Novel manganese oxides, production thereof, and use thereof

Abstract: A manganese oxide is provided which is represented by the general formula A x Mn₂O₄.nH₂O, where A is Zn, Mg, Ca, or Co; and if A is Zn, 0 ≦ x < 1 and n = 0; if A is Mg or Ca, 0 ≦ x ≦ 1 and and 0 ≦ n ≦ 20; and if A is Co, 0 < x ≦ 1 and n = 0. The manganese oxide has a spinel type, a layer type, amorphous type or both crystal structures of layer type and spinel type. The manganese oxide is produced by oxidizing a mixture of a hydroxide of A (where A is Zn, Mg, Ca, or Co) and manganese hydroxide in a molar ratio of 1:2 in an alkali solution, and then removing A by oxidation treatment. A lithium secondary battery is provided by use of a kind of the above manganese oxide for the positive electrode, exhibiting high output with high energy density and with satisfactory reversibility in charge-discharge cycle.

Description: The present invention relates to novel manganese oxides, a process for production of the manganese oxide, and a lithium secondary battery employing the manganese oxide. More particularly, the present invention relates to novel manganese oxides represented by the general formula A x Mn 2 O 4 ·nH 2 O (1) (where A is Zn, Mg, Ca, or Co; and if A is Zn, 0 ≦ x < 1 and n = 0; if A is Mg or Ca, 0 ≦ x < 1 and 0 ≦ n ≦ 20 ; and if A is Co, 0 < x < 1 and n = 0 obtainable by a) preparing an aqueous solution of an A salt and a two-valent Mn salt in a molar ratio of 1:2, adding an equal amount or more of an alkali solution, and oxidizing the resulting hydroxide mixture by bubbling air and/or oxygen into the solution to obtain an intermediate compound b) subjecting the obtained intermediate compound to an oxidation treatment without additional acid for attaining the +4 valence of Mn by chemical or electrochemical oxidation to obtain the compound of formula (1) said compound of formula (1) being not of a proton type, and having a crystal structure of spinel type, a layer type, or a spinel-and-layer combination type. The present invention also relates to a process for producing the novel manganese oxides, and use of some of the manganese oxides for a positive electrode of a lithium secondary battery. The manganese oxide having a spinel type or a layer type crystal structure has paths which enable migration of ions in the crystal structure, therefore being promising as a host compound. Additionally, the manganese atom is capable of varying its valency continuously and reversibly, and therefore it is promising as a material exhibiting both oxidation-reduction ability and host compound function. Accordingly, the manganese oxides having such properties are attracting attention for use as an active battery material, a catalyst, an adsorption material, and a magnetic material. On the other hand, a lithium secondary battery is promising as a new secondary battery because of its high output and high energy density. The manganese oxides have various compositions and various crystalline states. For example, manganese dioxide has crystal states of α type, β type, γ type, λ type, δ type, etc. The β type manganese dioxide forms a single chain by linking of MnO 2 octahedrons by sharing the edges three-dimensionally in the C-axis direction, and thereby develops a tunnel structure of (1 x 1) toward the (110) direction Similarly, the γ type manganese dioxide forms double chains of MnO 6 octahedron and the double chains are connected by sharing of the edges three-dimensionally in the C axis direction and has a tunnel structure of (1 x 2) along the (110) direction. α type manganese dioxide has a tunnel structure of (2 x 2) and (3 x 2) developed two-dimensionally. On the other hand, the λ type manganese dioxide has a tunnel structure formed by three-dimensional linking of (1 x 1)-tunnels and has a crystal state assignable to a spinel type crystal structure. The spinel type crystal structure has sites for cation at the octahedron position and the tetrahedron position of the close-packed oxygen atoms, and the sites are connected three-dimensionally. ZnMn 2 O 4 having a spinel type crystal structure is a compound in which Mn atoms occupy the octahedron sites and Zn atoms occupy the tetrahedron sites. If Zn is removed from this compound with the spinel type crystal structure kept unchanged, sites for accepting cation are formed in the crystal structure, and thereby the Mn atom is capable of varying its valency continuously and reversibly. Therefore the compound can be a material useful as a host compound. In particular, if Zn is completely eliminated from this compound, an λ type manganese dioxide is obtained in which 2/3 of the cation sites ore occupied by manganese atoms and 1/3 of the cation sites are unoccupied and vacant, and the cation-holding capacity becomes maximum. However, Zn has never been eliminated from ZnMnO 2 with its spinel structure maintained. Therefore, ZnMn 2 O 4 is used, as it is, only for limited application, for example, a raw material of ferrite and has not been used as a host compound. Although many other studies have been made on synthesis of λ type manganese dioxide of spinel type structure, the inherent function thereof has not been exhibited sufficiently. For example, the λ type manganese dioxide of the spinel structure reported so far is synthesized by heating a manganese spinel compound at a high temperature and treating the heated product with an acid. In JP-A-55-100224, manganese dioxide of spinel crystal structure is synthesized by mixing, in a solid state, β type manganese dioxide as a precursor and powdery Li 2 CO 3 in a ratio of Li:Mn = 1:2 (molar ratio), beating the mixture in the air at 850°C, treating the resulting LiMn 2 O 4 with sulfuric acid to remove Li. The inventors of the present invention, however, have found that the manganese dioxide of spinel structure obtained by the above procedure is a manganese compound of a proton-type spinel crystal structure in which Li is replaced by proton by ion-exchange since Li is removed by treating the precursor LiMn 2 O 4 with sulfuric acid. Accordingly, the compound is useful in only limited fields in comparison with inherent use of host compounds. In JP-A-62-270420, manganese dioxide of a spine, structure is synthesized by mixing an aqueous magnesium chloride solution and an aqueous manganese chloride solution in a ratio of Mg:Mn = 1:2 (molar ratio), adding ammonia to the mixture to form mixed hydroxides of Mg and Mn, collecting the product by filtration, heating the collected product at 700°C, and treating the resulting MgMn 2 O 4 with nitric acid to remove Mg. However, with this product also, the inherent function of manganese dioxide of the spinal structure is not exhibited for the same reason as mentioned above regarding LiMn 2 O 4 . δ type manganese dioxide is in a crystal state having MnO 6 octahedron linked two-dimensionally by sharing the edges, and has a layer structure in which Mn atoms are arranged in alternate layers of oxygen in hexogonal close-packed state. Manganese dioxide of a layer structure is found in nature as birnessite. As artificial birnessite, sodium birnessite is known which is synthesized by neutralizing an aqueous Mn 2+ solution with sodium hydroxide, and oxidizing the resulting manganese hydroxide by air at a low temperature of about 0°C. This oxide has a layer structure in which Mn atoms are arranged in alternate layers of hexagonally close-packed oxygen atoms, as described above. The layer structure is formed by electrostatic repulsion between oxygen atoms in Mn-lacking layers, and is usually stabilized by canceling the electrostatic repulsion by insertion of univalent cation such as sodium in the Mn-lacking layer. On insertion of the cation, water molecules are partly incorporated as hydration water. Thus the manganese oxide of a layer structure is capable of accepting cation between two layers formed by electrostatic repulsion between oxygens. Further, the layer structure is considered to be capable of accepting and releasing cation. Therefore, it is expected as a promising host compound. With prior art technique, however, the manganese oxide of layer structure needs to be synthesized at a low temperature of about 0°C as described above, which is not suitable for industrial production. By this reason, the manganess oxide of layer structure has not been practically used as a host compound. To solve this problem, US-A-4,520,005 discloses a birnessite having Bi or Pb inserted between the layers which is synthesized at an ordinary temperature in the co-presence of Bi ion or Pb ion with sodium ion in the solution. This disclosure intends to use the birnessite to a positive-electrode-active material of an alkali secondary battery, but it involves a problem that the production cost is high because of the use of expensive Bi or Pb. With such a background, the manganese oxide of layer structure has not been practically used as a host compound in spite of expectation. δ type manganese dioxide also exhibits a layer structure. However, it is considered as an aggregate of fine crystals having layer structure. δ type manganese dioxide is prepared, according to a generally known method, by mixing potassium permanganate with hydrochloric acid and heating the resulting precipitate at 250°C. However, known stable δ type manganese dioxide exists only in a narrow range of the oxidation degree of manganese from about 1.5 to about 1.8. This low oxidation degree of manganese prevents the material from exhibiting the performance expected from the crystal structure. On the other hand, with the progress of wireless electronic devices, development of a secondary battery of small size, light weight, and high energy density is strongly desired. To satisfy this desire, a lithium battery was proposed which employs, as the negative electrode, lithium or a material capable of occluding or releasing lithium. Oxides and sulfides of molybdenum, vanadium, niobium, titanium, nickel, cobalt, etc. were mainly studied as the positive electrode for the lithium secondary battery, but have not been practically used except for limited oxides or sulfides. Manganese oxide has been also studied as a promising positive electrode material. However, the manganese dioxide is applied only to the positive electrode of a lithium primary battery and has not been practically used as the positive electrode or a lithium secondary battery. Manganese dioxide, when used for the positive electrode of a lithium secondary battery, expands or shrinks in the crystal structure by incorporation of lithium ion into and release thereof from the crystal structure on charge and discharge. Therefore the manganese dioxides hitherto used for lithium primary batteries collapse in the crystal structure by the expansion and shrinkage of the crystal structure to result in remarkable drop of the discharge capacity, thereby not being applicable to secondary battery. Therefore manganese oxide is required which has crystal structure which does not collapse on cycles of incorporation-release of lithium ion in the crystal structure. With such a background, manganese oxides having lithium-receiving sites connected in a tunnel-like state in the crystal structure, such as of spinel structure and layer structure, are investigated in recent years. The manganese oxides of spinel type crystal structure have already been studied extensively. For example, the aforementioned LiMn 2 O 4 and the like have been proposed as the positive electrode material. The inventors of the present invention noticed that the manganese oxides of spinel type crystal structure disclosed so far are low in output power and usage percent, and are still unsatisfactory in reversibility in charge and discharge. On the other hand, a study on manganese oxide of layer type crystal structure or of δ type crystal structure in which lithium ion is considered to migrate more easily in the solid phase than in the one of spinel type crystal structure is almost none since a suitable material for the positive electrode of the secondary battery has not been obtained therefrom. The present invention intends to provide a novel manganese oxide which has a spinel type crystal structure, layer type structure, and a spinel-and-layer type crystal structure, and is promising material as a host compound for an active battery material, a catalyst, an adsorbent, and a magnetic material. The present invention also intends to provide a process for producing the above manganese oxide. The present invention further intends to provide a lithium secondary battery which employs some kind of the above oxides as the positive electrode, and exhibiting high output power, high energy density, and satisfactory reversibility of charge-discharge cycles. The manganese oxides of the present invention have a crystal structure of spinel type, a layer type, or a spinel-and-layer combination type, and are represented by the general formula A x Mn 2 O 4 ·nH 2 O (where A is Zn, Mg, Ca, or Co; and if A is Zn, 0 ≦ x ≦ 1 and n = 0; if A is Mg or Ca, 0 ≦ x < 1 and 0 ≦ n ≦ 20 ; and if A is Co, 0 < x < 1 and n = 0 obtainable by a) preparing an aqueous solution of an A salt and a two-valent Mn salt in a molar ratio of 1:2, adding an equal amount or more of an alkali solution, and oxidizing the resulting hydroxide mixture by bubbling air and/or oxygen into the solution to obtain an intermediate compound b) subjecting the obtained intermediate compound to an oxidation treatment without additional acid for attaining the +4 valence of Mn by chemical or electrochemical oxidation to obtain the compound of formula (1) said compound of formula (1) being not of a proton type. The process for producing the manganese oxides of the present invention comprises preparing an aqueous solution of an A salt and a two-valent Mn salt in a molar ratio of 1:2, adding an equal amount or more of an alkali solution, and oxidizing the resulting hydroxide mixture by bubbling air and/or oxygen into the solution to obtain an intermediate compound subjecting the obtained intermediate compound to an oxidation treatment without additional acid for attaining the +4 valence of Mn by chemical or electrochemical oxidation to remove A. The lithium secondary battery of the present invention employs one or more kinds of the above manganese oxides for the positive electrode. Fig. 1 is a cross-sectional view of a battery employed in the Examples and Comparative Examples. Fig. 2 is an X-ray diffraction pattern of ZnMn 2 O 4 prepared in Example 1. Fig. 3 is an X-ray diffraction pattern of Zn 0.67 Mn 2 O 4 prepared in Example 1. Fig. 4 is an X-ray diffraction pattern of Zn 0.33 Mn 2 O 4 prepared in Example 2. Fig. 5 is an X-ray diffraction pattern of Zn 0.1 Mn 2 O 4 prepared in Example 3. Fig. 6 is an X-ray diffraction pattern of manganese dioxide of spinel type crystal structure prepared in Example 4. Fig. 7 is an X-ray diffraction pattern of ZnMn 2 O 4 prepared in Example 5. Fig. 8 is an X-ray diffraction pattern of Zn 0.10 Mn 2 O 4 prepared in Example 5. Fig. 9 is an X-ray diffraction pattern of manganese dioxide of spinel type crystal structure prepared in Example 6. Fig. 10 is an X-ray diffraction pattern of manganese oxide having γ type crystal structure prepared in Comparative Example 1. Fig. 11 is an X-ray diffraction pattern of ZnMn 2 O 4 prepared in Comparative Example 2. Fig. 12 is an X-ray diffraction pattern of Zn 0.9 Mn 2 O 4 prepared in Comparative example 2. Fig. 13 is an X-ray diffraction pattern of MgMn 2 O 4 .16H 2 O prepared in Example 7. Fig. 14 is an X-ray diffraction pattern of Mg 0.08 Mn 2 O 4 .H 2 O prepared in Example 7. Fig. 15 is an X-ray diffraction pattern of MgMn 2 O 4 .14H 2 O prepared in Example 8. Fig. 16 is an X-ray diffraction pattern of Mg 0.06 Mn 2 O 4 .16H 2 O prepared in Example 8. Fig. 17 is an X-ray diffraction pattern of sodium birnessite prepared in Comparative Example 4. Fig. 18 is an X-ray diffraction pattern of sodium-free birnessite prepared in Comparative Example 4. Fig. 19 is an X-ray diffraction pattern of CaMn 2 O 4 .3H 2 O prepared in Example 9. Fig. 20 is an X-ray diffraction pattern of Ca 0.19 Mn 2 O 4 .0.4H 2 O prepared in Example 9. Fig. 21 is an X-ray diffraction pattern of CaMn 2 O 4 .2.3H 2 O prepared in Example 10. Fig. 22 is an X-ray diffraction pattern of Ca 0.10 Mn 2 O 4 .0.3H 2 O prepared in Example 10. Fig. 23 is an X-ray diffraction pattern of MnO 1.83 having δ type crystal structure prepared in Comparative Example 5. Fig. 24 is an X-ray diffraction pattern of CoMn 2 O 4 prepared in Example 11. Fig. 25 is an X-ray diffraction pattern of Co 0.80 Mn 2 O 4 prepared in Example 11. Fig. 26 is an X-ray diffraction pattern of CoMn 2 O 4 prepared in Example 12. Fig. 27 is an X-ray diffraction pattern of Co 0.70 Mn 2 O 4 prepared in Example 12. A manganese oxide of spinel type crystal structure represented by the general formula Zn x Mn 2 O 4 ( 0 ≦ x < 1 ) was found to be produced by removing Zn from ZnMn 2 O 4 by oxidation treatment of ZnMn 2 O 4 of a spinel type crystal structure. A lithium secondary battery exhibiting high output, high energy density, and satisfactory reversibility of change-discharge cycles is obtained by use of the manganese oxide as the positive electrode. In particular, starting from ZnMn 2 O 4 having a BET specific surface area of 10 m 2 /g or more, manganese dioxide of spinel type structure having a BET specific surface area of not less than 100 m 2 /g is obtained which is highly suitable for lithium secondary battery because of high output, high energy density and satisfactory reversibility of charge-discharge cycles. These surface area conditions are particularly preferred when x = 0. A manganese oxide having a novel layer structure having unknown novel interplanar spacing was found to be synthesized by oxidizing a mixture of magnesium hydroxide and manganese hydroxide in a Mg:Mn molar ratio of 1:2 in an agueous alkali solution. The interplanar spacing had not been known for an oxide of Mg:Mn molar ratio of 1:2. The interplanar spacing can readily be determined from the X-ray diffraction angle according to Bragg's equation. By removing Mg by oxidation treatment, a manganese oxide having a layer structure and represented by the general formula Mg x Mn 2 O 4 .nH 2 O( 0 ≦ x < 1, 0 ≦ n ≦ 20 ) was found to be obtained. With this magnesium oxide used for the positive electrode, a lithium secondary battery was constructed which exhibits high output, high energy density, and satisfactory reversibility of charge-discharge cycles. Another novel manganese oxide having unknown amorphous crystal structure was found to be synthesized by oxidizing a mixture of calcium hydroxide and manganese hydroxide in a Ca:Mn molar ratio of 1:2 in an aqueous alkali solution. The amorphous crystal structure had not been known for an oxide of Ca:Mg molar ratio of 1:2. By removing Ca by oxidation treatment, a manganese oxide was obtained which has δ type crystal structure and represented by the general formula Ca x Mn 2 O 4 .nH 2 O( 0 ≦ x < 1, 0 ≦ n ≦ 20 ). With this magnesium oxide used for the positive electrode, a lithium secondary battery was constructed which exhibited high output, high energy density, and satisfactory reversibility of charge-discharge cycles. A manganese oxide of a novel layer-and-spinel type crystal structure was found to be synthesized by oxidizing a mixture of cobalt hydroxide and manganese hydroxide in a Co:Mn molar ratio of 1:2 in an aqueous alkali solution. By removing Co by oxidation treatment, a manganese oxide having a layer-and-spinel type crystal structure and represented by the general formula Co x Mn 2 O 4 (0 < x < 1) was obtained. With this manganese oxide used for the positive electrode, a lithium secondary battery was constructed which exhibited high output, high energy density, and satisfactory reversibility of charge-discharge cycles. As described above, the present invention relates to a novel manganese oxide prepared as described above and having a crystal structure of spinel type, a layer type, or a spinel-and-layer combination type and being represented by the general formula A x Mn 2 O 4 nH 2 O (where A is Zn, Mg, Ca, or Co; and if A is Zn, 0 ≦ x < 1 and n = 0; if A is Mg or Ca 0 ≦ x < 1 and 0 ≦ n ≦ 20 ; and if A is Co, 0 < x <1 and n = 0), and also relates to a process for producing the novel manganese oxides, and a lithium secondary battery employing some of the manganese oxides for a positive electrode thereof. The manganese oxides represented by the general formula A x Mn 2 O 4 ·nH 2 O (where A is Zn, Mg, Ca, or Co; and if A is Zn, 0 ≦ x < 1 and n = 0; if A is Mg or Ca, 0 ≦ x < 1 and 0 ≦ n ≦ 20 ; and if A is Co, 0 < x < 1 and n = 0) have ion migration paths and ion accepting sites in the crystal structure, and is promising as a host compound. Further, the manganese atom is capable of varying its valency continuously and reversibly, so that the manganese oxide is also promising as a material exhibiting both a oxidation-reduction ability and a host compound function. The manganese oxide having a spinel type crystalline structure and being represented by the general formula Zn x Mn 2 O 4 ( 0 ≦ x < 1 ) is derived from a manganese oxide having a spinel type crystal structure and represented by ZnMn 2 O 4 by oxidation treatment to remove Zn. In particular, starting from ZnMn 2 O 4 having a BET specific surface area of 10 m 2 /g or more, manganese dioxide of spinal type structure having a BET specific surface area of not less than 100 m 2 /g is obtained which is assignable to λ type. When this latter condition is not the case, x is preferably 0 < x < 1 . The starting material, ZnMn 2 O 4 , is a manganese oxide of a normal spinel type crystal structure, and has Mn atoms at the octahedron positions of close-packed oxygen atoms and Zn atoms at the tetrahedron position. In the production of the spinel type crystalline structure by removal of Zn, it is important to oxidize mixed hydroxide of Mn and Zn in an aqueous alkali solution such as by aeration and oxidize the resulting ZnMn 2 O 4 . Specifically, to an aqueous solution containing a two-valent Mn salt and a Zn salt dissolved therein in a molar ratio of 2:1, an equal amount or more amount of an alkali solution is added, the resulting hydroxide may be oxidized by aeration, and the obtained ZnMn 2 O 4 is oxidized to achieve +4 valence of Mn. The obtained ZnMn 2 O 4 may be heat treated to raise its crystallinity. The aeration oxidation may be conducted by bubbling air and/or oxygen into the solution. The oxidation treatment for attaining +4 valence of Mn may be conducted, for example, by chemical oxidation or electrochemical oxidation. Specifically, in one method, ZnMn 2 O 4 is treated with a solution containing at least one oxidizing agent in an amount sufficient for oxidizing Mn to +4 valent. In another method, by using an electrolytic cell having an anode chamber and a cathode chamber separately, ZnMn 2 O 4 is suspended in the anode chamber and is oxidized on the anode. The oxidizing agent used for this oxidation is not specially limited. Any oxidizing agent may be used which is capable of oxidizing the Mn to +4 valence, including persulfate salts such as sodium persulfate, potassium persulfate, ammonium persulfate, etc., hydrogen peroxide, chlorine, and the like. Hitherto, treatment of ZnMn 2 O 4 with a mineral acid or an organic acid has been investigated as a method of removal of Zn. In this method, Mn dissolution occurs in addition to Zn removal, which gives a γ type manganese dioxide without maintaining the spinel type crystal structure, or otherwise gives a proton type of spinel type crystal structure in which H + is introduced in place of Zn even though Zn is partially removed. Accordingly, the compound having host compound function which is the object of the present is not obtainable by acid treatment. On the contrary, in the treatment of oxidizing Mn to +4 valence as in the present invention, the oxidation of Mn and the removal of Zn proceed simultaneously, giving manganese oxide having spinel structure from which Zn has been removed. Particularly in the case where ZnMn 2 O 4 having a BET specific surface area of 10 m 2 /g or more is used, the resulting manganese oxide is or an λ type assignable to the spinel crystal structure, having a BET specific surface area of 100 m 2 /g or more. The oxide, represented by the general formula MgMn 2 O 4 .nH 2 O ( 0 ≦ n ≦ 20 ) in which the Mg:Mn molar ratio is 1:2 does not have the spinel type crystal structure, but has a layer structure having interplanar spacings of 7.14 angstrom, etc. like sodium birnessite. In the layer structure, Mn atoms are arranged in alternate layers of oxygen atoms of a hexagonal close-packed state. In order to obtain the manganese oxide having Mg:Mn molar ratio of 1:2 of a layer structure, the important point is that a mixture of magnesium hydroxide and manganese oxide in a molar ratio of 1:2 is oxidized in an aqueous alkali solution. By oxidizing the hydroxide mixture in an aqueous alkali solution, the oxide of an Mg:Mn molar ratio of 1:2 having a layer structure is synthesizable although the detailed reason is not known. Since the Mg compound is inexpensive, and the synthesis is practiced at an ordinary temperature, the manganese oxide of a layer structure can be synthesized at low production cost. The oxidation may be conducted by aeration. Manganese hydroxide may be oxidized by bubbling air and/or oxygen into the solution, giving manganese oxide having a layer structure combined with Mg. The manganese oxide represented by the general formula Mg x Mn 2 O 4 ·nH 2 O ( 0 ≦ x < 1, 0 ≦ n ≦ 20 ) having a layer structure of the present invention is derived by removing a part or the whole of interlayer Mg atoms from the above manganese oxide represented by the general formula MgMn 2 O 4 ·nH 2 O ( 0 ≦ n ≦ 20 ) having a layer structure, and has a structure having vacant sites for receiving cations between the layers. The removal of Mg from MgMn 2 O 4 ·nH 2 O ( 0 ≦ n ≦ 20 ) is achievable in the same manner as in synthesis of a manganese oxide represented by the general formula Zn x Mn 2 O 4 ( 0 ≦ x < 1 ) having a spinel type crystal structure by an oxidation treatment to oxidize Mn to +4 valence. Hitherto, mineral acid treatment has been proposed. This method, however, gives layered manganese oxide of proton type formed by replacing cation between the layers with proton, therefore the product being limited in its uses. The oxide, represented by the general formula CaMn 2 O 4 ·nH 2 O ( 0 ≦ n ≦ 20 ) in which the Ca:Mn molar ratio is 1:2 does not have the spinel type crystal structure, but has an amorphous structure exhibiting only weak X-ray diffraction peaks near the positions of X-ray diffraction corresponding to interplanar spacing of 4.8 and 3.1 angstroms without other diffraction peaks and showing no specific crystalline symmetry. As already mentioned, the interplanar spacing is readily derivable from the position of X-ray diffraction peaks according to Bragg's equation. In order to obtain the manganese oxide having Ca:Mn molar ratio of 1:2 of an amorphous structure, the important point is that a mixture of calcium hydroxide and manganese hydroxide in a molar ratio of 1:2 is oxidized in an aqueous alkali solution. By oxidizing the hydroxide mixture in an aqueous alkali solution, the oxide of a Ca:Mn molar ratio of 1:2 having an amorphous structure is synthesizable although the detailed reason is not known. The oxidation may be conducted by aeration in the similar way for producing above MgMn 2 O 4 .nH 2 O. The hydroxide may be oxidized by bubbling air and/or oxygen into the solution, giving manganese oxide in an amorphous structure combined with Ca. The manganese oxide represented by the general formula Ca x Mn 2 O 4 .nH 2 O ( 0 ≦ x < 1, 0 ≦ n ≦ 20 ) having a δ type structure of the present invention is derived by removing a part or the whole of Ca atoms from the above amorphous manganese oxide represented by the general formula CaMn 2 O 4 nH 2 O ( 0 ≦ n ≦ 20 ). The removal of the calcium atoms in the amorphous manganese oxide of CaMn 2 O 4 .nH 2 O ( 0 ≦ n ≦ 20 ) is conducted by oxidation treatment for oxidizing Mn to +4 valence in the same manner as in synthesis of manganese oxide represented by Zn x Mn 2 O 4 ( 0 ≦ x < 1 ) having spinel crystal structure. The removal of a part or the whole of Ca atoms changes the amorphous crystal structure into a δ type structure having an undeveloped layer structure having vacant sites for cation between the layers, although the reason therefor is not known. Generally, the δ type structure is a layer structure having cations between layers. Removal of a part or the whole of the calcium is considered to form the vacant sites for cations between the layers. Conventionally, the removal of cation is carried out by mineral acid treatment. In the acid treatment, a high concentration of the mineral acid is required to remove Ca sufficiently, and the use of the high concentration of mineral acid accelerates a disproportionation reaction of Mn atoms to cause the crystal structure to change from δ type to γ type, so that the δ type crystal structure is not maintained in desired removal of Ca by acid treatment. Furthermore, the acid treatment gives layered manganese oxide of a proton type formed by replacing cations between the layers with protons therefore the product is not being used as a host compound, and is limited in its uses. The oxide, represented by the general formula CoMn 2 O 4 in which the Co:Mn molar ratio is 1:2 has a layer-and-spinel type crystal structure; that is, it is not only of a simple spinel type crystal structure, but also is of a compounded crystal structure with a layer structure having various interplanar spacing of 7.25 angstrom, etc. like sodium birnessite. In order to obtain the manganese oxide having Co:Mn molar ratio of 1:2 of a layer-and-spinel crystal structure, the important point is that a mixture of cobalt hydroxide and manganese hydroxide in a molar ratio of 1:2 is oxidized in an aqueous alkali solution. By oxidizing the hydroxide mixture in an aqueous alkali solution, the double oxide of a Co:Mn molar ratio of 1:2 having a layer-and-spinel structure is synthesizable although the detailed mechanism is not known. The oxidation may be conducted by aeration in the same manner as in synthesis of the aforementioned manganese oxide of the layer structure of the general formula of MgMn 2 O 4 .nH 2 O ( 0 ≦ n ≦ 20 ). The hydroxide may be oxidized by bubbling air and/or oxygen into the solution, giving manganese oxide in a layer-and-spinel structure combined with Co. The manganese oxide represented by the general formula Co x Mn 2 O 4 ( 0 < x < 1 ) having a layer-and-spinel structure of the present invention is derived by removing a part or the whole of the Co atoms from the above manganese oxide represented by the general formula CoMn 2 O 4 having a layer-and-spinel type crystal structure, and has a crystal structure in which cation transfer paths and cation receiving sites in a three-dimensional tunnel are formed by two-dimensional layers formed by electrostatic repulsion of oxygen atoms and closest packing of oxygen atoms. The oxide having such a crystal structure exhibits the characteristic function as described below. Generally, when cation is inserted into a layer structure like the layer crystal structure, the electrostatic repulsion of oxygen is suppressed by the charge of the cation to reduce the interlayer distance, whereas when the cation is released, the electrostatic repulsion of oxygen increases to cause expansion of the interlayer distance. On the contrary, when cation is inserted into a three-dimensional tunnel like in the spinel type structure the bond distance of the close-packed oxygen is expanded by the volume of the cation, and restores the bond distance of the original close-packed oxygen by release of the cation. In such a manner, the expansion and shrinkage of the crystal structure by insertion and release of the cation is just the opposite between the layer structure and the spinel structure. Accordingly, the manganese oxide having both crystal structures of a layer type and a spinel type represented by Co x Mn 2 O 4 ( 0 < x < 1 ) of the present invention exhibits extremely small expansion-shrinkage of crystal on insertion and release of cation. Therefore this manganese oxide when used as a host compound, is considered to function reversibly and stably. The removal of Co from CoMn 2 O 4 is achievable in the same manner as in the synthesis of a manganese oxide represented by the general formula Zn x Mn 2 O 4 ( 0 ≦ x < 1 ) having a spinel type crystal structure by an oxidation treatment to oxidize Mn to +4 valance. A lithium secondary battery exhibiting high output, high energy density, and satisfactory reversibility of charge-discharge cycles is provided by use, as the positive electrode of the battery, of some kind of the manganese oxide represented by the general formula A x Mn 2 O 4 .nH 2 O (where A is Zn, Mg, Ca, or Co; and if A is Zn, 0 ≦ x < 1 and n = 0; if A is Mg or Ca, 0 ≦ x < 1 and 0 ≦ n ≦ 20 ; and if A is Co, 0 < x < 1 and n = 0). The negative electrode of the lithium secondary battery of the present invention is made of lithium and/or a substance capable of receiving and releasing lithium, such as lithium metal, lithium-aluminum alloy, lithium-tin alloy, lithium-lead alloy, and carbonaceous materials capable of doping and releasing lithium ion electrochemically. The electrolyte employed in the lithium secondary battery of the present invention is not specially limited, and includes lithium salt solutions in an organic solvent such as carbonates, sulfolanes, lactones, and ethers, and lithium ion-conductive solid electrolytes. Fig. 1 illustrates a three-electrode cell construction having an electrode holder (1) , a positive electrode (2) , a reference electrode (3) , a negative electrode (counter-electrode) (4) , and a glass beaker (5) containing the electrolyte, which employs, as the positive electrode (2) , a manganese oxide represented by the general formula A x Mn 2 O 4 ·nH 2 O (where A is Zn, Mg, Ca, or Co; and if A is Zn, 0 ≦ x < 1 and n = 0; if A is Mg or Ca, 0 ≦ x < 1 and 0 ≦ n ≦ 20 ; and if A is Co, 0 < x < 1 and n = 0). The present invention is described in more detail by reference to examples without limiting the invention in any way. In the Examples and Comparative Examples, the X-ray diffraction patterns were measured under the conditions below: Apparatus: Philips PW-1729 X-ray: Cu-K α line Measurement mode: Step-scan Scanning: 0.02° per second (as 2 ) Time of measurement: 3 seconds Measured range: 3° to 70° (as 2 ) In Examples and Comparative Examples, the surface areas were measured by treating each sample in a nitrogen gas stream (flow rate 15 ml per min.) at 250°C for 40 minutes, and then subjecting it to measurement with the automatic surface area measurement apparatus (ASA-2000, made by Shibata Kagaku Kiki Kogyo) Example 1 Manganese oxide of a spinel type crystal structure represented by the general formula Zn x Mn 2 O 4 ( 0 < x < 1 ) was prepared by the procedures below. ( Preparation of ZnMn 2 O 4 ) Into an aqueous solution containing 0.52 mol/dm 3 of manganese sulfate and 0.26 mol/dm 3 of zinc sulfate, nitrogen gas was bubbled sufficiently. Then to 1 liter of the solution, 2 liters of 2.67 mol/dm 3 of sodium hydroxide solution was added at a rate of 10 ml/min. Then, to the solution, air was bubbled at a rate of 25 ml/min for three days. The resulting precipitate was washed with water, collected by filtration and dried at 70°C for 24 hours. The resulting compound was found to be ZnMn 2 O 4 from X-ray diffraction. Fig. 2 shows the X-ray diffraction pattern. The BET specific surface area was found to be 42 m 2 /g. ( Removal of Zn - preparation of Zn x Mn 2 O 4 ) 5 Grams of the powdery ZnMn 2 O 4 obtained above was added to 250 ml of aqueous 0.042 mol/dm 3 ammonium persulfate solution, and the mixture was stirred at 70°C for 4 hours. The product was washed with water, collected by filtration, and dried at 70°C for 24 hours. The resulting compound was found to be Zn 0.67 Mn 2 O 4 of a spinel type crystal structure from X-ray diffraction and composition analysis. The X-ray diffraction pattern is shown in Fig. 3, and the composition analysis result is shown in Table 1. ( Construction of Electric Cell ) The obtained manganese oxide having a spinel type crystalline structure, powdery carbon as the electroconductive material, and powdery polytetrafluoroethylene as a binder were mixed in a ratio of 88:7:5 by weight. 75 mg of this mixture was molded into a pellet of 8 mm in diameter by applying a pressure of 2 ton/cm 2 . This pellet was dried at 250°C for 2 hours. A three-electrode cell shown in Fig. 1 was constructed by using the pellet as the test electrode (2) , a lithium foil as the counter electrode (4) , a lithium piece cut out from the lithium foil as the reference electrode (3) , and 1 mol/dm 3 lithium perchlorate solution in propylene carbonate as the electrolyte. The three-electrode cell constructed as above was employed in cyclic voltammetry conducted at a potential scanning rate of 5 mV/sec and an electrode potential range of from 0.75 V to 5.0 V relative to the reference electrode by repeating oxidation-reduction. Table 2 shows the reduction capacity (equivalent to discharge capacity), the peak current value, and average output derived from the tenth cycle of the cyclic voltammetry. Example 2 Zn x Mn 2 O 4 was prepared in the see manner as in Example 1 except that 0.084 mol/dm 3 ammonium persulfate solution was used in the oxidation treatment. The resulting compound was found to be Zn 0.33 Mn 2 O 4 of a spinel type crystal structure from X-ray diffraction and composition analysis. The X-ray diffraction pattern is shown in Fig. 4, and the composition analysis result is shown in Table 1. A three-electrode cell was constructed in the same manner as in Example 1 except for using the above manganese oxide as the test electrode, and the cyclic voltammetry was conducted in the same manner as in Example 1. The results are shown in Table 2. Example 3 Zn x Mn 2 O 4 was prepared in the same manner as in Example 1 except that 0.5 mol/dm 3 ammonium persulfate solution was used in the oxidation treatment. The resulting compound was found to be Zn 0.10 Mn 2 O 4 of a spinel type crystal structure from X-ray diffraction and composition analysis. The X-ray diffraction pattern is shown in Fig 5, and the composition analysis result is shown in Table 1. A three-electrode cell was constructed in the same manner as in Example 1 except for using the above manganese oxide as the test electrode, and the cyclic voltammetry was conducted in the see manner as in Example 1. The results are shown in Table 2. Example 4 The same treatment was conducted in the same manner as in Example 1 except that 1.0 mol/dm 3 ammonium persulfate was used in the oxidation treatment. The resulting compound was found to be manganese dioxide of a spinel the crystal structure from X-ray diffraction and composition analysis. The X-ray diffraction pattern is shown in Fig. 6, and the composition analysis result is shown in Table 1. The BET surface area was 152 m 2 /g. A three-electrode cell was constructed in the same manner as in Example 1 except for using the above manganese oxide as the test electrode, and the cyclic voltammetry was conducted in the same manner as in Example 1. The results are shown in Table 2. Example 5 ZnMn 2 O 4 was prepared in the same manner as in Example 1 except for using an aqueous solution containing 0.52 mol/dm 3 manganese nitrate and 0.26 mol/dm 3 zinc nitrate. The resulting compound was found to be ZnMn 2 O 4 by X-ray diffraction. Fig. 7 shows the X-ray diffraction pattern. The BET specific surface area was 12 m 2 /g. Then the oxidation treatment was conducted in the same manner as in Example 1 except for using aqueous 0.5 mol/dm 3 ammonium persulfate solution. The resulting compound was found to be Zn 0.10 Mn 2 O 4 of a spinel type crystal structure from X-ray diffraction and composition analysis. The X-ray diffraction pattern is shown in Fig. 8, and the composition analysis result is shown in Table 1. A three-electrode cell was constructed in the same manner as in Example 1 except for using the above manganese oxide as the test electrode, and the cyclic voltammetry was conducted in the same manner as in Example 1. The results are shown in Table 2. Example 6 The same treatment was conducted in the same manner as in Example 5 except that 1.0 mol/dm 3 ammonium persulfate was used in the oxidation treatment. The resulting compound was found to be manganese dioxide of a spinel type crystal structure from X-ray diffraction and composition analysis. The X-ray diffraction pattern is shown in Fig. 9, and the composition analysis result is shown in Table 1. The BET surface area was 113 m 2 /g. A three-electrode cell was constructed in the same manner as in Example 1 except for using the above manganese oxide as the test electrode, and the cyclic voltammetry was conducted in the same manner as in Example 1. The results are shown in Table 2. Comparative Example 1 The treatment was conducted in the same manner as in Example 1 except that 250 ml of aqueous 0.1 mol/dm 3 sulfuric acid solution was used in the oxidation treatment. The resulting compound was found to be manganese oxide of a γ type crystal structure from X-ray diffraction. The X-ray diffraction pattern is shown in Fig. 10. The result of the composition analysis is shown in Table 1. Comparative Example 2 ZnMn 2 O 4 was prepared as follows. 15.8 Grams of powdery dimanganese trioxide (Mn 2 O 3 ) and 8.1 g of powdery zinc oxide (ZnO) was mixed enough with an agate mortar. The powdery mixture was heated at 850°C for 20 hours by means of an open-air type electric furnace. The resulting compound was found to be ZnMn 2 O 4 by X-ray diffraction and composition analysis. Fig. 11 shows the X-ray diffraction pattern, and Table 1 shows the result of the composition analysis. The BET specific surface area was 1 m 2 /g. Then the above compound was subjected to oxidation treatment in the same manner as in Example 4 to obtain Zn x Mn 2 O 4 . The resulting compound was found to be Zn 0.9 Mn 2 O 4 of a spinel type crystal structure. Fig. 12 shows the X-ray diffraction pattern, and Table 1 shows the result of the composition analysis. The BET specific surface area was 27 m 2 /g. A three-pole electrode was constructed in the same manner as in Example 1 except for using the above manganese oxide as the test electrode, and the cyclic voltammetry was conducted in the same manner as in Example 1. The results are shown in Table 2. Comparative Example 3 As a manganese oxide having a spinel type crystal structure, LiMn 2 O 4 was prepared as follows. 15.8 Grams of powdery dimanganese trioxide (Mn 2 O 3 ) and 1.5 g of powdery lithium oxide (Li 2 O) was mixed enough with an agate mortar. The powdery mixture was heated at 850°C for 20 hours by means of an open-air type electric furnace. The resulting compound was found to be LiMn 2 O 4 (lithium manganese spinel) by X-ray diffraction and composition analysis. A three-electrode cell was constructed in the same manner as in Example 1 except for using the above LiMn 2 O 4 as the test electrode 2, and the cyclic voltammetry was conducted in the same manner as in Example 1. The results are shown in Table 2. Example 7 A manganese oxide represented by the general formula Mg x Mn 2 O 4 .nH 2 O ( 0 ≦ x < 1, 0 ≦ n ≦ 20 ) was synthesized as follows. ( Preparation of Mg x Mn 2 O 4 .nH 2 O ( 0 ≦ n ≦ 20 ) Into an aqueous solution containing 0.52 mol/dm 3 of manganese sulfate and 0.26 mol/dm 3 of magnesium sulfate, nitrogen gas was bubbled sufficiently. Then, to 1 liter of the solution, 2 liters of 2.67 mol/dm 3 sodium hydroxide solution was added at a rate of 10 ml/min to obtain an aqueous alkali solution containing magnesium hydroxide and manganese hydroxide (molar ratio = 1:2). Then, to the solution, air was bubbled at a rate of 25 ml/min for three days. The resulting precipitate was washed with water, collected by filtration, and dried at 70°C for 24 hours. The resulting compound was found, by composition analysis, to be a manganese oxide containing Mg and Mn in a molar ratio of 1:2, having an oxidation degree of Mn of x=1.79 in terms of MnO x , and represented by the formula MgMn 2 O 4 .16H 2 O. According to X-ray diffraction measurement (X-ray source: Cu-K α line), this compound was found to be a manganese oxide of a layer structure having various interplanar spacings of 2.48, 2.51, 3.56, 5.00, 7.14 and 9.23 angstrom. Fig. 13 shows the X-ray diffraction pattern of this manganese oxide. Table 3 shows the X-ray diffraction pattern data of this compound. For comparison, X-ray diffraction pattern data of MgMn 2 O 4 described in ASTM card is shown also in Table 3. From Table 3, the manganese oxide is completely different from the known spinel type MgMn 2 O 4 . ( Removal of Mg - Preparation of Mg x Mn 2 O 4 .nH 2 O ( 0 ≦ x < 1, 0 ≦ n ≦ 20 ) 5 Grams of the powdery MgMn 2 O 4 .16H 2 O obtained above was added to 250 ml of aqueous 0.5 mol/dm 3 ammonium persulfate solution, and the mixture was stirred at 70°C for 4 hours. The product was washed with water, collected by filtration, and dried at 70°C for 24 hours. The resulting compound was found to be Mg 0.08 Mn 2 O 4 .H 2 O from composition analysis, and to have an oxidation degree of x = 1.95 in terms of MnO x by valence measurement of Mn. Further, this compound was found to have a layer type crystal structure maintaining the crystal structure of the starting material MgMn 2 O 4 .16H 2 O by X-ray diffraction measurement. The X-ray diffraction pattern is shown in Fig. 14 ( Construction of Electric Cell ) A three-pole cell was constructed in the same manner as in Example 1 except for using the above Mg 0.08 Mn 2 O 4 .H 2 O as the test electrode 2, and the cyclic voltammetry was conducted in the same manner as in Example 1. Table 4 shows the reduction capacity (equivalent to discharge capacity), the peak current value, and average output derived from the tenth cycle of the cyclic voltammetry. Example 8 A manganese oxide was synthesized in the same manner as in Example 7 except that 0.52 mol/dm 3 manganese nitrate solution and 0.26 mol/dm 3 magnesium nitrate solution were used and oxygen was bubbled in place of air. The resulting compound was found, by composition analysis, to be a manganese oxide containing Mg and Mn in a molar ratio of 1:2, having an oxidation degree of Mn of x=1.79 in terms of MnO x , and represented by the formula MgMn 2 O 4 .14H 2 O. According to X-ray diffraction measurement (X-ray source: Cu-K α line), this compound was found to be a manganese oxide of a layer structure having various interplanar spacings of 2.48, 2.51, 3.56, 5.00, 7.14 and 9.23 angstrom. Fig. 15 shows the X-ray diffraction pattern of this manganese oxide. Table 3 shows the X-ray diffraction pattern data of this compound. From Table 3, the manganese oxide is completely different from the known spinel type MgMn 2 O 4 . Then 5 g of powdery MgMn 2 O 4 .14H 2 O obtained above was further treated in the same manner as in Example 7 in order to remove Mg. The compound obtained by this treatment was a manganese oxide represented by composition formula, Mg 0.06 Mn 2 O 4 .0.8H 2 O, having an oxidation degree of Mn of x=1.95 in terms of MnO x . The X-ray diffraction pattern thereof is shown in Fig. 16. A three-electrode cell was constructed in the same manner as in Example 1 except for using the above Mg 0.06 Mn 2 O 4 .0.8H 2 O having a layer structure as the test electrode 2, and the cyclic voltammetry was conducted in the same manner as in Example 1. Table 4 shows the results. Comparative Example 4 Sodium birnessite having a layer structure was synthesized as follows. An aqueous 0.52 mol/dm 3 manganese nitrate solution was cooled to 0°C, and gaseous nitrogen was bubbled therein sufficiently. To 1 liter of this solution in a cooled state, 2 liters of 2.67 mol/dm 3 sodium hydroxide solution was added at a rate of 10 ml/min, and then air was bubbled therein at a rate of 25 ml/min for three days. The formed precipitate was washed with water and collected by filtration, and dried at 70°C for 24 hours. From composition analysis and X-ray diffraction measurement, the resulting compound was confirmed to be synthetic sodium birnessite (Na 4 Mn 14 O 27 .9H 2 O) having a layer structure. Fig. 17 shows the X-ray diffraction pattern of the obtained compound. Then 5 g of this powdery compound was added into 2 liters of an aqueous nitric acid solution of pH 2, and the mixture was stirred at 70°C for 16 hours. The product was washed with water, collected by filtration, and dried at 70°C for 24 hours. From composition analysis and X-ray diffraction measurement, the obtained compound was confirmed to be a sodium-free birnessite (Mn 7 O 13 .5H 2 O) having a layer structure. Fig. 18 shows the X-ray diffraction pattern thereof. A three-electrode cell was constructed in the same manner as in Example 1 except for using the above Mn 7 O 13 .5H 2 O having a layer structure as the test electrode 2, and the cyclic voltammetry was conducted in the same manner as in Example 1. Table 4 shows the results. Example 9 A manganese oxide represented by the general formula Ca x Mn 2 O 4 .nH 2 O ( 0 ≦ x < 1, 0 ≦ n ≦ 20 ) as follows. ( Preparation of CaMn 2 O 4 .nH 2 O ( 0 ≦ n ≦ 20 ) ) Into an aqueous solution containing 0.52 mol/dm 3 of manganese sulfate and 0.26 mol/dm 3 of calcium nitrate, nitrogen gas was bubbled sufficiently. To 1 liter of this solution, 2 liters of 2.67 mol/dm 3 sodium hydroxide solution was added at a rate of 10 ml/min to obtain an aqueous alkaline solution containing calcium hydroxide and manganese hydroxide (molar ratio = 1:2). Then, to the solution, air was bubbled at a rate of 25 ml/min for three days. The resulting precipitate was washed with water, collected by filtration, and dried at 70°C for 24 hours. The resulting compound was found, by composition analysis, to be a manganese oxide containing Ca and Mn in a molar ratio of 1:2, having an oxidation degree of Mn of x=1.75 in terms of MnO x , and represented by the formula CaMn 2 O 4 .3H 2 O. According to X-ray diffraction measurement (X-ray source: Cu-K α line), this compound was found to be an amorphous manganese oxide exhibiting broad and very weak X-ray diffraction peaks around the diffraction angles corresponding to the d-values of the interplanar spacings of 4.8 angstroms and 3.1 angstroms only. Fig. 19 shows the X-ray diffraction pattern of this manganese oxide. Table 5 shows the X-ray diffraction pattern data of this compound. For comparison, X-ray diffraction pattern data of CaMn 2 O 4 described in ASTM card is shown also in Table From Table 5, the manganese oxide is completely different from the known spinel type CaMn 2 O 4 . ( Removal of Ca - Preparation of Ca x Mn 2 O 4 .nH 2 O ( 0 ≦ x < 1, 0 ≦ n ≦ 20 ) 5 Grams of the powdery CaMn 2 O 4 .3H 2 O obtained above was added to 250 ml of aqueous 0.5 mol/dm 3 ammonium persulfate solution, and the mixture was stirred at 70°C for 4 hours. The product was washed with water, collected by filtration, and dried at 70°C for 24 hours. The resulting compound was found to be Ca 0.19 Mn 2 O 4 .0.4H 2 O from composition analysis, and to have an oxidation degree of x=1.80 in terms of MnO x by valence measurement of Mn. Further, this compound was found to have a δ type crystal structure by X-ray diffraction measurement. The X-ray diffraction pattern is shown in Fig. 20. ( Construction of Electric Cell ) A three-electrode cell was constructed in the same manner as in Example 1 except for using the above Ca 0.19 Mn 2 O 4 .0.4H 2 O as the test electrode 2, and the cyclic voltammetry was conducted in the same manner as in Example 1. Table 6 shows the reduction capacity (equivalent to discharge capacity), the peak current value, and average output derived from the tenth cycle of the cyclic voltammetry. Example 10 A manganese oxide was synthesized in the same manner as in Example 9 except that 0.52 mol/dm 3 manganese nitrate solution and 0.26 mol/dm 3 calcium nitrate solution were used and oxygen was bubbled in place of air. The resulting compound was found, by composition analysis, to be a manganese oxide containing Ca and Mn in a molar ratio of 1:2, having an oxidation degree of Mn of x=1.75 in terms of MnO x , and represented by the formula CaMn 2 O 4 .2.3H 2 O. According to X-ray diffraction measurement (X-ray source: Cu-K α line), this compound was found to be an amorphous manganese oxide exhibiting broad and very weak X-ray diffraction peaks around the diffraction angles corresponding to the d-values of the interplanar spacings of 4.8 angstroms and 3.1 angstroms only. Fig. 21 shows the X-ray diffraction pattern of this manganese oxide. Table 5 shows the X-ray diffraction pattern data of this compound. From Table 5, the manganese oxide is completely different from the known spinel type CaMn 2 O 4 . Then 5 g of powdery CaMn 2 O 4 .2.3H 2 O obtained above was further treated in the same manner as in Example 9. The compound obtained by this treatment was found to be a manganese oxide of δ type crystal structure represented by composition formula: Ca 0.10 Mn 2 O 4 .0.3H 2 O, having an oxidation degree of Mn of x=1.95 in terms of MnO x . The X-ray diffraction pattern thereof is shown in Fig. 22. A three-electrode cell was constructed in the same manner as in Example 1 except for using the above manganese oxide Ca 0.10 Mn 2 O 4 .0.3H 2 O having δ type crystal structure as the test electrode 2, and the cyclic voltammetry was conducted in the same manner as in Example 1. Table 6 shows the results. Comparative Example 5 A manganese oxide having δ type crystal structure was prepared as follows. One liter of aqueous 1 mol/dm 3 hydrochloric acid solution was added to 1 liter of aqueous 2 mol/dm 3 potassium permanganate solution, and the mixture was stirred for 2 hours. The formed precipitate was collected by filtration, and washed with water until the wishing water became neutral to obtain a reddish brown powder. The powder was dried at 70°C for 24 hours, and then treated at 250°C for 20 hours in the atmosphere. From composition analysis and X-ray diffraction of the powder, the resulting compound was found to be MnO 1.83 having δ type crystal structure. Fig. 23 shows the X-ray diffraction pattern thereof. Then 5 g of the obtained compound was further treated in the same manner as in Example 9, and was found to be an ordinary manganese oxide of δ type crystal structure. Example 11 A manganese oxide represented by the general formula Co x Mn 2 O 4 ( 0 < x < 1 ) was prepared as follows. ( Preparation of CoMn 2 O 4 ) Into 1 liter of an aqueous solution containing 0.52 mol/dm 3 of manganese sulfate and 0.26 mol/dm 3 of cobalt sulfate, nitrogen gas was bubbled sufficiently. To 1 liter of the solution, 2 liters of 2.67 mol/dm 3 sodium hydroxide solution was added at a rate of 10 ml/min to obtain an aqueous alkaline solution of containing cobalt hydroxide and manganese hydroxide (molar ratio = 1:2). Then, to the solution, air was bubbled at a rate of 25 ml/min for three days. The resulting precipitate was washed with water, collected by filtration, and dried at 70°C for 24 hours. The resulting compound was found, by composition analysis, to be a manganese oxide containing Co and Mn in a molar ratio of 1:2, and represented by the formula CoMn 2 O 4 . According to X-ray diffraction measurement, this compound was found to have crystal structure of both layer type and spinel type. Fig. 24 shows the X-ray diffraction pattern of this manganese oxide. ( Removal of Co - Preparation of Co x Mn 2 O 4 ( 0 < x < 1 ) ) 5 Grams of the powdery CoMn 2 O 4 obtained above was added to 250 ml of aqueous 0.5 mol/dm 3 ammonium persulfate solution, and the mixture was stirred at 70°C for 4 hours. The product was washed with water, collected by filtration, and dried at 70°C for 24 hours. The resulting compound was found to be Co 0.80 Mn 2 O 4 from composition analysis. Further, this compound was found to have both crystal structure of layer type and λ type belonging, to spinel type structure. The X-ray diffraction patted is shown in Fig. 25. ( Construction of Electric Cell ) A three-electrode cell was constructed in the same manner as in Example 1 except for using, as the test electrode 2, the above Co 0.20 Mn 2 O 4 having both crystal structures of layer type and λ type belonging to spinel structure, and the cyclic voltammetry was conducted in the sage manner as in Example 1. Table 7 shows the reduction capacity (equivalent to discharge capacity), the peak current value, and average output derived from tenth cycle of the cyclic voltammetry. Example 12 A manganese oxide was prepared in the same manner as in Example 11 except for using 0.52 mol/dm 3 manganese nitrate and 0.26 mol/dm 3 cobalt nitrate. From composition analysis, the resulting compound was found to be a manganese oxide containing Co and Mn in a molar ratio of 1:2 and represented by the composition formula CoMn 2 O 4 . From X-ray diffraction measurement, the compound was found to have both layer type crystal structure and spinel type crystal structure simultaneously. Fig. 26 shows the X-ray diffraction pattern. Then 5 g of powdery CoMn 2 O 4 obtained above was further treated in the same manner as in Example 11. The compound obtained by this treatment was found to be represented by the formula Co 0.70 Mn 2 O 4 . From the X-ray diffraction measurement, this compound has both crystal structure of layer type and λ type assignable to spinel structure. The X-ray diffraction pattern is shown in Fig. 27. A three-electrode cell was constructed in the same manner as in Example 1 except for using, as the test electrode 2, the above Co 0.70 Mn 2 O 4 having both crystal structures of layer type and λ type belonging to spinel structure, and the cyclic voltammetry was conducted in the same manner as in Example 1. Table 7 shows the reduction capacity (equivalent to discharge capacity), the peak current value, and average output derived from tenth cycle of the cyclic voltammetry. Comparative Example 6 CoMn 2 O 4 was prepared as follows. 15.8 Grams of powdery dimanganese trioxide (Mn 2 O 3 ) and 10.3 g of powdery basic cobalt carbonate (CoCO 3 .Co(OH) 2 ) was mixed enough with an agate mortar. The powdery mixture was heated at 900°C for 20 hours by means of an open-air type electric furnace. The resulting compound was found to be CoMn 2 O 4 having spinel type crystal structure by X-ray diffraction and composition analysis. As described above, the present intention provides novel manganese oxides which have a spinel type, a layer type, or spinel-and-layer type crystal structure. The novel manganese oxides of the present invention are useful as a host compound for a battery-active substance, a catalyst, an adsorbent, a magnetic material and the like. Further, a lithium secondary battery is provided by use of a kind of the above manganese oxide for the positive electrode, exhibiting high output with high energy density and with satisfactory reversibility in charge-discharge cycle. Result of Composition Analysis Mn (% by weight) Zn (% by weight) Composition Example 1 50.5 20.1 Zn 0.67 Mn 2 O 4 Example 2 56.2 11.1 Zn 0.33 Mn 2 O 4 Example 3 60.9 3.6 Zn 0.10 Mn 2 O 4 Example 4 63.2 - MnO 2 Example 5 60.9 3.6 Zn 0.10 Mn 2 O 4 Example 6 63.2 - MnO 2 Comparative Example 1 60.5 - Mno 2 Comparative Example 2 47.2 25.3 Zn 0.90 Mn 2 O 4 Evaluation of Battery Properties Reduction capacity (mAh/g) Peak current (mA/cm 2 ) Average output (mWh/cm 2 ) Example 1 10.5 10.1 14.7 Example 2 12.9 12.6 18.1 Example 3 14.3 14.8 21.6 Example 4 16.7 15.9 22.2 Example 5 14.9 15.0 21.7 Example 6 16.9 16.0 22.4 Comparative Example 1 3.6 3.6 4.7 Comparative Example 2 3.5 4.9 6.4 Comparative Example 3 4.8 6.7 6.2 Manganese oxide of Example 7 Manganese oxide of Example 8 MgMn 2 O 4 Interplanar spacing (angstrom) Relative intensity (Cu-K α line) Interplanar spacing (angstrom) Relative intensity (Cu-K α line) Interplanar spacing (angstrom) Relative intensity (Cuk α line) 9.23 7 9.22 7 4.88 100 7.14 100 7.13 100 3.06 20 5.00 10 5.00 10 2.86 10 3.56 35 3.65 35 2.74 70 2.51 5 2.51 5 2.47 90 2.43 6 2.42 6 2.44 20 Evaluation of Battery Properties Reduction capacity (mAh/g) Peak current (mA/cm 2 ) Average output (mWh/cm 2 ) Example 7 7.5 7.1 10.3 Example 8 9.3 8.8 12.7 Comparative Example 4 3.4 4.7 4.3 Manganese oxide of Example 9 Manganese oxide of Example 10 CaMn 2 O 4 Interplanar spacing (angstrom) Relative intensity (Cu-K α line) Interplanar spacing (angstrom) Relative intensity (Cu-K α line) Interplanar spacing (angstrom) Relative intensity (CuK α line) ca 4.8 - ca 4.8 - 4.82 30 ca 3.1 - ca 3.1 - 2.69 100 2.22 100 2.05 100 Evaluation of Battery Properties Reduction capacity (mAh/g) Peak current (mA/cm 2 ) Average output (mWh/cm 2 ) Example 9 7.2 8.5 11.7 Example 10 8.9 8.7 12.2 Evaluation of Battery Properties Reduction capacity (mAh/g) Peak current (mA/cm 2 ) Average output (mWh/cm 2 ) Example 11 12.9 12.6 18.1 Example 12 13.5 12.8 18.4

Claims: A manganese oxide represented by the general formula A x Mn 2 O 4 . nH 2 O wherein A is Zn, Mg, Ca or Co; if A is Zn, 0 ≤ X < 1 and n = 0; if A is Mg or Ca, 0 ≤ x < 1 and 0 ≤ n ≤ 20 ; and if A is Co, 0 < x < 1 and n = 0 obtainable by a) preparing an aqueous solution of an A salt and a two- valent Mn salt in a molar ratio of 1:2, adding an equal amount or more of an alkali solution, and oxidizing the resulting hydroxide mixture by bubbling air and/or oxygen into the solution to obtain an intermediate compound b) subjecting the obtained intermediate compound to an oxidation treatment without additional acid for attaining the +4 valence of Mn by chemical or electrochemical oxidation to obtain the compound of formula (1) said compound of formula (1) being not of a proton type. The manganese oxide of claim 1, wherein A is Zn, and the manganese oxide has a spinel type crystal structure. The manganese oxide of claim 1, wherein A is Mg, 0 ≦ x < 1 , and the manganese oxide has a layer type crystal structure. The manganese oxide of claim 1, wherein A is Ca, 0 ≦ x < 1 , and the manganese oxide has a δ type crystal structure. The manganese oxide of claim 1, wherein A is Co, 0 < x < 1 , and the manganese oxide has both crystal structures of layer type and spinel type. A process for producing manganese oxide of claim 1, comprising preparing an aqueous solution of an A salt and a two-valent Mn salt in a molar ratio of 1:2, adding an equal amount or more of an alkali solution, and oxidizing the resulting hydroxide mixture by bubbling air and/or oxygen into the solution to obtain an intermediate compound subjecting the obtained intermediate compound to an oxidation treatment without additional acid for attaining the +4 valence of Mn by chemical or electrochemical oxidation to remove A. A process according to claim 6 for producing manganese oxide of claim 1 or 2, wherein ZnMn 2 O 4 having BET surface area of 10 m 2 /g or more is used as the starting material, and Zn is removed therefrom by oxidation treatment. A manganese dioxide produced by the process of claim 7 having spinel type crystal structure, which has BET surface area of 100 m 2 /g or more, and a composition of x = 0. A lithium secondary battery, employing at least one of manganese oxides of claims 2, 3, 4, 5, and 8 as the positive electrode.

Inventors: MOURI TAKASHI, OKADA MASAKI, MOURI, TAKASHI, OKADA, MASAKI
Applicants: TOSOH CORP, TOSOH CORPORATION
Application year: 1993
Main classification: C01G45
IPC codes: C01G45, C01G51, H01M4, H01M10