A process for producing a spinel-type lithium manganese complex oxide for a cathode active material of a lithium secondary battery

Abstract: The invention provides a spinel-type lithium manganese complex oxide for a cathode active material of a lithium secondary battery, which is characterized in that said spinel-type lithium manganese complex oxide has an average particle diameter between 1 and 5 micrometers and a specific surface area between 2 and 10 m 2 /g. The invention also provides a process for producing the spinel-type lithium manganese compound oxide comprises the steps of 1)atomizing and pyrolyzing at least one of an aqueous solution and an alcohol solution of compounds containing metallic elements constituting a spinel-type lithium manganese complex oxide to obtain said compound oxide, and 2)annealing said spinel-type lithium manganese complex oxide to increase the average particle diameter thereof to between 1 and 5 micrometers and adjust the specific surface area thereof to between 2 and 10 m 2 /g. The spinel-type lithium manganese complex oxide which is useful as a cathode active material of a lithium secondary battery exhibits a large charge-discharge capacity and excellent charge-discharge cycle characteristics, and it can be used in a 4-V region secondary battery.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a process for producing a spinel-type lithium manganese complex oxide for a cathode active material of a lithium secondary battery and a process for producing thereof. The spinel-type lithium manganese complex oxide is useful as a cathode active material of a 4-V lithium secondary battery. Prior Art As a method of producing a spinel-type lithium manganese complex oxide which is used as a cathode active material of a lithium secondary battery, the following methods have been proposed so far. (a) Solid phase method in which powders of lithium carbonate and manganese dioxide are mixed with each other, and the mixture is calcined at approximately 800°C. (b) Melt-impregnation method in which low-melting lithium nitrate or lithium hydroxide is dipped in porous manganese dioxide, and calcined. (c) Method in which lithium nitrate and manganese nitrate are dissolved in water, and the solution is ultrasonically spray-pyrolyzed. As a lithium manganese complex oxide which is appropriate as a cathode active material of a lithium secondary battery, the following complex oxides have been proposed. (d) LiMn 2 O 4 (Japanese Patent Publication No. 21,431/1996) (e) Li x Mn 2 O 4 in which x is 0.9 ≤ x ≤ 1.1 excluding x = 1.0 (Japanese Patent Publication No. 21,382/1996) (f) Li 2 Mn 4 O 9 , Li 4 Mn 5 O 12 [J. Electrochem Soc., vol. 139, No. 2, pp. 363 - 366 (1992)] (g) Li x Mn 2 O y in which x and y are 1.0 x < 1.6, 4.0 < y < 4.8, and 8/3 + (4/3) x < y < 4.0 + (1/2)x [Japanese Laid-Open (Kokai) No. 2,921/1996] (h) Li 1+x Mn 2-x O 4 in which x is 0, 0.03, 0.05, 0.10, 0.22, 0.29 or 0.33 [Solid State Ionics 69 (1994), pp. 59 - 67] The above-mentioned methods have, however, involved the following problems. In the solid phase method (a), the powders of carbonate and oxide are used as starting materials, and it is, therefore, necessary to calcine the same at a relatively high temperature. Accordingly, a defective spinel such as an excess oxygen spinel tends to be formed. Further, it is impossible to uniformly mix the powders at a molecular level. For example, Li 2 MnO 3 and LiMnO 2 are sometimes formed other than intended LiMn 2 O 4 . In order to prevent formation of such complex oxides, the calcination for a long period of time has to be repeated several times while adjusting the concentration of oxygen. The melt-impregnation method (b) improves the uniform dispersibility of Li and Mn in comparison to the solid phase method. However, the porous manganese material is required as a starting material. In order to obtain this porous manganese material, a milling treatment is needed. For this milling treatment, a special milling device has to be used, and impurities are incorporated through a milling medium during the milling or due to abrasion of an inner wall of the milling device, decreasing qualities of the resulting complex oxide powder as a cathode active material or increasing the cost. Further, unless the calcination is conducted for a long period of time at a low temperature to control evaporation of the low-melting lithium starting material, the crystallinity of the resulting complex oxide is decreased. Accordingly, when the complex oxide is used as an active material of a secondary battery, the crystal structure collapses during the repetition of the charge-discharge cycle of the battery, decreasing the capacity of the secondary battery. Still further, when Mn is substituted with a cation having a low valence and having an ionic radium close to that of Mn, such as Fe, Co, Ni or Mg to improve the high-rate discharge or the charge-discharge cycle characteristics of the secondary battery, it is unescapable that the distribution of Mn and the substituent cation is non-uniform in this melt-impregnation method too. In the spray pyrolysis method (c), the elements constituting the spinel-type lithium manganese complex oxide can uniformly be mixed at the ionic level tu outstandingly increase the uniformity as compared to the melt-impregnation method. Further, since the step of milling the starting materials is dispensed with, incorporation of impurities formed during the milling step can be prevented advantageously. However, in this spray pyrolysis method, a series of steps of evaporation of the solvent and thermal-decomposition are conducted within a short period of a few seconds, with the result that the thermal history is extremely short as compared to that in the conventional calcination treatment and the crystallinity of the resulting complex oxide tends to deteriorate. Accordingly, when the complex oxide is used as an active material of a secondary battery, the crystal structure collapses during the repetition of the charge-discharge cycle of the battery to decrease the capacity of the secondary battery. Further, since the specific surface area of the resulting complex oxide is as large as tens of centares per gram, an electrolyte in contact with this complex oxide is decomposed, sometimes notably decreasing the charge-discharge cycle characteristics and the storage properties of the secondary battery. Still further, the compositions of the above-mentioned lithium manganese complex oxides are problematic in the following points. When the composition (d) is used as a cathode active material of a secondary battery, the capacity of the battery is decreased to 50% of the original capacity in tens of the charge-discharge cycles. When the composition (e) is used as a cathode active material of a secondary battery and x is 0.9 ≤ x < 1.0, the amount of the lithium ion taken out by the initial charge is decreased causing decrease of the capacity of the battery. When x is 1.0 < x ≤ 1.1, the crystal structure is changed from a cubic system of a 4-V region to a tetragonal system of a 3-V region through Jahn-Teller phase transition causing decrease of the capacity of the battery in the repetition of the charge-discharge cycle. The composition (f) has the operating potential of approximately 3.0 V region, and therefore cannot be used as a cathode active material of a 4-V region lithium battery. The composition (g) includes a composition close to the composition (f) of the 3-V region. In order to form these active materials, a manganese starting material having a specific surface area of from 5 to 50 m 2 /g is needed. However, a powder having a large specific surface area has a strong cohesive force and cannot uniformly be mixed with a lithium starting material. It is further necessary that after the manganese and lithium starting materials are mixed, the mixture is calcined at 500°C or less for 2 hours or more and then at 850°C or less for from 1 to 50 hours. As a result, the productivity is poor. In the composition (h), a part of a manganese site is substituted with lithium to control the Jahn-Teller phase transition and improve the cycle characteristics. Nevertheless, the mere substitution of manganese with 3.0-% lithium decreases the discharge capacity of approximately 20%. Summary of the Invention Accordingly, it is an object of the present invention to provide, upon solving the above-mentioned problems, a process for producing a spinel-type lithium manganese complex oxide for a cathode active material of a lithium secondary battery. The spinel-type lithium manganese complex oxide should be used as a cathode active material of a 4-V region lithium secondary battery having a large charge-discharge capacity and exhibiting excellent charge discharge cycle characteristics, and the spinel-type lithium manganese complex oxide. The present invention provides a process for producing a spinel-type lithium manganese complex oxide for a cathode active material of a lithium secondary battery, whereby said spinel-type lithium manganese complex oxide has an average particle diameter between 1 and 5 micrometers and a specific surface area between 2 and 10 m 2 /g. In the above spinel-type lithium manganese complex oxide, said spinel-type lithium manganese complex oxide may be represented by the formula Li(Mn 2-x Li x )O 4 , wherein x is 0 ≤ x ≤ 0.1 and more preferably, x is 0 < x < 0.02. In the above spinel-type lithium manganese complex oxide, said spinel-type lithium manganese complex oxide may represented by the formula Li(Mn 2-x Li x )O 4 , wherein x is 0 ≤ x ≤ 0.1, more preferably, x is 0 < x < 0.02, and Mn is partially substituted by Cr, Ni, Fe, Co, Mg or Li. This described composite complex oxide solves the above mentioned problem having a high surface activity which is appropriate as a cathode active material of a lithium secondary battery. The present invention provides a process for producing the spinel-type lithium manganese complex oxide as described above comprising the steps of: 1) atomizing and pyrolyzing at least one of an aqueous solution and an alcohol solution of compounds containing metallic salts constituting a spinel-type lithium manganese complex oxide to obtain said complex oxide, and 2) annealing said spinel-type lithium manganese complex oxide to increase the average particle diameter thereof to between 1 and 5 micrometers and adjust the specific surface area thereof to between 2 and 10 m 2 /g. In the above process, the atomizing and pyrolyzing temperature may be between 500 and 900°C and the annealing temperature may be between 600 and 850°C. In the above process, said complex oxides containing the metallic salts may be at least one of lithium nitrate, lithium acetate and lithium formate and at least one of manganese nitrate, manganese acetate and manganese formate. When the aqueous solution and/or the alcohol solution of the compounds containing the metallic elements constituting the spinel-type lithium manganese complex oxide is sprayed into a heating atmosphere, the pyrolysis occurs instantaneously to cause milling due to the self-chemical decomposition. Consequently, the fine complex oxide having the high surface activity can be formed. When this complex oxide is then annealed, the average particle diameter is increased to between 1 and 5 micrometers, and the specific surface area is adjusted to between 2 and 10 m 2 /g. Thus, the complex oxide with the high surface activity which is appropriate as a cathode active material of a lithium secondary battery can be obtained. The metallic elements constituting the spinel-type lithium manganese complex oxide are Li and Mn as well as substituents for improving charge/discharge characteristics such as Cr, Ni, Fe, Co, Mg and Li [this Li is a substituent of the Mn site (octahedral site) different from Li in the tetrahedral site] to be added as required for substitution of the Mn site. Typical examples of the water-soluble compounds comprising these metal elements include an acetate, a formate, a nitrate and a chloride. These compounds are much less costly than an organic compound in which a hydrogen ion in a molecule is substituted with a metallic ion, such as an alkoxide. With these compounds, the costs of starting materials can be reduced, and this is industrially advantageous. Thus, the process of the present invention can provide a uniform spinel-type lithium manganese complex oxide having an average particle diameter of between 1 and 5 mm and a specific surface area of between 2 and 10 m 2 /g. Accordingly, when this complex oxide, preferably the complex oxide represented by the formula Li(Mn 2-x Li x )O 4 in which x is 0 ≤ x ≤ 0.1, more preferably the complex oxide represented by the formula Li(Mn 2-x Li x )O 4 in which x is 0 < x < 0.02 is used as a cathode active material of a secondary battery, a lithium secondary battery which is excellent in the charge-discharge cycle characteristics and the storage characteristics can be obtained. The present invention is illustrated specifically by referring to the following embodiments. Brief Description of the Drawing Fig. 1 is a sectional view illustrating an example of a lithium secondary battery. EMBODIMENTS First, lithium nitrate, lithium acetate, lithium formate, manganese nitrate, manganese acetate and manganese formate were prepared as compounds of metallic salts constituting a lithium manganese complex oxide. In order to obtain a lithium manganese complex oxide represented by the formula Li(Mn 2-x Li x )O 4 (in which 0 ≤ x ≤ 0.1) shown in Table 1, these compounds were then accurately weighed, and charged into a container. Then, 1,000 ml of a mixed solutior of water and alcohol (volume ratio of 1:1) were added thereto, and they were dissolved therein while being stirred. Subsequently, this mixed solution was atomized and pyrolyzed at a rate of 1,200 ml/hr from a nozzle to a vertical-type caramic tube reactor adjusted to a predetermined temperature of from 400 to 900°C to obtain a powder of a complex oxide. The resulting complex oxide was then charged into an alumina plate, and annealed at a predetermined temperature of from 500 to 900°C for 2 hours. In this manner, lithium manganese complex oxides shown in Sample Nos. 1 to 26 in Table 1 were obtained. Note that the lithium manganese complex oxides of Sample Nos. 14 and 18 with asterisk are compounds not prepared in accordance with the process of the invention. A lithium manganese complex oxide of the formula LiMn 2 O 4 shown in Sample No. 27 in Table 1 was obtained by a melt-impregnation method for comparison. That is, lithium nitrate and electrolytic manganese dioxide were first prepared as starting materials. Then, 1 mol of lithium nitrate and 2 mols of electrolytic manganese dioxide were accurately weighed, and milled and mixed using a ball mill. The mixture was then calcined at 700°C for 2 hours to obtain the complex oxide. Photographs of the above-obtained complex oxide powders were taken by means of a scanning electron microscope (SEM) to measure the particle diameters thereof. Further, the specific surface areas of the complex oxides were measured by a nitrogen adsorption method. Still further, the identification of the complex oxides was conducted by an X-ray diffraction (XRD) analysis. The results are shown in Table 2. In Table 2, LM is short for spinel-type lithium manganese complex oxide, and Mo is short for Mn 2 O 3 . Sample No. Average particle diameter (micrometer) Specific surface area (m 2 /g) XRD analysis Phase 1 1.8 2.1 LM, MO 2 1.7 2.7 LM 3 1.9 3.0 LM 4 2.4 3.5 LM 5 2.4 3.6 LM 6 2.3 3.6 LM 7 2.3 3.7 LM 8 2.2 3.6 LM 9 2.3 3.7 LM 10 2.5 3.6 LM 11 2.3 3.6 LM 12 2.7 3.0 LM 13 2.4 2.3 LM *14 0.8 14.8 LM 15 1.6 8.4 LM 16 2.1 3.3 LM 17 1.9 2.5 LM *18 5.5 0.5 LM 19 1.4 2.9 LM 20 2.1 2.8 LM 21 1.4 3.4 LM 22 1.9 3.5 LM 23 2.4 2.3 LM 24 1.8 4.6 LM 25 2.1 3.6 LM 26 1.9 2.6 LM *27 3.4 1.8 LM Secondary batteries were prepared using the above-mentioned complex oxides as cathode active materials. That is, the powder of each above complex oxide, acetylene black as an electroconductive agent and polytetrafluoroethylene as a binder were kneaded, and the mixture was formed into a sheet. This sheet was pressed on an SUS mesh to obtain a cathode. Subsequently, as shown in Fig. 1, the above-mentioned cathode 3 and a lithium metal as a anode 4 are overlaid through a polypropylene separator 5 with the SUS mesh side of the cathode 3 outside, and the product was stored in a stainless steel cathode can 1 with the cathode 3 down. The separator 5 was immersed with an electrolyte. A solution of lithium perchlorate in a mixed solvent of propylene carbonate and 1,1-dimethoxyethane was used as the electrolyte. Thereafter, an opening of a cathode can 1 was sealed with a stainless steel negative electrode 2 through an insulation packing to complete a lithium secondary battery shown in Table 2. Then, the resulting lithium secondary battery was subjected to a charge-discharge test in 100 cycles under such conditions that a charge-discharge current density was 0.5 mA/cm 2 , a charge limit voltage was 4.3 V and a discharge cut off voltage was 3.0 V. The secondary battery after the completion of the charge-discharge cycle test was then dissembled, and the condition (presence or absence of peeling-off) of the cathode was visually observed. The results are shown in Table 3. Sample No. Discharge volume Peeling-off of a positive pole Initial After 100 cycles 1 86 65 No 2 128 126 No 3 130 127 No 4 133 122 No 5 131 124 No 6 130 126 No 7 130 128 No 8 129 126 No 9 128 125 No 10 122 118 No 11 115 112 No 12 96 95 No 13 136 132 No *14 133 113 Yes 15 138 136 No 16 140 136 No 17 136 135 No *18 102 98 No 19 128 125 No 20 129 125 No 21 127 125 No 22 129 126 No 23 135 131 No 24 137 135 No 25 139 135 No 26 135 134 No *27 115 103 No From the results in Tables 1 and 2, it is found that when the solution of the metallic elements constituting the lithium manganese complex oxide are atomized and pyrolyzed and then annealed, the complex oxide is obtained in which the average particle diameter is increased to between 1 and 5 mm and the specific surface area is adjusted to between 2 and 10 m 2 /g. Further, this complex oxide is a spinel-type lithium manganese complex oxide. When this complex oxide is used as a cathode material, the lithium secondary battery is obtained which is excellent in the initial capacity and the charge-discharge cycle characteristics and which is free from peeling-off of the battery. The specific pyrolyzing temperature is preferably between 500 and 900°C. When it is higher than 500°C, a single phase of a spinel-type lithium manganese complex oxide is obtained. The upper limit thereof is less than a temperature at which the spinel-type lithium manganese complex oxide is not pyrolyzed again. The specific annealing temperature is preferably between 600 and 850°C. That is, at the annealing temperature of between 600 and 850°C, the spinel-type lithium manganese complex oxide having the particle size appropriate as a cathode active material of a lithium secondary battery can be obtained. Upon comparison of Sample Nos. 4 to 12, when the amount x of substitution of manganese with lithium is 0 < x in the formula Li(Mn 2-x Li x )O 4 , the effect of controlling the Jahn-Teller phase transition is obtained, and the decrease in the capacity in the charge-discharge cycle is suppressed. Meanwhile, when the amount x of substitution is 0.10 or less, preferably less than 0.02, the higher initial capacity can be provided. Accordingly, x in the formula Li(Mn 2-x Li x )O 4 is preferably 0 ≤ x ≤ 0.10, preferably 0 < x < 0.02. In the above-mentioned Example, the compounds of the metallic elements constituting the complex oxide of the formula Li(Mn 2-x Li x )O 4 were nitrates, acetates or formates. However, the present invention is not limited thereto. That is, compounds which are dissolved in water or alcohol, such as chlorides, can also be used as required. Sample No. 25 in which lithium nitrate is used as a Li compound and magnesium formate as an Mn compound among the compounds of the metallic elements constituting the compound of the formula Li(Mn 2-x Li x )O 4 exhibits the higher discharge capacity than Sample No. 19 in which lithium acetate and manganese acetate are used or Sample No. 20 in which lithium formate and manganese formate are used, and the discharge capacity of Sample No. 25 is as high as that of Sample No. 16 in which lithium nitrate and manganese nitrate are used. When using lithium nitrate and manganese formate, the reaction represented by formula (1) occurs, and the amount of NO 2 formed is one-fifth that of NO 2 when using lithium nitrate and manganese nitrate that cause the reaction represented by formula (2). Thus, it is easy to treat a waste gas after the reaction. Accordingly, it is most preferable to use lithium nitrate and manganese formate as compounds of metallic elements constituting the complex oxide of the formula Li(Mn 2-x Li x )O 4 . LiNO 3 + 2Mn(HCOO) 2 + 2.5O 2 → LiMn 2 O 4 + 4CO 2 + NO 2 + 2H 2 O LiNO 3 + 2Mn(NO 3 ) 2 → LiMn 2 O 4 + 5NO 2 + 0.5O 2 When the spinel-type lithium manganese complex oxide is a compound oxide obtained by substituting a part of an Mn site of Li(Mn 2-x Li x )O 4 with Cr, Ni, Fe, Co or Mg, other than Li(Mn 2-x Li x )O 4 , the same effects can also be obtained.

Claims: A process for producing a spinel-type lithium manganese comple: oxide for a cathode active material of a lithium secondary battery, comprising the steps of: 1) atomizing and pyrolyzing at least one of an aqueous solution and an alcohol solution of compounds containing metallic elements constituting said spinel-type lithium manganese complex oxide to obtain said compound oxide, and 2) annealing said spinel-type lithium manganese complex oxide to increase the average particle diameter thereof to between 1 and 5 micrometers and adjust the specific surface area thereof to between 2 and 10 m 2 /g. The process according to Claim 2, characterized in that the atomizing and pyrolyzing temperature is between 500 and 900°C. The process according to Claim 1 or 2, characterized in that the annealing temperature is between 600 and 850°C. The process according to one of Claims 1 to 3, characterized in that said compounds containing the metallic elements are at least one of lithium nitrate, lithium acetate and lithium formate and at least one of manganese nitrate, manganese acetate and manganese formate. A process according to one of claims 1 to 4, characterized in that the spinel-type lithium manganese complex oxide is represented by the formula Li(Mn 2-x Li x )O 4 , where x is 0 ≤ x ≤ 0,1. The process according to one of claims 1 to 4, characterized in that the spinel-type lithium manganese complex oxide is represented by the formula Li(Mn 2-x Li x )O 4 , where x is J < x < 0,02. The process according to claim 5 or 6, characterized in that Mn is partially substituted by Cr, Ni, Fe, Co, or Mg.

Inventors: HATTORI KOJI, YAMASHITA YASUHISA, HATTORI, KOJI, YAMASHITA, YASUHISA
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1997
Main classification: H01M4
IPC codes: H01M10, C01G45, H01M4