Positive electrode active material for secondary battery, and secondary battery

Abstract

A positive electrode active material for a secondary battery, the positive electrode active material containing a lithium metal composite oxide having a rock salt-type crystal structure capable of belonging to space group Fm-3m, the lithium metal composite oxide containing at least Li and Mn, and the average aspect ratio of particles of the lithium metal composite oxide being less than 2.10.

Description

[0001] Cross-reference to Related Applications

[0002] This disclosure claims priority to Japanese Patent Application No. 2023-012189, filed with the Japan Patent Office on January 30, 2023, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure relates to a positive electrode active material for a secondary battery and a secondary battery. Background Art

[0004] 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 because of their high output power and high energy density. As a positive electrode active material for a lithium ion secondary battery, a composite oxide of lithium and a transition metal (e.g., cobalt) is used. By replacing a part of cobalt with nickel, high capacity can be achieved.

[0005] On the other hand, in recent years, in response to the demand for high energy density, Li-excess type lithium metal composite oxides based on a rock salt structure of Li 1-q , p , y , 1-x , q , <at 1+p , z , 1+x , 1+x Mn 1-x O2 have attracted attention.

[0006] Patent Document 1 discloses a positive electrode active material containing a lithium transition metal composite oxide having a crystal structure belonging to the space group Fm-3m and represented by the composition formula Li 1+x Nb y Me z A p O2 (Me is a transition metal containing Fe and / or Mn, 0 < x < 1,​​​​​​​​​​​​​​​​​​​In view of the above, one aspect of the present disclosure relates to a positive electrode active material for a secondary battery, comprising a lithium metal composite oxide having a rock salt-type crystal structure that can be attributed to the space group Fm-3m, wherein the lithium metal composite oxide contains at least Li and Mn, and the average aspect ratio of the particles of the lithium metal composite oxide is less than 2.10.

[0011] 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 includes the positive electrode active material for a secondary battery.

[0012] According to the present disclosure, a secondary battery with high energy density can be realized.

[0013] While the novel features of the present disclosure are described in the appended claims, the present invention both in terms of organization 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 accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a partially cutaway schematic perspective view of a secondary battery according to an embodiment of the present disclosure.

[0015] Figure 2 This is a graph showing the relationship between the discharge capacity per unit mass and the average value of the aspect ratio of lithium metal composite oxides. DETAILED DESCRIPTION

[0016] Hereinafter, embodiments of the present disclosure will be described with examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials are sometimes exemplified, but other numerical values ​​and materials may also be applied as long as the effects of the present disclosure can be obtained. In this specification, a description such as "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be replaced by "above numerical value A and below numerical value B". In the following description, when the lower limit and upper limit of numerical values ​​related to specific physical properties and conditions are exemplified, as long as the lower limit does not exceed the upper limit, any one of the exemplified lower limits and any one of the exemplified upper limits can be arbitrarily combined. When multiple materials are exemplified, one can be selected and used alone, or two or more can be used in combination.

[0017] Furthermore, the present disclosure includes combinations of matters described in two or more claims arbitrarily selected from the plurality of claims recited in the accompanying claims. In other words, as long as no technical contradiction arises, matters described in two or more claims arbitrarily selected from the plurality of claims recited in the accompanying claims may be combined.

[0018] In the following description, the term “comprise (or include, contain)” includes “comprise (or include),” “essentially consist of” and “consist of (composed of, formed of, made of, etc.)”.

[0019] Secondary batteries include at least non-aqueous electrolyte secondary batteries such as lithium ion batteries and lithium metal secondary batteries, and all-solid batteries using solid electrolytes.

[0020] The positive electrode active material for a secondary battery according to an embodiment of the present disclosure includes a lithium metal composite oxide (hereinafter also referred to as "lithium metal composite oxide (Fm)") having a rock salt-type 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 belonging to the space group Fm-3m, for example, a crystal structure similar to a rock salt structure represented by NaCl. In this crystal structure, oxygen atoms can be arranged at the anion site, and lithium atoms and metal atoms other than lithium can be randomly arranged at the cation site.

[0021] Lithium metal composite oxide (Fm) contains at least Li and Mn. 1+x Mn 1-x O2-based lithium metal composite oxides (Fm) are expected to exhibit high capacity.

[0022] Currently, some examples of synthesizing lithium metal composite oxides (Fm) at the laboratory level have been reported using a stirring device such as a ball mill to apply high shear force to a raw material mixture containing a lithium compound and a manganese compound.

[0023] On the other hand, when mass-producing lithium metal composite oxides (Fm), it is required to efficiently synthesize by calcining the raw material mixture. However, the particles of lithium metal composite oxides (Fm) synthesized by calcination are very hard and are often in a bulky state.

[0024] Crushing these extremely hard, bulk particles often results in insufficient discharge capacity. A detailed analysis of the relationship between particle shape and capacity revealed that the particle shape factor significantly influences capacity.

[0025] Generally speaking, the particles obtained by crushing the high block of hardness are easily formed into the non-uniform shape with sharp end points. In the case of particles with non-uniform shape, the part that is easily subjected to charge and discharge preferentially reacts, and the part that is difficult to receive charge and discharge is difficult to react. Therefore, it is believed that the charge and discharge reaction is carried out unevenly, and polarization also becomes larger. In addition, there is the more non-uniform the particle shape, the greater the tortuosity of the electron conduction path, and the higher the resistance tendency.

[0026] Furthermore, the capacity of lithium metal composite oxides (Fm) is more susceptible to particle shape factors than previously practical positive electrode active materials. In other words, when the particle shape of lithium metal composite oxides (Fm) is controlled to make them more suitable for charge and discharge, the capacity can be significantly improved. The reasons for this are not entirely clear, but it is speculated that this is due to the complex interplay of factors such as the low electronic conductivity of particles with a rock salt crystal structure belonging to the space group Fm-3m, the low internal strain of particles with high hardness, and the relatively smooth surface of the particles.

[0027] Specifically, the capacity of the synthesized lithium metal composite oxide (Fm) is significantly improved by making the average aspect ratio of the particles less than 2.1. By controlling the average aspect ratio of the particles to less than 2.10, the particle shape becomes uniform, and the difference in receptivity between the portion that is easily receptive to charge and discharge and the portion that is difficult to receptive to charge and discharge becomes smaller. As a result, it is believed that the charge and discharge reaction proceeds uniformly, polarization also becomes smaller, and the discharge capacity is improved.

[0028] In addition, lithium metal composite oxide (Fm) is a material with relatively low electron conductivity among the materials used as positive electrode active materials. Moreover, the more uneven the particle shape, the greater the tortuosity of the electron conduction path. In contrast, when the average aspect ratio of the particles is less than 2.10, the distance between the particles in the electrode becomes uniform and the tortuosity of the electron conduction path becomes smaller. This is believed to contribute significantly to the increase in capacity.

[0029] The average value of the aspect ratio of the particles only needs to be less than 2.10, but is preferably 2.09 or less, more preferably 1.94 or less, and still more preferably 1.80 or less or 1.70 or less.

[0030] (Aspect Ratio)

[0031] The aspect ratio is the value obtained by dividing the maximum diameter Dmax of a particle by the maximum diameter dmax in a direction perpendicular to the maximum diameter Dmax (maximum diameter Dmax / maximum diameter dmax). The maximum diameter Dmax and maximum diameter dmax of the lithium metal composite oxide (Fm) can be measured using particles of the lithium metal composite oxide (Fm) as a raw material powder before being made into a positive electrode, or the maximum diameter Dmax and maximum diameter dmax in the positive electrode can be measured. Because the lithium metal composite oxide (Fm) having a rock salt crystal structure is durable, the same measurement results can be obtained regardless of the measurement method. The maximum diameter Dmax and maximum diameter dmax in the positive electrode are measured using lithium metal composite oxide (Fm) particles observed in a scanning electron microscope (SEM) image of a cross section in the thickness direction of the positive electrode (hereinafter referred to as a "cross-sectional SEM image"). The particles observed in the cross-sectional SEM image of the positive electrode can also be a cross section of the particles. The maximum diameter Dmax and maximum diameter dmax of the raw material powder are measured using lithium metal composite oxide (Fm) particles observed in an SEM image of the raw material powder (hereinafter referred to as a "powder SEM image").

[0032] An example of a procedure for obtaining the average value of aspect ratios from a cross-sectional SEM image will be described below.

[0033] (1) Preparation of the positive electrode cross section

[0034] First, prepare the positive electrode of the measurement object. Typically, the positive electrode comprises a positive electrode collector and a positive electrode active material layer formed on the surface of the positive electrode collector. Such a positive electrode active material layer and the positive electrode collector are simultaneously cut along the thickness direction of the positive electrode to form a cross section. At this time, a thermosetting resin can also be filled into the positive electrode active material layer to solidify it. For example, a cross-sectional sample of the positive electrode active material layer can also be obtained by a CP (cross-section polishing) method, a FIB (focused ion beam) method, or the like.

[0035] The positive electrode to be measured was removed from a secondary battery with a depth of discharge (DOD) of 90% or greater. Depth of discharge (DOD) refers to the ratio of the discharged charge to the charge capacity of a fully charged battery. Furthermore, the charge capacity (i.e., the full charge capacity) obtained by charging a fully discharged battery (DOD = 100%) to a fully charged state (SOC = 100%, DOD = 0%) corresponds to the rated capacity. The voltage of a fully charged battery corresponds to the charge cutoff voltage. The voltage of a fully discharged battery corresponds to the discharge cutoff voltage.

[0036] (2) SEM image capture of cross-section

[0037] Next, a cross-sectional sample of the positive electrode (positive electrode active material layer) is observed using an SEM. Observation using the SEM is performed, for example, at a magnification of 500 to 3000 times. A cross-sectional SEM image is captured such that a region having a length of 30 μm or greater (preferably 40 μm or greater) in the plane direction of the positive electrode active material layer is observed.

[0038] (3) Image analysis

[0039] For each of 100 or more randomly selected lithium metal composite oxide (Fm) particles (particle cross-sections) in a cross-sectional SEM image, the ratio "maximum diameter Dmax / maximum diameter dmax" was measured as the aspect ratio. The average aspect ratio was calculated by taking the arithmetic average of the 100 or more obtained aspect ratios.

[0040] Image analysis software (eg, ImageJ) may be used for image analysis. In image analysis, the cross-sectional SEM image may be binarized so that particles of the lithium metal composite oxide (Fm) appear black (or white) and the rest appear white (or black).

[0041] In image analysis, elemental analysis using EDX (energy dispersive X-ray spectrometry) or EPMA (electron probe microanalyzer) can also be performed on the cross-sectional SEM image. Furthermore, a mapping image of the lithium metal composite oxide (Fm) can be obtained based on the analysis data. Using this mapping image, the "maximum diameter Dmax / maximum diameter dmax" can be measured as the aspect ratio for each of 100 or more randomly selected lithium metal composite oxide (Fm) particles (particle cross sections).

[0042] (Average particle size Dave (D50))

[0043] The average particle size Dave (D50 (median diameter) in the cumulative particle size distribution based on the number of particles) of the lithium metal composite oxide (Fm) can be obtained from the cumulative particle size distribution based on the number of particles (particle cross sections) of the lithium metal composite oxide (Fm) selected arbitrarily from the cross-sectional SEM image. First, the area of ​​each particle (particle cross section) is measured, and the diameter Dd of an equivalent sphere having the measured area (Sd) as the average cross-sectional area of ​​the sphere is obtained (Dd = 8√(Sd / π 3 )) and calculate the cumulative particle size distribution based on the number of these diameters (Dd). D50 (median diameter) is the diameter of the equivalent sphere when the cumulative number of equivalent spheres reaches 50%, starting from the smallest diameter value. D50 can be selected based on the electrode design. The average particle size D50 can be, for example, greater than 0.22 μm, 0.23 μm or greater, or 0.24 μm or greater.

[0044] Similarly, by accumulating from the smallest diameter value of the equivalent sphere, the diameter D10 of the equivalent sphere when the cumulative number reaches 10% and the diameter D90 of the equivalent sphere when the cumulative number reaches 90% are calculated, thereby calculating the "span value" defined by (D90-D10) / D50.

[0045] The span value in the number-based cumulative particle size distribution of the lithium metal composite oxide may satisfy, for example, 1.2 < (D90-D10) / D50, 1.4 < (D90-D10) / D50, or 1.5 < (D90-D10) / D50. The upper limit of the span value is not particularly limited, and (D90-D10) / D50 ≤ 8.4 may also be satisfied. The span value may satisfy 1.6 ≤ (D90-D10) / D50, 1.7 ≤ (D90-D10) / D50, or 1.8 ≤ (D90-D10) / D50.

[0046] D90 is greater than 0.46 μm, and may be 0.47 μm or greater, or 0.5 μm or greater. Such a lithium metal composite oxide (Fm) having a relatively large particle size can be obtained by pulverizing the lithium metal composite oxide (Fm) having a high hardness obtained by calcining the raw material mixture under appropriate conditions. Furthermore, the presence of particles having a large particle size facilitates increasing the electrode capacity.

[0047] Furthermore, because the lithium metal composite oxide (Fm) is highly hard and robust, the number-based cumulative particle size distribution obtained from 100 or more lithium metal composite oxide (Fm) particles (particle cross sections) randomly selected from a cross-sectional SEM image yields the same result as the number-based cumulative particle size distribution obtained from the lithium metal composite oxide (Fm) raw material powder for the electrode or the lithium metal composite oxide (Fm) powder separated and recovered from the positive electrode by decomposing a manufactured battery. The number-based cumulative particle size distribution of the raw material powder or the powder separated and recovered from the positive electrode can be obtained, for example, by converting the volume-based cumulative particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer.

[0048] (roundness)

[0049] Alternatively, the average value of the roundness of the particles can be controlled to replace the average value of the aspect ratio of the particles. Specifically, when the average value of the roundness of the particles is more than 0.55, the capacity is significantly improved. By controlling the average value of the roundness of the particles to more than 0.55, the particle shape becomes uniform, and the difference in the acceptability of the part that is easy to accept charge and discharge and the part that is difficult to accept charge and discharge becomes smaller. As a result, it is believed that the charge and discharge reaction is carried out uniformly, polarization also becomes smaller, and the discharge capacity improves.

[0050] Furthermore, when the particles are true spheres, the roundness is 1. Therefore, it can be assumed that the higher the roundness, the closer the particles are to true spheres. When the average roundness is set to 0.55 or higher, the particles are significantly closer to true spheres, the distances between particles within the electrode become more uniform, and the tortuosity of the electron conduction path is reduced, further contributing to increased capacity.

[0051] The average value of the circularity of the particles may be 0.55 or more, preferably 0.60 or more, more preferably 0.65 or more, and even more preferably 0.67 or more.

[0052] Here, circularity refers to the value obtained by dividing the area Sn of the observed particle by the area Sc of the circumscribed circle of the particle (Sn / Sc). Specifically, if Dmax, which will be described later, is used, circularity is calculated by "circularity = 4×Sn / π(Dmax) 2 "To calculate.

[0053] The roundness of the lithium metal composite oxide (Fm) can be measured using particles of the lithium metal composite oxide (Fm) as a raw material powder before becoming a positive electrode, or it can be measured in the positive electrode. Since the lithium metal composite oxide (Fm) having a rock salt type crystal structure is strong, the same measurement results can be obtained regardless of which measurement method is used. The roundness in the positive electrode is measured using the particles of the lithium metal composite oxide (Fm) observed in the above-mentioned cross-sectional SEM image. The particles observed in the cross-sectional SEM image of the positive electrode can also be the cross section of the particles. The roundness of the raw material powder is measured using the particles of the lithium metal composite oxide (Fm) observed in the SEM image of the raw material powder (hereinafter referred to as "powder SEM image").

[0054] The circularity ratio "Sn / Sc" is measured for each of 100 or more randomly selected lithium metal composite oxide (Fm) particles from a cross-sectional SEM image or a powder SEM image. The average Sn / Sc value is calculated by taking the arithmetic average of the obtained 100 or more Sn / Sc values.

[0055] The particles of lithium metal composite oxide (Fm) may contain impurity elements. For example, impurity elements may be mixed into the particles when crushing blocky particles with high hardness. When crushing blocky particles, a stirring device such as a bead mill or a ball mill is used that can apply a large shear force to the crushed object. In such a stirring device, a medium such as balls and beads is used. Since such a medium is composed of aluminum oxide, zirconium oxide, etc., the impurity elements may include, for example, Al, Zr, W, Fe, Cr, Ni, etc. However, the impurity elements tend to be present in the surface layer of the particles.

[0056] Here, "partial presence" means that the impurity element is present at a high concentration of CH within a depth of 5% of the particle diameter from the particle surface, while the impurity element is present or absent at a low concentration of CL (CH > CL) within a circular region with a radius within 5% of the particle diameter from the particle's center of gravity. In this case, CH ≥ 2CL or CH ≥ 5CL can be satisfied.

[0057] The presence of "predominant" impurity elements can also be confirmed by performing elemental analysis using EDX or EPMA on the cross-sectional SEM images described above. A mapping image of the impurity elements within the lithium metal composite oxide (Fm) particles can also be obtained based on the analysis data. Using this mapping image, each of ten or more randomly selected lithium metal composite oxide (Fm) particles is analyzed. If CH > CL, CH ≥ 2CL, or CH ≥ 5CL is satisfied in at least 80% of the particles, it can be determined that the impurity elements are preferentially present in the surface layer of the particles.

[0058] (Specific surface area)

[0059] The specific surface area of ​​the lithium metal composite oxide (Fm) is, for example, less than 21.9 m 2 / g, which can be 21m 2 / g or less, can be less than 13.2m 2 / g, can be 13m 2 / g or less, can be 11m 2 / g or less, or 10m 2 The specific surface area of ​​the lithium metal composite oxide (Fm) synthesized at the laboratory level by applying high shear force to the raw material mixture using a stirring device such as a ball mill becomes very large.

[0060] On the other hand, the lithium metal composite oxide (Fm) synthesized by calcination does not have a process caused by high shear force such as that caused by solid phase reaction, even if it is assumed that the particles or powders are obtained by crushing the block particles. Therefore, the specific surface area of ​​the lithium metal composite oxide (Fm) synthesized by calcination is, for example, 0.13 m 2 / g or more and less than 21.9m 2 / g, or 0.13m 2 / g or more and less than 13.2m 2 / g, easily becomes 6m 2 / g~12m 2 / g or 6m 2 / g~10m 2When the specific surface area is within the above range, the filling rate of the lithium metal composite oxide (Fm) in the positive electrode can be increased, and it is advantageous in terms of easily suppressing side reactions. If the specific surface area is too large, the surface state becomes unstable, and it may be difficult to suppress side reactions.

[0061] The specific surface area can be measured using the lithium metal composite oxide (Fm) as a raw material powder before being used as a positive electrode, or using the lithium metal composite oxide (Fm) separated from the positive electrode. In either method, substantially the same measurement results can be obtained.

[0062] Here, the specific surface area of ​​the lithium metal composite oxide (Fm) is measured by preparing a 0.20g to 0.25g sample of the lithium metal composite oxide (Fm), placing the sample in a measuring cell consisting of a glass tube for specific surface area measurement, and drying and degassing the interior of the measuring cell. Drying and degassing are performed at a pressure of 6.67 Pa and a temperature of 250°C ± 5°C for at least one hour. The mass of the sample in the measuring cell is then measured to the order of 0.1mg. The nitrogen adsorption of the sample at a temperature of -196°C is then measured using a specific surface area measuring apparatus. For example, the measuring apparatus used is the "TriStar II 3020," an automatic specific surface area / pore size distribution measuring apparatus manufactured by Shimadzu Corporation. Based on the adsorption measurement results, 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.

[0063] The lithium metal composite oxide (Fm) may contain an electropositive element M different from Li and Mn. Such an element M can be any electropositive element other than hydrogen, and 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 metals.

[0064] The element M may be, for example, at least one selected from the group consisting of Ti, Fe, Ge, Si, Ga, Ni, Co, Sn, Cu, Nb, Mo, Bi, V, Cr, Y, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Gd, Sm, Eu, Yb, Dy, Al, and Er. The lithium metal composite oxide (Fm) may contain any two or more elements selected from the above as the element M.

[0065] Even when the lithium metal composite oxide (Fm) contains the element M, the main component of the metal element other than Li is preferably Mn. The number of atoms b occupied by Mn in the lithium metal composite oxide (Fm) may be the largest among the numbers of atoms occupied by metals other than Li in the lithium metal composite oxide (Fm). The number of atoms of Mn may also be larger than the total number of atoms c of the metal elements other than Li and Mn. The ratio (b / c) of the number of atoms b of Mn in the lithium metal composite oxide (Fm) to the number of atoms c of the metal elements other than Li and Mn is, for example, greater than 1 and less than 15, preferably greater than 1 and less than 12, may be greater than 1 and less than 12, or may be greater than 2 and less than 12.

[0066] The lithium metal composite oxide (Fm) preferably contains at least Ti as the element M. The lithium metal composite oxide (Fm) containing Ti as the element M can exhibit a particularly high capacity. The reason for this is not entirely clear, but one reason is believed to be that in the lithium metal composite oxide, Ti can be used as a Ti with d orbital vacancy. 4+ In this case, it is believed that a rock salt type crystal structure with high symmetry, stability and high capacity is formed. In addition, it is believed that such a rock salt structure is not likely to become unstable even after repeated charge and discharge.

[0067] When the lithium metal composite oxide (Fm) contains Ti, the ratio of the number of Mn atoms to the number of Ti atoms in the lithium metal composite oxide: Mn / Ti may be 4 or more, 5 or more, 6 or more, and preferably 7 or more. In addition, Mn / Ti may be 70 or less, 30 or less, and preferably 15 or less.

[0068] The lithium metal composite oxide (Fm) may also be a fluoride oxide containing F. In the above-mentioned crystal structure, fluorine can replace the oxygen atom at the anion site. Thus, even when the lithium metal composite oxide (Fm) is in a Li excess state, the crystal structure becomes stable and a higher capacity can be obtained. In addition, the average discharge potential increases by the replacement of fluorine atoms. Furthermore, the Li excess state refers to a state in which the number of Li atoms in the lithium metal composite oxide (Fm) is greater than the total number of atoms of metal elements other than Li.

[0069] In lithium-excess lithium metal composite oxides (Fm), the configuration of Li at the cation sites is disordered, and the Li binding states are diverse. Consequently, the voltage distribution associated with Li release has a wide range. Consequently, it is difficult to utilize the lower end of the voltage distribution at low potentials as capacity. However, by introducing fluorine atoms, the voltage distribution associated with Li release shifts toward the higher potential side, making it easier to utilize the lower end as capacity. This further increases the usable capacity.

[0070] The lithium metal composite oxide (Fm) as an oxyfluoride can be represented by the compositional formula Li a Mn b M c O d F e . Here, 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 are satisfied.

[0071] The lithium metal composite oxide (Fm) can be represented by the compositional formula Li a Mn b Ti c M d O e F f . Here, 1 ≤ a ≤ 1.4, 0.3 ≤ b ≤ 0.9, 0 < c ≤ 0.5, 0 < d ≤ 0.25, 0 ≤ f ≤ 0.7, and 1.7 ≤ e + f ≤ 2 are satisfied. Alternatively, 1 ≤ a ≤ 1.4, 0.5 ≤ b ≤ 0.9, 0.02 < c ≤ 0.4, 0 ≤ f ≤ 0.67, and 1.7 ≤ e + f ≤ 2.2 are satisfied. In most cases, e + f is 2 or less, can be 1.94 or less, can be 1.9 or less, or can also be 1.8 or less.

[0072] As shown in the above compositional formula, a part of the oxygen atoms at the anion sites can also be replaced by fluorine atoms. Thereby, the state of Li excess (a > 1) becomes stable, and a high capacity can be obtained. In addition, as described above, the average discharge potential increases, and the available capacity further increases.

[0073] The lithium metal composite oxide (Fm) without fluorine can be represented by the compositional formula Li a Mn b M c O d . Here, 1 ≤ a < 1.4, 0.5 ≤ b < 0.75, 0 ≤ c ≤ 0.35, and 1.6 ≤ d ≤ 2 are satisfied.

[0074] Furthermore, the content rates of the elements constituting the lithium metal composite oxide (Fm) can be measured by an inductively coupled plasma optical emission spectrometry device (ICP - AES), an electron probe microanalyzer (EPMA), an energy - dispersive X - ray analyzer (EDX), or the like.

[0075] <Method for manufacturing lithium metal composite oxide (Fm)>

[0076] The method for producing the lithium metal composite oxide (Fm) is not limited, but it is preferably obtained by calcining a raw material mixture of elements constituting the lithium metal composite oxide (Fm). Calcination promotes the growth of crystals similar to a rock salt structure belonging to the space group Fm-3m, resulting in a lithium metal composite oxide with a large crystallite size. Subsequently, pulverization can be performed to control the crystallite size within a desired range.

[0077] As raw materials of elements constituting the lithium metal composite oxide, any of Mn compounds, compounds of element M, lithium compounds, fluorine compounds, titanium compounds, etc. can be selected and used. The types and mixing ratios of the raw materials can be appropriately selected according to the desired composition described above.

[0078] Examples of Mn compounds include Mn oxides such as MnO2 and Mn2O3, and manganese salts such as lithium manganate. Examples of compounds of element M include M salts such as oxides, fluoride oxides, and hydroxides. Examples of lithium compounds include lithium oxides such as Li2O, lithium salts such as LiOH, and lithium manganate (LiMnO2). Examples of fluorine compounds include fluoride salts such as lithium fluoride (LiF). Examples of titanium compounds include titanium oxides such as TiO2 and titanium salts such as lithium titanate.

[0079] The firing atmosphere of the raw material mixture may vary depending on the desired composition of the lithium metal composite oxide (Fm) and the type of raw materials, and may be, for example, an oxidizing atmosphere (eg, in air or in the presence of oxygen). Preferably, the gas in the atmosphere is allowed to flow.

[0080] The calcination 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 materials, and may be, for example, 700° C. or higher, preferably 900° C. or higher and 1300° C. or lower.

[0081] When the lithium metal composite oxide (Fm) obtained by calcination is in the form of bulk particles, the bulk particles may be crushed to obtain a desired aspect ratio or circularity. In this case, a stirring device such as a ball mill or a bead mill capable of applying a large shear force to the particles may be used.

[0082] There are many methods for controlling the shape of particles, and they are not limited to methods for controlling the shape of particles by crushing bulk particles. For example, the raw material mixture can be stirred while the raw material mixture is calcined. For example, a calcining furnace equipped with a fluidized bed can be used to stir the raw material mixture while the raw material mixture is calcined. In addition, as part of the raw material, particles close to spherical can also be used. For example, manganese compounds or manganese-titanium composite compounds close to spherical can also be used as raw materials.

[0083] Next, the secondary battery according to the embodiment of the present disclosure will be described in detail. The secondary battery includes, for example, the following positive electrode, negative electrode, electrolyte, and separator.

[0084] [positive electrode]

[0085] The positive electrode comprises 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. The positive electrode used in the secondary battery described above is used as the positive electrode. The positive electrode mixture layer can be formed, for example, by dispersing a positive electrode mixture containing a positive electrode active material, a binder, etc. in a dispersion medium to obtain a positive electrode slurry, applying the obtained positive electrode slurry to the surface of the positive electrode current collector, and drying the resulting slurry. The dried coating can also be rolled as needed. The positive electrode mixture layer can be formed on one surface of the positive electrode current collector or on both surfaces.

[0086] The positive electrode mixture layer contains a positive electrode active material as an essential component, and may contain a binder, a thickener, a conductive agent, a positive electrode additive, etc. as optional components. As the binder, thickener, and conductive agent, known materials can be used.

[0087] The positive electrode active material includes the lithium metal composite oxide (Fm) having a crystal structure similar to a rock salt structure belonging to the space group Fm-3m. The lithium metal composite oxide (Fm) is, for example, a secondary particle formed by agglomeration of multiple primary particles. The primary particles generally have a particle size of 0.01 μm to 1 μm.

[0088] Lithium metal composite oxide (Fm) can also be mixed with other known lithium metal oxides and used as a positive electrode active material. As other known lithium metal oxides, for example, 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 bLithium transition metal composite oxides such as LiO4, LiMePO4, and Li2MePO4F. Here, M is at least one selected from Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. Me contains at least one transition element (e.g., at least one selected from Mn, Fe, Co, and Ni). Wherein, 0 ≤ a ≤ 1.2, 0 ≤ b ≤ 0.9, and 2.0 ≤ c ≤ 2.3. Furthermore, the value a, which represents the molar ratio of lithium, increases or decreases with charge and discharge.

[0089] The shape and thickness of the positive electrode current collector can be selected from the shape and range corresponding to the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0090] [negative electrode]

[0091] The negative electrode may also 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 dispersing a negative electrode mixture containing a negative electrode active material, a binder, etc. in a dispersion medium to obtain a negative electrode slurry, applying the obtained negative electrode slurry to the surface of the negative electrode current collector, and drying the mixture. The dried coating may also be rolled as needed. In other words, the negative electrode active material may also be a mixture layer. Alternatively, lithium metal foil or lithium alloy foil may be attached to the negative electrode current collector. The negative electrode active material layer may be formed on one surface of the negative electrode current collector or on both surfaces.

[0092] The negative electrode active material layer contains a negative electrode active material as an essential component, and may contain a binder, a conductive agent, a thickener, etc. as optional components. As the binder, the conductive agent, and the thickener, known materials can be used.

[0093] As the negative electrode active material, materials that electrochemically absorb and release lithium ions, lithium metal, lithium alloys, etc. can be listed. As materials that electrochemically absorb and release lithium ions, carbon materials, alloy materials, etc. can be used. As carbon materials, for example, graphite, easily graphitized carbon (soft carbon), difficultly graphitized carbon (hard carbon), etc. can be exemplified. Among them, graphite with excellent charge and discharge stability and low irreversible capacity is preferred. As alloy materials, materials containing at least one metal that can form an alloy with lithium can be listed. For example, a composite material obtained by dispersing a lithium ion conductive phase and a silicon phase in the lithium ion conductive phase can also be used.

[0094] As the negative electrode current collector, a non-porous conductive substrate (metal foil, etc.) or a porous conductive substrate (mesh, net, punched plate, etc.) is used. Examples of the material of the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0095] [Electrolytes]

[0096] The electrolyte may be a liquid electrolyte (electrolyte), a gel electrolyte, or a solid electrolyte. The liquid electrolyte is, for example, an electrolyte solution comprising a non-aqueous solvent and a salt dissolved in the non-aqueous solvent. The salt concentration in the electrolyte solution is, for example, not less than 0.5 mol / L and not more than 2 mol / L. The electrolyte solution may also contain known additives.

[0097] Gel electrolytes contain 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 gels by absorbing the non-aqueous solvent. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, and polyethylene oxide.

[0098] As the solid electrolyte, for example, a material known in all-solid-state lithium-ion secondary batteries and the like (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halide-based solid electrolyte, etc.) is used.

[0099] For example, a liquid nonaqueous electrolyte is prepared by dissolving a salt in a nonaqueous solvent. The salt is an electrolyte salt that dissociates ions within the electrolyte, and may include, for example, a lithium salt. The electrolyte may contain various additives. While the electrolyte is typically used directly in a liquid state, it may also be in a state where its fluidity is restricted by, for example, a gelling agent.

[0100] As the non-aqueous solvent, for example, cyclic carbonates, chain carbonates, cyclic carboxylates, chain carboxylates etc. can be used. As cyclic carbonates, propylene carbonate (PC), ethylene carbonate (EC), vinylene carbonate (VC) etc. can be listed. As chain carbonates, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) etc. can be listed. In addition, as cyclic carboxylates, gamma-butyrolactone (GBL), gamma-valerolactone (GVL) etc. can be listed. As chain carboxylates, methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate (EP) etc. can be listed. The non-aqueous solvent can be used alone or in combination of two or more.

[0101] As lithium salts, for example, lithium salts containing chloric acid (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorinated acids (LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorinated imides (LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.). The lithium salts may be used alone or in combination of two or more.

[0102] The lithium salt concentration in the electrolyte can be from 1 mol / liter to 2 mol / liter, or from 1 mol / liter to 1.5 mol / liter. By controlling the lithium salt concentration within this range, an electrolyte with excellent ion conductivity and moderate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0103] [Diaphragm]

[0104] The separator is located between the positive and negative electrodes. It has high ion permeability and possesses appropriate mechanical strength and insulation properties. Microporous films, woven fabrics, and nonwoven fabrics can be used as separators. Polyolefins such as polypropylene and polyethylene are preferred materials for separators.

[0105] As an example of the structure of a secondary battery, an electrode group in which a positive electrode and a negative electrode are wound with a separator and a non-aqueous electrolyte is housed in an outer body can be cited. Alternatively, instead of a wound electrode group, another electrode group such as a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator can be used. The secondary battery can also be in any form, such as a cylindrical, square, coin-shaped, button-shaped, or laminated type.

[0106] Figure 1 This is a partially cutaway schematic perspective view of a prismatic secondary battery according to one embodiment of the present disclosure.

[0107] The battery comprises: a square battery case 4 with a bottom, an electrode group 1 housed in the battery case 4, and a non-aqueous electrolyte (not shown). The electrode group 1 has a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator between them. The negative electrode collector of the negative electrode is electrically connected to the negative terminal 6 provided on the sealing plate 5 via the negative electrode lead 3. The negative terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode collector of the positive electrode is electrically connected to the back of the sealing plate 5 via the positive electrode lead 2. That is, the positive electrode is electrically connected to the battery case 4 which also serves as the positive terminal. The periphery of the sealing plate 5 is fitted into the open end of the battery case 4, and the fitting portion is laser welded. There is an injection hole for the non-aqueous electrolyte in the sealing plate 5, which is blocked by the sealing plug 8 after the injection.

[0108] Furthermore, the structure of the secondary battery can be cylindrical, coin-shaped, button-shaped, etc., with a metal battery case, or a laminated battery with a battery case made of a laminate sheet serving as a laminate of a barrier layer and a resin sheet. In the present disclosure, the type and shape of the secondary battery are not particularly limited.

[0109] (Note)

[0110] Based on the above description, the following technology is disclosed.

[0111] (Technique 1)

[0112] A positive electrode active material for a secondary battery, comprising a lithium metal composite oxide having a rock salt type crystal structure that can be assigned to the space group Fm-3m,

[0113] The lithium metal composite oxide contains at least Li and Mn,

[0114] The average aspect ratio of the particles of the lithium metal composite oxide is less than 2.10.

[0115] (Technique 2)

[0116] According to the positive electrode active material for secondary batteries described in technology 1,

[0117] The lithium metal composite oxide contains an electropositive element M different from Li and Mn.

[0118] The M comprises at least one selected from the group consisting of Ti, Fe, Ge, Si, Ga, Ni, Co, Sn, Cu, Nb, Mo, Bi, V, Cr, Y, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Gd, Sm, Eu, Yb, Dy, Al and Er.

[0119] (Technique 3)

[0120] According to the secondary battery positive electrode active material of technique 2, the lithium metal composite oxide contains at least Ti as the M.

[0121] (Technique 4)

[0122] According to any one of techniques 1 to 3, the positive electrode active material for a secondary battery, wherein the lithium metal composite oxide is a fluoride oxide containing F.

[0123] (Technique 5)

[0124] The positive electrode active material for a secondary battery according to any one of techniques 1 to 4,

[0125] The lithium metal composite oxide has a composition formula of Li a Mn b M c O d F e express,

[0126] Satisfies 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.

[0127] (Technique 6)

[0128] The positive electrode active material for a secondary battery according to any one of techniques 1 to 5,

[0129] The lithium metal composite oxide has a composition formula of Li a Mn b M c O d express,

[0130] Satisfying 1≤a<1.4, 0.5≤b<0.75, 0≤c≤0.4, 1.6≤d≤2.

[0131] (Technique 7)

[0132] The secondary battery positive electrode active material according to any one of techniques 1 to 6, wherein D90 in the number-based cumulative particle size distribution of the lithium metal composite oxide is larger than 0.46 μm.

[0133] (Technique 8)

[0134] According to any one of techniques 1 to 7, the positive electrode active material for a secondary battery, wherein D50 in the number-based cumulative particle size distribution of the lithium metal composite oxide is 0.26 μm or less.

[0135] (Technique 9)

[0136] In the secondary battery positive electrode active material according to any one of techniques 1 to 8, (D90-D10) / D50 in the number-based cumulative particle size distribution of the lithium metal composite oxide satisfies 1.2<(D90-D10) / D50.

[0137] (Technique 10)

[0138] According to any one of the techniques 1 to 9, the positive electrode active material for a secondary battery, wherein the specific surface area of ​​the lithium metal composite oxide is less than 21.9 m 2 / g.

[0139] (Technology 11)

[0140] The positive electrode active material for a secondary battery according to any one of techniques 1 to 10,

[0141] The particles of the lithium metal composite oxide contain impurity elements,

[0142] The impurity elements are preferentially present in the surface layer of the particles.

[0143] (Technology 12)

[0144] The positive electrode active material for a secondary battery according to any one of techniques 1 to 11,

[0145] It is obtained by calcining a raw material mixture containing a lithium compound and a manganese compound at 700°C or above.

[0146] (Technology 13)

[0147] A secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive electrode and the negative electrode.

[0148] The positive electrode contains the positive electrode active material for a secondary battery according to any one of claims 1 to 12.

[0149] 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.

[0150] <Example>

[0151] [Manufacture of Positive Electrode]

[0152] A raw material mixture containing manganese oxide (Mn2O3), lithium carbonate (Li2CO3), and titanium oxide (TiO2) was fired at 950 °C for 10 hours to obtain bulk particles of a lithium-excess lithium metal composite oxide (Fm) satisfying Li a Mn b M c O d (1 < a < 1.4, 0.5 ≤ b < 0.75, 0.1 < c ≤ 0.4, and 1.6 ≤ d ≤ 2). The composition was determined by analyzing the lithium metal composite oxide (Fm) using ICP emission spectrometry.

[0153] Using a planetary ball mill, the obtained bulk particles of the lithium metal composite oxide (Fm) were pulverized under various conditions to obtain lithium metal composite oxides (Fm) having various aspect ratios. Taking the case of one example of the lithium metal composite oxide (Fm), the bulk particles of the lithium metal composite oxide (Fm) were put into a planetary ball mill (Premium-Line P7 manufactured by Fritsch, rotation speed: 150 to 300 rpm, container: 45 mL, balls: ZrO2 balls with a diameter of φ3 mm), and treated at room temperature for 12 hours or less in an air atmosphere. The BET specific surface areas of the lithium metal composite oxide (Fm) measured by the above method were all in the range of 7.2 m 2 / g to [8.8 m should be corrected to 8.8 m 2 / g. The D50 was 0.23 μm to 0.25 μm, the D90 was 0.49 μm to 0.56 μm, and the span value was 1.6 to 1.9.

[0154] The pulverized lithium metal composite oxide (Fm) was subjected to measurement and analysis of an X-ray diffraction pattern. From the number and positions of the XRD peaks, it was confirmed that it was a rock salt-type crystal structure belonging to the space group Fm-3m.

[0155] It should be noted that there is a small error in the original text where "8.8m " seems incomplete. It should probably be "8.8 m / g" as in the context. The translation has been adjusted accordingly.The obtained lithium metal composite oxide (Fm), acetylene black, and polyvinylidene fluoride were mixed at a solid content mass ratio of 7:2:1, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode slurry. The positive electrode slurry was then applied to a positive electrode current collector made of aluminum foil. The coated film was dried, compressed, and then cut into the desired electrode size to obtain a positive electrode. The average aspect ratio was then calculated using the above method based on SEM images of the positive electrode cross-sections.

[0156] [Preparation of electrolyte]

[0157] A non-aqueous electrolyte was prepared by adding LiPF6 as a lithium salt to a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) at a predetermined volume ratio.

[0158] [Preparation of test unit]

[0159] A test cell was fabricated using the aforementioned positive electrode and a negative counter electrode composed of lithium metal foil. The positive and negative counter electrodes were placed facing each other with a separator interposed therebetween to form an electrode assembly, which was then housed in a coin-shaped outer can. After injecting electrolyte into the outer can, the can was sealed to produce a coin-shaped secondary battery for testing.

[0160] [evaluate]

[0161] (Discharge capacity)

[0162] The secondary battery was charged at a constant current of 0.1C at room temperature until the battery voltage reached 4.95V. Thereafter, constant voltage charging was performed at a constant voltage of 4.95V until the current reached 0.01C. Afterwards, the battery was discharging at a constant current of 0.1C for 20 minutes until the battery voltage reached 2.5V, and the discharge capacity was measured. The relationship between the discharge capacity (mAh / g) per unit mass of the lithium metal composite oxide (Fm) and the average aspect ratio (average aspect ratio) is shown in FIG. Figure 2 .

[0163] Figure 2 It is shown that the discharge capacity is significantly improved by reducing the average value of the aspect ratio of the lithium metal composite oxide (Fm) to less than 2.1 (particularly 1.94 or less).

[0164] <Reference Example 1>

[0165] In the production of the positive electrode, a raw material mixture containing manganese oxide (Mn2O3), lithium carbonate (Li2CO3) and titanium oxide (TiO2) was put into the same planetary ball mill as above and treated at room temperature in an Ar atmosphere for 35 hours (the cycle of stopping for 10 minutes after running for 1 hour was set to 35 times), thereby obtaining a lithium-excess lithium metal composite oxide (Fm) with the same composition as the embodiment. The obtained lithium metal composite oxide (Fm) was measured and analyzed by X-ray diffraction pattern, and the results confirmed that it was a rock salt type crystal structure that can be attributed to the space group Fm-3m. D50 is 0.22μm, D90 is 0.46μm, the span value is 1.2, and the BET specific surface area is 13.2m 2 It is understood that the differences in physical properties from the examples are due to differences in the manufacturing methods.

[0166] Industrial applicability

[0167] The secondary battery cathode active material disclosed herein can provide a secondary battery having a high energy density. The secondary battery disclosed herein is useful as a main power source for mobile communication devices, portable electronic devices, and the like.

[0168] Although the present invention has been described in terms of its presently preferred embodiments, this disclosure should not be construed as limiting. Various modifications and variations will undoubtedly become apparent to those skilled in the art upon reading the above disclosure. Therefore, the appended claims should be construed to include all modifications and variations that do not depart from the true spirit and scope of the present invention.

[0169] Description of Reference Numerals

[0170] 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. A positive electrode active material for a secondary battery, comprising a lithium metal composite oxide having a rock salt type crystal structure that can be assigned to the space group Fm-3m, The lithium metal composite oxide contains at least Li and Mn, The average aspect ratio of the particles of the lithium metal composite oxide is less than 2.

10.

2. The positive electrode active material for a secondary battery according to claim 1, The lithium metal composite oxide contains an electropositive element M different from Li and Mn. The M comprises at least one selected from the group consisting of Ti, Fe, Ge, Si, Ga, Ni, Co, Sn, Cu, Nb, Mo, Bi, V, Cr, Y, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Gd, Sm, Eu, Yb, Dy, Al and Er. 3 . The positive electrode active material for a secondary battery according to claim 2 , wherein the lithium metal composite oxide contains at least Ti as the M. 4 . The positive electrode active material for a secondary battery according to claim 1 , wherein the lithium metal composite oxide is a fluoride oxide containing F.

5. The positive electrode active material for a secondary battery according to any one of claims 1 to 4, The lithium metal composite oxide has a composition formula of Li a Mn b M c O d F e express, Satisfies 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. The positive electrode active material for a secondary battery according to any one of claims 1 to 4, The lithium metal composite oxide has a composition formula of Li a Mn b M c O d express, Satisfying 1≤a<1.4, 0.5≤b<0.75, 0≤c≤0.4, 1.6≤d≤2. 7 . The positive electrode active material for a secondary battery according to claim 1 , wherein D90 in the number-based cumulative particle size distribution of the lithium metal composite oxide is larger than 0.46 μm. 8 . The positive electrode active material for a secondary battery according to claim 1 , wherein D50 in the number-based cumulative particle size distribution of the lithium metal composite oxide is 0.26 μm or less. 9 . The secondary battery positive electrode active material according to claim 1 , wherein (D90−D10) / D50 in the number-based cumulative particle size distribution of the lithium metal composite oxide satisfies 1.2<(D90−D10) / D50.

10. The positive electrode active material for a secondary battery according to any one of claims 1 to 4, wherein the specific surface area of ​​the lithium metal composite oxide is less than 21.9 m 2 / g.

11. The positive electrode active material for a secondary battery according to any one of claims 1 to 4, The particles of the lithium metal composite oxide contain impurity elements, The impurity elements are preferentially present in the surface layer of the particles. 12 . The positive electrode active material for a secondary battery according to claim 1 , which is obtained by calcining a raw material mixture containing a lithium compound and a manganese compound at 700° C. or higher.

13. A secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode. The positive electrode comprises the positive electrode active material for a secondary battery according to any one of claims 1 to 4.

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