Positive electrode for secondary batteries and secondary batteries

Abstract

This positive electrode for a secondary battery is provided with a positive electrode current collector and a positive electrode mixture layer that includes a positive electrode active material and is provided on the surface of the positive electrode current collector. The positive electrode active material contains a lithium-containing composite oxide having a layered structure, wherein 80 at% or more of the metals other than lithium is nickel. The positive electrode mixture layer includes a carbon-containing conductive auxiliary agent contained in a ratio of 1 part by mass or less with respect to 100 parts by mass of the positive electrode active material, wherein a material resistance Rm of the positive electrode mixture layer is at most 30 Ω•cm, and a ratio Rm / Rc of the material resistance Rm (Ω•cm) of the positive electrode mixture layer to an interface resistance Rc (Ω•cm2) between the positive electrode current collector and the positive electrode mixture layer is at least 200.

Description

[Technical Field]

[0001] This disclosure relates to secondary batteries, and more particularly to improvements to the positive electrode used in secondary batteries. [Background technology]

[0002] Secondary batteries, particularly lithium-ion secondary batteries, are expected to be used as power sources for small consumer applications, power storage devices, and electric vehicles due to their high output and high energy density. The positive electrode active material for lithium-ion secondary batteries is a composite oxide of lithium and a transition metal (e.g., cobalt). Higher capacity can be achieved by replacing some of the cobalt with nickel.

[0003] Furthermore, in recent years, in order to improve the energy density and durability of lithium-ion batteries, attempts have been made to increase the amount of positive electrode active material per unit area by using positive electrode active material with a large charge / discharge capacity in the positive electrode, and by compressing the large amount of positive electrode active material that is mounted.

[0004] Patent Document 1 proposes using non-aggregated composite oxide particles having a compressive strength of 250 MPa or more as the positive electrode active material in a non-aqueous electrolyte secondary battery. This makes it possible to suppress the decrease in capacity retention rate and the increase in resistance during charge-discharge cycles, even when using a positive electrode active material that includes composite oxide particles containing Ni, Co, and Li, and at least one of Mn and Al, in which the ratio of Ni to the total number of moles of metal elements excluding Li is 50 mol% or more.

[0005] Patent Document 2 describes a lithium-ion secondary battery, Li y Ni (1-x) M x It has been proposed that the cathode contains lithium nickel composite oxide and carbon nanotubes, represented by the formula shown, and that the ratio a / b, which is the ratio of the average length a of the carbon nanotubes to the average particle size b of the primary particles of the lithium nickel composite oxide, be 0.5 or greater. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2019 / 026629 Brochure [Patent Document 2] International Publication No. 2008 / 051667 Pamphlet [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] In recent years, there has been a growing demand for lithium-ion secondary batteries that achieve both high energy density and durability at an even higher level.

[0008] However, when using a lithium-transition metal composite oxide containing Ni as the positive electrode active material to achieve high capacity, the higher the Ni ratio in the lithium-transition metal composite oxide, the more Li is extracted from the positive electrode active material, which can modify the surface of the positive electrode active material and change it to a structure that makes it difficult to intercept and release Li ions. As a result, lithium ion movement is inhibited, and a significant decrease in cycle performance is likely to occur. [Means for solving the problem]

[0009] In view of the foregoing, one aspect of this disclosure comprises a positive electrode current collector and a positive electrode mixture layer comprising a positive electrode active material and provided on the surface of the positive electrode current collector, wherein the positive electrode active material has a layered structure and contains a lithium-containing composite oxide in which 80 atomic percent or more of the metals other than lithium are nickel, the positive electrode mixture layer contains a conductive additive containing carbon in a ratio of 1 part by mass or less per 100 parts by mass of the positive electrode active material, the material resistance Rm of the positive electrode mixture layer is 30 Ω·cm or less, and the interfacial resistance Rc(Ω·cm) between the positive electrode current collector and the positive electrode mixture layer of the material resistance Rm(Ω·cm) of the positive electrode mixture layer 2 The ratio Rm / Rc to ) is 200 or more, relating to the positive electrode for secondary batteries.

[0010] Another aspect of the present disclosure relates to a secondary battery having the above-described positive electrode for a secondary battery, a separator, a negative electrode facing the positive electrode for a secondary battery through the separator, and an electrolytic solution.

Advantages of the Invention

[0011] According to the present disclosure, a secondary battery having both a high energy density and high cycle characteristics can be realized. The novel features of the present invention are described in the appended claims. However, the present invention relates to both the configuration and the content, and will be better understood by the following detailed description in conjunction with the drawings, together with other objects and features of the present invention.

Brief Description of the Drawings

[0012] [Figure 1] It is a schematic perspective view of a part of a secondary battery according to an embodiment of the present disclosure, with a notch.

Embodiments for Carrying Out the Invention

[0013] The positive electrode for a secondary battery according to an embodiment of the present disclosure includes a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material and provided on the surface of the positive electrode current collector. The positive electrode active material includes a lithium-containing composite oxide having a layered structure and containing 80 atomic% or more of a metal other than lithium as nickel. The positive electrode mixture layer contains a conductive auxiliary agent containing carbon at a ratio of 1 part by mass or less with respect to 100 parts by mass of the positive electrode active material. The material resistance Rm of the positive electrode mixture layer is 30 Ω·cm or less. The ratio Rm / Rc of the material resistance Rm (Ω·cm) of the positive electrode mixture layer to the interfacial resistance Rc (Ω·cm 2 ) between the positive electrode current collector and the positive electrode mixture layer is 200 or more.

[0014] Here, the material resistance Rm means the resistivity (Ω·cm) of the positive electrode mixture containing the positive electrode active material and the conductive auxiliary agent, etc. The interfacial resistance Rc is the resistivity (Ω·cm 2 converted to a unit area of 1 cm of the interface between the positive electrode mixture and the core (current collector) material 2 ).

[0015] Generally, a positive electrode mixture is formed by preparing a slurry containing materials such as a positive electrode active material, a conductive assistant, a binder, and a solvent, applying the slurry to a core body such as a metal foil, and drying it. In the process of this drying, since the solvent moves outside the mixture by volatilization, materials such as the conductive assistant also move from near the core body along with its movement. Therefore, it is considered that the interfacial resistance Rc increases due to the decrease in the probability of the presence of the conductive assistant near the core body. The phenomenon that the conductive assistant moves from near the core body means that the conductive assistant in the positive electrode mixture is unevenly distributed. The uneven distribution of the conductive assistant ultimately becomes a factor that hinders the movement of lithium ions and inhibits electron conduction between the positive electrode active material particles, and finally causes non-uniform charge-discharge reactions. Especially when using a high-Ni positive electrode active material, although a large charge-discharge capacity can be expected and the battery capacity can be improved, on the other hand, since the amount of lithium released is large, the crystal structure becomes unstable, and it is particularly modified at the interface with the electrolyte, and the charge-discharge performance may deteriorate. Also, as conductive assistants with high electron conductivity, carbon nanotubes, graphene, carbon black with a small particle size, etc. are generally used. However, because these nanocarbons have a small particle structure, the steric hindrance between molecules is also small, and they move from near the core body during the drying of the positive electrode mixture and are likely to be unevenly distributed. Therefore, the smaller the interfacial resistance Rc and the material resistance Rm of the positive electrode mixture layer, the lower the potential variation between the positive electrode active material particles, and the non-uniform charge-discharge reaction is suppressed. Thus, the excessive progress of the charge-discharge reaction in a part of the region of the positive electrode mixture layer is suppressed, and the surface modification of the positive electrode active material is suppressed. As a result, the deterioration of the cycle characteristics is suppressed.

[0016] The cycle characteristics depend not only on the material resistance Rm of the positive electrode mixture layer, but also on the interfacial resistance Rc between the positive electrode current collector and the positive electrode mixture layer. Lower Rm and Rc are preferable as they suppress the deterioration of cycle characteristics. Generally, Rm and Rc can be lowered by increasing the content of the conductive additive. However, increasing the content of the conductive additive reduces the amount of positive electrode active material contained in the mixture layer, making it difficult to obtain high capacity. To maintain high capacity, it is preferable that the content of the conductive additive be 1 part by mass or less per 100 parts by mass of positive electrode active material. On the other hand, if the content of the conductive additive is 1 part by mass or less per 100 parts by mass of positive electrode active material, Rm and Rc will be high, and it may not be possible to maintain high cycle characteristics.

[0017] According to the positive electrode for secondary batteries of this embodiment, by setting the material resistance Rm of the positive electrode composite layer to a low value of 30 Ω·cm or less, and even 20 Ω·cm or less, and setting Rm / Rc to 200 or more, the cycle characteristics can be significantly improved even when the content of the conductive additive is 1 part by mass or less per 100 parts by mass of positive electrode active material, and the nickel ratio in the lithium-containing composite oxide is increased to 80% or more.

[0018] A positive electrode for a secondary battery using a nickel-containing lithium-containing composite oxide can maintain sufficiently high cycle characteristics if Rm is 30 Ω·cm or less and Rm / Rc is 200 or more. Rm / Rc is more preferably 500 or more. In this case, remarkably high cycle characteristics can be achieved. Rm / Rc may be 1000 or less, or 800 or less. The above upper and lower limits of Rm / Rc may be combined arbitrarily.

[0019] The material resistance Rm of the positive electrode mixture layer and the interfacial resistance Rc between the positive electrode current collector and the positive electrode mixture layer can both be measured simultaneously using, for example, the RM2610 electrode resistance measurement system manufactured by HIOKI E.E. CORPORATION. Rm and Rc are measured at multiple sufficiently distant locations (e.g., 10 or more locations) on the surface of the positive electrode mixture layer, and their average values ​​are calculated. Other measurement methods may be used if similar measurement results can be obtained.

[0020] The material resistance Rm of the positive electrode mixture layer and the interfacial resistance Rc between the positive electrode current collector and the positive electrode mixture layer can be controlled, for example, by changing the material and properties (particle size or fiber diameter, fiber length, etc.) and their content ratio of the conductive additive, the conditions for drying the mixture slurry, and the pressure when rolling the positive electrode mixture layer. Furthermore, using carbon fibers with long fiber lengths allows for surface contact between the positive electrode mixture layer and the positive electrode current collector, making it easier to reduce the interfacial resistance Rc.

[0021] A carbon-containing conductive additive is added to the positive electrode mixture layer as a conductive agent to enhance its conductivity by forming conductive paths between the particles of the positive electrode active material. The carbon-containing conductive additive may be conductive carbon particles such as carbon black, or conductive carbon fibers such as carbon nanotubes. Preferably, the carbon-containing conductive additive contains carbon nanotubes. Carbon nanotubes may account for 80% or more by mass of the carbon fibers. Carbon nanotubes are carbon fibers with a nanoscale fiber diameter, and by including carbon fibers with a small fiber diameter in the positive electrode mixture layer, resistance can be reduced even in small amounts. Furthermore, the decrease in liquid flowability is suppressed, and high cycle characteristics can be maintained even when a lithium-containing metal oxide with a Ni ratio of 0.8 or higher is used as the positive electrode active material.

[0022] The fiber length of the carbon nanotubes may be 1 μm or more. In this case, the aspect ratio (ratio of fiber length to outer diameter) of the carbon nanotubes, which are carbon fibers, becomes extremely large. Carbon fibers with a large aspect ratio make linear contact with the active material and current collector, rather than point contact. The highly conductive carbon fibers are interposed between the particles of the positive electrode active material, forming linear contact areas with the particles, thereby reducing the variation in potential between the positive electrode active material particles and suppressing non-uniformity of the charge-discharge reaction. Furthermore, the highly conductive carbon fibers form linear conductive paths between the particles of the positive electrode active material and the current collector, and also form linear contact areas with the current collector, significantly reducing the interfacial resistance Rc. Moreover, the interfacial resistance Rc can be further reduced by, for example, unevenly distributing the carbon fibers towards the current collector when forming the positive electrode mixture layer. On the other hand, carbon fibers with a large aspect ratio occupy only a small volume within the positive electrode mixture layer. Therefore, the carbon fibers are less likely to intervene in the gaps between positive electrode active material particles that should be filled with electrolyte, thus preventing a decrease in electrolyte flow. Furthermore, because they are fibrous, gaps for the electrolyte are more easily maintained even when the positive electrode active material is densely packed within the positive electrode mixture layer. As a result, the deterioration of cycle characteristics can be significantly suppressed.

[0023] In addition, since the carbon fibers occupy only a small volume within the positive electrode mixture layer, the majority of the space occupied by the positive electrode mixture layer excluding the carbon fibers can be occupied by the positive electrode active material. Therefore, by increasing the thickness of the positive electrode mixture layer and / or by compression, a high-capacity positive electrode can be easily obtained.

[0024] The carbon fiber content may be 1 part by mass or less per 100 parts by mass of positive electrode active material. The carbon fiber content may be 0.01 parts by mass or more and 1 part by mass or 0.02 parts by mass or more and 0.5 parts by mass per 100 parts by mass of positive electrode active material. The above carbon fiber content is a value based on the mass of the positive electrode active material in the discharged state.

[0025] Here, the fiber length of carbon fibers or carbon nanotubes refers to the average fiber length, which is determined by image analysis using a scanning electron microscope (SEM). The average fiber length of carbon fibers or carbon nanotubes can be determined, for example, by arbitrarily selecting several carbon fibers or carbon nanotubes (e.g., 100 to 1000), measuring their lengths and diameters, and averaging them. Furthermore, fiber length refers to the length when the fiber is in a straight line.

[0026] The fiber diameter (outer diameter) of carbon fibers or carbon nanotubes is, for example, 20 nm or less, but may also be 15 nm or less. Here, the fiber diameter of carbon fibers or carbon nanotubes refers to the average fiber diameter and is determined by image analysis using a scanning electron microscope (SEM). The average fiber diameter of carbon fibers or carbon nanotubes can be determined, for example, by arbitrarily selecting several carbon fibers or carbon nanotubes (e.g., 100 to 1000), measuring their fiber diameters, and averaging them. Furthermore, fiber diameter refers to the length in the direction perpendicular to the fiber length direction.

[0027] The proportion of positive electrode active material in the positive electrode mixture layer can be determined from a mixture sample obtained by disassembling a discharged secondary battery, washing the positive electrode with an organic solvent, and then peeling off only the mixture layer after vacuum drying. By performing thermal analysis such as TG-DTA on the mixture sample, the ratio of binder components and conductive material components other than the positive electrode active material can be calculated. If the binder components and conductive material components contain multiple types of carbon materials, the proportion of carbon fibers can be calculated by performing micro-Raman spectroscopy on a cross-section of the positive electrode mixture layer.

[0028] Carbon nanotubes can be single-walled, double-walled, or multi-walled, but carbon nanotubes with a fiber diameter of 20 nm or less are preferred because a large effect can be obtained with a small amount. The fiber length of the carbon nanotubes is preferably 1 μm or more from the viewpoint of ensuring electron conduction inside the positive electrode. On the other hand, there is no upper limit to the fiber length if they are properly arranged inside the positive electrode, but considering that the particle size of the positive electrode active material is generally between 1 μm and 20 μm, a length of about the same size is considered appropriate. In other words, the fiber length of the carbon nanotubes may be, for example, between 1 μm and 20 μm.

[0029] As a conductive additive, carbon fibers other than carbon nanotubes and / or conductive carbon particles may be used in mixture with carbon nanotubes. The fiber length of the carbon fibers other than carbon nanotubes is preferably 1 μm or more, and may be, for example, 1 μm to 20 μm. For example, when a plurality of carbon fibers (for example, 100 or more) are arbitrarily selected in the positive electrode mixture layer, the fiber length of 50% or more of these carbon fibers may be 1 μm or more, or 1 μm to 20 μm. The fiber length of 80% or more of the carbon fibers may be 1 μm or more, or 1 μm to 20 μm.

[0030] To obtain high capacity, the amount of positive electrode mixture layer applied per unit area on the surface of the positive electrode current collector is 250 g / m². 2 The above is also acceptable. According to this embodiment, even when the amount of positive electrode mixture and the amount of positive electrode active material are increased, high cycle characteristics can be obtained.

[0031] Furthermore, in order to obtain a high capacity, the proportion of nickel among the metals other than lithium in the lithium-containing composite oxide may be set to 85 atomic percent or more. According to this embodiment, even when the Ni ratio of the positive electrode active material is increased in this way, high cycle characteristics can be obtained.

[0032] Next, the secondary battery according to the embodiment of the present disclosure will be described in detail. The secondary battery includes, for example, a positive electrode, a negative electrode, an electrolyte, and a separator as follows.

[0033] [Positive Electrode] The positive electrode includes a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector and containing a positive electrode active material. As the positive electrode, the above-described positive electrode for a secondary battery is used. The positive electrode mixture layer can be formed, for example, by applying a positive electrode slurry in which a positive electrode mixture containing a positive electrode active material, a binder, etc. is dispersed in a dispersion medium onto the surface of the positive electrode current collector and drying it. The dried coating film may be rolled if necessary. The positive electrode mixture layer may be formed on one surface of the positive electrode current collector or on both surfaces.

[0034] The positive electrode mixture layer essentially contains a positive electrode active material and a conductive auxiliary agent containing carbon. As the conductive auxiliary agent containing carbon, the above-described materials (for example, carbon nanotubes) can be used. The conductive auxiliary agent containing carbon is contained in the positive electrode mixture layer at a ratio of 1 part by mass or less with respect to 100 parts by mass of the positive electrode active material. Another conductive auxiliary agent may be contained in the positive electrode mixture layer in addition to the conductive auxiliary agent containing carbon. The positive electrode mixture layer can contain, as optional components, a binder, a thickener, etc. Known materials can be used as the binder, the thickener, and another conductive auxiliary agent.

[0035] As the positive electrode active material, a lithium-containing composite oxide having a layered structure (for example, a rock salt-type crystal structure) containing lithium and a transition metal can be used. Specifically, the lithium-containing composite oxide is, for example, Li a Ni x M 1-xIt may also be a lithium-nickel composite oxide represented by O2 (where 0 < a ≤ 1.2, 0.8 ≤ x ≤ 1, and M contains at least one selected from the group consisting of Co, Al, Mn, Fe, Ti, Sr, Na, Mg, Ca, Sc, Y, Cu, Zn, Cr, and B). Among these, it is preferable that M contains at least one selected from the group consisting of Co, Mn, Al, and Fe. From the perspective of the stability of the crystal structure, it may contain Al as M. Note that the value of a indicating the molar ratio of lithium increases or decreases during charge and discharge. Specific examples of such composite oxides include lithium-nickel-cobalt-aluminum composite oxide (LiNi 0.9 Co 0.05 Al 0.05 O2, etc.).

[0036] Here, from the perspective of obtaining high capacity, it is desirable that the proportion of Ni in the metal elements other than Li contained in the lithium transition metal composite oxide is 80 atomic% or more. The proportion of Ni in the metal elements other than Li may be 85 atomic% or more, or may be 90 atomic% or more. The proportion of Ni in the metal elements other than Li is desirably, for example, 95 atomic% or less. When limiting the range, these upper and lower limits can be arbitrarily combined.

[0037] Hereinafter, a lithium transition metal composite oxide having a layered rock salt structure, containing Ni and at least one selected from the group consisting of Co, Mn, Fe, and Al, and having a proportion of Ni in the metal elements other than Li of 80 atomic% or more is also referred to as "composite oxide HN". Reversible insertion and desorption of Li ions are possible into the interlayer of the layered rock salt structure of the composite oxide HN.

[0038] Co, Mn, and Al contribute to the stabilization of the crystal structure of the composite oxide HN with a high Ni content. However, from the perspective of reducing manufacturing costs, it is more desirable that the Co content is lower. The composite oxide HN with a low Co content or without Co may contain Mn and Al.

[0039] The proportion of Co among the metal elements other than Li is preferably 20 atomic percent or less, more preferably 10 atomic percent or less or 5 atomic percent or less, and it is not necessary to include Co at all. From the viewpoint of stabilizing the crystal structure of the composite oxide HN, it is desirable to include 1 atomic percent or more or 1.5 atomic percent or more of Ni.

[0040] The proportion of Mn in the metal elements other than Li may be 10 atomic percent or less, or 5 atomic percent or less. The proportion of Mn in the metal elements other than Li may be 1 atomic percent or more, or 3 atomic percent or more, or 5 atomic percent or more. When limiting the range, these upper and lower limits can be combined arbitrarily.

[0041] The proportion of Al among metal elements other than Li may be 10 atomic percent or less, or 5 atomic percent or less. The proportion of Al among metal elements other than Li may be 1 atomic percent or more, 3 atomic percent or more, or 5 atomic percent or more. When limiting the range, these upper and lower limits can be combined arbitrarily.

[0042] A composite oxide HN is, for example, one with the formula: Li α Ni (1-x1-x2-y-z) Co x1 Mn x2 Al y M z O 2+β It is represented as follows: Element M is an element other than Li, Ni, Co, Mn, Fe, Al, and oxygen.

[0043] In the above formula, α, which represents the atomic ratio of lithium, is, for example, 0.95 ≤ α ≤ 1.05. However, α is the value at complete discharge. In (2 + β), which represents the atomic ratio of oxygen, β satisfies -0.05 ≤ β ≤ 0.05.

[0044] The atomic ratio of Ni, represented by 1-x1-x2-yz (=x), is 0.8 or greater, may be 0.85 or greater, may be 0.9 or greater, may be 0.92 or greater, or may be 0.95 or greater. Furthermore, the atomic ratio of Ni, represented by x, may be 0.98 or less, or 0.95 or less. When limiting the range, these upper and lower limits can be combined in any way.

[0045] x1, which indicates the atomic ratio of Co, is, for example, not more than 0.1 (0 ≤ x1 ≤ 0.1), and may be not more than 0.08, may be not more than 0.05, or may be not more than 0.01. When x1 is 0, the case where Co is below the detection limit is included.

[0046] x2, which indicates the atomic ratio of Mn, is, for example, not more than 0.1 (0 ≤ x2 ≤ 0.1), and may be not more than 0.08, may be not more than 0.05, or may be not more than 0.03. x2 may be not less than 0.01 or may be not less than 0.03. Mn contributes to the stabilization of the crystal structure of the composite oxide HN, and the composite oxide HN containing inexpensive Mn is advantageous for cost reduction. When limiting the range, these upper and lower limits can be arbitrarily combined.

[0047] y, which indicates the atomic ratio of Al, is, for example, not more than 0.1 (0 ≤ y ≤ 0.1), and may be not more than 0.08, may be not more than 0.05, or may be not more than 0.03. y may be not less than 0.01 or may be not less than 0.03. Al contributes to the stabilization of the crystal structure of the composite oxide HN. When limiting the range, these upper and lower limits can be arbitrarily combined.

[0048] z, which indicates the atomic ratio of element M, is, for example, 0 ≤ z ≤ 0.10, may be 0 < z ≤ 0.05, or may be 0.001 ≤ z ≤ 0.01. When limiting the range, these upper and lower limits can be arbitrarily combined.

[0049] Element M may be at least one selected from the group consisting of Ti, Zr, Nb, Mo, W, Zn, B, Si, Mg, Ca, Sr, Sc, and Y. Among them, when at least one selected from the group consisting of Nb, Sr, and Ca is contained in the composite oxide HN, it is considered that the surface structure of the composite oxide HN is stabilized, the resistance is reduced, and the elution of the metal is further suppressed. It is more effective that element M is unevenly distributed in the vicinity of the particle surface of the composite oxide HN.

[0050] The elemental content of the composite oxide HN can be measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES), electron probe microanalyzer (EPMA), or energy dispersive X-ray spectroscopy (EDX), among other methods.

[0051] The composite oxide HN is, for example, a secondary particle formed by the aggregation of multiple primary particles. The particle size of the primary particles is, for example, between 0.05 μm and 1 μm. The average particle size of the secondary particles of the composite oxide HN is, for example, between 3 μm and 30 μm, and may also be between 5 μm and 25 μm.

[0052] In this specification, the average particle size of secondary particles refers to the particle size at which the integrated volume value in the particle size distribution measured by laser diffraction scattering (volume-average particle size) becomes 50%. Such a particle size is sometimes referred to as D50. For the measuring device, for example, the "LA-750" manufactured by HORIBA, Ltd. can be used.

[0053] The positive electrode active material may contain lithium transition metal composite oxides other than composite oxide HN, but it is preferable that the proportion of composite oxide HN is high. The proportion of composite oxide HN in the positive electrode active material is, for example, 90% by mass or more, may be 95% by mass or more, or may be 100%.

[0054] The lithium-nickel composite oxide described above can increase its capacity by extracting more lithium ions from the oxide during charging as the Ni ratio x increases. However, the Ni in lithium-nickel composite oxides with increased capacity tends to have a higher valence state. As a result, the crystal structure tends to become unstable, especially in the fully charged state, and repeated charging and discharging can easily cause the surface of the active material particles to change (become inactive) into a crystal structure that makes reversible intercalation and release of lithium ions difficult. Consequently, the cycle performance tends to deteriorate. In particular, when adopting a configuration in which the thickness of the positive electrode composite layer is increased and / or the composite layer is compressed to increase the amount of positive electrode active material per unit area, the flow of lithium ions and / or electrons is easily obstructed during the charge-discharge reaction, and unevenness in the charge-discharge reaction is likely to occur. When unevenness occurs in the charge-discharge reaction, inactivation of the crystal structure progresses in some regions where the amount of lithium ion extraction is large due to excessive charging reaction, which may lead to a deterioration in cycle performance.

[0055] However, in the positive electrode for secondary batteries of this embodiment, by setting the material resistance Rm of the positive electrode mixture layer to 30 Ω·cm or less, and the ratio of Rm to the interfacial resistance Rc between the positive electrode current collector and the positive electrode mixture layer, Rm / Rc, to 200 or more, high cycle characteristics can be maintained even when a lithium-containing composite oxide with a large Ni ratio x is used. Therefore, a secondary battery with excellent cycle characteristics and high energy density can be realized.

[0056] The shape and thickness of the positive electrode current collector can be selected from the same shape and range as the negative electrode current collector. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0057] [Negative electrode] The negative electrode contains a negative electrode active material. The negative electrode typically comprises a negative electrode current collector and a layered negative electrode mixture (hereinafter referred to as the negative electrode mixture layer) held by the negative electrode current collector. The negative electrode mixture layer can be formed by applying a negative electrode slurry, in which the components of the negative electrode mixture are dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying it. The dried coating may be rolled if necessary.

[0058] The negative electrode mixture may contain a negative electrode active material as an essential component, and may also contain binders, thickeners, conductive agents, etc., as optional components.

[0059] (Negative electrode active material) As the negative electrode active material, metallic lithium, lithium alloys, etc., may be used, but materials capable of electrochemically intercalating and releasing lithium ions are preferably used. Examples of such materials include carbonaceous materials and Si-containing materials. The negative electrode may contain one type of negative electrode active material, or a combination of two or more types.

[0060] Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon). A single carbonaceous material may be used, or two or more may be used in combination. Among these, graphite is preferred as the carbonaceous material due to its excellent charge-discharge stability and low irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles.

[0061] Si-containing materials include elemental silicon, silicon alloys, silicon compounds (such as silicon oxides), and composite materials in which a silicon phase is dispersed within a lithium-ion conductive phase (matrix). Examples of silicon oxides include SiO2. x Examples include particles. x may be, for example, 0.5 ≤ x < 2, and also 0.8 ≤ x ≤ 1.6. As the lithium ion conducting phase, at least one selected from the group consisting of SiO2 phase, silicate phase, and carbon phase may be used.

[0062] For example, the materials exemplified for the positive electrode can be used as binders, thickeners, conductive agents, and dispersion media for the negative electrode slurry.

[0063] For example, a metal foil may be used as the negative electrode current collector. The negative electrode current collector may be porous. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy. The thickness of the negative electrode current collector is not particularly limited, but for example, it may be 1 to 50 μm, or 5 to 30 μm.

[0064] [Electrolyte] The electrolyte contains a solvent and a solute dissolved in the solvent. The solute is an electrolyte salt that undergoes ionic dissociation in the electrolyte. The solute may include, for example, a lithium salt. Components of the electrolyte other than the solvent and solute are additives. Various additives may be included in the electrolyte.

[0065] Any known material can be used as a solvent. Examples of solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC). Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of linear carboxylic acid esters include non-aqueous solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). Non-aqueous solvents may be used individually or in combination of two or more.

[0066] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10Lithium salts of fluorine-containing acids (such as LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (such as LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (such as LiCl, LiBr, LiI, etc.) can be used. Lithium salts may be used individually or in combination of two or more types.

[0067] The lithium salt concentration in the electrolyte may be between 1 mol / liter and 2 mol / liter, or between 1 mol / liter and 1.5 mol / liter. By controlling the lithium salt concentration within the above range, an electrolyte with excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0068] The electrolyte may contain other known additives. Examples of additives include 1,3-propanesalton, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0069] [Separator] A separator is interposed between the positive and negative electrodes. The separator has high ion permeability and possesses appropriate mechanical strength and insulating properties. Microporous thin films, woven fabrics, nonwoven fabrics, etc., can be used as separators. Polyolefins such as polypropylene and polyethylene are preferred as the material of the separator.

[0070] One example of the structure of a non-aqueous electrolyte secondary battery is a structure in which an electrode group, in which a positive electrode and a negative electrode are wound around each other with a separator, is housed together with the non-aqueous electrolyte in an outer casing. However, it is not limited to this, and other forms of electrode groups may be used. For example, a stacked electrode group in which the positive electrode and negative electrode are stacked with a separator in between may also be used. The form of the non-aqueous electrolyte secondary battery is also not limited, and may be cylindrical, prismatic, coin-type, button-type, laminate-type, etc.

[0071] Figure 1 is a schematic perspective view showing a portion of a rectangular non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure. The battery comprises a bottomed rectangular battery case 4, an electrode group 1 and a non-aqueous electrolyte (not shown) housed within the battery case 4. The electrode group 1 has a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed between them. The negative electrode current collector of the negative electrode is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector of the positive electrode is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. That is, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 fits into the open end of the battery case 4, and the fitting portion is laser welded. The sealing plate 5 has an injection hole for the non-aqueous electrolyte, which is sealed by a seal 8 after injection.

[0072] The present disclosure will be described in detail below based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0073] <Examples 1-7, Comparative Examples 1-4> [Fabrication of the negative electrode] A silicon composite material and graphite were mixed in a mass ratio of 5:95 and used as the negative electrode active material. The negative electrode active material, sodium carboxymethylcellulose (CMC-Na), styrene-butadiene rubber (SBR), and water were mixed in a predetermined mass ratio to prepare the negative electrode slurry. Next, the negative electrode slurry was applied to the surface of a copper foil, which was to be used as the negative electrode current collector. After the coating was dried, it was rolled to form negative electrode mixture layers on both sides of the copper foil.

[0074] [Fabrication of the positive electrode] A lithium-containing composite oxide shown in Table 1 was used as the positive electrode active material. The positive electrode active material, the conductive additive shown in Table 1, polyvinylidene fluoride, and N-methyl-2-pyrrolidone (NMP) were mixed in a predetermined mass ratio to prepare a positive electrode slurry. Next, the positive electrode slurry was applied to the surface of an aluminum foil, which served as the positive electrode current collector. After the coating was dried, the foil was rolled to form positive electrode mixture layers on both sides of the aluminum foil.

[0075] [Preparation of electrolyte solution] An electrolyte was prepared by adding LiPF6 as a lithium salt to a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7. The concentration of LiPF6 in the non-aqueous electrolyte was 1.0 mol / liter.

[0076] [Manufacturing of secondary batteries] An electrode assembly was fabricated by attaching lead tabs to each electrode and spirally winding the positive and negative electrodes via a separator so that the leads were located on the outermost periphery. The electrode assembly was inserted into an outer casing made of laminate film with aluminum foil as a barrier layer, vacuum-dried at 105°C for 2 hours, then a non-aqueous electrolyte was injected, and the opening of the outer casing was sealed to obtain a secondary battery.

[0077] Table 1 shows the composition of the lithium-containing composite oxide used as the positive electrode active material, the conductive additive and its amount added (parts by mass per 100 parts by mass of positive electrode active material), the coating amount per unit area of ​​the positive electrode mixture layer, and the Rm and Rm / Rc values ​​for Examples 1-7 and Comparative Examples 1-4. In Table 1, batteries A1-A7 correspond to Examples 1-7, and batteries B1-B4 correspond to Comparative Examples 1-4. The Rm and Rm / Rc values ​​listed in Table 1 were measured after charging and discharging the secondary battery once, disassembling the battery after discharge, washing the removed positive electrode with dimethyl carbonate solution, and vacuum drying.

[0078] In Table 1, CB in the conductive additive column represents carbon black (acetylene black, average particle size approximately 20 nm). Also in Table 1, CNT in the conductive additive column represents carbon nanotubes. The average diameter of the carbon nanotubes was approximately 10 nm, and those with the average fiber length shown in Table 1 were used.

[0079] [evaluation] (Initial charge / discharge) Each completed battery was placed in a 25°C environment and charged with a constant current of 0.5It until the voltage reached 4.2V. Then, it was charged with a constant voltage of 4.2V until the current reached 0.02It. Finally, it was discharged with a constant current of 1.0It until the voltage reached 2.5V, and the initial capacity C0 was determined. Charging and discharging were performed in a 25°C environment.

[0080] (durability) The rest period between charging and discharging was set to 10 minutes. The charge-discharge cycle was repeated 100 times under the above conditions in a 25°C environment, and the discharge capacity C1 at the 100th cycle was determined. The ratio R1 = C1 / C0 of the discharge capacity C1 to the initial discharge capacity C0 was defined as the capacity retention rate, and R1 × 100 was evaluated as an indicator of durability.

[0081] Table 1 shows the evaluation results for the durability of batteries A1-A7 and B1-B4. From Table 1, it can be seen that high durability can be achieved when the material resistance Rm of the positive electrode mixture layer is 30 Ω·cm or less (and even 20 Ω·cm or less) and the Rm / Rc ratio is 200 or more. In particular, batteries with 1 part by mass or less of carbon nanotubes added as a conductive additive have low Rm and Rm / Rc ratios and exhibit excellent durability.

[0082] [Table 1] [Industrial applicability]

[0083] The secondary battery described herein provides a secondary battery with high capacity and excellent cycle characteristics. The secondary battery described herein is useful as a main power source for mobile communication devices, portable electronic devices, and the like. Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention. [Explanation of symbols]

[0084] 1: Electrode group, 2: Positive lead, 3: Negative lead, 4: Battery case, 5: Sealing plate, 6: Negative terminal, 7: Gasket, 8: Sealing plug

Claims

1. The device comprises a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material and provided on the surface of the positive electrode current collector, The positive electrode active material has a layered structure and contains a lithium-containing composite oxide in which 80 atomic percent or more of the metals other than lithium is nickel. The positive electrode mixture layer contains a conductive additive containing carbon in a proportion of 1 part by mass or less per 100 parts by mass of the positive electrode active material. The material resistance Rm of the positive electrode mixture layer is 30 Ω·cm or less. The material resistance Rm (Ω·cm) of the positive electrode mixture layer is given by the interfacial resistance Rc (Ω·cm) between the positive electrode current collector and the positive electrode mixture layer. ２ The ratio Rm / Rc to ) is 200 or more. The aforementioned conductive additive includes carbon nanotubes, The fiber length of the carbon nanotube is 1 μm or more. A positive electrode for a secondary battery, wherein the carbon nanotube content in the positive electrode mixture layer is 0.01 parts by mass or more and 1 part by mass or less per 100 parts by mass of the positive electrode active material.

2. The positive electrode for a secondary battery according to claim 1, wherein the ratio Rm / Rc is 500 or more.

3. The lithium-containing composite oxide has the chemical formula Li ａ Ni ｘ M １－ｘ O ２ The positive electrode for a secondary battery according to claim 1 or 2, wherein the lithium nickel composite oxide is represented as (wherein 0 < a ≤ 1.2, 0.8 ≤ x ≤ 1, and M includes at least one selected from the group consisting of Co, Al, Mn, Fe, Ti, Sr, Na, Mg, Ca, Sc, Y, Cu, Zn, Cr, and B).

4. The positive electrode for a secondary battery according to claim 3, wherein x ≥ 0.85 in the chemical formula.

5. The positive electrode mixture layer is 250 g / m² ２ A positive electrode for a secondary battery according to any one of claims 1 to 4, wherein the above amount of mounting is provided on the surface of the positive electrode current collector.

6. A positive electrode for a secondary battery according to any one of claims 1 to 5, A secondary battery comprising a separator, a negative electrode facing the positive electrode for the secondary battery via the separator, and an electrolyte.

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