JP2024005520A - Positive electrode active material for lithium ion secondary battery and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material containing Li₂RuO₃ as Li transition metal composite oxide, which can achieve a high capacity and which has a high-level cycle characteristic.

SOLUTION: The present invention relates to a positive electrode active material for a Li ion secondary battery, which comprises an Li₂RuO₃ composite oxide having a layered rock salt-type crystal structure. The Li₂RuO₃ composite oxide to which the invention is applied has the following two requirements: (i) a diffraction peak of (003)-plane of an ilmenite type structure arises in an X-ray diffraction pattern after an initial full-charge operation; and (ii) after an initial full-charge operation, Ru ions of 20% up to 50% of Ru ions involved in the Li₂RuO₃ composite oxide transfer to Li ion sites. Suitable transfer of the Ru ions contributes to keeping of an interlayer distance allows a Li ion secondary battery to exhibit a high-level cycle characteristic. The Ru ion transfer is revealed from crystal structure analysis on X-ray diffraction patterns in respective stages in an initial charge process.

SELECTED DRAWING: Figure 10

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present invention relates to a positive electrode active material for Li-ion secondary batteries and a method for manufacturing the same. Specifically, the present invention relates to a positive electrode active material containing Li ₂ RuO ₃ which is a Li-excessive transition metal composite oxide, and which has high recyclability and high durability.

Li (lithium) ion secondary batteries have higher energy density than secondary batteries such as nickel-hydrogen storage batteries and nickel-cadmium storage batteries, and can be easily made smaller and lighter. For this reason, the scope of use of Li-ion secondary batteries is expanding, including small batteries used in portable electronic devices, and in-vehicle batteries such as hybrid vehicles (HV, PHV) and electric vehicles (EV). There is.

One of the factors that influences the battery characteristics of Li-ion secondary batteries is the positive electrode active material that is responsible for the electrochemical reaction of the positive electrode. As a positive electrode active material contained in a positive electrode of a Li-ion secondary battery, a Li transition metal composite oxide represented by LiMeO ₂ (Me: a metal element such as Co, Ni, Mn, etc.) has been the mainstream so far. In recent years, there has been a demand for smaller and lighter Li-ion secondary batteries for portable electronic devices and vehicles, as well as for increased discharge capacity. The use of Li transition metal composite oxides such as _{2MnO 3} -LiMO ₂ (M: a metal element such as Co, Ni, Mn, etc.) is being considered (Patent Document 1, etc.). In order to further increase the discharge capacity of Li-ion secondary batteries, improvements in the transition metals constituting Li-transition metal composite oxides have also been proposed. Note that such a Li transition metal composite oxide having a high Li content is sometimes referred to as a "Li-excess type".

As one of the Li-excessive transition metal composite oxides, a Li ₂ RuO ₃ composite oxide containing Ru (ruthenium) as a constituent element is considered to be promising. Ru is a metal element with high electronic conductivity, and causes reversible oxygen anion redox through electron transfer. Since this anion redox acts as charge compensation, it cooperates with charge compensation due to a change in the valence of Ru ions (Ru ⁴⁺ →Ru ⁵⁺ ) and contributes to high capacity as a positive electrode active material.

Furthermore, the advantageous characteristics of the Li ₂ RuO ₃ composite oxide include high cycle performance in which capacity deterioration due to increased charge/discharge cycles is suppressed. A charge/discharge cycle of a positive electrode active material made of a Li transition metal composite oxide is achieved by repeating desorption and insertion of Li ions from the composite oxide. However, during the desorption of Li ions during charging, the crystal structure may collapse due to the desorption of oxygen, resulting in a decrease in battery performance. This collapse of the crystal structure leads to a decrease in cycleability. Ru clearly has stronger covalent bonding properties than Ni, Co, and the like. Therefore, in the Li ₂ RuO ₃ composite oxide, oxygen desorption is difficult to proceed due to the strong covalent bond between Ru ions and oxide ions. This makes it difficult for oxygen desorption, which occurs in Li transition metal composite oxides composed of Mn and the like, to occur.

Furthermore, Non-Patent Document 2 also points out the movement (migration) of Ru ions during charging and discharging as a factor for the high cycleability of Li ₂ RuO ₃ composite oxide.

Japanese Patent Application Publication No. 2016-51504

Chemistry of Materials Vol.25(No.7), p1121-1131(2013). J.Phys.Chem.C2019,123,13491-13499

As described above, the Li ₂ RuO ₃ composite oxide to which Ru is applied as the transition metal is expected to be a positive electrode active material that can achieve not only a high capacity but also a high energy density due to high cycleability. However, there are still few studies on the application of Li ₂ RuO ₃ composite oxides to positive electrode active materials, and the mechanism by which the above-mentioned favorable characteristics are exhibited has not been fully elucidated. Furthermore, the structure and manufacturing process of Li ₂ RuO ₃ composite oxide to fully exhibit its characteristics have not been established.

The present invention was made against the above background, and relates to a positive electrode active material containing Li ₂ RuO ₃ as a Li transition metal composite oxide, which can exhibit suitable charge/discharge capacity and high cycle performance. The purpose is to clarify the composition and manufacturing process.

TECHNICAL FIELD The present invention relates to a positive electrode active material for Li ion secondary batteries that has a Li ₂ RuO ₃ composite oxide as a main component. Li ₂ RuO ₃ is a composite oxide having a layered rock salt crystal structure (O3 layered rock salt crystal structure). The O3 type layered rock salt crystal structure has a crystal structure consisting of regularly continuous layers such as O layer - Ru layer - O layer - Li layer - O layer - Ru layer - O layer, and Li is easily moved between the layers. It has a structure suitable as a positive electrode active material in which ions are present.

In order for the Li ₂ RuO ₃ composite oxide to exhibit its properties as expected, it is necessary to form a composite oxide that forms the above-mentioned regular layer structure over a wide range. The present inventors have diligently studied methods for synthesizing Li ₂ RuO ₃ composite oxide, and as a result, have discovered a Li ₂ RuO ₃ composite oxide with suitable characteristics. The present inventors discovered that this Li ₂ RuO ₃ composite oxide has a unique tendency in its X-ray diffraction pattern when it is fully charged for the first time from the state after manufacture.

That is, the present invention to solve the above problems provides a positive electrode active material for a Li ion secondary battery comprising a Li ₂ RuO ₃ composite oxide having a layered rock salt type crystal structure, wherein the Li ₂ RuO ₃ composite oxide satisfies the following conditions. A positive electrode active material for a Li-ion secondary battery is characterized by comprising (i) and (ii).
(i) A diffraction peak of the (003) plane of the ilmenite structure appears in the X-ray diffraction pattern after the first full charge.
(ii) During initial full charging After full charging, 20% or more and 50% or less of the Ru ions constituting the Li ₂ RuO ₃ composite oxide have moved to Li ion sites.

Further, as in the condition (i) above, the positive electrode active material for Li-ion secondary batteries according to the present invention has a diffraction peak of the (003) plane of the ilmenite structure in the X-ray diffraction pattern after the first full charge. It is expressed and maintains a layered structure. At this time, it is preferable that the positive electrode active material for a Li ion secondary battery made of the Li ₂ RuO ₃ composite oxide according to the present invention further satisfies the following condition (iii).
(iii) The crystal structure before the first full charge is a layered rock salt type crystal structure, and the diffraction peak of the (002) plane appears in the X-ray diffraction pattern at that time.
When 1 mol of Li ions are desorbed by initial charging, the crystal structure includes an ilmenite structure, and the X-ray diffraction pattern at that time shows the diffraction peak of the (002) plane, and Cu at the diffraction peak
The peak position (2θ) of the Kα rays is shifted by 1° or less with respect to the peak position (2θ) of the (002) plane before the first full charge.

Furthermore, in relation to the above condition (ii), in the present invention, it is preferable that the distance between Ru ions that have moved after the first full charge is 0.7 Å or more and 1.3 Å or less.

In the present invention, the ratio (I ₁₁₃ /I ₁₁₀ ) of the diffraction intensity I ₁₁₃ of the (113) plane to the diffraction intensity I ₁₁₀ of the (110) plane in the X-ray diffraction pattern after the first full charge is 0.5. It is preferable that the value is 1.0 or less.

The present invention also provides a method for producing the above-described positive electrode active material for Li-ion secondary batteries. This method for producing a positive electrode active material includes a mixing step of mixing a Li compound and a Ru compound to produce a precursor material, and a baking step of heating and baking the precursor material to form a Li ₂ RuO ₃ composite oxide. In the mixing step, the Li compound and the Ru compound are mixed until both the coefficient of variation CV _O of the O concentration and the coefficient of variation CV _Ru of the Ru concentration become 10% or less when a plurality of arbitrary locations of the precursor are analyzed for composition. The firing step is a step of heating the precursor at a temperature of 700° C. or more and 1000° C. or less.

As explained above, the present invention is a positive electrode active material for Li ion secondary batteries, which is made of a Li ₂ RuO ₃ composite oxide. The Li ₂ RuO ₃ composite oxide of the present invention causes appropriate migration of Ru ions during the charging and discharging process, suppresses structural collapse due to oxygen desorption, etc., and has excellent cyclability.

FIG. 3 is a diagram showing a constant current charge/discharge curve (first cycle) of a Li ₂ RuO ₃ composite oxide according to an embodiment of the present invention. FIG. 3 is a diagram showing an XRD diffraction pattern (Cu Kα ray) of a Li ₂ RuO ₃ composite oxide according to an embodiment of the present invention. FIG. 3 is a diagram showing the results of in-situ X-ray diffraction analysis performed on the Li ₂ RuO ₃ composite oxide of the embodiment of the present invention. FIG. 3 is a diagram showing synchrotron radiation XRD diffraction patterns at each stage of initial full charging of the Li ₂ RuO ₃ composite oxide according to the embodiment of the present invention. FIG. 2 is a model diagram illustrating a phase change from a host state to a 1 mol desorption state of the Li ₂ RuO ₃ composite oxide according to an embodiment of the present invention. The figure which shows the simulation result of _the diffraction pattern of 1 mol desorption state of _Li2RuO3 complex oxide of embodiment of this invention. FIG. 2 is a model diagram illustrating vacancy positions in a 2 mol desorption state (after full charge) of the Li ₂ RuO ₃ composite oxide of the embodiment of the present invention. FIG. 3 is a diagram showing simulation results of a diffraction pattern in a 2 mol desorption state of the Li ₂ RuO ₃ composite oxide according to the embodiment of the present invention. FIG. 2 is a model diagram illustrating a structural change due to the movement of Ru ions in a 2 mol desorption state of the Li ₂ RuO ₃ composite oxide according to the embodiment of the present invention. The figure which shows the simulation result of the XRD diffraction pattern when changing the migration rate of Ru ion in a 2 mol desorption state. FIG. 3 is a model diagram explaining the mode of movement of Ru ions in a 2 mol desorption state. The figure which shows the simulation result of the XRD diffraction pattern when changing the migration distance of Ru ion in a 2 mol desorption state. FIG. 3 is a model diagram illustrating changes in the crystal structure of the Li ₂ RuO ₃ composite oxide of the present invention at each stage of initial full charging. An SEM photograph of a precursor for producing a Li ₂ RuO ₃ composite oxide of an example. SEM images of Li ₂ RuO ₃ composite oxides of Examples 1 and 2. FIG. 3 is a diagram showing XRD diffraction patterns (Cu Kα rays) of Li ₂ RuO ₃ composite oxides of Examples 1 and 2. FIG. 3 is a diagram showing constant current charge/discharge curves (room temperature) of Li ₂ RuO ₃ composite oxides of Examples 1 and 2. 2 is a graph showing the relationship between the number of charge/discharge cycles and the discharge capacity of the Li ₂ RuO ₃ composite oxides of Examples 1 and 2. 3 is a diagram showing a constant current charge/discharge curve (50° C.) of the Li ₂ RuO ₃ composite oxide of Example 2. FIG. 3 is a graph showing the relationship between the discharge capacity and energy density with respect to the number of charge/discharge cycles of the Li ₂ RuO ₃ composite oxide of Example 2. The figure which shows the XRD diffraction pattern (Cu Kα ray) when the Li ₂ RuO ₃ composite oxide of the example is heated from room temperature to 500°C. FIG. 3 is a diagram showing constant current charge/discharge curves (room temperature) of Li ₂ RuO ₃ composite oxides of Comparative Examples 1 and 2. FIG. 3 is a diagram showing a synchrotron radiation XRD diffraction pattern of the Li ₂ RuO ₃ composite oxide of Comparative Example 1 after the first full charge.

(A) Positive electrode active material for Li-ion secondary batteries according to the present invention (A-1) Electrical properties of the positive electrode active material according to the present invention The following describes preferred positive electrode active materials for Li-ion secondary batteries according to the present invention. An embodiment will be described. FIG. 1 shows the constant current of the first cycle measured in an electrochemical cell using a Li ₂ RuO ₃ composite oxide as a positive electrode active material, which was manufactured as an embodiment of the present invention (Example 2 described later). This is a charge/discharge curve (voltage 2.2V-4.6V). The method for producing the positive electrode active material made of the Li ₂ RuO ₃ composite oxide according to the present invention and the method and conditions for measuring the constant current charge/discharge curve will be described in detail later.

In FIG. 1, in the positive electrode active material made of the Li ₂ RuO ₃ composite oxide of the present embodiment, the potential increases around the voltage of 4.0 V at the time of initial charging, and a potential plateau is observed around the voltage of 4.2 V. Regarding this potential change, the former potential rise around 4.0 V indicates desorption of 1 mol of Li ions, and the latter potential flatness is considered to be due to anion redox.

(A-2) In-situ X-ray diffraction analysis of the positive electrode active material according to the present invention In this embodiment, the state of the positive electrode active material made of Li ₂ RuO ₃ composite oxide is determined by (i) charging with reference to FIG. It is divided into four stages: before discharge (positive electrode active material after manufacturing), (ii) initial charging, (iii) when 1 mol of Li ions are desorbed, and (iv) after the first full charge. The structure was analyzed by in-situ X-ray diffraction analysis and synchrotron radiation X-ray diffraction analysis.

In this analysis, in-situ X-ray diffraction analysis was performed using a Rigaku in-situ The positive electrode active material from the stage (FIG. 1(i)) to the 1 mol Li ion desorption stage (FIG. 1(iii)) was analyzed in-situ.

FIG. 2 is an XRD diffraction pattern of the Li ₂ RuO ₃ composite oxide, which is the positive electrode active material of this embodiment, before charging (after manufacturing). FIG. 3 shows the results of in-situ X-ray diffraction analysis of the positive electrode active material during charging. In the figure, x on the vertical axis is the number of removed moles of Li ions. Although it can be seen that the peaks that appear differ as charging progresses, structural analysis based on the assignment of diffraction peaks will be explained in detail in the analysis results based on radiation X-ray diffraction, which will be described later. What should be noted here is the peak position of the (002) peak corresponding to the interlayer distance. From FIG. 3, even if the desorption of Li ions due to charging progresses, the shift amount of the peak position of the (002) peak is extremely small. Specifically, when 1 mol of Li is desorbed (Fig. 1 (iii)), the shift amount of the (002) peak is 2θ = 1 with respect to the (002) peak position in the state before charging (immediately after manufacture). ° or less (based on Cu Kα line). This peak shift is a change of about 0.24 Å in terms of interlayer distance. The Li ₂ RuO ₃ composite oxide of the present invention has Nevertheless, it shows that the change in the interlayer distance is small.As will be described later, the interlayer distance changes when 1 mol of Li is further desorbed from this state in which 1 mol of Li has been desorbed (i.e., after full charge). There is very little change in distance.One of the reasons why the positive electrode active material according to the present invention exhibits high characteristics is that the interlayer distance is maintained even when Li is desorbed.

(A-3) Structural analysis using radiation X-ray diffraction and its simulation Therefore, through precise analysis and analysis using radiation X-ray diffraction, the structure of the Li ₂ RuO ₃ composite oxide constituting the positive electrode active material according to the present invention was clarified. Make it. In the synchrotron radiation X-ray diffraction analysis, the electrodes in each stage of (i) to (iv) were recovered and cleaned, and then sealed in a glass capillary in an Ar atmosphere, and then irradiated with synchrotron radiation X-rays with a wavelength of 0.62 Å. The diffraction pattern was measured.

4(i) to (iv) are diffraction patterns actually measured by radiation X-ray diffraction at each stage of (i) to (iv) in FIG. 1 of the positive electrode active material of this embodiment. In Figure 4, in the Li ₂ RuO ₃ composite oxide before charging (after production), a (002) peak corresponding to the layered structure of the O3 type structure (layered rock salt type crystal structure) is observed around 2θ = 7°. (Figure 4(i)). Note that in the following, the positive electrode active material (Li ₂ RuO ₃ composite oxide) before the first charge may be referred to as a "host state." At the initial stage of charging (FIG. 4(ii)), no major change is observed in the crystal structure of the positive electrode active material.

The crystal structure of the positive electrode active material changes from around a voltage of 4.0 V at which 1 mol of Li is desorbed (FIG. 4 (iii)). In the positive electrode active material (LiRuO ₃ composite oxide) from which 1 mol of Li has been desorbed, a peak of Li ₂ RuO ₃ with an O3 type structure is partially observed, and a diffraction peak (14°) belonging to an O1 type structure is observed. (110) peak near 16°, (113) peak near 21°, and (116) peak near 21°) were observed. Furthermore, a (003) peak indicating a layered structure was observed. In this specification, the positive electrode active material (LiRuO ₃ composite oxide) from which 1 mol of Li has been desorbed may be referred to as a "1 mol desorbed state."

In the positive electrode active material after full charge (FIG. 4(iv)), a diffraction peak attributed to the O1 type structure and a (003) peak based on the layered structure are observed, similar to the 1 mol desorption state described above. Furthermore, in the diffraction pattern of the positive electrode active material after full charge, diffraction peaks derived from the superlattice structure are observed at around 7° and around 13°. Hereinafter, this positive electrode active material (RuO ₃ oxide) in which Li has been completely desorbed may be referred to as a "2 mol desorbed state."

Next, based on the diffraction patterns actually measured at each stage described above, the crystal structure at each stage from the host state through the 1 mol desorption state to the 2 mol desorption state will be examined. In this study, CrystalMaker (registered trademark: Hulinx Co., Ltd.), which is crystal/molecular structure analysis software, was used. In analyzing the crystal structure, a diffraction pattern was simulated using the software based on the estimated crystal structure, and the validity of the analysis result was evaluated by comparing it with the above-mentioned measured data.

First, the crystal structure of the positive electrode active material in a 1 mol desorption state will be discussed. Literature (H. Kobayashi et al.,
According to Solid State Ionics, 82, 25 (1995), it is reported that Li _0.9 RuO ₃ from which Li is removed from Li ₂ RuO ₃ takes an ilmenite type.
With reference to this reported example, the phase change from the host state to the 1 mol desorption state will be considered using the model shown in FIG. 5. In a 1 mol desorption state with an ilmenite structure, the two hexacoordinated Ru and Li share a surface, resulting in a structure in which Li shifts from its ideal position due to electrostatic repulsion and exists near the vacancy. is taking. At this time, the oxygen filling mode also changes, changing the crystal structure from an O3 type structure to an O1 type structure. When the diffraction pattern of the positive electrode active material in the 1 mol desorption state shown in this model is simulated, it becomes as shown in FIG. This simulation result agrees well with the actually measured diffraction pattern. This confirms that the crystal structure of the positive electrode active material (LiRuO ₃ composite oxide) of this embodiment in a 1 mol desorption state at the time of initial charging is an ilmenite type O1 type structure. As described above, the 1 mol desorption state of the positive electrode active material of this embodiment has a layered structure exhibiting a (003) peak, Li is held between the layers, and the interlayer distance is wide.

Based on the above analysis results for the 1 mol desorption state, the crystal structure of the positive electrode active material in the 2 mol desorption state in which Li is completely desorbed is analyzed.
As shown in the upper part of Figure 7, the arrangement of cations (Ru) in a normal ilmenite-type O1 stacked structure is such that the vacancy positions all occupy different sites as 1→2→3→1→2→3... It has become. A diffraction pattern simulated based on this hole model is shown in the middle part of FIG. Although the main peak of the diffraction pattern based on the hole model shown in the upper part of FIG. 7 matches the actually measured diffraction pattern, it cannot reproduce the diffraction peaks of the superlattice structure that should appear around 7° and around 13°.

Therefore, the simulation was performed by correcting the hole position and changing it to 1 → 2 → 2 → 1 → 2 → 2, etc. as shown in the lower part of FIG. As a result, as shown in the lower part of FIG. 8, the superlattice peak is reproduced and matches the measured data. However, in the actually measured diffraction pattern, when comparing the (110) peak intensity and the (113) peak intensity, the former is larger. On the other hand, in the diffraction pattern based on the simulation results, the magnitude relationship between the (110) peak intensity and the (113) peak intensity is reversed. The (110) peak and the (113) peak are unique peaks originating from the O1 type structure, but the magnitude relationship of these peak intensities is due to the crystal structure of the positive electrode active material in the 2 mol desorption state as well as the Ru This is thought to be caused by a change unique to the Li ₂ RuO ₃ composite oxide called migration.

From this, it becomes clear that it is necessary to consider the movement (migration) of Ru ions when examining the crystal structure of the positive electrode active material in the 2 mol desorption state. In other words, as shown in the lower part of FIG. 9, the positive electrode active material in a 2 mol desorption state is in a state in which Ru ions have migrated due to the desorption of Li from the ilmenite O1 structure. Assume that In the lower part of FIG. 9, it is assumed that Ru ions move on average to the 6-coordination sites of the Li layer. This movement of Ru ions to Li sites reduces interlayer repulsion, stabilizes the structure, and contributes to high recyclability.

When the migration rate is defined as the ratio of the transferred Ru ions to the total number of Ru ions constituting the Li ₂ RuO ₃ composite oxide of the present invention, when a simulation is performed with the migration rate ranging from 0% to 50%, the results are shown in FIG. 10. The XRD diffraction pattern shown is obtained. Referring to FIG. 10, when the Ru ion migration rate is 25%, the (110) peak intensity is significantly larger than the (113) peak intensity. From this, it is assumed that the magnitude relationship between the (110) peak intensity and the (113) peak intensity is the same as the actually measured diffraction pattern in 20% or more, and this becomes the lower limit of the Ru ion migration rate. On the other hand, when the transfer rate of Ru ions becomes 50%, the (003) peak becomes weak. This is understood to indicate the start of collapse of the layered structure. Therefore, the upper limit of the Ru ion migration rate is assumed to be 50% or less. From these, it is considered that in the positive electrode active material of the present invention, Ru ion migration occurs at a migration rate of 20% or more and 50% or less when fully charged.

Furthermore, for the arrangement of the Ru ions that have moved at this time, reference can be made to the study results regarding the positions of the remaining Li ions in a 1 mol desorbed state. As described above, in the ilmenite O1 structure in the 1 mol desorption state, the remaining 1 mol of Li receives electrostatic repulsion due to surface sharing with Ru and coordinates in a position distorted from its ideal position. It is thought that the arrangement of Ru that moves to the Li site in a 2 mol desorption state is similarly subjected to electrostatic repulsion due to surface sharing. In other words, it is considered that the Ru ions after movement also move to a distorted position that is deviated from the ideal position of the Li site. A model of the positional state of Ru ions based on this consideration is shown in FIG.

Therefore, by performing a simulation while adjusting the interval (relative distance) d between two Ru ions for Ru ions located at positions that have moved up and down from the ideal position due to this distortion, it is possible to obtain a diffraction pattern that is closer to the actually measured data. . FIG. 12 shows simulation results when the migration rate of Ru ions is assumed to be 40% and the distances d between the migrated Ru ions are set to 0 Å, 0.55 Å, and 1.10 Å.

Referring to FIG. 10 and FIG. 12, from FIG. 10, the migration rate of Ru ions is strongly related to the intensity of the (110) peak, and from FIG. 12, the distance d between Ru ions due to electrostatic repulsion is Strongly related to strength. In the positive electrode active material of this embodiment, by setting the distance d between Ru to 1.10 Å, the (113) peak intensity can be approximated to the actually measured data, and the intensity ratio with the (110) peak can also be determined from the actually measured data. is approximated by That is, in the positive electrode active material of this embodiment, when the transfer rate of Ru ions in a 2 mol desorption state is 40%, it is estimated that the distance d between the transferred Ru ions is 1.10 Å.

(A-4) Summary of structural changes in the cathode active material according to the present invention Based on the results of in-situ X-ray diffraction analysis and synchrotron radiation X-ray diffraction branching properties explained above, the cathode active material according to the present invention (Li ₂ FIG. 13 shows a summary of changes in the crystal structure of RuO ₃ ) at each stage before charging (host state), during the charging process (1 mol desorption state), and after full charge (2 mol desorption state).

The positive electrode active material of the present invention has an O3 type structure (rock salt type layered structure) before the first charge (after manufacture), and changes to an ilmenite type O1 type layered structure at the stage when 1 mol of Li ions are desorbed from the start of charging. Become. Then, due to the initial full charge, 1 mol of Li ions is further desorbed, causing migration of Ru ions. This migration of Ru ions suppresses the reduction in the interlayer distance and suppresses the collapse of the crystal structure due to oxygen desorption. The presence of an ilmenite structure after the first full charge is confirmed by the appearance of a (003) peak in its X-ray diffraction pattern. Further, the migration rate of Ru ions is estimated to be 25% or more and 50% or less from the magnitude relationship between the (110) peak intensity and the (113) peak intensity ((110)>(113)). From these, the above-mentioned conditions (i) and (ii) become clear. According to studies by the present inventors, the Ru migration rate under condition (ii) is more preferably 25% or more and 45% or less.

Furthermore, maintenance of the interlayer distance in the Li ₂ RuO ₃ composite oxide, which is the positive electrode active material of the present invention, is also inferred from the amount of shift of the (002) peak in the 1 mol desorption state. That is, the Li ₂ RuO ₃ composite oxide of the present invention includes an ilmenite structure as a crystal structure in a 1 mol desorption state, and also exhibits the (002) peak of the O3 structure. This (002) peak is shifted by 1° or less (Cu Kα line) with respect to the peak position (2θ) of the (002) peak before the first charge. From this, the above-mentioned condition (iii) becomes clear as a preferable condition. Incidentally, the reduction width of the interlayer distance based on this peak shift of 1° or less is preferably 0.23 Å or more and 0.25 Å or less.

Further, in the present invention, Ru ions move to Li sites at a transfer rate of 20% to 50% while maintaining the O1 type structure in a 1 mol desorption state. It is thought that the movement of Ru ions increases electrostatic repulsion between layers, contributing to stabilization of the structure. Furthermore, the moved Ru ions are in a distorted position relative to the theoretical position of the Li site. The movement of these Ru ions is estimated from the intensities of the (113) and (110) peaks in the diffraction pattern. According to studies by the present inventors, the ratio (I ₁₁₃ /I ₁₁₀ ) of the diffraction intensity I ₁₁₃ of the (113) plane to the diffraction intensity I ₁₁₀ of the (110) plane is 0.5 or more and 1.0 or less. Preferably. The range of the distance d between the moved Ru, defined based on the range of the intensity ratio (I ₁₁₀ /I ₁₁₃ ) of this (113) peak, is preferably 0.7 Å or more and 1.3 Å or less. .

Note that the interlayer distance maintained by the movement of Ru ions as described above is preferably 4.55 Å or more and 4.60 Å or less. The Li ₂ RuO ₃ composite oxide, which is the positive electrode active material according to the present invention, can maintain a wider interlayer distance than LiCoO ₂ , which has been known as a positive electrode active material for Li-ion secondary batteries. , which is considered to be associated with favorable properties.

(A-5) Crystal grain size of the positive electrode active material (Li ₂ RuO ₃ composite oxide) according to the present invention In the positive electrode active material according to the present invention, the average particle diameter of the Li ₂ RuO ₃ composite oxide is 0. The thickness is preferably 1 μm or more and 30 μm or less. Since a fine positive electrode active material with a diameter of less than 0.1 μm has an excessively large surface area, it is necessary to increase the amount of a binder when forming an electrode. The binder is a material that binds the positive electrode active material particles to each other or the positive electrode active material particles and the conductive material. As the amount of binder increases, the capacity per unit electrode weight decreases, so in order to obtain an appropriate surface area, the particle size of the positive electrode active material is preferably 0.1 μm or more. On the other hand, in a positive electrode active material having a particle size exceeding 30 μm, the resistance component of particle bulk resistance and interparticle resistance becomes large due to an increase in particle size and a decrease in surface area. As a result, the discharge capacity and the capacity retention rate due to charge/discharge cycles decrease, so it is preferable to use a positive electrode active material with a thickness of 30 μm or less. When considering cycle characteristics in addition to the above reasons, it is more preferable that the average particle size of the positive electrode active material is 0.5 μm or more.

(B) Method for manufacturing the positive electrode active material for Li-ion secondary batteries according to the present invention Next, a method for manufacturing the Li ₂ RuO ₃ composite oxide constituting the positive electrode active material according to the present invention will be described. The Li ₂ RuO ₃ composite oxide applied in the present invention has a basic process similar to the known manufacturing process of Li ₂ RuO ₃ composite oxide. A preferred method for producing the Li ₂ RuO ₃ composite oxide includes a method of producing a precursor by mixing a Li compound and a Ru compound, and producing a composite oxide by heating and baking the precursor at a high temperature. . However, the Li ₂ RuO ₃ composite oxide according to the present invention is required to have an appropriate layered crystal structure in order to exhibit the above-mentioned structural stability when charged and discharged as a positive electrode active material. That is, the method for producing a positive electrode active material according to the present invention includes a mixing step of mixing a Li compound and a Ru compound to produce a precursor, and heating and baking the precursor to produce a Li ₂ RuO ₃ composite oxide. The mixing step includes a firing step, and the mixing step includes adding Li until both the coefficient of variation CV _O of the O concentration and the coefficient of variation CV _Ru of the Ru concentration become 10% or less when the composition of arbitrary plural locations of the precursor material is analyzed. This is a step of mixing a compound and a Ru compound, and the firing step is a step of heating the precursor at a temperature of 700° C. or more and 1000° C. or less. Hereinafter, in the method for producing a positive electrode active material according to the present invention, the production process and firing process of the precursor material will be explained in detail, and the means for forming a Li ₂ RuO ₃ composite oxide with a suitable crystal structure will be mentioned. do.

As a raw material for forming a precursor of a composite oxide, Li compounds include Li carbonate, Li acetate, Li nitrate, Li hydroxide, Li chloride, Li sulfate, Li oxide, and the like. Among these, Li carbonate and Li oxide are preferred in consideration of stability and cost. As the Ru compound, Ru carbonate, Ru hydroxide, Ru oxyhydroxide, Ru acetate, Ru citric acid, Ru oxide, etc. can be used. For reasons of cost and stability, it is preferable to use an oxide as the Ru compound. In addition, both a non-hydrate (RuO ₂ ) and a hydrate (RuO ₂ .nH ₂ O) can be used as the Ru oxide.

In the production method of the present invention, first, the compounds serving as the above-mentioned raw materials are mixed, and a precursor material in which these are mixed is produced. Since the positive electrode active material according to the present invention is composed of a Li ₂ RuO ₃ composite oxide, it is preferable that the Li compound and the Ru compound are mixed at a molar ratio according to the theoretical composition of the composite oxide. However, depending on the type of Li compound, it may volatilize due to heating in the firing process. Therefore, the amount of the Li compound to be mixed is preferably increased by 1% or more and 10% or less relative to the mass corresponding to the target composition. The increase in the amount of Li compound is more preferably 1% or more and 8% or less. This is because if an excessive amount of Li compound is mixed, electrically inactive Li ₃ RuO ₄ may be partially generated, which leads to deterioration of the properties as a positive electrode active material.

In the mixing step, pulverization and mixing can be performed as necessary. When using a powdered raw material compound, if the particle size is large (15 μm or more), pulverization is performed to ensure uniformity of the precursor. When pulverizing is performed in the mixing step, a pulverizing device such as a ball mill, jet mill, rod mill, or sand mill can be used. Further, the pulverization may be carried out by either dry pulverization or wet pulverization. Preferably, wet pulverization is carried out using water or an organic solvent as a dispersion medium. Further, the precursor material after the mixing step may be subjected to granulation, pelletizing, etc., if necessary.

In order to produce the Li ₂ RuO ₃ composite oxide that becomes the positive electrode active material according to the present invention, it is necessary to improve the composition uniformity of the precursor material consisting of a mixture of Li compound and Ru compound in the above mixing step. required. Specific indicators of this compositional uniformity include the coefficient of variation of oxygen concentration CV _O and the coefficient of variation of Ru concentration CV _Ru Both must be 10% or less.

The coefficient of variation CV _O of the oxygen concentration of the precursor is calculated by calculating the average value (A _O ) and standard deviation (σ _O ) based on the _O concentration (C O ) measured at multiple locations of the precursor, and calculating CV _O = It can be calculated by (ρ _O /A _O )×100. Similarly, the coefficient of variation CV _Ru of the Ru concentration is determined by calculating the average value (A _Ru ) and standard deviation (σ _Ru ) from the Ru concentrations (C _Ru ) measured at multiple locations in the precursor material, and calculating CV _Ru = (ρ It can be calculated by _Ru /A _Ru )×100. The present invention requires that both CV _O and CV _Ru be 10% or less. When at least one of CV _{2 O} and CV _Ru exceeds 10%, the composition uniformity of the precursor is insufficient, making it difficult to obtain a suitable positive electrode active material according to the present invention.

The composition analysis of the precursor can be performed using various analytical methods such as electron probe microanalysis (EPMA), energy dispersive X-ray analysis (EDX), fluorescent X-ray analysis (FRX), and X-ray photoelectron spectroscopy (XPS). is applicable, and the concentrations of O and Ru are analyzed using a scheme according to each analysis method. Furthermore, when a plurality of arbitrary locations of the precursor are analyzed, it is preferable to set five or more locations for analysis.

By heating and baking the precursor material produced in the above steps, a Li ₂ RuO ₃ composite oxide, which becomes the positive electrode active material of the present invention, is produced. The heating temperature in the firing step is 700°C or more and 1000°C or less. At temperatures below 700°C, solid phase reactions for producing composite oxides are difficult to proceed. Moreover, when the temperature exceeds 1000° C., synthesis of Li ₂ RuO ₃ composite oxide is possible, but the positive electrode active material may have poor cycleability. This heating temperature is more preferably 800°C or more and 1000°C or less. In order to prevent unreacted raw material compounds (for example, Li carbonate, etc.) from remaining, the heat treatment time is preferably 1 hour or more and 48 hours or less. As a heating means for the firing step, general heat treatment equipment such as a fixed furnace such as an electric furnace or a batch furnace, a rotary furnace such as a rotary kiln, or a continuous furnace such as a roller hearth kiln can be used.

This firing step may be performed in the air or in a non-oxidizing atmosphere. However, if fired in a non-oxidizing atmosphere, stacking faults may be introduced into the Li ₂ RuO ₃ composite oxide depending on temperature conditions and the like. Therefore, the preferred atmosphere for the firing step is air or an oxygen-containing atmosphere.

Through the above mixing step and firing step, a positive electrode active material containing the Li ₂ RuO ₃ composite oxide can be manufactured. This Li transition metal composite oxide may be washed with deionized water and dried as appropriate. Further, in order to make the manufactured positive electrode active material into powder with a suitable particle size for use as a positive electrode of a Li secondary battery, post-treatment such as crushing and classification of the Li ₂ RuO ₃ composite oxide may be performed.

(C) Positive electrode and Li ion secondary battery for Li-ion secondary batteries to which the positive electrode active material according to the present invention is applied The positive electrode active material according to the present invention has the same configuration as a general Li-ion secondary battery, It can be used as a positive electrode for a Li-ion secondary battery and a Li-ion secondary battery.

A positive electrode for a Li-ion secondary battery is composed of components such as a conductive material and a binder in addition to the positive electrode active material according to the present invention. Examples of the conductive material include carbon powders such as graphite, acetylene black, and furnace black, and one or more conductive materials such as carbon whiskers, carbon fibers, metal powders, metal fibers, and conductive ceramic materials. It will be done. In addition, examples of the binder include one or two of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyethylene, polypropylene polyhexafluoropropylene, styrene-butadiene rubber, polyacrylonitrile, modified polyacrylonitrile, etc. There are more than one species.

The positive electrode is prepared by mixing each of these components with a solvent such as N-methylpyrrolidone, toluene, and water to prepare an electrode mixture, and applying the electrode mixture to a current collector (substrate) such as aluminum foil to form an electrode. It is manufactured by forming a mixture layer and then pressure-molding an electrode mixture layer.

Moreover, the Li ion secondary battery is constructed with the positive electrode containing the above-described positive electrode active material of the present invention, a negative electrode, an electrolyte, and a separator as main elements.

The negative electrode is composed of components such as a negative electrode active material, a conductive material, and a binder.
As the negative electrode active material, known materials such as carbon materials such as graphite and hard carbon, titanium-based materials such as Li titanate, and silicon-based materials such as silicon oxide can be used. The negative electrode active material is not particularly limited as long as it has a material and form that can absorb and release Li ions during charging and discharging. Further, other components constituting the negative electrode (negative electrode active material, conductive material, binder, etc.) can be the same as those of the positive electrode. The manufacturing process for the negative electrode is also similar to that for the positive electrode.

As for the electrolytic solution, materials with known configurations can also be used. The electrolytic solution is composed of an electrolyte and a solvent, and the electrolyte includes LiPF ₆ (lithium hexafluorophosphate), LiFSA (LiFSI: lithium bis(fluorosulfonyl)amide), and LiTFSI (lithium bis(trifluoromethanesulfonyl)imide). , LiClO ₄ (lithium perchlorate), LiBF ₄ (lithium tetrafluoroborate), etc. can be applied. In addition, examples of the solvent include EC (ethylene carbonate), PC (propylene carbonate), DMC (dimethyl carbonate), EMC (ethyl methyl carbonate), DEC (diethyl carbonate), TMP (trimethyl phosphate), etc. A single solvent or a mixture of solvents can be used.

In addition, other components of the Li-ion secondary battery include separators, terminals, insulating plates, battery cases (battery cans, battery lids), etc., but commonly used parts can be used for these parts. It is.

As an example of the present invention, the manufacturing process of the positive electrode active material according to the above-described embodiment, and the evaluation results of the electrical characteristics and recyclability of a Li ion secondary battery using the positive electrode active material will be described.

[Manufacture of positive electrode active material]
6 g of a precursor was prepared by mixing Li carbonate (Li ₂ CO ₃ ) powder and Ru oxide (hydrate: RuO ₂ .nH ₂ O) powder. In this example, only the Li carbonate powder is mixed in an amount 3% larger than the theoretical mass. As a mixing step for producing a precursor consisting of a mixed powder of Li carbonate and Ru oxide, the precursor was produced by pulverizing and mixing using a pestle and mortar for 10 minutes. In this example, five observation areas for compositional analysis by SEM-EDX were arbitrarily set, and the composition analysis was performed over the entire field of view of each observation area. Then, based on the composition analysis results (O concentration (C _O ), Ru concentration (C _Ru ): mass %) obtained in the five regions, the average value (A _O , A _Ru : mass %) and standard deviation were calculated. (ρ _O , ρ _Ru ) were determined, and the coefficients of variation (CV _O , CV _Ru ) of the O concentration and Ru concentration were calculated from them. The measurement results are shown in Table 1. Moreover, FIG. 14 is a SEM image of the precursor material of this example.

As confirmed in Table 1, in the precursor materials of Examples, both the coefficients of variation of O concentration and Ru concentration (CV _O , CV _Ru ) are 10% or less.

Next, the precursor produced by the above mixing step was compressed into pellets. Then, the pellet-shaped precursor was fired to produce a Li ₂ RuO ₃ composite oxide. The heating conditions for the firing step were as follows: heating in the air at a temperature increase rate of 10° C./min until the temperature reached 900° C., and after reaching 900° C., heating was maintained for 12 hours. After heating for 12 hours, it was cooled to room temperature in a furnace and the Li ₂ RuO ₃ composite oxide was taken out.

In this example, two types of Li ₂ RuO ₃ composite oxides were synthesized under the firing conditions of an air atmosphere, a heating temperature of 950° C. or 1000° C., and a heating time of 24 hours. FIG. 15 is a SEM image of these Li ₂ RuO ₃ composite oxides. Composite oxides with different particle sizes can be produced depending on the firing conditions, and when the firing conditions are 950° C. in the air for 24 hours, a Li ₂ RuO ₃ composite oxide with an average size of 2 μm is obtained (Example 1). On the other hand, when the firing conditions were 1000° C. in the air for 24 hours, a Li ₂ RuO ₃ composite oxide with an average thickness of 5 μm was synthesized (Example 2).

FIG. 16 shows the X-ray diffraction patterns (X-ray source: Cu Kα ray) of the Li ₂ RuO ₃ composite oxides of Examples 1 and 2. Although the Li ₂ RuO ₃ composite oxides of Examples 1 and 2 have different particle sizes, there is no large difference in peak position and peak intensity, and both are diffraction patterns of a single phase of Li ₂ RuO ₃ originating from an O3 type structure. It showed a pattern.

[Evaluation of electrochemical properties]
A constant current charge/discharge curve was measured for the positive electrode active material of this example, and the electrochemical characteristics were evaluated. In this evaluation test, the positive electrode was subjected to carbon composite treatment by mixing the positive electrode active material (AM) of this example, a conductive material (acetylene black: AB), and a binder (polyvinylidene fluoride: PVDF). using something. The configuration of the test equipment is as follows.
・Cell type: Bipolar electrochemical cell (TJ-AC: manufactured by Nippon Tomcell Co., Ltd.)
・Positive electrode: AM:AB:PVDF=80:10:10 (wt%)
・Counter electrode: Lithium metal ・Separator: Polyolefin porous membrane (Celgard 2500) + glass filter (GB-100R)
・Electrolyte: 1M-LiPF ₆

In the constant current charge/discharge test, the discharge capacity was measured at room temperature with a voltage range of 2.2 V to 4.6 V and a current density of 30 mA/cm ² at the first charge. Then, charging and discharging were performed for 5 cycles, 15 cycles, and 30 cycles, and the potential-capacity curve was measured.

FIG. 17 shows a constant current charge/discharge curve of the positive electrode active material (Li ₂ RuO ₃ composite oxide) of the present example (Examples 1 and 2). Moreover, FIG. 18 shows a graph showing the relationship between the number of cycles and the discharge capacity obtained from the results of the constant current charge/discharge test. Referring to these results, the positive electrode active materials of Examples 1 and 2 have initial discharge capacities exceeding 225 mAhg ⁻¹ and exhibit sufficient capacity. It can also be said that the capacity retention rate is at a high level. Example 1, which has a relatively small particle size, exhibits a high initial discharge capacity exceeding 250 mAhg ⁻¹ . Although a slight capacity deterioration is observed up to the 15th cycle, it can be confirmed that the battery can be stably charged and discharged up to 30 cycles without any capacity deterioration thereafter. On the other hand, although the initial discharge capacity of Example 2, which had a larger particle size, was lower than that of Example 1, it was confirmed that the capacity did not deteriorate at all up to 30 cycles and had excellent cycle characteristics. It can be said that the Li ₂ RuO ₃ composite oxide of the present invention exhibits sufficient discharge capacity and cycleability, although there are some differences in tendency depending on the particle size.

Next, for the positive electrode active material (Li ₂ RuO ₃ composite oxide) of Example 2, a constant current charge/discharge curve was measured at a test temperature of 50°C. The results are shown in FIG. 19, and the relationship between the discharge capacity and energy density with respect to the number of cycles is shown in FIG. 20 based on the results. In addition, FIG. 19 shows the results of the galvanostatic charge-discharge curve of Li _1.2 Ni _0.13 Co _0.13 Mn _0.54 O ₂ , which is a Mn-based lithium-excess positive electrode active material, measured under the same conditions. There is. In addition to the Mn-based lithium-excess positive electrode active material, FIG. 20 also shows the cyclability of the Ni-based positive electrode active material LiNi _0.815 Co _0.15 Al _0.035 O ₂ measured under the same conditions. The results are shown below.

19 and 20, the positive electrode active material of this example exhibits a high capacity approaching the theoretical capacity based on lithium under a high temperature condition of 50°C. Furthermore, the positive electrode active material of this example shows no capacity deterioration even after 100 cycles of charging and discharging, indicating that it exhibits extremely excellent recyclability. On the other hand, the Mn-based lithium-excess positive electrode active material (Li _1.2 Ni _0.13 Co _0.13 Mn _0.54 O ₂ ) has the highest energy density at the first full charge, but the increase in the number of cycles It is not possible to avoid a decrease in the emitted energy density due to . Further, the Ni-based positive electrode active material (LiNi _0.815 Co _0.15 Al _0.035 O ₂ ) is a positive electrode active material for Li-ion secondary batteries that has already been put into practical use. It can be seen that this conventional positive electrode active material is significantly inferior to the positive electrode active material of this example in terms of initial discharge capacity and cyclability at 50°C.

[Evaluation of thermal stability]
A heating test was conducted to examine the stability of the Li ₂ RuO ₃ composite oxide constituting the positive electrode active material of this example at high temperatures in more detail. In the heating test, in situ synchrotron radiation X-ray diffraction was performed while heating the sample using a nitrogen gas blowing device. After analysis at room temperature, heating and analysis were performed at 100°C intervals from 100°C to 500°C.

FIG. 21 shows a diffraction pattern when the Li ₂ RuO ₃ composite oxide of this example was heated from room temperature to 500°C. From FIG. 21, it can be seen that the Li ₂ RuO ₃ composite oxide maintains the O3 type layer structure from room temperature to 300°C. At 400° C., the (003) peak and superlattice disappear, so at this temperature, an irregular arrangement of cations occurs based on the O1 type structure. Since a RuO ₂ peak is clearly observed at 500° C., it is thought that decomposition due to oxygen elimination occurs here. Regarding the behavior of the Li ₂ RuO ₃ composite oxide of this example as described above, for example, it has been confirmed that LiNi-based (LiNiO ₂ etc.) decomposes due to oxygen elimination (NiO formation occurs at 250°C to 300°C). The Li ₂ RuO ₃ composite oxide of this example has a decomposition temperature 200° C. or more higher than that of the LiNi-based oxide, and can be said to be a cathode active material with extremely high thermal stability and durability.

From the above study results, it was confirmed that the positive electrode active material made of the Li ₂ RuO ₃ composite oxide according to the present invention has a suitable discharge capacity and high cyclability even at high temperatures. The reason why such excellent characteristics are exhibited is that, as mentioned above, the Li ₂ RuO ₃ composite oxide according to the present invention is capable of producing Ru ions even when 2 mol of Li is desorbed during the charging process. This is thought to be due to the fact that the decrease in interlayer distance is suppressed while maintaining the layered structure due to appropriate movement of , and collapse due to desorption of oxygen ions is suppressed.

Comparative example

As a comparative example to be compared with the positive electrode active material of the present invention, the following positive electrode active material was manufactured and its electrochemical properties were measured.

Comparative Example 1 : A precursor was produced by mixing the same Li carbonate powder and Ru oxide powder as in the example. In this comparative example, the same pestle and mortar as in the example were used in the mixing step, and the grinding and mixing time was set to 1 minute. The composition of the produced precursor was analyzed by SEM-EDX in the same manner as in the examples. The average value (A _O , A _Ru ) and standard deviation (ρ _O , ρ _Ru ) of the oxygen concentration of the precursor and the coefficient of variation of the O concentration and Ru concentration (CV _O , CV _Ru ) obtained from the results of composition analysis. are shown in Table 2.

As shown in Table 2, the precursor of Comparative Example 1 had both CV _O and CV _Ru exceeding 10%. Then, this precursor material was fired in the same manner as in Example 1 to produce a Li ₂ RuO ₃ composite oxide that would become the positive electrode active material of Comparative Example 1.

Comparative Example 2 : Using the same raw materials and mixing conditions as in the example, a precursor containing 10% or less of both CV _O and CV _Ru was produced. This precursor material was heated in the air at 1050° C. for 24 hours to produce a Li ₂ RuO ₃ composite oxide that would become the positive electrode active material of Comparative Example 1.

[Evaluation of electrochemical properties]
Constant current charge/discharge curves for the positive electrode active materials of Comparative Examples 1 and 2 were measured. The measuring device was the same as in Example 1, and the measurement conditions were room temperature, voltage range of 2.2 V to 4.6 V, current density of 30 mA/cm ^{2 ,} and the discharge capacity was measured at the first full charge. Then, 13 cycles of charging and discharging were performed, and a potential-capacity curve was measured. The measurement results are shown in FIG. 22. FIG. 22 also shows the constant current charge/discharge curve of Example 1 measured under the same conditions.

From FIG. 22, the positive electrode active material of Comparative Example 1 has an initial discharge capacity of less than 225 mAhg ⁻¹ , which is lower than that of the positive electrode active material of the example of the present application. In Comparative Example 1, both CV _O and CV _Ru were over 10% in the precursor material before firing to produce the Li ₂ RuO ₃ composite oxide, but the uniformity of this composition was poor. This is considered to be the result of On the other hand, Comparative Example 2 is a Li ₂ RuO ₃ composite oxide manufactured at a firing temperature of over 1000°C. The positive electrode active material of Comparative Example 2 has a lower initial discharge capacity than Comparative Example 1, and is also inferior in cycle characteristics.

FIG. 23 is a diffraction pattern of the positive electrode active material (Li ₂ RuO ₃ composite oxide) of Comparative Example 1 obtained by synchrotron radiation XRD after the first full charge. The Li ₂ RuO ₃ composite oxide of Comparative Example 1 has an O1 type structure after being fully charged, which is similar to the present invention. However, the intensity (I 110 ) of the ( ₁₁₀ ) peak is lower than the intensity (I 113 ) of the ( ₁₁₃ ) peak (I ₁₁₃ /I ₁₁₀ >1.0), which is the opposite of the present invention. Referring to FIG. 10, which examines the migration rate of Ru ions, the relationship between the peak intensities of (110) and (113) observed in Comparative Example 1 indicates that the migration rate of Ru ions is 0% or less than 20%. It suggests. The reason why the properties of the positive electrode active material of the comparative example are inferior to that of the present embodiment is considered to be because this movement of Ru ions does not occur or is insufficient. This tendency of the XRD diffraction pattern was also observed in Comparative Example 2 as well. In the Li ₂ RuO ₃ composite oxide according to the example of the present application, the interlayer distance is maintained due to the above-mentioned change in the crystal structure during the initial charging process and the appropriate movement of Ru ions, resulting in high capacity and cycleability. It can be said that it is an excellent positive electrode active material.

As explained above, in the positive electrode active material for Li ion secondary batteries according to the present invention, Li ₂ RuO ₃ has excellent cyclability due to structural changes during charging and discharging. The positive electrode active material according to the present invention can suitably be used as a positive electrode of a Li-ion secondary battery, and can be widely used in various small batteries, household power sources, vehicle batteries, and the like.

Claims (5)

In a positive electrode active material for a Li ion secondary battery comprising a Li ₂ RuO ₃ composite oxide having a layered rock salt type crystal structure, the Li ₂ RuO ₃ composite oxide satisfies the following conditions (i) and (ii). A positive electrode active material for Li-ion secondary batteries characterized by:
(i) A diffraction peak of the (003) plane of the ilmenite structure appears in the X-ray diffraction pattern after the first full charge.
(ii) After the first full charge, 20% to 50% of Ru ions constituting the Li ₂ RuO ₃ composite oxide move to Li ion sites. The positive electrode active material for a Li ion secondary battery according to claim 1, wherein the Li ₂ RuO ₃ composite oxide further satisfies the following condition (iii).
(iii) The crystal structure before the first full charge is a layered rock salt type crystal structure, and the diffraction peak of the (002) plane appears in the X-ray diffraction pattern at that time.
When 1 mol of Li ions are desorbed by initial charging, the crystal structure includes an ilmenite structure, and the X-ray diffraction pattern at that time shows the diffraction peak of the (002) plane, and The peak position (2θ) of the diffraction peak due to Cu Kα rays is shifted by 1° or less with respect to the peak position (2θ) of the (002) plane before the first charge. The positive electrode active material for a Li-ion secondary battery according to claim 1 or 2, wherein in condition (ii), the distance between Ru ions that have moved after the first full charge is 0.7 Å or more and 1.3 Å or less. The ratio (I ₁₁₃ /I 110 ) of the diffraction intensity I ₁₁₃ of the (113) plane to the diffraction intensity I ₁₁₀ of the ( ₁₁₀ ) plane in the X-ray diffraction pattern after the first full charge is 0.5 or more and 1.0 or less. A positive electrode active material for a Li ion secondary battery according to claim 1 or 2. A method for producing a positive electrode active material for a Li ion secondary battery according to claim 1, comprising:
a mixing step of mixing a Li compound and a Ru compound to produce a precursor;
A firing step of heating and firing the precursor to obtain a Li ₂ RuO ₃ composite oxide,
In the mixing step, the Li compound and the Ru compound are mixed until both the coefficient of variation CV _O of the O concentration and the coefficient of variation CV _Ru of the Ru concentration become 10% or less when the composition of arbitrary plural locations of the precursor material is analyzed. It is a mixing process,
The method for manufacturing a positive electrode active material for a Li ion secondary battery, wherein the firing step is a step of heating the precursor material at a temperature of 700° C. or higher and 1000° C. or lower.

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