Electrode active material, manufacturing method thereof, and secondary battery

Abstract

To provide an electrode active material, a manufacturing method thereof, and a secondary battery that use surface modification with metal oxide to prevent side reactions during charging and discharging and minimize deterioration of charging and discharging characteristics.SOLUTION: An electrode active material according to the present invention includes active material particles made of a compound containing a transition metal element and oxygen, and a surface modification layer containing a metal oxide that modifies the surface of the active material particles.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an electrode active material, a method for producing the same, and a secondary battery. [Background technology]

[0002] The transition metal oxides used as the positive electrode active material in magnesium secondary batteries are prone to side reactions, such as electrolyte and supporting salt decomposition, during charge and discharge, which can lead to deterioration of cycle characteristics. This is one of the reasons why the cycle life of the battery cannot be extended. To insert and extract magnesium ions into and from the transition metal oxide, the positive electrode active material is nanoparticle-shaped, and the increase in specific surface area caused by nanoparticle-shaping poses an obstacle to suppressing side reactions.

[0003] One method for improving the electrochemical properties of electrode active materials is surface modification. Surface modification is expected to suppress side reactions, such as oxidative and reductive decomposition of the electrolyte, that occur at the active sites on the electrode surface, thereby improving the stability of charge and discharge capacities. For example, there are reports of improving the properties of secondary batteries by coating the surface of a positive electrode active material with vanadium oxide or zirconium oxide (Patent Document 1, Non-Patent Documents 1 and 2). Furthermore, a technology has been disclosed in which organic phosphorus compounds, such as organic phosphonic acid, are used as surface-modifying compounds for the positive electrode active material to suppress deterioration in the charge and discharge properties of secondary batteries (Patent Document 2). Furthermore, iron has been reported to have the effect of suppressing the above-mentioned side reactions (Non-Patent Document 3). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 7109094 [Patent Document 2] Japanese Patent Publication No. 2022-024258 [Non-patent literature]

[0005] [Non-Patent Document 1] S.Doi, R.Ise, T.Mandai, Y.Oaki, S.Yagi, H.Imai, Langmuir, 36, 8537 (2020) [Non-patent document 2] N. Kitamura, T. Imura, N. Ishida, C. Ishibashi, Y. Idemoto, ACS Omega, 7, 46915 (2022) [Non-patent document 3] J. Han, S. Yagi, H. Takeuchi, M. Nakayama, T. Ichitsubo, J. Mater. Chem. A 9, 26401 (2021) Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in view of the above circumstances, and aims to provide an electrode active material that can prevent side reactions during charge and discharge and suppress deterioration of charge and discharge characteristics by surface modification using a metal oxide, a method for producing the same, and a secondary battery. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention employs the following means.

[0008] (1) An electrode active material according to one aspect of the present invention comprises active material particles made of a compound containing a transition metal element and oxygen, and a surface modification layer containing a metal oxide that modifies the surface of the active material particles.

[0009] (2) In the electrode active material described in (1) above, the compound may further contain magnesium.

[0010] (3) In the electrode active material according to either (1) or (2) above, the transition metal element in the active material particles is preferably an element selected from manganese, cobalt, nickel, and iron.

[0011] (4) In the electrode active material according to any one of (1) to (3) above, the metal element constituting the metal oxide contained in the surface modification layer is preferably an element selected from iron, cobalt, nickel, zinc, zirconium, tin, aluminum, and titanium.

[0012] (5) In the electrode active material described in either (1) or (2) above, it is preferable that the redox pair of the metal species of the metal oxide contained in the surface modification layer has a lower potential than the redox pair of the metal species of the metal oxide constituting the active material particles, and that the solubility of the lower-valent metal ion of each of the metal species constituting the redox pair is higher than the solubility of the higher-valent metal ion.

[0013] (6) A secondary battery according to one aspect of the present invention includes the electrode active material according to any one of (1) to (5) above as a positive electrode active material.

[0014] (7) In the secondary battery described in (6) above, the transition metal element is preferably iron or an element selected from metal elements having a higher oxidative decomposition activity for the electrolyte than iron.

[0015] (8) A method for manufacturing a secondary battery according to one embodiment of the present invention is a method for manufacturing an electrode active material according to any one of (1) to (5) above, and includes the steps of: a first suspension preparation step of suspending the active material particles in a solution containing oxalate ions to prepare a first suspension; a first powder obtaining step of drying the first suspension to obtain a first powder consisting of the active material particles having oxalate ions attached to their surfaces; a second suspension preparation step of putting the first powder into a solution containing metal elements that constitute the metal oxide to prepare a second suspension; a second powder obtaining step of drying the second suspension to obtain a second powder consisting of the active material particles having metal elements bonded to them via the oxalate ions; and a second powder heating step of heating the second powder.

[0016] (9) Another aspect of the present invention provides a method for producing a secondary battery, which is a method for producing an electrode active material according to any one of (1) to (5) above, and includes a third suspension preparation step of preparing a third suspension by suspending the active material particles in a solution containing divalent iron ions, a third powder obtaining step of drying the third suspension to obtain a third powder consisting of the active material particles having trivalent iron ions precipitated on their surfaces, and a third powder heating step of heating the third powder. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide an electrode active material that can prevent side reactions during charge and discharge and suppress deterioration of charge and discharge characteristics by surface modification using a metal oxide, a method for producing the same, and a secondary battery. [Brief explanation of the drawings]

[0018] [Figure 1] 1 shows Fourier transform infrared spectra obtained for the powders of electrode active materials of Example 1 and Comparative Examples 1 and 2. [Figure 2] 1 shows EDS spectra obtained for the electrode active material powders of Example 1 and Comparative Example 1. [Figure 3] (a) TEM image of an electrode active material particle having a surface modification layer in Example 1. (b) to (d) EDS mapping images of Mg, Mn, and Fe present near the surface of the same particle. [Figure 4] 1 shows charge-discharge curves obtained by carrying out a charge-discharge test on a composite electrode containing the electrode active material of Comparative Example 1. [Figure 5] 1 shows charge-discharge curves obtained by carrying out a charge-discharge test on a composite electrode containing the electrode active material of Example 1. [Figure 6] 1 is a graph showing EDS spectra obtained for the electrode active material powders of Examples 1 to 8 and Comparative Example 1. [Figure 7] 10 is a charge / discharge curve obtained by carrying out a charge / discharge test on a composite electrode containing the electrode active material of Comparative Example 3. [Figure 8] 10 shows charge-discharge curves obtained by carrying out a charge-discharge test on a composite electrode containing the electrode active material of Example 9. [Figure 9] 1 shows EDS spectra obtained for the powders of electrode active materials of Examples 10 to 12 and Comparative Example 1. [Figure 10] 10 shows EDS spectra obtained from dried powders of solutions obtained by centrifuging the third suspensions of the electrode active materials of Examples 10 to 12. [Figure 11] 10 is a charge / discharge curve obtained by carrying out a charge / discharge test on a composite electrode containing the electrode active material of Example 10. [Figure 12] 10 shows charge-discharge curves obtained by carrying out a charge-discharge test on a composite electrode containing the electrode active material of Example 11. [Figure 13] 10 is a charge / discharge curve obtained by carrying out a charge / discharge test on a composite electrode containing the electrode active material of Example 12. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, an electrode active material, a manufacturing method thereof, and a secondary battery according to an embodiment of the present invention will be described in detail with reference to the drawings. Note that the drawings used in the following description may show characteristic portions enlarged for convenience in order to make the characteristics easier to understand, and the dimensional ratios of each component may not necessarily be the same as in reality. Furthermore, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto. Appropriate modifications can be made within the scope of the present invention.

[0020] <Electrode active material> An electrode active material according to one embodiment of the present invention mainly comprises active material particles and a surface modification layer that modifies the surfaces of the active material particles.

[0021] The active material particles are composed of a compound containing a transition metal element such as manganese (Mn), cobalt (Co), nickel (Ni), iron (Fe), copper (Cu), titanium (Ti), vanadium (V), or chromium (Cr) and oxygen, capable of inserting and desorbing mobile ions, and reactive with the electrolyte. This compound may further contain magnesium (Mg). Examples of the mobile ions that can be inserted and desorbed include magnesium ions. In the electrode active material, multiple active material particles are aggregated and distributed in a porous secondary particle state.

[0022] Specific examples of compounds constituting the active material particles include magnesium manganese oxide (MgMn2O4), manganese dioxide (MnO2), etc. The crystal structure of manganese dioxide is not particularly limited, and may be an α-type (hollandite type).

[0023] In a porous electrode active material, the active material particles have a small particle size and a large specific surface area. Therefore, when the porous electrode active material is used as a secondary battery, side reactions such as oxidative decomposition of the electrolyte tend to occur on the surfaces of the active material particles. In this embodiment, these side reactions can be suppressed by modifying the surfaces of the active material particles, allowing the use of active material particles with various particle sizes (specific surface areas).

[0024] The surface modification layer is a thin film containing a metal oxide and having a dense and uniform thickness. The thickness of the surface modification layer is not particularly limited, but is preferably about 0.5 nm to 5 nm. The metal oxide of the surface modification layer is strongly chemically bonded to the surface of the active material particle. The surface modification layer present on the surface of the active material particle can be confirmed using a transmission electron microscope (TEM)-energy dispersive X-ray analysis (EDS), etc.

[0025] The metal oxide contained in the surface modification layer is not particularly limited, and examples thereof include iron, cobalt, nickel, zinc (Zn), zirconium (Zr), tin (Sn), aluminum (Al), and titanium. When the surface modification of the active material particles is performed using the oxidation-reduction method described below, the redox pair of the metal species of the metal oxide contained in the surface modification layer must have a lower potential than the redox pair of the metal species of the metal oxide constituting the active material particles. Furthermore, among the metal species constituting these redox pairs, the solubility of the metal ions with a relatively low valence must be high, and the solubility of the metal ions with a relatively high valence must be low. In other words, the solubility of the metal ions with a low valence must be higher than the solubility of the metal ions with a high valence. Examples of metal species having such redox pairs include iron (Fe 3+ / Fe 2+ (vs.) is preferred.

[0026] (Secondary battery) In the secondary battery of this embodiment, the positive electrode active material is the electrode active material described above. In the electrode active material of this embodiment, the surfaces of the active material particles are modified, which can prevent side reactions such as decomposition of the electrolyte and supporting salt during charge and discharge. Therefore, the transition metal element constituting the active material particles is not particularly limited, and may be iron or an element selected from metal elements having a higher electrolyte oxidative decomposition activity than iron.

[0027] [Method of manufacturing electrode active material with the aid of oxalic acid] The electrode active material of this embodiment can be produced by an oxalic acid-assisted production method (first production method). This production method mainly includes the following steps.

[0028] (First suspension preparation process) By adding powdered active material particles to a solution containing dissolved oxalic acid (C2H2O4) salt and stirring it, oxalate ions bond to the surface of the active material particles. Note that among the oxygen atoms in the oxalic acid molecules, those that are not bonded to the surface of the active material particles bond to counterions contained in the oxalate salt or hydrogen ions.

[0029] (First powder acquisition process) After washing the first suspension, it is dried at a predetermined temperature (for example, 60°C) to obtain a first powder consisting of active material particles with oxalate ions attached to the surface. The first powder is preferably washed with water or the like. Drying after obtaining the first powder is not necessary.

[0030] (Second suspension preparation process) The first powder is placed in a solution containing the metal elements that constitute the metal oxide and stirred to produce a second suspension in which active material particles having oxalate ions attached to their surfaces are suspended. In the second suspension, the metal elements are bonded to the surfaces of the suspended active material particles via the oxalate ions.

[0031] (Second powder acquisition process) The second suspension is dried at a predetermined temperature (for example, 60° C.) to obtain a second powder made of active material particles having metal elements bonded to the surfaces thereof via oxalate ions.

[0032] (Second powder heating process) The second powder is heated to a temperature at which the active material particles do not decompose, preferably at about 200°C to 400°C. This heating removes carbon dioxide from the oxalate ions on the surfaces of the active material particles. The metal element that was bonded to the oxalate ions directly bonds with oxygen on the surfaces of the active material particles to form a metal oxide, thereby forming a surface modification layer that modifies the surfaces of the active material particles, and the electrode active material of this embodiment is obtained.

[0033] [Method of manufacturing electrode active material by oxidation-reduction] When the redox pair of the metal species of the metal oxide contained in the surface modification layer has a lower potential than the redox pair of the metal species of the metal oxide constituting the active material particles, and when the solubility of the low-valent metal ions of the metal species constituting these redox pairs is high and the solubility of the high-valent metal ions is low, a production method based on redox (second production method) can also be used to produce the electrode active material particles of this embodiment. This production method mainly includes the following steps. Here, the metal oxide contained in the surface modification layer is iron oxide.

[0034] (Third suspension preparation process) Divalent iron ions (Fe 2+ The powder of active material particles is placed in a solution containing ferrous iron (Fe ), and the mixture is stirred to prepare a third suspension in which the active material particles are suspended. In the third suspension, the divalent iron ions that come into contact with the transition metal ions on the surface of the suspended active material particles are oxidized to trivalent iron ions (Fe ). 3+ ), and the transition metal ions that the divalent iron ions come into contact with are reduced. Trivalent iron ions have low solubility in the third suspension, while the low-valent transition metal ions produced by the oxidation-reduction reaction have high solubility in the third suspension. Therefore, the transition metal ions dissolve into the third solution, and the trivalent iron ions precipitate on the surfaces of the active material particles.

[0035] (Third powder acquisition process) The third suspension is dried at a predetermined temperature (for example, 80° C.) to obtain a third powder made of active material particles surface-modified with the precipitated iron ions.

[0036] (Third powder heating process) The third powder is heated at a temperature at which the active material particles do not decompose, preferably at 50° C. to 400° C. This heating removes unnecessary elements adhering to the third powder, thereby forming a surface modification layer that modifies the surfaces of the active material particles, and the electrode active material of this embodiment is obtained.

[0037] As described above, the active material particles constituting the electrode active material of this embodiment are compounds of transition metal elements that are reactive with an electrolyte, but are covered with a thin, dense, and uniform surface modification layer regardless of the size of their specific surface area. Therefore, when the electrode active material of this embodiment is used as a positive electrode active material of a secondary battery, side reactions during charge and discharge caused by contact between the transition metal elements inserted and desorbed by the active material particles and the electrolyte can be prevented, and charge and discharge characteristics can be stabilized. [Example]

[0038] The effects of the present invention will be more clearly understood from the following examples. Note that the present invention is not limited to the following examples and can be practiced with appropriate modifications within the scope of the present invention.

[0039] Example 1 The electrode active material of the present invention was produced using the oxalic acid-assisted method of the above embodiment in the following procedure.

[0040] A powder of active material particles (MgMn2O4) was prepared. The prepared active material particle powder (2.5 mmol) was added to an aqueous solution obtained by dissolving ammonium oxalate (1 mmol) in water (12 g), and the mixture was stirred at room temperature for approximately 3 hours to prepare a first suspension in which the active material particles were suspended.

[0041] The first suspension thus prepared was centrifuged, and the resulting precipitate was washed with water and centrifuged. The resulting precipitate was then dried at approximately 60°C for approximately 12 hours to obtain a first powder of active material particles having oxalate ions bound to their surfaces.

[0042] Methanol (12 g) and iron nitrate hexahydrate (Fe(NO3)3)·6H2O (0.5 mmol) were added to the entire amount of the first powder obtained, and the mixture was stirred at room temperature for approximately 3 hours to produce a second suspension containing active material particles with iron bound to the surface via oxalate ions.

[0043] After the second suspension was centrifuged, the resulting precipitate was washed with methanol and centrifuged twice, and the resulting precipitate was dried at approximately 60°C for approximately 12 hours to obtain a second powder of active material particles with iron bonded to the surface via oxalate ions.

[0044] The entire amount of the obtained second powder was heated to about 350°C over about 5 hours, and then held for about 5 hours for heat treatment to burn off the oxalate ions from the iron coordination bond moieties, thereby obtaining the electrode active material of the present invention.

[0045] Examples 2 to 8 An electrode active material of the present invention was produced under the same conditions as in Example 1, except that the metal salt added to the second suspension was changed to one of cobalt nitrate hexahydrate (Co(NO3)3)·6H2O), nickel nitrate hexahydrate (Ni(NO3)3)·6H2O), zinc nitrate hexahydrate (Zn(NO3)2)·6H2O), zirconyl nitrate dihydrate (ZrO(NO3)2·2H2O), tin chloride dihydrate (SnCl2·2H2O), aluminum nitrate nonahydrate (Al(NO3)3)·6H2O), and titanyl sulfate n-hydrate (TiOSO4·nH2O, n = 1 to 2).

[0046] Example 9 An electrode active material of the present invention was produced under the same conditions as in Example 1, except that α-MnO2 powder was used as the active material particles.

[0047] Example 10 The electrode active material of the present invention was produced by the oxidation-reduction method of the above embodiment in the following procedure.

[0048] A powder of active material particles (MgMn2O4) was prepared. 2+ The prepared active material particle powder (0.625 mmol) was added to a solution containing 0.125 mmol of ammonium iron (II) sulfate hexahydrate (Fe(NH4)2(SO4)2·6H2O) as a source of the active material, and the mixture was stirred at room temperature for approximately 3 hours to produce a third suspension in which the active material particles were suspended.

[0049] The prepared third suspension was centrifuged, and the resulting precipitate was washed with water and centrifuged twice. The resulting precipitate was dried at about 80°C for about 12 hours to form a precipitate with a surface of Fe. 3+ A third powder of active material particles modified with ions was obtained. This third powder was heated to about 350°C over about 5 hours and then maintained at that temperature for about 5 hours to perform a heat treatment, thereby removing any unnecessary elements adhering thereto, thereby obtaining an electrode active material of the present invention.

[0050] (Examples 11 and 12) An electrode active material of the present invention was produced under the same conditions as in Example 10, except that the amount of ammonium iron (II) sulfate hexahydrate added to the third suspension was changed to 0.25 mmol or 0.375 mmol.

[0051] (Comparative Example 1) A powder of the same active material particles (MgMn2O4) as in Example 1 was prepared. This powder was not surface-modified.

[0052] (Comparative Example 2) Surface-modified active material particles were produced in the same manner as in Example 1, except that heating after surface modification was not carried out.

[0053] (Comparative Example 3) A powder of the same active material particles (α-MnO2) as in Example 9 was prepared. This powder was not surface-modified.

[0054] Table 1 shows information on the electrode active materials of Examples 1 to 12 and Comparative Examples 1 to 3.

[0055] [Table 1]

[0056] [Evaluation of surface modification of MgMn2O4 with oxalic acid] The electrode active materials of Example 1 and Comparative Examples 1 and 2 were analyzed using Fourier transform infrared spectroscopy. Figure 1 shows the spectra showing the analysis results. In the spectrum of the electrode active material of Comparative Example 2, the peak at 1300-1700 cm -1 In the spectra of the electrode active materials of Comparative Example 1 and Example 1, an absorption peak indicating the presence of a carboxy group of oxalic acid is observed. In contrast, no absorption peak indicating the presence of a carboxy group of oxalic acid is observed. Comparison of the three spectra reveals that oxalate ions (carboxy groups) are attached to the surface of the electrode active material as a result of surface modification, and that these oxalate ions are removed by heating after surface modification.

[0057] EDS analysis was performed on the electrode active materials of Example 1 and Comparative Example 1. FIG. 2 shows spectra illustrating the analysis results. In the spectrum of the electrode active material of Comparative Example 1, only peaks indicating the presence of Mg, Mn, and O, which constitute the active material particles, are observed. In contrast, in the spectrum of the electrode active material of Example 1, in addition to peaks indicating the presence of Mg, Mn, and O, a peak indicating the presence of Fe, which was not contained in the active material particles, is observed. These results demonstrate that a film containing iron was formed on the surface of the active material particles by the surface modification of Example 1.

[0058] Figure 3(a) is a TEM image of an active material particle having a surface modification layer in the electrode active material of Example 1. Figures 3(b) to 3(d) are EDS mapping images of Mg, Mn, and Fe present near the particle surface. These images reveal that Fe, which constitutes the surface modification layer, is present on the active material particles containing Mg and Mn.

[0059] Charge-discharge tests were performed on the composite electrodes containing the electrode active materials of Comparative Example 1 and Example 1. The composite electrodes used in the charge-discharge tests were prepared by the following procedure. Approximately 6 mg of the electrode active material powder, acetylene black, and polytetrafluoroethylene (PTFE) were weighed out in a weight ratio of 60:30:10 and kneaded in an agate mortar. Approximately 2 mg of the obtained composite electrode was supported on an Al mesh and pressed to prepare a working electrode of the composite electrode.

[0060] The counter electrode was magnesium, the reference electrode was a silver wire immersed in a 0.01M silver nitrate and 0.1M Mg(TFSA)2 solution of triglyme (G3), and the electrolyte was a tetraglyme (G4)-based solvated ionic liquid (0.3M [Mg(G4)(TFSA)2] / Pyr13TFSA). Charge-discharge measurements were performed at 100°C. Charge-discharge measurements were performed in constant current mode, starting from the discharge side, with a current of 10 mAg. -1 The potential range was -1.6-0.9V vs. Ag / Ag + (1.0-3.5V vs. Mg / Mg 2+ ) and the charging capacity is 180mAhg -1 (MgMn2O4) or 200mAhg-1 (α-MnO2) was used to regulate the

[0061] 4 and 5 are graphs showing the charge / discharge curves for each cycle obtained for the composite electrodes made of the electrode active materials of Comparative Example 1 and Example 1, respectively. Comparing the two graphs reveals that the composite electrode of Example 1 significantly suppresses the decrease in discharge capacity and the increase in charging overvoltage with increasing cycle count, compared to the composite electrode of Comparative Example 1. The effect of suppressing the decrease in discharge capacity is particularly evident from the second cycle onwards. In the electrode active material of Example 1, the surface modification of the active material particles suppresses side reactions such as oxidative decomposition of the electrolyte. As a result, it is believed that the stability of the charge / discharge characteristics is significantly improved compared to when the surfaces of the active material particles are unmodified.

[0062] EDS analysis was performed on the electrode active materials of Examples 1 to 8 and Comparative Example 1. Figure 6 shows spectra showing the analysis results. In the spectra of the electrode active materials of Examples 1 to 8, in addition to peaks indicating the presence of Mg, Mn, and O that constitute the active material particles, peaks indicating the presence of Fe, Co, Ni, Zn, Zr, Sn, Al, and Ti, which were not contained in the active material particles, are observed. These results demonstrate that, regardless of which metal element is used, a film containing the respective metal can be formed to modify the surface of the active material particles.

[0063] [Evaluation of surface modification of α-MnO2 with the aid of oxalic acid] Charge-discharge tests were performed on the composite electrodes made of the electrode active materials of Comparative Example 3 and Example 9. Figures 7 and 8 are graphs showing the charge-discharge curves for each cycle obtained for the composite electrodes made of the electrode active materials of Comparative Example 3 and Example 9, respectively. Comparing the two graphs reveals that the composite electrode of Example 9 significantly suppresses the decrease in discharge capacity and the increase in charging overvoltage with increasing cycle count compared to the composite electrode of Comparative Example 3. The effect of suppressing the decrease in discharge capacity is particularly evident from the second cycle onwards. In the composite electrode of Example 9, the surface modification of the active material particles suppresses side reactions such as oxidative decomposition of the electrolyte. As a result, it is believed that the stability of the charge-discharge characteristics is significantly improved compared to when the surfaces of the active material particles are unmodified.

[0064] [Evaluation of MgMn2O4 surface modification by redox] EDS analysis was performed on the electrode active materials of Examples 10 to 12. Figure 9 shows the spectra of the analysis results. A peak indicating the presence of Fe is observed in all of the spectra of the electrode active materials of Examples 10 to 12. Comparing the three spectra, it can be seen that this peak increases in height in proportion to the amount of divalent iron ions in the solution in which the active material particles are suspended. From these results, it is believed that a method utilizing oxidation-reduction can also be used to form an iron-containing film to modify the surface of active material particles, and that the film formed becomes a stronger surface modification layer as the amount of divalent iron ions added increases.

[0065] In Examples 10 to 12, active material particles were suspended in a solution containing iron ions. After approximately three hours of suspension and stirring, the resulting third suspension was centrifuged to separate the active material particles from the supernatant. The supernatant was dried, and the remaining powder was subjected to EDS analysis. Figure 10 shows a spectrum illustrating the analysis results. In this spectrum, peaks indicating the presence of Mg, Mn, and O, which constitute the active material particles, are observed, but no peak for Fe is observed. This result indicates that the Fe contained in the initial solution was bound to the active material particles removed by centrifugation.

[0066] Charge-discharge tests were conducted on the composite electrodes made of the electrode active materials of Examples 10 to 12. Figures 11 to 13 are graphs showing the charge-discharge curves for each cycle obtained with the electrode active materials of Examples 10 to 12, respectively. It can be seen that, compared with the composite electrode of Comparative Example 1 (Figure 4), the decrease in discharge capacity and the increase in charging overvoltage with increasing cycle count are significantly suppressed for all composite electrodes. Furthermore, a comparison of the charge-discharge characteristics of Examples 10 to 12 reveals that the greater the amount of divalent iron ions in the solution suspending the active material particles, i.e., the greater the amount of iron ions contributing to surface modification, the more significantly the decrease in charge-discharge characteristics is suppressed.

Claims

1. active material particles made of a compound containing a transition metal element and oxygen; and a surface modification layer containing a metal oxide and modifying the surfaces of the active material particles.

2. 2. The electrode active material according to claim 1, wherein the compound further contains magnesium.

3. 3. The electrode active material according to claim 1, wherein the transition metal element in the active material particles is an element selected from the group consisting of manganese, cobalt, nickel, and iron.

4. 3. The electrode active material according to claim 1, wherein the metal element constituting the metal oxide contained in the surface modification layer is an element selected from iron, cobalt, nickel, zinc, zirconium, tin, aluminum, and titanium.

5. a redox pair of the metal species of the metal oxide contained in the surface modification layer has a lower potential than a redox pair of the metal species of the metal oxide constituting the active material particles, 3. The electrode active material according to claim 1, wherein, of the metal species constituting each of the redox pairs, the solubility of a low-valent metal ion is higher than the solubility of a high-valent metal ion.

6. A secondary battery comprising the electrode active material according to claim 1 or 2 as a positive electrode active material.

7. 7. The secondary battery according to claim 6, wherein the transition metal element is iron or an element selected from metal elements having an electrolyte oxidative decomposition activity higher than that of iron.

8. 3. The method for producing an electrode active material according to claim 1 or 2, a first suspension preparation step of preparing a first suspension by suspending the active material particles in a solution containing oxalate ions; a first powder obtaining step of drying the first suspension to obtain a first powder composed of the active material particles having oxalate ions attached to the surfaces thereof; a second suspension preparation step of preparing a second suspension by adding the first powder to a solution containing metal elements constituting the metal oxide; a second powder obtaining step of drying the second suspension to obtain a second powder composed of the active material particles having the metal element bonded thereto via the oxalate ions; a second powder heating step of heating the second powder.

9. 3. The method for producing an electrode active material according to claim 1 or 2, a third suspension preparation step of preparing a third suspension by suspending the active material particles in a solution containing divalent iron ions; a third powder obtaining step of drying the third suspension to obtain a third powder composed of the active material particles having trivalent iron ions precipitated on the surfaces thereof; a third powder heating step of heating the third powder.

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