Positive electrode for secondary battery, and secondary battery

Abstract

This positive electrode for a secondary battery comprises a positive electrode current collector and a positive electrode active material layer that is provided on the surface of the positive electrode current collector. The positive electrode active material layer contains, as a positive electrode active material, a lithium metal composite oxide having a rock-salt-type crystal structure that can belong to a space group Fm–3m. The lithium metal composite oxide contains at least Li and Mn. In a volume-based particle size distribution of primary particles of the lithium metal composite oxide derived from a cross-sectional image of the positive electrode active material layer, the difference (D90-D50) between the particle diameter D90 of a cumulative volume of 90% and the particle diameter D50 of a cumulative volume of 50% is less than 6.3 μm.

Description

Positive electrode for secondary battery and secondary battery CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims the benefit of priority to Japanese Patent Application No. 2023-193891, filed on November 14, 2023, in the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a positive electrode for a secondary battery and a secondary battery.

[0003] Secondary batteries, especially lithium-ion secondary batteries, have high power output and high energy density, and are expected to be used in small consumer applications, power storage devices, and electric vehicles. The cathode active material of lithium-ion secondary batteries is a composite oxide of lithium and a transition metal (e.g., cobalt). Replacing part of the cobalt with nickel can increase the battery capacity.

[0004] On the other hand, in recent years, in response to the demand for high energy density, the rock salt structure Li １＋ｘ Mn １－ｘ O ２ Li-excess lithium metal composite oxides based on the above have been attracting attention.

[0005] Patent Document 1 discloses a compound having a crystal structure that can be assigned to the space group Fm-3m and having the composition formula (1): Li １＋ｘ Nb ｙ Me ｚ A ｐ O ２ (Me is a transition metal including Fe and / or Mn, 0<x<1, 0<y<0.5, 0.25≦z<1, A is an element other than Nb and Me, and 0≦p≦0.2)

[0006] International Publication No. 2014 / 156153

[0007] In Patent Document 1, high capacity is possible by controlling the composition (i.e., by adding Nb), but the effect of improving capacity is insufficient, and there is still room for improvement.

[0008] In view of the above, one aspect of the present disclosure relates to a positive electrode for a secondary battery, including: a positive electrode current collector; and a positive electrode active material layer provided on a surface of the positive electrode current collector, wherein the positive electrode active material layer includes, as a positive electrode active material, a lithium metal composite oxide having a rock-salt crystal structure assignable to space group Fm-3m, the lithium metal composite oxide including at least Li and Mn, and wherein, in a volume-based particle size distribution of primary particles of the lithium metal composite oxide derived from a cross-sectional image of the positive electrode active material layer, the difference (D90 - D50) between the particle diameter D90 at 90% of the cumulative volume and the particle diameter D50 at 50% of the cumulative volume is less than 6.3 μm.

[0009] Another aspect of the present disclosure relates to a secondary battery including a positive electrode, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode is the above-mentioned positive electrode for the secondary battery.

[0010] According to the present disclosure, a secondary battery with a high energy density can be realized. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0011] 1 is a schematic perspective view of a secondary battery according to an embodiment of the present disclosure, with a portion cut away;

[0012] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits for specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more materials may be used in combination.

[0013] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0014] In the following description, the term "comprises (or includes)" encompasses "comprises (or includes)", "consists essentially of", and "consists of".

[0015] The secondary battery includes at least non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries and lithium metal secondary batteries, and all-solid-state batteries using a solid electrolyte.

[0016] A positive electrode for a secondary battery according to an embodiment of the present disclosure includes a positive electrode current collector and a positive electrode active material layer provided on the surface of the positive electrode current collector. The positive electrode active material layer includes, as a positive electrode active material, a lithium metal composite oxide (hereinafter also referred to as "lithium metal composite oxide (Fm)") having a rock-salt crystal structure that can be assigned to the space group Fm-3m. The lithium metal composite oxide (Fm) has a crystal structure based on a rock-salt structure that belongs to the space group Fm-3m, and has a crystal structure similar to the rock-salt structure represented by NaCl, for example. In such a crystal structure, oxygen atoms are arranged at the anion sites, and Li atoms and metal atoms other than Li can be irregularly arranged at the cation sites.

[0017] The lithium metal composite oxide (Fm) contains at least Li and Mn. １＋ｘ Mn １－ｘ O ２ The lithium metal composite oxide (Fm) based on this is expected to exhibit high capacity.

[0018] Mass production of lithium metal composite oxide (Fm) requires efficient synthesis by a solid-phase method in which a raw material mixture is calcined. However, the lithium metal composite oxide (Fm) particles synthesized by calcination are often large, hard, and aggregated particles. Particles obtained by crushing such hard aggregated particles often fail to exhibit sufficient discharge capacity.

[0019] On the other hand, detailed analysis of the relationship between the volume-based particle size distribution (hereinafter also referred to as "equivalent volume particle size distribution") of the primary particles of the lithium metal composite oxide derived from the cross-sectional image of the positive electrode active material layer and the capacity revealed that the particle size distribution has a strong influence on the capacity development. In particular, the developed capacity changes significantly depending on the difference (D90-D50) between the particle size D90 at 90% cumulative volume and the particle size D50 at 50% cumulative volume in the equivalent volume particle size distribution. Specifically, by controlling the difference (D90-D50) between the particle size D90 (hereinafter also referred to simply as "D90") and the particle size D50 (hereinafter also referred to simply as "D50") to less than 6.3 μm, a higher capacity positive electrode active material can be obtained.

[0020] Generally, particles obtained by crushing agglomerated particles with high hardness tend to have a large difference between D90 and D50 (D90 - D50). Particles with a particle size close to or less than D50 are particles that are easy to accept charge and discharge and are easy to develop capacity. On the other hand, particles with a particle size close to or exceeding D90 are particles that are relatively hard to accept charge and discharge and are hard to develop capacity. The smaller the difference between D90 and D50 (D90 - D50), the smaller the difference in the ease of accepting charge and discharge between the former particles and the latter particles. In this case, the charge and discharge reaction tends to proceed uniformly within the positive electrode active material layer, and polarization is less likely to increase.

[0021] Furthermore, the capacity of lithium metal composite oxide (Fm) is more susceptible to the difference between D90 and D50 (D90-D50) than conventionally used positive electrode active materials. Lithium metal composite oxide (Fm) exhibits a significant improvement in capacity when its particle size distribution is controlled to suit charge and discharge. While the reason for this is unclear, it is presumed that the rock-salt crystal structure particles belonging to the space group Fm-3m have a complex relationship between their relatively low electronic conductivity, their tendency to generate strain inside the particles, and their relatively smooth surface condition.

[0022] (Converted Volume Particle Size Distribution) The converted volume particle size distribution is a distribution obtained by converting the "area-based" distribution of primary particles of the lithium metal composite oxide derived from a cross-sectional image of the positive electrode active material layer (hereinafter also referred to as a "cross-sectional SEM image") into a "volume-based" distribution. By measuring the cross-sectional area Sd of the primary particles observed in the cross-sectional SEM image, the diameter Dd (Dd = 8√(Sd / π)) of an equivalent sphere having the measured area Sd as the average cross-sectional area is calculated. ３ ) and the volume (Vd) of an equivalent sphere with a diameter (Dd) can be calculated. By calculating the equivalent sphere volumes of all primary particles observed in the cross-sectional SEM image, the volume-based cumulative particle size distribution, i.e., the converted volume particle size distribution, can be calculated.

[0023] An example of a procedure for determining the reduced volume particle size distribution from a cross-sectional SEM image will be described below.

[0024] (1) Preparation of a Cross Section of a Positive Electrode First, a positive electrode to be measured is prepared. The positive electrode includes a positive electrode current collector and a positive electrode active material layer provided on the surface of the positive electrode current collector. The positive electrode active material layer and the positive electrode current collector are cut together along the thickness direction of the positive electrode to form a cross section. At this time, a thermosetting resin may be filled into the positive electrode active material layer and cured. For example, a cross section sample of the positive electrode active material layer may be obtained by a CP (cross section polisher) method, an FIB (focused ion beam) method, or the like.

[0025] The positive electrode to be measured is taken from a secondary battery with a depth of discharge (DOD) of 90% or more. Depth of discharge (DOD) is the ratio of the amount of discharged electricity to the amount of electricity in a fully charged battery. Note that the amount of electricity charged (i.e., the fully charged amount) when a battery in a fully discharged state (DOD = 100%) is charged to a fully charged state (SOC = 100%, DOD = 0%) corresponds to the rated capacity. The voltage of a battery in a fully charged state corresponds to the end-of-charge voltage. The voltage of a battery in a fully discharged state corresponds to the end-of-charge voltage.

[0026] (2) Taking a Cross-Sectional SEM Image Next, a cross-sectional sample of the positive electrode (positive electrode active material layer) is observed using an SEM. Observation using an SEM is performed, for example, at a magnification of 500 to 3000 times. The cross-sectional SEM image is taken so that a region having a length of 30 μm or more (preferably 40 μm or more) in the planar direction of the positive electrode active material layer (the direction along the surface of the positive electrode current collector) is observed.

[0027] (3) Image analysis For each of the cross sections of 100 or more arbitrarily selected primary particles of lithium metal composite oxide (Fm) in a cross-sectional SEM image, the area Sd of the cross section was measured, and the diameter Dd of the equivalent sphere (Dd = 8√(Sd / π ３ The volume-based cumulative particle size distribution of the obtained volumes (Vd) of 100 or more equivalent spheres is calculated as the converted volume particle size distribution.

[0028] The 100 or more primary particles may be selected from a central region in the thickness direction of the positive electrode active material layer and a region as far away as possible from the end portions in the surface direction of the positive electrode active material layer. Even if the particles are lithium metal composite oxide (Fm), secondary particles formed by aggregation of primary particles and particles that cannot be clearly distinguished as primary particles or secondary particles are excluded from the calculation of the converted volume particle size distribution. Secondary particles can be easily distinguished and excluded from the particle shape by visually observing a cross-sectional SEM image.

[0029] In the image analysis, image analysis software (e.g., ImageJ) may be used. In the image analysis, the cross-sectional SEM image may be binarized so that the primary particles of the lithium metal composite oxide (Fm) are black (or white) and the rest are white (or black).

[0030] (Particle diameter D50 at 50% cumulative volume) The D50 of the primary particles of the lithium metal composite oxide (Fm) is the diameter Dd at which the cumulative volume is 50% of the total when the volume is integrated starting from the particle with the smallest equivalent spherical diameter Dd in the converted volume particle size distribution. D50 may be selected according to the electrode design. D50 may be, for example, 0.05 μm or more, 0.10 μm or more, 0.20 μm or more, or 0.25 μm or more. D50 may be, for example, 8 μm or less, 2 μm or less, 1 μm or less, or 0.8 μm or less. D50 may be, for example, 0.05 μm or more and 8 μm or less, 0.10 μm or more and 2 μm or less, 0.20 μm or more and 1 μm or less, or 0.25 μm or more and 0.8 μm or less.

[0031] (Particle diameter D90 at 90% cumulative volume) Similarly, when integrating the volume of particles starting from the smallest equivalent sphere diameter Dd, the diameter of the equivalent sphere at which the cumulative volume is 90% of the total is D90. D90 may be, for example, greater than 0.46 μm, 0.47 μm or greater, or 0.5 μm or greater. D90 may be, for example, 11 μm or less, 5 μm or less, 2 μm or less, or 1 μm or less. A lithium metal composite oxide (Fm) having such a D90 can be obtained by pulverizing a high-hardness lithium metal composite oxide (Fm) obtained by firing a raw material mixture under appropriate conditions. D90 may be, for example, greater than 0.46 μm and 11 μm or less, 0.47 μm or more and 5 μm or less, 0.5 μm or more and 2 μm or less, or 0.5 μm or more and 1 μm or less.

[0032] (Difference between D90 and D50 (D90-D50)) The difference between D90 and D50 (D90-D50) may be 6.3 μm or less, but the smaller the better. The difference between D90 and D50 (D90-D50) may be 4.2 μm or less, 3.0 μm or less, 1.5 μm or less, or 1.0 μm or less. The lower limit of the difference between D90 and D50 (D90-D50) is not particularly limited, but may be, for example, 0.1 μm or more.

[0033] (Specific Surface Area) The specific surface area of ​​the lithium metal composite oxide (Fm) is, for example, 0.1 m ２ / g or more, 35m２ / g or less, and ２ / g or more, 31.2m ２ / g or less, and 3.8m ２ / g or more, 31.2m ２ / g or less, and 3.8m ２ / g or more, 20m ２ / g or less. When the specific surface area is within the above range, the reaction area of ​​the lithium metal composite oxide (Fm) becomes sufficiently large, and the capacity exhibited by the lithium metal composite oxide (Fm) per unit mass becomes significantly high. In addition, since the specific surface area is not too large, the filling rate of the lithium metal composite oxide (Fm) in the positive electrode active material layer can be increased. Furthermore, since the specific surface area is not too large, side reactions can be easily suppressed.

[0034] The specific surface area may be measured using the lithium metal composite oxide (Fm) as a raw material powder before it is made into a positive electrode, or may be measured using the lithium metal composite oxide (Fm) separated from the positive electrode. Either method will provide roughly similar measurement results.

[0035] Here, the specific surface area of ​​the lithium metal composite oxide (Fm) is measured by taking 0.20 g to 0.25 g of a lithium metal composite oxide (Fm) sample, placing the sample in a measurement cell composed of a glass tube for measuring specific surface area, and then drying and degassing the measurement cell. The drying and degassing is performed for at least one hour at a pressure of 6.67 Pa and a temperature of 250°C ± 5°C. The mass of the sample in the measurement cell is then measured to the nearest 0.1 mg. The amount of nitrogen adsorption of the sample at a temperature of -196°C is then measured using a specific surface area measurement device. For example, an automatic specific surface area / pore distribution measurement device "Tristar II 3020" manufactured by Shimadzu Corporation may be used as the measurement device. From the measurement results of the adsorption amount, the specific surface area of ​​the lithium metal composite oxide (Fm) is determined using the BET multipoint method within a partial pressure (relative pressure) range of 0.001 to 0.2. Furthermore, when the specific surface area is small, the amount of adsorption of krypton gas may be measured in order to measure the specific surface area with high accuracy.

[0036] The lattice constant a, which indicates the length of the crystal lattice in the a-axis direction of the lithium metal composite oxide (Fm), may be 4.050 Å or more and 4.160 Å or less. When the crystal lattice has such a lattice constant a, the effect of improving capacity may be more significant. Although the detailed reason is unknown, it is thought that when the lattice constant a is within the above range, the stability of the crystal structure is enhanced. When the specific surface area of ​​the lithium metal composite oxide (Fm) having a stable crystal structure is sufficiently increased, the effective reaction area becomes significantly larger, the capacity increases, and the capacity retention rate is also thought to be improved.

[0037] The crystal structure of the lithium metal composite oxide (Fm) is identified from the X-ray diffraction pattern measured using a powder X-ray diffractometer. For example, a desktop X-ray diffractometer "MiniFlex" manufactured by Rigaku Corporation can be used as the powder X-ray diffractometer. Furthermore, the lattice constant a of the lithium metal composite oxide (Fm) can be determined using, for example, the integrated powder X-ray analysis software "PDXL" manufactured by Rigaku Corporation. The X-ray source for the X-ray diffraction measurement is CuKα radiation, and the measurement range of 2θ is 10° to 100°.

[0038] The lattice constant a determined using the integrated powder X-ray analysis software "PDXL" may be followed by a numerical value in parentheses. The numerical value in parentheses indicates an error to the third decimal place. For example, in the case of 4.115(2) Å, the lattice constant a is 4.113 Å or more and 4.117 Å or less. In the present disclosure, the lattice constant a being 4.050 Å or more and 4.160 Å or less means that the lattice constant a is within the range of 4.050 Å or more and 4.160 Å or less within the above error range.

[0039] The lithium metal composite oxide (Fm) may further contain an electropositive element M different from Li and Mn. The electropositive element M may be any electropositive element other than hydrogen. The electropositive element M may be a metal element (including so-called metalloid elements). In other words, the lithium metal composite oxide (Fm) may be a lithium transition metal composite oxide containing at least three types of metals.

[0040] When the lithium metal composite oxide (Fm) contains the element M, it is preferable that the main component of the metal elements other than Li is Mn. The number of atoms b of Mn contained in the lithium metal composite oxide (Fm) may be the largest among the numbers of atoms of metal elements other than Li contained in the lithium metal composite oxide (Fm). The number of atoms of Mn may be equal to or greater than the number of atoms c of the total metal elements excluding Li and Mn (i.e., the electropositive element M). The ratio (b / c) of the number of atoms b of Mn to the number of atoms c of the electropositive element M contained in the lithium metal composite oxide is, for example, 1 or more and 15 or less, 1 or more and 12 or less, 1 or more and 12 or less, 2 or more and 12 or less, 3 or more and 12 or less, 4 or more and 12 or less, 5 or more and 12 or less, 6 or more and 12 or less, or 7 or more and 12 or less.

[0041] The positive element M may include at least one element selected from the group consisting of, for example, Fe, Ge, Si, Ga, Ni, Co, Sn, Cu, Nb, Mo, Bi, V, Cr, Y, Ti, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Gd, Sm, Eu, Yb, Dy, and Er. The lithium metal composite oxide (Fm) may include any two or more elements selected from the above group as the positive element M.

[0042] The lithium metal composite oxide (Fm) preferably contains at least Ti as the electropositive element M. The lithium metal composite oxide (Fm) containing Ti as the electropositive element M can exhibit particularly high capacity. One of the reasons for this is that in the lithium metal composite oxide (Fm), Ti has an empty d orbital. ４＋ This is thought to be because the crystal structure can exist in the form of a rock-salt salt. In this case, a rock-salt crystal structure with high symmetry, stability, and capacity is thought to be formed. Furthermore, such a rock-salt structure is thought to be less likely to become unstable even after repeated charge and discharge.

[0043] When the lithium metal composite oxide (Fm) contains Ti, the ratio of the number of Mn atoms to the number of Ti atoms (Mn / Ti) in the lithium metal composite oxide (Fm) may be 1 or more, 2 or more, 3 or more, 4 or more, 5 or more, 6 or more, or 7 or more. The Mn / Ti ratio may be 70 or less, 30 or less, 15 or less, or 12 or less. The proportion of Ti in the electropositive element M may be 25 atomic % or more, 50 atomic % or more, 80 atomic % or more, or 100 atomic %.

[0044] The lithium metal composite oxide (Fm) may be an oxyfluoride containing F. Fluorine can substitute oxygen atoms at the anion site in the crystal structure. This stabilizes the crystal structure and provides a higher capacity even when the lithium metal composite oxide (Fm) is in a Li-excess state. Furthermore, the substitution of fluorine atoms increases the average discharge potential. The Li-excess state refers to a state in which the number of Li atoms contained in the lithium metal composite oxide (Fm) is greater than the total number of atoms of metal elements other than Li.

[0045] In Li-excess lithium metal composite oxides (Fm), the arrangement of Li in the cation sites is irregular, resulting in a variety of Li bonding states. Therefore, the voltage distribution associated with Li release is wide. Therefore, it may be difficult to utilize the low-potential side (tail portion) of the voltage distribution as a capacity. However, the introduction of fluorine atoms shifts the voltage distribution associated with Li release toward the high-potential side. Therefore, it becomes easier to utilize the tail portion as a capacity. This further increases the available capacity.

[0046] Lithium metal composite oxide (Fm) has the composition formula Li ａ Mn ｂ M ｃ O ｄ F e Here, the following conditions are satisfied: 1≦a<1.4, 0.5≦b<0.9, 0≦c≦0.4, 1.33≦d≦2, 0≦e≦0.67, 1.7≦d+e≦2.2.

[0047] When the lithium metal composite oxide (Fm) contains a positive element M, the range of c is 0<c≦0.4, or may be 0<c≦0.3, or 0<c≦0.2. The lower limit of c may be, for example, 0.01≦c or 0.02≦c.

[0048] The content of the constituent elements of the lithium metal composite oxide (Fm) can be measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), an energy dispersive X-ray analyzer (EDX), or the like.

[0049] <Method for Producing Lithium Metal Composite Oxide (Fm)> The method for producing the lithium metal composite oxide (Fm) is not limited, but it is desirable to obtain it by calcining a raw material mixture containing the constituent elements of the lithium metal composite oxide (Fm). Calcination promotes the growth of crystals similar to the rock salt structure belonging to the space group Fm-3m. Thus, a lithium metal composite oxide (Fm) with large crystallite size is obtained. Thereafter, by carrying out a pulverization treatment under predetermined conditions, the equivalent volume particle size distribution, specific surface area, and the like may be controlled to desired states.

[0050] As raw materials for the constituent elements of the lithium metal composite oxide (Fm), Mn compounds, compounds of element M, lithium compounds, fluorine compounds, titanium compounds, etc. may be arbitrarily selected and used. The types and mixing ratios of the raw materials may be appropriately selected depending on the desired composition described above.

[0051] Examples of Mn compounds include MnO ２ , Mn ２ O ３ and manganese salts such as lithium manganate.

[0052] Examples of compounds of the electropositive element M include M salts such as oxides, acid fluorides, and hydroxides.

[0053] The lithium compound includes Li ２ Lithium oxides such as LiOH, LiMnO ２ ) and other lithium salts.

[0054] The fluorine compound may include fluorine salts such as lithium fluoride (LiF).

[0055] Examples of titanium compounds include TiO ２ and titanium salts such as lithium titanate.

[0056] The atmosphere for firing the raw material mixture may vary depending on the desired composition of the lithium metal composite oxide (Fm) and the types of raw materials. The firing atmosphere may be, for example, an oxidizing atmosphere (e.g., in air or in the presence of oxygen). It is desirable to circulate the atmospheric gas.

[0057] The firing temperature of the raw material mixture may vary depending on the desired composition of the lithium metal composite oxide (Fm) and the type of raw material. The firing temperature may be, for example, 700°C or higher, or 900°C or higher. The firing temperature is preferably 1300°C or lower.

[0058] The lithium metal composite oxide (Fm) obtained by calcination is usually in the form of aggregated particles. In this case, the aggregated particles can be pulverized. The pulverization conditions are selected so as to obtain a desired converted volume particle size distribution, specific surface area, etc. As the pulverization device, a stirring device capable of applying a large shear force to the aggregated particles, such as a ball mill or a bead mill, may be used.

[0059] There are various methods for controlling the equivalent volume particle size distribution, specific surface area, etc., and they are not limited to the method of controlling by crushing agglomerated particles. For example, the raw material mixture may be fired while stirring the raw material mixture. For example, the raw material mixture may be fired while stirring under various conditions using a firing furnace equipped with a fluidized bed. In this case, the particle shape tends to become closer to spherical, and the particle size distribution tends to become narrower and sharper. Control of the specific surface area also becomes easier.

[0060] Next, a secondary battery according to an 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 described below.

[0061] [Positive Electrode] The positive electrode includes a positive electrode current collector and a positive electrode active material layer provided on the surface of the positive electrode current collector. The positive electrode active material 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, to the surface of the positive electrode current collector and drying the slurry. In other words, the positive electrode active material layer may be a positive electrode mixture layer. The dried coating may be rolled as necessary. The positive electrode active material layer may be formed on one surface or both surfaces of the positive electrode current collector.

[0062] The positive electrode active material layer contains a positive electrode active material as an essential component, and may contain, as optional components, a binder, a thickener, a conductive agent, a positive electrode additive, etc. Known materials can be used as the binder, thickener, and conductive agent.

[0063] The positive electrode active material includes a lithium metal composite oxide (the above-mentioned lithium metal composite oxide (Fm)) having a rock-salt type crystal structure that can be assigned to the space group Fm-3m.

[0064] The lithium metal composite oxide (Fm) may be mixed with other known lithium metal oxides and used as a positive electrode active material. Examples of other known lithium metal oxides include Li a CoO2, Li a NiO2, Li a MnO2, Li a Co b Ni 1-b O2, Li a Co b M 1-b O c , Li a Ni 1-b M b O c , Li a Mn2O4, Li a Mn 2-b M b O 4、 LiMePO 4、Examples of lithium transition metal composite oxides include Li2MePO4F. Here, M is, for example, at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. Me contains at least a transition element. Me contains, for example, at least one selected from the group consisting of Mn, Fe, Co, and Ni. Here, 0≦a≦1.2, 0≦b≦0.9, and 2.0≦c≦2.3. The value a, which indicates the molar ratio of lithium, increases or decreases during charge and discharge.

[0065] The positive electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of the material for the positive electrode current collector include stainless steel, aluminum, an aluminum alloy, and titanium.

[0066] [Negative Electrode] The negative electrode may include, for example, a negative electrode current collector and a negative electrode active material layer formed on the surface of the negative electrode current collector. The negative electrode active material layer can be formed, for example, by applying a negative electrode slurry, in which a negative electrode mixture containing a negative electrode active material, a binder, etc. is dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the slurry. The dried coating may be rolled as necessary. In other words, the negative electrode active material layer may be a negative electrode mixture layer. Alternatively, a lithium metal foil or a lithium alloy foil may be attached to the negative electrode current collector. The negative electrode active material layer may be formed on one surface or both surfaces of the negative electrode current collector.

[0067] The negative electrode active material layer contains a negative electrode active material as an essential component, and may contain optional components such as a binder, a conductive agent, a thickener, etc. Known materials can be used as the binder, conductive agent, and thickener.

[0068] Examples of negative electrode active materials include materials that electrochemically absorb and release lithium ions, lithium metal, and lithium alloys. Examples of materials that electrochemically absorb and release lithium ions include carbon materials and alloy-based materials. Examples of carbon materials include graphite, easily graphitized carbon (soft carbon), and hardly graphitized carbon (hard carbon). Of these, graphite is preferred because of its excellent charge / discharge stability and low irreversible capacity. Examples of alloy-based materials include those containing at least one metal that can form an alloy with lithium. For example, a lithium ion conductive phase and a composite material in which a silicon phase is dispersed in the lithium ion conductive phase may be used.

[0069] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0070] [Electrolyte] The electrolyte may be a liquid electrolyte (electrolytic solution), a gel electrolyte, or a solid electrolyte. The liquid electrolyte (electrolytic solution) is, for example, a solution containing a non-aqueous solvent and a salt dissolved in the non-aqueous solvent. The concentration of the salt in the electrolytic solution is, for example, 0.5 mol / L or more and 2 mol / L or less. The electrolytic solution may contain known additives. The electrolyte may contain various additives.

[0071] The gel electrolyte contains a salt and a matrix polymer, or a salt, a non-aqueous solvent, and a matrix polymer. The matrix polymer is, for example, a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, polyether resin, and polyethylene oxide.

[0072] As the solid electrolyte, for example, a material known in all-solid-state lithium ion secondary batteries (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, etc.) can be used.

[0073] The electrolyte solution is prepared by dissolving a salt in a non-aqueous solvent. The salt is an electrolyte salt that ionically dissociates in the non-aqueous solvent. The salt may include a lithium salt. The electrolyte solution is usually used in a liquid state, but may have its fluidity restricted by a gelling agent or the like.

[0074] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), and vinylene carbonate (VC). Examples of chain carbonates 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 chain carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0075] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorine-containing acids (LiPF6, LiPF ２ O ２ , LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(CF4F9SO2), LiN(CF5SO2)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.), etc., can be used. One type of lithium salt may be used alone, or two or more types may be used in combination.

[0076] The concentration of the lithium salt in the electrolyte solution may be 1 mol / L or more and 2 mol / L or less, or 1 mol / L or more and 1.5 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte solution having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0077] [Separator] A separator is interposed between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator can be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0078] An example of the structure of a secondary battery is a structure in which an electrode group and a non-aqueous electrolyte are housed in an exterior body. The electrode group may be a wound type in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, or a laminated type in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween. Other forms of electrode groups may also be applied. In the present disclosure, the type, shape, etc. of the secondary battery are not particularly limited. The secondary battery may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0079] FIG. 1 is a schematic perspective view, partially cut away, of a prismatic secondary battery according to an embodiment of the present disclosure. The battery includes a bottomed prismatic battery case 4, an electrode group 1, and a nonaqueous electrolyte (not shown) housed within the battery case 4. The electrode group 1 includes a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed therebetween. The negative electrode current collector 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 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 is laser-welded. The sealing plate 5 has an injection hole for the nonaqueous electrolyte, which is closed with a seal plug 8 after injection.

[0080] The exterior body may be cylindrical, coin-shaped, button-shaped, or the like, having a metal battery case, or may be a laminate type having a battery case made of a laminate sheet that is a laminate of a barrier layer and a resin sheet.

[0081] (Additional Notes) The above description discloses the following technologies. (Technology 1) A positive electrode for a secondary battery, comprising: a positive electrode current collector; and a positive electrode active material layer provided on the surface of the positive electrode current collector, wherein the positive electrode active material layer contains, as a positive electrode active material, a lithium metal composite oxide having a rock-salt crystal structure assignable to space group Fm-3m, and the lithium metal composite oxide contains at least Li and Mn, and in a volume-based particle size distribution of primary particles of the lithium metal composite oxide derived from a cross-sectional image of the positive electrode active material layer, the difference (D90 - D50) between the particle diameter D90 at 90% of the cumulative volume and the particle diameter D50 at 50% of the cumulative volume is less than 6.3 μm. (Technology 2) A positive electrode for a secondary battery according to Technology 1, wherein the difference (D90 - D50) between the particle diameter D90 and the particle diameter D50 is 4.2 μm or less. (Technology 3) The positive electrode for a secondary battery according to Technology 1 or 2, wherein the lithium metal composite oxide further contains a positive element M different from Li and Mn, and wherein a ratio (b / c) of the number of atoms b of Mn contained in the lithium metal composite oxide to the number of atoms c of the positive element M is 1 or more and 15 or less. (Technology 4) The positive electrode for a secondary battery according to Technology 3, wherein the positive element M includes at least one selected from the group consisting of Fe, Ge, Si, Ga, Ni, Co, Sn, Cu, Nb, Mo, Bi, V, Cr, Y, Ti, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Gd, Sm, Eu, Yb, Dy, and Er. (Technology 5) The lithium metal composite oxide is a positive electrode for a secondary battery according to Technology 1 or 2, wherein the positive element M is at least one selected from the group consisting of Fe, Ge, Si, Ga, Ni, Co, Sn, Cu, Nb, Mo, Bi, V, Cr, Y, Ti, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Gd, Sm, Eu, Yb, Dy, and Er. ａ Mn ｂ M ｃ O ｄ F eand satisfying 1≦a<1.4, 0.5≦b<0.9, 0<c≦0.4, 1.33≦d≦2, 0≦e≦0.67, and 1.7≦d+e≦2.2. (Technology 6) A secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode for a secondary battery according to any one of Technologies 1 to 5.

[0082] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0083] Examples 1 to 5 and Comparative Example 1 [Fabrication of Positive Electrode] Manganese oxide (Mn ２ O ３ ), lithium carbonate (Li ２ CO ３ ) and titanium oxide (TiO ２ The raw material mixture containing Li was fired at 950°C for 10 hours. １.２２ Mn ０.６１ Ti ０.１７５ O ｄ Agglomerated particles of Li-excess lithium metal composite oxide (Fm) having the following composition were obtained. The composition was identified by analyzing the lithium metal composite oxide (Fm) by ICP atomic emission spectrometry. The ratio (b / c) of the number of Mn atoms b (=0.61) to the number of Ti (electropositive element M) atoms c (=0.175) contained in the lithium metal composite oxide (Fm) was 3.49.

[0084] The obtained aggregated particles of lithium metal composite oxide (Fm) were pulverized under various conditions to obtain lithium metal composite oxide (Fm) exhibiting equivalent volume particle size distributions with various D50 and D90.

[0085] As an example, in the case of lithium metal composite oxide (Fm), aggregate particles of lithium metal composite oxide (Fm) are milled in a planetary ball mill (Premium-Line P7 manufactured by Fritsch, rotation speed: 150 to 300 rpm, container: 45 mL, ball: φ3 mm ZrO ２ The mixture was placed in a ball (made of aluminum) and treated in an air atmosphere at room temperature for 12 hours or less.

[0086] The specific surface area of ​​the lithium metal composite oxide (Fm) measured by the method described above was 0.1 m ２ / g to 10m ２ The D50 was in the range of 0.2 μm to 0.5 μm, the D90 was in the range of 0.5 μm to 6.8 μm, and the difference between D90 and D50 (D90−D50) was in the range of 0.3 μm to 6.3 μm.

[0087] The X-ray diffraction pattern of the pulverized lithium metal composite oxide (Fm) was measured and analyzed. From the number and positions of XRD peaks, it was confirmed that the lithium metal composite oxide (Fm) had a rock-salt crystal structure that could be assigned to the space group Fm-3m. The lattice constant a of the lithium metal composite oxide (Fm) was 4.141 Å.

[0088] The resulting lithium metal composite oxide (Fm), acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 7:2:1 to form a positive electrode mixture, which was then dispersed in N-methyl-2-pyrrolidone (NMP) as a dispersion medium to prepare a positive electrode slurry. The positive electrode slurry was then applied to the surface of a positive electrode current collector made of aluminum foil, and the coating was dried and compressed to obtain a positive electrode plate. The positive electrode plate was then cut to a predetermined electrode size to obtain a positive electrode. Then, a cross-sectional SEM image of the positive electrode was taken using the method described above, and the D50 and D90 values ​​in the converted volume particle size distribution were calculated from the cross-sectional SEM image.

[0089] [Preparation of Electrolyte Solution] An electrolyte solution was prepared by adding LiPF6 as a lithium salt to a mixed solvent prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) at a predetermined volume ratio.

[0090] [Preparation of Test Cell] A test cell was prepared using the above-mentioned positive electrode and a lithium metal foil negative electrode. The positive electrode and negative electrode were arranged facing each other with a separator interposed therebetween to form an electrode assembly, and the electrode assembly was housed in a coin-shaped battery case (battery can). After pouring the electrolyte into the battery can, the battery can was sealed with a sealing plate to obtain a coin-shaped test secondary battery.

[0091] [Evaluation] (Discharge Capacity) The secondary battery was charged at a constant current of 0.1 C in a room temperature environment up to a battery voltage of 4.95 V, and then further charged at a constant voltage of 4.95 V until the current reached 0.01 C. After a 20-minute pause, the battery was discharged at a constant current of 0.1 C down to a battery voltage of 2.5 V, and the discharge capacity (mAh / g) was measured. Table 1 shows the relationship between the discharge capacity (mAh / g) per mass of the lithium metal composite oxide (Fm) and each parameter of the converted volume particle size distribution.

[0092] Batteries A1 to A5 are the batteries of Examples 1 to 5, respectively, and battery B1 is the battery of Comparative Example 1.

[0093]

[0094] Table 1 shows that when the difference between D90 and D50 (D90-D50) in the converted volume particle size distribution of the lithium metal composite oxide (Fm) is less than 6.3 μm (preferably 4.2 μm or less), the discharge capacity is significantly improved.

[0095] Examples 6 and 7 [Fabrication of Positive Electrode] Manganese oxide (Mn ２ O ３ ), lithium carbonate (Li ２ CO ３ ) and titanium oxide (TiO ２ The raw material mixture containing Li was fired at 950°C for 10 hours. １.２０ Mn ０.６３ Ti ０.１７５ O ｄ Agglomerated particles of Li-excess lithium metal composite oxide (Fm) having the following composition were obtained. The composition was identified by analyzing the lithium metal composite oxide (Fm) by ICP atomic emission spectrometry. The ratio (b / c) of the number of Mn atoms b (=0.63) to the number of Ti (electropositive element M) atoms c (=0.175) contained in the lithium metal composite oxide (Fm) was 3.6.

[0096] The obtained aggregated particles of lithium metal composite oxide (Fm) were pulverized under two conditions to obtain lithium metal composite oxide (Fm) showing equivalent volume particle size distributions with different D50 and D90.

[0097] The specific surface area of ​​the lithium metal composite oxide (Fm) measured by the method described above was 0.1 m ２ / g to 8.0m ２ The D50 was 0.54 μm or 7.7 μm, the D90 was 0.78 μm or 10.2 μm, and the difference between D90 and D50 (D90−D50) was 0.2 μm or 2.6 μm.

[0098] The X-ray diffraction pattern of the pulverized lithium metal composite oxide (Fm) was measured and analyzed. From the number and positions of XRD peaks, it was confirmed that the lithium metal composite oxide (Fm) had a rock-salt crystal structure that could be assigned to the space group Fm-3m. The lattice constant a of the lithium metal composite oxide (Fm) was 4.151 Å.

[0099] A positive electrode was obtained in the same manner as in Examples 1 to 5, except that the obtained lithium metal composite oxide (Fm) was used. Then, a cross-sectional SEM image of the positive electrode was taken using the method described above, and the D50 and D90 in the converted volume particle size distribution were calculated from the cross-sectional SEM image and evaluated in the same manner. Table 2 shows the relationship between the discharge capacity per mass (mAh / g) of the lithium metal composite oxide (Fm) and each parameter of the converted volume particle size distribution.

[0100] Batteries A6 and A7 are the batteries of Examples 6 and 7, respectively.

[0101]

[0102] Table 2 shows that even if the composition of the lithium metal composite oxide (Fm) is different, a high discharge capacity can be obtained when the difference between D90 and D50 in the converted volume particle size distribution (D90 - D50) is less than 6.3 μm. Furthermore, as D50 and D90 increase, the Li diffusion rate in the particle solid phase slows, reducing the utilization rate of the active material and raising concerns about a decrease in discharge capacity. However, even when D50 is actually relatively large, a high discharge capacity can be obtained when the difference between D90 and D50 (D90 - D50) is less than 6.3 μm. This is inferred from a comparison of Battery A6 and Battery B1.

[0103] The positive electrode for a secondary battery according to the present disclosure can provide a secondary battery with high energy density. The secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, and the like.

[0104] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0105] 1: electrode group, 2: positive electrode lead, 3: negative electrode lead, 4: battery case, 5: sealing plate, 6: negative electrode terminal, 7: gasket, 8: sealing plug

Claims

1. A positive electrode for a secondary battery comprising: a positive electrode current collector; and a positive electrode active material layer provided on a surface of the positive electrode current collector, wherein the positive electrode active material layer contains, as a positive electrode active material, a lithium metal composite oxide having a rock-salt crystal structure assignable to space group Fm-3m, the lithium metal composite oxide containing at least Li and Mn, and in a volume-based particle size distribution of primary particles of the lithium metal composite oxide derived from a cross-sectional image of the positive electrode active material layer, a difference (D90-D50) between a particle size D90 at 90% of a cumulative volume and a particle size D50 at 50% of a cumulative volume is less than 6.3 μm.

2. The positive electrode for a secondary battery according to claim 1, wherein the difference between the particle diameter D90 and the particle diameter D50 (D90-D50) is 4.2 μm or less.

3. The positive electrode for a secondary battery according to claim 1, wherein the lithium metal composite oxide further contains a positive element M different from Li and Mn, and a ratio (b / c) of the number of atoms b of Mn to the number of atoms c of the positive element M contained in the lithium metal composite oxide is 1 or more and 15 or less.

4. The positive electrode for a secondary battery according to claim 3, wherein the positive element M includes at least one selected from the group consisting of Fe, Ge, Si, Ga, Ni, Co, Sn, Cu, Nb, Mo, Bi, V, Cr, Y, Ti, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Gd, Sm, Eu, Yb, Dy, and Er.

5. The lithium metal composite oxide has the composition formula Li ａ Mn ｂ M ｃ O ｄ F e 4. The positive electrode for a secondary battery according to claim 3, which satisfies the following conditions: 1≦a<1.4, 0.5≦b<0.9, 0<c≦0.4, 1.33≦d≦2, 0≦e≦0.67, and 1.7≦d+e≦2.

2.

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