrode surface due to repeated charging and discharging, leading to short circuits and capacity reduction. As a result, cycle characteristics are insufficient. Moreover, because the internal resistance of such lithium metal secondary batteries tends to increase with repeated charging and discharging, rate characteristics also decrease.

[0012] Furthermore, methods have been developed to maintain a high voltage at the interface between the negative electrode and the separator by applying significant physical pressure to the battery in order to suppress the discrete growth of lithium metal during deposition in lithium metal secondary batteries. However, since applying such high voltage requires a large mechanical mechanism, the overall weight and volume of the battery increase, and the energy density decreases.

[0013] The present invention was made in view of the above-mentioned problems, and its purpose is to provide a lithium secondary battery with high energy density and excellent cycle characteristics or rate characteristics.

[0014] An embodiment of the present invention relates to a lithium secondary battery comprising: a positive electrode; a negative electrode, which does not have a negative electrode active material; a separator disposed between the positive and negative electrodes; and a buffer functional layer, wherein a fibrous or porous structure is formed on the surface of the separator opposite to the negative electrode and has ion conductivity; the positive electrode comprises a positive electrode active material and a lithium-containing compound that undergoes oxidation within the charge / discharge potential range of the positive electrode active material and substantially does not undergo reduction; and in the particle size distribution measured by laser diffraction scattering, the particle size D corresponding to 50% of the cumulative frequency of the lithium-containing compound is... 50 (S) is a particle size D corresponding to the 95% cumulative frequency of lithium-containing compounds, where 1.0 μm is greater than or equal to 20 μm. 95 (S) is 1.0μm or more and 30μm or less.

[0015] Because such lithium secondary batteries have high energy density, they are charged and discharged by having a negative electrode without negative electrode active material, by the deposition of lithium metal on the surface of the negative electrode and the electrolytic dissolution of the deposited lithium metal.

[0016] Furthermore, it is speculated that the buffer layer of the lithium secondary battery according to one embodiment of the present invention functions as a buffer layer to mitigate and suppress the volume expansion of the battery during charging and discharging.

[0017] Furthermore, the aforementioned lithium secondary battery has a lithium-containing compound as a positive electrode sacrificial agent at the positive electrode. This positive electrode sacrificial agent undergoes oxidation (i.e., releases lithium ions) during the initial charging of the lithium secondary battery, but substantially does not undergo reduction during subsequent discharge (i.e., no lithium-containing compound is formed before discharge). The lithium element from this lithium-containing compound remains as lithium metal on the negative electrode surface. Moreover, in the particle size distribution measured by laser diffraction scattering, the particle size D corresponding to 50% of the cumulative frequency of this positive electrode sacrificial agent... 50 (S) is a particle size D corresponding to 95% of the cumulative frequency, which is between 1.0 μm and 20 μm. 95 (S) is between 1.0 μm and 30 μm. Positive electrode sacrificial agents with such particle size can maintain a low interfacial resistance and more uniformly distribute lithium metal on the negative electrode surface.

[0018] Therefore, it is believed that during discharge, the lithium metal uniformly deposited on the negative electrode surface of the aforementioned lithium secondary battery is not completely dissolved, and even after discharge, some lithium metal remains on the negative electrode surface. Since this residual lithium metal becomes a base for further lithium metal deposition on the negative electrode surface during subsequent charging, lithium metal is more easily and uniformly deposited on the negative electrode surface during this charging process. Therefore, the aforementioned lithium secondary battery suppresses the growth of dendritic lithium metal on the negative electrode and exhibits excellent cycle characteristics.

[0019] Another embodiment of the present invention relates to a lithium secondary battery comprising: a positive electrode; a negative electrode, which does not have a negative electrode active material; a separator disposed between the positive and negative electrodes; and a buffer functional layer, wherein a fibrous or porous structure is formed on the surface of the separator opposite to the negative electrode and has ion conductivity; the positive electrode comprises a positive electrode active material and a lithium-containing compound that undergoes oxidation within the charge / discharge potential range of the positive electrode active material and substantially does not undergo reduction; and in the particle size distribution measured by laser diffraction scattering, the particle size corresponding to 50% of the cumulative frequency is set as D. 50 Under these conditions, the D of the positive electrode active material 50 (A) D with a particle size of 5.0 μm to 20 μm, used as a positive electrode active material 50 (A) D relative to lithium-containing compounds 50 (S) particle size ratio D 50 (A) / D 50 (S) is 2.0 or higher and 10.0 or lower.

[0020] Because such lithium secondary batteries have a negative electrode without negative electrode active material, a buffer functional layer, and a lithium-containing compound as a positive electrode sacrificial agent, they have high energy density and excellent cycle characteristics for the same reasons mentioned above.

[0021] Here, because the lithium-containing compound used as a positive electrode sacrificial agent has lower conductivity than the positive electrode active material, the internal resistance of the positive electrode as a whole tends to increase when a positive electrode sacrificial agent is added. On the other hand, in the particle size distribution of the aforementioned lithium secondary battery measured by laser diffraction scattering, the D of the positive electrode active material... 50 (A) is between 5.0 μm and 20 μm, due to the D of the positive electrode active material. 50 (A) D relative to lithium-containing compounds 50 (S) particle size ratio D 50 (A) / D 50 The S value is between 2.0 and 10.0, thus suppressing the contact between the positive electrode active materials, which is hindered by the positive electrode sacrificial agent. As a result, the conductivity within the positive electrode is high. Therefore, the internal resistance within the positive electrode of the above-mentioned lithium secondary battery is sufficiently low, and the rate characteristics are excellent.

[0022] In the above-mentioned positive electrode active material D 50 (A) Particle size is between 5.0 μm and 20 μm, with a particle size ratio D 50 (A) / D 50 In lithium secondary batteries with a S value of 2.0 to 10.0, the D of lithium compounds is... 50 (S) is preferably 1.0 μm or more and 10 μm or less. In this way, since the lithium metal deposited on the negative electrode surface is more uniform, the deposition of dendritic lithium metal on the negative electrode can be suppressed, resulting in better cycle characteristics.

[0023] In the above-mentioned positive electrode active material D 50 (A) Particle size is between 5.0 μm and 20 μm, with a particle size ratio D 50 (A) / D 50 In lithium secondary batteries where (S) is 2.0 to 10.0, the electrode density of the positive electrode is preferably 3.0 g / cc or higher. This allows for a further increase in the capacity of the lithium secondary battery.

[0024] The aforementioned lithium secondary battery preferably contains 1.0% to 15% by mass of the aforementioned positive electrode sacrificial agent relative to the total mass of the aforementioned positive electrode. In this manner, the positive electrode sacrificial agent functions more effectively and reliably, resulting in superior cycle characteristics for the lithium secondary battery.

[0025] The ratio of the irreversible capacity of the aforementioned lithium-containing compound to the cell capacity of the lithium secondary battery is preferably 1.0% to 30%. In this manner, since the residual lithium remaining on the negative electrode surface after discharge is in a more appropriate amount, the cycle characteristics and energy density of the lithium secondary battery are further improved.

[0026] The porosity of the buffer layer is preferably 50% or higher. In this way, the buffer layer functions more effectively and reliably, thus improving the cycle characteristics and energy density of the lithium-ion battery.

[0027] The buffer layer is preferably also conductive. When a lithium-ion secondary battery has a conductive buffer layer on the surface of the separator, lithium metal can be deposited not only on the surface of the negative electrode but also inside the fibrous or porous buffer layer, which has both ion conductivity and electrical conductivity. Therefore, the surface area of ​​the reaction field for the lithium metal deposition reaction increases, and the reaction rate of the lithium metal deposition reaction is slowly controlled. As a result, the growth of dendritic lithium metal on the negative electrode is further suppressed, and the cycle characteristics of the battery tend to be improved.

[0028] The aforementioned positive electrode sacrificial agent is preferably an iron-containing compound. In this manner, the cycle characteristics of the lithium-ion secondary battery become superior because the positive electrode sacrificial agent functions more effectively and reliably.

[0029] Invention Effects

[0030] According to the present invention, a lithium secondary battery with high energy density and excellent cycle characteristics or rate characteristics can be provided. Attached Figure Description

[0031] Figure 1 This is a schematic cross-sectional view of the lithium secondary battery involved in the first embodiment.

[0032] Figure 2 This is a schematic cross-sectional view of the use of the lithium secondary battery according to the first embodiment.

[0033] Figure 3 This is a schematic cross-sectional view of the buffer functional layer in the lithium secondary battery according to the first embodiment. (A) shows a fibrous buffer functional layer as an embodiment of the buffer functional layer, and (B) shows the deposition method of lithium metal in the fibrous buffer functional layer.

[0034] Figure 4 This is a schematic cross-sectional view of the buffer functional layer in the lithium secondary battery according to the first and second embodiments, showing one embodiment of the components constituting the fibrous buffer functional layer. Detailed Implementation

[0035] Hereinafter, embodiments of the present invention (hereinafter referred to as "this embodiment") will be described in detail with reference to the accompanying drawings. Furthermore, in the drawings, the same reference numerals are used to label the same elements, and repeated descriptions are omitted. Moreover, unless otherwise specified, positional relationships such as up, down, left, and right are based on the positional relationships shown in the drawings. Furthermore, the scale of the drawings is not limited to the scale shown in the illustrations.

[0036] [First Implementation Method]

[0037] (Lithium-ion rechargeable battery)

[0038] Figure 1 This is a schematic cross-sectional view of the lithium secondary battery according to the first embodiment. (Example) Figure 1 As shown, the lithium secondary battery 100 of the first embodiment includes a positive electrode 110, a negative electrode 140 without negative electrode active material, a separator 120 disposed between the positive electrode 110 and the negative electrode 140, and a buffer functional layer 130 formed on the surface of the negative electrode 140 opposite to the separator 120. The positive electrode 110 has a positive electrode current collector 150 on the surface opposite to the surface opposite to the separator 120.

[0039] (negative electrode)

[0040] The negative electrode 140 does not contain a negative electrode active material. In this specification, "negative electrode active material" refers to a substance that generates electrode reactions in the negative electrode, namely oxidation and reduction reactions. Specifically, lithium metal and host materials of lithium elements (lithium ions or lithium metal) are listed as negative electrode active materials in this embodiment. A host material of lithium elements refers to a substance provided to retain lithium ions or lithium metal on the negative electrode. There are no particular limitations on such retaining mechanisms; examples include intercalation, alloying, and absorption by metal clusters, with intercalation being typical.

[0041] Since the lithium secondary battery of this embodiment does not have negative electrode active material at the negative electrode before the initial charging of the battery, charging and discharging are carried out by depositing lithium metal on the negative electrode and electrolyzing the deposited lithium metal. Therefore, compared with lithium secondary batteries with negative electrode active material, the lithium secondary battery of this embodiment has a smaller overall volume and mass due to the reduction in the volume and mass of the negative electrode active material, and thus, in principle, a higher energy density.

[0042] In this embodiment of the lithium secondary battery 100, before the initial charging of the battery, the negative electrode 140 does not have negative electrode active material. Lithium metal is deposited on the negative electrode during battery charging, and the deposited lithium metal is electrolytically dissolved during battery discharge. Therefore, in this embodiment of the lithium secondary battery, the negative electrode functions as a negative electrode current collector.

[0043] When the lithium secondary battery 100 of this embodiment is compared with lithium-ion batteries (LIB) and lithium metal batteries (LMB), it differs in the following aspects.

[0044] In a lithium-ion battery (LIB), the negative electrode has a host material containing lithium elements (lithium ions or lithium metal). Lithium elements are filled into this material during battery charging, and the lithium elements are released through the host material to discharge the battery. The LIB differs from the lithium secondary battery 100 of this embodiment in that the negative electrode contains a host material containing lithium elements.

[0045] Lithium metal batteries (LMBs) are manufactured by having an electrode with lithium metal on its surface, or by using a single lithium metal cell as the negative electrode. That is, the difference between an LMB and the lithium secondary battery 100 of this embodiment is that, after battery assembly, i.e., before the initial charging of the battery, the negative electrode has lithium metal as the negative electrode active material. While LMBs use electrodes containing highly flammable and reactive lithium metal in their manufacturing, the lithium secondary battery 100 of this embodiment uses a negative electrode without lithium metal, resulting in superior safety and manufacturability.

[0046] In this specification, "not having negative electrode active material" means that the negative electrode 140 does not have or substantially does not have negative electrode active material. "Substantially not having negative electrode active material" means that the content of negative electrode active material in the negative electrode 140 is less than 10% by mass relative to the total negative electrode. The content of negative electrode active material in the negative electrode relative to the total negative electrode 140 is preferably less than 5.0% by mass, but can be less than 1.0% by mass, less than 0.1% by mass, or less than 0.0% by mass. By having the negative electrode 140 not having negative electrode active material or the content of negative electrode active material in the negative electrode 140 being within the above range, the energy density of the lithium secondary battery 100 is increased.

[0047] In this specification, "before initial charging" refers to the state of the battery from assembly to the first charging. Furthermore, "at the end of discharge" refers to the state of the battery with a voltage of 1.0V to 3.8V, preferably 1.0V to 3.0V.

[0048] In this specification, "a lithium secondary battery having a negative electrode without negative electrode active material" means that, before the initial charging of the battery, the negative electrode 140 does not have negative electrode active material. Therefore, the phrase "a negative electrode without negative electrode active material" can be translated as "a negative electrode without negative electrode active material before the initial charging of the battery," "regardless of the charging state of the battery, it does not have negative electrode active material other than lithium metal, and before the initial charging, it does not have a lithium metal negative electrode current collector," or "before the initial charging, it does not have a lithium metal negative electrode current collector," etc. Furthermore, "a lithium secondary battery having a negative electrode without negative electrode active material" can also be translated as an anode-free lithium battery, a zero-anode lithium battery, or a negative electrode-free lithium battery.

[0049] In this embodiment, the negative electrode 140 has a content of negative electrode active material other than lithium metal of 10% by mass or less relative to the total negative electrode, which is preferably 5.0% by mass or less, but can be 1.0% by mass or less, 0.1% by mass or less, 0.0% by mass or less, or even 0% by mass, regardless of the charging state of the battery.

[0050] Furthermore, in this embodiment, before initial charging, the lithium metal content of the negative electrode 140 is 10% by mass or less relative to the overall negative electrode, preferably 5.0% by mass or less, but can be 1.0% by mass or less, 0.1% by mass or less, 0.0% by mass or less, or even 0% by mass.

[0051] In this embodiment, when the battery voltage is 1.0V or higher and 3.5V or lower, the lithium metal content relative to the negative electrode 140 can be 10% by mass or lower (preferably 5.0% by mass or lower, or 1.0% by mass or lower); when the battery voltage is 1.0V or higher and 3.0V or lower, the lithium metal content relative to the negative electrode 140 can be 10% by mass or lower (preferably 5.0% by mass or lower, or 1.0% by mass or lower); or, when the battery voltage is 1.0V or higher and 2.5V or lower, the lithium metal content relative to the negative electrode 140 can also be 10% by mass or lower (preferably 5.0% by mass or lower, or 1.0% by mass or lower).

[0052] Furthermore, in the lithium secondary battery 100 of this embodiment, the mass M of lithium metal deposited on the negative electrode when the battery voltage is 3.0V is... 3.0 The mass M of lithium metal deposited on the negative electrode relative to the battery voltage of 4.2V. 4.2 The ratio M 3.0 / M 4.2 Preferably, it is 40% or less; more preferably, it is 38% or less; and even more preferably, it is 35% or less. Ratio M 3.0 / M 4.2 It can be 1.0% or higher, 2.0% or higher, 3.0% or higher, or 4.0% or higher.

[0053] Examples of negative electrode active materials used in this embodiment include lithium metal and alloys containing lithium metal, carbon-based materials, metal oxides and metals alloyed with lithium, and alloys containing such metals. The carbon-based materials are not particularly limited, and examples include graphene, graphite, hard carbon, mesoporous carbon, carbon nanotubes, and carbon nanotubes. The metal oxides are not particularly limited, and examples include titanium oxide compounds, tin oxide compounds, and cobalt oxide compounds. Metals alloyed with lithium include silicon, germanium, tin, lead, aluminum, and gallium.

[0054] As the negative electrode 140 in this embodiment, there are no particular limitations if it does not have a negative electrode active material but can be used as a current collector. Examples include at least one composition selected from the group consisting of Cu, Ni, Ti, Fe, and other metals that do not react with Li and their alloys, and stainless steel (SUS). Preferably, it is composed of at least one composition selected from the group consisting of Cu, Ni and their alloys, and stainless steel (SUS). When such a negative electrode is used, the energy density and productivity of the battery tend to be better. Furthermore, when SUS is used in the negative electrode, various conventionally known substances can be used as the type of SUS. The aforementioned negative electrode materials can be used alone or in combination of two or more. In addition, in this specification, "metals that do not react with Li" refers to metals that do not react with lithium ions or lithium metal under the operating conditions of a lithium secondary battery.

[0055] The average thickness of the negative electrode 140 is preferably 4 μm or more and 20 μm or less, more preferably 5 μm or more and 18 μm or less, and even more preferably 6 μm or more and 15 μm or less. In this manner, the volume occupied by the negative electrode 140 in the secondary battery 100 is reduced, thus further improving the energy density of the lithium secondary battery 100.

[0056] (positive electrode)

[0057] Because the positive electrode 110 contains positive electrode active material, the lithium secondary battery 100 has excellent stability and high output voltage.

[0058] In this specification, "positive electrode active material" refers to a substance that generates electrode reactions at the positive electrode, namely oxidation and reduction reactions. Specifically, lithium (typically lithium ions) host materials are listed as positive electrode active materials in this embodiment. There are no particular limitations on such positive electrode active materials; examples include metal oxides and metal phosphates. There are no particular limitations on the aforementioned metal oxides; examples include cobalt oxide compounds, manganese oxide compounds, and nickel oxide compounds. There are no particular limitations on the aforementioned metal phosphates; examples include iron phosphate compounds and cobalt phosphate compounds. Typical positive electrode active materials include LiCoO2 and LiNi. x Co y Mn Z O2(x+y+z=1), LiNi x Mn y O2 (x+y=1), LiNiO2, LiMn2O4, LiFePO4, LiCoPO4, LiFeOF, LiFeOF, LiNiOF, and TiS2. One or more of the above-mentioned positive electrode active materials may be used alone.

[0059] The positive electrode 110, in addition to the positive electrode active material, contains a lithium-containing compound (i.e., a positive electrode sacrificial agent) that undergoes oxidation within the charge / discharge potential range of the positive electrode active material but does not substantially undergo reduction. When the lithium secondary battery 100 equipped with such a positive electrode 110 is initially charged, both the positive electrode active material and the positive electrode sacrificial agent undergo oxidation while releasing lithium ions, releasing electrons to the negative electrode 140 via an external circuit. As a result, lithium ions from the positive electrode active material and the positive electrode sacrificial agent are deposited on the surface of the negative electrode. Furthermore, when the lithium secondary battery 100 is discharged after the initial charge (i.e., during initial discharge), the lithium metal deposited on the negative electrode surface electrolytically dissolves, and electrons move from the negative electrode 140 to the positive electrode 110 via an external circuit. Accompanying this, the positive electrode active material undergoes reduction while receiving lithium ions, while the positive electrode sacrificial agent does not substantially undergo reduction within the discharge potential range of the positive electrode active material, and is essentially impossible to return to its state before the oxidation reaction. In other words, the positive electrode 110 has a positive electrode sacrificial agent before the initial charge. Furthermore, "initial charge" refers to the first charging step after the battery is assembled.

[0060] Therefore, when the lithium secondary battery 100 is discharged after the initial charge, while lithium metal from the positive electrode active material is electrolytically dissolved from the negative electrode, almost all of the lithium metal from the positive electrode sacrificial agent remains on the negative electrode. Even after the battery is fully discharged, some lithium metal remains on the negative electrode. This residual lithium metal becomes a foothold for further lithium metal deposition on the negative electrode during the subsequent charging step after the initial discharge. Therefore, lithium metal is easily and uniformly deposited on the negative electrode during the charging step after the initial discharge. As a result, the lithium secondary battery 100 exhibits excellent cycle characteristics because it suppresses the growth of dendritic lithium metal on the negative electrode.

[0061] The positive electrode sacrificial agent in positive electrode 110 is a lithium-containing compound that undergoes oxidation within the charge / discharge potential range of the positive electrode active material but does not substantially undergo reduction. In this specification, "oxidation within the charge / discharge potential range of the positive electrode active material" means that within the charge / discharge potential range of the positive electrode active material, an oxidation reaction can occur to release lithium ions and electrons (including decomposition and release of lithium ions through oxidation). Furthermore, "not substantially undergoing reduction within the charge / discharge potential range of the positive electrode active material" means that within the charge / discharge potential range of the positive electrode active material, under reaction conditions normally understood by those skilled in the art, a reduction reaction to receive lithium ions and electrons is impossible or substantially impossible to generate them via a reduction reaction. "Reaction conditions normally understood by those skilled in the art" refers, for example, the conditions under which a lithium secondary battery is discharged. Furthermore, "the positive electrode sacrificial agent is substantially unable to undergo a reduction reaction to accept lithium ions and electrons or to be generated via a reduction reaction" means that a positive electrode sacrificial agent oxidized during battery charging with a capacity ratio of 80% or more (e.g., 80% or more, 85% or more, 90% or more, 95% or more, 99% or more, or 100%) cannot undergo a reduction reaction to accept lithium ions and electrons, or cannot be generated via a reduction reaction. Therefore, the initial discharge capacity of the positive electrode sacrificial agent relative to the initial charge capacity is less than 20% (e.g., less than 20%, less than 15%, less than 10%, less than 5%, less than 1%, or 0%).

[0062] In this specification, "charge and discharge potential range of the positive electrode active material" refers to the potential range within which the oxidation and reduction reactions of the positive electrode active material contained in the positive electrode 110 can occur. The specific value depends on the type of positive electrode active material contained in the positive electrode 110, but typically, it is relative to Li... +The reference electrode for Li is 2.5V or higher, 2.7V or higher, 3.0V or higher, 3.2V or higher, or 3.5V or higher, and below 4.5V, 4.4V, 4.3V, 4.2V, 4.1V or lower, or below 4.0V. The representative range for the charge / discharge potential of the positive electrode active material is 3.0V or higher and 4.2V or lower (vs. Li). + / Li reference electrode), whose upper and lower limits can be independently replaced with any of the above values. Furthermore, the positive electrode active material relative to Li + The charge / discharge potential range of the / Li reference electrode can be referenced to the operating voltage range of the lithium secondary battery 100. For example, when the operating voltage of the lithium secondary battery 100 is above 3.0V and below 4.2V, the positive electrode active material relative to Li + The charge / discharge potential range of the / Li reference electrode can be estimated to be above 3.0V and below 4.2V. That is, the positive electrode sacrificial agent can also be described as "a lithium-containing compound that undergoes oxidation within the operating voltage range of a lithium secondary battery and does not substantially undergo reduction".

[0063] Examples of positive electrode sacrificial agents are not particularly limited, but include lithium oxides such as Li₂O₂; lithium nitrides such as Li₃N; lithium sulfide solid solutions such as Li₂S-P₂S₅, Li₂S-LiCl, Li₂S-LiBr, and Li₂S-LiI; Li 1+x (Ti 1-y Fe y ) 1-x O2(0<x≦0.25, 0.4<y≦0.9), Li 2-x Ti 1-z Fe z O 3-y (0≦x<2, 0≦y≦1, 0.05≦z≦0.95), iron-containing lithium oxides such as Li5FeO4, etc. From the viewpoint of more effectively and reliably acting as a positive electrode sacrificial agent, it is preferable to use iron-containing lithium compounds, more preferably iron-containing lithium oxides, and even more preferably Li5FeO4. The aforementioned positive electrode sacrificial agents can be used alone or in combination of two or more. Furthermore, the aforementioned positive electrode sacrificial agents can be commercially available or manufactured using conventionally known methods.

[0064] The inventors' dedicated research has revealed that, unlike existing lithium-ion batteries that use lithium metal in the negative electrode or in the negative electrode of lithium metal batteries with a negative electrode active material, even when a positive electrode sacrificial agent as disclosed in this specification is added to the positive electrode of a lithium secondary battery using a negative electrode without a negative electrode active material, there are cases where sufficient effects cannot be obtained. Furthermore, the inventors have found that the positive electrode sacrificial agent contained in the positive electrode 110, in a particle size distribution measured by laser diffraction scattering, has a particle size D corresponding to 50% of the cumulative frequency. 50 (S) is a particle size of 1.0 μm to 20 μm, and D corresponds to 95% of the cumulative frequency. 95 When (S) is between 1.0 μm and 30 μm, it significantly improves the cycling characteristics of the positive electrode sacrificial agent. The main reason is speculated as follows, but the main reason is not limited to this.

[0065] When the particles of the positive electrode sacrificial agent are in the particle size distribution measured by laser diffraction scattering, the particle size D corresponding to 50% of the cumulative frequency... 50 (Hereinafter also referred to as "particle size D") 50 When the (S)”) is less than 1.0 μm, the interfacial resistance between the positive electrode sacrificial agent and other components constituting the positive electrode increases, the electrical resistance increases, and the conductivity of the positive electrode decreases. Therefore, it is considered that the function of the positive electrode sacrificial agent cannot be fully utilized, and it becomes difficult to improve the cycle characteristics.

[0066] Furthermore, when the particle size D of the positive electrode sacrificial agent 50 When the thickness (S) exceeds 20 μm, the positive electrode sacrificial agent becomes localized within the positive electrode, and lithium metal from the positive electrode sacrificial agent concentrates and precipitates in the negative electrode portion opposite to the localized positive electrode sacrificial agent. As a result, it is believed that lithium metal is precipitated unevenly on the negative electrode, that is, lithium metal grows into dendrites, which has a detrimental effect on the cycle characteristics of lithium secondary batteries.

[0067] In this embodiment, in the positive electrode sacrificial agent contained in the positive electrode 110, due to the particle size D 50 (S) is between 1.0 μm and 20 μm, so the aforementioned problems will not occur, effectively and reliably acting as a positive electrode sacrificial agent. Furthermore, the positive electrode sacrificial agent contained in the positive electrode 110, in the particle size distribution measured by laser diffraction scattering, has a particle size D corresponding to 95% of the cumulative frequency. 95 (S)(Hereinafter also referred to as "particle size D") 95 The particle size of the (S)”) is between 1.0 μm and 30 μm, so the particle size distribution of the positive electrode sacrificial agent is more uniform, inhibiting the growth of lithium metal into dendrites. As a result, it is believed that the above-mentioned positive electrode sacrificial agent can fully exert its effect.

[0068] The particle size D of the positive electrode sacrificial agent contained in positive electrode 110 50(S) is 1.0 μm to 20 μm. The particle size D of the positive electrode sacrificial agent contained in positive electrode 110 is... 50 (S) is preferably 2.0 μm or more, more preferably 3.0 μm or more, even more preferably 5.0 μm or more, and even more preferably 8.0 μm or more. Furthermore, the particle size D of the cathode sacrificial agent contained in the cathode 110 is... 50 (S) is preferably 18 μm or less, more preferably 15 μm or less, even more preferably 14 μm or less, and even more preferably 12 μm or less.

[0069] Furthermore, the particle size D of the cathode sacrificial agent contained in cathode 110 is... 95 (S) is 1.0 μm to 30 μm. The particle size D of the positive electrode sacrificial agent contained in positive electrode 110 is... 95 (S) is preferably 3.0 μm or more, more preferably 5.0 μm or more, even more preferably 8.0 μm or more, and even more preferably 10.0 μm or more. Furthermore, the particle size D of the positive electrode sacrificial agent contained in the positive electrode 110... 95 (S) is preferably 29 μm or less, more preferably 28 μm or less, even more preferably 27 μm or less, and even more preferably 26 μm or less.

[0070] The particle size distribution based on the laser diffraction scattering method described above can be measured using known methods. For example, it can also be measured using a particle size distribution measuring machine such as the Microtrac Bell MT3000EX. Furthermore, in the particle size distribution, the particle size DX corresponding to the cumulative frequency X% refers to the proportion of particles with a size of DX or less in the measured particle size distribution, which is X% of the total particle size.

[0071] The positive electrode 110 may also contain components other than the positive electrode active material and the positive electrode sacrificial agent. There are no particular limitations on such components; examples include, for instance, known conductive additives, binders, and solid electrolytes (such as polymer electrolytes, gel electrolytes, and inorganic solid electrolytes, typically polymer electrolytes or gel electrolytes). As a solid electrolyte, polymer electrolytes or gel electrolytes, as described later, may be used.

[0072] As conductive additives in the positive electrode 110, there are no particular limitations, and examples include carbon black, single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), carbon nanofibers (CF), and acetylene black. Furthermore, as binders, there are no particular limitations, and examples include polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, acrylic resin, and polyimide resin. One or more of the aforementioned conductive additives and binders may be used alone. In the gel electrolyte, the substances described later can be used.

[0073] The total content of the positive electrode active material and the positive electrode sacrificial agent in the positive electrode 110 relative to the total mass of the positive electrode 110 can be, for example, 50% by mass or more and 100% by mass or less. The total content of the positive electrode active material and the positive electrode sacrificial agent relative to the total mass of the positive electrode 110 is preferably 60% by mass or more, more preferably 70% by mass or more, further preferably 80% by mass or more, and even more preferably 90% by mass or more. The total content of the positive electrode active material and the positive electrode sacrificial agent relative to the total mass of the positive electrode 110 is preferably 100% by mass or less, more preferably 99% by mass or less, and even more preferably 98% by mass or less.

[0074] The content of the positive electrode sacrificial agent relative to the total mass of the positive electrode 110 can also be set to 1.0% by mass or more and 15% by mass or less. The content of the positive electrode sacrificial agent relative to the total mass of the positive electrode 110 is preferably 1.0% by mass or more, more preferably 2.0% by mass or more, and even more preferably 3.0% by mass or more. Furthermore, the content of the positive electrode sacrificial agent relative to the total mass of the positive electrode 110 can also preferably be 12% by mass or less, even more preferably 10% by mass or less, and 8.0% by mass or less. The above-mentioned content is particularly effective when the positive electrode sacrificial agent contains an iron-containing compound, and is especially effective when the positive electrode sacrificial agent contains a compound of Li5FeO4.

[0075] Furthermore, the content of the positive electrode sacrificial agent is preferably specified by the ratio of the irreversible capacity of the positive electrode sacrificial agent to the cell capacity of the lithium secondary battery 100. Here, "cell capacity of the lithium secondary battery" refers to the value obtained by calculating the total charging capacity of the positive electrode active material and the positive electrode sacrificial agent contained in the positive electrode 110. Specifically, the cell capacity of the lithium secondary battery 100 is calculated by multiplying the charging capacity density (mAh / g) obtained by charging and discharging a cell with a positive electrode active material or positive electrode sacrificial agent as the positive electrode and a lithium metal foil as the negative electrode at the driving voltage of the lithium secondary battery 100 (e.g., 3.0V or higher and 4.2V or lower) with the mass (g) of the positive electrode active material or positive electrode sacrificial agent contained in the positive electrode 110, and then summing the above products for all positive electrode active materials and positive electrode sacrificial agents contained in the positive electrode 110. Furthermore, "the irreversible capacity of the positive electrode sacrificial agent" refers to the irreversible capacity density A (mAh / g) obtained by charging and discharging a unit cell with the positive electrode sacrificial agent as the positive electrode and the lithium metal foil as the negative electrode at the driving voltage of the lithium secondary battery 100 (e.g., 3.0V or higher and 4.2V or lower), calculating the product of the irreversible capacity density and the mass (g) contained in the positive electrode 110, and obtaining the sum of the above products for all the positive electrode sacrificial agents contained in the positive electrode 110.

[0076] The ratio X of the irreversible capacity of the positive electrode sacrificial agent to the cell capacity of the lithium secondary battery 100 is taken as the irreversible capacity density A of each positive electrode sacrificial agent. j (mAh / g) and the content x in the positive electrode 110 j The sum of the products (mass%) relative to the charge capacity density A1 of each positive electrode active material and each positive electrode sacrificial agent. k (mAh / g) and the content x in the positive electrode 110 k The ratio of the sum of the products of (mass%) can also be calculated according to the following formula (1).

[0077] [Equation 1]

[0078]

[0079] Since the theoretical charge capacity density (mAh / g) and the theoretical irreversible capacity density (mAh / g) of each positive electrode active material and each positive electrode sacrificial agent are known, these known values ​​can also be used. The charge capacity density, discharge capacity density, and content of each positive electrode active material and each positive electrode sacrificial agent in the positive electrode 110 can be measured using existing known methods. The charge capacity density and discharge capacity density can be measured using the methods described in the examples. The content of the positive electrode active material and the positive electrode sacrificial agent in the positive electrode 110 can be measured, for example, by X-ray diffraction (XRD).

[0080] The content of the positive electrode sacrificial agent is preferably adjusted such that the ratio of the irreversible capacity of the positive electrode sacrificial agent to the cell capacity of the lithium secondary battery 100 is 1.0% to 40%, more preferably 2.0% to 38%, and even more preferably 3.0% to 35%. The ratio of the irreversible capacity of the positive electrode sacrificial agent to the cell capacity of the lithium secondary battery 100 can be either 4.0% to 33% or 8.0% to 20%. By adjusting the ratio of the irreversible capacity of the positive electrode sacrificial agent to the cell capacity of the lithium secondary battery 100, it is believed that the ratio of the residual lithium metal after initial discharge to the total amount of lithium metal deposited during initial charging can be controlled in the lithium secondary battery 100. Therefore, when the ratio of the aforementioned irreversible capacity is within the above range, the amount of residual lithium metal is considered appropriate, and the cycle characteristics and energy density of the lithium secondary battery 100 are more superior.

[0081] The content of the conductive additive relative to the overall positive electrode 110 can be, for example, 0.5% to 30% by mass, 1% to 20% by mass, or 1.5% to 10% by mass. The content of the binder relative to the overall positive electrode 110 can be, for example, 0.5% to 30% by mass, 1% to 20% by mass, or 1.5% to 10% by mass. The total content of the solid electrolyte relative to the overall positive electrode 110 can be, for example, 0.5% to 30% by mass, 1% to 20% by mass, or 1.5% to 10% by mass.

[0082] (Positive current collector)

[0083] A positive current collector 150 is formed on one side of the positive electrode 110. The positive current collector 150 is not particularly limited as long as it is a conductor that does not react with lithium ions in the battery. Aluminum is an example of such a positive current collector.

[0084] The average thickness of the positive electrode current collector 150 is preferably 4 μm or more and 20 μm or less, more preferably 5 μm or more and 18 μm or less, and even more preferably 6 μm or more and 15 μm or less. In this manner, because the volume occupied by the positive electrode current collector 150 in the lithium secondary battery 100 is reduced, the energy density of the lithium secondary battery 100 is further improved.

[0085] (Buffer layer)

[0086] like Figure 1 As shown, a buffer layer 130 is formed on the surface of the separator 120 opposite to the negative electrode 140. The buffer layer is a fibrous or porous material with ion conductivity. Since the buffer layer 130 is fibrous or porous, it has ion-conductive solid portions and porous portions formed by the gaps between these solid portions (the term "gap portion" is used interchangeably with "gap portion" in this specification). Furthermore, in this specification, the "solid portion" in the buffer layer refers to a portion containing a gel-like structure.

[0087] In existing lithium-ion batteries, lithium metal deposition is limited to the negative electrode surface, leading to battery expansion. The buffer layer 130 of the lithium-ion battery 100 of this embodiment has ion conductivity, serving as a buffer layer to prevent such volume expansion and also acting as an electrolyte for conducting lithium ions. In other words, the buffer layer 130 suppresses the increase in internal resistance and performs the aforementioned function of a buffer layer.

[0088] Furthermore, in the first embodiment, "lithium metal deposition on the negative electrode" means, unless otherwise specified, that lithium metal is deposited at at least one location: on the surface of the negative electrode, in the porous portion of the buffer functional layer, and on the surface of the solid electrolyte interphase (SEI) layer formed on the surface of the negative electrode (described later). Therefore, in the lithium secondary battery 100, lithium metal can be deposited, for example, on the surface of the negative electrode 140 (the interface between the negative electrode and the buffer functional layer) or inside the buffer functional layer 130 (the porous portion of the buffer functional layer).

[0089] The buffer layer 130 is not particularly limited to a fibrous or porous material with ion conductivity.

[0090] The components constituting the buffer layer are not limited to any substance capable of conducting ions; examples include polymer electrolytes or gel electrolytes containing inorganic or organic salts, with gel electrolytes being preferred. The components constituting the buffer layer are preferably polymers and substances containing lithium salts. Polymer electrolytes and gel electrolytes are preferred components constituting the buffer layer. Both polymer electrolytes and gel electrolytes are electrolytes containing polymers; in particular, substances that form a gel by containing an electrolyte or solvent are referred to as gel electrolytes.

[0091] Generally, there are no particular limitations on the materials constituting polymer electrolytes and gel electrolytes, especially for lithium-ion secondary batteries; well-known materials can be appropriately selected. There are no particular limitations on the polymers (resins) constituting polymer electrolytes or gel electrolytes; examples include resins such as polyethylene oxide (PEO) having ethylene oxide units in the main chain and / or side chains, acrylic resins, vinyl resins, ester resins, nylon resins, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polysiloxanes, polyphosphazenes, polymethyl methacrylate, polyamides, polyimides, aramids, polylactic acid, polyethylene, polystyrene, polyurethane, polypropylene, polybutene, polyacetal, polysulfone, and polytetrafluoroethylene. One or more of the above-mentioned resins may be used alone.

[0092] Salts included in polymer electrolytes or gel electrolytes include salts of Li, Na, K, Ca, and Mg. Lithium salts are not particularly limited, but examples include LiI, LiCl, LiBr, LiF, LiBF4, LiPF6, LiAsF6, LiSO3CF3, LiN(SO2F)2, LiN(SO2CF3)2, LiN(SO2CF3CF3)2, LiB(O2C2H4)2, LiB(O2C2H4)F2, LiB(OCOCF3)4, LiNO3, and Li2SO4. One or more of the above-mentioned salts or lithium salts may be used alone.

[0093] The resin-to-lithium salt ratio in polymer electrolytes or gel electrolytes can also be determined by the ratio of oxygen atoms in the resin to lithium atoms in the lithium salt (“Li” / “O”). In polymer electrolytes or gel electrolytes, the resin-to-lithium salt ratio can also be adjusted to, for example, 0.02 to 0.20, 0.03 to 0.15, or 0.04 to 0.12.

[0094] In addition to resin and salt, polymer electrolytes or gel electrolytes may also contain solvents that can be contained in lithium secondary batteries 100. Regarding the specific solvent, solvents that can be contained in the electrolyte described later can be used.

[0095] As one embodiment of the buffer layer 130, a fibrous buffer layer is listed. Figure 3 A schematic cross-sectional view of the fibrous buffer layer is shown in (A). Figure 3 The buffer layer 130 shown in (A) is composed of ion-conducting fibers 310. That is, in this embodiment, "the buffer layer is fibrous" means that the buffer layer has a solid portion and a porous portion formed by gaps between the solid portions, which includes or is composed of fibers. Furthermore, it is charged by the lithium secondary battery 100, such as... Figure 3 As shown in (B), it is presumed that lithium metal 320 is analyzed in the pores of the buffer functional layer 130. However, the precipitation of lithium metal is not limited to this.

[0096] exist Figure 4 An embodiment of the ion-conducting fiber 310 is shown as a schematic cross-sectional view in (C). Figure 4 As shown in (C), in one embodiment, the ion-conducting fiber 310 is composed of a fibrous ion-conducting layer 400. The ion-conducting layer 400 has the above-described configuration as a component constituting, for example, a buffer functional layer.

[0097] The average fiber diameter of the fibrous ion-conducting layer 400 is preferably 30 nm to 5000 nm, more preferably 50 nm to 2000 nm, even more preferably 70 nm to 1000 nm, and even more preferably 80 nm to 500 nm. With the average fiber diameter of the ion-conducting layer within the above range, the surface area of ​​the reaction field from which lithium metal can be deposited becomes more suitable, thus improving the cycle performance.

[0098] In other embodiments, Figure 3The buffer layer 130 of the lithium secondary battery 100 shown can also be porous. The porous buffer layer can also have, for example, a porous structure, especially an ion-conducting layer with interconnected pores.

[0099] The buffer layer, being fibrous or porous, has pores. The porosity of the buffer layer is not particularly limited, but by volume percentage, it is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more or 80% or more. With the porosity of the buffer layer within the above range, the surface area of ​​the reaction field where lithium metal can be deposited further increases, thus improving cycle characteristics. Furthermore, in this manner, it tends to more effectively and reliably suppress the volume expansion of the cell. The porosity of the buffer layer is not particularly limited, and by volume percentage, it can be 99% or less or 95% or less.

[0100] The average thickness of the buffer functional layer is preferably 100 μm or less, more preferably 50 μm or less, and even more preferably 30 μm or less. With the average thickness of the buffer functional layer within this range, the energy density of the battery is further improved because the volume occupied by the buffer functional layer 130 in the lithium secondary battery 100 is reduced. Furthermore, the average thickness of the buffer functional layer is preferably 1 μm or more, more preferably 4 μm or more, and even more preferably 7 μm or more. With the average thickness of the buffer functional layer within this range, the cycle characteristics tend to be further improved because the surface area of ​​the reaction field where lithium metal can be deposited is further increased. In addition, this approach tends to more effectively and reliably suppress the volume expansion of the cell.

[0101] The fiber diameter of the fibrous ion-conducting layer, the porosity of the buffer layer, and the thickness of the buffer layer can be measured using known methods. For example, the thickness of the buffer layer can be measured by etching the surface of the buffer layer with a focused ion beam (FIB) to expose its cross-section, and then observing the thickness of the buffer layer on the exposed cross-section using SEM or TEM.

[0102] The fiber diameter of the fibrous ion-conducting layer and the porosity of the buffer functional layer can be measured by observing the surface of the buffer functional layer using a transmission electron microscope. Furthermore, the porosity of the buffer functional layer can be calculated by performing binary analysis on the observed image of the buffer functional layer surface using image analysis software, determining the proportion of the buffer functional layer relative to the total area of ​​the image.

[0103] The above-mentioned measurement values ​​are calculated by averaging the measurement values ​​from three or more, preferably ten or more, measurements.

[0104] Furthermore, when the buffer functional layer contains a metal capable of reacting with lithium, the combined capacity of the negative electrode 140 and the buffer functional layer 130 is sufficiently small relative to the capacity of the positive electrode 110, for example, it may be less than 20%, less than 15%, less than 10%, or less than 5%. Moreover, the capacities of the positive electrode 110, the negative electrode 140, and the buffer functional layer 130 can be measured using conventionally known methods.

[0105] (Separator)

[0106] The separator 120 is a component used to prevent short circuits in the battery by isolating the positive electrode 110 from the negative electrode 140, and to ensure the ionic conductivity of lithium ions, which act as charge carriers between the positive and negative electrodes 110 and 140. In other words, the separator 120 has the functions of isolating the positive electrode 110 from the negative electrode 140 and ensuring the ionic conductivity of lithium ions. Such a separator can be used alone, or in combination with two or more components having the aforementioned functions. There are no particular limitations on the separator in terms of fulfilling the above functions; examples include porous components with insulating properties, polymer electrolytes, and gel electrolytes.

[0107] When the separator comprises a porous component with insulating properties, the component exhibits ionic conductivity by filling the pores of the component with an ionicly conductive material. Examples of filling materials include electrolytes, polymer electrolytes, and gel electrolytes, which will be described later.

[0108] The separator 120 can use one or more insulating porous components, polymer electrolytes, or gel electrolytes, either alone or in combination. However, when the separator 120 uses an insulating porous component alone, the lithium secondary battery 100 also needs to include an electrolyte.

[0109] The materials used to constitute the aforementioned insulating porous components are not particularly limited, but include, for example, insulating polymer materials, specifically polyethylene (PE) and polypropylene (PP). That is, the separator 120 can be a porous polyethylene (PE) film, a porous polypropylene (PP) film, or a laminated structure thereof.

[0110] The polymer electrolyte or gel electrolyte in the separator 120 can be used in the ion conduction layer of the buffer functional layer. The polymer electrolyte and gel electrolyte can contain the same polymers, salts, and other components.

[0111] The separator 120 may also be covered by a separator cover layer. The separator cover layer may cover both sides of the separator 120 or only one side. There are no particular limitations on whether the separator cover layer is a component that does not react with lithium ions; preferably, it is a layer that can firmly bond the separator 120 to layers adjacent to it. Such a separator cover layer is not particularly limited, and examples include adhesives containing, for instance, polyvinylidene fluoride (PVDF), styrene-butadiene rubber-carboxymethyl cellulose composite (SBR-CMC), polyacrylic acid (PAA), lithium polyacrylate (LI-PAA), polyimide (PI), polyamide-imide (PAI), and aramid fibers. Inorganic particles such as silica, alumina, titanium dioxide, zirconium oxide, magnesium oxide, magnesium hydroxide, and lithium nitrate may also be added to the separator cover layer in the aforementioned adhesives.

[0112] The average thickness of the separator 120 is preferably 20 μm or less, more preferably 18 μm or less, and even more preferably 15 μm or less. In this manner, the energy density of the lithium secondary battery 100 is further improved because the volume occupied by the separator 120 in the lithium secondary battery 100 is reduced. Furthermore, the average thickness of the separator 120 is preferably 5 μm or more, more preferably 7 μm or more, and even more preferably 10 μm or more. In this manner, the positive electrode 110 and the negative electrode 140 can be more reliably isolated, and short circuits in the battery can be more effectively suppressed.

[0113] (electrolyte)

[0114] The lithium secondary battery 100 preferably includes an electrolyte. The electrolyte can be impregnated in the separator 120 or sealed together with the lithium secondary battery 100 as a finished product. The electrolyte contains an electrolyte and a solvent, and is a solution with ion conductivity, acting as a conductive pathway for lithium ions. Therefore, the internal resistance of the lithium secondary battery 100 with an electrolyte is further reduced, and its energy density, capacity, and cycle characteristics are further improved.

[0115] The electrolyte is not particularly limited to salts, and examples include salts of Li, Na, K, Ca, and Mg. Lithium salts are preferred as the electrolyte. Lithium salts are not particularly limited, and examples include LiI, LiCl, LiBr, LiF, LiBF4, LiPF6, LiAsF6, LiSO3CF3, LiN(SO2F)2, LiN(SO2CF3)2, LiN(SO2CF3CF3)2, LiB(O2C2H4)2, LiB(O2C2H4)F2, LiB(OCOCF3)4, LiNO3, and Li2SO4. From the viewpoint of superior energy density, capacity, and cycle characteristics of the lithium secondary battery 100, LiN(SO2F)2 is preferred. Furthermore, one or more of the above-mentioned lithium salts may be used alone or in combination.

[0116] Examples of solvents include fluorinated solvents and non-fluorinated solvents. Fluorinated solvents are not particularly limited, but examples include 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, and 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether.

[0117] The aforementioned non-fluorinated solvents are not particularly limited, but include, for example, ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,2-dimethoxyethane, dimethoxyethane, dimethoxypropane, dimethoxybutane, diethylene glycol dimethyl ether, acetonitrile, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, propylene carbonate, vinyl chloride carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, trimethyl phosphate, triethyl phosphate, and 12-crown ether-4, etc.

[0118] The aforementioned fluorinated and non-fluorinated solvents can be used alone or in any combination of two or more in any proportion. There is no particular limitation on the ratio of the fluorinated solvent to the non-fluorinated solvent; for example, the ratio of the fluorinated solvent to the total solvent can be 0–100% by volume, and the ratio of the non-fluorinated solvent to the total solvent can be 0–100% by volume.

[0119] (Use of lithium secondary batteries)

[0120] exist Figure 2 The diagram illustrates one usage method of the lithium secondary battery according to this embodiment. The lithium secondary battery 200 has a positive terminal 220 and a negative terminal 210 respectively connected to the positive current collector 150 and the negative terminal 140 for connecting the lithium secondary battery 200 to an external circuit. The lithium secondary battery 200 is charged and discharged by connecting the negative terminal 210 to one end of the external circuit and the positive terminal 220 to the other end of the external circuit.

[0121] The lithium secondary battery 200 can also form a solid electrolyte interphase (SEI) layer at the interface between the buffer functional layer 130 and the separator 120 during initial charging. Alternatively, the SEI layer may not be formed, or it may be formed at the interface between the negative electrode 140 and the buffer functional layer 130. There are no particular limitations on the SEI layer formed, and it may contain, for example, inorganic compounds containing lithium and organic compounds containing lithium. The typical average thickness of the SEI layer is 1 nm to 10 μm.

[0122] Between the positive terminal 220 and the negative terminal 210, the lithium secondary battery 200 is charged by applying a voltage such that a current flows from the negative terminal 210 to the positive terminal 220 through an external circuit. During charging, lithium metal is deposited on the surface of the negative electrode. Furthermore, this lithium metal deposition occurs at at least one location: at the interface between the negative electrode 140 and the buffer layer 130, inside the buffer layer 130, and at the interface between the buffer layer 130 and the separator 120.

[0123] When the charged lithium secondary battery 200 is connected to the positive terminal 220 and the negative terminal 210, the lithium secondary battery 200 is discharged. This electrolytically dissolves the lithium metal deposits generated on the negative electrode surface.

[0124] (Manufacturing method of lithium secondary batteries)

[0125] As such Figure 1 The method for manufacturing the lithium secondary battery 100 shown is not particularly limited if it is a method that can manufacture a lithium secondary battery having the above-described structure. Examples of such methods are as follows.

[0126] The positive electrode 110 is formed on the positive electrode current collector 150 as follows. In addition to the aforementioned positive electrode active material and positive electrode sacrificial agent, a known conductive additive, a solid electrolyte, and a known binder are arbitrarily mixed to obtain a positive electrode mixture. The proportions of this mixture can be appropriately adjusted so that the contents of the positive electrode active material, positive electrode sacrificial agent, conductive additive, solid electrolyte, and binder are within the aforementioned ranges. Furthermore, by pre-measuring the charge capacity density of the positive electrode active material and the irreversible capacity density of the positive electrode sacrificial agent, the ratio of the irreversible capacity of the positive electrode sacrificial agent to the cell capacity of the lithium secondary battery 100 can be controlled by adjusting only the mass mixing ratio of the positive electrode active material and the positive electrode sacrificial agent. The obtained positive electrode mixture is coated onto one side of a metal foil (e.g., Al foil) having a specified thickness (e.g., 5 μm to 1 mm) serving as the positive electrode current collector and stamped. The resulting molded body is then cut to a specified size by punching to obtain the positive electrode 110.

[0127] Furthermore, the particle size of the positive electrode sacrificial agent can be controlled by known methods. Examples of such methods include those using a pulverizer such as a blade mill, jet mill, or ball mill. By setting the pulverization time based on the pulverizer to a long time, the particle size D can be further reduced. 50 (S) and particle size D 95 (S). From the viewpoint that particle size can be easily controlled, it is preferable to use a jet mill as a pulverizer.

[0128] Next, a separator 120 having the above-described configuration is prepared. The separator 120 can be manufactured using conventionally known methods or commercially available separators can be used.

[0129] Next, the negative electrode material, such as a metal foil with a diameter of 1 μm or more but less than 1 mm (e.g., electrolytic Cu foil), is cleaned with a solvent containing aminosulfonic acid, cut into a specified size, and then ultrasonically cleaned with ethanol and dried to obtain the negative electrode 140.

[0130] Next, the manufacturing method of the above-mentioned buffer functional layer 130 is not particularly limited as long as it can produce a fibrous or porous layer with ion conductivity, for example as follows.

[0131] Having such Figure 4 The fibrous buffer layer of the ion-conducting fiber 310, which is composed of the fibrous ion-conducting layer 400 as shown in (C), can be manufactured as follows.

[0132] First, a solution of the aforementioned resin (e.g., PVDF) dissolved in a suitable organic solvent (e.g., N-methylpyrrolidone) is applied to the surface of the pre-prepared separator 120 using a bar coater or doctor blade. Next, the separator 120 coated with the resin solution is immersed in a water bath and then allowed to dry thoroughly at room temperature, thereby forming a fibrous ion-conducting layer on the separator 120 (in addition, the ion-conducting layer can also function as an ion-conducting layer by injecting electrolyte during battery assembly), thus obtaining a fibrous buffer layer.

[0133] Furthermore, a porous buffer layer with a porous ion-conducting layer can be manufactured as follows.

[0134] By using a solution in which the above-mentioned resin (e.g., PVDF) is dissolved in a suitable solvent (e.g., N-methylpyrrolidone), a porous ion-conducting layer with interconnected pores is formed on the surface of the separator 120 by conventionally known methods (e.g., using methods to separate the phase from the solvent and using a foaming agent, etc.). (In addition, the ion-conducting layer can also perform ion-conducting functions by, for example, injecting electrolyte during battery assembly). Thus, a porous buffer layer can be obtained.

[0135] The positive electrode 110, the separator 120 forming the buffer functional layer 130, and the negative electrode 140, obtained as described above, are laminated in this order with the buffer functional layer 130 and the negative electrode 140 facing each other to obtain a laminated body. By sealing the obtained laminated body and the electrolyte together in a sealed container, a lithium secondary battery 100 can be obtained. There are no particular limitations on the sealed container; for example, a laminated film is provided.

[0136] [Second Implementation Method]

[0137] (Lithium-ion rechargeable battery)

[0138] The lithium secondary battery of the second embodiment, like the lithium secondary battery 100 of the first embodiment, includes a positive electrode, a negative electrode without negative electrode active material, a separator disposed between the positive and negative electrodes, and a buffer functional layer formed on the surface of the negative electrode opposite to the separator. The positive electrode has a positive current collector on the surface opposite to the surface opposite to the separator.

[0139] Except for the points described later, the configuration and preferred configuration of the positive current collector, positive electrode, separator, and negative electrode are the same as those of the lithium secondary battery 100 of the first embodiment. Regarding these configurations, the lithium secondary battery of the second embodiment achieves the same effect as the lithium secondary battery of the first embodiment or exhibits further performance. Furthermore, the lithium secondary battery of the second embodiment may also contain the electrolyte as described above, just like the lithium secondary battery 100.

[0140] (Buffer layer)

[0141] The buffer layer in the lithium secondary battery of the second embodiment is a fibrous or porous material with ion conductivity and electrical conductivity. That is, in this embodiment, the buffer layer is the buffer layer 130 of the first embodiment, which also has electrical conductivity.

[0142] Because the lithium secondary battery of the second embodiment has such a buffer layer, its cycle characteristics are superior compared to the lithium secondary battery of the first embodiment.

[0143] That is, since the buffer functional layer of this embodiment has both ionic conductivity and electrical conductivity, when charging the lithium secondary battery, electrons from the negative electrode and lithium ions from the separator and / or electrolyte are supplied to the surface and / or interior of the buffer functional layer. Furthermore, since the buffer functional layer of this embodiment is fibrous or porous, it has solid portions with both ionic conductivity and electrical conductivity, and porous portions formed by the gaps between these solid portions. Therefore, in the buffer functional layer of this embodiment, electrons and lithium ions supplied as described above react on the surface of the aforementioned solid portions, which are the interior of the buffer functional layer, and lithium metal is deposited in the porous portions (the surface of the solid portions). Furthermore, as described above, the "solid portion" in the buffer functional layer refers to a portion containing a gel-like structure.

[0144] In conventional lithium-ion batteries, lithium metal deposition is limited to the negative electrode surface, thus restricting its growth direction to from the negative electrode surface towards the separator, leading to dendritic growth. However, in the lithium-ion battery with the buffer layer of the second embodiment, as described above, lithium metal can be deposited not only on the negative electrode surface but also on the surface of the solid portion of the buffer layer, increasing the surface area of ​​the reaction field for the lithium metal deposition reaction. Consequently, in the lithium-ion battery of the second embodiment, since the reaction rate of the lithium metal deposition reaction is slowly controlled, it is presumed that anisotropic lithium metal growth can be more reliably suppressed, i.e., the formation of dendritic lithium metal can be suppressed. The inventors have discovered that in lithium-ion batteries where the positive electrode contains a positive electrode sacrificial agent, the effect of the positive electrode sacrificial agent is more significantly enhanced when a buffer layer with both ionic conductivity and electrical conductivity is introduced. It is speculated that this is because, during the initial charging, uniform lithium metal is deposited in the planar direction on the surface of the solid portion and the negative electrode surface, thus providing a foothold for lithium metal deposition during subsequent charging and inhibiting the growth of lithium metal into dendrites. However, the main reasons are not limited to the above.

[0145] Furthermore, in the second embodiment, "lithium metal deposition on the negative electrode" means, unless otherwise specified, that lithium metal is deposited at at least one location: the surface of the negative electrode, the surface of the solid portion of the buffer functional layer, and the surface of the SEI layer formed on the surface of the negative electrode and / or the solid portion of the buffer functional layer. Therefore, in the lithium secondary battery of the second embodiment, lithium metal can be deposited, for example, on the surface of the negative electrode (the interface between the negative electrode and the buffer functional layer) or inside the buffer functional layer (the surface of the solid portion of the buffer functional layer).

[0146] Examples of such buffering functional layers, without limitation, include materials that cover all or part of the surface of a fibrous or porous ion-conducting layer with a conductive layer; materials that cover all or part of the surface of a fibrous or porous conductive layer with an ion-conducting layer; and materials that interweave fibrous ion-conducting layers with fibrous conductive layers. As the ion-conducting layer, the same material as the ion-conducting layer 400 that may be present in the buffering functional layer 130 of the first embodiment can be used.

[0147] As a conductive layer, anything capable of conducting electrons is acceptable; examples include metal films. Non-limiting examples of metals that can be included in the conductive layer include SUS, Si, Sn, Sb, Al, Ni, Cu, Sn, Bi, Ag, Au, Pt, Pb, Zn, In, Bi-Sn, and In-Sn. From the viewpoint of improving affinity with lithium metal, Si, Sn, Zn, Bi, Ag, In, Pb, Sb, and Al are preferred as the metal included in the conductive layer. One or more of these metals can be used alone.

[0148] As one embodiment of the buffer function layer in the second embodiment, examples are listed that are used in conjunction with an embodiment of the buffer function layer in the first embodiment. Figure 3 A fibrous, buffer-like functional layer described in the same way. For example, with... Figure 3 Similarly, lithium metal can also be analyzed in the pores of the buffer functional layer. Furthermore, the fibrous buffer functional layer involved can also be composed of ion-conducting fibers 410, which are, for example, fibers with ion conductivity and electrical conductivity.

[0149] exist Figure 4 An embodiment of such an ion-conducting fiber 410 is shown as a schematic cross-sectional view in (D). Figure 4 As shown in (D), in one embodiment, the ion-conducting fiber 410 includes a fibrous ion-conducting layer 400 and a conductive layer 420 covering the surface of the ion-conducting layer 400. The ion-conducting layer 400 may have the configuration described above, for example, as an ion-conducting layer, and the conductive layer 420 may also have the configuration described above, for example, as a conductive layer.

[0150] The average thickness of the conductive layer 420 is preferably 1 nm to 300 nm, more preferably 5 nm to 200 nm, and even more preferably 10 nm to 150 nm. The average thickness of the conductive layer 420 may also be 10 nm to 100 nm. By ensuring the average thickness of the conductive layer is within the above range, the conductivity of the ion-conducting fibers can be more appropriately guaranteed, thus improving the cycle characteristics of the battery.

[0151] The average thickness and porosity of the buffer functional layer can be the same as those of the buffer functional layer 130 in the first embodiment.

[0152] The thickness of the conductive layer in the second embodiment can be measured using known measurement methods. It can be measured by observing the surface of the conductive layer using, for example, a transmission electron microscope. Alternatively, it can be measured by etching the surface of the conductive layer with a focused ion beam (FIB) to expose its cross-section, and then observing the thickness of the buffer layer in the exposed cross-section using SEM or TEM. Each measurement value is calculated by averaging the values ​​from three or more, preferably ten or more, measurements.

[0153] Furthermore, when the buffer functional layer contains a metal that can react with lithium, the combined capacity of the negative electrode and the buffer functional layer is sufficiently small relative to the capacity of the positive electrode, for example, less than 20%, less than 15%, less than 10%, or less than 5%.

[0154] (Manufacturing method of lithium secondary batteries)

[0155] In the manufacturing method of the lithium secondary battery of the second embodiment, the manufacturing of components other than the buffer functional layer and the assembly of each component can be carried out in the same manner as in the manufacturing method of the lithium secondary battery of the first embodiment.

[0156] The manufacturing method of the buffer functional layer with the above-mentioned conductive layer is not particularly limited as long as it can produce a fibrous or porous layer with ionic conductivity and electrical conductivity, for example, as follows.

[0157] like Figure 4 The fibrous buffer layer, as shown in (D), having fibrous ion-conducting fibers 410 with fibrous ion-conducting layer 400 and conductive layer 420 covering the surface of ion-conducting layer 400, can be manufactured as follows.

[0158] First, as described above, the separator coated with resin solution is immersed in a water bath and then thoroughly dried at room temperature, thereby forming a fibrous ion-conducting layer on the separator (in addition, the ion-conducting layer can also perform ion-conducting function by injecting electrolyte during battery assembly). Next, relative to the separator with the fibrous ion-conducting layer formed, a fibrous buffer layer can be obtained by evaporating a suitable metal (e.g., Ni) under vacuum conditions.

[0159] Furthermore, a porous buffer functional layer having a porous ion-conducting layer and a conductive layer covering the surface of the ion-conducting layer can be manufactured as follows.

[0160] First, as described above, a porous ion-conducting layer with interconnected pores is formed on the surface of the separator using existing known methods (the ion-conducting layer can also function as an ion conductor, for example, by injecting an electrolyte during battery assembly). Next, relative to the separator with the porous ion-conducting layer formed, a porous buffer layer can be obtained by evaporating a suitable metal (e.g., Ni) under vacuum conditions.

[0161] [Third Implementation Method]

[0162] (Lithium-ion rechargeable battery)

[0163] The lithium secondary battery of the third embodiment is the same as the lithium secondary battery 100, and includes a positive electrode, a negative electrode without negative electrode active material, a separator disposed between the positive and negative electrodes, and a buffer functional layer formed on the surface of the negative electrode opposite to the separator. The positive electrode has a positive electrode current collector on the surface opposite to the surface opposite to the separator.

[0164] Except for the points described later, the configuration and preferred configuration of the positive current collector, positive electrode, separator, buffer layer, and negative electrode are the same as those of the lithium secondary battery 100 of the first embodiment. Regarding these configurations, the lithium secondary battery of the third embodiment achieves the same effect as the lithium secondary battery 100 or exhibits better performance. Furthermore, the lithium secondary battery of the third embodiment may also contain the electrolyte as described above, just like the lithium secondary battery 100.

[0165] The positive electrode in the lithium secondary battery of the third embodiment is the same as that in the lithium secondary battery 100 described above, except that it includes a lithium-containing compound (positive electrode sacrificial agent) that undergoes oxidation within the charge / discharge potential range of the positive electrode active material and does not substantially undergo reduction. Here, the definitions, examples, and preferred methods of the components that can include the positive electrode active material, the positive electrode sacrificial agent, and other positive electrode components are the same as in the first embodiment.

[0166] The inventors' dedicated research has revealed that, in particle size distributions measured by laser diffraction scattering, the particle size corresponding to 50% of the cumulative frequency is defined as D. 50 Under these conditions, the D of the positive electrode active material 50 (A) D with a particle size of 5.0 μm to 20 μm and used as a positive electrode active material 50 (A) D relative to lithium-containing compounds 50 (S) particle size ratio D 50 (A) / D 50 When (S) is above 2.0 and below 10.0, the rate characteristics are particularly excellent. The main reason is speculated as follows, but the main reason is not limited to this.

[0167] The lithium secondary battery of the third embodiment, due to the D entering the positive electrode active material, 50 (A) has a particle size of 5.0 μm or larger and 20 μm or smaller and a particle size ratio of D. 50 (A) / D 50 (S) is precisely controlled within the range of 2.0 to 10.0, so the contact area between the positive electrode active materials is kept sufficiently high and the filling density of the positive electrode is increased. That is, it is believed that the positive electrode of the lithium secondary battery of the third embodiment includes positive electrode active materials that are in sufficient contact with each other to a degree that the internal resistance of the positive electrode is sufficiently small and a positive electrode sacrificial agent that is embedded in the gap of the positive electrode active materials. Since such a positive electrode has high energy density and low internal resistance, it is believed that the lithium secondary battery of the third embodiment has high energy density and excellent rate characteristics.

[0168] Furthermore, in this specification, "rate characteristics" means the ability to charge and discharge at high currents, and it is known that rate performance is excellent when the battery has low internal resistance. More specifically, it means that the discharge capacity when discharged at high speed (e.g., 3C) is maintained at a sufficiently high level compared to the discharge capacity when discharged at low speed (e.g., 0.1C). In this specification, "excellent rate characteristics" means, for example, that the discharge capacity when discharged at 3C is 60% or more, 65% or more, or 70% or more compared to the discharge capacity when discharged at 0.1C.

[0169] In the lithium secondary battery of the third embodiment, the particle size D of the positive electrode active material contained in the positive electrode is... 50 (A) 5.0 μm or more and 20 μm or less. The particle size D of the positive electrode active material contained in the positive electrode of this embodiment. 50 (A) Preferably, the particle size is 6.0 μm or more, more preferably 7.0 μm or more, even more preferably 8.0 μm or more, and even more preferably 9.0 μm or more. Furthermore, the particle size D of the positive electrode active material contained in the positive electrode of this embodiment... 50 (A) Preferably 19 μm or less, more preferably 18 μm or less, even more preferably 17 μm or less, and even more preferably 15 μm or less.

[0170] Furthermore, in the lithium secondary battery of the third embodiment, D serves as the positive electrode active material. 50 (A) D relative to the positive electrode sacrificial agent 50 (S) particle size ratio D 50 (A) / D 50 (S) is 2.0 to 10.0. This particle size ratio D 50 (A) / D 50(S) is preferably 2.5 or more, more preferably 3.0 or more, even more preferably 3.5 or more, and even more preferably 4.0 or more. Furthermore, this particle size ratio D... 50 (A) / D 50 (S) is preferably 9.5 or less, more preferably 9.0 or less, even more preferably 8.5 or less, and even more preferably 8.0 or less.

[0171] In the lithium secondary battery of the third embodiment, the positive electrode contains a positive electrode sacrificial agent, for example, with a particle size D. 50 (S) is 0.5 μm or more and 10 μm or less. The particle size D of the positive electrode sacrificial agent contained in the positive electrode of this embodiment is... 50 (S) can also be 1.0 μm or more, 1.5 μm or more, or 2.0 μm or more. Furthermore, the particle size D of the positive electrode sacrificial agent contained in the positive electrode of this embodiment... 50 (S) can also be 9.0 μm or less, 8.0 μm or less, 7.0 μm or less, or 6.0 μm or less. In this embodiment, the particle size D of the positive electrode sacrificial agent is... 50 (S) Within the above range, the cycle characteristics of the battery tend to be further improved.

[0172] In the lithium secondary battery of the third embodiment, the electrode density of the positive electrode is, for example, 3.0 g / cc or more. In this embodiment, the electrode density of the positive electrode may also be 3.2 g / cc or more, 3.3 g / cc or more, 3.4 g / cc or more, or 3.5 g / cc or more. In this embodiment, by setting the electrode density of the positive electrode within the above range, the energy density of the battery tends to be further improved because the filling density of the positive electrode is higher.

[0173] In this embodiment, "electrode density" refers to the mass contained per unit volume of an electrode. Therefore, g / cc or g / cm³ is used as the unit. 3 Electrode density depends on the density and arrangement of the materials constituting the electrode, such as g / mL. Therefore, in the positive electrode of this embodiment, the particle size of the positive electrode active material and the positive electrode sacrificial agent can be varied. In this embodiment, when the particle size ratio D... 50 (A) / D 50 As (S) increases, the electrode density of the positive electrode tends to increase. Furthermore, this can be controlled by adjusting the volume ratio of the positive electrode sacrificial agent to the positive electrode active material. Moreover, as the electrode density increases, the energy density of the lithium-ion secondary battery tends to increase further due to the increased capacity per unit volume.

[0174] In this embodiment, the content of the positive electrode active material, the positive electrode sacrificial agent, and other components that can be included in the positive electrode is the same as in the first embodiment. The D of the positive electrode active material... 50(A) D with a particle size of 5.0 μm to 20 μm and used as a positive electrode active material 50 (A) D relative to lithium-containing compounds 50 (S) particle size ratio D 50 (A) / D 50 When (S) is 2.0 or more and 10.0 or less, and the contents of the positive electrode sacrificial agent and the positive electrode active material are within the above range, the positive electrode filling density is increased, so it is preferred.

[0175] (Manufacturing method of lithium secondary batteries)

[0176] The lithium secondary battery of the third embodiment can be manufactured in the same way as the lithium secondary battery of the first embodiment. Furthermore, the control of the particle size of the positive electrode active material and the positive electrode sacrificial agent is also the same as in the manufacturing method of the lithium secondary battery of the first embodiment, and can be carried out using a pulverizer.

[0177] [Variation Example]

[0178] The above-described embodiments are illustrative of the present invention and are not intended to limit the present invention to these embodiments only. The present invention can be modified in various ways as long as it does not depart from its spirit.

[0179] For example, the lithium secondary battery of the third embodiment has the buffer function layer of the first embodiment, but the buffer function layer of the second embodiment can also be used as the buffer function layer. In this way, it is possible to provide a battery that has the excellent rate characteristics of the lithium secondary battery of the third embodiment and the even better cycle characteristics of the lithium secondary battery of the second embodiment.

[0180] The lithium secondary battery of this embodiment may or may not have a current collector on the surface of the negative electrode, which is arranged in contact with the negative electrode. There are no particular limitations on such a current collector; examples include materials that can be used in the negative electrode material. Furthermore, the lithium secondary battery of this embodiment may also not have a positive electrode current collector. In the case where the lithium secondary battery does not have a positive electrode current collector and a negative electrode current collector, the positive and negative electrodes themselves function as current collectors.

[0181] The lithium secondary battery of this embodiment can also have terminals for connection to an external circuit installed on the positive current collector and / or the negative electrode. For example, metal terminals (e.g., Al, Ni, etc.) with a diameter of 10 μm to 1 mm can be joined to one or both of the positive current collector and the negative electrode. As a joining method, existing known methods can be used, such as ultrasonic welding.

[0182] Furthermore, in this specification, "high energy density" or "high energy density" means a high capacity per unit of total volume or total mass, preferably 700Wh / L or more or 300Wh / kg or more, more preferably 800Wh / L or more or 350Wh / kg or more, and even more preferably 900Wh / L or more or 400Wh / kg or more.

[0183] Furthermore, in this specification, "excellent cycle characteristics" means that the rate of capacity reduction of the battery is low before and after a number of charge-discharge cycles conceivable in normal use. That is, it means that when comparing the first discharge capacity after the initial charge-discharge cycle with the capacity after a number of charge-discharge cycles conceivable in normal use, the capacity after the charge-discharge cycles is almost negligible compared to the first discharge capacity after the initial charge-discharge cycle. Here, "number of cycles conceivable in normal use" refers to, for example, 30, 50, 70, 100, 300, or 500 cycles, depending on the intended use of the lithium-ion battery. Furthermore, "the capacity after charge-discharge cycles is almost negligible compared to the first discharge capacity after the initial charge-discharge cycle" means, depending on the intended use of the lithium-ion battery, that the capacity after charge-discharge cycles is 60% or more, 65% or more, 70% or more, 75% or more, 80% or more, or 85% or more, depending on the intended use of the lithium-ion battery.

[0184] In this specification, the numerical ranges described as preferred ranges can also be replaced by any combination of the described upper and lower limits. For example, if a parameter is preferably 50 or more, more preferably 60 or more, more preferably 100 or less, or more preferably 90 or less, the parameter can also be any one of 50 or more and 100 or less, 50 or more and 90 or less, 60 or more and 100 or less, or 60 or more and 90 or less.

[0185] Furthermore, in this specification, the ion-conducting layer and the conductive layer are not limited to layered materials, but can also be fibrous, bulk, or porous. Therefore, the terms ion-conducting layer and conductive layer can also be replaced with ion-conducting phase and conductive phase, respectively.

[0186] Example

[0187] The present invention will now be described in more detail using examples and comparative examples. The present invention is not limited to the following examples.

[0188] [Making of Lithium-ion Secondary Batteries]

[0189] The manufacturing processes for lithium secondary batteries are carried out as follows.

[0190] (Preparation of the negative electrode)

[0191] After cleaning a 10μm electrolytic Cu foil with a solvent containing aminosulfonic acid, it is cut into the specified size, and then further ultrasonically cleaned with ethanol and dried to obtain the negative electrode.

[0192] (Preparation of the separator)

[0193] As separators, separators of a specified size, with 2μm polyvinylidene fluoride (PVDF) coated on both sides of a 12μm polyethylene microporous membrane, are prepared.

[0194] (The production of the positive electrode)

[0195] As a conductive additive, a mixture of 96 parts by mass of positive electrode active material and positive electrode sacrificial agent, 2 parts by mass of carbon black, and 2 parts by mass of polyvinylidene fluoride (PVDF) as a binder is coated onto one side of a 12 μm Al foil, which serves as the positive electrode current collector, and then stamped. The resulting molded body is then cut to a specified size by a die-cutting process to obtain the positive electrode.

[0196] LiNi is used as the positive electrode active material. 0.85 Co 0.12 Al 0.03 O2. Regarding Test Example 1 described later, the substances listed in Table 1 were used as positive electrode sacrificial agents. In Test Example 2 described later, Li5FeO4 was used as a positive electrode sacrificial agent. The irreversible capacity and particle size D of each positive electrode sacrificial agent used in Test Examples 1 and 2 will be... 50 (S), Particle size D 95 (S), content, and particle size D of the positive electrode active material 50 (A) and D 50 The ratio of (S) is shown in Tables 1 and 3. Furthermore, in each example, D... 50 Particle size and D 95 Particle size was measured using an MT3000EX manufactured by Microtrac Bell.

[0197] Furthermore, in Test Example 1, commercially available products were used for both Li₂O₂ and Li₃N. In Test Examples 1 and 2, Li₅FeO₄ was manufactured using the method described in Chemical Materials (Chem. Mater.) 2010, 22, 1263-1270. Specifically, a substance obtained by pulverizing and mixing LiOH·H₂O and Fe₂O₃ was calcined at 800°C for 72 hours under a nitrogen atmosphere to obtain the positive electrode sacrificial agent. Furthermore, the particle size of the prepared positive electrode active material and the positive electrode sacrificial agent was adjusted by pulverizing using a jet mill.

[0198] The mixing ratio of the positive electrode active material and the positive electrode sacrificial agent was adjusted so that the charge capacity density (mAh / g) of the positive electrode active material and the positive electrode sacrificial agent, and the irreversible capacity density A (mAh / g) of the positive electrode sacrificial agent were measured as follows, and the ratio of the irreversible capacity of the positive electrode sacrificial agent to the cell capacity of the battery became a specified value. In Test Example 1, the mixing ratio of the positive electrode active material and the positive electrode sacrificial agent was adjusted so that the ratio of the irreversible capacity of the positive electrode sacrificial agent to the cell capacity of the battery became the values ​​listed in Table 1 as "Addition Rate (Cell Capacity Ratio %)". The content of the positive electrode sacrificial agent relative to the total positive electrode of Test Example 1 is listed in Table 1 as "Addition Amount (Mass %)". Furthermore, in Test Example 2, the mixing ratio of the positive electrode active material and the positive electrode sacrificial agent was adjusted so that the irreversible capacity of the positive electrode sacrificial agent relative to the cell capacity of the battery became 10%. The content of the positive electrode sacrificial agent relative to the total positive electrode of Test Example 2 was 3.3 by mass.

[0199] Furthermore, in Test Examples 1 and 2, the total amount of positive electrode active material and positive electrode sacrificial agent was adjusted so that the unit cell capacity of the lithium secondary battery became 60mAh.

[0200] (Capacity measurement of cathode materials)

[0201] A slurry is prepared by mixing positive electrode active material or positive electrode sacrificial agent, PVDF, conductive additive, and N-methylpyrrolidone (NMP). This slurry is then coated onto aluminum foil, dried, and pressed. A test cell battery with a lithium metal counter electrode is fabricated at 0.2 mAh / cm². 2 After charging to a voltage of 4.2V, the voltage is discharged to a voltage of 3.0V. The charging capacity density (mAh / g) and / or irreversible capacity density A (mAh / g) are then calculated.

[0202] (Formation of a buffer layer)

[0203] A resin solution in which PVDF resin is dissolved in N-methylpyrrolidone (NMP) is applied to the separator using a bar coater. Next, the separator coated with the resin solution is immersed in a water bath and then thoroughly dried at room temperature to form a fibrous ion-conducting layer on the separator. (Furthermore, the ion-conducting layer performs ion-conducting function by injecting the electrolyte (a 4M LiN(SO2F)2(LFSI) in dimethoxyethane (DME) solution) described later during battery assembly.

[0204] The average diameter of the fibrous ion-conducting layer formed on the separator was measured using a scanning electron microscope (SEM) and found to be 100 nm.

[0205] Next, Ni was deposited under vacuum conditions relative to the separator in which the fibrous ion-conducting layer was formed. When the ion-conducting layer after Ni deposition was observed by SEM using an energy-dispersive X-ray analyzer (EDX), it was confirmed that Ni was distributed in a manner that covered the fibrous ion-conducting layer, and that the surface of the fibrous ion-conducting layer was covered by a conductive layer, thus confirming a fibrous buffer functional layer.

[0206] Furthermore, when the cross-section of the buffer functional layer was fabricated using FIB and observed by SEM, the average thickness of the buffer functional layer was 10 μm. When the buffer functional layer was observed using transmission electron microscopy, the average thickness of the Ni film serving as the conductive layer and the porosity of the buffer functional layer were 20 nm and 90%, respectively.

[0207] (Battery assembly)

[0208] Prepare a 4M solution of LiN(SO2F)2(LFSI) in dimethoxyethane (DME) as the electrolyte.

[0209] Next, the positive electrode, the separator formed by the buffer functional layer, and the negative electrode are laminated in this order to obtain a laminate. Furthermore, the lamination is performed with the buffer functional layer facing the negative electrode. Then, 100μm Al terminals and 100μm Ni terminals are ultrasonically welded to the positive current collector and the negative electrode, respectively, and the outer casing of the laminate is inserted. Next, the electrolyte is injected into the outer casing. By sealing the outer casing, a lithium secondary battery is obtained.

[0210] [Experimental Example 1]

[0211] (Examples 1-9)

[0212] Lithium-ion batteries were manufactured using the positive electrode sacrificial agents listed in Table 1, according to the method described above. Furthermore, the mixing ratio of the positive electrode active material and the positive electrode sacrificial agent was adjusted such that the ratio of the irreversible capacity of the positive electrode sacrificial agent to the cell capacity of the battery became the values ​​listed in Table 1 as "Addition Rate (Cell Capacity Ratio %)". Specifically, the content of the positive electrode sacrificial agent relative to the total positive electrode was adjusted to the values ​​listed in Table 1 as "Addition Amount (Mass %)".

[0213] (Comparative Example 1)

[0214] The lithium secondary battery was obtained in the same manner as in Example 1, except that no positive electrode sacrificial agent was used.

[0215] (Comparative Example 2)

[0216] Besides using D 50 Except for the positive electrode sacrificial agent (S) being 0.5 μm, a lithium secondary battery was obtained in the same manner as in Example 1.

[0217] (Comparative Examples 3-5)

[0218] Besides using D 50 (S) and D 95 Except for the positive electrode sacrificial agent whose (S) is the value recorded in Table 1, a lithium secondary battery was obtained in the same manner as in Example 1.

[0219] (Comparative Example 6)

[0220] Besides using D 50 (S) and D 95 Except for the positive electrode sacrificial agent of Li2O2 with values ​​of (S) recorded in Table 1, a lithium secondary battery was obtained in the same manner as in Example 8.

[0221] [Evaluation of energy density and cycle characteristics]

[0222] The energy density and cycle characteristics of the lithium secondary batteries fabricated in the various embodiments and comparative examples are evaluated as follows.

[0223] The manufactured lithium secondary battery has a capacity of 0.2 mAh / cm². 2 After charging (initial charging) until the voltage reaches 4.2V, the charge rate is 0.2mAh / cm³. 2 Discharge (initial discharge) until the voltage reaches 3.0V. Then, repeat the process 99 times at 1.0 mAh / cm³ at 25°C. 2 After charging to 4.2V, the rate is 1.0mAh / cm³. 2 The charge-discharge cycle was performed until the voltage reached 3.0V. For any embodiment and comparative example, the capacity obtained from the initial charge (initial capacity) was 60mAh. The capacity retention rate (%) was calculated as the ratio of the discharge capacity obtained from the discharge in the 100th charge-discharge cycle (counting the initial charge-discharge cycle as the first cycle) to the discharge capacity obtained from the discharge in the second charge-discharge cycle; this ratio was used as an indicator of cycle characteristics. A higher capacity retention rate indicates better cycle characteristics. The capacity retention rates for each example are shown in Table 1.

[0224] [Table 1]

[0225]

[0226] In Table 1, under the capacity maintenance rate, "unstable" indicates a state in which the value fluctuates significantly during the measurement of charge and discharge capacity, making it impossible to measure the charge and discharge capacity stably.

[0227] Table 1 shows that the added particle size D 50 (S) is 1.0 μm to 20 μm, and the particle size D 95Examples 1-9, which are positive electrode sacrificial agents with a diameter of 1.0 μm to 30 μm, operate stably compared to Comparative Examples 1-6, which are not, exhibiting high capacity retention and excellent cycle characteristics.

[0228] [Refer to Experimental Example 1]

[0229] As a reference example, a lithium-ion battery was fabricated by supporting a graphite material containing 10% by mass Si as the negative electrode active material on a 10 μm electrolytic Cu foil. The separator, positive electrode, buffer layer, and electrolyte were set to be the same as in Experimental Example 1. In addition, the amount and particle size of the positive electrode sacrificial agent were adjusted to the values ​​shown in Table 2.

[0230] The lithium-ion battery prepared as Reference Example 1 was measured for its cycle characteristics in the same manner as in Test Example 1. The results are shown in Table 2.

[0231] [Table 2]

[0232]

[0233] When comparing Reference Example 1 in Table 2 with Comparative Example 4 in Table 1, it can be seen that even using the same positive electrode, the lithium secondary battery of this embodiment, which has a negative electrode without a negative electrode active material, does not stably perform charge-discharge cycles, unlike the lithium-ion battery with a negative electrode having a negative electrode active material, which stably performs charge-discharge cycles. That is, it teaches that the lithium secondary battery of this embodiment requires a different positive electrode design than the lithium-ion battery with a conventional negative electrode having a negative electrode active material.

[0234] [Experimental Example 2]

[0235] (Examples 10-16)

[0236] The lithium secondary battery was manufactured in the same manner as in Example 1, except that it used a positive electrode containing a positive electrode active material and a positive electrode sacrificial agent having the characteristics described in Table 3. Furthermore, in Example 2, the mixing ratio of the positive electrode active material and the positive electrode sacrificial agent was adjusted so that the irreversible capacity of the positive electrode sacrificial agent was 10% of the cell capacity of the battery. The content of the positive electrode sacrificial agent relative to the total positive electrode of Example 2 was 3.3% by mass.

[0237] (Comparative Example 7)

[0238] The lithium secondary battery was obtained in the same manner as in Example 10, except that no positive electrode sacrificial agent was used.

[0239] (Comparative Examples 8-9)

[0240] In addition to using particle size D 50(S) is the same as the positive electrode sacrificial agent listed in Table 3, and a lithium secondary battery is obtained in the same manner as in Example 10.

[0241] (Comparative Examples 10-11)

[0242] In addition to using particle size D 50 (A) and particle size D 50 (S) is the same as in Example 12 except for the positive electrode active material and the positive electrode sacrificial agent, which are the values ​​listed in Table 3.

[0243] [Evaluation of Rate Characteristics]

[0244] The rate characteristics of the lithium secondary batteries manufactured in the various embodiments and comparative examples are evaluated as follows.

[0245] After the manufactured lithium-ion secondary battery was CC-charged to 4.2V at 3.0mA, it was CC-discharged sequentially at discharge rates of 0.05C, 0.1C, 0.5C, 1.0C, 2.0C, or 3.0C. The lower limit voltage was set to 3.0V. Between each discharge, it was CC-charged again to 4.2V at 3.0mA, and after charging, CC-discharged at the next discharge rate. The ratio of the discharge capacity at the 3.0C discharge rate to the discharge capacity at the 0.1C discharge rate was used as the rate characteristic (%), which is used as an indicator of the rate characteristic. Since the voltage drop based on internal resistance increases with increasing discharge current, and the discharge capacity tends to decrease, a higher rate characteristic value indicates a superior lithium-ion secondary battery in terms of rate characteristics.

[0246] [Table 3]

[0247]

[0248] Table 3 shows that the particle size D of the positive electrode active material 50 (A) has a particle size of 5.0 μm or more and 20 μm or less, and a particle size ratio of D 50 (A) / D 50 Compared with Comparative Examples 7 to 11, where (S) is 2.0 or higher and 10.0 or lower, Examples 10 to 16 have higher rate characteristics (%) and better rate characteristics.

[0249] Industrial applicability

[0250] Because the lithium secondary battery of the present invention has high energy density and excellent cycle or rate characteristics, it has industrial applicability as an energy storage device for use in a wide variety of applications.

[0251] Explanation of reference numerals in the attached figures

[0252] 100, 200… Lithium secondary battery, 110… Positive electrode, 120… Separator, 130… Buffer functional layer, 140… Negative electrode, 150… Positive current collector, 210… Negative terminal, 220… Positive terminal, 310… Ion conducting fiber, 320… Lithium metal, 400… Ion conducting layer, 410… Ion conductive fiber, 420… Conductive layer.

Claims

1. A lithium secondary battery, comprising: positive electrode; The negative electrode does not contain any negative electrode active material; A separator is disposed between the positive electrode and the negative electrode; and A buffer layer is formed on the surface of the separator opposite to the negative electrode, and has ionic conductivity. The positive electrode comprises a positive electrode active material and a lithium-containing compound that undergoes oxidation within the charge / discharge potential range of the positive electrode active material and does not substantially undergo reduction. In the particle size distribution measured by laser diffraction scattering, The particle size D of the lithium-containing compound corresponding to 50% of the cumulative frequency. 50 (S) is greater than 3.0 μm and less than 20 μm. The particle size D of the lithium-containing compound corresponding to a cumulative frequency of 95% is... 95 (S) is 1.0μm or more and 30μm or less.

2. A lithium secondary battery, comprising: positive electrode; The negative electrode does not contain any negative electrode active material; A separator is disposed between the positive electrode and the negative electrode; and A buffer layer is formed on the surface of the separator opposite to the negative electrode, and has ionic conductivity. The positive electrode comprises a positive electrode active material and a lithium-containing compound that undergoes oxidation within the charge / discharge potential range of the positive electrode active material and does not substantially undergo reduction. In the particle size distribution measured by laser diffraction scattering, the particle size corresponding to 50% of the cumulative frequency is set as D. 50 In this case, The positive electrode active material D 50 (A) 5.0μm or more and 20μm or less D, as the positive electrode active material 50 (A) D relative to the lithium-containing compound 50 (S) particle size ratio D 50 (A) / D 50 (S) is 2.0 or higher and 10.0 or lower. The particle size D of the lithium-containing compound corresponding to 50% of the cumulative frequency. 50 (S) is greater than 3.0 μm and less than 20 μm.

3. The lithium secondary battery according to claim 2, wherein, The lithium-containing compound D 50 (S) is greater than 3.0 μm and less than 10 μm.

4. The lithium secondary battery according to claim 2, wherein, The electrode density of the positive electrode is above 3.0 g / cc.

5. The lithium secondary battery according to any one of claims 1 to 4, wherein, The lithium-containing compound comprises 1.0% to 15% by mass relative to the total mass of the positive electrode.

6. The lithium secondary battery according to any one of claims 1 to 4, wherein, The irreversible capacity of the lithium-containing compound is more than 1.0% and less than 30% of the unit cell capacity of the lithium secondary battery.

7. The lithium secondary battery according to any one of claims 1 to 4, wherein, The porosity of the buffer functional layer is above 50%.

8. The lithium secondary battery according to any one of claims 1 to 4, wherein, The buffer layer is also conductive.

9. The lithium secondary battery according to any one of claims 1 to 4, wherein, The lithium-containing compound is an iron-containing compound.

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