POSITIVE ELECTRODE ACTIVE MATERIAL FOR Li-ION SECONDARY BATTERY, METHOD FOR PRODUCING THE SAME, POSITIVE ELECTRODE FOR Li-ION SECONDARY BATTERY, AND Li-ION SECONDARY BATTERY

Abstract

The present invention relates to a positive electrode active material for a Li-ion secondary battery containing a Li-transition metal composite oxide. This Li-transition metal composite oxide has a layered rock salt crystal structure, and is represented by a formula (1): (1?x)Li.sub.2RuO.sub.3xLiMnO.sub.2 (Mn is trivalent Mn, and x is a real number satisfying 0<x<1). In addition, when part of Ru and/or Mn of the Li-transition metal composite oxide is replaced with a metal M such as Ti, durability can be improved. According to the present invention, a reduced amount of Ru but a higher capacity can be achieved for a positive electrode active material containing Li.sub.2RuO.sub.3.

Claims

1. A positive electrode active material for a Li-ion secondary battery, comprising a Li-transition metal composite oxide, wherein the Li-transition metal composite oxide has a layered rock salt crystal structure, and is represented by: a formula (1): (1?x)Li.sub.2RuO.sub.3?xLiMnO.sub.2, wherein Mn is trivalent Mn, and x is a real number satisfying 0<x<1.

2. The positive electrode active material for a Li-ion secondary battery according to claim 1, wherein x in the formula (1) satisfies 0.1?x?0.9.

3. The positive electrode active material for a Li-ion secondary battery according to claim 1, wherein a primary particle size of the Li-transition metal composite oxide is 1 ?m or more and 50 ?m or less.

4. The positive electrode active material for a Li-ion secondary battery according to claim 1, wherein the Li-transition metal composite oxide has part of Ru and/or Mn replaced with a metal M, and is represented by: a formula (2): (1?y?z)Li.sub.2RuO.sub.3 yLiMnO.sub.2zLi.sub.aMO.sub.b: wherein the metal M is any one of Ti, Nb, Y, Zr, Hf, and Ta; y and z are real numbers satisfying 0<y+z<1; and regarding a, b and c, a=1 and b=4 for M of a monovalent metal, a=2 and b=3 for M of a tetravalent metal, and a=3 and b=4 for M of a pentavalent metal.

5. The positive electrode active material for a Li-ion secondary battery according to claim 4, wherein y+z in the formula (2) satisfies 0.1?y+z?0.9.

6. A method for producing the positive electrode active material for a Li-ion secondary battery defined in claim 1, comprising: a mixing step of mixing a Li compound, a Ru compound, and a trivalent Mn compound to produce a precursor substance; and a firing step of heating the precursor substance at 700? C. or more and 1100? C. or less to generate the Li-transition metal composite oxide, wherein the firing step is performed in a non-oxidizing atmosphere.

7. A method for producing the positive electrode active material for a Li-ion secondary battery defined in claim 4, comprising: a step of mixing a Li compound, a Ru compound, a trivalent Mn compound, and a compound of the metal M to produce a precursor substance; and a firing step of heating the precursor substance at 800? C. or more and 1100? C. or less to generate the Li-transition metal composite oxide, wherein the firing step is performed in a non-oxidizing atmosphere.

8. A positive electrode for a Li-ion secondary battery, comprising the positive electrode active material for a Li-ion secondary battery defined in claim 1.

9. A Li-ion secondary battery, comprising the positive electrode for a Li-ion secondary battery defined in claim 8.

10. The positive electrode active material for a Li-ion secondary battery according to claim 2, wherein a primary particle size of the Li-transition metal composite oxide is 1 ?m or more and 50 ?m or less.

11. The positive electrode active material for a Li-ion secondary battery according to claim 2, wherein the Li-transition metal composite oxide has part of Ru and/or Mn replaced with a metal M, and is represented by: a formula (2): (1?y?z)Li.sub.2RuO.sub.3yLiMnO.sub.2zLi.sub.aMO.sub.b: wherein the metal M is any one of Ti, Nb, Y, Zr, Hf, and Ta; y and z are real numbers satisfying 0<y+z<1; and regarding a, b and c, a=1 and b=4 for M of a monovalent metal, a=2 and b=3 for M of a tetravalent metal, and a=3 and b=4 for M of a pentavalent metal.

12. The positive electrode active material for a Li-ion secondary battery according to claim 3, wherein the Li-transition metal composite oxide has part of Ru and/or Mn replaced with a metal M, and is represented by: a formula (2): (1?y?z)Li.sub.2RuO.sub.3yLiMnO.sub.2zLi.sub.aMO.sub.b: wherein the metal M is any one of Ti, Nb, Y, Zr, Hf, and Ta; y and z are real numbers satisfying 0<y+z<1; and regarding a, b and c, a=1 and b=4 for M of a monovalent metal, a=2 and b=3 for M of a tetravalent metal, and a=3 and b=4 for M of a pentavalent metal.

13. A method for producing the positive electrode active material for a Li-ion secondary battery defined in claim 2, comprising: a mixing step of mixing a Li compound, a Ru compound, and a trivalent Mn compound to produce a precursor substance; and a firing step of heating the precursor substance at 700? C. or more and 1100? C. or less to generate the Li-transition metal composite oxide, wherein the firing step is performed in a non-oxidizing atmosphere.

14. A method for producing the positive electrode active material for a Li-ion secondary battery defined in claim 3, comprising: a mixing step of mixing a Li compound, a Ru compound, and a trivalent Mn compound to produce a precursor substance; and a firing step of heating the precursor substance at 700? C. or more and 1100? C. or less to generate the Li-transition metal composite oxide, wherein the firing step is performed in a non-oxidizing atmosphere.

15. A method for producing the positive electrode active material for a Li-ion secondary battery defined in claim 5, comprising: a step of mixing a Li compound, a Ru compound, a trivalent Mn compound, and a compound of the metal M to produce a precursor substance; and a firing step of heating the precursor substance at 800? C. or more and 1100? C. or less to generate the Li-transition metal composite oxide, wherein the firing step is performed in a non-oxidizing atmosphere.

16. A positive electrode for a Li-ion secondary battery, comprising the positive electrode active material for a Li-ion secondary battery defined in claim 2.

17. A positive electrode for a Li-ion secondary battery, comprising the positive electrode active material for a Li-ion secondary battery defined in claim 3.

18. A positive electrode for a Li-ion secondary battery, comprising the positive electrode active material for a Li-ion secondary battery defined in claim 4.

19. A positive electrode for a Li-ion secondary battery, comprising the positive electrode active material for a Li-ion secondary battery defined in claim 5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] FIG. 1 illustrates XRD profiles of Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2 (x=0.33), Li.sub.1.2Mn.sub.0.4Ru.sub.0.4O.sub.2 (x=0.5), and Li.sub.1.17Mn.sub.0.5Ru.sub.0.33O.sub.2 (x=0.6) produced in First Embodiment;

[0048] FIG. 2 illustrates SEM images of Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2 (x=0.33) and Li.sub.1.2Mn.sub.0.4Ru.sub.0.4O.sub.2 (x=0.5) produced in First Embodiment;

[0049] FIG. 3 illustrates an EDS profile of Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2 (x=0.33) produced in First Embodiment;

[0050] FIG. 4 illustrates graphs of results of a constant current charging/discharging test of Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2 (x=0.33), Li.sub.1.2Mn.sub.0.4Ru.sub.0.4O.sub.2 (x=0.5), and Li.sub.1.17Mn.sub.0.5Ru.sub.0.33O.sub.2 (x=0.6) produced in First Embodiment;

[0051] FIG. 5 is a diagram illustrating the relationship between a discharge capacity and cycle number obtained in the constant current charging/discharging test of Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2 (x=0.33), Li.sub.1.2Mn.sub.0.4Ru.sub.0.4O.sub.2 (x=0.5), and Li.sub.1.17Mn.sub.0.5Ru.sub.0.33O.sub.2 (x=0.6) produced in First Embodiment;

[0052] FIG. 6 is a diagram illustrating an X-ray absorption spectrum of a K absorption edge of Mn of Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2 (x=0.33) produced in First Embodiment obtained immediately after the production to charge/discharge process;

[0053] FIG. 7 illustrates XRD diffraction profiles of Li.sub.1.03Ru.sub.0.5Mn.sub.0.1Ti.sub.0.1O.sub.2 (y=z=0.14), and Li.sub.1.28Ru.sub.0.42Mn.sub.0.15Ti.sub.0.15O.sub.2 (y=z=0.21) produced in Second Embodiment;

[0054] FIG. 8 illustrates SEM images of Li.sub.1.3Ru.sub.0.5Mn.sub.0.1Ti.sub.0.1O.sub.2 (y=z=0.14), and Li.sub.1.28Ru.sub.0.42Mn.sub.0.15Ti.sub.0.15O.sub.2 (y=z=0.21) produced in Second Embodiment;

[0055] FIG. 9 illustrates graphs of results of a constant current charging/discharging test of Li.sub.1.3Ru.sub.0.5Mn.sub.0.1Ti.sub.0.1O.sub.2 (y=z=0.14), and Li.sub.1.28Ru.sub.0.42Mn.sub.0.15Ti.sub.0.15O.sub.2 (y=z=0.21) produced in Second Embodiment;

[0056] FIG. 10 illustrates graphs of results of a constant current charging/discharging test (50? C.) of First Embodiment (Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2) and Second Embodiment (Li.sub.1.28Ru.sub.0.42Mn.sub.0.15Ti.sub.0.15O.sub.2);

[0057] FIG. 11 illustrates graphs of results of a constant current charging/discharging test, performed in respective electrolytic solutions (LiPF.sub.6 and LiFSA), of Li.sub.1.17Mn.sub.0.5Ru.sub.0.33O.sub.2 (x=0.6) examined in Third Embodiment; and

[0058] FIG. 12 is a diagram illustrating the relationship between a discharge cycle and cycle number obtained in the constant current charging/discharging test, performed in the respective electrolytic solutions (LiPF.sub.6 and LiFSA), of Li.sub.1.7Mn.sub.0.5Ru.sub.0.33O.sub.2 (x=0.6) examined in Third Embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0059] First Embodiment: An embodiment of the present invention will now be described. In the present embodiment, a positive electrode active material containing a Li-transition metal composite oxide having a composition of the formula (1) in which part of Ru of Li.sub.2RuO.sub.3 was replaced with trivalent Mn (Mn.sup.3+) was produced, and subjected to structural analysis/composition analysis by XRD and the like. Then, the thus produced positive electrode active material was used in a positive electrode to produce a Li-ion secondary battery, and electrochemical properties thereof were evaluated.

[Production of Positive Electrode Active Material]

[0060] A Li carbonate (Li.sub.2CO.sub.3) powder, a Ru oxide (RuO.sub.2) powder, and a Mn oxide (Mn.sub.2O.sub.3) powder were mixed to produce a precursor substance. At this point, the masses of the respective raw material powders were adjusted to obtain x in the formula (1) of 0.33, 0.5, and 0.6, and thus, three precursor substances were produced. In the present embodiment, however, only the Li carbonate powder was mixed in an amount larger by 3% than the theoretical mass. This is for preventing Li carbonate from volatilizing at the time of firing at a high temperature. In a step of mixing the raw material compounds, a wet ball mill (volume: 45 mL, grinding medium: zirconia balls, 10 mm: 5 pcs5 mm: 10 pcs1 mm: 4 g) was used for grinding and mixing at a rotational speed of 300 rpm for 5 hours to produce the precursor substances. Each precursor substance resulting from the mixing step was compressed into a pellet.

[0061] Then, the pellet-shaped precursor substance was fired to obtain a composite oxide. As firing conditions in the firing step, heating was performed at a temperature increase rate of 10? C./min up to 900? C., and the heating was retained for 12 hours after reaching 900? C. During this heating process, an argon gas was caused to flow through a furnace to maintain the heating atmosphere at a non-oxidizing atmosphere. After the heating for 12 hours, the temperature was lowered by furnace cooling to room temperature, and a Li-transition metal composite oxide of the formula (1) (x=0.33, 0.5, or 0.6) was taken out.

[XRD Analysis]

[0062] The Li-transition metal composite oxides constituting the positive electrode active material produced as described above were subjected to XRD analysis to confirm the crystal structures. The XRD analysis was conducted with Bruker D2 PHASER used as a test apparatus, with a CuK? ray used as the X-ray source at a sweep speed of 22.5?/m in.

[0063] FIG. 1 illustrates XRD profiles of the three Li-transition metal composite oxides produced in the present embodiment, that is, Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2 (x=0.33), Li.sub.1.2Mn.sub.0.4Ru.sub.0.4O.sub.2 (x=0.5), and Li.sub.1.17Mn.sub.0.5Ru.sub.0.33O.sub.2 (x=0.6). FIG. 1 also illustrates, for reference, a profile of LiMnO.sub.2 and a profile of Li.sub.1.33Ru.sub.0.57O.sub.2 (Li.sub.2RuO.sub.3) having a layered rock salt crystal structure. It was confirmed, based on FIG. 1, that all of the three Li-transition metal composite oxides produced in the present embodiment (Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2 (x=0.33), Li.sub.1.2Mn.sub.0.4Ru.sub.0.4O.sub.2 (x=0.5), and Li.sub.1.7Mn.sub.0.5Ru.sub.0.33O.sub.2 (x=0.6)) have a crystal structure similar to that of Li.sub.2RuO.sub.3, namely, the layered rock salt crystal structure. Accordingly, it was confirmed that the crystal structure is not largely changed by replacement with trivalent Mn in Li.sub.2RuO.sub.3.

[0064] In the XRD profiles of the composite oxides obtained by the replacement with trivalent Mn of the present embodiment, no diffraction peak derived from LiMnO.sub.2 was observed. In addition, a superlattice line in the vicinity of 20?, which is observed in a composite oxide replaced with tetravalent Mn, was not also observed, and therefore, it was presumed that Mn having been replaced in Li.sub.2RuO.sub.3 was trivalent Mn.

[SEM-EDS Analysis]

[0065] Next, the Li-transition metal composite oxides produced in the present embodiment were observed with SEM, and subjected to EDS analysis. FIG. 2 illustrates SEM images of Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2 (x=0.33) and Li.sub.1.2Mn.sub.0.4Ru.sub.0.4O.sub.2 (x=0.5). It was confirmed, based on the SEM observation, that primary particles having an average particle size of about 1 ?m were observed in all the compositions, and that the compositions have favorable particle sizes. Besides, no difference was found in the particle size depending on the composition (coefficient x). FIG. 3 illustrates a result of the EDS of Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2 (x=0.33), and it is understood that Mn and Ru were present to be close to or superimposed on each other. It was thus confirmed that a solid solution of the respective constituting elements was synthesized in ?m size in the present embodiment.

[Evaluation of Electrochemical Properties]

[0066] Then, electrochemical properties of the positive electrode active materials produced in the present embodiment were evaluated. In this evaluation test, each positive electrode active material of the present embodiment was incorporated into a bipolar electrochemical cell (TJ-AC: manufactured by Japan Tomcell Limited Company), and the resultant was subjected to a constant current charging/discharging test. As a positive electrode, one obtained by mixing the positive electrode active material (AM) of the present embodiment, a conductive material (acetylene black: AB), and a binder (polyvinylidene fluoride: PVDF), and subjecting the resultant to a carbon compounding treatment was used. As a negative electrode, one obtained by mixing Li titanate used as a negative electrode active material (AM), AB and PVDF was used. The constitution of the test apparatus was as follows: [0067] Positive electrode: AM:AB:PVDF=76.5:13.5:10 (wt %) [0068] Negative electrode: lithium metal [0069] Separator: polyolefin porous film (Cell Guard 2500)+glass filter (GB-100R) [0070] Electrolytic solution (electrolyte/solvent): 1M-LiPF.sub.6/(EC:DMC=3:7)

[0071] In the constant current charging/discharging test, a discharge capacity was measured in the initial charge at room temperature or 28? C. in a voltage range of 2.0 V to 4.8 V at a current density of 0.1 mA/cm.sup.2. Then, the charge and discharge was performed for 5 cycles to 30 cycles to measure a potential-capacity curve.

[0072] FIG. 4 illustrates results of the constant current charging/discharging test of Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2 (x=0.33), Li.sub.1.2Mn.sub.0.4Ru.sub.0.4O.sub.2 (x=0.5), and Li.sub.1.17Mn.sub.0.5Ru.sub.0.33O.sub.2 (x=0.6) of the present embodiment, and Li.sub.2RuO.sub.3 (x=0) measured for reference. FIG. 5 illustrates graphs of the relationship between cycle number and a discharge capacity obtained based on the results of the constant current charging/discharging test. Referring to FIG. 4, in the positive electrode active material of the present embodiment, there was a potential plateau in the vicinity of a voltage of 4.2 V at the time of the initial charge. This potential plateau was also found in Li.sub.2RuO.sub.3 measured for reference, and is probably derived from similar anionic redox. The initial discharge capacity was 251 mAh/g in Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2 (x=0.33), was 245 mAh/g in Li.sub.1.2Mn.sub.0.4Ru.sub.0.4O.sub.2 (x=0.5), and 234 mAh/g in Li.sub.1.17Mn.sub.0.5Ru.sub.0.33O.sub.2 (x=0.6). Since the initial discharge capacity of Li.sub.2RuO.sub.3 measured for reference was 255 mAh/g, it is understood that all the positive electrode active materials of the present embodiment had favorable discharge capacities. Referring to FIG. 5, capacity retention after 30 cycles of Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2 (x=0.33) was 94%, capacity retention after 30 cycles of Li.sub.1.2Mn.sub.0.4Ru.sub.0.4O.sub.2 (x=0.5) was 92%, and capacity retention after 9 cycles of Li.sub.1.17Mn.sub.0.5Ru.sub.0.33O.sub.2 (x=0.6) was 99%, and it is thus deemed that these had excellent cycle characteristics.

[X-ray Absorption Spectroscopy Measurement]

[0073] Next, in order to confirm charge compensation mechanism in initial charge and discharge of Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2 (x=0.33) of the positive electrode active material of the present embodiment, the Li-transition metal composite oxides were subjected to X-ray absorption spectroscopy (XANES) measurement of K absorption edge of Mn at respective stages of before charge (immediately after production), initial charge, full charge, and full discharge. At this point, conditions and method for the constant current charging/discharging test were the same as those described above.

[0074] FIG. 6 illustrates XANES spectra of Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2 (x=0.33) at the respective stages. It is noted that lower diagrams of FIG. 6 indicate a point of the XANES measurement in the initial charge and discharge process. It is understood from FIG. 6 that Mn was oxidized because absorption edge energy was shifted at the stage of the initial charge (130 mA/h) toward a higher energy side as compared with that at the stage of before charging. Since this shift of the absorption edge energy continued up to full charge, it is presumed that Mn made a contribution to charge compensation even in the above-described potential plateau in the vicinity of 4.2 V. Then, the absorption edge energy shifted after full discharge toward a lower energy side. This indicates that Mn was reduced (Mn.sup.4+ to Mn.sup.3+) with the discharge. Therefore, it can be confirmed that the charge compensation of Mn reversibly proceeds in Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2 (x=0.33) replaced with trivalent Mn.

[0075] It was confirmed, based on the results of the constant current charging/discharging test (FIG. 5) and the results of examination on change in state of Mn (FIG. 6), that the positive electrode active material of the present embodiment has a high discharge capacity, and a favorable cycle life. In addition, it was confirmed that Mn (trivalent Mn) makes a contribution to these favorable electrochemical properties.

[0076] Second Embodiment: In the present embodiment, a positive electrode active material containing a Li-transition metal composite oxide having a composition of the formula (2) (a=2, and b=2) in which trivalent Mn (Mn.sup.3+) and Ti (Ti.sup.4+) were partly replaced in Li.sub.2RuO.sub.3 was produced. Then, the analysis and evaluation similar to those of First Embodiment were performed.

[Production of Positive Electrode Active Material]

[0077] A Li carbonate (Li.sub.2CO.sub.3) powder, a Ru oxide (RuO.sub.2) powder, a Mn oxide (Mn.sub.2O.sub.3) powder, and titanium oxide (TiO.sub.2: anatase type) were mixed to produce a precursor substance. In the present embodiment, the masses of the respective raw material powders were adjusted to obtain (y, z) in the formula (2) of (0.14, 0.14) and (0.21, 0.21), and thus, two precursor substances were produced. A step of mixing the raw material compounds was performed in the same manner as in First Embodiment.

[0078] Then, the pellet-shaped precursor substance was fired to obtain a composite oxide. The heating conditions in the firing step were the same as those employed in First Embodiment, and the firing was performed in an argon atmosphere. After the firing step, a Li-transition metal composite oxide of the formula (2) was taken out.

[XRD Analysis]

[0079] Each of the Li-transition metal composition oxides produced in the present embodiment was subjected to XRD analysis to confirm the crystal structure. The XRD analysis was performed in the same manner as in First Embodiment. FIG. 7 illustrates XRD profiles of the two Li-transition metal composite oxides produced in the present embodiment, that is, Li.sub.1.3Ru.sub.0.5Mn.sub.0.1Ti.sub.0.1O.sub.2 (y=z=0.14) and Li.sub.1.28Ru.sub.0.42Mn.sub.0.15Ti.sub.0.15O.sub.2 (y=z=0.21). It was confirmed based on FIG. 7 that the two Li-transition metal composite oxides produced in the present embodiment (Li.sub.1.3Ru.sub.0.5Mn.sub.0.1Ti.sub.0.1O.sub.2 (y=z=0.14), and Li.sub.1.28Ru.sub.0.42Mn.sub.0.15Ti.sub.0.15O.sub.2 (y=z=0.21)) both had a layered rock salt crystal structure similarly to Li.sub.2RuO.sub.3. Therefore, it was confirmed that the crystal structure is not largely changed by the replacement with Ti and the like.

[SEM-EDS Analysis]

[0080] Also in the present embodiment, the Li-transition metal composite oxides were subjected to SEM observation and EDS analysis. FIG. 8 illustrates SEM images of the respective composite oxides of the present embodiment. Also in the composite oxides of the present embodiment, primary particles having an average particle size of about 1 ?m were observed. It was confirmed based on results of the EDS analysis that the Li-transition metal composite oxide of the present embodiment is also a solid solution of the respective constituting elements.

[Evaluation of Electrochemical Properties]

[0081] Then, a constant current charging/discharging test was performed in order to evaluate the electrochemical properties of the produced positive electrode active material. A test apparatus and test conditions employed in the constant current charging/discharging test were the same as those employed in First Embodiment. In the present embodiment, the constant current charging/discharging test was performed by conducting 3 charge and discharge cycles at 50? C. in a voltage range of 2.0 V to 4.7 V at a current density of 0.1 mA/cm.sup.2, and a discharge capacity in the initial charge was measured.

[0082] FIG. 9 illustrates results of the constant current charging/discharging test of Li.sub.1.3Ru.sub.0.5Mn.sub.0.1Ti.sub.0.1O.sub.2 (y=z=0.14), and Li.sub.1.28Ru.sub.0.42Mn.sub.0.15Ti.sub.0.15O.sub.2 (y=z =0.21) of the present embodiment, and Li.sub.2RuO.sub.3 (y=z=0) measured for reference. It was confirmed based on FIG. 9 that the positive electrode active materials of the present embodiment both had an initial discharge capacity of 250 mAh/g or more, and had excellent cycle characteristics. Also in the present embodiment, a potential plateau derived from anionic redox was confirmed at the time of initial charge and discharge similarly to Li.sub.2RuO.sub.3.

[0083] Next, in order to confirm durability of the positive electrode active material of the present embodiment, Li.sub.1.28Ru.sub.0.42Mn.sub.0.15Ti.sub.0.15O.sub.2 (y=z=0.21) of the present embodiment and Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2 (x=0.33) that is the Li-transition metal composite oxide of First Embodiment obtained by the replacement with only Mn were subjected to a constant current charging/discharging test (cycle number: 5) at 50? C. for comparison. Test conditions were the same as those described above.

[0084] FIG. 10 illustrates results of this constant current discharging test. In both the Li-transition metal composite oxides, the initial discharge capacity was favorable, but in the Li-transition metal composition oxide of First Embodiment (Li.sub.1.25Mn.sub.0.25Ru.sub.0.5O.sub.2 (x=0.33)), the discharge capacity was lowered slightly with the increase of the cycle number in the second and later cycles. On the contrary, in the Li-transition metal composite oxide of Second Embodiment (Li.sub.1.28Ru.sub.0.42Mn.sub.0.15Ti.sub.0.15O.sub.2 (y=z=0.21)), the extent of lowering of the discharge capacity in the second and later cycles was very small. The Ru amount in the Li-transition metal composite oxide of Second Embodiment was smaller than that in First Embodiment. In spite, it was confirmed that the capacity lowering through charge and discharge cycles was reduced. This is probably because the composite oxide crystal was stabilized through the partial replacement with Ti.

[0085] Third Embodiment: In this embodiment, Li.sub.1.17Mn.sub.0.5Ru.sub.0.33O.sub.2 (x=0.6) of the Li-transition metal composite oxide examined in First Embodiment was evaluated for properties obtained when the constitution of the electrolyte was changed. Here, Li.sub.1.17Mn.sub.0.5Ru.sub.0.33O.sub.2 the same as that used in First Embodiment was incorporated into a bipolar electrochemical cell as the positive electrode active material to perform a constant current charging/discharging test. The constitution of a test apparatus in the constant current charging/discharging test was as follows, which are similar to those of First Embodiment: [0086] Positive electrode: AM:AB:PVDF=80:10:10 (wt %) [0087] Negative electrode: lithium metal [0088] Separator: polyolefin porous film (Cell Guard 2500)+glass filter (GB-100R)

[0089] As for the electrolytic solution, two electrolytic solutions of an electrolytic solution A using LiFSA as an electrolyte, and an electrolytic solution B using LiPF.sub.6 as an electrolyte as in First Embodiment, were examined as follows: [0090] Electrolytic solution A: 5.3 MLiFSA (electrolyte)/TMP (electrolytic solution) [0091] Electrolytic solution B: 1 MLiPF.sub.6 (electrolyte)/(EC:DMC=3:7 (electrolytic solution))

[0092] In the constant current charging/discharging test, a discharge capacity was measured in the initial charge at 28? C. in a voltage range of 2.2 V to 4.5 V at a current density of 0.1 mA/cm.sup.2. Then, 100 cycles of the charge and discharge was performed to measure a potential-capacity curve.

[0093] FIG. 11 illustrates results of the constant current charging/discharging test using the electrolytic solution A and the electrolytic solution B in the cells using Li.sub.1.17Mn.sub.0.5Ru.sub.0.33O.sub.2 (x=0.6) as the positive electrode active material. FIG. 12 is a graph illustrating the relationship between the cycle number and the discharge capacity obtained based on the results of the constant current charging/discharging test. It is understood from these drawings that as properties against an electrolytic solution obtained in using Li.sub.1.17Mn.sub.0.5Ru.sub.0.33O.sub.2 (x=0.6) as the positive electrode active material, the initial capacity was slightly higher in using the electrolytic solution B (LiPF6), but the capacity retention after the 100 cycles was higher in using the electrolytic solution A (LiFSA). It was thus confirmed that the Li.sub.2RuO.sub.3-based oxide replaced with trivalent Mn of the present invention has properties changed depending on the electrolytic solution (electrolyte). In the present embodiment, it is deemed that more excellent cycle property (capacity retention) was exhibited by applying LiFSA as the electrolyte. Therefore, it is deemed, in the present invention, that the electrolytic solution can be selected in accordance with property requirements of a Li-ion secondary battery to which the positive electrode active material is applied.

INDUSTRIAL APPLICABILITY

[0094] As described so far, a positive electrode active material for a Li-ion secondary battery of the present invention is obtained based on a Li.sub.2RuO.sub.3 Li-transition metal composite oxide, and a capacity is increased with a Ru amount reduced by replacement with trivalent Mn. According to the present invention, with the use amount of Ru reduced for reducing cost of the active material, favorable electrochemical properties of Li.sub.2RuO.sub.3 can be maintained.

[0095] The positive electrode active material of the present invention can be favorably applied to a positive electrode of a Li-ion secondary battery, and can be widely used in various types of small batteries, household power supplies, vehicle batteries and the like.